PROFE$$1ONA~
`ENG’rNEERTNG
`
`A top expert’s A-to-Z guide to membrane
`science, technology, & applications
`
`The ultimate skill-building source for
`engineers, chemists, and students
`
`Inside: Membrane Transport Theory, Reverse
`Osmosis, Carrier Facilitated Transport, and
`much more!
`
`ology
`and
`attons
`
`Richard W. Baker
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`GE-1008.001
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`

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`&,_.__
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`_.
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`_
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`W.
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`TP 159
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`B35
`2000
`COPY 2
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`GE-1008.002
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`GE-1008.003
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`GE-1008.003
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`
`~ernbrane
`Techne~ogy and
`Applicatiens
`
`GE-1008.004
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`

`
`Membrane
`Technology an
`Applications
`
`Richard W. Baker
`
`Membrane
`Technology and
`Research, Inc.
`
`Menlo Park,
`California
`
`McGraw-Hill
`New York San Francisco Washington, D.C. Auckland Bogotd
`Caracas Lisbon London Madrid Mexico City Milan
`Montreal New Delhi San Juan Singapore
`Sydney Tokyo Toronto
`
`GE-1008.005
`
`

`
`Cataloging-in-Publication Data for this title is on file with
`the Library of Congress
`
`oO
`
`McGraw-Hill
`
`A Division o[ The McGraw.Hill Companies
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`Copyright © 2000 by The McGmw-I-IiU Companies, Inc. All rights
`reserved. Printed in the United States of America. Except as permitted
`under the United States Copyright Act of 1976, no part of this publication
`may be reproduced or distributed in any form or by any means, or stored
`in a data base or retrieval system, without the prior written permission of
`the publisher.
`
`1234567890 AG~AGM 90432109
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`ISBN 0-07-135440-9
`
`The sponsoring editor of this book was Robert Esposito. The editing
`supervisor was Steven Melvin, and the production supervisor was Cheryl
`Souffrance. It was set in New Century Schoolbook per the MHC design by
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`Group composition unit in Hightstown, N.J.
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`Information contained in this work has been obtained by The McGraw-Hill
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`neither McGraw-Hill nor its authors guarantee the accuracy or completeness
`of any information published herein and neither McGraw-Hill nor its authors
`shall be responsible for any errors, omissions, or damages arising out of use of
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`
`GE-1008.006
`
`

`
`Contents
`
`Preface xi
`Acknowledgments xiii
`
`Chapter I Overview of Membrane Science
`
`Introduction
`Historical Development of Membranes
`Types of Memb[anes
`Isotropic Membranes
`Anisotropic Membranes
`Ceramic, Metal, and Liquid Membranes
`Membrane Processes
`References
`
`Chapter 2 Membrane Transport Theory
`
`Introduction
`The Solution-Diffusion Model
`Molecular Dynamics Simulations
`Concentration and Pressure Gradients In Membranes
`Application of the Solution-Diffusion Model to Specific Processes
`Evidence for the Solution-Diffusion Model
`Structure-Permeability Relationships in Solution-Diffusion Membranes
`Diffusion Coefficients
`Sorption Coefficients in Polymers
`Pore-Row Membranes
`Permeation in Ultraflltration and Microfiltration Membranes
`Knudsen Diffusion and Surface Diffusion In Microporous Membranes
`Transition Region
`Conclusions and Future Directions
`References
`
`Chapter 3 Membranes and Modules
`
`Introduction
`Isotropic Membranes
`Isotropic Nonporous Membranes
`Isotropic Microporous Membranes
`
`1
`2
`4
`4
`6
`6
`7
`13
`
`15
`
`15
`19
`19
`22
`27
`44
`47
`50
`58
`65
`68
`73
`80
`81
`83
`
`87
`
`88
`89
`89
`91
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`GE-1008.007
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`
`vi Contents
`
`Anisotropic Membranes
`Phase Separation Membranes
`Interracial Polymerization Membranes
`Solution-Coated Composite Membranes
`Other Anisotropic Membranes
`Repairing Membrane Defects
`Metal Membranes and Ceramic Membranes
`Metal Membranes
`Ceramic Membranes
`Liquid Membranes
`Hollow-Fiber Membrane
`Membrane Modules
`Plate-and-Frame Modules
`Tubular Modules
`Spiral-Wound Modules
`Hollow-Fiber Modules
`Module Selection
`Conclusions and Future Directions
`References
`
`Chapter 4 Concentration Polarization
`
`Introduction
`The Boundary Layer Film Model
`
`Determination of the Peclet Number
`Concentration Polarization in Liquid Separation Processes
`Concentration Polarization in Gas Separation Processes
`Conclusions and Future Directions
`References
`
`Chapter 5 Reverse Osmosis
`
`Introduction and History
`Theoretical Background
`Membranes and Materials
`Cellulosic Membranes
`Noncellulosic Polymer Membranes
`Composite Membranes
`Other Membrane Materials
`Reverse Osmosis Membrane Categories
`Membrane Selectivity
`Membrane Modules
`Membrane Fouling Control
`Membrane Cleaning
`Applications
`Brackish Water Desalination
`Seawater Desalination
`Ultrapure Water
`Wastewater Treatment
`Conclusions and Future Directions
`References
`
`95
`96
`114
`118
`121
`125
`127
`127
`127
`131
`132
`136
`138
`139
`141
`144
`147
`152
`152
`
`159
`
`162
`171
`173
`176
`179
`185
`186
`
`187
`
`187
`189
`193
`194
`197
`198
`200
`201
`205
`207
`208
`212
`214
`216
`217
`218
`219
`221
`222
`
`GE-1008.008
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`

`
`Chapter 6 Ultrafiltration
`
`Introduction and History
`Characterization of Ultrafiltration Membranes
`Concentration Polarization and Membrane Fouling
`Membrane Cleaning
`Membranes and Modules
`Membrane Matsdale
`¯
`Ultraflltration Modules
`System Design
`Batch Systems
`Continuous Systems
`Applications
`Electrocoat Paint
`Food Industry
`Oil-Water Emulsions
`Process Water and Product Recycling
`Biotechnology
`Conclusions and Future Directions
`References
`
`Chapter 7 Microflltration
`
`Introduction and History
`Background
`Types,of Membrane
`Membrane Charactedzatlon
`Mlcroflltratlon Membranes and Modules
`Process Deslgn
`Applications,
`Sterile Flltretlon of Pharmaceutlcels
`Sterllizatlon of Wlne and Beer
`Mlcroffitration in the Electronlce Industry
`Conclusions and Future Dlrectlons
`References
`
`Chapter 8 Gas Separation
`
`Introductlon and History
`Theoretical Background
`Membrane Msterlals and Structure
`Metal Membranes
`Polymeric Membranes
`Membrane Modules
`Process Deslgn
`Multlstep and Multistage System Designs
`Recycle Designs
`Applications
`Hydrogen Separations
`Oxygen/Nitrogen Separation
`Natural Gas Separations
`Vapor/Gas Separations
`
`Contents vii
`
`225
`
`225
`226
`230
`241
`244
`244
`245
`247
`247
`249
`251
`253
`254
`257
`258
`259
`260
`262
`
`265
`
`265
`267
`267
`268
`276
`279
`282
`283
`265
`285
`286
`286
`
`287
`
`287
`289
`296
`296
`298
`300
`301
`307
`3O9
`311
`311
`315
`321
`329
`
`GE-1008.009
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`

`
`viii Contents
`
`Dehydration of Air
`Conclusions and Future Directions
`References
`
`Chapter 9 Pervaporation
`
`Introduction and History
`Theoretical Background
`Membrane Materials and Modules
`Membrane Materials
`Membrane Modules
`Process Design
`Applications
`Solvent Dehydration
`Separation of Dissolved Organics from Water
`Separation of Organic Mixtures
`Conclusions and Future Directions
`References
`
`Chapter 10 Ion Exchange Membrane Processes--Electrodialysis
`
`Introduction and History
`Theoretical Background
`Transport through lon Exchange Membranes
`Chemistry of lon Exchange Membranes
`Homogeneous Membranes
`Heterogeneous Membranes
`Transport in Electrodialysls Membranes
`Concentration Polarization and Limiting Current Density
`Current Efficiency and Power Consumptlon
`System Design
`Applications
`Brackish Water Desalination
`Salt Recovery from Seawater
`Other Electrodlalysis Separation Applications
`Continuous Electrodeionization and Ultrapure Water
`Bipolar Membranes
`Conclusions and Future Directions
`References
`
`Chapter 11 Carrier-Facilitated Transport
`
`Introduction and History
`Coupled Transport
`Background
`Characteristics of Coupled Transport Membranes
`Coupled Transport Membranes
`Applications
`Facilitated Transport
`Background
`Process Designs
`Applications
`
`331
`332
`334
`
`337
`
`337
`340
`345.
`345
`350
`351
`354
`354
`358
`365
`369
`369
`
`373
`
`373
`377
`377
`380
`382
`384
`385
`385
`390
`392
`396
`396
`396
`397
`398
`401
`4O2
`4O3
`
`405
`
`4O5
`411
`411
`415
`420
`423
`425
`425
`429
`432
`
`GE-1008.010
`
`

`
`Conclusions end Future Directions
`References
`
`Chapter 12 Medical Applications of Membranes
`
`Introduction
`Hemodialysis
`Blood Oxygenators
`Controlled Drug Delivery
`Membrane Diffusion-Controlled Systems
`Biodegradable Systems
`Osmotic Systems
`References
`
`Chapter 13 Other Membrane Processes
`
`Introduction
`Dialysis
`Donnan Dialysis
`Charge Mosaic Membranes/Piezodlalysis
`Membrane Contactors and Membrane Distillation
`Applications of Membrane Contactors
`Liquid/Liquid Membrane Contactors (Membrane Distillation)
`Membrane Reactors
`Applications
`Conclusions and Future Directions
`References
`
`Appendix 499
`Index 511
`
`Contents ix
`
`437
`439
`
`443
`
`443
`444
`449
`450
`453
`459
`461
`467
`
`469
`
`469
`469
`472
`474
`477
`480
`483
`486
`489
`494
`495
`
`GE-1008.011
`
`

`
`Chapter
`
`3
`
`Membranes and Modules
`
`Introduction
`
`Isotropic Membranes
`
`Isotroplc nonporous membranes
`
`Solution casting
`
`Melt extruded film
`
`Isotropic microporous membranes
`Track-etch membranes
`
`Expanded-film membranes
`
`Template leaching
`Anlsotropic Membranes
`
`Phase separation membranes
`
`Polymer precipitation by water (the Loeb-Sourirajan proces~
`
`88
`
`89
`
`89
`
`89
`
`90
`
`91
`
`92
`
`92
`
`94
`
`95
`
`96
`
`97
`
`Empirical approach to membrane formation by water
`precipitation
`
`Theoretical approach to membrane formation
`
`Polymer precipitation by cooling
`
`Polymer precipitation by solvent evaporation
`
`Polymer precipitation by absorption of water vapor
`Interracial polymerization membranes
`
`Solution-coated composite membranes
`
`Other anisotropic membranes
`Plasma polymerization membranes
`
`Dynamically formed membranes
`
`Reactive surface treatment
`Repairing membrane defects
`
`Metal Membranes and Ceramic Membranes
`
`Metal membranes
`
`Ceramic membranes
`
`Metal oxide membranes
`
`Microporous carbon membranes
`
`Microporous glass membranes
`
`101
`
`104
`
`108
`
`110
`
`112
`
`114
`
`118
`
`121
`
`121
`
`124
`
`124
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`125
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`127
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`127
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`127
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`127
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`131
`
`131
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`87
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`GE-1008.012
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`

`
`88
`
`Chapter Three
`
`Liquid Membranes
`
`Hollow-Fiber Membranes
`
`Membrane Modules
`
`Plate-and-frame modules
`
`Tubular modules
`
`Spiral-wound modules
`
`Hollow-fiber modules
`
`Module selection
`
`Conclusions and Future Directions
`
`References
`
`Introduction
`
`131
`
`132
`
`136
`
`138
`
`139
`
`141
`
`144
`
`147
`
`152
`
`152
`
`The surge of interest in membrane separation processes that began
`in the late 1960s was prompted by two developments: .first, the abil-
`ity to produce high-flux, essentially defect-free membranes on a large
`scale, and second, the ability to form these membranes into compact,
`high-surface-area, economical membrane modules. These break-:
`throughs in membrane technology took place in the 1960s to early
`1970s, as part of the development of reverse osmosis and ultrafiltra-
`tion. Adaptation of the technology to other membrane processes took
`place in the 1980s.
`Several factors contribute to the successful fabrication of a high-per-.
`formance membrane module. First, membrane materials with the
`appropriate chemical, mechanical, and permeation properties must be’
`selected; this choice is very process-specific. However, once the mem-
`brane material has been selected, the technology required to fabricate
`this material into a robust, thin, defect-free membrane and then to
`package the membrane into an efficient, economical, high-surface-area
`module is similar for all membrane processes. Therefore, this chapter.:
`focuses on methods of forming membranes and membrane modules.
`The criteria used to select membrane materials for specific processes
`are described in the chapters covering each application.
`In this chapter, membrane preparation techniques are organized by
`membrane structure: isotropic membranes, anisotropic membranes,
`ceramic and metal membranes, and liquid membranes. Isotropic mem-
`branes have a uniform composition and structure throughout; such
`membranes can be porous or dense. Anisotr0pic (or asymmetric) mem-
`branes, however, consist of a number of layers, each with different
`structures and permeabilities. A typical anisotropic membrane has a
`relatively dense, thin surface layer supported on an open, much thick-
`er microporous substrate. The surface layer performs the separation
`and is the principal barrier to flow through the membrane. The open
`support layer provides mechanical strength. Ceramic and metal mem-
`branes can be either isotropic or anisotropic. However, these mem-
`
`GE-1008.013
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`

`
`Membranes and Modules 89
`
`branes are grouped separately from polymeric membranes because
`their preparation methods are so different.
`Liquid membranes are the final membrane category. The selective
`barrier in these membranes is a liquid phase, usually containing a dis-
`solved carrier that selectively reacts with a specific permeant to
`enhance its transport rate through the membrane. Liquid membranes
`are used almost exclusively in carrier-facilitated transport processes, so
`preparation of these membranes is covered in that chapter (Chap. 11).
`The membrane classification scheme described above works fairly
`well. However, a major membrane preparation technique, phase sepa-
`ration, also known as phase inversion, is used to make both isotropic
`and anisotropic membranes. This technique is covered under
`anisotropic membranes.
`
`Isotropic Membranes
`
`Isotropic nonporous membranes
`
`Dense, nonporous isotropic membranes are rarely used in membrane
`separation processes because the transmembrane flux through these rel-
`atively thick membranes is too low for practical separation processes.
`However, they are widely used in laboratory work to characterize mem-
`brane properties. In the laboratory, isotropic (dense) membranes are pre-
`pared by solution casting or thermal melt:pressing. The same techniques
`can be used on a larger scale to produce packaging material.
`
`Solution casting. Solution casting is commonly used to prepare small
`samples of membrane for laboratory characterization experiments. An
`even film of an appropriate polymer solution is spread across a flat
`plate with a casting knife. The casting knife consists of a steel blade,
`resting on two runners, arranged to form a precise gap between the
`blade and the plate onto which the film is cast. A typical handheld
`knife is shown in Fig. 3.1. After casting, the solution is left to stand,
`and the solvent evaporates to leave a thin, uniform polymer film. A
`detailed description of many types of hand-casting knives and simple
`casting machines is given in the book by Gardner and Sward.1
`The polymer solution used for solution casting should be sufficiently
`viscous to prevent it from running over the casting plate, so typical poly-
`mer concentrations are in the range of 15 to 20 wt %. Preferred solvents
`are moderately volatile liquids such as acetone, ethyl acetate, and cyclo-
`hexane. Films cast from these solutions are dry within a few hours.
`Solvents with high boiling points such as dimethyl formamide or N-
`methyl pyrrolidone are unsuitable for solution casting, because their low
`volatility requires long evaporation times. During an extended solvent
`evaporation time, the cast film can absorb sufficient atmospheric water
`
`GE-1008.014
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`

`
`90
`
`Chapter Three
`
`Wet wedge
`film
`drawdown
`
`Figure 3.1 A typical hand-casting knife. (Courtesy of Paul N. Gardner
`Company, Inc., Pompano Beach, FL.)
`
`to precipitate the polymer, producing a mottled, hazy surface. Very
`volatile solvents such as methylene chloride can also cause problems.
`Rapid evaporation of the solvent cools the casting solution, causing gela-
`tion of the polymer. The result is a fi]rn with a mottled, orange-peel,like
`surface. Smooth films can be obtained by covering the cast film with a
`glass plate raised 1 to 2 cm above the film to slow evaporation. When the
`solvent has completely evaporated, the dry film can be li~d f~om the
`glass plate. If the cast film adheres to the plate, soaking in a swelling
`nonsolvent such as water or alcohol will usually loosen the film.
`Solution-cast film is produced on a larger scale for medical applica-
`tions, battery separators, or other specialty uses with machinery of the
`type shown in Fig. 3.2.2 Viscous film is made by this technique. The
`solution is cast onto the surface of a rotating drum or a continuous pol-
`ished stainless steel belt. These machines are generally enclosed to
`control water vapor pickup by the film as it dries and to minimize sol-
`vent vapor losses to the atmosphere.
`
`Melt extruded film. Many polymers, including polyethylene, polypropy-
`lene, and nylons, do not dissolve in appropriate solvents at room tem-
`perature, so membranes cannot be made by solution casting. To.
`prepare small pieces of film, a laboratory press, as shown in Fig. 3.3,
`can be used. The polymer is compressed between two heated plates.
`Typically, a pressure of 2000 to 5000 lb/in2 is applied for i to 5 min, at
`
`GE-1008.015
`
`

`
`Membranes and Modules 91
`
`Solvent-
`laden air
`
`Casting
`solution
`
`Dry film take-up
`
`Solvent-
`~laden
`sir CasUng
`solution
`
`Figure 3.2 Machinery used to make solution-cast film on a commercial scale.
`
`rfilrn
`to
`take-up
`
`Figure 3.3 A typical laboratory
`press used to form melt-pressed
`membrane. (Courtesy of Carver,
`Inc., Wabash, IN.)
`
`a plate temperature just below the melting point of the polymer. Melt
`extrusion is also used on a very large scale to make dense films for
`packaging applications, either by extrusion as a sheet from a die or as
`blown film. Detailed descriptions of this equipment can be found in
`specialized monographs. A good overview is given in the article by
`Mackenzie in the Encyclopedia of Chemical Technology.2
`
`Isotropic microporous membranes
`
`Isotropic microporous membranes have much higher fluxes than
`isotropic dense membranes and are widely used as microfiltration mem-
`branes. Further significant uses are as inert spacers in battery and fuel
`cell applications and as the rate-controlling element in controlled drug
`delivery devices.
`
`GE-1008.016
`
`

`
`92
`
`Chapter Three
`
`The most important type of microporous membrane is formed by one
`of the phase separation techniques discussed in the next section; about
`one-half of the isotropic micropor0us membrane used is made in this
`way. The remaining types are made by various proprietary techniques,
`the more important of which are described below.
`
`Track-etch membranes. Track-etch membranes were developed by the
`General Electric Corporation Schenectady Laboratory.~ The two-step
`preparation process is illustrated in Fig. 3.4. First, a thin polymer film
`is irradiated with fission particles from a nuclear reactor or other radi-
`ation source. The massive particles pass through the film, breaking
`polymer chains and leaving behind a sensitized track of damaged poly-
`mer molecules. These tracks are much more susceptible to chemical
`attack than the base polymer material. So when the film is passed
`through a solution that etches the polymer, the film is preferentially
`etched along the sensitized nucleation tracks, thereby forming pores.
`The exposure time of the film to radiation determines the number of
`membrane pores; the etch time determines the pore diameter.4 A fea:
`ture of the track-etch preparation technique is that the pores are uni-
`form cylinders traversing the. membrane at right angles. The
`membrane tortuosity is, therefore, close to 1, and all pores have the
`same diameter. These membranes are almost a perfect screen filter;
`therefore, they are widely used to measure the number and type of sus-
`pended particles in air or water. A known volume of fluid is filtered
`through the membrane, and all particles larger than the pore diameter
`are captured on the surface of the membrane so they can be easily iden-
`tified and counted. To minimize the formation of doublet holes produced
`when two nucleation tracks are close together, the membrane porosity
`is usually kept relatively low, about 5 percent or less. This low porosity
`results in low fluxes. General Electric, the original developers of these
`membranes, assigned the technology to a spinoff company, Nu~lepore
`Corporation, in 1972.5 The company was subsequently acquired by
`Coming and then, in 1999, by Whatman Inc. Nuclepore® membranes
`remain the principle commercially available track-etch membranes.
`Polycarbonate or polyester films are usually used as the base mem-
`brane material and sodium hydroxide as the etching solution. Other
`materials can also be used; e.g., etched mica has been used in research
`studies.
`
`Expanded-film membranes. Expanded-film membranes are made from
`crystalline polymers by an orientation and annealing process. A num-
`ber of manufacturers produce porous membranes by this technique.
`The original development was due to a group at Celanese, now the
`Hoechst Celanese Separation Products Division, which made micro-
`
`GE-1008.017
`
`

`
`Membranes and Modules 93
`
`Step 1: Polycarbon~te rdm is exposed to
`cha~jed partk:k~s In a nuclear reactor
`
`Step 2: The Imck= left
`p~ ~
`
`’1
`lOpm
`
`Figure 3.4 Diagram of the two-step process to manufacture nucleation track mem-
`branes4 and photograph of resulting structure. (Photograph courtesy of Whatman Inc.
`Nuclepore® division.)
`
`porous polypropylene membranes by this process under the trade
`name Celgard.6 In the first step of the process, a highly oriented film
`is produced by extruding polypropylene at close to its melting point
`coupled with a very rapid drawdown. The crystallites in the semicrys-
`talline polymer are then aligned in the direction of orientation. After
`cooling and annealing, the film is stretched a second time, up to 300
`percent. During this second elongation, the amorphous regions
`between the crystallites are deformed, forming slitlike voids, 200 to
`2500/~ wide, between the polymer crystallites. The pore size of the
`membrane is controlled by the rate and extent of the second elongation
`step. The formation process is illustrated in Fig. 3.5. This type of mem-
`brane is also made from polytetrafluoroethylene film by W. L. Gore
`and sold under the trade name Gore-Tex.7 Expanded-film membrane
`was originally produced as rolled, flat sheets. More recently the
`process has also been adapted to the production of hollow fibers6;
`Hoechst Celanese produces this type of fiber on a large scale for use in
`blood oxygenator equipment (Chap. 12) and membrane contactors
`(Chap. 13). Gore-Tex polytetrafluoroethylene film is widely used as a
`water-vapor-permeable (i.e., breathable) but liquid-water-imperme-
`able fabric. The commercial success of these membranes has motivat-
`ed a number of other companies to produce similar materials.9,10
`
`GE-1008.018
`
`

`
`94
`
`Chapter Three
`
`Direction of
`unlaxial stretching
`~.
`
`Melt extrusion die
`
`Drawing
`
`region
`
`structure
`
`Stretching
`
`(a)
`
`!
`
`(b)
`
`Figure 3.5 (a) Preparation method of a typical expanded polypropy].ene fi!m
`membrane. (b) Scanning electron micrograph of the microdefects formed on
`uniaxial stretching of films; in this case Celgard®.6
`
`Template leaching. Template leaching is another method of producing
`isotropic microporous membranes from insoluble polymers such as
`polyethylene, polypropylene, and polytetrafluoroethylene. In this
`process a homogeneous melt is prepared from a mixture of the poly-
`meric membrane matrix material and a leachable component. To fine-
`
`GE-1008.019
`
`

`
`Membranes and Modules 95
`
`Heat ~
`
`Hopper
`
`Extruder
`
`. . . ~ Orier Guide rolls UDie
`wlnaup~) ~- ~
`Diluent
`supply
`
`Chill
`roll
`Figure 3.6 Flow schematic of a melt extruder system used to make
`polypropylene membranes by template leaching.13
`
`~
`Extraction
`
`ly disperse the leachable component in the polymer matrix, the mix-
`ture is often homogenized, extruded, and pelletized several times
`before final extrusion as a thin film. After formation of the film, the
`leachable component is removed with a suitable solvent, and a micro-
`porous membrane is formed,m13 The leachable component can be a sol-
`uble, low-molecular-weight solid, a liquid such as liquid paraffin, or
`even a polymeric material such as polystyrene. A drawing of a tem-
`plate leaching membrane production machine is shown in Fig. 3.6.
`
`Anisotropic Membranes
`
`Anisotropic membranes are layered structures in which the porosity,
`pore size, or even membrane composition changes from the top to the
`bottom surface of the membrane. Usually anisotropic membranes have
`a thin, selective layer supported on a much thicker, highly permeable
`microporous substrate. Because the selective layer is very thin, mem-
`brane fluxes are high. The microporous substrate provides the
`strength required for handling the membrane. The importance of
`anisotropic membranes was not recognized until Loeb and Sourirajan
`prepared the first high-flux, anisotropic reverse osmosis membranes
`by what is now known as the Loeb-Sourirajan technique.14 Hindsight
`makes it clear that some of the membranes produced in the 1930s and
`1940s were also anisotropic, but the importance of anisotropy was not
`realized at the time. Loeb and Sourirajan’s discovery was a critical
`breakthrough in membrane technology. Their anisotropic reverse
`osmosis membranes were an order of magnitude more permeable than
`the isotropic membranes produced previously from the same materi-
`als. For a number of years the Loeb-Sourirajan technique was the only
`method of making anisotropic membranes, but the demonstrated ben-
`efits of the anisotropic structure encouraged the development of other
`methods. Improvements in anisotropic membrane preparation meth-
`ods and properties were accelerated by the availability in the late
`!960s of the scanning electron microscope (SEM), which allowed the
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`Chapter Three
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`effects of changes in the membrane formation process on structure to
`be assessed easily.
`Membranes made by the Loeb-Sourirajan process consist of a single
`membrane material, but the porosity and pore size change in different
`layers of the membrane. Anisotropic membranes made by other tech-
`niques and used on a large scale often consist of layers of different
`materials which serve different functions. Important examples are
`¯ membranes made by the interfacial polymerization process discovered
`by Cadotte15 and the solution-coating processes developed by Ward,16
`Francis,~7 and Riley.is The following sections cover four types of
`anisotropic membranes:
`
`[] Phase separation membranes. This category includes membranes
`made by the Loeb-Sourirajan technique involving Precipitation of a
`casting solution by immersion in a nonsolvent (water) bath. Also cov-
`ered are a variety of related techniques such as precipitation by sol-
`vent evaporation, precipitation by absorption of water from the
`vapor phase, and precipitation by cooling.
`
`[] Interfacial¯ composite membranes. This type of anisotropic mem-
`brane is made by polymerizing an extremely thin layer of polymer at
`the surface of a microporous support polymer.
`
`[] Solution-coated composite membranes. To prepare these membranes,
`one or more thin, dense polymer layers are solution-coated onto the
`surface of a microporous support.
`
`[] Other anisotropic membranes. This category covers membranes
`made by a variety of specialized processes, such as plasma deposi-
`tion, in the laboratory or on a small industrial scale to prepare
`anisotropic membranes for specific applications.
`
`Phase separation membranes
`
`The Loeb-Sourirajan technique is now recognized as a special case of
`a more general class of membrane preparation process, best called the
`phase separation process, but sometimes called the phase inversion
`process or the polymer precipitation process. The term phase separa-
`tion describes the process most clearly, namely, changing a one-phase
`casting solution into two separate phases. In all phase separation
`processes, a liquid polymer solution is precipitated into two phases: a
`solid, polymer-rich phase that forms the matrix of the membrane and
`a liquid, polymer-poor phase that forms the membrane pores.
`Precipitation of the cast liquid polymer solution to form the
`anisotropic membrane can be achieved in several ways, as summa-
`rized in Table 3.1. Precipitation by immersion in a bath of water was
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`Membranes and Modules 97
`
`the technique discovered by Loeb and Sourirajan, but precipitation can
`also be caused by absorption of water from a humid atmosphere. A
`third method is to cast the film as a hot solution. As the cast film cools,
`a point is reached at which precipitation occurs to form a microporous
`structure; this method is called thermal gelation. Finally, evaporation
`of one of the solvents in the casting solution can be used to cause pre-
`cipitation. In this technique the casting solution consists of a polymer
`dissolved in a mixture of a volatile good solvent and a less volatile non-
`solvent (typically water or alcohol). When a film of the solution is cast
`and allowed to evaporate, the volatile good solvent evaporates first,
`then the film becomes enriched in the nonvolatile nonsolvent, and
`. finally it precipitates. Many combinations of these processes have also
`been developed. For example, a cast film placed in a humid atmos-
`phere can precipitate partly because of water vapor absorption but
`partly because of evaporation of one of the more volatile components.
`
`Polymer precipitation by water (the Loeb-Sourirajan process). The first
`phase separation membrane was developed at the University of
`California at Los Angeles (UCLA) from 1958 to 1960 by Sidney Loeb,
`then working on his master’s.degree, and Srinivasa Sourirajan, then a
`postdoctoral researcher. In their process, now called the Loeb-
`Sourirajan technique, precipitation is induced by immersing the cast
`film of polymer solution in a water bath. In the original Loeb-
`Sourirajan process, a solution containing 20 to 25 wt % cellulose
`acetate dissolved in a water-miscible solvent was cast as a thin film on
`a glass plate. The film was left to stand for 10 to 100 s to allow some of
`the solvent to evaporate, after which the film was immersed in a water
`bath to precipitate the film and form the membrane. The membrane
`
`TABLE 3.1 Phase Separation Membrane Preparation Procedures
`
`Water precipitation (the
`Loeb-Sourirajan process)
`
`Water vapor absorption
`
`Thermal gelation
`
`Solvent evaporation
`
`The cast polymer solution is immersed in a nonsolven~
`bath (typically water). Absorption of water and loss of
`solvent cause the film to rapidly precipitate from the top
`surface down.
`
`The cast polymer solution is placed in a humid
`atmesphere. Water vapor absorption causes the film to
`precipitate.
`
`The polymeric solution is cast hot. Cooling causes
`precipitation.
`
`A mixture of solvents is used to form the polymer casting
`solution. Evaporation of one of the solvents after casting
`changes the solution composition and causes
`precipitation.
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`98
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`Chapter Three
`
`was usually posttreated by annealing in a bath of hot water. The steps
`of the process are illustrated in Fig. 3.7.
`The Loeb-Sourirajan process remains by far the most important
`membrane preparation technique. The process is part of the overall
`membrane preparation procedure for almost all reverse osmosis and
`ultrafiltration and for many gas separation membranes. Reverse
`osmosis and gas separation membranes made by this technique con-
`sist of a completely dense top surface layer (the skin) on top of a micro-
`porous support structure. Ultrafiltration membranes, support
`membranes for solution coating, and interfacial polymerization mem-
`branes have the same general anisotropic structure; but the skin lay-
`er is very finely microporous, typically with pores in the 10- to
`200-/~-diameter range. Also the porous substrate of ultrafiltration
`membranes is usually more open, often consisting of large fingerlike
`cavities extending from just under the selective skin layer to the bot-
`tom surface of the membrane. Scanning electron micrographs of typi-
`cal sponge-structure reverse osmosis type and finger-structure
`ultrafiltration-type membrane