14
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.

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Page 1: Decanter Centrifuge Handbook

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.

Page 2: Decanter Centrifuge Handbook

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

Page 3: Decanter Centrifuge Handbook

Introduction 3

Liquids Feed Solids C~,m8 C~veyor Bowl Fl i# t

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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.

Page 4: Decanter Centrifuge Handbook

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.

Page 5: Decanter Centrifuge Handbook

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Page 6: Decanter Centrifuge Handbook

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Page 7: Decanter Centrifuge Handbook

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

Page 8: Decanter Centrifuge Handbook

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.

Page 9: Decanter Centrifuge Handbook

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.

Page 10: Decanter Centrifuge Handbook

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.

Page 11: Decanter Centrifuge Handbook

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

Page 12: Decanter Centrifuge Handbook

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.

Page 13: Decanter Centrifuge Handbook

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.

Page 14: Decanter Centrifuge Handbook

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