Adsorption of Chlorinated Volatile Organic Compounds in a Fixed Bed of Activated Carbon

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1 Korean J. Chem. Eng., 9(), 6-67 (2002) Adsorpton of Chlornated Volatle Organc Compounds n a Fxed Bed of Actvated Carbon Seung Ja Km, Sung Yong Cho and Tae Young Km Department of Envronmental Engneerng, Chunnam Natonal Unversty, Gwangju , Korea (Receved 9 June 200 accepted 28 August 200) Abstract Adsorpton sotherms of dchloromethane and,,2-trchloro-,2,2-trfluoroethane on an actvated carbon pellet, Nort B4, were studed. For these chemcals, the Sps equaton gave the best ft for the sngle component adsorpton sotherm. The adsorpton affnty on actvated carbon was greater for dchloromethane than that of,,2-trchloro-,2,2-trfluoroethane. An expermental and theoretcal study was made for the adsorpton of dchloromethane and,,2- trchloro-,2,2-trfluoroethane n a fxed bed. Expermental results were used to examne the effect of operaton varables, such as feed concentraton, flow rate and bed heght. Intrapartcle dffuson was able to be explaned by a surface dffuson mechansm. An adsorpton model based on the lnear drvng force approxmaton (LDFA) was found to be applcable to ft the expermental data. Key words: Adsorpton, Dchloromethane,,,2-trchloro-,2,2-trfluoroethane, Fxed Bed INTRODUCTION To whom correspondence should be addressed. E-mal: sjkm@chonnam.chonnam.ac.kr In recent years, the adsorpton process of removng and recoverng organc solvents n trace levels from ar has attracted specal nterest as a means of protectng the envronment from ar polluton. Volatle organc compounds (VOCs) are a famly of carboncontanng compounds whch are emtted or evaporated nto the atmosphere where they partcpate n photochemcal reactons [Jang et al., 998]. Some VOCs are toxc and carcnogenc. Most VOCs, to varyng degrees, contrbute to the formaton of ground level ozone. The VOCs whch may cause stratospherc ozone depleton are mostly a chlorne-contanng group of compounds known as chloroflourocarbons or CFCs. They release chlorne and other halogens nto the stratosphere gradually [Wolf, 992; Ccerone et al., 974; Cho and Cho, 996]. These compounds are partcularly effectve n destroyng the ozone even when present n small quanttes. VOCs are used as solvents n manufacturng of countless products. Some promnent examples of these products nclude crystallzed organc chemcals, prnted materals, pants and coatngs and dry-cleanng agents n magnetc tape and semconductor manufacturng facltes. One consequence of these manufacturng operatons s that large amounts of organcs are beng emtted to the atmosphere. VOCs have been mplcated as a major contrbuton to photochemcal smog, whch can cause haze, damage plant and anmal lfe, and lead to eye rrtaton and respratory problems [Ogura et al., 992]. Several technologes can be used for recoverng VOCs from gaseous wastes, but one of the most mportant and effectve methods for controllng the emsson of VOCs s the adsorpton process. Temperature swng adsorpton usng an actvated carbon bed s probably the most common adsorpton cyclc process for the separaton of strongly adsorbed speces. It has been wdely used n ndustres for removal and recovery of hydrocarbon and solvent vapors [Chhara et al., 2000]. In ths work, the removal of,,2-trchloro-,2,2-trfluoroethane (CFC-3) and dchloromethane (DCM) was studed theoretcally and expermentally. MATHEMATICAL MODEL A mathematcal model was developed that takes nto account the non-dealty of adsorbable speces n the adsorbed phase under equlbrum. Applyng the typcal assumptons [Hwang, 994] and takng the mass balance of the gas phase n a packed bed, the followng governng equaton for sothermal adsorpton s obtaned: c 2 c = D t L v c ε q ρ z 2 z ε p t In Eq. (), the gas-sold mass transfer rate can be expressed as the followng lnear drvng force (LDF) model for surface dffuson [Ten, 994; Lee and Moon, 200]: q 3k = k t s ( q q ) = f c r p ρ ( c ) p where 5D k s = s 2 r p As denoted by Eqs. (2) and (3), the LDF model s an approxmaton to the soluton of Fckan dffuson nsde a sphercal partcle. The expresson assumes that the mass transfer rate of adsorpton s proportonal to the dfference between the equlbrum concentraton and the bulk concentraton of the component. Snce the soluton of the LDF model s much smpler than the soluton of a dffuson model, we employed ths model n the present study. By ntroducng approprate dmensonless varables, Eq. () can be wrtten as follows: ζ = -- 2 ζ ζ α τ β S 2 S ( ζ ζ ) and the dmensonless varables are defned as follows: () (2) (3) (4) 6

2 62 S. J. Km et al.. Adsorpton Equlbrum The gravmetrc method s a well establshed technque for obtanng adsorpton equlbra of pure gases and vapors. 0. g of actvated carbon n the quartz basket s ventlated to remove mpurtes wthn the actvated carbon at 503 K for approxmately 4 hr by usng a vacuum pump. Adsorpton amounts are measured at dfferent temperatures of 303 K, 38 K, and 333 K. The adsorbent used for ths experment s Norbt B4 and the adsorbates are CFC-3 and DCM. The adsorpton data were obtaned by usng a hgh-pressure mcrobalance. Isotherms of CFC-3 and DCM on the actvated carbon at three dfferent temperatures are shown n Fgs. 2 and 3, respectvely. The sotherms of CFC-3 and DCM were fa- c ζ = ----, vt z vl 3k τ = ----, S = --, β = , α L L = f L ε r p v ε c 0 D L The assocated ntal condtons for 0<z<L s (5) ζ (z, 0)=0 (6) and the boundary condtons at z=0 and z=l for t>0 are: -- ζ β S z = 0 = ( ζ ζ ) z = 0 z = 0 + (7) ζ = 0 S z = L As prevous researchers [Moon and Ten, 987; Ten, 994; Wang and Ten, 982] ponted out n ther work on fxed-bed adsorpton, f the ntrapartcle dffuson s descrbed by a homogeneous dffuson model or a lumped parameter model (such as the LDF model), one needs to calculate the concentratons n both phases at the same tme. From Eq. (2), the followng dynamc condton could be obtaned: c + A q = B where r A = p ρ p k s and B (0) 3k = c + A q f EXPERIMENTAL The adsorpton system n ths study conssted of the actvated carbon (Nort B4, pellet type), and CFC-3 and DCM. The physcal propertes of the actvated carbon and bed characterstcs are gven n Tables and 2, respectvely. A schematc dagram of the fxed bed expermental setup s presented n Fg.. The apparatus was constructed wth stanless steel tubes. The adsorpton column lne s 0.42 cm ID and all other lnes are /4" ID. Pure ntrogen gas was sent to the solvent saturator. A constant temperature crculator Table. Physcal propertes of actvated carbon Actvated carbon Suppler Nort (B4) Pellet dameter (mm) 3.9 Pellet length (mm) 6.7 BET surface area (m 2 /g) 826 Mcropore fracton (< nm) 45.6% Porosty 0.42 from manufacture from ntrogen adsorpton at 77 K Table 2. Expermental condtons for a fxed bed adsorpton Varables Unt Range Bed length m Flow rate m/s Bed porosty Packng densty kg/m Bath temperature K (8) (9) Fg.. Flow dagram of adsorpton system. Flow meter Heatng band Pressure bottle 3 Way valve Water bath G.C. Column P.C. Thermo couple N 2 (Jeo Tech.) was attached to the pressure bottle n order to keep the temperature constant. The concentratons of solvents were determned from ther saturated vapor pressures by assumng vapor-lqud equlbrum state. In order to ncrease the mxng effcency of solvents, 6 mm glass beads were packed n the pressure bottle. The gas mxture of the saturated solvent vapor and the pure ntrogen gas was delvered to the adsorpton secton. The gas flow to the column was controlled by a flow meter that was precalbrated under the expermental pressure condtons. The gas mxture was heated to the expermental temperature n the preheatng col secton before beng charged to the fxed bed adsorber. The adsorber was made of a stanless steel tube n.03 cm ID. A K-type thermocouple was nstalled at nlet secton of the packed column, and the temperature of the system was montored contnuously on a dsplay recorder. The composton of the ext gas stream from the adsorpton secton was measured n the analyzer secton. For ths purpose, a gas chromatograph, GC-4B model (Shmadzu), equpped wth a hydrogen flame onzaton detector was used. RESULTS AND DISCUSSION January, 2002

3 Adsorpton of Chlornated VOCs n a Fxed Bed of Actvated Carbon 63 Table 3. Adsorpton equlbrum sotherm parameters of Cl 2 FC- CCl 2 F on actvated carbon Langmur Freundlch Sps Temp. [K] q m b k n q m b n Error (%) Error (%) Error (%) Fg. 2. Adsorpton equlbrum curves of Cl 2 FCCClF 2 on Nort B4 for dfferent temperatures. Table 4. Adsorpton equlbrum sotherm parameters of CH 2 Cl 2 on actvated carbon Langmur Freundlch Sps Temp. [K] q m b k n q m b n Error (%) Error (%) Error (%) ths pressure range, the affnty of the adsorbate toward the adsorbent s characterzed by a Henry s constant. The heats of adsorpton can be calculated from the Van t Hoff equaton, whch s defned as: ln p ---- p 2 cµ H = s R T T 2 () In the Henry law regon (C µ =K p P), Eq. () can be replaced by Eq. (2). Fg. 3. Adsorpton equlbrum curves of CH 2 Cl 2 on Nort B4 for dfferent temperatures. vorable types. Sngle-speces sotherm data were correlated by the well-known Langmur, Freundlch and Sps equatons. The parameters of each sotherm were obtaned by the least square fttng wth expermental data. These parameters and the average percent dfferences between the measured and calculated values are gven n Tables 3 and 4, respectvely. Among these sotherms, the Sps equaton wth three parameters s more approprate n predctng our data compared to the Langmur or Freundlch sotherms wth two parameters. Isotherms provde nformaton on the affnty of the adsorbent toward the adsorbate as well as the heat of adsorpton. The adsorpton amounts ncrease lnearly up to a pressure of 0.5 kpa. In Fg. 4. Temperature dependence of Henry s constant. Korean J. Chem. Eng.(Vol. 9, No. )

4 64 S. J. Km et al. The breakthrough curves for dfferent bed heghts are smlar n shape. Snce the breakthrough curve retans a constant pattern, t may be nterpreted that the adsorpton zone has a constant length. The effect of flow rate on adsorpton was studed and the results are shown n Fg. 8. Ths fgure shows that the breakthrough tme decreased wth ncreasng flow rate, and the breakthrough curves are slghtly steeper for hgher flow rates. These trends are consstent wth general behavor of a fxed bed adsorber. Snce the ntrapartcle dffusvty s usually ndependent of flow rate, ths behavor s due to the external flm mass transfer resstance. Ths ressln k p, = H s R k p, T T 2 (2) It can be seen from Fg. 4 that the Henry s constant for DCM s greater than that of CFC-3, and DCM s more strongly adsorbed on the actvated carbon than CFC-3. The heats of adsorpton are almost comparable, although that for DCM (5.9 kcal/mol) s slghtly greater than that for CFC-3 (5.0 kcal/mol). 2. Bed Adsorpton A column adsorber has been used as commercal equpment for separaton by adsorpton snce t gves a sharp breakthrough due to the dfference n affnty between gases and partcles [Park et al., 200; Hu et al., 200; Na et al., 200]. The breakthrough curves of all speces, n general, depend on adsorpton equlbrum, ntrapartcle mass transfer, and hydrodynamc condtons n the column. Therefore, t s reasonable to consder adsorpton equlbrum and mass transfer smultaneously n smulatng the adsorpton behavor n the fxed bed adsorber. The operatonal factors such as nput concentraton, flow rate and L/D rato are mportant n an adsorber desgnng and optmzaton. In ths work, breakthrough curves were obtaned under varous expermental condtons mentoned above. The expermental condtons are summarzed n Table 2. Fg. 5 shows the effect of flm mass transfer coeffcent, k f, on the breakthrough curve of CFC-3. As the value of the mass transfer coeffcent ncreases, the breakthrough curve becomes sharper and the breakthrough behavor s very senstve to k f value. Ths mples that the adsorpton equlbrum controls the breakthrough as the mass transfer resstance dwndles. k f for CFC-3 s approxmately 0 3 m/s. The breakthrough curves of CFC-3 and DCM at the same condtons are shown n Fg. 6. The breakthrough tme of CFC-3 s shorter than DCM because the affnty of CFC-3 s smaller than that for DCM. Ths fgure also shows that the predcted breakthrough curves by the LDFA model ncorporated wth the Sps equaton are ftted well wth the fxed bed data for sngle component systems. Fg. 7 shows the effect of bed heght on the CFC-3 adsorpton. Fg. 6. Comparson of breakthrough curves of Cl 2 FCCClF 2 and CH 2 Cl 2 adsorpton on Nort B4 (25 o C, L: 0.5 m, v: m/s). Fg. 7. Expermental and predcted breakthrough curves of Cl 2 - FCCClF 2 adsorpton for dfferent bed heghts (25 o C, v: m/s). Fg. 5. Effect of k f on sngle-spece breakthrough curves of Cl 2 - FCCClF 2. January, 2002

5 Adsorpton of Chlornated VOCs n a Fxed Bed of Actvated Carbon 65 R e are Schmdt and Reynolds numbers, respectvely. Molecular dffuson coeffcents, D m, for CFC-3 and DCM are obtaned by Fuller et al. [969]. Under the expermental condtons of ths study, the estmated values of axal dsperson coeffcent and external flm mass transfer coeffcent, and molecular dffuson n the fxed bed are lsted n Table 5. Fg. 9 shows the effect of the flow rate on the adsorpton of DCM n the fxed bed. As shown n ths fgure, the effluent concentraton ncreases wth the flow rate. Ths s due to the fact that at a hgher flow rate, the contact tme between the adsorbent and the gas phase n the bed s shorter. Fg. 0 llustrates the effect of nlet concentraton Fg. 8. Expermental and predcted breakthrough curves of Cl 2 - FCCClF 2 adsorpton for dfferent flow rates (25 o C, L: 0.2 m). tance decreased wth the flow rate, so that the length of the mass transfer zone s reduced, and a sharper breakthrough curve s generated. In modelng for a packed bed adsorber, the man parameters concernng the transport of adsorbates are the axal dsperson coeffcent and the external flm mass transfer coeffcent [Row and Cho, 990; Markovska et al., 200]. Usually, the axal dsperson coeffcent comes from the contrbuton of molecular dffuson and turbulent mxng arsng from the spttng and recombnaton of flows around the adsorbent partcle. In ths study, the axal dsperson coeffcent, D L, was calculated from the followng correlaton gven by Edwards and Rchardson [968]. Fg. 9. Expermental and predcted breakthrough curves of CH 2 Cl 2 adsorpton for dfferent flow rates (25 o C, L: 0.2 m). r = ε B Pe Re Sc βr Pe' + ε B Re Sc The constant values for ths calculaton are r =0.73, β=3.0, Pe' =2.0 (3) The flm mass transfer coeffcent, k f, n a fxed bed adsober s obtaned by Wakao and Funazkr [978]: 2k Sh = f Rp = Sc 3 D m Re 0.6 (4) where, D m s molecular dffusvty, S h s Sherwood number, S c and Table 5. Mass transfer parameters for a fxed bed model smulaton Parameters Symbols (unt) Cl 2 FCCClF 2 CH 2 Cl 2 Axal dsperson coeffcent D L (m 2 /s) Flm mass transfer coeffcent k f (m/s) Surface dffusvty D s (m 2 /s) Molecular dffusvty D m (m 2 /s) Fg. 0. Expermental and predcted breakthrough curves of CH 2 - Cl 2 adsorpton for dfferent nlet concentratons (L: 0.5 m, v: m/s). Korean J. Chem. Eng.(Vol. 9, No. )

6 66 S. J. Km et al. on expermental breakthrough curves for DCM. The breakthrough tme decreases wth the ncrease of nlet concentraton. The result can be explaned by the concept of the mass transfer zone (MTZ) velocty [Ruthven, 984]. The velocty of MTZ s a functon of ntersttal velocty, partcle densty, bed porosty, and the slope of the equlbrum sotherm. For a lnear sotherm adsorpton system, the velocty of MTZ s constant. Therefore the breakthrough tme s not affected by nlet concentratons at constant MTZ velocty. However, the adsorpton sotherm of CFC-3 on actvated carbon s favorable as shown n Fg. 2. As the nlet concentraton ncreases, the value of slope of the equlbrum sotherm decreases and the MTZ velocty ncreases. Therefore, the breakthrough tme becomes shorter under ths crcumstance. CONCLUSIONS Adsorpton capactes of solvent vapors such as,,2-trchloro-,2,2-trfluoroethane and dchloromethane on an actvated carbon pellet (Nort B4) were measured expermentally at varous temperatures. Sngle-speces equlbrum data show typcal type II sotherms. The Sps equaton gave the best ft for the sngle-component adsorpton of,,2-trchloro-,2,2-trfluoroethane and dchloromethane on actvated carbon. The adsorpton affnty on actvated carbon was greater for dchloromethane than that of,,2-trchloro-,2,2- trfluoroethane. Adsorpton behavor n the fxed-bed was studed n terms of flow rates, heghts of the bed, and concentratons of the adsorbates. The adsorpton processes of,,2-trchloro-,2,2-trfluoroethane and dchloromethane vapors on actvated carbon were found to be controlled by ntrapartcle mass transfer, and surface dffuson was a domnant ntrapartcle mass transfer mechansm. Adsorpton dynamcs of,,2-trchloro-,2,2-trfluoroethane and dchloromethane n the fxed-bed can be smulated by the LDFA (Lnear Drvng Force Approxmaton). ACKNOWLEDGEMENT Ths research was fnancally supported by the Korea Scence and Engneerng Foundaton (No ). A B NOMENCLATURE : dynamc condton of component : dynamc condton of component c : bulk phase concentraton of component [mol/m 3 ] c : equlbrum concentraton of component [mol/m 3 ] c µ : adsorbed concentraton on sold phase [mol/m 3 ] D L D s k f k p k s L p p e q q r p : axal dsperson coeffcent [m/s] : surface dffuson coeffcent [m 2 /s] : flm mass transfer coeffcent [m/s] : Henry s constant [mol/kg kpa] : LDF mass transfer coeffcent [/s] : bed length [m] : pressure [kpa] : Peclet number : adsorbed amount of component at equlbrum [mol/kg] : adsorbed amount of component at pure state [mol/kg] : partcle radus [m] S t T v z : dmensonless axal dstance : tme [hr] : temperature [K] : superfcal velocty [m/s] : axal dstance coordnate [m] Greek Letters α : dmensonless varable [-] β : dmensonless [-] ε : bed vod fracton [-] ζ : dmensonless concentraton [-] ρ p : partcle densty [kg/m 3 ] τ : dmensonless tme [-] REFERENCES Chhara, K., Mellot, C. F., Cheetham, A. K., Harms, S., Mangyo, H., Omote, M. and Kamyama, R., Molecular Smulaton for Adsorpton of Chlornated Hydrocarbon n Zoltes, Korean J. Chem. Eng., 7, 649 (200). Cho, S. Y. and Cho, D. K., Langmur Parameters for Adsorpton of Two Halogenated Chemcals on an Actvated Carbon Pellet, Korean J. Chem. Eng,. 3, 409 (996). Ccerone, R. J., Stolarsk, R. S. and Walters, S., Stratospherc Ozone Destructon by Man Made Chlorofluoromethanes, Scence, 85, 65 (974). Edwards, M. F. and Rchardson, J. F., Gas Dsperson n Parked Beds, Chem. Eng. Sc., 23, 09 (968). Hu, H. B., Yao, S. J., Zhu, Z. Q. and Hur, B. K., Comparson of the Adsorpton Characterstcs of Expanded Bed Adsorbent wth Conventonal Chromatographc Adsorbent, Korean J. Chem. Eng., 8, 357 (200). Hwang, K. S., Separaton of H 2 /CO 2, H 2 /CO Gas Mxtures by Pressure Swng Adsorpton and Fxed-Bed Dynamcs, Ph.D dssertaton, KAIST (994). Jang, B. H., Lee, S. S., Yeon, T. H. and Te, T. E., Effects of Base Metal Promoters n VOC Catalysts for Chlorocarbons and n-hexane Oxdaton, Korean J. Chem. Eng., 5, 56 (200). Lee, D. H. and Moon, H., Adsorpton Equlbrum of Heavy Metals on Natural Zeoltes, Korean J. Chem. Eng., 8, 625 (200). Markovska, L., Meshko, V. and Novesk, V., Adsorpton of Basc Dyes n a Fxed Bed Column, Korean J. Chem. Eng., 8, 90 (200). Moon, H. and Ten, C., Further Work on Multcomponent Adsorpton Equlbra Calculatons Based on the Ideal Adsorbed Soluton Theory, Ind. Eng. Chem. Res., 26, 2042 (987). Na, B. K., Koo, K. K., Eum, H. M., Lee, H. and Song, H. K., CO 2 Recovery from Flue Gas by PSA Process usng Actvated Carbon, Korean J. Chem. Eng., 8, 220 (200). Ogura, K., Kobayash, W., Mgta, C. T. and Kakum, K., Complete Photodecomposton of CFC-3, CHCl 3 and CCl 4 and Scavengng of Generated Reactve Speces, Envron. Technol., 3, 8 (992). Park, I. S., Kwak, C. and Hwang, Y. G., Frequency Response of Adsorpton of a Gas onto Bdsperse Pore-Stuctured Sold wth Modulaton of Inlet Molar Flow-Rate, Korean J. Chem. Eng., 8, 330 (200). Row, K. H., Cho, D. K. and Lee, Y. Y., Smulaton of the Combned Contnuous and Preparatve Separaton of Three Close-Bolng Com- January, 2002

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