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Decanter Centrifuge Handbook Alan Records 2001
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Decanter Centrifuge Handbook
1 st Edition
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Decanter Centrifuge Handbook
1st Edition
Alan Records Ken Sutherland
E L S E V I E R ADVANCED TECHNOLOGY
UK
USA
JAPAN
Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB. UK Elsevier Science Inc. 665 Avenue of the Americas, New York, NY 10010, USA Elsevier Science Japan, Tsunashima Building Annex, 3-20-12 Yushima, Bunkyo-ku, Tokyo 11 3, Japan
Copyright Q 2001 Elsevier Science Ltd.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers.
First edition 2001
Library of Congress Cataloging-in-Publication Data Records, Alan
Decanter centrifuge handbook / Alan Records,
Includes index. ISBN 1-8 5 6 1 7-369-0 (hardcover) 1. Centrifuges-Handbooks, manuals, etc. 2. Centrifugation-
Ken Suther1and.-1st ed. p. cm.
Handbooks, manuals, etc. I. Sutherland, Ken. 11. Title. QD54.C4 R43 2000 660' .2842-d~2 1 00-049 524
British Library Cataloguing in Publication Data A catalogue record for this title is available from the British Library.
ISBN 1 8 5 6 1 7 369 0
No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein.
Published by Elsevier Advanced Technology, The Boulevard, Langford Lane, Kidlington, Oxford OX5 l G B , UK Tel: +44(0) 1865 843000 Fax: +44(0) 1865 843971
Typeset by Variorum Publishing Ltd, Rugby
Transferred to digital printing 2005
Printed and bound by Antony Rowe Ltd, Eastboume
CONTENTS
Preface and Acknowledgements xiii
Chapter 1 Introduction 1.1 The Decanter Centrifuge
1.1.1 The basic decanter 1.1.2 Separation principle 1.1.3 Decanter applications The History of the Decanter 1.2.1 Origins 1.2.2 Machine and application development
1 .3 Decanter Manufacturers 1 .4 Present Trends 1.5 References
1.2
Chapter 2 Decanter Design 2.1 Basic Construction 2.2 Basic Components
2.2.1 Orientation 2.2.2 Flow 2.2.3 Materials of construction 2.2.4 Bowl
2.2.4.1 Front hub 2.2.4.2 Centrate weirs 2.2.4.3 Liner 2.2.4.4 Front hub bearing
2.2.5.1 Rear hub and bearings 2.2.5.2 Cake discharge 2.2.5.3 Liner
2.2.6.1 Conveyorhub 2.2.6.2 Flights 2.2.6.3 Feedzone
2.2.5 Beach
2.2.6 Conveyor
2 2 3 5 6 6 8
10 1 3 1 4
1 7 19 19 19 21 21 22 22 23 24 25 26 28 28 29 29 31 31
vi
2.2.7 2.2.8
2.2.9
2.2.10 2.2.11 2.2.12
2.2.6.4 2.2.6.5 2.2.6.6 Gearbox Frame 2.2.8.1 2.2.8.2 2.2.8.3 Casing 2.2.9.1 2.2.9.2 2.2.9.3 2.2.9.4 2.2.9.5
Floc/rinse zone Wear protection Conveyor bearings and seals
Bearing supports Feed tube Vibration isolators
Casing baffles Cake discharge Centrate discharge Casing seals Vents
Sub-frame Main drive Back-drive
33 33 34 36 37 38 38 39 40 4 1 4 1 42 42 42 4 3 43 45
2.3 Variations to Main Components 47 2.3.1
2.3.2 2.3.3 2.3.4
2.3.5
2.3.6
2.3.7 2.3.8
Orientation 47 2.3.1.1 Vertical vs. horizontal 47 2.3.1.2 Vertical decanter seals and bearings 49 2.3.1.3 Vertical decanter casing seal 51 Flow 51 Materials of construction 52 Bowl variants 54 2.3.4.1 Front hub 54 2.3.4.2 Centrate weirs 55 2.3.4.3 Liner 56 2.3.4.4 Main bearing 58 Beach 59 2.3.5.1 Rear hub 61 2.3.5.2 Cake discharge 61 2.3.5.3 Beach liner 64 Conveyor 64 2.3.6.1 Conveyor hub 66 2.3.6.2 Flights 66 2.3.6.3 Feedzone 6 7 2.3.6.4 Floc/rinse zone 69 2.3.6,s Wear protection 71 2.3.6.6 Bearings and seals 73 Gearbox 73 Frame 76 2.3.8.1 Bearing supports 76 2.3.8.2 Feed tube 76
2.3.9
2.3.10 2.3.11 2.3.12
2.3.8.3 Vibration isolators Casing 2.3.9.1 Baffles 2.3.9.2 Cake discharge 2.3.9.3 Centrate discharge 2.3.9.4 Casing seals 2.3.9.5 Vents Sub-frame Main drive Back-drive
2.4 Special Features 2.4.1
2.4.2 2.4.3 2.4.4
2.4.5 2.4.6
2.4.7
2.4.8 2.4.9 2.4.10
Basic construction 2.4.1.1 Screen-bowl decanter 2.4.1.2 Three-phase decanter 2.4.1.3 The countercurrent extractor
2.4.1.4 Decanters for temperature and
2.4.1.5 The cantilevered bowl 2.4.1.6 The “hubless” conveyor 2.4.1.7 Thickening decanter 2.4.1.8 The dual beach decanter Centripetal pump Skimmer pipe Centrate weir design 2.4.4.1 Cup dam 2.4.4.2 Notcheddam 2.4.4.3 Inflatable dam Noise suppression Bowl baffles 2.4.6.1 Cake baffledisc 2.4.6.2 Bafflecone 2.4.6.3 Floater disc 2.4.6.4 Conveying baffle 2.4.6.5 Longitudinal baffle Clarification enhancement 2.4.7.1 Quasi-axial flow 2.4.7.2 Fully axial flow 2.4.7.3 Vanes 2.4.7.4 Discs Conveyor rake Conveyor tiles Conveyor pitch 2.4.10.1 Variable pitch
decanter
pressure extremes
vii
77 77 77 78 79 79 80 80 80 82 86 86 86 86
89
90 90 90 90 92 9 3 95 96 96 96 97 97 99 99
100 101 102 103 104 104 105 105 106 107 108 109 109
viii
2.4.10.2 Reverse pitch Counterbalance and scraper flights 2.4.1 1
2.4.12 Feedzone 2.4.13 The reslurry collector 2.4.14 CIP 2.4.1 5 The Rotodiff 2.4.16 Power regeneration 2.4.1 7 2.4.18 Floating conveyor 2.4.19 Decanter controls
Dual main drive motor
2.5 References
Chapter 3 Applications 3.1 Application Classes 3.2 Application Analysis 3.3 Waste Sludge Processing
3.3.1 Industrial wastes 3.3.2 Water treatment sludges 3.3.3 Municipal sewage treatment
3.4 Energy Materials Production 3.5 Processed Fuels 3.6 Minerals Extraction and Processing 3.7 Food and Food By-products
3.7.1 3.7.2 Fish processing 3.7.3 Fruit andvegetable products 3.7.4 Other food processing
Meat and meat products processing
3.8 Beverages 3.9 The Chemicals Industry
3.9.1 Bulk inorganic chemicals 3.9.2 Bulk organic chemicals 3.9.3 Fine and household chemicals 3.9.4 Pharmaceutical and medicinal chemicals
3.10 Other Applications
Chapter 4 Decanter Theory 4.1 Basic Theories
4.1.1 Acceleration force 4.1.2 Differential 4.1.3 Conveyor torque 4.1.4 Process performance calculations
4.2 Particle Size Distribution 4.3 Clarification
4.3.1 Sigma theory
110 110 112 113 114 114 115 116 116 116 118
122 125 127 127 129 129 132 134 135 136 136 137 138 140 141 142 143 143 144 144 146
149 149 150 151 151 154 159 159
4.4 4.5 4.6 4.7
4.8 4.9
4.10 4.1 1
4.12
4.13
4.14 4.15
4.3.1.1 Usingsigma 4.3.2 Sigma enhancement 4.3.3 Flocculant requirement Classification Three-Phase Separation Thickening Conveying 4.7.1 TheBeta theory 4.7.2 Conveying on the beach 4.7.3 Dry solids conveying Conveyor Torque Dewatering and Washing 4.9.1 Solids dewatering 4.9.2 Washing 4.9.3 Solids compaction Dry Solids Operation Fluid Dynamics 4.1 1.1 Reynolds number 4.11.2 Moving layer 4.1 1.3 Cresting 4.1 1.4 Feed zone acceleration Power Consumption 4.12.1 Main motor sizing 4.12.2 Main motor acceleration Mechanical Design 4.1 3.1 Maximum bowl speed 4.1 3.2 Critical speeds 4.13.3 Liquid instability problems 4.13.4 Length/diameter ratio 4.13.5 Bearing life 4.13.6 Gearboxlife 4.13.7 Feedtube Nomenclature References
Chapter 5 Flocculation 5.1 The Principle of Flocculation 5.2 Polymer Solution Make-up
5.2.1 Dissolving solid polymers 5.2.2 Diluting dispersions 5.2.3 Final flocculant solution characteristics
5.3 Polymer Choice 5.4 Pretreatment 5.5 Admitting Flocculant to the Decanter
IX
165 166 167 168 170 173 175 175 176 177 179 180 180 181 185 186 192 192 194 194 195 196 197 198 200 200 202 203 204 204 206 206 208 213
217 220 220 221 222 225 229 230
X
5.6 Flocculant Suppliers 5.7 Low-Toxicity Polymers 5.8 Applications 5.9 Performance 5.10 References
Chapter 6 Test Work and Data 6.1 Test Equipment 6.2 Test Procedures 6.3 TestLog 6.4 SomeTest Data
6.4.1 Spent grain 6.4.2 Agricultural products 6.4.3 Lime sludge classification 6.4.4 Clay classification 6.4.5 Waste activated sludge thickening 6.4.6 Digested sludge thickening 6.4.7 Lactose washing 6.4.8 Coal tailings dewatering 6.4.9 Dry solids (DS) dewatering
Chapter 7 Calculations and Scaling 7.1 Basic Calculations 7.2 Three-Phase Calculations 7.3 Classification Calculations 7.4 Washing 7.5 The Probability Scale 7.6 7.7 7.8 Main Motor Sizing 7.9 DS Scaling
Scale-Up of Centrate Clarity Limiting Applications Simple Dewatering and Torque Scale-Up
Chapter 8 Instrumentation and Control 8.1 Decanter Plant Modules 8.2 Instrumentation
8.2.1 Flow meters 8.2.2 Solids concentration meters 8.2.3 Level probes 8.2.4 Speed probes 8.2.5 Temperature probes 8.2.6 Torque measurement 8.2.7 Timers 8.2.8 Counters 8.2.9 Electrical meters
233 235 236 237 241
245 248 2 52 255 255 258 259 261 263 265 267 269 269
284 288 291 294 298 300 302 306 308
317 319 319 319 320 321 32 1 321 321 322 322
XI
8.2.10 Bearing monitors
8.3.1 On/off devices 8.3.2 Variable output devices
8.3 Controlled Equipment
8.4 Controllers 8.5 Integrated Controller 8.6 CIP 8.7 References
Chapter 9 The Decanter Market 9.1 Market Characteristics 9.2 Market Trends 9.3 Market Size Estimates
9.3.1 Overall decanter market size 9.3.2 Regional market estimates 9.3.3 Application market estimates 9.3.4 Suppliers' market shares
Chapter 10 Suppliers' Data
Chapter 11 Glossary of Terms
322 323 323 324 325 328 3 30 331
3 34 335 336 336 337 337 338
3 3 9
3 6 3
Appendix 3 7 9
Index 41 3
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Preface and Acknowledgements
By virtue of its title, which involves the word "handbook", this book is intended, above all else, to be useful. Its aims include the explanation of the nature and methods of operation of the decanter centrifuge, and a description of the kind of performance that might be expected from a decanter.
The decanter centrifuge is a device for continuously separating particulate solids from a suspending liquid or liquids by sedimentation and decanting. As such, it is part of the general range of sedimenting, filtering and other mechanical equipment used for separation processes. A distinguished range of books exists that describes this complete spectrum of equipment, and the processes by which they operate. A previous book covers the whole range of centrifuges, both sedimenting (like the decanter) and filtering, but this is the first book to deal solely with the solid-bowl, scroll-discharge centrifuge, which is the decanter.
The book is aimed at all those for whom the decanter may be part of their studies, of their research, or of their working life. It is intended to be of value in undergraduate courses on filtration and separation, but it will also offer the practising engineer in end-user companies much that is of direct value to the daily job of designing, specifying or operating this sophisticatedly engineered, but very useful, piece of processing equipment. This handbook will find use in research establishments and equipment manufacturers ' engineering departments, as it gives guidance on basic design and operating features, some in regular use and some only recently introduced to the market.
This essentially practical text nevertheless covers the underlying theory of centrifugal sedimentation separations in some detail, which further extends its usefulness to the research or design engineer looking for new ideas.
The arrangement of the handbook follows a logical pattern: a general introduction, followed by technical descriptions of equipment features and the industrial uses of the decanter. Then comes the theory of the decanter 's design, and detailed descriptions of operational and test procedures. The book finishes with some marketing data, and descriptions of the equipment ranges of the main manufacturers.
xiv Preface and Acknowledgements
The authors (both Chartered Chemical Engineers) have a wealth of experience in the decanter business:
�9 Alan Records retired from a senior equipment application and development role with Alfa Laval, after almost a full lifetime's job involved with decanters, covering research, design, commissioning, operation and service, in a wide range of industrial applications; and
�9 Ken Sutherland, for a time Technical Manager for Sharpies, has later been heavily involved with the marketing aspects of separation equipment, including centrifuges.
The putting together of a book of this nature requires the help and co- operation of many individuals and organisations. The contributions, help, advice, work and kind permissions of those mentioned below are most gratefully acknowledged.
Lenny Shapiro and Jan Cederqvist contributed to the mechanical information, while Bert Guille assisted with the electrical content. The process data were obtained as a result of painstaking work in the field, often in far less than a salubrious environment, by numerous field engineers, our former colleagues, and in particular John Joyce, Betina Pedersen, and Keith Smith. Apologies are extended to all those not mentioned.
Denis Locke contributed to the work on many of the illustrations, professionally executed by Mike Nicklinson.
Graham Dawson, with the help of some of his former colleagues, advised on the section on flocculant technology. Keith Kernahan advised on the details of the Viscotherm equipment.
The Triton Electronic Company co-operated in providing photographs and details of their CST equipment.
The decanter centrifuge market is a highly competitive one, and thus manufacturers are, understandably, reticent in providing specific data and information on their products. Without such data and information, however, this book would be reduced in value. The authors are therefore especially grateful for the data supplied by the companies Alfa Laval. Baker Process (Bird Machine and Bird Humboldt), Broadbent. Centriquip, Centrisys, Flottweg. Gennaretti, Guinard, Hiller, Hutchison-Hayes, Noxon, Pennwalt India, Pieralisi, Siebtechnik, and Westfalia/Niro. Permission to reproduce sketches and drawings has been obtained from Alfa Laval, Bird Machine, Bird Humboldt, Broadbent, Centriquip, Centrisys, Cyclo, Flottweg, Noxon, Siebtechnik, Tomal, Viscotherm and Westfalia Separator.
Finally, gratitude is expressed to Bent Madsen and his colleagues for checking the early manuscripts. The book owes its origin to Nick Corner- Walker, then Director of Engineering with Alfa Laval, to whom the authors are indebted for the inspiration, for his personal support, and for putting the resources of a major manufacturer of decanters behind the venture. The
Preface and Acknowledgements x v
authors are very happy to acknowledge that debt here, but also to acknowledge the input from the other companies whose ideas and illustrations have been used at the appropriate parts of the text.
To these, and all of the other workers involved with the decanter for the 60 years of its effective operating history, the authors express their thanks.
Alan Records Ken Sutherland
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CHAPTER 1
Introduction
The decanter centrifuge has become a major processing tool in a wide range of liquid/solid separation applications. This handbook aims to be a thorough introduction to the design, performance and application of the decanter. It aims also to be a useful guide for the centrifuge engineer, both in equipment manufacturing companies and in the end-user companies, and their associated contractors and consultancies.
The handbook's first chapter introduces the reader to the decanter, to its history and to the manufacturing sector within which it is made. The contents of this chapter are intentionally brief, with major expansion of the topics covered in later chapters of the book.
1.1 The Decanter Centrifuge
The solid-bowl scroll-discharge centrifuge w now almost universally known as the decanter centrifuge has, indeed, become the workhorse of a wide range of liquid/solid separation activities. Its application to the dewatering of waste sludges has made it a most valuable tool in combating environmental pollution. This has made the decanter a well-known and widely appreciated piece of equipment.
1.1.1 The basic decanter
Although a complicated piece of machinery, the decanter centrifuge embodies a simple principle, that of the screw conveyor. In basic terms, the decanter comprises a solid cylindrical bowl, rotating at high speed. Inside the bowl is a scroll (screw conveyor) rotating at a slightly different speed. The differential speed between bowl and scroll provides the conveying motion to collect and remove the solids, which accumulate at the bowl wall.
A slurry of liquid and suspended solids is fed along the centre line. to some fixed position within the bowl, and is accelerated outwards to join the pond of liquid held on the bowl wall by the centrifugal force. This same force then causes the suspended solids to settle, and accumulate at the bowl wall. The clarified liquid then flows along the bowl, to leave at one end of it, over some kind of weir design, which sets the level of the liquid surface in the bowl.
The other end of the bowl is sloped inwards, towards the centre, thus providing a beach, up which the solids are conveyed, to be discharged from the bowl, at the top of the beach. Whilst the solids are conveyed up the beach, some, hopefully most, of the entrained liquid drains back into the pond, to join the liquid flow towards the far end.
The scroll usually is carried on a hollow axial hub, through which the slurry feed tube passes to the feed zone. The diameter, the number, and the pitch of the conveyor flights are chosen to match the needs of the slurry being treated as are the depth of the pond, the length of the bowl, the conveyor differential speed, and the angle of slope of the beach.
Most decanters operate with their axis horizontal, in which case they usually are mounted in substantial bearings at each end of the bowl. Vertical
Introduction 3
Liquids Feed Solids C~,m8 C~veyor Bowl Fl i# t
Dtsctmrl~ . t Zone Dischtr~
Feed
Tube
Figure 1.1. The main operating parts of a decanter centrifuge.
operation is possible, in which case the bowl is carried only on one set of bearings, at the top. If the decanter is short, then cantilevered horizontal operation is also possible, with bearings at one end only.
The rotating bowl is enclosed in a casing, which is divided to ensure that the discharged liquid (the "centrate") and solids cannot remix after separation.
The basic decanter is completed with a drive motor, usually electrical, and a gearbox, which controls the differential speed of the conveyor.
Aspects of the physical forms of the decanter in its different versions are described in Chapter 2.
1.1.2 Separation principle
The decanter operates mainly by sedimentation, a process causing the separation of suspended solids by virtue of their higher density than the liquid in which they are suspended. If the density difference is high, then gravity may provide sufficient driving force for the separation to occur in a reasonable time as is the case with large-tank clarifiers and clariflocculators, or with lamella and inclined-plate separators. If the difference in density is small, or the particle size is very small, then gravity separation would take too long, and the separation force must be augmented by the imposition of centrifugal forces many times that of gravity alone.
The centrifugal force may be imposed by virtue of the flow of the slurry, as in a hydrocyclone, or by means of mechanically driven rotation, as in the sedimenting centrifuge.
4 TheDecanterCentrifuge
There are several types of solid bowl sedimenting centrifuge, including:
�9 the tubular bowl centrifuge, mainly used for liquid/liquid separation, for which use any suspended solids would require cessation of operation for their removal (the tubular bowl centrifuge is also used for very difficult solid/liquid separations, where there is a low concentration of solids, which cannot be flocculated):
�9 the imperforate basket centrifuge, which is operated batch-wise for the removal of collected solids;
�9 the disc-stack centrifuge, originally developed for liquid/liquid sep- aration (cream from milk), but which has been improved to achieve continual solids removal (although, in most cases, not fully continuous), by a variety of devices at the outer periphery of the bowl; and
�9 the decanter.
The prime beneficial characteristic of the decanter in this spectrum of sedimentation equipment is its ability to remove separated solids from the separation zone on a fully continuous basis. It can operate, unattended, for weeks, if not months, at a time.
By comparison, therefore, with:
�9 gravity sedimentation the decanter can achieve separations that would be impossibly lengthy (or just impossible) in a clarifier or lamella separator, and it produces drier solids;
�9 h y d r o c y c l o n e s - the decanter has a much higher liquid capacity, can handle much higher slurry concentrations, and produces much drier solids:
�9 tubular bowl centrifuges the decanter offers higher capacities, the ability to handle concentrated slurries, and continuous operation:
�9 imperforate basket centrifuges the decanter operates continuously, can handle much higher solids concentrations, and produces much drier solids: and
�9 disc-stack c e n t r i f u g e s - the decanter is truly continuous in operation, can handle much higher solid concentrations in the feed slurry (although it cannot usually match the high centrifugal forces of the disc-stack designs, and so does not have the same clarification performance), and produces drier solids.
In addition to these other types of sedimentation centrifuge, the decanter competes effectively with several types of solids recovery f i l t e r - such as the plate-and-flame filter press, and the various types of band press, without
requiring the use of filter aids. The theory of the separation and dewatering behaviour of the decanter is
described in Chapter 4.
(It must be rcmcmbercd that there are many other types of industrial c:enIril'iigr, hul: t.tiese achieve sepratiori by riieans of filtration rather than sedimenl:;ii.ion -- :Ill.hoiigh the srreen-howl dwariler r:omhiries t.he two sepa r ii t.i on rn ~h a n isms . )
1.1.3 Decanter applications
Thc dccantcr cciitrifugc can be used for most types of liquidjstrlid separ;ition, and its ability to handle a wide rarige of feed slurry L.r,rir:entrol.iorrs odds to its general versatility.
11 C ~ I I be 11sed Tor t . 1 ~ clussificntian of' solids in Liquid suspc.nslon. where a single CUI. is required tielween IWC) sixes ofsolid particle (or, less often. bctweeii solids of differing density). I1 i s i i very gcind device for this purpose, and its early history inclnded developme,nt for thc kaoliii (china clay) iiidustry.
The drt:ant.er can be used for the clnrjficntiori of a liquid. it can be operated so as lo give a high degree of clarification, although it is not usually used to clocify o slurry that contains only a small amnunt. nf d i d s in suspensiun,
It (::in also be iwd in thc recovrr~j d a valuable scilid irom i t s suspcnsioii in a liquid, :ind li!Llowing such rcrovery it is capable o f \.wr.shing t.he recovcrcd solid free of the original mother liquor, and of drliqimrin[j ( d w n t e r i i i g ) the wrls1it.d solids to a high dcgree of dryri
Whcrc thc slurry is a waste needing treatment prior to sale dispos;iI, t h c decanter again can dewat.er such slurries to a high luvcl of ilryriess.
Finally the decariler c:in be opcratcd so as to act as il i h i r k m v - , producing a clcar liquid and i-l m o r e concentratcd slurry either in a manufacturing proccss, or i t 1 wsstc treatment.
This wide mnge of'potcntial uses. coupled with its continuous operalion, its ability tn accept a wide range of feed concentrations. and its arailahility in a wide raiigc o l fred capacities, t.<rgethc-r explain w h y the d ~ ~ i i ~ i t w centrifuge has become such a valuable pror:essing tool.
The main proccss applioal.ions of thc dccanter art. described in Uhapler '1. showirig what separation acliori i n required. while Iht: sper-itic design k3turt.s ol'the decanter t o suit thrse applications art. described in cach of the sections of' t h is Chap t cI,
1.2 The History of the Decanter . . .
The effective history of the decanter centrifiige occupies the second half of' the twentieth century, although it originated at the st;irt of that century. 'I'he tubular bowl and the disc centrifuges had. by then. been in usc for some time. and thcrc was. clcarly, soiiic inccntivc to producc a scntrifugc which would enablc the continuous rcmoval of solids which accumulated during t,he proc:essiriy o f ij liquitl/liquid mixture. The fdlc.iwirig discussion descrihes how that devel(Ipmerit. was achiwed, arid h o w h e modern decnnt,er evolved.
1.2.1 Origins
The first. patent rlrscrihirrg a d c v i w like the decanter was grantcd to Liedheck. a Swedish inventor (wilh r i o corporate assignee). in 1902 [ I 1, This was il vcrtically mounted device, wit,h ils tlrivc r~iolor situated below i t . and with the hrach section at thc top, discharging into ;in open s p x c in t.tit. casing. which also contained ths feed tank. Its rrsemblant:e t.o ;i veriiual basket oentrifugc is sLrong. but it is still astonishing to see Ihat 1,iedbeck's design. which was intended t o separate solids from two distinct liquids, contains all ill the main features of ij modern three-yhasc dccantcr. (An e;irlier paienl . by Stewart. working in ihtr sugar ir'idustry, looks similar to a decanter. in that it had a corivcyor i i i two parts, but is actually a filtering ccntril'uxe. wilh t h ~ conveyor scrolling solids across it screen.)
'I'his first dccanter patent iIppt!;ired a1 a time whcn Gustaf de 1 , ~ i w I ~ t.he inverilor or the continuous crc'mi separator, was active in dcvcloping this system. De 1,aval's first patent for a continuous ceriirifuuge. in thc form of ;I
roughly spherical bowl. was granted in 18 7 X [ 21. The key patent covering the disc stack was griirlted t.o vnn Btchtolshcim in 1888 3 ] + and was immediately bought by dc Laval's c:ompai.iy, which he joined. Quite soon i n this developrriciit i t was realised that a build up of solids in the howl wuuld be a serious disadvantage. and a number of ideas wcre developed to prrrilit thcir rcmoval. One such idea featured a slowly moving helical scraper (slowly. Lhat is, in relation 1.0 the bowl). mounted outside the disc stack -- but ihis design u7as dropped, because. rrf high mcchanical wear.
Introduction 7
Similar problems affected any solid bowl centrifuge (imperforate basket or tubular bowl), and it would seem that the decanter centrifuge designs available at the turn of the century would have been the answer to this problem. That these designs were not developed with commercial advantage may have been due to cost, and to the fact that the number of commercial applications for such a device was still quite small cream separation dominated the applications for continuous centrifuges, and this could manage without solid removal, except at a shut-down for cleaning.
Liedbeck's design was, admittedly, quite complex for the early 1900s. It featured a vertically mounted rotating assembly, with a cylindrical lower section, and a conical top part, the solids discharging over the edge of the beach, into a collector, and the two liquids leaving separately from the bottom. The drive mechanism was mounted directly below the rotating assembly, and was directly coupled to it, with a gearbox giving the differential speed to the conveyor screw. The drive shaft for the conveyor ran through that for the bowl, and the feed slurry entered the bowl at its bottom end. The drawing strongly resembles a vertical basket centrifuge, with the conveyor and beach added.
There was also, at that time, quite a lot of interest in the use of centrifuges capable of handling sludge for the dewatering of starch, and the German companies Uhland and Jahn made centrifuges that embodied the decanter principle for this purpose. Apart, however, from a slight resurgence of interest in starch processing in the 1920s, the decanter effectively vanished for 40 years.
It re-emerged at the end of the 193()s in the patent literature: Pecker (application February 1938, granted May 1942) [4], showing a conical decanter, with feed at the base of the cone. Then not again until the mid- 1940s: Ritsch, assigned to the Process Development Company (application September 1945, granted November 1950) [ 5 ], showing a conical bowl and a more sharply conical beach section, and intended to separate two solids, one settling and one floating, from the suspending liquid.
Meanwhile, the Sharpies Corporation was patenting basket and tubular bowl centrifuge developments, and the Bird Machine Company had not yet been formed. So the relatively sudden appearance on the market of conical bowl decanters by both companies in 1945/1946 , is, in retrospect, quite surprising. The Sharpies P-IO00 decanter had quite a rapid impact on the whale and fish oil market, so much so that AB Separator (not yet called Alfa Laval) was forced to copy the Sharpies machine, or lose a good market in the Norwegian fishing industry.
Bird Machine applied for a patent early in 1946 on kaolin production [ 6]. in which decanters were included merely as items in a flowsheet, as processing tools not otherwise described, while a patent granted in 194 7, but which had been filed in 1940 [7], showed a similar use in the cement industry, for classification by size. A December 1949 application by Milliken and Topping (also for Bird Machine) shows a three-section bowl, ready for solid washing on
8 TheHistoryoftheDecanter
a screen section [8]. The combination of the slow but sure economic recovery after World War
II, and the evidence of successful operations by Bird and Sharpies, brought a rush of competitors to the market. As well as Alfa Laval in Sweden, these included Kl6ckner, Krupp and Rahmesohl & Schmidt (Westfalia) in Germany, International Combustion in the UK, and Dorr Oliver in the USA.
1.2.2 Machine and application development
In the ensuing 55 years, since that first really commercial entry into the marketplace by Bird and Sharpies, rapid developments in both machine design and application technology have occurred. These developments were largely market-led, with demands from end users quickly converted into machine improvements. The main manufacturers had strong technical development and engineering groups, responsible for keeping their products both in line with market needs, and at least level with, if not ahead of, their competitors.
In relative terms, the greater changes occurred in the first 2 5 years of this period, up to 1970. This period saw the decanter expand into more than 100 applications within food and by-products processing, mining, energy materials and systems, chemical, petrochemical and pharmaceutical industries, and environmental engineering. The major manufacturers produced decanters suitable for a wide range of these applications, but there also grew smaller companies, specialising in just one industry, such as olive oil or starch.
Basic decanter performance (in terms of solids recovery, centrate quality and solids dryness) remained relatively little changed in this period, but improving engineering and materials of construction enabled the use of longer bowls to give greater feed capacity.
Of the major design variants (all described in Chapter 2), the screen-bowl design was developed by Bird in the mid-1940s. Sharpies produced the first vertical, pressurisable decanter in 1958, and three-phase operation came along in the early 1960s.
The main gearbox choices were all in place before 1970, while the potentially wearing surfaces of the conveyor could be protected by a range of treatments or materials, including the range of tungsten carbide tiles developed by Sharpies (tiles being small flat pieces of hard material fixed to the scrolling face of the conveyor flight).
As a separating device, dependent largely upon a combination of particle size and density to achieve good separation of solids from liquids, the decanter can show much better performance if the particles can be agglomerated before or during separation. The appearance onto the market of polymeric coagulating agents at the end of the 1960s quite revolutionised the use of the decanter for waste sludge dewatering, and offered great improvements in performance in other applications as well.
Introduction 9
In the subsequent 30 years, relatively few "new" applications have been developed, with the difficulty of achieving fully sanitary operation keeping the decanter from some of the food and biochemical operations. However, major changes have occurred in the refinement of machine design, and, therefore, performance.
These changes have been most apparent in the way in which the decanter handles the discharging solids, and especially in the achievement of the maximum dryness (least moisture content) in those solids. Thus there are restrictions available at the base of the conical section (such as Sharpies' "Centri-Seal" patent by Lee), operations with deep ponds, and improved control over conveyor speed and torque, leading to the "Dry Solids" decanter.
The availability of stronger stainless steels has enabled the production of very long bowls, as well as quite large diameter bowls. Very high rotational speeds are now available, giving separational forces of up to 10 000 times the force of gravity on smaller models.
Mechanical improvements include the ability to profile the protective tiles on the conveyor face, to use better bearings, and to use three-stage planetary gearboxes. Improved machine driving mechanisms include inverter drives, and the use of back-drives with power regeneration.
Alternative means for liquid discharge have been developed, largely from other centrifuge types, such as centripetal and skimmer pumps. Decanters have been fitted with discs to improve clarification performance, and sanitary performance has been improved by the development of clean-in-place (CIP) methods.
A very important system development has been the improvement in control methods, enabling the decanter to react automatically and quickly to changing feed conditions.
1.3 Decanter Manufacturers
By the mid-1940s, only two companies were working on decanters Bird Machine and Sharpies. In the ensuing 55 years, the number of manufacturers has increased many-fold, through a peak in numbers, with a decreasing number at the turn of another century. The 1990s trend in mergers and acquisitions, extending into the new century, has meant that, of the eight leading manufacturers mentioned at the end of Section 1.2.1, not one remains as an independent company, even if they exist at all.
The decanter centrifuge is simple in concept, but complicated in practice. It is therefore expensive to make, and has relatively low profit margins. It follows that a flesh entry into the marketplace needs strong corporate support, coupled with good engineering, a willingness to invest in a strong process engineering department, and possibly a niche market to target. One way to enter the business has been to establish working relationships with an existing manufacturer, as, for example, Broadbent did with Bird, and, later, with Tanabe, or as Tomoe did with Sharpies and then Alfa Laval.
That so many companies have entered the decanter business is a sign of the importance of the decanter to the process industries. It has not proved to be an easy manufacturing sector to stay in: at the time of writing three of the world's leading decanter manufacturers are for sale.
The list of significant former manufacturers of decanters is quite long, including:
�9 Comi Condor, Italy, recently stopped making decanters (but still makes other types of centrifuges).
�9 Dorr Oliver, USA, acquired first by Krauss Maffei, then by a Canadian investment company who sold the centrifuge interests to Alfa Laval, but the decanter range had already been dropped.
�9 International Combustion, UK, acquired by Rolls Royce and stopped making centrifuges altogether.
�9 Robatel, France, acquired by Rousselet, who decided not to continue the decanter range (but still make other types of centrifuges).
�9 Kl6ckner, Germany, K16ckner's KHD subsidiary, including Humboldt decanters, acquired by Baker Process/Bird Machine.
Introduction 11
Krupp, Germany, stopped making decanters many years ago. Sharpies, USA, acquired by Pennsalt /Pennwalt , then sold to Alfa Laval, and the decanter ranges merged.
In the late 1960s a small ripple in the decanter world was caused by a Danish engineer, Kruger, who developed the Total decanter, with good performance in waste sludge treatment. This was acquired by Niro, a spin-off from the Danish sugar company, but the Niro decanter has disappeared, following the company's acquisition by GEA, which merged the decanter business with Westfalia.
Other well known companies have been acquired over the years, but have kept their identity, as illustrated in the list below.
Despite these changes, the decanter manufacturing sector is still a large one. The following list gives the manufacturers believed to be operating in the year 2000, by country of ownership, with parent companies where these are known.
Denmark FFG Separation
France Guinard Centrifugation, owned by Andritz group (Austrian)
Germany Flottweg, owned by Krauss Maffei, which has been sold as Atecs by
Mannesmann to a consortium of Siemens and Bosch Hiller MAF SAMAG (Sangerhausen) Siebtechnik Westfalia, owned by GEA, itself owned by Metallgesellschaft
Greece Centrifugal Environment
India Pennwalt India
Italy Amenduni Cornello Gennaretti Officine Mecaniche Toscane (Athena) Pieralisi Group Rapanelli
lapan IHI Ishikawa-Harima Heavy Industries Kokusan Seiko Kotobuki Techrex
12 Decanter Manufacturers
Mitsubishi Kakoki Kaisha Sumitomo Tanabe Tomoe
S w e d e n Alfa Laval Separation, owned by Industri Kapital (Swedish) and Tetra
Laval (Swiss) Noxon, owned by Waterlink (USA)
S w i t z e r l a n d Chematec
UK Broadbent Centriquip
USA Bird (including Humboldt). owned by Baker Process Centrisys DMI Decanter Machine lnc Hutchison-Hayes
Of these 30 or so manufacturers, over 90% of the market is covered by just eight: Alfa Laval, Bird/Humboldt, Flottweg, Guinard, IHI. Pieralisi, Tomoe and Westfalia. Each of these market leaders has been in the business long enough to have developed a full range of decanters, making them capable of selling to the whole market. It is only among the smaller companies that any degree of market specialisation is seen, and that is mostly for olive oil processing.
The decanter manufacturers are described, together with the key features of their range of decanters, in Chapter 10.
1.4 Present Trends
The key features of decanter development described as occurring during the last 30 years are by no means fully developed. In capacity and size terms, there will still be larger, longer and faster machines to come. There is much still to be done in the development of control systems for the decanter, leading to complete automat ion of decanter processes, with associated telemetering.
As new production processes develop, in parallel with the derivation of new products, then the decanter will be adapted to keep pace with such changes. Oil production and refining will continue to be a challenge to the decanter manufacturer , especially as production moves into less hospitable zones. There is a wealth of food, protein, biochemical and pharmaceutical applications awaiting the efficient clean-in-place process for decanters, while the increasing demands on municipal and industrial waste t reatment will also add to the application range. If nuclear power returns to favour, then here also the decanter will have a part to play, especially once it is fully automated. The need to be able to process low-grade metal ores will also need help from the decanter.
1.5 References
1. A Liedbeck. Centrifugal apparatus. US Patent 750668, 27 October 1903 (patented in Sweden in 1902)
2. G de Laval. (AB Separator) Centrifugal creamer. US Patent 247804, 4 October 1881 (patented in Sweden in 1878)
3. C yon Bechtolsheim. (AB Separator) Centrifugal liquid separator. US Patent 432719, 12 May 1890 (patented in France in 1888)
4. J S Pecker. Centrifugal machine. US Patent 2283457, 19 February 1938 5. H P Ritsch. (Process Development Co) Centrifugal separator. US Patent
2528974, 19 September 1945 6. S C Lyons. (Bird Machine) Improving kaolin and products thereof. US
Patent 2524816~ 21 February 1946 7. F A Downes. (Bird Machine) Cement manufacture. US Patent 2424746,
25 September 1940 8. G A Milliken, K E Topping. (Bird Machine) Centrifugal separator. US
Patent 2600372, 16 December 1949
CHAPTER 2
Decanter Design The decanter centrifuge is, in principle, a relatively simple device, though far from simple to manufacture , being a rotat ing d rum with a screw conveyor in it; clarified liquid decants out of one end while dewatered solids are scrolled out of the other. The prime virtue of the decanter is its ability to remove quite high levels of suspended solids from a liquid, with a reasonably low level of retained liquids in the separated solids. The decanter can handle slurries containing solids occupying 100% of the volume of the slurry. So long as the slurry is pumpable, the decanter will handle it. The moisture content of many of the dewatered cakes from decanters is such tha t the cakes can be stacked without much seepage of moisture from the stack. Some cakes are free flowing and friable, while a few are pasty or clay-like.
This apparent simplicity of the decanter is, however, complicated by a very wide range of design variants. It is the aim of this chapter to describe both the basic operating elements of the decanter, and the range of design variants and special features (concentrat ing on those variat ions that affect operating performance).
16 Decanter Design
Gearbox Guard \
Conveyor Assembly Bowl Assembly \ Upper Casing
Gearbox .~... Assembly
Front ~ " Main Bearing Assembly
Torque Arm / (in place of Brake m upper illustration)
Lower Casing
Belt Guard (Upperl
Rear Mare Bearing Assembly
Feed Tube
~ . . ~ Drive Belts
Belt Guard (Lower)
Sub Frame
Torque Control Centrifuge Frame Main Drive Assembly
Fluid Coupling (when fitted)
Figure 2.1. A basic decanter centrifuge ( By courtesy of Alfa Laval ).
2.1 Basic Construction
The construct ion of a basic centrifuge with all its main components is shown in Figure 2.1. The heart of the decanter is the rotating assembly, which comprises a mainly cylindrical bowl housing an Archimedian screw conveyor, with a small clearance between it and the bowl. One end of the bowl is conical in shape, providing the means whereby solids can be removed from it. Affixed to one end of the bowl is usually a gearbox to effect a small differential speed between the conveyor and bowl.
The rotat ing assembly, usually horizontal, is supported by a bearing in a pillow block at either end of the assembly. Surrounding the bowl is a casing to collect at one end the clarified liquor and dewatered cake at the other. The bearing pillow blocks are mounted accurately on a rigid frame together with the casing.
Sometimes the frame is mounted on a sub-frame together with the drive motor, and where necessary a back-drive system, to control the gearbox pinion shaft, which will in turn control the conveyor-to-bowl differential speed. The back-drive system will be described later, but for the present it suffices to say that it is essentially a braking motor or similar device coupled to the gearbox pinion shaft. The main motor is offset from the bowl and drives the bowl by means of a set of V-belts. The back-drive can also be offset, in which case it would be connected with a timing belt. The timing belt is to facilitate more accurate speed control. However the back-drive system can also be mounted direct in line with the pinion of the gearbox.
The sub-frame assembly, or the main frame if there is not a sub-frame, usually is supported by vibration isolators.
The feed enters the bowl through ports in the conveyor, having entered the conveyor hub through a stat ionary coaxial feed tube projecting into the conveyor from a support, mounted on the main frame.
Feed slurry is metered through the feed tube into the rotat ing bowl. Suspended solids sediment to the bowl wall, where they are picked up by the conveyor and scrolled as a saturated cake to the conical end of the bowl, over the heel of cake which builds up in the small clearance between the bowl and conveyor. The resulting clarified liquor flows to the opposite end of the bowl and decants over weirs into the casing for discharge. The cake scrolls up the
18 Basic Construction
conical section of the bowl, the beach, before it falls into the casing for discharge.
The heel, the thin layer of process solids which builds up between bowl and conveyor, can progressively consolidate with coarser particles bedding themselves into it. This, while providing an aid to scrolling efficiency, can be an unwan ted source of abrasion for the conveyor. However, generally, there is a tendency for the heel to move, albeit at a much lower rate than the cake itself. Thus there is a tendency for the heel slowly, but continuously, to regenerate itself.
Materials of construction are impor tant considerations in the basic design. Most decanters are constructed with the parts in contact with the process in some form of stainless steel. Although some manufacturers successfully use carbon steel, others have not been as fortunate, due to severe corrosion and associated problems.
2.2 Basic Components
Many of the basic components of the decanter have been introduced in describing the construct ion of the decanter. These need to be described in more detail. The four major component assemblies are the rotat ing assembly, the flame and casing together, and the drive and back-drive assemblies.
The rotat ing assembly includes the bowl, beach, conveyor and gearbox. It is the most important (and expensive) part of the decanter, where all the work is done, and which contains the most sophisticated technology, both process and mechanical . For such a heavy component , weighing up to several tons and producing a force field of several thousand g, a high level of precision engineering is required, followed by precise balancing.
Bearings and seals used in the rotat ing assembly and gearbox are an important part of the decanter. Bearings in general have to be lubricated to work properly. To do this, seals separate the lubricated bearings from the process environment , both to protect the bearings and to avoid contaminat ion of the product or environment , by the lubricant. Seals are also needed to contain process liquids and vapours within the centrifuge casing. Seals are especially important where the process requires a positive pressure or vacuum, and where vapours are flammable or toxic.
2.2.1 Orientation
The rotational axis of the decanter can be horizontal or vertical. The vertical designs are most frequently used for special applications and are described in Section 2.3.1. Thus the horizontal design will be taken from here on as the basic design.
2.2.2 Flow
The flow of clarified liquor and cake in the decanter can be either co-current or countercurrent . In the co-current design, both solids and liquid travel in the same direction, axially, in the separating zone, with the clarified liquid diverting to the opposite end to the solids discharge through off-take channels. With the countercurrent design, solids and liquid travel in opposite directions,
20 Basic Components
axially, in the separating zone, and discharge at opposite ends. Both designs have strong proponents and arguments. For the moment, countercurrent flow is assumed, and co-current flow is discussed further in Section 2 .3 .2 .
Conventionally the front end of the decanter is the liquid discharge end and the solids end is referred to as the rear. The solids discharge is more usually referred to as the cake. While defining flow and positional conventions, it is worth mentioning that later in the book when discussing the interior of the bowl, terms such as "up", "over", and "bottom" for instance will be used. These terms relate to the centrifugal field, and thus "bottom" refers to the bowl wall, "up" and "over" mean towards the bowl axis.
- - - , ~ - a a 6 e t o o 6 0 e o o e e o q P o q ~ e o 0 0 o o o o o o 0 o 0 o o 0 o e o e 0 o o o
| ~ _ _ . ~a .aL I . I imAjp .OOOOOOOOOOOOlPtuu~ i ~ - " ~ L t e _ o o e e e ~ _ | " ~ , a u O O O 0 O O O - g ' q l , ~
_. �9 , ~ . e e o e o o e l ~ e , j , ~ " ' �9 =,~ �9 o o o e e o ' l P ~
" ~ p o o o o o 4
In / ...... ~ " i t . . . . . ; . . . . . . . . . . . m m m o m m m m m m
. . . . . . . . . . . . . . . . i _ ~ . . . . . . . . . . ,,,u, I Boo
. . . . . . . , 1 g u r t o o o o o . , A ~ - . - - , o o �9 �9 o o o �9 �9 . . - �9 �9 J h w - m m m __....-----.,,-,,,,,,,,,,,,,.,,,.,,,--,-,,,--,,----
Centrate Cake
Figure 2.2. Countercurrent flow.
F e e d
F e e d n
. . . . . . . . . . : : : ; ~ . : : : : : : : :::..~... ----'-.---4. dm~ o
- !
oooooo " ~ o o o o
" - 4 J o
C e n t r a t e Cake
Figure 2.3. Co-current flow.
DecanterDesign 21
2.2.3 Materials of construction
Materials of construct ion of the decanter are many and varied. It is more usual to make the contact parts, particularly in the rotating assembly, of some form of stainless steel. This is to avoid assembly problems and misal ignment due to corrosion on mating surfaces. This has to be avoided with high speed rotating equipment. Nevertheless, it must be said that there are many decanters in operation with bowls of carbon steel, where their manufacturer claims to be able to overcome , or avoid, corrosion. For stationary contact components there is no need for a high grade of stainless steel. When the process used is non-arduous, simple neoprene seals and gaskets will suffice. Supporting framework will be in ordinary or even cast steel. Materials of construct ion for the decanter are discussed in more detail in Section 2.3.3.
2.2.4 Bowl
The bowl in a modern decan ter is a cylindrical tube with a flange at either end, on which are bolted at one end the liquid discharge bowl hub. and, on the other end. the cake discharge hub, or the beach followed by the cake discharge hub. The first cylindrical bowls used a filler piece in the end of the bowl to form the beach. On modern bowls, particularly the larger ones, the beach is bolted to a flange at one end of the cylindrical section, al though with some overlap to provide mechanical location.
The thickness of the bowl wall is dictated by the material of construction used, the maximum speed at which the bowl will be rotated, and the maximum weight of process material, feed, centrate or cake, likely to be held in the bowl. Thus the density of the process materials in use can have a major effect on the safe working speed of the bowl.
I .
/ Front Hub Bowl Shell
i
Beach Rear Hub
Fignre 2.4. Basic bowl assembly.
22 Basic Components
The inside surface of the bowl can be plain machined. However, some effort is often made to encourage cake to stick to the bowl, to aid scrolling instead of slipping round with the conveyor. The means of doing this could be by knurl ing the inside of the bowl for instance. This can wear smooth relatively quickly. More often longitudinal ribs are welded, or a liner with similar ribs is fitted (see Section 2.2.4.3).
At each end of the bowl the outside bowl diameter can be increased to provide, if necessary, excess metal for removal during balancing. In particular, it can provide a position for machin ing grooves, which will mate with corresponding baffles in the casing. Together, the grooves and baffles form labyrinths to counteract cross-contaminat ion of the products discharging at either end of the casing.
2.2.4.1 Front hub
The front hub (the liquid discharge hub) bolts to one end of the bowl. It has an inner spindle to locate the conveyor, its bearing and seals, and an outer spindle for the fitting of the front main bearing and pillow block. Seals will also be fitted to the outer spindle as required. The discharging liquid is commonly known as the centrate.
Centrate Discharge Ports
Inner Spindle
Outer Spindle
Figure 2.5. A decanter fl'ont hub.
2.2.4.2 Centrate weirs
In a basic decanter the centra te discharges from the front hub over weirs. These weirs, sometimes called dam plates, cause a pond to form in the bowl. The level of liquid in the bowl, the distance from the bowl wall to the inner
Decanter Design 23
edge of the weir, is known as the pond height. The simplest form of weir is a rec tangular plate with slotted holes bolted to the outside face of the front hub. The pond height is adjusted by loosening the bolts, repositioning the plate and then re-securing the bolts (see Figure 2.6). Accurate location of the weir plates is necessary to enable adjus tment of the pond level to within, say, 1 mm or better. This has necessitated the development of better designs (see Section 2.3.4.2).
For best process control, the weir width needs to be maximised to reduce the level of cresting over the weir. The crest is the extra level of liquid above the weir inner edge, necessary to effect flow, as seen over weirs in rivers. This cresting varies with feed rate, but will be an inverse function of the weir length. Thus the larger this is, the smaller is the variat ion due to feed rate, or more properly, centrate flow changes.
Front Hub Weir Plate
l s
I #
!
!
Fixing Bolts
Adjustment Slot
/
Figure 2.6. A simple centrate weir.
2.2.4.3 Liner
A liner is a metal sheet rolled to spring into the shape of the inside of the bowl. On the inside surface of the sheet will be welded longitudinal strips. The liner is to combat erosion, but more part icularly to form a key for the settled cake to improve scrolling efficiency. The liner will be held in position in the bowl by tack, or spot, welds. On the smaller sizes of centrifuge the liner will be full length. On larger machines it can be full length, but sometimes it will cover only a partial length of the bowl from the beach junction forward to a little way past the feed zone.
The diameter of the conveyor and the profile of the larger end of the beach need to be adjusted to accommodate the liner. Thus the use of a liner should be decided before the centrifuge is built. Fitting a bowl liner is not an easy thing to do as an afterthought.
24 Basic Components
Bowl Shell Bowl Liner
Figure 2.7. A bo~vl shell with liner.
2.2.4.4 Front hub bearing
One of the two pillow block bearings is fitted to the front hub. It is supported in a housing and sealed with a non-contacting flinger, wind back and labyrinth parts on each side. The housing is accurately mounted to the main flame and aligned with the bearing housing at the opposite end of the rotor.
The bearing shown in Figure 2.9 is chosen to result in a long Lloh life (above 1 0 0 0 0 0 hours) at its speed and load conditions (see Section 4.13.4 for definition of Linch). Lubrication is usually by oil, static, circulating or mist. Circulating oil, while usually the most expensive, is the best and most reliable. Most actual bearing failures are due to lubrication failure or foreign contamination, not load. A circulating system flushes out contaminants and introduces only cooled, filtered oil to the bearing. The oil drains from each housing must be large enough to discharge the oil quickly, after it passes through the bearing.
Grubscrew Screw- Roller Screw- Holder- Seal O-ring- Screw ,-.,. - , \ End Plate Bearing Seal Holder Seal Disc Casing Seal t-unger L;over ~~1 i ] ~ - / \ ] ~ End Cover
Co ~ Outer Flinger glinger- End Plate- Pillow Block End Plate- Spacer- Seal Screw -
Bearing Bearing Bearing Seal Disc End Cover
Figure 2.8. Components of a main bearing assembl!t.
Decanter Design 25
Housing Fliiger
Figure 2.9. An alternative main bearing assembly.
Smaller decanters are often grease lubricated to reduce cost. The bearings of smaller decanters are often cylindrical roller, or ball type.
Bearing housing seals must have sufficient axial clearance to permit thermal expansion of the rotor, and at least one bearing must float axially.
The seal between the casing and front hub is usually a close clearance bushing. The space between the bearing housing and the casing is best vented to ensure separation of the oil and the process liquid. Most leakage is from outside air entering the casing, due to the slight vacuum produced by the rotat ing front hub.
If a positive seal is required, both axial mechanical and radial mechanical seals are used. A radial seal, which uses two split, floating, carbon rings with a gas buffer riding on a tungsten carbide coated runner, is an example of an advanced design, permitting axial movement and both high- and low- temperature operation.
2.2.5 Beach
The beach is the conical section at the end of the bowl, and is considered a part of the bowl assembly. The front hub and the beach together enable a pool of liquor to be held in the bowl. Being a component in contact with the process liquor, the beach will be fabricated in the same material as the bowl.
The beach will be flanged and bolted to the end of the bowl or inserted into the end of the bowl as a filler piece.
To the rearmost end of the beach is fitted the bowl's rear hub. There are a number of possible configurations involving these two components, to
facilitate the oaku discharge. 'l'lie discharge holes car1 be rourid, slotted or spccially shaped. 'I'lwse holes. i r ports. are genurally in the heach. but car1 hp i i l thc rear hub , or t J O t h .
The hiilf'inrluded iinglr ol' t.he cone shape o l ' th r heach i s commonly known ;is tht: hwch m g l e . A different beach iingle, o r ;i combination o"Igles in a compound beach, cvuld be selected lo t'acilitate better dryness, better washing. or pcrhaps casicr scrulling. dcpcnding upon thc prnrcss application. A brach angle of 8 to 10 dcgrccs is a common valuc choscn for many prnccsscs. 'I'he hcach is usually ribbcd or grooved to assist in cnnvcying thc solids. Alternative designs are dr:st:rihed i r i Sectiorl 2 . 3 . 5 .
2.2..5:1 R r a r huh and hearings
Thc rear hub bolts to the beach. The rear h u h m;iin hearings arid seals ;1rr similar to tha t ofthc front h u b (SCC Section 2.2.4 .)I), The rear h u h si1pport.s one end ot t he cnnveyor wllh ball. cylindrical rollcr or sphcrical roller bearings. A hall or roller hearing also resists the axial thrust ofthc convcyor. away from the beach. due 111 the solids conveying torque. All of the conveynr hearings are grease lubrici~ted and sealed. usually with elastomeric lip scals. This is possible sincc theseal rubbing veloci ly ir; low. due to thc low diffcrciitial spccd bctwccn the convcyor and howl, Care is required to ensure that centrifugal forcc docs
Decanter Design 27
Rear Bowl Hub
_ Q , - , . . a ,
i
J I i
_ _ _ _ . . . . . . . . . . . . 3
-1
Figure 2.11. A rear bowl hub.
Conveyor Rear Hub
Ill It Ill
Conveyor Thrust Beanng
i
i , , , ,
21 11 i "-q i
. . . . . . . .
Rear Bear
Rear Seals
Thrust Bearing Seals
,. ,. ~,,-, -, , - : ,4 ;2 ; , ; . . . . ' ; ~ , . . . . . . . .
I l l l l - - - " - " " - J
Figure 2.12. An alternative rear bo~,l hub and bearing assembly.
not separate the rubbing seal member from the contact surface. Larger decanters usually use a tension bar, to transfer the axial load to a bear- ing located in the driven pulley. This prevents the axial load produced by scrolling torque from being imposed on the bowl shell bolts. Where the axial load is resisted by bearings in the opposite end of the rotor, this load
28 Basic Components
must be added to the internal liquid and solids pressure load contained by the bowl shell bolts.
All well-designed decanters permit the re-greasing of the conveyor bearings without the requirement to disassemble the casing.
2.2.5.2 Cake discharge
The cake discharges at the rear of the beach, between the beach proper and the rear hub. In its simplest form, the cake discharge will be a series of radial holes around the beach end. These holes will usually be lined with some form of erosion protection, quite often in the form of a sintered tungsten carbide cylinder in a steel holder.
For process reasons it is important to have a defined cake discharge diameter. This is the diameter of the inner edge of the beach (radially, outer axially), over which the solids decant into the casing. Thus, prior to the discharge ports will be a ring or ledge providing a definite discharge level.
2.2.5.3 Liner
The beach surface is the most difficult section over which the cake has to be scrolled, being an incline in a field of a few thousand g. It is therefore common to provide a scrolling aid in the form of grooves or ribs. orientated axially. The grooves would be machined into the beach surface, whereas the ribs would be welded on or form a part of an inserted liner.
Figure 2.1 _3. Beach ribs and cake discharge apertures.
2.2.6 Cunveyor
The c(lIiveyor (or scroll) is in the form of'iiv Archirriedian screw. fitting insidc i,he beach and bowl bctwcen the two end hubs. with a small clearance of less than 2 mm radially. It has a iiurnbcr of functions. Not only does il. c:rlnvfiy I.he sol.ids, a f k r they form a cake, along thc cyliudrical bowl section and up t h e beitch, i f . also accepts the feed and ac.celeratcs it up to bowl speed. In its simplest form, t.he conveyor has a cyliiidrical central h u b with a sct of
flights welded onto it., t,u I'orrn one r:cmtiriuous helix. l'he conveyor bearings and associated seals are housed in bot.h ends ol'i1.s cenlral hub. Somewhere in between the bearings will be a charnbcr c,allcd the feed zone. sealed and isolated frotii both bearings,
In some applications, whcrc tlic solid partidcs are too fine to separate on l.heir own, it is rwcessary to use a flocculating aid. The flocculant can be added upslrearn of the decanter, but there are inany circumstances where, for best efficiency. it is adiiiitted in the bowl. On these occasions there will be an extra chamhw, t.he "flnc znne", huil t into the huh of the conveyor. M!herr nect.ssary this 11oc chamber car1 he used ;is i j rinse chiimber insl.e;id, t.o admit rinse liquor onto the scrolling cake.
2.2.6.1 Uorrveyor hub
This p x t cil'the conveyor is a substantial tubular st.ccl constructinn. 1 1 nay bc tapered at thc bcach cnd. It could. if necessary, he erilargerl in diameter a t ei1l-h end to take thc coiivtcyor bearings.
In each end of thc cotlvcynr h u l ~ wi l l he the conveyor bearings with their assnciatcd scals. A d j a m i t hi one olthe bearings (in thc basic dcsigii it will hc
30 Basic Components
Feed Port Rear Conveyor E, earlng Housing
Figure 2.1 5. A conveyor hub.
the front bearing) will be some form ofbushing. This bushing could be splined, keyed or specially shaped, e.g. lobed, to mate with the gearbox shaft, and so provide the conveyor drive.
The feed zone will be built into the hub to discharge at the start of the cylindrical section of the bowl adjacent to the beach. Next to the feed zone, a second chamber for flocculant or rinse may be fabricated within the hub. A buffer chamber between the feed and additive chamber will sometimes be built, with simple exit ports into the pond. By putting distance between the feed and the additive chamber bv use of the buffer chamber, there is less chance of the additive chamber being contaminated by feed material.
The natural vibrational frequency of the conveyor can be a limiting feature controlling the maximum speed of the centrifuge. This becomes especially critical when the L/D (length to diameter) ratio reaches 4.0 and more, and modern decanters are getting longer in order to give higher separating capacity. If the hub diameter gets smaller, the conveyor flexibility increases, thus lowering the natural frequency. Increasing the hub diameter will solve this problem, but with modern decanters using deeper ponds in many applications, the hub becomes immersed in the pond. Immersed hubs can result in more hydraulic turbulence, and thus lower separation due to friction on the liquor surface, and possible build-up on the hub due to a sticky floating phase. Surface non-concentricity results in mechanical vibration due to non- symmetrical buoyancy effects, so high precision is needed in geometry. Air flow and degassing of the feed stream become more complicated with submerged hubs. Some new designs avoid these problems by permitting small hubs designed with high stiffness and high natural frequency [1]. However, within the last decade, immersed hubs have been designed to float on the pond, considerably reducing potential vibration and enabling higher
speeds [ 2 ].
Decanter Design 31
2.2.6.2 Flights
The conveyor flights are fabricated from segments of annular discs suitably stretched and welded end to end, to form a regular helix. Naturally the helix profile has to be tapered to suit the beach section. Each section is welded in turn to the conveyor hub and then welded to the adjacent section. Double welding (both sides) with grinding afterwards is essential where hygiene is of importance. However double welding is common practice, even when hygiene is not required.
The flights will be normally perpendicular to the decanter axis or bowl wall. On the beach the flights will either remain at 90 ~ to the axis or will be at 90 ~ to the beach surface, depending upon the decanter manufacturer ' s choice.
It is not always appreciated what a complex shape the surface of a flight is. The usual pitch angle for a decanter is a little over 5 ~ . The pitch angle is the angle the tip of the flight subtends to a right circle of the bowl. To maintain the flight at a constant angle to the axis, the pitch angle of 5 ~ nearly doubles at the root of the flight.
If the flights are not to be protected from wear, then their tips will be ground smooth and perhaps chamfered, to provide a minimum of area in contact with the heel, to minimise torque.
2.2.6.3 Feed zone
There are a large number of designs, both complex and simple, for the feed zone. The feed enters the feed zone chamber from the feed tube. Once in the
II
Figure 2.16. A conveyor flight section before welding to the conveyor hub.
32 Basic Components
Accelerator Blades Feed Po~s
\
s ! s
Figure 2.17. A typical feed zone.
Feecl Pipe
feed zone, it has to be accelerated up to the bowl speed before spilling into the pond via the exit ports. To assist the feed up to speed, accelera tor vanes will sometimes be found on the " ta rge t" , the plate opposite the feed tube end. These vanes could be radial, at an angle to the radii, or curved.
In extreme cases of wear, parts inside the feed zone are hard surfaced or specific erosion resistant componen ts are used. Hard surfac ing is often used on the accelerator plate, par t icular ly on leading edges and the tips of the vanes. where most wear takes place. Some shaped accelerators have been made completely of u re thane rubber.
When the feed leaves the feed tube, in most cases it is at a high axial velocity. When it hits the ro ta t ing target, some splashing inevitably occurs. In fact a dense aerosol mist is often produced. At the back of the feed zone a tube is sometimes built in, to su r round the end of the feed tube. On the outside of this tube, small accelerator vanes are welded to accelerate and condense the mist and also to accelerate liquor up to speed, should the feed zone become flooded. Ideally, air is allowed to enter the feed zone from around the feed tube. It will be d rawn in by the fan effect of the feed zone and th rown out of the feed zone exit ports. The air would then pass along the bowl to exit over the centrate. This d raught helps to prevent splash back of feed
from the feed zone. The exit ports from the feed zone are themselves subject to a considerable
range of designs and innovat ions . It is not usual to have just one exit port. For symmetry and balance an even number of ports is usual , two, four, six or eight. The basic design has these ports fitted with tubu la r nozzles, often lined
with a ceramic wear protection. New feed zones have been introduced recently to reduce feed particle
attrition, by slowing and extending the accelerat ion time to bring the feed up to speed, and to reduce the inlet turbulence in the separa t ion zone.
Decanter Design 33
2.2.6.4 Floc/rinse z o n e
The floc or rinse zone is a c h a m b e r in the conveyor h u b beh ind the feed zone, somet imes separa ted by a buffer chambe r to minimise cross con tamina t ion . The required t h r o u g h p u t of this c h a m b e r is an order of magn i tude less t han tha t of the feed zone. Therefore, it does not require the same sophis t icat ion as
the feed zone, nor does it require erosion protect ion. From the floc zone, channe ls or tubes are provided to lead the f locculant
into the area of feed discharge to ensure an in t imate and economic mix at the appropr ia te point . Flocculant can be relat ively expensive, and on an effluent appl icat ion cont r ibu tes a large pe rcen tage to the total t r ea tmen t costs. It is therefore very impor t an t to ensu re tha t just the r ight a m o u n t of f locculant is used, and that no extra is requi red due to bypass ing.
As a rinse chamber , the exit pa ths are qui te different. General ly these will be on to the beach section, and even onto the dry beach section, if not at the junc t ion of wet and dry zones.
2.2.6.5 Wear protect ion
This is a very impor t an t topic and will be covered aga in in Section 2 .3 .6 .5 , and more fully in Section 2.4.9.
Various levels and grades of wea r protec t ion may be applied to the conveyor depending upon the application. The ma in areas on the conveyor requi r ing protect ion are the feed zone, flight scroll ing surfaces and the flight tips. Mechanical ly i n t e r changeab le wear inserts are more economica l to replace t h a n welded or bonded wear protect ion.
Pt / /
t~ I I i i ii I I II II ii
t I I I i I
;,, , 1
r ~ ~, /
Feed Zone Additive Chamber
Figure 2.18. An additive chamber.
34 Basic Components
Figure 2.19. Floc Addition.
Feed Tube
~/ - - ,. ~ 1 - J; U~ --
/ 1 / ," , / U , U .-,.-:,,, :..-:,-:::- ..... '
Figure 2.20. Rinsing.
2.2.6.6 Conveyor bearings and seals
Bearings are fitted into the ends of the conveyor and are generally grease packed. A seal will be used to retain the grease and a second outer seal will be used to prevent ingress of process fluids and solids into the bearing. Thus two seals are fitted back to back at each end. Grease channels have to be
Decanter Design 35
Areas of Protection
,--. p- - ~ - ~ - , , e ~ . , -
r ~ 1 7 6 ~ ' ' ~ ~ ~ .......... ~ _ ~ , , ,
�9 - o , , o o ~ [~ Figure 2.21. Areas of wear protection.
Figure 2.22. A typical conveyor bearing assembly with seals.
36 Basic Components
incorporated into the design of the conveyor hub and both bowl hubs, to facilitate the greasing of the conveyor bearings from the outside.
The differential rotation between the two races of each bearing is low and thus the life of these bearings should be good when adequately greased and sealed.
2.2.7 Gearbox
The gearbox is a major component of the rotating assembly, which creates the differential speed between the bowl and the conveyor.
There are two main types of gearboxes used on decanters. These are the epicyclic gearbox and the Cyclo gearbox, made by Sumitomo of Japan. However there are a number of decanters which have eliminated the gearbox by using a hydraulic system called a Rotodiff manufactured by the Swiss company Viscotherm. The Rotodiff and the Cyclo gearbox will be covered in more detail in Sections 2.3.7 and 2.4.15, respectively.
The epicyclic system consists of a pinion shaft and gear, which engages three planetary gears (mounted on carrier plates) which in turn engage a ring gear fixed to the gearbox casing. For the decanter the epicyclic gearbox involves two stages, although recently three stages have been in use. The carrier plate of the first stage holds a second pinion shaft carrying the sun gear for the second stage. The ratio of the gearbox is the product of the ratios for each stage. The maximum practical ratio for any stage is just over 1 3, giving a
1st Stage Ring Gear (Annulus)
100 I
I st Stage Sun Gear
2nd Stage Ring Gear (Annulus) 99
~ L Gearbox in ~ - operation
~ 1 Revolution
~ Output Spindle Shaft
"2nd Stage Sun Gear
m
Figure 2.2 3. A two stage epic!lclic,qearbox ( 1 O0 Ratio).
Decanter Design 37
m a x i m u m ratio for a two-stage epicyclic gearbox of 170 to 180. Three stage epicyclic gearboxes with ratios over 500 have been used on decanters.
If the central pinion shaft is held stat ionary, the differential speed between conveyor and bowl will be the bowl speed divided by the gearbox ratio. If the pinion shaft is allowed to rotate at some speed below the bowl speed, then the differential between bowl and conveyor will be the difference between bowl speed and pinion speed, divided by gearbox ratio. If the pinion speed is controlled by using a brake, or a variable speed motor, differential speed may be varied from close to maximum, when the brake is at its slowest speed, to nearly zero, when the brake is almost at bowl speed. Reducing the pinion speed below zero, i.e. by reversing, enables higher differential speeds to be obtained. Using an epicyclic gearbox causes the conveyor to rotate slower than the bowl, whereas it is normally faster when using a Cyclo gearbox. Generally the conveyor flight helix is "left handed" with an epicyclic gearbox and right handed with a Cyclo gearbox.
2.2.8 Frame
On smaller decanters, the frame has often been made from cast iron. More usually it is fabricated from steel channel or box sections. The flame needs to be a rigid support for the rotat ing assembly. The surfaces for the main bearing pillow blocks are accurately machined in the same plane, and in line, to ensure no end-to-end misal ignment of the rotat ing assembly, which would cause premature bearing failure. Some manufac turers fill part of the main flames of their larger machines with concrete effectively to form an inertia
J
Figure 2.24. Decanter n~ainJrame.
38 Basic Components
block, and for noise reduction. Some flames have been used as a reservoir for the lubricating oil for the main bearings.
The flame and casing (see Section 2.2.9) act as the link between the high g field of the rotating assembly and the s tat ionary area a round it.
2.2.8.1 Bearing supports
The main bearings with seals are mounted onto the main flame in pillow blocks. When oil lubricated, the pillow blocks will be piped to an oil system, which will include a circulating pump, an oil reservoir and cooler with associated pressure, flow, and tempera ture ins t rumenta t ion (see Section 2.2.4.4).
2.2.8.2 Feed tube
In its simplest form the feed tube is a plain cylindrical tube. A clamp or flange holds it on a support extension from the main flame. It extends to the feed zone and within a few centimetres of the accelerator in the feed zone.
Casing Seal Pillow Block
'j '~ i i I Pillow Block
3 ~x [[ii-Iti!:i i| I'lllll 'llllt i I!
Feed Tube
Figure 2.2 5. A main hearing and pillow block assemblH.
Decanter Design 39
Figure 2.26. Feed tube.
The geometry of many decanters is such tha t there is a risk of resonant vibration of the feed tube at frequencies around the bowl speed. To counter this, feed tubes have been made slightly tapered, and made of l ighter materials such as glass fibre and even carbon fibre, and sometimes composite material .
Entering the conveyor with the feed stream is a flow of leakage air, which passes through the clearance between the conveyor and the feed tube. This air flow must eventually be vented, and in those applications where odour or toxicity is an issue, minimising this air in-flow is important . A simple, lightly contacting lip seal is often used. On critical sealing processes, mechanical seals are required.
2.2.8.3 Vibration isolators
Out-of-balance forces in the rotat ing assembly are isolated from the ground by interposing vibration isolators between the f lame and ground. When a sub- flame is used the mounts are placed under the sub-flame.
Even with a well balanced machine , out-of-balance can occur when solids build up unevenly, when there is uneven wear, or when some unplanned mechanical movement occurs within the rotat ing assembly. Considerable oscillations of the rotating assembly can occur during run up and shut down, when the speed of the bowl passes through critical speeds. Owing to the presence of these isolators, all process connections to the decanter must be flexible. Likewise, oil lubrication connections must also be flexible.
Spring Carbon Seal Rings Spring O-ring Housing
Figure 2.27. A feed tube seal.
40 Basic Components
Fn S c r e w Su
.3h lo rop rene E l a s t o m e r
];i ;;J ,,,. - ,, ~-.-..-~/?..-.-.L ~ ; ~.; .-,..._~: ..~: :: :..~,. :, '. . .,.,-,-~-~ ..-:,_.-.4,.. . . . . . . . .-.--.~;-~;:;." .. ".
. . . . �9 "" F l oo r �9
Figure 2.28. A decanter vibration isolator.
2.2.9 Casing
T h e c a s i n g ac t s as t h e c o l l e c t o r for t h e p r o d u c t s d i s c h a r g e d f r o m t h e r o t a t i n g
a s s e m b l y , a n d c h a n n e l s t h e m to r e c e i v e r s for o n w a r d h a n d l i n g . T h e c a s i n g
m u s t , o b v i o u s l y , k e e p t h e s e p a r a t e d p r o d u c t s a p a r t .
Upper Casing
~
a
. . . . . . . . . . i i
Lower Casing
Figure 2.2 9. Decanter casing.
Decanter Design 41
Simply stated, this casing is a s tat ionary collector for cake at one end, and centrate at the other. There are many design variants and each manufacturer has its own recognisable design whether it be just its finish or its shape and functionality.
In its more usual format, a lid is hinged onto a bottom half and bolted with a flat rubber gasket between the two halves.
2.2.9.'1 Casing baffles
Inside, the casing is compartmenta]ised by several baffles, which are welded to the inside surface of the casing and fit very close to the rotating assembly. This close fit may be to the plain surface of the bow], or to a shoulder on the bowl, or into a labyr inth groove machined into the bowl.
2.2.9.2 Cake discharge
To cope with the wide range of rheologies of cakes experienced, the cake chute of the casing needs to be as open and large as possible without ledges, or any nar rowing of the opening. The cake discharges from the bowl at high velocity th rough 360 ~. It is necessary to ensure that this cake does not stick to the casing, and is directed down into the receiver.
Casing Baffles
\:,',, \
Figure 2.30. Casing baffles.
42 Basic Components
2.2.9.3 Centrate discharge
It is usual to cone down the centrate end of the casing in order to mate wi th an off take pipe. The pipe size needs to be sufficient to allow free flow of the copious quantit ies of air at this end. Alternatively a separate vent pipe will be introduced. The centrate tends to cont inue its circular motion on leaving the bowl and swirls around in the casing. It is therefore sometimes seen tha t the centrate discharge is offset from the centre line to take advantage of this tangential flow.
2.2.9.4 Casing seals
A flat gasket seals the two halves of the casing. This will be of, say, neoprene, Viton or silicone rubbers, depending upon the application. Sometimes these gaskets are moulded with various cross-sectional shapes to provide good location and secure positioning.
The clearance between the casing and the outer spindles of the bowl hubs can be open, or have fitted some form of seal depending upon the degree of sealing required. For atmospheric operation a simple surface contact of a PTFE ring often suffices (see Section 2.2.4.4).
Horizontal decanters, operating at a slight positive or negative pressure. require careful sealing when the casing is split, especially at the corners and the end. Some higher pressure designs avoid this problem by having the casing cylindrical, with disassembly in the axial direction, thus using simple O-ring seals. However, machine disassembly is then complicated and added floor space is required for main tenance .
Sealing process discharge vents, as well as feed lines, requires the use of commercial and special flexible connectors. These connectors must be carefully designed to limit the forces imposed on the centrifuge and modula te the forces t ransmit ted to the plant piping and structure. The material of these flexible connectors must be chosen to resist the process temperature, pressure and corrosive characteristics.
2.2.9.5 Vents
The bowl, rotating at high speed, acts like a fan, and drags large quant i t ies of air around with it in a turbulent motion. This turbulent motion of air, called windage, tends to move outwards due to centrifugal action, and in doing so drags air towards the centre line to replace it. Windage needs to be channel led and vented rather than suppressed. Suppressing windage can cause cross contaminat ion rather than prevent it. It can be advantageous to allow air into the centre compar tment to satisfy the windage created at either end of the bowl. In so doing the air travels from the centre outwards and thus helps to prevent escape of products from their designated discharge compar tments .
Decanter Design 43
If air is allowed in, then it has to be allowed out, and thus it is necessary to ensure that the product lines or receivers are vented and the lines are adequate in size to carry both product and the air flow.
Air flow ent ra ins a lot of the discharged centra te . This en t ra ined l iquor has a propensity to migrate a round baffles. Thus the bot tom of the casing is general ly slightly sloped to drain any spilled liquor back to the cent ra te . In th ickening applications, the slope will be in the opposite direction, as a little extra dirty liquor in a fluid cake is preferable to dirty liquor in a clean cent ra te .
2.2.10 Sub-frame
When the main f lame adequate ly fulfils its function, a sub-f lame is not used. The sub-f lame forms a base for the main f lame car ry ing the ro ta t ing assembly plus the main drive motor and back-drive system when used. It ensures tha t there is no u n w a n t e d differential movemen t wi th either of the drives and the rota t ing assembly. Vibration isolators are s trategical ly placed under the sub- f lame to share the load properly The use of a sub-f lame subjects the drive motor to more vibration, but at the same time it makes instal lat ion easier.
2.2.11 Main drive
The main centrifuge drive is usual ly an electrical motor moun ted on slide rails on the frame or sub-frame and connected to the centr ifuge drive pulley with V-belts. A purpose built belt guard will cover the two pulleys and belts. Motors on the larger decanters can consume a few hundred kilowatts of power. Such large motors are more often directly moun ted on the floor, in which case a special belt- tensioning device is incorporated to allow for the differential movement of the rota t ing assembly.
Figure 2.31. Sub-frame.
The mijin motor IS to i\CC<Aler;ltC i I high inertial load on start-up, Mrhcn thc howl is a1 speed, lht! rri:iiri r r i o l o r h:is 1 1 ) provide the power io ; irr~ler;~te t he process ma te r id lip t o speed. the power for scrolling Ihu citke. and most UF the braking yowcr for thc back-drivc systcm.
'I'here are various methods of starting a dccantcr with an inherently high inertinl Inad. A ~~ar-iahlt spccd drive might be considered. il' changes in rniirhiol! sprrrd :.irk! duciried t o bc iniportant due to varying process conditions. Howcver, a largc niiijority o f dt.ciiritt.r applications iise a fixed spced main drive motor. Once at speed. a decanter 's motor hi ls onc of the simplest duties. It is never subicctcd to cyclic o v e r l o i ~ d ~ , never suhjertcd 10 ct,nliriuous vibration a n d ncvcr subjccted to sevrry braking. clectrically or riieclia~~ically. I t i s seldnm stopped and restarted.
There is, however, one duty that Ilie maiii motor has t o perform tha t difkrs from most other drive niotrir applications. I t has to Iiavc the thermal capacity to accc1er;itc a high irierlial load to speed. smocithly ovcr an extended period of h e . without undue damage 10 rnoior wiridirigs by cxcessivc tcmpcraturc rises. 'I'his cannot be acliievrtl without certain limitations. such ;is thr rlurrlLer of starts from standstill to f u l l speed being liiiiitcd to a ccttain number within ;I fixed period of tiinc. Thc uscr's requirernrnts. such as installing thc motor stiirter reniotely. perhaps interlocking wil t i ot1it.r equipment startcrs and microprot.rssors, rxiay add to thc restricl ions imposed OII thc niain motor specification.
'l'lic tinic takcii lor t h e rnotor and dccmtci- to accelerate t.o rull speed is an important factor in t h e selection or the nlain motor. It is deprndent upon the inertia ul' the niotor and dccanter combined. the torqucjspccd characteristics of the motor, Lhe rcactivc torque from l.he decanter. windage tind friction ]osscs, and the s p e d at which the decanler is l o be run.
Decanter Design 45
Motors can be two-, four- or six-pole giving synchronous speeds of 3000, 1500 or 1000 rpm at 50 Hz, respectively. The most commonly used motor is the four-pole, which is a more usual standard in motors and is capable of being better balanced than the two-pole. Because of the low speed the six-pole motor would be an unusual choice.
The torque available from the main motor varies according to the method of start up, whether it is star-delta or direct-on-line. A motor connected in star produces a starting torque one third of that provided when starting direct-on-line. Direct-on-line starting torque can be two and a half times the motor's full load torque, with a starting current of six times full load current.
A standard motor has a very steep direct-on-line torque/speed curve characteristic, rising from a min imum of 180 -250% full load torque, to a maximum of 3 0 0 - 5 0 0 % full load torque. This maximum, called the pull out torque, occurs at about 85 % of full speed.
Main motors need starter overload and short circuit protection. High rupture fuses (HRC) will protect the motor against short circuit conditions, and will interrupt the electrical supply in milliseconds of the fault occurring. It is essential that fuses of this type are always fitted. Conventional overload protection, thermal or magnetic, can offer no protection to a motor with an extended acceleration time. Thermistor overload protection is the only true protection for a motor under these conditions. A thermistor is embedded in each of the motor's three windings and connected in series. The resistance of these thermistors is designed to increase rapidly at a set temperature, depending upon the insulation class of the motor. The thermistors are connected to an electronic amplifier control unit in the starter enclosure, and will trip the starter contacts when required. The device will not reset until the motor has sufficiently cooled.
In Europe the main motor is usually an AC motor, using a star-delta starter. An inverter for the main motor is becoming more common, particularly with the smaller decanters. The inverter enables a soft start, and allows speed adjustment for different process requirements. Inverter motors can cause unwanted electrical interference, and harmonic wave forms, on the main electrical supply lines. These problems can be minimised by using electrical filters and the latest advanced electrical technology.
2.2.12 Back-drive
The back-drive system is a means of controlling the speed of the gearbox pinion shaft (and thereby the conveyor differential speed) using, for instance, a motor or a brake. This could be offset from the gearbox shaft, in the same manner as the main drive, and connected by a belt. This belt would be a timing belt because of the accurate control required. Normally the back-drive is connected directly and in line with the gearbox pinion.
46 Basic Components
. ~ '1 ~1 '~I1~ I ~ '~ - ' " ~ "~ S ~ . . . . - ~ - - ~ ' "
! . - ~ - - ; ' ~ ~ ' ~ ! |i ~ ' ' ~ . . ~" 2- ;~* ' ; - - - ' " .,~oO~i i Figure 2.3 3. A decanter back-drive system.
The main component of the back-drive assembly can be an eddy current brake, inverter motor or a DC motor. The Viscotherm Rotodiff hydraulic conveyor drive is a variable speed device, powered by a fixed speed hydraulic motor.
2.3 Variations to Main Components
The basic components of the decanter have been described. It is now possible to discuss the alternative designs of these components.
2.3.1 Orientation
The orientation of the axis of rotation (horizontal or vertical) does not affect the process performance of a decanter. The g force is at right angles to the axis in both cases, so rotors that are equivalent in diameter, speed, geometry and design will give comparable separation.
There is one main alternative to the basic horizontal orientation and that is vertical. However, the Flottweg Company does have an inclined decanter, inclined at the angle of the beach, which facilitates full emptying of the bowl for clean-in-place processes.
The availability of inexpensive vertical decanters is very limited, such that in some circumstances one has to select a horizontal model even though a vertical one might have been preferred. The vertical decanter is designed and built primarily for high-temperature and/or pressurised operation. The vertical orientation lends itself to better and more reliable pressurisation, with only one end to be provided with rotational seals. The amount of engineering required for pressurisation and sealing makes the vertical decanter more expensive than the non-pressurised and open horizontal decanter.
2.3.1.7 Vertical vs. horizontal
Figure 2.35 compares the seal and structural differences of these types of decanters. While each type requires three slow speed seals for the conveyor and one slow speed seal for the gearbox, these seals, due to their very low surface rubbing speeds, do not cause problems if designed properly. The horizontal type requires two large high-speed seals to be mounted on the rotor, and one smaller high speed feed tube seal. The vertical type, in which the rotor and gearbox are pendulum suspended from a flexibly mounted spindle, requires only one high speed seal between the bearings in the spindle and the casing. There are no high speed bearings or seals at the bottom of the
48 Variations to Main Components
Rotary Joint for Gearbox Lube Oil Feed and Take-off
Drive Pulley
Spindle
Spindle Seal
Flexible Casing Seal
Gearbox
Upper Casing Wear Ring - ~ (used when fine abrasive J/ solids present in effluent) &
Effluent Discharge
Flexible Suspension System
Rubber / I s o l a t i o n Mounts
Upper Casing
Casing Vent
Conveyor ~.~ "~-Frame
Bowl
Feed Accelerator
Conveyor Wear Sleeve
Feed Tube
Solids Discharge Spray Ring
Abrasive Solids Wear Sleeve
Conveyor Support Washer
Conveyor Sleeve Bearing
Bowl Bumper
Lower Casing
Rinse Connection Feed Connection
Figure 2.34. A vertical decanter (By courtesy of Alfa Laval).
Decanter Design 49
Horizontal High Speed Seal Low Speed Seals
\ Flush Ve~n,t 1Flush y II ~ T High Speed Seals
Rinse
"ll'a .q �9 , ,,,, " .. ~ Feed
: Vibration I'', N~ ; ~ ~ '~ Low ~ _o~ .;c~. _; Isolators
,-,- Centrate Seals Solids
High Speq
Lo Spe Se~
Centr
Stationary Stationary Rigid ~ ~ Vibrating T
Solids
Figure 2.3 5. Comparison of vertical and horizontal decanters.
rotor, and no high speed feed tube seal needed. In addition, with the casing and flame of the horizontal decanter flexibly mounted and vibrating, all process and electric connections must be flexible, thus needing periodic attention. The vertical decanter f lame and casing are rigidly mounted. Therefore there is no need for a vibration isolator between it and the external connections. With no second bearing and seal on the vertical decanter, thermal expansion and al ignment issues are almost eliminated.
Almost all of the vertical decanters installed have been designed to meet the Unfired Pressure Vessel, Explosion Proof Code requirements and chemical industry piping, vessel lubrication and ins t rumenta t ion codes. Due to this, the cost of this design is higher than horizontal machines of equal process capacity. For those horizontal machines that meet the same envi ronment and code requirements, the cost is comparable.
In general, the emitted noise level of vertical machines is much lower than horizontal machines due to less vibrating surface exposed to the work area.
2 . 3 . 7 . 2 V e r t i c a l d e c a n t e r s e a l s a n d b e a r i n g s
The main high speed seal assembly used in vertical machines is usually a mechanical seal. This is mounted on the spindle assembly cartridge, which is separable from the gearbox by means of a tapered joint, thus allowing seal and bearing main tenance wi thout removal and disassembly of the main rotor. Maintenance can then be done in a clean environment . Most seals installed are t i tanium bellows with carbon nose, rubbing on a hard coated stainless steel or solid carbide or ceramic mat ing ring. The materials used are chosen to meet the most difficult of corrosive and tempera ture and pressure environments. Seal assemblies having two seals and one mat ing ring, and a buffer liquid between, are available.
50 Variations to Main Components
Figure 2.36. A vertical decanter seal and bearing assembly (By courtesy of Alfa Laval)
Decanter Design 51
It should be noted that the horizontal design will impose an axial load on one of the main bearings, equal to the process pressure times the feed tube seal area. The axial load (upward) on a vertical design reduces the axial bearing load due to the rotor weight. This reduction in bearing load is equal to the process pressure times the high-speed spindle seal area. In fact, with a high enough process pressure, the axial load can be near zero on the main bearing and can even push up on it. This load reversal must be considered in the design.
2.3.1.3 Vertical decanter casing seal
Since this design permits movement between the spindle housing and casing, a flexible static seal is required which is not needed in a horizontal machine. The device most used is a single reinforced elastomeric bellow, sufficient to resist the internal vapour pressure in the casing, and lined with PTFE to resist the corrosion of the process.
2.3.2 Flow
The main alternative to the countercurrent flow inside the bowl already described is co-current. In this design, the feed enters a feed chamber situated close to the bowl front hub, from a feed tube through the front main bearing. Both the cake and the clarifying liquor travel together towards the beach end. Prior to the beach, the clarified liquor decants into channels built onto the conveyor hub, dipping into the pond, which direct the centrate back to the front hub for discharge over adjustable weirs in the normal way, already described. In this design the gearbox is fitted to the rear of the bowl. The drive is usually at this end also.
The co-current design allows the shortest feed tube. It also means that the finer solids, settled the furthest distance from the feed zone, do not have to return under the turbulent area of the feed zone, and so risk being re- suspended. The co-current design requires horizontal channels built onto the conveyor hub, dipping into the pond, to direct clarified liquor back to the front hub. These can suffer from fines settlement. However, in the countercur rent design the majori ty of the solids are removed from the clarification zone early, allowing more room for clarification. Separation problems, which result in more solids in the centrate than usual, can necessitate the decanter to be shut down for rodding out of plugged return tubes. Turbulence at the entrance to the re turn tubes, near the solids discharge, can cause solids re-suspension. Despite these differences in behaviour, there are many thousands of decanters of each design successfully operating in the field, and both are currently sold. It thus can be concluded that performance differences are marginal . There are only a few applications where the supplier might claim the physical difference of his preferred design offers an advantage.
52 Variations to Main Components
Reinforced E l a s t o m e r ~ ~ ___
Pm F E ~~"~'~.~~ ~ ' ~ . ~ ~ - Liner-~ ~ ~ t i Casing
pindle
Figure 2.3 7. A vertical decanter casing seal (By courtesy of Alfa Laval).
2.3.3 Materials of construction
Each decanter manufac tu re r can choose from a range of construct ion materials, and will have its own preferred materials. Choice may be due to the manufac turer ' s own par t icular design, the process materials, the speed at
Decanter Design 53
I
Figure 2.3 8. A co-current decanter( BI! courtesy of KHD ).
which the bowl is to be rotated, or somet imes even the a r r a n g e m e n t s wi th the
par t icu lar steel supplier. The most critical parts, the contact par ts of the ro ta t ing assembly, are most
f requent ly made in one of the m a n y stainless steels. Never the less some m a n u f a c t u r e r s con t inue to supply bowls in ca rbon steel. The 316- type stainless steel is a common mater ia l for the smal ler decanters , but for the larger mach ines at h igher g-forces, mater ia l s wi th h igher s t reng th , such as the duplex stainless steels, must be used. Where high t empera tu res , or ext ra high speeds, or corrosive mater ials are in use, special cor ros ion- res i s tan t or t empera tu re - re s i s t an t materials may be used.
A c o m m o n reason for corrosion on decan te r s is the presence of chlorides in the process mater ial . The chlorides can cause pitt ing corrosion, crevice corrosion and stress corrosion of the decan te r parts in con tac t with the process. To avoid pitt ing and crevice corros ion in severe env i ronmen t s , more corrosion res is tant materials, normal ly special stainless steels wi th a h igher content of al loying elements, are used. In ext remely corrosive en v i r o n men t s , more exotic mater ials , such as the nickel-based Hastel loy or even t i t an ium, may be used.
Stress corros ion cracking (SCC) is a special corros ion form, wh ich is seen as f ine-branched t r a n s g r a n u l a r cracks in the mater ia l . SCC is caused by a combina t ion of mechan ica l stress, the presence of chlorides and elevated t empera tu re . At h igh t empera tu re and high mechan ica l stress, SCC of 316- type stainless steel can occur at chloride levels even below 1 0 0 0 ppm. The answer to this is to use a more cor ros ion- res i s tan t stainless steel or a duplex stainless steel. Duplex stainless steels no rmal ly do not show SCC at t empera tu re s below 100~
The fabr icat ion methods for the bowl are impor t an t w h e n cons ider ing its safe work ing speed. Some bowls are m a c h i n e d from a simple cast ingot and others from a special centrifugal casting. Some are even fabricated by rolling a
54 Variations to Main Components
sheet and welding. Quality control of casting, and especially rolled and welded parts, is critical.
In the case of contact parts that are not rotated, such as the casing, the specification of the steel need not be so stringent. Bowls in stainless steel and casings in a non-stainless steel are not unknown.
Materials for erosion protection are described in Section 2.3.6.5. Elastomers used for seals and gaskets again depend upon the envi ronment .
Neoprene is a standard material, but Viton and many other materials, even PTFE, are used when necessary.
In some parts of the decanter, such as bowl and beach liners and feed zone liners, they can be made of a similar material to the bowl, when they just act as a preferential sacrificial wear component , to protect a more expensive component.
In the cake discharge compar tment of the casing, linings of rubber, PTFE, PVC, steel and even Stellite have all been used variously to combat wear or to overcome cake sticking. Flexing of some materials here is encouraged to aid the desticking process.
2.3.4 Bowl variants
The bowl is a simple cylinder and therefore the variations possible are limited, a l though one of the first bowls made was fully conical and thus consti tuted both bowl and beach together.
Some of the longer bowls are made in sections, which are flanged and bolted together to give a set of s tandard bowl lengths. This allows common components with the shorter designs.
The machining of the inside diameter will depend upon whether a liner is to be fitted or not. The outside is machined to mate with baffles in the casing and with grooves when required to form a labyrinth with the casing baffles.
2.3.4.'1 Front hub
The design of the front (centrate discharge) hub varies considerably from machine to machine. However these variations are generally for mechanical considerations, having little effect, by themselves, on process performance.
The thickness of the hub is dictated by the mechanical s trength and stiffness requirements. When centrate discharge ports are drilled radially into it, as for some three-phase designs, extra thickness is needed. With many different designs for centrate discharge, which are incorporated on the front hub, the mechanical design is made to suit. The precise bearings specified for the inner and outer spindles of the hub will need their own mat ing diameters.
Devices added to the front hub, such as centripetal pumps, noise aba tement rings if used, and the gearbox, all require their own special designs and
fixings.
Decntiter Design 5 5
When simple weirs are used, then the hub will be machined with recesses to locate the weirs and their support plates. On the smaller diameter bowls the number of weir plates will probably be four, while Iarger models may take six or even eight.
2.3.4.2 Centrate weirs
An advance on the plain design (see Section 2.2.4.2) is to recess the weir plates into holes machined into the hub. A thicker cover plate would secure each weir plate. The outer edge of the weir and cover plate would be circular to locate in the recess. Different. pond depths are then obtained with different weir plates.
In the “moon” design the inside edge of the weir plate would be radiused to match the pond surface. The longer this edge is the better, as this minimises cresting, which facilitates easier process control. The cover plate would have its inner radius larger than the shallowest weir plate usable.
The deeper the weir plate the more important it is to secure it with a support plate. This is because the dam, being relatively thin because of the range of sizes necessary, can easily distort with the pressure of the pond at speed. The better cover plates will have a protruding lip fabricated on their inner edge. This is to ensure that the discharging centrate separates from the bowl at. the smallest possible radius. This minimises power consumption.
One alternative to the multi-plate weir is to have a complete circle split in two, t n facilitate fitting, and bolted over the ports. These hairrings have to be very thick to avoid distortion and loss of seal.
Another alternative is to have a complete circular plate n7itb slots cut at a specitic radius to cover the hub holes. Other sets of slots. maybe as many as threc. are cut at. alternativc radii but offset. circumferentially. To change the pond setting the plate is unboltcd and rotated. to engage another slot radius Over the hub holes. and then re-secured.
These last two variants tend to bc uneconomic unless i t is known that only a small number or pond settings will be requircd.
A further type of pond adjustment is provided by a set of close-fitting circular inserts in the bowl hub apertures. An eccentric hole is turned in each casting insert. By rotating the circular casting in its seat, an almost infinite range of pond settings can be obtained. The shortcoming of this design is that the overflow edge is locally circular, not matching the pond surface, such that a large crest forms over the weir. the height ofwhich varies considerably with the process rate. This does not allow good process control when needed.
When the weir height is set above the cake discharge height a considerable amount of feed can flow from the cake discharge at start up. until a cake seal is created. There are various ways of combating this. such as stopping and starting with a full bowl of solids, setting a minimal differential a t start up, and feeding at a low rate at the start. A better method is to use notched weir plates
56 Variations to Main Components
(see Section 2.4.4.2). This method of operating the decanter (liquid discharge radius smaller than the cake discharge radius) is sometimes known as "negative pond operat ion". It is used to improve scrolling efficiency, and to enhance the control of scrolling, by making use of the hydraulic head difference between the discharges.
2.3.4.3 Liner
Liners can sit inward of the bowl surface. Alternatively a recess can be machined into the bowl to take the thickness of the liner with its ribs. In the latter case, the option of whether to fit a liner or not can be taken after manufacture . However, in this case and operating without a liner, a much bigger heel results, which could give out of balance problems, should part of the heel dry out and break away after the bowl remains s ta t ionary for any length of time.
An al ternative to the ribbed liner, recently introduced, is an expanded metal sheet ("Expamet") fitted in a recessed bowl. This is now used frequently with success.
Instead of a liner, m a n y bowls have longitudinal ribs spot welded to the inside of the bowl. Another al ternative to a liner is knurling or roughening of the bowl, which is a simple procedure, but would normally be done only as a temporary measure as such surfaces will be quickly worn smooth.
Some applications do not require the assistance of a bowl liner as the process mater ia l has sufficient friction, even with a smooth machined bowl surface, to provide enough keying to allow an adequate scrolling efficiency. Using a liner on these applications could raise the scrolling torque to an unacceptable level.
Centrate Discharge
Figure 2.39. A centrate "moon"dam with cover plate.
Decanter Design 57
_Split Dams
_.. \
Figure 2.,10. A split dam.
Dam P l a t e ~ D ~ am Holder
Figure 2.41. A single piece dam.
2.3.4.4 Mdir i brar i r ig
Several different bearing types ;Jnd hwring combinnlions arc used for (lwarilers. A (:omrriort solution i s to hitvr ii deep groove balI-bcaring in onc
end. usually the drive end, which fixes the dccantcr in thc axial dircction and a cylindrical rrillcr bcarinE i n thc other cnd. which allows for therrrial expansion of thc bowl. Sornc manufacturers. i n particular on large dccanters. usc doublc spherical roller bearings. which havc high load capacity. [ f t he decanter has deep groove ball bearings. or other bearings which do not allow axial displacement, in both ends, the decariter tiiust be desiprtrd to acctmimodate the thermal expansion of the rotor.
When selecting milin bearings lor [Iec;jnters sevltral p;ir;jmet.rrs must he considered. The rn;iin par;imeters ;ire lhe rotat.ioria1 s ~ L ' c ' ~ , the radial 10;id. thc operating temperature anti the luhricatiori riiethod.
'l'he expected bcarlng lifc can bc calculatcd bascd on bcarinR data and thc dynamic arid static loads on thc bcarinE.
F o r a giver1 decanter. size a large convcyor hub shaft dianictcr is oftcn dcsirahle both to increase the stiffness or the rotor and to give space for feed pipe nnd splinr shafl.s. As t.he allowahle speed for a given bearing type dccrcases with increasing size, the hearing selettioil will he a compromise. An oil-lubricated bearing will have a higher allow able^ speed arid t.!ierr,fwr, o i l luhricatinri is often uscd for larger bearings and bearings with ;i high speed or rotitlion.
Decanter Design 59
Deep Groove Ball Bearing
Angular Contact Ball Bearing
Cylindrical Roller Bearing
Cylindrical Roller Bearing (Type NUP)
Spherical Roller Bearing (with Cylindrical Bore)
Figure 2.43. Some bearings used for supporting the rotating assembl!j.
The design of main bearings, their sealing, and their lubrication is a complicated task, which requires a high degree of expertise in bearings and decanter design.
See also Section 2.2.4.4.
2.3.5 Beach
There are many variants for the beach. A major beach parameter is the beach angle. Typically this is 8 -10 ~ semi-included cone angle. However, an angle as low as 5 ~ is not uncommon, nor are angles up to 20 ~ A few part icular special designs have been built with beach angles up to 45 ~ These special designs are used for thickening applications. Here the bulk of the solids are discharged through special nozzles in the bowl wall at the foot of the beach and only the coarsest solids discharge conventionally. In some designs the pitch of the
6 0 Variations to Main Components
10 ~
12"
6" I 10"
1 0 ~
11 ~
Figure 2.44. Some alternative beach designs.
Decanter Design 61
conveyor on the beach is reversed to convey any solids which pass the nozzles back to them.
Another variant of the beach angle is to have a compound angle. In this design, some distance from the foot of the beach the angle is increased or decreased resulting in a concave or convex design. The concave design is an at tempt to make a more gradual t ransi t ion from the horizontal bowl section to the final beach angle. The convex design is used to maximise the dry beach length where this is a limiting factor, and to facilitate conveying.
Rarely a completely, or near completely, curved concave contour is used. This is used for very difficult scrolling problems. The large increase in cost of fabrication of this beach and corresponding conveyor section inhibits the wide use of this design.
In some old designs a beach is not used at all. A simple flat rear hub is used with cake discharge holes. The conveyor is tapered as if there were a beach, and the cake then forms its own beach. This design can work reasonably well with stiff cohesive cakes, but very high conveyor torques can result.
Scrolling improvement and abrasion protection add to the beach variations. These include ribbing, grooving, tiling and liners.
The cake discharge is generally a part of the beach fabrication but this will be covered in Section 2.3.5.2.
2.3.5.'I Real hub
The precise design of the rear hub will vary from decanter to decanter and from manufac turer to manufacturer . There will be variations according to the method of lubrication of the rear conveyor bearing. The bowl design speed and the type of bearings used will affect the detailed design. These details will not affect process performance.
The main parameter , which will give rise to the major hub variations, is the design of the cake discharge. If the cake discharge is wholly in the beach wall then there is a simple a t t achment of the hub to the beach. Sometimes, however, the cake discharge is extended into the hub or is completely in the hub. The cake discharge variants are more fully described in the next section.
2.3.5.2 Cake discharge
The cake discharge is an important part of the decanter process design, especially because the ability to separate solids continuously is the prime reason for the existence of the decanter. An inadequate design will reduce process performance.
In the case of very high solids loading, the design that has a row of radial holes towards the end of the beach may be improved by the introduction of a
2.3.4.4 M d i r i k a r i i i g
Several differen1 bearing lypes ynrl h w r i n g cornbimabms arc uscd for c k r m l e r s . A cornrriori solution i s to have ii deep groove ball-bcaring in onc end. usuaily the drive end, which fixes the decanter in thc axial diruction and a ~yliiidrical rollcr bcaring iii thc other cnd. which allows for therrrial expansion of thc bowl. Sornc nianufacturcrs. i n particular on large dccanters. usc dnublc spherical roller benrings. which havc high Inad capacity. If t h e decanter has deep groove ball bearings. or other bearings which do not allow axial displacemeot. i n both ends, the decarikr must be desigrlt:d 1.0 a ~ ~ o ~ ~ i n i o d a t e the thermal expansion of the rotor.
When selecting milin bearings lor [lec;jnlers several p;ir;jmelers must bP considered. The m;iin p;ir;imeters ;ire I tie rolai.iorlal sped, Ihe radial load, thrr operating temprrotrlre and he luhricatiori riielhod.
'Ihc expected bcarlng life can bc calculatcd bascd on bearing data and thc dynamic arid static loads on thc bearing.
For a giver1 decanter size a large convcyor hub shaft diamctcr is oftcn dcsirablt. both to increase t he stiffness or the rotor and to give space for feed pipe and spline shaf1.s. As t.he allowahle speed for a given bearing type decreases with increasing size, lhe hearing selection will he a compromise. An nil-lubricated bcaring will have a higher iIllowilhle speed a r i d I.herefore oil li~hricatiori is often uwd for larger bearings iind bearings with ;i high speed or rot ;I I' . l o l l .
r-
L.
q~
....~
L
64 Variations to Main Components
cut-outs machined from around the outer end of the beach. An advance on this is to provide matching castellations on the rear hub to improve discharge area.
As designs have developed there has been a tendency to enlarge and shape holes and introduce castellations. On very large decanters the thickness of the beach wall in the area of the cake discharge is such as to allow the machining of exit ports of a helix (snail shell) shape to minimise contact of the cake with the surface of the discharge ports. This reduces wear in the area.
Some modern designs adopt a 360 ~ discharge method. Cake decants over the end of the beach th roughou t 360 ~ passing between support pillars fabricated axially on the outboard edge of the beach. These pillars enable the a t tachment of the rear hub. The pillars are restricted in length by mechanical strength considerations, but provide sufficient axial gap for free solids discharge.
In some applications it is found necessary to surround the cake discharge area with a s tat ionary collector, fixed to the casing, to prevent cake sticking to the main casing. The function of the collector is assisted by impellers, which are bolted to the outer surface of the beach and/or the rear hub. The impellers sweep clean the inside of the collector, al though they also add to the noise level of the decanter.
The type and design of hard surfacing add to the variants of cake discharge. Cast or sintered pieces of various ceramics and tungsten carbides can be fabricated to fit regular shapes such as circular holes and the face of the 360 ~ discharge. For irregular shapes, flame applied hard surfacing is used on the discharge area or on a steel casting made to fit the discharge. Alternatively a plain steel sacrificial component is used.
On the pillars of the 360 ~ discharge, half cylinders of sintered tungsten carbide are used. The latest development of this component, by Alfa Laval, is to use a saddle shape, which improves the wear life of the component.
2.3.5.3 Beach liner
While grooves or ribs are used on most beaches it is unusual to use a liner. However in extreme cases of erosion, a beach liner of say Stellite with its own Stellite ribs is used. Alternatively the beach can be covered with small carbide or alumina tiles like a Roman mosaic. Tiles of two different thicknesses allow
the formation of in-built ribs or grooves as desired.
2.3.6 Conveyor
The conveyor is a component that has a major influence on the process performance of the decanter. It thus has a large number of variations in design. In general the shape of the conveyor has to match the inside profile of the bowl and beach assembly, with from 0.5 to 2 mm radial clearance. The
DecanterDesign 65
i i I
Figzire 2.47. A beach liner.
exception is the beachless bowl where the taper of the conveyor is chosen for the part icular application and the process material itself forms its own "beach".
The large number of conveyor variants arise from the permutat ion of the different hubs, flight design, feed and floc zone types, and the different types and degrees of hard surfacing. Devices added to the conveyor, and the type of flow, two-phase, three-phase (see Section 2.4.1.2), co-current or countercurrent , add to the permutations.
The co-current flow conveyor will have its feed zone at the front (large diameter) end and some return channels will be built into the conveyor hub to lead the clarified centrate from the foot of the beach back to the front hub, between the exit ports of the feed zone. The countercurrent conveyor has its feed zone in the conveyor but abreast of the foot of the beach. Clarified liquor is free to flow around or through the conveyor flights to the front hub while the cake is scrolled to the rear, and up the beach.
The orientation of the bowl (vertical or horizontal) does not usually give rise to a variation in conveyor design.
The conveying action can induce a large axial thrust towards the end of the bowl opposite to the beach. On the larger horizontal decanters, Alfa Laval employs a tension bar on the conveyor to counterbalance this thrust and take off the load that could be placed on the front bowl hub. The tension bar consists of a substantial hollow bar bolted to the rear face of the conveyor. It projects th rough the spindle of the rear hub and is locked in position with a large nut outboard of the spindle. The hollow centre affords access for the feed tube.
66 Variations to Main Components
2.3.6.'1 Conveyor hub
The conveyor hub will vary according to the type and position of the feed and floc zones. Also the size and type of conveyor bearings and seals will make some differences to the engineer ing design of the ends of the hub. However, these last differences will not affect process performance. The diameter of the hub is usually minimised wi th in limits of, for example, mechanical s t rength and the size of the feed chamber to maximise pond volume. However, in some applications it is necessary to increase the hub size to shorten the flight height and so enhance the scrolling torque capacity of the conveyor.
To the hub will be a t tached any baffle discs or similar devices which the process needs. These are discussed in Section 2.4.6.
2.3.6.2 Flights
An al ternat ive to the solid helical flight is one with "windows" to allow axial flow of the clarified liquor from the feed zone area to the centrate discharge. The way these windows are manufac tu red can vary. The simplest design has holes cut out of the flights before they are welded onto the hub. A more sophisticated design (see also Section 2.4.7.1 ) has a ribbon flight welded to the extremities ofpillars hang ing from the conveyor hub.
The rake or cant of the flight is varied for some applications, as is the conveyor pitch or pitch angle (see Sections 2.4.8 and 2 .4 .10 ).
The vast majori ty of decanter conveyors have a single flight, or lead. There are some, however, which have two, three or more leads. A multi-lead conveyor would have the flights equispaced inter twined on the conveyor hub. When the multi-lead conveyors have solid flights, which is more usual, care has to be taken that the feed is distributed evenly among all the flights. Multi- lead conveyors can reduce the out-of-balance problems, which may result from the asymmetry of the single lead designs ( see also Section 2.4.1 1 ).
h r I
t,,
Figure 2.48. A co-current conveyor.
2.3.6.3 Fred m r w
Simplc variations in thc dcsign of thc fccd zonc iiicltldc thc number of exit ports. the number arid shape of the accelerator vanes and lie size 0 1 the chamber .
'l'he exit ports of the feed zone can be simple holes cut in the conveyor h u h , possibly hard surfaced. or with specially I 'abriuakd cylinders hohed into position. More sophisticated exit ports will be specially shaped to rriiriiniise contact with the process material. Sometirries, exit ~.ioules are used 10 0irec.t
68 Variations to Main Components
Second Flight First Flight
First Flight oeconu r~lgnl
Figure 2.52. A double lead conveyor.
feed onto the back of the conveyor flight (directional feed nozzles), to minimise disturbance in the pond.
The smaller the feed zone is relative to the feed rate, then the more sophisticated has to be its design, to ensure that the feed is accelerated to speed without flooding and consequent spillage back through the feed entrance. To this end, sometimes there will be seen accelerator blades to the rear of the feed zone. Also the entrance to the feed zone will sometimes have a tube fitted to surround the feed tube. This ensures a high velocity for the air sucked in around the feed tube to prevent escape of liquor from the feed chamber .
Directional Feed Nozzle
Directional Feed Nozzle
Figure 2.53. A directional feed nozzle.
Decanter Design 69
Round Feed Ports
Oval Feed Ports
Long Elongated Feed Ports
Figure 2.54. Some alternative designs of feed zones.
If large t ramp material enters the feed zone, from the feed tube, it may lodge in the feed zone if this zone conta ins subs tan t ia l dead areas. Thus, the walls and shape of the feed zone must be designed, not only to accelerate the feed adequately, but also so as to avoid plugging.
2.3.6.4 Floe~rinse zone
The variat ions of floc and rinse zones are ma in ly associated with size and how the rinse or flocculant d ischarges into the main process s tream. Simple accelerators on the chamber wall are somet imes fitted. The more efficient floc chambers will discharge the flocculant evenly adjacent to the feed d ischarge into the pond.
70 Variations to Main Components
F l o c c u l a n t Z o n e
F e e d P i p e I ) A d d i t i v e
Figure 2 .55. A floc zone.
' " " �9 . . . . . . ' " F e e d T u b e '
, ~ ~ l . ~- ; : ~ l ' " ' - " :': ~ " " ~ ",." " R i n s e N o z z l e s R i n s e , i ~ " - ,~ ,..~..ee~ r. ~ -:: : _ ~ ~ ~,. r _n -I
H " ~ . ,--~, ~u,~r.,Ch~~~.~/_/ \'\ " I~~
~'_.~..-t~.. ~~,~ ~~ ~ ~ ~. " ~':i."" .Po.d.,. , ' .... ": ' ..... .
' . " . . - " Rinse Rinse ~ 19 ~
-~-," . : - ' - : " ",~ ...... " : ~:.~-"~"'" " " "\ " "" ~ R i n s e Z o n e ~ - - - �9 "
Figure 2 .56. A rinse zone.
On the slower ro ta t ing bowls there is less need to mix the f locculant inside the bowl and flocculant is then admit ted into the feed zone t h r o u g h a flexible
pipe inside the feed tube, or directly into the feed line. The discharge from the floc zone is ei ther simply t h r o u g h holes in the
conveyor hub or via tubes or channe l s fabricated in it.
Decanter Design 71
As a rinse zone, discharge is at a different point from flocculant, often at the junct ion between the wet and the dry beach or spread over a finite length of wet and/or dry beach. Special spray nozzles are often used to spread and direct the spray, to improve rinse efficiency.
2.3.6.5 Wear protection
Wear protection is required on various parts of the rotat ing assembly when abrasive sludges are processed. It can also be required in the casing, where the discharged cake impacts.
Materials for hard surfacing where required are many and varied, and will depend upon where they are used and the degree of erosion expected. In the bowl, wear protection, when required, will be applied to the conveyor flight tips, the flight conveying surface, in the feed zone, and at the cake outlet. The flight tips can be coated with a flame-applied material, such as a metal alloy (nickel- or cobalt-based) conta ining tungsten carbide particles of varying size and shape, followed by a fusion process. Sometimes liners are fitted, for wear protection, to the inside of the bowl and occasionally to the beach surface. Fitted to the bowl or beach, the liner would double as a scrolling aid.
Colmonoy or Stellite, which are cobolt-based alloys, are alternative materials that are applied by flame spraying. While these can have higher
Figure 2.5 7. Some flight tile designs.
72 Variations to Main Components
Hard Surfacing on Flights
Flights
Figure 2.58. Flame applied hard surfacing to flight tips.
corrosion resistance they tend to be porous, and have less wear resistance than tungsten carbide. This is even though flame applied tungsten carbide is more porous than the cobalt alloys. Alternatively coatings can be applied by arc welding using special welding rods. Other application methods exist, such as plasma spraying and HVOF (high-velocity oxy fuel), but in general they will not be able to produce the same layer thickness as flame spraying. The wear resistance can, however, be higher.
Instead of coatings, special tiles can be affixed. These tiles can be specially shaped and profiled stainless steel, or even Hastelloy, backing plates brazed, silver soldered, or epoxy bonded with, say, plates of sintered tungsten carbide, a lumina or even more exotic material . The fixing method, ceramic to backing plate, depends upon the envi ronment in which they are to be used. The tile assemblies are carefully welded to the flight tip. Some tiles are even riveted in place. Where welding is involved care must be taken in using the correct weld- ing rods to prevent electrolytic corrosion, or in tergranular corrosion, in use.
Tungsten carbide is a common material used for tiles, but its exact manufactur ing method can give at least one order of magni tude difference in its abrasion resistance. Care therefore has to be taken when trading quali ty for costs. Tungsten carbide, when used as a tile, is a matr ix of tungsten carbide particles, sintered with an alloy binder. Care must be taken with the materials for the matrix in which they are sintered as these may corrode, leaving a porous honeycomb of the actual abrasion-resistant material , which readily collapses. The quality of the sinter depends upon the processing conditions, the binding alloy, and the composition and size distribution of the carbide.
Many other ceramics are used for hard surfacing, particularly alumina. Silicon carbide is another impor tant ceramic used for wear protection, which
is stronger and more wear resistant than alumina.
Decanter Desiqn 73
Besides ceramics (the most frequently used material), some types of rubber, such as urethane, can be used, mainly in the feed zone. One manufac tu re r has even used rubber on the conveyor flights.
Thus, the variat ions in hard surfacing follow from the type of mater ia l used, its extent and how it is applied.
Separate wear components are to be preferred wherever possible, as repair is more simple and inexpensive when high wear rates, higher than expected, are experienced.
2.3.6.6 Bearings and seals
The type of bearings used in the conveyor varies from manufac tu re r to manufacturer , and varies with the duty. For example, e levated- temperature operation could require a different bearing clearance rating. Similarly the materials used for the construction of the seals will vary with the application and the environment . Note that the conveyor seals are not only required to contain the lubricating medium but also to exclude process material . Some bearings used will be grease packed while others require forced oil lubrication.
2.3.7 Gearbox
The epicyclic gearbox is described in Section 2.2.7. The main al ternative is the Cyclo type gearbox. The German Cyclo Company was founded in 19 31 when Lorenz Braren invented the Cyclo gearbox. Shortly after this date Sumitomo of Japan licensed the device and eventually absorbed the Cyclo Company into its own group.
Another German company, Maun, manufactures gearboxes that work on similar principles. These use interlocking toothed gears, whereas the Cyclo gearboxes employ cycloid discs. The outer edges of the cycloid discs engage a series of rollers, situated on the inside of the gearbox casing. Because of the mechanical complexity of cycloid discs, it is easier to describe the Maun gearbox first.
Consider a gearwheel with external teeth rotating clockwise inside a slightly larger gearwheel with internal teeth. As the inner wheel processes around the inside of the outer wheel, it revolves anticlockwise on its own axis. An input shaft, on the axis of the outer gearwheel, with an eccentric disc central with the inner gearwheel, is fitted such that rotation of the shaft causes the inner gearwheel to travel around the outer gear. A bearing is fitted on the eccentric disc to carry the inner gearwheel.
A series of equispaced holes perforate the inner gearwheel at a fixed radius. An output shaft is fitted on the same axis as the input shaft, with a series of pins which engage in the aforementioned holes of the inner gearwheel. The holes in the inner gearwheel are such a size as to allow this gearwheel to engage the outer gearwheel at any position on the outer gearwheel 's teeth
74 Variations to Main Components
G Cycloid Disc 2 Centrifuclal Force from
Torque from Disc 1
Torque from Disc 2
Cycloid Disc
Centrifugal F Disc 1
Balance of Internal Forces
Figure 2.5 9. A C!lclo gearbox (By courtesy of C!lclo ).
without distorting the output shaft. Whatever are the relative positions of the two gearwheels, the holes and pins remain in contact. Note that Figure 2.59 depicts a Cyclo gearbox, where the teethed gear wheels of the Maun-type box are replaced by cycloid shaped discs.
Decanter Design 75
Conveyor D r l / / - "'-'"'- q [
Lobed Excentnc Wheels " Tracks
Figure 2.5 9.-(continued).
When used as a decanter gearbox, the input shaft, called the pinion shaft in decanter terminology, is held s ta t ionary or is braked to a speed below bowl speed. The body of the gearbox, holding the outer gearwheel, rotates at bowl speed. In consequence, the output shaft, which engages the conveyor, rotates at a slightly faster speed than the bowl.
In practice, the device as described so far would cause balancing problems. To overcome this situation, the input shaft has a double eccentric fitted with two inner gearwheels 180 ~ apart. The pins of the output shaft protrude th rough the holes of both inner gearwheels, so that contact of the hole of one gearwheel with a pin is 180 ~ around that pin from the contact of the hole of the other gearwheel (see Figure 2.59 ).
The Maun Company manufac tures a similar second design of gearbox with pins fixed to an end face of the gearbox casing, and the gearwheel with internal teeth as a part of the output shaft. This second design is such that the conveyor rotates slower than the bowl.
The Cyclo gearbox, as stated, uses cycloid discs (see Figure 2.59) in place of the inner gearwheels of the Maun gearbox. The outer gearwheel is replaced by a series of rollers on pins. There is one less roller than the number of lobes on the cycloid disc. The drive pins of the output shaft are also fitted with rollers.
The ratio of a Cyclo gearbox is from about 6 to a max imum of about 120. Therefore, for the decanter, many Cyclo gearboxes need only be single stage, which substantially reduces manufac tur ing costs.
It remains to be described how the output shaft of the Cyclo gearbox is a t tached to the two cycloid plates. The output shaft has another ring of pins and rollers on its inner end. These rollers engage in corresponding circular holes in the two cycloid plates. The holes in the cycloid plates are of such a diameter that each hole wall is always in contact with its corresponding roller in spite of the eccentric throw of the cycloid plate.
To calculate conveyor differential, the same relationships as described for the epicyclic gearbox are used, except, as already stated, the Cyclo differential is faster than the bowl speed whereas the epicyclic is slower.
76 Variations to Main Components
The Cyclo gearbox torque characteristics are different from those of the epicyclic. Its torque capability varies with differential speed while the maximum torque transmission of the epicyclic remains essentially constant for all differentials used. According to the manufacturer 's performance data. the torque capacity of the Cyclo gearbox approximately halves when differential is increased from minimum to maximum.
For both designs the conveyor torque equals the pinion torque times the gearbox ratio whatever the differential.
For a given torque rating the Cyclo gearbox tends to be slightly larger in diameter but shorter than the epicyclic gearbox. The Rotodiff (see Section 2.4.15) will be less heavy than either, which gives it an advantage when extra high torques are required. This is because rotodynamic and mechanical stress constraints limit the weight that can be cantilevered from the rotating assembly. When choosing a gearbox its weight for the required torque capacity is an important consideration.
2.3.8 Frame
The basic variations in flame design have already been discussed in Section 2.2.8. One further variant is one where the frame and lower half of the casing are fabricated together. The casing then becomes a part of the flame.
2.3.8.7 Bearing supports
The pillow blocks housing the main bearings are generally bolted and dowelled to the flame. Sometimes the blocks form an integral part of the flame. The bearings will have an interference fit in the pillow block. Alternatively the pillow block could be split allowing easy separation of the rotating assembly from the frame. Frame and bearing designs should permit interchangeability between rotating assemblies to permit reduced spare parts cost and lower maintenance time with multiple machine installations.
2.3.8.2 Feed tube
The main variant of the feed tube is one with a floc feature. In this design two tubes are held together concentrically. The inner tube takes the feed while the outer shorter tube is used for the flocculant or rinse. The flocculant or rinse leaves the outer tube via holes drilled radially at the sealed front end.
As discussed in Section 2.2.8.2, care has to be taken in avoiding resonance of the feed tube. The concentric floc tube helps with the rigidity. Tapering the feed tube and making it of lighter materials also help. The resonance frequency of the feed tube is reduced when feed is admitted to it. It is not unusual for the feed tube's resonance frequency to be higher than bowl speed when empty, and lower than bowl speed after feed starts to flow.
Decanter Design 77
F l o c c u l a n t
O | , ~
F l o c c u l a n t
Machined Surface
l j / ,
Locating Shoulder
Figure 2.60. A feed tube with floc feature.
Flocculent
~Feed
2.3.8.3 Vibration isolators
There are several types of vibration isolator in use. Some rely on the elasticity of rubber components, while others use coiled springs. It is important that each mount is rated for the share of the load it has to take. The rubber mounts would tend to be used for smaller machines, while the springs would be used for larger models. Damping features of vibration isolators limit the magnitude of vibration during the starting and stopping of the decanter.
2.3.9 Casing
As well as the standard casing, split and gasketted along the centre line, there is the single-piece casing that is fitted over a cantilevered rotating assembly (see Section 2.4.1.5).
A second variant is where the casing is made of two separate split casings, one for the liquor discharge and the other for the cake discharge. This type of casing has the advantage that cross contamination is eliminated completely. (The variant where the casing is integral with the flame has already been mentioned in Section 2.3.8.)
Cylindrical casings, with no gasket required, are also used, but these require axial disassembly from the ends. Special tools and greater floor space are then needed to remove the rotating assembly.
2.3.9.1 Baffles
The number of baffles used in the casing depends upon the manufacturer and the potential for cross-contamination of the two products relative to what is permissible. It would be unusual for there to be more than two baffles each for either product end.
The simpler and slower the decanter, the less is the baffling needed. Some baffles are fitted with a large clearance from the bowl while others penetrate grooves in the bowl to form labyrinths.
78 Variations to AIain Components
The method of sealing the two halves of each baffle also varies. In some there is no attempt to seal, leaving a gap the thickness of the main casing gasket. Alternatively the two halves of the baffles are overlapped and slightly sprung. A gasket at each baffle joint is unusual , but not unknown.
When the centrate discharges from the bowl a lot of turbulence occurs with splashing back onto the bowl. Some of this splashing may get into the baffle grooves causing an unacceptable level of cross contaminat ion. To prevent this, a gutter is sometimes fitted to the baffle adjacent to the centrate discharge.
2.3.9.2 Cake discharge
The variations in the casing cake discharge are mainly associated with aids introduced to maintain a smooth cake discharge without sticking.
Liners fitted to the inside of the casing are not unusual , either in the form of HDPE, PTFE or urethane rubber to prevent sticking, or ure thane rubber or, say, Stellite to counteract erosion.
Sometimes an inner casing is closely fitted outside the bowl discharge ports to form something akin to a racetrack (or "Rennbahn") . This can be aided by impellers, bolted on the outside of the bowl adjacent to the bowl discharge ports. The outlet of this inner casing or collector is strategically sized and
Baffle Gutter ,~ ,~
Figure 2.61. A casing baffle with guttering.
iI
Decanter Design 79
positioned to ensure that the discharge is directed at the main cas ing outlet. Racet racks with impellers add greatly to ambient noise.
On a very old design a similar device was used but, instead of impellers on the bowl, an externa l screw or conveyor was used.
As an a l ternat ive to the collector, a v ibrator or a reciprocat ing scraper inside the main casing has been used.
2.3.9.3 Centrate discharge
The alternative to free discharge of centrate into and from the casing is to use a pump or skimmer inside the bow]. These devices are discussed in Sections 2.4.2 and 2.4.3.
2.3.9.4 Casing seals
There are several possible var iat ions in the type of gaskett ing used, the types of mater ia l employed and the shape of the gaskett ing. From the simple flat gasket, there are then those with a circular section, sitting in a groove, to those of L or U shape located over the casing flange.
Care has to be taken with choice of materials, especially with non -aqueous liquors, to avoid swelling and deterioration.
Bowl
Cake Discharge
Figure 2.62. A cake collector.
80 Variations to Main Components
There are variations as to how the rotat ing assembly hubs are sealed through the casing ends. A plain bearing seal such as PTFE can be used. In more sophisticated designs, a rotating seal is used particularly where it is necessary to have a contained process. Mechanical seals are used when the casing needs to be pressurised. See also earlier sections, part icularly Section 2.2.4.4.
2.3.9.5 Vents
To control windage associated with the decanter, the centre compar tment of the casing is sometimes connected to a ventilation system. It is necessary to ensure that air is allowed into the centre compar tment and allowed free exit at either end. This helps to reduce the likelihood of cross contaminat ion of the two products.
2.3.10 Sub-frame
The simple function of the sub-frame does not allow much variat ion apart from whether or not to use one at all. The sub-frame on smaller designs provides a platform for mount ing main flame, main motor, and back-drive, enabling rigid connections before mount ing onto vibration isolators. Modern engineering designs allow easy mount ing of both motors, with flange mountings, onto the main frame.
2.3.11 Main drive
The variations in the main drive are in the choice of motor design. With modern electrical developments the number of choices has expanded. These
include:
�9 three-phase motor with fluid coupling and direct on-line start; �9 specially wound three-phase motor with star-delta starter;
�9 DC motor; �9 inverter motor: �9 high voltage three-phasemotor ; �9 soft start motor: and �9 hydraulic motor.
In processes using solvents, flameproof drives will be demanded. The fluid coupling, a slipping clutch type of design, is a device sometimes
mounted on the motor shaft allowing it to be started at full speed with minimal torque. Frictional drag from the fluid within the coupling slowly brings the rotating assembly up to speed with the release of heat within the fluid. It allows the use of a s tandard motor in spite of the high inertial load. Small
Decanter Design 81
Main Motor
Figure 2.63. A main motor with fluid coupling.
Fluid Coupling
I"I i i i Hi.4 i~ki i i iiii,.,,, l i I I I I ~ L J , P - II I I I1 , IL~LI II l i i z . , , i~G~ii I I "~-.l. . . . . . . - - I - - ~ " I I I I II iP'qt I ~ T T . . . . - - T - r u _ i - - - - - L JJ 11"~
I 1 I ,~1
, m . I . . . . . IIII11 I
tX I1 IP,~l l I ' J rll Imlll I
I II lllllll I , , I I I i lt1111 I I I I I IP':P, I I I ~ ~ I I ' k - - - - I m M I I I I I I I i l l I R i t U I I l i l T - " - I~ , t t l l ! I I I I I I li+~lll I I I I II I ll~ll I I I l i p I I ~ t l l I T PII I I~:'PII I
u i
G e a r b o x G u a r d G e a r b o x A s s e m b l y
fT . . . . . . . I _ , J ( ~ ' I m - - I I I I I l l
. . . . . . . . . . . . . I I I I - - ! 1 " 1 1 I ' 1 1 r I _ _ . . . . . . . . . . .
Upper ias lng
n Frame Vibration Mount
F l a n g e d M a i n M o t o r (Frame mounted)
Figure 2.64. A.flanged main motor mounted on main fl'ame.
decanters often use standard motors, but larger decanters need special motors or starters, s ince the inertia seen by the motor increases more rapidly than the power needed, making starting more difficult.
Some of the ways of m o u n t i n g the motor have already been discussed. These include m o u n t i n g oil slide rails, on a sub-f lame, m o u n t i n g on the f lame
82 Variations to Main Components
/// i..--.----
"'~" "q L~
,~..A. ~ ~ -~,
/
f . . . . . . . . . . F " ~
!I ~ i J ~".. I
Figure 2.6 5. A floor mounted main motor with belt tensioning device.
direct, and bolting to the ground direct with a special tensioning device to allow for movement of the centrifuge assembly on its vibration isolators. Flange mounting the drive motor onto the main flame is also an option.
2.3.12 Back-drive
There are many methods for controlling the conveyor differential. When variation of differential is not required the system could simply be a balanced arm on the gearbox pinion shaft, held stationary by resting against a sprung- loaded stop. With a differential requirement of something less than the maximum then the gearbox pinion shaft has to be rotated at a fixed speed. This can be done using a fixed speed three-phase motor with belt and pulleys. Alternatively a countershaft system is used whereby an idler shaft is rotated by a pulley and belt from the gearbox casing. Different belt and pulleys then drive the gearbox pinion shaft from the idler shaft.
Both these last two systems need a rotating clutch somewhere in the system to cater for overload situations. They also benefit from the ability to permit some change in the choice of differential by appropriate change of pulley sizes.
An advance on the fixed speed differential is the variable speed back-drive. This design uses variable speed motors. They may be variable mechanically
Decanter Design 83
with, say, split pulleys or special mechanical gearboxes. Otherwise they will use variable speed electrical motors, DC or inverter type or even a hydraulic motor.
A further alternative, which has been widely used, is the eddy current brake. This offers simplicity and cost effectiveness, but its disadvantage is that it cannot drive, nor can it regenerate power. The eddy current brake comprises a copper torque tube, rotating in a magnet field induced by an electric current through copper coils. Eddy currents are generated in the torque tube producing a braking torque. The extent of the braking torque is controlled by the amount of current which is allowed to flow through the coils.
Hydraulic conveyor drives are inherently variable (see Section 2.2.12).
Direction of Rotation
e Arm
~ Compression Torque Microswitch Link Control Lever
Figure 2.66. A torque overload arm S!lstem.
84 Variations to 2vlain Components
\
\
? L - L ; ; - - - - . . . .
. . . . . :;,,, ,,,.,. �9 ,.,..! J...- , .
", ' .": . z , ; >,;' " , . . ' " " ' "
Figz~re 2 . 6 7 . A coz~ntershaf t back-dr ive .
Decanter Design 83
Figure 2.68. Aneddy current brake back-drive system.
2.4 Special Features
It could be argued tha t many special features have been covered already and indeed at least some have been mentioned. It is considered that the variants already discussed are alternatives to do the same job as the basic component , but in a different way. The special features, now to be described, are designs that enable a unique task, or designs used by only a minori ty of manufacturers , or that are used for specific applications, or have enhanced efficiency in some respect.
2.4.1 Basic construction
There are a number of special var iants of the basic horizontal or vertical decanter for simple clarification and dewatering. Special designs are available for three-phase separation, for thickening applications, with a filtration section, for leaching, for washing, and those for operating at extra high temperature and pressure.
2.4.1.1 Screen-bowl decanter
The screen-bowl decanter is a basic decanter wi th an extra cylindrical section attached to the end of the beach. This extra cylindrical section is perforated with a bar screen fitted on the inside wall. The cake is scrolled from the beach and over the screen to undergo further dewatering by filtration. Some rinsing may be added to the screen. The conveyor itself wil l be extended with a cylindrical section to perform the scrolling over the screen. This decanter can be built in co-current and countercurrent format, but has a narrow range of applications. It has been used in the low-temperature separation of para- xy]ene, and in coal washeries. On abrasive applications the bar screen must be resistant to erosion. Hard surfaced and carbide variants of bar screen have been used for such applications.
2.4.1.2 Three-phase decanter
The three-phase decanter is a common variant for the separation of oil, water and solids. In these designs the cake is separated and removed as in the
Decanter Design 87
FRONT (Centrate Discharge)
Screen Section
I Decanting Section I REAR
Beach Section I (Cake Discharge)
Figure 2.69. A screen-bowl decanter.
two-phase design. Extra devices or baffles are required inside the bowl to separate and discharge separately the two liquids. The lighter liquid will discharge over the weirs already described in the normal way. The heavier liquid needs to decant under a reversed weir into a chamber in the hub and then can flow over and through nozzles of adjustable height.
Depending upon the relative quantities of the two liquids, it may be desired to decant the light phase from the nozzles and the heavy over the weir plates. This can be done by adjusting the relative heights of the weirs and nozzles, and altering the liquid/liquid baffle. Sometimes the weir ports in the front hub are alternately light and heavy, making a compact design. Half the weir ports only will be as for the two-phase design. The other half will have an inverted weir on the inside, to preferentially receive the heavy phase, with a dam plate on the outside to control the hydraulic head. The two separated liquids, with this latter design, have thus to be kept separate by appropriate baffling in the casing, allowing for the ever-present problem of windage.
To ensure separate collection of the two liquid phases, the discharge positions of the two phases need to be separated by a finite axial distance. To do this one of the discharges can be taken radially through the hub via drilled channels. If necessary a shroud is fitted to the bowl end to take the discharge further from the front.
Naturally, in three-phase designs there are many possibilities for variations with the fitting of special baffles, chambers and channels into the front end of the bowl but the principle is the same. However the use of a skimmer (see Section 2.4.3) in three-phase designs is very useful in controlling the equilibrium line between the two liquid phases while the bowl is at speed. The casing design is, of course, different.
88 Special Features
. .
. . . r . . , ' - - ,
Water Oil
Figure 2.70. A three-phase decanter.
Decanter Design 89
2.4.1.3 The countercurren t extractor decanter
This type of decanter is patented [3] and built by Westfalia, and is used for countercurrent extraction. It is similar to a three-phase decanter, except that the second liquid phase is purposely added separately to pass, in the bowl, through the first liquid phase. The two liquid phases are caused to run countercurrently in the bowl and the wanted product is leached from one phase into the other. Separate designs are available each for heavy and light raffinate. This type of decanter is often used in the pharmaceutical industry.
Main Bea,ring \
Counter-current- Separation Zone extraction Separation Zone
Centrtpetal Separating Disc ~ P u m p
Feed Tube
Cyclo Gear / ' \ I / Discharge (Extract)' Distributor Holes Regulating Ring Main Bearing
Discharge Distributor Conveyor
Main Bearing
Counter-current- Separat=on Zone extraction Separation Zone
- " l ' - - - - - ~ _ _ /Centripetal Purnp Separatmg Dtsc
F~dTu~
Cycio Gear
\
E -L \ Discharge (Extract Discharge Bowl Regulating Ring Main Bearing
l Conveyor Distrtbutor
Distnbutor Holes
Figure 2.71. Two t~lpes of a countercurrent extractor decanter (By cour.tesy of Westfalia ).
90 Special Features
2.4.1.4 Decanters for temperature and pressure extremes
Many processes, part icularly in the petrochemical industry, require the decanter to operate at elevated tempera tures and pressures, say up to 400~ and 10 atm. In these cases special designs and materials are used for bearing seals and lubricants. For the higher pressures, mechanical seals will be used.
These decanters are made in both horizontal and vertical construction, a l though those that supply the vertical design claim that this design is the most reliable for the application, while others would hotly dispute it.
2.4.7.5 The cantilevered bowl
In the early 1960s a range of horizontal bowl decanters was produced with just one bearing cartridge with two main bearings such that the bowl was cantilevered from these bearings. At least one manufac turer offers such a machine as interchangeable with a horizontal basket centrifuge, using the same drive assembly.
The bowls of these decanters generally have small length-to-diameter ratios, nearer 2 than 3. The design allows the free end to be open or at least very accessible. Maintenance, relatively, is much easier.
With large diameters, large conveyor hubs are possible allowing high conveyor torques. Generally these decanters have been used where clarification is easy and where high solids volume or torques are required.
2.4.1.6 The "'hubless " conveyor
A design has been patented (1992) by Alfa I, aval, which involves a decanter with a conveyor with a much reduced hub diameter [1]. The conveyor consists of a ribbon flight mounted onto a set of longitudinal radial vanes welded to the hub. The feed zone area is completely open. Thus, the vanes spanning the feed zone hold the front and rear sections of the conveyor together. The design allows the pond surface to be brought very close to the centre line. This allows much easier admission of the feed and the power consumption for accelerating the feed to bowl speed is much less. The main disadvantage is that the g-level at the pond surface is appreciably reduced.
2.4.'1.7 Thickening decanter
In a thickening operation, where all tha t is wanted is partial removal of some liquid, both of the two main flow options are possible, but the type of control with each could be different. One control option is to run the bowl with a pond level close to the level of solids discharge (conventionally called "neu t ra l " ) and to control the differential at a precise low level. A second option is to run the pond much shallower and operate the differential much higher and less
Decanter Design 91
Radial Vane Supports for Ribbon Conveyor Ribbon Conveyor
Ribbon Flight
P=.-- Radial Vanes
Section 'A' - 'A'
Figzlre 2.72. The "hubless" conve!lor.
precisely. The high differential prevents too m u c h drying and scrolls liquor out with the solids. In the first option some baffling on the conveyor (see Section 2.4.6.1) will probably be necessary.
However, if necessary, the cake could be allowed to over thicken, by setting the decanter to dewater, wi thout any need for fine control, with some feed by- pass mixed in with the cake at discharge, to reduce dryness to the desired level. This enhances overall capacity and can reduce overall flocculant consumption when used, as the by-pass requires no flocculant. This option is best used with those slurries that are more readily dewatered.
The thickening decanter will differ slightly from the dewatering decanter in the casing. In the thickening decanter, any contamina t ion into the centre compar tment needs to be drained to the cake end, rather than the centrate end as in the dewater ing design. In dewater ing the cake remains solid and will not flow and contamina te the centrate, whereas in the thickener both process streams are fluid, and any losses are better directed to the thickened cake.
The "B" range of decanters manufac tured by KHD (now Bird Humboldt) are unique in having one, or possibly two, nozzles in the bowl wall, allowing the discharge of the bulk of the thickened product. The remainder of the product is scrolled up the beach, which is much steeper than normal (as much as 45~ Discharge from the nozzle can be made intermit tent by providing a zero pitch to the conveyor for less than one tu rn at the nozzle location, thus covering the nozzle for a fraction of each conveyor turn.
92 Special Features
Figure 2.7 3. Nozzle discharge thickening decanter.
An interesting development [4] is offered by Bird, in which the conveyor in the clarifying zone, normally the cylindrical section, is replaced by rakes. Moreover, the clarifying section is made slightly conical, up to about 5 ~ semi- included angle, narrowing towards the front hub. The principle of this design is that the clarifying zone is free from turbulence created by a conventional conveyor, enabling the conveyor to be rotated faster to provide a higher than normal capacity. The sediment in a thickening application tends to behave similar to a liquid and thus in this decanter will fall down the 5 ~ slope into the path of the conveyor at the start of the beach (Figure 2.74).
2.4.1.8 The dual beach decanter
KHD (now Bird Humboldt) manufactures a decanter with a beach at each end of a short cylindrical section. This decanter is designed to separate light and heavy solids, particularly plastics, from a liquid. A baffle disc is mounted on the conveyor at the centre to prevent floating solids passing from the feed entry side to the heavy solids discharge end. On the feed side of the baffle, an extra small diameter flight section is added, with reversed pitch, between the main flight and conveyor hub (see Section 2.4.10.2). This extra flight is for scrolling light solids to the light solids beach. The clarified liquid decants into tubes, or nozzles, inserted into the bowl wall adjacent to the central baffle disc on the heavy solids side (Figure 2.76).
Decanter Design 93
Bowl
1tJ ~ Conveyor
I Rake Assembly -~ �9 i �9
h r - T I - 71- -~o~ol- ~ - J L
,! �9 l, II
Conveyor
Beach
FEED
Clarifier Heavier Phase Bowl
NO CONVEYOR
CLARIFIER -
CONVEYOR
BEACH
Figure 2.74. Tapered bowl thickening decanter ( Bzl courtesy of Bird).
Gearbox Adjustable Impeller
! �9 ,
Centrate Discharge
Solids
Figure 2.75. The Flottweg Sedicanter ( B!! r of Flottweg).
2.4.2 C e n t r i p e t a l p u m p
Discharge of clarified liquid from a decanter can be achieved in a completely different way using a centripetal pump. Fitted at the front hub, a centripetal pump converts the rotational velocity of the centrate to pressure on discharge from the bowl. Some control of pond height may be effected with the pump, while the bowl is at speed, using back pressure. Such a pump requires extra power. The centrifugal energy of the liquid is converted to kinetic energy for
94 Special Featz~res
i r~lF
Figz~re 2.76. A dztal beach decanter (By courtesz 3 of Bird Hzmzboldt ).
. . . . . . ~m ~,_.. ,,..:-:_ ~.
\ \
\ \ \
\ \
\
Liquid
Centripetal Pump
Figz~re 2.77. A centripetal pzmzp.
flow in the external pipe against a pressure head. It is capable of providing a pressurised flow to the following process operations with reduced air entrainment.
Decanter Design 95
2.4.3 Skimmer pipe
An alternative to the centripetal pump is the skimmer pipe, by means of which the centrate is skimmed or scooped from the pond surface in the bowl. The position of the skimmer in the bowl can be altered from the outside while the bowl is rotating, and so afford a means of pond depth adjus tment during operation. This is done using a lever on the outside of the casing at tached to a pipe positioned over and through an annular plate mounted on the front face of the front hub of the bowl. The skimmer pipe thus dips into the bowl pond
[
Regulating D~sc ~ - ~
Skimmer Pipe---.__..._
I
i �9 ! - J
I �9
! __J
I ! I
L ~ i
Sktmmer Pipe Radtal Adjustment Position
Collecting Chamber
Separating Plate
Regulating Tube
Figure 2.78. A skimmer pipe.
96 Special Features
inside the annu la r plate. The skimmer also has the penalty of requiring extra power for operation.
2.4.4 Centrate weir design
Over the years there have been many designs to facilitate op t imum centra te discharge rate and clarity. Some of these are described in Section 2.3.4.2. Other design endeavours have been made to enhance the area and length of the weir for discharge, to make height change easier or even while the bowl is at speed, and to overcome washout problems with very deep ponds during start-up.
2.4.4.1 Cup dam
An advance on the moon dam design (see Section 2.3.4.2) is the cup design. Here the whole dam bulges out from the hub in the shape of a part of a sphere. This maximises weir length.
2.4.4.2 Notched dam
The notched dam [5] is used to overcome washout from the cake discharge port during the start up of deep pond bowls. The inner edge of each dam plate is notched to bring the pond level below the cake discharge ]eve] at low flow rates. Thus, during start-up, the bow] is fed at low feed rate. When the cake has built up sumciently to reach the baffle and form a sea], the feed rate is increased to bring the pond level to its full height.
As an alternative to notching the dam, a small hole can be drilled in each
dam to produce the same effect.
Centrate �9 Discharge ,_~_/X//,///; ' //~
.... ] - 1 1 , , _ ~ ~ L . _ Z DaplaCover ~ ~ I
Cup Dam ~. y/~]
Figure 2.79. A cup dan1.
Decanter Design 97
A
C
E
B
D
F
Figure 2.80. Notched dams and equivalents.
2.4.4.3 Inflatable dam
Recently an inflatable dam has been patented [6]. Here a rubber toroid is inflated by the hydraulic pressure of a stream of control water fed under its surface.
The degree of inflation is managed by controlling the flow or head of control water. This inflatable dam sits just inside the hub to part cover the discharge ports. The control of the inflatable dam is useful in combating the washout at start up, previously described. Alternatively the dam can be used solely as a seal to be made after start up, with reliance on mechanical weir plates thereafter.
The inflatable darn is also useful in three-phase separation control, and in the control of thickening applications.
2.4.5 Noise suppression
Any protuberance on the bowl will induce noise, and the faster the bowl then the louder will be the noise. Impellers at the cake discharge are major producers of noise. Even bolt heads in the bowl can emit noise. Recessing the bolts helps to abate noise, but to improve the abatement one technique is to cover all major bolt heads with metal rings secured by much smaller flush bolts. These can be smaller as they do not have such a great load to bear. This action is more frequently taken on the larger high-speed decanters.
98 Special Features
m
Figure 2.81. Inflatable dam.
Decanters open to the atmosphere produce more noise than closed ones, and thus closing or sealing the casing outlets reduces noise levels.
Resonant vibration of various static components, part icularly in the casing, can produce irritating noises and thus care has to be taken to avoid flimsy and unsupported structures.
The ult imate noise suppression is the use of a purposely constructed noise suppression hood. These hoods can be very effective, but they do have
Decanter Desiqn 99
disadvantages apart from the cost. It can be a major exercise to initiate maintenance. There are advantages in being able to see and hear the machine to diagnose, or be alerted to, any problem.
2.4.6 Bowl baffles
The use of appropriately designed and positioned baffles, in a decanter bowl, can direct process streams in the directions required. Processes otherwise impossible are made possible, such as three-phase separation, and the use of pond levels above the solids discharge level. Baffles are used for both liquid and cake as well as floating solids.
2.4.6.1 Cake baffle disc
The cake baffle disc was patented more than 15 years ago by the then Pennwalt /Sharples company [ 7]. It has been variously known as the Lee disc (after the inventor), a BD disc, biological disc or a baffle disc. Together with a negative ring dam (pond level higher than the cake discharge) it was known at one time as Centriseal.
The baffle disc comprises a plain disc welded to the conveyor hub somewhere between the feed zone and the cake discharge. On countercurrent decanters the usual location for the disc is at the foot of the beach. However, more recently, the disc has been seen situated in several positions on the beach from bottom to top. Ideally there should be a minimum distance, say one pitch, between the disc and the feed zone. This is to avoid turbulence from incoming feed at the periphery of the disc.
The clearance between the tip of the disc and the conveyor outside diameter will be anything from a few millimetres up to about half of the pond
Cake Baffle D~sc
Figure 2.82. A cake baffle disc.
100 SpeciaI Features
depth. The clearance quoted is often disc tip to bowl wall. However, when the smaller clearances are used, the former has the most practical significance. This is because the difference between the two, which is the heel, is relatively static.
The function of the disc is to form a seal with the scrolling cake to prevent the escape of feed or clarified liquor when the liquid level is higher than the cake discharge level. This technique is often used in the dewatering of effluent sludges.
The baffle disc can experience considerable axial forces, and thus a substantial thickness needs to be used with good welding where it crosses the flight. In some instances a dog-leg is used where the disc is a t tached to the flight. This can produce unwanted paddling of cake, by virtue of the axial component of the dog-leg.
2.4.6.2 Baffle cone
The baffle cone has the same function as the cake baffle disc. In place of the disc at the foot of the beach, a cone the full length of the beach or a shortened one at the foot is substituted.
Feed is admitted to the bowl, inside the cone. This has the advantage of preventing disturbance to the cake scrolling past the feed zone. An extra
i . , i - t
, . . - - . , ,
WTI m - - -
r | - .
Figure 2 .83. A baffle cone.
I 8 1
w
leiitlire of'tht. cone i s that it allows tlocculant to bc addcd inside thc apex o f t h e cone. which ensiires ;in intimatc and uniform mix with the f e d as il lli)ws iiridtBr the feed zone. When using tlocculani thc mile has hern cititrwn t o produce Inrgc increases in cffificicncy over the siinplt. disc.
The gaps under cones arc usually around half t.he poiid tli?pl ti. Howcvcr. much smaller gaps havc bccn uscd in vegetahlr oil refining (in a thrcc-phasc dccaiitcr). Uniquely in vege~able oil refining a doi~blc coiiccnrric conr has hccn uscd where the caustic s o d a wash is fed hctwccn thc cones. ' Ihc wiisli exits a t thc pcriphcry or the cones under the surface ofthc oil .
'Ihis disc is the same as thc cake baffle disc hut titted closc 10 t h u co1itriil.e discharge. 11 is only siiited to thc ~ ~ ~ l ~ i t e r c u r r e i i t decanter.
As its [ i a [ ~ w implies, the tloatcr disc traps floating solids and prevents thcm dischiirging with the clarified centrate. 'I'hc cenlrate dips undcr thc disc before disr:h;irging, thus allowing solids with air attat:hetl lo h ; ~ v e marc time to de- aerate.
Floating solids build up on the surface ol' I h e pond bctwccn the floater disc: and the beach. Eventually thc floating solids build up sufficicnrly to be carried o n the back ofthc hcavy solids scrolling up l.hz he;ic,h.
The iloaterdisc can cause problems with venting of gas frnni the howl. gases ciitraincd or drawn in by thc fccd zunc. ' lo o ~ ~ r t ~ ~ t ~ ~ t this sorr~e snlall vent holcs can bc drilled at the root of thc disc above the pond surface.
2.4.6.4 C'onvPying baffle
The rwnveyiog hat'flc I H 1 is :I p;iteril by Alf;q h V i i I j 11 substitutes for t.he cake bafflc disc, I t consists of a similar disc. split along a radius and stretched to a largcr pitch than thc standat-d conveyor pitch. This largcr pitch can bc as much as doublc thc standard pitch. One end of thc conveying baffle is wcldcd 10 the coiivtfyiiip Pdce of the coiivevor while the other end is welded to the back face of' the conveyor, t .0 the rear of the first wrld. Thus the coiiveyirig baffle winds itself between [.he main coriveyor at a larger pit.ch eriabliiip the faster scrolling of the cakc nearcr the axis until it cnrnes up against. the rear o f the main flight.
If thc conveying flight is positioned nn t.he he;ir:h t.hen Ihe outside edge needs lo be (apered to suit the profile of thc beach. Sometimes it will be round that this t;iperirig is such that the clearancc bctwccn the baMe and the bowl or beach is made t.o reduct. lowards tiit: rear.
Thc longitudinal baffle 191 perfortiis tlre S ~ I W I'iinction as the rakc baffle disc hut in Corm is a rcctangular- plate fitted. h y wt!ldirig- between two adiaccni flights just bchind the feed znnc.
Wliilc his dtv icr i s simplr and eilsy to fit, it can be difficult to iisif. Rping longitudirially fitted. i t can C . ~ I I S P unwantcd paddling of t h e howl rontgnts, which intrrfm-rs w i t h thr hydraulic equilibrium i n the I I ( - I w I . Fur easy applications rnt!c:h:inlcnl simplicity Outweighs practical diRiculties.
this hijfflr has becn iiscd which is hiiiged. The idea behind this is lo h;ivr :iiilonialic adjustment of gap height to cope with varying solids hiids.
This Iornliit may haw problcms v v i t l i sealing of the sides, and very ntigittitrc porids. IS it could a l l o ~ w a ~ h o u t .
A hrrn
104 Specinl Fentrrrts
2.4.7 Clarification enhancement
Tn all but the last 10 or 1 5 years the majority of dccantcr applications have been limited hy [.heir ability to clarify the liquor. Scale up of thcsc centrifuges would bc bawd on thcir claritication ability. ' lhus, a lot o f I.he earlier dcvclopnieilt has bccn concentrated 011 enhancing clarificai.ion capacii y. Bowl speeds havc bccn incrcascd, bowls lcngthened and solids disr:hiirge areas increased to ensure solids chlokitkg does : i d inhibit clxificntinn rnlnme. Apart from thcsc obvious mcthods there are some 111eoh;tnic:;~l mm1ific:;jtions which can hclp,
Because o f the very high capdc:ii.ies possible through n rclatlvely smoll volrime. velocities in ii decanler can be very high. producing vcry high Reynolds numbers. Ti,rrbulent ilow i s not the bcst rcgimc to scttlc solids. $u incans h a w bccn dcvclopcd lo incrwsc thc clarification Icngth. reduce the cffcctive settling height and dccrcasc thc mean vclocity of the liquor to be clarificd, ' lhis fcaturc is :\Is0 known as Sigma cnhanccmcnt,
2.4,7:1 C!ud3;-axi;rl i l 0 l Y
With a riorrnal hrlical cwrlvrynr the clari f ied liqur:ir m u s t wind its way a round thc helix of the flight before discharge. The velocity ot't.he centra1.e i s thus much larger then i t would be i f i t could flow axially. Moreover tht! velocily must hc that much extra to overcome the screwing ellect ol'the ccinwyor.
To allow axial flow on iiiaiiy modern conveyors. holcs or "wrindows" are (:UI in ihr I1ight.s. '1'0 rnaintain a rcasonablc strcngth to the flight the open area [or flow is about 5 0 % of the cross-sectinnal area availahlc to t he depth of t he wiiidnws. Nntur;illy t he windows canriol he cul to the periphery of thc flight.
as a reasonable depth of flight must bc maintaincd to scroll thc cab. The nmoiinl ofribbon flight lek will dep~r id upon the scrnlllrig capacity required.
This qu;isi-;jxi;il Ilow Ceat.ure car1 he addetl afi.er t.he coriveyor is made alheii not without cosi. A hrt.ter desigri is possible wheri the Seeat.ure is iricluderl in !.he initial manufacture. A low-cost dcsign is possible by simply cutting out thc windows in thc flights before welding on, Wowcver an alternative dcsign i s to wcld pillws to thc conveyor hub, onto which is welded a ribbon flight. Becausc the pillars can bc iiiadc with greater strength a much greater open area for axial flow is possible.
?..1.7.2 Fully d a l lluw
This k a t iirc has thrt saint: f'uricl.icm ijs [.lie rl~~itsi-iixiiil tlow. In this design. longitudiiial radial vanes arc wclded t o thc conscyor hub. say 6 to 12 i n number. and a ribbon tlight is wclded on the outer edge of the vanes. 7'he viines extend Irom the leed zone to t'hc: front hub.
In this design axial tlow of liquor is assured wit,liniit any cirrurrifrrcritiaI ~ r t ' i l r e slip, The liquor cannot rotate around h e helix due t o the build-up of cake in lront ol'the flight scaling under thc vanes.
One problem with this dcvice is t.he Iwssiblc imc'vcn distribution nf flow b c t ~ c c u thc vanes. ' I 'o ~ v t r ~ ( ' m i s !his, ii small nuinbcr of hulcs in 1 1 i r viines will allow some flow bclween the rhanncls. I t i s iiiipnrtarit to rncike thc numbcrofchanncls rnatrh the number of'f'ccd ports in the crrnvcyor .
2.4.7..3 Vancs
In desigri this is similar to thc fully axial conveyor. exccpt that the vanes arc angled Lo the r a d i i nnd thcrc are many riinrP of thcm. I n present designs thc iiuniber o f vanes are 48 to 96 or imte depending iipoii the size uf decanter. They extend from just forward oCi.he feed zonc to the front huh.
'l'lie principle of thcsc vanes is (hiit they act like discs in ;i disc centrifuge separator. Ry virtue of the 17arie ariy,le the cffcctiye set.~liiig rlistancc fnr t.hy solids i s rnlirli rcduced.
106 Special Features
U "+U "L1 nU "El "El nU "U Beach Conveyor Assembly Removed
l:i~lure 2 .90 . ,tl~ (lm lh'd vt~ne conve!lor.
Theoretically the clarification capacity of a vaned conveyor can be doubled or increased even more. In practice the increase is somewhat less due to channelling of the centrate, and its not passing th rough the vanes with a uniform velocity distribution.
The vaned conveyor is not suitable for high solids loads or for those solids that tend to be at all sticky. The precise level of solids that is too high depends upon the settings of the decanter required. "Too h igh" is when the level of solids in the bowl start to reach into the vanes and impair their functioning. It is for those slurries with a small amoun t of very fine free-flowing solids, particularly inorganic solids, al though natural and artificial proteins have been processed very satisfactorily. This design is competitive with the more expensive high g, high-speed disc stack centrifuge, giving higher solids handling, drier cakes and acceptable clarities.
2.4.7.4 Discs
A decanter conveyor with a disc stack [10] has the same function as a conveyor with angled vanes. The disc stack comprises a stack of say 50 -100 conical discs very much the same as a stack of discs in a disc centrifuge. The
Decanter Design 10 7
Disk Stack
t | i / ! | i I , - " - I tDIsc Stack Assembly t ' / | I: ',1 I on Conveyor Hub t ' : | : , - I " - - " 1 ](less R,bbon Flights)
Figure 2.91. A decanter conveyor with disc stack.
stack is fitted around the conveyor hub at the front end, surrounded by a ribbon flight. The liquor is forced to flow between the discs from the periphery to the centre and then out over the front hub dams. The disc stack enhances the centrate clarity for a given capacity, or alternatively enhances the decanter 's capacity for a given centrate clarity.
2.4.8 Conveyor rake
A typical positive rake angle of a conveyor flight would be 1 ~ Sometimes this is automatical ly provided by the design of the tile wear protection. The reasoning behind such a raked flight is that it will tend to lift the cake from the bowl and in so doing reduce conveyor torque. Thus, this would not be used on soft cake/low torque applications but more on torque-producing, stiff, cohesive cakes.
108 SpeciaI Features
Variable Negative Rake Conveyor
Figure 2.92. A conveyor with variable negative rake (By courtesy oJNoxon ).
A negative rake would be used on soft sludges, those that tend not to produce sufficient friction for efficient scrolling. It is sometimes used to provide shearing of the cake. With moisture physically bound in some cakes, it is considered that some shearing of the cake will release some of the bound moisture. Notwiths tanding that, shearing of clay-like cakes such as effluent sludges is necessary to provide escape paths for released moisture. One manufacturer , Noxon. has patented a progressive (negat ive)raked conveyor to enhance cake dryness [1 1 ]. The rake of this conveyor increases from the front end of the conveyor for the full length to the cake discharge.
2.4.9 Conveyor tiles
There are many innovations with regard to conveyor tiles fitted to the flight tips. These are not only associated with the type of materials used, but also in the method by which they are secured. Many of these innovations are subject
to patents [12]. Ceramic or tungsten carbide plates are riveted, brazed, or epoxy bonded to
backing plates, which are then welded to the conveyor flight. The ceramics or tungsten carbide, while being highly abrasion resistant,
can be themselves highly abrasive. They are the materials used for cutt ing tools. Thus it is important that they are very carefully secured lest they break loose and cause dangerous wear to the bowl itself. As an added safety measure some ceramics are uniquely locked by dovetails onto their backing plates
before brazing or bonding or whatever [13]. The tips of the ceramics or tungsten carbide are generally chamfered to
provided min imum radial contact. The mount ing of the ceramic often,
purposely, provides a rake to the tile. A different form is the Alfa Laval plough tile [14], which, with its ploughing
action, provides an efficient turning over of the cake. Very high scrolling efficiencies are claimed for this tile with exceptional benefits for cake dryness on some applications. See Figure 2.5 7 for other tile designs.
Decanter Design 109
. ": . :
Figure 2.93. A plough tile.
2.4.10 Conveyor pitch
Generally manufacturers will have a standard pitch for each bowl diameter (about 5 ~ pitch angle) plus at least one other pitch generally smaller, a little more than half the standard. Very approximately these would be 30% and 20% of the inside bowl diameter, respectively. Some manufacturers have experimented with wider pitches.
2.4.10.1 Variable pitch
Many manufacturers have experimented with variable conveyor pitches. These have been used to enhance dryness in effluent dewatering. The pitch starts wide at the centrate discharge end, narrowing progressively along the
I ~l~-/lllll(((t~Zi~~,~_~ b/i(/ililo l ~ -
Figure 2.94. A variable pitch conveyor.
110 Special Featnres
Figure 2.9 5. The KHD dual pitch conveyor (By courtesy of Bird Hmnbohtt ).
bowl and, sometimes, up the beach. This theoretically provides an increasing pressure on the cake as it travels along the bowl.
A patent by KHD (now Bird Humboldt) specifies a standard pitch in the cylindrical section of the bowl with a nar rower pitch on the beach [15]. This obviates the need for a baffle at the foot of the beach as the nar row pitch produces a deeper cake forming the seal. The nar row pitch provides a back pressure on which the s tandard pitch can work.
An alternative use for the variable pitch is to improve centrate quality. For this the pitch of the flight adjacent to the front hub is increased, as much as double that for the rest of the conveyor. This design is particularly useful for applications in which the decanter can become heavily loaded with solids. The increased pitch tends to keep the centrate discharge region of the bowl relatively free of solids, thus reducing re-entra inment of solids as the centrate streams towards the weir.
2.4.10.2 Reverse pitch
A most novel design comes from Japan [16], and has been in existence for some time, but as far as is known has never been exploited to any large extent. This consists of a countercurrent decanter with an inner flight with a negative pitch. The driest cake wil l be found nearest the bowl and so the inner flight conveys the wetter cake in the opposite direction, to prolong its residence time. There wi l l be some reducing of the outer diameter of the inner flight to allow the forward cake flow to reduce to nothing towards the front hub.
2.4.11 Counterbalance and scraper flights
At the rear hub there is a finite clearance between the hub and the end of the conveyor. Depending upon the precise design, this clearance can be quite large. The reaction of the conveyor can shift the conveyor forward a further
Decanter Design 111
Reverse Flights
Figure 2.96. A reverse pitch conveyor.
Front Scraper Blade Extra Counter Balance Rear Scraper Blade
Figure 2.97. Scraper blade and counter balance flights.
small distance. With the designs that have radial holes in the beach section for cake discharge, the dead zone beyond the cake discharge can be even larger.
In this dead area, between the end of the conveyor and the inside face of the rear hub, cake can build up and become very compacted and eventually cause considerable wear. To combat this problem a small flight with negative pitch is often welded 180 ~ from the main flight end. This not only scrolls cake back into the path of the main flight, but also serves to counterbalance the non- symmetry of a single lead helical flight.
At the larger front end, some counter-balancing to the end of the flight is required, due to the non-symmetry of a single lead conveyor, which is often done by welding on a small flight section with positive pitch. This will be a 60 ~ segment with centre line positioned 90 ~ from the end of the main flight.
112 SpeciaI Features
However, part icularly on the larger bowls, this needs some special a t tent ion when balancing. While perfectly balanced on the balancing machine , considerable out of balance can occur when the bowl is filled with process material. This is caused by non-symmetr ica l buoyancy effects of single-lead conveyors. This has to be taken into account when dry balancing. Final balancing should be checked with the bowl filled with liquid.
2.4.12 Feed z o n e
There can almost be an infinite number of feed zone designs with different shaped inlets and outlets, different number of outlets, erosion protection, accelerators and linings.
One special feed zone design has come about from advanced development and a patent (Alfa Laval) for a decanter centrifuge [17] has been issued. This is the soft inlet feed zone. This is used on sludges with fine, discrete particles. It comprises an open volume around the feed tube. Feed builds up in the feed zone and is accelerated by vanes at a small radius. The combined action from the friction on the inside of the feed zone and the accelerat ing vanes will bring the feed up to bowl speed, before discharging into the pond. The design is such that the feed has to get to bowl speed before it can reach the feed zone discharge ports. This minimises shock, and thus minimises reduct ion of particle or agglomerate size.
Many feed zones have their discharge ports following the helix of the flight. This can allow build up of solids in dead areas. This is overcome with the "in- line" feed zone that is possible when using a baffle cone (see Section 2.4.6.2).
)
F igtire 2.9 8. A soft il~let feed zone.
Decanter Design 113
The feed ports can be symmetrically placed around the conveyor hub under the cone with floc ports equi-spaced between the feed ports (see Figure 2.80).
2.4.13 The reslurry collector
Sometimes it is necessary to reslurry the cake discharged from the decanter. This is generally for washing out contaminants from the cake, as in lactose production. It is sometimes done in effluent thickening, and when a greater dryness than is required is produced by the decanter for easy control. Then the cake is back mixed with a feed by-pass which reduces overall flocculant consumption, because the by-pass does not need flocculant. The by-passing material is, of course, 100% recovery.
The reslurrying can be done in a separate mixer after the decanter, but more often the slurrying liquor is added into the decanter cake discharge and, because of the turbulence therein, it reduces the mixing required thereafter. A more efficient system has been devised where the reslurrying liquor is added to an inner collector surrounding the cake discharge fitted with outer impellers. This provides very efficient mixing, obviating any need for further mixing outside the decanter.
Spray Nozzle Bars
Bowl
Reslurry Collector
Cake Discharge
Figure 2.99. A reslurry collector.
114 Special Features
2.4.14 CIP
The CIP feature is a very useful and necessary option for the decanter. It is used in many processes using foodstuffs, pharmaceuticals and biochemical products. To be able to clean a decanter to high levels of hygiene standard, without the need for dismantling, allows the use of decanters when otherwise it would be impossible.
There are varying degrees of sophistication offered for CIP. A donkey motor with a clutch may be fitted to the main motor to drive the bowl at low speed, slow enough to allow liquor to tumble inside the bowl, rather than rotate with the bowl. Sometimes a similar design but with a smaller motor is also fitted to the back-drive system. Today, however, the donkey motors have been mostly superseded by frequency inverter drives on the main drives to allow the necessary speed changes as required. During CIP the bowl and conveyor are rotated at this low speed for a set period after which there is the option to reverse the conveyor for a set period and then do the same thing in the reverse direction. During the CIP process cleaning or sterilising liquor is fed into the bowl and sprayed onto the outer surfaces of the bowl from spray bars fitted to the casing.
It is useful during the CIP process to be able to empty the bowl. To this end, spring-loaded drain valves will be fitted into the bowl wall. At speed, the valves close with centrifugal force against the spring. Below a set speed the spring force is greater than centrifugal force and the valves open.
2.4.15 The Rotodiff
The Rotodiff is a device supplied by Viscotherm of Z/irich, Switzerland. It is a device, a hydraulic system, which substitutes for both gearbox and back-drive system on a decanter. The Rotodiff, which is manufactured in a large number of sizes and torque ratings, is fitted in place of the gearbox. Depending on the specification, it could be one or more rows of pistons. For a specific torque rating the Rotodiff is lighter than a standard gearbox and thus the Rotodiff offers higher maximum torques for a given decanter size.
The Rotodiff is a sophisticated, rotating radial piston, hydraulic motor and rotary seal device, powered by a stationary variable speed pump unit and control system. The central system effects similar differential control to that possible with the standard gearbox and back-drive system. However, with a Rotodiff system, the differential speed is independent of the bowl speed. Being hydraulic, it can be easily made explosion proof, and systems are supplied in which both back-drive and main drive are hydraulic to satisfy explosion proof requirements. When using a Rotodiff, the differential can readily be reversed to assist with unblocking a bowl. However, this procedure must be done with care lest conveyor flights are bent at the front, pushing
against the front hub.
Decanter Design 115
/
, , - / , ,,
, / / / ' , ' ,,/. /
!' , '
. �9
/' / t ,"
�9 �9 :. -."7 j
.,/ ,.." / " i
O | , y , . ,,
'1,0
Z"
/ /
. - . . . :.
-..-...L~
"....
,.,,,~.
Figz~re 2.100. The Viscotherm Rotodiff s!lstem (B!t coz~rtes~ of Viscotherm).
Extra high pressure units ( 2 0 0 - 3 5 0 bar) have recently been developed, which enable reduced sizes and costs for the same duty. Naturally, maintenance requirements are those normally associated with hydraulic systems.
2.4.16 Power regeneration
Conventional back-drive systems on decanters perform a braking duty. As such many of them have the ability to regenerate power. Although an eddy current brake is unable to do this, AC, DC, and inverter motors, and hydraulic systems are. The braking process causes the electric motor to act as a generator and so returns power to the grid. In the case of the DC and inverter motor, the power regenerated is usually considered "dirty" unless electric filters are fitted which smooth out unwanted harmonics.
A better, though more expensive, approach is to fit DC or inverter drives to both main drive and back-drive and use for both a common DC bus. The power regeneration is then simple and clean, and the main motor then draws from
116 Special Features
the grid only the net power required by the decanter. This system is subject to a recent patent [18].
2.4.17 Dual main drive motor
The power required to accelerate the high inertia bowl to speed can be considerably higher than that required to rotate the bowl at speed. Thus, to start the decanter, the main drive motor need special consideration. This could be one of a number of alternatives, a soft start motor, a specially wound motor, a star-delta starter, or a motor with a fluid coupling. A patented alternative (Alfa Laval) to these methods, used on large decanters, is a dual set of motors [19]. Both motors are energised for start up and one is switched off when the bowl is at speed.
2.4.18 Floating conveyor
To get two c o n c e n t r i c masses rotat ing at different speeds, at 2000 to 3000 g or more, requires some sophisticated engineering and careful dedicated construction. Some decanters have now been built to produce as much as 10 000 g, requiring even higher levels of accuracy. A new approach, which has been successful, has been to fabricate the conveyor, such that it floats in the process material held in the bowl. The hub of the conveyor is made of thin- gauge steel and sealed to make the overall density of the conveyor less than unity. The feed zone of this design thus operates flooded. Floating conveyors avoid the natural frequency problem associated with long bowls.
2.4.19 Decanter controls
With modern electronic technology, a host of different controls is possible for the decanter, all of which can be integrated with the rest of the plant, with, if necessary, automat ion, remote control and telemetering.
The control of the main drive is relatively simple and straightforward. The control of the back-drive system has required dedicated development. Today the back-drive systems can control the decanter at a fixed differential, at a fixed torque or cake dryness or with some hybrid control system. The hybrid system, say, controls at a fixed differential, until a pre-set torque limit is reached and then at that torque level until the differential changes to a pre-set level.
With main drive and back-drive systems under control, it remains to control centrate quality. For this, a good centrate monitor is required, capable of assessing the level of suspended solids in the centrate. This has been difficult but there are a few reliable devices now available on the market [20]. The monitor is then coupled via a PID controller to the polymer pump speed control.
It is now common to couple the centrate and back-drive controls to a PLC together with signals to and from other parts of the decanter plant, such
Decanter Design 117
as the polymer make-up system, off-take conveyors, and pumps and feed tanks.
It is not a great step to go from here with a fully integrated system that controls and monitors the whole plant with safety warnings, level alarms, maintenance prompts and a complete regular costing and audit of the plant performance.
A feed solids monitor and flow meter will record the feed processed. The polymer plant is able to indicate polymer consumption. Electric meters will advise on power consumption. Feeding into the PLC costs of the various commodities allows the PLC to calculate running costs.
Various control algorithms will be available in the PLC to control the plant to a maximum running cost, a minimum cake dryness or, say, a min imum feed rate within certain limits of other parameters.
Many decanter plants operate today for many hours unattended and therefore there is a high demand for good automatic control systems.
\
,~ @
MBH ~ M ~ T T r
DIFFERENTIAL SPEED CONTROL
#: A l ~ l . ~ ena., ~ 2 o~
@ |
Figure 2.101. An Alfa Laval back-drive controller.
2.5 References
1 L Shapiro. (Alfa Laval) Decanter centrifuge with conveyor capable of high speed and higher flow rates. US Patent 5354255, 11 October 1994
2 N F Madsen. (Alfa Laval) Decanter Centrifuge. US Patent 4828541, 9 May 1989
3 H Hemfort. (Westfalia) Continuously operating solid-jacket counterflow centrifugal extractor. US Patent 4147293.3 April 1979; W Ostkamp, K H Brunner, F Wibbelt. (Westfalia) Continuously, completely jacketed, countercurrent centrifugal extractor. US Patent 4451247, 29 May 1984
4 A H Shapiro. (Bird Machine) Conveyorless clarifier. US Patent 5067939, 26 November 1991
5 P LaMontagne. (Pennwalt) Centrifuge employing variable height discharge weir. US Patent 4575370, 11 March 1986
6 J W Caldwell. (Alfa Laval) Inflatable dam for a decanter centrifuge. US Patent 5257968, 2 November 1993
7 C Y Lee. (Pennwalt) Centrifuge apparatus. US Patent 3795361, 5 March 1974
8 B Madsen. (Alfa Laval) Decanter centrifuge with helical- rib baffle. US Patent 6024686, 15 February 2000
9 R E High, A J Samways. (Pennwalt) Centrifuge apparatus. US Patent 3934792, 27 January 1976; R E High. Decanter centrifuge. (Decanter Pty). GB Patent 2182869, 28 May 1987
10 S Suzuki. (Kotobuki Techrex) Sedimentation centrifuge containing screw conveyor with fins. US Patent 5310399, 10 May 1994
11 L A Larson. (Noxon) Decanter centrifuge. World Patent WO93/22062, 11 November 1993; G L Grimwood, G C Grimwood. (Broadbent) Decanting centrifuges with improved compression. US Patent 5584791, 17 December 1996
12 F Brautigam. (Pennwalt)Centrifuge apparatus. US Patent 3764062, 9 October 1973; L Shapiro. (Pennwalt) Hard surfacing for a centrifuge conveyor. US Patent 4328925, 11 May 1982
13 D Locke, JW Trueman. (Alfa Laval) Screw conveyor for centrifuges. GB Patent 2273253, 15 June 1994
14 J W Caldwell. (Pennwalt) Conveyor flight configuration. US Patent 4449967, 22 May 1984
15 R Schilp, W Epper. (KHD) Solid bowl worm centrifuge. US Patent 5545119, 13 August 1996
Decanter design 119
20
16 IHI (Ishikawajima-Harima Heavy Industry), Utility Model JP 1880613 17 B Madsen. (Alfa Laval) Decanter centrifuge with energy dissipating inlet.
US Patent 5374234, 20 December 1994 18 J L Cooperstein. (Alfa Laval) Variable frequency centrifuge control. US
Patent 5203762, 20 April 1993 19 J L Cooperstein. (Alfa Laval) Decanter centrifuge having dual motor
drive. US Patent 5342279, 30 August 1994 J G Joyce. (Alfa Laval) Turbidity measurement. US Patent 5453832, 26 September 1995
This Page Intentionally Left Blank
CHAPTER 3
Applications The decanter centrifuge is a major item of processing equipment in m a n y industrial applications, and this chapter illustrates this range of use. The driving forces in the decanter marketplace result almost entirely from the needs of these industrial applications, so their importance to a handbook on the decanter is easy to see. The decanter has no place, however, in domestic, institutional or commercial (business) applications, which are covered by separation equipment of quite different kinds.
3.1 Application Classes
In addition to reviewing decanter applications by specific industrial end-use, it is necessary first to recognise that there are ways of looking at decanter applications in a more general sense. Mechanical liquid/solid or liquid/liquid separations serve two broad purposes:
�9 a utility function, such as the cleaning of cooling water or of a recycled hydraulic fluid; and
�9 a process function, such as the recovery of crystallised salt or the polishing of beer.
Utility applications are found th roughout the whole of the manufac tu r ing and service industries, and their characterist ics are dictated by the na ture of the application, and more often than not are independent of the na ture of the end-use sector, such as power generation, brewing or the production of pharmaceuticals, in which they are found. Utility applications are. on the whole, of similar levels of importance th roughout industry and commerce.
Process applications, on the other hand, are usually end-use sector specific, with important variations imposed by operating temperatures and pressures, by the corrosiveness of the liquid or abrasiveness of the solids in the system, and by the individual process needs, such as cake dryness or centrate clarity. Process applications may be an absolutely vital part of the end-use process, or may have only a small part to play.
Decanter applications certainly obey this differentiation. The number of different utility applications is small, a l though the main one, the dewater ing of waste sludges, is an enormous part of the decanter market. The great variety of decanter uses occurs in the very wide range of process applications.
This variety of end-use separation applications can be grouped into five broad types: clarification, classification, thickening, dewater ing and washing according to the main purpose of the separation process. It is, of course, very probable that more than one of these may be involved in a par t icular application.
The decanter separates suspended solids (or immiscible liquid droplets) from a liquid stream, so that one prime purpose for the decanter is the
Applications 123
clarification of this liquid s t ream as far as possible free from the suspended material . The decanter can be operated so as to give a high degree of clarification, a l though it is not always possible to achieve high degrees of clarification and of dewatering of the solids at the same time. It is not often tha t a decanter is used only to clarify a liquid, and it is not by any means the best equipment for clarifying a slurry that contains only a small a m o u n t of solids in suspension. Waste slurry processing, a l though not only a clarification process, does require the most effective removal of solids tha t is possible. In clarification mode, extra efforts must be made to ensure tha t no cross-contaminat ion between the exit streams is allowed.
As far as possible, then, clarification aims at a complete separation of solids from the liquid stream. The next purpose, by contrast, aims specifically to leave some solids in the exit liquid. In the classification of solids by a decanter , a slurry of solid particles of mixed particle size, or, less often, of mixed densities, is treated in such a way that a specific fraction is removed as separated solid, leaving a well-defined fraction of the original solids still in suspension. This mode of operation is particularly relevant to the processing of kaolin (china clay), and it also finds a place where the decanter is used to remove oversize material , ahead of a more efficient clarifier, which might interfere with the final separator 's operation (e.g. which might block the nozzles of a disc centrifuge). The decanter is a very efficient means of effecting classification by particle size.
The other intentionally partial separation occurs with the thickening of slurries, where only some of the suspending liquid is removed, to leave a thicker suspension to be discharged for subsequent processing. Thickening also can be used in waste sludge t reatment , where subsequent dewater ing is under taken by other types of equipment. It is also possible to achieve a final slurry thickness by dewatering the solids more than necessary, and then mixing them with feed liquid to the required degree, thus reducing the flocculant load. Sludges are thickened, ra ther than fully dewatered, if they are to be pumped or barged for final disposal.
The bulk of decanter centrifuge applications involve the recovery of the suspended solids, usually because the solids are wanted for subsequent processing, but also to make any subsequent t reatment easier or less costly. Where the next stage is not affected by the presence of some of the liquid phase, then the purpose of the separation is usually the efficient dewatering of the solids to as dry a state as is feasible and economically justifiable. This is certainly the case in the dewater ing of waste sludges, where the discharged solids will be dumped onto land, incinerated, or thermally dried (prior to use as fertilizer, or to incineration). The lowest water content in a sludge reduces the cost of transport, or of the energy needed for drying and incineration.
If the separated solids are to be treated chemically, or otherwise used in a subsequent process, such that purity is important , then they may need to be washed free of the original liquid. Washing of the separated solid can either be
124 Application Classes
done on the beach of a normal decanter or on the extended cylindrical screen section of a screen-bowl decanter. Washing on the beach is well suited to fibrous solids, and to most crystalline solids, whilst some crystals or hard granular materials (such as coal) can be washed efficiently on the screen-bowl section. If the solids are not easily dewatered, but need washing, then it may be necessary to reslurry the discharged solids, outside the decanter, and dewater them in a second machine.
In many cases of slurry dewatering, and especially where the slurry is a waste needing t rea tment prior to safe disposal, the decanter can dewater such slurries to a high level of dryness. The extra-dry solids decanter has become an important feature of the decanter market, both to allow it to compete for business with some designs of filter, and to provide a good means of processing waste sludges ahead of incinerat ion or land disposal.
A further modification of these five basic uses is the three-phase decanter, in which two immiscible liquids exist as well as the suspended solids that are involved in the separation. Here good clarification of both liquids is usually required, as well as "dewater ing" of the solids from both liquids.
3.2 Application Analysis
The actual uses of the decanter can be found in almost all of manufactur ing industry, in process terms, and beyond it as well for utility (mainly waste t reatment) uses. It is a help to see these application sectors in context, as is shown in Table 3.1. This table gives an a r rangement of the main items in the totality of human economic endeavour, as exemplified by the divisions of the UK Standard Industrial Classification. (This classification is similar to those used for various purposes by the statistical offices of the United Nations, the European Union, and the US and many other governments.)
The items in Table 3.1 have been selected to show the range of sectors of interest to the decanter centrifuge, with those of significant interest in bold text. The emboldened categories are those in which decanters play an important part in the process of that category. It should be remembered that decanters also have a vital role in the t reatment of waste slurries, which can arise from almost any of the manufac tur ing sectors. It is a moot point as to whether or not some waste sludge processing steps rank as "util i ty" or "process" applications, especially in the production of freshwater from raw water. Quite certainly, such t rea tment in the municipal waste t reatment sector is a "process" one, hence the highlighting of "Other communi ty services"
The emboldened categories in Table 3.1 do not imply that all parts of these sectors use the decanter, but a key use will be found somewhere in each one.
The remainder of this chapter is devoted to brief descriptions of these key applications, with reference to the features of the decanter that make it valuable for that application.
126 Application Analysis
Table 3.1. Standard industrial classification, 1992
SIC section Activity
CA CB
DA DB DC DD DE DF DG DH DI
DJ DK DL DM DN
Agriculture, hunting and forestry Fishing and fish farming Mining and quarrying
Energy materials Other mining and quarrying
Manufacturing Food products, beverages and tobacco Textiles and textile products Leather and leather products Wood and wood products Pulp, paper, printing and publishing Processed energy products Chemicals and chemical products Rubber and plastic products Other non-metall ic mineral products Basic metal production and fabrications Machinery and equipment Electrical equipment Transport equipment Other manufacturing
Electricity, gas and water production and supply Construction Wholesale and retail trade Hotels and restaurants Transport. storage and communication Financial services Other business services Public administration and defence Education Health and social work Other community services Domestic services
Source" National Statistics. Extracts from National Statistics are Crown Copyright and may
only be reproduced by permission.
3.3 Waste Sludge Processing
One of the largest applications for the decanter centrifuge, certainly in terms of machines sold, is the dewatering of waste slurries arising from industrial processing, from raw water t reatment , and from the t reatment of municipal sewage. Almost all industrial manufac tur ing processes generate liquid wastes containing suspended solids; even an industry as apparently unconnected with liquid processes in its operations as machinery manufacture. Machining uses complex liquids for cooling and lubricating machine tools, which then become contaminated with dirt and metal scrap.
Machine tool fluids are usually valuable enough to need recycling, and so they must be cleaned before re-use. This is also true of many industrial wastes, that the liquid, once cleaned, or the separated solids, once dewatered (and, perhaps, washed) can be recycled profitably. However, the main bulk of waste slurries have to be dewatered in order that the suspending liquid can be discharged to a river or lake wi thout polluting it, and the solids can be sent for final disposal in as small a bulk and as safe a condition as possible.
Waste sludges, once adequately dewatered, can be sent to landfill, or for soil improvement or fertiliser use, or to incineration. For most of these final destinations, a high degree of water removal is beneficial, and the decanter 's ability to achieve high drynesses has led to its wide acceptance in the treatment of waste slurries.
3.3.1 Industrial wastes
The wastes produced in manufactur ing processes vary in composition as widely as does the nature of the slurry-producing process. The range of modifications to the basic decanter structure enables it to cope with this wide variation in feed compositions.
Whether the ultimate destiny of the final sludge is to be recycled within the factory, or to be sent away for landfill, there is usually sound argument in favour of discharging it from the decanter in as dry a state as possible. The materials from which the decanter is made will have to be chosen well to match the likely corrosive or abrasive nature of the slurry components.
128 Waste Sludge Processing
The variety in the nature of sludges from industrial sources can be seen from the following list of manufacturing and other processes creating waste slurries in need of treatment:
manufacture of acetylene manufacture of aniline steel works' blast furnace operation processing of cellulose coal processing wastes extraction of coffee decortication (debarking) of wood flotation sludges from the de-inking of recycled waste paper dye and pigment processing wastes sludges from electrolytic and electrochemical processes fish transport water wet processing for the desulphurisation of flue gases foundry operations insecticide production iron ore scrubber slurries milk processing paper mill operations nuclear fuel and spent fuel processing slaughterhouse wastes tanneries manufacture of TV tubes
all of which, and more besides, use decanters to dewater the slurries. In particular, it should be noted that, like the dust scrubbers from iron making, an increasing demand for better air pollution prevention is increasing the number of wet scrubbers in operation. Each scrubber installation produces a slurry of the removed dust, which becomes a potential decanter application.
For any waste which is produced carrying an oily liquid as well as water, then the oil can be recovered separately from the water and the suspended solids, by the use of the three-phase decanter. This will occur especially in the oil refining and blending processes, but also in several food industry applications. The t reatment of refinery slop oils is an important decanter application.
The increasing need to treat sludges of all kinds, especially toxic ones, is leading to the development of total sludge treatment systems, which aim to destroy all organic matter and leave a neutral inorganic sludge. Wet air oxidation was the first of these, and this has recently been joined by Kemira's "Krepro" and Chematur 's "Aqua-Critox" processes (the latter using supercritical water oxidation). The wet sludges from these processes could be
Applications 129
sent to landfill, but they may be concentrated enough to justify recycling. The decanter would then be the means for the dewatering of the final sludge.
3.3.2 Water treatment sludges
The boundary between what is a direct-line-of-process slurry, and what is a "utility" application, is not easily drawn in the case of the treatment of raw water, surface or ground, to produce drinking water, or water for process use. Raw water is allowed time to settle out most of its suspended dirt, but coagulants may be used to hasten the process. The resultant slurry is, without doubt, a waste material, especially if the dirt content is high. If a high load of coagulant like alum is used in the treatment, then there may be a case for the recycling of the separated solids, for re-use, after suitable processing.
As well as the removal of sediment, waterworks may also soften the water, and may use lime for that purpose (as opposed to fixed bed ion exchange), again producing a sludge needing dewatering in a decanter.
Waste lime slurries are frequently recycled for conversion back to reusable material. Selective recovery of the calcium carbonate from the waste will enable such calcining to be undertaken without the heavy inert load of the unseparated waste.
For waterworks applications, decanters normally will be operated with polymer addition facilities, at alternative admission points. They will need full erosion protection for the flights, using tiles. They will have some kind of cake baffle, possibly a cone. and a variable speed back-drive, with good differential and torque control. They will be operated with deep neutral ponds, with axial flow (i.e. with flight windows, such that the liquid flows parallel to the axis, rather than around the helical space), and with provision for wash-out prevention at start-up.
3.3.3 Municipal sewage treatment
Probably the fastest growing, in market size terms, of all decanter applications, the treatment of sewage sludges is a vital part of the developed world's attempts to improve its relationship with its environment. Now that dumping of sewage sludges at sea has finally stopped, the sewage from 15- 20% of the world's population is now fully treated, giving rise to vast quantities of sludges.
The full sewage treatment process has three main stages:
�9 primary treatment, involving simple sedimentation, normally unaided; �9 secondary treatment, which is usually biological, over trickle beds, or in
an activated sludge process; and �9 tertiary treatment, which involves a range of polishing operations.
130 Waste Sludge Processing
There may be screens ahead of primary treatment (as well as grit and grease traps), from which the screenings may be macerated and added to the primary sludge. There will certainly be sludges produced by the primary and secondary treatment, and there may also be sludges from some of the tertiary processes. Many sewage treatment works use sludge digesters, which ferment the various sludges anaerobically, reducing the sludge volume considerably, but not eliminating it.
The dewatering tasks at a sewage treatment works can thus include:
�9 primary sludge, possibly mixed with screenings, which is a comparatively easy task:
�9 secondary (biological) sludge, which is not easily dewatered, and so is usually mixed with primary sludge, to give a third variety:
�9 mixed sludge; and �9 digester sludge.
The decanter can handle all of these sludges, with varying degrees of separation efficiency.
The use of coagulants and flocculants in the treatment of municipal sludges is now commonplace, and, indeed, the decanter would not be the efficient dewatering machine that it is today in the absence of modern, synthetic flocculants.
Secondary sludges are soft, and not easily dewatered, so that the role of the decanter in the processing of such sludges is more as a thickener, than as a dewatering tool, probably ahead of digestion. For this duty, the decanter will need full erosion protection, preferably with tiles on the conveyor flights. The decanter will operate at maximum speed with axial flow, with a deep neutral pond, and a pond setting just shallower than neutral. It should have a back- drive with good differential control, and a standard ratio, low torque gear box. Wash-out prevention and a cake baffle are necessary. An inflatable weir dam is a good idea, and can prevent wash-out. Sigma enhancement (i.e. the use of vanes, similar to the disc stack of a disc centrifuge) can be advantageous, but polymer addition may not be necessary, depending upon the degree of dryness
required. If digestion is not an option, then the secondary sludge could be mixed with
the primary sludge and screenings, and this mixed sludge (or primary on its own) can then be efficiently dewatered by the decanter. A resultant dryness of 18-22% can readily be achieved. With dry solids operation, this can be increased to 25-30%, or even 30-35% solids (an appearance of being solid). The relative proportions of primary to secondary sludge, and of municipal to industrial, will affect the final figures. Uncommonly, figures of 40-50% have
been experienced. The specification for a decanter to handle primary sludge alone, or mixed
primary and secondary, or digester sludges, would be as follows:
Applications 131
�9 polymer addition essential, with the ability to add internally; �9 full erosion protection essential, using flight tiles; �9 cakebaffle useful; �9 back-drive required, with good differential control; �9 s tandard bowl speed sufficient (g levels up to 2500 depending upon
precise design); �9 wash-out prevention necessary on start-up (such as notched weir
plates); �9 deep neutral pond (better performance than shallow pond); and �9 axial flow.
For the more abrasive sludges, a casing wear liner will probably be needed, while for the most abrasive sludges, hard surfacing should be added to back and front of the flights around the feed entry into the pond.
For dry solids operation, the above applies, plus:
�9 negative pond operation: �9 high-ratio (three-stage if necessary) high torque gearbox, with good
torque control: and �9 a cake baffle or restriction of some form, or its equivalent, is essential.
The processes, such as those already mentioned (in Section 3.3.1), which treat whole waste slurries and destroy all organic matter, will find a place in municipal sludge treatment once they become established, considerably extending the decanter market.
3.4 Energy Materials Production
The production of raw materials for energy g e n e r a t i o n - coal, oil and gas offers considerable scope for the use of decanters, at least for coal and oil. The manufacture of gas is a little used process now that natural gas is so freely available. When this phase passes, as gas availability reduces, then the gasification ofcoals and heavy oils will also offer good applications (as will the eventual need to convert coals into liquid fuels).
In coal processing, the mined coal must be washed free of impurities. The run-of-mine coal contains the clay and rock that are within the coal seam, as well as roof and floor material. Coal itself is a relatively low-density material, compared with the impurities, and can be separated by sedimentation in a liquid of appropriate density. An increasing proportion of the coal that is mined is crushed to quite small particle size, partly to enable the removal of as much sulphur as possible. For such small sizes, the decanter provides an excellent means of separation, also enabling the settlement fluid to be recycled.
Whatever the size of coal being processed, there will always be a need to separate residual coal from the washings, for which the decanter is a standard separating device (as well as for the final dewatering of waste liquids the coal tailings). Fine coal may also be separated bv means of froth flotation, and the dewatering of flotation concentrates is another good application for decanters.
In the production of petroleum or natural gas from underground sources, the initial well holes must be drilled, often into very deep strata. The search for new oil resources also requires the drilling of many trial holes, more often dry than successful. For such drilling, the drill head must be cooled and lubricated, and so a lubricating fluid (the "drilling mud") is pumped down to the drill bit. Not only does the drilling mud cool and lubricate the bit, but it also carries the rock cuttings back to the surface, it prevents water leaking into the well from water-bearing rock, and it prevents the collapse of the hole behind the drill. These duties require that the drilling mud be a complex liquid, basically a suspension of clay (bentonite) in water, but dosed with a range of special chemicals that allow it to cope with the range of rock compositions to be encountered, and to have the right density. The preparation of a drilling
Applications 133
mud is a size classification task, for which the decanter is very well suited. The used liquid is often too expensive to discard, so needs t rea tment for the removal of rock fragments for which duty, again, the decanter is a very good choice.
For either of these duties, the decanter will need to have full erosion protection, but can run at moderate or low speeds. It will also need an explosion proof motor (and back-drive equipment if this is used).
3.5 Processed Fuels
The production of na tura l gas results in an adaptable and easily used fuel, and further processing is often quite unnecessary. The processing of coals and oil, on the other hand, are major industries.
The carbonization (coking) of coal is a largely dry process, but the by- product tars and phenols do require cleaning, for which the decanter is used.
The modern petroleum refinery has a range of uses for the decanter, as well as the various waste s treams that need t reatment . One of the most impor tant is in the preparation and recovery of catalyst for cracking and reforming uses.
-
.
3.6 Minerals Extraction and Processing . - -. -~ -_
The extraction, iivd ~libseqiienl pro( ing, ol' metiilli(: ;in(l non-met.nl1ic minpr;ils :ire m:ijor applicat.iciri areas lor tho decarikr. This cl:issitic:ition obviously overlaps considerably with the production of inorganic chcmicals (CQvcrCd in Scction 3.9.1)- and the trcatment of calcium carbonatc differs littlc. whether it be intended as a chcmical or for other usos.
'I'ypical riiiiierals involving the decanter in ttieir procrssirig iricliidr [:ijl(.itin1 carbonate and chloride. clays, gypsuiii and potash slimes.
A sigiiificant part of the wct cxtractioii processes fcjr rriiriurals involves either the rernoval of oversixe rriaterials from ;1 suspension or the division of a suspensiori into two Erat:tioris by piirticlr sizc. Far thesc purposes. the decarilcr, opcratirig in classitic:iit inn mnde, i s an ideal proccssing tool.
Thc processing of china clay (kaolin) rcprcsents one of tlie r ~ l c l r s ~ uses for the dec-;inter. K;iolin occurs naturally as a fine white powdery deposit. (:omposed largely of the mineral kaolinilc, in ptirticle sizes iri t tic range from 0 . 1 to 1 0 pm or morc. I t i s uscd as a filler arid coiiting in the making of paper. iiS the basic malerial for cr ra i i i i c warc. as a filler (or robhcr , and in paints and m a n y other products. Each of thcse may need ;i cl:iy with i1 difl'crcnt particle sizc. and thc decanter is ablc lo provide the necessarv classificatiori o f particle sizc, a typical cut point beirig 2 pin. The kmlin is washed out idits rlidtive rock. largc rnatcrial is settled o u t . and t hen the slurry is sized i n a ctin1plc.a system of cyclones and decantrrs. It is ii largs applicatioii a n d :I good rnarket f(Jr decanlers.
Other minerals - rnet;illic or non-mctallic - arr processed similarly. sud1 as bentonite. ( -h i l lk , mica. and thoso rniiieriils produced I'm use 21s pigrlirnts i t1
paiiils. For these dutics. thc dccanter should be ablc lo operate a1 dilltrtnt speeds. arid h:iw ; I variablc spccd back-drive. I?uII wear protcciion is rssent inl . 'I'hc. use ol flocculants m a y be iirrrssiiry t'cir smie iiiirit.raI processing xtivities.
A great deal or mincral processiiig ends with n conctntrated slurry r ~ t the rcquired product, and this is usunlly drwiilcrcd to ii cohesive ci lkr in a decanter, and may be washed as well. on the beach of a standard dt:canter. uc in a scrccn-bowl machine.
3.7 Food and Food By-Products
In most of the applications discussed so far, cleanliness of operation has not been a vital aspect of the design. In those that remain, food, beverages, and some parts of the chemicals sector, sanitary operation is usually vital to the process. Thus, for most of the remaining applications, the decanter should have the clean-in-place facility built in to its design.
The production of foodstuffs, finished foods, and their by-products covers many different activities:
meat and meat products; fish and fish products; fruit and vegetables; vegetable and animal oils and fats; dairy products; grain milling and starch; prepared animal feeds: bread, pastry goods and cakes: sugar, cocoa and confectionery; pasta and other farinaceous products: tea and coffee: and soups and other food products.
with those components of the industry important to the decanter in bold type. As with other major industries, there are many components of the food
industry, using water in the processing, which give rise to liquid wastes carrying material in suspension. It is unlikely that these wastes can be processed for full recycle, but several can recover some value (for animal feed, for example). All will need dewatering, before final discharge. Where fat or oil are involved, as well as water, then the three-phase decanter can be used in the dewatering process.
3.7.1 Meat and meat products processing
The main processing of meat from the slaughterhouse to the butcher's counter has no use for decanters (other than in waste treatment), but they
Applications 13 7
have a major role in the processing of meat by-products. These are the non- meat (but not necessarily inedible) materials collected during the slaughtering process. Those that use the decanter in their subsequent treatment include:
�9 bones and rendered meat, for use as bone and meat meal in animal feeds and fertilizers;
* blood, for use as blood meal; �9 gelatin, obtained from high-collagen products such as pork snouts, pork
skin, and dried rendered bone, for use in confections and jellies; �9 edible fats are used as lard, tallow, shortenings, and cooking oils: and �9 inedible fats, used in soap and candle manufactur ing, and in various
industrial grease formulations.
Hormones and other pharmaceutical products such as insulin, heparin, and cortisone are obtained from various glands and tissues, while gelatin is also used for pharmaceutical preparations. These are discussed further in Section 3.9.4.
A major application lies in the rendering of animal fats, edible or inedible, wet or dry. Early rendering processes involved the heating of the raw fat to break the cell walls and release the fat as liquid, followed by the separation of the hot fat from the residual cell material (greaves). Several centrifugal separation processes were developed in the 1960s, for which the cells are ruptured in special disintegrators under close temperature control. The protein tissue is separated from the liquid phase in a decanter, following which a second centrifuge separates the fat from the aqueous protein l a y e r - or the whole separation may be undertaken in a three-phase decanter. Compared with conventional rendering, the centrifugal methods provide a higher yield of better-quality fat, and the separated protein has potential as an edible meat product, as opposed to its earlier use as animal feed or fertilizer.
A decanter for use in rendering processes would require full hard surfacing, and some dry beach, but would need to be simple and robust, to match the characteristics of the industry.
3.7.2 Fish processing
As with meat, the main production of whole fish does not offer any scope for the decanter, but the bulk production of fish meal certainly does. Fish meal is a coarsely ground powder, high in protein, made from the cooked tlesh of fish. Though formerly important as a fertilizer, fish meal is now primarily used in animal feed. The oily fish, such as menhaden, anchovy, herring, sand eel and pilchard, are the main source of fish meal and its companion product, fish oil. To be processed into meal, chopped fish is fed by a screw conveyor th rough long steam cookers. The cooked mash is then pressed to remove water and oil.
138 Food and Food By-Products
The press effluent is separated in a three-phase decanter into oil, water, and residual meal. In addition to its three-phase design, the fish meal decanter will have full erosion protection, with standard tiles, and a variable speed back-drive.
The fishing industry has to deal with a raw material with high nutrient content, but a low rate of utilisation of fish in normal fillet production. These factors, together with the growing worldwide awareness of the limited supply of natural fish stocks, have all led the fishing industry to develop procedures for better utilisation of the original fish, and for the development of edible products from underutilised species. Surimi was developed in Japan long ago, when it was discovered that washing minced fish flesh, followed by heating, resulted in a natural gelling of the flesh. Combined with other ingredients, the surimi could provide a range of "ready-to-eat" products. Modern surimi production involves continuously operating lines with automated machinery for heading, gutting, and deboning of the fish; mincing, washing, and pressing (to remove water); and heating of the flesh, or freezing of the flesh for later processing into various end-products. The decanter has a part to play in the treatment of the presswater, in an application of growing interest. Recently, Alfa Laval successfully introduced a decanter that can replace the press itself. This simplifies the process and reduces the footprint needed for the process line, which is critical for on board plants.
3.7.3 Fruit and vegetable products
The processing of fruits and vegetable material includes a number of operations where the decanter has a part to play. These include:
�9 the extraction of oils from seeds, such as rape. corn (maize), cottonseed. sunflower, safflower, groundnut (peanut), flax (linseed), castor, tung, soybean and sesame:
�9 the extraction of oils from fruits such as olive, palm (and palm kernel)
and coconut; �9 the extraction of starch from corn (maize), wheat, tapioca and potatoes: �9 the separation of gluten from starch; and �9 the canning of whole or cut fruit and vegetables.
Not included here are the processing of fruits and grains into juice or fermented beverages, which is covered in Section 3.8. nor the onward conversion of oils into soaps, which is covered in Section 3.9.3.
Vegetable seed oils are a very important component of the modern diet, and their production has increased to become a significant application for the decanter. The separated seeds (some, such as cottonseed or corn, being a by- product of other processing) are rendered, usually by mechanical grinding, but also by cooking, to form an oily paste. This paste may be mechanically
pressEd 1.0 uxtrude ihe oil (a1,id sorxie water), or separated directly in a decanter. After the oil i s extracted from the oilseeds, the residual meal. of c:nke, which has a high protein contcnt (high enough that it frequcnlly det.errrIiries t he valrlt: o r t.he o i l nrnp), is usually nsed i j s n protein concentrate to feed livcstock and poultry. If thc cakc is poisonous. as with castor beans and tung nuts, it is uscd as fertilincr.
Dccantcrs for vcgctable oil processing must be able 1.0 opera1.e at. eIeviIt.ed teI11peratures. 'rhey should have ;j (:ombinal.ion of' $1 dry beach with a bac.k- drive for setting the differerilial.
In the processing of olive oil. the degree oCpiirity ot'the oil is very important. such ihat. only the minimum of degradation can be allowed. 'The whole f r u i t . irlcluding the si.ones, ;ire c,nrnrninute,d in grinders or hammer mills. a i d 1 h e resullarii. paste is t.han stirred until the oil appears as large droplels. The nil ilnd olive juice arc then separated from the resir1il;il solids ("pomacc"), cithcr by a hydrnulir or mechaiiical press, or i n a derxn le r . The separated cakc is
ed, to extract further oil. which is used I'or industrial purposcs. rather t.tian cdihlc, and the oil/juicc mixture is scparated in a disc oentrifiigE. 4 more modcrn vcrsion of tl ic prirniiry separation uses a threc-phase dec:inter, to achicvc both scparatioris a1 once, with very little degradation in oil quality. The pomaco can he wiishrt!. in thc decanter, with s u r w recycled juice, to iiicrtasc the oil rerrioval, or the fccd liquor inay he diluled with watcr tbr the same purpose. 'lliwe is, in some parts of the industry, ;1 tendency now tu return to thc two-phasc dccariter.
Palm fruit is prweswi in much the sariit: wiiy as Ol i i i c s , cxcept i . h i i ~ the kerncls may be st:par;itrd first. thcn broken lo enablc thc palm kernel nil tn bc extracted separately. Thc palm fruit may be cooked before 1.11~. o i l is lihcrated.
'l'ht. olive oil applicatim is ;i very important one for the demntcr, and sotile duuanter rnariul'iicturinE compstriirs exist just to satisfy t . 1 ~ olive oil market. The olive arid palm oil dccenter will normally he of three-phasr: desigri (alt.hoiigh some processes now use a standard decanter, dischargirig 1 he juicc w i h I.he cake). I t will ust: rnoderatcly high bowl speeds ( to yrodut:o 2 500 t o SOOOg). and full crosion proIwtion (because {.he fruit is nc r f r h m , u g h l y cleaned. a n d beuausc the stoncs are quite i ihriisivc). It will prnbably have a
lixed s p ~ d back-drive. with tixcd capacity, ijlthough difkrerit.ial spccd utirilrt)l
is irn optional feature. Starch is primarily dcrivcd h r r l corn (maize). The cleancd grains ;Ire
soaked to soften them, t hcn mcchariically rendered to liberate ihc corn germ. which is thc oil coril;jining cornponerit.. The remainder ol' i he kerncl corlt.ilirls
starch. gluten and tlbrc. which ;ire progrcssively srparated. and the starch is then classified. Thc dccanter h;js somc part i r i hot.h the scparalion a n d c.lassi fic a t io r i s1.a gr s +
llaririeries largcly deal irr whole or cut fruit and vegctablcs, but. t.herc is a good applicet.inn for dccanters in wastc liquid prowising, yielding arl edible cake, which can bc rccgcled iri1.o !ood products.
140 Food and Food By-Products
3.7.4 Other food processing
Decanters are used in o ther parts of the food industry, for example:
�9 for separation, classification and washing in the production of casein and lactose in milk processing plants:
�9 for the preparat ion of edible protein from soybeans, and from single-cell protein fermentations;
�9 in the processing of coffee beans, and the production of ins tant tea and coffee; and
�9 in the production of soups.
Decanters in casein production will often have a centripetal pump discharge to combat foam, and axial flow, as well as clean-in-place systems. Lactose production uses a double-lead conveyor, wi th on-the-beach rinsing, and a reslurry rinse solids collector, as well, again, as CIP.
Protein separation and clarification employ erosion protection on the first stage extraction of the na tura l products, as well as m a x i m u m operating speeds, and a variable speed brake. Naturally, they also have a hygienic finish, and CIP. Sigma enhancemen t is sometimes employed.
3.8 Beverages
The beverage industry divides into two main parts ~ the making of alcoholic and non-alcoholic drinks. For non-alcoholic beverages, the main decanter application is in the separation of fruit and vegetable juices from their crushed pulp.
In alcoholic beverage production, from grain sources and from grapes and other fruit, there is a major decanter application in the separation of the processed grains from the liquid product. In the case of brewing, the fermented barley is separated for use as an animal feedstuff, as brewers' grains. From the distillation of fermented materials, the stillage from the bottom of the still is also processed to separate the spent grains for feedstuff use, as distillers' (dried) grains. The decanter is an efficient device in both applications for the production of a well-dewatered cake.
For brewing mash and stillage treatment, the decanter should have erosion protection, with special tiles to allow for the high operating temperatures in stillage processing. It should have directional feed nozzles on shallow pond machines, and a cake baffle with deeper ponds. Deep pond machines are increasingly being used, when higher capacities are employed, and when Sigma enhancement is used, to obtain maximum benefit from these characteristics. The decanter should operate at maximum bowl speed, and at close to neutral pond.
3.9 The Chemicals Industry
The chemicals sector covers a wide range of important application areas for the decanter. The full spectrum is:
industrial gases dyestuffs and p i g m e n t s fer t i l izers and n i t r o g e n compounds other basic i n o r g a n i c chemica l s p las t ics in p r i m a r y form synthetic rubber in primary form other basic o rgan i c chemica l s
pesticides and other a g r o - c h e m i c a l s pa in ts , v a r n i s h e s a n d o the r coa t ings , p r i n t i n g inks , and mas t i c s p h a r m a c e u t i c a l s , med ic ina l c h e m i c a l s a n d b o t a n i c a l products soap a n d d e t e r g e n t s , c l ean ing a n d po l i sh ing ma te r i a l s , p e r f u m e s and toiletries
explosives glues a n d ge la t ine e s sen t i a l oils photographic chemicals and materials recording media other miscellaneous chemicals
where, once again, the sectors of significant interest to the decanter, i.e. where most decanters are used, are in bold type. The first of the above groups of industries is concerned with bulk chemicals, and the other two with fine chemicals and pharmaceuticals. The whole chemicals sector represents a major market for the decanter, with over 200 different chemicals processed by its means although most of the applications are very similar. The petrochemical and plastics products are perhaps the most important in terms of numbers of machines sold.
Applications 14 3
3.9.1 Bulk inorganic chemicals
Apart from the production of industrial gases, the inorganic chemicals industry is full of decanter applications, a l though the boundary between wha t is an inorganic chemical and what is a mineral is not easily drawn. Many of the mineral applications already discussed find identical processes within the inorganic chemical sector. The decanter will be used either for dewater ing and washing of granular material and crystals, or for the classification of materials, where particle size is an important characterist ic of the product (such as in the manufac ture of pigments for paints).
Even the industrial gas sector may find a use for decanters if the "hydrogen economy" ever comes into being. The conversion of hydrogen-conta in ing raw materials could be big business in a decade or two.
The manufac ture of t i tanium dioxide, for example, by the sulphate route, involves the sulphuric acid digestion of the oxide ore (ilmenite), followed by the separation of unreacted solid from the resul tant slurry. This is an application for a decanter, made from corrosion-resistant steels, and equipped with full erosion protection. The produced dioxide needs a decanter again for wet size classification. Similar applications occur in the manufac ture of phosphoric acids. The decanter will need a high-torque, low- ratio gear box.
Among the other bulk inorganic chemicals whose manufac ture includes the use of a decanter are a luminium and magnes ium hydroxides, iron oxides and salts, calcium carbonate and other salts, silicon and silicates, caustic soda, sodium carbonate and other salts, graphite, u ran ium "yellow cake", gypsum, and zinc oxide and salts. Calcium carbonate is a part icularly difficult material to treat in a decanter, in that it needs very high torque to achieve an adequate level of cake dryness.
3.9.2 Bulk organic chemicals
Bulk organic chemicals include all the precursors to plastics, as well as the relatively simple chemicals such as acetone or benzene or organic acids. Also in this category, of course, are the major industries devoted to the manufac ture of polymeric materials of all kinds. (The term "petrochemicals" is also covered here to mean those chemicals made from petroleum as a raw material , a l though the products are the same as if made from any other raw material.)
Not surprisingly, the main decanter applications lie in the wet polymerisation of monomers to form:
polyolefins (polyethylene and polypropylene); polystyrene; and
polyvinyl and polyvinylidene polymers (acetate, alcohol and chloride).
144 The Chemicals Industry
The decanter is used to dewater the polymer granules, an operation which is often more difficult than it sounds. This is especially true with PVC, which can produce torsional vibrations ("chat ter") of the conveyor, overcome by fitting a very stiff conveyor. PVC separation decanters have multi-lead conveyors (at least two leads, sometimes three), with conveyors whose flights are polished, with chamfered edges. The machines operate with shallow ponds, and have a large capacity cake discharge on the beach. There is a high- torque, low-ratio gear box.
Poly vinyl alcohol uses decanters with beach rinsing, and a cake baffle, operating with a shallow pond. Polystyrene requires a high-speed decanter, also with polished and chamfered-edged conveyor flights. Operation is with a shallow pond, and there must be a close tolerance on the fit of the conveyor in the bowl, to prevent the entrapment of the plastic granules between the conveyor and the bowl. The casing venting must be such that there is no re- entrainment of moisture in the separated solids, which can have as little as 1% moisture.
Other petrochemicals, especially terephthalic acid and PTA, and nylon, as well as polyethylene and polypropylene, require flameproof equipment, and use the vertical decanter, which can be pressurised, and will operate at higher temperatures. These also have polished flights, and chamfered flight tips, without hard surfacing.
Another important organic chemical is p-xylene, whose production uses decanters suitable for low temperature operation, and fitted with a screen bowl.
3.9.3 Fine and household chemicals
The main application for decanters in the fine chemical sector is in the manufacture of soap and detergents. In soap manufacture, natural fats and oils (such as tallow or vegetable oils) are treated with hot alkali solution, such as caustic soda, producing a sodium fatty acid salt (the soap) and glycerin (or glycerol). If industrially produced fatty acids are used in place of natural fats or oils, the reaction with caustic soda yields soap and water instead of soap and glycerin. After the saponification reaction is complete, the suspension of soap in excess caustic soda solution is centrifuged to separate the two liquid phases and the residual solid material. Special designs have been developed which enable the washing of the oil within the bowl. Originally a tubular centrifuge application, this now provides a good market also for the three-
phase decanter.
3.9.4 Pharmaceutical and medicinal chemicals
A large proportion of the products of the pharmaceutical industry is made in batch processes, for which the decanter, quintessentially a continuous
Applications 145
machine, is not wholly suited. However, many materials are extracted from natural products, such as insulin or blood fractions, and the fermentation routes to antibiotics have long been continuous processes. These certainly do use the decanter.
As an example, insulin is made from animal pancreases, by rupture of the gland material by mincing. The minced material is extracted with acidified alcohol, and the gland residue is separated in a decanter, for further extraction with more acidified alcohol. The final alcoholic extract is clarified, and processed further, with eventual production by crystallization.
Many antibiotics, of which penicillin is one example, are made by fermentation of organic materials, and the decanter is used to separate the product liquor from the fermentation residues.
Blood, whether from humans (for transfers) or from animals (for blood meal fertilizer) has to be fractionated or dewatered. This is a good application for a decanter, provided that it is of hygienic design. Fractionation of blood plasma proteins requires very high-speed operation, and the 10 000 g machines have been developed expressly for this application.
Special machines have also been developed to permit the extraction of valuable materials from one liquid to another.
3.10 Other Applications
Many other individual applications for decanters exist, outside the classifications already covered. In the agricultural sector, decanters are used to dewater pig manure, as well as other farming wastes. In the mechanical engineering sector, as well as the cutting and lubricating oil recycle systems, there is the removal of accumulated impurities from electroplating and electrochemical machining solutions.
The recycle of waste oils (as opposed to their treatment prior to disposal) is becoming an important process for the decanter, as the cost of all kinds of oils has started once again to rise. The recycling of plastics is of rapidly growing concern, and the double beach (one at each end) decanter is used to separate plastic granules into two fractions by density.
CHAPTER 4
Decanter Theory The decanter centrifuge lends itself to a wide range of theoretical treatments, both process and mechanical. Reif and Stahl [1] observed that the decanter incorporates three "extensive fundamental problems", dewatering of the solids, clarification of the liquor, and conveyance of the cake produced. These three major subjects will be covered here, together with separate sections to deal with specific decanter processes, such as thickening, classification, three- phase separation and the latest technology of "dry solids" operation.
Separate sections concerning allied topics, such as particle size technology and fluid flow will also be included briefly. A few important mechanical aspects will also be covered, such as resonance, maximum bowl speed, and bearing and gearbox life.
Firstly some basic decanter theories will be expounded.
Vc
$
148 Decanter Theory
Fi~lure 4.1. D!tnamics of a particle movinq in a circle.
t i m e t - 0
4.1 Basic Theories . - _ ~ _ - -L --.-- --."-
As with every specialist subject. it i s easy for ihe cenlriTugt: erigirieer to ;qss~rne thiqt his basic t,heories are iiniversally iindersiood. To ensu re i.hat. (.his work is comptchcnsiblc to thc widcst possible readership. a few basic c.odccpts will bc covcred. Thcsc will includc 6-forcc, diffcrcntial spccd. and mass balances across the dccanter.
4.1.1 Acreleration force
'rhe celltrifugal acceler;it.ion Ibrce, commnnly known a s g-farce. is thc basic motive force for scparat.irig the solids from the liquid in any scdimeiiting cciitrifuge. 'I'hus, iri :I haridbook about t.he decantcr no apdogits arc needed wlieri covcririg g-force as the first basic concept.
Consider Pigiire 4.1. A particle. of mass H I . rotales at a tatigeril.iiil velocity. v ~ . , a i d arigular velocity, CJ, in a circle of radius, r . After a lime, t , t h r particle has moved to ii paint un thc circle radius, r . which subtends a n angle, -;. cvtiere 7 2 JI., from its position a t time f-0, Lhe extreme right or thc horixontal diameter oCi.ht! circle.
At time t , the horiznntal distance ofthe particle Iroiii the centre ol'the circle i s s, and at timc t=O it was r:
s = r .cos(7 ) (4.1)
The horizontal ;ic:celeration of the particle towards thc centre is I h t : second diffcrcntial of s:
(4- 3)
(4.4) = -w?r.ros(wt)
A t tirrie f.=O:
150 Basic Theories
d 2 S
dt 2 = --c02 r .cos(O) (4 .5)
= -co2r (4.6)
Thus, anything moving in a circle of radius r, at an angular velocity ~, will experience an acceleration towards the centre of the circle of ~2r.
In the centrifuge, it is the liquid that moves round in a circle, and the particles in suspension are free to move relative to the liquid. Thus, relative to the liquid, the suspended particles experience an acceleration, ~2r , radially outwards.
Thus, the gravitational force, F, on a particle of mass m, is the product of its mass and acceleration, where:
F ~- m r ~ 2 (4.7)
In centrifuge parlance the term "g" (or g-level) is often used. This is the number of times the acceleration in the centrifuge is greater than that due to gravity alone. To differentiate between "g", which is dimensionless, and the acceleration due to gravity, having dimensions of LT -2, the ratio of the two accelerations "g" will be denoted here by gc, and that due to gravity simply by g. Thus:
ad2t ' gr = (4.8)
g
Note that the g-level within a centrifuge will thus vary, proportional to the radius, throughout the depth of the liquid, and is proportional to rotational bowl speed squared. (To calculate gc using a simple expression, use rpm 2 x diameter in mm/1 .789 x 106; for example, rotating at 3000 rpm at 450 mm diameter, gc would be 32 x 4 5 0 / 1 . 7 8 9 - 2264.
4.1.2 Differential
The difference in rotational speed between the bowl and the conveyor is commonly referred to as the conveyor differential speed, N. Conveyor differential speed is calculated from a knowledge of the rotational bowl speed, S, the gearbox pinion speed, Sp, and the gearbox ratio, RGB:
N = ( S - Sp) (4.9) RGB
When an epicyclic gearbox is used, the conveyor rotates at a speed less than the bowl speed, while with a Cyclo gearbox the conveyor rotates at a speed above the bowl speed. This fact can have an effect on process performance
Decanter Theory 151
with short bowls, if the feed is not fully accelerated to bowl speed on entry [2]. With the conveyor rotating faster than the bowl, the liquor has to get up to speed to find its way to the liquor discharge hub. With conveyors rotat ing slower than the bowl, the liquor could wind its way around the helix of the conveyor to the centrate discharge ports, wi thout ever getting up to bowl speed. Note, to effect scrolling in the required direction, the flight helix on a conveyor using an epicyclic gearbox would be left-hand, and with a Cyclo gearbox it would be r ight-hand (assuming conventional equipment and operation).
With modern technology, the speeds of the bowl and gearbox pinion can be continuously measured with tachometers or proximity probes, and their signals fed to a simple PLC to work out, and even control, the differential. The PLC would need the gearbox ratio p rogrammed in to execute this duty.
4.1.3 Conveyor torque
Conveyor torque, T, is a measure of the force exerted by the conveyor in moving the separated solids th rough the bowl, up the beach and out of the decanter. It equals the pinion torque, T~,, times the gearbox ratio:
T = Rc~ x Tp (4.10)
It is not easy to measure conveyor torque directly, whereas pinion torque can usually be obtained from ins t rumenta t ion on the pinion braking system. This signal is often given to a PLC for control purposes. Conveyor torque is a vital measure in the control of modern decanter systems.
4.1.4 Process performance calculations
Consider the two-phase decanter separation system in Figure 4.2. Input is the feed at a rate of Qf, of density pf, with a solids fraction xf, and an additive, often a flocculant, of density pp, at a rate of Qp, with a solids fraction Xp. There are two products, cake at a volumetric tlow rate Q,~, at density p.~, and solids fraction xs, and a centrate at flow rate 01, at density pl, with a solids fraction of xl.
From measurements of some of the eight process parameters mentioned, it is required to assess the performance of the decanter. It is normal to monitor the feed rate, Qf, and the additive rate, Qp, with flow meters. Periodically, gravimetric analyses are conducted on samples of feed, cake, centrate and, if necessary, the additive. Performance is judged by how high is the solids recovery, R, and how low is the flocculant dose, PD, when this is used. Recovery is the percentage of solids in the feed that reports to the cake discharge. Flocculant dose, sometime referred to as polymer dose, is the amount of dry polymer used per unit dry solids in the feed, usually expressed as kg/t db (kilograms per tonne dry basis).
152 Basic Theories
Qf x,. qp
Q, 1 Q,
Xs X !
Figure 4.2. Decanter Mass Balance.
As an intermediate parameter in the calculations, it is necessary to calculate the centrate rate, Ol, by conducting a total and a solids mass balance across the decanter. Total mass balance:
@PS + GPp = O~p~ + O~pJ (4.11)
Solids mass balance:
Qfpyxf + QpppXp = QspsXs + Olp~xj (4.12)
Eliminating Qsps from equations (4.11) and (4.12):
OlPl - OfPf (Xs - xf ) + QPPP (Xs -- Xp) (xs- (Xs-
(4.13)
Recovery of solids is calculated by subtracting the percentage loss of solids in the centrate from 100. Thus:
Qlx,p,~ (4.14) R = l o o 1
Polymer dosage is given by:
PD = GXpPp (4.1 5)
Polymer dose levels are frequently expressed in kg/t db, with the dry basis measure applying to both solids rate and polymer rate.
During these calculations, one must take care with the units used. Volumetric flow rates are invariably measured and the density terms are often ignored as they are usually close to unity. However, the density terms must be used when density values are significantly above unity. The gravimetric analyses of the samples should all be total solids (i.e. samples are evaporated to dryness, and thus measure suspended and dissolved solids), which all except
Decanter Theory 153
the centrate generally are. However, centrates are generally analysed in terms of suspended solids. Any dissolved solids in the centrate cannot be considered a measure of a decanter 's inefficiency, as it is suspended solids which it separates, and any dissolved solids attached to the cake by virtue of its moisture content represents a bonus. Dissolved solids in the centrate are usually low and can be ignored, but when they are not, then they should be included in the equations when calculating centrate rate.
4.2 Particle Size Distribution
Very few process slurries contain particles of uniform size. A large proportion of slurries, processed by decanters, contain solids which have a particle size distribution which conforms closely to a logarithmic probability distribution. The logarithmic probability equation was derived by Hatch and Choate [3] in 1929:
dz Z exp[ {ln(d) - ln(d~) }2] = . - , ( 4 . 1 6 )
d{ln(d) } ~ In(a,,) 2 {ln(a.)}-
where Z is the total number of particles: d is the particle diameter: ag is the geometric standard deviation: dg is the geometric mean diameter: and z is the number of particles less than diameter d.
Integrating this equation gives the formula for a cumulative number distribution:
C,, - 5 + 5 erf x/21n(~rg) (4.17)
where Cn is the cumulative fraction of the number of particles below size d: and erfis a tabulated integral from - 1 to + 1.
It can be shown, by using equation (4.16), that the equation for the cumulative weight distribution is similar:
C,,. = ~ + ~erf x/2 In(;.) - x/2 (4.18)
where Cw is the cumulative weight or volume of particles below size d. Inverting and simplifying equation (4.18):
In(d) - al + a2erf -1 (2C,,. - 1) (4.19)
where al and a2 are constants, functions ofdg and ag.
Decanter Theory 155
Since Hatch and Choate first published their equation, special graph paper has been developed and printed whereby plotting the cumulative percent of particles by number or weight, oversize or undersize, against particle size, results in a straight line. The mathematics of the distribution are such that one can readily transfer between weight and number distributions, and even area and diameter distributions [4].
The diameter at the 50% line on the graph gives the geometr ic mean diameter for the type of distribution plotted, be it number, weight or whatever. The geometric standard deviation, which is the same for all types of distribution, is given by the size for which 8 4 . 1 3 % of the number , weight , etc., is smaller, divided by the geometric mean size:
d84,13 dso - - - - - ( 4 . 2 0 ) erq -- d5o dl 5.87
The relationship between the various means is given by"
dgu, - dge 3(in ~ (4.21)
)2 4 , - d.q e2On~ (4.22)
d ql - d~e (In (70)2 (4.2 3)
where dgw is the geometric mean for a weight distribution: dgs is the geometric mean for an area distribution" dg] is the geometric mean for a diameter distribution: and dg is the geometric mean for a number distribution.
Figure 4.3 shows a typical distribution plotted as a number, area, and weight distribution on the specially scaled graph paper. From a grapb such as this, the two basic parameters, ~rg and dg, can readily be obtained and from these more pertinent information can also be obtained.
For example, the total surface area. Av, of the solids ir~ the slurry can be calculated:
6 At -- dT,, .e} ~M~ ~-~ (4 .24)
This parameter is useful for paints and pigments, giving the covering power of the solids. It is also useful in assessing relative flocculant demand, as this is proportional to the surface area of the particles.
In general, the decanter removes the coarsest or densest particles from the slurry, leaving anly the finest or least dense in the centrate. Knowing the required percentage solids removal from the slurry, the recovery, one can read the desired cut point from the distribution graph. This then gives an appreciation of the feasibility of the desired separation. Experience will tell whether the decanter would be capable of achieving the required cut point. For instance, a cut point of 2-5 Bm would be feasible on a decanter with most
156 Particle Size Distribution
P a r t i c l e S i z e - M i c r o n s
8 II 1 I l l l l l l [ I I I I l I l l l l l l l l l ! 1 I | I I 1 1 | 1 1 1 1 1 1 1 1 | 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 t 1 1 1 1 1 1 1 1 1 1 1 1 I i 1 1 1 1 1 1 1 1 1 1 1 t l | I [ I I I I I I I I 1 1 1 1 l 11 !11111i1 1 - - 1 ? J I i l l I l l i l 11 I l l l [ l l l l I l l ! I I i l l | l l l l l l l l i l l l l l l l i l l l l l l l l | | l l l l l l l l l | l l l l l l l I | l l | l l | l l l l ! I 1 I l l l l l l l l l l l I ] 1 l l l l l l [ [ l - !
' ,- : : m : : ~ i i : : : : i i i i i i i i i i i i i ~ i i ~ i i i ~ i i ~ i iT i i i i ~ i i i i i i ~ :~ i i i i i i i i i i i i ! . ' . ~ ! ! W e i g h t D i s t r i b u t i o n i i t ~ i i i i ' : i ' ~ !
' I III',I II iiii',iil]iil]',iii]iiil}iiii',',',iii?iiii', - ii:~::~ i i:~iiii:,iiiiil i i iiii::::ii';iiii::ii~:iiiiii~iiiiiii~i::ii:~ii..A.~O.~:.~.u, ~ . . . . I ! ! ~
: : l l l l l l l I I l l l : : : l " : l l l : I : : l l l l : ' l l l l l l l l l l l ' , : : l l l l l l l ~ , ~ l l l l : l ' , l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [ [ | . . . . . . . . . . . , , . . . . .1| . . . . . . . . l : l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . " l l l l l l l l l l l l l l l l l l l l I l l l l l l l l l l l l l l l !%1111 F I
, i I i:]~!!!! ! ! ! ! ! ! ! ! ! ! ! f i l l I i i [ ] i ! ! i i i ! ! ! : ~ , ! ! ! ! ! ! : ~ ! [ ! ! i i i i i ! i ! i ~ N u m b e r o l a t r i b u t i o n i ! ! !!!!!!!! [ - | I ] [ I I 1 1 [ I ] [ ! 1 I I , I I ] 1 1 1 1 l i 1 1 I 1 1 1 1 1 J l p r ' ] [ l l l l l l l ] i J ~ r l l l 1 1 1 ] [ 1 1 1 [ l l l ] l l , l l J ~ r - [ l H I | [ I i i , ~ I i [ i i i i 11 I i 1 1 1 1 1 1 ' [ ] I I [ [ [ 1 : 1 1 1 1 11 ! 1 1 1 1 1 1 1 1 1 1 1 1 i [ | l l l J 4 1 r " i l l l [ l l l l l ] ~ 1 " l l ] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 L , F l l l l [ 1 1 | 1 1 1 [ I I ; I [ [ i 1 1 1 1 , 1 1 1 1 1 1 [ ~ 1 1 11111[ ~ I ! l l l l i l i l i I I I l l l l l l l l l I l i 1 I i l h ~ F l l l l l l l l l l l L ~ l l t l l l l l l l l l l l l l l i l l l J ' l l I l l 1 1 1 1 1 1 1 1 1 1 1 1 1 II I I 1 I l l I ! 1 1 1 1 1 1 1 1 1 1 - l i 1 l l l l l l l l 11 1 1 1 1 1 1 1 1 1 1 1 1 1 I I 1 2 P 1 1 1 1 1 1 1 l l l U ~ l l l l l l l l l i l l l l l l l l l l J , ~ l l l | l | l l l l l l I l | l l l l l l l l I i l I l l l l l l l l I 1 !1111111 ? ] I I i l l ' , l l l l l l l l l l l l l l ~ l l l l [ l ~ ' 3 1 1 1 1 1 1 1 J / l l l l l l l l l l l l l l l l l l i l l l L , K l l l l l l _ l l l l l l l l l l l l l l l l i l i I I I l l l l l l l l l l l l I I I i l l l l l l T ! ! I [ 11 ' , 1111 1 1 l l l l l l l t l t l l l ~ 1 F I l l l J ~ F I I I I l l l l l l l l l l l l l l l l l b l ~ l l t l l l l l l l l l l l l l l l t t l l l l i l l 1 l 1 1 I i l l t 1 1 1 1 1 1 I t 1 1 1 t 1 1 1 1 1 I I I 11111II I 11 I [ l l l l l l l l l l l ~ r 1 1 1 1 L J , ' F I I I l l l l l l l l l l l l l l l l l l L i # I l l l l l l l l l l l l l l l l l l l l l l l l l l l i l l I I I l l l I l l l l l l l 1 I I l l l l l i l l ~ i
o~ . . . . . : . . . . . . . . . . . . . . ,'1111 '1 b. ,~gl l l l : , l l l l l l l : l , l l l l ~ , l l l l l ; l l l l l l l l l l l ' , l l l l l l l l l l l l l l l ~ l l I : : : l l l l : l ~ l l l l I I l l l l l : l l ~ I o.I I I l 1111111 , I 1 1 1 [ l l l l l l l l l i ! i t l l l l l , , l [ l l l l , | l J I I r - l l l l i l ~ l , l l l l l l l l , l l l t l l | I | i l l l , 1 1 1 1 11 1 1 i 1 1 1 1 1 1 1 , 1 1 I I 1 l l l l l l ! ~ | n 7 I I I l l l l l l l 11 i I I I I I I I I l ] l l . ~ r I l l l l l l l l l l l l l l l l l ] , ~ " l l l l l l l l l l l l l l l l l l l l l l [ l l l l l l l l l l l l l l l l l l ] I 1 I l l I l l l l l l l l I I 1 I 1 1 1 1 1 1 1 ] |
.-" i ii~!iiii i i i i i i i i i i i i i i i i i !~ i / ! .L"4~i ! ! ! ! i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i [ i i i i i i i i i , i i i i i i i i i i : : i : ! i i i "iiii~.i ! 0 6 : I , ~ : l l l l l l I I l l : ; l l l l l l l l l l : ' l , ~ ' - , ~ l : ' , l l l l l l : t l l l l l l l l l l l l l : ; f | ; : l : ' , ' , l [ l l l l l l l l : l l l l l l l , l l l l : l ' , ' , ' , : l l l ' , l : : : I : : : l l l l l ~ |
: I ~ | l ' , ; l l l l | I [ l l ~ l ' , l l l ~ l l I ', l ~ " l ~ , l l ~ l l l l l ~ ; l l ~ l t l ~ l l ~ ; ' , l l ; ; l l l l l l l ~ ' , l l ' , l f i l l ~ l l ; l ' , t ' ~ l : l ~ : l l l l l I l l l ~ ; ' , ', l ~ l l l ; l l ~ : | .
o. i i i i l ~ i! i i i i i i i i i i i i i l ; ~ i i i i i i i i [ i i i i i i : i O ~ r i i -+i;Hiii i i~-4i--i i i i i i ;-Ft- 1 I 1 : I ] i l l l l 11 1 I I 1 1 1 1 1 1 II J ~ " i ! 1 I 1 1 1 1 , 1 1 1 1 1 1 1 1 , 1 1 1 1 1 / 1 1 1 1 , , 1 1 , 1 I , I I I 1 , 1 1 1 , , I ,1 I l l l l I 1 I , , I l l , , , I 1 I l l l l l , . . . . . . . . . . . . . . . . . . . . . t t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . l t l i t . . . . . . I . . . . . . t t . . . . I . . . . . . t f t . . . . . . . . . ~ 4 /
0 3 ! 1 : I111111 1 1 1 l l l l l l l l ] ] l l Iv I 1 ] l l l l l l l l l l , l l l l ] l l l l l l l l l l l l l l l l l l l l l l l l l l l l i , l l l l l i l l l l l l [ l ] 1 ! 1 1 1 1 1 1 1 1 I ] 1 1 I I i11111! I l / : I [ ~ ' ~ ' ' , l l : : I : ]' l : l l l : l l ~ l l l l ; : I l l l l : ' , ' , l : l l l ; : l l l l l l ~ l : l l : ; : : l ~ : l l l l l ' ~ : ; l : ' , l l l l l l l l l l l l l ' ' l : l l l l l l l l : l l I : l : ' , ' l : . ' . ~ : J I
1 �9 i 1 i i i i111 l l l i i i l l l l r 1 - T l i 1 l 1 [ 1 I I I l l i I I I 1 [ ' 1 11 I I [ L L I I I I I I I l I I I I I I I I ] I I I l I l ~ ] I l l l l l l 1 1 [ 1" I [ 1 I I 1 1 1 1 1 1 1 I [ [ I I I 111 [ I I
o~ I -t-i H-tfttt--tt--t t IH- t fH+Ht - - i ~ - 1 ~ + ~ . ~ ! i ~ H f ~ + f ~ - ~ . ~ + H ~ . ~ H f H ~ - H - ~ ~ - H H - ~ - ~ - ~ - - ~ - ~ - ~ - H I ~ f H ~ I I ii;;ii..) f--t--- _1 ! - - I ~ l l ! l l l l ; l [ l l l l l l l l l l ~ ~ I l l l l ~ l l l l llllll~ . . . . . l . . . . . ; l f l l l l l l l l l l [ l l ~ l . . . . . . . . . l l ; ~ [ l - . . . . . . I II~ . . . . . . . . . . . . . . . t I , , i~,: . . . . . . . . . . . . . . . . . . . . , , , , , , . . . . i ....... I f | | | , I f | I l l , . . . . . . . . . . . . . . . . l l l l : l l l l , , l ~ , , l I III; .... t11111'1 I I I I I I ! I I I I I 1[ 11JJ ] I [ 1 1 1 1 1 1 1 1 T I I 1 I I i l l l l l l l l l l l l l l l l l l l ] l l l l l l l l l l l I l l i l l l l l l l l l l l l l l l l l l l l l l ' [ l 1 1 ! 1 1 1 , 1 1 1 1 , 1 I [ i l I l l l l l l 1 1 | I 1 ! ~ L : I I I I 1 1 l 1 l l l l l l l l | l l I 1 I J l l l l l l l ; l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l ] l l l l l : 1 ] 1 1 I I I I I i l 1 1 1 1 1 1 I 1 I I I I I1 ! I I
i I I I I1 :1111 I I 1 1 1 1 1 1 1 1 [ 1 1 1 1 1 1 l l l l l l l l l I I l l l l l l l l l l [ l l l l l l l l l l l ] ~ l l l l l l l l l l l l l l l l l l l l l l l I I I 1 1 1 1 1 1 I I l 1 1 1 1 1 11 I I I l11! 1 I I l l l ; l l l l I I I l i l l l l l t l l t l 1 I l I [ l l l l l t l l l l i t t t l t l l l l i l l l l l l i l l t f l l l l l l l l l t i 1 1 1 1 1 1 1 I I I t | i l l f 1 1 1 1 1 1 1 1 1 1 1 1 1 I I f I t i 1 1 1 ; i i
t I I ~1111111 11 1 l l l l l l l I l l l l I 1 I I l l l l l l l l l l l l l l l l l l l l I l l l l I l l l l l l l l l l l l l l l l l l l I l l l l l l l l l l ' l l 1 1 I 1 1 1 I I I I I 1 1 | I I 1 I I I I I L ~ I i 1 : 1 1 1 I I I ! 11 I I I I I I I I F H I I I 1 l ~ l l l l l l l l l l l t l l t l l l l l l l l l l l l l l l l l l l l l l l l l l l I I l l [ l l l l t l l l l i l l I I I i l 1 1 I I I I I I I I 11 1 1 I I i l l i i l l ] I
o~ I 1 f i l l ! I ~ l ~ l l l t l , ,11,!I l l l l l I l ~ I l l~ I , t l I l . t l l l ~, :~ l I lu~ l~ l l~ I I~ l~ l~ l l I . I l l l l~ l~ ~I~ ~ l ~ l ~ l ~ l ~ . , l ~ ; + ~ . I o o l o0 1 o i O2 o s ~ 2 s Io ~o 3o ,IO Io II) Iio Io I s Iii I I I l l I I I I llelm
Cumulat ive % Unders ize
Figure 4.3. Cumulative weight, area and number log probabilit!l distributions.
slurries, but, say, less than O.1 pm would probably be impossible, however high the density of the particles.
The cut point size is the smallest particle size that has to be settled in the decanter. Technically 50% of particles of that size settle and 50% are lost in the centrate: above that size the separational efficiency increases and below it vice versa. In consequence, the size distributions in both the cake and the centrate will also exhibit logarithmic probability distributions.
Figure 4.4 depicts examples of weight frequency distributions for feed, cake and centrate. Note that the centrate and cake lines intersect at the cut point size, and at a frequency level half that of the feed at that size. Hence, there is a 50% split between cake and centrate at the cut point.
These frequency distributions are plotted as cumulat ive weight distribu-
tions in Figure 4. S.
16
14
12
10
8
6
4
2
0
10
158 Particle Size Distribution
P a r t i c l e S i z e - M i c r o n s lo
�9 I I iiiiiiii ii iiiiiiiiiiill i i iiiiiiiiiiiiiiii!iiiiiiiii!*,i!ii' i~!!!!!!! i i ! i i i i i i i i i i~~ 2~t~iiiiill i ii ~,iiiiii i i o _ I J I I i i i i l l l l ] l i i i l l l l l l l l l i 1 1 i i l l l l l l l l i l l l l l l i l l l l l l l l ] i l , i i ] I I ] I I I I I I I I I I I l I I I I A d " I d P T I [ ! I Idr t l l l l l i l l l l l l I 1 l l i L | l l [ ] | ? I l l l l l l l | ! ] 1 1 1 ] 1 1 1 1 1 1 1 1 1 I I I l ] l l l l l l l l l l l l l l l t l t l l l l ] l l i ; l l l l l l l l l l l l l l l l ~ l l l l l I.,SI I I I I I L I I I I I I I I 1 l l l l l l l l ] I
! i ! ! ! ! ! " " - ' ' ' ' t ~ ! i i ~ - ' ; . ; t ! ! ! ! i i ~ ! ! ~ . i i i i i ! ! ! i i i ~ i i i i . ! ! ! : ! ! , ! ! ! ! ! ! ! . . ! i ~ 2 ~ T F _ 7 . . r ~ i i . : : i i r - - ~ - i ! ! ! ; . i . ; ' i ! ! . ; i i i i " i i i i i i i !
�9 [ I i!iiiill i! i iiii~iiii!i ] i ! i i iiii~iiiiiiiiiiilHHiiiiiiiiiiiiii~g-~;~iiiiiilL-~ i i iiiiiiiiiiii i ii iiiii~ii i . ! [ !!!!!!!! i! [ l [ ! ! ! ! ! i i ' , i ! ! i i i ! !!!!! i i i ! !!!! l i i i ! !!!!![][ i l l i is i ~ i i i i ~ ! ! ! ! ! i ! !i ~!!!!!! !
. . . . . . . . . . . . I I l l l l | ; ' , l l l l l l l l l l l l l . . . . t t ] - ~ ~,~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . . . I ] i i ! ! ! ! ! ! i i I l l t l : : : i : : l : ] . . . . . . . . . . . . . . . . . . . . . . : : : : " : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : ~ : : : " : : I : : : : : :
i i i i i i i i i i i ! i i i i ! i i i i i l t t t . . . . . . . . . . . . . . . . ~ . . . . . . . . . * ' . . . . i i . . . . . . . . . . . . . . . . i i i . . . . . . . . . . . . . . . . . . . . . . . . . . 1 | [1111111 I I [ ] 1 1 1 1 1 1 1 | i 1 1 [ I I i l l l l l l l l l l ] l l l . , d ~ l i d l ~ l I I I ] I ' , I i : } I I I I I I I A P l l l I I I I I I I I I I | I l l l l l 1 I l l l l l l l l l l ] [ | [ i l l | f l l [ i 1
:1 l I i ; f i l l ; ; , ; l i l l i l l ; ; ; ; i l ; i i i i , ; , ; , ; ~ t l ; ; , ~ . . ~ - ; ; ~ , , " - ; ; ; ; ;iiii~i, : : : : : : : : : : : : : : : : : : : : : : : : : : : : , ; ; ; ; ; ; ; ; ; i ; ; ; ; ; ; ; ; ; ; ~ ; ; ; ; -- l - ,r l l ' : l l P , ' | - t | I I I ;H I I I l l I I : l ; ; l ; : ~3 , - - ' ~ , i l l , l ~ ; , ' , l l : l : [ l l l i l i I ' , I I ~ , , - ] I I ' , ' , t : : : :H ' , I I : : : : I ~ - : : " " i : i : : : : : : ~ ' , ~ , ' , : : ; ; ; : i ~ ~ :
~ ' ; " ~ : , : ,'T' t t l i t l l : l : ' , : ; ', : ; ; l : : ' 2 " , l ' r : ' ; : ' , , ' ' : ; ] ; : " ; ; l ' , ; l l l i l ! i " " - -m ' l l : ; ; ; : ; ;H ; ; ; l ; l l ; l ; ; ~ ; I ; ; ; i i " - : ; I : i ; ; ' ; : ; ; " : ' ' : ; ; ; t . . . l l l l l l h~ ~ p ~ - H ~ - H i ~ - H - H ~ - i - + - ~ ~ ` ~ ' ~ ' ' ~ ~ ~ ` ~ ' 4 q ' ~ ~ ' ~ ~ I I l l l l l l l l l l l ; l I I l l l l l l l l I : I ~ r i i I I I I I I , i P l I I I I ' , i i i i ~ I i ' , . , i . - ~ 'GL~ -% l ' , l l l l l l " i i i i i l l l l l ~ ~ " ; , ' , I I I I I I I I I I I I I I ' , ' , i i i i i I I I ' I I I I I I I I I I I I I I I ' ~ I I I I ' , I I I I I ' ' I
' i t lil~ii~ t I 1 ' - ' " " ' ~ . . . . . . . . i i t~l l l i l l i f i4.1. ,"i l . . . . . . . . . . . . . l l l i l~ :~ l i i l l l i i i i l l l i i I H i l i ! i I I l i l l l l l i I I l l l l l l l I I l u H e L y , , " - L - , I " I I I F e e d l l l l l l l l l H l ~ , - ' ~ ! H I C e n i r l l t e I I I I I I I I I I I I I I , , , I l l l l l l l l l l l l l I I I I I I I I I : ! l i ! 1 i l l i I I I i l [ l l i , l l ~ l 1 I l l l i i l [ i l i i l l ~ l l l l l l A r l l l l i l l l l , ~ l l i l l l l i i l l l l l l l l l l l l l l l I i I I i l I I I I I 1T l l l I I I I111111 I I 1 1111111 �9 I I 11 ] , i l i ' 11 I [ 1 I 11111 I I I i l 1 iJ~rl I ] ; ~1 i 111 l l l l l l l l l l l l l l l l l i l I 1 I I1 l I I l l l l 1 i l I 111 I i i I I i II11111 I I IA l~ r " l l l l l l l l 1 I l l l l l l l l l l l l l l l l l J~ r l l l l l l " : l l l ~ l ' . l l l l l l l l l l l l l l l l l l l l l l l I I I 1 I ! 111111 ] ! i ; I 11 I l l l l l l I I ] I I I I I I I l .A i~ r ] l l - IP r l l ' , l l t 1 I l l l l l l l l J i l J l l l l A~ l l l l l ] l l ! ! l l l l l ] l l l i l l l l l l l l l l l l l l l l l l l l I ! 1 I I I I 11111111 I 11 1111111 I
, , , , , , , , ~ , , . ~ , , , , , , , , , , , , , , , , I , , , , , , ~ ,, , , IL i l l l i , , , , , ,,,, 1
o , ,~ ,,,:::,,,, ,, ~,:~::',::,::, ,~,,:,',:iliil!~iii~iiiiiill I iiii:!!iiiiiiiiilb:!!!!!!!!iiiiiii i iiiiiiiiiiiii ii ~iiiiii i i I l ' l l l l l l l I I I I I I I I I I I I I I I I I I I I I I I , , ....... , , : , , . i i i~, , . . . . , , , , ...... , . . . . ~ . . . . o. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I I I I I I I I I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 07 ! ! !!!!_~ ! I ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! N ! ! ! ! ! ! ! ! ! ! ! ! ! - 7 ! ! ! ; ! ! i ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! l ! ! ! ! ! ! ! ! ! ! ! ! ! ! : ! ! ! ! ! ! ! ! ! ! . ! ' .. iJiii i i i~ ii:iiiiiiiiii~ii ! i iiiiiii~iiiiiiiiiiiiiiiiii:71iii!iiiiiiiiiiiiii~iiiiiiiii~il i i 111117,1111!111 iiiii~i i o, i I ! ! ! ! ! !~ !1 i i i ! ! ! ! ! ! !~ ! ! ! !!!!!!!!!!!!!!!!!!! l!!!!!!:!!!!! i!!!!!!!!!!!! i i!!!!!!!!!!! i: i i i ! !!!!!!!!!!!!! !! !!.!!!!!! ! !
I I I I I i i I I I I I I l l l l l l l l l i l l t I l l l l l l l l l l l l l l l l l l l l l l l l l l i { l l l i l i ~ l l l l l l l l l l lH i l l l l l l l l ~ l l l I I I I I I I I I ~ I I I : I I I I I ~ I i I I ~ I :
o, i i ' i i i ! ! ! ! ! t i i i i , i ! ! ! ! ! ! ! l i i i i i ! ! ! ! i i i i i i i i i i i i ! ! ! i i i i i : , ! i i i i iT!!!!!!!! i ! ! ! i ! i i i ! ! ! ! ! ! i l l ! i i i i i !!! i i!! i ! !! iiiili!! ! " f ] I I I I I I I I f I I I I I I I I I I [ I I I I l l l l l l l l l l l l l l l l l l l l l l l l l , : i l i l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l I I I I I I I I I I I I I I I I I I I I i t l i i I : / I l i l l l l 1 / i l l l l l l l l l i l l I 1 I l l l l l i i i l l i l l l l l l l i l l i i i l l i l l l l i l l l i l l i l l l l l l i l l l l l i l l l i l l l I 1 i l l l l l l i l i ! i I l i U l l ! l l l I �9 ! ! i11111 I 1 I l l ] l l l l l l l l l i 1 ! l l l l l l l l l l l l l l l l l l l l l l l l i ' . l l l l i I ' i l l l l l l l l l l l l l l l l l l l l l l l l I I 1 l l l l l i l l l l 1 1 I i i i i i i i i 1 i 1 �9 o.$ / t " ' l l l . : l i : ' . I I l l l ; l ' , i l i : i l ; i i i l l i i i i i i i i i l l l l i i i l l l l l l : ~ l ; l i l ; i i i i i i i i i i i l i i i i i i l l i i i i i l l i I l l l i l l ; i i i i i i ; I i i i i i i i i I ;
L ~ - I i i i i i i - i i i i i i i i i i i i i i i - i i - i - i i - i i i i i i i i i i i i i i i i i i i i i i i ~ ; i ~ - i i - i 4 i i i i i i 4 - i - i i - b i i ~ i i i i i - i i i i i i i i i- i , i i i i i i ~ i i i i i i i i i i i l i i . 4 I 7 I I ; I I I I I t i I I I I I I I I I I I I I l I l l l l l ; l l l l l l l l ' , l l l ; l l l ~ . l l l ; i l i l l l i ' , l l l ' , l l l l l l l I : l l " , ' , ' , ' , l l l ; l . ' I I l ' , l ' . l l l l t l l l I I I l l l l l ; l l I :
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001 00S 0.1 01 011 I ~' tt ';0 lie le 4O l e l 0 Se SO 'iS I I t l I l l i l l l t l l
C u m u l a t i v e % Unders i ze
Figure 4.5. Cumulative ~veight distributions for examples of feed, cake and centrate.
4.3 Clarification
The separation of solid particles and agglomerates from the suspending liquors, within a decanter centrifuge, has invited numerous theories. Few of these theories have provided exact results, which has given the opportunity for many more. This is no reflection on those providing the theories, as the system, let alone the processes used, are quite complex. In a decanter, liquid and solids flow in a helical path within a cylindrical vessel with a conical end. The rotational velocity can vary from the bowl wall to the pond surface. Many theories start by assuming discrete spherical particles that settle in a laminar flow regime, when often this is far from what actually happens. The expression of liquid from the cake can be as a result of one or more mechanisms. It could be by filtration, compaction, or simple drainage against the scrolling action.
Nevertheless over the years a number of theories have been proposed, which allow a reasonable fit wi th much of the data, or specific categories of data. This chapter gives some of the more common and usable theories.
4.3.1 Sigma theory
For more than 40 years the clarification of liquor in decanter centrifuges, in fact in most sedimentat ion-type centrifuges, has been characterised by the Sigma theory of Ambler [ 5]. This theory assumes that spherical particles settle in a laminar flow regime according to Stokes' law [6], in a cylindrical bowl rotat ing at constant angular velocity. Since the first publication by Ambler, several variations have been used which are approximations, or developments, of the original.
Refer to Figure 4.6.
Consider the smallest particle in the feed sludge that has to be separated, the cut point size, de. This particle has a density, Ps, and settles in a liquor of density, PL, and viscosity, r/L. The feed slurry enters the decanter at a rate of Qf, at a pond radius, rl, at point X at time t=0. By the time the particle traverses the clarifying length of the centrifuge, L, in time t=te, the particle must settle to a radius r 2, at point Y, the bowl internal radius, if it is to be collected by the conveyor. The centrifuge rotates at a constant angular
velocity, w. I t is assumcd that the fluid in the bowl also rotates unil'ormly at angular velocity w, and travels along llie howl in plug flow. It is furthcr assumed that the particle being considered is homogeneous and spherical. settling i n a lairiitiar flow regime.
At time t;f. the particle has a radial vclucity \',and a constant axial velocity dv,, It is also assumed that the particle travels a negligible distance from the pond sur!ace before it reaches its terminal velacltg.
The axial velocity is given by:
Thus: X L
t,, - - (rf - Qf
'I'he radial velocity at any time t is given by: 7
W- I' \Tr = \rs --
n
where 17, is t he Stokes settlirig velocity, given by:
(4.2 h )
(4.2 7)
(4.1 x)
where rl, is the parfiule size at the cut point; p s is the dcnsity of thc pilrticle; pL is the density d t h ~ liquid: and r/is the viscosity of thc liquid.
Now: d r
- dt ,? - - (4 .29 )
Siihstifutirig equation (4.29) into (4.27) and integrating between the limits of r=rl to rL ;ind !=(.I tot,:
I<lirninating t , from cquations (4.26) and (4.30):
'rllc terms (111 the right-haad side of equation (,4.3 1 j consist of vs. which is solely i1 limction o f t h e process material, nnd t he remainder olthe terms, that ;ire snlttly fiinct.ions ol' t.he ccnt.rifuge. Thew I;itt.er lerrns are cnllixtively knnwn as Sigma. I:, the c:larific:ation c:iipiic:ity ofthE: c:crilrifiige:
(4 .32)
Sigma has units of arcs. which is consisterit with rior~.r:erilriTi~gnl clarifying cquipment.
Frorncquations(4.31) and (4.52):
Bquatiuii (4.32) is \.he eqiiation For Sigma prcierred todijy. i I n d is particularly recorr~~iieridd when deep pond dccanlers ( r l ir.? '-'O,h5) are uscd. Whcii Amhler- first derived his formula. only shallow pnnds ( r , / r ! > 0 . 7 5 ) were wed. and he uscd different starting assumptions l'or his derivation.
With a shallow pond it is assumed that 11ir incomiiig iced distributes itselr cvenly t.hrvughuut the depth or the pond, iri the annular plant: of the puirit of rnlry. The theory thcn develops thc snmc cquatioiis in the some way, assuming that half ofthc particles of the srnallcst size that have l o be separatcd will be removed, This is consisterit wi1.h I he definition ofcut pnirli.
The last particle of the hall'ol'the smallcst particles 10 he setilcd will star[ a! il radiiis r, at thc fccd point, at which half of all particles will start inward arid Iialfoulward oftliis point ir'i this plane, S o :
whmce:
( 4 . 3 5 )
NOW substituI.ing equation (4.29) into e q u i l t i ~ n (4 .27) agaiti. hiil this timc integratirlg het.ween t h e limits of r = r l a i d r=rx and t=O t o t=t,:
(4.36)
Solving equation (4,361, eliminating rx using equalion 14.75) and eliminating t, using equation (4.26);
(4. .3 7)
uj = 2u,.c (4.3 8)
whcrc Z this time is the truc Arnblcr Sigma given by:
(4.39)
Cornpare 1: of rqiial.irm (4.3’4) with E of equaiion ( 4 . 3 2 ) . Nole I.hc extra numeral 2 in ryuatiori ( 4 . 3 X ) cornpared wilh equaiiori ( 4 . 3 3 ) . This is to he cxpcctcd if all thc particlcs that h;~vc to be separatcd h a w the advantage of starting at half pond depth!
I t will also be wen in the literature 171 that approximations are sometimes made for 1 tie logarithmic term in equation (4.321 to givc:
(4.40)
With shallow ponds it Is sometimes considered that the g-fnrce is cnnst.arit, iri
which case equation (4.27) would be rewritten;
Substituting equation (4.29) i n t o (4 .41 )and irikgraliag:
Eliminating t , I‘rorn equations (4,421 and (4.26) and rearrangirrg gives:
. v> 7 & 2 ( r l + Ul) (I-; .- r f )
2 Y . (r2 - h ) SJ =
(4.41)
(4.42)
(4.43)
Decanter Theory 163
= 7rLg:.(r2 + rl)Vs (4.44)
= 7rLg ,~.DA v. Vs (4 .4 5)
where g'c is the mean g-level in the pond; and DAy is the average pond
diameter. Alternatively equation (4.43) may be written:
V , (4.46) Os = ~-Tr Oc.V~
where Ar is the pond depth: and V is the pond volume. For this derivation of clarification capacity, it is readily deduced that"
W 2 r~ - r~ E = 7rL (4.47)
g r2 - - r l
In the graph of Figure 4.7 the various formulae for Sigma developed so far (equations (4.32), (4.39), (4.40) and (4.47)) are compared for various ratios ofr l / r2 . The common factor 7rLw2/g is removed and r2 is taken as unity, for the graphical comparison.
By means of Figure 4.7, a number of observations may be made. The expansion of the logarithmic term to give an easier formula for Ambler's Sigma is a very acceptable approximation. The even simpler formula last developed above is also acceptable for shallow ponds (radii ratio greater than 0.75). However, there is a significant difference for the formula used for deep ponds. Notice that with zero pond radius, the shallow pond versions of Sigma have finite Sigma values while the deep pond version has a zero value. This is because a particle starting at the centre line will experience no g to initiate its fall, while those which by definition start half way into the pond, or are subjected to a mean g throughout the pond will always have a finite settling rate. However for practical designs the radius ratio will always be appreciably over zero, generally in the range 0 .4-0.8 .
It will be seen from the various Sigma formulae that increasing the length of the bowl increases Sigma pro rata. Thus, in this respect Sigma is additive. Some like to include the Sigma value of the beach in their formula [8], especially when feeding on the beach. For this, using equations (4.46) and (4.40):
E=27rw 2 Lc r~+ r~ + ( r ~ + 3 r , , ' 2 + ) 48)
where Lk is the wetted beach axial length" and Lc is the cylindrical length of the bowl.
164 Clarification
2.50
2.00
U
~" 1.50 ell
DID
r ~
~. 1.00
m 0.50
0.00
o
a ' ' ~ 1
o O ~ ~
. r
I
!
o o
� 9
i I I 1
o v ~ J "
�9 I o a'~ I
i
' i I I I ...... Deep Pond. Equn. 4.32 I
I' Simplified. Equn. 4.40 [ , t
! . . . . A m b l e r . E q u n . 4 . 3 9 i I
. . . . Approximate. Equn. 4.47 l
i 1 ,
0 0.2 0.4 0.6 0.8 1
P o n d R a d i u s / B o w l R a d i u s
Fignre 4.7. Graph comparing the various.fornz.hu' for Sigma at vario.s pond depths.
Considering the assumptions used in the derivation, and the approximations used, one could question whether the use of the simpler equation (4.47) would not suffice, for use with shallower pond machines at least. It is the ratio of Sigma values which is used when scaling from one decanter size to another. Using equation (4.48), the ratio will be little affected as the extra term will increase by approximately the same ratio with geometrically similar machines.
Another expression, in place of Sigma, uses an empirical formula taking a nominal bowl radius, the ~ bowl radius, r~ (which equals three quarters of r2). This expression [2 ] is termed the "area equivalent", Ae~ and is determined by"
Ae3 = r2 L~ + cota (4.49)
where a is the beach angle ( half included). More often the abbreviated form is used, which ignores beach volume and
uses clarifying length"
(4.50)
h - 1 is then used as a scale-up factor, in p l i i c t : of Sigrna. Its usc is simply a mdttcr ofchoice arid habit.
A]] thc formulac? iridicate that a bcttcr clarifici1I.iori q i a c i t y is achicvcd at the shallowest pond depth, whereas in practice it is generally t.he opposite. 'rherefore, thc simple formula is gelierally considcred sufiicieot for p r ~ r i i c ~ l purposes. Howcver, when scalirig kern one macliinc to another, it is imperative t ,hat i:he sar~ie furmula is L I S ~ for hnt,h rnachiries. It is also rcc.orniiicnded that one should riot noritlally scale hrtwettri machiries of dissiniilar geornctry.
4.3.1.7 Usirig sigma
11 is unusual 10 use aiiy of tbcsc formulac 10 computc the cilpiicity of';] single machine. Their rriost effective and reliable use is in scaling data Irom OPC
geometrically sirIiilar machitie to aiiothcr and assessing rclative perform ii (1 ces ,
Eliminating \',Criir.i~cyuittiniis ( 4 . 2 8 ) and (4 . 3 3 ) :
( 4.. 5 1 &. 4 (Ps - 01, h/ - c 1X. l I
or
'I'aking Inger-ithmsoflinth sides nfsquat ior i (4 .5L):
l I l ( 9 L y ln(d , )
(4.52)
(4 . S '3 )
T I is known from equation ( 4 . 1 '7) that thc pcrccntage over nr uiidrr size is ii Ir)gari1hrnir: pi-ob;ihiliI.y i'i.inr.tion of pal-tick diameter. 'I 'hus. combining equations (4.1 9) and ( 4 . 5 3 1 , a logarithmic prohahi l i ly relationship between Qr/Y nnd solids recovcry is obtaincd:
I'liitting ur/C againsl solids recovery will f h u s give a good correlation [ 9 ] . l'lottirig on logarithrnic probability papw will produce a straight linc 11 01. 'l'he saiiie straight lirie is obtained for data from different decanters. prcfcrably
1 6 6 Clarification
of the same geometry. However, it must be noted that this only applies to process materials with solids exhibiting a skew Gaussian (logari thmic probability) distribution.
When scaling from one machine to another:
Qf2 Z2 = ~ (4.55)
where the subscripts 1 and 2 refer to centrifuges 1 and 2, respectively. When machines of different geometry are used then one needs to take into
account the relevant efficiency, (, of each design, when:
Qf2 __ ~2 Y~2 (4.56)
4.3.2 Sigma enhancement
The use of conical discs, or angled vanes, on the conveyor will theoretically enhance the Sigma value, the clarification capacity, of the centrifuge. To estimate the Sigma value of a stack of conical discs, the formula used for disc stack centrifuges may be employed [ 11 ]:
27rnD ~d 2 ED = .(r~ -- r~). cot O (4.5 7)
3 g
where ED is the Sigma value for the disc stack; nD is the number of discs; r ~ is the outside radius of the discs; and 0 is the half included angle of the discs.
The total Sigma value for the centrifuge is obtained by adding ED to the Sigma value calculated for the conveyor section between the feed zone and
the discs. There is little published on the effect of longitudinal angled vanes, but the
equation is derived in a similar fashion to tha t used for the disc stack
centrifuge:
n v L v w 2 ~ 7rLvw 2 Ev = 2g. c o t , (r~ - r~) -~- ~ r ~ g (4.58)
where Ev is the Sigma value for the vanes; Lv is the length of the vanes: nv is the number of vanes; and ~b is the angle between the vane and a radius.
If the vanes do not extend the full length between the feed zone and the centrate discharge, then the Sigma of the plain section needs to be added to Ev to obtain the total Sigma value for the centrifuge.
Caution is needed in using these extended Sigma values, part icularly for the angled vanes. This is because flow th rough the vanes or discs can channel , to
Decanter Theory 167
take the easiest path. This will reduce the effectiveness of the devices. Good designs, therefore, will endeavour to ensure even distribution of the flow across the vane and disc openings. Even then as liquor flows from the outer edge of vanes or discs, towards the centrifuge axis for discharge at the weir lips, considerable changes in kinetic energy occur. This can cause very complex flow patterns, turbulence and Coriolis effects.
4.3.3 Flocculant requirement
Chapter 5 is devoted to flocculants. However, it is appropriate at this junc tu re to mention the need for polymers in some process applications, par t icular ly effluent applications which are a large market for the decanter. In these applications, without a polymer flocculant, it would not be possible to employ a decanter. It is clear from equation (4.28), Stokes' law, that the settling velocity of a particle, Vs, is proportional to the square of its diameter. Thus doubling the particle diameter will increase vs by a factor of four. This results in greater separation efficiency. The objective of flocculants is to change the electrochemical forces on the surface of the particles, so as to bind them together such that they act as one large particle. Once flocculated, these particles must be handled carefully so as not to break them up mechanical ly . This is especially true when processing them in a decanter.
In most applications, the amount of polymer used is just sufficient to flocculate a sample of the feed. The amount necessary, as assessed in the laboratory, is generally the amount used in practice on the centrifuge, plus or minus a small fraction. However, recently there has been considerable development in decanters and their use in obtaining extra-dry cakes from compressible sludges, particularly effluents. In these instances, the consumption of polymer has increased considerably.
The amount of flocculant needed increases as the extent of dryness required in the cake increases, and it increases exponentially. The amoun t of flocculant required also increases with the feed rate to the centrifuge [ 12 ]. In practice, on a "dry solids" application, the polymer used will be two to three times tha t which would be used on a s tandard application with the same process material.
There has not been a theoretical formula proposed to quantify polymer demand. However, the available data suggest a format similar to equat ion (4.59):
P D - kl + k2.e (~:'-k~ (4.59)
where kl, k2, and k3 are constants.
Practical data can be very erratic, as it is easy to overdose when striving for extra dryness. When assessing the min imum polymer requirement, it is necessary carefully to adjust all operating parameters , to ensure performance is at the limit, without cont ingency levels added.
4.4 Classification
Classification, the fractionation or separation of particles by size, could be considered as merely inefficient clarification. The cut, or desired classification, is adjusted by altering the centrifuge's efficiency. This is most easily done by altering the feed rate or bowl speed. However, adjustment of pond depth or differential may, in certain circumstances, be used.
In a thick suspension hindered settling occurs, when there is a tendency for the larger particles, which should settle, to get held up by the dense concentration of the smaller particles. In these circumstances higher differentials could be used, to agitate the suspension and so release the heavier particles. The disadvantage of this is that the cake or "heavy fraction" tends to be wetter, as a result of the higher differential, and thus entrains larger quantities of the smaller particles. To correct this, a shallow pond is selected to allow release of liquid containing the smaller particles on the dry beach.
In some classification applications, the required cut point is very sharp and the rheology of both separated phases is such that they remain quite fluid. In this type of application the pond used would be relatively deep, and separation would be akin to a liquid/liquid separation, using a hydraulic balance under some form of baffle.
Very occasionally there will be found a classification application where it is required to separate two distinctly different particles, such as in the refining of minerals. In these cases the two different substances to be separated may have markedly different densities. This is particularly acceptable and quite advantageous when the denser material comprises the larger-sized particles. However, if this is not so, one must consider a combination of density and particle size for the cut point of each of the two substances, in relation to Stokes' law. One could visualise the situation of a large, low-density particle settling faster than a high-density, small particle. Thus for such a process to be feasible:
2 dc2h(Psh - Pf) > dc , (Ps l - Pl) (4.60)
where dch iS the required cut point size of the heavy fraction; dd is the required cut point size of the light fraction; Psh is the density of the heavy solids; and P~l is the density of the light solids.
Decanter Theory 169
Each of the two solid const i tuents will have their own size distribution from which a cut point size can be chosen to give the desired purity of product and yield.
Poor efficiencies can occur in some classification applications, due to na tura l agglomerat ion of particles. In these applications, the use of dispersants is quite common. Dispersants have the opposite effect to flocculants, and can be equally powerful.
4.5 Three-Phase Separation
Decanter three-philse operalion involves t:he sep;rrai:ion ol‘ two immiscible liquids from i t solid. The two immiscible liquids are generally nil and water. This could bc a wastc oil application or thc separation of a vcgctablc oil. such as palin or olivc oil.
To put tlic decanter into operation two weir heights or cquivalcnt h a w to he set relative lo the solids discharge level, as illustrated in Figure 4.X.
Firs1,ly the weir height, governed by radius r l , is set to fix the extent of the dry beach rcquired heforc solids arc dischargcd at radius r.+
The radius I’h has t.heri to he set. Lo create a hydraulic balance between the two liquid phases, to m:iiritairi Ihe equilibrium line at rr, where required.
Thc prcssurc at a n y radius. I’, in ii rotatirig ceritrifuge is given by ! I , . where:
(4.h 1 )
Thus, in the three phase ceritrifuge, thc pressure at the cquilihrium line i s P, where:
(4.h2)
where pl is t h e dcnsity of the light phasr; arid pi, i s the dcnsity nf the heavy phase.
The choice ole-line position dcpcnds upon R number orfactors. ‘I’lic volumc nf each phase in the bcrwl could bc chosen in propurtion to the volumes of each in t.he feed. ‘Thco approximiiiely the valuc of Q/C for each phase would he the same. Howcvcr. if the separa1.ion of one phasc from another i s relatively more difiicult than vicc versa. then extra vnlumc could bc given 1 . 0 orit: phase in the bowl to improve its clarification efticiericy. Alternatively. the purity or o r i f phase may be more important than the other. arid then bias would be given to
t,he morc important phase. Nevertheless. care has to be takcn in setting the tt- line in order not to allow breakthrough of one phiist: ii1t.o the other.
When thc flow rate o f one or morc of the phases is high, cresting over the weirs call movc the e-line cc-)nsiderablp. and adjusl.ment of the wcir hcights
Decanter Theor!!
171
Figfin, 4.8. Hydraulic bnlattw it1 thrcc-phase sepnmtioti.
172 Three-Phase Separation
will become necessary. Back-pressure from a centripetal pump or skimmer pipe will require recalculation of weir settings.
Working with three-phase separation will require revision of the formulae used for performance evaluation. Not only will there be interest in solids recovery and clarity (absence of solids), for both liquid phases, there will be an avid interest in what has happened to the oil. How free of water is the oil? What is the recovery of the oil? How much oil is left in the cake and the water phase?
In some of the three-phase applications, water is added to dislodge the oil from the solids. With water addition there are two input streams, feed and water, and three outlet streams, light liquid phase (oil), heavy liquid phase (water) and cake (solids). Each stream is analysed for the three elements, oil, water and solids. Four of the five streams are monitored for flow rate. The cake rate would be difficult to measure accurately.
To analyse performance, a mass balance across the centrifuge is performed for the three separate elements and the total mass, after which the cake rate is eliminated. Formulae are then developed for pertinent recoveries and purities. It is not necessary to develop these here as the pertinent formulae will depend on the application, and in any case the development is similar to that already shown for two-phase separation (see Section 4.1 ).
. . . .
. . .- 4.6 Thickening
'I'he decantcr thickening process is ;I part.ia1 removal of liquid froni a slurry, It is similar to clarification and cliis.;iI~caI.ion. The main difftrcncc is in thc wag t.tle (:cnirifuge is operated and controllcd. By definition t.here i s not the same interest iri petting t h e cakc SO dry. This oitcn cnnhles extra c i ~ p i ~ c i t y to he achieved.
SimiI:irIy to c:IassiIic:at.ioii. there ar-e two ways to opcratc thc decanter. Without i.he riced for close differential control. the pond IS set just below the c:~ke discharge level and the differential aild f w d ratc ~ I * C set hiEh. SO that with some cresting A wet r ake is virlually washed o u t . Thc afterriative is to sef lhr pond finely, iind il biltile disc or (:OIIC is hui l t i r i l o I h e umvtyur . Very ilcc:i1rilt.e c:nnt.rnl of' t h e difftrcntial urisiircs thii thickness or cakt. required. The pond setting is tincly tuncd to creatc a hydraulic halancc bctween the ccntrate and sllRhtly heavier cakc. Hawevcr, with the crcst which incvitably occurs with high throughputs. thc pond setting is usually slightly positivc. I n decanter terminology a positive pond is one wherc thc liquid discharge is at ii larger diamctcr t han thc solids dischargc, and a negative p r i d is the opposi t.e.
M a n y thickcning applications are with process materials that do not fully dewaler, or arc tlifiir:ult I(.) tlew;it,er. Neverthelcss, pcrhaps surprisingly. sume ofI.herni r:ari be Ihirkened without t he use nltlocculants. o r at Icasl with rrlur:h reduced yuaritii ies.
Occ;isioiinlly, process muterials arc rcquircd to be thickened that nnrrriiilly d e w a k r readily. With these. flocculant is usually rcquirecl. bu t ttiercr is more of ;I problem to prevent complctc dcwatwing.
Afiaio there a re two control options. One riiher L I S ~ S high diftcrcntials a n d dccp ponds or o w carefully controls the dirferential. A further nltcrnntivt. is 10 fully dcwater, without too dry a cake. and t hen back-mix wi1.h ;I feed by-pass with the c.ake discharge. This last mcthnd has advan1agr.s in rcduced overall polymcr consumption and high overall recovery ;is t.he by-pass is 1 oOw, rccovcry without polymer usage. 'I'ht! coril.rol lor this method is by mcalls of control 1 t 11 g t h c h y - pass rate .
'I'hicketiiiig control is a milt.t.er n i balancing thc volurnctric scrullirig rat.t! with the amoun t of solids being fed 1.0 the decanter. The ra tc of solids into i he decanter is the product of the feed rale. Or. and the l e d solids content. I,. The
174 Thickening
scrolling rate, Ss, of a particular decanter would be proportional to the differential, N. In a thickening process one might expect the scrolling rate to be an inverse function of cake solids content, because as the "cake" becomes thicker it gets more viscous and the scrolling efficiency reduces. Thus:
N s ~ - (4.63)
Xs
Dividing the scrolling rate by the solids feed rate gives an empirical "thickening factor", ~, where:
N = (4.64)
O.sxsx~ As the factor �9 is increased, cake dryness decreases and solids recoveries
increase, and vice versa. Very good correlations can be found between cake dryness and ~, and with solids recovery and �9 for fixed pond depths and fixed polymer dosages.
4.7 Conveying
In the past, the vast majority of decanter applications were limited by the clarification capacity of the centrifuge. With the development of the decanter, better designs, higher g-levels, and better knowledge of the technology, today many applications are governed by what happens at the other end of the machine.
Performance is now often limited by how efficiently the solids can be dewatered, and how efficiently and at what rate the solids can be discharged.
4.7.1 The Beta theory
The decanter capacity, related to a solids conveying limitation, is indicated by"
Oj p f 3 z f O( N P n l r 2 ( r 2 - rl) (4.65) psXs
where xf is the solids concentrat ion in the feed; Xs is the solids concentrat ion in the cake; pr is the density of the feed; Ps is the density of the cake; N is the differential between conveyor and bowl; P is the pitch of the conveyor: and nl is the number of leads or flights on the conveyor.
This is known as the Beta theory, derived by Vesilind [ 13 ], where:
f l - - 2 7 r N P n , r2 (r2 - r l ) (4.66)
Thus for solids conveying limitation, scaling from one machine to another:
Qf2 32 = ( 4 . 6 7 )
where the subscripts 1 and 2 refer to the centrifuges 1 and 2, respectively. The Beta value calculated by the above formula does not take into account
scrolling efficiency, nor the fact that the depth of solids between flights reduces to a min imum at the point of discharge. However, if scaling is between machines of similar geometry, efficiencies will be similar and dimensions of
176 Conveying
the flight area at discharge will be proportional to that in the cylindrical section. Thus, equation (4.65) will be valid.
One of the basic assumptions for equation (4.65) is that the cake fills the space between the flights, from the scrolling surface to the pond surface, throughout the length where capacity is being considered. With non-cohesive cakes, those that tend to be fluid or creamy, a head of cake touching the conveyor hub at discharge will not be possible and here the more appropriate formula would be, instead of equation (4.65):
Of pfxf cx NPnlr2 (4.68) PsXs
When thickening is involved, equation (4.68) is certainly more appropriate. If liquor levels are close to or above the solids discharge level, some fluid mechanics technology may need to be invoked with some attention to cresting heights.
4.7.2 Conveying on the beach
It is important in decanter work to have a good understanding of the factors that affect the mechanism and efficiency of scrolling [10], particularly up the beach.
Consider the general case of a single particle, mass m, being pushed up the beach, angle a, by a flight of the conveyor. Consider the forces on that particle (in Figure 4.9). Constructing a vector diagram of these forces, there is the normal force, FN, perpendicular to the face of the flight, the scroll friction at right-angles to this, the weight of the particle resolved down the beach parallel to the bowl axis, and finally the friction from the beach. The direction of the beach friction is indicative of the direction of travel of the particle up the beach, at angle 0 to the axis. It is worth considering what factors minimise 0 and thus maximise scrolling efficiency.
Reducing scroll friction by polishing and smoothing is one factor. Maximising beach friction is another: this is generally done by ribbing or grooving, which effectively polarises the friction, to stop slippage when cake rotates with the conveyor. This means that it is easier for the cake to slide up along the grooves, which are in line with the centrifuge axis, than to shear over itself at right-angles to the grooves. Reducing the acceleration force on the particle, the weight resolved down the beach, by reducing bowl speed or decreasing beach angle will improve scrolling efficiency but will have adverse effects on other features of the process. The same goes for reducing conveyor
pitch. It will be noticed that the weight resolved down the beach has a buoyancy
component. When the particle leaves the pond for the dry beach this buoyancy effect is lost. Instantaneously the weight resolved down the beach
Decanter Theory 177
o ~ ,\ Direction # of Motion ,,
r . ,, .m g .sin (x
F s
m ' g = ~d___~ (p_ pt)gr
m'g .s"
1 S w _._..Jl
~s - 1 + tan~-tanO - P
S.~ Best ~s = p = co,,,2~
Figure 4.9. Force Vector Diagram.
markedly increases. It can increase so much with some materials, that scrolling ceases. This is where the use of a baffle disc feature can be effective. A baffle disc is fitted onto the hub of the conveyor at the foot of the beach to restrict excessive cake flow, and the pond level is raised above the solids discharge level. Therefore, theoretically the cake is below the pond surface right up to the point of discharge, thus maintaining any beneficial buoyancy effects. Scrolling efficiency is also maintained, and enhanced by means of the differential hydraulic pressure across the disc.
4.7.3 Dry solids conveying
When a decanter is operated to obtain the driest cake from a compressible sludge, the decanter bowl will be virtually full of cake [9], from front to rear, with next to no volume of clear supernatant . The dryness of the cake, as will be seen in Section 4.9.3, with a constant torque, is inversely proportional to the volume of the cake.
In a dry solids decanter, there is generally a restriction, for example a baffle disc, against which the conveyor compresses the cake. Sometimes the cake discharge aperture forms the restriction or acts as an extra restriction in series
178 Conveying
with the baffle. The maximum possible throughput of the decanter is proportional to a combination of these restriction areas, to the conveyor differential and also to the cake dryness. It is proportional to cake dryness because the dryer the cake, the greater is its density and thus the mass per unit volume being conveyed. Thus:
QfR pfxf = PNAr (4.69) PsXs
where R is solids recovery: xf is feed solids content; Xs is cake solids content: Ar is a function of the area of the conveyor restriction; and P is conveyor pitch.
It is generally found that, with a properly operated dry solids decanter, the scrolling capacity at the point of maximum restriction, the smallest area, is greater than that calculated using equation (4.69), but less than that predicted by the equation at any other restriction in series.
4.8 Conveyor Torque
Whcn conveying solids in a "non dry solids" modc, thc majority of thc torquc is uscd in conveying solids up thc bcach. This tfirquc is given by [ 101:
(4.70)
where To is heel torque; A, is wel. area ol'heach: A, is dry iire:i rIl'he:ich; and k., is a corisl.arit. greater t han 1. deperiding upon t.hr type ot'snlici.
The heel torque is given by:
I'o hfm (4.71 )
whcrc A,, is the total surfacc ai-ca ofbowl and beach. In thc case of campressiblc cake and where the driest solids are needed, ii i s
neccssary to work ou t what pressure is required or1 the machirie hr ing scaled to. and rriodulate the dilTtlrerit.iai 1.0 produce the torque to providc: that. In this type of process. hydraulic pressure aids s c r o l h g , and the torque is not solely applied to thc Bcach section. 'I'liis is discussed more lully in Scctian 4.9.3.
4.9 Dewatering and Washing
The washing process is included in this section, as the extent of washing feasible will deperid upon the arriouiit of dewatering possible aflerwards.
4.9.1 Solids dewatering
To attempt to quantify the wtent. of dewatering within :i decanter i t i s necessary to appreciate the type of dewatcrina taking place. Even t.hen i t may be difficult. l'here arc a nuinbcr of ways that moisture separates frnrn t . h coke. It could be by fillr:ition t.hrough the cake back into the pond. oiicc thc cake has left the wet beach. 11 r:ouId hc hy simple drainage. through or ovcr thc calic bcd agoinst the scrnlling art ion of !.he conveyor. Squeezing out thc moistui-c, by compaction of thc cake. is another alt.ernat.ive.
In conventional dewatering, drying on ii dry heaoh is used frequently, where cotiveyor diffcrcntial is minimised wit.hiri ltie lirriits of the required scrolling capacity, arid the pond levcl is minirnised l o increasc dry beach length,
In h e dt!waiering of coiiipressiblc cakes, ;IS much pressure as pmsiblc is put on thc cake. before ndvrrsely affecting capacity or ccntrate clw-ily. In dry solids operation, Which will hr discussed in more dctail in other sections, i t has heen reported [ l ] that increasing cake height, and thus pressure in the centrifuge howl. improves dryiicss capahility. 'J'hus. for such applications. i t would he ndvaiilageous to maximise pond depih. '1'0 estimatc thc prcssure within the pond, refer t o equation (4.61
For 11 cnkc drairluge dewatering. scale up would be by nnc of the cake ( :or~veya~ice formulac. equ;jtions (4 .h5) o r (4.6K). and pond d e p h arid conveyor differential would he judiciously adjusted to ensure [.he resirletice timc in the crit.icnl areas was kcpt the s m i e ,
Thc coarser and rncire crystalline mi~t.erials will rely 011 intcrstitia! drairlagt., whcn it will be a matter of ensuring t.hat [.he solids coiivcyance ri1t.e 0 1 1 Ihe dry beach is less than the drninagc velocity. Herr t.he final moisture will be a l'iinotioii of the surface area o f the cake,
Firlcr ljut still particulate rrialerials will take advantage of rcsidence t.ime on the dry beach.
Decanter Theory 181
The more cohesive organic materials, such as clay-like substances and municipal effluents, can drain by decanta t ion with the squeezing action mentioned. Dewatering will commence under the liquor surface.
Thus, it will be appreciated that there cannot be a generalised equation for dewatering as there is for clarification. As with clarification though, special designs and devices can be incorporated, especially with respect to the conveyor, to enhance the dewater ing capabili ty of the decanter.
4.9.2 Washing
In some decanter applications it is required to remove, from the solids, some dissolved impurities in the liquid held in the cake. This is achieved by spraying rinse liquid onto the solids as they are conveyed up the beach. Admitting the rinse too far up the beach can cause problems, by washing the cake back into the pond, or producing too wet a cake. Washing too far down the beach risks poor washing efficiency, when rinse by-passes the cake, by s treaming over the surface of the supernatant .
To maximise rinsing efficiency, it is necessary to keep the cake flooded with rinse liquor, but not to add excess unless it flows through the cake, ra ther than over it. The ideal location for admitt ing the rinse, therefore, is at the junction between the wet and the dry beach. For opt imum location of the rinse nozzle(s) it is necessary to have a good appreciation of the cake profile around the wet and dry beach junction.
Consider the idealised system depicted in Figure 4.1(). Feed enters the system at rate Of with suspended solids content xf and dissolved impurity content yr. Cake is discharged at the right at rate 0,~ with solids content x's and impurities y~.
Qf Q,, x~: y~ x,.=o
y~-O
e~ _ 0 _ 0 a �9 -
0 u u | 0
QI X~: y~ QA
x , : y ,
Figure 4.10. Rinsing.
182 Dewatering and Washing
Rinse flows countercurrently at rate Qw with solids content Xw = 0 and impurities Yw = 0. The centrate, which includes the spent rinse, flows out at the left at rate Ql with solids content xl (which will be assumed equal to zero) and impurities content Yl. In the system considered, the cake remains completely flooded without excess liquor above it.
Impurity level in the feed is If where:
If = lO0.y/%wb = l o 0 . Y f % d b (4.72) xl
Similarly, in the cake:
Is = l o 0 . YS%db (4.73) Xs
Volumetric dry solids flow is QsD where:
Ps (4.74)
where ps is the solids density and ps is the cake density. The voidage flow in the cake at discharge is therefore Qv where
O,, = Os - OsD (4.75)
Assuming that the solids are impervious, and without surface adsorption, then Ow must equal or exceed Qv to remove all the impurity. If Qw is less than Qv then the impurity level of the solids as they emerge from the pond, Ie, will be proportional to the difference between these two figures, assuming plug flow:
le - - lO0. Yf. Qv -- Q " ~ ' ~0 (4.76) xs 0,,
After emergence from the pond, further dewatering takes place on the dry beach, assuming that there is a dry beach. Thus, the impurity level of the discharged cake will reduce to Is where
Ix = Ie o-Ss Ps(1 - xs) �9 " O , , "Pl
(4.77)
In practice solids are not impervious, and diffusion has to be relied upon to reduce impurity levels.
Consider a modification of Figure 4.10, as in Figure 4.11, to include
diffusion.
Of el
Decanter Theory 183
Ow c, = y,, = 0
Q, C 3 = C~
................................. _~ Diffusion
I
Figure 4.11. Rinsing with diffusion.
O. C2
The concentra t ions cl to c4 are the impuri ty concent ra t ions in the liquor, as shown in Figure 4.11. Thus:
c3 - (1 - xl) (4.78)
,l]s c2 =~1~ - x ~ ) (4.79)
Yf t'~ = (1 -- Xf) (4.80)
c4 - y, , , - 0 ( 4 . 8 1 )
Now:
(cs - c 4 ) . 0 , , . - ( c l - c 2 ) ( l - x . ~ ) . O s (4.82)
If the diffusion process is 100% efficient:
c3 - cl (4.83)
Subst i tut ing equations (4.81) and (4.83) into equation (4.82) and rearranging"
(] - x ~ ) G - 0 , , ,
r 1 6 2 (1 - x~)O~ (4.84)
However, the diffusion process is seldom, if ever, 100% efficient. The mass transfer factor, JD, for this type of si tuation is given by [ 14]"
1 8 4 Dewatering and Washing
hD ]D = - - - -(Sc) 0"67
l/c (4.85)
where Sc is the Schmidt number , a dimensionless group:
Sc = n pD
(4.86)
and h D iS the mass transfer coefficient; Uc is the superficial velocity of the rinse; 7/is the rinse viscosity; D is the diffusivity of the impurity: and p is the density of the rinse.
The mass transfer factor, JD. is a function of the modified Reynolds number, Rein, where:
uc4P (4.87) Re,, = (1 - e)r/
where d p is the characterist ic size of the bed, i.e. the typical mean pore size; and e is the cake voidage.
To estimate the total mass transfer of impurity, Na, the following basic equation is used:
Na = hDA,,Ac (4.88)
where Ac is the surface area of the cake bed: and Ac is the mean concentrat ion difference between the cake particle surface and the rinse.
From equation (4.88) it is seen tha t mass transfer will decrease as pond level increases, because the surface area of the bed, Ac, decreases. Washing efficiency should be unaffected by g-level, if all other parameter values are held constant, unless a higher g-level enables a lower residual moisture level in the cake, and thus a proportionally lower level of impurities. As differential is increased, the layer of cake becomes thinner, and therefore the superficial velocity of rinse liquor proportionally increases. However, the Reynolds number remains essentially constant as the characteristic size of the bed decreases, which in turn means tha t the mass transfer factor remains constant. Thus, from equat ion (4.85), the mass transfer coefficient will be proportional to superficial velocity. This means that washing efficiency should improve with differential as is found in practice.
The theory would suggest that washing efficiency should be unaffected by feed rate. However, there comes a point, when feed rate is increased, at which the thickness of the cake on the beach is such that the rinse cannot flood the bed because of the high g field. This then invalidates the theory. To mainta in a constant bed thickness as feed rate increases would require a pro ra ta increase
Decanter Theory 185
in bowl diameter. This suggests that the capacity of a decanter, limited by washing efficiency, would be proportional to its diameter.
4.9.3 Solids compaction
Many centrifuge manufacturers have expended significant development effort over the past 10 years or more to improve the cake product dryness from decanters when employed on compressible sludges, particularly effluent sludges. The efforts have not been wasted in that several suppliers now offer special ranges of decanters for "dry solids".
It has been found [15] that, in "dry solids" operation, the cake dryness produced is proportional to the torque developed by the conveyor or vice versa.
It has been shown [12] that:
where xs is cake dryness; T is cake yield stress: T is conveyor torque; and V is pond volume in bowl.
To improve dryness in a dry solids decanter, conveyor differential is reduced, and thus throughput has to be reduced, as a result of which the cake compacts and gets dryer, resulting in increased torque. Pushing this reduction too far will result in overspill of the solids into the centrate. The question for the centrifuge specialist is "what is the limit of dryness achievable, assuming that the practical torque limit of the centrifuge is not reached?" The maximum dryness achievable, without producing dirty centrate, will improve with bowl speed and pond depth and with reduced feed rate.
4.10 Dry Solids Operation - - --_
It has just beeii showti thal the perhrrnance of it dry solids dec,antcr is related la convcyclr torque achievable. bowl spzed, pond depth, and tlocculant usagc. Once h e ciikt: dryness has been fixcd. i t is uscful to be able to assess the maximum r:;ipacit,y possihlc on a givcn dccantcr.
It i s shown in Section 4 . 3 . 1 that thc clarification capacity of a decaiiler is thc pruduct of the Stokcs settling velocity (eyuation (4.28)) arid t.he Sigma value of the centrifuge (cquation (4.32)) . '['he Stiikes velocity is a functinn of the process material's physical parameters. while Sigma is A furit:l.ion o f mec:hiinisal t'eiitiires ol'i he cent,ril'iigc.
Equation (4,301 can be rearranged to give:
(4.90)
In :i dry solids decanter t,he howl is l'u11 [ I h] of solids and the assuiiiylicrtis made ror the derivation or Sigma art: hardly relevant. 'I'he solids are much coarser, heirig Ilocs, with very little distance. if any. to fall. arid thc scyararion process is one of solids comp:~c~.iori, with ~ h u clarified liquor having to Mter through cvcr-shrinking chiinnels. Corner-Walker [I(>] has used 1l'Arcy's equation to arrivc at the following lormula:
L) .I = K 1' c' (4.9 1)
whew K, is i\ function ( i f the process sludge parameters:
and k u v is i i I 1 average cake permcahility. Then:
(4.92)
Decanter Theory 187
where Z' is the Sigma scale-up value for the compaction process of "dry solids".
Note from equations (4.90) and (4.93):
~P_/ = { 1 -- ( r l / r 2 ) 2 } ~ (4.94)
An alternative approach to this version of compaction will now be made, which is based on conventional compaction theory [18] where processing capacity is found to be a function of volume rather than area.
Consider the compaction of a concentrated compressible sludge in a cylinder in a field of 1 g, as illustrated in Figure 4.12.
After time t the interface which develops between the settling sludge and the clear supernatant will be at a height H above the base of the cylinder. This interface will have a velocity v.
After an infinite time, settlement will cease when v will be zero, and the height of the interface will be at height H~. At this point the weight of the cake is no longer sufficient to express any more liquid from between the pores of the cake.
The rate of sedimentation is given [ 18] approximately by the expression:
dH v - - - - ~ ( H - H ~ ) (495)
dt
In this system, it is the compressive forces of the weight of the solids that are forcing the liquid at an ever-reducing rate, up through the ever-reducing spaces between the particles. As in most fluid flow systems, this flow rate will be proportional to g, as will be seen in the laboratory bottle spin centrifuge. The total volumetric flow of supernatant, O, which will equal the volumetric shrinkage of the cake, will be proportional to the cross-sectional area, A, of the cylinder. Introducing this area, and the proportionality constant, into equation (4.9 5), the following expression results:
Q = k s A ( H - H ~ ) (4.96)
where ks will have units ofT- ~ and is a constant for a given sludge.
\
Figure 4.12. Solids compaction in a 1 gfield.
188 Dry Solids Operation
The height components of equation (4.96), H and H:~. when multiplied by a density are equivalent to pressure heads.
Consider now compaction in the centrifuge. In the decanter centrifuge the geometry is quite different, as illustrated in Figure 4.13. Nevertheless, the equivalent pressure heads may be derived using equation (4.61 ) for pressure, Pr, developed at radius r within a centrifuge. In equation (4.61) an expression is given for the pressure developed at radius r with liquor above to a level of radius rl. In the present case, it is required to know the pressure of the head of cake, at the bowl wall radius, r2, with the surface of the cake at radius, !"1. Thus, r needs to be replaced by r 2, and rl by t in equation (4.61 ).
Thus, the term/-/in the 1 g mode will be equivalent in the centrifuge to:
~2
2g (r~ - ! "2) (4.97)
and H~ will be equivalent to: ~,2
2--g (r~ - r~) (4.98)
where r~ is the radius to which the sludge would settle in infinite time. Substituting equations (4.97) and (4 .98) into (4.96) the following equation
is obtained: ~d 2
(21 = k6A-~fl [(,'~ - r 2) - (r 2 -/'2,~)] (4.99)
where Q~ is the centrate rate that equals the cake volume reduction rate: and k~ is a constant for the sludge and system.
It is known that when a decanter is operated to its limit, it will be full of
solids, so: r = rl (4.100)
and thus: A = 2 7rrl L (4.101 )
where L is the length of the bowl where compaction takes place.
Figure 4.13. Compaction in the decanter.
Also cake dryness. the dry weighl. per unit weighl of cake expressed as a pcrccntagc. is. within the range ol’cake drynesses heing considered, very close to bcing inversely proportional to cake volume. This is because the cake dcnsity is close to unity, and so if for instance the cake is compressed to half its volume the dryness will bc very close to double that which it was before. ‘I’hus:
(4.1 (12) x,TxL(r; - r;) = . Y & T q r ; - r ; ! )
which redur:es I.U:
w h e r e xS is t.he operaling cake dryn,ess and X, is the dryness of the cake after infinite time. Wtiilr: we iire considcring thc continuous centrifugr s y s k r n , rx and T~ are hypothetical valucs. which occur after irifiriile lirne at‘ter Feed and discharges have been arrested,
Substituting equations (4. 100). (4.1 0 1 ) i tnd (4.103) into cquation (4.99):
(4.104)
(4.10 5 )
where I,’ is Itir pond volume. thc part nf t he howl in which compaction takes place. arid:
wheregI is thc crntrifuge g-level at thc poiid surface Thus, cquatinn (4.104) Irliiy he rewrittcn:
where Q, is thc cakc ratc:
(4.1 Oh)
(4.107)
(4 .1 08)
(4.109)
where R is the solids rccovery aiid xr is t.he solids fraction iri i.he Iked,
190 Dry Solids Operation
For acceptable performance:
R ~ 1 (4.110)
Substituting equations (4.11 O) and (4.109) into equation (4.108):
(4.111)
Substituting equation (4.111 ) into (4.10 7 ):
(4.112)
Thus, the highest possible feed rate, Qr, is proportional to g-level multiplied by volume times a function of xs, x~ and xr. In any single system, x~ and Xr will be constants. If xs is plotted as ordinate against Of/glV, then a line cutting the ordinate axis at a dryness of x~: will result, with a negative slope. Over the range of drynesses generally tested the line will be close to a straight line. The term gl V is referred to as "g-volume", or as g-Vol in equations, in the scaling- up calculations of Chapter 7.
With centrifuges of similar type and geometry, it should be possible to scale performance from one machine to another. Where geometries are dissimilar, the parameter x~: is liable to vary, as it will be a function of the g developed within the centrifuge and the depth of pond used. Shallow pond machines are not able to produce as much pressure at the bowl wall as deep pond machines and thus the ultimate dryness achievable will be less. Some fundamental work [19] has indicated the relationship between a compressible cake's dryness and its yield stress. As a moist cake is subjected to a stress, or pressure, there is an equilibrium level of moisture for each load value. The graphical results of this, for one particular sludge, are shown in Figure 4.14.
It cannot be said how closely this relationship is translatable to any other effluent, but it is anticipated that similar sludges will behave similarly, with perhaps some adjustments to the constants of the equation for the line. The slope of this line is 0.2 6. A figure of 0.2 5 will be taken here to formulate first- order assessments of performance. Thus approximately:
x~ cx ~ (4.113)
where Pc is the pressure to which the cake is subjected. This pressure will be proportional to density difference between solids and liquor, and also to cake voidage. Nevertheless, this pressure will be proportional to the total pressure, and so using total liquid pressures in comparing two systems will be valid, where the solids occupy the entire pond volume of the decanter.
Decanter Theory 191
2.00
1.80
1.60 A
1.40
1.20 o
0 o 1.oo
"6 0.8O
0.6o
0.40
0.20
o.o0
I F J i t~ i
i F
1 , ,1
J F
J f
1 1
1 I 0.00 1.00 200 300 4.00 5.00 600 7.00
Log10 (Yield Pressure Pa)
Figure 4.14. Cake !lield stress.
Thus, when comparing or scaling capacities of two decanters, the pressures at the bowl walls should be the same or the value of x~ should be adjusted in the light of the different pressures. Not only will the ordinate intercept change, but so will the slope of the line. The slope of the line could also be affected by changes in k6. which is constant for the sludge and system. If the geometry of the centrifuge changes, or the g-level, then this could affect the compaction enhancement effect of the conveyor enumerated by k6.
Equation (4.104) can be differentiated with respect to r] to find the optimum ratio of r ] / r 2 for maximising feed rate. The value of this optimum ratio is 1 / x/~ -- O. 5 8.
When polymer is used, as invariably is the case, a slight adjustment to equation (4.1 1 2 ) is needed. The subject of the equation then becomes the total flow to the decanter, Qt, where:
Ot = Q/+ Op (4.114)
where Qp is the polymer feed rate. The feed solids Xf needs to be replaced by x'f where"
, o~ (4 .115) �9
4Jl Fluid Dynamics
13y desigri t.he de(:ir~ler 1i;Indles very high throughputs relative to the small space i l oct:iipies. Mrjrpover. [.he tlow is nc11 simply in one end. and straight through and out thc othcr end. Flow can bc under. over and around baffles: it call bc a hellcal path around thc convcyor flights. or axially through holes in the conveyor, or a combiiiation ofboth.
The axiiil velocity of the feed into the decanter liiis l o he converted io a rg(ation:i,l velOr:ity in a very short time. This can I:LIUSP ct)risidrr:ihle turhulrnce. iInd help is required outside the Iced Z(IIIP t~ keep the howl contents up to t;pecd, i f not to get it fully to s p e d . Thc rotational speed of liquor at the pond surfacc can slipbclow that at thc bowl wall.
'1'0 maintain f l o w down thc bowl and ovcr thc wcirs. a hydrnulic hend builds lip wilh a Cresi.
Tri this secliori some of Ihese phenoniena will be exanlined morc closcly.
4.11.1 Reynolds number
Thc dcgrcc of turbulence in pipes a n d c.h;inriels is charar.terised by the valut: of the Keynolds number. For a pipc:
H T ~ ~ C ' T C ' Ill, i s thc pipe diarnrtrr: ai id I I is the velocity in the pipc. In thc decantcr with axial tlow;
Of 11 =
TT(I.4 - 1-4,
(4 .11 h)
(4 .1 17)
In EI channel, the pipe diaIneter. Up, i s substituted by ii hydruul ic mean diameter, &,:
4 A
P n
I l l (4.11 8 )
where A is the cross scctionaj area of the channel; ant1 p is [ h e wetled pcrimeter of the channel.
Thus, for n non-circular pipe (jr channel:
P lldr I I Rp = - 17
(4.1 19)
For an annulus;
d,,, = 2( r? r] ) (4 .120)
[lowever 11ie aririulus of the pond in a decanter does nut have its iririrr surfare "wetkd" , arid thus the hydraulic mean diameter becomes:
(4.123)
If thc vnlucs ford,,, from cquatinn (4,12 1 ) and velocity from equation (4.1 1 7 ) are introduced into t h e Reynolds numbcr in cquation [ ,4 . I IYI. this w t i u l r l imply axial flow. For helical flow:
1' = 1 ' + L(r.7 - 1 . 1 ) (4 .122)
Thus:
and:
(4.11 3 )
EqUatioIis (4.1 2 4 ) ;inti ( 4 , 1 2 5 1 ran bc substituted iri1.o equation (4.1 19) to find the K~'yrrold~ riurnher lor helical flow.
Once, the value ofthe Rtynolds number is known, for whattver type offlow is used, the level of turbulcncc can be asscssrd. With a Reynolds iiutiitwr
below 2000 ihr Ilow would bc laminar. I t will be lourid [ h a t the flow in many. il'noi most. of all practical cases is in the turbulent regime.
In a11 decanters, with solids iiioviiig radially o u t a n d liquid moving radially in, itc,c.elc.ration and dccc,leration occur. respectively. Without any mecliar1ic;tl
194 Fluid Dynamics
device to do this, viscous drag of the pond is the only means by which these actions can be accomplished. If the viscosity is low, considerable turbulence can occur, affecting cresting, interface location, stability, and sedimentation and re-entrainment of settled solids.
4.11.2 Moving layer In a pond of an operating decanter centrifuge, there often tend to be two distinct liquid layers. The upper or surface layer, the moving layer, moves rapidly and turbulently towards the discharge weirs. Under this moving layer, the pond is quiescent, allowing solids to settle under a laminar flow regime, and then to compact. This is a simplistic picture, as the shape of the conveyor and its movement adds to the complexity.
It is sometimes useful to estimate the depth of the moving layer to know when it is liable to disturb and re-entrain sedimented solids.
It will be appreciated that the thickness of the moving layer will depend upon whether the flow is axial or helical. Research has shown that for axial flow:
h,,, ~x (4.126) 4q;
where hm is the thickness of the moving layer. It will be seen that the formula is independent of path length, the clarifying
length. It has also been found that moving layer thickness closely follows cresting height (see Section 4.11.3). Thus, the shape, size and number of weir plates used can affect the moving layer thickness. The moving layer thicknesses found in helical flow are greater than those calculated using
equation (4.12 6).
4.11.3 Cresting
The level difference between centrate and cake discharges can be quite critical when optimising process performance. When the level difference is small, the degree and consistency of the centrate cresting can play an important part in
the process performance optimising. The crest height, the pond surface level above the actual weir height, is a
function of the centrate rate, the total weir width and centrifuge g-level, as well as physical constants of the liquor such as viscosity and density. Crest
height is hc, given by:
(4.127)
Decanter Theory 195
where co is a constant and generally approximately 0.415; and B is the total
length of weirs. This equation is derived from the Francis formula [20]. Experimental data show that, due to the interrupted nature of the discharge
weir, the calculated value of crest height needs to be increased by 35% for axial flow and 90% for helical flow. With a 360 ~ internal weir, B would be the full circumference, and thus would cause the least cresting.
4.11.4 Feed zone acceleration
Feed zones are designed to accept the m a x i m u m possible feed rate, and bring it up to bowl speed with the min imum of splashing and rejection. Bringing the feed up to the angular velocity of the bowl is not necessarily enough. As the process material flows out of the feed zone to the pond, it has a constant linear velocity fixed by its angular velocity at its point of exit from the feed zone. To main ta in its angular velocity extra linear velocity is required as the radius increases [21 ].
The power required to bring the feed material up to bowl speed at the pond surface is Pp, where:
PP - Or p.r ~2 ,~ ( 4 . 1 2 8 )
Power available in the feed stream at the pond surface is PA. where:
1 P a -- -~ Of Pf~X r-~ (4.129)
The power lost on entry is thus the difference between equations (4.124) and (4.125), and this is dissipated in heat and turbulence on entry. Thus to minimise turbulence and power loss, it is necessary to design the decanter with the pond surface as close as practicably possible to the centre line. Nevertheless, other process considerations may require the taking of a different view.
4.12 Power Consumption
The total power input required by a decanter centrifuge comprises a number of separate power components:
PT -- PP + PwI: + Ps + PB (4.130)
where PT is the total power required by the decanter: Pp is the power required to accelerate the process material to the bowl speed at the discharge radius; PWF is the power to overcome windage and friction: Ps is the power required for conveying: and PB is the power for braking.
From equation (4.128):
Pp - O.rpfw2,'d (4.131)
where rd is the process discharge radius. Naturally, if cake and centrate are discharged at different radii, then these
two power components have to be calculated separately. The windage and friction component is given by:
PWF = k7 + k8w + k,~,, '2 (4.132)
where k7, ks and k9 are constants. Pwv can be calculated with difficulty but is more generally derived practically in the factory by measuring the power absorbed for different bowl speeds. The conveying component is given by:
Ps = NT (4.133)
where N is the conveyor differential; and T is the conveyor torque. Similarly, the braking component is:
P B - - SpTp (4.134)
where Sp and Tp are the pinion speed and torque, respectively. In some types of backdrive the braking power can be regenerated, so that
the total power used is reduced.
Decanter Theory 197
4.12.1 Main motor sizing
The power of the main motor will be based on the ca lcu la t ion of PT from equat ion (4 .130) , while its physical size will be inf luenced by its s ta r t ing requi rements . Motor m a n u f a c t u r e r s rate their motors on the basis of the
m a x i m u m power delivered at the motor shaft, PM. This has to be grea ter t h a n PT to cater for frictional losses in the drive belts and fluid coupl ing, if used.
Thus, the motor power is PM:
PM .r = PT (4 .135)
where (D is the fluid drive efficiency; and (B is the efficiency of the drive belts. The power used, however , will be grea te r t han PM, due to losses in the
motor itself and losses in some control gear w h e n used, such as an inverter . Power is lost wi th in a motor due to a n u m b e r of factors, wh ich include:
�9 iron losses in the magne t i s ing mater ial , p roduc ing hea t in the moto r rotor and stator;
�9 friction in the rotor bearings; �9 energy needed to drive a cooling fan, in ternal ly a t t ached to the motor
shaft: �9 windage losses; and �9 copper losses (the power lost due to the res is tance of the windings ,
somet imes referred to as the I-'R losses).
These five factors combine to give a motor efficiency. CM, of less than uni ty. Extra power is also necessari ly supplied to the motor, w h e n the power factor
is less than unity. The power factor will never be unity, and is a me as u r e of how much the cur ren t lags or leads the applied voltage. It is measured as the cosine of the phase angle be tween cur ren t and voltage. W h e n an induct ion motor is connec ted to an AC electrical supply, w h e t h e r the motor does useful work or not. a cur ren t is d r a w n to excite the motor . This cur ren t , ins tan taneous ly on start-up, lags 90 ~ out of phase with the voltage, and is reactive current , or so-called idle or wat t less current . The power factor increases as the motor accelerates.
When the motor is put to work. it will take in addi t ion to its exci ta t ion current , a cu r ren t according to the a m o u n t of work to be done. The power factor will increase and will be m a x i m u m w h e n the mo to r works at its full power rat ing. Thus the power taken from the mains supply will be Pc where :
where Fp is the power factor.
FpPc.4~r = P~I (4 .136)
198 Power Consumption
To combat the anomaly of a low power factor, the installation of a capaci tance bank, ideally directly across the motor windings, causes the motor current to reach its m a x i m u m value closer to when the voltage does in the al ternat ing cycle. Therefore, a suitably designed capacitor added to an induction motor will reduce the lag of current, by any desired amount . Generally, in industry, because the cost of small capacitors is high, it is more economical and expedient to install large banks of capacitors at the supply source, and automatical ly switch in and out various sets of capacitors as the demand fluctuates. Moreover, a leading current, which is possible if the capacitor is too large, increases wattless current as much as a lagging current .
Motor manufacturers supply motors in s tandard increments of power. Thus, after power demand for the decanter is calculated, the next larger size is specified. Motor manufacturers can supply tables of efficiency and power factors for ranges of loading. Also available are performance curves for their motors, giving output torque against rotational speed. The selected size of motor, for economic reasons, needs to be as near as possible to the power demanded by the centrifuge.
Details of the installation need to be considered in the motor specification. These factors would include the ambient temperature , whether the installation is indoors or out, and whether any hazards exist, such as flammable materials in use, and whether the motor will need to be hosed down. The installed electrical services need to be assessed to ensure that they are adequate for the method of starting contemplated. It is important that supply cables be adequately sized to minimise voltage drops before reaching the main motor. The power supplied to the motor reduces proportionally to the square of any voltage drop. Nevertheless current will increase to compensate for the drop in voltage, increasing the heating and losses. Moreover regulations restrict voltage drops to a total of 4%.
If reduced voltage starting is used, it is important that the reduced start ing torque is never less than the sum of the friction and windage torque. Since the torque available to accelerate the bowl is equal to the difference between the motor torque and friction and windage torque, the motor may not reach full speed in a reasonable time, unless care is taken.
4.12.2 Main motor acceleration
Most decanter rotating assemblies have high inertias, which can require several minutes ' acceleration [22], or run-up time, ta. If the run-up time is too short, drive belts will slip and wear out prematurely, or even break. If the run- up time is too long, then the motor could overheat and burn out. The run-up
time is:
~M(IM + lP) (4.13 7) ta= (Ta- Ti)
Decanter Theory 199
where ]M is the inertia of the motor; lp is the inertia of the decanter at the motor; 02M is the motor speed; Ta is the motor torque; and Tl is the reactive
torque of the decanter. The decanter inertia is given by"
02 2 lp - -~M lo (4.138)
where ]D is the inertia of the rotat ing assembly. Both Ta and Tl vary with speed, and not linearly. Examples of motor and
decanter torque/speed curves are shown in Figure 4.15. To use equat ion (4.13 7), Ta and Tl are averaged over the speed range from zero to full motor
speed. Given the inertia at the motor shaft, the equations in this section are used to
determine whether the torque of the chosen motor is sufficient to accelerate the decanter bowl smoothly to speed, wi thout slippage of the belts. The thermal limits and the torque limit of the number of drive belts used and their cross-sections have to be checked, with the pre-set diameter of the smallest pulley taken into account. Causing the belts to slip will end in their failure, while producing copious amounts of dust in the belt guard, which could be an explosion hazard.
600
"O m 500 O . J
400 IJ.
0 300
C �9 200
o 1oo
'I . . . . . ~'--~- -~. . , "% -l 250
200 "0 m l ~ : . o
C~urrent DOL X i l i ' 15o - - - Current Star I ! I '
- - - " Torque DOL I t__ i t,~ t'--" "" Torque Star *-
loo #
. . . . . . . . . "~. . - . - ". ~ . . . . 0" " . 50
' Full Load ~ o d 0
0 20 40 60 80 100
M o t o r S p e e d % o f F u l l S p e e d
Figure 4.15. Motor and decanWr torque~speed curves.
4.13 Mechanical Design
The design of a good and reliable decanter centrifuge requires a thorough knowledge of most mechanical engineering disciplines such as machine dynamics, strength of materials, bearing design, and gearbox design. Some of the more important fundamental aspects of the mechanical design of decanter centrifuges will now be discussed.
The need for a careful mechanical design can be illustrated by examining the energy accumulated in a decanter centrifuge in operation. The rotational energy in a medium size decanter with rotational inertia of 50 kg m 2. rotating at 3600 rpm, will be 3.55 M]. This energy corresponds to the kinetic energy of a vehicle weighing 9.2 tons travelling at 100 km/h. Furthermore, it can be shown that the rotational energy of a decanter centrifuge will increase with the fourth power of the diameter, when the centrifugal force at the bowl wall, go, and the length/diameter ratio, )~, are kept constant. The ratio of the diameters of the largest to the smallest industrial bowl is over 10. Thus, the ratio between the rotational energy of the largest and the smallest decanter centrifuge on the market is over 104.
With the high energy involved, failure of one of the major rotating components of a decanter centrifuge can cause severe damage, both to the decanter and its surroundings. For all decanter designs a risk analysis, evaluating all possible failure modes, must be carried out. A European standard [23] deals with the foreseen risks for centrifuges in normal operating environments. The standard gives requirements for design, verification, and installation of centrifuges. The high risk of failure of a decanter requires a high quality, both of design and production, as well as periodic inspection during use, to ensure that unanticipated deterioration of materials of construction has not occurred. From the energy comparison above, it is seen that the risk increases with size, and a design which is adequate for a small laboratory- scale decanter, may be extremely dangerous on a large industrial scale.
thc matcrtal of the bowl shell. The cylindrical part of the bowl shell wiIl be, for normal decanter designs. the part of the bowl subjected to the highest stress levels.
The trlaxinlum pressure 011 the bowl shell is calculated using equation ( 4 I b 1 ) :
whew PLm is the maximum pressurc at thc bowl wall: and phq is the maxiniarn bulk density of process mstcrial evcr likely in tbc decantcr,
;is t.he dcnslty of the bowl rnatcrial. the average tangential stress i r i the howl sheli can bc, exprcssed. for a straiEht cylinder. as:
Defining t, as the wall thickness of the decanter bow1 shell and
(4.140)
where fJ2 is the aCt.iiill prrssure i1.t the bowl wall: and n1 is the mean t.angentia1 strcssin thebowl wall.
The fnrrnc~la is equivalent to the well-known pressure vesst.1 tormuiae For thiri cylindrical shclls. To ciisure safety againsl. f:iilure, thc tangential stress rr~ust be below B ccrtain allowable s h e w . According to the European EngineerinR l>irec,tivc. the tangcnt.ial stress shiill be kept bclow hh '% O T the yield strenEth and 44% of the uli.irna1.e strength of the bowl materinl, at the maximum allowcd opcrating teniperiilure, It is readily observed that the first term of cquation (4.14111 will decrease with tw, a n d the second tcrni will increase with t w , Unlike the sit.uation for a pressure vessel, simply intreesing the thicknm ofthe howl shell will not always reduce the risk of failure. On t h e graph of Ftgurc 4. l h . the relationship between the maximum oht.ein;tble g- farce. and the decankr lntcrnal diameter, tor differcnt process densitics is shown to il1ustr:Jt.r Ihls. Thc g-force is c.iilcnlatcd by assuming a bowl thickncss of 10uh of 1 tic! internal radius. 'I'he ;jllowablc stress is set. to 240 MPa, which corresponds to thc stress limil ol'a duplcx stainless s k e l i1t 1 w c .
Of course, ol.her howl materials, such as titanium and aluminium. will give d i fk ren t v a1 u es .
'I'he pressure insidc the bowl will also create an axial fnrcc. acting o n the cnd hubs. The maximum axial force, F,, is found by taking the mean pressurc in the pond as half the pressure c:IIcuhtCd from eq11ilt.ion (4*140), ;jnd multipl.yina by t he cross-sect.iana1 arca of t he pond. Thus:
r y (4.141)
On a l a rw dccanter. the axial form on the end hubs will bc greater than lo6 N. The cnd hubs, and axial fixings, on the rotor must tfiereforc be designed to
202 Mechanical Design
9 0 0 0 , ~ ~ - , - , ~ , " . ' ', : - P r o c e s s D e n s i t y 3 . 6 ~
8 0 0 0 " ~ .]L 1 t " t I O " ~ i , �9 ~ ~, ~, . . . . . . P r o c e s s D e n s i t y 1 . 2 t
ooo i ,x,i i'--..; i i i ! i ~ , ,, e o o o i i \ ! , r - . . i i ; ~,
', ! ~ ~ l I ~ , ~ 1 . '
4 0 0 0 t '. ! ~ , , " . = . ;
3 0 0 0 i ! I '; ~ t ! i I" " ' ' ' ~ ' ' " 2 ~ ~ . - . . = i I u ,
2 0 0 0 l 1 �9 ; ~ i , ~ " - - - ! , i I [ ~ ! t
1 0 0 0 ' ' ' ' . . . . . ' ~ , " : - �9 - I ~ ,
o ' ; I : . i 1 , I ' - i . ~ �9 ' -.
0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0 1 1 0 0 1 2 0 0
B o w l D i a m e t e r m m
Figure 4.16. Example of the relationship between bowl radius, max g-force and cake density for one material and one relative bowl shell thickness.
withstand this force with a sufficient safety margin. Components of the decanter may be subjected to several other design-dependent static loads, which must be considered by the designer. One example is the axial load, acting on the conveyor, caused by conveying the solids.
The decanter will also be subjected to cyclic loads, which can cause mechanical fatigue damage on both the rotat ing assembly and on the s ta t ionary parts. Among the cyclic loads which must be considered by the designer are the bending forces on the shafts, caused by the weight of the rotor, the loads from belt drives, unbalance forces from the rotor, and cyclic loads from frequent starts and stops, or in termit tent loading with process material . On complicated hub geometries, often it will be necessary to make a finite element calculation of the stresses to make a proper fatigue evaluat ion. The notch sensitivity and ductility of the material must be considered.
Quality assurance procedures during manufactur ing , such as X-raying of critical welds and die penetrant testing of castings, must be maintained.
4 . 1 3 . 2 C r i t i c a l s p e e d s
The na tura l frequencies and critical speeds of a decanter will depend on its actual configuration. A conventional decanter centrifuge consists of a f lame holding the double rotor - - conveyor and bowl - - in rigid bearings. The main motor can be at tached to the rotor f lame either by a rigid connect ion or flexibly through vibration isolators. Further the motor can be at tached to a sub-flame or to another part of the supporting structure. These factors are
more fully described in Section 2.1.
Decanter Theory 203
At speeds below operating speed the main flame and the rotor can be considered as one rigid body. If the decanter flame is mounted on soft vibration isolators the decanter assembly will have six natural frequencies and associated vibration modes below the operating speed. The natura l frequencies are determined by the spring stiffness of the vibration isolators and the mass and inertia of the total system. When the main motor also is supported on the decanter flame by vibration isolators, it will have six additional natural frequencies below the operating speed.
The important critical speed for a decanter is the lowest speed at which there is significant flexible deformation of the rotor. This speed is called the first rotor critical speed. Decanters will always have a certain unbalance, both due to the handling of solids from the process and due to wear on the rotor. Operating the decanter close to, or just above, the flexible critical speed of the rotor will result in high vibration levels and very high stresses in the rotor components. The critical value of the rotor speed will therefore be an upper limit for the operating speed, and the decanter must be operated below this speed with a safe margin.
The first rotor critical speed will mainly be a function of conveyor geometry, bowl geometry, gearbox weight, main bearing stiffness and conveyor bearing stiffness. The first rotor critical speed will decrease with the length of the decanter. The critical speed of a decanter can be calculated by using a finite element method and verified by measurements. It is normal practice to test decanters at a speed 15-20~ above operating speed, to verify the design integrity, and such an over-speed test can also reveal if the operating speed is close to a critical speed. These factors influencing the first rotor critical speed have been more fully covered by Madsen [24].
4.13.3 Liquid instability problems
Often very large vertical and horizontal vibrations are seen in some speed intervals on decanters when they are started and stopped with liquid inside the bowl. The vibration frequencies in the instability intervals correspond to rigid body natural frequencies of the decanter, but the vibrations are not caused by unbalances. Rather, they are due to interaction between the liquid inside the bowl and the decanter.
The vibrations which occur in some instability speed ranges are sub- synchronous, i.e. the vibration frequency is a fraction (normally about 0.7) of the actual operating speed. If, for example, the decanter is vibrating at an operating speed of 1000 rpm, the vibration frequency will be around 700 rpm.
The vibrations are usually harmless, but as very large forces could be acting on the foundations of the decanter, the manufacturer must supply information on the magnitude of these forces, and the foundation must be designed to withstand these forces. By having a constant flow of water to the decanter during starting and stopping, the instability vibrations can be suppressed.
204 Mechanical Design
The complicated dynamic phenomenon, which is related to all rotat ing cylinders with an internal annulus of liquid, has been dealt with in a number of publications [25, 26].
4.13.4 Length/diameter ratio
In general the bowl strength, the first rotor critical speed, and the m a x i m u m permissible speed of the main bearings control the m a x i m u m speed at which a decanter can be operated.
It is argued [24] that a long, slender decanter centrifuge will give advantages with respect to overall economy, power consumption and process performance. For a centrifuge with the L/D ratio above 3, the critical speed will often be the main factor controlling the m a x i m u m obtainable speed and it can therefore be desirable to increase the critical speed in order to obtain a high L/D ratio wi thout sacrificing the m a x i m u m operating speed.
When a decanter bowl, for calculat ion purposes, is approximated to a beam, its na tura l frequency is inversely proportional to the square of its length. In that g-force is proportional to the square of bowl speed, and it is necessary to keep resonance frequency above bowl speed, the max imum bowl speed is proportional to its length to the fourth power. To obtain g-forces in the range 20()0-3()O0, generally required for commercial decanters, the max imum length-to-diameter ratio, for the most frequently used designs, has to be restricted to a little over 4.0 [24].
In order to increase the critical speed of the rotor a number of different modifications can be made to the rotor system. By supporting one or both main bearings in a flexible pillow block the first critical speed of the rotor can be turned into a low speed rigid-body motion for the rotor. It can then be operated supercritically with respect to this critical speed. Other modifications are the floating conveyor and the separately supported gearbox.
These sorts of modifications have been utilised by Alfa Laval in producing a decanter with an L/D of over 5 which can operate with up to 10 000 g.
How these modifications extend the possible L/D ratio and clarification capacity was graphed by Madsen, for 250 mm diameter bowls, and reproduced in Figure 4.17.
4.13.5 Bearing life
One of the most frequent reasons for breakdown of decanters is failure of one of the main bearings. The operat ing conditions of decanters are often very arduous, and there can be a high load on the bearings. The failure of a main bearing on a properly designed decanter will not lead to a dangerous situation, but it can cause damage to other parts of the decanter, and
expensive downtime.
'I'he bearing lire is defiried as h e r iu rnhr~ of rrvolu~.ioris or number of' h o l m at constarit speed a bearing will operala bclore i t I'ijils. Acr:ortling to thc international statidard [ 2 71. and based on the assiimpliori that i he beoring will rail hy laligiie. t.he expected life of ii hearing is calculatcd by the sitiiple f( I rm 11 1 ;-I :
(4.1 4 2 )
whcrc L I r l is thc expccted life nieasiired i r i 10" rc-volutions: C is the dyriarriis load capacity of t h e hearing, a characteristic figurc for the hearing, determined by the manulac lurer in accordancc with thc IS(.) standard; Ce is the equivalent dynatn i r load, calculatcd f'ram the dynamic and static loads; and 11,'is a nurnhur depending upon thc hcaring type (e.g. for hall hearings w = 3 afid for rnllrr twaririgs \I' = 1 O/ 3 ) .
Both C a n d ar r expressed in a unit offoroc. 'l'licr I , , , ) lire is d s u sometilncs referred Lo a s t h e R l o l i fc.
For a macliine rohl. ing ;it ;I c,onstant speed. t i . in revolulions per minutc. tile expccted life cat1 be expressed in expected hours nfnpersiion, LIOh:
where L l o h is thc cxpcctcd life in hours; arid n is the number of rcvolutiotis per mi nu te.
206 Mechanical Design
This simple formula was developed around 1950 and was based on data for bearing fatigue failure. Since the formula was published, considerable progress has been made both in the knowledge of bearing failure, in bearing manufacturing, and in lubrication, which is not reflected by this formula. The formula is based on the assumption of fatigue failure, although it is in fact not the most frequent failure mode for bearings. More realistic methods for calculation of bearing life have since been developed, which both account for the improved quality of bearings, for the bearing environment and for the lubrication conditions. In 1977 the same ISO standard [27] introduced an adjusted life-rating Lloah formula:
Lloah--b,.b2.b3.(~--~)'" (4.144)
where bl is a constant accounting for reliability; b2 is a constant accounting for the material used; and b3 is a constant accounting for environmental conditions.
The key to avoiding failure of bearings is proper maintenance and lubrication. By monitoring and analysing vibrations measured with sensors directly on the bearing housings of a rotating machine, bearing faults can often be detected before they lead to failure. Several systems for detection of bearing faults, by continuous vibration monitoring, are available, and some decanter manufacturers offer their own specialised systems. For critical installations, and installations with several decanters, such monitoring systems can be a good investment, to avoid inconvenient bearing failures, damage to the machine, and unnecessary downtime.
4.13.6 Gearbox life
The decanter manufacturer will often quote the expected life of the gearbox. This will be based on the fatigue life of the gear teeth, which is proportional to the ninth power of the torque encountered. Thus, one has to be extremely careful not to overload the gearbox above its torque rating. An 8~ increase of torque over its rating will halve the expected life of the gearbox.
4.13.7 Feed tube
Each component of the decanter has its own natural frequency, even the stationary components, which could resonate sympathetically if this frequency is close to the bowl speed. The feed tube is a good example, being a long, thin tube. Apart from the inverse square relationship with length, resonance frequency is also proportional to the fourth power of diameter in its simplest form. Unless care is taken the feed tube can be caused to resonate like a tuning fork. The design engineer thus endeavours to maximise diameter and
Decanter Theory 207
minimise the length of the feed tube. Other techniques employed include tapering the feed tube and making it of lighter materials. Of course the double concentric tube used, when flocculant is added, helps to increase the natural frequency.
4.14 Nomenclature
Symbol
tl 1
A AB
A~ AD
Ae~_ 4
A~ Aw bl b, b3 B BI()
r
C1
C
C3
C4
C CE Cn
C Vr
d dc
dch
dcl dg
Description
Constant Constant Cross-sectional area Total surface area of bowl and beach Surface area of cake bed Dry beach area Centrifuge "area equivalent" Function of restriction area in the conveyor Wet beach area Constant Constant Constant Total length of weirs Expected bearing life in revolutions Weir discharge coefficient Impurity concentration in feed mother liquor Impurity concentration in cake moisture Impurity concentration in centrate Impurity concentration in wash liquor Dynamic load capacity of bearing Equivalent dynamic load of bearing Cumulative fraction by number of particles below sized Cumulative weight or volume of particles below size d Particle diameter Cut point size Required cut point size of the heavy fraction Required cut point size of the light fraction Geometric mean diameter (number basis)
Dimensions
L 2
L 2
L 2
L 2
L 2 L 2 L 2
L
MLT -2 MLT -2
Decanter Theory 209
dgl dgs dgw dm dp
d15.87 d so d84.13 D
DAy Dp e F FB FN Fs Fp
Fx g g~ gc I gl g-Vol hD hm H
If
1D ]D JM kav kl kl k2 ks k4 ks k6 k7 ks k9
Geometric mean diameter (length basis) Geometric mean diameter (area basis) Geometric mean diameter (weight /volume basis) Hydraulic mean diameter Characteristic size of cake bed 15.87% of all particles are below this diameter Median diameter 84 .13% of all particles are below this diameter Diffusivity Mean diameter of pond Pipe diameter Cake voidage Force Beach friction Normal force from conveyor flight Scroll friction Power factor Maximum axial force Acceleration due to gravity Acceleration number of times greater than gravity Mean value ofgc in pond Centrifuge g-level at the pond surface g-volume, product of g l and V Mass transfer coefficient Thickness of moving layer Height of interface at time t Height of interface after infinite settling time Impuri ty concentrat ion % in cake at exit from pond Impuri ty concentrat ion % in feed Impuri ty concentrat ion % in cake Mass transfer factor Inertia of the rotating assembly Inertia of the main motor Average cake permeability Constant Constant Constant Constant Constant Constant Constant Constant Constant Constant
L L L L L L L L L2T-1
L L
MLT -2 MLT -2 MLT -2 MLT -2
MLT -2 LT-2
L ~ LT-2
L L L
ML 2 ML 2 L 2
Z-1
T-1 ML2T-3 ML2T-2 ML2T-1
210 Nomenclature
Kp
L
Lc Lk Lv Llo Lxoh L a Oah
m
n
nD n v
nl N Na P P Pc PA PB Pc PD PM Pp Ps PT Pwr P2
P 2 m
Of Qf~ Of 2 Ol (2p (L
QsD (2,
Qw V
t" d
t" x
t" 1
F2
Product constant for compaction Clarifying length Cylindrical length of the bowl Wetted beach length Length of vanes on a conveyor Expected bearing life in revolutions Expected bearing life in hours Expected bearing life in h o u r s - adjusted formula Mass Revolutions per minute of bearing Number of discs in a disc stack Number of vanes in a vane stack Number of leads or flights on the conveyor Differential Mass transfer rate Perimeter length Conveyor pitch Pressure in the cake Power available in the feed stream in the bowl Power required for braking Power taken from mains supply Polymer dose Total power absorbed by the main motor Power required to accelerate the process stream Power required for conveying Total power absorbed by the decanter Power to overcome windage and friction Pressure at bowl wall Maximum pressure at bowl wall Volumetric flow rate of feed Volumetric flow rate of feed to decanter 1 Volumetric flow rate of feed to decanter 2 Volumetric flow rate of centrate Volumetric flow rate of flocculant Volumetric flow rate of cake Volumetric flow rate of solids in cake Total volumetric flow rate of flocculant and feed Volumetric flow rate of cake voidage Volumetric flow rate of rinse Radius Radius of discharge of process stream Radius used in Ambler Sigma derivation Pond radius Bowl inside radius
LT-a L L L L
T T M T-1
T - 1
MT-a L L ML-1T-2 ML2T-3 ML2T-3 ML2T- 3
ML2T - 3 ML2T -3 ML2T-3 ML2T - 3 ML2T- ~ ML-1T-2 ML-IT-2
L3T-1 L3T-1 L3T-1
L3T-1 L~T-1 L3T-1
L~T-1 L3T-a L3T-1
L3T-1 L L L L L
Decanter Theory 211
r3
too
R
Re Rein ROB S
Sl
82 S Sc Sp Ss t
te tw
T Ta Tl Tp To u
Uc
v
Va
Vc
Vr
Vs
V W
Xf X f t
Xl .Yp
~s
~Coo Yr
Yl Ys
Yw Z
Z
Ouside radius of a disc or vane stack Three quarters bowl radius Radius to which the sludge would settle in infinite time Solids recovery Reynolds number Modified Reynolds number Gearbox ratio Distance Distance Distance Bowl speed Schmidt number Pinion speed Volumetric scrolling rate Time Time for particle to traverse decanter Thickness of bowl shell Conveyor torque Motor torque Reactive torque of decanter Pinion torque Heel torque Velocity Superficial velocity Settling velocity of interface Axial velocity Tangential velocity Radial settling velocity Stokes settling velocity Pond volume Constant Solids fraction in feed Solids fraction in total feed and flocculant Solids fraction in centrate Solids fraction in flocculant Solids fraction in cake Solids fraction in cake after infinite compaction time Impurity fraction in feed Impurity fraction in centrate Impurity fraction in cake Impurity fraction in rinse Number of particles less than diameter d Total number of particles
L L L T-1
7 - 1
LST-1
T T L ML2T-2 ML2T-2 ML2T-2 ML2T-2 ML2T -2
LT-1 LT-1
LT-1 LT-1 LT-1 LT-1
LT-1 L 3
212 Nomenclature
/31 /32
-7 Ac Ar
CB ~D ~s
C, q r/L 0 2
71"
P Pb Pr Pl
PM Pp Ps PS Psh
Psi O'g
(7 t
s s
Y]2
T
0
~,0 M
Beach angle Beta value: the scrolling capacity Beta value; the scrolling capacity of centrifuge 1 Beta value; the scrolling capacity of centrifuge 2 Angle rotated after time t Mean concentration difference Pond depth Bowl separational efficiency Efficiency of the drive belts Fluid drive efficiency Scrolling efficiency Separational efficiency of decanter 1 Separational efficiency of decanter 2 Viscosity Viscosity of supernatant The half included angle of a disc stack Ratio of bowl length to diameter Pitch angle Universal constant Density Density of bowl material Density of feed Density of centrate Density of supernatant Maximum bulk density of process material Density of flocculant Density of cake Density of solids Density of the heavy solids Density of the light solids Geometric standard deviation Average tangential stress in bowl shell Sigma; equivalent settling area of centrifuge Sigma value for a disc stack Sigma value for a vane stack Modified Sigma for a compaction process Sigma for bowl 1 Sigma for bowl 2 Cake yield stress Cake path angle up the beach Angle between a vane and the radius Thickening factor Angular velocity Motor speed
o
L3T-a L3T-1 L3T-1 o
L
ML-1T-a ML-1T-a o
o
ML-3 ML- 3 ML-3 ML- ML-3 ML-3 ML-3 ML-3 ML- 3 ML-3 ML-3
ML-1T-2 L 2 L 2 L 2 L 2 L 2 L 2 ML-aT-2 o
o
L-3
T-1 T-1
4.15 References
1 F Reif, W Stahl. Transportation of moist solids in decanter centrifuges. Chem Eng Prog 85(11) (1989) 57-67
2 B Madsen. Flow and sedimentation in decanter centrifuges. IChemE Symposium Series 113 (1989) 301-17
3 T Hatch, S P Choate. Description of the size properties of non-uniform particulate substances. Harvard Engineering School, Publ. No. 35 (1928-29) 369--87
4 F A Records. The Performance of a 4" micronizer. AWRE series 0 reports Number O41/61, Feb. 1962
5 C M Ambler. The evaluation of centrifuge performance. Chem Eng Prog 48(3) (1952) 150-8
6 G G Stokes. On the effect of the internal friction of fluids on the motion pendulum. Trans Cam Phil Sot" 9 (1851) 8
7 G A Frampton. Evaluating the performance of industrial centrifuges. Chem Proc Eng 44(8)(1963) 402-12
8 C M Ambler. Theory of scaling up laboratory data for the sedimentation- type centrifuge. J Biochem MicrobiolTechnol Eng I (1959) 185-205
9 S Yano. Experimental studies of separational efficiencies in centrifugal sedimenters. Proceedings of the first China - J a p a n joint international conference on filtration and separation, China, Nov. 1991. Chinese Mechanical Engineering Society & Society of Chemical Engineering, Japan
l0 FA Records. Recent advances in sludge processing. Aqua Enviro, University of Leeds, 19 Nov. 1991
11 A Lavanchy, F W Keith. Centrifugal separation. Kirk Othmer Encyclo Chem Technol & Engng 2nd Edn,Vol. 4, p. 719
12 N Corner-Walker, FA Records. Filtration+Separation 37(8) (2000) 13 PA Vesilind. Scale-up of solid bowl centrifuge performance. ] Envirmlln Eng
Division, ASCE, April 1974 14 Coulson, Richardson. Chem Eng I (1962) 254 15 W W-F Leung. Torque requirement for high-solids centrifugal sludge
dewatering. Filtration+Separation 35 (1998) 883 (Figure lb) 16 NCorner-Walker. Filtration+Separation 37 (2000) 28-32 17 E A Relter, 1~ Schilp. Solid-bowl centrifuges for wastewater sludge
treatment. Filtration+Separation 31 (5) (1994) 18 Coulson, Richardson. Chem Eng 2 (1956) 515
214 References
19 20 21
22 23
24
25
26
27
] Eiken, B Madsen, ] Oppelstrup. Private communication Perry. Chemical Engineers' Handbook. 3 rd edition 409 W W-F Leung, A H Shapiro. Improved design of conical accelerators for decanter and pusher centrifuges. Filtration+Separation 33 (1996) 735 Electro Courier IV (1976), No. 2 European Standard EN12547. Centrifuges- common safety requirements, CEN 1999 N F Madsen. Slender decanter centrifuges. I Chem E Symposium Series 113 (1989) 281-99 J A Wolf Jr.Whirl dynamics of a rotor partially filled with liquid. ASME ]App Mech December (1968) 678-82 F Ehric. A state-of-the-art survey in rotor dynamics - nonlinear and self- excited vibration phenomena. Proceedings 2nd International Symposium on Transport Phenomena, Dynamics and Design of Rotating Machinery, Hemisphere Publishing Corporation (1989) ISO 281, Rolling bear ings- dynamic load and life ratings
CHAPTER 5
Flocculation
Formulae have been presented in Chapter 4, for estimating settling velocities of suspended particles. From these formulae it will be seen that the settling velocity is proportional to the second power of particle size. A simple calculation will show that, with very small organic particles, say less than 1 ()~m, settling rates will be very low, only a few centimetres per hour. With this sort of settling rate some enhancement is required. The use of a decanter centrifuge would increase the rate by a factor of a few thousand. However with particle sizes even lower, say nearer 2 or 3 gm, particularly with the lower density particles of natural materials, even this level of enhancement is not sufficient to allow efficient separation with a decanter centrifuge. This is when flocculant aids are necessary to increase the size of the particles.
Figure 5.1 Anautomatic Polymer make-up system (By courtsey of Tomal).
.c
5.1 The Principle of Flocculation
To form n picture or I.he ability of a sludge tu settlc, takc a sample in n ghss beirker ijnd allow i l to set1.k oil a 1)ench. under thc iilflucnce of gravity. As ;I rule of thumb. if a distinct interl'nct: develops heiween Lhe settling solids arid a clear supernatant, and good set.tltrment occ11rs within approximakly half an hour. then it is probable that the sludge will reidily separa1.e i r i a decarrter centrifuge. without any floc,c,ulaiit addition. By good settlement, is mcitnt the solids scttlina ~ J J least 70% of thc distance that t.hey would ;icbieoe, g iwn iniinitc timc.
I:ine particles in tiii aqueous susperisiori t i w mc-)st frrqueiitly coated with an electrical r:liargc [ 11. 'l'hrre are ii nurnhcr of reasons Tor 1 h c i elc!ctrir:al charges. mainly associated will1 the 1vater.s ability to ioriisc chcrriicals. Ioriisation, arid thus flocculaiion. ;ire not possible in non-aqueous suspensions.
The polarity of ' iht . (:hitrges on a mass of'suspended particlcs, normally the siirne thrniighout the mass, ciiuses the particlcs to rcpcl anc anothcr, The siiiallcr thc particlcs. tlicii thc greatcr is the effect of the repelling forces. Wherl particlcs iirc a s small as about 0.1 prri o r less. the suspensiori heromes cdloidi\l, and thcri no $cttlcmciit takes place.
Were these small particles ahle I O iippronch closer to one another. small molecular altractive fcirces, called Vijn drr W a a l s forces, would cvcntually becotne greater than the eltxtricill torces, allowinE agglomcration to occur. l'tiiis, iri sornt! flowulittion proccsscs. prc-trcatmetit is etriployed 1.0 hrcak down the repulsion Ihrres, to initiarc agglomeration. Simple agitation [a] uitn sorrieliii1t.s iriitia1.e agglomeri\t.ion. b u t rnorc gsncrally the electrical chargp on t he piirliclrs has to bc rcdiiccd or nuutralised by ptl, adjustnienl or by treating wi1.h a n inorganic coagulant.
Adjiishienl ol pH would bc by use of mineral acids or alkalis. This sornctirncs has thc added advantage of precipit,at.ing kinw,ant.ed diss~il\~ed sails. Howcvcr. thcrc arc timcs when salts are dissolved by I.he pH chiinge. This would bc a disadvantage if the purpose o f the treat.mrn1 is t o remove these substances. when dissolving them wnuld rnakt. ii niort- dific:dt.
Thc inorganic coagulants used are sa1t.s ol rniiltivillent metals such as calcium, iron or aluminium. These can i t t t.he sitme time adjust thc pH.
218 The Principle of Flocculation
sometimes more than required. Occasionally more complex chemicals, such as polyaluminium chloride (PAC), are used.
The processes just described are generally termed coagulation, and they achieve a certain degree of agglomeration by adjusting the environment within which the particles exist, and the nature of the particles themselves. Flocculation is a different process, in which the particles are actually bound together in a larger agglomerate, by electro-chemical attraction to a special flocculant molecule.
Coagulation pre-treatments can be sufficient on their own for some processes, such as gravity thickening, but for the decanter centrifuge the resultant agglomerates are neither large enough nor sufficiently strong to withstand the turbulent entry through the feed zone. For the decanter, therefore, a much stronger agglomeration is required. This is achieved by the use of flocculants after the pre-treatment, or, on most occasions, by using flocculants on their own.
Flocculants have been used in the process industries for more than a century. For instance, isinglass, a fish product, has been used in the settlement of lees in beer and wine making. Starch, tannin and chitin are also products which have been used for a very long time. However, it has been the development of polymers for use as flocculants in the past 40 years that has opened up a large market for decanter centrifuges.
The polymeric flocculants, also known as polyelectrolytes, are manufactured with a variety of ionic charges, both negatively charged anionic and positively charged cationic. They are also available in a large range of molecular weights, from a few hundred thousand to 30 million or more. The lowest molecular weight polymers tend to work by coating the suspended particles and neutralising their charge, to allow close approach to one another and thus agglomeration (i.e. a coagulation process). It is the much higher molecular weight polymers that have found wide use with the decanter centrifuge. These are mostly polyacrylamides. They are very long- chain molecules with charges along their length. The suspended particles attach themselves to these charged sites while other polymer molecules attach themselves to other sites on the same particles, and other particles as well. Thus a network of polymer molecules and suspended particles builds to form agglomerates or flocs. The bonding of these flocs is much stronger than in the case of the natural agglomerates, or those formed by pre-treatment with inorganic coagulants.
These long, linear polymer molecules can exhibit overdosing effects when excess polymer blocks new charged sites for further agglomeration. This results in reduced size and strength of flocs. New polymers have therefore been developed in the past decade, which are non-linear, with cross-linking between polymer chains. These polymers do not suffer from overdosing problems. Because of the cross-linking of the molecular chains tend to be more difficult to dissolve, and are therefore supplied as fine powder dispersions in
Flocculation 219
oil. Another advantage of the cross-linked polymer is its ability to reform flocs after they have been broken.
Polyelectrolytes are formed by co-polymerising various proportions of cationic or anionic monomers with nonionic monomers. The relative proportions of the ionic and nonionic monomers dictate the relative ionic charge strength of the final polymer, from fully cationic, to nonionic, to fully anionic. Rarely, polymers are made with both anionic and cationic charges.
5.2 Polymer Solution Make-Up
Flocculants from polymer suppliers come in several forms. A large amoun t is supplied in solid form, as powder, granules, or beads. An equally large amount is supplied as solutions, emulsions, or more commonly dispersions, in concentrat ions from around 15% up to 50% of polymer in the dispersant. The solid products require the most care when being dissolved. Without such care, pockets of undissolved powder will form, surrounded by a partially dissolved jelly-like mass, making further solution impossible. These pockets when small are sometimes known as "fish eyes".
5.2.1 Dissolving solid polymers
There are a number of ways to dissolve the solids, depending upon the quanti ty to be prepared. For the purpose of this discussion, three orders of magnitude of batch size will be considered. These are a quant i ty of 1 O0 to 200 ml for laboratory evaluat ion, 25 to 100 1 for small-scale decanter tests, and 1 m 3 or more for plant use.
The small laboratory samples may be prepared by slowly sprinkling a weighed powder sample onto a stirred measured volume of water, and then agitating the sample for up to half an hour. Wi thout meticulous care, quantities of the powder will agglomerate before dissolving, after which getting them into solution is impossible. To overcome this problem and aid dissolution, the aliquot of solids can be dispersed into a small quant i ty of volatile solvent, in which the polymer is insoluble. This is slowly poured into the measured quant i ty of water with stirring. The solvent is then allowed to evaporate, while the polymer dissolves into the water . Methanol is a common reagent used with acrylamide polymers. However, here again care has to be taken, as methanol will dissolve some polymers that are not based on
polyacrylamides. For the medium-size sample, the last method may be used with a little up-
scaling. This time the mixing vessel is a quarter-filled with water and stirred. The measured quant i ty of powder is then sprinkled into the remainder of the water as it is squirted, under pressure, into the stirred vessel, which is finally
Flocculation 221
made up to the required volume. A solvent dispersant can be used if necessary, but this has generally been unnecessary.
Usually, production quantit ies are made up automatically. The s tandard automatic make-up plant will consist of a mixing vessel, into which water is admitted at a constant rate. The solid polymer is metered out from a hygroscopically secure hopper, using a screw feeder, into the incoming stream of make-up water. Some automatic systems use an air blower to convey the polymer, entering the air stream via a venturi, to a mixer, where the water enters with a cyclone action to keep the powder away from the mixer walls. From the mixer the product falls into a stirred ageing vessel.
The physical nature of solid polymer is such that it requires a finite time, at least half an hour, to fully dissolve and for the molecular chain to unwind and become fully functional. Once made up in the mixing vessel, the solution is aged with gentle stirring for the requisite time, usually at least half an hour and up to one hour, before it is transferred to a second tank used for feeding to the decanter.
An example of an automatic make-up system is shown in Figure 5.1. An alternative automatic system uses a series of at least four stirred vessels
with overflow from one to another. A low-level probe in the final tank triggers the start of make-up in the first. The total volume of the four or more tanks ensures a mean residence time sufficient for the required ageing period. The shortcoming of this system is that ageing time is not uniform. The age of the solution at discharge will vary, from almost zero up to several times the mean. It is possible to calculate this age distribution [3]. The system is simple and offers fully continuous polymer solution make-up. If the size and number of tanks are chosen carefully, the age distribution of the preparation presents no serious problem, apart from the overall size of the system.
5.2.2 Diluting dispersions
To make up the dispersions is generally much easier. Two volumes have to be measured out, albeit of considerably different sizes. These are the water itself and the polymer as supplied, which as stated will be anything from 15 to 50% active ingredient. They both have to be measured or metered to ensure the correct ratio for the desired concentrat ion of the active ingredient. The use of a surface-active agent to aid dispersion is sometimes made. This is to ensure an intimate mix of the polymer in the water, by reducing surface tension. It also helps to minimise the formation of "fish eyes" small globules of undispersed polymer surrounded by a viscous layer of dissolved polymer of high concentration. This problem is not so prevalent with dispersions. Ageing is still required al though not necessarily for so long. The automatic systems make use of a small metering pump for the polymer dispersion. Some automatic systems are designed to handle both solid-grade polymers and dispersions.
222 Polymer Solution Make-Up
To meter the two flows, a flow meter, a variable orifice meter or eddy current or sonic flow meter, for the water is generally used, and a metering pump, generally a progressive cavity pump, is used for the polymer. Unlike for solid polymer make-up, for dispersions the make-up plant is the same basic design for all sizes unless it is to be made up by hand, which is possible for small test runs.
5.2.3 Final flocculant solution characteristics
Nothing has yet been said about the concentration of polymer required, as it is to be fed to the centrifuge. A level of O. 1% of active ingredient is very common and could be considered the norm. It is more common to quote solution strengths in terms of active ingredient, but polymers, which are supplied in solution or as a dispersion, are sometimes quoted in terms of percentage of "as supplied". Care must then be taken when assessing performance.
Because of the physical nature of the solid polymer, some suppliers add small amounts of chemicals to enhance the flow properties or aid dissolution. When assessing polymer dosage these additives also need to be taken into account. This again emphasises the need to record polymer dosage as kilograms of active ingredient per ton of dry solids in the feed, when assessing relative performance levels.
It should be appreciated that all solid-grade polymers will contain a small amount of moisture. Conversely to what has been said, this moisture ought to be included with active ingredient figures when measurements are made. For instance, sometimes it is found necessary to check the concentration of polymer solutions by gravimetric analysis, which is done by evaporating a sample to dryness. Then it should be remembered to analyse a sample of the raw polymer in the same way. This is so that the solution is recorded correctly as weight percent of the solid polymer as supplied, rather than as true active ingredient. Percentage active ingredient is satisfactory so long as the percentage active ingredient in the supply is known, and appreciated by those concerned.
Solid-grade, high molecular weight polyacrylamides can, reasonably easily, be made into solutions of up to about 1% by weight. Above this concentration, the solids become very difficult to dissolve. More normally the maximum strength of make-up is 0.5 to 0.7%. At these concentrations the solutions are extremely viscous, and they would then be further diluted before use, either in line on the way to the decanter or in a separate mixing tank. Two extreme examples of polymer viscosity are shown in Figure 5.2. These are not necessarily typical of the polymers specified, but just examples to show the viscosity extremes that could be encountered.
The viscosity of a polyelectrolyte solution is a measure of not only its concentration, but also of the molecular weight of the polymer. Moreover, violent agitation, or circulating the solution under pressure through an
Flocculation 223
12000 _ , - - ~omc; m~m MW; ~ c~. i
10000 - ~fiomc; m~m MW; ~ j +
8 0 0 0 ~ - '
"~ 6000 ~ ' I o
4 0 0 0 i t i
2000 ~ ' I m O ~ m m i m ~ ,
0 ...~ ..... ?'" i , I"'
0 0.2 0.4 0.6 0.8 1.2
i i ,
o - - " i
1
P o l y m e r C o n c e n t r a t i o n % w / w
Figure 5.2. Viscosity ranges of polyelectrolyte solutions.
orifice, will reduce the efficiency of the flocculant by breaking the molecular chains, which will reduce the viscosity of the solution.
Polymer solutions lose their potency with age. The more dilute the polymer, the quicker is the deterioration. Making up with hard water also makes deterioration more rapid. Thus, in preference, it is better to make up with soft water as concentrated as possible, and only dilute to the required dilution just before use. Figure 5.3 shows some results of work testing the effect of water hardness. The dewaterability of the sludge was gauged by the amount of supernatant produced after a set spin in a laboratory bottle centrifuge. Note the correlation of the fall in polymer solution viscosity with increased water hardness and resulting decrease in polymer efficiency.
The more dilute the polymer, the more efficiently it works, providing opt imum mixing with the process liquor. However the bigger the difference between the viscosities of the polymer solution and the process liquor, the more difficult is the mixing. The higher viscosity component tends not to break up and disperse so easily as the other liquor.
Another factor that affects the choice of the polymer concentrat ion is the quanti ty to be used, compared with the quanti ty of process liquor. Once the size of the decanter has to be increased to cope with the extra volumetric load of the flocculant solution, then dilution has gone too far. The flow of flocculant solution should be not more than, say, 10 or 1 5% of the feed rate, on this basis.
2 2 4 Polymer Solution Make-Up
160
140
120 g~
100
e ~
8 0 o
60 e ~
4 0
2 0
\ \
t
J i
{
I I 80
I J i ~ Viscosity" O. 1 % soln.
r
t . . . . . . . t , j
%--- . - , . , t T '
i !
75 ~ 4 ~
at , h a
70
65 y~
60
0 1 O0 2 0 0 3 0 0 4 0 0
Total Hardness TS mg/!
Figure 5.3. The effect of water hardness on polyelectrolyte solutions.
5.3 Polymer Choice
The first choice to make in selecting a polymeric flocculant is its ionic charge, whe the r cationic or anionic, or even nonionic. The pH and type of sludge dictate this. Anionic polymers are more effective in alkaline solution, while cationic polymers prefer acid solutions. In general, pH should be no more than one or two units away from 7. However, when using pre- t rea tment chemicals, the usable pH range can double, as some of the flocculants can be more effective in the presence of the chemical. Anionic and nonionic polymers are better for flocculating inorganic slurries, such as minerals. Cationic flocculants are more frequently used with organic sludges, such as sewage. Nonionic polymers would be the first choice with sludges which are very acidic.
In decanter applications, the higher molecular weight polymers predominate. Nevertheless, occasionally some lower molecular weight polymers are found very effective when used on some decanter centrifuges. Cross-linked polymers are finding frequent use on decanters, with both municipal and, particularly, industrial effluents.
When a sample of sludge arrives in the laboratory for flocculant assessment, there are a number of common tests which are conducted. Its settling rate under gravity is observed and its pH is measured. A small sample, about 50 ml, is spun in a laboratory test-tube centrifuge for a fixed time, say 5 to 10 minutes. The g-level at the tip of the centrifuge will be of the order of 1500 to 2000 g. The volume of sediment will be recorded.
The amoun t of solids in the feed will be analysed gravimetrically. Normally this is done as a total solids measure, by evaporat ing a weighed sample to dryness. However, occasionally the suspended solids content requires to be known. Then, besides a total solids analysis of the whole sample, a total solids content will be measured on a sample of superna tan t or filtrate, and the suspended solids will be obtained by difference. In many sludges, such as municipal effluents, the dissolved solids are 0.1 to 0.3% w/w or less, and a relatively small fraction of the total. However, in some other sludges, the dissolved solids are a significant fraction of the total, and differentiating between total and suspended solids is important when assessing the centrifuge performance. Centrifuges separate suspended solids, but cannot be
226 PolymerChoice
expected to remove dissolved solids. Using, for calculations, total solids in the feed and suspended solids in the centrate is common practice, enhancing the perceived separational efficiency of the centrifuge, but generally only marginally. However, this should not be done when the dissolved solids are high relative to the amount of suspended solids in the feed.
Other analyses on the sludge, such as particle size distribution measurement of the suspended solids, viscosity, density, and chemical and biological oxygen demand, are all done on particular sludges from time to time, but generally do not affect the choice of flocculant. They all help to build a picture of the sludge to be processed, help to compare the sludge with others from past experience, and will be useful for particular aspects of the separation. For example, size distribution is important in classification, and density is important when assessing the maximum safe operating speed of the centrifuge.
The analyses mentioned so far all help to form a picture of the sludge to compare with past experience. This experience will enable the choice of a range of likely polymers that might be effective. A selection of anything from 10 to 20 polymers would be made, and preferably fresh samples of the flocculant will be made. Alternatively, already prepared concentrated solutions will be diluted to provide, say, 100 ml samples at 0.1% concentration. Alternatively, for expediency and economy, old (but not too old) samples will be used initially, and then a narrower range of fresh polymer samples will be made up on the basis of the initial assessment. The initial assessment is made by adding aliquots of each polymer to separate measured quantities of sludge.
There are many ways of performing this initial polymer evaluation, both empirical and rigorous [4], each depending upon the preferences of the analyst, but many are similar with the same objective. One example follows.
The 10 or 20 polymer samples (10() ml) are placed in line on the bench. In front of each polymer container a 100 ml measuring cylinder of sludge is placed. A sample of approximately 20 ml of the first polymer is placed in a 200 ml beaker together with the sludge sample. These samples are mixed, by pouring them into a second 200 ml beaker and then successively pouring back and forth from one beaker to the other five or six times. This is repeated for each of the polymer samples in fresh beakers. Each of the products is then examined and assessed and compared. One will be looking for the largest stable floc with the cleanest supernatant . If no, or just poor, flocculation has taken place, a further 20 ml of each polymer is added and the procedure repeated. Naturally, the quantities mentioned would be adjusted before starting, given experience from earlier analyses. If flocculation after the second addition does not produce a result, a decision has to be made whether to restart the assessment with reduced polymer quantities because of overdosing, or whether to induce some granulat ion with a pre-treatment or
by pH adjustment.
Flocculation 227
From the initial assessment, a selection of, say, three to six polymers will be made. Any polymer which does not perform on the bench will not perform in the decanter. Any polymer which does perform on the bench is a candidate for the centrifuge, but it is not a guarantee that it will work well there.
The retest of the narrow range of polymers will be started as before with 1 O0 ml of sludge for each of the polymer samples. This time smaller samples of polymer are accurately measured by pipette and added successively to the sludge sample. The size of the polymer sample, 1, 2, or, say, 5 ml, is chosen depending upon the results of the initial assessment. At each addition, the size of the floc and the clarity of the supernatant are recorded. The number of pours to form the flocs could also be recorded.
From this retest, one to three polymers would be selected for further evaluation and/or testing on the decanter, on the basis of clearest supernatant, largest and strongest flocs, and the quickest formation of flocs. Any bench retesting would be to refine further the amount of polymer required, to see how strong the flocs are, how long they take to form, and whether they have the ability to reform once broken.
Further laboratory tests may be instituted to assess the dewaterability of the flocs produced. There are a number of techniques employed for this. A flocculated sample probably would be recentrifuged in the laboratory bottle spinner to observe any change in the settled volume.
A favoured device for assessing dewaterability is the CST, capillary suction test, apparatus [5,6], as depicted in Figure 5.4. A piece of filter paper is placed over two concentric circular electrodes embedded into the surface of a fiat Perspex plate. A small open-ended cylinder of about 10 ml capacity is placed at the centre of the filter paper concentric with the electrodes. A sample of flocculated sludge is poured into the cylinder, after which supernatant liquor seeps outwards through the pores of the filter paper. An electronic device detects when the second electrode is reached. A similar test on the untreated sludge may be conducted. A CST time of a few hundred seconds would be typical for a raw effluent sludge. A good flocculated sludge would give a CST time ofless than 15 seconds, or even below 10 seconds.
An alternative or adjunct to the CST is a laboratory filter press. A fixed pressure is applied to a cylinder of sludge and the quantity of filtrate is measured. The cake dryness and thickness are also measured and compared.
The ability of a sludge to dewater, which is what the CST and filter press are measuring, is a measure of the dryness achievable. In that different polymers give different CST values, it follows that different polymers can be instrumental in achieving different cake drynesses on the decanter. This is found to be so in practice.
228 Polymer Choice
�9 ~.-~:, m
Fiqure 5.4. A CST apparatus.
5.4 Pretreatment
If laboratory tests show that flocculation is difficult, impossible, or requires excessive quantities of flocculant, then a primary coagulant may be considered. This adds an extra dimension to the laboratory tests and will invariably require flocculant reassessment due to the resultant change of pH. Quite often, the ionic activity of the flocculant required would change, from cationic to anionic or vice versa.
Common coagulants that are used include aluminium sulphate, ferric sulphate and chloride, lime, and polyaluminium chloride, all except lime, being multivalent inorganic chemicals. The optimum amount of addition may cause the pH to move beyond the normal range to between 3 and l l. The optimum amount of coagulant, which could be in the range of 50-300 kg/ ton, can produce a matrix of fine, weak granules. However, the proof of the optimum quantity is whether good flocculation can then be achieved, and preferably at less cost than with the coagulant.
The use of a pre-treatment with a coagulant does have disadvantages apart from the extra cost. Extra equipment is required to control the addition. Some of the chemicals can be quite corrosive. An extra solids load is placed on the effluent system. Extra pH adjustment may be required for the clarified liquid after separation, and before disposal.
Another form of pre-treatment, which is rather unusual, is the addition of a small dose of anionic polymer to initiate granulation followed by a larger dose of cationic polymer. On the decanter centrifuge, the anionic polymer is added upstream of the centrifuge while the cationic polymer is added into the bowl. This type of treatment would not be chosen lightly as it would require duplication of all the make-up addition and control equipment.
5.5 Admitting Flocculant to the Decanter
The laboratory tests will advise the operator as to the quanti t ies of polymeric flocculant that will be required. The equipment will be set up capable of supplying in excess of this quan t i ty by at least 50% and perhaps 300% when dry solids operation is planned. It will be seen in subsequent chapters that three times the normal amount of polymer can be used in dry solids operation when the driest cakes are required.
When applying the flocculant to the sludge, there has to be good int imate mixing, otherwise excess polymer will be needed to ensure full flocculation. Sufficient time has to be allowed to ensure op t imum flocculation before set t lement commences, to maximise the use of the sedimentat ion and compaction zones in the decanter. Flocculation must not be too early, such tha t major breakage of the flocs occurs before sedimentat ion commences.
What is the normal or correct addition point varies depending upon the design of the centrifuge, the type of sludge being processed and how the centrifuge is being operated. For a slow-speed decanter it often suffices to admit the polymer into the feed zone. For the high-speed centrifuge it is mostly necessary to admit the flocculant into a separate chamber , the floc zone, for it to enter the pond separately and mix with the feed there.
Flocculant can be added before the centrifuge but, as just stated, with the higher speed centrifuges it is often necessary to add it inside the bowl to avoid break-up of the flocs on entry. Figure 5.5 shows the effect of inline dosing. In this part icular test some polymer was added in-line, and the more polymer added in-line the more was the total polymer needed to main ta in centrate clarity. It will be seen that cake dryness increases by much less than 1% for the addition of an extra 70% polymer. This is for a cationic polymer addition to digested sludge in a high speed 73 7 mm diameter bowl decanter. This is not mean t to infer that this happens universally. For a different process s t ream with a different design of machine , set differently, the reverse could occur. The graph is to demonstrate that the point of admission for the flocculant can be critical, and steps always must be taken to check it on new applications.
As another example, anionic polymers tend to be slower acting than the cationics and, even with the high-speed centrifuges, often have to be added
ups t ream of the centrifuge.
Flocculation 231
24
23.5
~: 23
~ ~2.~
e 22
~ ~ 21.5 1
a 21
20.5
20,
i , i i
io . ~ �9 1
f
t
I
2.00 0.00 4.00 6.00 8.00
In-Line Po lymer D o s e k ~ t db
'8 t ii -~ 17
16
15
o 14
13
~ 12 J .
O ~ 11
~ 10 o [- 9
Y
,.I'
/
10.00
0.00 2.00 4.00 6.00 8.00
In-Line Polymer D o s e kg/t db
Figure ~. 5. l~ffect qf in-line polynzer addition.
10.00
Adding the flocculant ups t r eam of the decan te r is usual ly done by tee ing
into the feed line, and any inefficiencies in mixing are overcome at en t ry into the centrifuge, t towever, considerable work has been done indicat ing the impor tance of mixing [7]. In some plants, in-line static mixers have been used. Some studies have been conduc ted into the g rowth and b reakage of tlocs, and
a method has been developed to measure the extent of floc breaking in a centr i fuge [8].
The cross-l inked polymers have the ability of reforming flocs, and do not greatly exhibit the p h e n o m e n o n of overdosing. These can often be added
ups t ream. Some operators prefer adding this polymer ups t ream, because it is
claimed tha t some breakage of flocs on ent ry to the centr i fuge is beneficial in releasing more water . This is confirmed by a lot of work [9-12] . wh ich has
232 Admitting Flocculant to the Decanter
shown correlations between agitation, floc size and floc density. It has been shown that floc density markedly decreases with increase in size. With dry solids operation, extra polymer allows extra dryness and/or extra capacity. The polymer requirement increases exponentially with capacity.
Flocculant requirement in dry solids operation is covered again in the next chapter , but no-one has given a satisfactory explanat ion as to why this requires extra polymer, and one can only guess. Extra dryness requires extra torque on the conveyor putt ing more stress on the cake. Flocs tend to be larger and stronger as more polymer is added. Excess flocculant could be beneficial in assisting with re-flocculation of any damaged and broken flocs.
The increased floc stability with increased polymer dosage is shown in the graph in Figure 5.6, where stability is measured by the number of pours, from one beaker to another , needed to break up the formed flocs.
The value of extra dryness varies from plant to plant and count ry to country. When thermal dewatering is required after centrifuging, it pays to dewater as much as possible mechanical ly in the decanter, which is much cheaper than thermal costs. However, if the cake is to be incinerated then there is no economic advantage in drying in the decanter much beyond its dryness when it will burn autothermically.
160
140
o 120
-~ 100 U
s _
m 80 0
"" 60 t _
o 40
20
i
" /
|
2 4 6 8 10 12
, , ,
!
i i I L
i |
14
Polymer D o s e kg/t db
Figure 5.6. Increase qffloc stability with polztmer dosage.
16
5.6 Flocculant Suppliers
There have been several large manufacturers of polyelectrolytes. However, the polyelectrolyte industry is no different to many others, with take-overs and buy-outs, such that today there are just four major manufacturers . These are Ciba (with the former Allied Colloids), Cytec (formerly Cyanamid with the former BTI company), Floerger, and Stockhausen. Nevertheless, there are a large number of other major companies supplying flocculants under their own brand names, having obtained supplies from one of the main manufacturers . These supplies can be to the manufacturers ' or their own specitication, and then perhaps blended to their own special recipes.
The flocculant market is a very competitive one. Flocculants are sold at several thousands of pounds sterling per ton, with some users purchasing several tons every few months. Having a product that performs at (). 1 or 0.2 kg/ton less than anyone else can be a major economic advantage. On the other hand a polymer which can be shown to produce 1 or 2% extra dryness or 1 or 2% extra recovery, in some fields, is well worth the extra cost of the polymer.
It would be useful for the reader to have a table of polymers for each supplier, with molecular weight, ionic charge and cost, with equivalences between the suppliers. However, apart from the commercial reticence of suppliers to participate, such a table would soon be out of date. New and more effective products are continually being introduced to the market. Turnover and competition enable modification of the price. Moreover. the determination of the very high molecular weights is difficult, and for some, conjectural. If one product enables good separational performance, most polymer suppliers will be able to offer their "equivalent". or what they would assess as something more efficient. Most suppliers will provide a user with an outline of their products, which are anionic and which are cationic, and their relative molecular weight. They will also make suggestions as to where the user should make a selection for the application. Most suppliers would offer a service, to bona fide users, to conduct a flocculant assessment on a process liquor sample from the user.
It is worthwhile noting that some suppliers market their products with different identification numbers and names in different countries.
234 Flocculant Suppliers
In some applications, a lower efficiency but m u c h lower cost may be preferable for the user, ra ther than a high-efficiency, high cost product. Each user must evaluate the economics of operat ional factors specific to the application.
5.7 Low-Toxicity Polymers
There is a demand for "non- toxic" flocculants for use in potable water t reatment and similar applications. The toxic component of polymers is the residual acrylamide monomer content. To enable a formal classification of low toxicity, the acrylamide content must be 0.025% or less for use in most European countries (0.05% in the USA). Some standard polymers meet this requirement, but extra work, and thus extra cost, is required to provide the certification and enforce the quality control.
When considering the quant i ty of monomer that contaminates the clarified liquor from a decanter, it needs to be appreciated that most of the polymer added to the system ends up in the cake. This is unless there is a vast amoun t of overdosing or there is little or no flocculation.
There has been interest in using flocculants for foodstuffs, part icularly animal feeds. However, because of the public concern about contaminat ion of the food chain, there has been very little use in this area.
Most manufacturers offer a range of low-toxicity-grade polymers.
5.8 Applications
A large percentage of flocculants used with decanter centrifuges is for the t reatment of municipal and industrial effluents. Nevertheless, there are other large applications such as mineral processing. Whereas in effluent t r ea tment polymer usage is generally in the range 2 .5-5 kg/t db, minerals require generally less than 1 kg/t db. In coal washing, for instance, 0 .2 -0 .3 kg/t db is not unknown. Dry solids operation on the decanter often demands m u c h more polymer, even as much as 15 -20 kg/t db for the driest cakes.
Surprisingly, good separation can be achieved in the decanter on a good (SVI, settled volume index, of 120 or less) activated municipal effluent. without the use of a tlocculant. This is possibly due to the homogenous nature of an activated sludge.
Industrial wastes include sludges from paper mills, de-inking plants, tanneries, creameries, potable waterworks and many food processing plants. For these the choice of polymer will vary from plant to plant, due to large variations in the make up of the sludges which can occur.
Most municipal sludges can be separated on the decanter, the most common of which are:
�9 primary; �9 co-settled humus, or activated, and primary; �9 mixed and secondary, aerobic or anaerobic digested; �9 oxidation ditch; �9 carousel: �9 frothflotated: and �9 whole aerated.
Specially treated sludges, such as biological and limed sludges, can also be
separated on the decanter.
5.9 Performance
There are many ways in which the performance of dosing flocculants may be represented graphically. Figure 5.7 show the results from a 737 mm diameter decanter in 1990, thickening a 2% digested sewage. It shows the effect of increasing polymer dose and changing feed rate on centrate quality. An alternative presentat ion is shown in Figure 5.8, where each line represents a part icular polymer dose level, and the effect of cake dryness with feed rate is examined. This work is again on digested sludge from a different plant using a 425 mm diameter decanter. In dry solids work it is more usual to plot dryness against polymer dosage for fixed feed rates, as in Figures 5.9 and 5.1 (). Figure 5.11 shows the effect of not only a change in feed rate, but a change in the polymer specification. This clearly demonstrates that the choice of polymer can influence the final dryness.
Figure 5.12 summarises the effects of the various variables of decanter dewatering with a flocculant.
8000
7000
6000 g~
5000 "0 e m I o
r~ 4000
3000
2000 -
1 0 0 0
T i
x
y= - 40
51)
.~______~--- "--65 m3/h
[" 85 m3/h]
�9 - t ~ X
I " I 0
2 2.5 3 3.5 z
P o l y m e r Dose kg/t db
1
4.5
Figure 5.7. Centrate solids against polymer dose for various feed rates.
238 Performance
32 l
30-
28
26
~ 2 4
m 22
2O
18
L
I
0 10 20 ;0
* 6 k e ~ m s kg/t db �9 10 kg/t r
Feed Rate m 3 / h
Figure 5.8. Cake dryness against feed rate for various polymer dosages.
40
28-
26
24
22
~ 2 0
,.~ m 18
16 ~-
14
0
I ( 1
1
t ......t- t O
"-"-4V-
2 4 6 $ 10 12 14
Polymer Dose k ~ t db
Figure 5.9. Cake dryness against polymer dosage: DS operation.
16
Flocculation 239
3 2 .
3 0 .
28
~ 26 ~ 2 4
I 22
.~ 20
~J 18
16 -
14-
, Y ' / _ . / / . , ,
~"t ] 1 i 1 I 1
/ . /
, h ' / / ' /
I �9 /
I
I f
t �9 7 m ] / h
�9 14 m3Pa
A 28 m3/h I l 1
0 2 4 6 8 10 12 14 16 18 20 22
Polymer Dose kg/t db
Figure 5.10. Cake dryness against polymer dosage for various feed rates.
30 ....
28
26 r ~
I
24
.,g
22
20
9
I ~ ~'
I t
1
I ....
4
I I
I
�9 20 m3/h: Polymer A
1 30 m3/h Polymer A
�9 20 m3/h Polymer B
�9 30 m3/h Polymer B ! 1
0 2 6 8 10 12 14 16 18 20
Polymer Dose k~t db
Figure 5.1 1. Cake dryness against polymer dosage for two feed rates and two polymers.
240 Performance
z
36
34
32
30
24
22
INCFLEASING
T C ! R Q U E
, \
I "
____~i ~ ~ . ~ ~
F E E D R A T E i
!
!
'i 16 . I .. !
0 2 4 6 8 10
P O L Y M E R D O S A G E
12
k g / t db.
Figure 5.12. Summary of effects of parameters in DS operation.
14
5.10 References
l G M Moody. Pre-treatment chemicals. Filtration+Separation, April 1995 2 R Hogg. The role of mechanical agitation in flocculation and dispersion of
mineral particles. XVIth International Mineral Proc. Congress, Stockholm, 1988
3 JAWWA. Jan 1975, p. 52 4 R Hogg, P Bunnail, H Suharyono. Chemical and physical variables in
polymer-induced flocculation. AIChemE Symposium Solid/Liquid Separation in Industry, Pittsburgh, July 1991
5 CST apparatus made by Triton Electronic, Bigods Hall, Great Dunmow, Essex, CM6 3BE, UK. J Institute of Water Pollution Control 2 (1968)
6 C P Werle, J T Novak, W R Knocke, J H Sherrard. Mixing intensity and polymer sludge conditioning. ] Env Eng 110 (5) (1984)
7 R Hogg, A C Maffei, D T Ray. Modelling of Flocculation Process for Dewatering System Controller, Society for Mining Metallurgy and Exploration, Littleton, CO. 199()
8 D J Bell, K H Brunner. A method for the evaluation of floc break-up in centrifuges. Filtration+Separation, July/August 1983
9 R C Klimpel, C Dirican, R Hogg. Measurement of agglomerate density in flocculated fine particle suspensions. Particulate Sci Technol 4 (1986) 45-59
10 R C Klimpel, R Hogg. Evaluation of floc structures. Colloids and Surfaces 5~ (1991) 279-88
11 R Hogg, R C Klimpel, D T Ray. Agglomerate structure in flocculated suspensions and its effect on sedimentation and dewatering. Minerals and Metallurgical Processing May (1987) 108
12 A DAndreadakis. Physical and chemical properties of activated sludge. Floc Wat Res 27 (12)(1993) 1707-14
This Page Intentionally Left Blank
CHAPTER 6
Test Work and Data
In spite of the copious theories for the decanter centrifuge, it is not possible to predict performance with any part icular process material simply from knowledge of pertinent physical parameter values, that is, wi thout prior knowledge of the process material 's behaviour on a decanter. Test work is essential on any new, or unknown, sludge and on any sludge, such as effluents, that can vary widely in composition and/or quality.
The decanter chosen for any test work could be a small laboratory machine, anyth ing from l O0 to 2 50 mm in diameter, or an industrial size, up to the full size expected. Occasionally, but rarely, a decanter larger than necessary is used, when the data need to be scaled down. This would occur when the larger machine happens to be more readily available. Occasionally again, when there is some previous experience available, it is sufficient to conduct a few laboratory analyses to compare with the previous experience before a performance prediction is given.
The main object of test work is to be able accurately to predict performance and to size equipment necessary for full-scale operation. Naturally, when installed, new equipment will need to be tested to confirm the performance predicted or guaranteed. Decanter manufacturers and specialists also conduct test work not only to demonstrate the advertised performance of the decanter, but also to assess new designs and features, and new process sludges.
The ancillary equipment required for a test plant is essentially the same type, whatever the size of the test decanter. However, it is easier to make do on the smaller size plant. While a bucket and s topwatch will suffice with a small plant, ins t rumenta t ion is preferred on the larger sizes. Similarly, small collector bins can be used with the smaller equipment, but some mechanised conveying system would be required on a large decanter, unless it could be mounted over a large hopper.
For a good test facility, one tha t will yield reliable data, it is necessary to have it as well ins t rumented as possible, with automated flow and measuremen t of feed and off-take streams. It is necessary to keep the operator
244 Test Work and Data
flee, to observe and record the performance, unfettered from having to control and conduct the husbandry of the plant. Such a system is described more fully in the next section.
Flocculant[ i i ~ow
Feed Pump
Make-U Tank
Polymer Pump
f '
Decanter
_
Cake
F(qure 6.1. A decanter test facilit~l.
l
/___k
te
6.1 Test Equipment
Figure 6.1 is a sketch of the flow and ins t rumenta t ion of a full test facility, including polymer make-up and addition equipment.
The main equipment around the decanter test machine includes the process feed supply, as well as the polymer make-up system, and the cake and centrate off-takes. Each pertinent piece of equipment is described in turn below.
The decanter needs to be steadfastly mounted, level, on a firm base, and sufficiently high to allow good access to couple up discharge off-take facilities. Within the decanter 's own start-up and control gear usually will be instruments for continuously monitoring the bowl speed, the brake speed, and the brake or conveyor torque. Ideally the conveyor torque or differential will be automatically controlled by a simple PLC controller.
The feed vessel needs to be stirred to ensure that a uniform sample is supplied to the decanter, unless one can be sure that the contents are uniform and no settlement will occur over the period of the test work. Often the vessel will be the process plant itself and all that is needed is a tee in the process pipeline.
The feed pump needs to be a variable speed metering type. Usually this is a progressive cavity type, al though with some sludges variable speed gear pumps have been used successfully. Valve-restricted centrifugal pumps have been used, but these have generally proved quite unsatisfactory. The same can be said of using a pressure head and restrictor. With slurries, intermittent build-up of solids occur on the upstream side of the valve, making control of the test work impossible. With the smaller decanter, only a small feed tank and stirrer are necessary when remote from the feed source. However, care needs to be taken to ensure that variations from batch to batch are minimal, or that the tank size is sufficient to reduce the number of feed batches necessary. A calibration of the feed pump is useful, to facilitate a speedy rate setting, and is a useful check of the flow meter calibration, but is not a recommended alternative, as most pumps will wear and lose their calibration with time.
The polymer make-up vessel, on larger automated plants, will be part of an integrated make-up system. It will have an automatic controller governing the raw polymer feeder rate, feeding from the hopper and controlling the
246 Test Equipment
feeder speed, the time the feeder is on, the polymer ageing t ime and transfer pump actuation. It will also act upon signals from the various level probes. The controller will further control the opening of the water valve, and count the batches of polymer used.
A second vessel receives the aged polymer, and acts as the polymer supply tank for the decanter. [n smaller test facilities the polymer will be made up manual ly and batchwise. In these smaller tests the size of the make-up vessel needs to be large enough to ensure a sufficient supply for several test runs. The polymer vessel could be duplicated to ensure not running out during a test. However, the practicalities of changing vessels during a test have to be considered. These include prevention of air getting into the feed line and the consistency between batches.
The polymer feed pump also needs to be of the variable speed and meter ing type. The progressive cavity type is the first choice, but peristaltic designs are quite acceptable. Piston pumps have been used, occasionally, with alleviators to smooth the flow. So long as the pulse frequency is high, there is not too much need for an alleviator. A good calibration of the polymer pump, whichever type is chosen, is very useful in conducting the tests, and can be an alternative to the flow meter, as the polymer pump does not so readily wear and lose calibration with polymer. The peristaltic pump is expected to wear, of course, as it can wear on the outside of its tubing.
On large-scale tests, it is necessary to employ a cont inuous device to remove the discharged cake from the test ~irea, because of the sheer volume which accumulates in a short time. This device will probably be a simple belt conveyor or perhaps a screw conveyor. Manually removed hoppers, or buckets, can suffice on the smaller tests.
Discharged centrate is usually directed to drain but on smaller facilities where ins t rumenta t ion is limited it may be necessary to resort to measur ing the centrate using a bucket and s topwatch technique.
The pipe work for the test rig needs to be adequately sized and engineered to ensure free flow of the process materials. Access must be allowed for sampling the four process streams, feed, polymer, centrate and cake. These four samples would be gravimetrically analysed for solids content.
There are a few alternatives to the system so far described, depending upon the part icular application. Some applications will not needthe polymer system. On thickening applications the cake will be fluid and thus a tank receiver will be required. For thickened cake an off-take pump, controlled by a level probe, could be used. It would be useful to monitor the thickened cake rate, a l though not essential. This could be done by measur ing the fill rate of the receiver.
Some polymer systems use liquid polymer. Then the polymer powder feeder would be replaced by a very small metering pump. This pump would need calibration for monitoring, as a flow meter is impracticable here.
Three-phase decanter tests generally do not require polymer, a l though some waste oil processes have used polymer. However, by definition, a second
Test Work and Data 247
liquid discharge is present. It is necessary to measure the feed flow and one of the two liquid discharge rates, usually the oil or light phase, to enable a mass balance across the decanter. In three-phase work, extra analyses are required, not only of the extra liquid discharge, but also for light phase content of feed, cake and the two liquid discharges.
In classification processes, particle size analyses are usually necessary on two or more of the streams. Where the decanter is used to separate ores, by virtue of density difference, e.g. calcium fluoride from barytes, chemical analyses are also conducted on two or more process streams.
The precise design of the test decanter needs to be adapted to the process to be tested. Oil/water separation requires a three-phase design, solvents require flameproof electrics, effluents need abrasion protection. While manufac tu re r s will have a pool of test decanters of basic design, it is essential that they are able to adapt them for special application testing. For instance, they will have special decanter conveyors that can be introduced, when necessary, and will be able to make small changes to them where necessary, such as the addition of a floater disc for floating solids. With a test decanter it is essential to be able to adjust bowl speed, differential and pond depth to suit par t icular applications or difficulties as they arise.
6.2 Test Procedures
The test equipment is put together as shown in Figure 6.1, or as near as possible to that, which the available facilities will allow. The ancillary equipment should be sized to cope with the maximum th roughput likely, down to a capacity of, say, half or less. of that desired or expected. A turn down ratio of 1 - 10 is desirable on the variable throughput capacity pumps.
The start-up of the test equipment will rely heavily on past experience with the process selected, or with a process considered similar or comparable. This experience will allow a selection of parameter values, to give the decanter the best possible start, so that the operator can be satisfied that a viable performance is possible. A preliminary run is conducted, to confirm this performance, after which a programme for the tests is planned. Commercially, test work is mostly conducted ad hoc, with a view to achieving a commercially successful objective as quickly as possible. Thereafter, small adjustments would be made to improve upon the success.
The parameters chosen for investigation will depend upon the objectives, the type of process and how close to the objectives the preliminary test comes. Also the time available will condition choices to be made. A technical test series would be planned differently from a commercial test, under conditions
where more time is usually available. With a given test decanter the number of variables at the disposal of the test
engineer is limited. With a simple dewatering application without flocculant
the main variables would be:
�9 pond depth: �9 feed rate; �9 bowlspeed; and �9 differential speed and/or conveyor torque.
When using flocculant the extra parameters would be:
�9 polymer type: �9 polymer dosage: �9 polymer strength; and �9 polymer addition point.
now1 speed as a pararnetcr for investigation is used sparjngly. and is generally broughi in frlr ir~vestigatiori last. ivhen earlier results havc suggested a benefit. i a chariging the bowl speed from thc standard.
Pond depth is usually fixed early in the test programme. and only chanjicd aftcr carly data have been analysed t.o show t.hat a pond dept.11 (:harlgr: woul(l be of bencfit. Nevcrthelcss, some tests will investigate a wide range of pond dcpths, whcn it has been difficult t.o achicve good centrnte good rake at
the same timc. Whctl illvcstigating a parameter. whosc value c,an be infinitely variable,
such as fccd rate, one should vary it logarithmically rather than ari~.hrrietically. For iristarice, I , 2 , 4. 8 . I h m'/h or 1 , 1.5 , 2 , . 3 , 5 , 7 , 10 rn"/h., rather thari, say, 2. 4, b, 8, 10. Hy this rneiiris a wider pictureof trends may be obtailled with fewer data points. For polymer dosage. it . is g:tjner:JIIy not possihle to be so precise, i is t,he riurnerictil valuc of the polgrricr dose is not kriown until a fk r the analyses hijvc been c,ornplctc. a:, fced solids uoricerltraliori can v:iry from test to test. Fnr a properly conducted lest a rriinirriurn o f l i v r dnta points a re rcquired for B graph. However, ifthere is not a well-ddined trend. then thcrc will be a iieed for several times that narnbcr of r i ; i ~ . a points.
I!errl rate is most often thc first paranwter to hr irivestigaled. Thc capacity o f the dccantcr is nf fundamental iiitercsl to t hc iiscr. Thc pond lcvul and h ~ ~ w l s p e d will havc hccii fixcd during I hr prelimimiry tcsting. If floccular~l is to be used, thcn a saft rinsing 1t.vt.l will have been detcrmincd and used duririg the prcliminary tests. afi.cr bench valuation of likely p o l y w r s lo bc uscd, 'I'he s d c dosing l w c l will bc such iis to pcrriiit a wide range of' k c t i ratcx and convcyor diffcrentials. without any appreriable deterioration i i i ccntrate quality. A series ol'lesls at. say . five differeIit l w d rates will bc cutidut1t.d ijnd smiplcs of feed. (xn1,riiIf: and cake will be taken lor solids aiialyscs.
Each test ruri will be conducted with. a s n w r as can he jvdgacl, tilt. samc flocculant dos:igc Icvcl. 'Ihe setting ol'diffcrcntial speed and/or lorquc h r each ruri w i l l depend upon the type of tcst bcing condur:ted. For a silllple drwatcring ~ i r s l , ;I tixed diffcrenlial speed may bc chosen. Ilowevcr, if it is thoi ipht that thc dccantvr may be solids capacity lirniled. thcn the same (tied r;itr/diRtrential speed ral.io could be chosen lor earh test . F(Jr dry solids oper;ilion. ;I tixcd torque would hc morc likely to he rhossn. AIterri~lilirtIy. for each test t hc minimurn difftrential is found. where thc hest dryness isarhievcd w i 1 h o II t dc t cr i or a t io n o C ce n t r t i t. c gu ah t y .
For each tcst run , the pararnelcr values are set , i ~ n d the decanter is allowed to run [tor a set lime after cquilibriurn has been achieved. 'I'ht. set time would ideally be a l.irrie siifficicnt for there to have hccn a miriiiriurri 0 1 three bowl voluinc uhariges, since achicving equilibrium. Thm. this time is calculated by dividing tc i t . ; i l howl holding capacity hy feed rate. or in the case of dry solids operation. by dividing the total bowl holding capacity by the volumetric cake discharge ralc, ;it its discharge dryness.
250 Test Procedures
Once the decanter has run at equilibrium for the desired time, sampling can commence. Sampling is often quite cursory, wi thout too much at tent ion to detail, and generally this does not detract from the result. However, nothing is lost in introducing a little method and science:
�9 always take the sample in the same order with the same time interval, taking the feed sample first;
�9 fill each sample jar with small aliquots over a period of a few seconds, and mix the sample before closing the lid of the sample jar: and
�9 do not leave off lids of sample jars, which would otherwise allow evaporation.
Once the first test series has been completed, the data should be graphed and examined to see whe ther an improvement in performance is required, or is likely to be achieved by altering one of the other parameters . For instance, if better dryness were required, the test series could be repeated with a lower differential, higher torque or higher bowl speed. For better cent ra te quality, a deeper pond might be chosen.
Once a set of data has been obtained which correlates, the flocculant consumption, if used, needs to be optimised. Alternative polymers might be examined if centrate quality had been difficult to main ta in or if the quant i ty needed was considered excessive. The relative flow rates of polymer solution, and feed, would be assessed to see whether the polymer concentra t ion needs to be adjusted to make it min imum strength, wi thout causing it to be a large fraction of the total flow. This should not be more than, say, 10 or 15%. Polymer tends to be most efficient when it is most dilute. Moreover it is easier to get a uniform mix of two liquids when they are both of comparable size. However the larger the volume the flocculant is, then the greater is the clarification capacity lost unnecessari ly to the clean flocculant. The location for admitt ing the polymer may be questioned, and considered for introduction further upstream, if flocculation in the centrate has been observed, or if extra dryness is required at the expense of extra polymer in dry solids work.
Having decided polymer type, concentrat ion and addition point, the operator can under take a series of tests of polymer dosage, at the opt imum feed rate found in the first series.
Other parameters may be tested as spot tests, or as series, depending upon objectives, time available and the results achieved so far.
Testing a three-phase decanter will take a slightly different course, because of the extra product stream. The prime objective in the prel iminary testing will be to fix the opt imum differential height, between the levels of the two liquid discharges, which can vary with feed rate. The opt imum differential height will be when the efficiency of separation of the two liquid phases is maximum. If differential height is too small, then heavy liquid phase content will be too high in the light phase discharge. With too large a differential height, light
Test Work and Data 2 51
phase content will be lost into the heavy phase discharge. Cresting over the weirs affects the opt imum differential between the weir heights of the two liquid discharges. Hence, changing feed rates will move the e-line in the bowl between the two phases, in or out, depending upon the relative proportions of the two phases in the feed. Thus the objective in prel iminary test work, wi th the three-phase machine, will be to fix the opt imum feed rate and weir heights, or at least fix the weir height for a limited range of feed rates. Conveyor differential would then be the main parameter tested, together wi th some of the process parameters such as feed temperature.
Thickening is another different test series. In the preliminary test work, the objective would be to set the pond level high enough to get good centrate, and at such a level that the solids discharge thickness can be controlled by conveyor differential speed adjustment . The test work proper would concentrate on a series of ranges of differential for fixed feed rates. Depending upon the initial results, adjus tment of pond depth or bowl speed may be considered. The use of flocculants in thickening, as usual, adds another dimension to the test work, and probably will widen the range of pond depths that can be considered. It will also enable higher feed rates.
Classification work is similar to thickening. No flocculant is used in classification but occasionally dispersants are used. The use of dispersants is not so complex as the use of flocculants. Dispersants are usually simply added to the feed tank. Turbulence on entering the decanter is an asset ra ther than a hindrance. The preliminary work will be to select weir height, such tha t conveyor differential is able to control cake dryness from wet to dry. The test work will be planned to investigate cut point and efficiency for a permuta t ion of feed rates and differentials. In classification, conveyor differential is a critical parameter. Too high a differential will create turbulence and carry over of coarse solids into the fines. It can also give a wet cake, where in some processes the wetness will contain fines. Too low a differential will increase hindered settling, allowing fines to be trapped amongst the coarse solids.
Testing different decanter designs is best done with two decanters running side-by-side, both being fed from the same feed tank. However, c i rcumstances do not always allow this, and then consecutive testing is employed. This always poses the question as to whether any differences found could be due to a change in feed quality. Feed quality is always questioned when performance- cannot be explained, and thus this parameter needs to be eliminated wherever possible. When testing different decanter designs, the performance of one usually will already be known. However, if possible, it should be re-tested alongside the new design, to guard against feed change effects or errors in the first series. The exact design of the tests will, naturally, depend upon the new design feature being investigated, and the objective of the new feature. A special conveyor pitch to improve capacity would concentrate on a test series permutat ing feed rate and differential. A different flocculant zone would invite a test programme series with a range of flocculant doses.
6.3 Test Log
It is a lways preferable to record too much data ra ther than too little. Data recorded and found of no value shortly after the tests, can prove invaluable in other work, years later. Parameter values of the decanter need to be recorded in full, part icularly of any special features that have been added after the original manufacture . Often these details will be encompassed, for brevity, if not confidentiality, in a serial number of the machine, plus the date of the test to fix the last modification date. All the design parameters discussed in Chapter 2 ought to be available for cross-referencing with the test data.
For the test series the following decanter details need to be known:
�9 Bowl inside diameter: �9 Conveyor pitch and number of leads: �9 Baffle diameter (if used) and position: �9 Clarifying length; �9 Cake discharge diameter: �9 Beach angle; �9 Conveyor hub diameter at cake discharge.
If some of these details are not known at the time of the test, recording the bowl and conveyor (if possible) serial numbers will enable the details to be found later. It is good practice to record serial numbers anyway, for later reference.
For each test run the following machine parameter values should be recorded where appropriate:
�9 Bowl speed; �9 Pond diameter; �9 Conveyor differential; �9 Light /heavy phase diameter (three-phase); �9 Conveyor torque; �9 Current and power of the motors.
Test Work and Data 2 5 3
From the decanter data, various pert inent other data can be calculated such as holding volume downs t ream of the baffle disc, scaling factors such as Sigma, and max imum scrolling rates.
Process data also need recording, such as feed source, and for each run the following need to be recorded:
�9 Feed %solids; �9 Centrate % solids; �9 Cake%solids; �9 Polymer % solids; �9 Feed rate; �9 Polymer rate; �9 Polymer addition point.
Any density measurements of any of the process streams are useful additions. For three-phase work, oil content of each phase needs to be
recorded, plus the solids analysis of the light phase product. From each set of test run data, the volumetric rates of each process s t ream
can be calculated as necessary, plus other pert inent figures such as torque/ volume for dry solids work, polymer dose kg/ ton db for flocculant work, g- volumes.
An example of a results sheet is shown in Figure 6.2. It is displayed to include polymer addition. For three-phase test work, oil analyses would need to be added, and data for two liquid discharge streams also would need to be recorded. The polymer addition would be omitted where none is used, but would be adapted for the use of rinse. Figure 6.2 is shown as a general result sheet to cover as wide a range of applications as possible. Naturally it can be, and is, adapted for special applications such as classification or, say, refining work, when interest, perhaps, will be more in size distributions and chemical analyses, respectively.
2 5 4 Test Log
R e s u l t s Shee t
M a c h i n e L o c a t i o n
P r o c e s s
Machine 1 .Run Number
/ /
�9 , , , ,
2.Date. 3.Time Machine Condit ions
.4.Bowl Speed rpm. 5.Pond dia ram.
,6.Conveyor diff rpm. ,7.Conveyor torque kNm
�9 . . , !
i .
,Feed Condit ions . . . .
,9.Feed 4n=.~Rate Solids m3/h" ~_ ' i , , , , . re~, ~ w/w d.s. , , ,
I ,Additive Condit ions
" ' 7 ,11 .Type. ; . . . . , ,12.Concentration %w/w. i . . . . ; - ,13.Addition point, i . . . . ,14.Rate m3/h. ,15.Dilution m~/Th. : ; .... :
I " I l
Product cond!t ions 116.Cake Solids %w/w. 117.Centrate Solids mg/I. A.Centrate Rate m3/h. B.Polymer O0se kg/tonne..,:
D.Solids Recoven/%w/w. E.C..ake Rate k~)/h w.b. F.Q/I; m ~ . . . .
G.TN N/cm 2 . . . .
, , ,
i i [
I _ ,i ,,
Figure 6.2. An example of a result sheet.
6.4 Some Test Data
Some actual test data are given in the next sections to cover as wide a range of applications as possible, and also to support various aspects of the theories developed in Chapter 4. In order not to overload the reader with too m a n y superfluous figures the tables of data concentra te on the pert inent figures. The information is gleaned from records covering several decades, and does not necessarily represent the max imum performance achievable today. In any case, most of the process materials will vary considerably in quali ty from plant to plant. Nevertheless, these data are useful in demonstra t ing the range of performances that have been achieved on the decanter centrifuge, and the trends of performance as parameter values have been altered. It also will provide material for Chapter 7, which is concerned with the scaling of test data.
6.4.1 Spent grain
Years ago, the spent grain from distilleries was a waste product. Today, thanks to the decanter, it is a valuable product used for animal feed. Moreover, the lower moisture content of the spent grain cake means better value to the farmer, and also means extra yield in the main distillery process. Reduced suspended solids in the centrate enables higher concentrat ions to be produced by the evaporators that follow the separation process.
Thus, apart from decanter capacity, the distillery is interested in low suspended solids in the centrate, and the best cake dryness.
The data are tabulated in Table A. 1 of the Appendix. In Figure 6.3 is plotted solids recovery against feed rate for three different
conveyor differentials. It can be seen that recovery reduces as feed rate increases. The lowest differential also causes a reduction in recovery. This is undoubtedly due to choking of the bowl. Figure 6.4 demonstra tes the reduction of cake dryness with increased differential. It also shows that better dryness, for a given differential, is achieved at higher feed rates. Thus cake dryness has to be balanced against the extent of recovery, for a given capacity.
Figures 6.5 and 6.6 show the effect of conveyor differential and feed rate on conveyor torque. From these graphs it can be appreciated that increasing feed
256 Some Test Data
90.0
85.0
80.0
=e 75.0
70.0 e,, w 65.0
,=,,=
60.0
55.0
50.0 0.0 2.0
e e
eDiff. 13.2 RPM t ==Diff. 18.2 RPM I �9 Diff. 23.2 RpMJ
A
,ih,
,qlp
4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0
Feed Rate mSlh
Figure 6.3. Graph - Recovery v Feed Rate - Spent Grain.
30.0
280
26.0
24.0 gll = E a 22.0
. I
t g
20.0 -[ �9 4.6 m31h l | �9 g.Om3/h
18.0 - �9 �9 18m3/h
16.0 ~, 10.0 12.0
i
I ' 14.0 16.0 18.0
@
1
20.0 22.0 24.0 26.0
Conveyor Differential R P M
Figure 6.4. Graph-Cake Dryness v Differential S p e e d - Spent Grain.
Test Work and Data 2 5 7
1.80
1.60
1.40
~ 1.20
1.00
~ 0.80
0.60
0.40
0.20
0.00 0.0
T T ~ X '
I 1 I 1 l&Diff" 23.2RPM~ t
I ! 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0
Feed Rate mS/h
Figure 6.5. Graph - Conveyor Torque v Feed Rate - Spent Grain.
1.80
1.60
1.40
E z 1.20 Q =I
" 1.00 I.- .~ 0.80
c 0.60 O tO
it ~ , I I
i -,t.l ~ \ i
L,,0m3,, / / ' ~ ~ - 4 - - ~ " ~ , o.4o / �9 9.0m3/h / / t ---- ~"
0.20 ~- & 13"8m3/h I 1 e18ma/h I /
o . o o | I I , 1 ,
10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0
Conveyor Differential RPM
Figure 6.6. Graph - Conveyor Torque v Differential Speed - Spent Grain.
258 Some Test Data
rate, and increasing cake dryness by reducing conveyor differential, will eventually increase the conveyor torque to the gearbox limit.
6.4.2 Agr icul tura l products
Various valuable food products can be extracted from fruits and vegetables by mashing and separation, leaving a cellulose by-product usable for animal fodder. Some of these fruits and vegetables lend themselves well to separation on a decanter centrifuge. The data tabulated in Table A.2 of the Appendix are from early development work on one such product.
In this process the centrate is the main product. Thus the cake needs to be as dry as possible to enhance yield of the product. The cake is compressible and thus "dry solids" technology could be applied.
The first ten runs were conducted on a laboratory decanter of 150 mm diameter. The following year, tests were conducted on a small plant with a pilot plant size decanter of 425 mm bowl diameter. The plant capacity was limited, but results (runs 13-29) were sufficiently encouraging to warrant work on a larger plant the following year (runs 31-34).
Figure 6.7 shows a graph of cake dryness against torque/bowl volume. Considering the time lapse between the tests, and the likelihood of variation in feed quality, the correlation is quite good. The graph in Figure 6.8 shows the variation in dryness with differential on the larger decanter, for a fixed feed rate.
24.0
22.0
20.0
18.0
16.0
at 14.0 e l
1 2 . 0
Q 10.0 0 ,Ig m 8.0
6 .0
4 .0
2 .0
0 .0
0 .00
L t I
J
/ . r
0.50 1.00
l I I
1 ' i !
~e 150mm Bowl Dia ~ i �9 425 mm Bowl Dia 1.5 m3/h
�9 �9 425mm Bowl Dia High Capacity, , , , . . . . . .
I I 1 ,
1.50 2.00 2.50 3.00
TorqueNolume Nlcm 2
Figure 6.7. Graph-Cake Dryness v Torque/Volume- Agricultural Product.
Test Work and Data 2 59
24.0
22.0
20.0
18.0
~ 16.o. 14.0
m m �9 12.0 r
a 10.0 m
== 8.0-
6.0
4.0
2.0
0.0 0.0
1~=1= ]
7k 1 1 I I I t
I 1 I
I
I I i I
1
' 1 J = lJ t ==1.5 m3/h 1 ! ! ! 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
Conveyor Differential RPM
Figure 6 .8 . Graph - Cake Dryness v Differential Speed - Agricultural Product.
The laboratory decanter was limited by its small gearbox torque, and by its relatively small pond depth. However, its performance was sufficiently encouraging to war ran t the larger scale tests and was confirmed in practice. The last laboratory test recorded stretched the limits of the decanter to demonstra te the feasibility of extra dryness. Because of the lower recoveries of the last three test runs, these are not included in the graphs. For the first pilot plant series, no centrate samples were analysed, but adjudged "good" and so a nominal figure is used for the sake of the calculations.
6.4.3 Lime sludge classification
There is interest in lime sludge classification in both wastewater and potable water t r ea tment plants. If the magnes ium hydroxide content of a lime sludge can be suitably reduced, the residual calcium carbonate can be recalcined to calcium oxide. Too much magnes ium hydroxide will prevent the slaking of the lime. The restrictions and rising cost of disposal of spent lime makes recovery and recycling attractive.
In the decanter separation process, it is required to obtain high calcium carbonate recovery, say above 80%, while keeping magnesium hydroxide recovery (in the cake) as low as possible, say below 50%.
Figures 6.9 and 6.10 show graphs of recovery of the two const i tuents against throughput , on a 150 mm diameter laboratory machine at two different pond levels. The difference of performance between deep and shallow
260 Some Test Data
100.0
90.0
80.0
70.0
60.0
o 50.0
40.0 '10
0 30.0
20.0
�9 m m
10.0
IL m
�9 150ram Bowl Dia Pond 1 4 3 : c a C 0 3
�9 150ram Bowl Dia Pond 143:Mg(OH)2 , .
0.0 I I 0.00 0.50 1.00 1.50 2.00 2.50
Feed Rate m3/h
Figure 6.9. Graph - Recovery v Feed Rate (Pond 14 3 ) - Lime Sludge Classification.
90.0
100.0 ,
80.0
70.0
60.0
50.0
n, 40.0 O
.m, 30.0
20.0
10.0
f v ~ . . @
l . , 1 l I
�9 150ram Bowl Dia Pond 130:CaCO3
�9 150mm Bowl Dia Pond 1 3 0 : M g ( O H ) 2 T 0.0 1
0.00 0.50 1.00 1.50 2.00 2.50
F e e d R a t e mS/h
Figure 6.10. Graph- Recovery v Feed Rate (Pond 1 3 0 ) - Lime Sludge Classification.
Test Work and Data 2 61
ponds, if any in this case, is marginal. The indications are that adequate performance at capacities beyond that tested is achievable.
Figure 6.11 is a similar graph for a pilot scale decanter, of 356 mm bowl diameter, where similar performances are shown.
Data for this work are tabulated in Table A. 3 of the Appendix.
6.4.4 Clay classification
Efficient separation of clay at very small particle sizes (1-2 lam) is required to produce good-quality products for the paper coating industry.
Data in Table A.4 of the Appendix are from work using a decanter with a 425 mm bowl diameter. The six runs shown in Table A.4 are a sample from a test series of nearly 100 runs. Large numbers of particle size analyses on feed and centrate solids were necessary to complete the work. Only an outline of the data is given here to demonstrate the principle of the data development, and this is shown in Figures 6.12 to 6.1 7.
Figure 6.12 is a graph, on log-probability scale, of feed and centrate particle size distributions for a run at 10 m S/h. From this graph are calculated frequency distributions for feed and centrate solids, knowing the solids recovery for the run. These are plotted on the graph in Figure 6.13, together with the cake size distribution, which is the difference between the first two curves. Note that the cut point for the run is indicated, and recorded, for where the cake and centrate lines intersect. This is the size at which there is a 5(): 50 split of particles shared between cake and centrate.
100.0
90.0
80.0
70.0 ae ~, 60.0
50.0
= 40.0 .,...
30.0
20.0
10.0
0.0 0.00
I
1 j
I I
I I
t 1
f
I i �9 3,56mm Bowl Dia Pond 286:CaCO3
I �9 356mm Bowl Dia Pond 286:Mg(OH)2 L _ _ _ . I
5.00 10.00 15.00 20.00 25.00
Feed R a t e m=lh
Figure 6. ] 1. Graph- Recovery v Feed Rate (Pond 286) - Lime Sludge Classification.
262 Some Test Data
=L
.= m e o 1: I Q.
10
0.1
/ /
0.0 0.02 0.2 1.5 7.5 25 50 75 92.5 C%
k+erf't((C-50)/50) Cumulative % Undersize
[ : Fenee~-~e
98.5 99.8 90.[]8 100
Figure 6.12. Graph - Particle Size v Cumulat ive % Undersize - Clay Classification.
70.0
60.0
50.0
u 40.0 r == g 30.0 \ , , .
20.0
10.0
--e--Feed ] --B-- Centrate !
Cake
O. 1 1 cut Point 10
Particle Size p
Figure 6.13. Graph - Size Frequency Distribution - Clay Classification.
Test Work and Data 2 6 3
The efficiency of separation for any particle size can be calculated by taking the ratio of the heights of cake and feed at the chosen size. The relationship between separational efficiency and particle size is shown on the graph in Figure 6.14 for 10 m3/h feed rate. This type of graph may be plotted for other capacities as well.
The recovery of total solids is plotted against feed rate on the graph in Figure 6.15. These data are required when calculating frequency distributions from the cumulative weight graphs. Once the cut points are obtained from sufficient runs at different feed rates, these can then be plotted against feed rate, or more properly against centrate rate, which is the determining parameter for cut point. Cut point against centrate rate is shown with the graph in Figure 6.16.
To complete the picture, the graph in Figure 6.17 indicates the sort of variation of product purity that could be expected with the material tested. Thus, from the graph it can be seen that at a rate of approximately 12 t/h from the size of decanter tested, 90% of particles in the centrate product will be less than 2 ~m and 70% less than 1 ~m.
6.4.5 Waste activated sludge thickening
Thickening waste activated sludge (WAS) is a common application for the decanter in the municipal waste industry. Unusually for the decanter in sewage applications, it often does not require polymer for good performance with this
::I. 0 i
_e .? I a.
10
0.1 0.0
1 1
i i 0 / 1 1
' / i ' / i
002 0.2 1.5 7.5
I / J
.X I
i ] i
i t l I
50 R%
25 75 92.5 98.5
�9 F ~ I ]
J i
9O.8 9O.98 IO0
k+erf'4((R-,50)/50) R e c o v e r y
Figure 6 .14 . Graph - Par t ic le Size v Recovery - Clay Classif icat ion.
70.0
60.0
50.0
~' 40.0
n, 30.0 m
,,,,1=
20.0
10.0
0.0
3.5
~ M
2 6 4 Some Test Data
5 10 15 20
Feed Rate mSlh
Fiqure 6 .15 . Graph - Recovery v Feed Rate - Clay Classif icat ion.
25
2.5
/ :1 ,,- 2 c ,.==, O
1.5 r
0.5
i 1 I
t 1
I i i A
I J
I 1
0 ~
0
" i
I t I 1 5 10 15 20
Centrate Rate t/h
Figure 6 . 1 6 . Graph - Cut Point v C e n t r a t e Rate - Clay Classif icat ion.
25
Test Work and Data 265
100
o 90
v
e~
o 80 o0
io
~ 6o
| ~ 5o o
40
i
i 1 micron !- - - 2 mic o I
5 10 15 20 Centrate Rate t/h
Fifl l lre 6 . 1 7 . G r a p h - Centrate Solids Composition v Centrate Rate - Clay Classification.
material. Invariably, a good decanter will not require polymer when thickening WAS if the settled volume index (SVI) is approximately 1 O() or less. The SVI should not be confused with the SSVI, the stirred settled volume index.
Table A. :3 in the Appendix contains a sample of data from a test series on thickening municipal WAS in a 7 3 7 mm diameter decanter bowl. Salient data from this table are plotted on the graphs in Figures 6.18 and 6.19. From these graphs it will be seen that conveyor differential controls both dryness and solids recovery. Feed rate also has a marked effect on both cake dryness and recovery. Increasing bowl speed with a slight decrease in pond depth makes the cake dryness a little more sensitive to conveyor differential change. Recovery is unaltered. Extra pond depth may have helped recovery, but this may have impaired dryness control.
6.4.6 Digested sludge thickening
Thickening digested sewage sludge requires different techniques from that required for WAS. Digested sludge requires polymer flocculant in the decanter, and because it is possible to over thicken, even dewater control has to be tighter. Cake dryness is controlled by the conveyor differential. The exact differential for a required cake dryness, or thickness, also depends upon the amount of solids (Qfxxr) being fed to the centrifuge. Scrolling efficiency is affected by the precise cake solids content being produced.
Thus a good way of correlating digested sludge thickening results from a decanter is to plot solids recovery, and cake solids, separately, against the empirical thickening factor qJ (N/[ Qf.xf.Xs)).
266 Some Test Data
100.0
95.0
90.0
m
...,. o r
l I
85.0 t
l 80.0 1
I f
70.0 i 0.0 2.0
1 I
8
4.0
[ 1 L
! I j ~
1 6.0 8.0
I ! I I ,, I I ,~ o
i I h
o, 2545 RPM; Pond 237; 40 m3/h [] 2545 RPM; Pond 237; 70 m3/h
i 1 1 I l , 10.0 12.0 14.0 16.0 18.0 20.0 22.0
Conveyor Differential RPM
Figure 6.18. Graph- Recovery v Different ial Speed - WAS T h i c k e n i n g .
6.0
5.0
4.0
l i t
3.0 L" a O
a 2.0 tO
I ! l ] i 1
t i I ; t I l z i i ! t
1.0 ~ I
0.0 t t
0.0 2.0 4.0
l . . L i �84 ' ~ i
( . ' ~ I I t \ . . i i
I I E l 1 I I 1 f ~ ' t t ! , I �9 2300 RPM; Pond 235; 40 m31h I
�9 2300 RPM; Pond 235; 70 m3/h i
i [ ] 2545 RPM; Pond 237; 70 m3/h t i 1
I ( i I I
6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0
Conveyor Differential RPM
Figure 6.19. Graph - Cake Dryness v Differential Speed - WAS T h i c k e n i n g .
Test Work and Data 267
Some decanter digested thickening data are tabulated in Table A.6 in the Appendix, and plotted on the graphs in Figures 6.20 and 6.21. While the correlations in this data set are not perfect, the trends are clearly visible. Examining the data shows that 2 .5 -3 .0 kg/t polymer is sufficient for this part icular sludge.
6.4.7 Lactose w a s h i n g
Production of lactose from milk requires the washing out of a number of impurities such as sulphate salts. This can be accomplished using the decanter centrifuge employing a rinse feature on the beach. Rinse is applied within the centrifuge th rough the outer tube of a concentric feed tube.
Brief data are given in Table A. 7 in the Appendix. These data are plotted in the two graphs in Figures 6.22 and 6.23. The work was conducted on two sizes of decanter, one of 356 mm bowl diameter, and the other of 600 mm bowl diameter. Figure 6.22 shows the washing of one specific impuri ty and indicates the effect of differential on the smaller machine. The second graph indicates the relative washing efficiencies of the two sizes of decanter, once optimised, with all analysed impurities averaged for several runs. This second graph will be used to demonstrate calculations in a later chapter. Notice that the initial impuri ty level of the larger machine is always the lowest. The reason for this is not known, but could be due to its longer beach, enabling a lower moisture level. Alternatively the smaller machine run at a higher g level
100.0
95.0
90.0
t ]
85.0
80.0
75.0
70.0 0.0
J i I I . . . A . ; : �9
I I I i I I 1 1
I I
1.0 2.0 3.0 5.0 7.0
L
i i
J 4.0
1
t 6.0 8.0
Psi (N/(Qx#~)} "100
Figure 6.20. Graph- Recovery v Factor Psi- Digested Sludge Thickening.
268 Some Test Data
18.0
16.0
14.0
~ 12.0
m 10.0 |
~ 8.0
o ,x a 6.0 ......
4.0
2.0
L I .I I
I I I �9 ! J I I
t i t
A
T ,e
J t J 1
I I I
l l 0 . 0 �84 ,
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 Psi {N/(Qx#~)} "100
Fiqure 6.21. Graph - Cake Dryness v Factor Psi - Digested Sludge Thickening.
i l I 1
4.s 1 t t
, I
~ 2.5 ~
m ! I
~. 1-5 ]4, 35,6mm Bowl Dia: 23 RPM Diff. I ! ] I
m 1 t n 356mm Bowl Dia: 36 RPM Diff.
�9 356mm Bowl Dia: 46 RPM Diff. ( 0.5 ' ie 600mm Bowl Dia: 49 RPM Diff. I
t J
0 I l I 0 2.5 5 7.5 10 12.5 15
Washing Rate (Wash /Feed * 100)
Fi~3ure 6.22. Graph- Specific Impurity v Washing Ra te -Lac tose Washing.
Test Work and Data 269
4.5
4
3.5
~ 3
~ 2 . 5 Q. E 2 el
1.5 Ir
0.5
I I P
I F-~o 356mm Bowl Dia 60 RPM Diff. ]J II 600mm Bowl Dia: 49 RPM Diff. j~ I
t
0 2.5 5 7.5 10 12.5 15 Washing Rate (Wash/Feed * 100)
Figure 6 .2 J. (Jraph - General Impurity v Washing Rate- Lactose Washing.
could have captured more fines, which, relatively, would attract a higher impurity level due to their larger specific surface area.
6.4.8 Coal railings dewatering
Sample data are given in Table A.8 in the Appendix. With such a small amount of data there is nothing to be graphed.
Coal tailings require the use of an anionic flocculant, albeit a relatively small amount, which is usually admitted upstream of the decanter, as in this case. The first four runs recorded were conducted at a standard bowl speed, giving just under 240()g gravitational field. The last six runs were when using a much lower gravitational field, under 1 ()OOg.
It will be seen that just as good a dryness was achieved at the low g level, with the benefit of reduced polymer usage and cleaner centrate. It is apparent that with this relatively heavy process material, the high g produced high torques and low scrolling efficiencies, even with extra pond depth. Lower differentials were possible at the lower g level, to compensate for the smaller g in obtaining the required cake dryness.
6.4.9 Dry solids (DS) dewatering
Obtaining extra-dry cake using a decanter (DS operation) is a relatively new technique for the decanter, having been developed only since approximately
2 70 Some Test Data
1985. Whereas, hitherto, dewater ing a digested sewage sludge would have produced a cake with, at best, 18 -20% solids, today well over 30% is possible.
When operating properly in DS mode, a decanter will be vir tually full of cake, from end to end, and from bowl wall to pond surface. The conveyor will be pushing the cake towards the discharge ports against a restriction, a baffle or a narrowing of the conveyor pitch, or against the nip between conveyor hub and the beach.
The first principle of DS operation to appreciate is tha t the volumetric wet cake rate is directly proportional to the conveyor differential. Thus, if the solids input rate doubles, say, then the differential has to double if the cake dryness is to remain the same. If the cake dryness increases, then the differential has to reduce proportionally to the decrease in volume of the cake. The graphs in Figures 6.24 and 6.25 demonstra te the proportionali ty of wet cake rate to differential, for six sizes of decanter. Note tha t the proportionali ty is not directly a function of the size of the decanter, but a function of the cross- sectional area available for scrolling, at the most restricted point.
If the pond level is not set sufficiently deep then the scrolling efficiency is impaired, as is demonstrated by the graphs in Figures 6.26 and 6.2 7. Once the pond is sufficiently deep, no extra scrolling capacity is obtained, as seen in Figure 6.2 7. Scrolling capacity is also dependent, as would be expected, on conveyor pitch. This is seen in Figure 6.28, where capacity is seen to increase a little more than the ratio of the pitches.
4.00 ...... I
3.50
,oo I / X l " = " ' J Y - I
~ _ ~ = 1 .x 1 .50 '"
]e 50mm Bowl Dia. .... 1
1.00 �9 425mm Bowl Dia.
�9 �9 450mm Bowl Dia. 0.50
0.00 0.0 2.0 4.0 6.0 8.0 10.0 12.0
C o n v e y o r D i f f e r e n t i a l R P M
Figure 6.24. Graph- C a k e R a t e v Di f fe ren t i a l S p e e d - DS.
14.0
Test Work and Data 2 71
16.0
14.0
12.0
�9 !~ 10.0 Q.
E 8.0
t 6.0 m o
4 .0
2.0
. . / I
E ~ J
Y OA J ~ "
S ,,,~
i O 575mm Bowl Dia. ! �9 737mm BowlDia. It& 1016mm Bowl Dia.
0.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
C o n v e y o r D i f f e r e n t i a l R P M
Figure 6 .25. Graph - Cake Ra te v Dif ferent ia l Speed - I)S.
0.20
0.16
" • 0.12
�9 0.08 m o
0.04
0.00
1 L ~ t I l I t i " / I
i . I , , / . ,
! i ! / I i l l J f l I I / I t I ~ . . - I / ! , J - ' ~ "
1 l / ~ i
i ~ 4)Pond 106
I I I l I J
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 C o n v e y o r D i f f e r e n t i a l R P M
Figure 6 . 2 6 . Graph - Cake R a t e v Dif ferent ia l Speed - DS.
272 Some Test Data
7.0-[ .... I
1 I 6.0 i |
5.0 . . . . . . B , , I
2 . 0 I -,, o. + i 'e P~na 2i~
0.0 ! I . ~ ~ 0.0 1.0 2.0 3.0 4.0 5.0 6.0
Conveyor Differential RPM 7.0
Figure 6.2 7. Graph- Cake Rate v Differential Speed - DS.
1.40
1.20
1.00
a. 0.80 Q
I~ 0.60 J I o
0.40
0.20
+ t t
t ,~ s
I I
I �9 t
, J O ,,
�9 127mm Pitch �9 200mm Pitch
,.+
0.00 ' 0.0 1.0 2.0 3.0 4.0 5.0 6.0
Conveyor Differential RPM
Figz~re 6.28. Graph-Cake Rate v Differential Speed-DS.
Test Work and Data 2 73
Scrolling rate is a volumetric rate function. As the graphs have been plotted with tph scrolling rate, some small variation can be expected from sludge to sludge, due to density changes, and this is seen in the graph in Figure 6.29.
The next feature or principle of DS operation is that cake dryness is proportional to the ratio of conveying torque/pond volume. At the limit, cake dryness will tail off, increasing only logarithmically with torque/volume. These relationships are shown in the graphs plotted in Figures 6 .30-6.32. Note that, on the same sludge, there is good correlation between different sizes of decanter, and different designs as diverse as those with different pitches.
The correlation of polymer dosage and cake dryness using DS data is covered in Chapter 5, with graphed examples in Figures 5.7-5.11. One further example is shown in Figure 6.33. With test work which is not so closely controlled it is difficult to obtain such good definition as shown here. In ad hoc test work polymer dosage is often kept high to ensure good centrate and maximum dryness. With this sort of data, one has to look at the maximum dryness achieved at each level of polymer dosage, before centrate quality is lost. At best a cloud of points will be seen on the graph with a well-defined upper limit, with the shape of the curves similar to those in Figures 5.9-5.11. Note that separate curves will be obtained for each feed rate.
Finally, for DS work a correlation is needed for cake dryness and centrifuge capacity. This is done by graphing cake dryness against the function feed rate/ g-volume. A line is then drawn through the points of maximum dryness. This line then defines the threshold between clean and dirty centrate. To enable
4.00
3.50
3.00
4~ 2.50 Q .
1~ 2.00 n,, 0 =c 1 5 0 r �9 o
1.00
I
!
I
0.50 �9 v m -
0o0 0.0 1.0 2.0
1 l !
t I I t i t
1 3.0
1
I I
J I I I t=
I
l !
I J
i I i I t
4.0 5.0 6.0 7.0 8.0
C o n v e y o r Di f ferent ia R P M
I I I I
I �9 DJgestecl~ DAF
�9 Primary ~--
9.0 10.0 11.0 12.0
Figure 6.2 9. Graph- Cake R a t e v Di f fe ren t i a l S p e e d - DS.
274 Some Test Data
27.0
25.0
j= 23.0
21.0
t t 19.0 E
a 17.0 o .lg m
ro 15.0
13.0
11.0 0.(I:)
I !
& &
_] �9 " I j r l , '
~ I l i l - v
0.50 1.00 1.50 1
2.00
i w
�9 150ram �9 l~Omm &425ram
1
2.50
&..
I i ....
Bowl Dia: 106ram Pond Br Dia: 110mm Pond Bowl Dia: 220mm Pond
l
3.00
~ Dia Dia
1 3.50 4.(]0
Conveyor Torque/Volume N/cm =
Figure 6.30. Graph-Cake Dryness v Torque/Volume- DS.
36.0
34.0
32.0
;~ 30.0 m t i ~ 28.0
O �9 26.0 ,al r (.)
24.0
22.0
20.0 ! 0.00
1 ; "' I ! "
t
I �9 - �9 ' i
i I , l - :~7~mm~0. : B.,..~,,. ~-' I - - " ~ i / I -~7~mr,, ~ D ' : ' ~ , , D"c I
i ,,, [ -I ! ~ Bowl_ Dia: B~ , ,e,Cone, ,,,, i ,
0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00
Conveyor Torque/Volume N/cm =
Figure 6.31. Graph-Cake Dryness v Torque/Volume- DS.
30.0
Test Work and Data 2 75
28.0 -
26.0
24.0 �9
i
22.0-
= �9 20.0 C
a 18.0 O I o 1 6 . 0 .
i
14.0 �9
12.0 �9
10.0 �9
0.00
;r . / f , u /
i
-, ! ,i
'0 127mm Pitch �9 200ram Pitch
L ! ,
2.00 3.00 4.00 5.00
Conveyor Torque/Volume Nlcm=
1.00
Figure 6.32. Graph -Cake Dryness v Torque/Volume- DS.
30.0
28.0
26.0
~ 24.0
~ 22.0 I
= 20.0 C
a 18.0 0
m o 16.0
1 4 . 0
10.0 [ 0.00
/ ' �9 /
5.00 10.00 15.00
Polymer Dose kg/t db
r , &
I.~sm~h 1- l=8 m3~ L
>9.3 m3/h [ A I
20.00 25.00
Figure 6. J J. Graph - Cake Dryness v Polymer Dose - DS.
276 Some Test Data
scale-up of data, the data for different sizes of decanter need to be coincident on the graph. This is not so for the two sizes plotted in Figure 6.34. This is because the slope of the line and the co-ordinate intercept are dependent upon the maximum dryness achievable, x~, which is a function of the depth of pond in the bowl. In the 152 mm diameter bowl the pond is very shallow. A better correlation is shown in Figure 6.35 for three different bowl designs, involving different beach angles and baffle types, and two different sizes.
The data series in the graph in Figure 6.35 are separated, for easier definition, in Figures 6 .36-6 .39. It can be seen from these that all but the smaller machine lines are coincident. The slight difference of the smaller one is probably due to its slightly reduced pond depth.
30.0
28.0
26.0
24.0
22.0 Ig
- 20.0
0 18.0 0 I I o 16.0
14.0
12.0
10.0 0.00
i
t 152mm Bowl Oia. !
n ~ ~ - ~ ] !: 425mm Bowl Dia. j - -
I - "t t
!
) J
--.-. . . . . .
0.05 O. 10 O. 15 0.20 0.25
Q4/g-Vol h "I
Figure 6.34. Graph - Cake Dryness v Feed Rate/g-Volume - DS.
Test Work and Data 2 77
35.0 " '
~~176 . . . . . . I "-. I.',J ~ . - "
25.0 . . . . . . . . . . . .
~ 2 0 . 0 .... t
m e 575mm Bowl DJa Type A I ! 0 u 575mm Bowl DJa Type B ) J
i ii
15.0 I , �9 575mm Bowl Dia Type C i ii
l I lo.o I . . . . .
0.00 0.05 0.10 0.15 0.20 0.25
Q~g-Vol h "4
Figure 6.3 5. Graph - Cake D r y n e s s v F e e d R a t e / g - V o l u m e - DS.
35.0
30.0
25.0 m =
e~ 2o .0
o
15.0
10.0 0.00
-t
I ~-~575mm B~ ~ Dia Type A 1
0.05 0.10 0.15 0.20 0.25
Ch/g-Vol h "4
Figure 6 . 3 6 . Graph - C a k e D r y n e s s v Feed R a t e / g - V o l u m e - I)S.
278 Some Test Data
35.0 . . . .
30.0
25.0 m =
a 20.0
m
15.0
10.0
0.00
)
I
! !m 575mm Bowl Oia Type B J I
0.05 O. 10 O. 15 0.20 0.25
Q4g-Vol h "t
Figt~re 6.37. Graph- Cake Dryness v Feed Rate /g-Volume- DS.
35.0
30.0
~e 25.0 m = E, 0 20.0 m u
15.0
10.0 0.00
1 I
, I 1 J
)
1
) I i
I i {" 5zsm~ ~,~, D~. ryp. c l l , , ]
0.05 0.10 0.15 0.20 0.25
Qg/g-Vol h 'n
Figure 6.38. Graph-Cake Dryness v Feed Rate/g-Volume-DS.
Test Work and Data 279
35.0
30.0
25.0 m =
o 20.0 0
m
15.0
10.0 0.00
i ! I "
0.05
" ' - " - t
J J t t I I ~ 425mm Bowl Dia Type A 1 J
J J J 0.10 0.15 0.20 0.25
Qt/g-Vol h "t
Figure 6.39. (;raph- Cake Dryness v Feed Rate /g-Volume - I)S.
This Page Intentionally Left Blank
CHAPTER 7
Calculations and Scaling
Each test run in any test programme requires a number of basic calculations depending upon the source of the data. The formulae required are given in Chapter 4. Which calculations are necessary depend upon wha t instrumentat ion is installed on the test facility. For instance, if centrate rate is measured then the feed rate needs to be calculated, and if the feed rate is measured then centrate rate has to be calculated. Both these rates are needed to calculate solids recovery, another of the calculations needed.
Some of the early models of decanter were equipped only to indicate gearbox pinion speed, rather than specifically conveyor differential. This meant that differential speed had to be calculated, and calculated using the nominal bowl speed without a bowl speed measurement , or with just a one-off measurement. With this calculation it has to be borne in mind that bowl speed can vary by 1 O0-2()0 rpm with change ofload, and belt slip, if any. This could lead to a major percentage error on differential calculation if the differential is low.
Early decanters using eddy current brakes were not able to indicate conveyor torque continuously. A reading of brake speed, and another ofbrake current, had to be recorded and used when referring to a brake calibration chart, to obtain the brake torque. Conveyor torque could be obtained by multiplying this figure by the gearbox ratio. A typical eddy current brake calibration chart is shown in Figure 7.1.
When flocculants are used the polymer dosage level has to be calculated. On special applications unique calculations will need to be performed. In three- phase work the mass balance needed to work out centrate rates is more complex. Oil recoveries and losses will need to be assessed. With three mass balances, solids, oil and water, some imbalance is to be expected due to experimental error, and care is needed to ensure that these errors are not allowed to affect the reliability of the result.
In classification work mass balances may be needed on a host of size ranges. When plotting cumulative data sets against size, the size used needs to be at
282 Calculations and Scaling
E Z o o "
p - o J< (I; =_ m
140.0 [
120.0.
i I -
l
1.75 Amps
- _ _ _
i
1.50 Amps
, 1.25 Amps I
/, 1 ~ ! I/ . . . . . . i . . . .
il , F ,! ~ 0,75Amps
, I jo 0
o.o , . ! I . .
100.0
80.0 ,
60.0
40,0
20.0
0 500 1000 1500 2000 2500 3000
Brake Speed RPM 3500
Figure 7.1. A typical Eddy Current Brake calibration.
Calculations and Scaling 283
one end of the size range, the end depending upon whether the data are cumulative undersize or oversize.
Test data may indicate that the size of test machine is adequate for the duty envisaged. Alternatively, the machine tested would be too big, which is not usual. More usually the test data have to be scaled to a larger size of decanter. When the data need to be scaled to another decanter size, other calculations may need to be performed for each run, such as conveyor torque/volume, feed rate/Sigma, wet solids ra te /conveyor differential speed, as well as the Sigma value itself if the data involve changes of bowl speed. As will be seen, these intermediate calculations help with the scale-up.
When contemplating the scaling of data, one has to consider what is the limitation to the performance on the test machine, because different limitations require different scaling techniques. Moreover if the scale-up factor is large, then the scaling may introduce another limitation, if the scale factor for the second limitation is smaller than the first.
The main decanter performance limitations are as follows:
�9 centrate clarity; �9 cakedryness -non-DS: �9 cake d ryness - DS: �9 scrolling volumetric capacity; �9 scrolling torque; �9 main motor power.
7.1 Basic Calculations
For a set of example calculations, which have to be conducted on each test run, the data from run 26 of Table A.6 in the Appendix will be used. Data"
Feed rate 19.6 m3/h Bowl speed 3150 rpm Feed solids 2.6% w / w Pinion speed 650 rpm Flocculant rate 0.98 m3/h Gearbox ratio 125 Polymer 0.13% w / w Bowl diameter 425 mm concentrat ion Centrate solids 1350 ppm Clarifying length 800 mm Cake solids 10.7% w / w Pond diameter 257 mm
It will be assumed that the densities of feed, polymer solution, and centrate are unity. Thus, from equation (4.13), c e n t r a t e rate"
( 1 0 . 7 - 2 . 6 8 ) ( 1 0 . 7 - 0 . 1 3 ) Qt = 19.6 + 0.98
(6.9 - 0.135) (10.7 - 0.135)
1 9 . 6 x 8 . 0 2 0 . 9 8 x 10.57 = +
10.565 10.565
= 14.88 + O.98
Ol ~" 15.9 m 3/h
From equation (4.14), r ecove ry :
(15.9x0.135) = 100(1 - 0 .0409)
R ~ 95.9%
From equation (4.15) p o l y m e r dosage:
0.98 x 0.13 PD--
19.6 x 2.68
PD- 2.43 kg/t.db
x 1000
Cnlrirlntians nvd ScnIing 2 8 5
where dh indicates dry basis and wb would indicate wet basis. Thus, in this case the polymer dose is quoted in kilograins of active polymer (dry) per ton of dry solids in t he feed.
From eqrialion (4.9), conveyor differential:
‘I’lie data in Table A.6 are fur a thickening application. For thickeriirig. ii is
lhrn equation (4.59), psi; sornelirnes useful LO calculate psi, the thickening factor.
2 0 19.6 x 2.68 x 10.7
* =
Note the mixture of units ( rpm divided hy tn3$l. For thc purist. the answer miry be indtiplicd by 2 x x MJ, tc> givt: uriit,s ofm-’ , Psi is uscd fur cornparison purposes. and therefore. 50 lorig a s [he same units arc uscd throughout, t.he choice of units is immatcriel.
Thc Sigma value fur llie decaqtes normally would be obtained horn the decantcr manufacturer. h u t Tor demonstration purposw A v;Iluc wqll be estlmatcd hcrc. Frntii cquaiion (4 ,# ) . centrifuge g-level:
> 21.25 = 3 3 0 . . x -
4 8 1
1:ro111 equation ( 4 . 3 2 ) . Sigma:
1 1 4 5 7 6 = 27900 x - ~
400 x 0 . 5 0 3 = z 7 v 0 0 x 569.45
c = 1.59 x 1o’cm’
286 Basic Calculations
Thus, 0 / ~ " 19.6
-
1.59 x 107
Q/E = 12.3 mm/h
x l O 0 0 x l O 0 0 x 10
For the purposes of demonstrat ion a pinion torque of 10 Nm will be assumed. In that this was a thickening application, where there would be little interest in torque measurement , the pinion torque, in all probability, would have been much lower. However, this figure is at the lower end of what would be experienced in a dewatering application.
From equation (4.10), c o n v e y o r torque:
T = 125 x 1 0 = 1 2 5 0 N m
T = 1.25 kNm
Clarifying volume:
V-- 71- 52 72 400 (42 - 2 5 ) x
3 = 71990 cm
V - 71.99 1
800
10
Thus, T~ V: 1.25 1000
T / V = x 71.99 1000
T / V - 1.74 N/cm 2
x 1 ()0
Q/g-Vol: 19.6 x 1000
Q/g-Vol = 2357 x 71.99
Q/g-Vol- 11.55 x 10 -2 h -1
Thus, the necessary factors have been calculated should a scale-up be required from this one set of data. Naturally, in a real situation calculations would not be conducted both for thickening and DS. They are done by way of example here. With a 10.7% cake, it is unlikely to be a DS application. However, were it to have been a DS application, and the dryness was considered adequate and, for instance, double the capacity was being sought, then a decanter with twice the g-volume would be required. The g-level would probably be chosen similar, thus the bowl volume would need to be double, and therefore to mainta in the same dryness a gearbox of twice the torque
Calculations and Scaling 2 8 7
would be required. More normally these calculations are conducted for each run, and performance levels plotted against them, such that the optimum performance can be chosen and scale-up is made from there.
7.2 Three-Phase Calculations
Similar calculat ions need to be conducted on any moni tored run in three- phase work. A sample set of calculat ions is given here. Data:
Feed rate 4 m 3/h Bowl diameter 42 5 mm Feed solids 39% w / w Oil discharge d iameter 2 70 mm Feed oil 2 4 % w / w Wate r discharge d iameter 2 7 4 m m Feed wate r 3 7% w / w Cake discharge d iameter 264 m m Rinse wate r rate 1 mS/h Bowl speed 3150 rpm Cake solids 50% w / w Clarifying length 750 mm Cake oil 6 % w / w Cake wa te r 44% w / w Oil rate 0.8 m3/h Effluent solids 4.4% w / w Oil wa te r 1% w / w Effluent oil 0.4% w / w Oil solids 1% w / w Effluent water 95.2% w / w Oil 98% w / w Oil density 0.85
First a total mass balance, followed by a solids mass balance, is conducted. One of the two u n k n o w n s , solids rate or effluent rate, is el iminated by subst i tu t ing from one equat ion into the other, and the two u n k n o w n s are calculated. Thus, from an equat ion similar to equat ion (4.11 ), the total mass
balance is:
FeedRate + W a t e r R a t e = CakeRate + Ef f luentRate + OilRate
4 + 1 = CakeRate + Eff luentRate + 0.8 x 0.85
whence
4.32 = CakeRate + Ef f luentRate
The solids mass ba lance is:
4 • 0.39 + 0 - 0.5 x CakeRate + 0.044 • EffluentRate + 0.8 • 0.85 x 0.01
1.56 = 0.5 x CakeRate + 0.044 • EffluentRate + 0.0068
Calculations and Scaling 289
whence 1 .5532 - 0.5 x C a k e R a t e + 0.044 x E f f l u e n t R a t e
By substitution" C a k e r a t e - 3 . 0 0 2 7 t /h Ef f luent ra te - 1 . 3 1 7 3 t /h
,-,3.o t /h ,,~1.3 t /h
This assumes that the densities of effluent, feed, and cake are all unity. This is not quite true, but within the experimental error associated with this type of work, this is acceptable.
R e c o v e r y o f s o l i d s
3 .0027 x 0.5 = x 1 0 0 ~ 9 6 . 2 %
4 x 0.39
R e c o v e r y o f oil
0.80 x 0.98
4 x 0 . 2 4 x 100 ,~ 81.7%
Oil l o s s in c a k e
3.002 7 x ().06
4 x 0.24 x 1 0 0 ~ , 1 8 . 8 %
Oil l o s s in w a t e r
1.3173 x 0 . 0 0 4
4 x ().24 x 1 O0 -~, 0.~/o
Note that the oil recovery and oil losses do not add up precisely to 10()%. This is due to experimental error, and sometimes can be much larger.
In three-phase work, it can be useful to determine the approximate position of the e-line (equilibrium line). Using equation (4.61 ):
0 " 8 5 x ~ 2 [ ~ 2 g ' 7 - ( 2 ~ 0 ) 2 ] - - l"Oxa~2 [ ~ 2 f f ' 7 - ( _ ~ ) 2 ]
which simplifies to"
0 . 8 5 ( r ~ - 1352 ) = 1 .O(r~- 1372 )
O.15r~ = 1 8 7 6 9 - 15491.25
V/ ~3277.75 r,. - - O. 1 5 ,~, 1 4 8 m m = 2 9 6 m m D i a
Thus, e - l i n e d iameter"
= 296 mm
290 Three-Phase Calculations
This gives a depth of oil of ( 2 9 6 - 2 7 0 ) / 2 - 13 m m over a wa te r depth of ( 4 2 5 - 2 9 6 ) / 2 - 64.5 mm.
Assuming it is just as easy to separa te the oil from the wa te r as it is to separate the wa te r from the oil, w h i c h is not necessari ly so, it is w o r t h ca lcula t ing the Q/E for each phase.
For the light phase, from equa t ion (4.32), Sigma:
E 7r x 750 x 3302 (2962 - 2702) - - - X
981 x 10 202 l n ( 2 9 6 / 2 7 0 )
1 4 7 1 6 = 2 6 1 5 5 . 9 x
4 0 0 x 0 . 0 9 1 9 : 1.05 • 107cm 2
Thus, light phase Q/E:
0.8 0 / z -
1.05 x 10-
Q / E - 0 . 7 6 mm/h x l O 0 0 x l O 0 0 x 10
For the heavy phase (flow rate - 1 .2018) , Sigma"
E = T r x 7 5 0 x 3302 x ( 4 2 5 2 - 2 9 6 2 )
981 x 10 2 0 2 1 n ( 4 2 5 / 2 9 6 )
93 O09 = 2 6 1 5 5 . 9 x
4 0 0 x 0 .361 7
= 2 6 1 5 5 . 9 x 6 4 2 . 8 6 -)
E = 1.68 x 107 cm-
Thus, heavy phase O/E:
1 .2018 Q/E- 1 . 6 8 x 107 • 1 0 0 0 • 1 0 0 0 • 10
O/E - 0.72 m m / h
With Q/E values being so similar for the two phases, it would seem tha t the differential pond sett ing is op t imum, barr ing any crest ing effects or back- pressure effects from any d ischarge device. If it were necessary , say, to improve the qual i ty of the oil phase at the expense of wa te r effluent quali ty, then the wate r d ischarge d iameter would need to be increased very slightly, to increase the depth of the oil level in the pond.
7.3 Classification Calculations _- -
As a11 example of thc calculations needed for a decanter test run On a cIassitic;jt.ion duty . the data uscd for Figures 6.1 2 - 6.1.7 will h e er~ipluyed. 'The dat.a are given in . arid adjacent to, Tablc 7 .1 .
The parl.icle size analyses frdkil Tablc 7 . 1 arc plott,ed in Figerr: h.1 2 . Prom this graph frequency dist.rihutions are calculated. 'I'he data abovc arc cc- tabulated, ciilculi~ting the percentage it1 each size interval, and dividing that percentage by the size inkrval . 'I'Iiese figures. thus calculatcd, give the rclativc frequcncy for each size interval. The freqrirric:ies iri t.hr c:eril.raf.c! distributioe arc thcn multiplied by (1 -solids recovery ;IS ii Itect.ion), to make the frequcncics in thc ccntratc distribution correspond to those in the feed. by virtue o f the particles lost in the cake. 'J'hese figures are tabulated in Table 7.2,
Table 7.1. PArl i i : lc s i x ;rn;rlyscs
Particlr Cum. 9:, r 1 1 1 1 r . '%I
sizc undcrsiec widcrsiac
(pn) feed centrale
1 I).O 0 7 . 4 0 9 ,!I 7 9.0 96.9 Y S . Y h 8.0 9f1.1 99.9 1 7 . 0 95.1) 99.88 6.0 9 3 . h 9 9 .7 h 5.U Y 1 . 3 9 Y . 5 3 4.0 88.0 99.00 3 . 0 X2.h 97.60 1 .o 7 2 .o 9 3 . 0 0 1.5 h 3 . ci H7.00 I ,I) 50.0 7 3 . 0 0 0.8 42.6 ~ 1 3 . 0 0 0.0 '3'1.6 49.00 0.4 2 2 . 0 30.00 0 . 3 1 5 . 2 1X.hO 0.2 8.0 8 . 2 0 0.1 2.5 1 . 2 0
_. .- .., . , , , . ,, , . ,- , -, --
.. . ~
. . . . . . ...
F w d rate Howl dlamr:tr:r iliirifylng lerlgth Cakc discharge diamcter Pond diameter Krrwl spct:d Solids rrtwvt.ry I ) i k r c n I i al SG of feed Feed solids Cakc solids C c r ~ t r . a ~ c u o l i d s
292 Classification Calculations
Table 7.2. Frequency distributions
Mean Size Feed Feed Centrate Centrate s i ze interval % in (%/~tm) % in (%/~tm) (~tm) (pm) interval interval
Centrate (%/~tmx(1-R))
9.50 1.0 0.5 0.5 0.01 0.01 0.01 8.50 1.0 0.8 0.8 0.03 0.03 0.02 7.50 1.0 1.1 1.1 0.05 0.05 0.03 6.50 1.0 1.4 1.4 0.12 0.12 0.07 5.50 1.0 2.3 2.3 0.23 0.23 0.13 4.50 1.0 3.3 3.3 0.53 0.53 0.31 3.50 1.0 5.4 5.4 1.40 1.40 0.82 2.50 1.0 10.6 10.6 4.60 4.60 2.70 1.75 0.5 8.4 16.8 6.00 12.00 7.03 1.25 O. 5 13.6 27.2 14.00 28.00 16.41 0.90 0.2 7.4 37.0 10.00 50.00 29.30 0.70 0.2 9.0 45.0 14.00 70.00 41.02 0.50 0.2 11.6 58.0 19.00 95.00 55.67 0.35 0.1 6.8 68.0 11.40 114.00 66.80 0.25 0.1 6.6 66.0 10.40 104.00 60.94 0.15 0.1 6.1 61.0 7.00 70.00 41.02 0.05 0.1 2.5 25.0 1.20 12.C)0 7.03
By tak ing the difference be tween the two f requency (%/lam) co lumns , the
size f r equency dis t r ibut ion for the cake is obta ined. This is s h o w n in Table 7.3.
The th ree f requency d is t r ibut ions are plotted on the g raph in Figure 6 .13 .
The cut point is ob ta ined as the part icle size at wh ich the cake and cen t r a t e
d is t r ibut ions intersect . As, at this point, the f requencies for the two d is t r ibu t ions are equal and total t h a t for the feed, it follows tha t this f requency
is half t ha t in the feed, wh ich is the definition of cut point. The cut poin t for each flow rate is similarly obta ined, after w h i c h the g r a p h
of flow rate agains t cut point may be plotted, as in Figure 6 .16. From the f requency dis t r ibut ions , the sepa ra t iona l efficiency for each and
any size m a y be ob ta ined by tak ing the ratio of the frequencies , cake to feed,
and mul t ip ly ing by 100. This can be done for each feed ra te tested. The
efficiency plot for 10 m 3/h is s h o w n in Figure 6 .14. Figure 6 .15 gives the recovery for each feed rate, w h i c h is needed to
ca lcula te the cen t ra te f requency curve , and t hus the cake f r equency curve.
~ un
u~
~
~ C
0
uq
u~
0 ~
~ C
C
~
0 0
,_,.
N
~.
i N
,..,.
,-t
0"
,_,~
0
,..,.
7.4 Washing
The data depicted in Figure 6.23 will be taken as the data source for a demonstrat ion calculation. The associated data are as follows:
Feed rate Feed impurity level Feed suspended solids Cake moisture content Wash rate
5500 kg/h 8 . 1 2 5 % w / w 3 7 . 5 % w / w 25% w/w 7.5%
Figure 4.11 (the rinsing with diffusion diagram) is reproduced as Figure 7.2, with specific figures, the derivations for which are given below.
W a s h rate:
0 W ---
7.5
1 0 0 x 5 5 0 0 - 412.5 kg/h
Assuming full recovery of solids. Dry C a k e Rate"
37.5 QsXs = x 5500 = 2062.5 kg/h.db
100
Q, =5500kg/h Q.=412.5~ c,=y. =0
Figure 7.2. Rinsing with diffusion-Mass flows.
Q.~2750k~ c2=?
Calculations andScaling 295
Wet c a k e rate:
100
( 1 0 0 - 2 5 ) x 2062 .5 - 2 7 5 0 kg /h .wb
Impurity l eve l of feed:
I f - 8.125%.wb
1 O0 = 8 .125 x
37.5
!1 - 2 1 . 6 7%.db
Based on the liquor in the feed, impur i ty c o n c e n t r a t i o n :
I()0 cl - 8 .125 x = 13.0%
( 1 0 0 - 37.5)
This last impur i ty level. 13 %. will be the same for the cake after decant ing the excess liquor, without rinse. However, based on the solids, c a k e impuri ty level ( w i t h o u t rinse):
( 2 7 5 ( ) - 2()62.5) I s - 21 .67 x
(5500 - 2062.S)
I~ -- 4.3 3%.db
Compare this figure with the ordinate intercept on the graph in Figure 6.2 3. It will be seen from this graph that this figure agrees extremely well with the practical result .
Cake m o i s t u r e d i scharge rate"
O.~ps(1 - xs) - 2 7 5 0 - 2062 .5 - 687 .5 kg/h
Rinse rate:
7.5 0 , , , - l o o X ~500
O, , , - 412 .5 kg/h
Thus, wi th perfect rinsing, c a k e impur i ty level w i th rinse:
(687.5 - 4 1 2 . 5 ) I s = x 4 . 3 3 = 1 .73%.db
687.5
296 Washing
This is the lowest impurity level possible with 7.5% rinse. In practice (Figure 6.23), for the smallest decanter tested, the level is 2.3%. Thus, r i n s i n g efficiency"
_- ( 4 . 3 3 - 2 . 3 ) x 100 ( 4 . 3 3 - 1 . 7 3 )
= 78%
To determine the effect tha t any possible parameter change may have, it is necessary to determine the effective mass transfer coefficient for these conditions.
Impurity c o n c e n t r a t i o n in m o i s t u r e of cake"
2 0 6 2 . 5 c2 = x 2 .3 = 6 . 9 %
6 8 7 . 5
Thence the concentrat ion in the centrate liquor can be obtained using equation (4.77).
Centrate l iquor impur i ty level:
( 1 3 - 6 .9 ) x 6 8 7 . 5 - 4 1 2 . 5 ( c 3 - O)
6 8 7 . 5 c3 = 6.1 x
412.5
= 10.17%
The logar i thmic c o n c e n t r a t i o n dif ference:
A c = ( 6 . 9 - 0 ) - ( 1 3 - 1 0 . 1 7 ) l n [ ( 6 . 9 - 0 ) / ( 1 3 - I O . 1 7 ) ]
A c - 4.57%
From equation (4.88)"
687.5 • ( 1 3 - 6.9) hDA,, --
IO0 x 4.57
= 9.18 kg/h .% conc.diff
The question that now could be posed is whether , by increasing conveyor differential on the smaller decanter, the cake impuri ty level could be lowered to match that achieved on the larger model, as shown in Figure 6.23; i.e. the impurity level needs to be lowered from 2.3% to 2.0%, still with 7.5% rinse
rate. Increasing the differential from 60 rpm to, say, 75 rpm, increases the
superficial velocity ofrinse over the cake by 75 /60 = 1.25.
This increase in siiprrfirial velocity will incrrijsr the mass transfer coefficient by thc Same proportion (see quat io i l (4.8 5 ) ) . Thus. revised h d , :
= 1.2 5 x 9.1 8 = 1 I . 5 ~ ~ / h . % ~ O t l C . d ~ /
The impurity lcvcls in the cake ciin now nnly be back calculated by iteration. 'I'ablc 7.4 gives figures obtained in the it.eraf,i()ri, from hid^ a resdt can he iriterpolakd. llsing l'able 7.4, it will bc seen that a diflerential of 7 5 rpm (1r)ok along the row for which hoAc is nearest to 1 1 . 5 ) only rcduccs thc cake impurii.y level frtim 2 . 3 to 2.1 7 . barely half thc reduction rcquired. To achieve the 2.03: tignre, t.he ccinveyor d i lken l i a l would need lo be doubled. assuming that. it. were prar:t.icahle.
'lable Z4. iterative clilculations for cake impurity levels
Cake Cake Mi ( k g / h ) Ceri t ml t: moisture impurity i% db) liquor iinpurity (%) impurity ('%, 1
5.2 1.71 i 3. h 1 1 3 . 0 0 5 , 5 1.8.5 5 1 . 5 0 12.50 h . 0 2.00 48.14 11.h; 6.5 2.1; 44.07 1O.H 1 7.0 2.33 4 1 . 1 5 10.00
Z . i O 17.H 7 0 . 1 7 n . o 2.67 14.36 8 . 3 3
... __ .. - . . . .
- , _ " .
- - f . 3
.. - .. - ..... . . . . . . .
0.011 '.Y
' ,ON '4.79 3.09 1 5 . 5 5 1 . Y 5 I 1 . J I 4 .72 H.74 5.4') (3.89 8 .53 4 .03
.............. -
7.5 The Probability Scale
Sometimes it is necessary to plot on a log-probability graph but the probability scale is not available as such in a spreadsheet. The probability scale can be calculated using the r ight -hand side of equation (4.19 t. The probability scale, therefore, will be proportional to e r f - l (2Cx-1 ) , where Cx is a percentage figure for which the scale is required.
The mathemat ica l term, erf(x), is a tabulated integral, which may be obtained from any good mathemat ica l book of tabulated t ranscendental functions.
For the spreadsheet a look-up table will have to be created of Cx against e r f -~(2C~- l ) , and a simple formula introduced to interpolate linearly between values. This is how the graphs in Figures 6.12 and 6.14 were created. The look-up table used is shown in Table 7.5.
The probability scale is frequently used in decanter work and it is useful to know how to create such a scale when a ready-made one is not available.
Calculations and Scaling 299
Table 7.5. Look-up table for Cx against erf- l(2Cx-1)
Percent Cx
0 0.01 0.05 0.1 0.2 0.5 1 2 5 10 20 30 40 50 6O 7O 80 9O 95 98 99 99.5 99.8 99.9 99.95 99.99 1 ()0
Er f - l (2Cx- 1)
0 .00 O.37 0 .68 0 .82 0 .97 1.18 1.36 1.55 1.84 2.05 2.33 2.63 2.82 3.00 3.18 3.37 3.68 3.95 4 .16 4.45 4.65 4.82 5 .()4 5.19 5.32 5.63 6.()()
7.6 Scale-Up Applications
of Centrate Clarity Limiting
Scaling up between two decanter sizes is generally best done when there is geometrical similarity between them. This means the same beach angle, the same conveyor pitch angle, and the same conveyor and feed zone designs. When there are differences then the scale-up may not be reliable.
A centrate clarity limiting application is characterised by a fall off in centrate quality, when feed rate is increased, independent of conveyor differential, once pond depth has been optimised. Spent wash dewatering, discussed in the previous chapter, is one such example.
Note the proviso concerning differential. Referring to Figure 6.3, the conveyor differential needed to be at least 18 rpm for the centrate to be unaffected, and raised even higher at the higher capacities.
To scale any of the capacities tested to another size of decanter, or the same decanter with a different bowl speed, the ratio of Sigma values simply would be
used. The best dryness achievable on the test machine is taken from Figure 6.4,
using the minimum differential necessary to achieve the best recovery, at the rate chosen, shown on the graph in Figure 6.3. Translating the optimum differential to the larger machine is usually not done by calculations, but by trial and error adjustment of differential, when commissioning the larger decanter. However, if necessary, calculations of cake scrolling rate and cake residence time may be made to ensure that the necessary differential range required on the larger machine is available.
The clay and lime classification applications, covered in Chapter 6, are also examples of data which would be scaled by Sigma ratios.
The lime classification is interesting in that it involves two materials of suspended solids with different densities. In this application it was required to produce a cake with less than 60% magnesium hydroxide, to prevent slagging during calcining. For economic reasons, it was necessary to recover at least 85% of the calcium hydroxide. From Figure 6.11 it will be seen that, on the test machine, any capacity between 12.5 and 20 mS/h would achieve the objective. This capacity range would be scaled proportionally to Sigma.
Calculations and Scaling 301
The lowest capacity would achieve 90% recovery of calcium hydroxide, and the highest 85 %.
7.7 Simple Dewatering and Torque Scale-Up
While the majority of applications will be scaled by Sigma, there are occasions when high conveyor torques are experienced during test work. Then it will be necessary to scale the torques experienced, to the proposed larger machine, not only to ensure that this will not be limiting but also to estimate the power required for the drive motor.
Some "dry solids" applications (covered in Section 7.9) will have their capacities limited by gearbox torque rating. When scaling these applications to larger decanter sizes, it is important that the torque rating available is greater than the scaled-up torque.
The graph in Figure 6.6 gives torque data for the spent wash dewatering application. These data need to be converted to a form suitable for scaling.
From equation (4.70) it will be seen that torque is proportional to feed rate and inversely proportional to differential. Therefore, the data of Figure 6.6 should be re-plotted as torque against feed rate/conveyor differential speed. This is done in Figure 7.3. Here it will be seen that most of the data, for the lowest two differentials tested, form a straight line 1.1 kNm/(m3/h/ rpm dill).
As an example, these torque data will be scaled to a decanter of 737 mm bowl diameter. Data:
Test decanter Large decanter Bowl speed rpm 3150 ? Bowl diameter mm 42 5 73 7 Pond diameter mm 261 480 Cake discharge diameter mm 264 483 Clarifying length mm 12 O0 22 60 Conveyor pitch mm 12 7 54 Wetted area of bowl m 2 2.2 6.7 Beach area m 2 0.5 1.4 Conveyor differential rpm 13 ? Feed rate m3/h 16 ?
1.80
1.60 �9
1.40 -
E z 1.20 .ar Q J =1
" 1 .00 - I -
0 .80 >"
0 .60
0.40
0.20 �9 I I
0.00 ,
0.00 0.20
Calculations and Scaling 303
I l
j . . /
t �9 Diff. 13.2 RPM ==Diff. 18.2 RPM
i& Diff. 23.2 RPM . . - - . - - - - - - _ _ ~ ] ~
J
0 .40 0 .60 0 .80 1.00 1.20 1.40
Feed R a t e / D i f f e r e n t i a l Q / N
Figure 7.3. Conveltor Torque vs. Feed Rate:D(fJerential Ratio.
Firstly the bowl speed for the la rger m a c h i n e needs to be ca lcu la ted ,
a s s u m i n g t ha t it will need to have the same g-level as the test m a c h i n e . For the test m a c h i n e , b o w l speed:
S - 3 3 0 r a d / s e c
g-level (equation (4.8)):
3302 4 2 5
g' 981 x 20
g,. = 2 3 5 7
For the la rger m a c h i n e , b o w l s p e e d :
2 3 5 7 x 9 8 1 x 20
S = 737
= 2 5 0 r a d s / s e c
S .~ 2 4 0 0 rpm
3 0 4 Simple Dewatering and Torque Scale-Up
The feed rate will be scaled by Sigma, which thus needs to be calculated for each machine, using equation (4.32). For the test machine, Sigma:
E 7r x 1200 x 3302 F,42s,[~,_yO__) 2_,261, ]i_2_ff) 2 ---- X
981 x 10 ln(425~ k 2 6 1 1
41849 .5 x 281 .26
0 .4876 E = 2.41 x 107 cm 2
For the larger model, Sigma:
E3 7r x 2260 x 2502 [ ( ~ ) 2 - (~~176 21 - - X
981 x 10 l n ( ~ )
45234 .5 x 781.9
0 .4288 E = 8.25 x 107 cm 2
Feed rate:
8.25 Of - 2.41 x 16 = 54.8 m3/h
Differential is the next parameter value to be calculated. In the absence of any other information, initially at least, the differential for the larger decanter would be fixed to have the same cake thickness, in the bowl, as the test machine. Cake thickness will be proportional to feed rate, and inversely proportional to conveyor pitch and differential. Thus, for the larger machine, conveyor differential:
54.8 127 425 N - • x x 1 3
16 254 737
N ~ 13 rpm
which, conveniently but coincidentally, is the same as the smaller machine. Torque is scaled up using equation (4.70). Conveniently again, there is no
dry beach, and so there is no need to use the factor k4. However, the heel torque needs to be estimated using equation (4.71). The heel torque for the smaller decanter is the ordinate intercept on the graph in Figure 7.3, which is approximately 0.3 kNm. Thus, for the larger decanter, hee l torque:
6.7 2357 737 To = 2.2 • 2357 • 425 • 0.3
To ~ 1.6 kNm
Calculations and Scaling 305
Torque on the test mach ine , at the specified feed rate, is 1.6 5 kNm.
On the la rger mach ine , torque above heel torque:
( T - To) = 54.8 x 425 16 737
( T - To) = 7.5 kNm
1.4 x 0.5 x (1 .65 - 0.3)
Total conveying torque"
T - 7.5 + 1.6 = 9.1 kNm
7.8 Main Motor Sizing
The data for the spent wash dewatering, used in the last section, will also be used to estimate the motor size required for the large decanter example.
Equations (4.130)-(4.134) are used to calculate the components of process power, except the power consumed to overcome windage and friction. This component would be obtained from the manufacturer of the decanter, or by measurement in the field.
The manufac turer supplies the figures of 4.4 and 3 3.6 kW, windage and friction, for the two sizes of decanter, operating at the speeds specified. The figure for the test decanter is not needed for calculations, but is given for comparison purposes.
The centrate and cake discharge at approximately the same diameter and therefore it is not necessary to work out the components of power for these two streams separately. Were the discharge diameters to be markedly different, then the rate for each s tream would need to be calculated, and equation (4.131 ) would then need to be used for each stream.
For the larger decanter, assuming process densities are close to unity, p r o c e s s a c c e l e r a t i o n p o w e r , substi tut ing the appropriate values calculated into equation (4.131) (w = 250, Qr = 54.8, rd=480/2/10):
Pp= 5 4 8 x 2 5 0 2 ( 4 8 0 0 ) 2 . x 2 x l
= 54.8 x 10 ~ W
1 0 0 0
3 6 0 0 x lOOx 100
PI'- 54.8 kW
From equation (4.133 ), c o n v e y i n g power : 27r
Ps = 13 x 9.1 x 1000 x 6--0
= 12.4 x 103 Nm/s
Ps = 12.4 kW
For the braking power, it is necessary to know the gearbox ratio, which will
be taken as 98 (from the manufacturer) .
Calculations and Scaling 307
From equat ion (4.9), p i n i o n speed:
Sp = 2 4 0 0 - ( 1 3 x 98)
S v = 1 1 2 6 r p m
and from equation (4.10), p i n i o n torque"
Tp = 9.1 x 1 0 0 0 / 9 8
Tp = 92.9 Nm
From equation (4.134), b r a k i n g power"
PB -- 92.9 x 1126 x
= 11 x 103Nm/s
P~-- 11 kW
27l"
60
From equation (4.130), the decanter requires at the bowl pulley, to ta l power"
P T - 3 3.6 + 54.8 + 12.4 + 11.0
PT-- l l l . 8 k W
The precise motor specification will depend upon a number of other factors such as the type of drive and starter, and how much contingency, or expansion, for which the user wants to cater. The losses between the motor and the centrifuge, such as belt friction, and fluid coupling losses if applicable, all need to be taken into account. However, the motor will be no smaller than 125 kW, the next standard size above the power so far calculated. If a much larger size of motor is chosen, the power factor for the motor efficiency will be reduced, imposing a greater penalty on the cost of electricity.
7.9 DS Scaling
Table A.9 in the Appendix gives a set of 14 data points for an unspecified effluent, which will be used to demonstrate scaling of DS data.
The requirement is to specify a size of decanter that can process the same sludge at 50 m3/h to give a cake of 30% dryness or better with the minimum of polymer usage, and good centrate. The salient data for the test machine and two larger machines worthy of consideration are given below.
Decanter Test No. 1 No. 2 Bowl diameter mm 42 5 575 737 Clarifying length mm 800 2000 1550 Bowl volume 1 86 385 460 Nom. scrolling rate tph / rpm 0.28 1.00 2.00 Max. conv. torque kNm 2.7 16 20 Gearbox ratio 125 267 254 Bowl speed rpm 3150 2900 2400 Cake discharge dia. mm 264 326 483 Pond dia. mm 252 306 463
The pond used for the test machine was 6 mm above neutral. For the larger machines 10 mm above neutral has been chosen as that is known to be a good
working level in practice. First the g-volume needs to be calculated for each model. Notice that, in
the g-Vol calculations for DS work, it is the g level at the pond surface that is used rather than g at the bowl wall, which is more commonly used in centrifuge work. Using equations (4.105) and (4.106), for the test machine,
g-volume:
(2 7r x 3 1 5 0 ) 2
g-Vol = 86 x 60
g-Vol = 120.2 m s
252 20 x 1000 x 981
Calculations and Scaling 309
For larger machine no. 1, g-volume:
-- ( 2 r r x 2 9 0 0 ) 2x g-Vol 385 x 60
g-Vol = 553.8 rn 3
3 0 6
20 x 1000 x 981
and for the larger machine no. 2, g-volume"
(2 7r x 2 400'~ 2 463
g-Vol-460x 6~ J X 2 0 x 1 0 0 0 x 9 8 1
g-Vol- 685.7 m 3
From these calculations it will be seen that the g-level at the pond surface on each machine is, respectively, 1400, 1440, and 1490, which are all very similar, indicating that scale up should be straightforward, providing the geometry of the test machine and the two larger machines are similar.
The data from the test decanter are plotted in graphs in Figures 7.4-7.7. The figures calculated above are used with these graphs to assess the likely performance of the larger machines proposed for the duty.
For Figure 7.4 scrolling rates are plotted against differential. This indicates that the test scrolling rate is 0.28 tph/rpm, which is what is expected. Therefore, no adjustment is needed for this parameter for the larger machines.
2.50
2.00
,Q
.= 1.50 Q.
Q
@ .x 1.00 q ro
0.50
0.00 0.0
I I I I
1 �9
J I /
I I I t
'ZI I
J I I
, I
O: ;cries1 ] I
2.0 4.0 6.0 8.0 10.0
Conveyor Differential RPM
Figure 7.4. Cake Rate vs. Differential.
310 DSScaling
In Figure 7.5 test data of cake dryness are plotted against torque/volume. It shows that a torque/volume figure of 2.0 N/cm 2 will be sufficient to produce a cake of 30%. Thus, for no. 1 machine, c o n v e y o r t o r q u e "
T = 2.0 x 3 8 5 / 1 0 0 = 7.7 k N m
and for no. 2 machine, c o n v e y o r t o r q u e "
T = 2.0 • 4 6 0 / 1 0 0 - 9.2 k N m
These torques are well within the capability of the size of machines selected. It should be noted that if drier cakes will be obtained in the future with
development of equipment, techniques or chemicals, the conveyor torque will increase. Reserve gearbox torque capacity will permit such improvement and also result in longer gearbox life.
The maximum capacity possible may be gauged from the graph in Figure 7.6. Here cake dryness test data are plotted against feed rate /g-volume. Above the line drawn on the graph, centrates are liable to be dirty, as were the centrates for the points with open symbols. Below the line good performance can be expected. For the required 50 m3/h on machine no. 1, O /g -vo lume :
34.0
32.0
30.0 ~e m
: 2 8 . 0
~ 28.o
24.0
22.0 I t i 20.0
0.00
I
, f I
l I
36.0
J
f l
1 0.50 1.00 1.50 2.00 2.50 3.00
Conveyor Torque/Volume Nlcm =
Figure 7.5. Dryness vs. Conveyor Torque:Volume Ration
3.50
Calculations and Scaling 311
g-Vol
O_ g-Vol
= 5 0 / 5 5 3 . 8 t7 -1
= 9.0 x 10 -2 h -1
and for m a c h i n e no. 2, O / g - v o l u m e "
g-Vol
0 g-Vol
= 5 0 / 6 8 5 . 7 h -1
= 7.3 x 10 -2 h -1
From Figure 7.6, it will be seen tha t for a 30% cake these two points are comfortably below the line. From this g r a p h also, it will be seen tha t 50 m3/h on mach ine no. 1 corresponds to abou t 11 m3/h on the test m a c h i n e and for no. 2, about 8.5 m 3/h on the test mach ine . This est imate is from k n o w i n g tha t the abscissa values for the points are 3, 5, 7, 10 m ~/h, etc.
The polymer dose r equ i remen t needs to be est imated now. The g raph in Figure 7.7 plots cake dryness agains t polymer dose, for two test ra tes of 7 and 15 m~/h. It is necessary to in terpola te along the 30% line for 11 and 8.5
40.0 I 1
38.0
36.0
34.0
32.0 lid lid �9 30.0 r
Q �9 2 8 . 0
I u 2 6 . 0
24.0
22.0 -
20.0 0.0
I I 1
I �9 I l ,
2 0 4.0 6.0
. '>..
/ /
i
Poo=" Centrate
-" ~
8 .0 10.0 1 2 . 0 14 .0 16 .0
Q~Jg-Vol l O ~ h "~
Figure 7.6. Cake Dryness vs Feed Rate:g-Vol Ratio.
�9
18.0 20.0
312 DS Scaling
m3/h. Ideally more data would be desirable, but from Table A.9 it is known that 31% cake was achieved at 10 m3/h at a polymer dose of 9.5 kg/t. Thus a rough estimate for the dosage on decanters no. 1 and no. 2 would be, respectively, 9 and 6 kg/t db. A further small test might be initiated to obtain more precise estimates. As is often the case, a choice needs to be made between capital and revenue expenditure. A larger and thus a more expensive machine will consume less polymer than the smaller machine.
The pond depth relative to the bowl diameter is larger for no. I decanter and smaller for no. 2. This would suggest there is some performance in hand with the estimate for no. 1 and suggest some caution with the estimate for no. 2. This would bring the performance levels for both closer, and thus the choice would probably be biased towards the smaller of the two.
It remains to check that the conveyor differentials needed for the two scaled-up machines are within the working range of the gearboxes specified. W e t c a k e rate:
OsPs = 50 x 3 /30 x 1.0 tph
Qsps = 5 tph
No. I di f ferent ia l"
N - 5 / 1 . 0 - 5 rpm
38.0
36.0
34.0
32.0
i 30.0 t Ir
~" 28.0 a
a 26.0 (..1
24.0
22.0
20.0 0.00 2.00
1
V
.S 7
4.00
f
. . . . t f
6.00 8.00 10.00
Polymer Dose kg/t db
I
12.00
JeT'm31h ! II 15m3/h
.1 14.00 16.00
Figure 7.7. Cake Dryness vs. Polymer Dose.
No. 1 d i f f e r e n t i a l range:
2 9 0 0 = 0 to = 0 to 1 0 . 9 rpm
267
Calculations and Scaling 313
No. 2 d i f f e r e n t i a l :
N - 5 /2 .0 = 2.5 rpm
No. 2 d i f f e r e n t i a l range:
2 4 0 0 = 0 to = 0 to 9.4 r p m
254
This concludes two very satisfactory scale-ups. The scale-ups have shown that the required dryness can be achieved with both of the two larger machines at the desired capacity. The calculations have shown what conveyor torques and what differentials would be needed on each machine.
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CHAPTER 8
Instrumentation and Control
The first production decanters were virtually devoid of ins t rumenta t ion and control, apart from the main motor starter. Today, ins t rumenta t ion and controls are many [ l ], and can be quite sophisticated. The present tendency is for full automation, to minimise the need for h u m a n intervention, and reduce labour costs. Improved safety standards have encouraged the development of some useful, and reliable, instruments . The development of small, affordable controllers themselves has enabled the introduct ion of some much needed process instruments. Hitherto an expensive process ins t rument could not be justified to be used merely as a monitor.
When a decanter is automated, au tomat ion of a lot of the associated equipment is also necessary, together with interlocking. For instance, it would be inadequate to have a decanter operat ing automatical ly una t tended if failure of the cake off-take system could occur wi thout communica t ion of the fact to the decanter control system.
Figure 8.1 depicts an ins t rument and flow diagram for a decanter plant using flocculant, with al ternative cake discharge flows for thickening and dewatering. It is not possible to cover every eventual i ty with one diagram, but this one covers the majority of usual situations. The ins t rumenta t ion shown is not necessarily always used, but is tha t which the plant engineer would consider useful, were it possible. The equipment that could be controlled automatically, or is controlled in s tandard plants, is marked. Each of these possible instruments will be discussed in turn, after outlining the various modules of a decanter plant.
!
[ ~
6 fell I"+ | [ | + |
~
ljy~ j | I + II
1~--~ (~l Thickening DewateHng
. , . . . ,
Kt? C Control Input E Electrical Amps/Watts L Level n Count P Pressure Q Flow Rate s Speed t Time T Torque �9 Solids Concentration O !Temperature
Fi#ure 8.1. An instrument and.flow alia#ram for a decanter plant.
8.1 Decanter Plant Modules
A fully equipped decanter centrifuge plant will normally have several distinct
modules within it:
�9 the flocculant system; �9 the process slurry feed system; �9 the decanter itself; �9 the centrate off-take system; and �9 the cake discharge system.
The flocculant system for a decanter plant, particularly for the larger plants, is usually supplied as a separate entity, with its own control system. Some of these control systems can be quite sophisticated, with dosing controlled from a feed solids concentra t ion sensor. Nevertheless, there is no reason why this control system could not be coupled into the main control system.
There are a number of types of polymer make-up system. The one represented in Figure 8.1 is the usual dual tank batch make-up system for solid-grade polymers. It basically comprises a powder hopper with a screw feeder, discharging into a stirred vessel. The volume of water is controlled by level probes in this vessel. The contents are stirred for a fixed time, to allow the polymer to dissolve and age to its full potency. After the required ageing time, it is automatical ly transferred to the polymer supply vessel when actuated by a low-level signal from this second tank. The polymer pump is controlled from the decanter control system.
The feed will be supplied from the main plant. This could simply be a tee into a pipeline of the plant, or more usually from a storage tank. A variable speed pump, usually a progressive cavity type, feeds the process slurry to the centrifuge. The rate is fixed manua l ly or by a plant controller.
The decanter system itself hardly needs further description. The main motor and back-drive motors are the main control inputs. Larger decanters may have a separate oil lubrication system for the main bearings, in which oil flows, temperatures and pressures are monitored.
The centrate off-take system is generally a large pipe to drain, or to a receiver vessel. Occasionally the decanter will be fitted with a centripetal or
318 Decanter Plant Modules
skimmer pump, when a pressurised discharge will occur, which may have to be released below the liquid product surface in the receiver, to prevent or reduce foaming. In three-phase decanters a second light phase discharge will be present, the flow of which will also need measuring.
Dewatered cake is often discharged onto a belt conveyor, straight into a hopper, or perhaps into a screw conveyor or elevator. Where decanters employ negative pond operation, ponds deeper than the cake discharge level, unwanted liquid discharge from the cake outlet can occur during start-up. This can produce an unpleasant mess on belts, causing them to slip, and will contaminate the product. This is sometimes prevented using notched weir plates, or special start-up and shut down procedures. Alternatively, devices are fitted under the cake discharge to feed the wet cake back to the feed vessel. These devices could be, alternatively, a flap diverter, or a hopper that is automatically moved under the discharge at start-up. The unwanted liquor discharge is then pumped back to the feed tank. A further alternative is to angle the belt conveyor, such that liquid flows back down the belt into a hopper, while solids convey upwards on the belt. With all these devices some flush may be required after the wash-out has ceased.
Thickened cake discharge can simply be into a hopper which is emptied by a pump actuated by level probes in the hopper. However, modern technology often requires the discharge to be monitored for solids content, if not rate. For this a small stirred buffer tank is used. A sample from this tank is pumped and recycled continuously to provide a continuous sample. The stirred buffer tank is sized to smooth out major fluctuations which can occur in the decanter discharge, due to hold-up in the casing.
8.2 Instrumentation
This section is separated into the various categories of instruments , such as flow meters, solids concentrat ion meters and timers. Lastly, controllers will be
covered.
8.2.1 Flow meters
Flow meters for aqueous slurries are reliable, accurate and seldom require adjustment after initial calibration. Moreover they are amenable for connection to PLCs, computers and controllers. The most common models used on decanter plants are eddy current and ultrasonic type. Flow meters are used on the feed line after the feed pump and similarly on the polymer line.
Flow measurement is employed on the oil lubrication lines, but is usually of the rotameter, or variable orifice type. This means that they are used for indication only, and are not readily coupled into the control system, unless simply as alarm features.
Ideally, a flow meter should be fitted on the thickened cake recycle line. This is because the solids monitor works on the principle that the cake solids concentrat ion is a function of viscosity, which in turn is monitored as a pressure drop when flowing. Thus the flow rate also affects the pressure drop, and therefore must be kept constant . However, often it is found that plant users rely on the constant rate from a metering pump, making periodic adjustments to flow or calibration should the pump wear.
The total flow of thickened cake is usually obtained by calculation, but a check can be made by measur ing the time intervals between discharge of the sump tank. This would be how the oil flow is measured from a three-phase decanter.
If dilution water is used, this is generally measured with a ro tameter variable orifice meter. However if this flow has to be integrated into a control system then an electronic method, as used for the feed, will be necessary.
8.2.2 Solids concentration meters
These monitors tend to be the most expensive instruments , but enable the most sophisticated type of process control. Without them "live" measurement
320 Instrumentation
of, for example, solids recovery, polymer dosage, cake and centrate rates, and product quality would not be possible. Some laboratory analyses take a few hours to perform, by which time the plant could be way off the control desired.
A few companies offer devices that can cont inuously measure solids content of feed flows. Various principles have been used, including the coriolis effect and the use of a radioactive source. The method using a radioactive source has proved reliable, but there is resistance to using it where a watercourse is involved, and moreover there are stringent regulations with regard to the disposal of the instrument once it is at the end of its useful life. Nevertheless the suppliers natural ly offer a comprehensive service. Light reflection or transmission is another method that is in use.
Centrate solids concentrat ion measurement is an important parameter for decanter control. Several such instruments are available to measure in this range. However, one problem presented by centrate from a decanter, on many applications, is the copious production of bubbles or foam in the flow. These bubbles are read, by many instruments, as solids, thus preventing the use of such devices. De-aeration of a sample flow of the centrate has met with a modicum of success. Some decanter manufacturers developing their own ins t rument [2] have obtained more success.
Continuous measurement of solids, or moisture content, of dewatered cake, as far as is known, has not been practised on decanters yet. However, infrared devices, which can measure moisture content of products on conveyor belts, when positioned about 30 cm above the belt. have been reported.
The solids content of the flocculant solution would be a useful parameter in any integrated control system. However, the solids are dissolved, and the concentrat ions involved are a fraction of 1%, with an accuracy requirement probably down to 0.001%. Moreover viscosity of solutions covers a wide range, and is dependent upon a number of other factors, not the least of which is temperature. Therefore, it is not practicable to measure polymer concentrat ion directly. However, there is no reason why the powder feeder could not be calibrated for the powder in use, and its on-time measured. With liquid polymer make-up, the on-time of a calibrated raw liquid polymer pump
would be measured.
8.2.3 Level probes
There is no great need to measure levels in the plant, but merely to have an indication of whether a tank or hopper is empty, full, or in between. This is achieved by conductivity, or sonic, probes. They are fitted to the two polymer tanks, to initiate a new batch make-up, and to actuate transfer before the polymer supply tank empties. Probes could be employed in the polymer powder hopper to guard against running out during operation. Smaller plants will not use powder probes, and rely on a system using several days' supply.
Instrumentation and Control 321
8.2.4 Speed probes
It is part icularly necessary to measure the speed of rotat ion of the decanter bowl and the gearbox pinion shaft. Occasionally a t achometer will be built into the braking device. More generally, bowl and pinion speeds are measured by proximity probes, acting on a protuberance or castellation, on a spigot, hub or shaft.
The speeds of the feed and polymer pump, and also the cake sample pump, are useful though not absolutely necessary to measure, as comparing this speed with a calibration speed will indicate the onset of wear.
Measuring the speed of the polymer screw feeder has already been mentioned.
Another useful speed monitor would be on the solids conveyor driven shaft. All that is needed here is an indication tha t the shaft has stopped, for instance if the belt should break. In a non-at tended plant it is essential to know if the off-take system ceases to function, so that the feed may be arrested.
It is worth noting that the majori ty of downt ime of a decanter plant is caused by failures in ancillary equipment, rather than the decanter itself.
8.2.5 Temperature probes
The temperature of the lubricating oil from the bearings is usually measured with thermocouples. The temperature of the feed is only measured if this is an operating parameter . The temperature of motor windings are usually monitored by thermistors, connected to a safety cut-out system in the motor control gear. Obtaining a direct reading of motor winding tempera ture would be unusual .
8.2.6 Torque measurement
Conveyor torque today is an essential part of decanter control. However, direct reading of conveyor torque is very difficult to achieve. Even direct reading of pinion torque is difficult, but could be done using strain gauges on the pinion shaft. However, the most usual method is to use a calibration of the braking device. The control device for the brake will give a read-out, on request, of the braking torque.
8.2.7 Timers
Timers are integral parts of some of the control systems. They are used in the starter of the main motor, to switch from star to delta operation. They are used in control systems, for the sequential start-up and shut-down of ancillary equipment. Timers are used for the ageing of the polymer, and the on-time of
322 Instrumentation
the feeder. A timer would be used to measure the fill time of the cake sump, to check cake rate.
However, a l though run-t ime meters can be fitted to most motors, this is usually, if at all, only on the main motor.
8.2.8 Counters
Counters are used to count batches of polymer made up, to keep an overall check on usage. Cumulative flow is often found in electronic flow meters, to keep account of total flows through the plant.
8.2.9 Electrical meters
The current to the main motor is often monitored to prevent overloading. It also gives an indication of the power being consumed, a l though a better device for this is the wattmeter .
8.2.10 Bearing monitors
Interest is now being placed in instruments that monitor the health of bearings in operation. Premature failure can be predicted before expensive damage occurs. These ins t ruments are not yet in wide use.
8.3 Controlled Equipment
The control strategy for a decanter plant often will hinge on experience, the user's requirements, what is available and the extent of control required. One of the main decisions to make is regarding the flocculant control. The option for flocculant control is whether to have a feed-forward control, requiring a feed solids meter, or whether to have feedback control using a centrate solids monitor. With feed-forward control, the flocculant rate is modulated according to the level of solids in the feed. The ratio of flocculant to feed solids may have to be trimmed occasionally should the quality of the feed vary. With feedback control the control performance is independent of feed quality. Nevertheless some centrate monitors can be badly affected by aeration and foam which can occur with some polymers and feeds. The extent of the sophistication of the control will depend upon how much of the plant is required to be incorporated into the decanter system. The good functioning of feed tank levels, off-take pumps and conveyors all may need to be brought into the strategy with appropriate interlock controls.
To devise a control strategy for a decanter plant, it is necessary to know what devices are available to the controller. These maybe on/off devices, or devices which can be varied in output by the controller.
8.3.1 On/off devices
These will include the stirrers in the polymer system and thickened cake sump. Also included will be complete module systems, such as the polymer system, the oil lubrication system, and perhaps the cake off-take system. The decanter main motor is also a controlled on/off device, al though a variable speed main motor can be employed.
The pumps actuated by the level probes on the polymer system, and the sump discharge, are also on/off devices, as are the belt conveyor, the cake diverter, and the polymer screw feeder.
In a completely manual plant, even the feed and polymer pumps could be on/off, and merely controlled on or offby safety interlocks.
324 Controlled Equipment
8.3.2 Variable output devices
These are mainly the feed and polymer pumps, and the decanter brake torque or speed. However, in special cases, the actual bowl speed could be a part of a control strategy. The pond depth itself, using the inflatable dam, could be used in a thickening control strategy.
The polymer feeder could be used in a control system, if wide ranges of feed concentrat ion were to be anticipated. As far as is known, this has not yet been used.
8.4 Controllers
Modern electronic technology has provided industry with a wide choice of small, user-friendly, cost-effective controllers, with proportional integral and derivative (PID) control action. These can be used individually on the input flows, or integrated into a master controller. It would not be unusual to have one each on the feed and polymer pumps. A signal from the flow meter would be supplied to the controller, which would adjust the speed of the pump to give a flow agreeing with the set point entered by the operator. The set point could be set alternatively by a master controller.
A separate controller is supplied with each polymer make-up system. When energised, it will, according to inputted set points, control the on-times of make-up water and polymer feeder. It will control the ageing time in the make- up tank, when the stirrer is switched on and when switched off. Transfer of aged polymer will only be allowed when the supply tank is below a certain level, when the transfer pump is energised, and afterwards de-energised. This is a simple but very effective system. There are some variations from manufac ture r to manufacturer . The polymer is very hygroscopic and difficult to dissolve, and if not handled properly, can create an inordinate mess. One manufac ture r supplies an air blower to transfer the dry powder into a cyclone wett ing chamber, to minimise the onset of glue-like deposits in the lines.
The polymer control system can be augmented with a feed solids meter, to give "feed-forward" control, fixing the kg/t db polymer usage to an operator set point.
The main motor controller is a separate controller, and depends upon the type of installation and motor. The motor could be AC, DC or inverter type. Rarely, it could be a hydraulic motor. The starter could be DOL (direct-on- line), particularly if a fluid coupling is fitted, it could be a soft-start inverter system, or a DC system. With an inverter system thc back-drive, also an inverter type, could be connected th rough the DC bus to allow power regeneration. The starter itself could be actuated by a separate master system. Undoubtedly there will be interlocks with the starter, to cause it to de-energise with certain scenarios.
All the controllers are important, but the most important controller for the process is the one controlling the gearbox pinion shaft brake. This PLC
326 Controllers
(programmable logic computer) will be required to control the brake, either to give a set conveyor differential, or a set output torque. Whilst this duty, as specified, seems simple, the overall duty expected makes it, internally, quite complex. In certain circumstances it is required to control speeds close to zero and even to reverse speed. It is expected to be suitable for the complete range of a manufacturer ' s decanters, and yet expected to control each within safe limits. Moreover it needs to be appreciated that reducing output torque allows a lower differential, which increases torque! Thus to allow a higher conveyor torque, the controller effectively has to reduce its output torque. Nevertheless, excellent controls have been established on several thousands of installations.
A good brake controller will be required to indicate:
�9 Bowl speed: �9 Conveyor differential speed: �9 Brake or conveyor torque; �9 Torquehigh/ low alarm; �9 Differentialhigh/low alarm: �9 Status (start-up or running); �9 Mode of control (torque/differential): �9 Set point.
Access is needed to the operating parameters, with an encrypted code to prevent unauthorised tampering. One such controller is shown in Figure 8.2.
Only after using such an instrument can the extent of the needs for such a device be appreciated.
The operating parameters may include:
�9 Entry code; �9 Modes permitted: �9 Upper and lower alarm limits; �9 Set points: �9 Setpointl imits; �9 PID settings: �9 Secondary PID settings for two-stage control; �9 Sense of alarms (normally on or off): �9 Gearbox ratio; �9 Pulses per revolution for probes; �9 Pulley ratios for speed recalculations; �9 Control ramp rate; �9 Calibration of external signals; �9 Parameters for transmission of data; �9 Parameters for computer communicat ion; �9 Brake torque/current calibration reference.
Instrumentation and Control 327
This is a very brief synopsis of wha t could be 60 or more separa te
parameters .
0 0 @ 0 0 0 0 0 0 @ 0 0 0 0 0 0 ~.: Alfa Laval
Figure 8.2. An Alfa Laval Automatic Backdrive Controller (,4 BC).
8.5 Integrated Controller
With separate controllers already provided to control separate functions of the plant, it is an obvious next step to integrate them into one master controller, or to supply a master controller to supervise the individual controllers. This is being demanded increasingly. Some large plants demand a central remote control room, with mimic diagrams, all controlled by one central, large industrial computer.
Some decanter manufacturers already have their own integrated controller, all with varying degrees of sophistication. Some of the duties of an integrated controller are described below.
All the signals available, shown in the diagram in Figure 8.1, need to be continuously fed to the controller and converted to digital figures. It should be possible to display any of these figures on request.
The figures then need to be processed, according to the relationships in Chapter 4, to provide figures of:
�9 Solids recovery; �9 Polymer dosage: �9 Torque/volume; �9 Feed rate/g-volume; �9 Centrate rate; �9 Cake rate; �9 Cake rate/differential; �9 Power usage on the decanter and the total.
These should all be displayable. A cost display should be possible, once application itemised cost data are
inputted. The data required for an effluent would include the cost of power at various times of the day, cost of effluent disposal, polymer cost, and cake disposal cost. Other costs that may be included would be, for example, amortisation of capital. The processor would then work out the plant running
costs for display, or periodic print out. The controller processor would have in-built control algorithms for the
plant manager to select. Control could be to minimise overall cost, maximise
Instrumentation and Control 329
dryness, minimise revenue costs, or maximise throughput . It could also be on the basis of keeping the feed tank down to a certain level. Priorities would need to be set for the various performance factors, such as solids recovery, dryness, cost and throughput . Maximum and min imum levels for each would need to be set.
The controller would be set on a cont inuous loop to conduct the calculations, perform control adjustments, display and if necessary print results, and act as an annuncia tor for alarms and maintenance schedules.
The control method could be a simple "hill climbing" technique where small adjustments of one variable at a time are made, and performance checked. The adjustment continues so long as performance improves and a step back is made once a deterioration is detected. The next variable is then adjusted in the same way. Adjustment steps could then be reduced once all variables have been used. The process is then repeated.
An alternative control method, which is a newly developed technique, uses a technique called fuzzy logic [ 3 ].
8.6 CIP
The equipment used for the CIP feature is described in Section 2 .4 .14. A small PLC, or an adjunct to one of the s tandard controllers, is required to supervise the CIP operation. An operator giving the start command to the PLC or equivalent will initiate the CIP sequence of events. The feed will be stopped. Then the main motor and back-drive system will be de-energised, and allowed to coast down to the required CIP speed, when the main drive donkey motor will be energised and take over to rotate the bowl at such a speed as to generate slightly less than lg (about 70% of lg). The more sophisticated systems will also have a donkey motor to rotate the gearbox pinion.
Timers will control the duration of the low-speed running and other timers will open valves to admit cleaning fluid into the bowl and into the spray bars on the casing. Some systems will periodically reverse the back-drive donkey motor to reverse the conveyor differential. This feature must be used with caution, as reversing the scrolling could jam solids between the front bowl hub and the conveyor, and ult imately bend conveyor flights. The program in the PLC will dictate the duration of the CIP, the durat ion of each phase, and how many times the conveyor, and if necessary the bowl, are reversed. The program will also dictate when, and for how long, the cleaning tluids are
applied. The CIP feature is a valuable asset in food and pharmaceut ica l processing.
The ability to keep the decanter clean and hygienic, wi thout the need to dismantle it, has enabled the use of decanters in processes h i ther to impossible. Decanters can be run for many months without dismantling, wi th acceptable
standards of cleanliness. With suitable designs of decanter, the CIP procedure can be used, where
necessary, for sterilisation, instead of, or with, chemical cleaning.
8.7 References
l W Leung, P Wardell, L Hales. (Baker Hughes Inc.) Method and apparatus for controlling and monitoring continuous feed centrifuge. US Patent 5948271, 1 December 1995
2 J G Joyce (Alfa Laval) Turbidity measurement. US Patent 5453832, 6 March 1991
3 C yon Altrock, B Krause. Fuzzy logic application note: optimization of a water treatment system, http://www.fuzzytech.com/e.a.dek.htm
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CHAPTER 9
The Decanter Market
In a total world market for liquid/solid separat.iriri eqiiiprnent. of' about $ 6 billioris (coveririg ;ill appliciitions, domestic and institutional as well a s hdUStrial), the decanter has come to be an important cornponeat. with ;t
market share of ahoui 1 0 ' K (11' that figurc. This chapter looks briefly at. lhe market hi- decanters, tiow i l is made up, and how it is expeckd I,{) develop.
9.1 Market Characteristics
The decanter centrifuge is an impor tant processing tool, but is by no means cheap, so the decision to invest in a new decanter is one that has to be taken with care. The market is character ised by the presence in it of a few large suppliers, with many years of experience and with wide ranges of types of decanter available. There is then a group of smaller, general suppliers, plus a handful of niche market suppliers (mostly to olive oil production and similar applications). There is certainly enough experience available in the market place, to enable any potential purchaser to obtain satisfactory quotat ions for a new machine from a number of competitive suppliers.
The purchase of a new decanter is very strongly influenced by the intended process duty, and almost all such purchases are made only after careful analysis by the supplier of the required performance, and. possibly, after some kind of trial with the customer 's process liquor. Trials may involve the installation of a temporary test decanter, and ancil lary plant, as a static or mobile rig. Such a test may be for an extensive period, to cover all the likely variations in process slurry characteristics. The test rig could be a full size, pilot scale, or laboratory installation. Experience of a part icular application by the supplier makes the selection process one which can be approached with confidence, and the potential purchaser would do well to enquire as to the level of relevant experience available.
The major manufacturers have sales or subsidiary company offices in most, if not all, the larger industrial countries, and local to many of the large decanter markets. These offices are usually staffed with very competent sales engineers, able to convert the supplier's weal th of experience into a preliminary quotation quite easily. Such a start must usually then be followed
by the trial process already mentioned.
9.2 Market Trends
As is shown in some detail in this handbook, the decanter is an extremely versatile processing device, by virtue of the many different items of its make- up that can be changed to suit the process needs. In this way, the decanter has been able to meet a wide range of process challenges over the past half- century.
The main trend in the market place can thus be expected to be a steady improvement in detailed design, to enable the decanter to meet further such challenges. These improvements will spread to all the main suppliers, so that the choice ofdecanter source will still remain wide.
The major application growth will continue to be in the processing of waste slurries, and this duty requires as inexpensive a machine as possible, a l though coupled with quite advanced specifications, in order to achieve high dryness figures in the discharged solids.
The trends identitied in Chapters 1 and 2 will impact on the market, but the major marketing input continues to be to let the world of the process industries know what a useful thing the decanter is. and how it can solve so many liquid/solid processing problems.
9.3 Market Size Estimates
The estimation of the size of the decanter market is beset by the usual problems faced by any at tempt to enumera te a market: definition of scope and avoidance of double count ing being two of the most difficult. Currency variations can have a marked effect on size estimation, especially where historical data are being extrapolated.
Market size est imations may be approached from two directions: top-down and bottom-up.
The downwards approach starts with nat ional or internat ional data, for trade and production, and uses these to derive components of the market. This method is made difficult by the lack of common identity among categories of data, and by the omission, certainly from national production data, of most small companies. The existence of a single supplier in a nat ional market may also be a sufficient reason to omit the figures from published statistics.
The upwards approach starts with individual components of the part icular market, and aggregates them to arrive at an overall figure. These components may be the sales into particular end-uses, or the sales by individual manufacturers . It is in this method that the problems of scope and double counting are most likely to occur. Many companies, for instance, do not differentiate between machine-only sales, and all of the ancillary work that is done to make up a final sales contract, or of the size of after-sales work.
The result of the inadequacies of each of these two approaches is that both have to be used together, to derive an estimate that appears to satisfy both. There may then be, as is the case with the decanter, other published market size estimates, which can be used to corroborate the results of the direct analyses. These published data rarely agree very closely, once (and if) a common basis can be established, but they do give outer limits to a calculated figure, and present some confidence as to the results of the work.
9.3.1 Overall decanter market size
By means, then, of the methods just outlined, a total world market for the decanter centrifuge has been derived of $625 millions for 2000, at a mid-year value of the US dollar. This figure has a probable accuracy of + 10%.
it relates to the final salc to thc end-user. at the pricc paid by that customer: i t covers the sale or whole decanlers only, :ind r i o t any aft.er-s;ilrs work; i t covers f.he sale a1. the i.irne of t h e whole machine of ;iny si;jnd;ird supply of sp:ire parts, but not of any spares for that machine said later: i d
it ex~liides ill1 xiditional equipment sold with thc dccantcr that is not necessary fur the safe and cficient operatioil o r the riiacliirw.
4
This salcs valuc corresponds to a tigurc ir i the regioii o f 1500 10 .3000 Ibr I he number of decantercelltrifuges to he sold in 20( ' )0 .
It is expected that the decaliter rnarkei, which hiis hcen growing qu i te strongly in size sirice the eiid o f t he recession o l t h c early 1 Y Y O s , will continue t h i s growth p;ittrrri, Over the next tive pears. indccd. t l ~ c markct is t-xprt.tet1 tu grow at brlwyerii 4 iind 4.5% pcr aiiiiuiii (i.c. cnmbrtably iri cixrt?ss ol' thc expected iiicreasr in gross dnint?stic product tigurcs).
3 j . l ' i L for all water arid w;ir;te water t rea tment . industrial as M C I I a s rri i i n I ci pal : (1. 3% lor fuel malt'rral extraction atid processing; 19.1 'XI for food arid b rvwigc proccssing:
4 1 3.4'%, for minerah arid hulk inorganic chcrnicala: I 1 0.4'K for h l h organic chcinicals and petrochemicals: I 8.i'%, fur finechuinirals and pharmaceutical?; iind
a 7.[1'% for other applications.
338 Market Size Estimates
9.3.4 Suppliers' market shares
The suppliers of decanters to the world market are mentioned in Chapter 1, which includes a list of most of the manufac turers known to be producing decanters in 2000. If a l lowance is made for the companies not listed in Chapter 1 (believed to be all small ones), then the major holdings of marke t share are:
�9 36.8% by Alfa Laval and Tomoe; �9 16.8% by Baker Process (Bird Machine and Bird Humboldt); �9 9 .6%byFlot tweg; �9 8.8% by Westfalia; and �9 6.4% by Pieralisi.
Shares of between 1.5 and 3.0~ are held by Broadbent (with Tanabe), Guinard, and Siebtechnik. and of between 0.5 and 1.5~ by Amenduni , Centriquip, Centrisys. Hiller, Hutchison-Hayes. Noxon, and Pennwal t India. This leaves a market share of 6.1% held by the other, unspecified companies. It can be seen that the five largest companies hold almost four-fifths of the total market.
CHAPTER 10
Suppliers' Data
This chapter lists the main rnari~1I'a~:~i~rt:rs ofd(:r:arllers, together with details of thcir company structiirc and of their ranges of decanter centrifiige. This is lint an cxhaustivr list. but it includes data alrcady in the public domain, issued by manufacturers in their brochures. and augmented by data supplied by some ofthc companies Ibr thc purposcs of'this book.
'L'hc coveragc hcre is intcndcd to be that of all of the main manufacturers. plus as ~iiaiij' othcrs a s could bc located. 1Pcit each r i ian~ifa~turer are given the sii l icnl r i1cl .s i i l>(>11i i1.s Iieatiq~iiirters irrirl ut ticr ilrlrlresses, plus its mariufat:tutirig rurlpci. arid othcr iriforrn;ilion d u s t : to t h e rtwdtir. lktails ofits drcarilcr rriodels art! I hrri t i~hulu~et l .
'I'his iriforniu~ivri is prvvirled lu enitble the reader to develop some parnmctcrs ofchoicc when a ncw purchase of zi decanter is to be undertaker]. The data glvcn should not, howcwr , bc uscd Tor design iiiid spec-ilicaliori purposes. bu t niorc for initial studies a s t o what c o i ~ l d lie p s s i t ~ l r . o r frir corn par is(> n stud i cs. F i 11 a 1 rcc onim e 11 d a t i o 11 s s 11 o u Id ;I 1 w ;I y s hr! so 11 g h t from thc prcftrrcd suppliers,
Thc ent.rics arc in :Ilph~ibetic:il nrrlcr, and no attempt has been rnadc by the authors tu be rritriclivc in any cntry. IJnder the heading "company uwnership", rriutition is rn:itic! of m:ijor clwncrr;lIip by another soriipariy. or 01 thc cxistonce of major cqiiity holders. Otherwisc. ownership is assuo~ed t.o hr private o r public: sharc ownership. according to t h t b type oTc.orrlp:iny.
khtrics undcr "othcr main biisiacsscs" refer t,o oI,}ier rjon-dscanter aotivitirs ofthc riameti c.ornpilny. whilc "othcr company c'oriricctioris" refbr to business associations spccific l o the decanter husiriess.
The data given under thc hcading "decailler sales" art! eithttr siipplicd by t.hr ~niin~Il'ilct1irer in qiiestion. or cstimated by t h y authors. Set. Chapter 9 lor discussion on market size estimation.
In the model tabulation. all combinations of (iritcrrial) howl diameter and length tire includcd ;IS far as possible. ' Ihe column headed "howl length" rcfcrs to the cylindrical bowl lerigl.11 or ihe sedimentation zonc. whitc the
340 Suppliers'Data
"total inside length" includes the beach, a length for the cake discharge, and the screen section in screen-bowl machines.
The column headed "beach dia" refers to the diameter at the cake discharge. The "total liquid volume" is the bowl's liquid holding capacity with the pond set at neutral. The maximum bowl speed tabulated is that quoted by the manufacturer for a maximum process density of 1.2 kg/1, unless otherwise stated.
The location of the feed zone dictates the clarifying length. Often this is located at the foot of the beach, but this is by no means universal and is seldom so on co-current designs. This feature, together with the design and location of baffles and restrictions, is considered an intimate design detail by most manufacturers and is only known to the users after purchase.
The ranges of gearbox ratios and torque ratings are those that are known to have been used. or which have been quoted by the manufacturer. The maximum and minimum ratios do not necessarily correspond to the maximum and minimum torque ratings, respectively. A specific ratio and torque within the range cannot arbitrarily be quoted. For precise figures the supplier must be approached directly.
In the column "design", "A" corresponds to counter-current operation, while "B" signifies co-current: "H" refers to horizontal mounting, while "V" is vertical. A and B together, or H and V together, signify that the size is available in both variants.
A reasonably complete set of data is given for the range of decanters made by Alfa Laval, which company decided to support the production of this handbook by making free of its design data, so as to enable the reader to put much of the body of the text into some real context. Other manufacturers did not feel able to support this gesture, and so the data supplied are considerably fewer, but at least give some idea of the relative strengths of the production
ranges.
Alfa Laval, Sweden
Name of company Alfa Laval Separation AB
Headquarters address Hans Stahles vfig, S- 14 7 8() Tumba, Sweden Phone: (+46)8 5306 5000: Fax: (+46) 8 5303 3589
UK address Doman Road, Camberley, Surrey GU 15 3DN Phone: 01276 6 3 3 8 3 : F a x : 0 1 2 7 6 61088
US address 955 Mearns Road, Warminster, PA 18974, USA Phone: (+ 1) 215-443 4000: Fax: (+ 1) 21:5-443 4112
c'oiii p ii n y CI IV n er s ti i p hl ;i j or i t J' h o Id i n g h y 1 n d I I st r i K ;i p i t a I ( S w cdcn (Switzttrl;ind 1
'I' y pc of c 0 m pa I? 1' S pec i t i I i s t cen t r i f'u ge m a nu Ca c t u re r . Th c S har plcs Corpora t i c m now par 1 of
Alt'a Laval, was cine of the world's first f w o major manuhaclurcrs uf d cca 11 t c'r s
IXsc stack ccntrifugcs, tuhular bowl ccntrifuE,cs. filtration cciitrifugcs Alfa Laval company also has 'I'hermal arid I k w divisions
Associat.ecl manufacturer: 'Iomoe. 'I'okyo. J apari
h ti la 11 cc' by Te t rii La I: a 1
Othcr main busiiicsscs
( ) t h t'r corn pa I I y con 11 ec t io 11s
k c a r i t e r iriariufacturing sites C;irnhPrley. U K Coprn tiagen (Suhorg). llerlmatk W ;i rm i 11 s I E r 1. J S A Pune, India
Decanter range Over h O models. i1viiil:ible in range or v;iriarits. with rhree-phase and scrccn-bowl designs. plus vertical tiiachincs
Over 1000 uni t s sold a n n u a l l y . Sales in region of$130 illillions i'orccasl for 3000. inchding I h o s r suld by 'I'omoe.
Very wide range nf applications. strong in wastc treatnicnt
Dccantcr salcs
Sales specialisatioti
Suppliers'Data 343
Machine details Model ref 8o~,1
dta , (mm)
I 152 2 152 3 152 4 18-1 5 230 6 250 7 249 8 310 9 353 I0 353 I1 353 12 353 13 353 14 353 i 5 356 16 356 17 356 18 356 19 356 20 356 21 356 22 356 23 356 24 425 25 425 2e 27 28
425 425 425
2~ 450 .3(3 450 31 450 32 4�82 33 451) 34 450 35 4")8 30 418 37 480 38 480 30 508 40 50g 41 549 42 575 43 575 4.4 575 45 010 46 610 47 670 48 620 49 6"10 50 610 51 615 52 615 53 635 54 ~35 55 635 56 73 .) 57 737 58 73") 59 73 "~ 60 73 "~ 61 889 62 1016 63 1016 64 1016 65 l l) lb 60 1016 67 12"0
Bowl Beach a n g ~ 'Beach dta : Total ms)de Tota'l bqu:d length' {deg ) (ram) length (nun) vol ' ( ! ) ~mm) . . . . . . .
10 (I 110 353 2 I00 112 353 2 10 0 119 50~ 3 10 0 121 430 5 8 0 ! 42 7 0 0 12 10 0 180 550 tO 8 2 102 i 275 3h 8 2 12.8 1627 8,2 8 5 202 860 "~ 5 20 0 198 86O 47 8 5 202 1160 55 20 0 198 1160 67 8 5 202 1460 74 =-~30 108 87 1460 10 0 264 572 I 8 I0 0 241 572 20 I0 o 264 704 28 I011 241 794 32 I 0 0 264 1264 49 10 0 241 1264 57 17 0 212 12o4 ~0 5 3 254 1264 47 100 200 12o4 68 I 0 0 264 1264 86 II 0 230 1204 98 15 0 108 1264 109 I00 2O4 1772 130 15 O 208 1772 104, g5 266 !, , t~ I I I 20 0 258 1460 137 21) t'q 258 IglO 1~5 20 0 258 2360 23~ 8 5 266 1910 15 n 8 5 266 2~60 2O4 B ") 274 1984 l ~7 8 2 274 2448 246 10 0 203 2035 212 20 0 263 2t)'~5 233 I0 0 381 i186 93 I0 0 3gl 1946 151 It, 0 349 2266 " t2 10 0 326 2 ~ 0 380 20 0 326 2440 ~,~15 20 3 326 3015 "86 l0 O 381 1930 2")8 15 0 285 1030 358
1850 8 0 17~ 247 8 0 375 7640 4(~5 16 0 352 1845 3%4 16 0 352 2615 503 I(] 0 483 16-'6 187 I I 0 381 1676 2.58 10 0 483 2286 272 17 0 391 2256 .~2 IC o 483 35O 100
2946 55o 2337 365
tO 0 483 2397 46l 15 o 343 2737 62t IO o 483 30.18 634 15 0 343 30,tg go I 113 0 572 3302 10(34 I0 Ot "~556 1104
14~6 762
IC o 610 3556 15 (1 406 3556 1934 1(2 0 610 44 ;0 1900 15 0 400 4470 2255 -)
. . . . . . . [5 o _ 50~, __5334 . . . . ~'~ 2')
Max bowl Power load Gearbox mu0 Gearbox torque speed ffpm) range tkW) range range (kNm}
6000 95-98 tq 2-0 9 2 6O00 98 0
80(IO 95 0 o bOO0 95 0 9 62O0 25-55 (19 �9 tO00 94 0 9 8500 92 I 5 76r 92 1 5 4000 57-159 2 5-3 5 4000 57-159 2 5-3.5 4000 57-159 2 5-3.5 4000 57-159 2 5-3 5 4(~X) 57-159 2 5-3 5 4000 57-159 2 5-3 5 4000 52-125 8-2 7 4000 52-125 8-2 7 4000 52-125 B-2,7 4O00 52-i~ ,'1-2 7 4flO0 52-125 B-2 7 4000 52-125 8-2 7 4000 52- I LS ~-2 7 40(~ 52-]25 .R-2 7 4000 52-125 fl-2 7 31250 52-125 8-2 7 3,b50 52-125 8-2 7 3~50 52-125 B-2 7 3650 52-125 8--2 7 3050 52-I 2b 8-2 7 3250 57 3250 57-159 5--6 0 3250 57-159 2.5-0.0 2900 57 ISq 2 5 - 0 0 3250 57-150 2 5-6 0 2qO0 169 6 0 2")00 57-50 t 5-B 0 2700 57- 59 t .5-g 0 3650 57-380 ~, 5 - I0 0 �9 M1511 9"I -380 "LS-10 0 3130 49-98 10 2 3350 49-98 10 2 I050 91,), 10 2900 207 380 I0 0--i6 0 2000 267 380 I0 0 16 0 2t,hO 2127- %36 16 O- 16 0 285O 49 98 I0 2 2g50 49--Q~ IC 27,~; 4(i 7 0 370(I 45 7 0 270O 45 7.0 27(R) 45 7 o %(I~K) 48--325 20, 0--24 0 3000 48-325 2t} (I.-24 U 3000 48-325 20 0-24 0 30[')0 48-'~25 20, 0--24 0 2700 48-325 20 0--24 0 2700 80 34 0 _'27~1 48--325 2C o. 24 O 271g) 48-325 20 (I,-24 0 2800 48 325 20 0--24 0 ?6(141 48- 325 2(3 0--24 t) 2150 81~95 20 .)-.3-1 0 2(hX) 't0--95 2C 4-34 0 20:kq 47-80 20, &-3.4 0 20LX) 80-95 2G 4-34 0 23',,)0 60.--484 ~,4 0 56 6 21',)0 4?-95 20 4 .,:')o 47-o5 . . . . . . . ?n _4 . . . .
Mac~nc desqztr
A/H MH A/a,' A/H MH
A/H MH A/H A/H A/H N H A/H A/H A/H A/H A/H MH/V A/H A/H/V A/H A/H A/H
A/H ~ H A/H A/H A/H AJH A/)! A~H A/H A/H
A/H A/H A/H A/H A/H
A/H A/H ~ H
.VII A/14 ndH A/lt adH
A/H~" A/H A/H A/ll A/WV Jdtt
MH All! A/tt A/H A/I! A/H A/It g/H A,'tt
~H .....
In this. and all ()tiler tables of data in Chapter 1 (). the following notes apply: 1. The column headed "bowl length" refers to the internal, cylindrical bowl length, of the
sedimentation zone. while the "total inside length" includes the beach, a length for the cake discharge, and the screen section in screen-bowl machines.
2. The column headed "beach dia" refers to the diameter at the cake discharge.
~. The "total liquid volume" is the bowl's liquid holding capacity with the pond set at neutral. 4. In the column "Design". "A" corresponds to counter-current operation, while "B" signifies
co-current: "H" refers to horizontal mounting, while "V" is vertical. A and B together, or H and V together, signify that the size is avai lable in both variants .
344 Sz~ppliers' Data
Amenduni, Italy
Name of company Amenduni Nicola Srl
Headquarters address Via delle Mimose 3. Z I, 1-70026 Modugno (BA), Italy Phone: (+39) 080 531 4910: Fax: (+39)080 531 4920
UK address None
US address None
Company ownership Private company
Type of company Specialist equipment manufacturer
Other main businesses Other company connections
None Decanter manufacturing sites
Modugno Decanter range
Four models with two diameters Decanter sales
Sales in region of $4 millions forecast for 20()() Sales specialisation
Olive oil extraction Machine details
Mndel ret. Bowl dla. Bowl Beach Beach Total nu (mm j length ~ an~ie dta." mstde
,ram) (dog. t (mn)) length [nun)
Total hqmd Max bowl Power load Gearbox Gearbox Machine vu i 'q ] ~ .~peed qrpnn~range (kW') rauo t~lrque des0gn ~
range rangc (kNm)
I 9(~) .393 1()41 2. 9,01 430 14()() 3 992/C "430 1670 4 ~)21S 450 1930
A,q-I M H M H .AdI-I
Bird Humboldt, Germany
Name of company Bird Humboldt
Headquarters address Dillenburger Strasse 100, D- 5110 5 K61n. Germany Phone: (+49) 0221 822 6500; Fax: (+49) 0221 822 6169
UK address Euroby Ltd
Suppliers'Data 345
31 High Street. Wootton Bassett, Wilts SN4 7AF Phone: 01793 848104 : Fax: 01793 8 4 8 1 0 4
US address Bird Machine Co Inc 1 O0 Neponset Street, South Walpole, MA 020 71 Phone: (+1) 508-668 O400: Fax: (+1) 508-668 6855
Company ownership Subsidiary of Baker Hughes group, as part of Baker Process
Type of company Specialist equipment manufac tu re r
Other main businesses Waste water t rea tment systems, thermal sludge t rea tment ("Centr idry" disc and rotary driers), filter presses, gravity separat ion systems
Other company connections Within Baker Process, sister company to Bird Machine, South Walpole, USA
Decanter manufac tur ing sites Cologne, Germany
Decanter range The range of supply is that of the KHD/Humboldt Wedag company. acquired by Bird Machine Co. There are a dozen basic models, available in several different bowl lengths, with co-current as well as counter -current tlow, three-phase and screen-bowl versions. Also includes "Censor" Twin beach machine, for separation of solids by density
Decanter sales For 2()()() these are estimated at $1 ()5 millions, to cover all Baker Process decanter sales
Sales specialisation
Very wide range of applications, special success in waste t rea tment Machine details
Mendel Bowl d,a Bowl Bcach Bench Total Tota l hqu ,d Max bowl Power ioa-d Gea rbox G e a r b o x Macn ,ne ref n,~ (ram, length' angle din. z Inside ~,ol. ~ {I) speed ~tpm~range ( k W ) rauo Iorque des lgn a
(ram) ( d e g ) (ram) length range rdnge
t mm~ ., ~ kNm)
I ] 5o A/El " 2'q) 22 A/H
350 I (~51) ~4(~) 15/3. t} .a~tl-I
�9 ~ 451) I ~50 3000 18.5/'~5 M H
5 530 1590 2r 22/75 M H
5a 530 2 2 8 0 2200 30/75 A/H
6(~) I ~2 M H
7 ~50 22~0 2500 451L6~) AJH
7a 650 2800 2300 551160 A/H
,~ 750 A / H
q 76O 250 A/H
I 0 900 300 A/I-!
1 i 1000 400 A / H I 2 1 I IX~ 40(I A/I-!
I ~ t 21)0 400 A/I'I
I..4 141~t . 500 A / H . . . . . .
346 Suppliers'Data
Broadbent, UK
Name of company Thomas Broadbent & Sons Ltd
Headquarters address Queen Street South. Huddersfield, W Yorks HD ] 3EA Phone: O1484 422111; Fax: 01484 516142
UK address As above
US address Broadbent Incorporated PO Box 18 5249, 2684 Gravel Drive. Fort Worth, TX 76118 Phone: (+1) 817-595 2411; Fax: (+1) 817-595 0415
Company ownership Publically owned
Type of company Specialist equipment manufacturing company
Other main businesses Batch and continuous basket centrifuges: laundry equipment
Other company connections Associated manufacturer: Tanabe. Tokyo, Japan
Decanter manufactur ing sites Huddersfield, UK
Decanter range Standard machines including three-phase separators, plus screen-bowl designs Standard machines as 17 models, in 1 () different diameters
Decanter sales Sales in region of $20 millions forecast for 2()00, including those sold by Tanabe
Sales specialisation Company history heavily sugar oriented. Decanters sold strongly into the chemicals sector, and for waste treatment, especially in flue-gas desulphurisation processes
Suppliers'Data 347
Machine details
Figure 10.2. A Broadbent centrifzLqe installation.
Model Bowl d0a ref no (ram)
Bog l Beach Beach Total length I anglc dna : ms]de (,rim) qdeg.) ( ram) length
(ram)
I t 5 0 3 (x)
2 225 7(~)
450 7(10
4 45o i I00
5 5sO ! 375
6 6tW) 9.51) i btW) 15o0
t~(x} 2100 9 750 I ~75
I(I "'~(] 220t)
I I o 0 0 I ),ILK)
12 othl 2500
I ~ l(kO0 1500
14 1200 21013
15 120O 30(0")
16 1200 36(K) 17 I 4(X) 18f10
Tota l hquid Max bowl Power load Gearbox Gcarbox Machine vol ' ( I ) speed (rpm)range tkW) ratm torquc destgn 4
range range (kNm)
5/7 5
7 5115
15130
15130
30/60
~0/60
30/t)()
10/75
%011 I(1
55/ I I(.) r
] 10 / ! 5(} 75~00
1101350
I I (11350 1101350
I I 0/350
Centriquip, UK
Name of company Centriquip Limited
Headquarters address Derby Road. Clay Cross. Derbys $4 5 9AG Phone: 01246 252600; Fax: 01246 252601
UK address As above
US address
348 Suppliers' Data
None Company ownership
Private company Type of company
Specialist decanter manufacturer Other main businesses
None Other company connections
None Decanter manufacturing sites
Clay Cross, UK Decanter range
A range of models, based on three sizes of machine, with variable specifications, to suit c u s t o m e r ~ i n d u s t r y requirements
Decanter sales Sales in region of $6 millions forecast for 2000
Sales specialisation Waste sludge treatment for industrial and municipal applications
Figure 10. 3. A Cetztriquip centrilzlge itlstallation.
Suppliers'Data 349
Machine details
Model rcf . . . . Bowl Bowl B e ~ t ; "" Beach-dia? Total' inside ro'tal liquid Max bowl P . . . . Gearbox Gearbt, x Mach,ne dla. length I angle (dell.) (ram) ienllth (ram) vol. ~ ( i ) speed load ratio torque design ~ (mm) (mm) (rpm) range range range
(kW) LkNm)
I CO3(~) ] 8.5
2 CQ40r 45
3 CQ5 _f~, . . . . 45
Centrisys, USA
Name of company Centrisys Corporation
Headquarters address 9586 58th Place. Kenosha. WI :33144. USA Phone: (+1) 262-654 6006; Fax:(+1) 262-654 6063
UK address David Hollier Ltd Upperthorpe, Westwoodside. Doncaster DN9 2AQ
US address As above
Company ownership Private company
Type ofcompany Specialist decanter manufacturer
Other main businesses Service facility for all major brands of centrifuge
Other company connections None
Decanter manufacturing sites Kenosha, USA
Decanter range
Six models, available as two- and three-phase machines, with variable specifications
Decanter sales Sales in region of $ 7 millions forecast for 2()()()
Sales specialisation Water and waste water sludges
3 50 Suppliers'Data
Machine details
"Iviodc! ref no. "Bowl dia. "'Bowl ' Beach Imgic -"Beach ~a ~ Tocli reside Tor hqmd voi ' Max bowl Pow.~" C_~ar~ Gearbox ~bc]uac" (ram) length' (de S ~ (mitt) lcngth (ram) ( } A speed (rpm) load nmgc rauo range tm'que rtnlp: design'
( m m ~ . . . . ( , k w ) . . . i ~ m . ) .
1 CS 10-,t 250 830 15 160 1000 27 6000 "7 5/18 5 I 0-,I..4 AJ]-[
2 CS 1,1-4 350 1040 15 215 1450 68 4500 |8.5/30 2.0--8 9 A/H
3 CS ls-a 450 L280 15 260 1800 14.8 3600 22/45 4 4--8.9 A/H
4 CS 21-4 330 1880 15 330 2300 387 3200 30/56 4.4-12,9 AJH 5 C5 26.-4 660 2340 15 410 2650 531 2830 56/! 12 8.9--16 7 AJH
6 CS 30-4 7~0 2700 15 475 3050 814 2650 112/186 8 9-35.0 A/H
t
"~ ~, i ~ ' '
,~-,
k',
Figure 10.4. A Centris!ls mobih' lest rig.
Flottwcg, Germany
Name of company Plottwcg GrnhFl
Headquarters address rndustriestrasse 6-8, n-X413 7 Vilsbibutrg, Cermiiny Phone: (+49)08741 301-0;I:ax:(+49)US741 3 0 1 3 0 0
Krauss-Maffci (UK) Ltd E u r o p Houlcvard. Gemini nusirless ['ark. Warrington, Cheshire WA.5 5TR Phone: c) 1 9 2 5 h44 1 OI) : Fax: 0 1 9 2.5 G 44 1 7 X
US address Krauss Maffei ('orporation, Process l'cchnology Llivision P()Box h270 , Florence, K Y 41022-h270
A rrieiiibcr of thc Krauss-Mnffri Grniip. withiil Atecs. owned by the Siemens-Bosch consortium (Crrmany 1
A specialist decantcr manufacturer
I k l t presses a rc supylicd by Flollweg. a n d combined systcims with thr! disc st:lcIi r.rntril'uges madu h y Vcroncsi
Closti ;issocialiori with VeIonesi inside tlic Krauss-Maffci group
V i lshi bu rg , Germ i i n y
Thirtccii modcls. based on eight bowl diametcrs, available with variable spcciticalinns. including thrsc-phasc operation, and dcsigiis For optimal sludgc thickcning and high dry solids (ipcratiun TlwrC arc four distinct modcl rangcs, cacb with its ow11 particular process sepii ra t i II 11 c a pa hi 1 i t y :
Uk' iiddrcss
Company ownership
'I'yyc of colllp~llly
( )I h I: r 111 ai 11 b usi n csscs
Other r:r)mpijny ~or~r i ec l io r i s
I)rca n ter mar I 11 I'a c t 11 ring sites
I k c a n t e r r a n g e
I1ec;inter si i les
Sales specialisation Sales in region oI5hO miillions 1orcc;ist lor 2 0 0 0
Very wide range or ;ipplir:;itions, n o piirticular speciality, althouEh not17 emphiisising wiisle treiilment
3 5 2 Suppliers'Data
Machine details
Model ref. no ROW[ dia. Bowl "" (ram) length t
(mm)
I Z 23-3 23O
2 Z 23-4 230
3 Z 32-3 32O 4 Z 32..4 320
5 Z 40-3 420
6 Z 40-4 420
7 KVZ-51 500
g Z53-4 5"~0
9 Z 6E-2 620
l0 Z 6E-3 620
l I Z 6E-4 620
12 Z 73-4 730
13 Z 92-4 920
Beach Bea'cla dr'a? Total Total liquid Max bowl angle (deg.) (mm) inside vol. 3 (I) speed (rpm)
length (ram)
Power " C_learbox Gearbox Machine load ratio range range (kW)
7.5/15
7.5/18.5
15130 |5/30
22/45
22/45
30190
22/55
45/132
451132
45t 132
37/110
90/250
torque desngtl 4 range (kNm)
Figzlre 10. 5. A Flottwe~.I cetztrffzlge installation.
Gennaretti, Italy
Name of company Genaretti SpA
Headquarters address Via Roncaglia 10, 1-60035 Jesi (AN), Italy Phone: (+39) 0731 200200: Fax" (+39) 0731 2 0 0 1 2 4
Suppliers'Data 353
UK address None
US address None
Company ownership Private company
Type of company Specialist decanter manufacturer
Other main businesses None
Other company connections None
Decanter manufacturing sites Jesi. Italy
Decanter range Two basic models, with a range of specifications for different applications
Decanter sales Sales in region of $4 millions fl~recast fl)r 2()()()
Sales specialisation Range of industrial applications, but mainly involved in animal and vegetable products
Machine details ,_
Model ref. nt:. Bowl dr.',. Bc~wl Be:,ch Beach dla.: Total (ttun) length ~ angle ~deg.) {Jmn) inside
(nun) length ( mm )
1357 2 5(X)
Total hquid Max howl Power Gearbox Gcarb~x Machine vol ~ ( i ) .,,peed ( rpml load ralm torque destsn'
range range range (kW) ~'l, N m l
4(X)() 15130 A/H 3(X)O 45155 A/H
Guinard, France
Name of cmupany Guinard Centrifugation
Headquarters address 2-4 ave de l'Europe. F-7814() V61izy, France P h o n e : ( + 3 3 ) ] 3926( )55( ) :Fax : (+33) l 3926()56()
UK address Andritz Ltd, Unit B, Sheepbridge Centre, Sheepbridge Lane, Chesterfield, Derbys $41 9RX Phone" ()1246 26()66(): Fax" ()1246 26()76()
US address Andritz-Ruthner Inc 1010 Commercial Blvd South, Arlington. TX 76001 Phone: (+1) 817-465 5611" Fax: (+1) 817-468 3961
354 Sz~ppliers' Data
Company ownership Subsidiary of Andritz group (Austria)
Type of company Specialist separation equipment manufacturer
Other main businesses Press belt filters, disc stack centrifuges, basket centrifuges.
Other company connections Only within Andritz group
Decanter manufacturing sites Chateauroux, France
Decanter range Seven basic models, with some design variability
Decanter sales Sales in region of $12 millions forecast for 2 OO0
Sales specialisation Coupled with Andritz equipment, strongly oriented towards waste treatment
Machine details
Model tel nt~. Bo~.'l daa Bowl Beach Beach dta : Total (nm11 length ~ angle ~deg ) (nun} inside
i rnm I length (ram)
Total 'hqu'Jd Max bowl P~)~,er Gearbox G e ~ , x Math,he vol.' ( 1 ~ speed (rpml luad ratto torque cteslgn'
rant~e range range (kW) (kNml
1 D2 2b{) A/[-I.
2 D3 A/H 3 D a A/H 4 D5 A/H
5 D6 A/H 6 D7 A./H 7 DI() ItKIO 4201) 2 twJ~ ) AJH
Hiller, Germany
Name of company Hiller GmbH
Headquarters address Schwalbenholzstrasse 2. D-84 ] 37 Vilsbiburg, Germany Phone: (+49)08741 48~ Fax: (+49)(]874] 4304
IJK address Dirk European Holdings Ltd 29-3 ] Woodchurch Lane, Prenton. Birkenhead. Wirral CH42 9PJ Phone" 0151-608 8552" Fax" O151-608 7579
US address None
Company ownership Type of company
Specialist decanter manufacturer, until 1998 made exclusively for
Humboldt
Suppliers'Data 355
Other main businesses Hydraulic motors for decanters
Other company connections None outside Dirk group
Decanter manufacturing sites Vilsbiburg, Germany
Decanter range Five models in two basic types
Decanter sales Manufacturing capacity for 200 centrifuges per year Estimated sales for 2000 of $8 millions
Sales specialisation Food (especially olive oil) and waste treatment
Machine details
Model ref no. Bov,'i dta. Bowl Bcach Beach d=a 2 Total (ran:) tenglh t angle (deg.) (ram) mslde
I ram) length [ n m l )
Total h q m d Max bowl Power Gearbox Gearbox Mach ine vol. ~ ( l ) speed ( rpm) load ratm torque des ign 4
range range lange (kW) (kNm~
I D31 308 135q 4500 I I A/H
2 D37 "~72 1634 4~K) 18 5/22 A/H
3 D45 450 ! qtr 35(X1 3LV37 A/H
4 D54 544 2359 3200 37155 A/H
5 D6~ b60 2847 2900 75/9() A/H
Hutchison-Hayes, USA
Name of company Hutchison-Hayes Separators lnc
Headquarters address 352() East Belt. Houston. TX 77()1 5. USA Phone: (+1) 71 3-455 96()(): Fax: (+1) 71 3-455 7753
UK address None
[IS address As above
Company ownership Private company
Type of company Specialist engineering company
Other main businesses Reconditions centrifuges, supplies range of disc stack centrifuges
Other company connections None
Decanter manufacturing sites Houston, USA
356 Suppliers'Data
Decanter range Five models in two different diameters
Decanter sales Sales in region of $ 5 millions forecast for 2 000
Sales specialisation Food industry (dairy and milk, meat and fish processing, edible oils), fuels and lube oils, petrochemicals
Machine details
M, ldel ret no. Bowl d,a Br Beach Beach dia." Total Total liquid Max bowl Power Gem 'box Gearbox Machine (mm~ Icngth f angle ldeg.) Cram) inside vo[ ~ { I i speed (rpm)load ratio torque design 4
I mm ) length range range range (.ram, j (kW; . . . . (kNm)
11430 356 762 10.0 ~ ' , ~
2 1433 356 838 8.5 A/H
4 1456 356 1422 8.5 A/H
5 5500 406 1397 10 0 A/H
Noxon, Sweden
Name of company Noxon AB
Headquarters address Sisj6 Kullegata 6. S-421 32 Vfistra Fr61unda. Sweden Phone: (+46)31 748 5400; Fax: (+46)31 748 5419
UK address Waterlink (UK)Ltd Prickwillow Road. Ely. Cambs CB 7 4TX Phone: ()1353 645700: Fax: 1353 645702
US address Waterlink Inc 410() Holiday Street N W. Canton. OH 44 718-2 532 Phone: (+ 1 ) 330-649 4000: Fax: (+ 1) 3 30-649 4008
Company ownership Part of Waterlink Inc. through European Water and Division/Waterlink AB
Type of company Specialist decanter manufacturer
Other main businesses None
Other company connections None outside the Waterlink group
Decanter manufacturing sites Kungsbacka. Sweden
Wastewater
..
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358 Suppliers'Data
Sales specialisation Wide industrial coverage, inherited from Sharples progenitor
Machine details
"ModeL r~f .o Bowl cha Bowl Beach ~mgle Beach ~a . : Total ,ns,~ Tolal Max bok, I pow~ load Gearbox Gearbox Machine (ram; length' (deg) (ram; length (ram) hqmd vol ~ speed rrpm) raage (kW) mtm range torque r'aagc design'
, Imm) ,,, ( I ) . .
i P-600 152 tO 119 353 2 6000 95
2 P- 1500 250 l 0 180 559 10 4000 94
3 P-2fR)O 356 10 264 572 IA 4000 52-125
4 P-3000 356 i0 264 794 28 4000 52-125
5 P-3400 156 I0 264 1264 49 40(0 52-125
6 PM 20000 356 I0 241 794 32 40,30 52-125
7 PM 30000 356 10 241 1264 57 4000 52-125
8 PM 35000 42~ 10 264 1264 86 3650 52-125
(kNm)
0.2 .~IH
0.9 A/H
I g-24 A/H
I .R-2.4 A/H
1.8-2.4 A/H
I &-2.4 A/H
1.8-2.4 A/H
I g -24 AsH
Pieralisi, Italy
Name of company Gruppo Pieralisi
Headquarters address Via Don Battistini 1. I-6()0 3 5 ]esi {AN), Italy Phone: (+39)()731 2311;Fax: (+39)0731 231239
UK address Kirton Engineering Ltd Old Station Close. Shepshed, Leics LE 12 9 NI Phone'()1 509 5()4565" Fax" ()15()9 60()()1 ]
US address None
Company ownership Private company
Type of company Specialist engineering company
Other main businesses Vertical basket centrifuges, belt press filters
Other company connections None
Decanter manufacturing sites lesi, Italy
Decanter range Four basic models, with a number of specification variables
Decanter sales Sales in region of $40 millions forecast for 2000
Sales specialisation Primarily an olive oil decanter maker, which has diversified into other industry sectors
Suppliers'Data 359
Machine details
Model ref. no. Brwl Bowl Beach dia. Icngth ~ angle (ram) qmm) (deg.)
. . . . . l B.;y, 23., _~64 2 Baby 2 232 773
3 FP61R)/M 353 925
4 FP600 RS/M 353 1228
5 FP600 2RS&t 353 1525
6 Jumbo I 470 1189
7 Jumbo 2 470 1589
8 Jumbo 3 470 1998
9 Jumbo 4 470 2406
I0 Giant I 700 205 I ! ! Giant 2 700 25 ! 3
12 Grant ~ 7()(I 2975
Beach Total Lnslde Total din." length hquid (mm) (mm) vol. ~ (I)
Max Power Gearbox Gearbnx Machine bowl load n~tio torque design 4 speed range range range
�9 (.rpm) (kW) (kNml
520O 5.5 A/I-I
5200 7.5 A/H
4100 ! 1 A/H
�9 I 1 0 0 1 i A/H
41 O0 i 5 AIH
3350 30 AJH
3350 37 AJH
3350 45 A/H
335O 45 A/H
2000 55 A/H
2000 75 A/H
2000 90 A/H
Siebtechnik, Germany
Name of company Siebtechnik GmbH
Headquarters address Platanenallee 46. I)-4 54 78 Mfilheim an der Ruhr. Germany Phone: (+49) ()2()8 58()1-()(): Fax: (+49) ()2()8 58()1 3()()
IlK address TEMA (Machinery)I,td 3 Great Central Way, Woodford Halse, Northants NN 11 3 PZ Phone" ()1 327 2626()()' Fax" ()1 327 262571
US address Tema Systems Inc 78()6 Redsky 1)rive. Cincinnati, OH 4 5249 Phone: (+1) 51 3-489 7811: Fax: (+1) 51 3-489 4817
Company ownership Private company
Type of company
Specialist engineering company, for mechanical separation equipment Other main businesses
Horizontal conical basket centrifuges (sliding discharge, worm discharge and vibratory), pusher centrifuges, screening equipment and size reduction equiprnenl
Other company connections None
Decanter manufactur ing sites Mfilheim. Germany Cincinnati, USA
360 Suppliers'Data
v %
B ~
i
B ~ m ~
Figure 10.6 A Siebtechnik centrifuge installation.
Decanter range Six different models of decanter-type centrifuge: tunnel design, pedestal bearing design, and overhung-mounted decanters, plus decanters with second larger diameter cone (Twin-Cone Decanter), and larger diameter screening drum (Turbo-Screen Decanter), as well as an overhung design worm-screen centrifuge with a decanting area ("Conthick"). Some models with cantilevered bowl are interchangeable with conical basket filtration machines.
Decanter sales Sales in region of $12 millions forecast for 2000, including those sold by Tema
Sales specialisation Wide range of industrial applications, especially within the chemical and pharmaceutical industries, as well as industrial waste water applications. Major focus on individually designed centrifuges based on application requirements
Suppliers 'Data 361
Machine details
Model Bowl Bowl Beach Beach Total Total Max ref no. dia. length I angle dia. " inside liquid l~,wl load
(ram) (ram) (dog.) lmm) length vol. 3 ( 1 ) speed range (ram) (rpm) (kW)
Tunnel design (TSt, pedestal bearing design (DZ) and overhung-mounted (T3YF) 1 210
2 3(~}
3 3o0
4 420
5 500
6 600
7 710
8 850
9 1000
Twin-Cone (TWC; and Turbo-Screen (TSD) i 0 300/450
1 I 50t)/'/00
12 6O0/75O
13 8(~)/I 0 ~
Conthwk r 14 240/180
15 320/240
! 6 4501340
17 600145 (.)
! 8 800/600 19 I ()()()17 50 20 12001000
Power Gearbox Gearbox Machine ratio range torque design 4
range (kNm)
Westfalia, Germany Name of company
Westfalia Separator AG Headquarters address
Werner-Habig-Strasse 1. D- 593()20elde. Germany Phone: ( + 4 9 ) 0 2 5 2 2 77-()" Fax: (+49t ()2522 7 7 2 4 8 8
tJK address Habig House. Old Wolverton. Milton Keynes MK 12 SPY Phone: ()19()8 313366:Fax: ()19()8 3 1 1 3 8 4
[JS address 10() Fairway Court. Northvale. N] ()764 7 Phone: (+1) 201-767 39()0:Fax: (+1) 201-767 3416
Company ownership Part of GEA Mechanical Separation Division. itself a subsidiary of Metallgesellschaft
Type of company Specialist centrifuge manufacturer
Other main businesses Disc stack centrifuges
362 S z t p p l i e r s ' D a t a
Other company connections None
Decanter manufacturing sites Oelde, Germany Niederahr, Germany Chfiteau-Thierry, France
Decanter range Three ranges of three, four, and five models respectively, but new range appearing
Decanter sales Sales in region of $ 5 5 millions forecast for 2 000
Sales specialisation Very wide industrial application, with modern emphasis on waste treatment
Machine details
Model ref m, Bowl dia. Bo'wl ' ' Beach angle Beach dia. 2 Total Total hquid Max b~ Power (rnnt) length ~ (dcg.i (mml inside "~ol." ( ! ) speed (rpm) load ratio Iorq tie
~'mm) length range rnnge range , _. (nwn) . (kW) (kNm~
I CB300
2 CA450
3 CA505
4 CA755
5 MD43. 4(X) 1211(I
6 MD44 400 I r
7 MD54 500 20(KI
HIM3 4L~ 1200
v HD44 400 16tKb
1o HD54 5r 2000
I I i tD8 ~ ~(l~) 2400 12 HD84 ~s(M) 3200 l 3 AD0504 3(~.) i 2(1(I 14 ADi22!) 458 2015
15 AD 2040 510 2(14o
44(X) 18 5
3(Xk') 35
35(X) 9O
Gc,'u'0ox Gearbox Machine design 4
A/F! A/H A]H AIH AJF1 A/H
A/H AJI-I A/H A/H A/H A lE A/H
CHAPTER 11 .- --.. -
Glossary of Terms
A(.(-t*lpr;llor
Arrylamide
Active bafilc Anionic
Annunciiitot
Anti-vibration mounl
Arithmetic mean
fiXitIt flCJ%
The plate. or target. with vanes in the lked zone. opposite the feed lube exil, which accelerates the ikctd strtslrn up to bowl speed An org:anic chemical. wt1ic.h when polymerised acls as :I Ilo~~r~ul;lling agent A syntmym used f’or conveylnl: baffle A description fur the ionic charge of ;I polyclcctrc~lytc. and which is ncgalive Arrangcmcnt of indicators OH a PlL’or intcgratcd contrtjllcr to display r)periitintl:ll
condition and fuIlctionirlg oflhta pl;lnt A mer:h;inic;~l device, fitted undct- the dccantrr, to isolal~~ any vibrations of’thc decanter from tlic surrounrling environment. Also called ;I vibration isolalot Themcan ofa set ofr~ numhcrs. ohtaincd by ;Idding all the numbers togt’lh(~r and dividing
by 11 The now ofrl;Iriiicd liquid along the cylindrical part ofthr howl when it flnws parallel with the axis, ruablcd by windows or holt>s irl the conveyor flights (as distinct Cram the norma\ helical Ilow partcrn) A systc+m (electrical. hydraulir rrr mechanical) IO t4’rt.t control althc conveyor ditkrential S[W’l
364 Glossar!! qf Terms
Backing plate
Baffle
Baffle cone
Baffle disc
BD Beach
Beach angle
Beach length Beach liner
Beta Bowl
Bowl assembly
The support plate of steel, compatible with the process and for welding to the flight tip, on which a ceramic tile is mounted, by brazing or bonding A restriction, generally in the form of a disc or cone, on the hub of the conveyor, used to impede the movement of cake or solids, or to redirect the movement of the liquid flow. It can also be a plate, compar tmenta l i s ing the centrifuge casing, minimising cross- contaminat ion of discharged products A conical baffle built onto the conveyor hub, shrouding the feed zone. and impeding the flow of cake travelling from the bowl's cylindrical section onto the beach. Sometimes called a BD cone A disc built onto the conveyor hub to baffle scrolled cake. Also known as a cake baffle Baffle disc Conical section of the rotating assembly, up which the cake is scrolled before discharge. Sometimes referred to as the extension Semi-included angle of the beach cone. It is the angle subtended by the beach inner surface to the decanter axis The axial length of the beach A covering applied to the inner surface of the beach, which may be plain, roughened, ribbed or grooved, to aid scrolling or for wear protection A scrolling capacity function The hollow cylindrical and conical shell of the decanter rotating assembly. Alternatively. it is the vessel, including hubs and the beach. which holds the process being separated. As such. it excludes the conveyor and gearbox. Sometimes the whole rotat ing assembly is referred to as the bowl or bowl assembly. For example, "Mount the bowl in the frame", means get the rotat ing assembly mounted in the casing, onto its frame The assembled bowl, hubs, beach and conveyor with main bearings and pillow
blocks
rlakr baffle disc
A formed met.iil sheet. inserted into thcborv! cglilider. to act as a scrotlirig aid, or for wear protection. It could he Cull or part c p f I.hr cglitidrical length. I t could be plain. ribbed, gnjljved or w i h ii special surface finish 'rhe t ~ o l l o ~ , ~ cylirider section whirh fits around the c:orivryvr '['he inside surfacc oTthc bowl shcll A compnrtmcnt within the convcyt)r h u h between fccd and flooculakit zones. l o prevent splashing from tht! feed chamber into the flocculant chamber Synonymous wit11 b i l k rbaniber The sedirnented residuc from thc centrifuged
iz disc m t w n f d on t he C O ~ ~ V C ~ O ~ hub. genrrally at the ]unction betwccn the bowl shell and bcach, to restrici tlie flow ofcakc. and provide a back pressure. i t can also bc littcd tinywhcru up [lie h r x - h o r intu the c y 1 in d r ic al sect i ( )n Thc minitriurri pr twurc that can cxprtss mnisiurc frtim t h u cake. The cakc yield stress d u p c d s iipon the solidsct}nccntr~tir,n in thc caks, and increascs rapidly iis that concentration increases Often used instcad d * ' r a k t I . ' . h i t whcn i t is. i t usually itifdrs a positive ri3ke. e.nabling litting of the cake. Srr? rake A short horizontal decantel wirh i.hr maih bearirigs pliir-ed at t h e same end ot't hc bowl , su u h t h a t 1.h r rot i1 t i n g assciiibl y is cii n t i I C V U rcd from t hosc bearings A test, or apparatus. whir:h a s s ~ ~ s s c s thc drw;itrrability oC;i sludgti by measut'irrg i I s ;tbilily and speed to release moisture. Often used to asscss the eKe,ctivrness of floccu lants 'I'he plate or shcct rrietal housing for the bowl ;~sscmbly. which collects. and kceps wp;irate, t.hc discharges from the hcach and liqiiiil outlet A plate welded inside the casing to coinparttrit.ul.;ilise it and kccp separ;! te the d i ffc rcn t bo w 1 d i sr h a r g es
ret:d
Cake yicltl strcss
t'iln I
C'iintilevcrcd clcciinter
C' a pi 1 liar y si I I' I inn test
366 Glossary of Terms
Casing gutter
Cationic
Centrate Centrifugal force
Centripetal force Centripetal pump
Ceramic
Chatter
CIP
Clarification enhancement
Clarifying length
A ledge welded to the inner edge of the upper half of a casing baffle, to prevent splashings runn ing down the baffle surface and onto the bowl A description for the ionic charge of a polyelectrolyte, and which is positive The clarified liquor discharged from a decanter The gravitat ional force produced by rotation at speed The reactive force to a centrifugal force A pump fitted to a decanter 's front hub, which converts centrifugal energy in the centrate to pressure energy, and thus causes flow under pressure in a discharge pipe line A non-organic, non-metallic compound. Often a refractory, used for lining furnaces, but in decanter work is used for its abrasion resistance. For example, a lumina, silicon nitride Torsional vibrations of the dynamic system consisting ofbowl, conveyor and the friction between the conveyor and the product. This may result in metal fatigue and premature failure of gearboxes Clean-in-place. The features on a decanter which allow it to be cleaned hygienically without the need for dismantling. The operation of such features. This involves operating the decanter for set periods at just below 1 g, forward and backwards, and flushing it with a cleaning agent. The outside of the bowl and the inside of the casing are cleaned by spray nozzles Longitudinal vanes welded to a decanter conveyor, at an angle to its radii, from the conveyor hub to the conveyor ribbon flight. These increase the theoretical Sigma of the decanter. Alternatively discs can be substituted for the vanes. Also known as Sigma enhancement The distance between the middle of the feed zone exit and the front hub
Classification
ronr Coriveying baffle
Corivcyor
Colivtayor cliffcrcntial
C'onvtsynr h u b
Co 11 n t crs h aft 1)a t:k -4 rive
l'resting
CSI' Curnuliitivc distrihutinn
y'hc dc.cantcr process whcrcby a fccd stream is separ;;~ed in1.o t.wo streams containing particles otdistinctly different sizes. or of (1 i ( k e n t I: ci m posi t in n by s i r t us of density difkrcncc A shroud placed over the cake discharge from thc bcacli, to direct the cake flow and prercnt sticking to the casing
A pr(Jpriei;iry type ofhard surfacing materi;d A cake whit:h can bcsqueezed to express moisture More oftcn rcfcrred to a s [.he bench A complcx forin of cake baRle, comprising a flight section with larger pitch. sirctching from thr front o rone main flight to the r w r of mother The dcvicc that movcs the scttlt~l solids in thc bowl The differtwcr in speed belwtrrn howl and cu nvcyor 'I'hc central supporling tube ol'the coriveyor. whic:h hoL1sC'S thc krd iind floccularil or rinse zii r i PS, i i nd t hc co 11 vr y or bc a r i ti g a SSP m b 1 i cs
The rotational reaction uf t h e conwyor to thP resistance c ' d c a k t ~ movoriic'rii, exprwsed i l l
wrms offorw t.imcs rad ius at which the I'orcc is acting A back-drivc systciii whereby the pinion shaft
w h i c h in tiirn i s tolaled by a p u l l ~ y 011 rhe main rotating assrrnhly. I t cflects :i tixcd diffcrciitial. hut this run he changed by dtcririg the sizes oi'thc pulleys Thc l t ~ r I ofpo11d ;iho\rc the weir pIatc 1)eigIit. bcing th;it nCccssilry to p r o d u t y thc dist.h;lrg:l. flow 10 match t t ic input rate See capillary suclion tcsl A particle sizc dislribution. I r it by weight. v ~ l u u i e . surface or cross-sectional area, number 01' whalevcr. the percentage for a parlicular size is thc percentage of thc distribution wliich is below or above that size
1s rotated by i l bell ctJ1lrlWted 10 a shaft.
Cyclo I'ylindricnl lcngth
Darn Dam plarc
Dewatering
Tjifferrntinl ni 1 11 t i on water
Dirnciisioiilcss group
13isr.s
1)uublt. lead
Uovutail t i l e
A hulbous weir plate, which archcs out from the front hub surfact to provide extra weir edge lcngth A term allotted to a particular type of gearbox Thc distance froin the root oCtht: beach lo the fl'Orlt hub Short for dam plate Weir plate. OW of ij set of plates boitcd to thc froat h b b to coiilrol the pond depth l h e decanter process whereby the solids arc sep;ir;iled from the siispendinE liquor. to give H low-moisture cake and ii ccntratc with low yolids contkvt Con w y u r difkrcn tia 1 Water used to rcducc the strength orthe floccularnt Solution. gcncratly addcd i n the flocculant line A set Oiproccss parameters. w h i ~ h , rvher i va riou st\! mu 1 ti plied together andiut d i Bided. 11 as ii o d i m e n sion s , p ro1.i d i n g c'o I i s i st r 11 t II 11 i t Y
:ire iist?d
A nnzzleftted tn rachof theex i t p c i r t s o f t h t lwd zone. a r i d l i l t e d radially. Thr exit o i e a ~ h norile faces :ixi:illy or tangentially. s u c h that thc feed from the feed zone discharFcs onto thc back 01- Front face of the adjacent flight or \aiigeniiaIIv intu the helical channel. towards the rear o r front of the bowl C'oriicaI plates iitted around thc ctmvcyoi- h u h towards the ctiiitrat.e discharge to e i i h a n c ~ cliirili(;;itiori L:ilpii[;it.y. A ribhi)[\ [light is iittiiched 1 0 the o u ~ s i d r periphery of t h e discs Two cquispaced ccrntinuo~ir tlighlson lhr cn 11 vc y o r h 11 b A typp of'tilr sonstruc:tiori. Tlir cerainic wear plate has a triangular spigot on its hack h c c raating with a triangular rcccss on the front face ofrhe backing platc to which it is bonded or brazed. 11 provitks support arid extra safety should t h e bond or braze fail
Dry beach
US
ns m o ~ i e 135 operation Uual angle bench
L)uplex steel
Epicyclic
Er f
A spring-loadcd plug titted to t.hr h > w l d1d3 usually in pairs fitted 1 8 0 , apart. It is used for c ~ p work to allow the liowl ID drairi durirtg clealling, opening at low h v l speed and r:losing as spccd increases The portjo11 of the beach ahove t.he lrvet OL'
thc pond Ury solids. The description of a dccantrr dcsign. its method ofoperation or itscake product. whereby extra-dry cake is iichieved The control mcthod for US opera! ion Operating ti dccanter in t?S mode A beitch which has two distinct semi-included ;~nglcs . such ttiar. ifs cross-scction is co111:ave or convc'x A decailtcr with a hviich at both etids. I:sed tor separating riglit z i id heavy solids from ii suspension h type ol'strcl uscd for twwl c:onstructiori. whcrt: high chloride. ct:)riiciil and etrviitrd t ~ ~ ~ ~ p e r i i t u r c s cxist in the fccd or wlirre high stt'erigth is r c q u i r d A circular wcir 1)1;1tc with a11 ccc-rntric hole. l iwally fitled in rqual numbers with Tour or iriiirr to a set. Hy rotating t h v pliitc in its recess, (1 i Fierent pcr n r l heights m :I y be o b t ai tird . Sr)mctimes cillled a universitl dam A description ol'thc inorc! common type of gParbnn. built with two or thrce st ugcs. Each stage consists of a "sun" fit'ar\vhcel s 11 rro u I 1 ded by two (.I r t h r w ' pl a r I r t ' ' geai*wliec+ls rulinirll: on i i tmr11t.d track 011 ihc ii'liier wall oftlir gearbox. The planer wheels arc mountcd on a c;irricr plale to whiuli is centrally mounted t.hc piiiiori shaft for the sun wheel of the nexi. stage or 10 engage L ~ P
conveyor itself A mathematical lunction. called on error fuiicl ion, wheTe:
Integratkon is not possible m;ithcmatically. and t.hercfore t he function has to be tahulatcd
370 Glossary ofTerms
Feed chamber Feed nozzle
Feed tube
Feed zone
Flight
Flight tip Floater disc
Floating conveyor
Floc chamber Flocculant
Flocculant chamber
Floc feed tube
Flush water
Frame
Synonymous with feed zone A short tube, often lined with erosion protection, fitted to each exit of the feed zone A pipe secured to the frame and inserted along the bowl axis to the feed zone, to carry the feed stream into the centrifuge A chamber in the hub of the conveyor, where the feed is received from the feed tube and brought up to speed, before discharging into the pond The metal plate welded radially to the conveyor to provide the scrolling surface of the conveyor. It is one cont inuous plate wound end to end on the conveyor hub. Erroneously, one 360 ~ turn of flight is often considered one flight. One talks of "adjacent flights", meaning the part of the flight 360 ~ from the last one The outer periphery of the flight A disc fitted at the front end of the conveyor hub, dipping in the pond, to prevent floating solids discharging with the centrate A conveyor whose overall density is less than the suspending liquor of the feed. It thus floats in the pond. Used on some 10 ()00g decanters. Also called a swimming conveyor Same as flocculant chamber A reagent which causes fine particles in suspension to agglomerate A compar tment behind the feed zone in the conveyor hub, sometimes separated by a buffer chamber, used for receiving the flocculant before passing into the pond. Sometimes referred to as the polymer chamber or polymer zone. In certain designs it can double as a rinse
chamber A feed tube with a concentric outer tube for conveying flocculant or rinse into the bowl The water used to wash out the decanter on shutt ing down The cast iron, or fabricated steel, base on which the rotating assembly is mounted
Friction factor
Froril
Front hiib
Full axial flow
(;a u ssia I 1 d i s t r i b u I i (11 i
Geom u t r i c s t a I I d ;I rd drvi;i\ inn g-Forw
g-l,evel
Heel
A dimcnsionlcss ~ r o u p . iiivolving fric.tion, fluid velocity and derisity, used in fluid rriechariic:s a d related 1.0 Reynrllds number Conventionally the end n l the d e c a n k r where t~he c e n t r a k discharges The tlange, or end plate, which encloses thc howl a t the centrate end, and includes the spindles. which suppnrt thc front main and front conveyor bearings. It also supports the ccntratc weir platcs Flclw along the bowl which is axial, a n d cannot deviate f r w it. This fl(iw is achieved bet,wem radial vaiics or Sigrrla eiihancernent v a n e s A partic:lrr size dist r ibut ion havinE a symmetricitl bell shape about Ihc mean. Sornctimcs known as the nnrmal or i lorrr~al
probability d ist r ihu t ion. 'I't 1 t: rri ;I t h em at I cs [or thc CLIWL' wcrc dcveIoped hy thc mat hcma t ici a n r'k hloivrc. hu l also associated wit11 i t were ol,tit:r gr ra t mathcmaticians. ( i a u s s arid 1,;ipl;icc An cncl(ised metal cylinder- rnnl:lining a system ofgears irnlliurstid in oil, Ti is inciunted on onc C k l d of t h u rotating ;issrmhlv iind e k c t s the rntaticiri crf the conveyor ui a slightly differtiit speed to thrbowl Sce "chattcr" Tho i n w r i u f a set of'n nuiiilxrs, ohloined 13s multiplying all the nurribers logether and taking t h e 11th root A special sCilodi\rd d tv ia t io~ i used in a skcw Caussian distributiuri The outward tudial forcc produccd hy rotation in a circlc Tlic numhcr o1'rimes t1iecentrilug;il ticld is greater than earth's gravity The ;ixial slots machined in the bowl wall or beach to emourage keying to assis1 scroll ing rfficicncg The thin layer ol' ixke betwccn Lowl/hach and CoklVcyOr d w to thc necessary rnechailicaI clcarance
372 Glossary of Terms
Heel torque
High baffle
Hindered settling
Horizontal decanter
Hub
Hubless conveyor
Impellers
Inertia
Inflatable dam
Intlatable seal
In-line dosing
Integrated controller
Inverter
The conveyor torque developed by the heel after feeding and the bowl has emptied, apart from the heel Named after the inventor, R High. Another name for the longitudinal cake baffle, between two flights The settling of particles in a fluid, when they are so close that they impede one another ' s settling rate A decanter whose axis is horizontal when installed The central part of the conveyor. Alternatively the end flanges of the bowl/beach, each having a central spindle for supporting main bearings on the outside and conveyor bearings on the inside A conveyor which has no central hub. or just a very small diameter one. The flights are held on radial vanes joined onto a small diameter pipe support Small blades affixed to the outside of the beach at the cake discharge, to impel the cake away from the discharge area. or around and out of a collector The property of a rotating body, which opposes any change of rotational speed. A function used to calculate run-up times of the rotat ing bodies An elastomeric device fitted inside the front hub, to control pond depth by hydraulic inflation Similar to an inflatable dam, but used to control wash-out by acting on valve ports at a larger radius than the pond surface Admitt ing flocculant in the feed line somewhere before the decanter A PLC or industrial computer used to supervise the overall running of the decanter plant, either directly or by supervising other controllers An electrical device which modifies the mains electrical frequency to control the speed of an electrical motor
lrlverter motor
1,;lrnhda
Lcad
l.,og probability
M r i i r i frame M ii ss t r a n s fc i- c oc Ffici c 11 t
hl c t cr i 11 g p LI 1'11 p
Oil nozzle
orriega
374 Glossary of Terms
pH
Pillow block Pinion shaft
Pinion torque Pitch
Pitch angle
Plough tile
PLC
Polyacrylamide Polyelectrolyte Polymer
Polymer chamber Polymer dosage Polymer dose
Polymer utilisation baffle
Polymer zone Pond
Pond level Pond volume
Power factor
A physical chemistry term used to denote the acidity or alkalinity of an aqueous liquor. Numerically equal to the logarithm of the negative of the molar concentration of the hydrogen ions The housing for a main bearing The small shaft of the gearbox to which is attached a torque arm or a back-drive motor or brake. Sometimes referred to as the sun wheel shaft or input shaft The torque on the pinion shaft The distance between consecutive sections of the conveyor flight 90 ~ less the angle subtended by the flight tip and the conveyor axis. Alternatively it is the arctangent of it x conveyor diameter/pitch A tile with a plough-shaped profile, used for lifting and rotating the cake while scrolling Programmable logic computer. Used to control various decanter plant devices A type of flocculant A generic name for a class of flocculants In decanter work synonymous with flocculant, unless it refers to a particular decanter feed See flocculant chamber Synonymous with polymer dose The amount of polymer used per unit solids in the feed A disc fitted to the conveyor hub just down stream of the feed zone. It dips slightly in the pond and prevents the flocculant skimming o v e r the surface of the pond with poor utilisation See flocculant chamber The process liquor held in the bowl or the volume occupied by it The radial depth of liquor in the bowl The actual volume of the pond. In DS work it sometimes means just the volume between front hub and cake baffle The degree by which electrical current leads or lags the voltage. Numerically it is the cosine of the phase angle
Glossary of Terms 375
Probability
Psi
Racetrack
Rake
Rear
Rear hub
Recovery
Rennbahn Reslurry collector
Reverse pitch
Reynolds number
Ribbon flight
Ribs
For the purpose of this book it is a part icular mathemat ica l scale used in particle size distribution work A factor used in thickening work to indicate the control position between high dryness, poor centrate, and wet cake, good centrate A close-fitting collector for the solids phase. Also called a Rennbahn The angle the flight subtends to a perpendicular from the axis. Can be positive, which lifts the cake, then sometimes called cant, or it can be negative when cake will be pressed against the bowl wall. In some designs the flights are "canted" in the beach section, to provide zero rake on the beach; i.e. the flights are perpendicular to the beach. Tiles are designed to give the conveyor flight tips a cant to improve scrolling efficiency Conventionally the end of the decanter where the cake discharges The flange, or end plate, which encloses the bowl at the cake end, and includes the spindles which support the rear main and rear conveyor bearings The percentage of the suspended solids in the feed which reports to the cake discharge See racetrack A collector with rinse nozzle connections to slurry the cake on exit from the bowl. Used for counter-current washing in some processes Refers to a conveyor with an inner flight as well as the main outer flight. The inner flight has opposite pitch, so as to scroll surface cake towards the front hub to increase its residence time in the bowl A dimensionless group, used in fluid dynamics, heat and mass transfer A flight of small height, welded to pillars or vanes at tached to the conveyor hub Metal strips sometimes welded, or spot-welded, axially to the bowl or beach, or onto a liner thereon. Used sometimes in place of grooves
376 Glossary of Terms
Ring dam Rinse
Rinse chamber
Rinse zone Rotodiff
Saddle
Schmidt number Screen bowl
Screw Scroll Semi-included angle
Sigma
Sigma enhancemen t
Single piece dam
Skew Gaussian distribution Skimmer pump
Soft inlet
Spigot
Another term for weir plate Fluid, usually water, applied to the cake on the beach to wash out unwan ted impurities The compar tment in the hub of the conveyor behind the feed chamber into which the rinse is applied before spraying onto the cake. A buffer chamber is sometimes interposed between rinse and feed chambers. The floc chamber is sometimes used as a rinse chamber Same as rinse chamber The name used by the Viscotherm Company for their hydraulic gearbox A wear insert placed over the castellations of the beach that form the cake discharge. In profile it has the shape of a horse 's saddle. The shape spreads the wearing area A dimensionless group used in mass transfer A decanter which has a perforated bowl section on the end of the beach Synonymous with conveyor Synonymous with conveyor Half the apex angle of a cone. Used to describe the beach angle A scale-up factor for decanters, indicating the clarification ability A means of increasing the clarification capacity of a bowl, using discs or vanes t see clarification enhancement) A type of weir plate, made in one piece, which covers all centrate discharge points to the same level A log-probability distribution A special pipe a t t achment close to the bowl front hub. which enables centra te to be skimmed off the pond surface, obviating the need for a weir plate. The skimmer can be adjusted while the bowl is at speed, which effectively alters the pond height A type of feed zone that enables feed to be brought up to speed and enter the pond wi thout turbulence A short raised step to afford mat ing in a recess of another component
Spindle
Split bcach Split dam
Standard deviation
Stellite Stokcs settling vclocity
Sub-frame
SlIIlM~tleei stlart Supccficial vclocity
'I'nrque Torque overload a r r ~ i
'I'tiple lead Ilniversal darn IJpper rasing
A sinall solid or hollow tube on the axis of a r:ompc:)rierlt which rnt,ates. Siiialler than a hub A diial ;Inglc beach A type ofweirp1;il.r for t h e whole front hI ih ,
madc in two identical halves A rnathcinatical tcrm to indicate the spread of a sct of numbcrs. Thc dlfferciic,c: bclwccii each number and the arithmetic niean of them all, is squared. All the squared figures are tlieri
added together and divided by ltle t.ot.al number, Finally thv square root i s taken of Ihisnumber A propriet.ary name for i t type OF hard surfacing Thc tcriiiiiial falling vclucity uf'a particle in a gravitational ficld of I g H S predicted by Stokcs' law A fabricated frame f i t t d iindt?r t . h ~ niairi frame to support i t arid thc rriairi r I i o l r r and hark- drive systems Svnoriynious will1 pinion shalt Thc avcragc t~uloci ty of a fluid through it hr:d of particles. averaged over t h y wt~iolt. a r u r > T t11r bod See fliratirig cwriveyor A cylindrical bar ai1actietl t o one r n d ola corivryor on lrlrarr decanters. tensioned to support the t h r u s t reaction of thc conrcyor caused by the torquc nccdcd to scroll tht. cake A decantcr prnccss whcrc dewatcring t.:ikrs p l a ~ t . but the cake phase re~iiiiiris liquid A decanter. or process. in which the feed is split into three phases, gent.riiI\y ii cake and a light a n d heilvy liquid A repl;ic-r;iblc assembly fixed to tliu ct invryor llight tips 10 comh;it erosion Tangential fbtcc times distance An :irm which can be mounted on a gearhox pinion shaft, which is kcpt at rest bv a torque overload mcchanism. 'Hie n i r c h a n ism releases thc ar1u when an overroad o(:(:urs Three separate flights on a conveyor liuh Another name for an eccentric dam The uppcr half ofa casing. usually hinged to allow access lo the bowl
378 Glossary of Terms
Van der Waal forces
Vanes
Vertical decanter
Vibration isolator Viscotherm
Wash Wash out
Wear insert
Weir Weir plate
Wet beach
Windage
Windage and friction
Window
Name given to small inter-particulate forces at the molecular level Longitudinal plates fixed to the conveyor hub supporting the flights. Used for full axial flow, the hubless conveyor, and Sigma enhancement A decanter whose axis is vertical when installed Same as anti-vibration mount Name of the company which supplies the Rotodiff Same as rinse The unwanted discharge of feed from the cake discharge ports A fabrication which is inserted into another to combat erosion. Found in the feed zone around the cake discharge and other areas prone to wear The discharge of liquid over a lip A replaceable plate on the front hub face to control the pond height That part of the beach which is below the pond level The flow of air induced by the rotat ion of the centrifuge Name given to the power component arising from windage and the various frictional forces in the decanter system A hole in a conveyor flight, which allows the clarified liquor to flow axially (otherwise, it would flow helically, between the flights)
Appendix
This ;ippt?ndix l.ahulai.es all the data used i r i the previous chapt.ers, arid sorneof'whir:h i s f.he sour(:t: data for graphs iisetl i r i iht: illustrnlioris.
380 Appendix
Table A.1 Spent grain data used in Section 6.4.1
Decanter Bowl Diameter mm 425 1200 Clarifyin 9 Length mm
Process Distilliary Spent Wash
,l,
Machine Condltlons 4.Bowl Speed rpm. 51Pond dia ram.
3150 261
61Conveyor diff' rpm. 7.Conveyor torque kNm
Feed Conditions 9.Feed Rate m3/h.
23.2 0.37
3150 3150 3150 261 261 261 18.2 13.2 23.2 0.871 0.66 0.50
3150 3150 261 261
, ,
18.2 13.2 0.87 1.06
10.Feed Solids % w/w d.s.
Product Conditions 16.Cake Solids %w/w. 18.8 7.Centrate Solids m~/I. 11100
A.Centrate Rate m3/h. 3.5
4.6 4.61 4.6 9.0 9.0 5.50 5.44 '-" 4.15 3.96 4.30
9.6 4.48
24.8 23.5 16.6 22.2 25.6 13200 11000 10600 10700 12900
3.8 4.0 7.3 7.6 7.8
D.Solids Recovery %w/w. E.Cake Rate k~/h w.b.
84.8 80.0 77.1 78.2 78.9 75.0 1.14 0.81 0.63 1.68 1.38 1.18
Appendix 381
Table A.] (contd.)
Decanter Bowl Diameter mm I 425 Clarifvino Lenoth mml 1200 Clari~in~l Len~lth mm
Process Distilliary Spent Wash
Machine i .Run Number Machine Conditions 4.Bowl Speed rpm. 5.Pond dia mm. 6.Conveyor diff' rpm. 7.Conveyor torque kNm
Feed Conditions 9.Feed Rate m3/h. 10.Feed Solids % w/w d.s.
Product Conditions 16.Cake Solids %w/w. 17.Centrate Solids m~l. A.Centrate Rate m3/h.
D.Sohds Recovery %w/w. E.Cake Rate k~/h w.b.
3150 3150 3150 261 261 261
23.2 18.2 13.2 ,
0.60 0.99 1.56
24.5 13300 ..... i"1.6
77.4 2.16
26.3 14600
11.8
75.0 1.97
27.9 15400
12.4
66.4 1.36
3150 261
23.2 0.75
21.9 14700
15.9
66.9 2.15
3150 261 18.2 1.37
25.5 17200
16.3
60.2 1.67
1 |
12 |
3150" 261 13.2 1.75
,
18.0 3.81
28.1 18000
16'6
56.4 1.38
382 Appendix
Table A.2 Agricultural product data used in Section 6.4.2
Decanter Bowl Dia. mm 150 Clarifying Length mm 220
Process Agricultural Product
Machine 1 .Run Number Machine Conditions 4.Bowl Speed rpm. 5.Pond dia mm. 6.Conveyor diff' rpm. 7.Conveyor torque kNm
5OOO1 5OOO 112 i 112 5.01 3.0
O.Ol i ..... O.Ol
I Feed Conditions
Product Conditions l~6.Cake Solids %w/w. 11.4 ~17.Centrate Solids m~l/i. 600 A.Centrate Rate m3/h. 0.2
. ,
D.Solids Recovery %W/W. E.Cake Rate k~/h w.b: F.Q/T. mm/h G.TN N/cm2
,,
93.0 0.02 1.27 0.48
10.7 650 0.3
92.4 0.02 1.41 0.41
,.sooo 112! 1.5
0.01
10.9 680 0.3
92.7 0.02 1.42 0.59
~ooo 112 1.5
D.01'
12.3 730 0.1
92.1 0.01 0.63 0.51
50OO I 5O00 1121 112 3.01 s.o
0.011 0.01
11.0 660 0.1
92.9 0.01 0.63 0.28
9.1 680 0.1
92.8 0.01 0.64 0.35
Appendix 383
Table A.2 (contd.)
Decanter Bowl Dia. mm 150 Clarifying Length mm 220
Process Agricultural Product
Machlne 1 .Run Number Machlne Conditions 4.Bowl Speed rpm. &Pond dia mm. &Conveyor diff" rpm.
5OOO 112 3.0
0.01 7.Conveyor torque kNm
Feed Conditions 9.Feed Rate m3/h. 0.4 10.Feed Solids % w/w d.s. 0.87
Product Conditions 16.Cake Solids %w/w. 9.9 17.Centrate Solids mg/I. 650 A.Centrate Rate m3/h. 0.3
D.Solids Recovery %w/w. 93.1 E.Cake Rate kg/h w.b. 0.03 F.Q/Z mm/h 1.81 G.T/V N/cm2 0.51
5000 112 5.0
0.02
13.0 3990
0.8
68.0 0.05! 4.28 0.72,
5OOO 112
10.0 0.01
12.0 1880
0.7
85.3 0.07 4.09 0.60
5000 1121 5.0
0,03
18.0 1890
1.1
86.1 0.07 6.21 1.15
12 =
Table A.2 (contd.)
425
Agricuitural Product
Decanter Bowl Dia. mm ..... Clarifying Length mm 800
Process ~
384 Appendix
Machine 1 .Run Number
M a c h i n e Conditions . . . . . .
4.Bowl Speed rpm, :5.Pond dia mm. 6,Conve}/or diff' rpm, 7.Conveyor torque kNm
~Feed Conditions
Product Conditions 16.Cake Solids %w/w. 17,Centrate Solids mg/I, A.Centrate Rate m3/h.
D.Solids Recover}/%w/w. E.Cake Rate kg/h w.b, F.Q/~ mm/h G.T/V N/cm2
3400 3400 3400 3400! 3400 3400 267 267 267 267; 267 267 7.0 7.0 5,0 5.0 3,0 3,0
0,29 0.29 0.36 0.36 0.35 '6,54
,27 127 , 4 3 154
1.0 1 .I . 1 .I I . I
98,7 98.7 98.7 98,6 98,6 98.6 0.48 0,44 0.4A 0.39 0.36 0.36 0,60 0.60 0,60 0.60 0,60 0.60 0.33 0.33 0.42 0.42 0.41 0.62
Appendix 385
Table A.2 (contd.)
Decanter Bowl Dia. mmJ 425 800 Clarifying Length mm
Process Agricultural Product
Machine 1.Run Number Machine Conditions 4.Bowl Speed rpm. 5.Pond dia ram. 6.Conveyor diff' rpm. 7.Conveyor torque kNm
Feed Conditions 9.Feed Rate m3/h, 10.Feed Solids % w/w d.s,
Product Conditions 16.Cake Solids %w/w. 17.Centrate Solids mg/I. A.Centrate Rate m3/h.
D,Solids Recovery %w/w. I E,Cake Rate kg/h w.b. F,Q/~ mm/h G.T/V N/cm2
21 2 t 2 i 21 2 i 2 19 20 21 22 23 24
3400 3400 3400 3400 3400 3400 267 267 ! 262 262 262 262 1.0 1,0 3.0 1,0 0.5 1,0
1.34 1,21 0,37 1,21 2.31 1.50
1.5 3,74
19.1 700 1,2
98,5! 0,29: 0.60 1.55
1,5 3.74
19.3 700 1.2
98,5 0.29 0.60 1.41
1,5 3.74
14,9 700 1.1
98.6 0.37 0,60 0,44
1,5 3.74
18.4 700 1.2
98.5 0,30 0.60 ~ 1,41
1,5 3,74
20.9 700 1.2
98.5 0,26 0,60 ! 2.68
1.5 3.74
19.1 700 1.2
98.5 0.29 0.60 1.74
386 Appendix
Table A.2 (contd.)
Decanter Bowl Dia. mm 425 . ,
Clarifying Length mm 800 Process Agricultural Product
Machine I .Run Number Machine Conditions 4.Bowl Speed rpm. 3400 3400 3400 3400 3400 5.Pond dia ram. 262 262 262 262 262 6.Conveyor diff' rpm, 1.0i 0.6 0.7 0.8 0.9 7.Conveyor torque kNm 1.50 2.06 2,10 1,00 1.00
Feed Conditions
Product Conditions 16.Cake Solids %w/w. 20.0 17.Centrate Solids mg/l. 700 A.Centrate Rate m31h, 1.2
D.Solids Recovery %wlw, 98.5 E,Cake Rate kglh w,b, 0.28 F,QIZ: mmlh 0,60 G.T/V N/cm2 1,74
22.9 700 1.3
98.4 0.24 0,60 2.39
23.3 700 1.3
98.4 0.24 0.60 2.44
19.9 700 1.2
98,5 0.28 0.60 1.16
20.5 700 1.2
98.5 0.27! 0.60 1.16
0 30
Appendix 387
Table A.2 (contd.)
Decanter Bowl Dia. mm Clarifying Length mm
Process
425 800
Agricultural Product
Machine . ,
1.Run Number Machine Conditions 4.Bowl Speed rpm. 5,Pond dia ram, 6,Conveyor diff' rpm. 7.Conveyor torque kNm
iFeed Conditions
16.Cake Solids %w/w. 17,Centrate Solids mg/I. A.Centrate Rate m3/h.
D.Solids Recovery %w/w. _E.Cake Rate kg/h w,b. F.Q/Z mm/h G,T/V N/cm2
3400 268 18.0 2.25
20.9 600 ~ 12.2!
98.1 1.83 5.60 2.61
3400 268 18.0 2,25
20.1 5OO 9,3
98,4 1.47! 4,321 2.61
3400 268 12.0 1.50
18.2 390 9.3
98.8 1.66 4.40 1.74
3400 268 6.0
0.75
15.6 340 9.1
99.0 1.94 4.40 0.87
388 Appendix
Table A.3 Lime sludge classification data used in Section 6.4.3
Decanter Bowl Diameter mm 150 170 Clarifying Len~h
Procen Lime Sludge Classification
Machine , 1J 1.Run Number 1 I Machine Conditions 4.Bowl Speed rpmo , 5000 5.Pond dia mm. 143
Feed Conditions 9.Feed Rate m3/h. 0.6 I
I Product Conditions D.CaC03 Solids Recovery %w/w. 92.5 F.M~I(OH)2 Solids Recovery %w/.w. 92.0
,,
5000 5000 143 143
92.5J 87.5 64.0 ! 55.0
5OOO �9 143
1.2 I
.,87.5 I 51 .oi
I
5OOO 143
86.5 45.5
1 6
5000 143
1.5
86.5 45.0
Appendix 389
Table A.3 (contd.)
Decanter Bowl Diameter mm Cladfyin~l Len~Ith
Process
150 170
Lime Sludge Classification
Machine 1.Run Number Machine Conditions 4.Bowl Speed rpm. 5.Pond dia ram.
Feed Conditions 9.Feed Rate m3/h.
Product Conditions D.CaC03 Solids Recovery %w/w. F.Mg(OH)2 Solids Recovery %w/w.
~1 ~1 ~1 1;I ,:1 5000 5000 5000 5000 5000
143 143 143 143 143
"l ~0] ~01 ~i ~! ~9.0 81.5 81.5 79.0 79.0 :10.0 42.0 54.0 46.0 40.0
390 Appendix
Table A.3 (contd.)
Decanter Bowl Diameter mm Clarifying Length
Proceu
150 170
Lime Sludge Classification
IMachine 1 .Run Number Machine Conditions 4.Bowl Speed rpm. 5.Pond dia mm.
Feed Conditions 9.Feed Rate m3/h.
Product Conditions D.CaC03 Solids Recovery %w/w, F,Mg(OH)2 Solids Recovery %w/w.
,3o 5000 5000 5000 5000
130 130 130
96.0 94.0 91.5 87.0 85.0 43.0 47.0 53.0 55.0 45.0
1 18
5OOO 130
1.4
83.0 51.0
Appendix 391
Table A.3 (contd.)
Decantei Bowl Diameter mm i 150 c la .~ i .~ Le.~th
Process 17O
, , .
Lime Sludge Classification
L , ,
Machine 1.Run Number
!Machine Conditions 14.Bow,I S~eed r pm . "
5.Pond dia ram.
Feed Conditions , ,,
9.Feed Rate m3/h.
Product Conditions' D.CaCO3 Solids Recovery %w/w.
|F.Mg(OH)2 Solids Recovery %w/w. I ! !
1 1 1
5ooo 5o00 " ~,o6o 130 130 130
1 221 .I I
85.0 8O.0 33.0 t . . . . . 38.0 !
79.0 ... 35.'0 t
392 Appendix
Table A.3 (contd./
Decanter Bowl Diameter mm 356 870 Clarifying Length
Proce• Lime Sludge Classification
Machine I .Run Number Machine Conditions 4.Bowl Speed rpm. 5.Pond dia ram.
Feed Conditions 9.Feed Rate m3/h.
Product Conditions D.CaCO3 Solids Recovery %w/w. F.Mg(OH)2 Solids Recovery %w/w.
3250 286
37j 97.8 73.0
8:~'J
94.0 83.0
325oj 286l
1 9"8 !
93.5 84.O
21 21 2 28 29 30
3 2 ~ I 32sol 325o 2861 2861 286
,. I I
_ I
56.0 61 55.0 I . ,
Appendix 393
Table A.3 (contd.)
Decanter Bowl Diameter mm 356 870 Clarifyin~ Length
Proceu Lime Sludge Classification
Machine 1.Run Number Machine Conditions 4.Bowl Speed rpm. ~5.Pond dia ram.
Feed Conditions 9.Feed Rate m3/h.
Product Conditions D.CaCO3 Solids Recovery %w/w, F.Mcj"(OH)2 Solids Recovery %w/w,
2
3250 286
17'7 I
88.sl 52.0
3250 286
20"4 I
86.0 48.0
394 Appendix
Table A.4 Clay classification data used in Section 6.4.4
Decanter Bowl Diameter mm . 425 ! Clarifying Length mm 780
Process Clay Classification
Machine 1 .Run Number Machine Conditions 4.Bowl Speed rpm. 3150 5.Pond dia ram. 232 6.Conveyor diff' rpm, 32
;Feed C:ondltlon$ 9.Feed Rate m3/h. 16.0 10.Feed Solids % w/w d.s. 21.4 11.%<21~ 73
50 18.5
12.%<1p 14.Rate t/h.
Product Conditions 16.Cake Solids %w/w. 58.8 17.Centrate Solids % w/w 16.0
..
A.Centrate Rate m3/h. 17.8 B.%<2F 85 i C%< 1 t~ 60 D.Solids Recovery %w/w. 34.7
.,,
E.Cake Rate t/h w.b. 2.33 F,Cake S.G, 1,58 G.Yield <2is 76.00
232 228 228 226 32 32 51 32
20.0 12.0 8.0 6.0 .=
20.3 19.0 18.5 13.8 73 68 68 70 50 50 50 49
22.9 13.6 9.0 6.6 ..
60.1 17.0 23.8
83 64
22.0 1.71 1.6[]
86.00
59.3 58.8 14.0 12.8 13.5 8.7
89 89 70 74
35.0 40.0 1.51 1,13 1.58 1.58
85.50 78.7C
58.8 7.7 6.5 97! 80
51.0 0.79 1.59
68.00
3150 226
32 . . . . .
10.0 13.8
69 48
10.9
58.8 ... . .
8.8 11.4
94 76
42.C] 1.1g 1.5~
78.0C
Appendix 3 9 5
Table A.5 WAS thickening data used in Section 6.4.5
Decanter Bowl Diameter mm: 737 ' Clarlfying Length mm I 1550
Process! WAS WAS Thickening
"Machine ' 1 ,Run Number 'Machine Conditions 4.Bowl Speed rpm. " 2300 2300 2300 5,Pond dia mm. ' 345 345 '345 6,Conveyor diff' rpm. 20.0 10.0 15.0i
!
7.Conveyor torque kNm - .
|
Feed Conditions r9.Feed Rate m3/h.
, ,
lO.Feed Solids % w/w d.s.
Additive Conditions 11.Type. : 14.Rate m3/h. 15.Dilution m3/h.
Product Conditions 16.Cake Solids %w/w 17.Centrate Solids m~l/l. A.Centrate Rate m31h. B.Polymer Dose kg/tonne.
D.Solids Recovery %w/w, E.Cake Rate k~l/h w,b. F.Q/.~ mm/h
_
4o.o l :~o.o I i oo t
,o I o I O0 O0 olo olo
, ,
3,5 1210 28.7 0.0
92,0 ~i.32 6.36
5.8 1770 33.7 0.0
,.
85.9 6.34 6.36
'~0.01 .,1 .o9 t
'" 0 I O0 olo
5.4 1320 32.7 0.0
90.1 7.30 6.36
~3001 345, 15.0 i
.0.00, |
70.0 ! 1tl I
0
2.8 .28~ 47.1
0.0
82.6 22.85 11.13
23001 345 I
. =
5.0 . , .
0.11
70.0~ 1.14[
0 I O0 _ 010
4.3 " 3660
56.3 i, 0.0
74.2 13.70
"'11.13
2300 345 10.0
_
0.61
70.0 1,15
0 0.0 0,0
3.3- 2920 49.9
O.0
81.9 20.10 11.13
3 9 6 Appendix
Table A.5 (contd.)
Decanter Bowl Diameter mm Clarifying Length mm
Process
737 1550 WAS Thickening
Machine 1 .Run Number Machine Conditions 4.Bowl Speed rpm. 5,Pond dia ram. ,,
6.Conveyor diff' rpm, 7.Conveyor torque kNm
2545 347 20,0 0.86
Feed Conditions [9.Feed Rate m3/h. 40.0 r
10.Feed Solids % w/w d.s. 1.37
Additive Conditions 11.Type. 0 114.Rate m3/h. 0.0 15.Dilution m3/h. 0.0
Product Conditions 16.Cake Solids %wlw. 17.Centrate Solids mg/l. A.Centrate Rate m3/h, B.Polymer Dose kg/tonne.
D.Solids Recovery %w/w. E.Cake Rate kg/h w.b. F.Q/E. mm/h
4.9 1690 29.8 0.0
90.8 10.18 5.19
2545 347 15.0 0.61
0 0,0 0.0
3.5 3550 47.3
0.0
82.5 22,74 9.09
2545 347 10.0 0.53
0 0.0 0.0
4.3 3850 52.7 0.0
78.7 17.26 9,09
2545 347 18,0 0.85
0 0.0 0.0
3 . 4 �84
3460 46.4 0.0
83.5 23.62 9.09
2545 347 12.0 0.56
0 0.0 0.0
4.1 3770 50.9
0.0
80.1 19,12 9.09
2545 347 18.0 0.94
40.0 1.22
0 0.0 0.0
5.9 1500 32.6
0.0
90.0 7.42 5.19
Appendi.x" 3 9 7
Table A.5 (contd.)
Decanter Bowl Diameter mm Clarifyin~l Length mm
Process
737 1550 WAS Thickening
Machine ~1 ,Run Number
!
Machine Conditions 4.Bowl Speed rpm. , 2545 2545 2545 2545 5.Pond dia ram. 3471 347 347 347
i
6.Conveyor dil l ' rpm. 20.0 15.0 22.0 21.0 ,7.Conveyor torque kNm ~ 0.87 0.87
J Feed Conditions 9.Feed Rate m3/h. 110.Feed Solids % w /w d.s.
0.99
Additive Conditions 11 .Type. , 0
;14.Rate m3/h. 0.0 i15.Dilution m3/h. 0,0 1 1
, Product Conditions 16.Cake Solids %w/w. 4.9
|
17.Centrate Solids mcj/I. , 920 A.Centrate Rate m3/h. 31.5 B.Polymer Dose k~/tonne, , 0.0
0 0.0 0.0
3.4 1680 50.2
0,0
0 0.0 O0
36 666 28.4 0.0:
0 0.0 0.0
3.9 770
29.7 0.0
2545 347 19.8
o o 0.0 0.0 0.0
5,0 1030 31,8
0.0
2545 347 19.8
!
4O.O |
1.24 |
|
0.0
5,4 1170 31.4
0.0
D.Solids r~ecovery %w/w. i 93,5 88.8 95.7 94.6 92.6 92.6 E.Cake Rate kg/h w.b. 8.52 19.81 11.60 10.31 8.18 8.55
. . . . .
F.Q/:~ mm/h 5.19 9.09! 5 19 5.19 5.19 5.19 E
398 Appendix
Table A.6 Digested sludge thickening data used in Sections 6.4.6 and 7.1
Decanter Bowl Diameter mm Clarifying Length mm
425 8OO
. . . . .
Process Thickening Digested Sewage
J Machine Conditions 4.Bowl Speed rpm. 3150
Feed Conditions 9.Feed Rat( 10.Feed Sol
J 5.Pond dia mm. 257 6.Conveyor diff' rpm. 12.0
3150 3150 257 257 13.0 15.0
3150 3150, 3150! 257 257 257 14.0 14.0 14.0
Additive Conditions l i . T y p e . . _ Z63 Z63 Z63 Z63 Z63 Z63 12.Concentration %w/w. o. I 0 o, I 0 o, I 0 O. I 0 o. I 0 o, I 0
. . . . , .
13.Addltion point. _ B B B B B B 14.Rate m3/h. 1.0 1.0 1.3 0.8 1.0 1,5
,,
Product Conditions n
16.Coke Solids %w/w. ,,,
17.Centrate Solids mg/l. .....
A.Centrate Rate m3/h. B.: Pol y mer Dose kg/tonne.
. . .
D.SolIds Recovery .%.w/w. E.Cake Rate kg/h w.b. i~'.Q/~ mm/h G.Psi * 100
14.5 1400 15.5 3.3
92.8 1.93
5 67
10,8 6.4 8.7 , ,
700 1000 1000 ,,.
14.6 12.8 14.1 3.2 3.9 2.4
. .
96.7 96.1 95.5 , ,
2.77 4.86 3.42 . . . .
5.67 5.67 5.81 _ _
3.92 7.30 5.15 , .
9.7 11.5 1450 675 14.7, 16.6 3.1 4,7
93.3 96,7 3.07 2.68 5.81 5.81 4.54 3.85
Appendix 399
Table A.6 (contd.)
Decanter Bowl Diameter mm 425 Clarifying Length mm
Process 800 Thickening Digested Sewage
I
Machine l.Run Number Machine Conditions 4. Bowl Speed rpm. 3150 5.Pond dia mm, 257 6.Conveyor diff' rpm. 15.0
Feed Conditions 9.Feed Rate m3/h. 10.Feed Solids % w/w d.s.
Additive Condltions 111~.Type. Z63 ! 12.Concentration %w/w. 0.10 :13,Addition point. B ~14.Rate m3/h. 0.8
Product Conditions _
116.Cake Solids %w/w. 8.1 17.CenlTate Solids m~l/I. 1425 A.Centrate Rate m3/h. 15,6 B.Polymer Dose k~l/tonne. 2.5 1 D.Solids Recovery %w/w. _ 94.1 E.Cake Rate k~l/h w.b. . 3.84 IF.Q/~ mm/h 5.81 rG.PsJ " I00 5.61
3150 3150 3150 3150 257 257 257 257 12.0 16.4 16,4 14.0
Z63 0.10
B 0.7
9.3 1350 13,8 2.6
94.0 2.52 4.4g 5,14
Z63 010
B 1,3
104 1450 11.1 2.9
93.9 3.86 7.1g 3.69
Z63 0.10
B 1.0
7.6 1150 18.2 2.9
95.1 4.36 6.29 6.18
3150 257 13.01
I
Z63 Z63 0.10 0.10
B B 0.8 0.8
7.0 950 14.8 2,6
96.3 4,03 5.12 6.85
7.6 1150 112"59.
94.6 3.26 5.12
400 Appendix
Table A.6 (contd.)
Decanter Bowl Diameter mm 425 . . . . .
Clarifying Length mm 800 Process Thickening Digested Sewage
Machine I I .Run Number 13 Machine Conditions 4.Bowl Speed rpm. 3150 ! I ! 3 501 ! 5.Pond dia ram. 2571 2571 2571 2571 2571 257 6.Conveyor dill' rpm. 20.01 17.6 i 16.0 ! 15.01 13.0 ! 12.0
Feed Conditions
Addltive Conditions 1.Typel Z63 Z63 Z63 Z63 Z63 Z63 1
12.Concentration %w/w. 0. I 0 0. I 0 0. I 0 0.10 0. I 0 0. I 0 13.Addltlon point. B B B B BI B 14.Rate m3/h. 1.6 1.4 1.1 0.9 0.7 0.7
Product Conditions 16.Cake Solids %w/w, 10.5 9.6 9,2 8.2 6,8 12.0 17.Centrate Solids m~I/I. ' 1300 1000 1000 1000 950 1100 A.Centrate Rate m3/h, 21.4 17.5 15.9 12.9 10.4 i' 1.9:
k~l/tonne. 2.5 3.6 2.8 2.9 2.9 2.8 B.Polymer Dose
D.Solids Recovery %w/w. 95,6 95.4 95.9 95,7 96.0 94.9 E.Cake Rate k~l/h w.b. 5.80 3.821 4,03 3.51 3.45 2.02 F.Q/~ mm/h 8.85 6.91 6.50 5.39 4.56 4.56
G.Psi * 100 3.01 4.79 4.49 6.03 7.79 3.92
Appendix 40 ]
Table A.6 (contd.)
Decanter Bowl Diameter mm Clarifying Length mm
Process
425 80O Thickening Digested Sewage
Machine 1.Run Number Machlne Conditions 4.Bowl Speed rpm. !5.Pond dla mm. 6.Conveyor diff' rpm.
I 24
Feed Conditions 9.Feed Rate m3/h. 19.0 10.Feed Solids % w/w d.s. 2.17
Addltlve Cor~llflons
3150 3150 3150 31501 3150 3150 257 257 257 257 257 257 15.0 15.0 15.0 15.01 15.0 13.0
11.Type. Z92 Z92 12.Concentration %w/w. 0.13 0.13 13.Addition point. B B 14.Rate m3/h. 1.0 1.0
Product Condltlons 16.Cake Solids %w/w. 12.0 9.3 17.Centrate Solids mg/l. 1250 1050 A.Centrate Rate m3/h. 16.7 14.3 B.Polymer Dose kg/tonne. 3.0 3.7
D.SolIds Recovery %w/w. 94.9 95.5
16.8 1.73
Z92 Z92 Z92 Z92 0.13 0.131 0,13 0.13
B B B B 1.3 1.5 0.8 0.5
11.7 750 15,3 4.8
9.7 850 15.2 6.0
8,4 1100 13.8! 2.91
I E.Cake Rate k~l/h w.b. 3.25 3.46 I F.Q/~ mm/h 8.57 5.81 G.Psi ~ 100 3.03 4.78
96.5 2.69 5.81 3.91
8.3 950 14.0 2.2
95.9 95.3 95.4 3.11 3.70 3.32 5.81 5.81 5.81 4.94 5.51 5.37
402 Appendix
Table A.6 (contd.)
Decanter Bowl Diameter mm 425 800 Clarifying Length mm
Process Thickening Digested Sewage
Machine 1.Run Number Machine Conditions 4..Bowl Speed rpm. 3150 31 50 31 50 i5.Pond dia mm, 257 257 257 6.Conveyor dlff' rpm. 21,5 20.0 20,0
Feed Conditi-0ns 9.Feed Rate m3/h. 19'.2 10.Feed Solids % w/w d.s. 2.61
Addltlve Condlfions 11 .Type. Z92 12.Concentration %w/w. 0.13 13.Addition point. B 14.Rate m3/h. 0.8
Product Conditions 16.Cake Solids %w/w. 9.1 17.CentTate Solids mg/I. 1800
A.Centrate Rate m3/h. 14.7 B.Polymer Dose kg/tonne. 1.9
D.Solids Recovery %w/w. 94.'7 ! E.Cake Rate kg/h w.b. 5.18 F.Q/~, mm/h 6.64
. . . .
G.Psi * 100 4.70 . .
Z92 0.13
B 1.0
I0.7~ 1350 15,9 2.4
95.9 4.70 6.78 3.56
Z92i 0.13
B 1.1
9.6 1500 17.4 2.6
95.1 5.28 7.47 3.88
3150 257
21.0
Z92 0.13
B 1.3
8.0 1450 18.6 2.9
94.9 6.26 ,=,
8.16 4.93
3150 257
21.5
Z92 0.13
B 1.4
6.9 1600 20.0
3.6
93.5 6.56 8.71 6.34
3150 257 16.0
18.0 1.91
Z92 0.13
B 0.8
8.5 1400 14.9 2.7
93.9 3.80 6.22 5.50
Appendix 40 3
Table A.6 (contd.)
Decanter Bowl Diameter mm Clarlfyln~ Length mm
Process
425 800 Thickening Digested Sewage
Machine 1.Run Number Machine Conditions 4.Bowl Speed rpm. 31 50 31 50 31 50 5,Pond dla mm. 257 257 257 6.Conveyor diff' rpm. 14.0 15,0 16.0
Feed conditions 9.Feed Rate m3/h. 15.2 10.Feed Solids % w/w d.s. 1.81
Additive Conditions 11 .Type. Z92 12.Concentration %w/w. 0.13 13.Addition point. B 14.Rate m3/h. 0.6
Product Conditions 16.Cake Solids %w/w. 6.9 17.Centrate Solids mg/I, 1500 A.Centrate Rate m3/h. 12.1 B.Polymer Dose kg/tonne. 2.7
D.Sollds Recovery %w/w. 93.4 E.Cake Rate kg/h w.b. 3.69 F.Q/~ mm/h 5.25 G.Psi " 100 7.35
Z92 0.13
B 1.1
12.6 1450 18.7 3.2
93.7 3.18 7.19 2.78
Z92 0.13
B 1.3
12.2 1250 18.7 3.6
94.6 3.36 7.19 3.02
3150 257 17.0
Z92 0.13
B 1.3
12.6 1100 19.0 3.8
94.9 3.07 7.19 3.29
3150 257 18.0
Z92 0.13
B 1.3
8.9 1200 17,6 3.7
95.0 4.46 7.19 4.85
1 36
3150 257 19,0
20.8 2.33
Z92 0.13
B 1.3
7,9 95O 16.1 3.2
96.9 5.95 7.19 4.98
404 Appendix
Table A.6 (contd.)
Decanter Bowl Diameter mm Clarifying Length mm
425 800
Process Thickening Digested Sewage
[ , I
Machine Conditions 4.Bowl Speed rpm. 5.Pond dia ram. 6.Conveyor diff',.,rpm.
31 50 31 50 3150 3150 257 257 257 257 20.0 10.0 8.0 12.0
3150 257~ 12.0
1 42
3150 257 12.0
Feed Conditions
Additive Conditions 11.Type. 12.Concentration %w/w. 13.Addition point. 14.Rate m3/h. "
Product Conditions 16.Cake Solids %w/w. 17.Centrate Solids mg/I. A.Centrate Rate m3/h. B.Polymer Dose k~l/tonne.
D.Solids Recovery %w/w. E.Cake Rate kg/h w.b. F.Q/E mm/h G.Psi ~ 100
Z92 Z92 Z92; ... .
0.13 0.10 0.10 B B B
1.3 0.5 0.5
6,9 " 12.41 14.3 1150 2150 3000 16.6 9.3~ 10.0 4,0 1.6 1.8
95. I 93.8 89.2i 5.40 2.40 1.70 7.19 3.87 3.87 7.44 2.53 2.02
Z92 0.10
B 1.3
14.1 4300 17.3 2.7
84.0 ..
2.74 ..,
6.50 1.83
,..
Z92 0.10
B 1.0
13.0 3300 15.2 2.5
87.7 2.72 5.88 2.26
Z92 0.10
B 1.3
13.8 2400 15.7 ~ 3.3
~.~ 2.48 5.88 2.27
Appendix 40 5
Table A.6 (contd.)
Decanter Bowl Diameter mm Clarifying Length mm
Process
425 800 Thickening Digested Sewage
Machine ~.Run Number Machine Conditions [4.Bowl Speed rpm.
1 43
3150 3150 5.Pond dia ram. 257 257
,,, .
6.Convey..0r diff' rpm. 10.0 11.0:
Feed Conditions , , ,
9.Feed Rate m3/h. 15.3 10.Feed Solids % w/w d.s. 2.16
Additive Conditions 11.Type. Z92 Z92 -i 2.Concentration %w/w, 0.10 0.10 13.Addition point. B B 14.Rate m3/h. 0.8 0.8
Product Conditions 16.Cake Solids %w/w. 13.0 13.6
.,.
17.Centrate Solids m~l/I. 38001 1700 A.Centrate Rate m3/h. 13.91 12.9 B.Polymer Dose k~l/tonne.
D.Sollds I~ecovery %w/w.
2.3 2.7
84.0 92.2 E.Cake Rate k~l/h w.b. 2.11 1.88 F.Q/Z mm/h 5.2g 4.84 G.Psl " 100 2.33 2.91
3150 257 10.0
Z63 0.10
B 0.8
14.5 2250 13.0 2.7
89.3 1.68
4.84 2.52
3150 257 10.0
Z63 0.10
B 0.8
13,5 1200 11,0 3.0
94.6 1.72
4.15 3.01
3150 257 9.0
Z63 0.10
B 1.0
15,0 1600! 11.6 4.4
91.9 1.40
4.15 2.63
3150 257 8.0
12.0 1.91
Z63 ... .
0.10 B
1.0
15.2 1600 11.6 4.4
'91.9 1.38
4.15 2.30
406 Appendix
Table A.7 Lactose washing data used in Section 6.4.7
Bowl Dia. Mm Wash - ' ~ . ~ e r e n t i a l
iO/o of Feed 0
2.5 5
7.5 10
12.5 15
356 23RPM
Specific Washing .i
356 356 36RPM 46RPM
General washing "
" 4 .3 4 3 .8 3.'3 4.3 318 4.1 3.7 3.4 2.9 3.5 3.1 3.7 3.2 3.1 2.4 2.8 2.5 3.6 3 2.7 1.9 2.3 2 3.1 2.7 2.5 1.8 2 1.7
3 2.5 2.1 1.7 1.8 1.6 1.7 1.5
i|
600 356 600 49RPM 60RPM 49RPM
Appendix 407
Table A.8 Coal tailings data discussed in Section 6.4.8
Decanter Bowl Diameter mm 425 Clarifyin~l Len~Ith mm 560
Process Coal Tailings Dewatering
Machine I 1.Run Number I Machine Conditions 4.Bowl Speed rpm. 3150 3150 3150 3150: 5.Pond dia ram. 264 264 264 264 6.Conveyor diff' rpm. 39.0 39.0 51.0 41.0[ 7.Conveyor torque kNm 0.99 0.57 1.04 0.36
Feed Conditions 9,Feed Rate m3/h. 8.0 10,Feed Solids % w/w d.s, 35.40
Additive Conditions 11 .Type. M3127 12.Concentration %w/w, 0.10 13.Addition point. D 14.Rate m3/h, 1.2 15.Dilution m3/h. 0,0
Product Conditions
M3127 0.10
D 1.8 0.0
16,Cake Solids %w/w, 66,0 62.3 17,Centrate Solids m~l/I. 78000 1210 A.Centrate Rate m3/h, 5,6 5,3 B.Polymer Dose k~/tonne, 0.4 0.8
D.Solids Recovery %w/w. 84.7 99.7 E.Cake Rate k~l/h w,b. 3.63 3,48 F.Q/s mm/h 5,37 4,70 G,T/V N/cm2 1.15 0,66
M3127l 0.10
D 1.5 0.0:
64,4 1920
6,4 0,6
99.5 4.15 6.04 1,21
M3127 0.10
D 1.2 0.0
63.0! 1300
5.6 0,5
99.7 3.54 5.37 0.42
408 Appendix
Table A.8 (contd.)
Decanter Bowl Diameter mm .
425 560 _ Clarifying Length mm
Process Coal Tailings Dewatering
I
Machine I .Run Number Machine Conditions
[4.Bowl Speed rpm. 5.Pond dia ram. 6.Conveyor dill' rpm, 7.Conveyor torque kNm
Feed Conditions 9.Feed Rate m3/h. 10.Feed Solids % w/w d,s,
2000 2000 2000 2(~30 2000 2000 270 270 270 270 270 270
26.0 26.0: 26.0 26.0 26.0 26.0 0.73 1.30 0,73 0,68 0.78 0,60
Additive Conditions 11 .Type. M3127 M3127 12.Concentration %w/w. 0.10 0,10 13,Addition point. D D 14.Rate m3/h. 1.5 1.5 15.Dilution m3/h. 0.0 0.0
, ,
Product Conditions 16.Cake Solids %w/w. 61.5 61.4 17.Centrate Solids mg/l. 480 510
,,
A,Centrate Rate m3/h. 4.6 5.5 B.Polymer Dose k~l/tonne. .. 0.5 0.6
99.9 4,13
14.99 0.85
,.L
D.Solids Recovery %w/w. 99,9 99.9 E.Cake Rate kg/h w.b. 4.85 4.23 F.Q/Z: mm/h 13.33 11.66 G.T/V N/cm2 0.85 1.51
. . . . .
'1'213127" M3 i ;27 M3127 M3127 ..
0.10 0,10 0.10 0,10 �9 ,
D D D D 1,5 1.2 1.2 1.2
' o.0 " 0.0 o.o 0.o
64.5 ~.8 65.2 63,2 560 320 165 350 5.4 4,8 4.4 6.9 0.6 0.4 0.31 0.4
,,,
~.9 ~00.0 99,9 4.65 5,10 4.86
13.33 0.00 0,00 0.79 0.91 0,70
APl;endi.v 4(]9
Table A.9 Data for DS scaling used in Section 7.9
Decanter Bowl Diameter 425 .... J Clarifying Length 800 ,
Process Unspecified Effluent
Machine 1.Run Number Machine Conditions 4. Bowl Speed rpm. 3150 3150 5.Pond dia mm. 252 252 6.Conveyor diff' rpm. 1.0 1.6 7.Conveyor torque kNm 2.70 2.70
Feed C~ondition$ 9.Feed Rate m3/h. 3.0 5.0 10.Feed Solids % w/w d.s. 3.11 3.21
AdditiVe Conditions . . . .
11.Type. Z78FS40 Z78FS40 12.Concentration %w/w. 0.15 0.15 13.Addition point. B B 14.Rate m3/h. 0.5 0.8 15.Dilution m3/h. 0.0 0.0
_ =
: Product Conditions 16.Cake Solids %w/w.
, .
17.Centrate Solids mg/I. A,Centrate Rate m3/h. B.Polymer Dose kg/tonne.
D.SolicJs Recovery %w/w. E,Cake Rate kg/h w.b. F.Q/Z mm/h
. ,
G.T/V N/cm 2 H Q/~-,.V.ol *100h "1
I
34.0 ,,
550 3.2 7.2
98,1 0.27 1.40 3.14 2.50
34.2 425 5.4 7.7
98,6 0.46 2.33 3.14 4.16
3150 3150 3150 3150 252 252 252 252 2.2 3.3 5.7 8.7
2,30 1.80 1.10 0,40 . . . . . .
7.0 10.0 15.0 22.0 2,99 2.95 3.05 2.88
Z78FS40 Z78F.~40 Z78FS40 Z78FS40 . . .
0.15 0.15 0.15 0.15 , ,
B B B B 1.2 "i ,9 3,0 . 4.8 0.0 0.0 0.0 0.o
33.0 485 7.6 8.6
98.2 0.62 3.26 2.67
,.,
5.82
31.1 28.2: 620 495! 10,9 16.4 9.5 9.8
_ _
97.7 0.93 4.66 2.09 8.32
_
98.2 1.59 6.99 1.28
12.48
25.3 650 24.3 11.4
10.25 ~ 0.461 18.36
410 Appendix
Table A.9 (contd.)
Decanter Bowl Diameter 425 800 Clarifying Length
Process Unspecified Effluent
I
Machine Condltions 4.Bowl Speed rpm. 31 50 5.Pond dia ram. 252
3150 3150 3150 252 252 252
31"50 252 2.7
0.80 ,,
3150 252 2.4' 6.Conveyor diff' rpm. 6.7 6,0 5.4 5.0
7.Conveyor torque kNm 0.10 0.60 1.50 1.60 1.40
Feed Conditions 9.Feed Rate m3/h. 15.0 15.0 15.0 15.0 7.0 10.Feed Solids % w/w d.s. 2.97 2.97 3.14 3.07 2.91
7,0 2.84
Additive Conditions 11.Type. Z78FS40 Z78FS40 Z78FS40 Z78FS40 Z78FS40 12.Concentration %w/w. 0.15 0.15 0.15 0.15 0.15 13.Addition point. B B B B B 14.Rate m3/h. 1.4 2.3 4.2 4.6 0.3
.. . . .
15.Dilution m3/h. 0.0 0.0 0.0 0.0 0.0
Z78FS40 0.15
B 0.4 0.0
Product Conditions 16.Cake Solids %w/w. , 24.1 26.2 29.5 30.1 2'7..2 17.Centrate Solids m~t/I. 800 445 1400 200C 590 A.Centrate Rate m3/h. 14.6 15,6 17.7 18.2 6.6
"" 4.7 7.7 ~ 13.4 15.0! 2.4 B.Polymer Dose k~l/tonne. .
29.1 380 6.7 3.2
D.Sollds Recovery %w/w. 97,4 98.4 94.7 92.1 98.1 98.7 E.Cake Rate k~I/h w.b. 1.80 1.67 1.51 1.41 0,.73 F.Q/~. mm/h 6.99 6.99 6.99 6.99 3.26 G,T/V N/cm 2 0.12 0,70 1.74 1.86 0.93 H Q/~l-Vol *lOOh "~ 12.48 12.48 12.48 12.48 5.82
0.67 3.26 1.63 5.82
Appendix 411
Table A.9 (contd.)
Decanter Bowl Diameter 425 Clarifying Length
Process 800
Unspecified 15ffluent
I Machine Conditions I 4.Bowl Speed rpm. 3150 5.Pond dia ram. 252 6.Conveyor diff' rpm. 2.4 7.Conveyor torque kNm 2.00
Feed Conditions
3150 252 2.4
2.50
9.Feed Rate m3/h. 7.0 7.0 10.Feed Solids % w/w d.s. 3.02 3.04
Additive Conditions 11.Type. Z78FS40 Z78FS40 12.Concentration %w/w. 0.15 0.15 13.Addition point. B B 14.Rate m3/h. 0.8 1.7 15.Dilution m3/h. 0.0 0.0
=,,
Product Conditions 16.Cake Solids %w/w. 31.5 33.8 17.Centrate Solids mg/i. 425 1500 A.Centrate Rate m3/h. 7.2 8.1 B.Polymer Dose k~l/tonne. 6.0 11.8
D.Solids Recovery %w/w. 98.6 E.Cake Rate k~l/h w.b. 0.66 IF.Q/y mm/h 3.26 G.T/V N/cm 2 2.32 H O/g-Vol * 100h "1 5.82
94.3 0.59
This Page Intentionally Left Blank
Editorial Index
A abrasion see wear protection AC rnotors 45 .11 5. 197. 325 acceleration force 149-5() accelerator blades/vanes 32 .68 acetone 143 acetylene 128 acidic sludges 22 5 acrylamide content, polymers 2 35:see also
polyacrilamide additive chamber 29. :~ 3.34 additives, to polymeric tlocculants 222 agglomeration 21 7 agitation 21 7 agricultural products, test data 258 -9 agricultural wastes 146 agro-chemicals 142 air in-tlow, feed tube ~9: see also windage air pollution prevention 12 8 alarm systems 11 7. 319 alcoholic beverages 141 : see also spent grain alumina tiles 6 4 . 7 2 alunlinium 2() 1 aluminium (coagulant) 129.21 7- 1 8 aluminium hydroxide 14 aluminium sulphate 229 aniline 128 animal fats 1 37 animal feed136. 1 4 1 . 2 3 5 . 2 5 5 , 2 5 8 antibiotics 14 5 applications 5.1 3. 121---45
classes 1 2 2 - 4 . 1 2 :, early 7-9 market estimates ~ ~ 7
Aqua-Critox 128 arc welding 72 area equivalent 164-5 automatic control 1 1 6 - 1 7 . 3 1 5 . ~2 3- 3() axial tlow see flow. axial axial load/ thrust 27-8 . "50.65.201
B buck-drive 1 7 . 4 5 - - 6 . 8 2 - 4 . 1 2 9 , 1 3 1 , 1 9 6
control 116. 321. 3 2 5 - 7 . 3 3 ( )
baffle cone l ( ) ( I - l ( ) l . 112 -13 . 129. 173 bat'fle discs 66 .92 .99-1( ) ( ) . 1()1-3. 173,
1 7 7 . 2 5 3 baffle guttering 78 baffles
casing 4 1 . 7 7 - 8 conveying 1 ()2- 3 conveyor 9 1 . 9 2 . 9 9 - 1 ()3.27(l hinged 1()3 longitudinal 1 () 3 in three-phase separation 87 8 see also cake baffles: grooves
bar screen 86 beach 2 . 2 5 - 8 . 5 9 - 6 4
angle 26. 47. 59-61. 164 .276 . 3()() conveying theory 176-7 , 1 79 dual beach 92. 146 liner 2 8 . 5 4 . 6 4 . 7 1 pitch 11 () Sigma value 163 wall thickness 64 washing 124.1 35.181
bearing supports, frame 38 bearings 2, 1 7 . 5 8 - 9 . 9 ( )
conveyor 29 - 3(). 3 4 - 6 . 7 3 failure 2()4- 6. 322 frame 37-8, 76 front hub 24 - 5 life 2 ( )4 6 monitor ing 322 rear hub 2~- 8, s tandards 2()6 vertical decanters 49- 5()
belts 1 7 .45 , 199. 321. ~23 bentonite 1 32.1 35 Beta theory 175-6 beverages 141 : see also spent grain biochemical industry 9.1 3 .114 biological disc 99 biological sludge 2 36 Bird Machine Company 7 .8 blast furnace operation 128
4 1 4 Index
blood 145 bolt heads 97 botanical products 142 bowl 2, 7, 9, 2 1 - 5 . 5 4 - 9 . 2 76
baffles 99-103 casing 2 fabrication 54 hubs, see front hub: rear hub liner 22 .23 .54 , 56-8, 71 shell loading 200-202 special variants 86-95, 104 speed 200-202. 2 4 7 . 2 4 8 , 2 4 9 , 2 6 5 , 2 8 1 .
303 unblocking 114 wall/nozzles 2.17, 2 0 . 2 1 - 2 . 5 6 - 7 , 91.29 wear protection 71
braking power, calculation 306-7 braking torque 83. 321. 324. 326 braking see also back-drive brewing 141 : see also spent grain bubbles, in flow 320 buffer chamber 30 buffer tank 318
C cake 17-18
dryness 108.1 ~)9-- 10. 1 4 3 . 1 7 3 - 4 . 1 7 7 - 8.18(), 227.2 37 and tlocculants 2 30.2 32 test work 255 see also dry solids
reslurrying 11 3 shearing 108 sticking 54 .64 .78
cake baffles 99-1( )0 .129 .130 , 131 cake collectors 78--9 cake discharge 2 6 . 2 8 . 5 6 . 6 1 - 4 , 78-9, 113,
246 casing41, 54 diameter 28 .177 height 56 liners 78 system/module 318
cake rate calculation 289 cake size frequency distribution 292. 293 calcining 129,300 calcium Icoagulant)217-18 calcium carbonate 135,143, 259 calcium hydroxide 300-301 canning 138, 139 capacitors 198 capillary suction test (CST) 22 7-8 carbide 49, 64 carbon fibre 39 carbon steel 18.21, 53 casein 140 casing 17, 18, 38.40--43, 54, 7 7 - 8
baffles 41, 77-8, 87
seals42, 51, 52, 79-80 wear resistance 71, 131
cast iron 37 castellation 64 casting, of bowl 54 catalyst preparation and recovery 134 caustic soda 101.143, 144 cellulose 128, 258 cement industry 7 centrate 19-20
monitoring 116. 320,323 quality 110, 116-17 ,237 , 250,300-3()1 rate calculation 152 ,284
centrate discharge by centripetal pump 93-4 casing 42, 43 .79 and floater disc 1 () 1-2 system/module 317-18 test facility 246. 247
centrate weirs see front hub weirs/dams centrifugal force 2 .3 .27 : see also g-force centrifuges 4 .7
special variants 86-93 standards 200
centripetal pump 9.55, 9 3 - 4 . 1 7 2 . 3 1 7 Centriseal 9 .99 ceramic materials 3 2 . 4 9 . 6 4 . 7 2 . I t)8 chalk 1 35 chemicals industry 142- 5 china clay tkaolinl 7. 123.1 35 chitin 218 chlorides, in process material 53 CIP(clean-in-placel 9.1 3.47, 114, 1 36.
14~). 33O clarification 5.9~). 92.12 3
conveyorless 92 enhancement 1 ()4-7 scale-up 30()-3()1 theory 159-67 volume calculation 286
classification 5, 123, 139 .226 calculations 2 8 1 - 2 . 2 9 1 - 3 and scaling up 3~)()--3()1 test work 2 4 7 . 2 5 9 - ~6 theory 168-9 see also particle size
clays 132.1 34. 181 .261-3 . 300 clean-in-place (CIP) 9, 13, 47, 114.136.
140 .330 co-current flow 19 -20 .51 -3 , 65 coagulants 7. 1 2 9 , 1 3 0 . 2 1 7 - 1 8 . 2 2 9 coal processing wastes 128, 132.134 coal tailings 269 coal washing 86,124. 132. 236 coating materials 71-3 coatings industry 142 cobalt-based alloys 71 codes of practice/standards 4 9 , 2 0 0 . 2 0 6
coffee production processes 128 .140 Colmonoy 71 compaction theory 18 5 .186-91 conductivity probes 320 control algorithms 117, 328-9 control systems 9 . 1 1 6 - 1 7 . 3 1 5 . 3 2 3- 3()
test equipment 24 5-7 controllers 116 .315 , 32 5-7
integrated 32 7-8 conveying theory 175-8 conveyor 2 9 - 3 6 . 6 4 - 7 3 . 144. 2 4 7 , 3 0 0
tloating 1 1 6 conveyor baffles 9 1 . 9 2 . 9 9 - 1 0 3 . 270 conveyor bearings 2 9 - 3 0 . 3 4 - 6 , 61, 73 conveyor differential speed 150-51, 196,
251. 255. 265.27() calculation 150 ,281 . 285 and scaling up 3()(). 3()2.3()4.3()9, 312-
13 conveyor flights 2 . 2 9 - 3 1 , 3 7 . 6 6 - 7 . 9 2
clarification enhancement 104-9 wear protection 7 1 - 3 . 1 1 1 . 129.13().
131. 144: see also tiles conveyor hub 1 7.29- 3(). 58.66.9()
clarification e n h a n c e m e n t 1 0 5 - 9 . 1 6 6 - 7 comeyor pitch 31.92.1 ( )9-12 .251.27() .
3()() conveyor rake 1()7 8 conveyor seals 29. 34 6 .73 convevortorque 1()7. 1 85 .245. 255-8 .
27 1. 326 calculation 1 51. 286 measurenlent 151. 1 79. 286. 321 and scaling up 3{)2-5. ~; 1() see also scrolling torque
conveyor vanes 1( )5-7 .166 Coriolis effect 167. 32() corrosion-resistant materials 53 - 4 . 7 2 . 1 4 3 cortisone 1 37 costing 11 7. 328-9 counter balancing 1 l ( ) - I 2 countercurrent flow 19-2(), 51-2 .65 , 89.
1()1. l 1() counters 322
countershaft system 82 .84 cream processing 4 . 7 . 2 36 cresting 2 3. 176. 192. 194-5. 251 crystalline solids 124, 180 cut point 155-6 . 2 6 1 - 3 , 2 9 2 cyclic loads 2()2 Cyclo gearbox 36. ~ 7 .73, 7 4 . 7 5 - 6
conveyor differential speed 15(I-51
D
dam plates see front hub weirs/dams D'Arcy's equation 186 l)C motors 46. 8(). 83 .115 . 325 De Laval. G. 6
Index 4 1 5
de-aeration, centrate solids concentrat ion measurement 320
de-inking plants 1 2 8 . 2 3 6 decanter plants, instrumentat ion 315-22 decanters 4 .7
orientation 2 . 1 9 . 4 7 - 5 1 , 9 0 special variants 86-93 , 24 7 standards 200
deliquoring see dewatering Denmark. manufacturers 11 density 2 ,146 , 152. 168 ,194 . 226 desulphurisation processes 128 detergents industry 142 dewaterability 22 7 dewatering 5 . 9 1 . 1 2 3
coal tailings, test data 269 flotation concentrates 1 32 in mineral processing 13 5 scaling up 3(12-5 solids 18()-81. 269-79 : see also dr)' solids spent grain 141. 255-8.3()() . 3()2 test procedures 2 4 8 , 2 4 9 theory 18()-81 see also waste sludge processing
differential height 2 5(I-51 differential speed 15()-51. 247 .2 58: seealso
conveyor differential speed diffusion 182- 3 digester sludges 1 3(}-~ 1. 265 -8 direct-on-line motors 45.8(). 325 directional ff'ed nozzles 67. 141 disc-stack centrifuges 4.1 ()6- 7 discharge vents/ports 17, 32 .42 - ~. 67 8.
112-13 dispersants (use in classification) 169.25 l dispersions (polymeric tlocculant ~ 22(). 221-
)
distilleries 141. 255 donkey motors 114.33() drain valves 114 drainage, dewatering by 18(). 181 drilling mud I 32-3 drive bells 199 drive motor 2.1 7 . 4 3 - 5 . 8 ( I - 8 2
acceleration 4 4 - 5 . 1 9 8 -9 control 116. 32 3. 325 dual 116 installation 198 mounting 811, 81- 2 sizing 197-8, 306-7 speed 44 - 5 timers 321-2 torque 44-5: see also braking torque drysolids (DS) 9 , 1 8 6 - 9 1 . 335 basic calcualtions 286 -7 compaction theory 185 conveying theory 1 7 7 - 8 . 1 7 9 tlocculant addition 167.22() . 2 38.24()
4 1 6 Index
scaling up 302. 308-13 test work 249 ,258 ,265 . 269-79
dyestuffs and pigments 128, 135. 141. 142, 155
E e-line (equilibrium line t calculations 170.
289-90 eddy current brake 46.83, 85 .115
calculations 281,283 eddy current flow meter 319 effluent
flocculants 33, 167.236 municipal 129-31. 2 2 5 . 236. 263-5 rate calculation 289 test facility 2 4 7
thickening 113, 130. 263-8 see also dewatering: waste sludge
elastomers 51, 54 electrical meters 322
electrolytic and e l e c t r o c h e m i c a l sludges 128, 146
energy materials production 132-3 equilibrium line (e-line) calculations 1 70.
289-9() erf 1 54. 298-9 erosion protection see hard surfacing: tiles European Engineering Directive 2{)1 European standards 2{){) Expamet 56
F failure, mechanical 2()() 207 fats. rendering 137 fatty acids 144 feed ports 17. 32 feed pump 245 feed quality 251 feed rate 1 8 4 . 2 4 8 . 2 4 9 , 2 5 1
and scaling up 3()2 test work 248,249. 251. 255. 263. 292
feed tube 17, 31, 3 8 - 9 . 5 1 . 7 6 - 7 tloc feature 76. 207 resonance 206-7 rinse feature 267 seals 39
feed vessel 245.318 feed zone 29, 30, 31-2, 6 7 - 9 , 3 0 0
acceleration 195 admitting flocculants 230, 231 liner 54 special variants 112-13 test equipment 245 ,246 wear protection 71-3
fermented materials 141 ferric chloride 229 ferric sulphate 229 fertilizers 127, 142, 145
fibrous solids 124 filters (presses) 4 .22 7 filtration 86. 180 first critical rotor speed 203 fish eyes 220, 221 fish oil industry 7 fish processing 137-8 fish transport water 128 flame spraying 71-2 floater disc 101-2 floating conveyor 116 floc feature, feed tube 76.230, 231 floc zone 29 .30 , 34 .69 -70 .230 . 251 flocculant system/module 317 flocculants 29, 30.33, 34 .69-71. 101.130.
1 3 5 , 2 1 7 - 1 8 admitting to decanter 230-32. 248 performance 237-40 selection 22 5-8. 248.2 5{) solids content measurement 320 suppliers 2 33-4 see also polymeric tlocculants
flocculation 215 -40 applications 2 36 pre-treatment 217-18 theory 1 6 7 . 1 7 3 . 2 1 7 - 1 9 see also flocculants
flotation concentrates 1 32 flow 1 9 - 2 0 . 5 1 - 3 . 1 6 7
axial 104-5. 192. 193. 194 sewage treatment 12'). 1 3(). 131
co-current 19-20, 5 l - 3 countercurrent 19- 2(). 51-2 fully axial 1 ()5 helical 192,193. 194 laminar 193. 194 pressurised 94 quasi-axial 1{)4-5
flow diagram, decanter plant 315, 316 tlow meters 1 51, 1 5 2 . 2 2 2 . 3 1 9 . 3 2 2 fluid coupling 80-81 ,116 . 307. 325 fluid dynamics 176 .192-5 food and food by-products 9 . 1 3 . 1 1 4 . 1 2 8 .
1 3 6 - 4 0 . 2 3 5 . 2 3 6 . 2 5 8 - 9 , 3 3 0 foundry operations 128 fractionation 145" see also classification frame 1 7, 3 7 - 4 0 . 7 6 - 7
bearings 37 -8 .76 France 10. 11 frequency distributions 291-3 frequency inverter drives 114 friction, beach 176 friction power component 196, 197, 198,
306 front hub 2 2 - 5 . 5 4 - 5 . 9 3
weirs/dams 17.22-3 , 55-6, 5 7 . 8 7 . 9 6 - 7 froth flotation 132.236 fruit and vegetable products 138-9 .141,
2 58-9" set' also vegetable oils fuses, high rupture (HRC)45 fuzzy logic 329
G g-force 149-50 g-level calculation 150. 285,286. 303.
308-9 gasification 1 32 gaskets 21.42, 54.78: see also seals gearbox 3.8, 9, 17 .19 .36 -7 , 55, 73-6. 131
and back-drive 4 5, 325-6 Cyclo 3 6 . 3 7 . 7 3 . 7 4 . 7 5 - 6
conveyor differential speed 150-51 epicyclic 3 6 - 7 . 4 5 . 7 5 . 7 6
conveyor differential speed 15()-51 hydraulic 36.46 life 206.31 () Maun type 73- 5 ratio 3()6-7 Rotodiff 36.76
gelatinle) 1 37. 142 Germany 1 (). 11 glass fibre 39 glues 142 gluten extraction 1 38.1 ~9 grain, dewatering see spent grain graphite 14 gravimetric analyses 1 51.152- 3. 222 ,225 -
6 gravitational field 2 69 gravity separation 3.4 Greece 11 grooves and baffles 22, 28, 64, 176: seealso
baffles gypsum 1 34.14 3
t t
hard surt'acing 6 4 . 7 1 - 3.1 31.1 37 hard water 22 3. 224 Hastelloy 53.72 HI)PE 78 heel 17.18
torque 1 79 heparin 1 37 high rupture fuses {ttRC) 45 high-temperature applications 47.53 4 . 7 3 .
9(1. 141 high-velocity oxide fuel iHV()Ft 72 horizontal decanters see orientation hormones 137 hubs. see conveyor hub: front hub: rear hub HVOF (high-velocity oxide fuel) 72 hydraulic balance, three-phase separation
1 7 O - 7 1
hydraulic conveyor drives 46.83 hydraulic motors 80; see also Rotodiff hydrocyclones 3.4
Index 4 1 7
hydrogen economy 143
I impellers 78 -9 .97 imperforate basket centrifuges 4, 7 impurity calculations 1 8 2 - 4 . 2 6 7 - 9 , 2 9 4 - 7 India 11 industrial classification 125-6 industrial wastes processing 12 7 -9 .236 inertia 198 -9 ,200 infrared instrumentation 320 inline dosing 23(), 231 inorganic chemicals 142, 143 ,229 inorganic sludges, flocculation 225 inorganic solids, processing 106 insecticides 128 instability 203 instrumentation 9 .116-17 .31 5- 3()
test equipment 245-7 insulin I 37. 145 inverter motors 4 5 . 8 3 . 1 1 5 . 3 2 5 ionic activity 217. 229 iron coagulants 21 7-18. 229 iron compounds, manufacture 14 3 iron ore scrubber slurries 128 isinglass 218 IS() standards, bearings 2()6 Italy 1(). 11
I Japan 11 12
K kaolin (china clay} 7.12 3.1 35 knurling/ribbing 22 .28 .56 -7 , 64.1 76 Krepro 128
L labvrinths 78 lactose 14(). 267-9 landfill 127.129 Lee disc 99 level probes 32() Liedbeck. A. 6.7 light retlection/transmission
instrumentation 32() lime ~coagulant ) 2 2 9
lime sludge/slurry 129.2 36.2 59-61. 3()() liners
beach 28 .54 .64 .71 bowl 22.23, 54 .56-8 .71 cake discharge 78 feed zone 54
logarithmic probability 154. 165-6. 298-9 low-temperature applications 86 ,144 lubrication 19, 2 4 - 5 . 2 7 - 8 . 5 9 . 9 0 . 206,
317,319 monitoring 317,319. 321
4 1 8 Index
pillow blocks 38 systems 24, 38, 323
M machine tool fluids 127 magnesium hydroxide 143.2 59. 300 main drive see drive motor maintenance 117. 206 manufacturers 10-12. 334. 3 3 7 , 3 3 9 - 6 2
early decanters 6-11 polymeric flocculants 233
market 333-8 mass balance calculations 152. 2 4 7 . 2 8 1 - 2 .
288 mastics 142 materials
ofconstruction 9 . 1 8 . 2 1 . 3 7 . 3 9 . 4 9 . 5 3 - 4 .64
for hard surfacing 7 1 - 3 . 1 3 1 . 1 3 7 Maun type gearbox 73-5 meat/meat products 1 2 8 . 1 3 6 - 7 mechanical design 2()~-2(~7 mechanical engineering applications 146 medicinal chemicals 1 4 2 . 1 4 4 - 5: see also
pharmaceuticals meters 116, 319-22: see also tlow meters methanol 22() mica 1 35 milk and cream 4 .7 .128 .14 ( ) . 2 3 6 . 2 6 7 - 9 minerals extraction and processing 1 35.168
tlocculants 22 5 .236 modules 3 1 7 - 1 8 . 3 2 3 molecular weight, tlocculants 218 .2 33 monitoring 116-17, 2 ( )6 .317-22
test facility 24 5-7 motors
AC46. 115. 197. 325 DC46 .8 ( ) , 83 ,115 . 325 direct-on-line 45.8(), 325 donkey 114. 330 hydraulic 8(): see also Rotodiff inverter 4 5 . 8 3 . 1 1 5 . 325 soft start 80 .116 . 325 star-delta 45 .80 , 116 starter 44 .45 . 325 three-phase 80 variable speed 82- 3.323 see also back-drive: drive motor (main
drivel mounting, drive motor 8(). 81-2 mud. drilling 1 32-3 multi-lead conveyors 66 ,144 municipal sewage treatment 129- 31.225.
2 3 6 , 2 6 3 - 5
N negative pond operation 56.131, 318 negative ring dam 99
neoprene 2 1 , 4 2 . 5 4 nickel-based alloys 71 noise 38, 4 9 . 5 5 . 7 9 , 97-9 nomenclature 208 -12 nuclear power processes 13 .128 nylon 144
O oil. water and solids, separation 8 6 - 8 . 1 7 0 -
72; see also three-phase decanters: waste oils
oil production and refining 13, 128, 132. 134
oils, vegetable 101. 138-9. 142, 17(). 334 olive oil 139, 170 .334 on/offdevices 32 3-4 ore processing 1 3 . 1 2 8 . 1 4 3 organic chemicals 142 .143, 181 organic sludges, flocculation 225 orientation 2, 19, 47-51 ,9 ( ) overload protection 45 .82 oxidation treatments 128
P paints 135. 142 .155 palm fruit oil 139.17() paper mill operations 128.2 36 p a r a - x y l e n e 8 6 . 1 4 4 particle dvnamics 148 .149 - 5() particle s ize154-8 . 168 9. 2 2 6 . 2 4 7 . 2 6 1 -
3.291 : see also classification patents, early 6-9 Pecker. ].S. 7 penicillin 14 5 performance calculations 1 51 - 3 . 1 7 2 . 2 8 1 -
313 perfumes 142 pesticides 142 petrochemical industry 9t). 1 32 -3 .142 ,
143-4 pH 21 7 - 1 8 . 2 2 5 , 2 2 6 . 229 pharmaceuticals 89 .114 .1 37. 142. 144-5,
33O phenols 134 phosphoric acids 143 PI1) controller 116 .32 5 pigments and dyestuffs 128.1 3 5 . 1 4 1 . 1 5 5 pillow blocks 37, 76.2()4 pipe work. test facility 246 pitch angle see conveyor pitch pitting 53 plasma spraying 72
plastics processing 92 ,142 . 143-4. 146 PLC (programmable logic computerl 116-
1 7 , 1 5 1 . 2 4 5 . 325-6. 330 polyacrylamide flocculants 2 1 8 - 1 9 , 2 2 0 polyaluminium chloride (PACt 218. 229 polyelectrolytes see polymeric flocculants
polymeric flocculants 2 1 8 - 1 9 , 2 2 0 addition 7 ,129 . 130.131. 1 9 1 , 2 4 8 , 2 5 3 .
265 concentrat ion measurement 320 consumption measurement 117 cross-linked 2 1 8 - 1 9 . 2 2 5 . 2 3 1 dosage 1 5 1 - 2 . 1 5 5 , 167, 222, 231-2.
237- -40 ,248 ,273 . 317 calculation 152.281. 284-5 and scaling up 311-12
electrical charges 217-19 , 225 monitoring 320.321. 322, 323-4 selection 225-8 solution make-up 22(I-24, 317 solution strengths 222-4 test facility 2 4 5 - 7 , 2 4 8 - 5 1
polymerisation processes 14 3-4 polyolefins 14 3, 144 polystyrene 143, 144 polyvinyis 143, 144 pond 2 . 2 2 - 3 pond settings 2 3 . 5 5 - 6 . 9 3 . 1 9 ( ) . 247 ,248 .
249. 259, 29(). :112 deep 141, 161. 259-61. 265 negative 56.99, 131,318 neutral 9(). 129.13(). 1 ]1. 141 shallow 9(), 1 3(), 131. 141,163.19() .
259-61 pond volume 2 7 ports 1 7. 32 .42 - 3.67 - 8 . 1 1 2 - 1 3 potash 1 35 power consumption 117. 196-9. 3(16-7.
322 power losses 195, 197, ](17 power regeneration 115-16, 325 pre-liocculation treatment 21 7-18, 2 2 5 .
226 ,229 : see also coagulants pressurised applications 47, 49, 90 pressurised tlow 94 printing inks 1 2 8 , 1 4 2 . 2 ] 6 probability, logarithmic 154, 1 6 5 - 6 , 2 9 8 - 9 probes 320-21 process acceleration power 306 process applications 122-4 process data. test log 253 process instruments 315-16. ] 19--22 process performance see performance
calculations proportional integral and derivative (PIDI
controller 116. 325 protein processing 1 0 6 . 1 3 7 , 1 4 0 proximity probes 321 psi (thickening factor) 1 7 4 , 2 6 5 - 8 , 2 8 5 P T A 144 PTFE42.51 , 54.78, 80 pumps 9 . 3 8 . 2 4 5 - 7 . 3 1 7 - 1 8 . 319. 323. 324
centripetal 9, 55, 93-4, 172 .317 speeds 321
Index 4 1 9
purity 123--4; see also impurity calculations PVC linings 54 PVC separation 144
R racetrack (Rennbahn178-9 radioactive source instrumentation 320 rakes, in clarifying zone 92, 1 (17-8 raw water t reatment 12 7 .129 re-suspension 51, 52 rear hub 2 5 . 2 6 - 8 . 6 1 , 1 1 0 - 1 2 recovery, oil 289: see also solids, recovery recycling 127, 128. 129. 1 3 9 , 1 4 6 . 2 3 6 ,
259 Rennbahn (racetrack) 78-9 rendering 137 reslurry 11 3, 124. 140 resonance 38-9 , 76, 98.2()6 results sheet 2 53-4 reverse pitch 92.11(1 Reynolds number 1 0 4 . 1 8 4 . 1 9 2 - 4 ribbing 22 .28 , 56-7 .64 , 1 76 rinse nozzles 181 rinse zone 29.3(). 3 3. 34. 69 -71 rinsing, see washing risk analysis 2()() Ritsch, H.P. 7 rotameters 319 rotating assembly 17.19, 21- 37 Rotodiff 36, 7 6 . 1 1 4 - 1 5 rubber 54, 7 3, 77.78. 142
g
safety 117, 2()(), 2()6, 315 sampling 2 5() sanitary performance 9, 136 saponitication 144 scaling 2 5 3 , 2 7 6 , 2 8 2 . 3()()-] 1 3 Schmidt number 184 scraper blades 11 O- 12 screen-bowl decanter 86, 124, 135 screens, sewage treatment 13() scroll I screw conveyer) 2: set' also conveyor scrolling aids/efficiency 23 .28 , 56.58, 71,
1 7 6 . 1 7 9 , 2 6 5 . 269.27() scrolling rate 1 7 3 - 4 . 2 5 3 . 273. 3()(), ]()9 scrolling reversal 3 ]() scrolling torque 28 .58 .66 : see also conveyor
torque seals 19, 21, 54, 59.9(1
beach 26 -8 bowl 21.22.25 casing 42, 5 1 . 7 8 . 7 9 - 8 0 conveyor 29, 34-6, 73 feed tube 39 vertical decanters 49-51 see also gaskets
sedimentation principle 3
420 Index
separation 3-5, 8 . 5 2 , 1 2 2 - 4 low-temperature 86 partial 123 three-phase 86-8 , 89, 9 7 , 1 0 1 , 1 2 4
settled volume index (SVI) 265 settling
ability 217 velocity 1 5 9 - 6 1 , 2 1 5 see also clarification
sewage t reatment 1 2 9 - 3 1 , 2 2 5, 2 3 6 , 2 6 3 - 5: see also effluent
Sharpies Corporation 7 . 8 . 9 short circuit protection 45 Sigma 159-6 7, 2 53
calculation 1 6 1 , 2 8 5 - 6 . 290 and "dr), solids" 186-7 enhancement 104 .130 . 140. 141. 166-7 and scaling up 3()(). 304
silicon carbide 72 silicon/silicates, processing 14 3 skimmer pipe 8 7 . 9 5 - 6 . 1 7 2 skimmer pump 9 . 3 1 8 slaughterhouse wastes 1 2 8 . 1 3 6 - 7 sludge, digesters 13(): see also waste sludge slurry dewatering 124 soap industry 1 4 2 , 1 4 4 soft start motor 8(). 116. 325 soil improvement 127 solids compaction .see dry solids solids concentrat ion meters 319 -2() solids rec(wery 5. 2 5 5 - 6 3 ,265
rate calculation 1 52. 284 solvents 8(). 24 7 sonic probes 32() soybean protein 14() speeds 2
critical 2()2-3. 204 measurement 1 51. 2 4 5 - 7 , 3 2 1
spent grain 141. 255-8 , 3()(). 302. 3()6 spindles 22 .42 spot tests 25() spray nozzles, floc zone 71 stainless steels 9, 1 8 . 2 1 . 4 9 . 5 3 . 7 2
duplex 53 .54 Standard Industrial Classitication 125-6 standards/codes of practice 49.2(1(). 2()6 star-delta motors 4 5, 8(). 116 starch, as tlocculant 218 starch extraction 1 3 8 . 1 3 9 starter motors 44, 45, 325 steel 29 .37 , 64" see also carbon steel"
stainless steel steel works 128 Stellite 5 4 . 6 4 . 7 1 , 78 sterilising 114 .330 : see also CIP stillage t reatment 141 stirred settled volume index (SSVI) 265 stirring 317 .318 , 24 5, 32 3
Stokes 'Law 1 5 9 , 1 6 0 . 167. 168. 186 stress corrosion cracking (SCC) 53 -4 s tresses 2 0 0 - 2 0 7
sub-frame 17, 39, 4 3, 80 sulphate salts 267 supercritical water oxidation 128 suppliers 334, 33 7: see also manufac ture rs surfacing 64, 71-3 . 131. 137 surimi 138 Sweden 12 Switzerland 12
T t a c h o m e t e r 321 tanneries 128, 2 36 tannin, as flocculant 218 tars 134 tea, instant 140 telemetering 116 temperature extremes see high- temperature :
low-temperature temperature probes 32 l tension bar 2 7 - 8 . 6 5 terephthalic acid 144 test data 2 55-79 test equipment 24 3- 7 test log 2 52-4 test procedures 248 - -31 .334 thermistors 4 5. 321 thermocouples 321 thickening 5, 6 1 . 9 ( ) - 9 2 . 9 7 . 1 2 3
basic calculations 2 8 4 - 7 effluent 113.1 3(). 2 6 3 - 8 factor { psi) 1 7 4 . 2 6 5 - 8 . 2 8 5 test work 2 51 theory 1 73 -4
three-phase decanters 8 6 - 8 . 8 9 . 9 7 . 1 ()1. 124 ,128 . 136, 1 39. 144 calculations 2 8 1 , 2 8 8 - 9 ( ) ins t rumentat ion 319 separation theory 17()-72 testwork 2 4 7 . 2 5()-"51.253
three-phase motors 8() tiles 6 4 . 7 2 , 1 0 8 . 129, 13() timers 321-2. 330 timing belts 1 7, 45 t i tanium dioxide manufac ture 14 :~ t i tanium materials 4 9 . 5 3 . 201 torque 27, 44 -5 , 1 79: see also b rak ing
torque: conveyor torque toxic sludge t reatment 128 -9 toxicity, of polymeric flocculants 2 35 trials 334: see also test procedures tubular bowl centrifuges 4, 7 tungsten carbide 28, 6 4 . 7 1 , 7 2 . 108 turbulence 5 2 . 7 8 . 9 2 , 1 ( ) 4 . 1 1 3 , 1 6 7 . 1 9 2 -
5 .251 TV tube manufac ture 128
two-phase decanter, process performance calculations 151-3
U UK 10 .12 ultrasonic flow meter 319 uranium "yellow cake" 143 ure thane / syn the t ic rubber 73, 78 .142 USA 10 .12 utility applications 122
V Van der Waals forces 217 vanes, conveyor hub 1 0 5 - 7 . 1 6 6 variable orifice meters 319 variable ou tput devices 324 variable speed motors 8 2 - 3 . 3 2 3 varnishes 142 vegetable and fruit products 1 3 8 - 9 . 1 4 1 .
258 -9 vegetable oils 101, 1 38-9. 142 ,170 . 334 vents 4 2 - 3 . 8 0 . 1 ( )2 .112 -13 vertical decanters 2 - 3 . 1 9 . 4 7- 51 vibration 3(). 39 .77 .2 ( )3 -4 : seealso
resonance vibration isolators 1 7.39-4() . 43,8(). 82.
2O 3 vibration monitor ing 2()6 viscosity 194, 319
oft locculants 222- 3. 32() Viton 54
Index 421
Von Bechtolsheim, C. 6
tV wash-out prevention 129, 130, 131 washing 5 . 1 2 3 - 4 . 1 3 5 . 1 8 1 - 5
calculations 2 9 4 - 7 test data 2 6 7 - 9 see also rinse nozzles: rinse zone
waste oils processing/recycling 146 .170 . 247
waste sludge 8, 1 3 . 1 2 2 . 1 2 3 . 1 2 7 - 3 1 , 2 5 9 - 6 7 . 3 3 5 waste activated sludge (WASt 2 6 3 - 5 see also effluent
water addition 172 water hardness 223. 224 water (potable). t reatment 12 7, 129, 236.
259 wattmeter 322 wear protection 3 3 . 5 4 . 6 4 . 7 1 - 3 . 1 31. 247:
see also conveyor flights weight distribution 154-8 weirs (centrate) see front hub weirs/dams welding, double 31 welding rods 72 w indage42 -3 .8 ( ) . 87. 196. 3()6
and friction power component 196 .197 . 198. 3()6
wood. decortication 128
Z zinc oxide and salts 14 3
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