methylene chloride adsorption reference book · 2018-06-13 · the bet equation was used to...

North Carolina State University Department of Chemical Engineering

Spring, 1997

Methylene Chloride Adsorption Reference Book

CHE 45 1 Group Number 2 Matt Bobo

Paige Langenbach John Chambard Kurt Loughlin

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References

I ) Tsai, W.T., Chang, C.Y. (1994). “Adsorption of Methylene Chloride Vapor on Activated Carbons”. Journal of Chemical Technology and Biotechnology, v61, pp. 145-15.1.

2) Fang, C.S., Khor, S.L. (1989). “Reduction of Volatile Organic Compounds in Aqueous Solutions Through Air Stripping and Gas-Phase Carbon Adsorption”. Environmental Progress, v8, n4, pp.270-278.

3) Eissman, R.N., LeVan, M.D. (1993). “Coadsorption of Organic Compounds and Water Vapor on BPL Activated Carbon. 2. 1,1,2-Trichloro- 1,2,2-trifluoroethane and Dichloromethane”. Industrial Chemical Engineering Research, v32, pp. 2752-2157.

4) Quinones, I., Guiochon, G. (1996). “Derivation and Application of a Jovanovic- Freudlich Isotherm Model for Single Component Adsorption on Heterogeneous Surfaces”. Joumal of Colloid and Interface Science, v183, pp.57-61.

5) Parmele, C.S., O’Connell, W.L., Basdekis, H.S. (1979). “Gas phase adsorption cuts pollution, recovers solvents”. Chemical Engineering, December 3 1, pp.58-70.

6) Gong, R., Keener, T.C. (1993). “A Qualitative Analysis of the Effects of Water Vapor on Multi-Component Vapor-Phase Carbon Adsorption”. Journal of the Air Waste Management Association, v43, pp.864-872.

7) Chambard, J., Lehmden, T. (1996). “Engineering Study on Carbon Adsorption Unit”. GlaxoWellcome Company memorandum.

8) Brandett, G.W. (1987). Handbook of Dehumidification Technology. Butterwerths pp. 165-175.

9) Gardner, A. William. (1971) Industrial Drving. Leonard Hill pp. 285-295.

10) Walas, Stanley. (1987) “Rules of Thumb” Chemical Ene-. pp. 75-79

11) Ulrich, Gael D. (1984) A Guide to Chemical Engineering Process Desien and Economics. pp.120,431.

12) Schweiger’, Thomas A.J. Levan, M. Douglas. (1993) “Steam Regeneration of Solvent Adsorbers” Industrial Engineering Chemical Research. vol. 32 pp. 2418- 2429.

13) Schweige? , Thomas A.J. (1995) “Effects of Water Residues on Solvent Adsorption Cycles” Industrial Engineering Chemical Research. vol. 34 pp. 283-287.

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14) Boppart, Stephen. (1995) “Get the Most From Activated-Carbon Systems” E n v i r o n m e n t a l . May-June pp. 12-14.

29

Reference Summary

J%L '..a2 -__ h4

7 u p u n d IuJ,&U

N/R - not reported N/A - not applicable

Adsorption of Methylene Chloride Vapor on Activated Carbons \\/cn-Tien Tsai & Ching-Yuan Chang* (;r;ldu;ite Insti tute of Envi ronmenta l Engineering, Na t iona l Taiwan University, 71 Chou-Shan Road. '1;iipei 106. Taiwan

,<:.<L,ivcd 17 January 1994; accepted II April 1994)

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Abstract: A laboratory investigation on the adsorption of hazardous methylene chloride (METH) vapor on the commercial activated carbons BPL and PCB, which were made from bituminous coal and coconut shell. respectively, was conducted at 283, 293. 303, and 313K. The physical properties and surface functional groups of the two activated carbons were also measured and compared with each other. The experimental results indicate that the adsorption capacity of carbon PCB is slightly higher than that 01 carbon BPL. It was found that the Langmuir, Freundlich, and Dubinin-Radushkevich adsorption equations were well fitted by the measured adsorption data. The values of the parameters of the adsorption equations were determined for the two adsorbents. The physical properties (e.& micropore volume) of the adsorbents are consistent with the parameters obtained from the adsorption results.

Key words: adsorption isotherm. methylene chloride, granular activated carbon.

N 0 TAT ION

:\diorption potential (J mol-') :\<Isorbate concentration (mol m-') .Adsorbate liquid density (g cm-') 1';irameter of Freundlich adsorption isotherm [mol kg- ' (m' mol-')''"] Adsorption equilibrium constant of Langmuir isotherm (m' mol-') Molecular weight of adsorbate (g mol- ' ) :?dsorption intensity defined by Freundlich isotherm (dimensionless) Iltfractive index of adsorbate liquid (dimensionless) Equilibrium pressure of adsorbate vapor (Pa) Eicc:ronic polarization of adsorbate (dimensionlcss) Saturated vapor pressure or adsorbate (Pa) Adsorption capacity (mol k g - ' ) itdsorption capacity at equilibrium (mol kg- ' )

remperature ( K ) -\dsorption capacity (cm' g- '1 Active pore volume (cm' g- ' )

pllfinity coefficient (dimensionlas) StrKtural constant (dimcnsionlcss)

Gas constant ( = & 3 l 4 J mol- ' K - ' 1

I INTRODUCTION

Methylene chloride (METH), also known as dichloromethane, is a colorless, volatile non-flammable liquid with a penetrating, ether-like odor.' Its vapor density is three times that of air. The compound is an sicellent solvent and is especially attractive as a cleanins solvent because of its emectiveness and non-flammability. I t has been widely used as a blowing agent of polyurethane (PU) flexible foam and as a solvent for many applications. including photoresist stripping, aerosol formulations. and to a large extent in paint stripping formulations. I t \vas also formerly used as an extraction solvent in food and pharmaceutical processing where its high volarilit! is desirable.' The annual consumption of this soI\~enI has exceeded 500000 metric tons world-wide,' and a v e r 3 ~ e s 8000 metric tons in Taiwan."

The current American Conference of Governmsnral Industrial Hygienists (ACGIH) threshold limit v3luc (TLV) for METH is SO ppmv as an 8 h time-ueishtsd average (TWA).' Repeated contact with METH may result in dermatitis. The liquid and vapor of METH are irritating to the eyes. skin, and upper respirator). tract. Symptoms of exposure .include Iieadaclis.~~dirrinsjs. nause;~. giddiness, stupor, irritability, and tinelin: in thc limbs:" METH is-rcndily takcn-up -through inh:iixtion

. .

.i W -T. T.$ai, C: Y. Chany I! 4

J iilso absorbed to ii considcrable extent through the 11. Siiicc 1985 the eniphasls of METH toxicity has itered around its carcinogenic eFTects.' The ACGIH i classified it as a suspected human carcinogen (Group ). Bccausc of the toxicity of METH, it has been listed onc of the hazardous air pollutants or air toxics in US Clean Air Act Amendments ( C A A A ) of 1990, and

:of tlic toxic and priority pollutants in many environ- ntal and workplace regulations.' kco rd ing to the US Environmental Protection mcy's (US EPA) Toxic Release Inventory (TRI), ZTH emissions are the largest source of carinogenic ,anics in the atmosphere. During 1990, about 46 142 tric tons of the compound were released to the air.9 der Title I l l of the CAAA, the organic vapors are to be ulatcd. requiring their emission sources to install ximum achievable control technologies (MACTs). rrent MACTs for organic vapors are condensation, 'neration, carbon adsorption, and liquid absorption. these technologies, carbon adsorption is a very

imon one because it oRers some advantages over the ers. The advantages include the possibility of the >very of raw materials or pure products for recycling, high removal efficiency (i.e. ~ 9 5 % ) at low inlet

centrations, and the low fuel/energy costs.lO.ll n this study, the adsorption isotherms of two different vated carbons and METH vapor were investigated. : applicability of various common adsorption iso- -ms was tested. In addition, this paper describes the racterization ofactivated carbons by means of various ameters such as BET surface area, pore volume, pore

distribution, and surface functional groups. The tionships between the observed values of the adsorp-

I isotherm parameters and the physical/chemical perties of activated carbons are also discussed.

2 EXPERIMENTAL

Materials

> dinerent kinds of the commercial granular activated )oris (GACs) made from bituminous coal (BPL) and )nut shell (PCB) wi th a particle size of I2 x 30 mesh supplied by Calgon Carbon Co. (Pittsburgh, USA)

:used in thisstudy. Theactivnted carbons were sieved iesh ranges of 20 x 30 (average particle diameter of 8 mm), and dried at 393'K for a t least I O li bcforc g uscd in the experiments. METH, supplied by linckrodt Co. (Kcntucky, USA) was over 99.9% pure was uscd without any further purific;ition.

Surface characterization measurements

surfzcc charactcrization~of carbons BI'L and I'CB mc;isurcd by using precise instrumentid tocliniqiies

I h C IpilrticIc 'and true densities: UET surhcc t i roa,

pore volume and surface functional groups. The particle density was measured by a mercury displacement method using a mercury porosimeter (Autopore I I 9200; Micro- meritics, lnc.. GA. USA).The truedensity wasdetermined by a helium displacement method using a pycnometer (AccuPyc 1330; Micromeritics, Inc., GA, USA). There- fore. the particle porosity can be computed from the particle density and true density.12

The BET surface area, total pore volume and micropore volume of GACs were determined by nitrogen adsorption/desorption apparatus ASAP 2000 (Micro- meritics, Inc.,GA, USA), usingacontinuousflow method. The BET equation was used to calculate the surface area of G A G . The Kelvin equation and t-plot method were used to calculate the total pore volume and micropore volume, rcspectively.''

I n order to obtain the appropriate information of surface functional groups from the Fourier transform infrared (FTIR), spectroscopy analysis using a Bomen DA3.02 system (Bomen Inc., Quebec, Canada), the spectra were measured with KBr discs containing only about 0.05-0 I wt% of the fine, ground activated carbons. The FTIR instrument was purged to minimize the inter- ference of the air during the measurement of scanning. The spectra were obtained with 2048 scans at 4 cm-l resolution. The FTIR spectroscopy measurements were also used to confirm the functional groups on the surface of the GACs observed by the X-ray photoelectron spectroscopy (XPS) measurements. The XPS experiments were performed with a VG ESCALAB-210 (UK) system using a MgK, X-ray source in a vacuum of mbar. Samples were prepared for XPS analysis by grinding milligram quantities of the carbons, and their carbon and oaygen contents determined. The carbon Is photoelectron peak, which was used as a reference for the chemical shift, was assumed to have a binding energy (BE) value of 254-6 eV.

2.3 Adsorption apparatus and methods

The apparatus used for the measurement of adsorption isotherms is shown in Fig. I . The dried (i 3% relative humidity) and purified air was metered by using two rotiimeters with needle valves. The constant flowrate of gas through ;in activated carbon column of 1.5 cm i.d. resulted i n ii linear velocity of about 0-21 m s - ' . For all exporinicnts. the mass of tlic activated carbon was about 2-4 s. I t w a s packed in a carbon bed to a height of 2-S . . 5 6 cm. The adsorption temperatures. performed at 28.;. 293, 303. ;ind 3 13 K, were maintained by a refrigerated circulating constant beth ( k 0 . I K accuracy). The experi- inii'nts wcre conductcd iit inlet concentrations of METH vapors r:ingingfroin4.16 x lO-'to 5.32 x 10.' mol m-' (k 10020(x) ppmv).

Small amounts of the clllucnt stis of METHiai r were wiihdr;iwii into ii gas-tight micro-syringe and measured I)!' gxs i.liroiii:it~~gr:~pliy (GC. Hcwlett-I'nckard 5890

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Arlsorp!ioii of mrtlrjilme chloride uupor on acriuated carbons I47

Fig. 1. Schematic diagram of experimental apparatus. I. Gas pump; 2. si l ica gel column; 3, carbon column; 4, particulate filter; 5. needle valve; 6, rotameter; 7, nozzle; 8. glass bed mixer; 9, temperature/relative humidity ( R H ) detector; IO, sampling port (to GC);

I I, air cylinder; 12, M E T H vapor generator; 13. constant temperature bath; 14, adsorption column; 15, three-way valve.

Series 11). A 20.32 cm x 0-3175 cm (8' x 1/8") stainless steel (SS) column packed with 1% SP-1000 on 60/80 Carbopak B (Supelco Co., USA) was used with the flame ionizat ion detector (FID) for the analysis o f the sample. When the effluent concentration oftheadsorbate reached the influent concentration, the concentration measurement of the effluent gas was stopped. The gas flow through the activated carbon column was continued for an addi t ional 30-60 m in to ensure that adsorption equi l ibr ium was reached. The sample tube was then removed from the experimental system, wiped, and the exterior dried. The adsorbed amount o f the METH vapor in equi l ibr ium with the inlet concentrat ionat thespecified adsorption temperature was determined from the mass increase of the carbon bed.

3 RESULTS AND DISCUSSION

3.1 Stirface characterization of GACs

Table I lists the main physical properties of the activated carbons uscd a s adsorbents i n this study. The surlace :trca values of 102s and 1012 m' E - ' were obtained by nitrogcn adsorption using tlie BET equiit ion for carbons I3PL i i n d PCB. respectively. The micropore volume of cxrbon PCB (0.41 cnn' g - ' ) is greater th:in that 0fc;trbon I3PL ( 0 2 9 ciii' f ' ) atid t h u s the micropore surface area ofc:irbon PCB (Si.3 in: g - ' ) i s Iiirger than tha t ofcarbon IjI'L (621 i l l L : - I ) . B u t the toti i l pore volume and porosity o f carhoii B P L ( 0 . 6 0 c m ' g - ' :tiid 6P;J arc larger t l i iui those olc.arhon B P L (0.52 ciii.' :- and 60%). I.'roiii tlic nitrogen :idsorption--dcsorptioin isotl icrins tinc;isiircd ;it 77 K. i t wns ohrervcd th:it t l ic iictivittetl c:irhoiis e:zliihitcd tylic 1% hysteresis loops . b ; i s d on t l ic ilcnocr c la~s i f ic i i lk i i i . ' ' This w x s ;in iiidic:ition of thc

TABLE I Physical Properties ofCommercial Activated Carbons BPLand

PCB Used in this Study

Physical properties" B P L PCB

Total surlace areab (N2, BET method) 1028 I012 (m2 9 - l )

Micropore surface areab ( m ' g - ' ) 62 I 853 Total pore volumeb (cm'g- '1 0.59XX 0.5175 Micropore volumeb (cm' g - ' ) 0.2855 0.4069 Particle density (Hg displacement 0-7949 08776

True density (He displacement method) 2-2350 2.2178

Porosity' 0.6443 0.6043

" A t least two measurements for each sample. 'Measured lor surface areas and pore volumes of pores (total pores as well as micropores ( < 2 0 nm)) in the range 01 1-5-350 nm because of instrumental limitation. ' Computed from particle density and true density.

method) (g cm-')

(gem-')

wide mouth- l ike shape o f the pore as well as of the considerable micropore volume. The pore size distr i- butions obtained for the activated carbons reveal one maximum :it ii pore diameter of about 2.0nm. This supports the fact t l i i i t most o f the pore volume arises from pores wi th porc diameters smaller than 2.5 nm and tliiit the micropores arc responsible for a large propor t ion or the surface area.

Thc F T I R spectra of carbons B P L and PCB are given i t i Fig. 2. Absorption bands are observed in the regions 1760- I71(1. 1640- 1620, 1470- 1450, 1400- 1350, 1290- 1260. i tnd I180-1000cm~!. Most o f thess.bands~haus i t l s o heen rcported by other investigators for dill'crcnt mrhm ~iiolerials. ' l~l '~~"J The most significant absorption . .

18 W.-T. Tsoi, C: Y. Cliang

2500 660 WAVENUMBER (cm-')

Fig. 2. FTIR spectra or activated carbons BPL and PCB.

I I !9L 292 290 288 286 28b 282 280 278 276 2 X

Binding Energy (eV)

g. 3. Carbon I s chemical shift data in XPS spectra: (a) carbon PCB, (b) carbon BPL.

md occurs in the I 180- 1000 cm ~ region." This region characteristic o f the C-0 stretching vibrations, and ost probably arises from the phenolic and/or carboxylic 'uctures. Figure 3 compares the XPS spectra of carbons B P L .d PCB. The C I s spectra have asymmetric peak shapes owing l ong tails at the higher binding energy sides tl ic ma in peak. Three peaks are observed, which occur shifts 010.9-1.7, 2.9-3.1. and 5-0-5-1 eV from the main a k . These may be attributed to the C-0 (phenolic/ .droxyl), C=O (carbonyl), and COOH (carboxylic) nctionalities, respectively."-" I t can be seen that the ak assigned to C-0 For the carbon B P L occurs at il

if1 vitlue slightly higher than that for the carbon PCB.

! Adsorption isotherms

IC adsorption or M E T H vapor o n GACs :it \'ariotis nperatures may be l i t t rd by t l ie linear rorm of t l i e Ingniuir . Freundl ich. and Duhinin-Il;iduslii,cvich I~-R) The isotherm equations arc givcn low. Thc IL;inginuir isotl icrni c:in hc rcprescntid l ) ? ~

01 I 0 20 40 60 80 100 120 140 16.3

1IC (m3 mol-') Fig. 4. Langmuir plots of METH adsorption on carbon BPL

at various temperatures.

-203 K +293 K *303 K -313 K

I 0 20 40 60 80 100 120 140 160

1IC (m3 molt')

Fig. 5. Langmuir plots of METH adsorption on carbon PCB at various temperatures.

where q i s the amount adsorbed per uni t mass of the adsorbent, i.e. adsorption capacity. qm i s the adsorption capacity at monolayer saturation, C i s the adsorbate equi l ibr ium concentration, and K i s the adsorption equi l ibr ium constant. Figures 4 and 5 present the Langmuir isotherm fits ( l / q vs IIC) of METH to the measured adsorption data for carbons B P L and PCB, respectively. The Langmuir isotherm appears to fit the data reasonably well. A least square method has been used to give the Langmuir isotherm parameters of qm and K ofcarbons B P L a n d PCB. Theadsorpt ion capacity of carbon PCB is seen to be slightly higher than that of carbon B P L (Table 2). The values or K dccrease wi th increasing temperature, indicating that the adsorbents have a higher adsorption af in i ty at lower temperatures.

The Freundlich isotherm was used in the Form

In q = In k + ( I j r i ) In C ( 2 )

where k and 11 are empirical constants. and q mid C are :IS prcviously deiincd. In gcncriil. 11 has ii v:tluc greater than unity, mid 1/11 represents the intensity factor of the ;idsorptioii.~A~liighcr~valiic of k incrciiscs t l ic edsorption ~- c:ipacity of the adsorbent.'" Figiircs 6 i i n d 7 prcsciit the

Adsorptioii oJ methylei ie chloride vapor on ocrivored curbons I49

1.5

TABLE 2 Langmuir Piirameters lor Adsorption of METH on Activated

Carbons iit Various Temperatures

Adsor.henl ,Irl.snrpl iori Loiigmlcir pnrot,!eiers r l Ie.rtipernfurc -~

( K ) %,, K (llto/k(/-’) (,n’l1l,1l-’~

f-283K *293K +303K *313K -

BPL 283 3.208 293 2.875 303 2.7 I4 313 2504

PCB 283 4406 293 3.826 303 3.920 313 3.566

83.39 0.966 56.67 0.976 3888 0.976 21.63 0.992

83.08 0954 58.85 0.940 3309 0.994 19.44 0.955

1 -

U c -

-5.5 -5 -4.5 -4 -3.5 -3 -2.5 -2

In C Fig. 6. Freundlich plots of M E T H adsorption on carbon BPL

at various temperatures.

2 f- 283 K +293 K * 303 K -313 K

1.5-

(5

C -

TABLE 3 I:rcundlich Par:inielrrs for Adsorption 01 METH on Activated

Carbons at Various Temperatures

BPL 283 293 303 313

PCB 283 293 303 313

8-503 8.002 7,175 9.479

11,692 11.948 10-598 10-371

2.515 0.998 2.250 0-99 I 2. I 34 0.993 1.504 0999

2,547 0-9 9 5 2,137 0-982 2.012 0997 1-668 0.970

straight l ines were obtained. The Freundlich parameters, k and n, were computed and are given in Table 3. I t i s seen that the value of n decreases with increasing temperature. The values o f the exponent I I were in the range of 1-3, indicating slightly Favorable adsorption.” Also, the values of I, of carbon PCB are larger than those ofcarbon B P L a t thesame temperature. This isconsistent wi th the results based on the Langmuir analysis. i.e. that carbon PCB has slightly higher capacity and favorability for METH vapor than carbon BPL.

According to the Dub in in -Po lany i theory.” the adsorption isotherm of vapor can be represented b! the following D-R equation:

( 3 )

A = RTln(P,/P) (-1)

mass of carbon, LV, i s the active pore volume oi the

is a n affinity coeficienr which permits comparison oi the

In w = In W, - ti(Aifl)’

where CV is the amount of adsorbate adsorbed per uni t

carbon, ti i s a constant related to the pore structiire. /!

adsorption potential of the test adsorbate wi th that of a reference adsorbate, R is the gas constant. T i s t l i c absolute temperature. Po i s the saturation vapor p x j s i i r c at T, P i s the adsorbate equi l ibr ium pressurc. m d ~I i s defined as adsorption potential. The affinity coef?iiisnt /i for the adsorption of an adsorbate on act iwted a3rbon may be approximated by the rat io of the ds i t ro i i i c polarization (P,) of t l ie adsorbate [P, (vapor)] t~ ili.it of the reference adsorbate (i.e. benzene) [ I ; (ref)] iLv pol:lr organic adsorbatcs better t11:1n by t l ie inoliir \oii:mc o r p;tr;tclior mcthods.’” The approximnt ion Bites

.~

L

-

Adsorption I ? / iiiethylene chloride uapor on activated carboils 151

7. Agcncy forToxicSubstances;ind Discase Rcgistry (ATSDR), To.~iroloqicoI I’ro/ile.fiir Meiltylette Chloride. At lmta. USA, 1989.

8. Kokoszk:i. L.C. & Flood, J. W.. Enuironmeninl M~utmgcrncni Ilsrdbook - ~ T o . ~ i c Clieniical Moreriols and Wasies. Marccl Dekkcr, Ncw York. NY. USA, 1989.

9. Dcmpscy. C. R.. A comparison of organic emissions From hazardous wastc incinerators versus the 1990 toxic rclcasc invenlory air releases. J . Air Wusie Manaye. Assoc.. 43 (1993) 1374-9.

I O . Ruddy, E. N. &Carroll. L. A,, Select the best VOC control strategy. Chetn. Eng. Proyras, 89(7) (1993) 28-35.

I I . Spivey. J. I., Recovery of volatile organics from small industrial sources. Ettuiroi~tne~iial Proyress, l (1988) 31 -40.

I?. Lowcll, S. & Shields. J . E.. Powder SurJace Aren and I ’ormiiy, 3rd edn. Chapman and Hall, London, UK. 1989.

11. Gregg. S. J . & Sing, K. S. W.. Adsorption, SurJace Area am1 Purosiiy. 2nd edn. Academic Press, London, UK. 1982.

14. Ishizaki. C. & Marti. I., Surface oxide structures on a commerciiil activated carbon. Carbon, 19 (1981) 409-12.

I S . OReilly. J. M. & Mosher. R. A., Functional groups in carbon black by FTlR spectroscopy. Carbon, 21 (1983) 47-51.

16. Richardson. J. T., Principles oJ Caialysi Deuelopniem. Plenum Press, New York, NY, USA, 1989.

17. Friedel. R. A. & Hofer, L. J. E., Spectral characterization of activated carbon. J. Phys. Chem., 14 (1970) 2921-2.

18. Friedel. R. A. & Carlson. G. L., DiRerent carbonaceous materials and their infra-red and Raman spectra: reassign- ments lor coal spectra. Fuel. 51 (1972) 194-8.

19. Meldrum. B.J. & Rochester. C. H., I n situ infrared study of the surface oxidation of activated carbon in oxygen and carbon dioxide. J . Chem. Soc. Faraday Trnns.. 86 (1990) 861-5.

20. Coltliiip. N. B. & Daly. L. H., Iniroduciim to 1,frarp.d o d I<ontori Spcciroscopy, 3rd edn. Academic Press. San Diepo. CA. USA, 1990.

?I. Ishitani, A..Application of X-ray photoelectron spectroscopy to surface analysis of carbon fber. Cnrbon, 19 (1951) 269-75.

22. Takahagi.T.& Ishitani;A.,XPSstudies by use ofthedi-tal diflercncc spectrum technique of functional groups on rhe surface of carbon fiber. Carbon. 11 (1984) 43-6.

23. Bradley. P. H.. Ling, X. & Sutherland. I., An investigation of carbon fiber surface chemistry and reactivity baed on XPS and surface free energy. Carbon, 31 (1993) I 115-20.

Process. John Wiley. New York, NY, USA, 1984. 25. Suzuki, M., Adsorpiion Engineering. Elsevier, Amsterdam.

The Netherlands, 1990. 26. Abo-Elela, S. 1. & El-Dib, M. A,, Color remosal \ ia

adsorption wood shaving. Sci. Total Enuiroti.. 66 119S7) 269-73.

27. Treybal, R. E., Mass TransJer Operaiions. 2nd edn. McGraw-Hill, New York. NY, USA, 1980.

28. Dubinin, M. M., Porous structure and adsorption propnies of active carbons. In Chemistry and Physics o/ Carbon. ed P. N. Walker Marcel Dekker, New York, NY, USA. 1966.

29. Reucroft, P. J., Simpson. W. H. & Jonas, L. A., Sorption properties of activated carbons. J. Phys. Chem.. 15 (1971) 3526-31.

30. Trout, D.. Breysse, P. N., Hall, T., Corn, M. & Risb!. T.. Determination of organic vapor respiratory cartridge variability in terms of degree of activation of the carbon and cartridge packing density. Ani. lid Hyg. dssoc. J . . 41 (1986) 491-6.

24. Ruthven, D. M.. Principles of Adsorption and Adsorpiion -

-

Reduction of Volatile Organic Compounds in Aqueous Solutions Through Air Stripping

and Gas-Phase Carbon Adsorption ,:: L.

C. S. Fang and Sok-Leng Khor

Aqueous solution of volatile organic compounds (VOC's) is a waste stream of many chemical process industries. Conventionally,

it is treated with a costly liquid-phase activated carbon adsorption process. A n alternative process is studied, which is the combination of air stripping

and gas-phase activated carbon adsorption. The removal of VOC's from waste-water can be as high as 99.8%, using a packed column for air stripping at room temperature. These VOC's include vinyl chloride, carbon tetrachloride, trichlorethylene, 1,l-dichloroethane, toluene,

chloroform, 1,I,I-trichloroethane, benzene and xylene. These VOC's are separated from air and recovered through a carbon adsorption-

regeneration cycle. The cost of VOC removal and recovery is in the range from $0.457 to $0.899 per 1000 liters.

eleven VOCs which represented 79.5% of the i organic wastes. They are listed in Table 1. Also the are five v°C's reported by Other

tries. Fourteen Of these sixteen "OCS listed in

Dilute aqueous solution of volatile organic compounds

dustries, and often is a serious environmental concern. Conventionally,,it is treated with a liquid-phase activated are priority pollutants. carbon adsorption. process. The process is reportedly effective in removing VOCs from water [I], but its cost is high. The combination ofair stripping and gas-phase activated carbon is an alternative process, and it appears to be efficient and cost-effective. Before any pilot-plant test, a preliminaiy design study w a s conducted to examine the technology and its cost.. The results of the study are pre-

(VOC'~) is g stream of ma,ly chemical process in- present in waste streams ofmany chemical v o c e s

HENRY'S LAW CONSTANT

Henry's law describes the distribution of a com in the gas and liquid phases at equilibrium CO Henw's law, in cO"onlY used units, is:

IC COMPOUNDS

. , . . .

L

Others:

Ik,,Ze"e* Clethyl chloride' Ilichloromethane* \crylonitrile*

Cyclohexane

15.513 13,355 2.395 1.775

823 67 61 5.5

Relative Volatility

25°C

125.0 3.353 4.843

~ ~ .. 3.150 9.650 1.207 3.825 R 9S7

3.8 9.751 3.0 11.51 2.5 5.650 34.006 (total organic wastes identifcd: 42,774)

4.037 179.3 18.15 4.487 4.139

%PA pritirify pdutnnir .

It also can be written in other terms. For example, if

y, = m xi (2)

PROCESS DESCRIPTION mole fraction is used,

Figure 1 shows the process flow diagram o f the process to be shldied. The~process includes two parts: (1) air stripping operation, and (2) gas-phase activated carbon adsorption, including spent carbon regeneration. Equip- merit used in the air shipping includes a feed pump, air stripping column, blower and air filter.

The VOC laden ai: from the air stripping column is fed to one of two carbon adsorption columns to separate VOC's from air. Air after adsorption is vented to the atmosphere, while the VOC's are captured and collected in the adsorption column. When the concentration ofVOC's in air reaches the break-through point, the feed is switched to the other column which has regenerated carbon. Spent carbon is regenerated either by hot air, electrical-resistance heater, or microwave radiation.

y, = mole fraction of the component i in gas-phase xi = mole fraction of the component in liquid-phase m = Henv's law constant, dimensionless Henry's law constant is a good indication whether a

VOC can be removed from an aqueous solution by air stripping. Henry's law constants of eighteen VOC's are shown in Table 2. These values were reported by Thi- bodeaux [31. Dilling [41, and Mackay [5].

Henry's. law constant for methyl ethyl ketone was reported to be 2.97 X m3 ahnlg-mole, which appears to be too low. When this value is used in design calculations, the results are, less than reliable.

. ~~~

TABLE 2. HENRY'S L%w CONSTANT, 25°C L

Henry's Law Constant

Hc, n1J-Rh/g-lll"le m H,*. atni

3,175

0.0286 185

0.00577 58.1

12.4

0.0117 88.8 Trichloroethylene '

'(0.409) 32.2 2.0 3.5

6.8 ,224

November,1989 : 271

Figure I . Process flow diagram

AIR STRIPPING

Air stripping of an aqueous solution is i i well e s t a b lislied process. llecently, i t has been used tu remove various VOC's from the drinking water supply in a number of communities [6, 7,8]. Basically, i t is an operation providing inter-phase mass transfer of VOC from water to air. It can be accomplished with a tank and spargers, tray column, or packed column. In this work, both types of columns were studied; however, only the results of packed columns are reported here. The diagram given by Kavanaugh and Trussell [9] shows a comparison of tray and packed columns in the stripping operation. When Henry's law constants given in Table 2 are applied to this diagram, it is found that the diagram recommends a packed column for air stripping of most VOC's.

In air stripping of VOC's from drinking water supplies, the contaminated air is vented to the atmosphere. The more air is used in stripping, the VOC concentration in the effluent air of the stripping column is lower, which makes the operation look better. However, the VOC concentration in the effluent air from the strippin$ column in industrial wastewater treatment is too high to be dis- charged directly to the atmosphere. Dilution is not considered as a solution in industrial operations.

In this study, the contaminated air will be treated. Therefore, the less air is used in stripping, the cost of treahnent of contaminated air will be less. T h e minimum gas-to-liquid ratio is equal to the reciprocal of Henry's law constant in [ I O ] :

5: .., The actual value of gas-to-liquid ratio is

1.5 to 6 times the minimum; that ' is: (CIL) ,,,j,, = t h u , where n = 1.5 to 6. The values liquid ratio of vilrious VOCs are shnwo in Table 3 stripping of tricliloriietliylene and perchloroethylene f

the drinking wnter supply of the city of San Bemar;l$#, CA, the gas-to-liquid ratio is more than three magnlbd&i of order greater than the minimum In. nc%c, - . _ . .

Also shown in Table 3 is liquid loading, which g&= flow late of liqriid fed to the iiir stripping column e.; by the cross-sectional area of the column. It varies &ii; 288 literslminhn' C7.1 gpmlft') for Vinyl chloride to.6710 literlminlni2 (165 gpnilfr?) for methyl ethyl ketone.~rh; high liquid loadings for several VOC's are due to their low Henry's law constant.

The height of the packed bed is determined as foll&jY'

2 = N g r ~ ' H ~ L 2 j i j .: where .: .:,:.

[ l l ] : '....', ..+

CI .~,

Z = height of packed bed N,,,L = number of overall transfer units (NTU),

on liquid phase ,: ,... ~. HI,.,. = height of overall transfer units (HTU), based

on liquid phase

TAULE 3. GAS-LIQIJIU f iT I0 ANU LOAUINC OF STRIPPING COLUhlN

Gas-Liq. Ratio Liq. Loading Pressure Drop ( VOC', SCMlliter liter/min/m2 c m ~ 9 0 %I

Vinyl Chloride 1.2-Dichloroethane Carbon Tetrachloride Triuhlaroethvlene ,~ 1.1-Dichloraethane Toluene Methyl ethyl ketone Chloroform Acetone Acrolein 1.1,l-Trichlaraethnne 1.1.2-Trichloraethane Benzene Acrylonitrile 0-Xylene m-Xylene p-Xylene

1.64 x 10-3

5.39 x 10-3 7.85 x 10-3

0.0749

0.0159 0.0138 1.167

0.0239 0.298 0.554

5.16 X IO-> 0.101 0.0168 0.144 0.0175 0.0159 0.0138

288.48

724.47 488.95 985.24 860.15

3,058.8

6,712.1 1.395.6 5.461.6 6,263.1

3,587.7 1,030.5 4,230.7 1,068.8

686.98

980.35 865.04

272 .November, 1989

'ne for , + Irdino, ~

itudes

is the ;,

ivided s from : 3 6710 ?. The > their

nllows :.

(4) ::

. based

3.

.. . . .p

.-.;-.A&.. TAULE4. AI$$~TIUPPIN(: COLUMN , .~ & 'Waste gpm, VOC Reduction: 500 to 1 mdl, Operating Tcmp. = 25-c

',I' Stripping Column

Installed NTU Height Diiunctor C,,lumn cost

_. HkL m N,,,,. m 111 0 voc's . . - __ ~

Vinyl Chloride 1.2-Dichloraethnnr Carhon Tetrachloride Trichloroethylene L1-Dichloroethane Toluene Methyl ethyl ketone Chloroform ,\cetane Acrolein I.l.l-Tiichlnrocth;Ine 1.1,2-Trichloroethane Benzene Acrylonitrile ,,-Xylene m-Xylene i,-Xylene

2.96 1.96 2.56 3.36 2.12 2.43 0.192 2.05 1.29 0.576 2.68

~2.01 2.15 1.92 2.43 2.49

7.24 7.00 7.90 7.90 7.90 7.90

L5.36 7.90

15.36 15.36 7.90 7.90 7.90

11.05 7.90 7.90

21.40 15.45 20.21 26.61 16.75 19.25 22.21 16.21 19.75 8.85

21.20 15.90 17.00 21.20 1!1.25 19.71

4.09 1.26 2.58 3.14 2.21 2.37 0.85 1.86 0.94 0.87 2.65 1.16 2.16 1.07 2.12 2.22

2.59 7.90 20.50 2.3,

3~3.900 13 1.400 261.800 384.10(1 192.000 226,800

18,600 146.600 94.900 42.800

277.800 110.000 I91.600 117.500 206.500 221.800 238.200

Hc = 0.011 *,, (S,)$'.5

L*,, = liquid loading based on mass flow rate, Kgh"S

fl = (kLlLl/lLu)u.16

fJ = (u,JuJo-'

Values of +,,, $A, and & were given by Sinnott graphically [121. They are converted to mathematical equations.

. . i . . . .

. ... . . . . . . . . . - .

I .. 3 3 5 8 8

1.0 t u

-4 U

E xn 0.8

f rn o.6

0.4

so that they can be used in computer programs in this work. These mathematical equatious will be presented later.

The diameter of the packed column is given by half of the cross-sectional area of the column at its flooding condition. The flooding condition is given by Peter and Tim- merhaus [I31 graphically. Again, the graph is converted to a mathematical equation for computer calculations.

Using these equations, the diameter and height of the air stripping column for various VOC's were calculated and are listed in Table 4.

Pressure drop across the column was also calculated. It is small, less than 4.6 m of water (15 psi). A low pressure blower is sufficient to deliver the needed air in the column.

100 I I

c

-7.187481 X 10"

:. 3 :

for gar-phose HTU for Bed Saddler .? sinnow [12])" ' " ' .~ .::. ;:+

November, 1989 . 273 .I .:,p . * ... .._ . ,. .... . , ..; & , ..t.. .,., ." . . , .I:.

,d to, as

(15) i

ked col-

... , .

COST OF AIR STRIPPING

Using the equations and computation procedures described above, equipment sizing and cost estimates were performed for an air stripping facility of 1000 gpni wastewater capacity and 99.8% VOC removal, from 500 mgll to 1 nigll. The operating temperature is 25°C (77"F), and pressure is I atm.

The sizes and costs of stripping columns for v' '1llOUS .' chlorinated hydrocarbons, toluene and benzene are shown in Table 4. Cost data given by Cnthrie [I41 and Peters-Timmerhaus [I31 are used. The costs are for the second quarter of 1988. The column costs include the costs of a shell, support and packing material. Table 5

Utility Cost $/?.eel

9.800 38.300 IO.80L) I2.600 12:270 10.600 I8.-100 10.820 81.620 l i , 100

l lG.100 IH;200 li.000 15.700

Labor Cost $/year

57.500 57,500 57,500 57,500 57,500 57.500 57.500 57,500 57.500 57,500 57,500 57,500 57.500 57,500

Total Stripping Cost $/year $/IO00 liters

153.100 0.141 170.500 0.156 131,600 0.121 159,300 0.146 1211,700 0.114 128,800 0.118 118,800 0.109 134,700 0.124 203.600 0.187 129;900 0.119

134,300 0.123 257,900 0.237

135.400 0.124 136,300 0.125

VOC'S

shows the sizes and costs of the supporting facilities which include a feed pump, blower and air filter.

Table 6 shows the required capital investment, or the total installed equipment cost, for the air stripping of various VOC's. It includes the costs ofair stripping, pumping and air supply. The utility cost is calculated, based on lOe/KW-hr. T h e labor cost is based on two semi-skilled operators at $25,000 per year, plus 15% fringe benefits (total $28,750 per year). It is assumed that the facility is operated 16 hours per day, 300 days per year, and the service time of the facility is 5 years. It varies from $118,800 to $257,900 per year, or from $0.109 to $0.237 per 1000 liters ($0.39 to $0.90 per 1000 gallons) of VOC aqueous solution.

TABLE 7. SIZES OFAOSOHPTlON COLUMNS Operation Time = 8 hrs; Superficial Velocity = 0.55 Wsec

VOC Concentration (ppm by "VI.)

Influent Efluent

Vinyl Chloride Carhan Tetrachloride Trichloroethylene I,l-Dichloroethane Toluene Chlorofom, l.l,l-Trichloroethane Benzene "-Xylene m-Xylene p-Xylene

TU r ~ w e r t 111 (I, It, rsntltiply Ihy 3 . m ~ .

98,434 13,257 10.697 71030 8,726 3,901 15,992 8,461 5,991 6,590 7,537

Carbon Bed Diameter, D Length, H

m m ~

i

0.93 13.96 1.62 5.43 1.95 3.81 2.78 2.58 3.39 1.58 2.85 2.90 2.77 2.59

. : Ctwhnn tetrachloride [I61

Bmzene[lS] n-Hexane

.... ~ , .

. .

TAULE 8. F H E U N U L I C I I - E ~ U A T I ~ N CONSTANTS g organic

(g carbon) (mm Hg)" k

, " - 0.2928 ~~~. 0.1114 0.2367 0.134-

n

0.20999 0.36462 0.1780 0.08032-

1.95 2.22 1.37 5.60 1.83 1.81 1.97 2.23

Temp., "C

37.8 1 0 0 ... 33.3 25.0

November, 1989 275

CARBON ADSORPTION

Gas-phase activated carbon adsorption has been used for two OK three decades in the solvent recovery and odor control, removing alcohols, benzene, paraffins, trichloroethylene, chloroform and carbon tetrachloride from air streams (15, 16, 17). A recent field study by Nelson, e t al., reported a high removal of VOC's from air using gas- phase carbon adsorption [18].

Air carrying VOC's stripped from aqueous solution is fed to an activated carbon adsorption column. The concentration of VOC in air varies from 3,901 ppm to 98,434 ppm by volume, as shown in Table 7. The concentration of VOC's in ef luent air after adsorption is determined by the adsorption equilibrium. It is approximately 3 ppm for carbon tetrachloride, according to Parmele [16].

The adsomtion eouilibrium data of benzene and car- I>oii tetrachloride were reportad I)y Coolidge [19] and Par- mrle 1161. renvcctivclv. 'l'huse data were uscd to size the adsorption coiumn. .

the adsorption equilibrium. The equation is: The Freundlich isotherm equation is used to express

x = k p" (17) where

X = weight ratio, grams of VOC adsorbed per gram of activated carbon

p k,n = Freundlich-equation constants

= partial pressure of VOC in gas-phase, mm Hg

The values of k and n for carbon tetrachloride, benzene and n-hexane are shown in Table 8. The values of k and n of n-hexane are estimated from k's and n's of ethane, pro- pane and butane.

Adsorption equilibrium data of other VOC's are not available. Therefore, k and n of n-hexane are used, as an approximation, in sizing adsorption columns for these VOC's. There are two reasons to use n-hexane: thev are: . . the normal hoiling point otii-hexane is comparable to that of n i a w VOC's and rhe valurs of k and 11 of n-hexane give a conservative design,

Based on the concept of fixed-bed mass-transfer and Freondlich isotherm equation, the depth of th qiiired carbon bed is obtained as follows

L -

where

& VcA

= molecular weight of VO = flow rate of contaminate

PI> = bulk density of carbon bed, g/

tion (18).

COST OF ADSORPTION

activated carbon

TABLE 10. COSTOFSPENTCARBON REGENERATION

Cast of Regeneration Cost of Make-up Carbon voc $/year $/loo0 liter

Carban tetrachloride 174,500 0.1GO Benzene 316,400 0.291 Others 400,600 0.367

.276 November, 1989 Environmental Prog

1 4

!'$

0 ' 6 3 ) 1 3 0

er zone the re-

' I.,,

(18) .

orptionii;; ty of aii,i. point of ftlsec):

various 1Equa-:. ...

7;. I,

.d,%*

REGENERATION

Regeneration of spent carbon is a supporting operation, which can be accomplished as a part of the process, or by xu off-site commercial operator through a contract.

Spent carbon can be regenerated by hot air [20] nr electrical-resistan6e heater [ZL]. ,Microwave regeneration is also a possibility [ZZ]. However, the conventi~innl steam regeneration is not considered, since it produces contaminated water.

Marquess and Nell, Inc. (New York) reported that their electrical-resistance heater can regenerate spent carbon at 13eIKg (6Ulh) with only 4% carbon loss. Without com- mitting to any specific regeneration method, the cost of regeneration and carbon loss are assumed to be, respectively, 22#/kg ( l O $ / I b ) and 10%. Based on these assumptions, the cost of regeneration operation is calculated and shown in Table 10. For carbon tetrachloride, bewane and other VOC's, it is, respectively, $0.320, $0.582, and $0.734 per 1000 liter ($1.21, $2.20, and $2.78/1000 gal) of wastewater.

MICROWAVE REGENERATION

The concept of using microwave radiation to regenerate granular spent carbon was first suggested by Schulin [23] in 1971. It was followed by more than a halfdozen of patent awards in Japan, the United Kingdom, and the United States. However, there is no commercial application reported, and the literature in the public domain on this subject is scarce.

Two microwave frequencies, 915 and 2450 MHz, are popular in industrial heating and domestic cooking. When microwave radiation is applied to polar molecules, which includes all VOC's, molecules are heated by the induced molecular rotations, and separated from the carbon.

In laboratory tests [22], samples of 100 grams granular carbon loaded with acetone were placed in a domestic (Kenmore) microwave oven, and received microwave radiation for 48 minutes. The weight loss was monitored during the radiation. The weight loss represented the removal of acetone from spent carbon The results are shown in Figure 6. All three tests showed the desorption of acetone, as shown by the three curves in the figure. The same microwave regeneration experiment was repeated for n-hexane. The results are also shown in Figure 6. The rate of desorption for n-hexane is slower than that of acetone. The dielectric constants of n-hexane and acetone are, respectively, 1.89 and 20.7 at the room temperature [24]. Therefore, the slower de'somtion rate of n- hexane is expected.

Granular Pctivated

I I I 10 20 30 40

T i m Of l4icrcwaw mdiation, -"tees

Figure 6. Microwave desorption.

The experiments show that microwave radiation can remove VOC's from spent carbon with very little purge gas, if any. This is the advantage of microwave regeneration over other types of carbon regeneration. The energy consumption of microwave regeneration is 7.3 KWhr per Kg (11,230 Btdlb) of acetone.

CONCLUSIONS ..

Neither air stripping nor gas-phase carbon adsorption of volatile organic compounds is new, but their combination shows a potential to remove VOC's from aqueous solutions economically, particularly from dilute solutions. Water involved in activated carbon is much less in gas- phase adsorption than that in liquid-phase adsorption. It makes regeneration of spent carbon easier and the recovery of VOC's from aqueous solutions possible.

The case of 3,785 literdmin. (1000 gpm) of aqueous solution containing 500mg/l of VOC was studied. The 99.8% of VOC can be removed at costs ranging from $0.457 to $0.899 per 1000 liters of solution, depending on the VOC involved, as shown in Table 11. The cost can be reduced, if the credit for the reuse of treated water is considered. The recovered VOC's are in high concentrations, which can be recycled. The required capital investment is low, in the range of $580,000 to $840,000.

.

TABLE 11. TOTALCOSTOFVOC STIUPPINCANU RECOVERY

Cost of Owration. $/loo0 liters

voc Air Stripping

Vinyl Chloride Carbon tetmchloride Trichloroethylene 1.1-Dichloroethane Tnluene Chloroform 1,l.l-Tri~hlorneth;n~ Benzene Il-xylene m-Xylene p-Xylene .

0.141 0.121 0.146 0.114 0.118 0.109 0.124 0.119 0.123 0.124 0.125

Adsorption

0.0242 0.0157 0.0194 0.0123 0.0120 0.0156 0.0223 0.0120 0.0121 0.0123 0.0120

Regeneraion and Make-up Carbon Total -

(0.734) 0.320

(0.734) (0.734) (0.734) (0.734) (0.734) 0.582

(0.734) (0.734) (0.734)

-' 0.899 0.457 0.899 0.860 0.864

'l 0.859 :.~' 0.880 -15 0.713 . , .. 0.869 .?,0.570. ':'0.871 .. .

-November, 1989-*': <277 ,... . Environmental Progress (Vol. 8, No. 4)

.n. I . <;itisti, D. il.. I< . :\. Cotiw;ty. iiwl C. 1'. L;twson, , s ~ ~ ~ ~ ~ ~ ~

C:wlmn Adsorption o f I'ctl.nchciiiicals," J . \v~,-E, vel. ,~ No. 5. 1,. 947-065 (lvlay 1984).

2. Jiicolms. J.. i\. J. Eiiglai~le. Jr.. iind C. D'AmouV, - L ~ ~ ~ ~ ~ ~

Survey and Disposid Technolagics." Rep,,,t for ~ ~ , , i ~ ~ -

.'I. T1,i ldcaur . L. J . , Clieiiiody,iet,iics, Johu Wiley (1979).

. I . Ililling, \V. L.. "lnterphnse Transfer Prncerscs 11,- E , , " , ~ ~ , Sci. 6 Tech., Vd. 11. No. 4. I). 405409 (Apr. 1977).

S . hlnckev. D. iind P. J . Leinoncn. "Rate of Evaporation 'of Low-Solubility Contaminants from Water nodies to Atmor. phere," Encirm~. Sci. C- Tech., V d . 9, No. 13, p. 1178.1180 (Dec. 1975).

Problem." Il',itcrlE,igirieeriiiU 6 Mnne~emctz t , p. 20-45 (May 1986).

ACKNOWLEGMENT ......

This work was suppnr t ed hy a grant from tlrc I l epa r t - Inllnrtry-Resnnrce I\ecovcr\. and UY-Pro<l,lct Collvenlon inen t of Environmental Quality, t he State OS L~ii i is i : tn:~ The suppor t is greatly appreciated.

NOTATION

A = cross-sectional iirea ot 'dsorptior C I I I U I I I I I . tm' or It' c, = concentration ofcomponcnt i i n liquid ~II:ISC, nwll D = ~ol i imi i diameter, in1 o r ft D,. = dittiision coefficient ofliquid, mYs 01 I F l s D,, = diffiision coefficient of vapor, mz/s o r It%

7. Nowood, S. G., "Air Stripper Cleans Up Chlorinated g$ Ji = liquid viscosity correction Factor Ja = liquid density correction factor drocarbonr," Pollution Equipment News, 1988. 4 . /d = liquid surface tension correction Iictor 8. W~,terl~nginEeriIlg & Monngewwlt , p. 22-23 (Jan. 1985). 1. .'

= gar How rate, kghr-m' or Ibmilrr-ft9 9. Kavnnaugh. ivl. C. and R. R. Trussell. "Air Stripping a Treatment Process," paper presented at the AWWA ~ n n d H<; = height ofa gas-phase transfer unit, on1 01' ft

. .,I.#:: Ht. 5 height ofa liquid-phase transfer unit, m or ft H,,,L = height ofan overall liquid-phase transfer unit, m or ft 10. Eckenfelder, Jr., W. W., Principles OJ IVuter Quolity Man.

Kt = percentage flooding correction factor 11. Tmybal, R. E., Moss-TrartsJer Operution, 3rd ed., L = liquid flow rate, kg/hr-m* or Ibm/hr-ft2 Hill (1980). L = depth of carbon adsorption bed. m or ft 12. Sinnott. R. K., Cliemicol Engineering. Val. 6, J. M. L*," = liquid,mass flow rate per unit column cross-sectional and J. F. Richardson. Pergamon (1983).

711 = Henry's Law constant, dimensionless M.. = molecular weight ofVOC. gig-mole or Ibm/lb-mole n = Freundlich equation constant N,,,L = number of overall liquid-phase transfer units p = partial pressure of VOC in gas-phase, mm Hg pi = partial pressure ofcomponent i in gas-phase, atm P,*,, = total pressure in adsorption column, mm Hg S = stripping factor fSch = liquid Schmidt number = (~dp,D, . ) fSsc). = vapor Schmidt number = (F,JplDI) Vcb = How rate of contaminated air, Ib-mole of aidmin

Hoard of Regents. Fell. IOXS.

I<. Willcy, H . H. iind R. B. \Villi:um, "Ways to Tackle the

~

Conference, St. Louis. MO (1981). k = Freundlich equation constant ngeelent, c n I P U ~ . (1980). . .

area, kglm2s

Xi,. = mole fraction ofsolute in liquid-phase at entering condi- \'- ciency for ~ ~ 1 1 scale carbon ~ d ~ ~ ~ ~ i ~ ~ systems,- e Progress, Val. 4, No. 1, p. 14-19 (Feb. 1985). tion, dimensionless

tinn, dimensionless

tion, dimensionless

dimensionless

sorption bed

'\ Y,, = mole fraction of solute in gas-phase at entering condi-

Y,,,,, = mole fraction ofsolute in gas-phase at leaving condition.

!I,.(,<: = mole fraction of VOC i n the feed to the carbon t d

Z = height of stripping column. m or ft

York, NY.

Samples," American Lab.. p. 48-57 (July 1984). Greek Letters

+,, +,, p,,

= i i m for H ~ ; for neri saddies - factor for H I . for Urd Sitddles = viscosity of liquid, kdm-r or Ilmdtt-aec

.a,

, , Environmental Progress ( V ~ I

Jnd. Eng. Cticm. Res. 1993, 32, 2752-2757

ldsorption of Organic Compounds and Water Vapor on SPL .ivated Carbon. 2. 1,1,2-Trichloro-1,2,2-trifluoroethane and hloromethane

Roy N. E i s s m a n n ' a n d M. D o u g l a s LeVan '

Dcporfment o/ Chemicol Engineering, Uniucrsily of Virginia, Charlottcsuille. Virginia 22903-2442

A novel volumetric apparatus is used to measure equilibria for mixed vapors of halocarbons and water coadsorbed on BPL activated carbon. Pure-component 1,1,2-trichloro-l,2,2-trifluoroethane (CFC-113) and dichloromethane (methylene chloride) isotherms are obtained at 0, 25, 50, 75, and 100 "Cover wide ranges of pressure. A separate isotherm measured for CFC-113 a t 50 "C indicates the presence of pure component hysteresis in the mesopore s t ructure of the activated carbon a t high halocarbon loadings. Mixture equilibria are measured for CFC-113iwater systems at 25 and 100 "C and for dichloromethane/water systems a t 25 "C. All components exhibit hysteresis, and the halocarbon partial pressure increases as t h e water loading is increased a t constant halocarbon loading. Results for ha locarbonhater mixtures together with previous resultsfor hydrocarhon/water mixtures show t h a t the apparent total pore volume filled near saturat ion is dependent on t h e adsorption temperature and the solubility of the organic compound in water.

duet i o n orofluorocarbons and other halocarbons are used as !rants, solvents, foaming agents, blowing agents, and :tinguishing chemicals. Concern over the environ- .I and health consequences of using these materials .isen very significantly in recent years. design adsorption processes to recover these and compounds effectively, adsorption equilibria are ed. Water is known to have a significant effect on irium adsorbed-phase organic loadings in many ations such as the use of carbon filters to remove c contaminants from humid gas streams, fixed beds uce levels of undesirable compounds in closed nments, and adsorption columns tha t have a steam ,ration cycle. The knowledge of how water vapor ~ the adsorption properties of the activated carbon ow for more efficient design of adsorption systems :h applications. i paper is concerned with the determination of ition equilibria for two popular halocarbon sol- 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) and romethane (methylene chloride). Pure-component 'ms are measured over a wide range of temperatures essures, and the coadsorption of wnter vapor over nnges of composition is considered. This paper s the work of Rudisill et al. (1992), which was on )or phase adsorption of hexane, acetone, water, and :arbon/water mixtures on BPL nctivated carbon. aper gives a thorough review of prior work on the tion of water vapor, with nnd without n competing : species, on activated carbon. In an ndditional Y published study using BPL nctivnted cnrbon, (1992) mensured ndsorption isotherms for water dsorbed on nanogiam microparticles ofBPLcarbon ier ndsorbents by Ievitnting them in electric fields; ngree rensonably well with previous mensurements. e resenrch has been performed on ndsorption of .bans, even to obtnin pure-component isotherms.

the more recent pnpers, Kodnma et a / . (1992) .ed isotherms of CFC-113 on 13): zeolite. mont- nite, and silicn gel a t 25 "C nnd on nctivnted cnrbon

hor to whom correspondcnce should b ~ , nddrcseed. c n t oddress: Du Ponl Chcmicols. Dccpwoter, NJ.

, , , . . , . , , . , , . , - .

from 15 to 30 "C. Also, Kuo et ol. (1991) and Kuo and Hines (1992) measured isotherms for chlorocarhons adsorbed on silica gel from 15 to 25 "C. Concerning reactivities, Barrer and Brook (1953) reported that flu- oromethanes react with chabazite. Similarly, Kumar (1982) found tha t dichlorodifluoromethane (CFC-12) reacts with 4Azeolite, reducing the intraparticle diffusion coefficient for adsorbed molecules.

The interpretation of adsorption equilibria for vapors of organic compounds and water coadsorbed on activated carbon poses interesting challenges. Even water adsorbed separately raises theoretical questions concerning mechanism and hysteresis as discussed by Rudisill et ol. (1992). When water and a hydrophobic organic compound are coadsorbed in chemically and structurally heterogeneous microporous carbons, the problem becomes much more complex. This paper reports results of an experimental investigation. Discussion of results is limited to trends reflected in the data and comparisons with previously reported results. Methods developed to predictadsorption equilibria should be consistent with these trends.

Experimental Sect ion

Appara tus . The apparatus used in this work has been described in detail by Rudisill et al. (1992). Briefly, t h e apparatus is housed in an environmental chamber with programmable temperature control. A magnetic pump is used to recirculate gns through a closed loop of known volume which contains a bed of cnrbon. A known nmoullt of material is injected into the loop, and the vnpor phnsr is sampled using n gas chromntogrnph.to determine its concentrntion nnd whenequilibium isreached. Amaterinl halnncc is then made to determine the nmountof mnLerin1 adsorbed on the nctivnted cnrbon.

There nre severnl distinct ndvantnges to using this t.YPc ofnpparnt.us. Ipirst, the system tempernturc cnn IJC varied widely so thntisothcrmu over n wide mtlgeof tempernLures (0-125 "c) cnn he mcnsured. Second,tllenppnrntusU'orks equnlly well for both pure components nnd mixtures. includinjisyslcms thntshow hysteresis. Third,equilibrium cnn be uchieved rapidly. Finnlly, equilibrium from very IOU, pressures t o sni,urntion pressures can mensurcd nccurntely. 'h: system volume and tile nmountufcnrhon in the system cnn be ndjusted to ensurr thnt the nmOunL

. ",'v<b;, " 'I .the

. j . .

rinl ndsorbed on the carbon is n significnnt frnction otnl material in the loop. Thus, when the vnpor- oncentrntion is measured directly, the error in the adsorbed determined by mutcrinl balance is smnll.

Mnterials. The ndsurliiites used iii t l i i v experiment.

grade), dichloromethnne 139.5 mol 1)ure H1'I.C nnd dis:illed, deionized wi~ter.

cnrbun uwd u n s G X IF mesh'l'ype Bl'L. i ~ c i i v n t e d (Calgon Cnrbon Cur['., lot No. 1814.1)

~ p c r a t i n c Procedures . 1:rrr t l i e mcasur(:mLnl LI pdrt compoiienr hnlocnrbon iwtherin>. ttic \ ~ U L I I I . of the 1 0 ~ 1 . a was kept smn11. 100 2 cm". 'l'hi experinieiit iw+n Ov

, regenerating the activnred carhon IL remove nn\ conlau.. Danm adsorbed on it This was nccomplished hy heat ing

:'*.. .nitrogen flow through the bed of 2-4 I,/min until tht bed

?showed no chnnpe in weight. The bed wns then p l ; ~ t d i n the aypnrntus, nnd n small amount of Iinl~,carhm was injected. Thesystem \%'a> hented to 100°(: nnd held until equilihrium u'ns esmhlished ns determined by sampling the gas phase. The temperature was then lowered to 75 "C and the sampling repeated. Then, the temperature WBS dropped t u 50 "C, then 2 5 T , nnd finally 0 "C. Another injection was made, and the system wns heated hack up M 100 "C. This procedure was repcnted until the regim in which hysteresis uccurs in mesopores wns reached. An experiment resembling those for uater condsorption was also performed t o determine the sizt and extent of [lie hysteresis loop t h n r exists for pure-component hslocnr. bons.

For mixtures of halocarbon nnd w n t w , i t 1% \'cry imponant [hot the experiment be set up so thnt changes insystem tempernturt willsignificnntly affect the loading of the adsorbnte on the cnrbon. This 15 doni: 1.0 L-nsurc that adsorption and desorption points are corrtctl? measured. We seek 10 approach equilibrium viu e p3tn dong which both components are either adsorbing or desorbing. The varinhlcs which can he virried so thnt this can occur nre the system volume nnd the nmount of carbun

-used. Since wr are studying a mixture in which one compound ismmich morestrongly adsorbed thmtheorher , the ratio of cnrhon nmount I L syst1.m volume wi ia w c h that most o f t h e stronl:ly ndsorbing hiJocarhoii was in rhe :adsorbed phase,nnd hctwem 20''; and 807 of the w a k I \ adsorbing water uiis in the ids(.rhed ~ ~ h a > e '' In these experiments. the loiidinl: of the halnc;rrbm wi js heldconstnnt mid the Ionding of wnter wiis var:ed Lver tht .Complete range uf reducrd wnter pnrtinl pressure. l'hv WnLerivotherrn wasiriensurcd todvtermine the exte i i t iind size of the hysteresis loop. T h e mensurenient of thi.. isostcric mixture iwttivrni ib possilile since even fur S16nificnntclinnges in t l ir hnlucnrtwn wpor concmtriii inn

hnlocnrtoii uhicli desurbt. Trim (hi curbon is sinall to the totid iiniount uf ti&cX~Jc,il ;dsurl)ed

Additionnl injections or Iidociirtmi CUI (x ~ u i d ( . . 11 necessars. tu ini~iiit~,in n coitstiint hiding. I These experinicrm ncre performed by f i r> r i i iJ( ' ( 11111:

the hnlocarboii .nt<. t h e loop. After ni i i i 1 i i r . l p u r l . component metrsurenwnt IVIIS wnipl?tcd, wutcr wits 11..

Jectedand thesysiem hentad I,et\rien 10rind 2.1 '~ ' ;sI icwt. the temperature ofintercsi. lie system w i s t t m c o ( ~ ( ~ 'lowly (2-1 oc ~ i , I,. the temperiiture <s inlcrt,.st uiid \ w l ( i ~hereUrltil equilibr1iiir \ w s cs tnb l i~ l ied . 'l'hv \%pur }iriiis(

was nampltd for h e cui.centrntton- (,i h 1 1 1 i 1~i,I(~c6arIwi. %d wntcr. ~ n t e r i u l Iulnnces w u ~ d t l icn I,( niiide 1.1 $itermine tlie 1ondiiBgI. 01 Ir( . t t l c(,i~lpuuljds ' ~ ' l i c s p i t 11.

p , & h e i i cooled I)( t w ~ i ~ i i d 2~~ ? ( ' I d o t v t h . icn:

c

b the fixed bed of carbon in an oven at 1 5 1 ~ 1 7 5 " C with ir

Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 2753

G , , ~ ~ . I 0 c

PCFC-113 k P a j Figure 1. Isotherms for CFC-113 adsorbed an B P L activated carbon. Vertical lines indiate saturation pressures.

n , I . I "

P"cn! (LPaj /::.I ., . . ' { . .,*, - . Figure2 lsothermsfordichloromerhaneadsorhed onBPLactivated carhon. Vertical lines indicate saturation pressures.

perature of interest, hented slowly (2-4 "C/h) back up to the temperature of interest, and held until equilibrium was established. The vapor phase was sampled ngnin to determine thedesorption branchof the isotherm. Another injection of wnter wns made, and this procedure wns repented until the vapor phase wns saturated.

Resul t s a n d Discussion

I Pure-Component Halocnrbon Isotherms. Isotherms I for CFC-113 and dichloromethane at 0,25,50,75, and 100 ' "C nre shown in Figures 1 and 2, respectively. T h e dntn

nre plotted ns the amount adsorbed versus the pressure, which is shown on n lognrithmic scnle over mnny decades. Verticnl lines indicate saturntion pressures. T h e results show thnt the nmount ndsorbed. nCFc.l13 or nDCM, is relntively constnnt for the data at the different temper- nturesnsisosteresnre measuredat lowlondings. Butonce thc vnpor-phnse concentration becomes large enough. dropping the temperature results in noticenbly grenter loudings for n given amount of halocarbon in the loop. TO dcl.crminc whether BPL cnrbon produces iype 1 or

l w e 4 isotherms for a hnlocnrbon, nnothcr experiment wfis pcrfornied. 'l'his experiment tested for hysteresis in the CFC-11:J isotherm ut 50 "C. If hysteresis is prenent, then the isutlierni is uf the t.ype 4 clnusificntion, meaning

Ind. Eng. Chem. Res.. 1'01. 32, No. 11, 1993

6 I

1 I

3 6 B 12 15

I @ ' 0

nit, (mollkg)

Figure 4. CFC-113 loading ffi n function of water loading at 25 "c forerperimenrswithnc~c,~~= 0.746md 1.477moWkg. Filledsquarer denote adsorption, and open squares denote desorption. The solid lines represent the average CFC-I13 loadings.

3 0 ~ 1 J

I

>

10

0 0.2 0.4 0.6 0.8

pi. Figure 5. Water isotherms at 25 "C for CFC-113 loading of ~ C F C B I ~ = 0 (top). 0.746. and 1.477 (bottom) mollkg. Filled squares denote adsorption. and open squares denote desorption.

figure is the pure water isotherm of Rudisill et al. (1992). T h e figure shows thnt the size of the hysteresis loop decreasesdramaticallyns the CFC-113 loading isincrensed. Cnlculntions indicate thnt, using apparent densities determined from pure-component adsorption, the micropore volume filled by the mixture near saturation of the vapor phase is significantly less than 100 5%. For n c w 113 = 0.746 mollkg, 83% of the pore volume is available for wnter adsorption but only 71 9b of tha t volume is filled by water. For ncFc.I13 = 1.477 mollkg, 66% of the port volume is available nnd only 357;. of thnt is filled.

Figure 6 shows the dependence of the equilibrium CFC- 113 pnrtinl pressure a t 25 " C on the amount of wfllc: ndsorhed for the two londings o f CFC-113. While the pnrtinl pressure increases with the wnter londing and is pnth dependent. the trend is not dramatic. A pattern can be noted in F iwre 6a thnt is present in nlmost nll oi our dntn. Spccificnlly. for condsorption ofwater and an orgnnlc compound of moderote volntilit.y, the hysteresis o h s e r d for the orgnnic compound is in the snmr direction ns thnt forwuter. I n (rthcrwords, higherlondings (orlower pnrtjn' pressures) for both CFC-113 and ~ a t c r occur (111 ~ I w desorption ~irunch of the isotherm.

Wnter isotherms n t 100 "C nre shown in Figure 7 for CFC-113 Inndings Of0.68 & 0.12 nnd 1.41 & 0.18 mol/kE

.~

L

of ca: shift1 isoth,

,,, teres . , . loadi .~',:: t h e r

FZ 02 51 F 0

I

99LZ

L aql

i e

Ind. Eng. Chem. Res., Vol. 32, No. 11,1993 2767

L i t e r a t u r e Cited

Barrer, R M.;Brwk D. W. Sorption and Reactivity of Simple Organic' Molecules in Cbabazite. Trom. Fnradoy Soc. 1963,49,940?)48.

Dubinii. M. M. Porous Structure and Adsorption Properti- of Active Carbons. In Chemistry ond Physics o/ Carbon; Walker, P. L.. Ed.; Marcel Dekker: New York, 1966; pp 51-120.

Dubinin, M. M. Water Vapor Adsorption and the Microporous Structures of Carbonaceous Adsorbente. Carbon 1980,18,355- 364.

Gregg, S. J.; Sing, K. S. W. Adrorptian. Svr/ace Area and Porosity; Academic Press, 1962.

Kodama. K.; Kaguei, S.; Wakao, N. Bat& Adsorption of R i d - rotrifluorcethane (Freon-113) onto Activated Carbon-Surface Diffusivity and Pore Diffusivity. Can. 3. Chem. Eng. 1992, 70, 244-249.

Kumar. R. Effect of Freon-12 Exposure on the Sieving Property of 4A Zeolite. Con. J . Chem. Eng. 1982,60,577-578.

Kuo, S.-L.; Hines, A. L. Adsorption of l,l,l-Richloroethane and Tetrachloroethylene on Silica Gel. J . Chem. Eng. Dnto 1992,37, 1-3.

Kuo,S.-L.;Hines,A.L.:Dural,N. H. CorrelatianofMethylChloride, Methylene Chloride, Chloroform. and Carbon Tetrachloride Data on Silica Gel. Sep. Sei. Technol. 1991,26, 1077-1091.

Okazaki, M.; Tamon, H.; Toei, R. Prediction of Binary Adsorption Equilibria of Solvent and Water Vapor on Activated Carbon. J . Chem. Eng. Jpn. 1918.11, 2W215.

Rubel, G. 0. Water Isotherm Measurements for Micropartieles of Carbon. Carbon 1992.30. 1007-1011.

&hysteresis than CFC-113 a t 26 "C. Rudisill the oppositetemperature and solubility effects, .j et al. (1978) report the opposite solubility

IS

apparatus was used to measure vapors of CFC-113 and dichloromethane

parately and with water on BPL activated wide ranges of temperature and pressure. All , exhibited hysteresis, and the halocarbon #sure increased as the water loading was

I and temperature have significant effects on ading tha t can be achieved for halocarbon/ ue adsorption. An increase in temperature is gnificantly increase the extent to which the e can be filled at apparent pure-component ikewise, an increase in solubility also tends to ore complete pore filling, thus increasing the ing. m/water coadsorption tends todowfor higher both components on the desorption branch of n. eresis loop found for CFC-113 indicates th'e aignifieantmesoporosity in BPL carbon. This s further complication to attempts to model organic compounds and water adsorbed on

ubon.

lgment

itefultothe US. Army ERDEC for thesupport uch.

Rudisill, E. N.; Hacskaylo, J. J., LeVan, M. D. Coahorption of Hydrocarbons and Water on BPL Activated Carbon. Ind. Eng. Chem. Res. 1992, 31.1122-1130.

Received for reuiew January 21,1993 Reuised manuscript receiued July 6, 1993

Accepted July 13,1993.

*Abstract published in Advance ACS Abstracts, October 1, 1993.

4 new Jovanovic-Freundlich isotherm model is derived for de- ihing single-component adsorption equilibria on heterogeneous .j,,;:,s. The equation is obtained by assuming that the rate of Irc:Ise ofthe fraction of the surface unoccupied by the adsorbate ,lecules is proportional to a certain power of the partial pressure t h e adsorbate. The equation reduces to thc Jovanovic equation ,en the surface becomes homogeneous. At low pressures, the jation reduces to the Freundlich isotherm but at high pressures, nonolayer coverage is achieved. This model has been applied :cessfully to the description of the adsorption behavior of a ies of chlorinated hydrocarbons on a microporous silicagel, at rerent temperatures. The monolayer capacity and the heteroge- I! parameter exhibit a weak temperature dependence. The third rameter of the model decreases exponentially with increasing nperature. The fit of the experimental data to the new model scribed is shown to be better than the comparable fits to classical therms used for heterogeneous surfaces. The energy distribution iction corresponding to the model for Langmuir local adsorption iavior was derived using the Sips procedure and evaluated nu- .rically in a few selec ed cases This distribution is an exponen-

'if?. Words: adsorption; Freundlich isotherm; isotherm: isotherm iiicl; Jovanovic isotherm.

I decay. o 1996 *radc "i rrrrr , : inC

INTRODUCTION

The use of adsorption processcs Tor 1:irgc-scale scp;ir:i- ns or purifications is becoming conimon i n industry. e recent development O S iiidiistrii~l-sc;ile Iprcparative riiinatography is contributing 1 0 further incrc:isc the i n -

~ titiice of adsorption-hascd processcs. Thc desizn and : optimization of an iinplcment;itioii o f ;in adsorption- sed separation process rcquirc tlic ;ihility to c1i:iriictcrizc Zurately the adsorption equilihria i i i v o I \ u l iind their de- IldCIiCe on the cxpcrimcnt;il conditi i~ns ( I ) . Although, principle. the coiiipclitivc isotlicrms ofi i l l tlic (ccd c o n -

ponents are required: there are mcthods to derive reasonable approximations of tliesc coinpctitivc isotlierms irom single-component isotherms. These methods i i l low considerable reduction i n the time and cost required by the data acquisition. Their accuracy depends greatly on the availability of accurate correlation for the single-coinpo- nent isotherm data ( I . 2 ) . However, except i n rare cases involving homogeneous surfaces (e.g., graphitized carbon black, zeolites), the single-component isotherms cannot be calculated from first principles but must be measured. Then' arises the problem of selecting a proper isotherm model for correlating these data ( 2 ) .

Theoretical models of single-component adsorption isotherms have been derived using a variety of kinetic. statistical mechanical, or thermodynamic approaches (2, 3) . These fundamental studies assume the solid sui-face to be honioge- neous. Most of the adsorbents used in current practice. such as activated carbons, silica gels. alumina, and even zeolites exhibit surface and structural heterogeneities ( 3 ) . For such surfaces, isotherm models are derived from a fundamental integral equation relating the experimental isotherm. the adsorption energy distribution, and the local isoLherm ( 3 ). Independently. a number OS empirical and semi-empirical isotherm models have been s u g p t e d IO iiccount Tor the adsorption behavior on heterogeneous surC;ices ( 3 ) . Single- component isotherms have becn derived Sroni ii dil'lcrential cquation relating surlllcc coverage : i d pressure. This iip-

proach was first suggested by Schmidt ( 4 ) . latcr cnpmdcd by T6th (5 ) and Misra ( 6 ) . :ind rcccntly applied again by T6th (7 ) . I t allows the derivation of ~Iicriiiodyn~niically consistent isotherms for both Iiomogcneous :ind ieierogcneous surl;lccs.

I n the present paper. wc apply the Iaitcr ;ippro;icIi to dcrivc ii semiempirical Jov;iiiovic-Frcuiidlicli is(~tIicr~ii model for thc monol:iyer. sinFlc-componcnt iidsorptiwi on lictcnigc- I ~ C O U S surfiiccs. Wc usc this modcl IO :iccoiiiit Sor cxpcriiiicii- ti11 iidsorptioii data Ipiiblishcd rcccntly. rcgiirdiiig cliloriniitcd Iiytlrnciirhiiiis oii silica gel, :it diilcrctii icmpcr:iiurc~ (8 , 9 ) , :ind wc coinpiirc tlic quality ( 1 1 tlic rcprcsciiiiiiioii olitiiincd

I W . ~ ~ ~ ~ ~ I W S I X c~y,,;:ll, I l ~ ~ ' J 0 I>) A<:,,l,w,,r I"\. I,/(

A l l iigliir 181 I ~ . ~ > ~ ~ X ~ ~ ~ C I I I I I I 181 :twy l,iiiii I ( . W I Y C ~ ~

d k c - Pr:usnitz K p / [ I -,iKpYI IIIII + ( K I T ] I1 4. i K p r ~ Kpl l I l l 1 (19) ub i ,~ in -Rndur l~kev ic l~ c- - I1 1121 (20)

l jole, For homogeneous-surface Inwdels. 0 IS h e Iocd s u c l ~ c ~ . co\,el;ige. (I: lor the olhci models. i t is l l ic o~ernl l c o v c r a ~ e . 11,: w e Eq. I I I h' is the I' e n r ~ cons13n~, and are [he adsorbale-adsorbate interiicimn p;~ranieleri. 1 1 is the Ileterofeneil) parameter. ' 8 , h. illid i. arc olhcr p r m K t c r ~ which ??+ ,pear i n same models.

,it)> a yarietp of other theoretical and semiempirical iso- (2 . 3 ) . A last approach involves tlie assumption of a local

are the choices of the IWO models. ierln models previously reported ( 5 , 10. I 1 ) . and an overall isotherm model ( 3 ) . I t is also arbitrary, as

We note that Langmuir ( 13) had already proposed de- THEORY

An heterogeneous surface is characterized by a distribu- ,on of adsorption sites which have different adsorption ener- ies. For homotattic sites, a local adsorption isotherm model j assumed. This model refers to an homogeneous surface. .he experimental or global isotherm is obtained (3 ) by the ntegral equation

8 ,@) = - = B ( p . t)F(t)de. [ I 1 q s J"=

where B , ( p ) is the overall surface coverage (excess adsorp- ion) of the heterogeneous surface by the monolayer of ad- ;orbate. a function of the partial pressure p of the adsorbate n [he hulk, q is the amount of adsorbate adsorbed ill equilib- .iuni per unit amount of adsorbent. q. is the monolayer cepac- ty, B(p, e ) is the local adsorption isotherm lor honio11;Llic ;ires with an adsorption energy c. and F ( c ) is the :idsorption 2ncrgy distribution. Equation [ I ] is the fundamcntal equation i n the theory of adsorption on heterogencous surfaces. Be- zause this Fredholm integral equation is ill-posed, i t docs inct have a unique solution ( 3 ) . Attempts at the derivation of lhe adsorption energy dislribulion froni lhc cxpcriment;il isotherm. using numerical solutions and :in assumed Ioc:tl isotherm. have not led IO convincing results hut li;ivc demon- strated t l ie difficulties, usually undcrestitn:lled. (IS collcclinf proper experimental d:ita ( 12). Another ;ippro;tcli consists i l l dcrivinf closed-form intcgra~s or Eg. I I ] . :issuming simple Iunc1ion:tl dependcnccs for H ( p . C ) :itid I : ( < ) :ind litliii: cx- pcrimcntal rcsuIts to thcsc cqu:itiotis i n im effort i o idcntily ttle rC~Cv:inI [lar;trnetcrs. A nwnbcr 01' such s(dtilio11s arc IisIcd i n T;thlc I . This list is h y IIO In1c:irls cotiiprc1Icnsivc

scribing the adsorption on an heterogeneous surface as the sum of a finite number of classical Langmuir isotherm (Eq. [ 3 ] , Table 1 ), each of them characterizing adsorption on an homogeneous patch of surface. This, in effect, introduces a discrete energy distribution. The biLangmuir isotherm, introduced by Graham (21) and used by Laub ( 2 2 ) in gas chromatography, is a particular case of this approach. I t has proven to be extremely successful in the description of the adsolption of enantiomers on chiral phases ( 2 3 , 24) . This result is explained by the physical nature of these interactions. Both enantiomers undergo the same nonselective interactions with most patches of surrace, u,hile enanlioselecti\~e interactions are hi&hly selective, but take place i n well-defined sites which are localized and relatively 1;ir Ironi each other, ensuring the lack of adsorbate-adsorbale ititeidctions on the enantioselective sites. Furtherniorc. adsoi-ptioii cnerg- ics on these sites are high, so saturation is itchicved at low wlues of the concentratioii and the activity cocllicienw remain practically constant.

A significant number of overall isolheriiis 0 1 adsorption on hetcrogeneous surfaces wcre originally proposcd lor empirical or semiempirical rcasons. witli t l ic solc aim 01 describing accurately and 2s simply i ts possihlc the ;idsorption hcliavior of fr i l l systems. Ex:miples ilicsc tntidcls x c tlie R-cundlicli ( 1 7 ) , tlic Langinuir-I;rruiidlicli ( IS ) . tlic T6th ( 5 ) . the IhJkc-Pr:iusnitz (19) , atid tlic L)ubiiiiii-R;idush: kcvicli (20) equations. These isotlicriii iiiodcls wcrc dcrivci 10 m c Eq. 1 I ] IO cdculatc the ;~dsiirption ciicrgy distrihutior lroiii :I ciiiiibiniition llr ;I Ioc:il imtl ;ut overall ;dwrpiiiir tsollicriii. Mosl studies coticcrtiiiig pliysic;il xlstirptiiiii or Iiclcrofcncous surfitccs wcm c:trricti oi i i t i !Ag 111r I . : I I I ~ I I I U ~ I

I

-

L

A

NE\\' JO\'ANO\'IC-~IIEIINDLICH ISOTHERM MODEL 59

ere b i s an adjustable parameter and 11 i s the heterogeneity ameter [ 3 ] . The U N I L A N equation i s based on a uni form tribution o f the adsorption energies and a local Langniuir therm ( I O ) . I t s equation is

ere C i s an adjustable parameter, which, i n principle, iuld be equal to I / K , where K i s the low-pressure equil ib- n constant or Henry constant. Note that the heterogeneity ameter, I,, i s a parameter o f the adsorption isotherm but ,Is0 a parameter o f the adsorption energy distribution ( 3 ) I, thus, i s not the same for different models. For the mod- which are reducible to the Langmuir model (e&, Lang- ir-Freundlich, Toth, generalized Freundlich, Redlich- erson models), I, is between 0 and 1 . For other models s., Freundlich. UNILAN, Dubinin) which can fit also type isotherm dam, I I may take negative values. For some dels (e&. Freundlich. Langmuir-Freundlich. Toth, Jovii- Zic-Freundlich, Redlich-Peterson models), the surface ioniogeneous when I I i s equal to I. For other models $., U N I L A N ) , tlie stdace i s homogeneous when I , i s In1 to 0. Squations 1131 and [ 141 reduce to tlie Langmuir cqualion values OS I I equal to I and 0. respectively. M i s m (2.5)

d the Jovnnovic niodcl ( 14) IO represcnl the local adsorp- 1 isolliertii in tlie solution OS Eq. [I]. in combin:itioii wi th %renl energy distributions. At low pressures, or for smal l ucs OS I,. the solutiolis I-educe to tl ic Shlygin-Frumkin i ) or FI-cundlich ( 17) isothcrnis, respectively. 11 scciiis I tllc use o f citlici- t l ic Jov;inovic or lite Langniuir iiiodcls ilccount for thc Ioc:il beltilvior OS l l ie ;tdsorb:tle pct-inits ! 10 obkiit i s i m i l x ovcriill isotlicrms Sor : ~ t i ; i l o g ~ ~ i t ~ citcr;y I r i b~ t i o i i s ( 3 ) . Jiiroitlcc :~ttd I'iotrowsk:~ (27 j haw (111-

t:tincd l l ic Sollowing ovcril l l adsorption isotltcrni l o r :I J O K - tniivic Ioc:iI isotlierm, using :I y-Suiiction t i 1 clmactcrizc the distriliutioti O S tlic Hcnry cii i ist~int, I;. on tlic lic~crogcncous surl;lcc

wlicrc p and y arc [Iic p;ir;imclcrs o f tltc eiicrgy distrihutioti. This model. lhowc\,cr. has not been widely applicd.

Rccently. I i incs e r ol. ( I I ) dcvclopcd i t i i o\w:tll adsorp- t i w isotlici-ni niodcl i n wl i icl i the local isollicrin i s given by the Jov:ino\~ic equation (Eq. 14 1 , Table 1 ) and the energy distt-ibutior i s represented by a Morse type distribution. They obtained the equation

where K , , K2, and K, are the parameters of the energy distribution. K2 i s usually very small. W e note that if we neglect it; Eq. [I61 becomes identical wi th the Langmuir isotherm, &h K = l / K , .

The derivation o f adsorption isotherm equations from d i f - ferential equations has been reported for a long time. I n spite of several important papers published this century (4-7), i t i s not a most popular approach. Init ially, Schmidt (4) proposed a semi-empirical treatment o f physical adsorption and derived a single-component adsorption isotherm using the relationship

where x i s the amount OS component adsorbed at equil ibrium. C i s tlie concentration oSthe component i n the bulk. S i s the inaxiinuni aniount adsorbable. and D I s a constant. Integra- l io i i O S Eq. 1171 gives

1181 I = S(1 - c - ).

Later. T6th (.5) derived several adsorption isotl icri i i models using (me ol tlic equations

,K

~

L

60 QUIRONES AND GUIOCHON

Later, Misra ( 6 ) derived severill other adsorption i~otlieriii iiiodels froin the differential equation

_ - do - K ( I - H ) ' , 4, 1211 whcre cl~(H, p ) is ii Suiiction OS both tlic surf :w coverage

and the partial prcssurc O S tlic :~dsorhcnt. Accordingly. we consider the following equation

whcre k is a constant. The general solution oSEq 121 1 is Eq. [ I O ] (Table I ). When 1. > I ~ certain theoretic:il conditions of Inononiolecular ;idsorptioii are met. Howevcr. Eq. 12 I I is empirical. When k assuiiies viilucs of 0. 1. or 2, Eq. 1211 reduces to the Henry (2) . Jovanovic (14), or Langmuir

between Eq. [ I S ] and the Jovanovic model ( 14), considering the proporiionality between q and x on the one hand, between

~ l ~ ( H , p ) = ( I - N ) X ( H , / J ) . I271

This rel ; l t ,ons~l ip ci,ll hc rCM,riIICIl ils

( 13) isotherms, respectively. I t is woi-th noting the similarity - d ( I - 8 ) ( I - d)dp

h ( 8 . IJ) . 1281

7. and S on the other. However, the concept of monolayer adsorption was first introduced by Langmuir ( 13) while Eq. [ 4 ] was originally derived by Jovanovic (14). using a kinetic ipproach. and later by Jaroniec (28) , using statistical ther- nodynamics.

Misra (6) considered that the functional dependence of :he variation of the surface coverage with respect to the idsorbate partial pressure at constant temperature is given IS

Equation [2S] was selected because X ( 0 , p ) has a physical meaning. I t is the rate of decrease of the fraction of the surface which is unoccupied by the adsorbate molecules. Most of the classical isotherms can be analyzed using Eq. [28] . The functions X(0. p ) were derived for these isotherms on homogeneous surfaces (Henry ( 2 ) . Langmuir ( 13). Jova-' novic (14), quadratic (15), and Fowler (16)) and on heterogeneous surfaces (Freundlich ( 17). Langmuir-Freundlich ( I S ) , Toth ( 5 ) . Radke-Prausnitz (19). and Dubinin-Ra- dushkevich (20)) . They are listed in Table I . Among these equations, we can distinguish those which do not take the adsorbate-adsorbate interactions into account (Henw, Langmuir, and Jovanovic) and those which do (Fowler and quadratic). We also observe that for all models which reduce- )r rather, considering the ProPortion of uncovered surface

\rea, to the Henry law at low partial pressure, the A-function, X(0, p ) . tends toward K when p tends toward 0. Thus, the limit of the A-function for low values of p (hence of 8 ) is, K. the low pressure equilibrium conslant. This conclusion

- K / ' ( I - a ) . [23] dQ dr, _ -

<quation [21] is one of the simplest forms of Eq. 1231. kcently, T6th (7 ) proposed the derivation of thermodynain- cally consistent isotherm equations for homogeneous or het- :rogeneous surfaces from one of the differential equations

/here Il,(Q) and & ( p ) are functions or the surhce coverage xi0 or the partial pressure. respectively, which are derived -om an incorrect.isotherni model. previously obtained c n - irically (7) . It is clcnr that Eqs. (191 and 1201 can be :duced to Eqs. (241 : ~nd 1251, respectively. Consideratioii r 4 s . 1241 and 1251 suggests thai a more gencrd relation- lip is

is also obvious from Eq. [2S]. Isotherms which are explicit with respect to pressure give

values of A ( H,p) which depend only on the adsorbate partial pressure. Implicit models (e&, the Fowler isotherm) give values of h ( 8 . p ) which are functions of both H and p . For ~

several models (Langniuir. Fowler, quadratic. and Misra), : h(H. p ) decreases with illcreasing partial pressure of t h e ' . adsorbnte. However, in the case of the Jovanovic i so thenn~; h(R. p ) remains constant and equal to K . For the other ;: isotherms, tlic relationship is more complex and depends on $.

this aspect must be discussed for specific cases. I t will be analyzed latcr.

Consideration of thc A-functions in Table I suggests try;

. .

f

thc specific values or the parameters of these models. Thus, 11 :i

ing the following function j

+ h(M. p ) = o i ~ ( q i ) " - ' , 1291.

:in cxpressioii which is also found i n the nunieraior of the Frcundlich cqu:Itioii. On l l ic other Iland. the numemior o f the Liingiiiuir- Frcundlich cqu:itioii is also siniiliir io tlic Illis of.

i

N E W JOVANOVIC-I;REUNDLICH ISOTHERM MODEL 61

I ) . Substitiitioii o l Eq. 1 29 I inlo E q I28 1 iiiid i i i tcgw ,,cs

O , ( , > ) = I - <.Fl.W 130 I

odcl ciui be consiilcrcd tis ii c ~ m h i i i : i t i ~ i i o i t l ic Jov:i- ~ n d t l ic Freondlich i s o ~ l i c r ~ i i ~i iodcIs. For I, = I. i1

; to t l ie Jov;movic isotlicrm 1or I i om i~ fc i i c~ ius sur- r0r hrsc villues o i tlic piirtiiil lircssurcs. /f,(p) tcnds tinit); ii nionoliiycr co\'cr:ige i s iicliicved. For low of the partial ~ i rcssurc~. the isoIIicriii cquation i s

1 u i t to tl ie Fi-euiidlicli isotlicriii I 17 1 . Accordinfly. ,del ciiiinot account for ii Henry liiw ( 2 ) region. On ier hand, the coverase riitio i s given iis :in enplici l ,n of tlie partial pressure of tlie adsorbite. which i s :I :,-able advantage for the dererniinatioii o f isolherms chromatographic methods of frontal analysis or pulse ateau (29) or for their use i n tlie calculation of elution s or other concentration signals at high concentrations. ionlinear conditions (30). Note that Eq. 1301 i s equiv- J a cumulative Weibul l distribution (42).

new model can be used 10 represent experimental rncerning adsorption isotherms on heterogeneous sur- omposed o f several subsurfaces which are themselves ;eneous. so the overall isotherm i s the sum of several sions such as Eq. [ 3 0 ] , each one written for one of bsurfaces. L i k e other inodels developed for the de- In of monolayer adsorption on heterogeneous surfaces it lateral interactions, this model can, i n principle, be ed to describe monolayer, sinFle-gas adsorption wi th interactions, multilayer single-gas adsorption without

2 lateral interactions, the adsorption of mult icompo- as mixtures, or the adsorption o f multisolute dilute Sns (3) . For example, the extension of tlie model to )e competitive adsorption can be accomplished using 5 of the adsorbed solution lheory ( 3 I ) or the inelhod )ed by Jaroniec e! nl. (32). integral equation [I] ciin be solvcd :iniilytically wi th

I to [he energy distribution l i i i ict ion i n the p;irlicular n which one assumes :I L; inp iu i r Iociil adsorption ,111 and a global ;idsorption isolhcriii given b y Eq. [ 301 dul ion is derived usin; the procedure suggested hy IS. 33) and was recently suiiiiii:irizcd by Jiironicc ( 3 ) crlse. Eq. [ I ] i s rewritten :is

where the constant o i s the same piiranicter as in Eq. 1 301 I t s temperature dependence i s defined iis

~ 3 5 1 [I = K"e'.""

where K" i s the preexponential factor, R the ideal gas constant, and T the absolute temperature. The pardmeter < i s defined as

I. 1361 = e<c-<:,ul:T -

The relationship between the global adsorption isotherm and the function <I(<) i s given by

while the energy distribution function i s relaled to the function QJ(c ) by

\r,(<) = R T F ( < ) . 1-18]

Considering Eqs. [301, 1341. iiiid 137). we see th31 i l i e function @ ( E ) for the Jovanovic-Freundlicli isotlicrni model I S

,I,(:) = 1 - [ , - I : - ' l ' 1391

Combining Eqs. [ 33 1 and 1 39 I giws tl ic v:ilue o f ihc function qf(c) ,

.,<(,=.,,-? - (,- 'c,-, .- , ,- , , 1401

c

2ii; wr, =

Using Eulcr's fur i i iul i i3 E(]. 1 40 I ciin hc triii islbriiicd inio

! QUINCJNLS AND GUIOCHON

TABLE 2 mar^ of thc Adsorption Data An;ilseed i n This S t i i d ~

C H S I 293 CH,CI

Cl.l2C1? 2% CHCII 2% CHCI, 29.3 CHCI, 29s CCI. 288 CCI, ? Y 1 CCI, 298 C2HCI, 28s CIH,CI, 293 C;H,CI, 298 C?CI, ?S8 C,CL 293 C?CI, 298

I S 19) 17 (91 I S 19) l j 191 15 I91 IS (9)

,er of data points in the original work plus lhc coordinate origin e outlier in the case of syriem 2.

iting Eqs. [36] and [38] into Eq. [41] gives the final ion for the adsorption energy distribution

.tribution includes an exponential decay at high ad- energies and a maximum value for E = L., which is

RESULTS

dsorption data used in the present work to illustrate isotherm model described above. in Eq. 1301. and its potential usefulness were reported by Hines PI

) . 2 These data regard the adsorption ofscve~-;tl chlori- 'drocarbons on a microporous silica gel at differenl .ures (see Table 2 ) . The surface should be consid- leterogeneous because the heat of adsorption tends ise with increasing conceniration of the compound: 'hese experimental d;tte were fitted to t l ie Tdth ( 5 ) . 4 ( I O ) . and Hines ( I I ) models of isotlicrtii ;tdsol-p-

tion 1 0 ciinipxc the pcrlorm;incc of ~licsc tii(idc1s i n :iccount:: ing for tltcsc ditlit with t h i t l of the l i ~ ~ ~ t i t ~ i \ , i c - l ~ r c i i i i d l i c h IiiodcI (Eq. [X)]). Thc T611i i ~ n d Uiiiliiii IitodcIs \vert chosen i n this wwk bccousc ilicy are cimsidcrcd :IS t l ic hcst :ill-

itround thrcc-ciinst;tnt isotlicrm i i iot lcls Iiir iiiicroporous ad. siirbcnls iiiid thcy wcrc tliosc s ~ l c c ~ c t l Iiir 1l1c \ystcttiiltic correliitioits of llic I:irgc d;it:i b : t t t ~ of x w r p ~ i ~ ~ i i isotlicnns on lhctcrogcticous surlhces ( I ) . The I-lilies iiiodcl is one of llic 1110~1 rcccit~ modcls for h c t c r o ~ c t i c ~ ~ i t ~ :~c lsor l i c~ i ts which itssitnics t1t:ti tlic local ;tdsorpiioii iwtheriii (III lhonwtitttic silcs is dcscribcd by the Sovano\,ic iiiodcl ( 1 4 ) .

The ~ioiiliticiir regression ;in;ilysis of (t ic i i iodcls uw carried out using ii fitting proccdure b x c d oii h4;irquilnlt algo- rilhiii (341, which niiniiiiizes the residual s l i m i i f t h e squares of the diflerences between the experinienr;tl data and the model calculations. The estimates of the Iiiiiiltiieiers present in tlie models are given at the asymptotic 95% conlidence interval. Conveniional use of the Fisher disiribution is not possible in this case because there is only one data point for each value of the partial pressure. As suggested i n (351, for each model and each set of experimental data. the Fisher parameter was calculated according to

where

9Exn,i are the experimental values of the solid phase con-

qLIP IS the mean value of the data, q.,p., for a given system, 9<., is the estimate of the solid phase concenti.:ttioii of the

/ is the number of adjusted parameters i n the inodel. and in is the number of experimental data for a given system.

Equation 1441 is different from the convcnrional Fisher equation. The second factor i n its RHS contains ilie sum of residuals in the denominator. Thus, the higher Fc,,,, the better Lhe model correlates the expet-iment;tl diita. The first factor in the RHS of Eq. ( 4 4 1 decreases with increasing nunibcr of p;tr:tmcters of ( l ie model. This cqu:itioii :ttlows the comp;lrison of models having diflcrcni iiuiiihers of paritmeters (35 ) .

Tables 3-6 sumni:Irizc the results 01' i l ic ~non1iiic:w rcgres- sioii :tn:tlysis of [lie models evaluaied i i i this sludy. These tnbles report ilic parnmeler csiimates. their conlidcncc intervals, and (lie Fisher v:tIues for cacti sei of cxpcriiiicniill d:tta.

dcviatiriiis ( A A D ) produced tip !he Jo\,:tIi(ivic-~rcutldlich ~iiodcl. i n ordcs to compwc ilicsc dcvi:itiotts witlt IItosc ~ e i i - crated hy 11ic l i i of ~ I i e d:ttit to ihc Ltngiiiuir IiiodcI ( I3 1 imtl

the V:ic:iticy Solution Miidcl lmscd t i i t i l i c iisc 111 i l ic IyI(iry- IHu:gins cqti:ilioii (36) lor ~Iic : ic i ivi iy ciiclficiciii I.cporicd

centration of the adsorbate for a given system. -.

adsorbate by a given model,

For systclns 13 1 0 18, wc c;llcul;ltcd thc :Ivel';lgc :lllsoIule

I .: I 4

'' I)i,

NEW JOVANO\'IC-PREUNDLICI~ ISOTHERM MODEL 63

TAI3LE 3" Itegression Results for the Jo\,anovic-~reundlicl, Model''

IC111

'I. ,I x IO? l:~.,,,

I 5 . w f lI .2l~ 0.42 I O.Il5 0.74 f 0.113 1517.0 5.77 f 0.38 0.38 f O.ll6 11.77 f 0.lIh S5O.I 5.36 f 0.28 ll.30 f ll.Il5 I1.XII f O.Il5 xs2.s

4 5.26 I 0.2') 2.4 1 0 . 4 0.6h f 0.00 4111.7 5 . I2 f 0.33 2.11 f 11.4 (1.71 f 0.OS 2 3 w >

1) 5.02 f 0 . 4 2 I .6 I 0.4 0.7l1 f 0.IO 152.l> 7 j 6 7 1 0 . 1 4 8.3 f 1.3 l1.6l1 f l1.06 3.74l1 S 3.59 f 0.17 7.2 I 1.4 11.62 f 0.07 332.8 [I .7.51 f 11.19 5.8 f 1.2 0.65 fO . I l9 229.6

I 2.78 f 0.0S6 11.6 f 1.4 0.76 f 0.06 547,s 2 2.72 2 o.oss 9.6 2 a s 0.69 2 o.04 923.2 3 3.35 f 0.041 32 f 4.1 0.60 f 0.06 392.8 J 3.33 f 0.062 25 1 2.8 0.57 f 0.06 250.6

3.33 f 0.052 14 f 1.2 0.5s I 0.04 4s9. I 2.95 f 0.062 260 f 28 0.37 2 0.03 1530.0

I1 2.91 f O.Il72 13.0 I 1.3 0.64 f 0.04 1154.6

7 2 . ~ 7 2 0.047 20s 17 0.41 f 0.03 1638.7 S 2.76 2 0.090 144 2 24 0.48 f 0.06 160.2

Units: 9.. mmolelg: (I. (mm Hg).'. v and F,,,,. dimensionless. Model equation. Eq. 1301.

Hines ( 9 ) . The AAD were calculated according to the xession

[451 I 9eip.l - 9j.i I 9exp.i

AAD = I00

TABLE 4' Repression Results for the Toth Model

I 2 3 4 5 6 7 S 9 I1 I 2 3 4 5 6 7 S

8.1 f 1.5 7.5 f 1.6 7.1 f 1 . 1 7.0 f I .7 6.7 1 1.7 0.6 f 2 4.4 f 0.8 4.3 f 0.8 4.1 f 0.9 3.4 f 0.4 3.2 f 0.4 3.2 I 0 . 3

3.45 f 0.06 3.6 f 0.1s 3.7 L 0.2 3.3 f 0.2 3.2 f 0.2 3. I f 0.4

30 f 33

x5 i 110 5 z 4.5 S f IO 9 2 14 2 1 1.5

66 2 ion

2.5 2 2 3.5 t 4 2.1 f 0.9 3.4 f 2 3.4 f 1.8 1.4 f 0 . 3 1.4 f 11.5 1.7 f 0.6

11.19 f 0.010 0.22 f O.Il1 x

0.69 3 0. I 9 0.8n f 0.3

0.SY f 0.2

0.05 1 0 . 4 0.60 f 0.24 0.63 f 0.26 0.6s t 0.36 0.69 f 0. I 7

0.76 f O . l X 0.99 f 0.13 0.77 f 0.14 0.6') I o.i2 0.45 1 0.00 0.4~) 1 0. IO 0.53 1 0.1s

n.xs 2 o.2

0.66 2 0.3

o.xi z w

007.46 4s11.92 555.46 I77.h I 123.1 I S4.69

162.12 144.90 95.llll

225.70 287.39 643.86 235.911 269.36 x')2.09 56O.Il5

UII. I7

378.2s

TABLE S" Regression Results lor the U N I L A N Madcl

I 2 3 3 5 0 1 S 9 I I 1 I1 I2 13 14 I S

7.4 I 1.2 7.1 1 1.2 (1,s I t o 6.3 1 1 . 1 0.c f 1 . 1 5.C) f I .? 4.11 f 11.4 4.0 f 11.5 3.9 f 0.5 3.2 f 0.2 3. I f 0.2 3.0 f 0.19

3.45 f 0.04 3.4s f 0.0s 3.54 f 0.10

1.6 1 1 1 ' ) 1 . 1 f I.! I .AI i 0.0 2 . 0 2 1 .1 I.(, 1 1.7 I . I , f I .h 2.1 I I . 0 2 . 0 I 1.0 1.8 I 1.4 1.7 f 0.7 1.3 I 1.0 1.4 f 0.7 0.4 f I .7

1.63 I 0.6 1.9 f 0.5

2 2 6 f % 243 1 'Jb 20s f s2

35 1 I'J 4 2 f 2 0 51 1 3 1 1

8 f 2.7 9 f .xi, I I f 5.1 5.3 f I.? 011 f 1.5 7.2 f 1.4 I.? flI.11 2.3 1 0..3 4.0 I 11.5

16 3.03 f 0.06 3.4 I 0.4 0.16 f 11.l115 1336.l11 17 2.97 I 0.07 3.0 f 0.4 0.22 f ll.Il2 92S.I') 18 2.9 2 0.15 2.6 i 0.8 (1.35 f lI.OS 106.5s

" Dimensions: as in Table 3. with the addirian of C. 8wn Hg

Table 7 reports the AAD values calculated according to Eq. 1451 for the selected models. Figure I shows tlie best f i t of the Jovanovic-Freundlich model to the adsorption data of methyl chloride. Table 8 and Fig. 2 illustrate the temperature dependence of the parameter a in Eq. 1301. according to the linear form of Eq. [35] for the six chlorinated hydro- cai-bons studied. Figure 3 shows the pressure dependence of the function X(B, p ) for the Jovanovic-Freundlicli (Eq. [30])andT6th(Eq. [13])models inthecaseofchlorometh- ane at 298 K. Figure 4 represents the energy distribution functions calculated according to Eq. [42] for dichloromethane at 288. 293, and 298 K using the parameters 0lT;lbles 2, 3. and 8. Figure 5 shows the adsorption energy distribution\ calculated according to Eq. [42 ] for the six chloriiinted hydrocarbons studied at 288 K.

DISCUSSION

The rcsulls presented i n Tables 3-6 show 1hat tlic p;ir:ime- icrs of tlic Jovanovic-Frcundlich isotherm model were idcii- tilicd with a smaller error than those of all the other iiiodels studied. The Fisher values obtained for all tlie systems s l i d ied arc largcr for the regression ofthe Jovanovic-Frcundlich isolhcrni model than for those of tlie T6ih and UNILAN models, willi the exception of I.I,I-trichIoroctIiii~ie :it 7SS K. showing th:il the datn l i t better to [lie lirsi tliiiii to the ollicr two niodcls. Note that tlie l i t of tlic d;ii:t for I, I, I - I~chIOroc111ane :11 288 K to [lie T6th and UNILAN i i iodel\ is : I I S O better lliiiii their li t to Hines model. Cotiilxirison <)I 1111. rcsitlls 0ht;liiied willi IIK Jo\~:iii i i\~ic-Frcuii(llicli iintl 11ic

QUINONES ANI) GUIOCIION

TABLE 6" Ilegrcssion Results for (lie Hincs el 01. Model

6.78 f 11.03 6.69 1 11.117 0.49 2 11.116 5.79 : 11.2s 5.77 i 11..73 S.Ocj i 11.45 3.8s f IJ. I 3 3.86 J 0.18 3.7s 1 11.20 3. I I 2 0.03

21111 1 4.5 224 2 9 252 f s

37 2 IO 4 5 1 15 5 9 2 26 9 f 1.7

I 2 f 5.4 13 2 (1

5.7 f 0.5

2.') f 1.0 2.11 I 11. I'J

1.34 f 0.15 Ill I 52 s 1 4(1 I1 I 57

49 I .SI 3 3 t 112 311 J 4 5 34 t 5

-1 .5 1 1 1 7 -2.2 1 11,s -I.') 2 I 1 4 -11.4 2 2 -0.5 I 3 11.115 f 4

-1I.ti2 r 1.5 -11.36 I 1.7 -0.92 f 1.5 -11.42 J 11.2

51721 1494: 11137:

461 2% 141 54: 301 22s

4SI: I 3.03 2 0.09 6.6 i 1.5 I2 J 9 41.22 2 0.5 56' z 2.96 f 0.05 7.9 f 1.0 13 2 7 -0.25 f (I..? I 2 4

1 " < * " " C I < + , " I9 * Jl,,,,,, ,I ,> - I ,", 11.1, I, 0"L . ~ . Y , - ll"" ,..a - 2.7 \ I - I lYl,"

I 3.46 f 0.06 3.5 2 2.0 0 i 590 11.00 f 0,s 351 ? A E + nrm < " + I n " + ,<n ,I ",> - n I n.,

I,."" - "." 74, >.-" - ".UT "." - ,.- " - ,.," 6 2.98 f 0.05 0.04 2 0.04 -23 2 29' 1.09 2 0.6 180: 7 2.91 i 0.02 0.00 i 0.04 11 i 4900" 0.81 f 0.2 5 8 3 8 2.85 ? 0.08 0.00 i 0.13 0 Z 264000" 0.88 z 0.6 23:

Dimensions: 8s in Table 3. with the addition of K , . K,. mm Hg. K,. dimensionless. In this case the parameter KI is not multiplied by 10,000.

ies models shows that the performance of the Hines model upenor for a majority of systems ( 1 to 3, 10, 12 to 15, and 16) . By contrast, the values of the Fisher parameters close for several other systems (4 to 9, 11, and 16),

icating similar model performance. It seems that the ori- of the isotherm was not included in the initial regression

the data (8, 9) . If we include this point in the data set. vever. our estimates of the parameters of the Hines iso- rm model differ from those reported initially (6 , 9 ) . It suld be noted that the errors associated with the parameter mates are higher for the Hines model. especially for K2

I K3, than for the Jovanovic-Freundlich model. This latter del is also simpler than the former, having three paranie- ~ instead of four. The results of Table I show that the .D values are lower for the Jovanovic-Freundlich model

TABLE 7 .verage Absolute Deviations Observed with the Jovanovic- undlich Model (Eq. [301), the Langmuir Model (Eq. 131 and . (9)) , and the Vacancy Solution Model Based on the Flory- ggins Activity Coefficient Equation (VSM-FH, Ref. ( 9 ) )

em

Eq. 1.101 Eq. 131 V S H - F H

I I .22 I .06 2.05 1 I ,911 2.54 3.1.7 5 1.52 3.21 2.1s

0.61 3.25 2.33 7 11.67 3.44 2.22 ; I.SS 3.s7 4.47

than those reported (9) for the Langmuir model and for thl Vacancy Solution Model ( 3 6 ) . The only exception is fo l,l,l-trichloroethane at 268 K. In this case, the adsorptio~ data f i t better to the Langmuir model. All these results dem onstrate the suitability of the Jovanovic-Freiindlich is0 therm model 10 correlate the adsorption data studied here.

The Toth, UNILAN, and Hines models reduce 10 th, Henry law at low pressures, whereas the Jovanovic-Freund

6 1

NE\\' JOVANOVIC-I-'RElINDl.lCli ISOTI-IEIIM MODEL 65

I -

1 -

TABLE 8 Rcgrcssion Rcsulls for tlic Temperalure Dcpcndcncc

of llic a Paramcicr (Eq. (351)

s,a,<lard I<ej!russinll I l i l l C 111 A*' c., crmr cllcriiciclll

- 1 O f 9 I? 1 2 3 11.11178 0 . Y X X K X -16 2 111 29 I 24 11.11187 O.~J'J78?? -13 f 17 26 2 42 l1.1l326 lIJJ9Xl80 - 1 1 I I S ?I f 38 0.1129s l ~ , ~ J 9 l l l 2 ' ~ -33 f 7 1 8 2 17 0.0130 lI.V'J98SS

:I :I, I,

CI , -17 2 311 43 f 71 11.0556 0.99 I 744

1 CHXI 4 ccu 2 CH2CI2 5 C2H3CD

3 CHCI3 6 C2CU

nodel does not. Apparently, the experimental data avail- do not allow sufficient coverage of the Henry law re- . The success of the Jovanovic-Freundlich model to :r correlate the data may be associated with the fact these data are concentrated mainly in the regions of mediate and high coverage ratio, so the main drawback e Jovanovic-Freundlich model, its inability to correctly lduce the Henry law region, has little influence on the ity of the data fit observed. Besides, it is known that three-constant isotherm equation is unable to provide a ise fit of the experimental data at both high and low ice coverages ( I ) . The fact that the Jovanovic-Freund- model does not reduce to the Henry law is a drawback :h has a cenain imponance for a variety of reasons, )ugh it shares this property with other imponant isotherm els widely applied to describe adsorption isotherms on rogeneous surfaces. Such is the case, among others, of Freundlich (17), Langmuir-Freundlich (18). general- Freundlich (33) . Marczewski-Jaroniec ( 3 7 ) , Dubi-

-Radushkevich (20). Dubinin-Astakhov (38). and

I I

-1

x 10.' 5

i

Fritz-Schlunder (39) models. None of them reduces to the Henry law at low pressures.

The impossibility of accounting for the low partial pressure, Henry law region of the isotherm is a drawback of these isotherm models which cannot be ignored nor corrected for single-component problems. However, there is an important application of single-components i n which this difficulty can be alleviated; it is ihe modeling of competitive isotherms. In this case, a new approach has been recently proposed (40). based on models of the adsorbed solution theory. It allows ignoring single-component behavior in the Henry law region and gives results which are as good as the classical ones. Thus, i t removes the sensitivity of adsorbed solution theory models to the type of isotherm models used to fit single-component equilibrium data in the Henry law reZion. In compensation. the method introduces new paranie- ters that must be evaluated from binary equilibrium data, thus eliminating the possibility of predicting multicomponent equilibrium isotherms from sinZle-componenl data. This limitation is relative, however. because multicomponent data must always be generated. as tests 01 tlic model and, in l l i e case of actuiil adsorbed solution theory models. Tor corrclating adsorbed phase activity coefficients (3 I ).

In this case, there are two possibilities, either to iiicasurc single-component isotherms down to [lie Henry law ree' w i n or to measure binary equilibrium under constant lola1 prcssurc. From 11 practical pain! 01 view, the sccond solution is ol ren bcttcr. espccially when strongly ;idsorbcd compounds :irc concerncd. Furthcriiiorc. tlic cxperiiiiciital " A d s 01 tlcicI- miniition OS niulticoniponcnt cquilibrium isotIIcIriiis for ho!h gas iiiixturcs iind soliitioiis liiivc hccn sigiiific;iiilly iiiipsovcd i n rccciii ycass. i i ; i inly hcc;iusc 0 1 thc av:iil;ihili~y of coin-

! OUIk'ONES ANU GUIOCHON

I. 193. and 29s K .

Adsorption energy disrribuliotw c:ilcul;ilcd for dicliloromellxme

ata acquisition and processing (41 ) . Mol-eover. when ve been acquired in the Henry law region, it is possi- :orrelate them separately. importance of models that f i t single-component ad- n data well at high concentrations stems from the m observation that competitive adsorption behavior :r unimportant at low concentrations ( I ) and that ferent theories now available for the calculation of imponent isotherms provide good estimates at low ;es, while no model is completely satisfactory at high [rations (31) . Thus, high concentrations is the range :h it is desirable to find models accounting well for :omponent data, provided that these models can be ithin the framework of the adsorbed solution theory

esults of the regression analyses presented in Tables )w a slight variation (usually a decrease but. i i i a few n increase) of the monolayer capacity with increasing iture for all the compounds studied. However. the in1 error observed in the determination ofthis parani- :S not allow a definitive conclusion regarding this -his situation is the same as lor tlie results previously I (8, 9) . However, the Jo\.:ino\,ic-Frculidlicli iso- ives the smallest value 01 ;ill isntlieriii models for lolayer capacity. mperature dependence of tlie hcterogcncity paramc- 01 well defined. given tlic values of tlic cstiiiiaies 1 at the different tempcraiures end tliosc of the errors 'lie only exception is tlic trcntl cxhihiicd by the Iici- ity parameter detci-mined froin ilic l i t n1 thc d ;m for chloroethane to tlic T6th niotlel. At 2XS I < . !lie \.:duc d is close to unity. suygcstilig i l l i l l ttic L:tngxiiiiir 11s)' correlate tlicsc d a ~ :IS wcI1 :IS 111111~ complex I n fact. tliis rcsiili w;is olit:linccl prcviiiiisly hy I.liiics

(9) . :IS show11 i n T;ihlc 7 . Ncvcrtliclcss. tlic surfiicc is not Iioiiiogeneous. ;is indicnictl by llic dccrcasing trend 0 1 the v;iliics oftlic isosteric Iiciits ofadsorpti~rn (9 ) . Tlic st:itistical sinii1;irity beiwccn tlic \,:1111cs (11 1111: lictcriigciicily 1i;ir:iine- tcrs dctcriiiiiicd Iron1 tlic l ~ ~ ~ ~ ~ ~ i ~ ~ ~ ~ i c - l ~ r ~ ~ ~ ~ i t l l i c l ~ i i i i t l tlic T6th isotlicrni models (see T;iblc?. .i iiiitl 4 ) should l)c iioted, The ~iclcrogcncily p ~ i ~ l l n e t ~ ~ \ O f l l l C . ~ ~ J ~ ~ l l i ~ l ~ ~ ~ ~ - ~ ~ C l l l i d ~ ~ c h iiiotlcl iirc nearly indcpcnilciit 0 1 tlic icinpcr;itiire. The telii- Ipcr:iiurc dcpendcncc (11 tlic Iicicrngcliciiy p:iriiiiictcr is not c1c:irly dclined and l l i c tcnipclillurc dcpciidciicc of the ad- sorptinii energy distribution is :iIso s n i i i l l ( 3 ) . Milily empirical :ind semiempiric;il adsirrptioii isnilicriii cquiitioiis for heterogeneous surlaces produce adsorption energy distributions t l i a i we temperature independent ( 3 )

As shown in Fig. 4, there is :I great similarity between tlie adsorption energy distributions derived for dichloronieth. ane at the different temperatures studied. This result obtained with the Jovanovic-Freundlich model wiis also obtained by Hines el a/ . (8 ) . The values obtained for the heterogeneity parameter of the different compounds are close. The heterogeneity parameter of the Jovanovic-Freundlich model shows near independence of the nature of the compound. This confirms that silicagel exhibits the sanie degree of heterogeneity for the majority of the chlorinated hydrocarbons studied, in agreement with previous results regarding their adsorption energy distribution (8 ) . The distributions derived in this work, using Eq. [42], are similar in shape for five of the six compounds (Fig. S). There is a shift along the energy axis corresponding to the different values obtained for E, (Table 8). This result is again similar to the one reported previously ( S ) . The values obtained for the heterogeneity parameter of reirachloroethylene are different from those of a11 the other chlorinated hydrocarbons, which may be explained by the fact that i t is the only unsaturated com-

0.2 ' \ I

p,mnd "I' l l l C

UNIL/ Tlic C I

cllrdin; (listiibl

The lclllpc; ( I ! l l l C

T l i C S I

HowC' ,re r:ii fimieu - points of K" mlue mii i i i l

a. "'q ' i l ie

sure o lich i! studie faster the Jc functi libriul

nuiiie functi towar result Henr: creasl mode

x ' ,r

NE\\' JOVAl\ 'O\ ' lC-~I I~I INI) I . ICH ISOTI-IERM hIODEI. 67

Illit] studicd. Thc s:mic rcsull is dcrivccl frnm 11ic iinalysis llle values derivcd lroni llic rcgrcssioii of l l ic T6ih iintl J ~ L A N models 10 the ;!dsorplion (I:ilii o f this conl~~nili ld.

,.ncrgy distribution c;llcul:itcd lor this cotiipnund :IC- rllinf to Eq. 1421 docs iiot li;tvc tlic siitiic sliiipc iis llic ,Iributions of the oilier live coiiipoiinds (I?:. SI. ~l~~ l,ilrilmctcr o dccrcascs cxponcntiully wiili iticrcasin: l l l ,~f i~i i i re , as secii i i l Table 8 and Fig. 2. Largc v:IIucs :lie corrcliilioti coefficients wcrc obtained i n all CBSCS. .. s~alld:~rcl error of the csliiiiiition (cnrrclatior I is s i i i i i l l .

I,\rcvcr. the errors tmadc on thc cstinixtcs ( 1 1 A''' and (,,

: rather larfe because the correlations of thesc I W O p:l- neters are based on the use o f three expcrinienlal daki inis, resulting i n only one degree of Irecdom. The values K" are low. as expected ( 3 ) . For each compound. (he lue of ea derived from the regression is close to the liimuni value of the isosteric heat of adsorption (8, 9) . .-cquired by theory ( 3 ) . :lie A-function is plotted in Fig. 3 versus the partial pres-

re of the adsorbate. It is higher for the Jovanovic-Freund- h isotherm than for the T6th isotherm within the range idied. On the other hand, the function A(8, p ) decreases iter with increasing pressure for the Toth model than for : Jovanovic-Freundlich isotherm. At low pressures, the nction tends toward a limit equal to the low pressure equi- rium constant for the T6th isotherm. while it tends toward ".Y the Jovanovic-Freundlich isotherm (see Eq. [29] and ,tierical values of u and v in Table 3) . The fact that the nction A(8, p ) for the Jovanovic-Freundlich model tends sard infinity when the pressure tends toward 0 may be a ;ult of the fact that this equation does not reduce to the mry law a[ low pressures. However, the .k-funclion de- x e s with increasing pressure in a similar way for both 3dels.

ACKNOWLEDGMENTS

rllir work was supported in pnrl by UNDP projecl CUB/91/001. I.Q. nkr Ihc Labornloire dc GCnic Chimique ( U R A CNRS 192) or INP- 'SIGC. TOUIOUS~. F ~ I ~ C C rot suppon. and e ~ p c c i ; ~ ~ ~ ? Pror. D ~ . A . I\I. Ihclm. and ocknowlcdger Eng. RciiC Lcgrd from CQF. H:ib;m;~ Cuh;~ )>is Ihclp in the prcparaiion or campuicr gr:sphr.

REFERENCES

~~IlcnLucIa, D. P.. and Myerr. A. L.. "Adsarpliirn Eqii l i lv i iwn D:iiii ~~l:mdbook." Prcnlicc Hall. E n g l c ~ d Clifls, 1989. ~ u l h v c n . D. M.. "Principlcr or Adsorplion :!lid Adnirpliw, Pniccrrc\." \~'i~ey-llllerscicnce. N e u York. I9S4. J:lronicc. M.. and Madc). R.. "Physic:tl A ~ S O ~ ~ > I I W > 011 I i e ! c r c , g w w t s Solids." Elnevicr. Amslcrdnm. 1998. S c h i d l . G. C.. Z. 1 ' 1 , ~ Clrru. 77. 641 ( I'Jl I ) .

29. Dandi. F.. Gannord. M.-F.. and Guiochon. C.. .I. ColLrid Irircrjrrce Sci. 62,316 (1977).

30. Rouchon. P.. Schonaucr. M.. Valenlin. P.. Vidal-M~jd;ir. C.. ;tnd &io- chon. G.. J. P18j.r. C l t m 89, 2076 ( 1985 )

31. Scholl. S.. Schnchtl. M.. Sievers. U.. Sc1iwei~li;m. P.. :and Mersmonn. A.. Clmn. Dig . Tcechnol. 14, 31 1.1 1'ZI ).

32. Jaroniec. M.. Narkiewicz. J.. and Rudzinski. W.. J. Colloid Iriicrf,,cr

Charles S. Pamele, Wilber1 L. OConneli, and Harold S. Basdekis, Hydrosczmce, Inc

0 \'ci!atilc organics are used as solvents in the manu- f a c t u r ~ of countless products. Some prominent esam- P l Q of these products include crystallized organic h n i c a i s , printed materials, paints and coatings, and dYc lean ihg flids-to namc just a few.

One consequence of thcse manufacturing operations t h t large amounts 0 1 organics are being ciriitted to

atniospticrc e a c ~ ? vcar. stricter air-~ollution laws

ventilation air in a work area make it economically attractive 10 reuse this air. But before the air can be recycled, the concentration of organicr must be reduced to limits set by OSHA guidelines or fire protection codes.

Role of adsorption Vapor-phase activated-carbon adsorption has gaincd

f;ivor as a " A d or recovering valuablc solvcnts from ;itniospIicric emissions [ / I . This is especially true for-

applications where more-conventional techniques a unable to produce the required low outlet concent1 tions at reasonable cost.

Condensers-another nondestructive pollutia control option-are also effective in reducing hea emissions, but they sometimes cannot achieve I1

enough discharge levels. Condenser-outlet concenti tions are often limited to above 10,000 to 20,000 ppm due to the vapor pressures of the organics at the tempi ature of the cooling water. Better recoveries can achieved with refrigerated condensers, but this option usually limited to applications where water is not pr' ent to foul the condenser with ice.

Scrubbers using nonvolatile organics as the scru bing medium are another option. However, they i usually limited technically and economically to apF cations in which the spent scrubbing fluid can be reus directly without the absorbed organics having to removed. Desorption requirements are often prohil tivcly expensive because the residual organic concentt tion in the liquor must be extremely low for it to suitable for reuse.

(In some cases, emissions from large solvent-stora tanks have been controlled by scrubbing with the SL cooled solvent itself. Package systems for this patent technoloS).-U.S. patent 3,981,156-are offered E c o l o g Control, Inc., Houston, Tes.)

Although condensers and scrubbers are not elfecti in controlling dilutc organic cmissions, fume incine; tion is. However, i t is usually a more expensive cont method than rccovery technologies, because the orgai that is destroycd has value. Fume incineration can compctitivc wit11 mcthods that require additional se, ,ration cquipmcnt to cflcct a suitable rccovcry.

I\ Y " I " I / I Y . ~ ~ l , I l i l l l C I , , ' \ ; \ / t i l \ l l lTl l Li lCt l d~.<,,Lsl,o,,! ,IIC :,r,icIc. . .

ladcn air st reak downward through a fixed-c bed. Upflow design is generally avoided because c p;irticles can bccome entrained i n the exhaust a t h superficial vclocitics. Granular carbon is usually vored for vapor-phasc applications because it is not sd:i. ' casily entrained as powdercd carbon, and offers lo&;^^.. .i pressure drop across the bcd. +.?, . I

Spent carbon is usually regeneratcd with low-pressure steam. This drives off the ad ganic, which is usually recovered by conde vapors and separating the organic from the either decantation or distillation. The counterc upflow pattern favors high removal efficienc cnt quality.

rem. Feed to the adsorber is pretreated to remov

(high inlet concentrations or high-boiling c since these &in hamper performance.

Fig. I shows a flowsheet for a typical fixed-bed

Accumulations of solid materials can cause a n

removed upstream of the adsorbers. Because ope costs are directly proportional to the weight of or comoounds to be adsorbed and desorbed. Dretrea

centrations.

<

I

!

ally less than 10 ppm, as shown in Zone 1 of F,' I Containment of breakthrough emissions (ZO?

This is done by avoiding premature breakthroug by sending material emitted during breakthroug another vessel hooked up in series.

cling the noncondensables from the condenser ba the inlet of the online adsorber.

Containment of organics exhausted during th ing and drying cycle (Zone 4). This cycle prepa carbon bcd for renewed service.

In order to acliievc thk optimum interaction b adsorption and regeneration, i t is important to stand the factors that affect t he performance of vated carbon during these two operations.

Adsorption performance As yet tliere is no method t l m satisfactorily p

tlie perforniancc of adsorption systenis from the

. . , .. ...:

1 1 0 I. Time

t i l .

;. :,, . t!;cr should consult Ref. [2, 3, 41.

:. :. aspects. Readers who want to consider these aspects fur-

The adsorption of vapors onto activated carbon is controlled by the properties of both the carbon and the adsorbate, and also by the conditions under which they are contacted. The phenomenon is generally believed to result from the diffusion of vapor molecules into the surface of the carbon. These molecules are retained at the surface in the liquid state, because of intermolecular or Van der Waals forces. As the temperature falls, or as the partial pressure of

the vapor above the carbon rises, the average time that a molecule resides on the surface increases. So does the

$, fraction of the available surface covered by the adsorb- ,. ate. In this early stage, the adsorption can be described ' by the following equation, developed by Freundlich (31:

v = kP""

where k and n are Freundlich constants. A log-log plot of V, the liquid volume adsorbed, as a function of P, the partial pressure of the vapor, yields a straight line.

However, the carbon surface is not uniform and consists of sites whose activities vary. More-active sites will 8. $ become occupied firs and, as the activities of the re-

8- . .! maining available sites decrease, the adsorption e n e r g & 4 will change. This is why experimentally determined iso-

... fherms will deviate from Eq. ( I ) . The Freundlich equation also applies to chemisorp-

tion, where the adsorbed material undergoes a chemical reaction a t the surface. This phenomenon is not common in air-pollution-control applications, except when an impregnated carbon is used.

Eq. 1 rarely applies to systems in which the carbon is regenerated in place. Economics dictate that most regeneration processes do not drivc OR enough adsorbate 10 reach the region wIict.c the equation is valid.

The most accepted thcory describing adsorption in h e smallest pores is Polyanyi's potential theory [.'I. It holds that for a fixcd amount of adsorbed material, the free energy of adsorption, AF, is a constant and is a h c t i o n of the rclativc

.- ,

(1)

I,rcSSurc (r/ iJu):

where: T = absolute temperature, Po = vapor pressure, P = partial pressure of vapor, and R = ideal gas constant.

This relationship makes it possible to calculate from one measured isotherm the relative vapor pressures of a material in equilbrium with carbon at a variety of temperatures. These data are usually presented as a series of isotherms (Fig. 3). Isotherms can also be predicted by comparisons with homologous materials, as well as by modification [6J of Eq. (2).

Choosing an adsorbent The physical structure of activated carbon is not

known in detail, but i t is probably composed of ran- domly distributed carbon pores, between which lies a complex network of irregular interconnected passages. The pores.range in diameter down' to a few angstroms, and provide a surface area of around 1,000 m2 per gram of carbon. The volume of pores at each diameter is a n important variable that affects carbon performance (Fig. 4).

Since adsorption takes place at the carbon-gas interface, the surface area of the carbon is probably the most important factor to consider. Generally, the higher the surface area, the higher the adsorption capacity for all compounds. However, the surface area must be available in the proper range of pore sizes. If too much of the area is available in pores smaller than 5 A, many solvent molecules will be unable to penetrate the pores. That area of the carbon will be essentially unavailable for adsorption.

For most pollution-control applications, the surface area of pores whose diameters range between 5 and 50 .A yields good efficiency, because the relative pressure (/'/Po) of the vapor is usually too low for thc larger pores to become filled.

At high relativc pressurcs, however, the total pore volume becomes important becausc the macropores also becomc active.

The adsorption of bcnzcnc, for example, has been shown to be aRccied by pore size distribution [7]. At high benzcne concentrations, carbons in which largc porcs predominaic liiivc higher capacitics than those in which nrcdium or smilll porcr predominate. B u t at low

i i

some solvent on the carbon, even a freshly re$'

or vapors that will not be adsorbcd. If the inlet con&:<. .:' higher

water is o and cooled bcd \vi11 have a n equilibrium Concentr&~~: ..i: tively cor¶

10 30 100 1.000 10.000 100.000

concentrations, the large-pore carbon has a lower capacity.

Although the larger pores are active only at high partial pressures, they serve as passageways, leading the vapor to the smaller pores in the interior OF the carbon. This phenomenon is most important during regeneration, when the molecular flux is highest.

In general, the nonpolar surface of activated carbon makes it moreefficient for nonpolar organics than for polar ones. Silica gel, alumina, and molecular sieves, on the other hand, have polar surfaces and adsorb polar materials more efficiently. The polar adsorbents are rarely used for air pollution control because of their unfavorable competition for water.

Recently, it has become possible to alter the type of oxide formed on the carbon surface, in order to make it somewhat more efficient in capturing polar materials at low loadings. Some carbons, especially those of vegeta- ble origin, are more efficient for polar materials because their surface is more hydrophilic.

Adsorber operation Fig. 5 shows a profile of the adsorbate vapor concen-

tration along the axis of a fixed bed. Note that a portion of the bed, L,, is saturated, that there is a relatively short active-transfer zone, Lz, where most of the adsorption is taking place, and that a portion of the bed is unused.

T h e transfer zone m o v a through the bed until breakthrough occurs, at which time it starts to leave the bed. A bed that is shorter than the transfer zone will have essentially no capacity. Similarly, the longer the bed, the smaller the fractional capacity lost.,

The applicability of activated carbon for solvent recovery or air pollution control is determined to a large extent by the characteristics o l the adsorbate-princi- pally its volatility and polarity. From Eq. (Z), the a- pacity of carbon is proportional to the natural log of P/P,,, the relative saturation or vapor pressure, and therefore falls off considerably at low solvent concentrations.

Since'standard in-place regeneration always leaves

62

~__... trations arc low and vary widely, the bed will actuau" Y

This difficulty has bccn conf"d by data taken from full-scale adsor-bers used by a client to recover car. bon tetrachloride from a product dryer exhaust (Fig. 6 ) . The operation OS this instalhtion differs from that elsewhere, in that one bcd, A, is in seivice during the first 30 min of the dryer cyclc and is regenerated after each batch; the sec6nd bed, U, is in'service during the last 100 min or so of the cycle, but is regenerated either daily or after three dryer cycles.

Therdore, the inlet concentration in bed A is always high, as indicated by data for three successive a d s o p tion cycles (Fig. 6a). The inlet concentration at bed .B varies more widely (Fig. Gb), because of the decr&iG<' amount of solvent left in the batch. In fact, the inlet concentration a t bed B actually drops below the outlet concentration at the end of the second batch.

These data show that better than 99% removal.& achieved by bed A. In bed B, because the outlet content is more a function of operating conditions in the.+ bon, the removal efficiency of the bed is directly related to the inlet concentration.

tration is frequently set so that the vapor content d o s not exceed 25% of the lower explosion limits. Conken- tration may also be limited because of a reduction in capacity, or because of safety problems posed by high bed temperatures produced by the heat of adsorption; The maximum practical inlet concentration is usually about 10,000 ppm, and higher concentrations mustfre; quently be reduced by condensation or dilution aheid of the adsorption step.

Because granular carbon is a good thermal insulator, : it inhibits heat dissipation from the interior of the;G&J: to the walls of the container. The heat of adsorption (which is approximately the same as the heat of,?n:; densation) can usually be kept from buildine up b d - ' .

add organics to the air. . .

:I

If flammable vapon are present, the upper co

,-.;*,<

show watc and loadi.

Fig. 7 i

c a n be a I watcr exh bon bed is kictory pe d i ed to tl n)idity lo"

Less-vol carbon. T: l ow levels. organic m during rep '

materials practical I gas or ste:

Convenl The ke)

regenerab I ? h) dict: be used ii

characten: using thrc restricted) applicatio

Regenei by changi about a lo the tempe hances de

Low eq introducir: place the I the carbo]

Vapor-F atcd using and hot g the desoq

_ . . . . . lowing the temperatuk of th'e air stream to go up$& extent of the temperature rise can be estimated~by$$fi

;, .:?& suming adiabatic thermal equilibrium. ..:cL.,+A.l . ,.

Frequently, the carbon is left partly wet with con@$ i The &I

downstrea sate after the regeneration steo. SubseQuent evapora:?

aqueous p

.,...... : tion of the water-dissipates so& of the i ea t of adso$;i~ fion step i tion and limits the rise in bed temperature.

This tactic is helpful when the inlet conce high. However, it does impose a greater resist collection of the organic and may cause proble a very high capture efficiency (of, say, 99%) i

The influence of adsorbate volatility on is indicated by the term In (P/P,) in Eq. (2). at the normal commercial operatins temperatu to 55"C, carbon has insufficient capacity. l o pounds that boil below 0°C to makeadsorption materials practical.

Moreover, although the lower-boiling m be captured efficiently at reduced temperatures, bon's capacity for these materials is freque when competing water vapor is present at hi tive-saturation pressurcs. Fig. 7 shows that

water is only weakly adsorbed by carbon, it can effcc- lively coinpctc with marginally adsorbable orsanics ;,I

$ higher relativc humidities. (Rectilinear axes a,-c used to watcr loading data at adsorbatc c o n c c n t r a t i ~ r ~ ~

;,nd loadings near zero.) Fig. 7 also illustratcs that carbon loading 01 water

cnri bc a (unction of the type 01‘ carbon uscd. and th;,t water cshibits a hystercsis 01 desorption. Tlius, i l a carbon bed is exposed to a high water co~icent ra t io~~, siltis- I ;~xory performance cannot be achieved until tlic bcd is dried to the point where its exhaust has a rclativc h c i - ri!idity lower than 40 to 50%.

Less-volatile materials are more strongly adsorbed on carbon. They can usually be removed lrom air to very low levels. However, the affinity of the carbon for t he organic may make these materials difficult to remove during regeneration. Again, experience has shown that materials whose boiling point is above 200°C are im- practical to remove by in-place regeneration using hot :as or steam.

Conventional regeneration The key to the effectiveness of activated carbon is its

regenerability. Short adsorption cycles (often less than 12 h) dictate that in-place nondestructive regeneration be used in order to avoid the carbon losses that are characteristic of thermal techniques. Therefore, systems using throwaway carbon or thermal regeneration are restricted to low inlet concentrations, or to odor control applications that have extended adsorption cycles.

Regeneration, or desorption, is usually accomplished by changing the conditions in the adsorber to bring about a lower equilibrium-loading capacity. Increasing [he temperature or decreasing the partial pressures enhances desorption.

Low equilibrium loadings can also be achieved by introducing a more strongly adsorbed material to dis- place the previously adsorbed compounds, or by eluting the carbon with a suitable solvent.

Vapor-phase adsorption systems are usually regenerated using steam, hot gas, or a combination of vacuum and hot gas. Solvent regeneration is usually limited to the desorption of nonvolatile organics collected from aqueous phases.

T h e choice of regeneration method depends on the downstream use for the desorbed organics. The adsorption step itself serves only to concentrate the organics lor subsequent collection or destruction by other techniques. Although condensing them from the regenerating fluid is the customary approach, incinerating them in the regenerant can be a cost-effective alternative to direct incineration of dilute fumes-if the savings in capital and energy that result from using a smaller tume incinerator oRset the expense of the adsorption system.

Steam regeneration Direct steaming of the carbon bed is the most widely

u e d regeneration technique, because it is cheap and simple. Steam is very effective in raising the bed t c n - Pcrature quickly, andyit is a more concentrated sourcc 01 heat than hot gas. It is a readily available, condrnsa- b k fluid from which,many organics can be easily scpa-

rated and recovered. And, as previously mentioned, the high concentration of water promotes desorption of the organics.

Fig. 8 illustrates how desorption profiles change as a function of the amount of steam or other regenerant used. These profiles are ‘not simply a reversal of the pro- gression shown in Fig. 5.

To begin with, the individual curves do not represent equal time intervals or equal amounts of regenerating agent. The desorption requirements to achieve profiles 1 and 2 are relatively small. Increasingly larger amounts of regenerating agent are required to produce profiles 3, 4 and 5 . Regeneration of a bed is usually halted once profile 4 or 5 is achieved. A “heel” is always left on the carbon bed because complete desorption is technically difficult to achieve and economically im- practical.

The amount of steam required for regeneration depends on how much material is to be desorbed from the bed. A fixed amount is required to raise the bed to its regeneration temperature and provide the heat of desorption. A certain flow is also required to reduce the partial pressure of the adsorbate and carry the organics out of the bed. Passing steam through the bed lor a long time produces a n equilibrium between the xeam and the heel.

Optimizing steam consumption Fig. 9 shows a typical relationship between steam

consumption and desorption efficiency (relative to what would be desorbed with exhaustive steaming). T h e curves were developed lor methylene chloride and an intermediate (boiling point, 110°C) produced by a Hoffmann-La Roche, Inc. pharmaceutical proccss.

The data show that during the initial heatup pet-iod, nothing comes out of the adsorber. After this period, substantial amounts of adsorbed material are released, until a plateau is attained. This occurs when an equilibrium is reached between the remaining adsorbed organic and the prevailing operating conditions.

I t is not cost-effective to try for 100% dcsorption. Ac- ceptable working capacities can be acliieyed without consuming so much steam. For solvent recovery sys- t e m , a r c q u i r e ~ e ~ ~ ~ a f . 0 . ~ 5 ~ ~ ~ . 3 5 !b or s t C Z n 7 i S T carbon usually ~~ has been ~ ~ c i ~ d . This requircment rcprcscnts ai! opiimization betwccii the cost 01 steam

L , . > . . ... . . . . . , VAI'OI~-PIIMli noso, ,~on

during cooling depends upon the amount of heel i n turn depends upon how much steam was u reach hirh removal efficiencics. it is necessary to corit

20,000 10.000 20,000 10.000 1,000 2.0W 500

Flowrate, cfm Feed concentrotton. ppm

Capital charges at 24% $96.000 $50.400 $96,000 $50.400

_ _ _ _ _ _

U f l l l t l C S

Steam a t 0.3 Ibllb 01 carbon 40.300 40,300 20,100 20.100 and a t $4/1.000 Ib

and et S0.04/kWh

stcam and 81 $0.10/1,000 gal

Electrieityar 5 hpl1.000cfm 25.100 12,500 25.100 12,500

Cooling watcr at 4.2 g a l h 4.200 4.200 2,100 2.100

'

~

Carbon replacement at 4-yr 9,000 4,500 9,000 4.500 : lifetime and at Slllb - _ _ _ _ _ _ _ _ .

~

Total SI 74.600 $1 11,900 $152.300 $89,600

and the cost of bigger beds to compensate for reduced working capacity.

T h e shape of the desorption efficiency curve depends upon what is adsorbed, the loading capacity, and the regeneration conditions. Saturated steam at pressures of u p t q m i s usually used for regeneration, although pressures o f .up to 50 psig are feasible. Superheated steam is not. usually advantageous, because the adsorbed organics may react at elevated temperatures. Often, these reactions decrease the life of the carbon bed. Also, the extra cost of superheated steam is not usually justified by the marginal improvement in desorption.

'The rate of steaming should be limited to -/ (min)(ft2) to prevent the carbon from being shifted in the bed. Such shifts can cause flow distribution problems.

T h e heel that is left on the carbon influences the equilibrium and dynamics of the next adsorption cycle. Any optimization of steam requirements will have to take into account the impact on effluent concentration during the next adsorption cycle. Data are generally un-' available, but the concentrations will probably take the qualitative form shown in Fig. 10, in which no values for steam usage have been assigned to the abscissa.

Importance of cooling and drying

.

.

Aiter regeneration, a hot, wet carbon bed will not remove organics from air eKectively because, as previously discussed, high temperature and humidity do not favor complete adsorption. If a separate cooling/drying cycle is not included, the concentration at the outlet will at first nearly equal that at the inlet, then fall to thc baseline concent~mtion as thc bcd is cooled and dried by the incoming gas.

T h e laboratory data in Fig. 12 show that, when air with a velocity .of 40 fpm is used for cooling, a W C I I - insulated, Ill-in.-dccp bed will cool in approximately 10 min.

T h e wmx" or organics crucrgins from 111c outlet

. .

., these organics, usually by recycling tl1c cxha bed currently in the adsorption stagc.

To do this, the beds and blower must be si vidc Cor the additional time and flow capacity re To route thc coolins and dryins air to the 0 t h additional valves and piping should be inslalled t he beds can be operated in series. The pipins n Cor rccyclins of the cooling/drying exhaust is vided in many existing systems. Tllc added cap can usually be justified only when high captur cics are desired.

should be able to handle this recycling. Carbo inents will not he greatly aKected,since the organics tha t are recycled from one adsorber is usually no more than 3 to 5% of the total adso duty. The additional load of water and organics generally not exceed normal process variati ever, the additional water may pose a problem if!' relative humidity of the vapor-laden air flow is a too high.

A properly designed and operated adsorption syst

Alternative adsorber designs Conventional steam-regenerated fixed-bed .syst

quiring development of One problem has

pecially carbon steel. Although stainless ceptable alternative for many tain crevice corrosion when exposed to ch solvents. (Many of these materials hydrolyze slig generate. HCI.)

Steel adsorbers with inexpensive linings h not performed acceptably in this service beca hole leaks. Another difficulty with liners is coefficients of thermal expansion diKer from that. substrate, making for poor bonding in se have frequent thermal cycles.

partial pressure of the adsorbate by sweeping it the condenscr.

To circumvent these problems, other appro being developed. For example, one firm tems Div. (Harleysville, Pa.); has develop resencrated system that changes the regeneratio ditions so as to reduce corrosion. Direc

The system shown in Fig. 1 1 controls emission a pill-coating operation. Regeneration is acco by :I combination of convective heating to

mcnt crnbcdded in thc carbon. Thc

Batch 3 w f l e l

Batch 1 outlet

0 40 80 120 160 200 240 280 301

evacuated to 1 m m H g absolute in order to promote desorption. The desorbed organics are removed from the adsorber by a closed-loop purge stream, and are then separated by a refrigerated condenser. Noncondens- a l k s are recirculated back to the adsorber so that the organics will not be emitted.

Vacuum regeneration can offer a cost-effective alternative when direct injection of steam demands the use of more expensive materials of contruction. It may also be called for when steam is not effective because the boiling point of the adsorbate is too high or because polymerization of the adsorbate is promoted by temperature and/or water. (This applies to vinyl chloride monomer emissions from PVC plants.)

The decision to use vacuum regeneration is determined by the tradeoff between costs for materials of construction and related steam system requirements versus the added equipment costs for a vacuum pump, refrigeration unit, and the slightly stronger adsorber- vessel design.

Shown in-Fig. 14 is the Purasiv HR system, which represents a different approach to adsorption that has been developed by Kurcha Chemical Co. of Japan and marketed in the U S . by Union Carbide. Only one ves- S(.l is used. The top of the vessel is a Ruidizcd-bed adsorption section; the bottom is a moving, dense-bed regenerating section. Carbon flow is from top to bottom.

The concept. of a fluidized-bed adsorber was made

possible by development of a spherical activated carbon produced from molten petroleum pitch. The shape and hardness of this new carbon were specified so as to avoid problems of carbon attrition.

Solvent-laden air enters below the fluidized-bed section, rises countercurrently through the trays of the fluidized-bed section, and exits out the top. Spent carbon drops continuously into the desorption section, where it forms a dense bed that is preheated by the heat exchanger.

The hot carbon is pursed with regenerating fluid, steam or nitrogen, which carries the desorbed materials to the condenser. When nitrogen is used, noncondensables leaving the condenser are recirculated through a secondary adsorption section between the adsorbing and desorbing sections. This reduces the concentration of organics in the nitrogen before it is recycled to the bottom of the desorbing section.

Desorbed carbon that emerges from. the desorption section is cooled by another heat exchanger and blown back to the top of the adsorption section for another pass through thc system.

Nitrogcn is usually used instead of steam with water- miscible solvents. This eliminates the need for addi- ' nal unit operations to separate the oxanics from the

is not g e n c r a n ~ ~ 6 F i & e of the danger - -. _ _ ~

poscd bf thc prcscnce of oxygen and organic vapor; in contact with a catalyst (the carbon). /J

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... , . . : ,. . ~ j , . , ,, . ,

0.60 , ,

A Darco carbon 8 Typical of Pittiburgh BP or

Union Carbide SXA whon

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e S o d o n DO",CO S*l,am SWd"." Y - 0aum.n, No. PB.221.13B. PP. 4.14

< 0.40 -

m

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D 51 0.20 - I

: 0.10-

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Relative humidity. IP/P,l

The main advantages of this scheme arise from the single vessel design [8 ] . Energy savings over fixed-bed designs are likewise anticipated because the vessels are not alternately heated and cooled. Savings in construction material costs may also be possible, since the equipment is not exposed to such corrosive conditions in the desorption section. The need for peripheral equipment such as valves, timers and controllers is said to be reduced.

Design data and scaleup One of the most important design parameters is the

working capacity of the carbon in a particular service. Isotherms for the more common volatile organics are

available from the literature, vendors or carbon manu- facturers. It should be remembered that these curves describe the equilibrium capacity that was measured under specific conditions of temperature, partial pressure, carbon type, and relative humidity. Working capacities are always less than isotherm capacities, because the isotherm conditions are rarely duplicated in the real world.

Fortunately, significant scaleup experience exists for straightforward applications involving the more common organics. A general rule of thumb is to use one-half the isotherm loading at the design temperature and partial pressure. If isotherm data are not available in the anticipated concentration ranges, laboratory evalu- ations are necessary. Isotherms can be measured for single-component systems. However, if thc adsorption performance o r a mixture is in question, one should make a Series of column runs to measure the working capacity Of the carbon under anticipated scaleup conditions.

Laboratory runs should be made in a carbon tied that is.18 to 24 in. dcep-thc normal depth of full-scale

Lenqth. in. a

- units, The diameter should be large enough to elimi- nate wall effects. The system should be designed to minimize losses of volatile materials. It must be 1arg.e enough that the loss of several grams of material wo not greatly affect material balance calculations.

As in any miniplant or pilot-plant evaluation, co tions in the laboratory must accurately reflect the antic:, ipated full-scale conditions. Accurate characterizatibd of the emission that will be controlled is essential. Oh": ously, the flow and concentration of primary maten,al to be adsorbed should be known. But other materials that might be present should also be identified so that a full Understanding of the service can be reach

The importance of this understanding wa strated during the above-mentioned Ho Roche project, whose purpose was to control emissions bearing methylene chloride. Analysis of the emissions revealed the presence of a volatile intermedia

f steam would be needed to effectively desorb quently, process changes were made so as to mini&e, the amount of intermediate emitted. This tactic fo'ie stalled adverse effects on the adsorbers' performa removing methylene chloride.

The influence of water vapor on adsorption readv been noted. When desien data are generated..+

. .

cated by the data in Fig. 9, an uneconomical .

-

bu t th< wlatilc vaporii other s would : - be ads<

Ther since ti enough tion cyc to prev adsorb, t l i e org metric

ally ch: ity mu: spent i though installe change put. If

APPl

K l l O U g k

a n ade Pres:

lection I I

the laboratory, it is important to measure the r humidity so that differences between laboratory tions and plant conditions are fully defined. One recognize that plant emissions rarely have the lo rive humidity (-20%) that is characteristic o f t ratory compressed air used to approximate a n If high-humidity conditions are anticipated, th sion should be humidified.

Full-scale design A crucial aspect of designing a full-scale syste

accurate definition of the adsorber duty. The tot rate and the amount of the emitted organic m predicted as accurately as possible, bccause the ' the adsorben will be determined from these

Overall material balances based on monthly. r i a will often yield only an order-or-magnit mate. Measuring thc weight lost when a batch

.. Y 100, _ ,

I 0 1 2 3 4 5 6 7 8 9

I Steam consumption. Ib of rteamllb of carbon

b u t the assumption that all the weight lost represents volatile organics should be verified. Water, entrained or vaporized product, and lint are only three examples of other sources of misleading weight losses. Such losses would give high estimates of the amount of material to be adsorbed.

"ere is usually some flexibility in sizinK an adsorber. since two factors will be balanced: (1) ;here must be enough carbon for a n adequate and effective adsorp- rion cycle, and (2) cross-sectional area must be sufficient to prevent excessive pressure drop across the bed. Thus, 'ldsorber sizes are limited either by the mass flowrate of the organics and the working capacity, or by the volumetric Rowrate and the depth of the bed. A p p l i c a t i o n s limited by adsorption capacity are usually characterized by high inlet concentrations. Capac- ity must be large enough to allow adequate time for the spent adsorber to be desorbed, cooled and dried. Al- though systems with adsorption cycles of l h have been installed, they provide little flexibility for operational changes that improve performance or increase through- /)ut. If possible, new systems should be sized to contain GX@E"-Fol-arlem~of adsorption, normally,$ an adequate time for the carbon to be r-erated/

Pressure-drop c o n S i ~ e r a i i o ~ ~ ~ ' u s ~ ~ l y - ~ o ~ e r n the selection of bed height and cross-sectional area. Fig. 13 shows how pressure drop is influenced by linear velocity ind bed depth. If a centrifugal fan is used to move the iir, the allowable pressure drop is limited to 10 to 20 in. If water, and cross-sectional areas are designed for lin- 'ar velocities ofS&to U f p m . Bed depths then usually ange from 12 IO 36 in. The appropriate vessel size c a r ,e determined straightfomardly from the volume of :arbon, linear velocity, bed depth, and pressure d-

If inlet concentrations are high, the cross-sectional m a dictated by the volume of carbon needed usually esults in a linear velocity at the low end of the r a n g e s 0 100 fpm. Vertical cylindrical vessels are favored for mall flows. For large flows, horizontal vessels 0 R . r an .Iternative to fabricating short, large-diameter vertical usels.

Some additional design flexibility is available if the mission is under pressure before discharge. This is thc ase in many oxidation reactions that use air as the mrce of oxygen. Higher linear velocities and/or dccpcr

- .

..

-. . _.__

beds are possible when a higher pressure drop can be taken across the adsorber. The advantage is a smaller adsorption system, since more of the volume inside the vessel is filled with carbon.

Flowrate poses a restriction when it is high and the inlet concentrations are low (usually below 1,000 ppm). This situation often occurs when ventilation is needed insidethee ' anic concentrations below t e e m a n d a t e d -~&-- -- ~. ~~ by fire and/or in L e codes. Dilute emissions also occur in odor control, and in installations specified to meet workplace concentrations set by OSHA regulations.

In these cases, adsorbers are sized primarily for handling a large flow a t a low pressure drop. If the linear velocities must be limited to 100 fpm, the vessel is usually much larger than that indicated by the adsorption duty. Even though bed depths of 6 in. or less will yield an adsorption cycle of 2 h or more, deeper beds are often used if the pressure drop will allow it. The incre- mental cost of adding more carbon to a large vessel is more than offset by the added flexibility that a longer adsorption cycle gives.

Safety Activated carbon or its irppurities catalyze the de-

composition of some organics, such as ketones, alde- hydes and possibly organic acids and esten..These exothermic decomposition reactions can generate localized hot spots and/or bed fires within an adsorber if the heat

. is allowed to build up. These hazards will crop up if the (low is low and the inlet concentrations are high, or if an adsorber is left dormant without being completely regenerated.

A fire hazard can also exist during tlie virgin operating cycle, because there is little water present on thc carbon to act as a heat sink and because more material is adsorbed during this cycle tlian during othcr cycles.

These considerations do not mean that vapor-phase adsorption should not be selccted for thcsc potentially

r

Air filter and condenser

1"lCt

I I I I

P. " . "

hazirdous applications, but rather that safety and performance require compensation for hazards in the adsorber design and operation. For example, the careful choice of activated carbon and operating procedures has enabled General Tire and Rubber Co. to recover methyl ethyl-ketone from its fabric coating operations for over 15 years without incident [9 ] .

To reduce the hazard of bed fires, the following procedures [IO] are usually recommended:

1. The bed should never be left dormant unless i t has been thoroughly regenerated.

2. Design parameters should be set so as to avoid high inlet concentrations and low flow.

3. Instrumentation, including alarms, should be installed to monitor the temperature change across the adsorber bed and the outlet CO/CO, concentrations. The instrumentation should siznal the first signs of decomposition, so that any acceleration leading to a bed fire can be forestalled.

4. A deluge system should be provided to water-cool the bed in the wen t of an excursion.

5. A virgin bed should be steamed before the first adsorption cycle. Residual condensate will remove heat.

Cost factors T h e example of a fixed-bed system will be used here

to give some illustration of the associated costs. T h e system will be designed to handle 20,000 cfm of

air with a pollutant concentration of 1,000 ppm (a 25 : 75 mixture of toluene and hexane). Major items include a blower, two or more carbon beds, a condenser, a decanter, interconnecting piping and valves, and automatic controls.

Instrumentation that automatically switches the bcds between the adsorption and regeneration modes will either follow a tinled cycle or detect organic brcakthrough at the outlet. An adsorption cycle based on time is most erective when the emission profile is pre- dictable: When- the profile is variablc, a breakthrough

Cooling time. min

monitor becomes cost-effective because steam wasted in regenerating a partly loaded carbon

The highest-cost items of an adsorption syste the adsorbers, blowers and automatic valves, which all dependent on the feed-gas Rowrate.

The amount of carbon required for tbe beds will have an effect on capital costs in terms of blower sure drop, bed size, and condenser size. Vendors that the purchase price of a n adsorption system from $10 to $15/cfm. Installation cost ranges from 100% of the purchase price, assuming that there, ready some ductwork to the adsorber and that u t hookups are available at the ba t t ev limits. ' S mounted units in the 5,000-cfm range or below available off the shelf, and can provide a saving custom-built systems.

Materials of construction stronelv influence th

costs fc replace around ciation trative I

irrials i

The exampl chase p The ste. Ib per I carbon superfic IS assun which I

The i horseDo " ,

tern cost. The purchase costs cited above do not to systems that require protection against the pr of HCI, generated, for example, by hydrolysis of a rinated solvent. Metals that corrode at a high o rate or pit rapidly or sustain crevice corrosion a

loss of production. Valve selection is especially important for ap

Systems in excess of 10,000 cfm need large handle the flow, and frequently require more t

to reduce installation costs by using inexpensi which will not be leaktight.

The annual costs for a system include thc fi

CWWCAL I ~ N C I K I X I U N C micmm:n 51. ~m

E (4

costs for steam, electricity, cooling water, and carbon ,; replacement. The annual capital charges are normally

around 24% of the installed cost, which includes: depre- ciation and interest at 1590; taxes; insurance; adminis- 1 trative charges at 4%; and maintenance, labor and ma- :.erials a t 5%.

T h e installed cost of the 20,000-cfm system in this example will be estimated a t $400,000, based on a pur-

.; chase price of SlO/cfm and a n installation factor of 2. 7 The steam consumption for such a system is around 0.3

Ib per Ib of carbon. It will require about 18,000 Ib of $ carbon per bed, based on 3-ft-deep beds whose design 2 superficial velocities are 100 fpm. The carbon capacity

is assumed to be 0.0625 'Ib of organic/lb of carbon, i: which means that the adsorption cycle will be 4.5 h. I' T h e amount of electricity used is estimated from the '4 horsepower requirements of the blower. Cooling-water :I usage depends on the amount ofsteam used for regener- .ij ation; it was calculated here to be 4.2 gal/lb of steam. '4 Carbon replacement costs are based on a 4-yr life-

time. Actual lifetimes have ranged from 1 yr to at least I O yr, depending on how effectively materials other than the primary organic were excluded from the bed.

Total annual cost for this carbon adsorption system is S174,600 (Table I). This cost is equivalent to SO.OE/lb of organic lost, or about 60% of the purchase price of the organic.

Three other cases are given in Table I to illustrate the impact that a reduction in total flow or organic flow will have on operating costs. Given the assumptions noted above, decreasing the flowrate by 50% has a bigger impact on the operating costs than reducing the mass flowrate of organics by 50%. Cost savings of 50% are projected if both flowrate and mass flowrate of or- wnics are cut by 50%.

tichieving high efliciencics The concentration profile in FiS. 2 shows that high

removal efficicncics require good cmuent quality and

i . .

<:

':]

adequate adsorption capacity. Enough capacity must be provided to prevent premature breakthrough and to limit emissions from a hot, wet adsorber during the cooling/drying period.

Maintenance of good effluent quality requires efiec- tive desorption, and the close control of materials (such as other organics and water) and conditions (such as temperature) that impair performance.

Prevention of premature breakthrough requires a favorable balance between the adsorption capacity dictated by the mass flowrate of organics, and the speed and effectiveness with which the adsorber can be desorbed, cooled and dried.

To avert the discharge of organics from a hot, wet adsorber, piping must be arranged so that exhaust can be recycled to an online adsorber. There must be sufficient time in the total adsorption cycle for the bed to be cooled and dried.

Removal efficiencies of 95 to 99% can be achieved, but it must be recognized that the systemis comprised of three closely related process operations: adsorption, desorption, and cooling/drying. The idea that adsorption systems are black boxes to be installed on the roof and run with no supervision must give way to a n understanding of all the factors that affect operation.

When designing new systems, one should size the adsorben large enough tn allow sufficient time for effective desorption and cooling/drying. Take into account increases in flow and/or inlet concentration so that they will not adversely affect performance, Make provisions so that more steam can be furnished by increasing the steaming time and/or the steam flowrate.

Improving the performance of existing systcms not desisned for high eEciencies will often prove more difficult than designing a new system. The most important step IS to verify that thc adsorption section is operating properly. If it is, take stcps to define clearly the cause of the poor emuent quality or the premature brcak- ihrough.

I.

I f there is insufficient adsorption capacity, look for ways to rcduce the mass Ilowl-atc of organics to thc nd- sorlxrs, as well as ways to improve the desorption cff- cicncy. The mass flowrate of organics can bc reduced by minimizing the air Row through vessels that contain volatile materials, by installing efficient ventilating hoods to collect fugitive cmissions, and by vcrifying the effcctivcness of such prctrcatmcnt equipment as condensers, filters, scrubbers and coolers.

For many cxisting systems, opportunity to improvc desorption efficiency probably exists. I f thc amount of steam originally specified was dictated by economic considerations, i t may have been insuficicnt for attain- ing high removal efficiencies. In some cases, the Row of steam supplied for the regeneration cycle may have fallen even lower due to the failure of steam valves or instrumentation.

Unless desorption efficiency data similar to those in Fig. 9 show that enough steam is already being used, cast about for ways to furnish more. Increasing the length of the steam cycle (if time permits) or increasing the steam flowrate (if the size of the condenser permits) are the two simplest means. If the desorption cycle is initiated by a timer, consider changing to a control system that detects breakthrough.

If the existing syitem does not have a cooling/dryiog cycle, evaluate methods for including one. Although the total adsorption cycle will have to be changed, the main task will be installing the piping, and possibly a second blower, to move ambient air through the cooling/ drying bed and into the inlet of the online adsorber vessel.

If additional steam has proved sufficient to reduce the residual organic content, it may be cost-effective to install the equipment necessary to recycle the exhaust from cooling and drying.

When the effluent quality remains poor, despite apparently adequate desorption and cooling/drying, it can usually be traced to a n accumulation of other organics on the carbon. The accumulation is often unde- tected until performance can only be improved by changing the carbon. Sometimes, more steam will improve the desorption efficiency enough to extend the life of the carbon. However, if the extra steam cost is more than the cost of changing the carbon, an optimum can be reached between the cost of steam and the cost of changing the carbon.

Finally, do not overlook the possibility of modifying the emission source as an option for improving performance. Often, a solution to the problem is unnecessarily constrained by a n arbitrary decision that the process

VA~~OK.I.IIA.~E AL>SOKITION

cannot be changed. It has been Hydroscience'i;,,? ence that thc most cost-ckctivc solution is usual1 combination of D ~ O C C S S chances that red

~

capacity, and modifcalions that upsrade op performance. . .

Acknowledgement T l ~ c authors wish to cxpress their gratitude to Hydro.

science's clients and to the vendors of carbon and car. bon-adsorption equipment for information that was provided for this arlick. Apprcciation is also extended to Ms. Virginia Hamrick for her valuable contributions in editing the manuscript.

Illustrations were contributed by the following firms: photo, top p. 58-Union Carbide Corp., New Yo&,

tom p. 58-Calgon Corp., Pittsburgh, Pa.; photo, p,, 59-Vic Manufacturing Co., Minneapolis, Minn.; Fig.. I I-Met-Pro Systems Div., Harleysville, Pa.; Fig. 14-- Union Carbide Corp.

References ~

N.Y., and Polaroid Corp., Waltham, Mass.; photo, bot- -

J. H. Mrmnm, E

I. Andcnon. R.. tl 01.. Solvcnl Rccovcr/ Pryr OR, Bvrirvrr Wek, Apr.'18,

2. U.S. EPA Packagc Sorption Dcvicc Syncm Study, Documcnt

3. Kovach. J. L.. Car.Pharc Adsorption and Air Purificarion, Ch

1977. p. 138.

21.138. April 1973.

"Carbon Adsorption Handbook." ed. by P. N. ChcrcmisinoA md" F. Ellcrburch. Ann Arbor Scicncc Publilh.cn. Ann Arbor, Mid., 1978;

4. Smirck, M., and Ccmy, S., "Aclivc Carbon," Elwvicr, Amsi , London-Ncw York, 1970. 5. Ibid, pp. 103-107. 6. Ibid, p. 119. 7. Kovach. J: L., op. ri~. p. 341. 8. Chandrarckhar. R., and Yon, C. M.. Fluidizcd Bed Accivatcd

orp pi ion. p a p pracntcd at thc Symposium on Textile lndus ogy, Williamsburg, Va.. Dcc. 5-8. 1'378.

C. Parmclc, Om. 5. 1979.

.' ' -

9. Lang, \V.. General Tire and Rubbcr CO.. pcmonal communicatio

10. Zanicrch, R.. "Solvcnf Rccaucry from Low Conccnlraiion

Gravurc Printers. spomorrdjointly by Gravure Racarch Inn. Tcchnical k n . . Louiruillc, Ky.. Scpi. 26, 1979.

seminar On ~ ~ i ~ i ~ ~ conLroi ror Publication, ~ ~ & ~ ~ i ~ ~ a

The authors

L

control and product rccovcry will bccomc ihc final ins!allmcnt has appcarcd. "Liq-

lor product klcanup ,and w&tc L _ , . I . . .

70 CIIEMICAL

Charles 5. Psrmele Wilben L. OConnsll Harold S.'BI

Char1.r s. Ibrmclc i s a

http://Publilh.cn

Runzhi Gong and Tim C. Keener :

Civil and Environmental Engineering Dept. University of Cincinnati

Cincinnati, Ohio ~ ..

,

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x i . . e _ ,. 7

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The effects ofwatervapor on binaryvapor adsorption of tolueneand methylene chlorlde by activated carbon were investigated on a bench-scale experimental system. Three levels of relative humidity (1J,@ and 9,o percent) in conjunction with differen! concentrations of individual adsorbates (from 400 to 1200 ppmv) were tested by tracing :he breakthrough curves of each adsorbate eluted from a fixed-bed adsorber. The adsorption capacities of :he activated carbcdested far each adsorbate under thevarious conditionswere obtained from

al'sc;p!ion by ~ct ivated carbon i s one of the :iItcrnative techniques for the removal ofV0C:i. tlowever.forthecxampleslis!ed cailicr. ihc relative humidity o f the a i r stieams .IIC hish. sometimes approaching the watcr v:mm caturnuon conditions

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . , : . . . . . . . . . . .... " ...... ;.. .,I ,..; .. ."..11.1... ..... .,". .,;". ...... _1 .. ,.,.._ .. c, ... j increasing reladve humidity. h e shape of breakthroish curves was asymmetrically ~ distorted and the width df .ilte breakthrough curves was broadened for toluene and

steepenedformethytenechloride.Theadsorptioncapacltiesfor both tolueneandmethylene j chloride were decreased with the increase of relative humidity The maanitude oithe effect

I

,,, , .>.e

', :: ..-.

takcc ,,,... .. ~ that has alrcaciy been adsorbed by the

adsorbent can considerably influence the i I eificicncy o' , ~ g a n i c adsorption from

....... : . . . . . ."%.- .. :./ .......... <-:,,:;!::::>

system using activated carbon. the effccts of relative humidity ( R H ) on the vapor adsorption process, in addilion lo other factors. must be taken into account. I t has

. !

j organic,. the effect of water v3por may I in the genericcondikons: (t)wet adsorbatetesls. j detractfromtheovera11adsorptionwp~city. I repone (2) we! adsorbent tests. and (5) a I For example, a i r stripping h a been shown ~ consid combination of 1 and 2.' In the wet 1 to be an cwnomiwl method for removing achiev adsorbntc tests (or the use humidity tests)'

~ contaminated groundwater and for the 1 estimates that publicly owned sewaF,e ! introduc-d concurrently into adrywrbon I preuea!ment of some types oi aqueous treatment facilities ( P O ~ V S ) emit mL,rr 1 bed. In the wet adsorbent tests (or the

j industrial wales. However. the stripping than 26 mijlior: Ibslycar of VOG inlo llle I preconditioning tests)' Ihe dry adsorbate i units'exhaustgaesmaywuseairpollution , a1mospherc.l c m i ~ i o n s o f v o a i is introduced into a wet carbon bed which I p rob tems i f i i ownl ro lmerh~~eemployed i produce potential llealth for ~ has he-n cquilibratcd io a cenain level Of

I before discharge. Another enamplc is thc 1 tre3rmencf~ciliiy personnel and ~ rehlive humidit:, hefore !he beginning of ~ activated sludge process which is widely 1 public in the surrounding areas. vapor adiorptian. Tlle third type is the wet

~ adsorbate and wet adsorbent test which , i combines thefirsttwosituationstotcstthe !

i ! , , 1 ! we! adsorbate on the wet adsorbenl. Most ' , Im"llcatI"n.2

j volatile organic compounds (VOCs) from Ihebioreiracrorywmponmls.~~eU.S,EpA ; the adsorbate and water vapor are

~

, , i /

.... ~ I of thc iwcstigations which ltave been

been conj,clcdwith asinglc adsorbaic orpntc compound.:ln general.

, ~ I T i l l e l l l o f t h e C l e a n A i r A c t A m e n d m e n l s 0 f 1 9 9 0 w i l l e s ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ a v a i l a b l e c o n t r o ! , ieponcd ; i tocnnology requirements (MACT) for a wide range of toxic air po i~ums. vapor phase I i

< i ! carbon adsorption will undoubtedlv be rewired lor vlnlrolliro Some of ih" V O I ~ M ~

i .

i

-

hetypeoiadsorbntes.theirpnnia1 pry-- .surcb ind the level o i relative humidity." niormation related 10 the effect of water ,apron multicomponent vapor adsorption ias been reported by Grant.' The wet idsorbateandwctadsorbenttesLs(RH=80 ~ r c c n t ) for two ternary mixtures of nraftins, aromalia and halowrbons were onducted in their study. It w a postulated hat [he adsorbed water reduces the pore +T :i*>:iiIahle Cor :idsorption on :I one 111 we vulumc basis. They prcscnlcd ;i nmdc! lased on the Polanyi adsorption potenlial heorywhich proved adequate to predictthe .ffect 0fmoisNreon break1hroughwpaci:ies if a mixture containing low concentrations bfdichloroethylene. n-hexane and toluene. iutdid not predict the breakthroughwpacity tfthe relativelysoluble methylene chloride n a temary mixture which included n- ieptane and toluene.

The investigation of single vapor dsorption under the influence of water 'apor provides a basic understanding of hceffcctofwatervapor on Ihc adsorption Ifindividualadsorbales. For more precise lcsigii purposes, more iniormation is !ceded on the effects of water vapor on nulticoniponent adsorption. With this in nilid. this project iocuscdon Ihc ef iecsof e la l ive humidity on the dynamic process 1 1 binary org;inic vapor adsorption.

:xperimenlal Methodology Dynaniic ;idsorption of vapor mixtures

8 1 Iduenc :~od nict~iy~enc chloride with ilrious conccniration~oiwater vapor Ilvcslig;iicd by tracing t i le c(,,III1~ctc

brcakthrough curvcsgcnrrated by charging J Elr:cd bed o i pellc!izcd activated carbon in a bench-scale expximental system. The vapors were analyzed by means of gas chromatography in order 10 distinguish bctwccn the breakthrough periods foreach

cun,puund. L ;IC W C ' I *cmiu;Itc : C s h . h C I

conductcd under conditions which wer considered to beofpractiwl interesl.Thes included the adsorption temperalure, th amount o f adsorbent used, and th regeneration conditions. !,

1W

80

s

3 c ? m

40

20

0

AIR & WASTE. Vol. 4 3 . June 1993 * 865

, - //' a ;I i *.I , 80 4 / j -

U 01 0" 1 L .. !

0

/

200 -. 4c€ 600 Adsorpttonlhe, mills,

Trials were pcrformcd undcr threc differcntrclativc humiditics(RH = 15,65, and 90 percent. with 15 percent RH as the control) in conjunctiun with threc diffcrent . d w > r i x u c ~ i n l ~ u c n t IIIIICC:~~~:IIIOII\. r m ~ i n s from -Ill0 10 E 0 0 ppm. In addition. iour single vapor adsorption tests were conducted to provide iniormation for comparisons. The amount of adsorbent usedineachtrial was lOOOgrams,resulting in a bed depth of approximately 32.5 cm (13.78 inches). A i r f low through the adsorberwasO.jI+O.Ol m'/min.(lS.15 t 0.5 scfm), corresponding to a superficial airvelocityof65 mlmin. (195 fpm)andan air retention time in the carbon bed o l 1.3 sec. The k e d a i r pressure was 20-23 psis and the temperature was j7.5"C + 2°C.

The adsorbent used in this study was pelletized grade JXC activated carbon (4x6 ,Mesh) manuiactured by the Witco Cllcmical Corporation. The carbon was round 10 hmc 3 BET s u r i x x arc3 oi lpproximately l0JO m2/g by analysis. rile lctivatcd carbon was prccondit~oned :I[

105°C for 24 hours to rcmuve impurities Uhich may havc been adsorbed. storcd in

polarity, adsorbability and solubility) and bccause they arc widely used in industry

adjustcdbyihc flowrateolthehumidified air clrcam andlor the water tcmperarure in thc humidifier. The relnt iv~ humidity wa; mcasurcd hy wet bulb and dry bult; thcrmom~:tcrs 2 , t!lc i..:..: . ; [ [ ! . e tl+o:~cr;

mcthylenc chloride were pumped at an accurate and constant flowrate by a F M ~ h b p u m p (Model RP-G?O. FGI Fluid Metering Inc.) and injccted through an air atomizing nozzlc (114 J-11-ss. Spraying Systems Co.) into a mixing tank. ne adsorbate l iquid w a s atomized and immcdiatcly evaporated. and was1 intimatcly mixed with the incoming airlo, form a humidified. adsorbate loaded challenge stre:lm. The desired concentralions of adsorbate vapor in the a i r stream were obtained by changing the flowrate of the adsorbate liquid injection pump while the now rates and pressure of the carrier gas and the stomizing gas were maintained constant.

The adsorption proccss was conducted i n t w o s t a i n l e ~ ~ ~ t e e l tanksoC10cm. (4.20 in.) dinmcicr 2nd 5r) CT !?!I in.) heickt. ;;x,.<G '.ut;, , i i l i : l ;:..;:;., <ii . , i i i i"tC) l i

carbontoa heightofj2cm. (12.5in.).The tlow direction was downward to avoid fluidization ofthe bed. Tlle pressure drop through the bed was 10 psis. The ;Idsorption tcsts were not iniliared until a l l of [he test conditions had achievec'thcir required steady statc w lues . D e a i ; : the adsorbatc conccnlration of the air s;rcxn was very sensitive to the change of pressure

L iqu id mixtures of to luene

i.!!\!rIIIImc:il3i i 'r i1tcC:IoIl :\::::cp '!'!,c bcnch-scale experimental system was comprised of five sections: (1) carrier gas prctrealmcnt. (2) humidity generation. (j) adsorbate vaporgeneration. (4) adsorption. and (5) adsorbent regeneration. A flow chart of the experimental system is illustrated in Figure 1.Thecompressed air (EO p i g ) from the laboratory air supply system was pretreated to control the pressure and remove impurities such as oil. water vapor and particles. A regulalGr was uscd 10 rcduce the iiir prcssure from EO psis to 40 psis, 31 which point the air flow was purified by m o i l rcmoval filter ( M o d e l 42036, D:tvton Electr ic Manufacturing Co.) 2nd a drycr (Modcl XOj-02-000 114 desiccant dryer. Fauvcr Company. Inc.). which could remove a i l oi l :iud water and a11 sulimicron particles down tu 0.3 microc sizc. The various :lir humidities were achicvcd Ihv ciivcrtintv .I

. - . : V C S S I ~ T C drop Curio; :>i. s y s t e ~ up t i i c s a m c ~ s t h ~ t i n t n ~ a c s o r p t i o n t ~ ~ ~ "I . i % V O

identical adsorpcrs were connected in p m l l e l to allow for this bypass mode,:of operation. One adsorber was used as the lest bed whereas the other served only.as the back-upbed toequilibrate thechallenge' stream each time the system was staned.; Tlie exhausted adsorber was regenerated using steam with a pressure ofjO-JOpsig. (saturated vapor) at the :ateofthree pounds of steam per pound oforsanic adsorbates. After steam regencratiun. prior to the inilialion or 3 new adsorption cycle. the hot. wet adsorbcr was dried and cooled 10 thc operational t c m p c n u r c 11y blowing i t u,ith cleaned and d m t i .,ir.

Grab samples o i air wcic manually tnkcn by inserting 5 or ! O ml prcssurc-lock gassyringes(CA-2.Su~lco, Inc.)through lllcscplaoSs~mplin_rportsat thc inlet and thc (iutlct of the :idsorbcrs which were a sw:igclock Tcc conncctinn containing :I

tlicrniogrccn scp i ;~ from s u p ~ l c o Corp. 'The s ; i m p l c wcrc injcctcd into the gas chrt~matogr;ipl~ 3s so011 :IS possible ( ICSS

111;111 one minute). ,\ modcl Sigma I gas c l i r o m a t o ~ r ; ~ p t t (r;c) with 3 fl:imc ioni%~tiuc ~ C I C C I O ~ (FID) w : ~ uscd 10

~ ~ ~ c r m i r i c tlrc adsorbate concentr;itions. ;lie tic column (MR 51539 Supclco. ;upelco Inc.) was Y' x lis" stainless steel ,ackcd with 100/120 chromosorb PAW :o:ited with IO percent TCEP. The carrier :as for the GC was nitrogcr, 31 3U ml pcr ninutc. The injector tcmperaturc was !00T and the detector temperature was !5O"C. The GC operated at an initial emperature of 70°C 2nd incrcased 10°C icr minute up to 115°C. Liquid standards Yere prcpared by dissolvitig different mountsoftolucne and methylene chloride 'Certified. Fisher Scientific) in a solvent if carbon disulfide (Technical GRA, MCB.). The model Sigma 1 GC Contains 1 microprocessor-controlled console to :stablish analyzer chromatographic conditions, collect and reduce dala from chromatography, and printout the analysis reports. Thus, each analysis was executed according to an analysis method which was generated to determine the analyzer conditions and data handling. The chromatogr~m and data report. including peak areas and peak retention times. were

printedplotter.

. . p , ,:lLL.duul ,:,,l~,r;,,!:!c:,,;; i:,:,,> :::: ::::::::A

Discussion Frpm the data collected in this study.

breai through curves were plotted based on L,C ratio a i the adsorbate oul lcl conchtration to the inlet concentration (C iCprcent ) versus the adsorption time (t . k 5 . L F ipre 2 lhrough Figure 7 il;; . L' t I i i . schrc: i~thr ; ,u~hcurvcs~~nder varioas test conditions. Figure 3 illustrates the effect o f relalive humidity on CH:CI, and C, H, respectively, for single component adsorption at relat ive humiditiesof I5 and 90 percent. Fiyres3, 4 and 5 show the results of binary vapor experiments at relative humidities of 15. 65 and 90 percent. respectively.

The adsorption apacity was calculated from the areas determined by the breakthrough curves as determined by polynomial cume fitting? As shown in Figure 6. the hatched area corresponds to thcamount of toluene adsorbed during the binary adsorption process. The adsorption capacity for tol:1ene can be calculated hy the following relalionship:

q, = C,, A, Q M (24.5 x IO6 W)

(1) wherc: q, = xlsorptioncqncity. gmmstolucnc

adsorbcd/gram o f :ICI~V:IIC~

nrbon. C,, = inlct concentration of tolucnc.

ppmv. ,\, = : I T C ~ intcgratcd from tlic cn-

ordinatc to thc hrc:ikthrough

curve, representing the amauni ofllietolueneadsorbed. in un i lo i timc, min.

0 =carrier gas flowrate 31 room tcmperature and prcssure, I/min.

W = wcigiit of~~L i \a tc ; caibci. i: :?.< fixed bed. gm.

M =molar weight of the adsorbate.

Due to the rollover phenomenon (a typical phenomenon observed in multi- componenl adsorption. which wi l l be explained later), the net adsorption of methylene chloride in the fixed-bed i s equal to the difference between the adsorbed methylene chloride and the desotbcd methylene chloride. As shown in Figure 7, the adsorbed amount is represented by the hatched rectangular area (A") and [he desorbed amount by (he area under the breakthrough curve (Ad). The calculation formula for methylene chloride is given by the following:

qm = (Ad-Ad) C, 0 M Q4.5 x lo6 W)

L (2) i i i ic : : .

q = adsorption capacit,y, g m methylenc~;;tl~~deigm activated carbon

C, = inletconcentrationofmelhylene chloride, ppmv.

i'hc valucs 01 A\, anti /Id wc-IC ubtaitkcc by ni im~rical intcgration." The results o f the calculation for the adsorption capacity of each camponcnt are listed in Table I.

Water Vapor Effect on Breainiirougn Curve

The effects of water vapor on the shapes of breakthrough curves for the adsorbates studied are shown in Figures 8 and 9. The breakthrough curves of both methylene chloride and toluene were shifted and were distorted with increasing water vapor. These breakthrough curves possess an asymmetric character, with the curve's front portion being steeperthan that ofthe rear. This indicates that at high relative humid i ty , the method to calculate adsorption capacity based on 50 percent breakthrough time'." may result in error. Therefore, area integration ofthe complete breakthrough curve is recommended in order to obtain more accurate adsorption capacity for these conditions.

The width of the breakthrough curves myhc.m.-nsllrcd hythcdiffcrence beween thcsaiuration iimc(ts, 2nd l h c ~ x c a ~ : l ; ~ c g time(Q, asshown in Fiyre.6.The relative positions of I$ and $ are arbitrary. In this study, the value of 1% and $were taken as 90 percent and 10 percent, respectively.

AIR & WASTE. Vol. 4 3 . June 1993.867

I t i s ~ f i : , : ~ m i ~ n c c IC . : ~ ) I C t h a : w i ~ o . i i ~ ~ relative humidity increased from 15 ~

pcrcent to 90 percent. the width of the

wider and flatter, whereas those of ~

methylene chloride are narrowed x,~! I steepened. For example. when the relative humidity increases from 15 percent to 90 , percent, the value of t -t for toluene at the i i b liigher concentration increases from 80 to

~

110 minutes . and a t the lower I concentration, from 170 to 390 minutes. For methylenechloride the width decreases

concentration and from 110 to 65 minutes I a1 the lower concentration. This indicates I that the effectsof relative humidity on the I breakthrough curves of toluene aie ~

different from those of methylene chloride. Toexplain fhe influence ofwatervapor

on the breakthroughcurves, i t isnecessary ~

tu understand the dynamic process of multicomponentadsorprion in a fixed bed.

Breakthrough curves observed from a dynamic adsorption process are actually a 1 ‘eflection of a moving mass transfer zone IMTZ) in a fixcd bed. The $width of thc

brcakthrough curves of toluet~c bccnmc 1

I

I from 90 to 40 minutes at the higher I 1

i

i

80.00

40.00

0.00

Adsorption Tim (mins.) . .

I , . . =-. he MTZ is due Lo the distribution of Idsorption velocitiesamong the adsorbate 1 F b r a 6. lllustrallve dlawino lor the amount 01 toluene adsorbed in binarymgor tests.

~nolecules. In a microscopic view, the adsoption velocity of individual adsorbate

I

0 200 400 Adsorption Time, mins.

1

600

moleculescm beaffected by many random factors: the kinetic energy distribution among the adsorbate molecules, the irregular shape. s ize and pore path 01 the Zdsorbcnt, and the inhomogeneous channels around the adsorbent particles, etc. As a result. some adsorbate molecules move relatively quickly and some muve relatively slowly due to the differences in their kinetic energy and encountered resistance. Also, someadsorbate molecules can be adsorbed on the adsorbent surface faster than others. depending on the probability of an effective collision with the active adsorption site. As a result, the adsorbed molecules entering the adsorber at the same time may be adsorbed at different depths of the adsorber.Thercfore, there is a finite layer (i.e., a ME‘) in a n adsorberon which theadsorbed molecules distribute ovcracenain thickness. Thus, i t :an be concluded that the thickness of a MTZ, and i n turn. the width of 3

xeakthrough curve, is affected by the lame lactors that cause the variation of the Idsorption vclocily amon5 the adsorbate nolecules.

For a specified adsorber and &AS lowrate. such as in this study, the only ?mors that affect the thickness of t h c 4TZ are the change of thc adsorption :apacity for lhc adsorbate. and the

86’3.June 1993 *Val. 43 AIR h WASTE

i s~; I I ,~~ ;tnd driving forccs for mass nsfcr. The thickness L.' !!IC M T Z wil l :rcase with 3 decrease in the resistance. increase in the driving force. and an -cast in adsorption capacity. i n tile casc o i , , , " i ,~~~. , i , , ,~~, ,~~i

iorption. more lhan one MTZ exists in : fixed bed due to the differences in the 'fusion rates 2nd the adsorbabilities of :adsorbates. Tlicse MTZs may overlap separate.dcpendingon the components' ,pertics. In senerd. the MTZ of the :akly :idsorbed component localcsahrad that of the more strongly adsorbed

mponent. This is due to two reasons. le is that the less adsorbed components ncrally have smallzrmolccularsize and .vel weight compared to the more ongly adsorbed components so their ifusion velociticsaregreater than that of : latter. T h e second reason is that the ire strongly adsorbed components can ;place the lessadsorbed component from : adsorption s i te they have already cupied and thus, force them to desorb d move down the bed to find new

iki! ,i: ', -',<, >I !!!Z

:a!dy adsorbed component results in 8 nccntration increase of that component the gas phase because this increased ncentration at any time is equal to its le t concentration plus its desorbed ncentration. Therefore, the t!!cakthrough w e of thc less s t rong i~ . adsorbed mponent elutes earlier thxt that o f the >re strongly adsorbed coriponent. and pears with a hump at wbi$ the outlet nccntiation is higher '!.. . the inlet inczitntion. 'This has 1:: ,.::illcd tiic dlover" phenomenon." * I Based on the previous discussion, i t

n be inferred [hat in multicomponent sorption, the thickness of the MTZ or c width of the breakthrough curve rends increase for the more strongly adsorbed mponent and decrease for the less sorbed component. Since the MTZ of a is adsorbed component locates ahead of a t o f a more strongly adsorbed mponent. the mass transfer resistance the more strongly adsorbed component creases, due to counter diffusion from sorption o f the less strongly adsorbed smponent. and the change of the surface d pore path of the adsorbent which is a su l t of the less strongly adsorbed 8mponent already adsorbed. Thus the icknessoftheMTZofthcmorestrongly lsorbed component increases. Also. the "muration of a less strongly adsorbed "ponent entering t l ic MTZ is higl~er an its inlet concentration. due to ~IIC

' 1 lover phe n 0 men on. The h i s h e r incentration implies a grcatcr driving rce of mass transfer and ;I llighcr lsorption capdcity. As a result. t l lc M I Z

uccomcs mume u r r u w cunlparcd I(; i1 , i

slngle component adsorption casc. Thsreforc, the effect of water vapor on

the shape ofbrcaklkrough curves oforganic vapors may depend on the eluting order o f

KL:K K;;: kc-. :i!c . : i ~ ; - ~ . c n t !h:d i f water i s considered as an :idsorbatc component. Increasing the watcr vapor concentration w i l l result in wider breakthrough curves lor the components which elute from the adsorbent bed latcr than water. and narrowcr breakthrough curvesfor those clutingmore quickly than water.

Theelution order ismainly determined by the adsorbability of the components. Since waterisa polar molecule. itsaffinity to 3 non-polar surfacc adsorbent. such as activatcd carbon, is much smaller than thatoftheorganiccomponents. Inaddition. the molecular size and weisht o f water is generally smaller than that o f most organic components. Thus, in the concurrent adsorption o f water and organic components, water is likely to elute much earlier than other organic components.

:n I h i v ~ t \ i < I \ . in Iuc*c i s !I: -.rlorc strongly ausurocd wr~ipui~ci ir \ + X I I ; ~ . . ~ . . .T boiling point and lower p o l x i t i when compared to both methylene chloride and water.Thercfore. i t isapparent that toluene should be the l as t component (after methylene chloride and water) to elute

-

'-vi thebed.As3 rc,u;..;;iaddit~onlothc influence of mcthylcnc chloride. thc breakthrough curve oftoluene was further broadened due to the presence of water vapor. Thisbroadening ofthe breakthrough ,::\T.'C is caosed bv the reduction of the mass transfer rate and thc availability of active adsorption sites. The decrease of the mass transfer rate of toluenc can be attributed by nvo factors. T h e counter diffusion of the desorbed molecules in the gas phaseobviously restricts the migration of toluene to the surfacc of the adsorbent. In addition, the condensation of water in the micropores also produces an extra mass transfer resistance fora hvdroohobic , . adsorbate. if a water film is formed.

k- The effect o f water vaoor on the breakthrough of methylene chloride in this study may be explained by the assumption that methylene chloride elutes beforewater due to itsweaker adsorbability and its similar polarity to that of water. Although the molecular weight of water (IS) is smaller than that of methylene chloride (85), the adsorbability of water m i o the sorhent (activated carbon) a t the

pcrccn,) becomes significant [ the relative @r&sures of methylenc chloride under these test conditions are much smaller (< 0.0032) than that o f water (> 0.65)]. Therefore, i t seems possible for the water

,.,_.._. .._,,,,.. ~ r c ia , i \c :.cii:iu,.- ,-'.-;.,:<::: ..? -

40

0 m 400 Adsorption Time (mini.)

AIR 8 WASTE. Val 4 3 . June 1993.869

OELO'O SZLO'O

0010'0

28000 8E10.0 8L 10'0

k 6 2 0 ' 0

, , j C!Z3'?

'00s

008

0021

EPS 0011

01s

? l e 890 1

06

06

06

s9 s9 s1

SL S l

1811'0

2lE1'0 LSPL'O 5811'0

SOEL'O 8SSl.O '

1091'0

9191'0

OW

OSL OOLL 08P SPO 1

OEP OlL

5001

06

06

06

s9

59

S l

s t S I

:se component mixtures. k n important nclusionoblained from thisstudy is that

effect o f water vapor on mulli- niponcnt vaDor adsorption could. be :her different t h m l l ial of the singlc sorbate case.

1 n c I u s i o n s The presence of high concentrationsof

ater vapor resul ts i n an ear l ier cakthrough forthetwoadsorbatestesled. luene and methylene chloride, and an .ymmetr ical dis tor t ion of the eakthrough curves. The method which ;es only one point on the breakthrough irve (50 percent breakthrough point) 10 ilculate the adsorption capacity o f isorbent, may not be suitable for lsorption tests designed tosludy the effect I water vapor. In order to obtain an =urate measurement of the adsorption !pacity of carbon, an area integration of le complete breakthrough curve is :commended to calculate the capacity of le adsorbate adsorbed for conditions of iyh wI:itivc hiimidilv.

__ ._ - . .-

<..n ..>. '. -. 1 J*

0 200 400 600 Adsorption Time, minS.

:urves in multicomponent vapor ad- iorption will depend on the elution order ,f the adsorbates from the bed.The width ,f breakthrourh curves of those eluted :xlier than t he w t c r tcnus to bc rcduccd. Nhile that of those componenls eluted ater than water may be increased.

The adsorption capacity of activated :arbon for toluene and methylenechloride is reduced by the presence ofwaler vapor. Bulthe magnitudeofthis effect is different, dependingon theadsorbatewnoentrations. Toluene is subject to a strong water vapor effect at its lower concentration, while methylene chloride is subject to a strong effect at its higher concentration. The tendency o f the water vapor effect increasing with increasing adsorbate concentration may be possible for those organicadsorbateswhich have a molecular polarity close to that of water.

T h e e f f ec t of water vapor on multicomponent adsorption is more' compliwted than that for single component adsorption. The experimental data show that lor multicomponent adsorption. the adverse effect of water vapor on the adsorption capacity is not as significant as that for single component adsorption. T h e r e f o r e , any at tempt to predict multicomponent adsorption based on thc data ofsingle component adsorption under -

AIR & WASTE. Vol. 4 3 . June 1993.871

I!,:: tnllu,:nc:: ~f Ihigl; Iwruidi!y n u y I~:I

Additional cxpcrimcninl work is suggcstcd with various typcs of organic ndsorbatcs to model thc waicr effect. cspccially on tltosc orS3nicsWhiclicont:iil1 Iunciional groups posscssing somcspeci;tl propcrlies. such as higher molecular

prow sttcccssiui.

grcaicr water solubilily. This would Iic vcry uscful in he

design of adsorplion systcms which could ~ take intoconsidcration Ihc cffecl olwaicr I vapor on mullicomponcnl adsorp(ion.

Acknowledgment The work prcsented in this paper was

parlially supported by [lie U.S. 1 EnvironmcnralProtec~ionAgenc~through I conlracl number 68-03-4038 l o the

Univcrsily of Cincinnati. This support is i grcally acknowledgcd.Thisarticle has not been subject lo [he agency's review and i therefore docs no1 nccessarily reflect [he

1 views of the qency, and no official I endorsement should be inferred.

1 References I . Li. llm,,d,,, '"cp"', I" cimgrcl, L," (lac

OischargeofHaurdous W:tslealo Publicly Owned Trcztmcnt Works." EPAl53U-SW- i 86-WJ. February 1986. I

6 .

7.

I

L.A IO^^;,^. , -mtC efrecl mol~~ur( . (111 the ;Idrurplionofchlorolorm by aci iv i ied ciwhon." Ani. Ind. l lyg . AIIOC. 1. 46:ZO (1985). C.O. Nclnun. e! 31.. "Respiritor cartridge cificicncy studies: "ii. Errcct or rclaiivc humidhy :md tcmpcmurr." Am. Ind. IIyp. :II.TUC. J . 37: (1976). M.O. W U ~ C ~ . TI,^ erfcCts I c ~ . ~ ~ y c

lhmidi ly on Ihe vapor phase adsorption of lrichlorocthylcne by rctivricd carbon.",4m. hid Ilgg. r l s w ) ~ 1. 46585 (1985). Y . Takcuchi. E. Furuya."Prcdic!ion of thc Brerkthrough in Sulvcni Recovcry in an Aclivatcd Carbon 0cd:'paper prcrenicd PI

Ihc Engincering Foundation Conference. Fundmenids of Adsolpiion. Bavaria. W. Germany, May 19K3. pp. 629-636. M.0.Wcmcr.N.L. Wintcrr."AReviewof M d e l s Dwcloped IO PrcdictGsvour Phaw Aclivaled Cubon Adrorplion of Organic Compounds." CRC Criticrl Rsviews in Environmental Control. Vol. 16. Issue 4. IY88. pp. 327-356. R.1. Granl. e l al.. " T h e E f f m of Relriivc Humidity on the Adsorption of Water- Immiscible Organic Vapors on Accivaled C~rbon."papcrprescn,edalthe Enginccring Foundation Conference. Fundamcnials of Ad~orplion. Bavaria. W. Germany, May i W 3 , pn 21'8.227 ,\I. L,l~l.d~~.~~l, C I . i , . . ' . , . , .< : . ;<L,d ,cd

Environmental Fate o f I29 Priorsly I 'UIIU~~~S.' ' EP,\-JJlI/J-79-II2Y..b. US. EPA. Washington. D.C.. 1979.

'1 R ~ o n g . " E l f e c i r o f i ~ c I a i ~ v c Humidity on Mullicomponenl Vapor Adsorption of Tulucnc and Methylcne Chloride b y A c l i v a c d Carbon,'' Marlers' Thesis, Univerriiy afCincinnali. 1989. C. Nunze. hf. Korurko. 0. Danicl."Effcct of Humidity on C.,:hon hdsnrpf ion Performance in Rcmuving Organics from Cunlaminiled Air Slrcam~," Papcr No. 89. 19.2. p r c s c n e d ~ t the 82nd Annual Meeting & Exhibit ion of !hc A i r and Waste Managemmi Associalion. Anaheim. CA, lune 25-30, 19x9.

11. R.T. Ysng. GurSepororlon by Adsorption Procerrer. Buucnuonh Publishers. 1987.

12. 1.Joseph. D.S. Bcach1er.M. Lrrlie."Control of Gascour Emissions." EPA 450R-81- W6. May 1981.

IO.

About the Authors The authors are with the Civil and

Environmental Engineering Dept.. 741 Ealdwin Hall (ML 071). University of Cincinna+Cincinnati. OH45221 -0071. This ,.... - ...... ..~ ","..,r le,;^

..IUIIIiL. - " J u r , i l . I L L " . " , +", 8 "

on July f4, 1990. The revised manuscript was received on M a r 6 4. 1993.

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US.& 319 I MEMORANDUM GLAXOWELLCOME CO.

From: PMO Engineering John Chambard Tom von Lehmden

Subject:Eneineer Studv on Carbon Adsorption Unit Methylene Chloride Recovery Digitalis - Building 5

Date: February 22, 1996

SUMMARY

On February 16th. Tom von Lehmden, Richard Singletary, and John Chambard conducted a test on the Methylene Chloride Carbon Adsorption Unit at Digitalis in Building 5. The test consisted of the following:

* Closing the valve on the recycle stream, thus preventing condensed desorption vapors from entering the inlet of the active carbon adsorption bed.

* Collecting data from the outlet of the active carbon adsorption bed using the Methylene Chloride analyzer. Data was collected for the full 76 min. cycle.

From this test, it is evident that the carbon bed is not effective. The analyzer recorded 180 ppm at 45 minutes into the adsorption cycle. Fifty ppm is the limit allowed by the permit for releasing methylene chloride into the atmosphere. It is recommended that more testing be conducted to determine the cause of poor performance for this Carbon Adsorption Unit.

INTRODUCTION

The Carbon Adsorption Unit (CAU) uses carbon, the adsorbent, to adsorb Methylene Chloride, the adsorbate. The first level of carbon near the inlet to the bed begins to adsorb the methylene chloride . As this level of carbon adsorbs to its capacity, the next level of carbon begins to adsorb the methylene chloride. At this point the ratio of outlet concentration (c) over inlet concentration (co) should be approximately zero. The carbon will continue to adsorb until the carbon comes close to its capacity. When the carbon is near its loading capacity, the outlet concentration begins to register traces o f methylene chloride. Rule of thumb indicates that once the ratio (c/c,,) is 0.05, the carbon bed is considered to.be at the break point. This point represents the maximum that can be desorbed efficiently. Beyond this point, the bed is inefficient and more energy is required to desorb the bed properly. In Figure 1, the breakthrough curve may be seen. At time 4, the break point is reached and the bed should switch to the desorption phase [Geankopolis, 7021. There are two recommended operating conditions in running a carbon bed. First, the bed should be hot during desorption. Second, the bed should be cool during adsorption.

2 m 3

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EQUIPMENT

The existing system consist of two carbon beds which altemate adsorbing and desorbing. Figure 2 shows an abbreviated view of the CAU process piping. Solvent laden air is combined with a recycle stream and put through the CAU. The air stream is then exhausted into the environment Steam is injected into the bed to desorb the carbon. The steam laden with the methylene chloride is condensed and separated in a decanter. The methylene chloride that did not condense is sent back in a recycle stream to the CAU. After the steam cycle, air is blown on the carbon to cool it for the adsorption phase. The adsorption cycle is completed in 76 minutes. While on bed is adsorbing, the other bed is desorbing. Steam is injected on to the bed for 40 minutes, then the bed is dryed and cooled for 35 minutes.

There is an analyzer which is utilized to determine the concentration of the outlet stream from the Carbon Adsorption Unit. Another analyzer may be used if necessary.

I/

SP - Current sampling poinu for vapor analyzer SP . hporcd Sampling poinu (vapor & liquid) Ambient

Rsycle

S ‘ S * -@ Condenser Condenser i - 7 D cc a n i c r

Solvent Laden Air

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RESULTS

Tom von Lehmden, Richard Singletary and John Chambard conducted a test on the CAU. First, the bed to be tested during the adsorption cycle was desorbed for approximately 2 hours. An analyzer sampling tube was connected directly between the bed outlet and the methylene chloride analyzer. Next, the recycle valve was closed to prevent the recycle stream to combined with the solvent laden &. This would provide an environment to test the adsorption effectiveness of the carbon bed when handling only the load from the building. Data was collected for the complete 76 minute cycle. A peak was observed at the beginning of the cycle. The peak was determined to be caused by the bed being to hot to adsorp and was immediately cooled by the inlet stream.. The concentration dropped to 18 ppm at 5 minutes into the cycle. The concentration never dropped below this 18 ppm. The concentration steadily increased. Between 10 and 15 minutes, the concentration reached 50 ppm. Ffty ppm is the limit allowed by the permit to release medylene chloride into the atmosphere. The concentration continued to rise. At 45 minutes, the concentration reached 183 ppm. This is the highest point, and the point at which the bed is no longer adsorbing. The tailing off at the end is contributed by the decrease of concentration on the inlet. Figure 3, shows the graph of the data collected. Actual data appears in Appendix A.

. . . . ..

Data C-allecled horn Test wlo Recycle Stream

0 1D 20 33 A 0 50 60 70 80

n m (mi") ; Figure 3

Data was collected at an earlier date by LYM Dutton and Richard Singletaxy. Their test included the recycle stream and, their sample point was on the inlet to the bed. TO determine the effect of the recycle stream, the recycle valve was closed at 19 minutes into the cycle. Figure 4 shows the decrease in the concentration when the valve was closed. The valve was reopened at 23 minutes into the cycle.

3

The temperature was also included in data collection. Figure 5 shows the temperature of the bed while the recycle valve was open. Adsorption is an exothermic process and is the cause of the temperature increase in Figure 5. Actual data appears in Appendix B.

RECOMMENDATIONS

First, we need to find the problem: Overloading the G e t Inefficient desorption. (tempkime) Fouled carbon Ineffective condensers Bed too hot during adsorption (inlet cooling or cool down) Mechanical failure

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Identifying the problem will involve further testing and commitment from all departments. There are at least four tests which we should consider conducting:

1. Determine the status of the current carbon beds and the operation of the entire

2. Determine the purity of the carbon 3. Determine the effect of the recycle stream on the CAU 4. Determine an efficient desorption process

system

Current omrations: * First Test:.

1. Establish a carbon bed in its original condition. This will involve

2. Monitor and collect data on the inlet, outlet and recycle streams

3. Monitor and collect data on the inlet and outlet streams during multiple '

4. Repeat the test for reproducibility.

untreated emissions or production shutdown.

during multiple adsorption and desorption cycles.

adsorption and desorption cycles without recycle stream.

* Material balances should be done around the carbon beds and condensers. Determine how much methylene chloride is adsorbed on the carbon beds and desorbed off the beds.

* Establish break through point on a clean bed (maximum loading). * Compare current operation data to design criteria and carbon activity. * Venfy that all valves are seating properly.

w: * Sample carbon for adsorption efficiency analysis (high efficiency sites may be

clogged). Send sample to Calgon. Based on the analysis have VardCalgon provide desorption criteria to regain original efficiency.

* Verify that the correct carbon is being used for this application * Will any additional carbon help? Supposedly previous investigation determined

* Identify contaminants that may be obstructing the high efficiency sites. Would a maximum of 6 inches of carbon could be added.

filtration of the steam help?

Recvcle: * Material balance around condensers to determine efficiency. * Use a solenoid to altemate samples from solvent laden air and recycle stream

Desorption: * Determine the pressure rating of the vessel. Can vacuum be used in the

* Discuss with Vara options for desorbing the bed to its original condition. * Long term consideration: Can control strategy be triggered by outlet

desorption phase to improve its efficiency?

concentration, as opposed to time interval, to prevent overloading the carbon (Vara programming, additional hardware cost?).

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Resoonsiblities:

* Test Plan - ETS will develop a detailed plan. 1

1 PMO Engineering will review plan/ PMO Mgr approve plan

Environmental Services notify state of plan.

**Timing - one week

* Run Test - ETS I PMO Engineering I PMO Operations

**Timing - four weeks

/

* Results - ETS will compile the final report.

Arrange a meeting to determine the final scope of RES 33052

** Timing - two weeks

APPENDIX A

6

APPENDIX B

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2418 Ind. Eng. Chem. Res. 1993, 32, 2418-2429

Steam Regeneration of Solvent Adsorbers -et- 4

Thomas A. J. Schweigert and M. Douglas.LeVan' Department of Chemical Engineering, University of Virginia, Charlottesuille, Virginia 22903-2442

We report results of modeling and experimental studies on steam regeneration of activated carbon beds with adsorbed n-hexane. Experiments are performed using a well instrumented pilot-scale apparatus with a nearly adiabatic adsorption column. We vary the initial loading of n-hexane, the steam flow rate, and the flow direction, and measure temperature profiles within the bed and effluent concentrations and flow rate. Experiments indicate the presence of a sharply condensing steam flow with drastic changes in velocity in the bed, the development of liquid phases within the bed, and a wave character with fronts separated by plateau regions. Mathematical models are developed based on finite differences and orthogonal collocation on finite elements. The models predict theobservedvelocityvariations, thedevelopment of theliquidphases,and the wavecharacter of the process.

Introduction

Steam regeneratdn of an adsorbent bed is carried out by passing live steam through a bed of exhausted adsorbent, usually activated carbon with adsorbed solvent. It is commonly used industrially, yet little fundamental research of either a theoretical or experimental nature has been performed to improve our understanding of the process. Unoptimized steam requirements for solvent recovery systems have been found to vary from 0.77 to 20 kg of steam per kilogram of solvent recovered (LeVan and Schweiger, 1991). Improved understanding can be elf- pected to lead to improved design methods and optimi- zationof existing processes through minimization of steam requirements. Also, better understanding is important because regeneration fixes the state of the adsorber when it is placed in service. This means regeneration determines the capacity of the adsorber to recover solvent and also, because of residual solvent loading, contributes to the level of solvent emissions during service.

is nn effective regenerating agent for activated carbon adsorbers used for solvent recovery in temperature swing cycles because, with ita high heat content, it quickly raises the temperature of the adsorbent to desorb the

i solvent for pore volume of the adsorbent to enhance ' desorption. The flow of steam purges from the adsorber the desorbed solvent, which is easily condensed and purified for reuse. After steaming and perhaps drying, the carbon is ready for service again.

While many models for adsorption and desorption by a pressure reduction or by purge with a hot inert gas have been developed, steam regeneration has been given little attention. The principal reason for this is most likely that models for steam regeneration require the treatment of adsorption equilibrium for hydrocarbon-water mixtures, which is not well understood, and the treatment of fluid velocity for a condensing flow. With steam regeneration, the condensing steam flow results in interstitial velocities that drop from the inlet value to almost zero during the initial part of the cycle step. Most past studies have fitted experimental regeneration data with empirical equations or used overly simplified mathematical models; e.g., the adsorber is preheated to avoid condensing flow (El-Rifai et al., 197% Anikeeva et al., 1976; Dubinin et al., 1978;

Author to whom correspondence should be addressed. t Present address: Du Pont de Nemours, (Luxembourg) S.A.,

0888-5885/93/2632-2418$04 OO/O

I"-- solvent. Further, adsorbed water competa with the

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Grand Duchy of Luxembourg.

Scamehorn, 1979; Lukm and Egorov, 1979,1984; Kisarov et al., 1980; Subbotin and Kaslmikov, 1980; Capelle et al., 1983; Jedrzejak and Paderewski, 1988).

Other cqntributions have involved both theoretical and experimental efforts. Schork and Fair (1988) present experimental results for a small-scale adsorber. The wall of their apparatus is massive and the steam flow rate is fast; thus, their results suggest the adaorber can be modeled as a single, well-stirred tank. Ustinov and co-workers (Ustinov et al., 1982,1985; Ustinov, 1986) simplified the mathematics by treatmg steam as a gas which condenses, rather than adsorbs, on the carbon; with this model they explain the fronts of condensing vapors that emerge from an adsorber during regeneration. They present no experimental results.

Models and experimental results for steam regeneration of an adsorber uniformly loaded with n-hexane were recently presented hy the authors (LeVan and Schweiger, 1991). We extend that work here, showing more experimental results, presenting an alternative numerical solution, and improving significantly the adsorption equilibria in light of new data. These new data (Rudisill et al., 1992) allow quantitative prediction of water adsorption in

Models

In our recent paper (LeVan and Schweiger, 1991). we presented two models for steam regeneration of a carbon bed based on discretizing the bed length into stages (or m i x i cells). One modelassumedlocalequilibrium, while the other added maas-transfer resistances. We observed that mass transfer imposes no significant limitation since the propagation of changes in concentration and temperature is slow. Furthermore, it bas been identified that fluidmechanics, throughflowmaldistribution, can bemore significant than mass- and heat-transfer resistances in determining observed rate behavior (Schweiger, 1991). Thus, in the formulation of our current models, we assume local equilibrium. Two models are presented here, the original equilibrium stage model and a model solved by orthogonal collocation on finite elements. Both models treat an adsorber as a column containing a fixed bed and well-mixed unpacked volumes at the inlet and outlet. The conservation equations are material and energy balances for the fluid and stationary phases. The energy balance contains a term for energy loss through the wall of the

0 1993 American Chemical Society

packed bed, and the heat capacity of the wall is lumped with that of the column packing.

The two models differ in how spatial derivatives are treated. The stage model writes spatial derivatives by backward finite differences, which is mathematically equivalent to describing the fiied bed as a cascade of well- mixed cells. The orthogonal collocation model fits variables to polynomial functions in the axial direction, which are differentiated to obtain the spatial derivatives. As a consequence, the conservation equations for the two methods differ on two points. First, the stage model is first order, whereas the collocation model bas second derivatives in the form of an axial dispersion term to allow implementation of two boundary conditions, which the method requires. Second, the stage model solves directly for velocity a t constant pressure, whereas our collocation model includes pressure as a variable and requires an additional equation to link pressure and velocity.

Conservation and Rate Equations. For the collocation model, the material and energy balances are the following.

adsorbate material b&ce:

water material balance:

Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993 2419

voidage e' is the local void fraction in the bed including intraparticle pore volume. It is calculated from

(9) where x is the particle porosity and +A and +ware specific volumes of n-hexane and water in the stationary phase.

For the collocation model, the pressure gradient is used to determine the velocity in the packed bed through the Blake-Kozeny equation

8 = 60 + (1 - @x - &(+A + +W)

For the stage model, the material and energy balances are given by eqs 1-4 but without the dispersion terms on the right-hand sides. Also, rather than use pressure as a variable in the stage model, we assume that at any time the bed is at a constant pressure throughout and solve for velocity directly as described below.

The wall of the end tanks, on which water and solvent may condense and in which energy accumulates and through which it is transferred, is assumed to be in contact with the bulk vapor through a mass- and heat-transfer f i b . For both models, rate equations for this process are the following:

condensation of adsorbate:

-= awA k(c,-ci) at

overall material balance:

energy balance:

~ f = hf -P (5)

(6) hf = (cACpU + cwCPw; + c,Cpd(T- Td)

UB = ( C p , u + + qwCPwJ(T- Td) - s," XA(qA',%Tmf) dqA' - s,'" Xw(q.&w'.Td) dqw' (7)

Since internal energy is a state variable, u. is calculated by following a thermodynamic path. The above equation is for a path in which the solvent and then the water are adsorbed at a reference temperature and then heated to the current systemtemperature. XA(qA, qw, 2'1 and Xw(qA, qw, T, are the isosteric heats of adsorption of the solvent and water. For the solvent, this is calculated from its adsorption isotherms using

evaluated at T,r. The latent heat of adsorption of water is taken to be independent of loading (Dubinin, 1980) and set equal to the heat of vaporization of water. Liquid heat capacities are used for the adsorbed components. The

_- - k(cw - cL) at

accumulation and transfer of energy

uw,(Td - T,b) (13)

Boundary Conditions. The boundary conditions at the column ends arise from the material and energy balances for the end tanks. For the collocation model, we stipulate that concentration, temperature, and fluxes are continuous across the boundary between well-mixed end tanks and the packed bed (Ramkrishna and Amundson, 1974;ParulekarandRamkriihna, 1982,1984). Atthe bed inlet, z = 0, we have

where uI.-o is obtained from the overall material balance

2420 Ind. Eng. Chem. Res., Vol. 32, No. 10,1993

WAandww,theamountofadsorbakandwatercondensate in the tank respectively, are restricted to nonnegative values. At the bed outlet, z = lb, we write

Aehr(ul,, -0Lk) (21)

where ulOut is obtained from the overall material balance:

The pressure at the outlet is fixed. For the stage model, flux is continuous across the bed

inlet and outlet planes. The boundary conditions serve as the equations for a tank at the inlet emptying into the adsorber and for a tank at the outlet into which the adsorber empties, respectively. At the column inlet, z = 0, we have

and at the column outlet, z = lb, we write

8% - V - - -h(T - Twd) + Af((uh&,k) - (UI,,&) (28) -U at

where uL-0 and ulOut are again obtained from overall material balances given by eqs 18 and 22.

Numerical values of parameters used in the model are given in the Appendix.

Adsorption Equilibria. A mathematical description of adsorption equilibria over wide ranges of temperature and composition is needed. For the pure components, n-hexane and water, we have used correlations of the data of Rudisill e t al. (1992). All of these data were measured for the same lot of Calgon Type BPL activated carbon that was used in the adsorher for the steam regeneration experiments reported here.

Rudisill et al. (1992) identified two important trends in their data that we take into account in constructing an empirical representation of adsorption equilibria. These trends were not included in the adsorption equilibrium model used in our previous paper (Schweiger and LeVan, 1991). First, theshapeofthe waterisothermchangeswith temperature. Specifically, as temperature increases, the normally sharp upturn in water adsorption becomes blunt and occurs a t higher relative pressures. Second, the

interaction between solvent and water in the ad phase is significant, with adsorbed water able to d solvent.

Our principal concern for the adsorption equilibhh correlation is for accurate prediction over the path found in the steam regeneration experiments. As we will shes; this path begins at room temperature with modera& solvent loading, passes through moderate temperat& with high solvent and low water loading, and finishes high temperatures with low solvent and moderate to high water loading.

The correlations used, based o are given in the Appendix., For assume that the apparent pore vo

atlow loading,weincludeanempiricalscalingofthe effect of water on the volatility of the solvent at low solvent loading. While the form of our scaling function Iackqa theoretical basis, it allows representation of the available data fairly well and is numerically efficient in the steaih regeneration models.

Solution Method. For bot were exprlnded using the chain of time derivatives of qA, qw,

derivatives were then integrated numerically. models, the equation set was integrated using method solver LSODE (Hindmarsh, 1980).

Experiments

ature, concentrat


I

Figure 1. Apparatus for ateam regeneration.

effective heat capacity of the entire adsorption column is 50 % greater than the heat capacity of the adsorbent alone.

Temperatures in the bed are measured with thermocouples located at one-quarter bed length intervals, with the thermocouples located at the radial position where the area of the inner cylindrical region equals the area of the outer annular region &e., a t r/rwd = l/dZ). Two, thermocouples are placed at midlength, one positioned to give the core temperature and one positioned to give the wall temperature. (Radial temperature gradients were foundto be small.) Anadditionalthermocouple isinstalled in the feed line to measure the temperature of the column feed. The thermocouples for which temperature breakthrough curves are reported below are just upstream of theretainingscreens.near thetopofthecolumnforupflow and near the bottom for downflow. All thermocouples are connected to a multipoint recorder and temperatures are measured continuously.

Mostly, flows are downward for loading the bed with hydrocarbon and upward forsteaming, and theapparatus iseasilymodifiedtoreverse the flowdirections. Thesteam flow rate from an electric steam generator is set with a needle valve and metered with a rotameter mounted in a thermostated hot oil bath to prevent condensation. The bath also houses a second rotameter for measurement of the flow rate of the effluent. AU lines carrying steam or concentrated solvent vapors

are heat-traced using electrical heating tapes to prevent condensation. The carbon is dried prior to loading with solvent, and dry air is used in the feed makeup. Gas samples are continuously drawn from the column outlet and analyzed using a gas chromatograph with a thermal conductivity detector. Effluent concentration measurement is automated with sampling every 2.5 min.

The column heat capacity and wall heat-transfer coefficient were determined experimentally by heating the column with an inert gas. Axial dispersion coefficients for the adsorher were estimated from tracer experiments. Additional details concerning the apparatus and operating procedures are given by Schweiger (1989).

Results Resdtsoffive experimentsare reported here. Theinitial

conditions and feeds are given in Table I. We vary the

Table I. Exbriments initial condition

experiment ~

p* (K) (moVkg) (moVkg) ___ T 9A' qap

1 basewe 0.1 302 3.03 0.008 2 high flow 0.1 300 3.04 0.008 3 downflow 0.1 298 3.05 0.008 4 low load 0.01 302 2.45 0.008 5 highloLd 0.8 300 3.56 0.W8

feed G Th

@/a) (K) 0.021 384 0.041 390 0.020 388 0.0205 400 0.021 400

a Predicted.

initial loading of n-hexane on the carbon, the steam flow rate, and the flow direction. Flow of steam was upward except for experiment 3. In all experiments the steam fed to the column was at atmospheric pressure and slightly superheated. We show breakthrough curves predicted by the stage model using 50 stages for all experiments and predictions of the collocation model for selected experi- menta. Velocities predicted by bothmodels oscillate with time and will be shown for only selected experiments.

For the fist three experiments, the bed was loaded by passing air 10% saturated with n-hexane (Le., PA PA/PA = 0.1) through the column at ambient temperature untilequilihrium was reached. For experiment I, the base w e , steam was then passed upward through the column at atmospheric pressure and at a flow rate of 0.021 g/b, corresponding to 0.85 superficial bed volumes of steam per minute at feed conditions. Bed profiles predicted for experiment 1, obtained using the collocation model after 35 superficial bed volumes of steam have been passed into the column, are shown in Figure 2. The bed profdes show various fronts and plateaus; these will be more sharply defined in stage model predictions. Note the plateau at 334 K, which wil l be described in detail below. The front a t { = 0.7 that precedes this plateau removes the inert gas from the bed.

Experimental breakthrough curves for experiment 1 are showninFigure3. Plottedasafunctionoftimearesolvent and water effluent mole fractions from the column and temperature at the outlet thermocouple just inside the bed. The solid lines are predictions of the stage model, andthepointsareexperimentalresults. The breakthrough curves show the emergence of two principal zones. At first the conditions at the outlet are the initial conditions,

-

2422 Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993

0 0.2 0.4 0.6 0.8 1 .o

- - 0 " " I " " I " 8 7 a I-' " ' 280

0 0.2 0.4 0.6 0.8 1 .o

.>

,

c Fuure 2. Bed proffie8 for experiment 1 (base m e ) predicted by collocationmodel. (top)Loadingsandtempereture. (bottom)Vapor- phase mole fractions and velocity.

with a gentle purge of inert. After about 60 min of steaming, a wave of desorbed solvent emerges at a temperature of 334 K. After about 110 min, when most of the solvent has been desorbed, the solvent concentration drops rapidly. Note, however, that the second wave here is not as sharp as predicted; the thermocouples positioned along the bed showed this wave to be relatively sharp at

= 0.76, only becoming diffuse at the bed outlet (Schweiger, 1989). A third, very slow moving wave is predicted to still be in the bed near its inlet; it is this wave that essentially fds the pores of the carbon with water.

For experiment 2 the steam flow rate was 1.7 superficial bed volumes per minute, double the base-case flow rate, with the direction of steam flow still upward. Figure 4 shows bed profdes predicted by the stage model using 100 stages after 40 bed volumes of steam have been passed into the column. Three distinct regions are apparent. Beginniigat the bedinlet. the first regioncontainsresidual solvent and steam at high temperature, with the pores of the carbon nearly f N i g with water very close to the bed inlet. In the second region, between t = 0.4 and 0.76, the solvent has rolled up to a great extent and forms a plateau. The void fraction of the bed is 0.4. On this plateau, e', the total local voidage of the bed, including that within the particles, is only 0.38; thus, the volume of the solvent and water exceed the pore volume of the carbon and some of the n-hexane and water exist as bulk liquid in the

280 0 50 100 150

t (min)

Figure 3. Breakthrough curves for experiment 1 (base Predictions by stage model. (top) Vapor-ph mole fracti column outlet. (bottom) Temperature et bed outlet.

interstitialvoids ofthe packing. In fact, in the and throughout the experiments, we wil l cont this plateau with a temperature of 334 K. temperature a t whichthe sum ofthe pure com

influenced by the steaming and thus preserves the init condition.

fust 30 min as inert gas is purged from the bed alo small amounts of n-hexane and water. The veloc

plateau, with the predicted value oscillating for discussed below. With the emergence of the seco from the bed, temperature continues to increase slightly superheated steam leaving the bed.


4 0.6

r Figure 4. Bed profdes for experiment 2 (high velocity) prediaed by stage model. (top) Loadings and temperature. (bottom) Velocity and total voidage.

Experiment 3 used the initial conditions and steam feed and flow rate of the base case, but the steam flow was downward. The flow direction was changed to determine if the liquid phase formed is immobile or if it trickles downward through the packing. Breakthrough curves are shown in Figures 7 and 8with stage and Collocation madel predictions, respectively. The transitions for this experiment are much sharper than those observed for S k d n g the bedwithupflowinthebasecaae,andtheoutletvelwity on the steam distillation plateau is slightly higher. Wave fronts predicted by the stage model are smoother than those obtained with the collocation model. The velocity oscillations characteristic of our models are readily apparent.

For experiments 4 and 5 the bed was equilibrated at ambienttemperaturewithairl% and80% saturatedwith n-hexane, respectively. Experimental measurements and stage model predictions are shown in Figures 9 and 10. For the lower loading, the steam distillation plateau is relatively short and shows some variation in measured temperature and concentration, although the quantity of adsorbate and liquid that develops is still predicted to be enough to overflow the pore structure. For the higher loading, this plateau is extended because of the greater quantity of solvent removed; the solvent rollup quickly overflows the pore structure of the adsorbent. Velocities predicted for these experiments (not shown) show oscillations on the steam distillation plateau that are less than

280 20 40 60 BO 100 120 0

t (min)

0 I , , , , I I I I I I I I I I I I I I I I 0 20 40 60 80 100 120

t (min) Figure 6. Breakthrough curves for experiment 2 (high velocity). Predictions by stage model. (top) Vapor-phaee mole fractions at column outlet. (middle) Temperature at bed outlet. (bottom) Velocity at column outlet.

average for the low initial loading and greater than average for the high initial loading. For experiments 4 and 5, despite the large difference in

initial loading of n-hexane, 2.45 mol& compared to 3.56 movkg, the solvent residue (or "heel") remaining on the carbon after regeneration is essentially the same. Stage model predictions of these are shown in Figure 11 after regeneration with 260 bed volumes of steam. Experiment ,

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t (min)

0 20 40 60 80 100 120

1 (min)

0 0 20 40 60 80 100 120

t (min) Figure 6. Breakthrough curves for experiment 2 (high velocity). Predictions by colloeation model (top) Vapor-phase mole fraction8 at column outlet. (middle) Temperature at bed outlet. (bottom) Velocity at column outlet.

5, which began with more n-hexane adsorbed, finishes with leas. The small difference between the curves is due to the water competing with the solvent for pore volume; more water must condense to provide the heat to desorb the larger quanitity of solvent a t the high initial loading. In the experiments, approximately 15% more water is predictedtoadsorbforthe higher initialn-hexaneloading.

0 50 100 150 2w

t (min)

280 0 50 100 150 200

I (min) . .. i . .. . ... 1.0 1 1 1 ~ ~ , , , , ~ , , , , ~ , 1 , ,

L

0.8 -

0.6 - -5

0.4 -

0.2 -

0 l , , , , I , , , , 0 50 100 150 200

t (min) Figure 7. Breakthrough curvea for experiment 3 (downflow). Predictiona by stage model. (top) Vapor-phase mole fractiona at column outlet. (middle) Temperature at bed outlet. (bottom) Velocity at column outlet.

Discussion

Thermal regeneration of an adsorption bed usingsteam is qualitatively different than thermal regeneration using a hot noncondensible purge gas. Heat losaes from a vessel undergoing regeneration with a noncondensible gas arp manifested by a temperature drop and lower find ted- perature reached, whereas with steam regeneration, the


l . O C I ' ' ' I ' ' ' ' I ' '

t (min)

400 - L

360 380 t 340 - t b-

320

,

300

0 0 50 100 150 200

t (min) Figure 8. Breakthrough e w e s for experiment 3 (downflow). Predictions hy wllwation model. (top) Vapor-phase mole fractions at column outlet. (middle) Temperature at bed outlet. (bottom) Velocity at wlumn outlet.

temperature ia not affected significantly hut condensation Oecm within the vessel at thew&. The velocity of the effluent also differs considerably. With hot purge gas regeneration, the carrier leaves the bed throughout the heating step with roughly the same molar flow rate that I t entered the bed. In contrast, with steam regeneration, the Steam adsorbs and condenses in the bed. Only aslow

1 .o

0.8

0.6 3 2.

1 0.4

0.2

0 0 50 100 150 200

t (mm)

.

0 50 100 150 200

t (min)

Figures. Breakthroughnweaforerpariment4 (lowinitialloadingl. Predictions hy stage model. (top) Vapor-phme mole fractions at wlumn outlet. (bottom) Temperature at bed outlet.

purgeofinertgaswithsolventandwateratinitialcondition values leaves the bed until many bed volumes of steam have been fed to the bed, at which point the first front leaving the bed effectively sweeps all inert gas from the bed. Of course, there are also other ohvious qualitative differences between steam and hot purge gas regeneration such as the added competition for adsorbent pore volume by adsorbed water, the large latent heat effects for condensing steam, etc.

Most of the energy provided hy the steam goes Meat the carboa, the vessel, and any residues of solvent and water. OnlydYiiU~ &e-fourtW~-6n~third of the total energy goas to desorb solvent. Specifically, for ow base case, with steam fed to the bed until just after the steam distillation plateau has been removed, approximately62% of the energy has gone into heating the carbon (excluding the vessel), 32% has gone toward desorbing n-hexane, and 16% has gone into warming the solvent residue (ignoring any preadsorbed water). If the heat capacity of the vessel is included in the calculations, then the percentage of the energy used to desorb the solvent drops to 26%.

From the standpoint of an energy analysis, the amount of water that adsorbs and condenses can be thought of as being determined largely by energy needs. Indeed, if water did not adsorb hut only condensed, we would observe some of the same qualitative behavior, especially the early trends. The effect of water adsorption is most influential

-

2426 Ind. Eng. Chem. Res., Vol. 32,.NO. 10; 1993

J-- l . O C ' I I I , I I I I , I I I , I I , '$

3 2.

3 0.4

0.2

0 I , > , , I , I , I

0 50 100 150 200

t (min)

380

-3 300

280 0 50 100 150 200

t (min)

Figure 10. Breakthrough curves for experiment 6 (h ih initial loading). Predictions by stage model. (top) Vapor-phase mole fradiona at column outlet. (bottom) Temperature at bed outlet.

0 0 0.2 0.4 0.6 0.8 1.0

6 Figure 11. Residual loadings of n-hexane (A) and water (W) in bed at end of heating with 260 bed volumes of steam an predicted by stage model for experiment 4 (low initial loading) and experiment 6 (high initial loading).

after the steam distillation zone has passed. In particular, treatment of water adsorption gives a more accurate prediction of final profiles of bed temperatures and loadings. Its inclusion in models is necessary for quantitative prediction.

The wave character of steam regeneration obse experimentally is predicted by the models. inch velocity variations and the formation of a liquid ph within the bed. Both models give fairly good predictio of the behavior and their results differ only a little. 0 two models, the stage model is the faster computation An explanation for why the solvent heel after

eration is essentially independent of initial loading i simple. During regeneration the solvent rollup o the pore structure, substantially erasing an initial loading. All of the cases we identical steam distillation platea loadings areapproximately4.5andO.5 and the temperature is 334 K. If this

,

then it is unremarkable that one sho

determine somewhat the amount of water ads wi l l determine the length of the steam distillati

assumed that the shape of qw vs PIP, was indepen consequence our simulat

the water isotherm changes with temperature, and isotherm used here reflects this dependence.

Viewing the amount of water adsorbed from standpoint of energy needs with the relative pressure water fixed by ita isotherm, if the isotherm is inc sharp in the model, then the loading of adsorbe determined by energy needs gives too low a pressure, which results computationally in a h temperature because the sum of the solvent and water is assumed that adsorbe of solvent adsorbed. dicted in our previo increasing the volatility of the adsorbed solvent.

Condensate Flow. The most notable difference between the experimentalandpredicted breakthroughcurves is the difficulty the models have in matching the transition after the steam distillation plateau for experiment 1. In experiment 2, performed at a higher steam flow rate, and in experiment 3, with steam downflow, the transition was much sharper. These anomalies can be explained only by the formation and downward trickling of condensate in the column. As mentioned previously, for experiment 1 thermocouples a t other locations in the bed (e.g., {= 0.76) showed the wave to be much sharper prior to reaching the top of the bed (Schweiger, 1989). In experiment 2, the increase steam flow rate reduced the time for heat losses and for condensation and trickling to become severe. The sharpeningofthe wave withincreasingvelocity iscontrary to conventionalmass-transfer models, occurring only when axial molecular diffusion is controlling, which is not the case here. In experiment 3, downward flow facilitates drainage, rather than the refluxing and flow maldistri- bution that are likely occurring with upflow.

Ind. Eng. Chem. Res., Vol. 32, No. 10,1993 2427 used to analyze the experimental results. Both are of the local equilibrium type; rate models give similar results (Schweiger, 1989; LeVan and Schweiger, 1991) since rates of intraparticle mass and heat transfer are fast. The models require no fitted parameters and predict break- throughcurvea that are inreasonahlygdagrmentwith experimental breakthrough curves.

Three zones are predicted and observed under normal operating conditions: a plateau on which the initial condition including inert gas is purged at low flow rate from the bed, a plateau on which steam distillation of the water-immiscible solventoccura, andalargelysteampurge leading ultimately to the feed condition. Most of the solvent is removed during the elution of the second zone. The vapor-phase concentration of solvent drops rapidly after this steam distillation plateau has passed, signaling the end of effective regeneration. As a practical consideration, in experiments only a weak dependence on flow direction was found, indicating that the primary mechanism for movement of solvent is vaporization with downstream adsorptionlcondensation rather than any bulk flow caused by trickling.

The extentofregenerationis determined by thequantity of steam passed through the adsorber. The solvent heel remaining in a well-regenerated adsorber a t the end of steaming is largely independent of the initial loading.

For models for steam regeneration of activated carbons to predict performance representative of a real system, the adsorption equilibrium relations, and particularly the interaction between adsorbed water and solvent, must be reasonably accurate. While the current adsorption equilibria impzoves on our previous work, it remains a critical point to the usefulness of the model.

Acknowledgment is made to the donors of the Pe- troleum Research Fund, administered by the American Chemical Society, for the support of this research.

Nomenclature A = cross-sectional area of column, mz c = fluid-phase concentration, moVm8 C, = heat capacity, J/(mol K) C,,M = heat capacity of adsorption bed, J/(kg K) C,,," = heat capacity of end mixing cell, J/K d, = diameter of adsorbent particle, m D = column diameter, m DL = axial dispersion coefficient, mZ/s G = maas flow rate of feed to column, gls h+ = enthalpy of fluid phase, J/m3 h = heat-transfer coefficient, 8-1

k = mass-transfer coefficient, ma/(kg 8 ) lb = bed length, m M = molecular weight, kglmol P = pressure, Pa Pe = Peclet number for mass dispersion, .ovol&, PeT = Peclet number for energy dispersion, C O U O ~ ~ X L q = adsorhed/condensed phase concentration, moVkg t = time, s T = temperature of fluid phase, K ur = internal energy of fluid phase, J/m3 u. = internal energy of stationary phase, J/kg U = wall heat-transfer coefficient, J/(m2 s K) U,U = end mixing cell wall heat-transfer coefficient, J/(K s) u = interstitial fluid velocity, m/s U* = dimensionless flow rate, c u / ( ~ u , ) V," = volume of end mixing cells, m3 ui = condensate in mixing cell, mol y = fluid-phase mole fraction z = axial coordinate. m

i

-

Moreover, flow direction through the development of the liquid phase affects transitions differently. For steamingupward, trickling of theliquid phase wouldaffect the second transition zone, where mostly n-hexane would fall into a hot region. There, it would quickly vaporize and he pushed back up into the bed. For steaming downward, the trickling would affect the first transition zone, where n-hexane would fall into a cold region. There it would tend to be wicked into the pore structure of the ,-arbon and be adsorbed.

We expect trickling to be most pronounced at the wall, where heat transfer to the vessel wall promotes the formation of additional condensate. Also, in comparison with a large industrial bed, our apparatus has a large wall area compared to its volume. For the hrgerbeds, trickling at the wall may be of leaser importance.

Our models assume that condensate in the bed is stationary. It is not allowed to trickle; instead, it moves by vaporization and recondensation. The possibility that condensate in the top end region was dripping back into the bed in the base-case experiment and causing the broadening of the secbnd transition was investigated by modifying the stage model. The model was changed to let 80 '3, of the liquid condensate drip back into the last stage. The results of this test showed no significant difference in the breakthrough curves, indicating that the observed effects of condensation and trickling are due to condensation and trickling in the bed and not predominantly in

to heat residues, particularly of water, canbecome hie., We predict if an adsorbent has a significant water residue that is not removed before regeneration, water may continue to accumulate with successivecycles as moreand more steam adsorbs and condenses to heat the residue. With upflow steaming, significant trickling and refluxing can beexpectedtooccur,and theadsorbermay effectively flood. By implication, the relative humidity of the feed is important in determining the water residue, as is the operating temperature. If the regenerating steam is near saturation, the carbon on the steam-feed step will fill with water as it reaches equilibrium with the feed. Although this wave propagates through an adsorber slowly, it deposits water which may cause flooding problems later if it is not removed.

Velocity Oscillations. Our models predict that the velocity on the steam distillation plateau is reasonably constant as shown in the bed profiles in Figures 2 and 4. However, this velocity rises and falls with time as shown in Figures 5-8. The number of oscillations is proportional to the number of stages or collocation points. The stage model, with no upstream propagation of effects, shows a largespikein thevelwityas thesteamdistillationplateau exits theadsorber. Thisspikeisdueto therapidexpansion of gas and vaporization of condensed solvent in the end mixing cell as it is heated by the wave of steam. Similar expansion effecta cause the prior oscillations as the second transition, ending the steam distillation plateau, passes through stages or past collocation points.

Conclusions

We have reported experimental data for steam regeneration obtained using an apparatus designed to enable measurement of concentrations, temperatures, and flow rates under flexible operating conditions. We know of no other data of this kind.

Two mathematical models, a stage model and an orthogonal collocation on finite elements model, have been

2428 Ind. Eng. Chem. Res., Vol. 32, No. 10,1993

Greek Letters c = volumetric gas fraction in interstices of bed e,, = void fraction of packing c' = total volumetric gas fraction including pore volume f = dimensionless axial coordinate, Z/Ib 0 = fractional loading, 4/40 A = heat of desorption, J/mol XL = thermal diffusion/dispersion coefficient, m2/s p = viscosity of fluid phase, kg/(m s) Pb = bulk density of bed, kg/ma p1 = liquid density, kg/m3 4 = volume adsorbed based on liquid density, qM/pl, mVkg x = particle porosity

Subcripts A = solvent f = fluid phase I = inert gas in = feed value 1 = adsorbate/liquid W = water

Superscripts r = relative to saturation s = saturation

L

Appendix

Model Parameters

= 0.584 m D = 0.073 m V,U = 0.OOO 32 m3 U = 0.944 J/(m2 s K) U,u = 0.0174 J/(s K) d, = 2.67" p b = 480 kg/ma q = 0.4 x = 0.5 k = 1 m3/s h = 6.4 X 10' kJ/(m3 8 )

Pe = 60 PeT = 60

,

T.mb = 296-303 K T,r = 298 K C p , ~ = 1.52 X 10s J/(kg K) Cp,-" = 127.0 J /K C p u = 158.0 J/(mol K) CPwf = 36.9 J/(mol K) CP1 = 29.24 J/(mol K) CP,j = 216.0 J/(mol K) Cpwr = 75.4 J/(mol K) AW = 4.07 X lo' J/mol p~ = 9.4 X 108 kg/(m 8) a t 105 OC pw = 12.7 X 1lP kg/(m 8 ) at 105 OC pl = 21.9 X 1lP kg/(m s) at 105 O C

1987): Adsorpt ion Equi l ibr ia . Pure n-hexane (Hacskaylo,

B + b 1 0 1' - A) for e, 5 1

(29) C + T In PA (MPa) = A + In 8,-

lnP , (MPa) = A -- for e,? 1 (30)

withA = 6.98946,B = 2737.59, C = -46.87, and bl = 3356.89. 4; = 477.4 X 10-8 m$/kg. PAJ = 654.71 - 0.9735(T- T,&

C + T

Pure water:

In Pw (MPa) = A' -

A' = A + In 8, + al(l - Sw) + a2( l -8w)2 + a,(l- + a4(i - ew)4

(12 = 4.8123, as = -7.5767, a4 = 5.9743, and bl = 43 4' - 406.0 X 1V m3/kg. p w , ~ = 997.01 -0.5221(T

%&tures: The equations above are used to p partial pressures empirically using values of eA given by

$A + (1 - ~ X P ( + . Z ~ ~ , ~ ) M ~

Literature Cited

iter Vapor Adsorption and th bonaceoun Adnorbents. Carbon

of Volatile Solvenk from Active Carbons by Saturated Zh. R i k l . Khim. 1979.52,2248-2263.

Lukin, V. D.; Egorov,A V. Mathematical Modelofthe Nonisothed Desorption of Volatile Solvents by Saturated Water Vapor from Active Carbns. Zh. Prikl. Khim. 1984.57, 2711-2716.

Pdekar , S. J.; Rnmhishna, D. A Spectral-Tbeoretic View of Axid Dispernion Models. Residence Time Distribution Theory in Chemical Engineering; Petha, A,, Noble, R. D.. Ma.; Verlas Chemie: Berlin, 1982; pp 113-146.

Parulekar, 5. J.; Ramkrishna, D. Analynisof Axial Dispersion S&mS with General Boundary Conditions. Chem. Eng. Sci. 1984,39, 1671-1611.

Ramkrinhna. D.; Amundaon, N. R. Stirred Pota, Tubular Reactors, andself-AdjointOperabrs. Chem.Eng. Sci. 1974,29.1363-1361.

.'

Rudisill, E. N.; Haeakaylo. J. J.; LeVan, M. D. &adsorption of Hydrouubons nrid Water on BPL Activated Carbon. Ind. Eng. Chem. Res. 1992,31,1122-1130.

Scamehorn, J. F. Removal of Vinyl Chloride from Gaseous Streams by Adsorption on Activated Carbon. Ind. Eng. Chem. Process n.,* nor,. 1919. IR. 21&217. --".I--. -_... ...- ~

Schork, J. M.; Fair, J. R. Steaming of Activated Carbon Beds. Ind.

Schweiger. T. A. J. Ph.D. Dissertation, University of Virginia, 1989. Schweiner. T. A. J. Pollution Reduced Through CFD-Guided

Eng. Chem. Res. 1988,27,1&(6-1541.

Modkration to Flow in Carbon Adsorption Solvent Recovery Syatem. Indualrial Application8 of Fluid Mechnnics-1991; Morrow, T. 8.. MeraFmll, L. R., Shes, S. A,, Eda.; ASME Fluida Engineedng Division: New York, 1991; VoL 132, pp 19-23.

Suhbotin, A. I.;Kashniov,Y. V. Deaorptionof CertainHydxoearbons from Active Carbon by Steam. Zh. Prikl. Khim. 1980,53,63-61.

Ind. Eng. Chem. Rea, Vol. 32, No. 10,1993 2429 Ustinov, E. A. Dynamiea of Desorption of Organic Substancea by

Wnter Vapor. Zh. Prikl, Khim. 1986,59,796801. Uatinov,E.A;Seballo,A.A,Plachenov,T. G. ModelingofDesorption

of Organic Substances from a Fixed Adsorbent Bed hy Steam. Zh. Prikl. Khim. 1982.55. 116-122.

Ustinov, E. A.; Sehallo, A. A, Kisarov, V. M.; Plachenov. T. G. Relationships in Adiabatic Deaorption of Organic Substances by SteaminRecuperationProcessen. Zh.prikl. Khim. 1986.58,803- 808.

Received far review February 24, 1993 Revised manuscript received June 24,1993

Accepted July 2,1993.

Abstract publiahed in Aduance ACS Abstracts, September 15, 1993.

I d . Eng. Chem. Res. 1995,34, 283-287 283

Effects of Water Residues on Solvent Adsorption Cycles Thomas A. J. Schweigert DuPont de Nemours (Luxembourg) SA., L-2984 Luxembourg, Grand Duchy of Luxembourg

The effects of water residues on solvent adsorption cycles are investigated with a mathematical model which has been experimentally verified for steam regeneration. Adsorbers for recovering hexane vapors from humid air streams are simulated in cycles during which they are alternately loaded to 5% breakthrough and regenerated with a fured amount of steam. The temperature of the feed determines whether a cycle is stable or unstable. An unstable cycle accumulates water, leading to flooding of the adsorber. Rapid changes in outlet temperature, flow rate, and solvent concentration occur immediately after steam regeneration, when the feed of solvent-laden air is reintroduced.

Introduction Because of its ability to adsorb organic solvents from

dilute gas streams, activated carbon is widely used for solvent recovery. Usually, the gas to be treated is humid. While activated carbon's capacity for solvent adsorption is high, it is recognized that it decreases as humidity increases. Adsorbed water is a hindrance t o solvent adsorption, and studies have probed the effect of preadsorbed and coadsorbed water on adsorption equilibria and on adsorption in small tixed beds (Doong and Yang, 1988; Keener and zhou, 1990; Okasaki et al., 1978; Petrova and Nikolaev, 1982a,b; Rudisill et al., 1992).

Since carbon adsorption solvent recovery systems are generally regenerated with steam, the presence of water is deliberate and unavoidable. The advantages and disadvantages of preadsorbed water are debated in the literature. Naujokas (1985) regards preadsorbed water as important for preventing bed fires and dissipating the heat of adsorption of the solvent. Parmele et al. (1979) and Mastroianni and Rochelle (1985) indicate thal cooling and drying of an adsorber after steam regeneration are necessary for it to perform well when placed in service and that they reduce corrosion and prevent solvent degradation. Scamehom (1979) studied the drying step with a simple model but concluded that no simple mechanism can describe the drying process. He found that drying is slow without heating. Ustinov (1986) models the drying of a steam-regenerated adsorber to optimize energy consumption.

In our recent paper on steam regeneration of fixed bed solvent adsorbers (Schweiger and LeVan, 1993), we predicted that if an adsorbent has a significant water residue that is not removed before regeneration, water may continue to accumulate with successive cycles.

Solvent Recovery Cycles

Adsorbers for solvent recovery are operated in cycles where the adsorbent is alternately loaded and regenerated. Regeneration with steam deposits water on the adsorbent, and this water influences the adsorber's ability to recover solvent. This paper examines the advantages and disadvantages of residual water to adsoher dynamics using a numerical simulation of solvent recovery cycles in which an adsorber is fed humid solvent-laden air until breakthrough and regenerated with a fixed amount of steam.

'Present address: E. I. DuPont de Nemours & Co., InC. Richmond, VA 23261.

Table 1. Cycle and Adsorber Parameters case 1 case 2

adsorber feed temperature ("C) 25 45 ppm hexane 20 000 20 000

breakthrough (%) 5 5

ppm water 15 400 15 400 relative humidity (%) 50 16

regenerating steam temperature ("C) 100.05 100.05 pressure ( E a ) 10 1.3 2 5 101.325 no. bed volumes 300 300

heat capacity (Jkg ' C ) 1050 1050 density <kg/m? 480 480

adsorber properties

Simulation. The simulation is made using a model presented in our previous paper (Schweiger and LeVan, 1993) in which we verify the model's ability to predict experimental measurements of a steam-regenerated adsorber loaded with hexane. Here the stage model with 50 stages is used. The same adsorption equilibria are used for hexane and water on type BPL carbon fitted to the data of Rudisill et al. (1992). Local equilibrium is assumed during loading and regeneration. We observed that mass transfer imposes no limitation during steam regeneration (LeVan and Schweiger, 1991). NO end cells are used. The adsorber is adiabatic; the vessel wall stores and transmits no heat.

The cycles have two steps. The adsorbers are alternately loaded and regenerated. The feed conditions and parameters of the adsorbers for the cases investigated are listed in Table 1. The adsorbers begin uniformly loaded with the feed and are then regenerated with 300 superficial bed volumes of steam. Afterwards, the adsorbers are loaded to 5% breakthrough before regeneration.

Results and Discussion

Flooding. Figures 1 and 2 illustrate the importance of the feed temperature on the stability of an adsorption cycle. The figures show the water loading versus bed depth for two adsorbers operated in cycles which are identical in all respects except for the temperature of the feed. Figure 1 is for an adsorber whose feed is at 25 "C; it shows the water loading on the carbon a t the end of steaming after 1, 2, 8, 15, 21, and 24 cycles. Figure 2 is for an adsorber whose feed is a t 45 "C; it shows the water loading a t the end of steaming after 1, 2, 5, and 8 cycles.

0888-5885/95/2634-0283$09.00/0 0 1995 American Chemical Society


\ I

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 I fraction of bed depth

Figure 1. Bed profiles of the water loading at the end of steaming for case 1 a h r (a) 1 cycle, (h) 2 cycles, (c) 8 cycles, (d) 15 cycles, (e) 21 cycles, and (0 24 cycles.

I 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

fraction of bed depth Figure 2. Bed profiles of the water loading at the end of teaming for case 2 8 h r (a) 1 cycle, (b) 2 cycles, (c) 5 cycles, and (df8 cycles.

The two cases demonstrate the importance of removing the water that adsorbs and condenses during regeneration. For case 1, the removal of water is incomplete and more accumulates on the carbon with each steaming. The water accumulation encroaches from the side of the steam injection, shrinking the region dried by the feed. In the wet part of the bed the solvent my& compete with water to adsorb. After eight cycles, more than half of the bed is saturated with water. At saturation the adsorbent's micropores are full. Any additional water deposits by condensation and overflows into the macropores of the adsorbent. After 21 cycles, water overflows the macropores and begins filling the space between the adsorbent particles. Duringthe 25th cycle, the space between the particles fills completely withliquid in part of the bed. The simulation fails a t this point since there is no open space for gas flow. In practice, flooding would be indicated by a steep increase in the pressure drop across the adsorber.

By contrast, case 2 is stable. The bed profiles for each step converge after eight cycles, so that they are indistinguishable from one cycle to the next. The adsorber is dry by the time it reaches breakthrough. Unlike case 1, no water accumulates. The dip in water loading is caused by the temperature in the adsorber a t the beginning of regeneration. A thermal wave precedes the solvent during loading and a t breakthrough has reached the adsorber's outlet, making it wanner than the feed. Less water adsorbs to warm this region to the regeneration temperature. Both case 1 and 2 manifest a short region with a high water loading on the side of the steam injection. The carbon here is slowly coming to equilibrium with the saturated steam used to regenerate the bed.

The difference between the two cases is that the feed in case 2 is 20 "C warmer than the feed in case 1.

2 - 0 1W 200 300 400 500 600 700 8 superficial column volumes of feed '.;

Figure 3. Amount of water in the adsorber during a loading for case 2.

"C reduces the energy requirement

capacity to 1.5 moykg. Thus, the amount needea for solvent desorption is cut by 15%. water adsorbs,'less has to be removed.

Normally it is desirable to operate an ad

is placed back in service for solvent reco reintroduction of nonadsorbing gases upsets lihrium and causes a

adsorbed and vapor phases.

the residues. Figure 3 shows the average amount of water on

carbon during a typical loading step for case 2. ordinate is the number of superficial bed volume solvent-laden air fed into the adsorber. Drying is.

drying, heat stored in the carbon provide desorb water. The stored energy is lim

- E, 900 6 703- 2 500- 3 4w- e 300-

.%ea-

x m -

Y

I

j I 5 10 t

-

0 10 20 30 40 50 60 70 80 90 1W column volumes of feed

Figure 5. Relative flow rate at the column outlet for the first 100 bed volumes of feed during a typical loading for case 2.

is provided by the warmth of the feed, assisted by the heat of adsorption of the solvent.

The temperature, relative flow rate, and solvent concentration at the column outlet during the loading step change quickly as the feed is reintroduced to a regenerated adsorber. These variables are plotted in Figures 4-6, respectively. Figure 4 shows the adsorber cooling from the temperature of regeneration to a temperature below that of the feed. The drive to maintain equilibrium cools the carbon by a mechanism similar to evaporative cooling, except that the water is desorbing, rather than evaporating. Most of the cooling takes place with the first few bed volumes of air feed, and the outlet temperature is within 10 "C of its minimum after 80 bed volumes. Unless the feed is very humid or the carbon is very dry, an adsorber will reach a minimum temperature below that of the feed. The temperature at the outlet rises above the feed temperature only when water is adsorbing either from a humid feed or from rollup of desorbed water or when the thermal wave from the heat of adsorption of the solvent emerges at the end of the loading step.

The relative flow rate at the outlet is shown in Figure 5. Initially, the volume of water which desorbs to dilute the nonadsorbing gas in the feed is so large that the outlet velocity is over 30 times the inlet velocity. As the carbon cools, less water is added to the vapor phase, and the outlet velocity approaches the inlet velocity. This surge in gas flow is brief but may be substantial enough to fluidize the carbon bed if the air flow is upwards.

Figure 6 shows a spike of solvent emitted as the adsorber cools and then a rise at the end to the breakthrough concentration. The spike is caused by the residue of solvent on the carbon at the end of regeneration. Since mass transfer resistances are neglected in this study, equilibrium determines the solvent concen-

6 200 3 1w ,

: a / . r. .-

M <IO M : 'i; 8 . ; 3 : p 6


I l e m now

airnow vluraled steam - ..... - Superheated swam -\ -

- \

I. ,_...-.. steam now 120 ._.' llo - undried p_ -

70

_.'

.. 40

30 20 L 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 O.!

Figure 9. Bed temperature profile with and without pred after 600 bed volumes of feed.

lost because of the high temperature of the carb the region dominated by the thermal wave fror solvent's heat of adsorption. Bed temperature pr for case 2 with and without predrying are shox Figure 9. Without a water residue to moderat temperature, the adsorber reaches 113 "C after 50 volumes of feed. With a water residue, the adsoi ,temperature remains low. The temperature falls sh where the thermal wave encounters the remain( the water residue.

fractionof bed depth

~~

-

0 10 20 30 40 50 60 70 80 90 100 column volumes of feed

Figure 8. Solvent concentration at the outlet during the first 100 bed volumes of feed after regeneration with saturated and superheated steam.The adsorbers have case 2 initid conhtions prior to regeneration.

Regenerating with superheated steam is one way to reduce the high water loading at the steam injection side of the adsorber. While saturated steam tends to fill the pores of the carbon, superheated steam has a lower equilibrium loading. Figure 7 compares the water loading on the adsorber in case 2 &r regeneration with saturated and superheated steam. The superheated steam is at 115 "C and atmospheric pressure. The penetration of the bed by superheated steam is too slow to offer an advantage for the desorption of solvent for the entire bed, as was found by Carpenter (1983), but superheated steam reduces the buildup of water on the end of the carbon bed and enhances desorption of the solvent residue responsible for the spike. However, this does not lead to lower emissions.

Figure 8 shows the solvent emissions after regeneration with saturated and superheated steam. Losses are higher from the adsorber regenerated with superheated steam, despite the low water and solvent residues on the end of the adsorber. This is because the water residue is also responsible for the rapid cooling of the adsorber. The adsorber regenerated with superheated steam cools more slowly than the adsorber regenerated with saturated steam. The advantage of a low water residue to solvent equilibrium is offset by the volatility of the solvent at a higher temperature.

Although a water residue poses certain problems, it offers the advantage of moderating the temperature rise from the heat of adsorption of the solvent. The heat of adsorption of the solvent drives off water, drying the carbon, rather than creating a large thermal wave. Completely drying an adsorber before it is returned to service reduces its performance. For example, if the adsorber in case 2 is dried with 2000 bed volumes of solvent-free feed before being placed in service, it will have 40% less capacity to breakthrough. Capacity is

Conclusions

A numerical simulation of adsorption cycles waz to show the advantages and disadvantages of rer

~ water on solvent adsorbers. Large accumulatic water result when drying is unsatisfactory. With cycle more steam adsorbs and condenses to hes water residue, leading finally to water overtlowir pore structure of the adsorbent and flooding th sorber. The temperature and moisture content 4 feed determines whether a cycle will be stable o flood.

High adsorber temperatures and low capac, breakthrough result when drying is too comple controlled residue of water limits the temperatur from the heat of adsorption of a solvent. Control residue may be obtained by manipulating the ope] conditions of the adsorber or by adding a seg drying step. Regenerating with superheated : reduces water residues in the region immed downstream of the steam injection.

Rapid changes occur as an adsorber returns to s after steam regeneration. These include a dr temperature, a high outlet flow rate, and an em of solvent. The emission is caused by residual SI on the carbon, rather than leaking of solvent fro feed.

Applications of this work extend to the desig optimization of solvent recovery systems. Adso systems are often designed based upon small scale The accumulation of water shown here will not a in a bench scale study run for only a few cycles.

Multibed adsorption systems generally feed t fluent from the primary adsorber into another ad. to reduce solvent emissions. For such systen principal source of solvent losses is the spike asso with the cooling of the adsorber after regener Reducing solvent residues and optimizing wate dues a t the outlet of an adsorber will lead to solvent emissions.

-

Literature Cited carpenter, B. M. A Model for Steam Regeneration of Activated

Carbon Fixed Beds. Master's thesis, University of Virginia, 1983.

~ ~ ~ ~ g , S. J.; Yang, R. T. Adsorption of Mixtures of Water Vapor and Hydrocarbons by Activated Carbon Beds: Thermodynamic Model far Equilibrium Adsorption and Adsorber Dpamica. MChE Symp. Ser. No. 259 1988,83,87.

seener, T. C.; Zhou, D. Prediction of Activated C a r b n Adsorption % performance Under High Relative Humidity Conditions. Enui- Ton. Prog. 1990, 9 (4). 40-46.

Levan, M. D.; Schweiger, T. A. J. Steam Regeneration of Adsorp- tion Beds: Theory and Experiments. In Fundamentals',of Adsorption; Mersmann, A. B., Scholl, S. E., Eds.; United Engineering Trustees: New York, 1991; pp 487-496.

Mastroianni. M. L.; Rochelle, S. G. Improvements i n Acetone Adsorption Efficiency. Enuiron. Prog. 1985,4 (l), 7-13.

Naujokas, A. A. Spontaneous Combustion of Carbon Beds, Plant/ Oper. Pmg. 1986,4 (2), 120-126.

Okasaki, M.; Tamon, H.; Toei, R. Prediction of Binary Adsorption Equilibria of Solvent and Water Vapor on Activated Carbon. J. Chem. Eng. Jpn. 1978, I1 (3), 209-215.

psrmele, C. S.; O'Connell, W. L.; Basdekis, H. S. Vapor-phase Adsorption Cuts Pollution, Recovers Solvents. Chem. Eng. 1979, Dec. 31, 58-70.

Petmva, N. I.; Nikolaev, K M. Adsorption of Certain Chlorhydro- carbons by Commercial Active Carbons in Presence of Water Vapor, I. Zh. Prikl. Win. (Leningmdl 1982a, 55, 1517-1521.

,-'. 9 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 287

Ruthven, M. D. Principles ofAdsorptwn and AdPorptwn Processes; JohnWiley & Sons: New York, 1984.

Seamehom, J. F. Removal of Vmyl Chloride fiom Gaseous Streams by Adsorption on Activated Carbon. Ind. Eng. Chem. P m e s s Des. Dew 1979,18,210-217.

Schweiger, T. A. J.; LeVan, M. D. Steam Regeneration of Solvent Adsorbern. Ind. Eng. Chem. Res. 1993,32,2418-2429.

Ustinov, E. A. Simulating Adiabatic Adsorbent Drying in Recu- peration. Zh. Prikl. Khim. (Leningrad) 19S6,59, 801-807.

Receiued for reuiew January 24, 1994 Reuised manuscript received May 2,1994

Accepted September 12, 1994e

IE9400412

e Abstract published in Aduame ACS Abstmcb, November 15,1994. ,

,

. . ,

any manufacturing operations use organic solvents that evaporate in the course of their M transportation, storage

and use. Adsorption systems using activated carbon have long been used to recover these airbome solvents and other organic vapors. Adsorption systems are an economical way to reclaim solvents for reuse. They are also an effective way to comply with pollution-control regulations that govem volatile organic compounds (VOCs) in industrial exhaust streams, and vapors collected during soil and site remediati0n.l

Activated carbon can be made from any carbonaceous material, including lig- nite, bituminous coal, ceconut shell and peat. More than 150 different grades of powdered, granular and extruded carbon are available.

For most air-pollution-control applications, carbon is activated by subjecting the starting material to controlled char- ring in a kiln, extruding it into pellets and activating it with steam at roughly 1,Mx)"C. During activation, the carbon pellets develop an intricate pore structure, which may have a total intemal surface area of up to 1,500 m2/g. It is this intemal pore structure that gives activated carbon its purifying capacity.

When contaminated gas or air passes through an activated carbon bed, the im- punties (or adsorbates) diffuse into the carbon pores where they are adsorbed onto the surface by attractive forces similar to van der Waals forces. The attraction to the carbon surface is so strong that the organic vapors eventually condense in the pores. This allows impurities in a vapor-phase exhaust stream to be removed and temporarily stored (ad-

sorbed) in the carbon pellets. During operation, solvent adsorption

continues until the bed becomes saturated. Once the bed has reached capacity, it can no longer adsorb vapors in the inlet stream, so unadsorbed vapors are allowed to pass through the bed and exit in the exhaust stream, perhaps exceeding allowable emission levels.

At th is point, the carbon must be either replaced or regenerated. This decision depends on the duration of adsorption, the resulting carbon consumption, and whether or not the collected solvents are valuable enough to wmant recovery for reuse (as opposed to collection for dis- posal or destruction).

To recover solvents adsorbed on the carbon, energy must he applied to over- come the solvents' attraction to the pore surface. This is most commonly done by applying heat, by means of steam, hot inert gas or direct heating. Desorption can also be carried out by changing the equilibrium conditions, by applying a vacuum or purging the adsorbates from the carhon pores using a gas stream. The solvent vapors coming off the carbon will be concentrated, and can be condensed and recovered as a liquid.

Kinetics of solvent recovery A variety of systems are in use today, including fixed and fluidized beds, and ro- tary adsorption wheels? Due to its flexibility, simplicity and ease of use, the most commonly used solvent-recovery system is a fixed-bed adsorber with direct stream generation.

For continuous operation, carbon-adsorption systems are typically operated on a cyclic basis using at least two beds. With such a two-bed configuration, a monitor indicates when the first bed has

INOW Don't miss the second pan of this fealure repon. Adsorption-capacity data for 283 organic compounds.

For a related anicle, see Recover VoCs using aciivaied carbon, E n ~ m m l o I E n @ ~ n g a supplement to Chem. Eng., February 1993, pp. 6-12, and The proven pracess of carban adsorption, Clzem. Eng., July 1994,

pp. 16-20.

pp. 94-95.

reached saturated conditions, by detecting the organic compound in the adsorher effluent. Once breakthrough is de- tected, the first bed is switched off for regeneration, and the vapor-laden stream is diverted to the fresh bed. This ensures that one bed is alwoys operating while the other is being regenerated (Figure 4). In other systems, timers are used rather than monitors to regulate the switch between cycles.

The amount of solvent vapors that the carbon can adsorb depends on the type of activated carbon, the nature and concentration of the adsorbate and the process conditions. The equilibrium loading capacity (A) is a function of the process conditions, primarily adsorbate concentration (C) and temperature (T):

A=KC,T) The solvent-recovery and carbon-

desorption process is best illustrated using a set of adsorption isotherms (Fig- ure 1). An adsorption isotherm shows the relationship between the critical parameters and the equilibrium adsorption capacity.

The initial loading capacity (ILC) of the carbon is determined by the process conditions - temperature and concentration -of the solvent-laden air (Step 1 in Figure 1). When the first bed has reached its adsorption capacity, unadsorbed emissions will be allowed to es- cape. A monitor detecting unadsorbed pollutants in the exhaust stream triggers the vapor-ladin inlet stream to be redirected to a second bed, and the first bed is switched to desorption mode.

To facilitate desorption, steam is introduced to the top of the first bed (Step 2 of Figure 1). During the first few minutes, all of the steam will condense on the vessel walls and the carbon. As the bed temperature approaches the steam saturation temperature, desorption begins (Step 3).

During desorption, a portion of the steam condenses on the bed, equalizing

12 ENVIRONMENTAL ENGINEERING WORLD I MAY-JUNE 1995

the heat lost to radiation and providing the energy required to desorb the collected organics. However, most of the steam flows through the bed, flushing adsorbates out of the carbon pores. As the steam drives the solvent out of the carbon pores, the loading capacity of the carbon drops (Step 4).

Once desorption is complete, there is a small, residual amount of organic invari- ably left on the carbon. This residual loading capacity (RLC; Step 5 ) depends strongly on the amount of steam used, and the duration of the desorption cycle.

The difference between the initial loading concentration (ILC) and the residual loading capacity (RLC) is commonly referred to as the effective loading capacity, and is shown as the shaded region in Figure 2. Since not all the solvent is removed from the bed, the ILC will be comprised ofthe solvent adsorbed on the next cycle (effective loading) and what was left on the bed after the last regeneration (RLC).

Adsorption is an exothermic phenom- ena, so adsorption capacity falls as the temperature of the bed or the inlet air stream rises. If the bed is not allowed to cool sufficiently following the regeneration cycle, emission "bleed" levels in the exhaust stream will be initially high (Step 6). As the bed cools, adsorption capacity rises, reducing emissions. Once desorption is complete and the bed has cooled, the freshly regenerated bed is put on standby until the other bed reaches breakthrough and needs to be regenerated.

The intricate porestructure of activated carbon gives it enormous capacityto adsorb organic vapors from process exhaust streams

Analogous to the relationship between temperam, inlet concenhation and the re- sultant adsorptive capacity discussed above, a converse relationship relates the temperam 0 and the residual loading capacity (A) to the concentration of emissions leaving the bed (C).

C = f (AT) Simply stated, the more completely the

xd has been regenerated, and the lower the emperam is during the subsequent adsorption cycle, the lower the "bleed" emissions from the bed will be.

Countercurrent flow is most often used juring desorption, to optimize the regener- ition of the carbon near the bed outlet. The U C at the bed outlet will typically be

much lower than the average RLC (Figure 2). It is the RLC at the outlet side of the bed - not the average RLC -that will determine the continuous adsorption capacity during the following adsorption cycle.

Steam use typically comprises the largest portion of the operating costs. Therefore, optimization ofthe steam- to-solvent ratio is critical to the efficient operation of a carbon-based system.

As mentioned, the regeneration cycle can be broken down into three phases: * All the steam condenses on the bed and vessel walls, heating the bed. Solvent is not yet driven out of the bed * The steam purges the solvent out of the carbon pores, and the solvent

is recovered *Additional steaming purifies the bed, but the amount of recovered solvent dimin- ishes rapidly

Figure 3 shows how the effective loading capacity of a carbon bed changes as the steam-to-solvent ratio varies during steam regeneration. By optimizing the steam-to-solvent ratio, the operating costs of the installation can be kept to a mini".

It is possible that even under optimized regeneration cycle times, the residual, "bleed" emissions from the bed may stiU be too high. Rather than simply increasing the length of the steam cycle, altemative carbon types should be investigated. Car- bon types with different activities may result in lower steam-to-solvent ratios. Check with an activated carbon supplier to ensure that your application is using the most suitable carbon type.

Figure 4 shows a typical, dual-bed ad-

ENVIRONMENTAL ENGINEERING WORLD I MAY-JUNE 1995 13 !

recovery unit

sorption unit, designed for continuous op eration. The vapor-laden inlet stream is di. rectal up through Bed 1, where by the intricate network of pores, it collects entrained organic vapors. A pre-condition- ing step may be necessary if the temperature of the inlet stream is higher than the design threshold for the carbon bed, or il the relative humidity exceeds 60%. Aftel passing through the carbon bed, the purified air can be vented to the atmosphere 01 fed back to the process.

Meanwhile, Bed 2, which was previously loaded with adsorbates, is regenerated with steam. The steam-adsorbate mixture driven out of the bed during regeneration flows through a condenser and a condensate cooler to the separation unit.

In the case of adsorbates that are not miscible with water, separation can be carried out with a simple gravimetric decanter. For water-miscible compounds, distillation may be needed to separate the phases.

In either case, the adsorbate phase is recovered for reuse and the water phase is stripped to remove the last traces of any dissolved solvents. The stripper offgas is mixed with the vapor-laden inlet stream, and returned to the carbon-adsorption unit.

As discussed earlier, if the regenerated bed is brought online directly after regeneration, the elevated temperature of the carbon may create an emissions peak. If this is not tolerable because of environmental or safety concems, a procedure known as emission cut hack (ECB) can be implemented.

The ECB procedure entails cooling the bed with a small amount of air, typically a slip stream comprising 20 to 30% of the airflow through the system. This speeds bed cooling, and collects bleed vapors. Cooling air exiting the hot bed will be enriched in adsorbate and must be redirected to the preconditioner or the adsorber. Once the bed has cooled sufficiently (usually IO to 20 minutes), it can be brought back online.

With each steam- regeneration cycle, residual water may collect in the bed. It is critical that this water be removed; if it were to in-

- Chiller,

mist eliminator, and heater for

preconditioning crease with every cycle, more and more steam would be needed to heat the bed enough to purge collected water and organics, increasing costs.

To facilitate drying of the carbon, a heat sink can be installed beneath the carbon. Such an apparatus, typically a bed of high-silica-ceramic balls, stores heat during the steam cycle. This heat raises the temperature of the cooling air slightly, to increasing its drying capacity. System design should ensure that all water collected during steam regenera- .tion is removed during the following cooling and adsorption cycle.,

General design parameters The dimensions of the carbon bed are influenced by the operating requirements, such as total air flow, maximum accept- able pressure drop, concentration of adsorbates and the desired removal efficiency. The supetiicial design velocities through the bed are typically up to 60 Wmin of gas for a 3-mm carbon pellet, and up to 80 Wmin for a 4-mm carbon pellet. Higher velocities are possible, but may create higher pressure drops.

Given the total airflow and design velocity, the surface area of the bed can be determined. For small airflows, vertical cylindrical beds are widely used. For higher flowrates, horizontal cylindrical beds may offer a more economical design.

The bed depth strongly depends on the amount of solvent to be adsorbed and the required removal efficiency. In essence, greater carbon volumes provide greater solvent adsorption capacity, and better re-

14 ENVIRONMENTAL ENGINEERING WORLD I MAY-JUNE 1995

Recovered solvent

Vapor-laden I , air inlet Wastewater

moval eftlciencies. Deeper beds also offer longer cycle times and lower steam-to-solvent ratios. Typical bed depths range from 2 to 3.5 ft.

Last but not least, not all activated carbons are the same. What may be good for one set of process conditions or mix of organics may not necessarily be best for another. Use of the wrong carbon type may compromise collection efficiencies, resulting in higher breakthrough emissions and greater steam consumption.

While activated carbon typically loses some of its adsorptive capacity under normal use over time, improper design and operation of solvent-recovery systems are more often to blame for poor recovery efficiencies and high operating costs. Careful examination of the process conditions and selection of the ideal cycle times are critical to optimizing the performance of a carbon-based adsorption system.

Author Stephen Boppart is a senior applications specialist for Notit Americas, Inc., a multi-

ing a wide range of activated carbonsfor air andwater treatment, food, chemical and pharmaceutical processes and catalyst support (1050 Crown Painte Parkway. Suite 1500, Atlanta. GA 30338 [el: 404

national corporation supply-

6043853). Boppart holds a B.S. in'chemisuy from the Universily of British Columbia (Van- couver. B.C.) and has eight years of experience in solvent-recovely in Europe and the U S He has received two patenw for emission-abatement technology, and is a member of the Air & Waste Management Asn. and AIChE.

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