Oxidative Coupling of Methane: Reactor Performance and Operating Conditions

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1 20 h European Symposum on Compuer Aded Process Engneerng ESCAPE20 S. Perucc and G. Buzz Ferrars (Edors) 2010 Elsever B.V. All rghs reserved. Oxdave Couplng of Mehane: Reacor Performance and Operang Condons Sanslav Jašo, a Hamd Reza Godn, a Harvey Arellano-Garca, a Güner Wozny a a Beln Insue of Technology, Srasse des 17. Jun 135, Berln, sanslavjaso@malbox.u-berln.de Absrac In hs work, he achevable performance n case of desred-produc selecvy, yeld and converson are evaluaed sysemacally for dfferen reacors n order o fnd he opmum range of operaon for he OCM process. Ths approach s appled o a nonsohermal plug flow reacor and a nonsohermal porous packed bed membrane reacor usng dfferen ypes of caalyss n he wde range of operang condons. Moreover, a fludzed bed reacor s also consdered. The resuls show ha rackng he opmum area of operaon has a monoonc drecon under some range of operang condons, whereas reflecs a qualave rade-offs under some oher ranges of operang condons. For all nvesgaed reacor conceps he lkelhood of opmal operang condons are found, and he bes correspondng performance for all of hem are repored. Keywords: Oxdave couplng of mehane, reacor desgn, membrane reacor, fludzed bed reacor 1. Inroducon The producon of C 2 hydrocarbons (.e. ehylene + ehane) hrough he oxdave couplng of mehane (OCM) has been prohbed from commercal pracce due o low overall yeld. In parcular, oday's caalyss exhb eher hgh selecvy (>70%) coupled wh low converson (<5%) or hgh converson (>75%) wh low selecvy (<15%) [1]. Ths mples ha an opmum rao of converson - selecvy exs, n order o acheve he hghes possble yeld of C 2 hydrocarbons. Many dfferen reacor conceps were proposed for he oxdave couplng of mehane, for nsance : couner curren smulaed movng bed reacor [2], sold oxde fuel cell reacor [3], caalyc dense membrane reacor [3,4], fludzed bed reacor [5,6], porous membrane recor [7,8], fxed bed reacor [5]. All of hose reacor conceps are havng her advanages and drawbacks. However, only he las hree reacor ypes have he poenal o be exploed ndusrally. Therefore only hose hree reacor ypes wll be nvesgaed n hs sudy. Fxed bed reacor represens a sae of he ar n he ndusry, and has o be examned n deal. Several sudes on hs reacor ype were done, and he bgges drawback are severe ho spos, formed as a consequence of he poor hea removal from he hghly exohermal reacon. Feed dluon s necessary f any applcaon s expeced. Fludzed bed reacor has been nvesgaed only brefly [5,6,9], and showed smlar performance lke he fxed bed reacor. The bgges advanage of he fludzed bed reacor s he sohermal operaon and a possbly o operae usng undlued feeds [6,9]. Ths opon s very aracve for ndusry (even n he case of lower selecvy) because cosly separaon of nrogen and mehane s prevened, and he downsreamng process s smplfed.

2 S.Jašo e al. Membrane reacor concep allows fne dsrbuon of one reacan no he reacor, usng a porous membrane as a gas dsrbuor. Fne oxygen dsrbuon has been proven o gve enchased selecvy even wh sgnfcan mehane converson. However, feed dluon and reacor sze are man drawbacks of hs reacor ype. Feed dluon s necessary because of he safey reasons as well as for low hea ransfer raes. Reacon raes are approxmaely one order of magnude smaller compared o fxed and fludzed bed reacor, herefore much bgger reacor s needed o acheve same converson level. In hs work hree reacor conceps are compared usng a same graphcal approach for obanng nformaon abou her opmal performance. Moreover same knec model [10] for caalys La 2 O 3 /CaO s used n order o gve comprehensve comparson of hese reacor conceps. 2. Modelng 2.1. Fxed bed reacor Fxed bed reacor was smulaed usng a sandard pseudo homogeneous plug flow reacor model : df = rc, A (1) dl Where F (mol/s) s he molar flow-rae for componen and A (m 2 ) s he cross seconal area of he ubular reacor. Boh sohermal and nonsohermal smulaons were performed wh a ypcal 2:1 mehane o oxygen feed rao and a reacor of 7 mm dameer, wh a flow rae of 10 cm n 3 /s. Addonally, nonsohermal smulaon was done addng a dfferenal reacor hea balance no he sysem of equaons Membrane reacor Membrane reacor model s based on pseudo-homogeneous one dmensonal flow. No radal and axal dffuson was aken no accoun. Ths was possble only due o he small reacor dameer of 7 mm used n he smulaon. The reacor model equaons are: df dl s df dl = r c, = N π d A N π d + r g, A s ( Tube ( Shell sde) sde) Where F (mol/s) s he molar flow-rae for componen n he ube and he shell sde of he membrane reacor, and A (m 2 ) s he cross seconal area of he ubular reacor. N represens he mass ranspor of he componens hrough he membrane. Reacon occurs n boh ube and shell sde of he reacor. More deals of hs model can be found n [11]. In case of membrane reacor, hea balance equaons smlar o Eq. (2) were added n order o smulae nonsohermal reacor behavor smlarly lke for he plug flow reacor Fludzed bed reacor Fludzed bed reacor was smulaed usng a modfed wo phase model suggesed by Werher e al.[12] : [ u u (1 ε )] dc mf b + k g, a ( C C e, ) = ε b r ( C, T, ε bb ) dh (3) dc e, [ u (1 ε )] = k a ( C C ) + (1 ε ) r ( C, T, ε ) mf b dh g, b e, e (2)

3 Oxdave Couplng of Mehane :Reacor Performance and Operang Condons Here C (mol/m 3 ) represens he concenraon of componen n bubbles and emulson, ε represens porosy, k g, s he mass ransfer beween bubbles and emulson and r s reacon rae. Dameer of he smulaed fludzed bed reacor s 40 mm, for he Geldar A group of parcles of 110 µm dameer. The range of operaon of he fludzed bed reacor les beween fludzaon numbers of 3-15, herefore n he bubblng regme. More deals abou he reacor model appled n hs sudy can be found n [12] 3. Resuls and dscusson Fxed bed reacor has been smulaed for he sohermal and nonsohermal case n order o nvesgae he achevable performance of hs reacor ype. Only wo varables are nfluencng he performance of hs reacor ype: nle composon whch can be represened as nle rao of mehane o oxygen, and emperaure of he feed. For he sohermal case emperaure n he reacor was kep consan and equal o he nle emperaure. For he nonsohermal smulaon hs emperaure was only an nle boundary condon, and he hea balance deermned emperaure a oher pons n he reacor. The smulaon resuls for he sohermal case are represened n he fgure 1. Fg. 1 Performance of he sohermal fxed bed reacor As can be seen from he Fg.1, here s a range of opmal operang condons for hs reacor ype. These operang condons are emperaure of 810 C and nle mehane o oxygen rao of 1.2. The maxmum yeld of hgher hydrocarbons obaned by smulaon accouns for 20% as repored by expermens as well [5]. Nonsohermal smulaon shows he necessy o use dlued feeds n order o effcenly remove hea from he reacor. Wall hea ransfer coeffcen was calculaed by correlaon of L and Fnlayson (1977), (260 W/mK) wh envronmen emperaure of fxed o 600 K. When envronmen emperaure was kep same lke he nle emperaure (lke for he oven), emperaure runaway was nevable for any appled condon. I can be concluded ha sgnfcan dluon of he caalys bed and dluon wh nrogen s requred n order o avod ho-spo formaon n he fxed bed reacor. On he oher hand where dluon of he feed or he caalys surpasses ceran crcal value, ho spos do no appear and emperaure n he reacor drops monoonously. Here we can emphasze ha conrollng he emperaure n he fxed bed s almos an mpossble ask. Reason for hs s very fas and exohermal reacon, whch s self-accelerang when hea balance s dsurbed. Consequenly ho spos of more han 300K are observed.

4 S.Jašo e al. Fg. 2 Influence of caalys dluon (lef) and dluon by nrogen (rgh) on he ho spo formaon n he fxed bed reacor Smlar nvesgaons were conduced for he membrane reacor. Several varables such as flow of he feed n he ube sde and n he shell sde were manpulaed, as well as oxygen concenraon n he shell, emperaure and he membrane hckness. We concluded ha he hckes membrane nvesgaed (0.1 mm) provded he bes overall reacor performance. Moreover, equal flow rae of he ube and he shell sream allowed hgher yeld n comparson o oher nvesgaed flow raes. Influence of he shell oxygen fracon and he nle emperaure on he yeld can be seen on he fgure 3. Fg. 3 Influence of he emperaure and he oxygen rao on he performance of he porous membrane reacor Porous membrane reacor gave remarkably hgh yelds of over 50%. I should be noed ha here are no expermenal nvesgaons for use of undlued feeds and such hgh membrane hckness o esfy wheher smulaed resuls are reasonable or no. Hghes yeld acheved so far n a porous membrane reacor s 27.5 % obaned by Lu e al [8]. Nonsohermal smulaon for he porous membrane reacor showed some dsadvanages for membrane reacor applcaon. In mos cases emperaure runaway was observed, smlarly lke for he fxed bed reacor. However, because of he slower reacon rae n he membrane ube, was possble o manan ho spo formaon below desrable level

5 Oxdave Couplng of Mehane :Reacor Performance and Operang Condons by manpulang he feed dluon and membrane hckness (fg. 4). Durng hs nvesgaon, ube and shell flow rae were kep consan 4 cm n 3 /s wh a sochomercal rao of mehane/oxygen and nle emperaure of 750 C. I can be noced ha membrane reacor has o be operaed wh sgnfcan feed dluon and wh a hck membrane n order o provde sohermal operaon. Ths leads o a sgnfcan ncrease n reacor sze requred for necessary converson level. Fg. 4 Nonsohermal smulaon of he membrane reacor : nfluence of he feed dluon (lef, δ = 0.1 mm) and membrane hckness (rgh, 75% dluon) on he ho spo formaon Fnally, fludzed bed reacor was nvesgaed under dfferen operang condons. Manpulaon of he reacon emperaure, feed composon and he fludzaon velocy showed nfluence of hese varables on he reacor performance. Increase n fludzaon velocy showed monoonous ncrease n yeld and selecvy, mos probably because reacon fron was moved away from he dsrbuor zone o he bulk, and allowed bubble mass ransfer o play an mporan role. For furher smulaon only he hghes flow rae of 15 u mf was appled. Resuls are shown on he fgure 5. Fg. 5 Influence of he reacon emperaure and feed composon on he performance of he fludzed bed reacor

6 S.Jašo e al. The hghes yeld n he fludzed bed of 26% s n reasonable agreemen wh expermens [6,9]. Nonsohrermal smulaon for he fludzed bed was no appled, because mos of he auhors of expermenal sudes on he OCM n he fludzed bed reacor have repored sohermal behavor, even when hey used undlued feed [6,9]. We can herefore adop he fac ha he fludzed bed reacor provdes he possbly o operae under sohermal operaon under all smulaed condons. 4. Conclusons A model-based performance analyss of fxed bed, fludzed bed, and porous membrane reacor were conduced. Smulaon resuls undoubedly show ha fxed bed reacor can no be used ndusrally, even when hgh hea ransfer raes are appled. Ths reacor ype has no conrol over he reacon hea n case of small dluon, and behaves almos deally adabacally. C 2+ yelds of 20% are no sasfacory for an ndusral applcaon. In conras o hs, he membrane reacor offers he possbly o ncrease he yeld by fne oxygen dsrbuon hrough he membrane. However, applcaon of undlued feeds and hn membranes resul n ho spos of over 100 K. Even hough a yeld of over 50% yeld s achevable for such condons, hese are only heorecal resuls. For an sohermal operaon hck membranes and over 80% feed dluon are necessary. The fludzed bed reacor shows an mproved performance n comparson o he fxed bed reacor. Alhough yelds of 26% are sll below ndusral requremens, he use of undlued feeds and an sohermal operaon are he man advanages of hs reacor ype. A furher economcal analyss of he membrane reacor and/or he fludzed bed reacor ncludng he downsreamng process s however necessary n order o answer defnvely he queson whch s he mos suable OCM reacor concep. For hs purpose, a mn-plan s currenly beng bul a he Berln Insue of Technology. 5. Acknowledgemens The auhors acknowledge suppor from he Cluser of Excellence "Unfyng Conceps n Caalyss" coordnaed by he Technsche Unversä Berln and funded by he German Research Foundaon - Deusche Forschungsgemenschaf. References [1] G.Huchngs, n E.E. Wolf (Ed.), Mehane Converson by Oxdave Processes : Fundamenal and Engneerng Aspecs, Van Nosrand Renhold, New York, 1992, pp [2] A.L. Tonkovch, R.W. Carr, R. Ars, R, Scence 262 (1993) 221. [3] W. Kakpong, T. Tagawa, S. Goo,S. Assabumrungra, K. Slpasup, P. Praserhdam, Chem. Eng. J., 115 (2005) 63 [4] S. Bhaa, C.Y. Then, A. R. Mohamed, Chem. Eng. J. 148 (2009) 525 [5] L. Mlezko, M. Baerns, Fuel Processng Technology, 42 (1995) 217. [6] L. Mlezko, U. Pannek, Ind. Eng. Chem. Res. 35 (1996), [7] Y.K. Kao, L. Le, Y.S. Ln, Caal. Today, 82 (2003) 255 [8] Y. Lu, A.G. Dxon, W.R. Moser, Y.H. Ma, Chem. Eng, Sc.,55 (2000) [9] K. T. Do, J. H. Edwards, R. J. Tyler, Can. J. Chem. Eng. 73 (1995) 327 [10] Z. Sansch, L. Mleczko, M. Baerns, Ind. Eng. Chem. Res. 36 (1997) [11] H. R. Godn, H. Arellano-Garca, M. Omdkhah, and G. Wozny, proceedngs of The 19h European Symposum on Compuer Aded Process Engneerng (ESCAPE19), Cracow, Poland, 1, pp. (2009) 123 [12] Werher, J, Ind. Eng. Chem. Res. 43 (2004), 5593