Modeling of CO 2. Adsorption on Activated Carbon and 13X Zeolite via Vacuum Swing Adsorption. IOP Conference Series: Materials Science and Engineering

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1 IOP Conference Seres: Materals Scence and Engneerng PAPER OPEN ACCESS Modelng of CO Adsorpton on and 3X Zeolte va Vacuum Swng Adsorpton To cte ths artcle: M H Zarghampoor et al 7 IOP Conf. Ser.: Mater. Sc. Eng. 6 4 Vew the artcle onlne for updates and enhancements. Related content - Study of CO adsorpton capacty of mesoporous carbon and actvated carbon modfed by trethylenetetramne (TETA) I Sulstant, Y K Krsnand and I Moenandar - The usage of actvated carbon from teak sawdust (tectona grands l.f.) and zeolte for the adsorpton of Cr(VI) and ts analyss usng sold-phase spectrophotometry (sps) S Saputro, M Masykur, L Mahardan et al. - Synthess of Amno-Impregnated ZIF-8 for CO Adsorpton Nur Hafza bnt Yahya, Yn Fong Yeong and L Sze La Ths content was downloaded from IP address on 4//8 at :6

2 Modelng of CO Adsorpton on and 3X Zeolte va Vacuum Swng Adsorpton M H Zarghampoor, M Mozaffaran,*, M Soleman and M Takht Ravanch Department of Chemcal Engneerng, Amrkabr Unversty of Technology, No. 44, Hafez Ave., P.O.Box: , Tehran, Iran. Catalyst Research Group, Petrochemcal Research and Technology Company, Natonal Petrochemcal Company, Tehran, Iran Emal: mozaffaran@aut.ac.r Abstract. Due to carbon doxde role n global warmng, low CO emsson lmts have been establshed n recent years. Ths has led to a varety of studes on CO removal approaches. In ths study, a VSA cycle consstng of two packed beds s consdered for CO removal from flue gas. An atmospherc stream contanng % CO and 8% N s fed to the beds at 5 C. Two adsorbents, namely and actvated carbon were selected to compare ther performance. Due to the monolayer adsorpton of CO and N on these adsorbents, the Toth sotherm was used for equlbrum adsorpton estmaton. A quas-second order model was consdered for the mass transfer rate predcton due to low CO concentraton. The modelng results showed that the average absolute devaton for equlbrum adsorpton capacty predcton was %, and the CO breakthrough tme curve was predcted wth less than.5% devaton. Based on the results, the VSA cycle tme for zeolte 3X bed wll be 3.5 tmes of the actvated carbon bed. Another advantage of s that n each process cycle, 8% of adsorbent wll be used, whle only 74% of actvated carbon n beds s used. The advantage of actvated carbon bed s ts better regeneraton capablty, snce the actvated carbon wll be regenerated 5% more than zeolte 3X at a vacuum pressure of.bar.. Introducton The ncrease n greenhouse gas emsson has created many envronmental problems. Carbon doxde s one of the greenhouse gases wth the greatest mpact on global warmng []. In recent years, the Internatonal Energy Agency has presented a roadmap to reduce CO emsson, develop sensble separaton technologes and lmt the ncrease n Earth s temperature to C from 3 to 5. However; t s expected that the Earth s temperature wll rse by 4 C durng ths perod[]due to ndustral developments, the ncrease n fuel consumpton and low CO cost. Numerous studes have been conducted n recent years on trends and developments of CO separaton processes [3, 4] leadng to suggested processes such as absorpton, adsorpton, membrane and refrgeraton. The selecton of each process depends on varous factors such as operatng pressure, feed gas temperature, product purty and feed gas mpurtes. The comparson of these processes has ndcated that amne absorpton s the most wdely used process n ndustres. However; the applcaton of ths process s not recommended due to corroson problems and hgh solvent regeneraton cost. Smlar problems n the refrgeraton Content from ths work may be used under the terms of the Creatve Commons Attrbuton 3. lcence. Any further dstrbuton of ths work must mantan attrbuton to the author(s) and the ttle of the work, journal ctaton and DOI. Publshed under lcence by Ltd

3 process such as low lfe tme, plastzaton problems, foulng n membranes and hgh energy requrements have attracted the researchers to the adsorpton process n recent years. In order to study the performance of adsorbers n CO removal, expermental studes on dfferent adsorbers such as actvated carbon, zeolte 5A and zeolte 3X have been performed [5, 6]. Adsorbent capacty and selectvty are the most mportant factors n the process performance and effcency. It should be noted that the applcaton of very strong adsorbents wll make the regeneraton process more dffcult. Adsorbent regeneraton can be performed by heat n TSA [7], pressure n PSA [8], and electrcal energy n ESA [9] processes. Vacuum can be used as the necessary drvng force for regeneraton when the gas feed pressure s near the atmospherc pressure. Ths process s known as vacuum swng adsorpton (VSA). The requred energy for regeneraton n VSA process s -.5 MJ/kg CO [9]. Ths lower energy requrement for the VSA process compared to TSA process wth 3-4 MJ/kg CO [7]and PSA process wth -3 MJ/kg CO [] has drawn the attenton of researchers to the VSA process n recent years. The am of ths study s to evaluate and compare the performance of zeolte 3X and actvated carbon for CO removal from a CO /N mxture n an adsorpton bed wth VSA cycle. For ths purpose, the expermental data of equlbrum sotherms and adsorpton bed parameters of Dantas and her colleagues were used[, ]. Fnally, n order to study the process lmtatons, a comprehensve revew of the effectve parameters such as feed condtons, vacuum pressure, purge to gas rato and cycle tme, and ther effect on the recovery and performance of CO productvty are studed.. Mathematcal Modelng Each VSA process cycle conssts of four steps ncludng bed pressurzaton, adsorpton, blow-down and purgng. To mprove the process performance, blow-down and purgng steps are carred out n the opposte drecton of adsorpton step[3]. Fgure shows an overvew of the pressure profle n a VSA cycle. As can be seen, the flow n the adsorpton step s downward to elmnate the possblty of adsorbent fludzaton. Also, the pressure profle s assumed to be lnear n the pressurzaton and blowdown steps. The adsorpton process mathematcal modellng predcts the unsteady state behavour of the adsorpton bed. In order to acheve ths goal, a set of partal dfferental equatons ncludng the mass, momentum and energy balance, along wth one ordnary dfferental equaton are consdered and solved smultaneously to predct the mass transfer rate [4, 5]. Fgure. Pressure profle durng a VSA cycle In order to provde a mathematcal model, the followng assumptons are consdered:

4 . The adsorpton bed operates under an sothermal condton.. The radal concentraton gradents are neglgble. 3. The pressure drop n the bed s calculated based on the Ergun equaton. 4. The feed and purge gas condtons reman constant durng the process. 5. The flow pattern s assumed to behave as an axally dspersed plug flow model. 6. The mass transfer rate s represented by a quas-second order model. 7. The gas behavour s consdered to be deal. The flow model for the adsorpton process s based on packed beds. The mass balance equaton ncludes axal dsperson, convecton, accumulaton n gas phase, and rate of adsorpton. C C b q Dz uc p z z t b t () -3 - wherec (mol.m ) s the adsorbate concentraton of component n the flud phase, u (m.s ) s the flud velocty, z (m) represents the dstance along the bed, t (s) s tme, s the bed vod fracton, b s the partcle densty and - -3 p (kg.m ) q (mol.kg ) s the average concentraton n adsorbent partcle. - The axal dsperson coeffcent Dz (m.s ) s defned by an emprcal correlaton as a functon of Reynolds and Schmdt number. bd D m z.5sc Re () - where Dm (m.s ) s molecular dffusvty of component and Sc and Re are the Schmdt and Reynolds numbers. The mass transfer rate s defned based on dfferent adsorpton knetcs. When the feed concentraton s low, t s recommended to use a quas-second order model[6]. Snce CO concentraton n flue gas s 8 to 5 percent, t s recommended that a quas-second order model be used to predct the mass transfer rate. The general form of ths model s defned n Equaton 3. q * K s, q q (3) t where K s the quas-second order mass transfer coeffcent and * s, (kg.mol.s ) q (mol.kg ) s the adsorbed phase concentraton n equlbrum wth the flud phase concentraton. When the adsorpton of all materals except component s neglgble, the equlbrum adsorpton can be estmated wth ther pure sotherms at ther partal pressure. Due to mono-layer adsorpton of CO and N on zeolte 3X and actvated carbon, the Toth sotherm s recommended for equlbrum adsorpton predcton. Ths equaton s defned as follows: q * max q K eq P n Keq P n (4) max - where q (mol.kg ) s the maxmum amount adsorbed at equlbrum, K s an - eq (bar ) equlbrum constant, P (bar) s the total pressure and n s the Toth parameter. By passng the flud through the empty spaces between adsorbent partcles, a pressure drop may take place n the flud. The Ergun equaton was used to predct ths pressure drop: P 5.75 u z d d g b b g u 3 3 b p b p (5) 3

5 where s the gas vscosty, d (m) - g (Pa.s ) p s the partcle dameter and -3 g (kg.m ) s the gas densty. In order to evaluate the VSA cycle performance n CO separaton, t s necessary to calculate ts emsson to the atmosphere and the recovery. These two parameters are defned n Equatons 6 and 7. t adsorpton CO Emmson (-purge/feed rato).flow dt (6) CO CO obtaned n adsorpton -CO used n purgng CO Recovery= (7) CO fed durng pressurzaton & adsorpton In order to predct the VSA cycle behavour, the above equatons must be solved smultaneously. The set of partal dfferental equatons are dscretzed n the length drecton and solved by MATLAB wth the ODE5s method. To evaluate the VSA performance n CO reducton, the expermental data by Dantas et.al was used [, ]. Therefore, an atmospherc mxture of % CO and 8% N s ntroduced to the cycle at 5 C wth a flow rate of 3 L.mn -. The adsorbent propertes and bed parameters are reported n Tables and. Table. and actvated carbon propertes Surface area [m /g] Surface area [m /g] 53 Mcropore volume [cm 3 /g].7 Mcropore volume [cm 3 /g].97 Partcle densty [g/cm 3 ].3 Partcle densty [g/cm 3 ].4 Partcle porosty.37 Partcle porosty.46 Partcle dameter [mm].9 Partcle dameter [mm] 3.8 Table.Adsorpton bed parameters Bed Length [cm] 83 Bed Dameter [cm] Bed vod fracton.5 Column wall thckness [mm].5 Column wall densty [kg.m -3 ] 78 Bed weght [kg] Results and Dscusson The adsorpton equlbrum determnaton s one of the major problems n predctng the VSA process behavour. Fgure shows the results of CO and N equlbrum adsorpton predcton wth the Toth model. The results ndcate that the maxmum devaton error for equlbrum adsorpton capacty predcton s.5% for CO and % for N. As can be seen, the selecton of s much better due to ts hgher capacty at low pressures. 4

6 q [mol.kg - ].5 CO on N on CO on N on p [bar] Fgure. CO and N adsorpton equlbrum on and actvated carbon at 5 C; the sold lnes represent the results of the Toth model. Fgure 3 shows breakthrough curve whch s plotted to determne the proper tme to carry out the adsorpton cycle. Addtonally, fgure 3 ndcates the results for modelng the VSA process n packed actvated carbon and zeolte 3X beds. Accordng to ths fgure; N s adsorbed n the bed at frst, and then the CO molecules wll replace N due to the compettve adsorpton nature of CO and N. Ths phenomenon causes an overshootng peak on the N breakthrough curve. The breakthrough tme for zeolte 3X bed s 3.5 tmes greater than the one for actvated carbon bed. An ncrease n breakthrough tme reduces the necessary tmes for bed regeneraton and mproves the recovery. The results show that the average devaton error for breakthrough tme predcton curve s.5%...8 C/C.6.4. CO Breakthrough curve N Breakthrough curve Tme [mn] Fgure 3. Breakthrough curve of CO /N mxture on zeolte 3X(N :,CO : ) and actvated carbon(n :,CO : ); lnes are obtaned by a quas-second order model. As an addtonal advantage, more parts of zeolte 3X adsorbent are consumed n each cycle of the process. Fgure 4 demonstrates the avalable area n a VSA cycle for two adsorbents. In the adsorpton step, the process should be stopped at break pont tme due to CO emsson lmtaton. At ths tme, the mass transfer zone reaches the bottom of the bed. The shorter the length of mass transfer zone, the greater the amount of consumed adsorbent. The mass transfer zone n the actvated carbon bed s.5 tmes of zeolte 3X bed. The results show that at the end of the adsorpton step, 93% of zeolte 3X 5

7 adsorbent s saturated, but only 85% of actvated carbon s saturated at break pont tme. If the process s contnued to saturate more actvated carbons, the CO emsson to atmosphere wll ncrease sgnfcantly. The advantage of actvated carbon s the smplcty of ts regeneraton. At the end of blowdown to. bar and purgng 5% of N, 89% of adsorbent wll be regenerated. However, at the end of ths step only 87% of zeolte 3X s regenerated. The reason s related to the strong nteracton between CO molecules and zeolte 3X adsorbent. Ths strong nteracton causes a better and selectve separaton wth more energy cost requrement for regeneraton. As can be seen n fgure 4, n each cycle of the process, 8% of zeolte 3X adsorbent wll be consumed, as opposed to only 74% of adsorbent n the actvated carbon beds. Dmensonless CO molar concentraton End of Adsorpton Step Avalable Area In VSA Process End End of of Purgng Step Dmensonless Bed Length Fgure 4. Adsorbent consumpton n each cycle of VSA process for zeolte 3X and actvated carbon; Sold lne: actvated carbon, Dash lnes: zeolte 3X. Snce adsorpton s an exothermc process, an ncrease n temperature wll reduce the adsorbent capacty. Therefore, t s recommended to keep the temperature as low as possble n all adsorpton processes to have the maxmum avalable adsorbent capacty. Fgure 5 shows the effect of temperature on the adsorpton process. As can be seen, a rse of temperature from 5 C to C wll reduce the breakthrough tme by 4% n a zeolte 3X bed and by 65% n an actvated carbon bed. Breakthrough Tme [mn] Temperature [ C] Fgure 5. Temperature effects on breakthrough tme of zeolte 3X and actvated carbon beds. 6

8 As can be seen n Fgure 6, a 33% ncrease n feed flow rate wll decrease the breakthrough tme by 5% n a zeolte 3X bed. However, ths feed flow rate ncrease wll reduce the break pont tme by 3% n an actvated carbon bed. Breakthrough Tme [mn] Feed Flow Rate [lpm] Fgure 6. Feed flowrate effect on breakthrough tme of zeolte 3X and actvated carbon beds. Fgure 7 shows the effect of CO feed concentraton on the breakthrough tme. An ncrease n CO concentraton wll reduce the break pont tme and ncrease the number of regeneraton tmes whch wll cause adsorbents eroson and ncrease the pressure drop. A 5% ncrease n CO concentraton wll reduce the break pont tme by 55% n a zeolte 3X bed, whereas ths wll only decrease the break pont tme by 6% n an actvated carbon bed. Therefore, the feed concentraton effect on actvated carbon s less than the effect on a zeolte 3X bed Breakthrough Tme [mn] Mole fracton Fgure 7. CO feed concentraton effect on breakthrough tme of zeolte 3X and actvated carbon beds. One of the most mportant ponts n the desgn of a VSA process s ts cycle tme. Fgure 8 shows the effect of cycle tme on CO emsson and ts recovery. In ths process, 4% of cycle tme s consdered for the adsorpton step. A reducton n cycle tme ncreases the amount of CO emsson to atmosphere. The maxmum cycle tme s determned by the CO emsson lmt. If the process s desgned to mnmze CO emsson, the maxmum cycle tme wll become 8 mnutes for zeolte 3X beds. However, the maxmum cycle tme n actvated carbon beds wll be only mnutes. 7

9 Another advantage of zeolte 3X s ts hgher CO recovery. For a zeolte 3X bed wth 8 mnutes cycle tme the recovery s 98%, whereas the CO recovery for an actvated carbon bed wth a cycle tme of mnutes s 95% CO molar emsson (mol) %Recovery Cycle Tme,mn Fgure 8. Cycle tme effect on CO emsson to atmosphere and ts recovery; Sold lne: CO recovery, Dash lne: CO molar emsson. A vacuum pump creates the requred drvng force for adsorbent regeneraton n all VSA processes. Snce the vacuum pump consumes energy, t s necessary to determne the requred vacuum pressure. Fgure 9 shows the effect of vacuum pressure n a bed regeneraton process. As can be seen, any reducton of vacuum pressure ncreases the bed regeneraton. It should be noted that both the zeolte 3X and actvated carbon beds regenerated completely at mmhg vacuum pressure even though reachng ths degree of vacuum s not cost effectve. The regeneraton behavour of zeolte 3X showed that most of adsorbent regenerates at a very low pressure. The regeneraton of 9% of zeolte 3X adsorbent requred a vacuum pressure less than. bar. However, ths amount of regeneraton s possble wth vacuum pressure equal to.6 bar n actvated carbon beds. 98 % Bed Regenerated Depressurzaton Pressure [bar] Fgure 9. Vacuum pressure effect on CO emsson and bed regeneraton. The applcaton of purge gas at the end of bed regeneraton s another mportant factor n determnng the process performance. As s shown n fgure, ncreasng the purge to feed rato mproves the bed regeneraton wth lttle effect on CO recovery. It should be noted that an ncrease n purge to feed rato 8

10 wll reduce the qualty of CO product at the end of the process. In the case where the N product s mportant, an ncrease n purge to feed rato wll decrease N productvty and ts recovery % Bed Regenerated Purge to Feed Rato Fgure. Purge to feed rato effect on bed regeneraton. 4. Conclusons In order to evaluate the VSA performance n CO removal, a process consstng of two adsorpton beds was modelled. The equlbrum adsorptons were predcted by Toth sotherm. The comparson of the expermental results and the model showed that the maxmum devaton errors for equlbrum adsorpton capacty predcton are.5% for CO and % for N. Then the beds were modelled wth a quas-second order model and the results showed that the average devaton error for breakthrough tme predcton curves was.5%. The results of the analyss for effectve parameters can be summarzed as follows: The breakthrough tme for the zeolte 3X bed s 3.5 tmes greater than the tme for actvated carbon bed. Ths wll reduce the necessary tmes for bed regeneraton and mprove the recovery. In each cycle of the process, 93% of zeolte 3X s saturated at the end of the adsorpton step. Addtonally, at the end of blow-down to. bar and purgng 5% of product N, 87% of them wll be regenerated. In fact 8% of adsorbent are used n each cycle. However, 85% of adsorbent are saturated at the end of adsorpton step n an actvated carbon bed and 89% of them wll be regenerated at the end of purgng. Ths phenomenon showed that only 74% of adsorbers are used n each cycle. A temperature rse of 5 C wll reduce the break pont tme by 6% n the actvated carbon bed. However, ths wll decrease the tme by 4% n a zeolte 3X bed. Doublng the feed flow rate wll decrease the break pont tme n the zeolte 3X bed by 5%, whereas ths wll reduce the tme by 56% n an actvated carbon bed. A 5% ncrease n CO feed concentraton reduces the breakthrough tme by 55% n a zeolte 3X bed. However, ths concentraton change wll decrease the tme by only 6% n an actvated carbon bed. In the case of feedng the system by several sources wth dfferent concentraton, f the process steps are changed based on a tme schedule rather than the usual CO breakthrough, then t s better to use an actvated carbon bed rather than one wth zeolte 3X. Due to the strong nteracton between CO and zeolte 3X molecules and more adsorpton capacty at a low pressure, most of adsorbent wll be regenerated at low pressure. The regeneraton of 9% of adsorbent requres a vacuum pressure of.bar n a zeolte 3X bed. However, ths amount of regeneraton requres a vacuum pressure of.6bar n an actvated carbon bed. 9

11 A % ncrease n purge gas wll ncrease the bed regeneraton by %. It s recommended that purge gas be used as lttle as possble. An ncrease n purge gas wll reduce the CO product purty and N recovery. References [] Songolzadeh M, et al. 4 Carbon doxde separaton from flue gases: a technologcal revew emphaszng reducton n greenhouse gas emssons, The Sc. World J.. [] IEA, 3, Technology Roadmap: Carbon capture and storage. [3] Mondal M.K., et al. Progress and trends n CO capture/separaton technologes: A revew, Energy, Energy 46() 43. [4] Yang H., et al., 8, Progress n carbon doxde separaton and capture: a revew, J. Env. Sc (Chna) () 4. [5] Dantas T L P, et al. 8, Adsorção de CO e N Sobre Carvão Atvado e Zeólta 3X: Isotermas de Equlíbro Através de Meddas Gravmétrcas, 7 Encontro Braslero sobre Adsorção, 5. [6] Khazrae S et al. 3, Expermental Study for Equlbrum Adsorpton of Carbon Doxde and Ntrogen on 5A and 3X Molecular Seve, Internatonal Revew of Chemcal Engneerng (IRECHE) Int. Rev. of Che. Eng. (IRECHE) 5 4. [7] Ntamoah A et al. 6, CO Capture by Temperature Swng Adsorpton: Use of Hot CO-Rch Gas for Regeneraton, I&EC Research 55(3) 73. [8] Shen C, et al., Adsorpton equlbra and knetcs of CO and N on actvated carbon beds, Che. Eng. J. 6() 398. [9] Zhang J, et al. 8, Effect of process parameters on power requrements of vacuum swng adsorpton technology for CO capture from flue gas,energy Converson and Management, Eng. Con. and Mng. 49() 346. [] Koga T, et al., Study on CO Recovery Systems by Pressure Swng Adsorpton under Hgh Pressure Condton, J. of Novel Carbon Resources 3 6. [] Dantas T L P et al., Carbon doxde-ntrogen separaton through adsorpton on actvated carbon n a fxed bed, Che. Eng. J. 69. [] Dantas T L P et al., Carbon doxde ntrogen separaton through pressure swng adsorpton, Che. Eng. J. 7(-3) 698. [3] Farooq S et al. 989, Numercal smulaton of a pressure swng adsorpton oxygen unt, Che. Eng. Sc. 44() 89 [4] Faruque Hasan M M, et al., Modelng, Smulaton, and Optmzaton of Postcombuston CO Capture for Varable Feed Concentraton and Flow Rate,. Pressure Swng Adsorpton and Vacuum Swng Adsorpton Processes, I&EC Research 5(48) [5] Ruthven D M, et al. 994, Pressure Swng Adsorpton, VCH Pub. Inc., Chapter 3. [6] Azzan S 4, Knetc models of sorpton: a theoretcal analyss, J. of Collod and Interface Sc. 76() 47.