Simulation of Surface Chemical Reactions in a Monolith Channel for Hydrogen Production

Transcription

1 Except fom the Poceedgs of the COMSOL Confeence 008 Hannove Smulaton of Suface Chemcal Reactons a Monolth Channel fo Hydogen Poducton N. Pacheco *,1, D. Pavone 1,. Sula 1, J.L. Houzelot and E. Schae 1 IFP-Lyon, ENSIC *Coespondg autho: IFP-Lyon, BP3, Solaze, Fance, nuno-manuel.pacheco-magalhaes@fp.f Abstact: Ths pape tends to show a model of a monolthc eacto fo the autothemal efomg pocess (ATR). Ths pocess uses hydocabons (fossl fuels o bofuels) to poduce H. The ATR chemcal eactons take place on the suface of monolth channels coated wth a catalyst. The sothemal ATR eacto s modeled usg catalytc suface chemcal eactons that volve 13 sold speces and 7 gas speces. To solve the model, two numecal technques ae compaed: the Suface Model usg weak fom uatons (SM) and a standad th Volume Model (VM). Ths pape shows that the suface modelg (SM) s the bette way to calculate the monolth catalytc eactons. Because ths smulato s a pelmay veson not yet expementally valdated, and ode to vefy the smulato esult consstency, esults ae compaed wth themodynamc ulbums calculated by COMSOL. The conclusons ae that the esults gven by the dynamc smulato ae ageement wth the themodynamc ulbums. eywods: Autothemal Refomg, Monolth, Methane, Hydogen, Suface Chemcal Reacton, Weak Fom, Themodynamc Equlbum 1. Intoducton IFP has been developg economc and clean pocesses fo hydogen poducton fo seveal yeas. [1] Two ma souces ae consdeed: hydocabons and bomass. The hydogen s nowadays consdeed as a pomsg enegy souce fo the futue. It appeas as an teestg way to lmt the use of fossl fuels and t could educe the emssons of geenhouse gases. Howeve, the hydogen s not a pmay souce of enegy and theefoe t must be poduced hence, the hydogen poducton tself must use low emssons of CO technques. One effcent way to poduce H s va Autothemal Refomg (ATR). Ths pocess s maly based thee chemcal eactons: CH + + H O CO 3H O CO + H O + H O CO H (1) I CH + () I CO + (3) I Howeve, ths chemcal eacton system needs to be catalyzed fo the pocess optmzaton. To hold the catalyst, IFP decded to use monolths the eacto (Fgue 1). Fgue 1. Monolth examples Fom a pocess pot of vew, the monolth offes advantages lke low pessue dop and no fe patcle poducton. In addton, fom a eseach pot of vew, the monolth channel geomety s well-known and hence can be pefectly descbed COMSOL, whch s not the case fo andom loadg packed beds.. Objectve The am of ths wok s to buld an ATR monolth eacto smulato usg COMSOL. It wll take multphyscs to account, maly mass balances cludg a suface chemcal eacton mechansm volvg chemcal eactons, 7 gas speces and 13 adsobed speces. The monolth consdeed ths wok has deposted ove ts walls a washcoat mpegnated wth catalyst stes. Ths chemcally actve egon wll be modeled consdeg t numecally as a suface (SM) o as a th volume (VM). The objectve s to use the best numecal way to smulate such a th secton of catalyst. In addton, the themodynamc ulbums wll be calculated COMSOL to be compaed

2 wth the numecal smulaton as an altenatve to the expemental data that ae not yet avalable. Fally, ths smulato wll be helpful to mpove the pefomance of the ma pocess, to choose the ght geomety fo the monolth channels and to deve knowledge ab suface chemcal eactons and mass/enegy tansfe between gas and sold. The smulato benefts fom pevous modelg eseach developed at IFP. [] 3. Monolth Model Descpton 3.1 Geomety A monolthc eacto s modeled consdeg only one channel. To smplfy calculatons, ths channel s assumed to be a cyldcal tube. Theefoe, an axal symmetc geomety s employed. The catalytc laye s modeled usg the two appoaches pesented Fgue : one models catalyst as a suface (SM) whle the othe one a th volume (VM). z Gas Outlet Gas Phase Gas Inlet Catalyst Suface Channel wall Fgue. Two monolth channel geometes: catalytc laye as a suface ( o as a th volume (. The geometc paametes ae pesented Table 1: Table 1. Geometc paametes Channel adus Washcoat thckness Length Gas Outlet Gas Phase Gas Inlet Catalyst Laye Channel wall mm mm 00 mm 3. Mass Tanspot In the gas phase, the convecton and dffuson of all the 7 gas speces (N, H, O, CO, CO, H O and CH ) ae consdeed, hence the "Convecton and Dffuson" applcaton of "Chemcal Engeeg" module s used (). Fo the gas speces, the dffuson coeffcent s assumed to be constant and ual to 10-5 m /s. Only catalytc eactons ae consdeed, theefoe any chemcal eacton occus between gas speces. In the sold phase, the model consdes the chemcal eactons and the dffuson fo the 7 gas speces. No convecton exsts the sold phase. The Deutschmanns [3] mechansm s used wth suface chemcal eactons, volvg 13 suface adsobed speces and the 7 gas speces. The 13 suface adsobed speces ae: H Os, OHs, Hs, Os, Cs, COs, CO s, CH s, CH 3 s, CH s, CHs, HCOs and N. No poosty effects ae consdeed ths wok. The mass balance uaton s defed as: C t ( D C + C u) R k + = k k. () I Whee C k epesents the concentaton of the spece k, R s the souce tem defed as: R = ν (5) I = k ν j C (6) I Whee ν s the stochometc coeffcent, s the tansfomaton ate, k s the ketc constant and C j s the concentaton of the spece j. Sold catalytc modelg Dependg on the way the geomety s modeled (SM o VM), two dffeent COMSOL applcatons ae used fo the mass balance uaton the sold phase: Bounday Weak Fom o Dffuson. Fo the catalytc suface modelg (SM), the "Bounday Weak Fom" fom the "PDE Modes" s used, whee the mass balance uaton fo the sold phase weak fom s: j weak = c_test * R (7) I dweak = c_test*c_tme (8) I

3 Fo the standad modelg of catalysts as a th volume (VM), the Dffuson applcaton fom Chemcal Engeeg Module s employed. Hee, the dffuson coeffcent s 0 fo adsobed speces. The suface eacton mechansm s conveted to a volume eacton mechansm consevg the total numbe of catalytc stes. Theefoe, all the ketc constants ae dvded by the catalyst thckness. 3.3 Bounday Condtons The bounday condtons fo the gas phase ae shown Fgue 3. In the case of the suface modelg (SM) the souce tem fo the suface s conveted to flux bounday condton fo the gas phase. In the othe modelg case (VM) boundaes ae obvous. Convectve Axe Fgue 3. Bounday condtons fo gas phase the suface case ( and contutes fo the volume case (. 3. Othe Consdeatons Fo the hydodynamcs, the model uses the well known soluton fo lama Nave-Stokes uatons.e. the paabolc pofle. Consuently, the velocty feld s: u u z = 0 = v Axe j 1 Convectve R Insulaton (9) I Whee, v j s 0.8 m/s, s the adal coodate and R the tenal channel adus. The model s solved usg the dect UMFPAC method the tme dependent mode, consdeg that all tal values ae ual to 0.. Themodynamc Equlbum The themodynamc ulbums ae helpful to compae wth numecal solutons when expemental data s not avalable because the themodynamc mposes the chemcal conveson lmts. The calculaton obeys to: 1. the atomc mass balance (fou atoms then, fou atomc mass balances (10) - (13)). to the mmum of Gbbs enegy (ulbum condton (1) and (15)) 3. the assumpton that oxygen s totally conveted F F F ( N ) F ( N ) ( O ) + F O) = F ( CO ) + F ( CO) + F O) O) + F ( CH ) = F O) + F ( CH ) + F ) F = (10) ( CH ) = F ( CH ) + F ( CO ) + F ( CO) (11) (1) (13) The ulbum constants ae tempeatue dependant (16).The paametes uaton (16) ae pesented Table fo the eactons. = e a + b+ c.log T CH 3 H PCO P ( 1) = (1) P P CO H O PCO P H ( 3) = (15) P P H O 3 ( T) + d.t+ e.t + f.t (16) Ths calculaton s set as "Global Equatons" COMSOL.

4 Table. Equlbum coeffcents Coeffcent Reacton (1) Reacton (3) a b c d E E-3 e -1.17E E-7 f 9.795E-11 1.E-10 Howeve, ths uaton set can have multple solutons especally ones wth negatve concentatons. To avod ths poblem and to get only one soluton set, we swtched the COMSOL unknowns fom C (concentatons) to log(c). Hence, when we get the soluton, we ae ceta that C = exp(log(c )) s postve. Ths technque woks vey well and has the advantage to gve se to homogeneous unknown scalg. 5. Results 5.1 Suface (SM) and Volume Modelg (VM) compason To smplfy, the smulaton pesented hee just volves O. O eacts along the suface and poduces two adsobed Os. The fal smulaton tme s 1s, shot enough to catch the font the mddle of the channel. Fo these two cases (SM and VM), Fgue exhbts smla pofles of oxygen concentaton the gas phase. Ths poves that the two models ae uvalent. Howeve, a slght delay between oxygen font waves can be obseved. It s due to the catalyst gas poosty that s not smulated the SM case but acts as a sk the volume case (VM). Because the poosty gas volume the catalyst s vey low, the dscepancy the two kds of smulaton s lght. Fom global smulaton pot of vew, unde the same condtons, the th volume case (VM) takes much moe tme to calculate the soluton than the suface one (SM). Futhe moe, wheneve usg the whole chemcal eacton set the smulato becomes numecally stable leadg to vey long smulaton tmes. Consuently, the ogal suface modelg case (SM) wll be used fo the next calculatons. 5. ATR Reacto Model Results These esults wee smulated a dynamc mode untl acheves the statonay state (ab 100s). The followg condtons wee taken to account: Table 3. Injecton Condtons Gas Total Flow.69E-07 m 3 /s Mola Facton O CH H O N The mesh used s quadangula wth 100 cells z decton and 5 cells decton wth an element ato of 5 fom the wall towads the z axs. The concentaton pofles and the let concentatons ae shown Fgue 5. H O CH H O CO CO N 9.5% 0% <0.1% 0.5% 9.5% 0.5% 0.0% Hgh O Low O Fgue. Oxygen concentaton pofle the suface case (, and the volume case (. (Whte le epesents 10% of O jecton value) Low Concentaton Hgh Fgue 5. Concentaton pofles the monolth channel. Almost all of the conveson can be obseved the vey begng of the eacto.

5 Lookg at the O pofle, we can say that oxygen conveson () s fast and total. Ths fact, plus the cease of H O and CO pove that the combuston s the fastest eacton. Ths was expected because the combuston s moe spontaneous than steam efomg (1) o wate gas shft (3). The methane conveson s completed latte the channel by steam efomg eacton (Eq 1), poducg H. Fally, the wate gas shft eacton occus though the ente eacto, poducg moe H and consumg the unwanted CO. The Fgue 6 pesents the suface mola facton fo the thee ma speces, the othe speces ae not shown because they ae less than 1%. commecal softwae to ensue that the ulbums ae well calculated. The eacto eaches the ulbum state at the eacto fst half, whch means that only 10cm ae necessay to obta the maxmum conveson. The esults fom the smulaton agee wth the themodynamc ulbum calculatons. 6. Conclusons Fom the modellg pot of vew, t can be concluded that the suface modellg (SM) that uses weak fom uatons, gves good esults. The monolthc ATR eacto sothemal condton seems to be coectly descbed usg a chemcal eacton mechansm of eactons and volvg 0 dffeent speces. Moe ove the uaton set leads to convesons that ae close to the themodynamc ulbums. Fally, the themodynamc calculaton eveals to be a useful tool sce t can be quckly compaed wth the smulaton esults, whle watg fo expemental data. Howeve, some numecal tcks need to be taken to easly mplement ths kd of calculaton COMSOL. 7. Acknowledgments Fgue 6. Mole facton of adsobed speces the sold phase Fom numecal esult we can obseve that moe than 50% of catalytc stes ae fee. In tems of coveage ato, the most sgnfcant speces ae the Os, the Hs and the COs. Consuently, t can be sad that all eactons whee these speces ae volved ae vey sgnfcant. 5.3 Themodynamc ulbum compason The themodynamc ulbums ae calculated to be compaed wth smulaton esults. Themodynamc ulbums can be calculated COMSOL, as explaed paagaph. We have vefed these esults wth The authos thank ANRT fo the fancal suppot. Nuno Pacheco s gateful to IFP and ENSIC fo povdg a doctoal gant. 8. Refeence 1. IFP - Hydogen Poducton and Tanspotaton, (01/09/08). Pavone, CO captue by means of chemcal loopg combuston, Comsol Multphyscs Confeence, , (005) 3. Deutschmann, O., Tsche, S., Schaedel, B., and Mae, L., Suface eactons: Refomg and oxdaton of methane on nckel, ml, (01/03/06)