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Membrane Processing
The Society of Dairy Technology (SDT) has joined with Wiley-Blackwell to produce a seriesof technical dairy-related handbooks providing an invaluable resource for all those involvedin the dairy industry, from practitioners to technologists, working in both traditional andmodern large-scale dairy operations. For information regarding the SDT, please contactMaurice Walton, Executive Director, Society of Dairy Technology, PO Box 12, Appleby inWestmorland, CA16 6YJ, UK. email: [email protected]
Other volumes in the Society of Dairy Technology book series:
Probiotic Dairy Products (ISBN 978 1 4051 2124 8)Fermented Milks (ISBN 978 0 6320 6458 8)Brined Cheeses (ISBN 978 1 4051 2460 7)Structure of Dairy Products (ISBN 978 1 4051 2975 6)Cleaning-in-Place (ISBN 978 1 4051 5503 8)Milk Processing and Quality Management (ISBN 978 1 4051 4530 5)Dairy Fats (ISBN 978 1 4051 5090 3)Dairy Powders and Concentrated Products (ISBN 978 1 4051 5764 3)Technology of Cheesemaking, Second Edition (ISBN 978 1 4051 8298 0)Processed Cheese and Analogues (ISBN 978 1 4051 8642 1)
Membrane Processing
Dairy and Beverage Applications
Edited by
A.Y. TamimeConsultant in Dairy Science and Technology, Ayr, UK
A John Wiley & Sons, Ltd., Publication
This edition first published 2013 © 2013 by Blackwell Publishing Ltd.
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Contents
Preface to the Technical Series xiiiPreface xvContributors xvii
1 Development of Membrane Processes 1K. Smith
1.1 Historical background 11.2 Basic principles of membrane separations 3
1.2.1 Depth versus screen filters 31.2.2 Isotropic versus anisotropic membranes 41.2.3 Cross-flow filtration 51.2.4 Requirements of membrane processes 7
1.3 Types of membrane separations 81.3.1 Reverse osmosis 81.3.2 Nanofiltration 81.3.3 Ultrafiltration 91.3.4 Microfiltration 9
1.4 Theory of membrane transport 91.4.1 Transport models 91.4.2 Reverse osmosis/nanofiltration membranes 101.4.3 Ultrafiltration/microfiltration membranes 11
1.5 Factors affecting membrane separations 111.5.1 Factors affecting reverse osmosis/nanofiltration separations 111.5.2 Factors affecting ultrafiltration/microfiltration separations 121.5.3 System parameters 13
1.6 General characteristics of membrane processes 131.6.1 Retention and rejection 131.6.2 Pore size 141.6.3 Molecular weight cut-off 141.6.4 Flux 141.6.5 Concentration factor 151.6.6 Membrane life 15
1.7 Conclusion and future development 15Suggested literature 15
vi Contents
2 Principles of Membrane Filtration 17A. Hausmann, M.C. Duke and T. Demmer
2.1 Introduction and definitions 172.1.1 Membrane processes 172.1.2 Definitions of membrane processes 18
2.2 Membrane properties based on materials 242.2.1 Membrane structure 242.2.2 Material properties 26
2.3 Flux behaviour in pressure-driven membrane operations 292.3.1 Modelling flux behaviour 302.3.2 Influence of chemical potential on the reverse osmosis
process 352.4 Effects of feed characteristics and operating parameter on separation
efficiency 372.4.1 Effects of feed components 372.4.2 Effects of operating parameters 40
2.5 Cross-flow systems 432.5.1 Background 432.5.2 Single-pass versus feed-and-bleed operation 43
2.6 Recent membrane processes following different operating principles 442.6.1 Forward osmosis 442.6.2 Osmotic distillation 452.6.3 Membrane distillation 46
2.7 Conclusions 47References 47
3 Commercial Membrane Technology 52K. Smith
3.1 Introduction: polymers used in membrane manufacture 523.1.1 Cellulose acetate 523.1.2 Polysulphone/polyethersulphone 533.1.3 Polyamide 543.1.4 Polyvinylidene fluoride 553.1.5 Thin-film composites 55
3.2 Other materials used for membranes 563.2.1 Ceramic membranes 563.2.2 Metallic membranes 57
3.3 Membrane configuration 583.3.1 Spiral-wound 593.3.2 Tubular 613.3.3 Hollow fibre 633.3.4 Plate and frame 64
3.4 Modes of operation 653.4.1 Diafiltration 66
Contents vii
3.4.2 Batch design 673.4.3 Continuous design 69
3.5 Conclusion and future developments 71Suggested literature 71
4 Membrane Fouling, Cleaning and Disinfection 73L.L.A. Koh, M. Ashokkumar and S.E. Kentish
4.1 Introduction 734.2 Flux reduction 73
4.2.1 Membrane resistance 744.2.2 Concentration polarisation 744.2.3 Fouling 804.2.4 Fouling in the beverage industry 834.2.5 Fouling in the dairy industry 83
4.3 Membrane cleaning and disinfection 844.3.1 Cleaning methods 844.3.2 Chemical cleaning factors 874.3.3 Disinfection 954.3.4 Cleaning procedures 954.3.5 Chemical cleaning agents recovery and reuse 97
4.4 Recent developments 984.5 Conclusions 994.6 Nomenclature 100
References 102
5 General Application for the Treatment of Effluent and Reuseof Wastewater 107N.A. Milne and S.R. Gray
5.1 General wastewater quality issues 1075.2 General wastewater treatment 108
5.2.1 Primary treatment: solids, fats, oils and grease removal 1105.2.2 Secondary treatment: biological treatment and the membrane
bioreactor 1105.2.3 Tertiary treatment: disinfection 1155.2.4 Desalination: nanofiltration and reverse osmosis 116
5.3 Water reuse 1175.4 Conclusions and future applications 123
References 124
6 Liquid Milk Processing 128G. Gesan-Guiziou
6.1 Introduction 1286.2 On-farm concentration of milk 1286.3 Protein standardisation by ultrafiltration 130
6.3.1 Advantages of protein standardisation 131
viii Contents
6.3.2 Regulatory aspects 1326.3.3 Process involved 133
6.4 Removal of bacteria by microfiltration 1346.4.1 Microfiltration process: operating conditions
and performances 1346.4.2 Industrial applications 137
6.5 Fractionation of fat 1386.6 Removal of somatic cells by microfiltration 1396.7 Conclusions and future trends 140
References 140
7 Membrane Processing of Fermented Milks 143B. Ozer and A.Y. Tamime
7.1 Introduction 1437.2 Microflora of the starter cultures 1447.3 Patterns of production and consumption 1457.4 Manufacturing practice of gel-type (set and stirred) products 145
7.4.1 Mesophilic–lactic fermentations 1457.4.2 Thermophilic–lactic fermentations 1487.4.3 Yeast–lactic fermentations 1517.4.4 Mould–lactic fermentations 152
7.5 Manufacturing practice of concentrated products 1527.5.1 Background 1527.5.2 Concentrated yoghurt 1537.5.3 Shrikhand and chakka 1567.5.4 Ymer 1567.5.5 Skyr 158
7.6 Quality control 1587.6.1 Compositional quality 1587.6.2 Microbiological quality 1677.6.3 Organoleptic properties 168
7.7 Conclusion 169References 170
8 Cheese 176V.V. Mistry
8.1 Background 1768.2 Properties of membrane processed concentrates 177
8.2.1 Buffering capacity 1778.2.2 Rheology of concentrated milks 1788.2.3 Rennet coagulation 178
8.3 Applications of ultrafiltration in cheesemaking 1788.3.1 Protein standardisation 178
Contents ix
8.3.2 Medium or intermediate concentrated retentates 1798.3.3 Liquid pre-cheeses concept 1808.3.4 Application of ultrafiltration for fresh and soft cheeses 184
8.4 Cheese quality 1858.5 Applications of microfiltration in cheesemaking 186
8.5.1 Removal of bacteria 1868.5.2 Casein standardisation 1878.5.3 αs-/β-casein ratio adjustment by microfiltration 1878.5.4 Recovery of fat and brine 188
8.6 Nanofiltration 1888.7 Milk protein concentrates 1898.8 Future potential 189
References 190
9 Whey Processing 193L. Ramchandran and T. Vasiljevic
9.1 Introduction 1939.2 Whey: components, their functionality and uses 1939.3 Problems of traditional whey processing 1959.4 Membranes in whey processing 196
9.4.1 Microfiltration 1979.4.2 Ultrafiltration 1989.4.3 Diafiltration 1999.4.4 Nanofiltration and reverse osmosis 2009.4.5 Electrodialysis and other related processes 2009.4.6 Integrated processes 204
9.5 Conclusions 204References 205
10 Concentrated Milk and Powders 208G. Gesan-Guiziou
10.1 Introduction 20810.2 Concentrated milks and powders 208
10.2.1 Background 20810.2.2 Production of concentrated whole milk and powder 20910.2.3 Production of concentrated skimmed milk and powder 21110.2.4 Applications of reverse osmosis concentrated milks 21510.2.5 Dulce de Leche 217
10.3 Milk protein concentrates 21810.3.1 Manufacture of milk protein concentrates 21810.3.2 Applications of milk protein concentrates 219
10.4 Conclusion and future trends 222References 222
x Contents
11 Further Applications of Membrane Filtration in Dairy Processing 225J.A. O’Mahony and J.J. Tuohy
11.1 Introduction 22511.2 Fractionation of milk proteins using membranes 226
11.2.1 Separation of casein and whey proteins in milk 22611.2.2 Fractionation of individual casein proteins 22911.2.3 Fractionation of individual whey proteins 23211.2.4 Fractionation of milk protein hydrolysates 23311.2.5 Enrichment of osteopontin from milk/whey 23811.2.6 Production of microparticulated whey protein 23911.2.7 Isolation and enrichment of growth factors from milk/whey 240
11.3 Fractionation of milk fat using membranes 24011.3.1 Isolation and enrichment of native milk fat globules 24011.3.2 Isolation and enrichment of milk fat globule membrane 24211.3.3 Removal of phospholipids from liquid whey 24311.3.4 Filter sterilisation of polyunsaturated fatty acids 244
11.4 Fractionation of milk carbohydrates using membranes 24511.4.1 Isolation and purification of bovine milk oligosaccharides 24511.4.2 Filter sterilisation of lactase 24711.4.3 Lactic acid removal and purification 247
11.5 Fractionation of milk salts using membranes 24811.5.1 Demineralisation using membranes 24811.5.2 Demineralisation using electrodialysis 249
11.6 Conclusions and future trends 251References 253
12 Fruit Juices 262A. Cassano
12.1 Introduction 26212.1.1 General Background 26212.1.2 Background to manufacturing practice 262
12.2 Fruit juice clarification by microfiltration and ultrafiltration 26512.2.1 Microfiltration 26512.2.2 Ultrafiltration 26512.2.3 Selection of microfiltration and ultrafiltration membranes 266
12.3 Membrane fouling and membrane cleaning 26612.3.1 Membrane fouling 26612.3.2 Methods of reducing membrane fouling 26712.3.3 Methods of fouling treatment 268
12.4 Performance of microfiltration and ultrafiltration membranes 26912.5 Process configurations 27312.6 Quality of the clarified juices 27412.7 Integrated processes 27612.8 Conclusions and future development 277
References 277
Contents xi
13 Beer and Cider 281J. Bergin and J.J. Tuohy
13.1 Introduction 28113.2 Beer brewing process 282
13.2.1 Milling 28313.2.2 Mashing 28413.2.3 Wort separation 28413.2.4 Boiling 28713.2.5 Trub separation 28713.2.6 Fermentation 28813.2.7 Clarification 28913.2.8 Beer make-up 29013.2.9 Packaging and microbiological stabilisation 291
13.3 Cidermaking process 29213.3.1 Juice extraction and formulation 29213.3.2 Fermentation 29313.3.3 Racking and maturation 29313.3.4 Blending, filtration and packaging 293
13.4 Membrane applications in the brewing process 29413.4.1 Wort separation 29513.4.2 Beer filtration and stabilisation 298
13.5 Membrane applications in cidermaking 30013.5.1 Background 30013.5.2 Cider clarification 301
13.6 Membrane applications common to brewing and cidermaking 30213.6.1 Yeast separation and product recovery 30213.6.2 Microbiological stabilisation 30413.6.3 Gas standardisation using membranes 30513.6.4 Water recovery/cleaning-in-place systems 30813.6.5 Alcohol removal for non- or low-alcohol products and malt
beverage production 30913.7 Future opportunities 311
References 313
14 Wine 316K. Grainger
14.1 Background 31614.2 Clarification and filtration methods 318
14.2.1 Traditional methods in common use 31814.2.2 Membrane filtration 31914.2.3 Cross-flow microfiltration 320
14.3 Membrane fouling 32214.4 Must correction, wine correction and alcohol reduction
using membrane technologies 32214.4.1 Reverse osmosis 322
xii Contents
14.4.2 Ultrafiltration 32414.4.3 Wine correction: reducing alcohol content 32414.4.4 Wine correction: removing acetic acid 32514.4.5 Wine correction: removal of taints 326
14.5 Wine stabilisation and pH adjustment 32714.5.1 Tartrate stabilisation 32714.5.2 pH adjustment 328
14.6 Conclusions and future developments 328References 330
15 Application of Membrane Technology in Vinegar 334F. Lopez
15.1 Introduction 33415.2 Process of vinegar making 33515.3 Membrane technology in the production of vinegar 33615.4 Conclusions 338
References 338
Index 339
Preface to the Technical Series
For more than 60 years, the Society of Dairy Technology (SDT) has sought to provideeducation and training in the dairy field, disseminating knowledge and fostering personaldevelopment through symposia, conferences, residential courses, publications, and its jour-nal, the International Journal of Dairy Technology (previously published as the Journal ofthe Society of Dairy Technology).
In recent years, there have been significant advances in our understanding of milk sys-tems, probably the most complex natural food available to man. At the same time, improve-ments in process technology have been accompanied by massive changes in the scale ofmany milk processing operations, and the manufacture a wide range of dairy and otherrelated products.
The Society has embarked on a project with Wiley-Blackwell to produce a TechnicalSeries of dairy-related books to provide an invaluable source of information for practicingdairy scientists and technologists, covering the range from small enterprises to modernlarge-scale operation. This eleventh volume in this series, on Membrane Processing –Dairy and Beverages Applications , provides a timely and comprehensive update of theprinciples and practices involved in this technology. The commercial exploitation of mem-brane technology is developed further in its applications to the processing of milk andmilk products, plus a wide range of non-dairy beverages, where membranes provide novelopportunities for separation, fractionation and concentration.
Andrew WilbeyChairman of the Publications Committee, SDT
Preface
In the last two decades, there have been significant developments in membrane filtrationprocesses for the dairy and beverage industries. The filtration systems can be classifiedinto four main groups: reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) andmicrofiltration (MF). The primary objective of this book is to assess critically the pool ofscientific knowledge available to the dairy and beverages industry, as a tool for processand product innovation, quality improvement and safety.
Appraisals of the key technical aspects of membrane processing stages to control andmaintain the consistency of dairy and beverages products are also included. Although thishas produced some overlap in the coverage of membrane processes, I have felt justifiedin allowing this overlap because it emphasises the prime importance of processing in thepreparation of milks and beverages, to achieve the desired quality and consistency of theseproducts for the end-user.
Generally, the book is divided into three main parts. Part I, which consists of fivechapters, reviews the principals, developments and designs of membrane processes thatare mainly used in commercial dairy and beverage applications. Successful applications ofmembrane processes in the food industry and requirements pertaining to ensure hygienicconditions in the equipment are reviewed in Chapter 4, and Chapter 5 details the aspectsof food industrial effluent treatment, recovery of detergent from cleaning-in-place (CIP)systems and reuse of wastewater.
The application of membrane technology in the manufacture of different dairy productscould be briefly summarised as follows: (a) MF for the removal of bacteria and sporesfrom skimmed milk for the production of an extended shelf-life product; (b) UF is used toconcentrate the fat and protein components in milk, which can be used for standardisationof the protein content prior to cheesemaking and also to concentrate the fermentate forthe production of concentrated yoghurt (labneh); in addition, a wide range of cheeses arecommercially manufactured from ultrafiltrated milk (e.g. Quarg, Feta); (c) reduction of saltsin milk is achieved using NF, electrodialysis (ED) has been used for whey; (d) combinedapplications of MF, UF and NF have been used to fractionate milk components and, insome instances, the composition of cow’s milk has been modified to be similar to mare’smilk; (e) RO is widely used as a lower energy alternative to evaporation where low degreesof concentration are needed.
Part II of the book provides information on the applications of membrane processes inthe manufacture of dairy products. This ranges from on-farm concentration of milk as apre-treatment for cheesemaking to fractionation of milk and whey to provide a wide rangeof ingredients for food and other applications.
xvi Preface
Part III considers membrane applications during the manufacture of fruit juices, beer andcider, wine and vinegar. These include concentration, deacidification and dealcoholisationprocesses.
Dairy and beverage processors are encouraged to use this book as a reference in theapplication of membranes, both to aid the creation of novel products and to improve theirprocess economics.
A.Y. Tamime
Contributors
Editor
Dr. A.Y. TamimeDairy Science & Technology Consultant24 Queens TerraceAyr KA7 1DXScotland – United KingdomTel. +44 (0)1292 265498Fax +44 (0)1292 265498Mobile +44 (0)7980 278950E-mail: [email protected]
Contributors
Dr M. AshokkumarUniversity of MelbourneParticulate Fluids ProcessingCentre School of ChemistryVictoria 3010AustraliaTel. +61 (0)3 8344 7090Fax: +61 (0)3 9347–5180E-mail: [email protected]
Dr A. CassanoIstituto per la Tecnologia delle Membrane
(ITM-CNR)Universita della Calabriavia P. Bucci, 17/C87030 Rende (CS)ItalyTel. + 39 0984 492067 or 492014Fax + 39 0984 402103E-mail: [email protected]
Dr-Ing T. DemmerGoethering 56D-85570 Markt SchwabenGermany
Tel. +49 8121 40891E-mail: [email protected]
Dr M. DukeVictoria UniversityInstitute for Sustainability and InnovationPO Box 14428Victoria 8001AustraliaTel. +61 (0)3 9919 7682Fax: +61 (0)3 9919 7696E-mail: [email protected]
Dr G. Gesan-GuiziouDirector of Research INRAUMR1253 Science and Technologie du Lait
et de L’oeufINRA – Agrocampus Ouest65 Rue de Saint Brieuc35042 Rennes CedexFranceTel. +33 (0)2 23 48 53 25Fax: +33 (0)2 23 48 53 50E-mail: Genevieve.Gesan-Guiziou@rennes
.inra.fr
Dr K. Graingerc/o Tarsus HotelDaventry RoadSouthamWarks CV47 1NWEngland – United KingdomMobile +44 (0)7956 004855E-mail: [email protected]
Dr S. GrayVictoria UniversityInstitute for Sustainability and InnovationPO Box 14428
xviii Contributors
Victoria 8001AustraliaTel. +61 (0)3 9919 8097Fax: +61 (0)3 9919 7696E-mail: [email protected]
Dr A. HausmannVictoria UniversityInstitute for Sustainability and InnovationPO Box 14428Victoria 8001AustraliaTel. +61 (0)3 9919 7690Fax: +61 (0)3 9919 7696E-mail: [email protected]
Dr S.E. KentishUniversity of MelbourneParticulate Fluids Processing CentreDepartment of Chemical and Biomolecular
EngineeringVictoria 3010AustraliaTel. +61 (0)3 8344 6682Fax: +61 (0)3 8344 4153E-mail: [email protected]
Ms L.L.A. KohUniversity of MelbourneDepartment of Chemical and Biomolecular
EngineeringVictoria 3010AustraliaTel. +61 3 8344 6682Fax: +61 3 8344 4153E-mail: [email protected]
Dr F. LopezUniversitat Rovira i VirgiliFacultat d’EnologiaDepartament d’Enginyeria Quımicaav. Paısos Catalans 2643007-TarragonaSpainTel. 34 977 558 503Fax: 34 977 559 621E-mail: [email protected]
Dr N. MilneVictoria UniversityInstitute for Sustainability and InnovationPO Box 14428
Victoria 8001AustraliaTel. +61 (0)3 9919 7646Fax: +61 (0)3 9919 7696E-mail: [email protected]
Dr V.V. MistrySouth Dakota State UniversityDairy Science DepartmentBrookings SD 57007United States of AmericaTel. + 1 (0)605 688 5731Fax +1 (0)605 688 6276E-mail: [email protected]
Dr L. RamchandranVictoria UniversityInstitute for Sustainability and InnovationPO Box 14428Victoria 8001AustraliaTel. +61 (0)3 9919 7684Fax: +61 (0)3 9919 7696E-mail: [email protected]
Dr S. O’MahonyUniversity College CorkSchool of Food & Nutritional SciencesCorkRepublic of IrelandTel. + 353 (0)21 4903625E-mail: [email protected]
Dr B. OzerAbant Izzet Baysal UniversityFaculty of Engineering and ArchitectureDepartment of Food Engineering14280 BoluTurkeyE-mail: [email protected]
Dr K. SmithDairy Processing TechnologistWisconsin Centre for Dairy Research1605 Linden DriveMadison, WI 53706–1565United States of AmericaTel. +1 (0) 608 265 9605E-mail: [email protected]
Contributors xix
Dr S.J. Tuohy2e Technical Development Ltd.Hollybrook HouseCorrinFermoyCo. CorkRepublic of IrelandTel. : + 353 (0)25 31144Mobile: + 353 (0)87 2657706E-mail: [email protected]
Dr T. VasiljevicVictoria UniversitySchool of Biomedical and Health SciencesWerribee campusVictoria 8001AustraliaTel. +61 (0)3 9919 8062Fax: +61 (0)3 9919 8284E-mail: [email protected]
1 Development of Membrane Processes
K. Smith
1.1 Historical background
The ability of membranes to separate water from solutes has been known since 1748,when Abbe Nolet experimented with the movement of water through a semi-permeablemembrane. Depending on the reference, either Abbe Nolet or Dutorchet coined the wordosmosis to describe the process. Throughout the 18th and 19th centuries, membranes wereused exclusively for laboratory applications, and often consisted of sausage casings madefrom animal intestines or the bladders of pigs, cattle or fish.
The first synthetic membranes were produced by Fick in 1855, and appear to havebeen made of nitrocellulose. Membranes based on cellulose were known as collodionand had the advantages of reproducible characteristics compared with the previously usedanimal-based membranes. Bechhold further advanced the process for manufacturing col-lodion membranes when he developed methods for controlling pore size and measuringpore diameters in 1907. He is generally credited with first using the term ultrafiltration(UF). In addition, Richard Zsigmondy at the University of Gottingen, Germany, patenteda membrane filter in 1918 that was referred to as a cold ultrafilter. His work becomes thebasis of the membrane filters produced by Sartorius GmbH.
Collodion membranes produced by the Sartorius GmgH of Germany became commer-cially available in 1927. The primary use of membranes until the 1940s was the removal ofmicro-organisms and particles from liquids and gases and research applications. There wasa critical need to test drinking water in Europe for microbial content following the SecondWorld War, and membranes were developed that could rapidly filter water and capture anymicro-organisms on the membrane surface, where they could quickly be enumerated todetermine the safety of the water for human consumption.
In addition to the separation of relatively large particles from water, there was interestin developing membranes that could desalinate sea or brackish water. The term reverseosmosis (RO) had been coined in 1931 when a patent was issued for desalting water;however, the available membranes could not withstand the pressures required.
Although many improvements were made in the following years, including the use ofother polymers for constructing membranes, membranes were limited to laboratory andsmall specialised industrial applications. Factors limiting the use of membranes includeda lack of reliability, being too slow, not sufficiently selective and cost.
Membrane Processing: Dairy and Beverage Applications, First Edition. A. Y. Tamime.© 2013 Blackwell Publishing Ltd. Published 2013 by Blackwell Publishing Ltd.
2 Membrane Processing – Dairy and Beverage Applications
A breakthrough came in the early 1960s when Sourirajan and Loeb developed a processfor making high-flux, defect-free membranes capable of desalinating water. Researchers atthe time believed the best approach to improving flux would be to reduce the thicknessand thereby the resistance to flow of the membrane. Sourirajan and Loeb attempted toproduce such membranes by taking existing cellulose acetate membranes and heating themwhile submersed in water in a process known as annealing. They expected the membranepores would increase in size by such a process, but instead the pores became smaller andthe membrane more dense. When they attempted the same process with cellulose acetateUF membranes, they discovered not only did the pores become smaller but the ability ofthe membrane to reject salt increased, as did flux. The flux improvement was such that themembranes could be a practical way to desalinate water.
The annealing process of Sourirajan and Loeb had created an anisotropic or asymmetricmembrane. Anisotropic membranes have different behaviour depending on which sideof the membrane is used for the separation. Although this type of membrane had beenseen over 100 years earlier with natural membranes, it had not been reproduced with thesynthetic variety.
The key to the anisotropic membrane of Sourirajan and Loeb was the thin ‘skin’ onone surface of the membrane. The skin typically was approximately 0.1–0.2 μm thickand had a dense structure whereas the remainder of the membrane had a very porousopen structure. The thickness of the membrane essentially determined the flux and so byreducing the effective separating distance from 100–200 μm to 0.1–0.2 μm the rate ofliquid crossing the membrane dramatically improved, but because of the small pores in theskin the rejection of salt remained high.
Many changes in the production of membranes occurred during the 1960s, 1970s and1980s. By continuing the work of Sourirajan and Loeb, others were able to developadditional methods for producing membranes. Initial membrane modules were plate-and-frame (Danish Sugar Corporation) or hollow fibre (Amicon) designs, but membranesin formats, such as spiral-wound and tubular (Abcor), were introduced shortly afterwards.The thickness of the separating layer was further reduced to less than 0.1 μm. Largeplants using RO, UF and microfiltration (MF) were operating around the worldby 1980.
Cellulose acetate remained the material of choice until the mid-1970s, when methodsof producing composite membranes for water desalination were developed. By combiningpolysulphone and polyamide, composite membranes had the advantage of high salt rejec-tion combined with good water flux and increased resistance to temperature and chemicals.Nanofiltration (NF) or ‘loose RO’ membranes became available in the mid-1980s. The NFmembranes operated at lower pressures than RO systems, and were able to permeate mono-valent ions. They found immediate application in producing ultrapure water by permeatingtrace salts from water produced by RO.
In addition, membranes made from inorganic materials, such as zirconium and tita-nium dioxide, became commercially available in the mid-1980s. Membranes made fromthese materials are referred to as mineral or ceramic and are available in tubular formfor UF and MF. Union Carbide (USA) and Societe de Fabrication d’Elements (France)used carbon tubes covered with zirconium oxide for their inorganic membranes. LaterCeraver (France) used a ceramic base with aluminium oxide. Chemical and temperatureresistance were the significant advantages of ceramic membranes. It was originally thought
Development of Membrane Processes 3
that such membranes had an unlimited life, but subsequent experience has shown this is notthe case.
Advancements in membrane composition and design along with operation of mem-brane systems have continued. A wide variety of membrane polymers and designs havebeen adapted for RO, NF, UF and MF, resulting in many commercial applications. Thefeasibility of membrane-based applications depends chiefly on the ability of the filtrationprocess to economically produce an acceptable product. Membrane pore size distribution,selectivity, operating conditions, membrane life, capital and operating costs become impor-tant economic considerations. These parameters are in turn influenced by many factors,such as the membrane polymer, element configuration and system design.
1.2 Basic principles of membrane separations
Membrane filtration is a pressure-driven separation process using semi-permeablemembranes. The size of membrane pores and the pressure used indicate whether the termRO, NF, UF or MF is used for a given separation. RO and NF systems use the highest pres-sures and membranes with the smallest pores, whereas MF has the lowest operatingpressures and membranes with the largest pores. UF is intermediate in pressure used andmembrane pore size.
1.2.1 Depth versus screen filters
In the past, filtration processes relied on depth filters. This type of filter has fibres or beadsin a mesh-like structure. Particles in the feed solution become trapped or adsorbed withinthe filter network, which eventually clogs the filter, thereby resulting in replacement of thefilter (Fig. 1.1). By contrast, screen type filters generally rely on pores, with the size andshape of the pores determining passage of particles. Pores are more rigid and uniform andhave a more narrowly defined size than mesh openings in a depth filter. Components notable to pass through pores remain on the membrane surface and, therefore, do not typicallybecome trapped within the membrane structure (Fig. 1.2). Because the fouling materialsremain on the surface, internal fouling decreases and the membrane can be reused.
Path of filtered liquid through membrane
Fig. 1.1 Depth filter with particles entrapped within the membrane structure.
4 Membrane Processing – Dairy and Beverage Applications
Path of filtered liquid through membrane
Fig. 1.2 Screen-type membrane separates particles at the membrane surface.
1.2.2 Isotropic versus anisotropic membranes
Membranes can have several types of internal structure. Terms, such as microporous,non-porous, isotropic and anisotropic, refer to the structure of the membrane. Typically,membranes are either isotropic or anisotropic. Microporous and non-porous refer toisotropic membrane structure. An isotropic membrane will have a relatively uniformstructure (Fig. 1.3), i.e. the size of the pores is similar throughout the membrane. Themembrane, therefore, does not have a top or bottom layer, rather the membrane propertiesare uniform in direction. Isotropic membranes generally act as depth filters and, therefore,retain particles within the internal structure resulting in plugging and reduced flux.
Microporous and non-porous membranes typically are isotropic. Microporous mem-brane structure can resemble a traditional filter; however, the microporous membrane hasextremely small pores. Materials are rejected at the surface, trapped within the membraneor pass through pores unhindered, depending on particle size and size of the pores. A non-porous membrane will not have visible pores and materials move by diffusion through themembrane.
An anisotropic or asymmetric membrane has pores that differ in size depending on theirlocation within the membrane (Fig. 1.4). Typically, anisotropic membranes will have a thin,dense skin supported by a thicker and a more porous substructure layer. The thin top layerprovides high selectivity, whereas the porous bottom layer has good flux. Membranes usedfor commercial separations in the food industry are typically anisotropic.
Fig. 1.3 Structure of an isotropic membrane.
Development of Membrane Processes 5
Fig. 1.4 Structure of an anisotropic membrane.
1.2.3 Cross-flow filtration
RO, NF, UF and MF systems all involve cross-flow filtration, which can be comparedto the traditional method of perpendicular filtration. In traditional filtration (Fig. 1.5),the entire feed stream passes through the filtering media, i.e. the incoming stream flowsperpendicularly to the filter with the filter retaining any trapped solids. The result is afiltered stream with solids trapped on and within the filter.
In cross-flow filtration (Fig. 1.6), the feed stream passes parallel to the membrane.Some of the incoming feed stream and particles will cross the membrane into the permeatesection, whereas the other portion with the concentrated solids is the retentate stream.At any time only some of the water and particles will cross the membrane into thepermeate stream, unlike traditional filtration where most particles are trapped after onepass through the filter. Because the feed stream flows parallel to the membrane rather
Feed stream
Filtered stream
Filter
Fig. 1.5 Traditional filtration with perpendicular flow.
6 Membrane Processing – Dairy and Beverage Applications
Permeate
Feed
Membrane
Membrane
Retentate
Permeate
Fig. 1.6 Cross-flow filtration.
than perpendicular to it, cross-flow filtration is self-cleaning by comparison. Solutes andparticles are continually swept along and away from the membrane surface by the reten-tate stream, thereby allowing longer operating times without cleaning than is possible withtraditional filtration.
The affect of cross-flow permeate flow and thickness of the fouling or cake layer canbe seen in Fig. 1.7. In perpendicular filtration, the flow of permeate is reduced as thethickness of the material on the surface of the filter, i.e. the thickness of the cake layer,increases over time. With cross-flow filtration, however, the thickness of the material onthe membrane is limited by action of the feed stream sweeping across the surface of the
Perpendicular filtration
Time Time
Cross-flow filtration
Cake thickness
Cake thickness
Permeate flux
Permeate flux
Feed stream
Filter
Filtered stream
Feed
Permeate
Permeate
Membrane
Membrane
Retentate
Fig. 1.7 Effect of perpendicular and cross-flow filtration on flux and cake thickness.
Development of Membrane Processes 7
membrane. Because the thickness of the deposited material is limited, permeate flow ismaintained at a higher level throughout filtering.
1.2.4 Requirements of membrane processes
The shared characteristics of membrane processes are pressure-driven processes usingsemi-permeable membranes. Pressure is used to reverse the direction of the osmosis pro-cess, while differences in membrane permeability determine separation of molecules. Theprocess of osmosis is illustrated in Fig. 1.8. Solutions containing two different concentra-tions of dissolved materials are separated by a membrane that will allow only water to cross
Semi-permeable membrane
More concentrated solution
Less concentrated solution
Initial solutions
Semi-permeable membrane
More concentrated solution
Less concentrated solution
Osmosis
Semi-permeable membrane
More concentrated solution
Less concentrated solution
Reverse osmosis
(a)
(b) (c)
Solution will rise to level where head is equal to apparent osmotic pressure
Pressure
Fig. 1.8 The processes of osmosis and reverse osmosis.
8 Membrane Processing – Dairy and Beverage Applications
(Fig. 1.8a). Nature will try to equalise the concentration of the two solutions. Since thedissolved material cannot cross the membrane, water must flow from the solution of lowerconcentration to the solution at the higher concentration (Fig. 1.8b). The flow of water willcontinue until the solutions are of equal concentration or no more water is available. Thedifference in the height of water in the corresponding tubes is a result of the movement ofwater from lower to higher concentration. The final water level in the more concentratedsolution compared with the original level is equal to the apparent osmotic pressure.
In the process of ‘reverse’ osmosis, pressure is used to force water to flow in theopposite direction (Fig. 1.8c). Enough pressure must be applied to overcome the apparentosmotic pressure of the more concentrated solution before water can flow from the moreconcentrated to the less concentrated side. In doing so, the more concentrated side becomeseven more concentrated through the loss of water. It is this ability to concentrate andseparate that is taken advantage of in commercial membrane separations.
Another shared characteristic is the use of semi-permeable membranes. Membranescan be distinguished from filters by the size of the particulates that are separated. Byconvention, filters generally separate particulates that are greater than 1–10 μm in size,whereas membranes separate smaller particles. Semi-permeable refers to the ability toseparate some particles from other particles.
1.3 Types of membrane separations
The classification of membranes as RO, NF, UF and MF is somewhat arbitrary, andhas considerable overlap between categories. Generally, RO/NF membranes will retainmolecules in the ionic size range, UF membranes will separate macromolecules, and MFwill retain particles of micron size. Because RO, NF, UF and MF membranes differ in thesize of molecules they separate, the osmotic pressure involved is considerably differentbetween the processes. RO, which retains the smallest molecules, has the highest osmoticpressure to overcome and, therefore, requires the highest operating pressure. A range from1.38 to 8.28 MPa is common for RO, 1.03 to 2.76 MPa for NF, 0.21 to 1.03 MPa for UF,and MF requires only from 0.07 to 0.69 MPa.
1.3.1 Reverse osmosis
RO membranes generally retain all compounds allowing only water to cross into thepermeate. There are exceptions to this general statement and, at times, relatively largemolecules may pass into the permeate. RO membranes can, therefore, either concentratea feed stream (retentate stream) through removal of water or produce very pure water(permeate stream).
1.3.2 Nanofiltration
NF membranes are very similar to RO membranes with the exception that NF membraneswill allow the passage of monovalent ions into the permeate. NF membranes are veryeffective at concentrating materials in the feed stream since only monovalent ions are
