Modeling Open-Flow Steam Reforming of Methanol over Cu/ZnO/Al 2 O 3 Catalyst in an Axisymmetric Reactor

Transcription

1 Journal of Aled Flud Mechancs, Vol. 8, No. 1,. 33-4, 015. Avalable onlne at ISSN , EISSN Modelng Oen-Flow Steam Reformng of Methanol over Cu/ZnO/Al O 3 Catalyst n an Axsymmetrc Reactor L.Pacheco 1, D. Della-Valle 1,, O. Le Corre 3,C. Habch 4, T. Lemenand 1, H. Peerhossan 5 1 LUNAM Unversté, Laboratore de Thermocnétque de Nantes, CNRS UMR 6607, Nantes, France ONIRIS, B.P. 85 F-443, France 3 GEPEA, Ecole des Mnes de Nantes, CNRS-UMR 6144, B.P. 07 F-44307, France 4 Energy and Thermo-Fluds Grou, School of Engneerng, Lebanese Internatonal Unversty LIU, Berut, Lebanon 5 Unv. Pars Dderot, Sorbonne Pars Cté, Insttut des Energes de Deman (IED), Pars, France Corresondng Author Emal: hassan.eerhossan@unv-ars-dderot.fr (Receved May 6, 013; acceted March 17, 014) ABSTRACT Ths aer descrbes a CFD study of the steam-reformng rocess (SRP) of methanol n a short seudo-contact tme reactor of fxed bed tye, n ax-symmetrc condtons. The SRP s mortant sake for hydrogen roducton, and the desgn /scale-u/control of the ndustral rocesses n the future are suorted by a relable knowledge and redcton of the catalytc reacton. The dffculty of determnng the reacton scheme and the assocated constants s wellknown, due to the necessty of dentfyng the reacton knetcs n urely chemcal regme, meanng wth a erfect homogenety and flow ndeendence. Practcally these deal condtons, albet assumed, are not fulflled so that the ntrnsc chemcal knetcs s not reached. For the case of SRP, we have attemted here to valdate the Peley s model by a numercal modellng reroducng exactly the local condtons n the exermental duct, accountng for gradents n the cross secton. The numercal results show the same trends than the exermental one, but wth a slght shft of 0% as a consequence of the reactor heterogenety. Ths result seems accetable to valdate the use of the Peeley s model for further studes n other tyes of comlex flow reactors. Keywords: Methanol; Bo-methanol; Steam reformng; Hydrogen roducton; Multfunctonal heat exchanger. NOMENCLATURE C secfc heat C s actve ste concentratons of tye D dameter d catalytc bead sze E actvaton energy F molar flow h enthaly H adsorton heat k knetc constants k K L m cat M r r re-exonental term n Arrhenus equaton adsorton constants reactor length catalyst mass molar mass ressure reactor rado rate of reacton or roducton of seces Subscrts cat catalyst chemcal seces R reactor wall wall R gas constant S reacton heat S entroy of adsorton S artcle secfc surface S g secfc surface T temerature u velocty comonent n the axal drecton V velocty comonent n the radal drecton v velocty vector W catalysts mass Y mass fracton z length reactor Greek letters thermal conductvty vscosty ρ densty acked bed orosty Acronym MD methanol-decomoston reacton SRM steam-reformng reacton WGS water-gas shft reacton

2 1. INTRODUCTION Restrctons on the use of fossl-based energy sources are ever becomng more strngent. The Euroean Unon s 008 CLIMAT lan ams at a 0% reducton of CO emssons and consumton of rmary energy sources, and a 0% ncrease n energy roducton from renewable sources. Hydrogen s among the solutons consdered n the context of decarbonsed electrcty roducton. However, mlementng ths technology stll nvolves severe socoeconomc and safety-related dffcultes. The sgnfcant storage roblems n usng hydrogen for transort alcatons must be solved before t s acceted as a wdely used transortaton fuel. In addton, hydrogen s not a rmary energy source but an energy vector: chemcal reactons must be used to extract t from hydrocarbons. Hydrogen can be roduced from several hydrogen-rch rmary fuels such as methanol, natural gas, ethanol or gasolne. These fuels are abundant, esecally methane (and hence methanol, snce t can be roduced from methane). Actually, the electrcty roduced by fuel cells can be qualfed of green electrcty f the fuel comes from the bomass, to comensate for the carbon budget. Several reactons can be envsaged for hydrogen roducton from the above rmary fuels: steam reformng, an endothermc reacton wth ure water that roduces hydrogen as well as carbon monoxde and carbon doxde. artal oxdaton, an exothermc reacton wth oxygen that roduces hydrogen as well as carbon monoxde. Table 1 shows the rncal characterstcs of some hydrogen roducton rocesses for several reactants rch n hydrogen. The molar fracton of hydrogen s mortant for fuel-cell alcatons snce cell effcency strongly deends on hydrogen urty, so that the artal oxdaton reacton shown n table 1 s not recommended for hydrogen roducton. Snce carbon monoxde s a oson, hydrogen roducton by methanol steam reformng s more arorate for ths alcaton because ts rate of CO roducton s two orders of magntude smaller than n the other hydrogen roducton modes. However, n other rocesses of hydrogen roducton, t s necessary to add a gas urfcaton ste before the fuel cell wth a water-gas shft reacton to transform the carbon monoxde to carbon doxde. Snce ths reacton occurs at 473 K, t s necessary to add another reactor for gas urfcaton. Ths reactor s not needed f methanol steam reformng s used for hydrogen generaton. Table 1 also shows that the methanol steam reformng s the only rocess n whch the reactons occur at moderate temeratures (less than 800 K). Hgher reacton temeratures consume more energy and requre more exensve materals. In addton, extra safety recautons are needed. Another advantage of methanol steam reformng s ts low sulfur content. In fact, all the rocesses consdered Table 1 Characterstcs of dfferent rocesses for hydrogen roducton Prmary fuel Hydrogen molar fracton (dry fracton %mol) CO molar fracton n roduct (%mol) Reacton temerature (K) Steam reformng of : Methane Methanol Ethanol Gasolne, gasol, fuel Partal oxydaton of : Methane Gasolne, desel, fuel for hydrogen roducton utlze catalysts. Many catalysts used n the vaor hase are senstve to murtes. Sulfur s resent n most gas fuels and s also a oson, so that t must be removed before beng ntroduced nto the hydrogen roducton rocess. Snce catalysts are used n several stages of methanol roducton, the sulfur can be removed n early stages, leavng methanol a sulfur-free lqud rmary fuel. For all these reasons, methanol (bo-methanol) s a very advantageous fuel for hydrogen roducton by chemcal converson. Indeed, the C/H rato for methanol s 0.5, comared to 0.7 for gasolne, 0.5 for desel and 0.5 for methane. Furthermore, methanol does not contan carbon-carbon bonds. Methanol steam reformng makes t ossble to roduce hydrogen wth hgh converson rates at low temeratures ( K) wth less byroduct (CO) formaton than other fuels. 1.1 Prevous catalyst work The methanol steam reformng rocess (SRP) has been studed extensvely. Prevous work has suggested that the SRP can be adequately modeled usng the overall reacton: methanol steam reformng (SRM), methanol decomoston (MD) and water-gas shft (WGS), see Dümelmann and Baker (199) Peley et al. (199a-b). Catalyst erformance has been studed n terms of both ncreased CH 3 OH converson and decreased CO roducton. The steam-reformng rocess (SRP) of methanol occurs over Cu or Pd/Zn alloy catalysts at low temeratures (00 C to 300 C), see Zhao et al. (007). These dfferent catalysts have dfferent advantages and drawbacks. Coer-based catalysts have been wdely studed n the SRM reacton. Ranganathan et al. (005) showed that the most sutable catalyst for SRP s Cu/ZnO/Al O 3. However, ths catalyst s unstable at hgh temeratures (aroxmately 300 C) and s very suscetble to thermal snterng va a surface mgraton rocess. Coer-based catalysts normally contan oxdes that ncrease stablty, manly by hyscally searatng the coer crystalltes. In addton, synergetc effects have 34

3 been observed, e.g. alloy formaton, morhology changes or the nducton of stran n the coer crystallne structure (for Cu over ZnO), see Zhao et al. (007). The catalyst rearaton method affects erformance (e.g. CO selectvty, catalyst stablty). The coer co-rectaton method s the most commonly used method of catalyst rearaton, see Gunter et al. (1995). Takezawa and Iwasa (1997) studed the SRP and catalytc erformance of coer and grou VIII metals (Cu, N, Rh, Pt and Pd) on SO ; hgher erformance (n selectvty and actvty) was obtaned by usng Cu/SO. Later, usng an mregnaton catalyst method, Takezawa and Iwasa (1997) and Iwasa et al. (1993) studed the selectvty for the SRP of Pd catalysts on dfferent suorts (ZnO, Al O 3, SO Nb O 5, Nd O 3, La O 3 and ZrO ), concludng that coer-based catalysts and Pd/ZnO offered hgh erformance for SRP. They also noted that Pd/Zn alloys ncrease the erformance of Pd/ZnO catalysts for the SRP and that the Pd/Zn alloys exhbt excellent thermal stablty. Lndström and Pettersson (001,00), used coerbased catalysts on an Al O 3 suort to comare the catalytc actvty of romoters Cr, Zn and Zr. They showed that large coer content n the catalyst ncreases H and CO selectvty excet for Cu/Zn, where the actvty s almost nvarant and the romoter Zn has a greater affnty to CO selectvty and methanol converson than Cr, Zr romoters. They also showed that the addton of a Zr romoter ncreases methanol converson. Lndström and Pettersson (001) comared the stablty of coer-based catalysts Cu/ZrO and a commercal Cu/ZnO/Al O 3 catalyst, fndng that the zrconum- doed catalysts show hgh CO selectvty but H roducton s decreased. Takeguch et al. (00) comared coer-based and alladum-based catalysts for SRP at dfferent reacton temeratures wth two rearaton methods, mregnaton and co-rectaton. Exermental tests showed that coer-based catalysts over ZnO/Al O 3 have the greatest H roducton rate (hghest methanol converson) and hgh CO selectvty, see Agrell (003), Guchard (007), Purnama et al. (004a), Zhao et al. (007). Fgure 1 comares the dfferent catalysts for H roducton rate as a functon of temerature. 1. Knetc mechansms n methanol steam reformng Knetc mechansms n methanol-steam reformng on Cu/ZnO/Al O 3 have also been studed extensvely. Jang et al. (1993a-b) used a U-tube reactor (I.D. 6 mm) mantaned at atmosherc ressure and constant temerature. They develoed a reacton mechansm n SRM by regresson analyss of methanol synthess n whch the adsorton of CH 3 OH and H sgnfcantly affects the reacton rate; CO has no effect. They suggested a rocess that s 100% selectve for CO n whch the rates of WGS and MD reactons are neglgble, and concluded that CO has no nfluence on the reacton rates and affrmed that the SRP s 100% selectve for CO and H and that CO s not detected for temeratures lower than 533 K. Further, they suggested Fg. 1. Comarson of catalytc erformance for SRM reactons: ( ) mregnaton catalyst; ( ) corectaton catalyst, see Lndstrom and Pettersson (001-b) that CO s not the ntermedate roduct of the SRP rocess over coer-based catalysts. They found that water-gas shft reacton takes lace under the oeratonal condtons of the SRP. Ther nfrared studes showed that comettve adsorton takes lace between CH 3 OH and CO by whch the CH 3 OH s referentally adsorbed. Agrell et al. (00) used a tubular quartz reactor (I.D. 6 mm) at atmosherc ressure wth helum as the carrer gas. They assumed sothermal reacton condtons and neglgble transfer resstance. Agrell (003) neglected the MD reacton and attrbuted CO formaton to a reversble WGS reacton. He suggested that the CO s a secondary roduct formed at hgher temeratures by a reversble WGS reacton and confrmed that the WGS reacton s favored by a lower seudo-contact tme and a hgher temerature. Jang et al. (1993a-b) had also develoed a model to redct the rates of formaton of CO and H, but ths model dd not redct CO formaton; they showed CO formaton exermentally at low temeratures. The model roosed by Purnama et al. (004b), on the other hand, redcted CO formaton and showed that t can be nfluenced by catalyst artcle sze. They used a tubular stanless steel reactor (I.D. 10 mm) at atmosherc ressure and sothermal condtons to study the catalytc actvty and selectvty of Cu/ZrO and commercal CuO/ZnO/Al O 3 n long-duraton tests and showed that methanol converson s more favored by CuO/ZnO/Al O 3 catalyst. Agrell et al. (00) and Purnama et al. (004b) reorted neglgble CO roducton by the reversble WGS reacton and MD reacton. Table shows ther dfferent exermental condtons. The densty consdered s 1100 kg.m -3 and the orosty calculated for body-centered cubc ackng s 0.3. Several dfferent knetc mechansms have been roosed for SRP, see Jang et al (1993a-b), Peley et al. (199a-b), Purnama et al. (004b). Dümelmann and Baker (199) measured the SRM and MD reacton rates under varous oeratng condtons that mantaned the WGS reacton n equlbrum, adjustng the artal ressure of CO. Under these condtons, they concluded 35

4 Table Exermental condtons n ltterature Jang et al Peley et al Agrell 003 Purnama et al. 004 Catalyst Cu/ZnO/AL O 3 BASF S3-85 BASF K3-110 G-66 MR Süd-chme 50 % Cu S g [m.kg -1 ] 83 x x x x 10 3 W cat [kg] 0.5 x x x x 10-6 d [mm] D R [mm] T wall [K] P [bar] H O/CH 3 OH N [m 3 mn -1 ] x that methanol s converted rncally to CO and that the MD reacton roduces a neglgble amount of CO. They suggested that the MD reacton rate s lower than that of the WGS reacton. 1. Objectve of the study Peley et al. (199a-b) roosed one of the most relable and comrehensble reacton networks for modelng SRP n whch SRM, MD and WGS reactons are ncluded. Ths model shows that there s one tye of catalyst ste for the MD reacton and another tye for the SRM and WGS reactons. Exerments by Peley et al. (199a-b) confrmed the results n Dümelmann and Baker (199), Jang et al. (1993a-b). Here we develo Peley s et al. (199a-b) reacton mechansm usng numercal ntegraton of a lug-flow tubular reactor model, assumng sothermal wall condtons and a ure methanol-steam feed. In Peley s et al. (199a-b) model, these knetc mechansms were develoed by measurng the CH 3 OH converson ercentage at the reactor entrance and ext (whch was assumed sothermal). Thus the temerature gradents n the reacton zone and the effect of reactant flow on the reactons were not consdered. In ths work we take these effects nto consderaton and show that they affect the converson rates of the dfferent seces. The knetc mechansm roosed by Peley et al. (199a-b) resent the advantage of redctng CO formaton, whch s not generally taken nto account. The numercal smulatons resented here were erformed to reroduce Peley s exerments n a fxed-bed reactor made u of shercal beads coated wth catalyst. Comarson between smulaton and exermental results oens the dscusson of the chemcal model. The model can also be used to exlore favourable condtons for hgh CH 3 OH converson rates and low by-roduct formaton. MATHEMATICAL AND NUMERICAL MODELS.1 Conservaton equatons The conservaton equatons are solved for lamnar flow, gnorng gravtatonal external body forces. In order to descrbe the comlex reformng rocess n a comuter model, a number of assumtons on the hyscal condtons are made: 1. the fxed-bed (bead-bed) catalyst s modelled by a orous medum and treated as a seudohomogeneous medum wth gven homogenzed hyscal roertes. flow s axsymmetrc, lamnar and steady 3. bed orosty s constant n axal and radal drectons (sotroc) 4. gases are assumed ncomressble 5. the chemcal reacton occurs only at the bead surface 6. catalyst deactvaton s neglected 7. heatng effect of vscous dssaton s neglected 8. reactor wall and fxed bed temerature are assumed constant 9. hyscal roertes are thermo-deendent 10. The hyscal model conssts of the followng set of equatons, for mass, momentum and energy balance: contnuty equaton: v (1) 0 ressure losses: L 3 d 3.51 u energy equaton: v h T S 3 d u () (3) where S s the reacton heat: S o h C dt r, (4) M wth ρ the densty, v the velocty vector, the ressure, the vscosty, the orosty of the catalytc bed, L the reactor length, h the enthaly, T the o temerature, λ the thermal conductvty, h the standard formaton enthaly of seces, Y the mass fracton of seces, and r the rate of roducton or consumton of seces. The secfc enthaly s defned as: T h Y h wth h c, dt (5) Tref where T ref s the reference temerature and C, s the secfc heat at constant ressure. 36

5 . Chemcal model The rate of roducton or consumton of each seces r s comuted usng Peley s reacton mechansm (Peley et al. 199a-b), whch can be reduced to a set of three chemcal reactons descrbng the SRP of methanol. The rate of converson SR and the CO formaton are redcted by the water-gas-shft (WGS) and methanol decomoston reactons (MD). These reactons are resented n the followng, assocated wth ther relatve rogress rate: Methanol steam reacton (SRM): CH3OH 3H H O CO (6) H CO ksrk CH3O 1 CH3OH CS1CS1 as g ksr H O r SR (7) DEN 1 KH 1 H wth: 0.5 DEN H KCH3O1 CH3OH K HCOO CO H KOH (1) H O Methanol decomoston reacton (MD): CH3 OH CO H (8) H CO kmdk CH3O CH3OH CS CS as g k MD r MD H KCH3OCH3OH KOH H O 1 KH H (9) Water gas shft reacton (WGS): CO H O CO H (10) 0.5 H CO kwgs KOH 1 H CO H O CS1Sg K WGS r WGS (11) DEN where the knetc constants are calculated wth the Arrhenus law: k k ex E R T (1) and the adsorton constants are calculated wth the Vant Hoff s law: K ex S R H R T (13) In ractce, we can consder that the reactons take lace at the surface of the catalyst artcles. In seudohomogeneous orous meda, ths feature s taken nto account by a secfc surface (S g ) coated wth the catalyst and actve ste concentratons (C s, C sa ) n the reacton rates, eqs. (7), (9), (11). The arameters of the knetc rate exresson for methanol steam reformng (SRM), methanol decomoston (MD) and water gas shft (WGS) reactons are gven n table 3. The methanol converson s based on the carbon balance, see Peley et al. (199a-b): F 3 1 % conv 1 CH OH *100 F F F CH 3OH CO CO (14) Table 3 Knetc constants for steam reformng of methanol, Peley et al. (199a-b) Equlbrum constants k [mol.m².s -1 ] or S [J.mol -1.K -1 ] E or H [kj.mol -1 ] k SR 7.4 x k WGS 5.9 x k MD 3.8 x K CH3O K HCOO K OH K H K CH3O() 30-0 K OH() 30-0 K H() C s1 = C s 7.5 x 10-6 [mol.m - ] C s1a = C sa 1.5 x 10-5 [mol.m - ].3 Boundary condtons At the reactor nlet (z = 0), the flow s assumed homogeneous n temerature, comoston, and velocty, wth no radal velocty. At the reactor ext (z = 140 mm), ressure s assumed to be the ambent ressure. The wall temerature s fxed by Drchlet temerature condton, whle n the core, a heat flux s assumed between the beads and the gas mxture. Ths flux s conventonally gven by a Nusselt number, taken here equal to for solate shere dffuson mode, and a flm temerature whch s the average between the bead and the gas temeratures. The stablty n tme of the bead temerature s ensured by the value of the Bot number, whch s here about 0.08, enough below the usual crtcal value of 0.1. Ths mles that the beads are heated more radly by the wall, than cooled by the gases..4 Smulaton condtons Smulatons are carred out for the condtons of Peley s exerments n the orous bead bed geometry as shown n fgure. The exermental tubular reactor studed by Peley et al. (199a-b) s receded and followed by non-catalytc orous meda, each 50 mm long to avod end effects. The catalytc zone of the reactor s 40 mm long, and ts dameter s.1 mm. The three zones are modelled. The reacton zone contans g of catalyst. The orosty of the body-centered cubc ackng s assumed constant at the nomnal value of 0.3. Nevertheless, the latter value s not reorted n Peley s study, so that we have to carry out a senstvty study on ths arameter. Geometry, catalyst roertes, hyscal roertes and oeratng condtons are summarzed n tables and 4. 37

6 Table 4 Physcal roertes of coer-based catalyst ρ cat [kg cat.m -3 ] λ cat [W.m -.K -1 ] 0.3 C cat [J.kg -1.K -1 ] 550 Fg.. Tubular reactor model for a constantorosty fxed catalytc bed.5 Numercal method The numercal smulatons were carred out usng the comutatonal flud dynamcs code Fluent (Fluent, 001, webste), couled wth external subroutnes (userdefned functons, UDFs) develoed n C++ to mlement Peley s chemcal model. A segregated solver, second-order uwnd dscretzaton scheme wth steady and mlct lnearzaton was used, see Patankar (1980). Pressure-velocty coulng s acheved by the SIMPLE algorthm. The grd ndeendence was examned n relmnary test runs, wth the oeratng condtons gven by Peley et al. (199a-b), as resented n table. The mesh sze effect was evaluated for the methanol steam reformng reactor at a temerature of 553 K, wth number of grd cells 75600, and The fracton mole devatons of CH 3 OH and H for grds and are 5% and 6%, and for the grds and are 0.4% and 1.9% resectvely, as resented n table 5. In the resent study a grd of cells was chosen for the smulaton. To determne an adequate convergence crteron, a seres of flow smulatons were carred out for sto-crteron values rangng from 10-3 to Beyond the value of 10-6, no sgnfcant changes were observed n the velocty feld, temerature gradents and reactons rates, and thus 10-6 s used as the convergence crteron for all smulatons. 3. RESULTS AND DISCUSSION 3.1 Senstvty of the orosty In Peley et al. (199a-b), the bed catalyst orosty s not reorted, and t was hence necessary to valdate the choce of a reference value of 0.3 (body-centered cubc ackng). As the catalyst secfc surface s varyng wth the orosty n the reactor, two assumtons were made for ths test the orosty arameter: the catalyst mass s ket constant (varable ste actve densty) the actve ste densty s consdered constant (varable catalyst mass) Fg. 3. Porosty sensblty for the methanol converson (%) and hydrogen roducton (%) Table 6 Catalytc mass and artcle secfc surface for dfferent orosty values ε m cat [g] S [m -1 ] The secfc ste actve densty s functon of the catalyst mass dstrbuted on the surface formed by the bed artcles, eqs 15 and 16. mcat catsecf (15) S where the artcle secfc surface (S ) s defned as: S 6(1 ) (16) d The nomnal oeraton condtons are taken, meanng W/F CH3OH = kg.s.mol -1 and the wall temerature at 553 K. The orosty s vared n the range [0.88, 0.35] corresondng to ±10% of the reference value. The fgure 3 shows that the global CH 3 OH converson (%) and H roducton (% mol) senstvty s low n our case to orosty arameter, less that 3% and 0.01% for CH 3 OH converson and H roducton resectvely. The table 6 shows the corresondent values of the catalytc mass and artcle secfc surface for dfferent orosty values. 3. Model trends: satal evolutons Fgure 4 shows the reacton rate rofles for SRM, MD, and WGS reacton rates at dfferent z-axs locatons. In the entrance to the catalytc zone (0 < z < 40 mm) we observe, as far as the methanol concentraton s maxmum, a hgh reacton rate for MD (fg 4b), whch s resonsble for CO roducton. Further downstream n the catalytc zone, as CH 3 OH converson becomes sgnfcant, the artal ressure of the roducts (H, CO 38

7 Table 5 Mol fracton of CH 3 OH and H for the varous grd tests at dfferent z-axs locatons grd numbers 1E H mole fracton and CO) ncreases and thus the MD reacton rate dramatcally decreases (fg 4b), a trend that s even stronger for SRM (fg 4a). However, the combned effect of these two reductons s to ncrease the cumulatve amount of CO roduced n the catalytc art of the reactor, as s shown n fgure 5d. Ths observaton s n agreement wth the exermental results n Dümelmann and Baker (199) and Cho and Stenger (00). Snce CO s rncally roduced from the MD reacton, ts rate of roducton s also strong n ths zone. However, t should be recalled that the CO selectvty s only %. (a) Fgure 4c also shows that the WGS s weak at the begnnng of the catalytc zone and reaches ts maxmum at the end of ths zone (long contact tme). Ths s due to the fact that at the begnnng of the catalytc zone the methanol concentraton s hgh and therefore ts rate of absorton by the catalyst s also hgh. However, further downstream the CO concentraton ncreases whle the methanol concentraton decreases (fg 5a and 5d) and thus CO absorton by the catalyst s large enough to favour the WGS reacton. Comarson of fgures 4c and 5a shows that hgh WGS rate takes lace where the CH 3 OH concentraton s low snce the condtons are favourable for CO absorton. (b) Fgures 4b and 4c show also that the rates of WGS and MD reactons are stronger near the reactor walls (r = mm), where the temerature s hgh due to the wall heatng. Ths observaton (whch was n fact exected) shows the mortance of takng thermo-fluddynamc effects nto account n the reacton smulatons. Fgure 6 shows that the ressure losses are aroxmately kpa, less than % of the ambent ressure, justfyng the assumton of sobarc condtons. (c) Fg. 4. Profle lots of rate of reacton (kmol.m -3.s -1 ) at dfferent z-axs locatons. H O/CH 3 OH rato of 1.36; CH 3 OH molar feed rate s mol.s Model trends: converson rate Numercal results on the rate of CH 3 OH converson as a functon of seudo-contact tme W/F CH3OH (catalyst mass over CH 3 OH molar flow rato) are lotted n fgure 7. Fgure 8 shows the rate of CH 3 OH converson as a functon of W/F CH3OH for two dfferent wall temeratures (533 K and 513 K), a H O/CH 3 OH rato of 1, at 1 bar rocess ressure. Agan, the exermental and numercal results follow the same trends, but the 39

8 (a) Fg. 6. Profle lots of absolute ressure (kpa), H O/CH 3 OH rato of 1.36; CH 3 OH molar feed rate s mol.s -1. (b) Fg. 7. Methanol converson as a functon of seudocontact tme W/F CH3OH for Peley et al. (199a-b) exerments and resent numercal redctons. T wall 553 K, H O/CH 3 OH rato of 1.36, 1.16 bar (c) Fg. 8. Methanol converson as a functon of seudocontact tme W/F CH3OH for Peley et al. (199a-b) exerments and resent numercal redctons. T wall 533 K, 513 K, H O/CH 3 OH rato of 1, 1 bar (d) Fg. 5. Profle lots of the molar concentraton for selected seces (kmol.m -3 ), H O/CH 3 OH rato of 1.36; CH 3 OH molar feed rate s mol.s -1 absolute values for the methanol converson rate are slghtly dfferent. Fgure 8 also show that for a gven contact tme, the hgher the reactor temerature the hgher the converson rate. Both fgures 7 and 8 show that ncreasng W/F CH3OH s advantageous for methanol converson. At the same 40

9 tme, t s shown n Agrell et al. (00, 003) that the reversble WGS reactons are also favoured by the ncrease n W/F CH3OH and consequently there s an ncrease n CO roducton. Other arameters also can affect the rate of methanol converson: the addton of water n the ntal gas mxture ncreases the SRM reacton rate and decreases that of the MD reacton, leadng to a reducton n CO roducton. However, t should be notced that the addton of water ncreases the flow rate n the reactor and hence decreases the contact (resdence) tme. Nevertheless, the global decrease n methanol converson due to these two ooste effects s less than 5%. The overall effect s that the water addton decreases CO formaton, a ostve result for hydrogen roducton amed at fuelcell alcatons. Moreover, extra water s favourable for catalytc actvty and decreases catalytc deactvaton see Amhlett et al. (1994), Jang et al. (1993a-b). 3.4 Comarson wth the exermental results and dscusson of chemcal model The fgure 7 comares exermental data from Peley et al. (199a-b) wth the resent numercal redctons for reactor wall temerature 553 K, H O/CH 3 OH rato 1.36 and rocess ressure 1.16 bar. Ths fgure shows that the numercal results follow the same trend as Peley s exermental results, but wth a smaller redcted methanol converson rates than n the exermental results. The man feature s that the dscreancy can be attrbuted to the dfference between the real local temerature and the global one used n Peley s comutaton, snce the temerature s the most nfluental arameter n CH 3 OH converson, see Amhlett et al. (1994). Fgure 9 shows the Δ- temerature (ΔT) curve along the catalytc art of the reactor. Peley et al. (199a-b) took the reacton temerature as equal to the temerature 1 cm ustream of the entrance to the catalytc bed. Ths assumton was justfed by the fact that n hs exerments the temerature dfference between the reactor wall and the center (ustream of the entrance to the catalytc zone) was less than 1 K; he thus assumed sothermal condtons and no radal temerature and concentraton gradents even n the catalytc zone. However, as fgure 9 shows, the temerature dfference between the wall and the center of reactor n the catalytc zone s around 6 K, contradctng the sothermal assumton. The hgh ΔT zone n the mddle of the catalytc duct fgure 9 shows that the SRP endothermc reacton sgnfcantly lowers the temerature. The table 7 shows the senstvty of knetc and adsorton rates basng on the maxmal temerature dfference n the cross secton ΔT, at the reference temerature of 553 K. As s shown n equatons (7), (9) and (11), the artal ressures also affect the adsorton constants ther nfluence over the rate s not drectly roortonal. The knetc constants k SR, k MD and k WGS resent a relatve varaton of %, 19% and 33% resectvely from the maxmal ΔT. Ths varaton ntroduces a 10-0% bas (systematc error) n the results, as shown n fgures 7 and 8. The knetc constants are establshed by Peley et al (199a-b) by nverse method assumng a homogeneous temerature Fg. 9. The Δ-temerature between the catalyst bed and the furnace wall, H O/CH 3 OH rato of 1.36; CH 3 OH molar feed rate s mol.s -1 n the reactor, so ths assumton leads to a roblem of accuracy. Here we show that aroxmately 1% of temerature gradent gves a 10-0% dscreancy on the converson rates, what can quanttatvely exlan the sgnfcant devaton between the numercal results and the exerments. Nevertheless, the numercal redctons resent consstent trend wth the exermental results, justfyng the mlementaton of ths knetc model n the study of SRP ntensfcaton. 4. CONCLUSION In ths work, Peley s mechansm (199a-b) has been used n a numercal model for redctng the rate of methanol converson as a functon of seudo-contact tme. An external subroutne was develoed n Fluent to mlement the reacton model. The numercal model allowed redcton of the converson rate and selectvty, couled wth the ressure and temerature dstrbutons along the catalytc zone of a tubular reactor. The model ermts a better understandng of the local rocesses, by the dentfcaton of roducton (MD) and consumton (WGS) zones of CO by-roducts; the MD and WGS reactons are accelerated n hgh-temerature regons next to the wall. The MD reacton occurs more quckly wth a short seudo contact-tme, unlke the WGS reacton, whch occurs more quckly for hgh contact tmes. Ths feature s exlaned by adsorton cometton between CH 3 OH and CO. The numercal results show that the temerature dstrbuton s hghly non-unform n the catalytc zone, thus rulng out the assumton of an sothermal reacton. Actually for numerous chemcal reactons, temerature s a key arameter n the SRP. We have demonstrated that the assumton of full homogenety n the Peley s model gves a 0% shft n the methanol converson when used n the fxed-bed model. Deendng on the fnal urose, ether the numercal smulaton can be used to mrove the accuracy of the re-factors n Peley s model, or these results can be consdered as a satsfactory reresentaton of the ntrnsc reacton system to be used n the reactng flow modellng for methanol steam reformng. 41

10 ACKNOWLEDGEMENT The authors would lke to thank Dr. Pascal Fongarland and Dr. Domnque Tarlet for contrbutng deas. LP would lke to acknowledge the fnancal suort of the Consel Regonal des Pays de la Lore. REFERENCES Agrell J., H. Brgersson, M. Boutonnet (00), Steam reformng of methanol over a Cu/ZnO/AlO3 catalyst: a knetc and strateges for suresson of CO formaton, J. of Power Sources 106, Agrell J. (003), Develoment of methanol-reformng catalysts for fuel cell vehcles. PhD thess, Royal Insttute of Technology, Stockholm. Amhlett J.C., K.A.M. Creber, J.M. Davs, R.F. Mann, B.A. Peley, D.M. Stokes (1994), Hydrogen roducton by steam reformng of methanol for olymer electrolyte fuel cells, Int. J. of Hydrogen Energy 19, Cho Y. and H.G. Stenger (00), Fuel cell hydrogen from methanol on a commercal Cu/ZnO/ ALO3, Al. Catal. B: Envron. 38, Dümelmann R. and A. Baker (199), Crtera for gradentless oeraton of nternal recycle reactors, Chem. Eng. Sc. 47, Fluent 6.3.6, Coyrght Fluent Incororated, Lebanon, NH, 001, htt:// Guchard A. (007), Étude exérmentale et modélsaton d échangeurs comacts multfonctonnels en réacton catalytque. PhD thess, Insttute Natonal Polytechnque de Toulouse 007. Gunter M., T. Ressler, R.E. Jentoft, B. Bems (1995) Redox behavour of coer oxde/znc oxde catalyst n the steam reformng of methanol studed by n stu x-ray dffracton and absorton sectroscoy, J. of Catal. 03, Iwasa N., S. Kudo, H. Takahash, S. Masuda (1993), Hghly selectve suorted Pd catalysts for steam reformng of methanol, Catal. Lett. 19, Jang C.J., D.L. Trmm, M.S. Wanwrght (1993a), Knetc study of steam reformng of methanol over coer-based catalysts, Al. Catal. A: Gen. 93, Jang C.J., D.L. Trmm, M.S. Wanwrght (1993b), Knetc mechansm for the reacton between methanol and water over a Cu/ZnO/AlO3, Al. Catal. A: Gen. 97, Karm A., J. Bravo, A. Datye (005), Nonsothermalty n acked-bed reactors for steam reformng of methanol, Al. Catal. A: Gen. 8, Lndström B. and L. Pettersson (001), Hydrogen generaton by steam reformng of methanol over coer-based catalyst for fuel cell alcatons, Int. J. of Hydrogen Energy 6, Lndström B. and L. Pettersson (00), Steam reformng of methanol over coer-based monolths: the effects of zrcona dong, J. of Power Sources 106, Patankar S.V. (1980), Numercal Heat Transfer and Flud Flow, Seres n Com. Meth. n Mech. and Therm. Sc., McGraw-Hll. Peley B.A., J.C. Amhlett, L.M. Kearns, R.F. Mann (1991a), Methanol-steam reformng on Cu/ZnO/ALO3. Part 1: the reacton network, Al. Catal. A: Gen. 179,1-9. Peley B.A., J.C. Amhlett, L.M. Kearns, R.F. Mann (1991b), Methanol-steam reformng on Cu/ZnO/ALO3 catalysts. Part : A comrehensve knetc model, Al. Catal. A: Gen 179, Purnama H., F. Grgsdes, T. Ressler, J.H. Schattka, R.A. Caruso, R. Schomäcker, R. Schlögl (004a), Actvty and selectvty of a nanostructured CuO/ZrO catalyst n the steam reformng of methanol, Catal. Lett. 94 Nos 1-. Purnama H., T. Ressler, R.E. Jentoft, H. Soerjanto, R. Schlögl, R. Schomäcker (004b), CO formaton/selectvty for steam reformng of methanol wth a commercal CuO/ZnO/AlO3 catalyst, Al. Catal. A: Gen. 59, Ranganathan E.S., S.K. Bej, L.T. Thomson (005), A methanol steam reformng over Pd/ZnO and Pd/CeO catalysts, Al. Catal. 89, Takeguch T., Y. Kn, M. Inoue, K. Eguch (00), Steam reformng of methanol on coer catalysts suorted on large-surface-area ZnAlO3, Catal. Lett. 83 Nos 1-. Takezawa N. and N. Iwasa (1997), Steam reformng and dehydrogenaton of methanol: dfference n the catalytc functons of coer and grou VIII metals, Catal. Today 36, Zhao T., K-D. Krever, T. Van-Nguyen (007), Advances n Fuel Cells, Elsever Ltd 4