Microreactors technology for hydrogen purification: Effect of the catalytic layer thickness on CuOx/CeO2-coated microchannel reactors for the PROX reaction

1 Mic o eac o s echnology o hyd ogen pu i ica ion: E ec o he ca aly ic laye hickness on CuOx/CeO2-coa ed mic ochannel eac o s o he PROX eac ion O.H. Laguna*, M. González Cas año, M.A. Cen eno, J.A. Od iozola Ins i u o de Ciencia de Ma e iales de Se illa, Cen o Mix o CSIC – Uni e sidad de Se illa, A enida Amé ico Vespucio 49, 41092 Se ille, Spain *Co esponding au ho e-mail: [email protected] Keywo ds Mic o eac o ; P ocess in ensi ica ion; CO-PROX; Ca aly ic laye hickness Abs ac Two blocks o mic o eac o s composed by 100 mic ochannels and coa ed, espec i ely, wi h 150 and 300 mg o a CuOx/CeO2 ca alys , we e p epa ed and es ed in he p e e en ial oxida ion o CO in p esence o H2 (PROX). The deposi ion o di e en amoun o ca alys esul ed in di e en ca aly ic laye hicknesses hus modi ying he ca aly ic pe o mances o he mic o eac o . The e alua ion o he main eac ion a iables ( he space eloci y, he O2- o-CO a io and he p esence o H2O and/o CO2 in he s eam) was pe o med o e bo h mic o eac o s and compa ed o ha o he pa en powde ca alys . The leas loaded mic o eac o , wi h a coa ing hickness a ound 10 µm, p esen ed he highes CO con e sion and selec i i y le els a empe a u es below 160 ºC. This esul e idences i) he imp o emen o he ca aly ic pe o mances go by he s uc u a ion o he powde ca alys and ii) he impo ance o he selec ion o he adequa e hickness o he ca aly ic laye on he mic o eac o , which ha e no o exceed and op imal alue. An adequa e coa ing hickness allows minimizing he mass and hea anspo limi a ions, hus esul ing in he enhancemen o he ca aly ic pe o mance du ing he PROX eac ion. 2 1. In oduc ion Cu en ly, he need o al e na i e ene gy sou ces mo i es he de elopmen o new echnologies based on uels di e en o hose based in pe oleum. In his sense, many e o s ha e been ocused in he use o H2 o anspo and po able applica ions [1, 2]. Howe e he p oduc ion o H2 is a mul i-s age p ocess including e o ming o hyd ogen-con aining uels, such as hyd oca bons and alcohols, ollowed by successi e cleaning up s eps o up- aking he CO, such us he high and low empe a u e wa e -gas-shi (HT and LT-WGS) and he p e e en ial oxida ion o CO (PROX). These p ocesses should educe he CO le els a ound 1% in he case o he WGS, and hen below 50 ppm wi h he PROX, in o de o ensu e a longe li e ime o he elec ochemical de ices whe e he H2 uelled is con e ed in elec ic powe . In his sense, highly ac i e ca alys s in he CO aba emen eac ions a e equi ed. Ac ually many sys ems wi h di e en suppo s and ac i e phases ha e been es ed in CO oxida ions eac ions [2-7]. The eal applica ion o he H2 echnology in au omo i e and po able de ices s ongly depends on he inco po a ion o he di e en eac ions in ol ed in o he engine o hese sys ems. Besides his, he size o he compac uel p ocesso s whe e he di e en eac ions occu mus be adequa e o he e iciency du ing he anspo [1]. In his ega d, he mic o eac o s a e an in e es ing op ion i hei di e se ad an ages a e conside ed [8, 9]. The mic o eac o s can be conside ed as small scale chemical eac o s ha achie e high con e sion a es wi hin a educed olume [10, 11]. These de ices allow he p ocess in ensi ica ion because hey a e able o enhance he hea ing exchange, being especially a ac i e o exo he mic eac ions whe e a s ic empe a u e con ol has o be achie ed. Mo eo e , he o al low o he eeding and he p essu e in he inle can be conside ably inc eased, compa ed wi h hose o he common packed-bed eac o s. In addi ion, a mo e di ec ansi ion om he labo a o y o he indus ial p oduc ion scale is easily achie able o mic o eac o s [8, 9]. Se e al wo ks ha e p esen ed in e es ing p og esses on he applica ion o mic o eac o s in di e en eac ions [12, 13]. Fo example, Jang e al. [14] 3 s udied a mic o-me hanol s eam e o me and es ablished ha he geome y o he mani old and he channel size, among o he a iables, had a c ucial in luence on he e iciency and consequen ly mus be op imized o e e y mic o eac o . This ag ees wi h he esul s p esen ed by Ma hieu-Po in e al. [10] ha analysed, by a nume ical simula ion s udy, he op imal geome y o ca aly ic mic o eac o s. They conclude ha he compe i ion be ween di usion, ad ec ion and kine ic phenomena dic a e he design o he mic o eac o s. In his sense, some adimensional numbe s mus be conside ed, such us he Schmid and he Bejan numbe ha a e ela ed o he beha iou o he luid in o he channels. Addi ionally, i was poin ed ou ha he simula ion o he mic o eac o beha iou equi es he op imiza ion o he mass o ca alys in oduced in o he mic ochannels and o he o al low used, in o de o achie e he adequa e aspec a io ha imp o es he maximum con e sion. Among he di e en a iables conside ed du ing he designing and manu ac u ing o mic o eac o s, he op imiza ion o he amoun o loaded ca alys is a capi al pa ame e o ob aining he maximum ca aly ic pe o mance and consequen ly, he con ol o he ca aly ic laye hickness esul s de e minan . This a iable may s ongly in luence he hea and mass anspo phenomena on he ca aly ic beha iou o he mic o eac o . In his sense, he modi ica ion o he p oduc s dis ibu ion by he a ia ion o he selec i i y and he dec easing o he hea anspo could be no iceable i he ca aly ic laye hickness is ou side he op imal alue. The e icien eleasing o hea du ing he eac ion is c ucial o exo he mic p ocesses such as PROX. Mic o eac o s may imp o e he eleasing o he hea p oduced in such exo he mal eac ions a oiding he o ma ion o ho spo s and allowing a s ic con ol o he eac ion empe a u e. Fo ins ance, ecen ly Llo ca e al. [15] ha e p esen ed he s udy o a silicon mic omonoli h con aining ca. 40000 egula channels o 3.3 µm in diame e pe squa e millime e con aining Au/TiO2 as he ca alys s coa ed in he walls o he channels, o he PROX eac ion. The au ho s demons a ed ha al hough hei silicon mic o eac o did no achie e high CO con e sions le els a low empe a u es (150 – 240 ºC), his sys em was conside ably mo e e icien han a con en ional 4 400 cpsi (cells pe squa e inch) co die i e monoli hs, which p esen ed a poo hea ans e du ing he p ocess. Ouyang e al. [16] ha e showed he lowe adial and axial empe a u e g adien s in mic o eac o s compa ed o hose o packed bed eac o s, which minimize he ex end o he e e se WGS eac ion, and a ou highe CO con e sions. Howe e , i he amoun o ca alys coa ed in he walls o he mic ochannels is e y high, his be e hea eleasing a e may be less ob ious, and he occu ence o some mass anspo limi a ions inside he ca aly ic laye canno be ully disca ded. This p oblem was s udied by Po emkin e al. [17] o e a mic o eac o loaded wi h a 5 w .% Cu/CeO2-X ca alys . The au ho s p oposed he calcula ion o an in e nal e ec i eness ac o (ηCO), de e mined as he a io o he eac ion a e calcula ed assuming he exis ence o mass ans e limi a ions o ha calcula ed in he assump ion o ze o mass ans e limi a ions. The analysis o he ηCO ac o o di e en hickness o ca aly ic laye s sugges ed ha he lowes con ibu ion o mass anspo limi a ions in mic o eac o s is p oduced when he hickness does no exceed ~20 µm. Ob iously, we mus ake in o accoun ha his alue is app oxima e and only alid o he speci ic ca alys used in ha wo k, since he ex u al p ope ies o he ca alys (po e size dis ibu ion, e c.) may con ol he kind and a e o di usion o he eagen s and p oduc s along he channels. F om all abo e, i is clea ha he de elopmen o mic o eac o s s ill equi es addi ional s udies whe e many a iables ha e o be op imized in o de o gene a e a comple e knowledge abou he designing, manu ac u ing and applying o mic o eac o s as ca aly ic sys ems. The p esen wo k p e ends o analyse di e en ca aly ic expe imen al esul s ob ained wi h wo blocks o mic o eac o s, composed by 100 mic ochannels, coa ed wi h di e en o al amoun s o a CuOx/CeO2 ca alys (150 and 300 mg) o he PROX eac ion. 2. Ma e ials and me hods 2.1. Syn hesis o he CuOx/CeO2 ca alys and manu ac u ing o he mic o eac o s 5 Recen ly we ha e published he expe imen al p ocedu es o he syn hesis and he cha ac e iza ion by means o N2 adso p ion-deso p ion, XRD, Raman spec oscopy and H2-TPR s udies o he CuOx/CeO2 powde ca alys [18, 19]. The manu ac u ing p ocess o ou mic o eac o p o o ype coa ed wi h 300 mg o ca alys [4, 18-20] is summa ized in Figu e 1. In he p esen wo k, his mic o eac o is named as MR300 and will be compa ed wi h a new block coa ed wi h 150 mg o ca alys , deno ed as MR150. 2.2. Ca aly ic ac i i y measu emen s P io o e e y ca aly ic es he ca alys s we e ea ed unde 30 mL/min o al low o 21% O2 in N2 a 300 ºC o 2 h. Fo he powde ca alys , he PROX eac ion was ca ied ou in a ixed-bed cylind ical s ainless s eel eac o wi h in e nal diame e o 9 mm unde he expe imen al condi ions p esen ed in [21]. Fo hese expe imen s, 100 mg o ca alys (pa icle size= 100 < ϕ < 200 μm) we e employed and dilu ed wi h c ushed glass (in he same pa icle size) o ming a bed o abou 50 mm in leng h. The CO con e sion and he selec i i y o CO con e sion we e calcula ed acco ding o Eq. 1 and Eq. 2, espec i ely. Fin and Fou e e o mola low a es a he eac o inle and ou le , espec i ely [4, 20]. CO con e sion (%) in CO ou CO in CO F FF , ,, 100)(    (Eq.1) Selec i i y o CO con e sion (%) )(2 100)( ,, ,, 22 ou OinO ou CO in CO FF FF     (Eq.2) In he case o mic o eac o s, he same eac ion se -up was used; howe e he cylind ical eac o was eplaced by wo s ainless s eel cases connec ing he inle and ou le posi ions o he mic ochannels, and allowing he con ac be ween he coa ed walls o he channels and he eed-s eam acco ding o ha desc ibed in [4, 20]. The empe a u e was con inuously moni o ed by 4 K- ype he mocouples. Two o hem we e placed in con ac wi h he me allic block a 6 he inle and ou le posi ions o some cen al mic ochannels. The o he wo senso s eco ded he empe a u e a la e al posi ions in he walls o he mic o eac o [4]. The eed-s eam composi ions and he λ alues used in he ca aly ic es s a e p esen ed in Table 1. The λ alue was calcula ed o e e y mix u e o eac ion acco ding o Eq. 3 [22].   .% .%2 2 CO ol olO    (Eq. 3) A i s compa ison o he ca aly ic pe o mances o he h ee ca alys s s udied, he powde and he MR150 and MR300 mic o eac o s, was ca ied ou employing he eed-s eam A and main aining he low- o-ca alys weigh a io in 60 L·h-1·g-1. Addi ionally, he in luence o he space eloci y (30, 60 and 120 L·h- 1·g-1) was analyzed o bo h mic o eac o s applying he eed-s eam B. The in luence o he λ alue was also e alua ed by using he eed-s eams B, C and D a a cons an space eloci y o 60 L·h-1·g-1. Finally, he e ec o he p esence o CO2 and/o H2O was s udied wi h he eed-s eam composi ions A, E, F, and G a a cons an space eloci y o 60 L·h-1·g-1. 3. Resul s and discussion 3.1. Compa ison o he CuOx/CeO2 powde ca alys and he mic o eac o s MR150 and MR300 o he PROX eac ion The ca aly ic esul s ob ained o he CuOx/CeO2 powde ca alys and o he wo mic o eac o s unde a space eloci y o 60 L·h-1·g-1 a e p esen ed in Figu e 2. In all h ee ca alys s, he CO con e sion inc eases wi h empe a u e nea o ull con e sion a 160ºC and abo e (Figu e 2A). Howe e a empe a u es below 160 ºC, di e en beha iou s a e app eciable. MR150 p esen s he highes 7 ca aly ic ac i i y while he MR300 exhibi s he lowes CO con e sion le els. The powde ca alys p esen s an in e media e beha iou . The selec i i y o CO con e sion (Figu e 2B) dec eases wi h he empe a u e in he h ee cases. Bo h mic o eac o s p esen simila selec i i ies each o he bu highe han ha o he powde ca alys , which sugges s ha he H2 consump ion eac ions (H2 oxida ion and/o R-WGS) would be inhibi ed in hem. F om he e, i is clea ha he mic o eac o loaded wi h he lowes amoun o ca alys (MR150) p esen s be e ca aly ic pe o mances han hose o he powde ca alys . Besides his, al hough bo h mic o eac o s has a simila selec i i y o CO2, he highes CO con e sion le els showed by he lowes loaded mic o eac o (MR150), makes i he sys em wi h he bes pe o mance. The obse ed di e ences in CO con e sion and selec i i y among he h ee e alua ed sys ems may be a ibu ed o he exis ence and di e en ex ension o mass and hea anspo phenomena. The highes ca aly ic ac i i y o he MR150 mic o eac o , mus be ela ed wi h he bes e iciency in he elease o he hea p oduced by he exo he mic eac ions implied (CO and H2 oxida ions and WGS). This allows a be e he mal con ol in o he mic ochannels and ensu es he iso he mal condi ions a e e y e alua ed empe a u e, a oiding he ho spo s o ma ion. Consequen ly, al hough he di e ence be ween he appa en ac i a ion ene gies o he CO and H2 oxida ion eac ions is no so big (36.9 and 110 kJ/mol espec i ely) [21], he achie ed s ic con ol o he empe a u e in he MR150 delays he appea ance o he H2 oxida ion eac ion, especially a lowe empe a u es. Ou esul s a e in good ag eemen wi h he highly e icien hea ans e ha has been widely desc ibed o o he mic o eac o s employed in he PROX eac ion, some o hem coa ed wi h CuOx/CeO2 ca alys s [16, 23, 24]. Fo ins ance Sny niko e al. [25] ha e s udied a coppe -ce ium oxide ca alys o he PROX eac ion as a powde ca alys in a ixed-bed eac o and coa ed in mic o eac o s. They ha e p oposed ha in he case o mic o eac o s, he eleasing o he hea p oduced du ing he oxida ion eac ion is e y as because o he di ec con ac be ween he ca aly ic laye and he me al subs a e. Howe e , in he case o he ixed-bed 8 eac o , he p esence o qua z, which is used as diluen , may p esen hea anspo limi a ions due o i s a he low he mal conduc i i y. A simila e ec could occu in ou case o he powde ca alys s, since i was es ed in a ixed- bed eac o and dilu ed wi h g ound glass. Besides his, he supe io ca aly ic beha iou o he MR150 mic o eac o espec o ha o he powde ca alys may be also associa ed o he diminu ion o he mass ans e limi a ions in he s uc u ed sys ems. P obably, he con ac be ween he ca alys and he eac ion mix u e is di e en in bo h si ua ions and, addi ionally, he p esence o g ound glass wi h he powde ca alys in he ixed- bed eac o also ep esen s an addi ional ba ie o he adequa e mass anspo . Despi e he ad an age o he MR150 s uc u ed sys em, he lowe ca aly ic pe o mance a empe a u es below 160ºC o he MR300, e en in e io o ha o he powde ca alys highligh s he need o op imize he amoun o coa ed ca alys in he mic o eac o s, ha consequen ly means he need o op imize he hickness o he ca aly ic laye . As said in he in oduc ion sec ion, he ele ance o op imizing o he ca aly ic laye hickness was emphasized by Po emkin e al. [17] who demons a ed ha ha op imum alues o he in e nal e ec i eness ac o o he CO oxida ion, ηCO, which assu es no mass limi a ions, o he PROX eac ion o e a mic o eac o coa ed wi h a Cu/CeO2-x ca alys in he 170–230ºC empe a u e ange, a e achie ed when he washcoa ed hickness did no exceed 20 µm. 3.2. E ec o he space eloci y o e he mic o eac o s In o de o e alua e he in luence o he space eloci y on he ca aly ic pe o mances o he s uc u ed sys ems, h ee o al low o weigh o ca alys a ios we e s udied (30, 60 and 120 L·h-1·g-1), keeping he same composi ion o he eac i e eed and wi h a λ = 1 alue. The esul s o CO con e sion and selec i i y o CO oxida ion a e p esen ed in Figu e 3. 9 The MR150 mic o eac o p esen s a maximum in he CO con e sion a a ound 140 ºC wha e e he space eloci y used, al hough he alue o such maximum con e sion dec eased when inc easing he space eloci y. Mo eo e , highe he space eloci y o he expe imen , lowe he ca aly ic ac i i y o he sys em a empe a u es below 140 ºC. On he o he hand, he MR300 mic o eac o shows a ema kable inc easing o he CO con e sion wi h he empe a u e, no ma e he space eloci y, up o 160 ºC. A highe empe a u es, he ca aly ic ac i i y ends o s abilize. In e e y case, he con e sion cu es shi o highe empe a u es when inc easing he space eloci y. The block coa ed wi h 300 mg o ca alys exhibi s highe con e sions han hose o he MR150 mic o eac o a high empe a u es. Howe e , in his empe a u e ange, he MR150 p ese es a supe io selec i i y o CO oxida ion. Fo bo h mic o eac o s, he inc emen o he space eloci y esul s in he dec ease o he ca aly ic ac i i y, especially a empe a u es below 160 ºC. Conside ing ha maldis ibu ions o eed-s eam in o he channels o he s udied mic ochannels blocks may be disca ded acco ding o p e ious CFD s udies ca ied ou o designing he mic o eac o s and he cases o inse ing hem in he eac ion sys em [26], he low CO con e sion shown by bo h blocks, especially a low empe a u es occu s because he ac i e si es a e wo king unde demanding condi ions (a high amoun o CO molecules o be con e ed pe uni o ime and an insu icien he mal ac i a ion), showing hei ull po en ial. Sny niko e al. [23] epo ed simila obse a ions s udying mic o eac o s coa ed wi h Cu/CeO2-x o he PROX eac ion. They concluded ha , when he low a e is inc eased, he minimum ou le CO concen a ion is inc eased and he op imum eac o wo king empe a u e, co esponding o he minimum CO concen a ion emission, was shi ed o highe alues. Conce ning he selec i i y o CO oxida ion (Figu e 3B), a gene al iew allows es ablishing ha he MR150 mic o eac o p esen s highe selec i i ies han hose o MR300 in all cases, especially a empe a u es abo e 160 ºC. Howe e , he modi ica ion o he space eloci y seems o p oduce an opposi e e ec in e e y mic o eac o . In he case o he MR150, when he space eloci y is inc eased, he selec i i y also inc eased. Howe e , o he MR300 16 [6] J.L. Ayas uy, A. Iglesias-González, M.A. Gu ié ez-O iz, Chemical Enginee ing Jou nal, 244 (2014) 372-381. [7] P.-Y. Peng, I. Jin, T.C.K. Yang, C.-M. Huang, Chemical Enginee ing Jou nal, 251 (2014) 228-235. [8] K. Jahnisch, V. Hessel, H. Lowe, M. Bae ns, Angew. Chem.-In . Edi ., 43 (2004) 406-446. [9] G. Kolb, Chem. Eng. P ocess., 65 (2013) 1-44. [10] F. 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Feed-s eam composi ions o he di e en ca aly ic ac i i y es s Feed- S eam CO ol.% O2 ol.% H2 ol.% N2 ol.% CO2 ol.% H2O ol.% λ A 1.0 1.0 50.0 48.0 -- -- 2.0 B 2.0 1.0 50.0 47.0 -- -- 1.0 C 2.0 1.5 50.0 46.5 -- -- 1.5 D 2.0 3.0 50.0 45.0 -- -- 3.0 E 1.0 1.0 50.0 46.0 2.0 -- 2.0 F 1.0 1.0 50.0 38.0 -- 10 2.0 G 1.0 1.0 50.0 36.0 2.0 10 2.0 19 Figu e Cap ions Figu e 1. Manu ac u ing o he mic o eac o p o o ype: (a) Mic omilled pla es; (b) Joining o he pla es; c) Final mic ochannels block Figu e 2. Ca aly ic ac i i y o he h ee e alua ed sys ems (CuOx/CeO2 powde , MR150 and MR300) in he PROX eac ion: (A) CO con e sion; B) Selec i i y o CO oxida ion Figu e 3. Ca aly ic ac i i y o he mic o eac o s (MR150 and MR300) modi ying he space eloci y: (A) CO con e sion; (B) Selec i i y o CO oxida ion Figu e 4. Ca aly ic ac i i y du ing he PROX eac ion o MR150 and MR300 modi ying he λ alue in he eed-s eam: (A) CO oxida ion; (B) Selec i i y o CO oxida ion Figu e 5. Ca aly ic ac i i y o he MR150 and MR300 including CO2 and/o H2O in he eed-s eam: (A) CO oxida ion; (B) Selec i i y o CO oxida ion Figu e 6. E ec i eness ac o as a unc ion o he empe a u e du ing he PROX eac ion o he CuOx/CeO2 powde ca alys s, MR150 and MR300 20 Figu e 1 21 Figu e 2  120 140 160 180 200 0 10 20 30 40 50 60 70 80 90 100 CuOx/CeO2 (powde ) MR150 MR300 Con e sion o CO (%) Tempe a u e (ºC) (B) (A) Space eloci y = 60 L·h-1·g-1 Feed-s eam A (Table 1)  = 2 120 140 160 180 200 0 10 20 30 40 50 60 70 80 90 100 Space eloci y = 60 L·h-1·g-1 Feed-s eam A (Table 1)  = 2 CuOx/CeO2 (powde ) MR150 MR300 Selec i i y o CO oxida ion (%) Tempe a u e (ºC) 22 Figu e 3  120 140 160 180 200 0 10 20 30 40 50 60 70 80 90 100 30L·h-1·g-1 60L·h-1·g-1 120 L·h-1·g-1 CO con e sion (%) Tempe a u e (ºC) = 1 Feed-s eam B (Table 1) MR150 MR300 120 140 160 180 200 0 10 20 30 40 50 60 70 80 90 100 = 1 Feed-s eam B (Table 1) 30L·h-1·g-1 60L·h-1·g-1 120 L·h-1·g-1 Selec i i y o CO oxida ion Tempe a u e (ºC) (A) (B) MR150 MR300 23 Figu e 4  120 140 160 180 200 0 10 20 30 40 50 60 70 80 90 100 CO con e sion (%) Tempe a u e ºC  = 1.5  = 2.0  = 3.0 MR150 MR300 60 L· h -1 · g -1 120 140 160 180 200 0 10 20 30 40 50 60 70 80 90 100 60 L· h -1 · g -1 Selec i i y o CO oxida ion (%) Tempe a u e ºC  = 1.5  = 2.0  = 3.0 MR150 MR300 (A) (B) 24 Figu e 5  125 150 175 200 225 250 0 10 20 30 40 50 60 70 80 90 100 H2O CO2MR300 CO con e sion (%) Tempe a u e (ºC) MR150 0% 0% 2% 0% 0% 10% 2% 10% 60 L·h-1·g-1  = 2 125 150 175 200 225 250 0 10 20 30 40 50 60 70 80 90 100 60 L·h-1·g-1  = 2 Selec i i y o CO oxida ion (%) Tempe a u e (ºC) CO2H2OMR150 MR300 0% 0% 2% 0% 0% 10% 2% 10% (A) (B) 25 Figu e 6  120 140 160 180 200 0 1 CO Tempe a u e (ºC) CuOx/CeO2 (powde ) MR150 MR300