29 Fixed Bed Adsorption of Acetone and Ammonia Onto Oxidized Activated Carbon Fibers

8/9/2019 29 Fixed Bed Adsorption of Acetone and Ammonia Onto Oxidized Activated Carbon Fibers

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F i x e d B e d A d s o rp t i o n o f Ac e t o n e a n d Am m o n i a o n t o O x id i z e d

A c t i v a t e d C a r b o n F i b e r s

C hr i s t i a n L . Ma ng u n, R i c ha r d D . B r a a t z , * J a m e s E c o no m y , a nd A l l e n J . H a l l

Department of Chemical Engineering, 600 South Mathews Avenue, University of Illinois at

Urbana-Cham paign, Urbana, Illinois 61801-3792, and Department of Materials S cience and Engineering,

University of Illinois at Urbana-Champaign, 1304 West Green Street, Urbana, Illinois 61801

The tailoring of the pore surface chemistry of activated carbon fibers is shown to be an effectivemethod for increasing the removal efficiencies of various contam inants under a fixed bedconfiguration. An oxidation treatment with nitric and sulfuric acids resulted in a significantincrease in the adsorption capacities and breakthrough times. The adsorption kinetics weredescribed by a homogeneous surface diffusion model. Although the effective diffusion coefficientswere a ctua lly r educed by th e oxidation process, the improvement in equilibrium adsorptioncapacities more than compensated, to result in the overall improved breakthrough times.

1 . I n t r o d u c t i o n

One of the undesirable features of the modern dayworld is widespread conta minat ion associated with the

release of chemicals into the environment. In recentyears th ere ha s been a significant amount of work doneto quantify the harmful effect of these trace contami-nants. Societys growing concern has led many govern-ments to establish stricter standards for clean air andwater, m any of them in th e par ts-per-billion (ppb) range.This necessitates t he developmen t of improved meth odsfor cont rolling th e release of toxic cont am inan ts a nd t hedesign of new ma terials t ailored to selectively r emovea wide range of contaminants and permit for recoveryand reuse. Industry ha s a lso recognized the potentialeconomic benefit that can be a chieved through therecycling of chem icals in stea d of disposal. On e t echnol-ogy which is attractive for this purpose is the use of dryadsorbents such as activated carbons.

Commercially a vailable a ctivated carbon granu les(AC Gs ) have been an i ndus t r y s t andar d for m anyyears.1-4 They are used as adsorbents to purify pollutedwaste streams because of their low cost and becauset hey have been det er m i ned t o be t he bes t avai l abl etechnology for the removal of m any contam inants.However, ACGs suffer from a num ber of drawbacksincluding poor selectivity, slow kinetics, the need forexpensive conta inment systems, less th an 100% workingcapacity, a nd costly reactivation. Perhaps the mostserious problem is t he absence of a comprehensiveund ersta nding a s to th e effect of pore size and chemistryon the adsorption properties. Such knowledge wouldallow t he tailoring of activated carbons for t he contr ol

of specific contaminants. Another problem is the lackof design flexibility when using gra nulat ed carbons.There is a definite need for the development of improvedmaterials designed specifically for a wide range of environmen tal problems.

To address some of these disadvanta ges, Economy an dLin 5,6 reported on the development of phenolic-based

activat ed car bon fibers (ACFs), which displayed sig-nificantly improved adsorption capabilities over thoseof ACGs. Today, these ACFs are commercially availablefr om N ippon K ynol and ar e pr epar ed by t he dir ect

activation of a cross-linked phenolic fiber at 800-1000C using a mixture of steam and carbon dioxide. TheACFs ha ve the a dded a dvan ta ge of simplified cont ain -ment , capability for in situ electrical reactivation, a ndgreater rates of adsorption over those of conventionalgranu lated activated carbons due to much better cont actefficiency.5,7

Historically, th e ACFs ha ve only been used in n ichemar kets du e to their high cost, poor a brasion resistan ce,and limited adsorption versatility. This is because theadsorption chara cteristics a re contr olled to a consider-able extent by the acidic nature of the surface. Recentlysome new approaches have been reported to eliminateor reduce the dr awba cks associated with th e ACFs.7-11

In order to t ailor the ACFs for a dsorption of specificcontam inants, they were chemically converted to anitr ided, hydrogenated, chlorinated, or more highlyoxidized su rface.8,9 The problem of poor abrasion resis-tance has been partially addressed by activating theprecursor phenolic fibers at temperatures of 400-45 0C in air .7 This greatly improves both the activationyields (lower cost) and the abrasion resistance. Theseand related studies 12 have indicated the usefulness oftailored ACFs for gas separ ation a nd for th e rem ovalof very low levels of contamina nts in t he ppb ra nge.

T he goal of kinet ic dat a anal ys is i s t o develop apredictive model that can be used for designing com-mercial-scale adsorbers. Although initial experimenta lstu dies with ACFs ha ve been promising, there a re very

few studies that construct a phenomenological modelof their adsorption kinetics.12 This is in contrast to thecase for ACGs, where methods for analyzing adsorptionkinetics have been extensively developed over the pasttwo decades.13-18 The adsorpt ion kinet ics for ACGs a reusua lly chara cterized by t he homogeneous surface dif-fusion model, which assumes that the adsorbate mol-ecules creep along the internal pore surfaces while inan adsorbed state.18,19 An altern ative is the pore diffu-sion m odel, in which th e adsorbat e is assum ed to diffusethrough a f luid phase within the pores.19 The flux ofadsorbate through the adsorbent is affected by pore

* To whom all correspondence should be addressed at theDepartm ent of Chemical En gineering. Telephone: 1-217-333-5073. Fax: 1-217-333-5052. E-mail: braa [email protected].

Department of Materials Science and Engineering.

3499Ind. Eng. Chem. Res. 1999, 38 , 3499-3504

10.1021/ie990064m CCC: $18.00 1999 American Chem ical SocietyPublished on Web 08/07/1999


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tortuosity, constriction, a nd branching. In a real a dsor-bent, however, these pore structure chara cteristicscannot be est i m at ed or m eas ur ed i n a r eas onablyaccura te ma nn er. Instead, th e flux is assumed t o occurin the radial dimension, and an effective surfacediffusion coefficient is estimated from experimentaldata.14,15 Another mode of mass-transfer resistance thatcan be significant for some adsorbent systems is thatof an external f i lm surrounding the adsorbent. I t hasbeen found that the external m ass-tran sfer resistancecan be neglected for many adsorbent systems, includingthose with activated carbon as the adsorbent.20-22 On eof the goals of this study is determine how well suchmodels repres ent th e performa nce of ACFs.

The p otent ial of ta ilored ACFs motivat ed th is fixedbed s t udy t o det er m i ne t hei r ut i li t y i n an appli eds ys t em . I t w a s of in t er e st t o in ve st ig a te h ow t h eadsorption kinet ics would be a ffected by chemical t reat -ment of the ACFs. Another goal of this st udy was todetermine if the homogeneous su rface diffusion modelwould be adequa te for modeling diffusion in ACFs.Although four major types of chemical treatment forACFs have been developed,8,9 for specificity the studyfocuses on the adsorption of acetone and ammonia onto

unt reat ed a nd oxidized ACFs. Acetone r epresents apolar organic molecule, and ammonia a basic molecule.The breakthrough curves are measured, and the effec-tive diffusion coefficients ar e comput ed. Th is is followedby discussion and conclusions.

2 . Ma t e r i a l s a n d M e t h o d s

2 .1 . Ac t i v a t e d C a r b o n F i b e r s . Activated carbonfibers can be made from a variety of precursors includ-ing pitch, polyacrylonitrile, and phenolics.12,23 For thisstu dy, the ACFs were obta ined from Nippon Kynol inJapan. These f ibers are made from a phenol-formal-dehyde resin which has been melt-spun and then acid-cured t o form a fully cross-linked amorphous polymer

structure. This precursor fiber, known as Kynol, is thencarbonized and activated using the combustion byprod-ucts of liquefied petroleum (mostly water and carbondioxide). The fibers can be p roduced in a ra nge of surfacear eas bet w een 600 and 2500 m 2 /g, depending on thetime and temperature of a ctivation. The microporestructure has been shown to be very uniform acrossapproximat ely 95% of the fiber diam eter core by scan-ning tun neling microscopy.24,25 In addition, activationalso results in t he initial fiber diam eter being redu cedfrom approximately 16 m t o 9-15 m, depending onthe degree of activation. The fibers used in this studyconsisted of a series of ACFs with increasin g sur facear ea (increa sing degre e of bur noff): ACC-5092-10 (ACF-10), ACC-5092-15 (ACF-15), and ACC-5092-25 (ACF-25).

All ACFs used in this stu dy came from th e sameproduction batch, as identified by Nippon Kynol. Alltrea ted ACF-10s were oxidized in the same appa rat usat the same t ime; half were used for acetone experi-ment s, and ha lf for the am monia experiments. A similarmethod was used for ACF-15 and ACF-25. RegeneratedACFs were n ot used in any of the experiments.

2.2. Characterization of Physical Structure: SEMT e c h n i q u e s . A commercial scanning electron micro-scope (Hitachi model S800) was u sed to measure thefiber diameters of t he ACFs. The fibers were cutperpendicularly, and several photographs were takenof the cross-sectional surface. The individual fiber

diameters were then measured by hand and reportedas an average value. The accelerating voltage used foral l r uns w as 20 keV, and t he fi el d el ect r on s our ceprovided a resolution of 2 nm. The fiber diameters weremeasured to be 15.4 m for ACF-10, 13.75 for ACF-15,and 11.25 for ACF-25.

2.3. E ffect of C hemi cal Modi fi cati on: E l emental

A nal ysi s. The carbon edge at oms can be combined witha number of different atoms, depending on the activa-

tion conditions and post-treatm ent. Conversely, het-eroatoms can be removed upon heat treatment. Sincethe ACFs ar e produced by a high-tempera tu re oxidat iveetching process, the pore surface tends to be a cidic dueto th e pr esence of car boxylic acid an d ph enolic hydroxylgroups. This a cidic surface chemistry can be modifiedusing a variety of reactions, often at high temper atu res(700-900 C).8,9 In particular, t he acidity of the poresurface can be increased by treatment with a strongoxidizer s uch as a m i xt ur e of ni t r i c and s ulphur icacids;8,9 an ACF tha t un dergoes this treatment is saidto be oxidized. Conversely, a high-temperature treat-m ent w it h am m oni a yields a s ur face r i ch i n bas icnitrogen groups9 which has shown enhanced adsorptionof acidic gases su ch a s H Cl.26

A model CE440 elementa l an alyzer ut ilizes a combus-t i on t echni que i n w hich t he car bon, hydr ogen, andnitrogen weight percentages are determined. The sampleis burned in an oxygen atmosphere at approximately960 C while the amounts of evolved carbon (CO2),hydrogen (H 2O), and nitrogen (N 2) ar e m eas ur ed byth erma l conductivity detectors. Oxygen was deter minedby mass difference, assuming only these elements arepresent.

2.4. Structu ral Properti es: N i trogen A dsorpti onI s o t h e r m s . Nitrogen adsorption isotherms were usedto characterize the surface area and micropore volume.A Coulter Omn isorb (Hialeah , Florida) was u sed for t hevolumetric measurement of the nitrogen adsorption

i sot her m at 77 K (for det ail s, s ee r ef 26). T ype Iisoth erms, wh ich ar e indicative of microporous car bons,were observed. This data was used to calculate the BETsurface area u sing the standar d method,3,27 applied overthe relative pressure range 0.01-20. The microporevolume was calculated by a pplying the Dubinin-Ra -dushkevich equation to the experimental nitrogen iso-t her m .28,29

2.5. Fi xed B ed A nal ysi s. A flame ionization detector(FID) of a Hewlett-Packard gas chromatograph model5890 (HPCG) was u sed to monitor th e concentra tion ofthe outlet gas stream for acetone and ammonia. Theflow rate was set at 50 mL/min using a Tylan Generalflow controller for all experiments. Sample preparation

consisted of cutting the ACF samples into eight 1 in.diameter circles. These were mounted into a stainlesssteel cylinder using rings as spacers between each fiberlayer, with the distance between each fiber layer being6 .3 3 m m . T h e r in gs w er e t h e n com p r es se d b y a ninternal spring. The inlet s tream hit a perpendicularplate immediat ely upon en tering th e cylinder t o promotea constant velocity profile upon entering the fixed bed.

After loadin g, a flow ra te of 50 mL/min of nitrogen a ta temperature of 100 C was used to remove adsorbedwater and other impurit ies. After 12 h, the bed wascooled to 25 C an d h eld isother mally during the entirebreakthrough experiment. To ensure th at the detectorwas operating within a linear detection regime, calibra-

3500 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999


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tions were performed that showed that the concentra-tion and signal output were linearly related with R 2 g0.999.

The concentra tion of acetone in the inlet stream was1000 ppmv for all experiments. The concentration ofammonia used was 980 ppmv for all experiments. Oncethe data were collected, the signal was normalized witht he hi ghes t plat eau s ignal t o obt ain a nor m al izedconcentration curve (C/C0) and t he t i m e val ues w er enormalized by the bed weight.

3 . T h e o r y

In the homogeneous surface diffusion model,18,19 t h eintra par ticle ma terial bala nce for a cylindrical activatedcarbon fiber is

where Ds is the effective surface diffusion coefficient,q(z,t,R ) is th e adsorbate concentrat ion in the adsorbent,

z is the distance downstream from the inlet of the fixedbed, r is the radial position in the fiber, and t is time.

This a ssumes diffusion only in the radial direction,w hich i s a good as s um pt i on, s ince t he as pect r at i o>10 000 and STM images indicate that the material isuniform.

An adsorbate ma terial balance in the gas phase is

where C(z,t) is th e concentra tion of adsorbate in the bulkfluid, is the external void fraction, v is the interstitialfluid velocity, FA is the adsorbent density, and R is theradius of an activated carbon fiber. This balance as-sumes negligible a xial dispersion, consta nt velocitythr ough th e fixed bed, and n egligible film mass-tra nsfer

resistance. The validity of the last assumption wasconfirmed by compar ing n umerical pr edictions withexperimental data and has been found in many otheradsorption studies on a ctivated car bon.20,21 The constantvelocity assumption is valid because of the low adsorbateconcentration in the bulk fluid and the short length ofthe fixed bed.

The fixed bed is initially adsorbate free, which givesthe initial conditions

where L is the length of the fixed bed. The boundaryconditions are

an d

where the two parameters q0 a n d M are fit t o equilib-rium data, P(z,t) is th e part ial pressure of the a dsorbatein the gas phase, and P0 is the satu ration pressure. Thislast boundary condition is the Dubinin equation, whichrelates equilibrium adsorbate concentrations betweenthe adsorbent and the gas phase.28,29

The partial pressure P(z,t) is related to the concentra-tion in th e gas phase C(z,t) using the ideal gas law. The

coupled pa rt ial differential equ at ions were solved usingthe numerical method of l ines.30,31 A sparse Gearsm et hod w as us ed t o s ol ve t he r es ult i ng s yst em of ordinar y differential equ at ions (LSODES).32,33 For eachbreakthough curve, the one parameter, the effective

diffusion coefficient, was fit using nonlinear least squaresparameter estimation,34 with the optimization per-formed using the complex algorithm (DBCPOL 35).36,37

In a ll results reported here, the simulated breakt hroughcurve using the above model with the least squaresoptimal effective diffusion coefficient gave a good fit toexperimental data.

4 . R e s u l t s

4.1. C haract eri zati o n of A C Fs. Table 1 gives theelemental a nalysis results for the ACFs used in thefixed beds. ACFs r eceiving t he trea tmen t with nitric/sulfuric acid for 1 h are referred to as being oxidized.This treatment raised the oxygen content from 5 wt

% up to values as high as 28 wt %. The higher surfaceareas allowed more room for functional groups, resultingin an increasing amount of oxygen. Table 2 shows asignificant decrease in both the surface area and themicropore volume of the ACF samples after oxidation.This decrease in su rface area might seem to imply thatthe micropores are being blocked by oxygen groups onthe surface. However, the su rface ar ea decrease can beexplained by the additional weight of the oxygen groups.Since t he added functional groups are not part of thecarbona ceous microstru cture, th ey do not contr ibute t othe surface area, resulting in an artificial lowering oft he s ur f ace ar ea val ue. W hen t he s ur f ace ar eas ar enorma lized for t he a dded ma ss of th e functiona l groups,the surface areas do not change significant ly. The bulkdensity of each fiber is also reported in Table 2, whichis required for solving t he modeling equa tions.

The adsorption equilibrium parameters in eq 5 for thetwo adsorbates an d th e ACFs ar e reported in Ta bles 3and 4.

4 . 2 . B r e a k t h r o u g h C u r v e s . 4 . 2 . 1 . A c e t o n e . Th ebreakthrough curves are shown in Figures 1 and 2 forthe un trea ted a nd oxidized ACFs. Consistent with anearlier study which did not consider chemical tailoring, 38

the ACF with the largest surface area does not neces-sarily adsorb the most acetone. Both ACF-15 and ACF-25 benefited by the oxidation treatment through in-creased breakth rough times an d adsorption capacities.The polar molecule acetone is more attracted to the

T a b l e 1 . E l e m e n t a l A n a l y s i s f o r A c t i v a t e d C a r bo n F i b e r s

sa m ple %C %H %N %O

ACF -10 u n t r ea t ed 92.41 0.76 0.21 6.62ACF -10 oxidized 70.61 0.83 0.89 21.47ACF -15 u n t r ea t ed 95.23 0.24 0.20 4.33ACF -15 oxidized 69.20 1.89 0.95 25.39ACF -25 u n t r ea t ed 95.61 0.15 0.22 4.02ACF -25 oxidized 68.30 1.57 0.90 27.92

T a b l e 2 . P h y s i c a l P a r a m e t e r s f o r th e A c t i v a t e d C a r bo n

F i b e r s

sampleB.E.T.S.A

(m 2/g)micropore

volume (mL/g)bulk density

(g/cm3)

ACF -10 u n t r ea t ed 760 0.328 0.4226ACF -10 oxidized 557 0.235 0.4771ACF -15 u n t r ea t ed 1670 0.721 0.3161ACF -15 oxidized 1236 0.571 0.3533ACF -25 u n t r ea t ed 2158 0.864 0.2786ACF -25 oxidized 1496 0.562 0.3079q

t)

Ds

r

r(rq

r) (1)

C

t+ v

C

z+

2FADs(1 - )

R [q

r|r)R ) 0 (2)

q(z,0 ,r) ) C(z,0) ) 0, r [0, R ], z [0, L ] (3)

C(0,t) ) C0, tg 0 (4)

log10 q(z ,t,R ) ) log10 q0 - M log102

(P0/P(z,t)) (5 )

Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3501


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oxidized ACFs through hydrogen bonding and dipoleinteractions with the surface.9 However, the oxidationprocess was actually detrimental for the breakthroughtimes of ACF-10. The small pores (high overlap inpotential) of the u ntr eated ACF-10 alrea dy cause mostof the micropore volume to be utilized. The added weightof the oxygen groups only serves to decrease t he adsorp-tion capacity due to loss of micropore volume. At thisconcentration, the oxidized ACF-15 was the best forremoving acetone.

I n al l r es ul t s r epor t ed her e, t he s i m ul at ed br eak-through curve using the above model with the leastsqua res optimal effective diffusion coefficient gave a

fairly good fit t o experiment al da ta (see Figure 1 for arepresentative fit). The effective diffusion coefficientsfor the un trea ted a nd oxidized ACFs ar e reported inTable 5. The oxidized ACFs have smaller effectivediffusion coefficients tha n the unt reat ed ACFs. Thiscould be because adsorption utilizing chemical func-tional groups requires a more site specific mechan ismthan if it was pure physical adsorption (van der Waals);that is , the contaminants may not be effectively ad-sorbed until they locate a group to bindwith. Anotherpossibility is t ha t the addition of so man y groups onto

the pore walls could result in constriction; that is, thepores are effectively smaller. Also, in the case of surfacediffusion, the presence of numerous chemical groupscould int erfere with the movement of molecules a longthe pore walls. The oxidized ACFs have increasedbreakthrough t imes in spite of the reduced effectivediffusion coefficients. The improvement in equilibriumadsorption capacities caused by the oxidation processmore than compensates for the reduction in effectivediffus ion coefficients .

This data complements the study39 which consider edthe adsorption of acetone on an ACF in a similar fixedbed configura tion. Lordgooei et al.39 concluded that themain reason an ACF gave longer breakthr ough t imesthan a packed bed of ACGs was the ACFs greateradsorption capacity. Although the results suggestedsomewhat faster mass-tran sfer kinetics for the ACFt han t he A C G , t hi s w as not t he dom i nant f act or i ncausing the greatly different breakthrough times. Ad-ditiona l results of this study over ref 39 were th e use ofa homogeneous surface diffusion model to characterizethe adsorption kinetics, and the experiments and analy-sis of how chemical tailoring a ffects the breakt hroughtimes, adsorption equilibria, an d a dsorption k inetics.

4.2.2. Ammon ia. The breakthrough curves are shownin Figures 3 a nd 4. The u ntr eated ACFs do not adsorban appreciable amount of ammonia, thus giving veryshort breakthrough curves. ACF-10 h as the highestadsorption capacity du e to its smaller pore size, which

T a b l e 3 . A d s o r p t i o n E q u i l i br i u m P a r a m e t e r s f o r Ac e t o n e

sampleq0 (mg of adsorbate/

g of adsorbate) M

ACF -10 u n t r ea t ed 2.6428 0.03774ACF -10 oxidized 2.5057 0.02891ACF -15 u n t r ea t ed 2.9861 0.06732ACF -15 oxidized 2.8393 0.04515ACF -25 u n t r ea t ed 2.9563 0.07529ACF -25 oxidized 2.8550 0.05040

T a b l e 4 . A d s o r p t i o n E q u i l i b r i u m P a r a m e t e r s f o rA m m o n i a

sampleq0 (mg of adsorbate/

g of adsorbate) M

ACF -10 u n t r ea t ed 0.9711 0.08533ACF -10 oxidized 1.6080 0.02577ACF -15 u n t r ea t ed 0.8480 0.08533ACF -15 oxidized 1.8207 0.02988ACF -25 u n t r ea t ed 0.7853 0.08533ACF -25 oxidized 1.8903 0.03456

F i g ur e 1 . Normalized breakt hrough curves for acetone onunt reat ed ACFs.

F i g ur e 2 . Normalized breakt hrough curves for acetone onoxidized ACFs.

T a b l e 5 . E f fe c t i v e D i f f u s i o n C o e f f i c i e n t s f o r A d s o r p t i o n

o f A c e t o n e

fiber t ype 1014Ds (m 2/s)

ACF -10 u n t r ea t ed 2.8ACF -10 oxidized 2.3ACF -15 u n t r ea t ed 3.9ACF -15 oxidized 2.9ACF -25 u n t r ea t ed 4.0ACF -25 oxidized 2.4

F i g u r e 3 . Normalized breakthrough curves for a mmonia onunt reat ed ACFs.



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is m ore effective for r emoval of gases. Following oxida-tion treatment, the breakthrough t imes increase tre-mendously, from 20 to 800 min/g in the case of ACF-15. This improvement is due to th e interaction of thebasic ammonia molecule with the acidic functionalgroups. After oxidation, the most effective fiber is ACF-25. From Table 1, oxidized ACF-25 has the highestamount of oxygen, followed closely by oxidized ACF-15an d th en by oxidized ACF-10. As this sam e tren d is seenin the breakthrough curves (Figure 4), it appears thatt he ads or pt ion capaci t y s cales w it h t he am ount of

oxygen present.The effective diffusion coefficients for the untreated

an d oxidized ACFs ar e report ed in Ta ble 6. Again, th eoxidized ACFs ha ve sma ller effective diffusion coef-ficients than the untr eated ACFs. As was found foracetone, th e improvement in equilibrium adsorptioncapacities caused by the oxidation process more thancompensates for the reduction in the effective diffusioncoefficients, resulting in increased breakthrough times.

The results qualitatively agree with an earlier paperwhich studied the adsorption of ammonia and amineson ACFs impregnated by t ransit ion metal halides.40

From a careful consideration of the batch adsorptiondata in Figure 3-17 of ref 40, it can be determined t hat

the impregnation of an ACF with CuCl2 slowed theadsorption of a mmonia while greatly increasing t heACFs adsorpt ion capa city. Quan tita tively, th e impreg-nat ion process increased th e a dsorption capacity of theACF by approximately a factor of 10 compared to theH NO3/H 2SO 4 process which increased the adsorptioncapacity by a factor of 15-30, depending on t he su rfacearea of the ACF.

5 . C o n c l u s i o n s

The tailoring of pore surface chemistry was shown tobe an effect i ve m et hod for i ncr eas ing t he r em ovalefficiencies of acetone and ammonia under a fixed bedconfigur at ion. F or acetone, the oxidized ACF-15 ha d t he

right combina tion of pore size and p ore volume t o makeit the most effective adsorbent. Adsorption of ammoniawas poor on unt reat ed ACFs. The oxidation t reat mentresulted in a tremendous increase in the adsorptioncapacities and breakthrough times, which scaled withthe amount of oxygen groups present. In both cases, theeffective diffusion coefficients were reduced by theoxidation process, but the improvement in equilibriumadsorption capacities more th an compensat ed, to resultin th e overall improved brea kth rough t imes. The fixedbed system ga ve steep concent ra tion profiles consistentwith a lack of channeling.

This appear s to be the first study which constr ucts aphenomenological m odel for a fixed bed of activatedcarbon fibers. The phenomenological model was usedto chara cterize h ow the oxidation process a ffected theadsorption kinetics. For a ll ACFs r eported here, t hesimulated breakthrough curve using the homogeneoussurface diffusion model with the least squa res optimaleffective diffusion coefficient gave a good fit to experi-mental data. As mentioned in section 3, including anexternal mass-transfer coefficient in the model did notsignificantly improve th e fit to experimenta l dat a, whichis consistent with m an y adsorption studies for a ctivat ed

carbon granules. However, this assumption would haveto be re-evaluated during the design of a production-scale fixed bed ads orber u sing ACFs.

A c k n o w l e d g m e n t

The work by the first a nd th ird aut hors was supportedby the National Science Foundation under Award No.DMR-9208545. The second aut hor acknowledges th esupport of the DuPont Young Faculty Award.

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F i g u r e 4 . Normalized breakthr ough curves for a mmonia onoxidized ACFs.

T a b l e 6 . E f fe c t i v e D i f f u s i o n C o e f f i c i e n t s f o r A d s o r p t io n

o f A m m o n i a

fiber type D (m 2/s)

ACF -10 u n t r ea t ed 7.5 10-13

ACF -10 oxidized 6.0 10-14

ACF -15 u n t r ea t ed 7.7 10-13

ACF -15 oxidized 5.4 10-14ACF -25 u n t r ea t ed 5.5 10-13

ACF -25 oxidized 4.1 10-14

Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3503


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Received for review January 27, 1999Revised m anu script received Ju ne 22, 1999

Accepted Ju ne 29, 1999

IE990064M


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29 Fixed Bed Adsorption of Acetone and Ammonia Onto Oxidized Activated Carbon Fibers