Adsorption Engineering

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AdsorptionEngineeringMOTOYUKI SUZUKlProfessor, Institute of Industrial Science, University of Tokyo, Tokyo 1

K O D A N S H A iqqn E I CC\ I ICD


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Copublkhed byKODANSHA LTD., TokyoandELSEVIER SCIENCE PUB LISHE RS B. V., Amsterdame x c h iv e sales rights in JopmrK O D A N S H A LTD .12-21. Otowa 2-chome, Bunkyo-ku. T o k y o ll 2 , Ja pa nfor the US.A. and CanadaELSEVIER SCIENCE PUBLISHING COMPANY. INC.655 Avenue of the Americas, New York, NY 10010, U.S.A.for the resr ofrhe worldELSEVIER SCIENCE PUBLISHERS B. V.25 Sa ra Burgerhanstraat. P.O. Box 21 1, 1000 AE Am sterdam , Th e Netherlands

Library o f Congress C a t a l o g l n g - i n - P u b l i c a t i o n Data

Suzukl. Motoyukl. 1941-Adsorptton englneerlng / Motoyukl Suzukl.p. cn. -- (Chemical englneerlng monographs : vol. 25)Includes bibllographical referencesISBN 0-444-98802-5 (U.S.)

1. Adsorptlon. I. Tttle. 11. Series Chemical e n g l n e e r l n gnonographs : v . 25.TP156.A35S89 1989660'.28423--dc20 89-23532CIP

ISBN 0-444-98802-5 (Vol. 25)ISBN 0-444-41 295-6 (Series)

IS BN 4-06-201 485-8 (Japan )Copyright @ 1990 by Koda nsha Ltd.All rights reserved.No part of this book may be reproduced in any form, by photostat, microfilm.retrieval system, or any other means, w ithout the written permission of KodanshaLtd. (except in the case of brief quotation for criticism or review).

Printed in Japan


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CHEMICAL ENGINEERING MONOGRAPHSAdvisory Editor: Professor S.W. CHURCHILL, Department of ChemicalEngineering, University of Pennsylvania, Philadelphia, PA 19104, U.S.AVol. 1 Polymer Eng ineering (Williams)Vol. 2 Filtration Post-Treatment (Wakema n)Vol. 3 Multicomponent Diffusion (Cussler)Vol. 4 Transport in Porous Catalysts (Jackson)Vol. 5


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Preface1 was introduced to adsorption when 1 spen t two years (19691 1971)with Professor J. M. Smith at the Universityof California, Davis,studying application of chromatographic methods to determine rate

processes in adsorption colum ns. After returning to the lnstitute ofIndustrial Science, University of Tokyo, 1 joined Professor KunitaroKawazoe's adsorption research gr ou p where pioneering work in adsorp tionengineering had been condu cted, and star ted research on development ofadso rption technology for water pollution control.Since then I have had many opp ortu nitie s t o acquire valuable ideas o nadsorption problems not only from Professor Kawazoe but fromnumerous other senior colleagues including Professor Toshinaga Kawaiof K anagaw a University, Pro fessor Yasushi Takeuchi of M eiji Universityand the late Professor Hiroshi Takahashi of I.I.S., University ofTokyo.In the laboratory, I was fortunate to have many good collaboratorsincluding Dr. Kazuyuki Chihara, Dr. Dragoslav M. Misic, ProfessorByun-Rin Ch o, D r. Akiyoshi Sa ko da , Dr . Ki-Sung Ha and other students.T he technical assistance of Mr. T osh iro M iyazaki and Mr. Tak ao Fujii inlab ora tory w ork was invaluable. Th is volume was written based on thework of this gro up and 1 am very gra tefu l to these colleagues and t o m anyothers not listed here.In preparing the manuscript, I repeatedly felt that much workrem ains t o be do ne in this field and th a t many directions of research arewaiting fo r newcom ers t o seek out. Because of my imperfect knowledgeand experience, many important problems which require discussion arenot included. If possible these should be treated in a future edition. Ittook me far longer than expected at the beginning to prepare thema nuscript fo r this m ono gra ph mainly du e to idleness. Mr. IppeiOhta of Kodansha Scientific Ltd. diligently prodded me. Withouthis energy this book would never have been completed. I also had tospend much time working at home and I a m very grateful t o my patientand gentle w ife, Keiko, t o whom I dedicate this volume.TokyoDecember 1989 Motoyuki Suzuk i


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Fluld 975 3 Batch Adsorptlon In a Batch with Finrte Volume 1065 4 Adsorptlon rn a Vessel wlth Co n t~ n u o u s low 1175 5 Flurd-to-Partrcle Mass Tr an sfe r In a Vessel 118

6 Kinetics of Adsorption in a Column-ChromatographicAnalysis .. . .. ... . . .. .. .. . . . .. . ..I256 I Fu nda me ntal Relations 1266 2 Analysts of Ch rom atog rap h~cElutlon Curves 1276 3 Method of Mom ent 1286 4 Ex te ns ~o n f the Method of M oment t o M ore Complex Systems 1356 5 Comparrson wlth Slmpler Models 1446 6 Other Methods for Handllng Chromatograph~cCurves 148

7 Kinetics of Adsorp tion in a Column-Breakthrough Curves ..I517 1 Llnear Isotherm System s-Solut~on t o the General Model 1527 2 Llne ar Isoth erm System-Slmple Models 1567 3 Nonlinear Isotherm Systems-Constant Pa tter n Adsorp tlonProfile 1587 4 Numerrcal Solutrons for Nonlinear Systems 1707 5 Breakthrough of M ult~c om pon entAd sorba te Systems 1727 6 D ~s pe rslo n nd Mass Transfer Pa ram eters In Packed Beds 179

8 Heat Effect In Adsorption Operation . .. ... .. . . ..I878 1 Effect of Heat Generation on Adsorptlon Rate Measurement by aSrngle Partrcle M ethod 1878 2 Basic Models of Heat Transfer In Packed Beds 1908 3 Heat Transfer Param eters rn Packed Beds 1938 4 Chro ma tograp hic Stud y of H eat Transfer In Packed Beds of

Adsorbents 1978 5 Ad rab a t~ cAdsorptlon rn a Column 2018 6 A dso rptlo n wrth Heat Transfer Thro ugh the Wall 203

9 Regeneration of Spent Adsorbent . . .. .. . . ...2059 1 Th erm al Desorptlon in Gas Phase 2069 2 Chemlcal Desorptlon from a Column 2089 3 Thermal Regeneration of Spent Actrvated Carbon from WaterTreatment 214


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10 C h r o m a t o g r a p h i c S e p a r a t io n . . . . 22910.1 Basic Relations of Chromatographic Elution Curves n Linear IsothermSystems 22910.2 Separation of the Neighboring Peaks 23210.3 Large Volum e Pulses 23310.4 Elution with Concentration Grad ient Ca rrier 23610.5 Chromatog raphy for Large-scale Separation 238

1 1 Pressu re Sw ing Adso rp t i on . . .. . . . . . 245I I . I General Schem a of PSA Operation 24611.2 Equilibrium Theory for PSA Criteria 25011.3 Num erical So lution of Nonequilibrium PS A Model 25311.4 Simplified Solu tion of Dynam ic Steady State Prome from ContinuousCountercu rrent Flow Model 25911.5 Mass Transfer Coefficient ln Rapid Cyclic Adsorption and

Desorption 26711.6 PSA Based on Difference of Adsorp tion Ra tes 271

12 A d s o r p ti o n f o r E n e r g y T r a n s p o r t .. . . . .. .. 27512.1 Princ iple of Adsorptio n Cooling 27512.2 Choice of Adsorbate-Adsorbent System 27712.3 Analysis of H eat an d Mass Tra nsfe r in a Small-scale AdsorptionCooling Unit Utilizing So lar Heat 28012.4 Heat Pump Utilizing Heat of Adsorption 287

Index 291


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Introduction

Understanding of engineering design m ethods of adsorption systems isan important aspect of process engineering design not only in thechemical industry but also in the fields of environmental pollution controland energy utilization. Moreover, adsorp tion is coming t o be regarded asa practicable separation method for purification or bulk separation innewly developed materia l pr od uc tion processes of, for example, high-techmaterials and biochemical and biomedical products.Advances in chemical engineering principles such as transfer rateprocesses and process dynamics a nd accumulation of quantitative da ta inth e field of adsorp tion, together with the development of easily accessiblemicrocomputers, have combined to enable the development of anintegra ted curriculum of adsorpt ion engineering.Th e first book on engineering design of adsorption app aratus in Japan ,written by Professo r Kunita ro K awazoe (1957), was published as pa rt ofth e "Advanced Series in Chem ical Engineering." The contents included:I. Adsorbents, 2. Industrial Adsorbers, 3. Adsorption Equilibrium, 4.Ad sorp tion Rate, 5. Co ntac t Filtration Adsorption, 6. Moving BedAdsorp tion , 7. Fixed Bed Adsorption , and 8. Fluidized Bed Adsorption.F o r m any adsorption design engineers, this was the only textbook for along time. Late r developm ents in the field have been published in

Kagaku-kogaku-benran Chemical Engineering Handbook edited by theSociety of Chemical Engineers, Japan). This is a good contrast withPerry's Chemical Engineers' Handbook up to 6th editions where theworks of the school of the late Professor Theodore Vermeulen wereintroduced.A n um ber of volumes have been written on adsorbents and adsorption.Th e one by professor D. M . Ruth ven (1985) is a good compilation of thechemical engineering work cond ucted in this field. Extensive work ondiffusion in zeolite by the author and his collaborators are summarizedand a complete collection of mathematical analyses in the literature areuseful fo r readers initiating adv anc ed studies in this field. Also ProfessorR . T. Yang (1986) published a book focusing on gas separation by


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adsorptio n, which is also a good reference for chem ical engineers.The purpose of the present volume is to provide grad uate students andchemical process engineers an overall understanding of the chemicalengineering principles related to adsorptio n processes. Balanced rathe rthan detailed discussions a re attempted.Chapter 2 gives brief p icture of the adso rben ts frequently used in actualprocesses. Surface characteristics and pore structures of adsorbents arethe main properties in determining adsorption equilibrium and rateproperties which ar e needed for plant design. New adsorbents are

continuously being developed, introducing new applications for adsorptiontechnology.Chapter 3 introduces the concepts of adsorption equilibrium with theprimary purpose of discussing the applicabilities and limits of somesimple expressions which ar e used in later sections o n design of adsorbers.Adsorption equilibrium is the fund am ental fac tor in designing adsorptionoperations.Chapter 4 is an attempt to provide an global view of diffusionphenomena in adsorbent particles, another important aspect of adsorp-tion engineering.Chapter 5 deals mainly with batch adsorption kinetics in a vessel.Dete rmina tion of intraparticle diffusion param eters shou ld be don e witha simple kinetic system w here no oth er rate processes are involved. F o rthis purpose measurement of concentration change in the finite bathwhere adso rptio n takes place is the most effective me thod. Con centration

change curves derived for nonlinear isotherm systems as well as for alinear isotherm system ar e presented for convenient determination of rateparameters. These discussions are also applicable to the analysis anddesign of adso rption op erat ion in a vessel o r differential reactor.Chapter 6 introduces another powerful technique for determining therate parameters involved in a n adsorption colum n. T he principle ofchromatographic measurement implicitly contains many fundamentalconcepts concerning dyna mic performance of a column reactor. Th emathematical treatments introduced in this chapter can easily beextended to cover more complicated dynamic operations.Chapter 7 gives the basic relation s used for calcu!ation of breakthroughcurves in an adso rpti on colu m n. Th e discusssion focuses on simplertreatment with overall mass transfer parameter. Th ere ar e many rigoroussolutions to fu nda m ent al equa tion s but in m ost indu strial designs, while aquick estimation method is preferable at the same time the effects of manyparam eters need to be clarified. Cons tant pattern development is animportant characteristic in the case of nonlinear (favorable) isothermsystems simplifying design calculation.


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Heat effects in adsorption processes are discussed in Chapter 8.Adsorp tion is accompanied by heat generation, and adsorption equilibriumand rate ar e dependent on tempera ture. This coupling effect brings ab ou tcomplex but interesting problems.Chapter 9 is devoted to methods for the regeneration of spent adsorbents.Since adsorption separation is a transient technique, regeneration ofadsorbents after the period of adsorption is an important part of anadso rption purification o r separation system. Recovery of valuableadsorbates will also become increasingly important.

In Ch ap ter 10, chrom atographic op eratio ns on the industrial scale iscons idered . Developm ent of this ar ea is especially needed in the area offine prod uc ts separation such a s required in biotechnological processes.Much improvement of adsorbents and new operation schemes areexpected in this field.Chapter 1 introduces a bulk separation technique, pressure swingadso rption (P SA ). This method has become very sophisticated andcomplex. Th e chapter attem pts to define fundamental ideas inconsidering these attractive processes.In Chapter 12, one unique application of adsorption for energyutilization purposes is introduced. F o r refrigeration, cooling an d heatpumping application of adsorption phenomena has been attempted.Fu ndam enta l ideas on these app lication are discussed.As described above, Chapters 2 thro ug h 4 deal with the fun dam entalsof adsorption phenomena which are necessary to understand theoperation and design of basic adsorption operations introduced inChapters 5 to 7. Chapters 8 and 9 are fundamental topics specific toadsorption operations and Chapters 10, 11 and 12 introduce basic ideason the practical and rather new applications of adsorption phenomena.The reader can start from any chapter of interest and refer to thefundamentals if necessary.


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2Porou s Adsorbents

Physical adsorption is caused mainly by van der Waals force andelectrostatic force between adsorbate molecules and the atoms whichcom pose the adsorb ent surface. Th us adso rbe nts are characterized firstby sur face prope rties such as surface area and polarity.A large specific surface area is preferable for providing largead sor ptio n capacity, b ut th e creation of a large internal surface area in alimited volume inevitably gives rise to large numbers of small sized poresbetween ad so rp tio n surfaces. Th e size of micropore determines theaccessibility of ads orb ate molecules to the adsorp tion surface so the poresize distribution of micropore is another important property forcharacterizing adsorptivity of adsorbents.Also some adsorbents have larger pores in addition to microporeswhich result f ro m gr anu latio n of fine powde rs o r fine crystals into pelletsor originate in the texture of raw m aterials. These pores calledmacropore s are several micrometers in size. M acropo res function asdiffusion paths of adsorbate molecules from outside the granule to themicropores in fine powders and crystals. Adsorbents containingmacropores and micropores are often said to have "bidispersed" porestructures.

Po re size distrib utio ns of typical adso rbe nts a re shown in Fig. 2.1.Surface pola rity corre spon ds to affinity w ith polar substances such aswater. Po la r ads orb en ts are thus called "hydrophilic" and aluminosili-cates such as zeolites, por ous a lumina , silica gel o r silica-alumina areexamples of ad so rbe nts of this type.On the o the r h an d, nonp olar a dsorb ents are generally "hydrophobic."Carbonaceous adsorbents, polymer adsorbents and silicalite are typicalnonpolar adso rben ts. These adsorbents have mo re affinity with oil thanwater.Popular adsorbents in commercial use are reviewed in the followingsections. M ea su rem en t techniques fo r pore size distributions are alsobriefly introduce d in t he la ter sections of this cha pte r.


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(a) Actlvatcd carbonsFig. 2.1.a. Pore size distnbution of ty p~ ca l ctivated carbons.

Pore rad~us,r, ( A )(b) Acttvated carbons

Fig. 2.1.brecover Pore slze dtstr~but~onf ty p~ ca l act~vated carbons for solven


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Activated Carbon 7

10 20 50 100 200 500 10Pore r a d ~ u s , ( a )Fig 2 I c Pore slze distributions of Sllica gel (A ) Type A, (B) Type B

F I ~ 1 d Pore size dlstrlbut~onsof Alumma pellets w ~ t h ~lferent elletizingpressures Pellet density 1 0 5 8 , 2 0 68 3 0 8 3 4 1 02 g /cm'


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Pore rad~us. , ( A )h g 2 l e Pore slze dlstnb ut~on s of Molecular acv e zeo l~te 5 A (A )Davtdson 2-100 (B~nderlcss). (B) Dav~dsonRegular.

2.1. Activated CarbonAct~vated carbons are the microporous carbonaceous adsorbentswhose history can be traced back to 1600 B.C. when wood chars wereused for medicinal purposes in Egypt. In Japan, a well for undergroundwater equlpped with a charcoal filter at the bottom was found at an oldshrine (Kashiwara Jingu, Nara) constructed in the 13th centuryA. D. InEurope, wood char and later bone char were used for refining beetsugar, a practice started in France because of the blockade against theContinent during the Napoleonic era. In the 20th century, during theWorld Wars, he need to develop gas masks stimulated rapid growth In

adsorption research. Many books have been published on activatedcarbon and ~ t spplications (Araki, 1932; Bailleul et at!, 1962; Hassler,1974; Mantell, 1951; Mattson and Mark, 1971; Tanso Zairyo-Gakkai,1975.2.1.1. Preparation of activated carbons

Commercially available activated carbons are prepared from carbon-containing source materials such as coal (anthracite or brown coal),lignite, wood, nut shell, petroleum and sometimes synthetic highpolymersThese materials are first pyrolyzed and carbonized at several hundreddegrees centigrade During thls process the volatlle fract~on nd lowmolecular products of pyrolys~s re removed and the res~dual arbona-


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Activated Corbon 9ceous material undergo the following activation process by usingoxidizing gases, such as steam at above 800C or carbon dioxide athigher temperatures. Micropores are formed during the activationprocess. The yield of activated carbon from raw materials is in mostcases less than 50% and sometimes below 10%.Carbonization and activation can also be performed using inorganicchemicals such as zinc chloride or phosphoric acid, which is known tohave a catalytic effect on pyrolytic condensation of carbohydrates. Thenreaction proceeds at lower temperatures and increases yield of charduring carbonization. In this process, precursor of micropore is formedwhen carbonization takes place around fine crystals of inorganic saltand washing of the salt by acid after carbonization produces microporeswhich are larger in diameter than those formed by gas phase activation.This method provides the larger micropores preferable for theadsorption of larger molecules.

2.1.2 Features of activated carbonsA. microporesMicropores, where most adsorption takes place, are in the form oftwo-dimensional spaces between two graphite-like walls, two-dimen-sional crystallite planes composed of carbon atoms. The distancebetween the two neighboring planes of graphite is 3.76 A (0.376 nm), butin the case of activated carbons which have a rather disorderedcrystallite structure (turbostratic structure), this figure must be largersince adsorbate molecules are not accessible otherwise (Fig. 2.2).B. su fac e oxide groupsMost activated carbons contain some oxygen complexes which arisefrom either source materials or from chemical adsorption of air(oxidation) during the activation stage or during storage after activation.Surface oxides add a polar nature to activated carbons, e.g. hydrophicity,acidity and negative [-potential.Oxygen complexes on the surface exist mainly in the form of fourdifferent acidic surface oxides: I) strong carboxylic groups, 11) weakcarboxylic groups which exist as lactone groups combined with theneighboring carbonyl groups, 111) phenolic groups and 1V) carbonylgroups which form lactone groups with carboxyl groups of Type I1(Fig. 2.3).

Distinguishing these acidic oxides is possible by multibasic titration(Boehm et a!., 1964), titration with alkaline solutions of different


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Fig. 2.2. Graph~te tructure (a ,b ) and turbostratic structure (c). Cenceptual~llustration f granular activated carbon (d).streng th. Fo r instance, so diu m bicarbonate, NaHCO3, whose pK, valueis 6.37, neutralizes surface oxides of group (I). Sodium carbonate,Na lCO , (pK.=10.25), can be used for titr ation of grou ps (I) and (II),sodium hyd roxid e, Na OH (pk;=15.74), for grou ps (I), (11) and (111) andsod ium methoxide, NaOC2H5 (pR =20.5 8), for groups (I), (II), (111) and(IV). Hence it is possible to determ ine the am ou nt of each surface oxidegroup from the difference in titration values.Th ere a re several other f or m s of oxides including basic grou ps such ascyclic ether groups . T he basic character of activated carbon isemphasized when activation is conducted at higher temp eratures.Surfa ce oxide groups can be removed by heat treatm ent of carbo ns inan inert atmosphere or under vacuum (Puri et of., 1962. 1964,1966). Evolution of COz is observed at temperatures below 600C andsurface acidity 1s closely related to the amount of the evolved C02.


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Actiwted Carbon I I

COOHCOOH

Open lYPe Lactonc typeFig. 2.3 . Surface oxides on carbon surface. I : Carboxyl group, a : Removedby 200C, b : Removed above 325OC. 11 : Carboxyl group which exists aslactol group, 111 : Phenolic hydroxyl group, IV : Carbonyl group.

Above 600C, the evolving C O is considered t o correspond t o the basicfunctional grou ps on the carbon suiface.C. ashesActivated carbon also contains to some extent ashes derived fromsta rting ma terials. The amo unt of ash ranges from 1% t o 12%. ,Ash esconsist m ainly of silica, alumina, iron, alkaline and alkaline ea rth metals.T he fun ctio ns of these ashes are no t qu antitatively clarified but som e ofthem are 1) increasing hydrophilicity of activated carbon, which isad van tag eou s when P A C is used for water treatment, 2) catalytic effectsof alkaline, alkaline earth and some other metals such as iron duringactivation or regeneration step which modifies pore size distribution tolarger pore range, and s o on. Acid soluble ashes can be removed bywashing with weak acid.2.1.3. P o w d e r e d a c ti va te d c a r b o n ( P A C )

Activated ca rbon s in comm ercial use are mainly in two forms: powderform and g ran ula r or pelletized form. Powdered activated carbons(PAC) in most cases are produced from wood in the form of saw dust.The average size of PA C is in the range of 15 to 25 pm and the geom etricalsta nd ard deviation is between 0.15 to 0.266. Th is particle size assuresthat intraparticle diffusion will not be the rate limiting step; thus theadsorption operation is designed from the view point of selection of


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contacting apparatus, mixing of PAC with liquid, separation of PACafter contacting and disposal o r regeneration if possible afte r usage.The major industrial uses of PAC are decolorization in foodprocesses, such as sugar refinery, oil production and sodium glutamateproduction as well as wine prep aration. Recently, considerable PA C isused in water treatment for both drinking water and wastewatertreatment.In the use of PAC in water phase, surface charge of the carbonpowder becomes an important factor since it affects ease of coagulation

sedimentation or filtration fo r separatio n of PA C from the bulk liquidafte r adsorp tion. Surface charge can be detected by [-potential orcolloid titrations. Measurement of [-potential for several commerciallyavailable PA Cs showed th at it varies considerably by sample and th at itis also very dependent on pH of the solution, suggesting that theexistence of dissociative functional groups (oxide groups) is playing anim po rtan t role. Th e effects of the acid washing for remo ving solubleashes and the successive heat treatment to remove surface acidic oxides

Fig 2 4 Change of [-potentla1 of PAC by a c ~ dreatment (HCI I N) and heattreatme nt (900C, N >stream) P A C Hokuetsu Tanso, wood cha r( R ep r o du c ed w ~ t h ermission by S u z u k ~ ,M and Ch~hara ,K , Wafer R e s , 22.630 (1988))


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Acfivaed Carbon 13

on [-potential are clearly shown in Fig. 2.4. Powdered activated carbonsproduced from saw dust are used and 5-potential calculated from theelectromobility of th e powders at different levels of pH are illustrated inthe figure. Presence of both acid soluble ashes and surface oxidesincreases the negative c harge of the particles; this is no t desirable whencoagulation of the powders is needed.When PAC is used, not only is its adsorbability utilized but itssurface charg e may also cooperate as a coagulant for colloidal fractionsin the liquid phase. In this case, however, the regeneration of PA C maybecome r ath er difficult. Th is is one of the reaso ns why spent P A C is inmost cases dum ped r ath er than regenerated f or repeated use.2.1.4. G r a n u l a r a c t iv a te d c a r b o n (GAC)

Granular activated carbons (GAC) are either in the form of crushedgranules (coal o r shell) o r in the pelletized form prepared by granulationof pulverized powders using binders such as coal tar pitch. G A Cproduced from petroleum pitch is prepared by activation of the sphericalbeads prepared from the pitch.Size of granules differ depending on the application. Fo r gas phaseadsorption, cylindrically extruded pellets of between 4 t o 6 m m o rcrushed and sieved granules of 4/ 8 mesh to lo/20 mesh are often used.The m ain app licatio ns in gas phase are solvent recovery, air purification,gas purification, flue gas desulfurization and bulk gas separation.In the case of liquid phase adsorption, intraparticle diffusion oftenbecomes the rate de termin ing step of a dsorp tion so smaller particles offor example, 12 /42 mesh a re advantageous f ro m this point of view, butope rational requ irements such as ease of handling , low pressure dro p inthe adsorption bed, little elutriation or abrasion during back washingand so o n define the low er limit of the particle size. Decolorization insuga r refinery, removal of organic substances, o d or and trace pollutantsin drinking water treatment, and advanced wastewater treatment aremajor applications of liquid phase adsorption.

Spent GAC in most applications is regenerated by a thermal method.Detailed discussions are given in Chap ter 9.2 . 1 . C arb on molecu la r s ieves

Size of micropore of the activated carbon is determined d uringpyrolyzing and activation steps. Thus small and defined micropores thathave molecular sieving effects can be prepared by using proper startingmaterials and selecting conditions such as carbonizing temperature,


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activation temp erature and time o r properties of binders for granulation.The main applications of activated carbon with molecular sievingability are separation of nitrogen and oxygen in air on the basis ofdifference of diffusion rates of these gases in small micropores, andadjustment of fragrance of winery products where only small moleculesare removed by ads orp tion in liquid phase.Ca rbo n molecular sieve (CM S, o r Molecular Sieving Carbon, M SC) isan interesting material as a model of activated carbons since it has auniform and narrow m icropore size distribution. Th e Dubinin-AstakhovEquation for adsorption isotherms of various gases was tested usingM SC (Kawazoe et a/., 1974), and later this equation was extended forad sor bents with microp ore size distributions (Sak od a and S uzuk i, 1983)and for isotherm relation in the low pressure range. Also, chrom atographicmeasurement of Henry's constants and micropore diffusivities weremade for M SC (Chihara et al., 1978); these gave clear re lations betweenheat of adsorption and activation energy of diffusion in micropores ofMSC.2.1.6. Act iva ted c a r bo n fibe r

Synthetic fibers such as phenolic resin (K yno l R), polyacrylic resin(PAN) and viscose rayon are carbonized at high temperatures in inertatmosphere and activated carbon fiber (ACF) is then prepared bycareful activation. Recently the carbon fibers prepared by spinning fro mmesophase carbon melt derived from coal tar pitch are being furtheractivated to provide A C F at less cost.M ost A C Fs have fiber diameter of 7 to 15 pm , which is even smallerthan powdered activated carbon. Hence the intrafiber diffusion becomesvery fast an d the overall adsorp tion rate is controlled in the case of A C Fbed, by longitudinal diffusion rate in the bed (Suzuk i and Sohn,1987).A C F is supplied in the form of fiber mat, cloth and cut fibrous chip ofvarious sizes. A C F and cellulose composite sheet is also available.Application of sheet adsorbent is found in the field of air treatmentsuch as solvent recovery. Application in water treatm ent is und erdevelopment in several areas such as chlorinated organics and odorouscomponents removal during city water purification from deterioratedwater sources such as eutrophicated lake water and polluted under-ground water (Sakod a et a!., 1988).


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S111ca nd Alwnina I 5

2.2. Silica and AluminaPure silica, Si02, is naturally a chemically inactive non-polar material

like quartz but when it has a hydroxyl functional group (silanol group),the surface becomes very polar and hydrophilic (Fig. 2.5).

Silica gel is the adsorbent particle prepared by coagulation of acolloidal solution of silicic acid (3 to 5 nm) followed by controlled

Fig. 2.5. Surface hydroxyl group on s ll~ ca urface.

Fig 2 6 Ty p~c al xamples of ads orp t~o n ~othc rm s f water vapor on Silicagel type A and B and ac ttve a l u m n a


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dehydration. Liquid sodium silicates are neutralized by sulfuric acidand the mixture is then coagulated to form hydrogel. The gel is washedto remove the sodium sulfate formed during the neutralization reaction.Then it is dried, crushed and sieved. Spherical silica gel particles areprepared by spray drying of the hydrogel in hot air.Silica gels of two types of pore size distribution are frequently usedfor commercial purposes. Type A and B have different shapes ofadsorption isotherms of water vapor (Fig. 2.6). This differenceoriginates from the fact that type A is controlled to form pores of2.013.0 nm while Type B has larger pores of about 7.0 nm. Internalsurface areas are about 650 m2/g (Type A) and 450 m2/g Type B).Silica gel contains about 0.04 to 0.06 gso/g of combined water afterheating at 350C and if it loses this water, it is no longer hydrophilic andloses adsorption capacity of water.The main application is dehumidification of gases such as air andhydrocarbons. Type A is suitable for ordinary drying but Type B ismore suitable for use at relative humidity higher than 50%.2.2.2. Active alumina

Aluminum oxides have several crystal forms. Active alumina (porousalumina) used as an adsorbent is mainly y-alumina. Specific surfacearea is in the range of I50 and 500 m2/g with pore radius of I5 to 60A(1.5 to 6 nm), depending on how they are prepared. Porosity ranges from0.4 to 0.76 which gives particle density of 1.8 to 0.8 g/cm3.Porous alumina particles are produced by dehydration of aluminahydrates, in most cases alumina trihydrate, Alz033H20, at controlledtemperature conditions to about 6% of moisture content.Active alumina is also used as a drying agent and the typicaladsorption isotherm of water vapor is included in Fig. 2.6. It is also em-ployed for removal of polar gases from hydrocarbon streams.

2.3. ZeoliteZeolite (the word derives from a Greek word zeein meaning to boil) isan aluminosilicate mineral which swells and evolves steam under theblowpipe. More than 30 kinds of zeolite crystals have been found innatural mines. Many types can be synthesized industrially.Crystalline structures are composed of tetrahedral units, at the centerof which a silicon (Si) atom is located with four oxygen atomssurrounding it. Several units construct secondary units, which are


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Frg 2 7 Several fundam ental unlts of S I atom s In Zeolrte structuresNerghborrng S I atoms are connected through oxygen atom not shown In thefigurellsted In Fig 2 7 Arrangement of these secondary unrts forms regularcrystalllne structures of zeol~tes

Regular crystalllne structures prov~deunlque adsorpt~oncharacter-~stlcs For example, m the case of zeol~te ype A, e~ght odal~te nits,(a) In Fig 2 8, form a cublc cell whose unlt s ~des 12 32 A and eachsodal~te nlt 1s located at the corner Neighboring un~ts re connectedthrough the D4R unlt (Fig 2 7 d) rn the form of (c) In Fig 2 8 and theresulting erght-membered rlng connecting ne~ghbonng cells controlsaccesslbll~ty f adsorbate molecules Into the cell where adsorpt~onakesplace In the case of zeol~te ype X or Y sodal~te nlts are connectedthrough D6R (Flg 2 7 e) unlts and form the unlt structure shown byF I ~8 e A cell In the unlt shown by F I ~8 f has four openlngscomposed of I 2-membered nngs

SI atoms In tetrahedral unlts can be replaced by alumlnum (Al) Ionwhlch results m a deficit of positive valence, requlrlng the addit~on fcatlons such as alkallne or alkallne earth Ions corresponding to thenumber of Al atoms These catlons are easlly exchangeable and the slzeand properties of these Ions mod~fy dsorption character~stlcs f zeol~tesslnce they affect the sue of the wlndow between the cells2 3 1 Natural zeol~te

Zeollte mlned I n Japan and nelghborlng countries 1s llm~ted n typeand only cl~nopt~lolltend rnorden~te re now excavated (Mlnato 1967)


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(a) So da l~te nrt (&cage) (SI, A1 atoms (b) Sodallte un lt ( w ~ t h xygen atoms)are shown)

(c ) Type A structure unlt Composed (d) M ~c ro po re el l In Type A un lt-aof erght sodalrte un lts (o nly SI, A1 cage, gas molecules enter throughatoms are shown, black circles are on e~ght-membered ngsthe back)


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(e) Type X and Y structure unit, composed of 10 sodal~teunits connected by W R nits

(I) M~cro po recell in Type X(Y) unlt, gas moleculesenter through twelve-membered rlngs

F I ~ R Ftructurc unlt of sodalite ( (a) and (b)). Type A ((c) and (d)). X(Y)((e) and (I))Zeolitec


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The main uses of natural zeolite as adsorbent are as drying agents,deodorants, adsorbents for air separation, ion exchangers for waterpurification especially for removing ammonium ion and heavy metalions and fo r water softening, soil upgrading and so on.Suzuki and Ha (1985) showed that clinoptilolite has good adsorp-tion selectivity of ammonium ion and obtained the adsorption equilibriumand rate of ammonium exchange.2.3.3. Sy nth etic zeolite

Some zeolitic crystal structures can be synthesized by hydrothermal

R g 2 9 Re lat ~o ns between effectlve pore size of Zeo l~ tesA and X andLennard-Jones klnet~c ~ameter,o Reproduced w ~ t h ermlsslon from Breck,D W . Zeolrre Molecular Sieve-srmcture, Chernrsrry and Use, J Wlley andSons, N ew York (1974)(Reproduced w ~ t h ermlss~onby Ruthven, M . Prmcples of Adrorprron & AdrProcesses. I I , Wlley (1985))


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reaction in autoclaves. M uch of the literature is devoted to clarifyingcrystal structure and synthesis (Barrer 1968, Flanigan and Sand 1971,Breck 1973, Meier and Uytterhoeven 1973, Katzer 1977, Barrer 1982,Olson and Bisio 1984, R am oa el al. 1984, Drza j el al. 1985, Murakamiel al. 1986).A limited nurnber of synthetic zeolites are currently used ascom mercial adsorbents i.e., Type A and Type X.In th e crystal structures of Type A, show n in Fig. 2.8, exchangeableca tions are located n ear the w indow between neighboring cells. Th e 4Atype zeolite contains Na ion at this site, which permits the entry ofmolecules smaller than 4 A. This effect is called the molecular sievingeffect, and is schematically illustrated in Fig. 2.9. If the K ion, w hich islarger than the Na ion, is introduced to this position, effective window(aperture) size becomes 3 A and only Hz0 and NH3 can penetrateth roug h the window. Then this type is called 3A zeolite. O n the otherhand, if a Ca ion, which has two valences, is introduced, the effectiveapertu re becomes larger. T he zeolite of this type is called 5A zeolite.Type X zeolite has much larger windows made up of 12-memberedrings (Fig. 2.8). and is usually called 13X zeolite.

2.4. Other Adsorbents2.4.1. Bone char

In sugar refineries, bone char has been commonly used as anadsor be nt fo r decolorizing an d refining sugar since the 19th century.Bone char is believed to have basically the same adsorption character-istics a s activated carbo ns. But in addition to the ion exchan ge abilitiesderived from the main constituent, calcium hydroxy apatite functionalgroups from animal matter may render superior adsorption ability forremoving color, od or and taste.Dry bones free of flesh together with fat and oil of animals crushed,screened and freed from miscellaneous foreign elements are put in anairtight iron retort and heated at 600-900C for ab ou t 8 h. Thevolatile gases evolved by this process contain ammonium, tar and non-condensible gas. Th e remaining ch ar is cooled in inert atmosp here thentak en ou t fo r further crus hing and screening. The yield of bone char isabo ut 60%.Calcium hydroxyapatite is an interesting adsorbent which collectsca tion s of heavy metals suc h as lead and cadmium. Suzu ki (1985)suggested that hydroxyapatite is capable of ion exchange not only with


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cations but also with anio ns such as fluoride ion. T he same effects maybe expected in part of the hyd roxyapatite com posing bone chars.2.4.2. Metal oxides

Metal oxides appear to be simple inorganic materials having well-defined chemical struc tures . However, as far as adsorption on oxidesurface is concerned, surface properties, which is largely dependent onhow the ox ide is prepa red, reveal very com plicated functions. This iswell known in the pre pa ratio n of m etal oxide catalysts.Metal oxides developed fo r industrial adsorb ents include magnesiumoxide, titanium ox ide, zirconium oxide an d cerium oxide. Magnesiumoxide (magnesia) is used for removing polar molecules such as color,acids, and sulfide comp oun ds fro m gasoline and solvent for dryc leanin gpurposes. Also it is effective in removing silica from water, andmagnesium trisilicate is used as a medicinal adsorbent. Ikari (1978)showed that magnesium hydroxide is a good adsorbent for phosphateremoval in the advanced treatment of wastewaters.Oxides of four valence metals, titanium, zirconium and cerium,sometimes show selective adsorption characteristics in removing anionsfrom water phases. Hydrou s titanium oxide is know n to be a selectiveadsorbent for recovering uranium in seawater, which is present in theform of carbonyl complex in co ncentrations as low as 3.2 ppb.Zirconium oxide in a monohydrated form is found to adsorbphosphate ion from wastewaters (Suzuki and Fujii, 1987). Cerium oxideis effective in adsorption of fluo ride ion in industria l wastewaters.

2.5. Measurement of Pore-related Properties2.5.1. Porosity

Porosity of adsorbents is determined by several alternative methods.When true solid density, p, (glcm]), is known, total porosity, EI (-), orspecific pore volume, v, (cm3/g), is calculated from p, and particledensity, pp g/cm3),as

Particle density, p,, can be determined using a mercury pycnometer byassuming that mercury does not enter any pore of the porous sample.


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Measurement of Pore-related Propertres 23When the sam ple is first evacuated an d th e evacuated sample is exposedto mercury at atm ospheric pressure, P., then m ercury can penetratepores with radius larger than 7.5 pm. M ost porous adsorbe nts haveintraparticle pores smaller than this and one atmosphere is enough to fillthe void between particles with mercury. Thus, by com paring the weightof the empty glass pycnometer filled with mercury, W,, and that of th epycnometer with the sample of weight, W,, filled with mercury afterevacuation, W,, th e particle density , p,, is calculated as

p, = particle weight] par ticle volume= Wsl[(Wm- W, + W,)lpnJ (2-3)

where p~~ s the density of mercury at th e measurement temperature.True solid densities, p,, are often foun d in the literature. But someporous bodies are no t easy to find in the tables o r to make an estimationbecause they have confined pores which are not accessible from outside.In such cases, the "true" density must be determined by directmeasurement.One practical method is to use the same method as described formeasuring particle density using mercury, except that mercury isreplaced by a liquid which penetrates pores. W ater and organic solventsare often used . The same equa tion can be used by replacing p, with thedensity of the liquid employed.Another metho d is to employ a helium densitometer (Fig. 2.10). Th ismethod is based o n the principle of PV=constant at con stan ttemperature. A known weight, W,, of po rou s sample is put in to thenevacuated from a closed vessel of volume VI. Next nonadsorbable gasessuch as helium are introduced to pressure P1, the valve closed and thevolume of the vessel changed by moving the piston in the connectedcylinder. Fr om the displacement of the piston, the change in volume,AV, and chang e in pressure, AP, are ob tained. Then the volume of thesample, V,, in which helium can no t penetra te is obtained as

F I ~ 10 D~sp lacemcn type pycnomeler (h el ~ um ensf lometer )


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and true density, p,, is defined as

Obviously, adsorbable or condensable gases cannot be employed in thismethod.2.5.2. Po re size distribution

Th e m ost comm on m ethods to determine pore size distributions arethe A. mercury penetration method, B. nitrogen adsorption method,and C. molecular probe method.A . mercury penetration methodBy applying pressure, P, o mercury surrounding a porous body,mercury p enetrates into the pores whose radii are larger than r given bythe following equation

r = - 20 cos ePwhere a represents the surface tension of mercury, 470dynelcrn(298 K), 8 is the contact angle between mercury and the sample

Electr~cbr~dgePressure

Constanttemperature bath

Flg 2 1 1 Mercury penetration apparatus.


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Measurement of Pore-related Properties 25

surface , which is usually taken as 140C. Then

An example of a mercury porosimeter is illustrated in Fig. 2.11. A smallam ou nt of a sample is put in t o the penetometer, then evacuated, afterwhich mercury is introduced from the separa te vessel. Then thepenetometer is set in the pressure vessel shown in the figure and highpressure is applied by oil pressure pump. Mercury penetration isdetected from displacement of meniscus a t the burette of thepenetometer. Th e pore size distribution is calculated f ro m the pressure-penetration volume relation.An ordinary mercury porosimeter generates pressure as high as3000 kg /cm 2, which makes it possible to determine pore size distribu-tions down to r-25 A. Usually, this method is suitable for determininglarger pores such as macropores of activated carbons.B. nitrogen adsorption merhodWhen nitrogen adsorption is carried out at liquid nitrogen tempera-ture (-195.8OC=77.34 K), nitrogen adsorption on the surface andcapillary condensation of nitrogen in the pores take place (Fig. 2.12).The thickness of adsorbed layer on the surface, t, and the size of the porewhere condensation happens, rt, depend on the partial pressure ofnitrogen. Th us adsorption isotherm can be converted to the pore sizedistrib ution by assuming prop er relations between both t and r, and thepartial pressure, p.There are several equations proposed for the relation between r and

( d ! ( b )Ftg 2 I2 Concept of pore size measurement by the adsorpt~onmethod


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the pressure, but Halsey's relation is given here as one of the mostfundam ental relations.i (A) = 4.3[5/ln(p,/p)]'13

where p, is the satura tion pressure, which is the atmospheric pressure inthis case.F o r the capillary condens ation radius, rt, Kelvin rad ius is derived as

r , (A) = -9.53 /In@ Ips)by assuming the contact angle of nitrogen 0 is given as cos 8=1 and t hesurface tension u=8.85 dynelcm.Adsorption isotherm of nitrogen at liquid nitrogen temperature isdetermined by the grav imetric method (Fig. 2.13), o r the constantvolum e method (Fig. 2.14). F ro m the adsorption isotherm of nitrogenshown in Fig. 2.15, the pore size distribution is calculated by Dollimoremethod (1964) and show n in Fig. 2.16. Cumulative po re size distribu-tion is sometimes preferred when change of pore size distribution is in-volved, for example during thermal regeneration or activation of ac-tivated carbons.For microporous adsorbents such as carbon molecular sieves,Dollimore's method is not advisable since the Kelvin equation is no

Fig 2 13 Gravlmetr lc measurement of nr trogen ad sor pt~ on sotherms a t l lq u~ dnltrogcn temp erature


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Pressurecontroller- - DctccterN z c y l ~ n d e r vwand evacuation i

Vacuum

Liqu~dnitrogentank

Pump

Frg 2 14 Automatrc adsorptron system for nltrogen (constant volume type),Carlo Erba

I

Ftg 2 15 Adsorptron ~solhermof nltrogen at Ilquld nltrogen temperature(- 195 8'C)

MVacuum I

1

longer val~dwhen p ore s u e 1s close to molecu lar s u e of adsorbate(Dolllmore and Heal, 1964)

For the purpose of descr~b~ngore size d~ st rib ut io n n thls range,Horvath and Kawazoe (1983) proposed a method based on potenttal func-


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Pore radius, r ( A )Fig 2.16 a Cumulative pore volume (below 100 A) FS 400.

Pore radtus, r ( A )Fig 2 16 b Differential. pore size d~stributlon urve, FS 400.


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tion in the pore.For obtaining an average potential in the slit-like pore between thetwo layers of carbon, adsorption potential, an expression of adsorbate-adsorben t and adsorbate-adsorba te interactions resulted in the followingequation.

where I represents the distance between the nuclei of the two layers, NAand N., respectively, are the number of molecules per unit area ofadso rbate a n d th e num ber of atom s composing the surface layer per unitarea, A, and AA are the constants in the Lennard-Jones potentialfunction defined as

where m is the mass of electron, c is the velocity of light, a and xrepresent the polarizability and the magnetic susceptibility of adsorbentatom (suffix, a) and an adsorbate molecule (suffix, A) and K is theAvogadro number. d is defined as

where d, is the diameter of an adsorbent atom and dA is that of anadsorbate molecule.a is the d istance between an adso rbate molecule and adsorbent at omwhere interaction energy becomes zero and Everett and Pow1 (1976) gavethe equ ation as follows.

By taking proper values for the nitrogen-carbon system as shown inTABLE.1, the final form becomes


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TABLE 1 Phys~cal ropert~es or Pore Size D~ stnbu t lon alculat~on y Horvath-KawazoeMethod(1983)Carbon N~trogcn

(Reproduced w ~ t h ermlsslon by Horvath, G and Kawazoe K , J Chem Eng Japan16,47 2 (1983))

T A B L E 2 Values of Corresponding @/pa). (1-d.) P a ~ r sAccord~ng o Horvath andKawazoe and DolllmoreRela t~ve Effect~ve ore suepressure [nmlP / P ~ - 1 Horvath-Kawazoe model (l-d.) Dolllmorc model

1 4 6 x 1 0 ' 0 4 -6 47x10 1 0 43 -2 39x10 6 0 46 -1 05x10 * 0 5 -1 54x10-4 0 6 -8 86x10 0 7 -2 95x10 3 0 8 -2 22x10 2 I I 1 164 6 1 x 1 0 1 1 3 1 327 59XIC 2 I S 1 463 15x10 I 3 0 2 237 24x10 1 10 0 5 0 9

(Reproduced w ~ t h ermlsslon by Horvath G and Kawazoe, K J Chcm Eng Japan.16,473 (1983))where 1 u ln I(F ro m t he e q u a t ~ o n ,he relatlon between p an d I is unlquely definedand thus the re latlon between th e effect~v e ore slze (I-d.) and p is alsoobtalned as sho w n In TAB LE2 Th e relatlon IS also plotted in Fig. 2 17In the table an d figure, the pore s u e calculated by Doll~more'smethod 1sincluded Both methods appro ach pore size of aroun d 13 4 I( andp/p,=O 05, and Howath and Kawazoe suggested that their methodshould be a p p l~ e d o m easurements below p/p,=O 5 and that Dolllmore'smethod should be used above thls relatlve pressure

Pore size dlstrlbutlons of carbon molecular sleve determlned by thlsmethod are sho w n In Flg 2 18, where W - 1s th e m axlm um pore volum edetermlned f ro m the am ount adsorbed at p/p,=O 9


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Measurement o j Pore-related Properties 31

Fig. 2.17. Pore size vs. pressure.(Reproduced with permission by Horvath, G. and Kawazoc. K., J. Ckm figJqpms 16,473 (1983)).

10-5-U. 1.0-?!P

0. I

Effective pore size (nm)

- x DollimoreHorvath-Kawazoe

+x+a7X-+++ i

Fig. 2.18. Effective pore size distributions of carbon molecular sievescalculated by Horvath-Kawazoe method.(Reproduced with permission by Horvath, G. and Kawazoe, K., J. Chem. fig.Japun. 16,474 (1983)).

10-7 10-6 10-5 1W 4 lo-' 10-2 10-1 1.0Relalive pressure P/P. ( - )

C. molecular probe methodAs is understood from Fig. 2.9, fo r small pores which have molecularsieving abilities, it is not possible t o d etermine p ore size distribu tion bynitroge n adsorp tion method . In these cases, the most direct determina-tio n o f the effective po re size is the m olecular probe method.For zeolites, size of entering molecules is determined by assuming thatth e po re s are cylindrical. In th e case of activated ca rbons, especially inthe case of C M S, thickness of the molecules decides the adso rbability ofthe molecules to the micropore since the m icropores are considered to betwo-dimensional.

For several CMS's, this method is applied and comparison of pore


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Micropore size ( A )Fig. 2.19. Micropore size distributions of carbon molecular sieve adsorbentsdetermined by the molecular probe method.

sizes is tried (Fig. 2.19). Ads orpt ion from satura ted pressures of fiveorgan ic solvents, c ar bon disulfide (thickness of 3.7 A), methylenedichloride (4.0 A), ethyl iod ide (4.3 A), chloro form (4.6 A) an d cyclo-hex ane (5.1 A), were tried an d fro m the am ount ads orbed of eachcom pone nt, the pore volume for t he micropores larger than this size canbe determined.2.5.3. Surface area

lnternal surface area of microporous adsorbents is often used as oneof the measures to describe the degree of developm ent of pores. Theconcept of B.E.T. adsorpt ion isotherm (Eq. 3-12), where th e amountadsorb ed by monom olecular coverage, q,, is defined, gives the specificsurface area by assuming the molecular sectional area of nitrogen t o be16.2 A2/mo1ecu1e,which corr espo nds t o 9.76X104 m 2 /mol o r 4.35 m2/Ncc.T o de te rmine q,,, from the experimentally obtained d a ta of isotherm,so-called BET plot of pr/[q (l-p,)] versus pr is made as shown inFig. 3.7. Th en from the slope a nd the intersect of the straigh t lineobtained in the range of 0.35


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References 33

be calculated but the physical meanlng and importance may be reduced.For microporous adsorbents, micropore volumes and size distributionsdetermined by the molecular probe method or the nitrogen adsorptionmethod, if possible, may be more informative than the surface area.

Arakl, T ,Karrer~ansoActrvc Carbon), Maruzen. Tokyo (1932) ( ~ napancsc)Badlcul, G ,K Bratzlcr, W Hcrbert and W Vollmer, A kl rw Kohle und Ihrc Induc~nelleVenvendung,4th Ed , Fc rd~ na nd nke, Stuttgart (1962)Barrcr, R M , ydro~hemralChemrrtry of ZPoIr~es,Acadcmrc Press, Ncw York (1982)Barrcr, R M (ed ). Molenrlo r Swws, R oc of lhe 1st In1 Zeolrle Confirence, Soc Chem Ind(1968)Boehm, H P ,Advances m Calalysu, 16, 179 (1964)Boehm, H P , E Drchl, W Hcck and R Sappok, Angew Chem , nt E d , 3,669 (1964)Breck, D W , Zeohte Mo Imrlor Stew -Stm ure, Chemutry and Use. John W~lcyandSons. Ncw York (1974)Chlhara, K .M Suzuk l and K Kawazoe, AICh E Journal, 24 ,279 (1982)Dollrmore, D and G R Hcal, J Appl Chem . 14,109 (1964)

D a a j , B . S Hocc var and S Pcjovnlk (cds). Zeolrtes-Synlhesrr Slnrrrure. Technologyand App lrcatton. Elscvlcr, Amsterdam (1985)Everett, D H and J C Powl, J Chem Soc, Faradoy T ra m , 1 72 , 619 (1976)Flanrgan. E M and L B San d (eds ), M o k ~ h r rew ZPolrtes-I 8 11, Proc of the 2ndInt Zeolrtc Confcrcncc, Adv m Chemrrtry Serws 101 & 102. ACS (1971)Hasslcr, J W , r tf ia t ro n wtlh Actrvared Carbon, Chcm P u b , New York (1974)How ath, Gcza and K Kawazoe, J Chem Eng Japan, 16, 470 (1983)Hara. N and H Takahash1 (cds ). Zeorarlo (2iwhtes), Kodansha, Tokyo (1975) (rnJapanese)Ikarl, Y . agaku t o Kogyo, 3 1 , 4 (1978) (In Japanese)Katzcr, J R (cd ), Molecular Swws 11, Proc of thc 4th In t Zeolrtc Confcmncc, ACSSymposrum Scnes, 40,ACS (1977)Kawazoc, K ,T K a w a ~ Y Eguchl and K Itoga. J Chem Eng Japan. 7, 158 (1974)Man tcll , C L . drorptron, 2nd Ed .McGraw-H111, New York (1951)Mattson. J S and Mark, J r ,H B .Actrwted Carbon, Marcel Dekka, Ncw York (1971)Mcrer, W M and J B Uyttcrhoevcn(cds ). Molecular Swws, Proc of the 3rd IntZcol~ tcConfcrcnce, Adv In ChcmrstrySerlcs 121, ACS (1973)Mlnato. H , Zeorarto to sono Rryou (Zeolne and rts Applrcatronr), Chapter 2, G~hodo,Tokyo (1967) (m Japancsc)Murakamr, Y .A 11j1maand J W Ward (eds ), New lkwlopmmts m Zoolrte Scwnceand Technology, Proc of the 7th Int Zeollte Conference. Kodansha-Elsev~er,Tokyo/Amstcrdam (1986)Olson, D and A B w o (eds ), Rceeedrngs of the Surlh Intornatrom I ZE OLITEConference, Buttern orth, London (1984)Purl, B R (ed ). Roceedrngs of the 5th Confirence on Carbon, vol 1. 165 (1962)Pun, B R and R C Bansal, Carbon, 1,451 ,457 (1964)Pun, B R .Carbon. 4, 391 (1966)Ramoa, F A E Rodr~gues .L D Rollmann and C Naccache(eds ). Zeobtes Screnceand Technology,N A TO AS1 Serlcs E-80 N~ jhoffPub1 The Hague (1984)Sakoda. A and M Su zu k~ , Chem Eng Japan. 15.279 (1982)Sakoda, A , K Kawazoc and M Suzukr Wafer Re~earch21, 717 (1987)

S m ~ s e k ,M and S Cerny Acrrbe Corbon Manufaclure Roperttes and ApplrcaltonsElsev~cr,London (1970)


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Suzukl, M and K -S Ha, J. Chem Eng. Japan. 17, 139 (1984).Suzuh, M and J.-E Sohn, 159th ACS Annual M eeting, Denver(1987).Suzukl, M . and T FUJI],R oc 4th APCChE '87, 675, S~ngap ore 1987).Suzukl, M and K Chlhara, WaferResearch. 22, 627 (1988).S u z u k ~ , , K. lshlgak ~ nd N. Ayuzawa, Chem. fig. C o m m w . , 34.143 (1985).Tanso-za~ryo-gakka~. arserfan. Kcro f o Ouyou (Acttva ted Carbon. FundarnenfaLr andApplrcanons), Kodansha, Tokyo (1975) ( ~ napanese)Zeora ~to to sono R ~ y o u Henshuu Ilnka~, Zeoraifo f o sono Riyou (Zeolite and rfsApplrcafions), Glhodo, Tokyo (1967) (in Japanese).


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Adsorption Equilibrium

In practical op era tions , max imum capacity of adsorbent canno t be fullyutilized because of mass transfer effects involved in actual fluid-solidcontac ting processes. In ord er t o estima te practical or dynamic adsorptioncapacity, however, it is essential, first of all, to have information onadsorptio n equilibrium. Then kineticanalyses a re conducted based on rateprocesses depending o n types of contactin g processes. The most typical ofthe rate steps in solid adsorbents is the intraparticle diffusion which istreated in the next chapter.Since adsorption equilibrium is the most fundamental property, anumber of studies have been conducted to determine I) the amount ofspecies adsorbed under a given set of conditions (concentration andtemperature) or 2) how selective adsorption takes place when two o r moreadsorba ble components coexist. There are many empirical an d theoreticalappro aches. Only several simple relations , however, can be applied in latertrea tments on kinetic description of adsorptio n. These relations ar esometimes insufficient for predicting adsorption isotherms under a newset of operating conditions. Thu s more sophisticated trials on soundthermodynamics o r on substantial models have been proposed by manyauthors.A basic review is given here. Fo r more detailed discussions refer to Rossand Olivier (1964) and Ruthven (1984). A recent publication by Myers(1988) also gives adsorption equilibrium data available in the literature.

3.1. Equilibrium RelationsW hen an adso rben t is in contact w ith the surrounding fluid of a certaincomposition, adsorptio n takes place a nd a fte r a sufficiently long time, theadsorb ent and the surro un din g fluid reach equilibrium. In this state theam ou nt of the component adsorbed on the surface malnly of the m icroporeof the ad sorb ent is determined a s shown in Fig. 3.1. The relation between

amount adsorbed, q, and concentration In the fluid phase. C. at


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Conccntratlon, C or Pressure, pFig 3 1 Adsorpt~on~sotherrns

Ternperaturn, T

F I ~ 2 Adsorption Isosteres.temperature, T, is called the adsorption isotherm at T.

The relation between concentration and temperature yielding a givenamount adsorbed, q , is called the adsorption isostere (Fig. 3 2).C = C ( T ) for q (3-2)


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Adsorprion lsorherms 37

Adsorption isotherm s ar e described in many m athematical forms, someof which are based on a simplified physical picture of adsorption anddesorption , while othe rs ar e purely empirical and intended t o correlate theexperimental d at a in simple equations with two o r at most threeempiricalparameters: th e m ore the num ber of empirical parameters, the better the fitbetween experim ental da ta and the empirical equation. But empiricalequ ation s unrelated to physical factors d o not have practical significancesince they do not allow extrapolation beyond the range of variables forwhich the parameters have been determined.

3.2. Adsorption Isotherms3.2.1. S u r f a c e a d s o r p t i o n

The simplest model of adsorpt ion o n a surface is that in which localizedadsorption takes place on an energetically uniform surface without anyinteraction between adsorbed molecules. When surface coverage o rfractiona l filling of the micropore is 0 (=q/qo) and the partial pressurein the gas phase, p, which is to be replaced by C(= p/RT) when thecon cen tration in the fluid ph ase is used, the adsorp tion ra te is expressed ask.p(l - 0) assum ing first or de r kinetics with desorption rate given as kd0.Then equilibration of adsorption rate and desorption rate gives theequilibrium relation as

The above relation is given by Langmuir (1918) and K = k./kd is calledthe adsorption equilibrium constant.Th e above eq ua tion is called the Langmuir isotherm. When thea m ou ntadsorbed, q, is far smaller compared with th e adsorp tion capacity of theadsorbent, go, Eq. (3-3) is reduced to the Henry type equation;

Further, when the concentration is high enough, p > I / K, thenadsorption sites are saturated and


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The above equation is modified when interaction between adsorbingmolecules are taken into account. Fowler and Guggenheim (1939) gave

p = I(- ' exp (2uOlkT)K 1 - 8where 2u represents pair interaction energy (positive for repulsion andnegative for attraction), and k is the Boltsmann constant.When adsorbed molecules are free to move on the adsorbent surface(mobile adsorption), the Langmuir equation is modified to

When mobile adsorption with interaction is considered, the following isderived.

Fig. 3.3 shows deviation of the isotherm relation from the Langmuir

Flg 3 3 Effcct of moblle ad sor pt ~o n nd interaction o f adsorbed molecules o nshape o f Isotherm


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Adtorptlon Isotherms 39

equation due to mobile adsorption and interaction between molecules.Suwanayuen and Dann er (1980) introduced the nonideality ofadsorbed phase by considering the adsorbed phase as a mixture ofadsorba te and vacancies. T he activity of vacancies is used to describe thenonideality and the Wilson equation is employed to express activitycoefficient involving two pa ram eter s for a single component system.

where and A31 are Wilson's parameters for surface interactionbetween adsorbate and vacancy.Another typical example of the isotherms frequenlly employed is theFreundllch type equation (Freundlich, 1926).

F I ~ 4 Examples of Freun dllch plot Aqueous phase adsorption of s~nglecomponout organlc ac~dson actlvaled carbon. FS-400 at 298 K


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This equa tion is often considered to be anem pir ica leq ua tion . It is possibleto interpret this equation theoretically in terms of adsorption on anenergetically heterog eneo us surface as described below. This form can alsobe related to the Dubinin-Astakov equation, which is derived foradso rption of the micropore filling type (Dub inin an d A stakov, 1970).Examples of corre lation of adso rption d ata take n in aqueous phase areshown in Fig. 3.4. This equa tion fits well with the experimental da ta for alimited range of concentrations.The Freundlich equation does not satisfy the conditions given by Eqs.(3-5) and (3-6) because it gives no limit of adsorption capacity, makingthe am ou nt adso rbed go to infinity when concentration increases. It isonly applicable below the saturation concentration (solubility or satura-tion vapor pressure) where condensation or crystallization occurs andadsorption phenomena are no more significant.At extremely low concentrations, the Henry type equation (Eq. (3-3))usually becomes valid. Rad ke and Pra usn itz (1972) formulated thefollowing equation, which combines the Freundlich equation with theHenry type equation.

This equation contains three empirical constants and is useful in

a p-Chlorophenolo p-Cresolo Acetone

Fig. 3 5 . Ad sor pt~ on rom aqueous solut ion at 2SC.(Reproduced w ~ t h ermlsslon by Radke, C J and Prausn~tz . M ..l nd . Eng. ChemFundamenralr. 11. 447 ( 1972))


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Adrorption Isotherms 4 1

correlating isotherm data obtained in a wide range ofconcentrations. Anexample is shown in Fig. 3.5.Another useful expression is the Toth equation (Toth, 1971) whichcontains also three parameters.

The equation reduces to the Henry type at low concentrations (pressures)

KP ( - )Fig. 3.6. Toth equations for different values of r .

Fig 3 7 BET plot of gas phase adsorpt~on sotherm.


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an d appro ache s satura tio n limit at high pressures. When the param eter, t,is unity, the above e qua tion is identical to the Langm uir equation. Fig. 3.6shows the effect of para meter, I , on the shape of the isotherm.When adsorption takes place in multilayers, adsorption on theadsorbent surface and abo ve the adsorbed molecules is considered t o be

based on different attractive forces. Monolayer ad sor pti on is formed bythe sam e concept as the Langm uir type adsorption while adso rptio n abovemonolayers is equivalent to condensation of the adsorbate molecules,giving rise to the B ET (B runau er, Em mett and Teller, 1938) equa tion

where p, is the relative pressure (=p fp,) and q, represents the amou ntadsorbed by monomolecular coverage on the surface. From nitrogen adsorp-tion at liquid nitrogen temperature, the surface area of the adsorbentis determined by converting q, t o the surface area. In most cases, q, isobtained from the B ET plo t of the adsorption da ta as shown in Fig. 3.7. Itgives a straight line in the range 0.05


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Adsorption Isotherms 43

Fig. 3 8. Adsorpt~on haracteristic curve.(Reproduced with permission by Kawai, T., Ph. D. i'lzeses (Unrv ofp 34 (1976)). Tokyo, 1976).

W ( A ) is the adsorption characteristic curve originally introduced byPolanyi (1914) an d Berenyi (1920). Adsorp tion of benzene on activa tedcarbon is plotted in Fig. 3.8 in the form of the characteristic curve.Dubinin (1960) assumed a distribution of the Gaussian type for thecharacteristic curve and derived the following, which is called Dubinin-Radushkevich equation.

Adsorption equilibrium relation of benzene on two types of activatedcarbon a re plotted by the Langmuir plot and the Dubinin plot in Fig. 3.9(a)an d (b). Ap parently, the Dub inin equ at ion gives a better regression t o thedata of Hasz (1969).Later this equation was generalized by Dubinin and Astakhov (1970)to the following form.

In this expression E is the characteristic energy of adsorption an d obtainedfrom adsorption potential A a t W/ O= e-1. The parameter n in the


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( a ) Langmuir plot (b ) Dubinin plotFig. 3.9. Correlation of equilibrium data on activated carbon (Columbia NX C),data by Hasz, source: Kawai, 1976.(Reproduced with permission by Kawai, T.. Seiken Koushukai Text N o. 3, 7,Seisan Gijutu Shoureikai (1977)).

Dubinin-Astakhov equation was originally considered to have integer value,and n = 1 ,2 and 3 respectively correspo nds to adsorp tion o n the surface,in micropores and ultramicropores where adsorbed molecules lose one,two o r three degrees of freedom. Fo r nonpola r adsorba tes, the simplifiedestimates given in TABLE.1 have been proposed.Kawazoe and Kawai (1974) tried to examine applicability of theDubinin-Astakhov (D-A) equation to equilibrium data of molecularsieve carbon (MS C). Since the D-A equation can be written as

It is possible to determine n and E by plotting the left hand side of Eq. (3-18)versus In A provided WQ is known. An exam ple is given in Fig. 3.10 fo rbenzene on MSC 5A (Kawazoe er a!., 1971). WO an be estimated fromthe limit of adsorption and is considered to correspond to the microporevolume of the adsorbent. Density of the adsorbed phase, p, is necessary toconvert the amount adsorbed to the volume filled by adsorbate. Foradsorption below critical temperature, liquid density at the sametemperature can be used for p but above critical temperature, thehypothetical density estimated from the Dubinin-Nikolaev equation is


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T A B L E 1 Parameters of Dubtntn-Astakhov Equations tn Relatton to ratto of pore s~ze,D, and molecular stte, d AH 0 represents heatof vaporttattonAdsorptton stte Rafto n E Examples of a adsorptton systems Three types of adsorptton sttes

D/d>5 I Carbon black-benzene,( I ) Surface I Stltca gel-hydrocarbon3

2 Acttvated carbon-C OI, benzene( I I ) Mtcropore hydrocarbon, ram gas etc

(111) Ultram~cropore 3 > Dld 3 -AH0 MSCtrhane, Acttvated carbon ym0 b(Columbta LC)-saturated hydrocarbon - B2111 S

(Reproduced wtth permtsston by Kawatoe, K and Kawat, T, etsan Kenkyu. 22 , 493 (1970)) 8s33


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Fig 3 10 In In (WO/) vs In A plot according t o Eq. (3-18).(Reproduced w ~ t h ermlsslon by Kawazoe. K., Astakhov. V A , K aw a ~ ,T andE g u c h ~ ,Y ., Kagaku Kogaku. 35. 1009 ( 197 1 )).recommended.

where pb is the density of liquid at normal boiling point, Tb, and po isthe density of adsorbed phase at critical temperature, T,. M and b aremolecular weight and van der Waals con stant. Characteristic valuesof the adsorption of various gases on MSC-5A are shown in TABLE.2.Cha racteristic energy of ad sorption was correlated by paracho r a s shownin Fig. 3.11 (Kawai and Kawazoe, 1975).

Also from T A B L E.2 on e can see that a parameter, n , is not necessarily aninteger. n may be a function of relative magnitude of adsorba te molecularsize and micropore size.Fr om this point of view, an d Suz uki and Sa ko da (1982) tried t o extendthe D -A equation to include adsorb ent which has appa rent m icropore size


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TABLE 2 Characteristic Values of Adsorption on M S C SAWo nAdsorbate E[CC /gl [-I [ca l l moll

I nitrogen2 carbon d~ox~de3 oxygen4 hydrogen5 neon6 argon7 krypton8 xenon9 methane

10 ethyleneI I ethaneI2 propylenc13 n-butane14 n-hcxancI5 benzene16 ethyl acetate17 p-xylcne18 trichlorocthylene19 tetrahydrofuran20 methykne chloride2 1 cyclohexane22 aatone23 carbon disulfidc24 methanol25 ethanol26 n-butanol27 acctlc acid28 pyrlglnc.. -

(Reproduced w~thpcrmlsslon by Kawazoc. K , Kawai. T , Eguch~,Y and Itoga, K ,J Chem Eng Jqm. 7 , 160 (1974))

d ~ s t n b u t ~ o n .n thls case n and E are assumed to be fun ct~ on s f d lD , theratlo of molecule slze to pore s ue , and the ad so rp t~ onsotherm 1s given InIntegral form.

where f( D ) 1s the denslty d ls tr ~ b u t~ o nunctlon of mlcropore slze n( d/ D)and E (d / D) were deterrnlned uslng MSC 5A and 7A w ~ t h enon, ethyleneand ethane as ca l~ b ra tl o n ases The results ar e glven In Flg. 3 12 (a)and (b). Fo r ord lnary actlvated carbons it is difficult t o determine thes t r ~ c t unctional form of f( D ) Assumtng normal dlstribut~ on or f( D )from the mean pore sue 55 and the square root of the varlance o for thede ns ~t y u nc t~ on f pore su e, f( D ) for the commercial actlvated carbonwas o bta ~n ed rom the a ds or p t~ on sotherm measurement


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parachorFig. 3.11. Correlation of characteristic energy of adsorption and parachor(F or key refer to TABLE.2).(Reproduced with permission by Kawazoe, K.. Kawai.T., guchi. Y. and Itoga, K.,J. Chem. Eng. Japan. 7, 161 (1974)).

A typical resu lt is sho wn in Fig. 3.13. M icro pore size of ordinary activatedcarbon fo r gas phase ad sorp tion is believed t o range from 0.5 to 1.5 nm.One obvious deficiency of the D-A equation is that it does not ap-proach the Hen ry type equation at lower concentrations. According tochromatographic measurement using helium gas as a carrier (Chihara,Su zuki and Kawazoe, 1978), the adso rpt ion equilibrium coefficient of theHenry type equation, which was assumed to hold at an extreme of C=O,could be determined fo r M SC 5A. T he results are shown in Fig. 3.14. Inthis experiment, since helium gas exists in large excess compared withadsorba ble trace r gas, the co adso rption effec t of helium may not have beennegligible, making it possible to assume the Henry type isotherm fo r thetracer gases.S ak od a and Suzu ki (1983) measured th e adsorption isotherm of xenonon M S C 5A in a w ide pressure range (4X 1V4-IX 102 To rr) in the absen ceof other components and assumed that below the point where the D-Aequation ha s a tangent that goes through the origin, the Henry equationcan be used instead of the D-A eq ua tio n, as show n in Fig. 3.15. If this


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Adsorptron Is ot he rm 49

0 XenonA Ethylene"1 0 Ethane \

( a ) Varrat~onof n w ~ th hange of D/d fromMSC samples

0 Ethane5 1 0s 0 9 2 0 2 5

n ( - )( b ) Plots o f E/EIdHo versus n for MSC samples

Fig. 3 12. Relations among EI A H a n and ratio of pore s ~ z e nd moleculard~ameter,D / d(Reproduced w ~ th cmusslon by Suzuki. K and Sakoda, A., J Chcm. Eng. apan, 7,283 (1982)).assu mption is valid, then the adsorption equilibrium constant o ft he Henrytype equation is related to the constants involved in the D-A equation as

Transience occurs at

and the fractional amount adsorbed at this point isW / O ex p [ - ( A / E )" /( n- l) ] (3-24)

For adsorption of xeno n o n MSC 5A at room temperature, E, and n given


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Fig 3 13 Normal distribution curves of carbon C Curve (I) f rom ~ s o t he r m fxenon, Curve (11) from Isotherm of ethylene, Cur ve (111) from ~s o th er m fethane(Reproduced with permisston by Suzuki. M and Sakoda, A , J Chem Eng Japun. IS284 (1982))

Flg 3 14 van 't H ofr s plot of ad so rpt ~o n qul l lbr~ um onstants(Reproduced by permlsslon by Chlhara. K , uzukl , M an d Kawazoe, K , IChEJournal, 24, 24 1 (1978))


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Heat of Adsorpiton 5 1

Fig 3 15 Adsorpt~onsotherms of xenon on MS C 5A at0 Ca nd 19OC Dottedllnes and so11d lines correspond t o the Henry type equatlon and the D -Aequatlon. respect~vely(Reproduced with perrnlsston by Sakoda, A and Su zu k~ , ,J Chem Eng Japon16, 157 (1982) )

In T A B L E2 lndlcate that the tr an s~ en ce rom the D-A equatlon to theHenry equatlon occurs at around Wl WO 0 01It should be added that the D-A equatlon when n=l IS reduced to theF r eu n d l ~ chype equatlon

Then parameters In Eq (3-10) correspond t o the parameters In Eq (3-25)as follows

3.3. Heat of AdsorptionA d so r p t ~ o n IS accompanled by evolut~onof heat slnce adsorbatemolecules are more stablllzed on the adsorbent surface than In the bulkphase Adsorp t~onS accom panled by phase change and thus depending onthe occaslon ~tmay lnvolve mechanical work Fo r thls reason, theam ount

of heat evolution by unlt adsorpt~ondepends on the system adopted


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(R os s and Olivier, 1964). Fr om a practical stan dpoin t, the imp ortantdefinitions of heat of adsorptio n ar e "differential heat of adsorpt ionnand"isosteric heat of adsorption."Differential heat of adsorption, Qdlrr, is defined as heat evolution whenun it adsorp tion takes place in an isolated system. This heat is directlymeasurable by calorimeter.lsosteric heat of adsorption, Q,,, is defined from isotherms at differenttem peratur es by Eq. (3-29). Qa is bigger than Qdlr since it requ ires workequivalent to p V( =RT) .

Q,c is related to adsorption isotherms at different temperatures by thevan? Hoff equation

Fro m experimental isotherms a t temperature Ti and Tz,Qsl s obtained as

For the Henry equation and the Langmuir equation, Q,, is related to theequilibrium constant, K, a s

When adsorption sites are energetically homogeneous and when there isno interaction between adsorbed molecules, the heat of adsorption isindependent of the am ou nt adsorbe d. However, when the adso rben tsurface is composed of a n um ber of patches having different energy levels,or when interaction am on g adsorb ed molecules can not be neglected, theheat of adsorption varies with the surface coverage. W hether variationof heat of adsorption is due to surface heterogeneity or to the interac-tion among adsorbed molecules is hard to distinguish in some cases.Here no distinction between the two mechanisms is made and onlyphenomenolog~cal ariation is considered.Variation of heat of ad so rpt ion can be described two ways. One is by


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Heat of Adrorption 53

defining the spectral density of adsorp tion sites where heat ofadsorptio n isQ,f (Q) . Then

Or more directly, it is possible to describe the heat of adsorption as afunction of amount adsorbed q, QJq).The relation between f(Q) and e(q ) is a s follows: Q(q) is convertedso that the am ou nt adso rbed , q , is written as an explicit function of Q,q(Q), and then

When energy distribu tion, f(Q ), is given, the corresponding form ofadso rption isotherm can be estimated by assuming tha t the heterogeneoussurface consists of small homogeneous patches (small areas) an d tha t theadsorption isotherm on homogeneous surface holds o n each small patch.Fo r example , Langrnuir type isotherm relation (Eq. (3-3) with Eq. (3-32))can be assumed to hold on each patch.

where dq is the amount adsorbed on the patch having the adsorptionenergy Q. Then th e total isotherm equ ation is given as

This equa tion is solved for a given f(Q) using Stieltjes transform . Butfor the sake of simplicity Roginsky's approximation is attractive, i.e.,assumption of the Langrnuir type isotherm on each patch is furthersimplified t o a stepwise isotherm a s

Then the integral in Eq. (3-36) is simplified to


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Equilibrium concentratlon. C (mol/ml)Fig. 3.16. Summary of adsorption isotherms of propionic acid on activatedcarbon H G R 5 13 from aqueous solution measured at 283,293, 303 and 3 13 K.Hollow circles are measured points at 303 K .

where

Then if f(Q) is given, corresponding adsorption isotherm is easilycalculated.Also from an isotherm relation, q ( p ) , pectraldensity function ,flQ), canbe calculated from the following equation.

As a typical example of an adsorption isotherm in aqueous phase.adsorption of propionic acid on activated carbon is shown in Fig. 3.16.At higher conce ntratio ns isotherms ca n be correlated by the F reundlichtype equ ation , but a t low coverage ( 9 ~ 0 . 1 mol/g), the slope of isothermsbecomes steeper and seems to reach Henry's type relation. Fro m isothermsat different temperatures, isosteric heat of adsorption was obtained as afunction of the am ou nt adsorbed a s sho wn in Figs. 3.17 an d 3.18. Athigher coverage, measured isosteric heat of ads orp tion seems to decreasewith increasing am oun t adso rbed.


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Hear of Adrorptlon 55

Fig 3 17 Adsorpt~onsosteres lor determln ~ngsostcrlc heal ofa dso rp t~o n romEq 3-30(Reproduced w ~ t h ermrsslon by Suzukl, M and FUJII. ,AlChE Journal, 28,383 ( 1982))

This functional form is consistent with the Freundlich isotherm and theFreundlich exponent n~ determined by experiment are compared withQo/ RT in TABLE.3. Agreement between them is reasonable.At low coverage, the Freundllch isotherm assumes existence of sites withvery high heat of adsorption. For measured equilibrium dat a, the Radkeand Prausnitz equation can be used for correlating low coverage data.Tem perature dependence of K gives heat of a d so rp t~ o n t initial coverage.This Q,,.o was 4.6 J / m o l and is shown in Fig. 3.18 by the arrow . FromFlgs 3.16 and 3.18, tran sition from Henry type isotherm and Freundlichtype isotherm can be said to oc cur a t around 4X 10-lmol/kg.


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Amount adsorbed. q (mole/kg)Ftg 3 18 lso ste r~c eat of adsorption Qstr plotted against amount adsorbed q(Reproduced w ~ t h ermlsston by Suzukl, M and FUJII,T , AIChE Journal 28,384 (1982))

T A B L E3 Constants of Freundl~chType Equatlon q=kcl lmApphcd to the isotherm results for q> lW1 rnol/kgTemperature (K) k n QoIRT

283 0 55 2 74 3 0 0293 0 49 2 64303 0 43 2 52313 0 32 2 45 2 71(Reproduced wrth permission by Suzukl, M and FUJII,T . AIChE Journal. 28. 383( 1982))

3.4. Adsorption Isotherms for Multicomponent SystemsWhen two o r m ore ad sorba ble components exist with the possibil~ty foccupying the sa me a ds or pt ~ onites, isotherm relationships become m orecomplex. The srmplest is extension of the Langm uir type rsotherm byassuming no inte ractio n between adsorbin g molecules. In the case of twocomponents, the extended Langmurr isotherm (Markham and Benton,1931) is given a s

Thls equatron enables qurck est ~m atro n of equilibrium relatrons of


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Adsorprron Isotherms for Multtcomponent Systems 57

multicomponent adsorption from Langmuir parameters determinedfrom th e single compon ent isotherm of each component. Eq. ( 3 4 2 )is thermodyn amically consistent when 901= 9 0 2 holds. Furtherm ore, theequa tion c an be applied without s ignif icant erro r to a combination ofdifferent values of q o if the co mpon ents a re similar in natureand follow theLangm uir isotherm relation.

Fr om Eq. (342 ), i t follows that the separa tion factor for a mixture oftwo components can be given directly by the ratio of the equilibriumconstants.

This relation holds independent of concentration an d can naturally beextended to a n arbi t rary combination of components.

Lewis er 01. (1950) showed that fo r adsorp tion of two components with aconstant total pressure of P = pl +p2, the following relation holdsbetween the a mo un t adsorbed of each componen t , 91and q2.where q0 I and q02ar e the am oun ts of pure co mpon ents adsorbed a t pressurepa I = P n d p02 = P. Th e above relation is derived from Eq. (3 42 ). F orads orp tion of mixtu res of hydrocarbons, Eq. (3 4 5 ) is valid since a,)becomes relatively con stan t independent of concentration. Th e exam plesof measurement by Lewis et al. (1950) are shown in Fig. 3.19.

The extended Langmuir (Markham-Benton) isotherm has limitedapplicability especially for liquid phase adsorption, since even single-com po nen t isotherms in liquid phase ar e rarely explained by the Langm uirequation . Th ere have been several trials to extend the Freundlich typeequ atio n to mi xtu re isotherms. Fritz an d Schliinder (1974) gave thefollowing equation .

These types of frequently found equations involve problems concerninginconsistency with singlecomponent isotherm data and lack ofther mod yna mic bac kgro und . However since they employ relatively largenumb ers of e m p lr ~ c al arameters, final fit with the experimental dat a


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Adsorption Isotherms for Mulficomponenf Systems 59

The Raoult law is al so applied to the relation between mole fraction in gasphase and adsorbed phase.

where yi and x, are mole fraction of the i-th com ponent in gas phase and inadsorbed phase, respectively.

T o obtain the equilibrium am ou nts adsorbed which correspond to a givenset of gas phase concentra tion s, first a guess of surface pressure R A / R T sma de and thenpol an dp 02 are obtained. Then from pt,p2 ,p0 I andp02, XIan d x l are obtained by Eq. (3-49). This procedure is repeated until Eq. (3-50) is satisfied. Then from q 0 an d q02determined fo rm pO and pO z,q ~ c a nbe determined by Eq. (3-48). which then gives q~and q2 from XI and x lpreviously obtained. T he itera tion procedure is minimized by employingasm all computer. W hen iso therm s for single com pon ents can be expressedby analytical equations, the integral of Eq. (3-47) can be determinedanalytically. Fo r instance, if the Freundlich equ at ion can be applied in awide range, then the integral becomes

which greatly simplifies the procedure. But it should be kept in mind tha tunless single-component isotherms especially fo r the weaker com ponentare determined in a wide range of amounts adsorbed, this simplifiedtreatment may result in considerable deviation.Th e IA S model is practical for predicting binary isotherms from single-com ponent isotherm data . As an example, isotherms of propane andbutane on activated car bo n ar e shown in Fig. 3.20. The assumption ofideal adsorbed solution may need careful consideration in some caseswhere a combination of two components forms an azeotropic mixture in theadsorbed phase as reported by Glessner and Meyers (1969).The IAS theory is also applicable in the case of adsorption from aqueoussolution, a s shown by R ad ke and Prausnitz (1972b).Th e vacancy solution model was extended to describe mix ture adsorptionisotherm (Suwanayuen and Danner, 1980).


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-/ olid lines are from Myers and Prawitr method0 100 200

Butane pressure, pb (Torr)Fig. 3.20. Adsorp tion isotherm s of propan eand butane mixture onactivatcdcarbon; amou nt adsorb ed o f propan e and butane for propane pressures 0.76,1 14 and 228 Torr.(Reproduced with permission by Suzuki, M.eraL, FundmnenraLrofAdcouprionEngineering Fo und ation, 622 (1985).

3.5. Adsorption Isotherms of Unknown MixturesIn the case of water treatment by adsorption, mixtures of organicsusually become involved and phenomenological parameters are used to

express water qualities. Th ese parameten are COD (chemical oxygendemand), BOD (biochemical oxygen demand) o r T O C (total organiccarbon), which correspond to the weighted sum of concentrations ofcomponent organics. Since each organic co m po ne nt has differentadsorbability, the weighted tota l sometimes appe ars strange in adso rptioncharacteristics.For the sake of simplicity. the adsorption of an aqueous solution ofCOD which consists of two organic com pone nts o n activated carbo n isconsidered here. One of the components, whose con cen tration is C,, is notadsorbed at all while the other component of concentration Cz has anadsorp tion isotherm of the Freund lich type, q = kC21J",when it exists as asingle component. C O D of the mixture, COD,,s the weighted sum ofCI nd C2.


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measured Isotherm will become as shown by do tted lines in Flg 3.21.The difference I n Isotherms by the two me tho ds whlch never occurs Inslng leco m po ne nt m easurement 1s du e to the dllutlon of the unadsorbedcom pone nt in the latter case. In oth er wo rds, the existence of anunad sorbab le com po ne nt may be checked by me asu nn g isotherms by thesetwo different methods.

Bercnyl, M ,Z Physrk Chemre, 94, 628 (1920). 105, 55 (1923)Brunauer, S . H. Emmett and E Teller, J Am Chem Soc. 60. 309 (1938)Chihara, K , M Suzuki and K Kawazoe, A IC h E / o u m l . 24, 237 (1978)Dublnln M M , Chemistry and Physrcs of Corbon, vo1 2, p 51 Marcel Dckker,New York (1966)Dubinln, M M , Chem Rev. 60, 265 (1960)Dublnm, M M , V A Astakhov, 2nd hr C o d on MoWar-Stew Zeo l re (1970)Fowler, R H and E A Guggenheim, Slarisrical 77rermodymmics, Cambridge Un~venltyPress, Ca mb ndg c (1939)Freundlich, H , Collotd and Cqpillary Chemwrry, Mathucn. London, pp 110-134 (1926)Fntz, W and E U Schlundcr, Chem Eng Scr. 29, 1279 (1974)Glersner A J and A L Myers. Chem Eng Progr Sympo Ser. 96, Vol 65, 73 (1969)Hasz. J W and C A Barrere. J r . Chem fig Progr Sympo Ser. 96, Vol 65,48(1969)Kawazoe, K , T Kawal, Sewn Kcnkyu. 22, 491 (1970) (In Japan ese)Kawazoe, K . T Kawai, Y Eguchl and K Itoga, J Chem Eng Japan. 7. 158 (1974)Kawazoe, K . V A Astakhov, T Kawal and Y Eg uch ~,Kagaku Kogaku. 35, 1006 (1971)( ~ n apanese)Langmuir, 1, Chem Soc. 40, 1361 (1918)Lewis, W K ,E R Glllll and B Chcrtow and W P Cadog en, In d Eng C k m , 42. 1319

(1950)Markham E C and A F Benton, / Am C k m S oc. 53, 497 (1931)Myers, A L , Database on Adrorprron Equrlrbrtum, in preparation (1988)Myers. A L and J M Prausnltz, AlChE Journal 11, 121 (1965)Polanyi, M , Verh Deur Chem. 57, 106(1914)Radke, C J and J M Prau sn~tz ,nd Eng Chem. Fundom.. 11, 445 (1972a)Radke, C J and J M Prausnitz, AI Ch E Journal, 18, 761 (1972b)Ross, S and J P Ohv ier. On Ph~srcalAdrorprron. Intersclence (1964)Ruthven, D M . Pnncples of Adsorprron and Adtorprron Processes, John Wrley & Sons,New York (1984)Sakoda, A and M Suzukl. J Chem Eng Japan. 16, 156 (1983)Suzuki. M and A Sakoda, J Chem Eng Japan. 15. 279 (1982)Suzukl. M . M Horl and K Kawazoe, Fundamenrols of Adsorption. 619 Eng Fou ndatlon,

N Y (1985)Suwanayuen S and R P Danner. AI Ch E Joumol. 26, 68. 76 (1980)Suzukt. M and T Fujll. AlChE Journal. 28 380 (1982)Toth, J , Arra Chrm Acad Srr Hung, 69 31 1 (1971)


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4Diffusion in Porous Particles

M ost of th e adsorben ts com mercially used are porous particles. Fo rlarge adsorption capacity, large surface area is preferable, as a resultlarge num be rs of fine pores, as fine as possible, are needed. Adsorbatemolecules come from outside adsorbent particles and diffuse into theparticle t o fully utilize the ad sorp tion sites. Depending o n the structureof the a dsorb en t, several different types of diffusion mechanisms becomedom inan t and sometimes two o r three of them com

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