Membranes for Energy Applications E-Book

Young Moo Lee

• Herausgeber: Wiley-VCH GmbH
• Kategorie: Fachliteratur
• Sprache: Englisch
• Veröffentlichungsjahr: 2024

Seitenzahl: 620

Beschreibung

Provides important information and recent advances on membranes technology fundamentals as well as diverse important energy applications.

Das E-Book Membranes for Energy Applications wird angeboten von Wiley-VCH GmbH und wurde mit folgenden Begriffen kategorisiert:
Chemical Engineering, Chemische Verfahrenstechnik, Energie, Energy, Hydrogen, Batteries & Fuel Cells, Membran, Polymer processing, Polymer Science & Technology, Polymerverarbeitung, Polymerwissenschaft u. -technologie, Process Engineering, Prozesssteuerung, Wasserstoff, Batterien u. Brennstoffzellen

Leseprobe

Table of Contents

Cover

Table of Contents

Title Page

Copyright

Preface

Acknowledgments

1 Introduction

1.1 Energy and Membranes

1.2 Brief History of Membrane Technology

References

2 Fundamentals of Membrane Technology

2.1 Introduction

2.2 Definition of Terms

2.3 Membrane Materials

2.4 Basic Principles of Membrane Preparation

2.5 Membrane Fabrication

2.6 Membrane Module Fabrication

References

3 Membranes in Gas Separation for Energy and Environment

3.1 Introduction

3.2 Basic Principles of Gas Separation in Polymer Membranes

3.3 Limitations of Gas Separations Using Polymer Membranes

3.4 Polymer Membrane Materials

3.5 Membrane Gas Separation Applications

3.6 Conclusions and Future Perspectives

References

4 Membranes for Fuel Cell

4.1 Introduction

4.2 Basic Electrochemical Principles

4.3 Membranes in Proton Exchange Membrane Fuel Cell

4.4 Membranes in Direct Methanol Fuel Cell

4.5 Membranes in Anion Exchange Membrane Fuel Cell

4.6 Anion Exchange Ionomers

4.7 Fuel Cell Vehicle Market

4.8 Conclusions and Future Perspectives

References

5 Membranes in Energy Storage System

5.1 Introduction

5.2 Requirements of Li‐Ion Battery Separators

5.3 Fabrication of Separator

5.4 Gel Polymer Electrolytes

5.5 Polymers for Separators and Polymer Electrolytes

5.6 Next‐Generation Li Battery

5.7 Conclusions and Future Perspectives

References

6 Membranes in Hydrogen Production by Water Electrolysis

6.1 Introduction

6.2 Alkaline Water Electrolysis

6.3 Proton Exchange Membrane Water Electrolysis

6.4 Alkaline Exchange Membrane Water Electrolysis

6.5 Conclusions and Future Perspectives

References

7 Membranes for Power Generation

7.1 Water Energy Nexus and Membranes

7.2 Concept of Osmotic Power

7.3 Energy Obtained from PRO

7.4 Membranes for Pressure‐Retarded Osmosis

7.5 Hybrid Systems with Membrane Distillation and Others

7.6 Conclusions and Future Perspectives

References

Index

End User License Agreement

List of Tables

Chapter 2

Table 2.1 Categories of membranes.

Table 2.2 List of rubbery polymers and glassy polymers.

Chapter 3

Table 3.1 Different diffusion mechanisms of gas through a polymer membrane....

Table 3.2 Permeabilities of oxygen and carbon dioxide through selected poly...

Table 3.3 Physical properties of simple gases [40].

Table 3.4 Overview of “upper bound” parameter of

k

and

n

.

Table 3.5 Commercially available polyimides in membrane technology and thei...

Table 3.6 Physical properties and gas transport properties of amorphous per...

Table 3.7 Structure–property relationships of poly(amide‐

b

‐ethylene oxide) ...

Table 3.8 Physical properties and permeation properties of polymers with in...

Table 3.9 Permeabilities and ideal selectivities of selected polymers in ai...

Table 3.10 Selected hydrogen separation membranes.

Table 3.11 Comparison among three representative CO

2

separation processes....

Table 3.12 Biogas composition from different sources [312].

Chapter 4

Table 4.1 for the reaction H

2

 + O

2

 → H

2

O at various temperatures.

Table 4.2 , maximum EMF (or reversible open circuit voltage), and efficien...

Table 4.3 Typical fluoropolymers and hydrocarbon polymer for radiation graf...

Table 4.4 Conditions of radiation grafting for ion exchange membrane from c...

Chapter 5

Table 5.1 Ideal value of typical parameters for lithium‐ion battery separato...

Table 5.3 Summary of polyolefin and fluoropolymer‐based separators for lithi...

Table 5.3 Data for several electrochemical reactions in Li‐based batteries....

Chapter 6

Table 6.1 Development of water electrolysis technology.

Table 6.2 Comparison of water electrolysis technologies.

Table 6.3 Physical and electrochemical properties of commercially available...

Table 6.4 Critical properties of typical AEIs.

List of Illustrations

Chapter 1

Figure 1.1 Global primary energy consumption by source.

Chapter 2

Figure 2.1 The mass transport across a membrane as a function of the membran...

Figure 2.2 The structure of the various membranes.

Figure 2.3 The shapes of membranes.

Figure 2.4 The mass transport through a synthetic membrane.

X

1

A

and

X

2

A

repr...

Figure 2.5 The mass flux through porous membranes and a dense membrane. (a) ...

Figure 2.6 The flux and the separation of the components A and B in a membra...

Figure 2.7 Membrane development consist of (i) development of membrane mater...

Figure 2.8 The change of the free volume in a polymer.

Figure 2.9 A flow chart illustrating preparation processes of inorganic memb...

Figure 2.10 Schematic diagram of a composite ceramic membrane. (a) Top layer...

Figure 2.11 Three‐component phase diagram showing the formation of a porous ...

Figure 2.12 Characteristic phase diagram of thermally induced phase separati...

Figure 2.13 Phase diagram of thermally induced phase separation determined b...

Figure 2.14 (a) Polymer membrane exhibits inherent defeats in selective laye...

Figure 2.15 Schematic of (a) asymmetric hollow fiber spinning process using ...

Figure 2.16 Dual spinneret and triple spinneret.

Figure 2.17 Schematic of (a) triple hollow fiber spinning process using (b) ...

Figure 2.18 Schematic of (a) asymmetric hollow fiber spinning process using ...

Figure 2.19 Schematic of (a) asymmetric hollow fiber spinning process using ...

Figure 2.20 Schematic of (a) polymer supports membrane production and (b) th...

Figure 2.21 Schematic of hollow fiber membrane module.

Figure 2.22 Schematic of spiral wound module.

Figure 2.23 Schematic of fuel cell stack. Bipolar plate and gaskets are plac...

Figure 2.24 Schematic of lithium‐air battery pouch for stacks. Source: Adapt...

Chapter 3

Figure 3.1 Representative sorption phenomena of gases in polymer membranes. ...

Figure 3.2 Schematic representative of gas permeation steps across polymer m...

Figure 3.3 Graphical representation of the time lag determination from perme...

Figure 3.4 Changes in gas permeability (

P

, Barrer, 10

−10

 cm

3

((STP) cm

2

Figure 3.5 Schematic representation of the relationship between the polymer ...

Figure 3.6 Schematic representation of a dual‐mode sorption analysis using E...

Figure 3.7 Correlation of (a) diffusion, (b) solubility, and (c) permeabilit...

Figure 3.8 Upper bound line of CO

2

permeability and CO

2

/CH

4

permselectivity ...

Figure 3.9 Comparison of slopes of ln

α

A/B

versus ln

P

A

plots, (a) λA/B...

Figure 3.10 Chemical structure of cellulose derivative membrane. When R is h...

Figure 3.11 Schematic of (a) Loeb–Sourirajan membrane with defects. (b) Defe...

Figure 3.12 D3 or D4 monomer and poly(dialkyl siloxe) where R = CH

3

, C

2

H

5

et...

Figure 3.13 For a PDMS membrane. (a) Mixed‐gas hydrogen, methane, ethane, pr...

Figure 3.14 Chemical structures of disubstituted polyacetylenes: (a) PTMSP, ...

Figure 3.15 Chemical structures of Teflon (a) AF‐2400, (b) AF‐1600, (c) Cyto...

Figure 3.16 Chemical structure of poly[2,2′‐(m‐phenylene)‐5,5′‐bibenzimidazo...

Figure 3.17 H

2

permeance and H

2

selectivity over CO

2

and N

2

as a function of...

Figure 3.18 The ratio of penetrant permeability in the nanocomposite to that...

Figure 3.19 Examples of 2D materials. (a) Graphene oxide [30], (b) metal org...

Figure 3.20 Schematic illustration of the exfoliation and dispersion process...

Figure 3.21 Structure of poly(amide‐

b

‐ethylene oxide) (PEBAX

®

).

Figure 3.22 Change of CO

2

permeability with the amount of crystalline region...

Figure 3.23 Typical polymers with intrinsic microporosity: (a) PIM‐1 and (b)...

Figure 3.24 Molecular concept of PIMs in which structural units are fused to...

Figure 3.25 Chains of microporous polymers that have a contorted ladder stru...

Figure 3.26 Mechanism of thermal rearrangement: (a) hydroxyl‐containing poly...

Figure 3.27 TR polymers containing shape‐persistent units.

Figure 3.28 Strategies for improving microporosity in polymeric membrane mat...

Figure 3.29 Molecular simulation of dihedral angle distributions for bridged...

Figure 3.30 Upper bound curve of oxygen permeability and selectivities of ox...

Figure 3.31 Schematic representation of (a) vacuum and (b) pressure mode for...

Figure 3.32 Single‐, two‐, and three‐step designs for nitrogen production fr...

Figure 3.33 Nitrogen recovery as a function of product nitrogen concentratio...

Figure 3.34 Approximate competitive range of current membrane nitrogen produ...

Figure 3.35 Simplified flow schematic of the PRISM membrane system to recove...

Figure 3.36 Hydrogen recovery from a hydrotreater used to lower the molecula...

Figure 3.37 Schematic of channel‐based and carrier‐based membranes toward ol...

Figure 3.38 (a) Ultrathin ZIF‐8 layer with a thickness less than 1 μm can be...

Figure 3.39 Supported liquid membrane concept for facilitated transport of o...

Figure 3.40 (a) Ethylene/ethane, (b) propylene/propane separation performanc...

Figure 3.41 Potential application of membranes in CO

2

separation: (a) post‐c...

Figure 3.42 Schematic of a membrane capture process using selective exhaust ...

Figure 3.43 A simplified block diagram of an integrated gas turbine‐membrane...

Figure 3.44 A picture comparing the size and footprint of the MTR Polaris CO

Figure 3.45 (a) Schematic of the cross‐sectional structure of PolyActive mul...

Figure 3.46 Stability plot of the SW module tested with actual flue gas at N...

Figure 3.47 A simplified process flow diagram illustrating the impact of a s...

Figure 3.48 Schematic diagram of a 1000 ton/day cement plant fitted with a m...

Figure 3.49 UOP Separex membrane systems to treat 680 MMscfd (217 Nm

3

 s

−1

...

Figure 3.50 Cynara membrane plant (Natco Group) to recover acid gas from pet...

Figure 3.51 Driving force generation alternatives: (a) Feed compression, in ...

Figure 3.52 Propylene/nitrogen recovery unit. Silicone rubber propylene‐sele...

Chapter 4

Figure 4.1 First fuel cell experiments by William Grove in 1839.

Figure 4.2 Basic scheme of anode–electrolyte–cathode construction of a fuel ...

Figure 4.3 Schematic of acid electrolyte fuel cell. Proton ions pass through...

Figure 4.4 Schematic of alkaline electrolyte fuel cell. Hydroxyl ions pass t...

Figure 4.5 Schematic of a battery. A battery is like “warehouse,” depleting ...

Figure 4.6 Schematic of combustion engine and fuel cell stacks.

Figure 4.7 Schematic of fuel cell types depending on the electrochemical rea...

Figure 4.8 Graph showing the current‐voltage (

i

–

V

) curve. Dashed line repres...

Figure 4.9 Typical

i

–

V

–

P

curve. A maximum power density curve is call peak p...

Figure 4.10 Effect of activation overvoltage on fuel cell performance. React...

Figure 4.11 The effect of increasing pressure on the voltage–current graph o...

Figure 4.12 Different water movements to, within, and from the electrolyte o...

Figure 4.13 Schematic of a proton exchange membrane fuel cell.

Figure 4.14 Nafion, Aquivion, and 3 M perfluorosulfonic acid membranes and i...

Figure 4.15 Cluster‐network model for Nafion membranes. The polymeric ions a...

Figure 4.16 Radiation‐grafted polymer membranes.

Figure 4.17 (a) Polarization curve and (b) power density on H

2

/O

2

for ETFE‐

g

Figure 4.18 Proton conductivity as a function of relative humidity at 50 °C ...

Figure 4.19 Schematic of the importance of ion‐conducting channels to improv...

Figure 4.20 Chemical structure of sulfonated poly(arylene ether sulfone)‐bas...

Figure 4.21 AFM images of block copolymers. (a) 14a (IEC = 1.29–1.32 meq g

−1

...

Figure 4.22 Chemical structure of comb‐shaped SPAES and simplified illustrat...

Figure 4.23 Chemical structures and morphology images of (a) XESPAE85 (IEC =...

Figure 4.24 Fuel cell performance of pristine and end group cross‐linked mem...

Figure 4.25 (a) Schematic drawing of nano‐cracks on polymer membrane and (b)...

Figure 4.26 Schematic of single‐cell composed of membrane, catalyst layer, a...

Figure 4.27 Schematic of a direct methanol fuel cell.

Figure 4.28 Schematic of anion exchange membrane fuel cell. Source: Chen and...

Figure 4.29 Degradation mechanism of typical quaternary ammonium groups.

Figure 4.30 Most studied and efficient cationic groups.

Figure 4.31 (a) The

t

1/2

of different QA groups under

λ

 = 4.8 condition...

Figure 4.32 Chemical structure of aromatic ether containing polymers and pol...

Figure 4.33 Chemical structure of polybenzimidazole and poly(styrene‐ehtylen...

Figure 4.34 Chemical structure of polynorbonene and cross‐linked polynorbone...

Figure 4.35 (a) Polarization and power density curves for AEMFCs with a Type...

Figure 4.36 Chemical structure of polyphenylene prepared by ROMP polymerizat...

Figure 4.37 Hydroxide conductivity change of quaternized Diels‐Alder polyphe...

Figure 4.38 Preparation of acid‐catalyzed polymer with aryl‐ether‐free piper...

Figure 4.39 Examples of ether‐free poly(aryl piperidinium) for anion exchang...

Figure 4.40 Chemical structure of poly(fluorenyl terphenyl piperidinium) (PF...

Figure 4.41 The power density of AEMFCs based on Pt‐Ru/C anode with back pre...

Figure 4.42 Polarization curves of PDTP-based AEMFC with back pressure. Sour...

Figure 4.43 Radiation‐grafted anion exchange membranes on ETFE or PE‐based s...

Figure 4.44 (a) H

2

/O

2

AEMFC performance at 80 °C for the radiation‐grafted A...

Figure 4.45 Current–voltage (filled) and current‐power density (empty) curve...

Figure 4.46 Schematic diagram of the electrode structures made by (a) thin f...

Figure 4.47 (a) Fuel cell vehicle Nexo produced by Hyundai Motor Company and...

Chapter 5

Figure 5.1 (a) Schematic of lithium‐ion battery (LIB) consisting of the anod...

Figure 5.2 (a) The worldwide battery market from 1990 to 2015 [6]. (b) Marke...

Figure 5.3 Battery separator. Source: THE ELEC/ http://www.thelec.net/news/a...

Figure 5.4 Schematic of fabrication process of microporous membranes from (a...

Figure 5.5 SEM images of microporous membrane separators prepared by (a) PP ...

Figure 5.6 Scanning electron microscopes of Celgard 2325 (PP/PE/PP) separato...

Figure 5.7 Sem images of the electrospun membrane coextruded with PVDF‐CTFE ...

Figure 5.8 Ceramic coated microporous separator. Source: From Mohamed Alamgi...

Figure 5.9 SEM images of Al

2

O

3

/PVDF‐HFP binder (50/50, w.w in acetone as a s...

Figure 5.10 Schematic representation of Li‐ion, non‐aqueous and aqueous Li‐O

Figure 5.11 Framework of garnet‐type solid electrolytes Li

5

La

3

M

2

O

12

(M = Ta,...

Figure 5.12 Structure of a LiPON based thin‐film ASSB. Source: Reproduced wi...

Figure 5.13 Comparison of conventional LIB and all‐solid‐state lithium, batt...

Figure 5.14 Composition and possible design concepts of high energy ASSB wit...

Chapter 6

Figure 6.1 The schematic of green hydrogen energy system.

Figure 6.2 KOH‐circulating alkaline electrolyzer consisting of a PGM‐free el...

Figure 6.3 Schematic of PEMWE. Electrodes use precious‐metal group catalysts...

Figure 6.4 Chemical structure of Nafion, Aquivion, and 3M perfluorosulfonic ...

Figure 6.5 (a) Comparison of the water electrolysis polarization behavior fo...

Figure 6.6 Schematic diagram of the structure and fabrication process for th...

Figure 6.7 Effect of Nafion ionomer loadings in wt% using 50 mm Nafion 212 m...

Figure 6.8 GenHy5000 PEM water electrolyzer (5.0 Nm

3

 H

2

/h) for the storage o...

Figure 6.9 Schematic of AEMWE. Anion exchange membrane (AEM) separates the P...

Figure 6.10 The applied voltage breakdown for (a) DI water, (b) 0.01 M KOH, ...

Figure 6.11 General chemical structure of Aeomion PBI polymer.

Figure 6.12 Structure of Sustainion membrane.

Figure 6.13 The voltage needed to maintain 1 A cm

−2

current in an AEM ...

Figure 6.14 Structure of Orion TM1 polymer.

Figure 6.15 Structure of PAP‐TP polymer or PiperION polymer.

Figure 6.16 Long‐term stability performance of water‐fed AEMs at 200 and 500...

Figure 6.17 Performance comparison of electrolyzers from three commercially ...

Figure 6.18 Structure of PFTP polymer.

Figure 6.19 The effect of AEMs on AEMWE performance. (a) Polarization curve,...

Figure 6.20 Structure of HTMA‐DAPP.

Figure 6.21 The performance of MEAs employing PGM‐free catalysts (the magnif...

Figure 6.22 (a) Durability of the MEAs using the TMA‐70 ionomer at 85 and 60...

Figure 6.23 Chemical structure of crosslinked poly(norbonene).

Figure 6.24 Examples of six‐ and five‐membered ring attached to piperidinium...

Figure 6.25 Structure of PDPF‐Pip and PDPF‐bisPip polymer.

Figure 6.26 Various voltage behaviors of AEMWEs for 100 hours extended‐term ...

Figure 6.27 (a) Chemical structure of crosslinked PNB ionomer for self‐adhes...

Chapter 7

Figure 7.1 Schematic diagram of the PRO. Source: Logan and Elimelech [9]/Spr...

Figure 7.2 Schematics of four osmotic processes. (a) Osmosis, (b) pressure‐r...

Figure 7.3 Direction of water flux as a function of applied pressure in FO, ...

Figure 7.4 Illustration of concentration profiles for an asymmetric TFC memb...

Figure 7.5 Magnitude and direction of water flux for FO, RO, and PRO. The ma...

Figure 7.6 A closed‐loop system in which synthetic low‐ and high‐concentrati...

Figure 7.7 Working principle of RED. Source: Logan and Elimelech [9]/Springe...

Figure 7.8 Surface morphologies of the top layer (a) before and (b) after in...

Figure 7.9 Fabrication procedure of TFC membrane by interfacial polymerizati...

Figure 7.10 SEM images of the fabricated TFC membranes (i.e. TFC200) before ...

Figure 7.11 (a) Illustration of the preparation procedure of thermally rearr...

Figure 7.12 Power density of the TFC membranes with different treatments usi...

Figure 7.13 PRO performance and specific salt flux profiles of (a and b) TR‐...

Figure 7.14 Schematic illustration representing the configuration of (a) a t...

Figure 7.15 Schematic illustration of RO‐MD‐PRO hybrid system Source: Kim et...

Figure 7.16 Schematic diagram of osmotic heat engine. Source: Hickenbottom e...

Guide

Cover

Table of Contents

Title Page

Copyright

Preface

Acknowledgments

Begin Reading

Index

End User License Agreement

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Membranes for Energy Applications

Young Moo Lee

Author

Prof. Young Moo LeeHanyang UniversityDepartment of Energy EngineeringSeoul 04763South Korea

Cover Image: © Young Moo Lee

All books published by WILEY‐VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Library of Congress Card No.: applied for

British Library Cataloguing‐in‐Publication DataA catalogue record for this book is available from the British Library.

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© 2024 WILEY‐VCH GmbH, Boschstraße 12, 69469 Weinheim, Germany

All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.

Print ISBN: 978‐3‐527‐34764‐3ePDF ISBN: 978‐3‐527‐34765‐0ePub ISBN: 978‐3‐527‐34766‐7oBook ISBN: 978‐3‐527‐82726‐8

Preface

There are many books published in general membranes and processes, but there is no book published in membranes for energy applications. These books are fragmented; there are books and review papers on transport of gas, ions, and water separately, but there are no books touching these three very important small molecules for separation or their transportation. As there are shortages in energy sources from natural resources, many devices using alternative and renewable energy or energy storage are now a hot topic in recent years. Such fuel cell vehicles or smartphone/electrical vehicles should have fuel cell stacks or Li‐ion batteries that equip with membranes to selectively separate ions and/or water. Here, membrane materials and membrane morphology or properties play an important role for electrochemical performance of the mentioned energy devices and provide a pivotal role in safety issues of these devices. Failure to block the transport of ions will cause explosion of devices such as smart phones. Fuels are also important for these energy devices. Such gases as hydrogen or oxygen can be used for these energy devices where membranes are also very important component in producing these fuels that need to be studied. Many processes involve membranes for fuel production. In addition to hydrogen gases as fuel for these devices, salinity in water can be used for energy generation as well if the devices are properly designed. Here, membranes should be mechanically strong enough to withstand very high applied pressure to overcome the osmotic pressure of saline water as a feed for membranes.

This book provides important information and recent advances on membranes for energy production. It starts from the history of membrane science and technology, fundamentals of membrane technology including principles of membrane formation and principle behind, will introduce on the gas separation using membrane technology, introduce membranes for ion transport or separation realized in energy generation and storage, and power generation, and conclude the future direction and outlook of the membrane technology in energy application and industry. Readers of this book will benefit not only on the application of membranes in energy industry, but also on the principles or theories behind this important application including the transport of small molecules such as gas, ions, and water. Also, the future direction of this topic will be covered at the end so that readers can advance the standard life of human beings in the future.

First chapter introduces an energy issue and a brief history of synthetic membranes and their technology and the existing concepts and phenomenon to commercial use. It will then describe how membrane separations compare with the other separation methods. Finally, it will bring the reader to the current state‐of‐the art membrane technology.

Chapter 2 is intended to bring‐up the reader to what membranes are, how transport occurs through the membranes, what are synthetic polymer and inorganic membrane materials, how membrane and membrane modules are fabricated, how one selects which is the best membrane/membrane module for a given separation application, and finally how simple and complex membrane processes are developed.

In Chapter 3, the issue is how membrane processes can aid in energy and environmental issues. Membrane systems for air separation will be described, and enriched oxygen air combustion will be described and their advantages over the current systems in curbing greenhouse effects will be stated. Membrane processes for natural gas upgrading and carbon‐dioxide capture and pervaporation for biofuel production will be described.

The problem included in Chapter 4 is to find membrane applications in the energy conversion using fuel cell devices. Electrochemical processes for the conversion of various fuels (hydrogen, methane, and methanol) will be described and concepts and equipment for the various fuel cells will be presented. Membranes as well as the importance of polymeric binders for catalysts will be discussed. Advantages and disadvantages of the various fuel cells and their applicability will be presented including the present status on fuel cell vehicles.

In Chapter 5, the problem addressed is the use of membranes in energy storage devices such as lithium‐ion batteries. The basic principle of these devices will be described, and the needs associated with the membranes in these applications will be pointed out. Then, the various concepts and membranes and their use will be described. Membrane separators in new and emerging areas will also be covered.

The subject discussed in Chapter 6 is how to provide clean fuel for transportation and other applications for our future society. Hydrogen is regarded as a new energy source in future society. The key challenge is how membranes can play a role in production, purification, and storage and safe operation of hydrogen. Hydrogen is the main target fuels form the reaction between water and catalyst and this chapter provides insights how the membranes are playing a role in water electrolysis.

In Chapter 7, clean water supply is going to be a challenge for the coming generations. Here, the question posed is that how we can generate/recover energy while producing clean water from the saline and brackish waters. Followed by the importance of water‐energy nexus in produced water, the concept of osmotic pressure and its exploitation for energy generation will be discussed.

April 2024

Young Moo Lee

Seoul, South Korea

Acknowledgments

It is difficult to credit all individuals who have contributed to the preparation of this book. Instead of including a long list of names, I would like to express our sincere thanks to colleagues and former PhD students from the Membrane Laboratory of Hanyang University, particularly, Drs. Jun Tae Jung, Chuan Hu, and Sun Ju Moon who helped coordinate our activities during editing, and friends in other universities and companies who provided valuable suggestions and discussions. I would like to thank the publisher for their dedication and hard work for their supporting role in realizing this ambitious project.

April 2024

Young Moo Lee

Seoul, South Korea

1Introduction

1.1 Energy and Membranes

Energy crisis, water and food shortage, environmental pollution, and so on are important problems of human beings for the next 50 years. Energy crisis is the single most important problem because of an increase in energy usage from the technological development and its influence on energy toward water, food shortage, and environmental pollution. According to International Energy Outlook 2021 (International Energy Association 2021) from the United States Energy Information Administration (EIA), the world energy consumption will grow by nearly 50% between 2020 and 2050. Most of this growth comes from regions where strong economic growth is driving demand, particularly in Asia. Demand for all fuels increased, with fossil fuels meeting nearly 70% of the growth for the second year running. As a result, global energy‐related CO2 emissions rose by 1.7% to 33 gigatons (Gt) in 2018. Coal use in power generation alone surpassed 10 Gt, accounting for a third of total global CO2 emission. Most of that came from newly built coal‐fired power plants in developing countries [1]. As the amount of energy consumption increased enormously, in particular, after the Second World War, there was a worldwide issue on energy consumption and production that raised intensive environmental issues (see Figure 1.1). The largest portion of the energy sources are fossil fuels such as coal, oil, and natural gas that cover more than 80% of all the energy sources.

To solve the energy crisis, various efforts have been made to economically produce energy and use it efficiently. From fossil fuels to renewable energy, various kinds of resources have been utilized to produce energy. Fossil fuels are hydrocarbons, primarily coal, petroleum, or natural gas, generated from buried combustible geologic deposits of organic materials that have been converted to petroleum, coal, and natural gas by heat and pressure in the earth's crust over hundreds of millions of years. Because of their origin, fossil fuels have a high carbon content, resulting in a high heat content. According to the Statistical Review of World Energy from British Petroleum in 2020, the main primary energy sources worldwide consisted of coal (24%), petroleum (33%), and natural gas (27%), amounting to an 84% share for fossil fuels in primary energy consumption in the world [2]. Fossil fuel is the most economic resource to produce energy; however, it is required to reduce the share of fossil fuel due to emission of CO2 inducing global climate change. Currently, technology innovation is applied as a means to decrease the environmental impact of coal combustion by increasing the boiler efficiency, co‐combustion with biomass or carbon capture, utilization, and storage [3]. Nuclear power is one of the candidates to replace fossil fuels owing to its low price for energy production. However, it is hard to completely replace fossil fuels with nuclear power due to its drawbacks such as uncertain accidents and the production of radioactive nuclear wastes. Especially, the three nuclear disasters (that is, Three Mile Island in 1979, Chernobyl in 1986, and Fukushima in 2011) caused by human mistakes and/or natural disasters such as flood and earthquake make it difficult to expand the number of nuclear power plants. Renewable energy including solar, wind, and geothermal energy has been developed during the last decades with an infinite potential; however, they require further research to overcome their limitations such as low energy density, intermittent energy production, and so on. As such, each energy resource has strengths and possibilities; however, it also has weaknesses to be solved.

Figure 1.1 Global primary energy consumption by source.

Source: Our World in Data based on Vaclav Smil (2017) and BP Statistical Review of World Energy

Let us briefly look at the conversion of fossil fuels to electrical energy. Thermal power plants use coal, petroleum, and natural gas as fuel. According to the species and phases of the fuel, transportation of the fuel to the plant and post‐ and pre‐treatment of the fuel are different resulting in the application usage. However, the principle of generating electricity from fossil fuels is the same. The steam is produced at a high pressure in the steam boiler after burning fuel in boiler furnaces. Superheated steam then enters into the turbine and rotates the turbine blades. The turbine is mechanically coupled with alternator that its rotor will rotate with the rotation of turbine blades. After entering into turbine, the steam pressure suddenly falls and the corresponding volume of the steam increases. After imparting energy to the turbine rotor, the steam passes out of the turbine blades into the condenser. In the condenser, cold water is circulated with the help of a pump which condenses the low‐pressure wet steam. This condensed water is further supplied to a low‐pressure water heater where the low‐pressure steam increases the temperature of this feed water and it is again heated at high pressure. The chemical energy in fossil fuels is converted to thermal energy through carbon and hydroxide combining with oxygen during the combustion. Heat energy boils pressurized water to steam, and the steam travels through a mechanical turbine, causing it to rotate converting thermal energy to electric energy, i.e. electricity.

Pulverized coal (less than 5 cm) is burned in a boiler where water boils to steam. Converted steam is used to operate turbines for electronic generators. Compared to thermal power plants using other fuel types, coal requires a specific fuel processing and ash disposal. Flue gas from the combustion of fossil fuels contains carbon dioxide (CO2) and water vapor, as well as pollutants such as nitrogen oxides (NOx), sulfur oxides (SOx), and, for coal‐fired power plants, mercury, traces of other metals, and fly ash. Solid waste ash from coal‐fired boilers should be removed. Gas‐fired power plants, which burn natural gas (mainly methane) to generate electricity, produce a quarter of the world's electricity; however, they are also a significant part of global greenhouse gas emission resulting in global warming.

As the majority of current global primary energy relies on fossil fuels, the energy system is the source of approximately two‐thirds of global CO2 emissions. As methane and other short‐lived climate pollutant emissions are believed to be severely underestimated, it is likely that energy production and use are the source of an even greater share of emissions.

Meanwhile, membrane technology, which has been able to be utilized with various kinds of energy application, has been developed to solve the water issues and the problems of energy production. For example, in energy production using fossil fuels, membrane technology can be utilized to reduce the environmental impact from the emission of carbon dioxide. Membranes can separate the carbon dioxide from the ash inducing environmental issues and prevent them from being discharged to the outside. Moreover, membrane technology is also used as an integral part of the energy production in various ways. For example, in a fuel cell application, membrane performance determines the overall system performance including power density and long‐term stability of the fuel cells. In energy production using salinity gradients and osmotic pressure, membranes enable the harvesting of blue energy from the ocean, induced by the chemical potential difference using the diffusion difference of water and salt.

1.2 Brief History of Membrane Technology

Membrane technology has been known for a long time. The first report on gas separation was made by Mitchell [4], who observed the separation of gases through membranes. He noticed that gases had different permeabilities in experiments using rubber balloons exposed to various gas atmospheres. Graham [5, 6] reported in 1829 about different rates of permeation in experiments where a wet pig bladder inflated when stored in CO2 environment. Fick established the theoretical equations for diffusion through membranes. Graham proposed the concept of solution–diffusion as the permeability process in polymeric membranes in 1866 [7]. Daynes [8] established a time lag method to determine diffusion coefficients employing the unsteady‐state solution of Fick's second law to mathematically determine the diffusion coefficient from the extrapolation of the steady‐state flux to the time axis. Barrer and Rideal [9] noted the temperature‐dependent Arrhenius equation of gas permeability and diffusion of gases. Although most of these scientific advances have enlightened the membrane community, historic advances in gas and liquid separation membranes occurred in 1960 with the discovery of asymmetric cellulose acetate membranes with a selective layer thickness <1 μm for reverse osmosis by Loeb and Sourirajan [10–12]. Before this discovery, dense cellulose acetate membranes separated water and salts but were too thick to produce sufficient water flux to meet the commercial needs [13]. The production of asymmetric cellulose acetate membranes spurred the commercialization of reverse osmosis membrane and later led to gas separation membrane [14, 15]. A thin polyamide layer of 0.2 μm thickness can be coated on top of a microporous polysulfone support layer with a thickness of 200 μm to produce a thin‐film composite membrane for reverse osmosis and nanofiltration [16]. The same concept of thin‐film composite membrane can be applied to produce integrally skinned asymmetric membrane by depositing or coating selective dense layer on top of a porous sublayer [17].

The first commercial gas separation membrane system was introduced by Hennis and Tripodi [18, 19] at Monsanto to produce hydrogen in an ammonia synthesis plant. Membrane‐based gas separation is being considered for carbon capture and storage of flue gas evolved from coal‐fired power plants to meet the requirements of global climate change [20, 21].

The market for polymer membranes for gas and liquid separation has been expanding annually. The sales of membrane gas separation systems were approximately US$ 846 in 2019 and projected to be US$ 1132 million in 2024 at a CAGR of 6%. The worldwide sales of all synthetic membranes are estimated to be over US$ 5.4 billion as of 2019 and projected to grow to US$ 8.5 billion by 2024, at a CAGR of 9.0%. These numbers exclude the membranes for energy generation applications such as lithium‐ion batteries and fuel cells. The main reasons for the fast growth of membrane markets and sales include consumer demand for higher quality products, increased regulatory pressures, deteriorating natural resources, and the need for environmental and economic sustainability. The major drivers for the membrane market include increasing population, raising awareness about wastewater reuse, and rapid industrialization. Also, the shift from chemical treatment of water to physical treatment of water, strict regulations regarding water treatment and discharge, and changing climate dynamics in terms of the amount of precipitation are also driving the membrane market. The future market is expected to expand, and further growth of this technology is anticipated for the next 10 years or so.

It is interesting to note that energy can be produced using membranes, for example, by using hydrogen as a fuel. Fuel cell concept was first demonstrated by William Grove in 1839 using an experiment applying hydrogen and oxygen [22]. Water is electrolyzed into hydrogen and oxygen by passing an electric current through it. A fuel cell acts as an engine as it is a device that converts chemical energy into useful work, whereas a combustion engine converts the chemical energy of the fuel and oxidizer into mechanical work. The first fuel cell was developed by General Electric in the United States for the US Gemini space program in the early 1960s [23]. The fuel cell was expected to provide electricity and water for an animal orbiting in space in a satellite. The development of the space program continued with the incorporation of a new polymer, Nafion. DuPont and General Electric looked into the application of Nafion that received much attention due to its use as an electrolyte in the proton exchange membrane fuel cell (PEMFC) [24].

The same Nafion membrane was used in proton exchange membrane water electrolysis (PEMWE). The first PEMWE using this polymer began to be commercialized in 1978 [25]. Because of the use of expensive catalyst and membrane, an alternative to PEMWE was proposed, i.e. anion exchange membrane water electrolysis (AEMWE). Although many membrane materials have been tested, large‐scale commercial AEMWE needs more time to appear.

We are living in an age of electrical vehicles. About 10% of the new cars are electrical vehicles equipped with lithium‐ion batteries. European countries will ban the cars equipped with internal combustion engines by 2030, and this trend is moving forward to other countries. The proportion of electrical vehicles in total automobile market will exceed 30% by 2030, where LIB demand accounts to be 1293 GWh [26]. Batteries are made of many components such as cathode, anode, electrolyte, and separator. Separator is a porous membrane made mainly of polyolefins. Separator production in 2025 is estimated to be 2.7 billion m2 with a CAGR of 12% [27]. Separators are made by dry process and wet process.

1.2.1 Current State‐of‐the‐Art Membrane Technology

Membrane‐based gas separation is currently being used to separate hydrogen from ammonia plants, oxygen from air, nitrogen from air, CO2 from nitrogen stream or methane mixture, volatile organic compounds from petrochemical plants, water from organic vapors, and isopropanol from semi‐conductor plants. For these applications, membranes are in hollow fiber or flat sheet forms that are constructed to make modules. Polysulfone, polyimide, and cellulose acetate are the most widely used membrane materials in industry while many microporous materials are under investigation mainly in academia.

PEMFC is currently utilized in fuel cell automobiles or vehicles while direct methanol fuel cell is used in forklifts. These membranes use perfluorosulfonic acid (PFSA) membranes. Solid oxide fuel cell or metal ceramic fuel cell is used in electricity generation for households or small residential areas.

PEMWE is currently being used in generating hydrogen. This technology also uses PFSA as a membrane. Proton exchange membranes are incorporated to fabricate membrane electrode assembly where catalysts are coated on each side of the membrane. To reduce the cost of membrane and catalyst in PEMWE, AEMWE is evolved these days. A lot of progress has been made in AEMWE which will soon be commercially available.

Separator is an exploring market and has been widely used in LIB manufacturing. Polyethylene or polypropylene has been used to manufacture LIB separators. Ceramic coating technology is widely used to increase the thermal stability of the separator. Other high‐performance polymers are considered for separator materials, but the cost of the separator hinders the widespread of these materials for separator. Dry extension and wet manufacturing technology are the two major technologies widely used in the separator manufacturing industry.

Fuel cells and battery separators use membranes in flat sheet type which can be fabricated in stack or module form.

Throughout this book, we will introduce various kinds of membrane technologies for energy application, from their history to the recent development. We will also describe how membrane separations compare with other separation methods, and bring the readers to the current state of the art.

References

 1

IEA (2018). Renewables 2018. International Energy Agency (IEA), Paris, France, pp. 1–10.

 2

Looney, B. (2020). bp Statistical Review of World Energy June 2020.

 3

Barnes, D.I. (2015). Understanding pulverised coal, biomass and waste combustion–a brief overview.

Applied Thermal Engineering

74: 89–95.

 4

Mitchell, J.K. (1995). On the penetrativeness of fluids.

Journal of Membrane Science

100: 11–16.

 5

Graham, T. (1995). On the law of the diffusion of gases.

Journal of Membrane Science

100: 17–21.

 6

Graham, T. Notice of the Singular Inflation of a Bladder (Reprinted from Quarterly Journal of Science, No II, Pg 88, 1829).

Journal of Membrane Science

100 (1995): 9–9.

 7

Graham, T. (1867). On the absorption and dialytic separation of gases by colloid septa.

Journal of the Franklin Institute

83: 39–41.

 8

Daynes, H.A. (1920). The process of diffusion through a rubber membrane.

Proceedings of the Royal Society of London Series A, Containing Papers of a Mathematical and Physical Character

97: 286–307.

 9

Barrer, R. and Rideal, E.K. (1939). Permeation, diffusion and solution of gases in organic polymers.

Transactions of the Faraday Society

35: 628–643.

10

Loeb, S. and Sourirajan, S. (1963). Sea Water Demineralization by Means of an Osmotic Membrane. In:

Saline Water Conversion—II

, 117–132. ACS Publications.

11

Riley, R., Gardner, J., and Merten, U. (1964). Cellulose acetate membranes: electron microscopy of structure.

Science

143: 801–803.

12

Loeb, S. (1965). Desalination research in California.

Science

147: 1241–1242.

13

Reid, C. and Breton, E. (1959). Water and ion flow across cellulosic membranes.

Journal of Applied Polymer Science

1: 133–143.

14

Glater, J. (1998). The early history of reverse osmosis membrane development.

Desalination

117: 297–309.

15

Strathmann, H., Kock, K., Amar, P., and Baker, R.W. (1975). The formation mechanism of asymmetric membranes.

Desalination

16: 179–203.

16

Cadotte, J.E., Petersen, R.J., Larson, R.E., and Erickson, E.E. (1980). A new thin‐film composite seawater reverse osmosis membrane.

Desalination

32: 25–31.

17

Shenvi, S.S., Isloor, A.M., and Ismail, A.F. (2015). A review on RO membrane technology: developments and challenges.

Desalination

368: 10–26.

18

Henis, J.M. and Tripodi, M.K. (1980). Multicomponent membranes for gas separations, Google Patents.

19

Henis, J.M. and Tripodi, M.K. (1983). The developing technology of gas separating membranes.

Science

220: 11–17.

20

Wilberforce, T., Baroutaji, A., Soudan, B. et al. (2019). Outlook of carbon capture technology and challenges.

Science of The Total Environment

657: 56–72.

21

Favre, E. (2011). Membrane processes and postcombustion carbon dioxide capture: challenges and prospects.

Chemical Engineering Journal

171: 782–793.

22

Grove, W.R. (1839). XXIV. On voltaic series and the combination of gases by platinum.

The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science

14: 127–130.

23

Barbir, F. (2012).

PEM Fuel Cells: Theory and Practice

. Academic press.

24

Banerjee, S. and Curtin, D.E. (2004). Nafion

®

perfluorinated membranes in fuel cells.

Journal of Fluorine Chemistry

125: 1211–1216.

25

Millet, P., Dragoe, D., Grigoriev, S. et al. (2009). GenHyPEM: a research program on PEM water electrolysis supported by the European Commission.

International Journal of Hydrogen Energy

34: 4974–4982.

26

Berckmans, G., Messagie, M., Smekens, J. et al. (2017). Cost projection of state of the art lithium‐ion batteries for electric vehicles up to 2030.

Energies

10: 1314.

27

Lebedeva, N., Di Persio, F., and Boon‐Brett, L. (2016). Lithium ion battery value chain and related opportunities for Europe, European Commission, Luxembourg.

2Fundamentals of Membrane Technology

2.1 Introduction

Membranes applied in energy application is associated with synthetic membrane processes such as gas separation, ion transport processes, fuel cell, and energy storage differ widely as far as the applied driving forces for mass transport are concerned. In membrane science, fundamental aspects of mass transport can be described with thermodynamic and kinetic relations at the membrane/bulk solution interface. Therefore, in this chapter, before discussing further on the membranes in energy applications, the definition of terms used in membrane science are provided, such as permeability, perm‐selectivity, rejection, and so on. Then, membrane structures such as porous and homogeneous or symmetric and asymmetric membrane and their function in membrane processes are described. Finally, the basic thermodynamic, kinetic, and electrochemical relationships relevant to the description of mass transport phenomena in membranes and membrane processes are treated.

2.2 Definition of Terms

Membrane performance in each application is determined by a number of components and processes such as membranes, modules, and operation parameters or engineering aspects of a process. These components and parameters and their functions are described and defined.

2.2.1 The Membrane and Its Function

It is difficult to define a precise and complete definition of a membrane that covers all aspects of membranes. In a general term, a membrane separates two phases and controls the transport of various components from one phase into the other in a specific manner. A membrane may have a dense and homogeneous structure or can have pores. Membrane may be formed from organic, inorganic, carbon, or metallic materials and their hybrid materials. It may be neutral or it may give positive or negative charges, or functional groups with specific binding capabilities. Its thickness can be less than 100 nm or more than several millimeters. The electrical area membrane resistance of a membrane may vary from more than 106 Ω cm2 to less than Ω cm2.

Figure 2.1 The mass transport across a membrane as a function of the membrane permeability (P), the driving force, and the membrane thickness (I).

The separation of a mixture in a membrane process is resulting from different transport rates of different components through the membrane. The transport rate of a component through a membrane is determined by the driving force or forces acting on the component in question and the permeability of the membrane matrix. The transport rate is inversely proportional to the thickness of the membrane as described in Figure 2.1 which shows the flux J of a component i from phase 1 into phase 2 through a membrane as a function of a driving force, permeability, and the membrane thickness.

The flux of a component through a membrane can be resulted from a convective bulk flow of matter through defined pores in the membrane or diffusion of individual components through a dense membrane. The permeability of a membrane for a given component is determined by the mobility and concentration of the component in the membrane matrix. Driving forces in membrane processes are gradients in the electrochemical potential of a component. Thus, activity, hydrostatic pressure, electrical potential, and temperature gradients can lead to a diffusive transport of individual molecules through the membrane. In porous membranes, a hydrostatic pressure difference between membranes can also result in convective mass transport.

2.2.2 Membrane Materials and Structure

Synthetic membranes used in energy applications show various physical structure and can be classified into the following three categories.

Porous membrane

Homogeneous and dense membrane

Dense membranes carrying electrical charges or selective functional groups

Structure of membranes may be

Isotropic or symmetric

Anisotropic or asymmetric

The materials used for the preparation of membranes can be

Polymers

Ceramics

Carbons

Metals

Hybrids

The membranes can be manufactured as

Flat sheets

Hollow fibers

2.2.2.1 Symmetric and Asymmetric Membranes

Note that symmetric membranes are used mainly in energy applications such as in battery separator, fuel cell, water electrolysis, and so on. In symmetric and isotropic structure, the transport properties and the structure are identical over the entire cross‐section of the membrane and its flux is directly proportional to the membrane thickness (see Figure 2.2). The thickness of symmetric membranes is typically between 20 and 200 μm. An asymmetric membrane consists of a 0.1–5 μm thick skin layer on a highly porous 100–300 μm thick substructure. The skin layer is the actual selective barrier of the asymmetric membrane. The nature of the material and/or the size of pores in the skin‐layer determines the separation characteristics. The porous sublayer is only a support for the thin and fragile skin and has little effect on the separation characteristics or the mass transfer rate of the membrane. Asymmetric membranes are used primarily in pressure‐driven membrane processes such as gas separation and reverse osmosis or nanofiltration. High flux and good mechanical properties are the advantages of asymmetric membranes. To prepare the asymmetric membranes, two techniques are typically used. One is based on a phase inversion process that leads to the integral skin with asymmetric structure and porous substructure. The other is a composite membrane structure where a thin layer is cast on a porous sublayer in a two‐step process. In this case, skin layer and support structures are typically made from different materials.

Figure 2.2 The structure of the various membranes.

Table 2.1 Categories of membranes.

Type

Pore size (nm)

Macroporous

>

50

Mesoporous

2–50

Microporous

<

2

Dense

—

Source: Adapted from Koros et al. [1].

2.2.2.2 Porous Membranes

A porous structure is a very simple form of a membrane, which resembles the conventional filter. These membranes consist of a solid matrix with defined pores with diameters ranging from less than 1 nm to more than 10 μm. The size of the species to be separated is the key to determine the pore size of the membrane to be used and the related membrane process. As listed in Table 2.1, porous membranes with average pore diameters larger than 50 nm are classified as macroporous, and those with average pore diameters in the intermediate range between 2 and 50 nm are classified as mesoporous. Membranes with average pore diameter between 0.2 and 2 nm are classified as microporous. Some of the membranes used in energy application such as in battery separator are microporous.

Porous membranes can be made from various materials such as polymers, ceramics, graphite, and metal or metal oxides. Their structure may be symmetric or asymmetric depending on the application. For example, the structure of a typical battery separator is porous, whereas that of a gas separation membrane is asymmetric with a skin layer with a thickness of 100 nm. The techniques for the preparation of porous membranes can vary and include phase inversion method and sol–gel conversion techniques.

2.2.2.3 Homogeneous Dense Membranes

A homogeneous membrane is a dense membrane through which a mixture of molecules is transported by a pressure, a concentration, or an electrical potential gradient. Separation of various components of a mixture is directly dependent on their transport rates within the membrane, which is determined by their diffusivities and concentrations in the membrane matrix and the driving forces acting on the individual components. Therefore, dense membranes are often called as solution–diffusion type membranes if they are used in gas separation. Dense membranes are typically made from various materials such as polymers, metals, ceramics, and their hybrid. Dense membranes may carry positive or negative electrical charges or functional groups. Since the mass transport in dense membranes is based on diffusion, their permeability is rather low. They are usually used to separate components with similar size. Gas separation is a typical membrane process using dense asymmetric membranes with small thickness. Dense asymmetric structure on the porous substructure provides high fluxes of small gas component.

2.2.2.4 Ion Exchange Membranes

Ion exchange membranes carries positive or negative electrical charges or functional groups in dense structures. Two different types of ion exchange membranes are present: (i) anion‐exchange membranes which carry positively charged groups fixed to the polymer matrix and (ii) cation exchange membranes that contain negatively charged groups fixed to the polymer matrix. In a cation exchange membrane, the fixed anions are in electrical equilibrium with mobile cations in the interstices of polymer. In contrast, the mobile anions are more or less completely excluded from the cation exchange membrane due to the electrical charge, which is the same as that of the fixed ions. This exclusion of the anions means that a cation exchange membrane only permits transfer of cations. Anion exchange membranes carry positively charges fixed on the polymer matrix. Therefore, they exclude all cations and are permeable only to anions. Polymeric ion exchange membranes are mainly used in fuel cell and water electrolysis.

2.2.2.5 Membrane Shapes

Industrial membrane process requires membranes to be in a certain geometrical shape. The most suitable membrane geometry in each separation process depends on the application and is determined by the performance as well as by manufacturing and operating costs. The most used membrane geometries in energy application are illustrated in Figure 2.3.

Flat sheet membranes which are 0.5–2 m wide with any length can be in modules which are used in membrane separation processes. Their structure can be symmetric or asymmetric with the actual separation barrier on the surface of the membrane. Flat sheet membranes are laminated to a porous polyester nonwoven fiber for additional mechanical strength. In laboratory and medical applications, small‐scale membrane modules are often used without any support structure.

Figure 2.3 The shapes of membranes.

Hollow fiber membranes have an outer diameter of 0.05–0.5 mm and any length and a wall thickness of 0.01–0.1 mm. Hollow fiber membranes have an asymmetric structure with the selective barrier layer. Skin layer may be on the outside or inside of hollow fibers depending on the applications. Hollow fiber membranes have the advantage of providing an extremely large membrane area per unit module volume and their production costs are relatively low.

2.2.3 Mass Transport in Membranes

Membrane separation processes are resulted from the differences in the fluxes of individual chemical species through the membrane. The transport rate is determined by the driving forces or forces applied on the various components and their mobility and concentration in the membrane. In symmetrical ion exchange membranes, the driving forces are differences in electrochemical potentials of the different components in the two phases separated by the membrane. The synthetic membrane itself does not provide any energy for the transport of mass or energy. This is the difference between biological membrane and synthetic membrane. Biological membrane participates actively in the transport of various components providing required energy by a chemical reaction within the membrane structure. Therefore, biological membranes are capable of active transport whereas synthetic membranes are only capable of passive transport.

The size and the physical structure of the membrane material determine the mobility of a component in the membrane, while the chemical compatibility between the permeating component and the membrane material determine the concentration of the solute in the membrane. In the membrane separation processes, different forms of mass transport occurs. The most general form of mass transport through a membrane is illustrated in Figure 2.4. The membrane separates two phases and acts as barrier that reduces the flux of various components according to their chemical nature, size, and their electrical charges. The driving force for the transport of a component A across the membrane is a difference in the Gibb's free energy of component A (ΔGA) in the two phases separated by the membrane, or the difference in the electrochemical potential of the component in the two phases.

The mass transport in a dense and a porous membrane is shown in Figure 2.5a,b. Figure 2.5a illustrates the transport of two components of the same or similar size, such as two small gas molecules, by diffusion through a dense membrane. One component permeates through the membrane under a driving force of a chemical potential while the other is retained. The transport of a component in a dense membrane is determined by two terms, that is, the mobility and the concentration of the component in the membrane matrix. The mobility of a component is a kinetic factor which is inversely proportional to its size whereas the concentration is a thermodynamic factor which is directly proportional to its solubility in the membrane material. When two components have the same size, their mobility is the same and a difference in the membrane permeability of the two components is the result of a difference in their solubility in the membrane material. Thus, two components with the same size can be separated by a dense membrane if they have different solubility in the membrane matrix.

Figure 2.4 The mass transport through a synthetic membrane. X1A and X2A represent the Gibbs free energy levels of a component A in phase 1 and phase 2.

Figure 2.5 The mass flux through porous membranes and a dense membrane. (a) The viscous flow of two components at different size through porous membrane with a different pore size. (b) The diffusive flux of two components of identical size but different solubility a dense membrane.

Figure 2.5