Preface xiii

Symbols xv

1 Introduction 1

1.1 Overview of Membrane Science and Technology 1

1.2 History of Membrane Science and Technology 4

1.3 Advantages and Limitations of Membrane Processes 7

1.4 The Membrane-Based Industry: Its Structure and Markets 9

1.5 Future Developments in Membrane Science and Technology 12

1.5.1 Biological Membranes 14

1.6 Summary 16

Recommended Reading 16

References 17

2 Fundamentals 19

2.1 Introduction 19

2.2 Definition of Terms 19

2.2.1 The Membrane and Its Function 19

2.2.2 Membrane Materials and Membrane Structures 21

2.2.2.1 Symmetric and Asymmetric Membranes 22

2.2.2.2 Porous Membranes 23

2.2.2.3 Homogeneous Dense Membranes 23

2.2.2.4 Ion-Exchange Membranes 24

2.2.2.5 Liquid Membranes 24

2.2.2.6 Fixed Carrier Membranes 24

2.2.2.7 Other Membranes 25

2.2.2.8 Membrane Geometries 25

2.2.3 Mass Transport in Membranes 27

2.2.4 Membrane Separation Properties 31

2.2.5 Definition of Various Membrane Processes 33

2.2.5.1 Pressure-Driven Membrane Processes 34

2.2.5.2 Activity and Concentration Gradient Driven Membrane Processes 35

2.2.5.3 Electrical Potential and Electrochemical Potential Driven Processes 36

2.3 Fundamentals of Mass Transport in Membranes and Membrane Processes 37

2.3.1 Basic Thermodynamic Relationships with Relevance to Membrane Processes 37

2.3.2 Basic Electrochemical Relationships with Relevance to Membrane Processes 42

2.3.2.1 Electron and Ion Conductivity and Ohm's Law 42

2.3.2.2 Ion Conductivity, Ion Mobility, and Drift Speed 43

2.3.2.3 Coulomb's Law and the Electric Field Effect on Ions in Solution 45

2.3.2.4 The Electric Field Effect in Electrolyte Solutions and the Debye-Hückel Theory 46

2.3.2.5 Electrical Dipoles and Intermolecular Forces 48

2.3.3 Chemical and Electrochemical Equilibrium in Membrane Systems 49

2.3.3.1 Water Dissociation Equilibrium and the pH- and pK Values of Acids and Bases 49

2.3.3.2 Osmotic Equilibrium, Osmotic Pressure, Osmosis, and Reverse Osmosis 51

2.3.3.3 The Electrochemical Equilibrium and the Donnan Potential between a Membrane and a Solution 54

2.3.3.4 The Donnan Exclusion of the Co-ions 55

2.3.4 Fluxes and Driving Forces in Membrane Processes 57

2.3.4.1 Viscous Flow through Porous Membranes 58

2.3.4.2 Diffusion in Liquids and Dense Membranes 59

2.3.4.3 Diffusion in Solid or Dense Materials 63

2.3.4.4 Ion Flux and Electrical Current 65

2.3.4.5 Diffusion of Ions in an Electrolyte Solution 66

2.3.4.6 Ion Mobility and Ion Radius in Aqueous Solutions 67

2.3.4.7 Migration of Ions and the Electrical Current 68

2.3.4.8 The Transport Number and the Permselectivity of Ion-exchange Membranes 69

2.3.4.9 Interdependence of Fluxes and Driving Forces 70

2.3.4.10 Gas Flux through Porous Membranes, the Knudsen and Surface Diffusion and Molecular Sieving 71

2.3.4.11 Surface Diffusion and Capillary Condensation of Gases 73

2.4 Mathematical Description of Mass Transport in Membranes 74

2.4.1 Mass Transport Described by the Thermodynamics of Irreversible Processes 75

2.4.2 Mass Transport Described by the Stefan-Maxwell Equations 77

2.4.3 Membrane Mass Transport Models 79

2.4.3.1 The Solution-Diffusion Model 79

2.4.3.2 The Pore Flow Model and the Membrane Cut-off 84

References 87

3 Membrane Preparation and Characterization 89

3.1 Introduction 89

3.2 Membrane Materials 89

3.2.1 Polymeric Membrane Materials 90

3.2.1.1 The Physical State of a Polymer 90

3.2.1.2 Crystallinity and Glass Transition Temperature 92

3.2.1.3 The Glass Transition Temperature and the Free Volume 93

3.2.1.4 Molecular Weight of a Polymer Chain 94

3.2.1.5 Macroscopic Structures of Polymers 95

3.2.1.6 Polymer Chain Interaction and Its Effect on Physical Properties 97

3.2.1.7 The Chemical Structure of the Polymer and Its Effect on Polymer Properties 98

3.2.2 Inorganic Membrane Materials 100

3.2.2.1 Metal Membranes 100

3.2.2.2 Glass Membranes 101

3.2.2.3 Carbon Membranes 101

3.2.2.4 Metal Oxide Membranes 102

3.2.3 Liquid Membrane Materials 103

3.3 Preparation of Membranes 104

3.3.1 Preparation of Symmetric Porous Membranes 104

3.3.1.1 Isotropic Membranes Made by Sintering of Powders, Stretching of Films, and Template Leaching 106

3.3.1.2 Membranes Made by Pressing and Sintering of Polymer Powders 106

3.3.1.3 Membranes Made by Stretching a Polymer Film of Partial Crystallinity 107

3.3.1.4 Membranes Made by Track-Etching 108

3.3.1.5 Membranes Made by Micro-Lithography and Etching Techniques 109

3.3.1.6 Glass Membranes Made by Template Leaching 112

3.3.1.7 Porous Graphite Membranes Made by Pyrolyzing Polymer Structures 112

3.3.1.8 Symmetric Porous Polymer Membranes Made by Phase Inversion Techniques 112

3.3.2 Preparation of Asymmetric Membranes 114

3.3.2.1 Preparation of Integral Asymmetric Membranes 115

3.3.3 Practical Membrane Preparation by Phase Inversion 117

3.3.3.1 Temperature-Induced Membrane Preparation 117

3.3.3.2 Diffusion-Induced Membrane Preparation 118

3.3.4 Phenomenological Description of the Phase Separation Process 124

3.3.4.1 Temperature-Induced Phase Separation Process 125

3.3.4.2 Thermodynamics of a Temperature-Induced Phase Separation of a Two-Component Mixture 126

3.3.4.3 The Diffusion-Induced Phase Separation Process 133

3.3.4.4 Structures of Asymmetric Membranes Obtained by Phase Inversion 136

3.3.4.5 Identification of Various Process Parameters in the Preparation of Phase Inversion Membranes 136

3.3.4.6 General Observation Concerning the Structure of Phase Inversion Membranes 137

3.3.4.7 The Selection of a Polymer/Solvent/Precipitant System for the Preparation of Membranes 144

3.3.4.8 Membrane Pre- and Post-Precipitation Treatment 148

3.3.5 Preparation of Composite Membranes 149

3.3.5.1 Techniques Used for the Preparation of Polymeric Composite Membranes 151

3.3.6 Preparation of Inorganic Membranes 155

3.3.6.1 Suspension Coating and the Sol-Gel Process 157

3.3.6.2 Perovskite Membranes 158

3.3.6.3 Zeolite Membranes 159

3.3.6.4 Porous Carbon Membranes 160

3.3.6.5 Porous Glass Membranes 161

3.3.7 Preparation of Homogeneous Solid Membranes 161

3.3.7.1 Preparation of Liquid Membranes 162

3.3.7.2 Preparation of Ion-Exchange Membranes 164

3.4 Membrane Characterization 170

3.4.1 Characterization of Porous Membranes 171

3.4.1.1 Techniques using Microscopy 172

3.4.1.2 Determination of Micro- and Ultrafiltration Membrane Fluxes 173

3.4.1.3 Membrane Retention and Molecular Weight Cut-Off 175

3.4.1.4 The Bacterial Challenge Test 178

3.4.2 Membrane Pore Size Determination 178

3.4.2.1 Air/Liquid and Liquid/Liquid Displacement 179

3.4.2.2 The Bubble Point Method and Gas Liquid Porosimetry 180

3.4.2.3 Liquid/Liquid Displacement 182

3.4.2.4 Permporometry 185

3.4.2.5 Thermoporometry 188

3.4.3 Characterization of Dense Membranes 189

3.4.3.1 Determination of Diffusivity in Dense Membranes 190

3.4.3.2 Long-Term Stability of Membranes 193

3.4.4 Determination of Electrochemical Properties of Membranes 193

3.4.4.1 Hydraulic Permeability of Ion-Exchange Membranes 194

3.4.4.2 The Fixed Charge Density of Ion-Exchange Membranes 194

3.4.4.3 Determination of the Electrical Resistance of Ion-Exchange Membranes 195

3.4.4.4 Membrane Resistance Measurements by Impedance Spectroscopy 198

3.4.4.5 Permselectivity of Ion-Exchange Membranes 203

3.4.4.6 Membrane Permeation Selectivity for Different Counter-ions 206

3.4.4.7 Water Transport in Ion-Exchange Membranes 207

3.4.4.8 Characterization of Special Property Ion-Exchange Membranes 209

3.4.4.9 The Mechanical Properties of Membranes 209

References 210

4 Principles of Membrane Separation Processes 213

4.1 Introduction 213

4.2 The Principle of Membrane Filtration Processes 214

4.2.1 The Principle of Microfiltration 216

4.2.2 The Principle of Ultrafiltration 219

4.2.3 The Principle of Nanofiltration 223

4.2.4 The Principle of Reverse Osmosis 229

4.2.4.1 The Reverse Osmosis Mass Transport Described by the Solution-Diffusion Model 230

4.2.4.2 Reverse Osmosis Transport Described by the Phenomenological Equations 234

4.2.4.3 The Water and Salt Distribution in a Polymer Matrix and the Cluster Function 239

4.3 The Principle of Gas and Vapor Separation 239

4.3.1 Gas Separation by Knudsen Diffusion 240

4.3.2 Gas Separation by Surface Diffusion and Molecular Sieving 241

4.3.3 Gas Transport in a Dense Polymer Matrix 243

4.3.4 The Principle of Pervaporation 254

4.3.4.1 Material Selection for the Preparation of Pervaporation Membranes 259

4.4 The Principle of Dialysis 261

4.4.1 Mass Transport of Components Carrying No Electrical Charges in Dialysis 262

4.4.2 Dialysis Mass Transport of Electrolytes in a Membrane without Fixed Ions 264

4.4.3 Dialysis of Electrolytes with Ion-Exchange Membranes 266

4.5 The Principle of Electromembrane Processes 268

4.5.1 Electrodialysis and Related Processes 269

4.5.1.1 Mass Transport in Electrodialysis 270

4.5.1.2 Electrical Current and Ion Fluxes in Electrodialysis 272