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GAS ADSORPTION EQUILIBRIA Experimental Methods and Adsorptive Isotherms

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GAS ADSORPTION EQUILIBRIA Experimental Methods and Adsorptive Isotherms J��rgen U. Keller Reiner Staudt Universit��t Siegen Germany Springer

eBook ISBN: 0-387-23598-1 Print ISBN: 0-387-23597-3 Print ��2005 Springer Science + Business Media, Inc. All rights reserved No part of this eBook may be reproduced or transmitted in any form or by any means, electronic, mechanical, recording, or otherwise, without written consent from the Publisher Created in the United States of America Boston ��2005 Springer Science + Business Media, Inc. Visit Springer's eBookstore at: http://ebooks.kluweronline.com and the Springer Global Website Online at: http://www.springeronline.com

CONTENTS Preface xi Acknowledgements xiii Introduction 1. 2. 3. 4. Introduction 1 1 Gas Adsorption Processes in Separation Technology 2 Experimental Methods 6 What Is Not Considered 10 References 11 Chapter 1: BASIC CONCEPTS 17 1. 2. 3. 4. Introduction 17 Adsorption Phenomena 18 Sorbent Materials 25 Characterization of Porous Solids 31 4.1 4.2 4.3 Mercury Intrusion Porosimetry 32 Helium Measurements 34 Gas Adsorption 43 5. Mass and Volume of Adsorbed Phases 52 5.1 Models for the Void Volume of a Sorbent Material and the Volume of a Sorbate 56 5.2 Outline of Calorimetric-Dielectric Measurements of Absolute Masses of Adsorbates 66 6. List of Symbols 70 References 72 Chapter 2: VOLUMETRY / MANOMETRY 79 1. 2. Introduction 79 Volumetric Measurement of Pure Gas Adsorption Equilibria (N = 1) 81 2.1 2.2 2.3 2.4 Experimental 81 Theory 82 Uncertainties or Errors of Measurements 85 Example 87 3. 4. Thermovolumetry 88 Volumetric Measurement of Multicomponent Gas Adsorption Equilibria (N 1) 91

vi 4.1 4.2 4.3 4.4 Experimental 91 92 94 95 Theory Uncertainties or Errors of Measurements Example 5. Volumetric ��� Calorimetric Measurements The Sensor Gas Calorimeter (SGC) 97 99 5.1 5.2 5.3 Experimental Outline of Theory and Calibration 103 106 109 109 109 111 114 Example 6. Pros and Cons of Volumetry / Manometry 6.1 6.2 Advantages Disadvantages 7. List of Symbols References Chapter 3: GRAVIMETRY 117 117 120 120 120 122 127 129 129 131 134 135 153 1. 2. Introduction Gravimetric Measurements of Pure Gas Adsorption Equilibria (N = 1) 2.1 Two Beam Balances 2.1.1 2.1.2 2.1.3 Experimental Theory Uncertainties or Errors of Measurements 2.2 Single Beam Balances 2.2.1 2.2.2 2.2.3 Experimental Theory Uncertainties or Errors of Measurements 2.3 Examples 3. 4. Thermogravimetry Gravimetric Measurement of Multicomponent Gas Adsorption Equilibria (N 1) 157 157 158 161 162 167 167 170 171 175 4.1 4.2 4.3 4.4 Experimental Theory Uncertainties or Errors of Measurement Examples 5. Pros and Cons of Gravimetry 5.1 5.2 Advantages Disadvantages 6. List of Symbols and Abbreviations References

Contents vii Chapter 4: VOLUMETRIC ��� GRAVIMETRIC MEASUREMENTS 181 1. 2. Introduction 181 Volumetric ��� Gravimetric Measurements of Binary Coadsorption Equilibria 182 182 185 191 193 2.1 2.2 2.3 2.4 Experimental Theory Uncertainties or Errors of Measurement Examples 3. Densimetric ��� Gravimetric Measurements of Binary Coadsorption Equilibria 205 205 208 213 214 3.1 3.2 3.3 3.4 3.5 Experimental Theory Uncertainties or Errors of Measurement Example Densimetric-Volumetric Measurements of Binary Coadsorption Equilibria 218 222 3.6 Volumetric-Densimetric Measurements of Wall Adsorption 4. Pros and Cons of Volumetric-Gravimetric Measurements of Binary Coadsorption Equilibria 225 225 226 4.1 4.2 4.3 Advantages Disadvantages Comparison of Densimetric-Gravimetric and Densi- metric-Volumetric Binary Coadsorption Measurements 227 229 232 5. List of Symbols and Abbreviations References Chapter 5: OSCILLOMETRY 235 235 1. 2. Introduction Measurement of Pure Gas Adsorption Equilibria (N = 1) by Slow Oscillations of a Rotational Pendulum 237 237 240 241 243 251 252 2.1 2.2 Experimental Outline of Theory 2.2.1 2.2.2 The Motion of the Pendulum in Vacuum The Motion of the Pendulum in Sorptive Gas 2.3 2.4 Uncertainties or Errors of Measurement Examples 3. Oszillometric - Gravimetric Measurements of Gas Absorption in Swelling Materials 256 256 257 260 263 3.1 3.2 3.3 3.4 Introductory Remarks Experimental Theory Example

viii 4. Oscillometric ��� Manometric Measurements of Gas Absorption in Swelling Materials 265 265 266 271 272 275 275 275 277 282 4.1 4.2 4.3 4.4 Introductory Remarks Experimental Theory Example 5. Pros and Cons of Oscillometry 5.1 5.2 Advantages Disadvantages 6. List of Symbols References Chapter 6: IMPEDANCE SPECTROSCOPY 287 287 289 289 299 299 302 1. 2. Introduction Dielectric Measurements of Gas Adsorption Systems 2.1 2.2 Experimental Theory 2.2.1 2.2.2 2.2.3 Basic Concepts Polarization of Dielectrics Models for the Complex Permittivity of Dielectric Sorbent-Sorbate Systems 306 2.3 Uncertainties of Dielectric Measurements of Adsorption Systems 316 318 2.4 Examples 3. Dielectric-Manometric and Dielectric-Gravimetric Measurements of Pure Gas Adsorption Equilibria 332 332 336 342 349 349 350 351 353 3.1 3.2 3.3 Experimental Examples Impedance Measurements in Adsorption Reactors 4. Pros and Cons of Impedance Spectroscopy 4.1 4.2 Advantages Disadvantages 5. List of Symbols References Chapter 7: ADSORPTION ISOTHERMS 359 359 363 363 363 372 377 1. 2. Introduction Simple Molecular Isotherms 2.1 Langmuir Adsorption Isotherm 2.1.1 2.1.2 2.1.3 Classical Form Heterogeneous Surfaces Admolecules with Interactions

Contents ix 3. Empirical Isotherms 382 382 384 386 387 391 393 394 394 395 402 404 407 3.1 3.2 3.3 3.4 3.5 3.6 Freundlich-Ostwald-Boedecker (FOB) Virial Expansions Toth���s Isotherm Brunauer-Emmett-Teller Isotherm (BET) Dubinin-Polanyi Theory Integral Equation Approach 4. Thermodynamic Isotherms 4.1 4.2 Gibbs���s Approach Internal Variable Approach 5. 6. Conclusions List of Symbols References Subject Index 415 421 Author Index

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PREFACE This book is intended to present for the first time experimental methods to measure equilibria states of pure and mixed gases being adsorbed on the surface of solid materials. It has been written for engineers and scientists from industry and academia who are interested in adsorption based gas separation processes and/or in using gas adsorption for characterization of the porosity of solid materials. This book is the result of a fruitful collaboration of a theoretician (JUK) and an experimentalist (RS) over more than twelve years in the field of gas adsorption systems at the Institute of Fluid- and Thermodynamics (IFT) at the University of Siegen, Siegen, Germany. This collaboration resulted in the development of several new methods to measure not only pure gas adsorption, but gas mixture or coadsorption equilibria on inert porous solids. Also several new theoretical results could be achieved leading to new types of so-called adsorption isotherms based on the concepts of molecular association and ��� phenomenologically speaking ��� on that of thermodynamic phases of fractal dimension. Naturally, results of international collaboration of the authors over the years (1980-2000) also are included. Both, traditional and new measurement methods for gas adsorption equilibria are presented in Chaps. 2-6 and elucidated by quite a number of experimental data sets, most of them having been measured in our laboratories. Special emphasis is given to uncertainties of data and pros and cons of all measurement methods are given to the best of our knowledge. Also the basic concepts underlying interpretation of measurements and calculations of adsorbed masses from measurement signals, are discussed in Chap. 1.

xii In publishing this book the authors hope to contribute to the development of effective and reliable methods to measure pure gas and gas mixture adsorption equilibria preventing young (and old) experimenters from doing all the mistakes we have done during our laboratory work*) making experimental gas adsorption data measured today in many laboratories all over the world more easily comparable to each other, as methods and procedures should be come more and more similar and possibly also will be standardized (IUPAC) in the years to come. In view of the complexity of interaction of molecules from fluid, i. e. gaseous or liquid phases with the atoms of the surface of a solid material the authors have put their emphasis on experimental measurement methods approaching especially mixture adsorption phenomena. Of course we are well aware that simulation of adsorption systems based on molecular models is making considerable progress. This especially is promoted by still growing computer capacities and new and powerful software and simulation programs. However, reality is in experiment, not in computer���s silica. There only our present knowledge and model of physical-chemical reality can be reflected. Nevertheless, we expect in future a combination of highly selective chosen key experiments and computer simulations to be the most effective way to make progress in the complex field of gas mixture adsorption equilibria and probably also in some neighboring fields like adsorption kinetics. However, all these interesting fields of adsorption science including applications of adsorption phenomena to chemical engineering are not considered here but left to other authors. In view of space limitations neither all of the experimental details and tricks of the various measurement methods nor all of the analytic arguments of the underlying theories could be presented. If readers do have questions they are cordially invited to approach the authors, namely for the former RS**) for the later JUK**). *) **) A true experimenter pursues his goal till everything in the lab is ruined. Often only then he becomes aware that nobody has taken notes of what was done and what has really happened (W. Sibbertsen, 1990). keller@ift.maschinenbau.uni-siegen.de Staudt@inc.uni-leipzig.de

Preface xiii As we are well aware of the fact that not many readers do have time to read a book like this cover to cover, we always have tried to present the material in nearly self-contained separate chapters. For this reason we also have provided the literature separately for each chapter being aware of the fact that some books and papers on gas adsorption may have been cited more than once. Acknowledgements It is now our pleasure to express our grateful thanks to all of our undergraduate and graduate students who have been engaged in project work at IFT in the field of gas adsorption during the years (1984-2004). Among them especially the contributions of F. Dreisbach, N. Iossifova, H. Rave, M. Seelbach and M. Tomalla are highly appreciated. Thanks for cooperation and discussions at international conferences (FOA, COPS, PBCAST) and at private meetings are due to our colleagues W. Arlt G. V. Baron W. Bongartz St. Brandani J. Cyprian D. D. Do J. Fritzsche L. Fuller U. von Gemmingen A. Guillot Ch. Haynes R. He K.-D. Henning U. Hoffmann M. Jaroniec J. K��rger K. Kaneko H. von Kienle K. S. Knaebel K.-F. Krebs E. Krumm M. D. Le Van H.-W. L��sch F. Metz P. Monson A. L. Myers A. W. Neimark B. Roehl-Kuhn J. Rouquerol W. Rudzinski D. M. Ruthven A. Sakoda M. Sakuth Sh. Sircar F. Stoeckli D. Sunderer M. Suzuki O. Talu M. Thommes K. Unger R. T. Yang H. Yoshida Li Zhou W. Zimmermann Special thanks are given to W. A. Steele, College Park, for reading the manuscript of the book and helping to improve the English wording. Thanks are also due to J. M. Prausnitz, Berkeley and J. A. Clark, Ann Arbor, USA, for reading chapter 3, Gravimetry, of the manuscript and contributing valuable hints and remarks to its contents. Special tribute is also paid to K. S. W. Sing, Exeter, UK for several stimulating lectures given at IFT during (1992-1998) and also for discussions on fundamental aspects of gas adsorption systems.

xiv Cordial thanks are also given to our colleague and friend Prof. h. c. Erich Robens, Friedrichsdorf and Mainz, for fruitful and interesting discussions over many years on the porosity of solids and also for valuable hints to experimental measurement procedures. Several people have contributed to realize this monography by processing the manuscript: Mrs U. Schilk did the excellent typing and formatting of the text with never ending patience and Mr M. U. G��bel did the art work, contributing also many ideas to Figures and Diagrams. Both of them are given our sincere thanks for devotion and dedication to this work. Last not least we would like to express our gratitude to the Publishers, especially to Mrs C. Day and Mrs D. Doherty for providing useful information in layout and styling of the manuscript and for several encouraging e-mails and notes. Siegen - Weidenau Leipzig J. U. Keller R. Staudt