Membrane Process Design Using Residue Curve Maps

By Mark Peters, David Glasser, Diane Hildebrandt and Shehzaad Kauchali

Design and Synthesis of Membrane Separation Processes provides a novel method of design and synthesis for membrane separation. While the main focus of the book is given to gas separation and pervaporation membranes, the theory has been developed in such a way that it is general and valid for any type of membrane.    

The method, which uses a graphical technique, allows one to calculate and visualize the change in composition of the retentate (non-permeate) phase. This graphical approach is based on Membrane Residue Curve Maps. One of the strengths of this approach is that it is exactly analogous to the method of Residue Curve Maps that has proved so successful in distillation system synthesis and design.

• Language: English
• Publisher: Wiley
• Release date: Apr 20, 2011
• ISBN: 9780470922835

Book preview

Preface

It is the intention of this book to introduce the reader to new, exciting and novel methods of designing and synthesizing membrane-based separation processes. Initially developed by the authors in fulfillment of postgraduate (PhD) research, these methods provide a unique way of analyzing membrane systems. This book is a monograph (of sorts) documenting the various aspects of the research done. Some of this work has already appeared in reputable scientific journals and has also been presented at numerous conferences. By capturing this work in the form of a book, rather than a thesis only, it is hoped that the research presented here will generate interest, planting a seed that will grow.

While selective membranes are useful in that they are able to perform high-purity separations, they can be very costly to fabricate. However, the work displayed here shows that basic, nonselective membranes are also a viable option for achieving useful separations. What's more, the nonselective membranes are more robust—making them cheaper and allowing for a reconsideration of the design procedure.

The contents of this book are aimed at undergraduate and postgraduate students, research academia, engineers and scientists in industry involved in the process design, as well as membrane specialists. The book explains the ideas and conceptualizes them by incorporating tutorials and worked examples. A computer-aided program, entitled MemWorX, written by the authors and fellow postgraduate students, is included to assist the reader with the contents of the book. MemWorX is not intended as a design tool, but rather a learning aid, and should be used in a supplementary manner with the book material.

Mark Peters

David Glasser

Diane Hildebrandt

Shehzaad Kauchali

Johannesburg, South Africa

Acknowledgments

We, the authors, wish to thank the following postgraduate students for their efforts and significant contribution to the preparation of the book and CD-ROM: Craig Griffiths, Neil Stacey, Chan Yee Ma, Aristoklis Hadjitheodorou, Ronald Abbas, Nik Felbab, and Daniel Beneke. Their inputs have been extremely valuable, enhancing the overall outcome of this work. A special thanks to Darryn van Niekerk for his contribution in preparing the cover artwork.

Several organizations have supported this work, both directly and indirectly, and we are grateful to them. These include the University of the Witwatersrand, Johannesburg, South Africa; The Academic and Non-Fiction Authors' Association of South Africa (ANFASA); The National Research Foundation (NRF) of South Africa; and Sasol Technology for their financial contributions.

Mark Peters

David Glasser

Diane Hildebrandt

Shehzaad Kauchali

Notation

Scalar quantities represented in italics.

Vector quantities represented in bold italics.

Symbols

Greek Letters

Subscripts

Superscripts

Abbreviations

About the Authors

MARK PETERS obtained his undergraduate degree in Chemical Engineering cum laude in 2003 and his PhD in 2008 from the University of the Witwatersrand, Johannesburg, South Africa. The topic of his thesis, entailing the development of graphical tools for membrane process design, has formed the basis of this book. Mark has spent time at the University of Illinois at Chicago (UIC), Chicago, Ilinois, USA as a research student. He has authored four scientific articles in the field of membrane separation and has presented work at numerous internationally recognized conferences. He previously worked as a research process engineer at Sasol Technology, focusing on Low Temperature Fischer–Tropsch (LTFT) Gas-to-Liquids (GTL) conversion. He is currently a Separations Consultant and Research engineer at the Centre of Material and Process Synthesis (COMPS), based at the University of the Witwatersrand.

DAVID GLASSER is a personal Professor of Chemical Engineering and director of the Centre of Material and Process Synthesis (COMPS) at the University of the Witwatersrand, Johannesburg, South Africa. He obtained his BSc (Chemical Engineering) from the University of Cape Town and his PhD from Imperial College in London. Along with Diane Hildebrandt, he pioneered the work in the Attainable Region (AR) approach for process synthesis and optimization. He has been awarded an A1 rating as a scientist in South Africa, by the National Research Foundation, the central research-funding organization for the country. He has been awarded the Bill Neale–May Gold Medal by the South African Institution of Chemical Engineers (SAIChE), as well as the Science for Society Gold Medal of the Academy of Sciences of South Africa for his research work. He has also been awarded the inaugural Harry Oppenheimer Gold Medal and Fellowship. He has authored or coauthored more than 100 scientific papers and was editor-in-chief of the new book Series on Chemical Engineering and Technology, published by Kluwer Academic Publishers of The Netherlands. He has authored a chapter published in Handbook of Heat and Mass Transfer Volume 4, Advances in Reactor Design and Combustion Science. He has also served as President of the South African Institution of Chemical Engineers. He has worked in a very wide range of research areas, including optimization, chemical reactors, distillation, and process synthesis.

DIANE HILDEBRANDT is the codirector for the Centre of Material and Process Synthesis (COMPS) at the University of the Witwatersrand, Johannesburg, South Africa. She obtained her BSc, MSc, and PhD from the University of the Witwatersrand. She has authored or coauthored over 70 scientific papers and has supervised 40 postgraduate students. She has been both a plenary speaker and invited speaker at numerous local and international conferences. In 1998, Diane became the first woman in South Africa to be made a full professor of Chemical Engineering when she was appointed as the Unilever Professor of Reaction Engineering at the University of the Witwatersrand. In 2003, she became the first woman professor of chemical technology in The Netherlands when she was appointed as a part time Professor of Process Synthesis, University of Twente, The Netherlands. In 2005, she was recognized as a world leader in her area of research when she was awarded an A rating by the National Research Foundation. She has been the recipient of numerous awards, including the Bill Neale–May Gold Medal from the South African Institute of Chemical Engineers (SAIChE) in 2000, and Distinguished Women Scientists Award presented by the Department of Science and Technology (DST) South Africa. Most notably, in 2009, she was the winner of the African Union Scientist of the year award. She has worked at Chamber of Mines, Sasol and the University of Potchefstroom and has spent a sabbatical at Princeton.

SHEHZAAD KAUCHALI obtained his PhD at the School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, South Africa. He is currently a full time senior academic and the director of the Gasification Technology & Research Group at the university. Shehzaad's thesis developed process synthesis tools for reactor and separation networks. He has developed expertise in the areas of reactor network synthesis, hybrid membrane–distillation design and gasification technologies. Shehzaad has spent ten months at Carnegie Mellon University in Pittsburg, Pennsylvania USA as a research scholar. Shehzaad completed a sabbatical, in the capacity of visiting professor, at the Indian Institute of Technology (IITB), Powai, Mumbai, India. Shehzaad has coauthored over ten publications in the field of chemical engineering and has cosupervised some theses for masters and doctoral degrees. Shehzaad has been a consultant with the Centre of Material and Process Synthesis (COMPS) and has consulted for Sasol, AECI, de Beers Diamonds, Element Six, Pratley, and the Paraffin Association of Southern Africa.

Chapter 1

Introduction

Separation processes are fundamentally important in the chemical industry. It is inevitable that during any chemical process, be it continuous or batch, the need for effective separation will arise. There are a variety of separation options available. However, distillation has proved to be the most effective and commonly used method, especially for the separation of mixtures containing compounds with relatively low molecular weights, such as organic substances. Other efficient separation techniques are absorption and liquid–liquid (solvent) extraction. In recent decades, membrane permeation has come to the fore as a successful method of separating mixtures, both gaseous and liquid.

One would like to have available a technique to select the appropriate method of separation to achieve the required product specifications. This chapter will begin to address this need, laying the foundation for the rest of the book.

Membranes have been developed for various separation applications. Examples of these include, among others, reverse osmosis, electrodialysis, pervaporation, and gas separation. Rautenbach and Albrecht (1989) discuss each of these. The aim of this book is not to reiterate what numerous texts have discussed previously. Rather, the reader is referred to this, and other texts (such as Geankoplis, 1993; Hoffman, 2003; Drioli and Giorno, 2009), which give more detailed appraisals of each individual membrane application. In order to demonstrate design and synthesis techniques, only diffusion membranes (e.g., gas separation and pervaporation membranes) will be considered in this book, but the method developed can be adapted and applied to the other kinds of membranes.

In diffusion membrane separation, a high-pressure fluid mixture comes into contact with a membrane, which preferentially permeates certain components of the mixture. The separation is achieved by maintaining a lower pressure (sometimes vacuum) on the downstream, or permeate, side of the membrane. The remaining high-pressure fluid is known as the retentate. Figure 1.1a,b depicts basic batch and continuous diffusion membrane separation units, respectively. A more detailed discussion of membrane process operation is given in the book where appropriate. Gas separation involves the diffusion of a gaseous mixture, whereas pervaporation is a separation process where one component in a liquid mixture is preferentially transported through the membrane and is evaporated on the downstream side, thus leaving as a vapor. These processes are discussed in more detail in Chapter 2.

Figure 1.1 (a) Batch and (b) continuous diffusion membrane units.

The conventional way of analyzing membrane separators is to ask what permeate composition can be achieved for a particular feed, as in the experiments conducted by Van Hoof et al. (2004) and Lu et al. (2002). Furthermore, the flux of a particular component through the membrane is also reported as a function of the feed in these and similar experiments. However, it must be remembered that the flux of any of the components may not necessarily remain the same and may vary as permeation proceeds down the length of the membrane (continuous operation). Therefore, the conventional information, although accurate, is insufficient, especially when it comes to designing industrial-scale membrane separators, as well as sequencing of such equipment.

When examining how the other, more established, separation processes are analyzed, it can be seen that the methods used for membranes are somewhat ineffective. In distillation, as well as single-stage flash separations, one never reports how either the top or bottom products are related to the feed, but rather how they are related to each other. A similar kind of analysis is conducted when designing solvent extraction circuits—one requires the equilibrium data that relates the aqueous phase to the organic phase.

Relating the two product streams to each other allows one to design multiple flash units, or cascades, as well as countercurrent liquid-extraction circuits and distillation columns. The design of such reflux cascades would be impossible if either one of the product compositions were related to the feed entering any of the units within the cascade. It is necessary to analyze membrane separators in a similar manner—one needs to investigate how the permeate composition is related to that of the retentate.

The relationship of the permeate to the retentate has been modeled mathematically. For example, Eliceche et al. (2002) model a pervaporation unit for a binary separation—the analysis was carried out by considering the flux of each component through the membrane and solving simultaneous mass and energy balance equations. Stephan et al. (1995), on the other hand, describe the permeation by a simple dual-mode transport model, and make use of Henry's law to relate the permeate composition to the composition of the retentate. A thermodynamic and more fundamental approach is given in the article by Wijmans and Baker (1995). In this review, the concentration and pressure gradients in the membrane are described using chemical potentials as the fundamental starting point. By appropriately modeling the chemical potentials, and by making use of Fick's law of diffusion, Wijmans and Baker (1995) are able to model the permeation for the various types of membranes, including gas separation and pervaporation. The details of the resulting equation for gas separation are discussed later in Section 2.1.1. Figure 1.2 gives a basic sketch of membrane permeation.

Figure 1.2 Membrane permeation. Differing permeabilities provide the driving force for separation.

Although the information in these models is correct, it is somewhat difficult to interpret and utilize them for design purposes. It is therefore the aim of this book to formulate a graphical technique that can incorporate the appropriate models in order to interpret, analyze, and design membrane separators in a convenient and efficient manner.

Conventionally, it was believed that residue curve maps (RCMs), and their binary equivalent (i.e., x–y plots), were suitable only for equilibrium-based separations and could not be used for the representation of kinetically based processes (Fien and Liu, 1994). This is discussed further in Chapter 3. However, as will be shown, the differential equations that describe a residue curve are merely a combination of mass balance equations. Because of this, the inherent nature of RCMs is such that they can be used for equilibrium-based, as well as nonequilibrium-based processes. This now allows one to consider kinetically based processes, such as reactive distillation (Barbosa and Doherty, 1988; Doherty and Malone, 2001; Huang et al., 2004) as well as membrane separation processes.

This book guides the reader through the development of graphical tools for nonreacting membrane systems. It is necessary to mathematically describe the flux of material through a membrane, as detailed in Chapter 2. The models derived and discussed are used throughout the text. Chapter 3 introduces the concept of membrane plots for various systems, and from Chapter 4 onwards the various applications of these maps are explored.

The MemWorX Package

A mathematical computer program, coded in Matlab®, entitled MemWorX, was especially developed for this book. A CD-ROM containing the MemWorX program is available for the reader and is to be used to aid understanding of the material covered in this book. Throughout the book, where appropriate, references are made to MemWorX, giving basic steps on how to generate plots shown in the book and, where necessary, allow users to produce their own plots. For details on how to install and run MemWorX, the reader is referred to Appendix A, which also includes a step-by-step guide to producing plots using MemWorX.

The majority of figures displayed throughout the book can be reproduced using MemWorX, and, where appropriate, the reader is prompted to attempt to reproduce the figure being referred to (or similar) using MemWorX. The symbol, shown alongside, will indicate when MemWorX should be used. Should any problems be encountered, Table A.4 in Appendix A lists the MemWorX parameters used to produce each figure, according to the tutorial number as listed in the book. This table can be regarded as the solutions to each of the tutorials involving the MemWorX package.

Chapter 2

Permeation Modeling

In order to generate