Kinetic Modeling of Methanol to Olefins (MTO) Process on SAPO-34 Catalyst

Transcription

1 Simultion nd Optimiztion Chin Petroleum Processing nd Petrochemicl Technology 203, Vol., No. 3, pp September 30, 203 Kinetic Modeling of Methnol to Olefins (MTO) Process on SAPO-34 Ctlyst Pu Jinglong; Weng Huixin (Est Chin University of Science nd Technology, Shnghi ) Abstrct: A kinetic model of MTO process over the SAPO-34 ctlyst considering the effect of ter nd coke deposition hs been proposed. The model tkes into ccount three steps of the MTO rection in hich the products cover lumped components. The ter in the feed not only reduces the concentrtion of methnol but lso llevites the dectivtion of SAPO-34 ctlyst. The kinetic prmeters hve been estimted by the lest squre method. It hs been proved tht the clculted vlues in the kinetic model re in good greement ith the experimentl vlues. Key ords: methnol to olefin process (MTO); kinetic modeling; SAPO-34; dectivtion Introduction The rpid development of the ntionl economy leds to rising demnd for ethylene nd propylene, hich re the bsic chemicl r mterils. The methnol to olefins (MTO) process provides ne route for the production of ethylene nd propylene. Compred ith the trditionl ethylene nd propylene production process (stem crcking of NGL or nphth), MTO hs some dvntges, becuse MTO cn provide ide nd flexible rnge of ethylene nd propylene rtios to meet the mrket needs []. The industril implementtion of MTO process t high methnol conversion hs been possible thnks to the high ctivity nd selectivity of the SAPO-34 ctlyst [2]. The kinetic model for MTO process hs been studied by mny reserchers becuse of the importnce of MTO process in the production of loer olefins. Considering the effect of coke on the MTO process, Bos, et l. proposed kinetic model consisting of 2 rections involving 6 component lumps plus coke [3]. The kinetic model grees ell ith the crbon pool mechnism proposed by Dhl nd Kolboe [4]. Gyubo, et l. simplified the model of Bos by eliminting the sloer steps nd studied the effect of ter in the rection in order tht the model could be esily used in the design of the rector for optimized production of loer olefins []. In this pper, the kinetic model for the MTO process is studied on SAPO-34 ctlyst in fixed bed rector, hich hs tken into considertion the effects of ctlyst dectivtion by ter nd coke deposition. The ne kinetic model, hich covers 3 steps for the formtion of olefins in the MTO process, hs lumped products. The rection rte equtions hve been introduced ccording to the proposed rection netork, nd the prmeters of the model re estimted by the lest squre method. 2 MTO Kinetic Modeling 2. MTO rection netork The rection of MTO process is composed of three steps, viz.: () dehydrtion of methnol to dimethyl ether (DME); (2) formtion of intermedite species, [CH 2 ]; nd (3) formtion of olefins nd other prffins [4]. The overll rection pth my be represented by: CH OH CH OCH [ CH ] olefins Since the source of the olefins is uncertin, the methnol nd DME re regrded s one lumped component clled oxygentes [6]. The products ethylene, propylene nd butene re lumped s C2, C3, nd C4, respectively. Since the mount of methne, CO, nd CO 2 in the products is smll, they re lumped s C. The ne proposed kinetic Recieved dte: ; Accepted dte: Corresponding Author: Prof. Weng Huixin, Telephone: ; E-mil: hxeng@ecust.edu.cn. 86

2 Pu Jinglong, et l. Kinetic Modeling of Methnol to Olefins (MTO) Process on SAPO-34 Ctlyst model for MTO process grees ell ith the Hydrocrbon Pool mechnism [4] hich is shon in igure. igure Rection netork for MTO process over SAPO-34 The kinetic scheme includes 6 first-order nd one secondorder rections. Rection 6 is the second order one, in hich the formtion rte of ethylene from propylene depends on the concentrtion of propylene nd methnol in the medium [3]. Upon considering both the ccurcy nd simplicity of the model, the secondry rection of butene is ignored becuse of the smll mount of butene compred ith propylene in the products. According to the kinetic scheme shon in igure, the rection rte equtions cn be obtined esily, hich re formulted s function of the mss frction (expressed on ter-free bsis, i ) of the corresponding rectnts. A d W = ( ki ) i= () d W = k (2) 2 d W = ( k2 A + k6 A 3) θ (3) 3 d W = ( k3 A k6 A 3 k7 3) θ (4) 4 d W = k4 () d W = ( k A + k7 3) θ (6) In the model, θ is function tht quntifies the effect of ter in the rection medium; is the ctivity of SAPO- 34 ctlyst; k i is the rection rte constnt of ech rection, hich follos the reltionship of Arrhenius eqution: k i = k e 0i Ei RT 2.2 Dectivtion of SAPO-34 ctlyst Coke deposition is extrordinrily rpid in the MTO process, becuse under more severe conditions coke deposition rte equting to 3 4% of the methnol mount in the feed is mesured in the first minute of time on strem []. The ctivity of the SAPO-34 ctlyst declines ith increse of the time on strem. Moreover, the ctivity of SAPO-34 ctlyst chnges ith the rection temperture, spce velocity, nd the degree of methnol dilution by ter in the feed. Consequently, the coke induced dectivtion of ctlyst cnnot be overlooked especilly ith respect to the fresh SAPO-34 ctlyst. The ctivity of SAPO-34 ctlyst is defined by the rtio beteen the rection rte of ctlyst t given time on strem, r, nd the rection rte tht the fresh ctlyst ould hve t tht time under the sme rection conditions, r f : r = r f (8) The dectivtion rte eqution cn be expressed by the folloing reltionship, hich indictes tht the dectivtion of SAPO-34 ctlyst complies ith the prllel dectivtion mechnism: d = dt d kd A θ d (9) In this eqution, k d is the dectivtion rte constnt hich follos the reltionship of Arrhenius eqution, nd θ d is function tht quntifies the ttenuting effect of ter in the rection medium on coke deposition of SAPO-34 ctlyst. 2.3 Effect of ter in the kinetic model The presence of ter in feed hs gret effect on the rection rte in the MTO process. Compred ith the methnol in the ter-free feed, the presence of ter decreses the rte of rection becuse of the dilution of the methnol concentrtion in feed. The effect of ter resistnce cn be quntified by the function θ: θ = + K (0) In the eqution, K is prmeter tht quntifies the ter resistnce for the rection rte, nd is the ter content in the rection medium hich is the eight frction (on (7) 87

3 Chin Petroleum Processing nd Petrochemicl Technology 203,(3):86-90 ter-free bsis) of ter in the rection medium. On the other hnd, the presence of ter in feed lso hs significnt effect of ttenuting coke induced dectivtion of ctlyst. This result cn be ttributed to the competition of ter ginst coke deposition on the cid sites of SAPO-34 ctlyst. Wter cn be dsorbed on the cid sites of SAPO-34 ctlyst prior to the coke deposition becuse ter is polr molecule [7]. The inhibition effect of ter on coke deposition llevites the dectivtion of the SAPO-34 ctlyst hich cn be quntified by the function θ [8] d : θ d n Kd = + () here K d is prmeter tht qulifies the ter resistnce ginst the coke deposition on SAPO-34 ctlyst. The ter content in the rection medium,, is sum of the ter in feed, 0, nd the ter formed, f. = 0 + f (2) The ter formed in the rection medium hs reltionship ith the oxygente content, A, hich cn be expressed by the folloing eqution [9] : 2 3 = (3) f A A A 3 Estimtion of Kinetic Prmeters 3. Experimentl dt on SAPO-34 The experimentl dt on the prmeter estimtion ere obtined from Qi [0], hose experiments ere conducted in fixed bed rector using the fresh SAPO-34 ctlyst. The temperture of the rection s 37, 400, 42, 40, nd 47, respectively, nd the spce velocity over the ctlyst vried from 3.84 h - to 3.9 h - (grms of methnol fed into the rector in n hour versus grms of ctlyst), hile the totl rection pressure s kept t tmospheric pressure. The rtio of ter nd methnol in feed s 0 nd in order to investigte the effect of ter on the rection. The time on stem of the process s equl to. minutes. 3.2 Estimtion procedure The procedure for estimting the kinetic model involves non-liner prmeter estimtion problems. The ordinry differentil equtions ere solved by the fourth-order method of Lung-Kutt, in hich the initil conditions ere: (W/) 0 =0, A0 =, nd i0 =0 (i=, 2, 3, 4, ). After the concentrtion of ech component t the outlet s clculted, the prmeters in the kinetic model ere estimted by the lest squre method in order to minimize the objective function, EO. nl nexp 2 i j i= j = ( i clc j ),, EO = (4) nlnex p here i,j is the experimentl vlue of eight frctions for i lump in the experimentl point j, nd i(clc),j is the clculted vlue, heres n l is the number of lumps, nd n exp is the number of experimentl points. 4 Results nd Discussion 4. Kinetic prmeters The prmeters of the kinetic model re estimted nd re presented in Tble, in hich the corresponding vlue of the objective function, EO, is Tble Prmeters of the kinetic model of MTO over SAPO-34 ctlys Kinetic constnts, h - k i0, h - E i, J/mol Prmeters of dectivtion nd functions of ttenution by ter k d=.0 k K =.0 k k k k k k d θ = + K The verge reltive errors nd the mximum reltive errors of clculted vlues re listed in Tble 2. Since the desired products in the MTO process minly cover ethylene nd propylene, the emphsis of the study is to predict the ethylene nd propylene yields. As shon in Tble 2, the mximum reltive error of ethylene yield is.83% nd its verge reltive error is 4.0%. The mximum reltive error of propylene yield is.8%, ith the verge reltive error reching 3.4%. The verge reltive errors of ll the lumped components re less thn 0%. Consequently, the clculted vlues of the kinetic model θ d = +. 88

4 Pu Jinglong, et l. Kinetic Modeling of Methnol to Olefins (MTO) Process on SAPO-34 Ctlyst re relible. Tble 2 Reltive errors of ech lumped component Lumped components C C2 C3 C4 C Averge reltive error, % Mximum reltive error, % Sttisticl test of the model The sttisticl tests of the kinetic model, including the decisive index, the ρ 2 tests nd the tests, re conducted. The significnt level, α in the test is %, hich evlutes the devition for the men regression. The results of the sttisticl tests confirm tht ρ 2 >0.9 nd >0 t, s shon in Tble 3. Consequently, the kinetic model nd the estimted prmeters re relible. Tble 3 Sttisticl test of the kinetic model ρ 2 t 0 t The clculted vlues of ech lumped component re compred ith the experimentl vlues s shon in igure 2. It cn be seen from igure 2 tht the dt points re lmost symmetriclly distributed on both sides of the digonl line. It is proved tht the clculted vlues re in good greement ith the experimentl vlues. igure 2 Comprison beteen clculted vlues nd experimentl vlues 89

5 Chin Petroleum Processing nd Petrochemicl Technology 203,(3):86-90 Conclusions In this study, ne kinetic model for MTO process in fixed bed rector s built by considering the effect of ter nd coke deposition. The effect of ter in the feed on the rection is relted ith dilution of methnol nd ttenution of the SAPO-34 ctlyst dectivtion. Since the increse of the coke deposition on fresh SAPO-34 ctlyst is rpid t the beginning of the rection, the dectivtion of the SAPO-34 ctlyst must be considered. The prmeters of the kinetic model re estimted by the lest squre method nd the clculted vlues re compred ith the experimentl vlues. The result hs verified tht the clculted vlues re in good greement ith the experimentl vlues, ith the verge reltive errors of ll lumped components being less thn 0%. So the kinetic model hs good cpbility to predict the performnce of the lumped components in fixed bed rector. Nomenclture ctivity of the SAPO-34 ctlyst d order of dectivtion k i rte constnt of the rection, h - k d rte constnt of dectivtion, h - A mss frction of methnol (ter-free bsis) i mss frction of component i (ter-free bsis) W eight of SAPO-34 ctlyst, g mss flo rte of methnol in the feed, g/h K prmeters tht quntifies the ter resistnce mss frction of ter in the rection medium (ter-free bsis) k 0i pre-exponentil fctor, h - E i ctivtion energy, J R molr gs constnt, J mol - K - t time on strem, min T thermodynmic temperture, K f mss frction of ter formed in the rection medium (ter-free bsis) 0 mss frction of ter in the feed (ter-free bsis) EO the objective function θ function of ttenution by ter responsible for the min rection θ d function of ttenution by ter responsible for the dectivtion References [] Njfbdi A T, temi S, Sohrbi M, et l. Kinetic modeling nd optimiztion of the operting condition of MTO process on SAPO-34 ctlyst[j]. Journl of Industril nd Engineering Chemistry, 202, 8 (): [2] roment G, Dehertog W J H, Mrchi A J. Zeolite ctlysis in the conversion of methnol into olefins[j]. Ctlysis, 992, 9 (): -64 [3] Bos A N R, Tromp P J J, Akse H N. Conversion of methnol to loer olefins kinetic modeling, rector simultion nd selection[j]. Ind Eng Chem Res, 99, 34(): [4] Dhl I M, Kolboe S. On the rection mechnism for propene formtion in the MTO rection over SAPO-34[J]. Ctl Lett, 993, 20: [] Gyubo A G, Aguyo A T, Sánchez del Cmpo A E, et l. Kinetic modeling of methnol trnsformtion into olefins on SAPO-34 ctlyst[j]. Ind Eng Chem Res, 2000, 39(2): [6] Chen D, Grønvold A, Moljord K, et l. Methnol conversion to light olefins over SAPO-34: rection netork nd dectivtion kinetics[j]. Ind Eng Chem Res, 2007, 46(2): [7] Wng Geng, Tng Yu, ue Zhenxin. Recent progress in methnol to olefins technology[j]. Lioning Chemicl Industry, 20, 40 (7): (in Chinese) [8] Gyubo A G, Aguyo A T, Alonso A, et l. Rection scheme nd kinetic modelling for the MTO process over SAPO- 8 ctlyst[j]. Ctlysis Tody, 200, 06: 2-7 [9] Sánchez del Cmpo A E. Kinetic modelling of the trnsformtion of methnol into light olefins on SAPO-34 ctlyst[d]. University of the Bsque Country, Bilbo, Spin, 997 [0] Qi Guozhen, ie Ziku, Chen Qingling. Kinetics of methnol to olefins over fresh SAPO-34 ctlyst[j]. Chemicl Rection Engineering nd Technology, 202, 28 (6): 06-2 (in Chinese) 90