Natural Gas Conversion in Monolithic Catalysts: Interaction of Chemical Reactions and Transport Phenomena

Transcription

1 Natural Gas Converson VI, Studes n Surface Scence and Catalyss 136, E. Iglesa, J.J. Spvey, T.H. Flesch (eds.), p , Elsever, 21. Natural Gas Converson n Monolthc Catalysts: Interacton of Chemcal Reactons and Transport Phenomena Olaf Deutschmann, Renate Schwedernoch, Luba I. Maer, Danel Chatterjee Interdscplnary Center for Scentfc Computng (IWR), Hedelberg Unversty Im Neuenhemer Feld 368, D-6912 Hedelberg, Germany Abstract The nteracton of transport and netcs n catalytc monolths used for natural gas converson s studed expermentally and numercally. The paper focuses on a precse flow feld agreement between experment and model. Therefore, we use extruded monolths wth rectangular channel cross-secton and a three-dmensonal Naver-Stoes smulaton ncludng detaled reacton mechansms and a heat balance. Latter also accounts for heat conductng channel walls and external heat loss. If a washcoat s used, a set of one-dmensonal reacton-dffuson equatons s addtonally appled for modelng the transport and heterogeneous reactons n the washcoat. Partal oxdaton of methane to synthess gas on rhodum coated monolths has been studed as example. 1. INTRODUCTION Monolthc catalysts are often appled for natural gas converson processes such as partal oxdaton of lght alanes [1, 2] and catalytc combuston [3, 4]. In partcular at short contact tmes, a complex nteracton of transport and reacton netcs can occur. Chemcal reactons can not only tae place on the catalytc surface but also n the gas phase as was shown for partal oxdaton of methane on rhodum at elevated pressure [5] and oxy-dehydrogenaton of ethane on platnum at atmospherc pressure [6]. In those studes, computatonal tools for the numercal smulaton of heterogeneous reactve flows were developed and appled to a twodmensonal smulaton of a sngle-channel n a foam monolth. In that wor, the complex shape of the pores of the foam monolth was descrbed by usng the smplfed model of a straght tube. A three-dmensonal smulaton of the catalytc partal oxdaton of methane to synthess gas n a wre gauze confguraton has been prevously performed by de Smet et al. [7] but a smple surface reacton model was used. The understandng of the detals of the reactor behavor demands a better agreement between expermental and modeled flow feld wthout neglectng the complex chemstry. Therefore, our present wor focuses on settng up an adequate experment where we use extruded monolths wth straght channels wth rectangular cross-secton. The model then apples a three-dmensonal flow feld descrpton coupled wth detaled reacton mechansms and an enthalpy governng equaton that ncludes a heat conductng channel wall.

2 The nner walls of the sngle channels of extruded monolths are frequently coated wth a thn layer of washcoat to enlarge the surface. Here, dffuson of reactants and products to and from the catalytc actve centers n the washcoat can lmt the total heterogeneous reacton rate [8]. Therefore, a set of one-dmensonal reacton-dffuson equatons s appled to account for washcoat dffuson n the numercal smulaton of the monolthc catalyst. As an example we wll dscuss experments and numercal smulatons carred out for the partal oxdaton of methane to synthess gas on a rhodum coated extruded monolth. 2. EXPERIMENTAL The expermental set up was desgned n a way that allows the applcaton of detaled models for the physcal and chemcal processes occurrng n the reactor. Experments were carred out n a tubular quartz reactor, 25 cm long and 2.6 cm n (nner) dameter. The tube contans a 1cm long extruded monolth wth a well-defned rectangular cross-secton (1mm x 1mm) of ts channels. The ceramc monolth made of corderte s coated wth the noble metal rhodum by saturaton wth an acdc aqueous soluton of Rh 2 (SO 4 ) 3, 24 hours dryng at 1 C, reducton n at 5 C, and calcnaton n ar at 5 C for 18 hours. Metal loadngs are 1 to 3% Rh by weght. Energy dspersve X-ray spectroscopy (EDX) pctures revealed that no sulfur compounds are left on the surface from the mpregnaton process. For the nvestgaton of washcoat dffuson, corderte monolths pre-coated wth an alumna washcoat are used. The entre reactor can be run ether auto-thermally or temperature-controlled by a furnace. The gas temperature at the ext of the catalytc monolth s determned by a thermocouple placed nsde a thn quartz tube to prevent catalytc reactons. Because of heat losses to the ambence, the measured ext temperature was always sgnfcantly lower than the adabatc reactor temperature would be CO.8 O 2 12 T pea Selectvty O Converson CH p [ ] CO 2 T ext Inlet mole rato CH 4 /O Inlet mole rato CH 4 /O 2 8 Fg. 1. Effect of methane/oxygen rato on selectvty, converson, and pea (smulaton) and outlet temperature; symbols = experment, lnes = smulaton. No washcoat s used.

3 The product composton s determned by gas chromatography (TCD, FID) and by quadrupole mass spectroscopy. The latter one can also be used for transent measurements, for nstants for studes of gnton and extncton phenomena. The reactor s operated at atmospherc pressure (1.1 bar). The total feed flow rate corresponds to a resdence tme of few mllseconds. Methane/oxygen mxtures were fed dluted by argon. In the experment, we have been studyng the effect of composton, flow rate, dluton, and preheat on selectvty and converson. Exemplary, Fg. 1 exhbts selectvty, converson, and outlet temperature as functon of the methane/oxygen rato. A good agreement s shown between expermentally determned and numercally predcted data, the latter acheved by the model dscussed below. 3. MODELING THE MONOLITH CHANNEL Even though the expermental measurements reveal that sgnfcant heat loss occurs, we smply assume for the model that every channel of the monolth behaves essentally ale. Thus radal profles over the monolth as a whole are neglected, and only one sngle channel has to be analyzed. The flow wthn these small dameter channels s lamnar. Because an objectve of ths study s an approprate agreement between expermental confguraton and flow feld model, we solve the three-dmensonal Naver-Stoes equatons for the smulaton of the rectangular shaped channel. These equatons are coupled wth an energy conservaton equaton and an addtonal conservaton equaton for each chemcal speces. The energy conservaton equaton accounts for heat transport by convecton and conducton n the gas phase, heat release due to chemcal reactons n the gas phase and on the catalytc surface, and heat conducton n the channel walls. Furthermore, we added an external heat loss term at the outer boundary of the channel wall to account for the expermentally occurrng heat loss. The temperature-dependent external heat loss was specfed so that the predcted outlet temperature agrees wth the expermentally measured temperature. Because selectvty and converson n catalytc partal oxdaton of lght alanes strongly depend on the spatal temperature profle, the detaled descrpton of the energy balance s crucal for the understandng of the reacton. The chemcal reactons are modeled by detaled reacton schemes for homogeneous as well as heterogeneous reactons. In the heterogeneous reacton model we apply the mean feld approxmaton. That means that the adsorbates are assumed to be randomly dstrbuted on the surface, whch s vewed as beng unform. The state of the catalytc surface s descrbed by the temperature T and a set of surface coverages Θ, both dependng on the macroscopc poston n the reactor, but averagng over mcroscopc local fluctuatons. Balance equatons are establshed to couple the surface processes wth the surroundng reactve flow. The producton rates s& of surface and gas phase speces (due to adsorpton and desorpton) s then wrtten as Ks = 1 N g + N = 1 ν s s& = ν ( c ) (1) f

4 wthk s = number of surface reactons ncludng adsorpton and desorpton, ν, ν = stochometrc coeffcents, f = forward rate coeffcent, N ( ) g N s = number of gaseous (surface) speces, c = concentraton of speces, whch s gven n mol cm -2 for adsorbed speces. Because the bndng states of adsorpton on the surface vary wth the surface coverage of all adsorbed speces, the expresson for the rate coeffcent becomes complex: f Ea N ε Θ β s = A T exp = Θ µ RT exp (2) 1 RT A = preexponental factor, wth reacton. Coeffcents µ and β = temperature exponent, and ε E a = actvaton energy of descrbe the dependence of the rate coeffcents on the surface coverage of speces. For adsorpton reactons, stcng coeffcents are commonly used. They are converted to conventonal rate coeffcents by S RT ads = (3) Γ τ 2π M f wth S = ntal stcng coeffcent, Γ = surface ste densty n mol cm -2, τ = number of stes occuped by the adsorbng speces, M = molar mass of speces. Whle the surface ste densty can be estmated from the catalyst materal, the nowledge of the rato of the actve catalytc surface area to geometrcal surface area s essental for the model. An exact value for ths rato has to be determned expermentally. In the smulaton dscussed here we smply use a value of unty, and the surface ste densty for rhodum s set to be mol cm -2. In spte of numerous surface scence studes on, CO and hydrocarbon oxdaton there s stll a substantal lac n netc data. Nevertheless, several surface reacton mechansms wth assocated rate expressons have been publshed for complete and partal oxdaton on noble metal catalysts n the last decade. Even though the mechansms are often based on few expermental data, whch were acheved for a lmted range of condtons, they led to a better understandng of the process. In the present study, the surface chemstry s descrbed by a detaled surface reacton mechansm that s under development for the descrpton of partal as well as complete oxdaton of methane on rhodum [9]. The mechansm conssts of 38 reactons among 6 gas phase speces and further 11 adsorbed speces, as shown n Table 1. Because the surface coverage s low for the condtons chosen n the present study, the dependence of the rate coeffcents on the surface coverage was neglected. However, t may become mportant at dfferent condtons. For more detals, we refer to a forthcomng paper n whch the establshment of the reacton mechansm wll be dscussed [9]. We would le to note that the present study does not focus on the development of the surface reacton mechansm but rather

5 on ts applcaton n mult-dmensonal smulatons that allow to descrbe the reactor behavor as adequate as possble. TABLE 1: SURFACE REACTION MECHANISM A E a (1) + 2 Rh(s) 2 H(s) s.c. (2) O Rh(s) 2 O(s) s.c. (3) CH 4 + Rh(s) CH 4 (s) s.c. (4) O + Rh(s) O(s) s.c. (5) CO 2 + Rh(s) CO 2 (s) s.c. (6) CO + Rh(s) CO(s) s.c. (7) 2 H(s) 2 Rh(s) (8) 2 O(s) 2 Rh(s) + O (9) O(s) O + Rh(s) (1) CO(s) CO + Rh(s) (11) CO 2 (s) CO 2 + Rh(s) (12) CH 4 (s) CH 4 + Rh(s) (13) O(s) + H(s) OH(s) + Rh(s) (14) OH(s) + Rh(s) O(s) + H(s) (15) H(s) + OH(s) O(s) + Rh(s) (16) Rh(s) + O(s) H(s) + OH(s) (17) OH(s) + OH(s) O(s) + O(s) (18) O(s) + O(s) OH(s) + OH(s) (19) C(s) + O(s) CO(s) + Rh(s) (2) CO(s) + Rh(s) C(s) + O(s) (21) CO(s) + O(s) CO 2 (s) + Rh(s) (22) CO 2 (s) + Rh(s) CO(s) + O(s) (23) CH 4 (s) + Rh(s) CH 3 (s) + H(s) (24) CH 3 (s) + H(s) CH 4 (s) + Rh(s) (25) CH 3 (s) + Rh(s) C (s) + H(s) (26) C (s) + H(s) CH 3 (s) + Rh(s) (27) C (s) + Rh(s) CH(s) + H(s) (28) CH(s) + H(s) C (s) + Rh(s) (29) CH(s) + Rh(s) C(s) + H(s) (3) C(s) + H(s) CH(s) + Rh(s) (31) CH 4 (s) + O(s) CH 3 (s) + OH(s) (32) CH 3 (s) + OH(s) CH 4 (s) + O(s) (33) CH 3 (s) + O(s) C (s) + OH(s) (34) C (s) + OH(s) CH 3 (s) + O(s) (35) C (s) + O(s) CH(s) + OH(s) (36) CH(s) + OH(s) C (s) + O(s) (37) CH(s) + O(s) C(s) + OH(s) (38) C(s) + OH(s) CH(s) + O(s) The unts of A are gven n terms of [mol, cm, s] and of E a n [J/mol]. s.c. = ntal stcng coeffcent. If the nner walls of the sngle channels are coated wth a thn layer of washcoat the catalytc actve surface area can easly be ncreased by a factor of n comparson to the geometrcal surface area of the nner wall of the monolth channel. Dffuson and reacton

6 wthn the washcoat pores leads to local concentraton gradents that have to be resolved to calculate the total heterogeneous reacton rate. Therefore, we apply a set of one-dmensonal reacton-dffuson equatons to calculate the speces concentratons and surface coverages wthn the washcoat as functon of the dstance (r) from the gas-washcoat nterface: j γ s& =. (4) r Here, s& = local molar reacton rate of gas phase speces due to adsorpton and desorpton, γ = actve catalytc surface area / washcoat volume, j = mass dffuson flux of speces. Dependng on the washcoat structure molecular or Knudsen dffuson coeffcents have to be used. The number of coupled nonlnear reacton-dffuson equatons equals the number of gas phase speces. In addton to ths equaton set, one addtonal algebrac equaton, s& =, has to be solved for each surface speces. The equaton smply says that the surface coverage has to be constant when the steady state s reached. The numercal smulaton s based on the computatonal flud dynamcs code FLUENT [1] whch was coupled to the chemstry tool DETCHEM [5, 11] va FLUENT s nterface for user defned subroutnes. DETCHEM models the chemcal processes n the gas phase and on the surface ncludng the surface coverage calculaton based on mult-step chemcal reacton mechansms. Addtonally, several washcoat models can be ncluded [12]. Due to symmetry, only an eghth of the channel cross secton has to be smulated, for numercal reasons we smulated a quarter nstead. 4. RESULTS AND DISCUSSION The sngle channel of the catalytc monolth s smulated under condtons as chosen n the experment. The methane/oxygen mxture, dluted by 75 vol.% argon, flows at 3 K and 1.1 bar wth a unform velocty of.26 m/s (correspondng to 7 slpm over the whole monolth) n the rectangular shaped monolth channel. The smulated channel s 1.1 cm n length wth the frst mllmeter beng non-catalytc. In the smulaton and n the experment, the reacton has to be gnted. In the experment the reactor s nsde a furnace whch s heated up to ntate the reacton; after gnton the furnace s swtched off, the reactor s operated auto-thermally. In the smulaton, the channel wall s gven a suffcently hgh temperature to gnte the reacton. Then, a heat conductng wall s assumed ncludng external heat loss. In Fgure 1, a good agreement s shown between expermentally determned and computed selectvty and converson. The syngas selectvty and methane converson are lower than the data reported by the Schmdt group [1] due to sgnfcant heat loss n the reactor. The computed pea temperature s much hgher than the ext temperature, not only due to heat loss but also due to endothermc steam reformng. The three-dmensonal smulaton ncludng the detaled reacton mechansms allows us to study ths behavor n more detal. In prevous studes [5], we used the surface reacton mechansm proposed by Hcman and Schmdt n ther poneerng wor n 1993 [1], n whch

7 steam and CO 2 reformng s not sgnfcant. Because of new expermental studes [9], at least steam reformng seems to be an mportant reacton step. Among other reasons, ths fact led to the development of the revsed reacton mechansm. In Fgure 2, the profles of the speces mass fractons for a CH 4 /O 2 vol. rato of 1.8 reveal fast O 2 consumpton at the catalyst leadng edge whle CH 4 s consumed over almost the whole length of the reactor. Complete oxdaton taes place at the catalyst entrance only, where O 2 s stll avalable. Because CH 4 as well as re-adsorbed O decompose nto atoms on the surface (Table 1), t cannot be dstngushed between drect partal oxdaton of CH 4 to or steam reformng. At least further downstream s formed va steam reformng only. In contrast to that, CO 2 reformng does not occur. The temperature profles exhbt the strong axal and radal gradents due to chemcal reactons and heat transport. 7.3E-2 8.E-2 CH 4 3.2E-2 O 2 1.8E-5 6.3E-3 4.5E-2 O 6.E-2 CO 1.9E-2 CO 2 1.2E+3 T(K) 3.E+2 Fg. 2. Speces mass fractons and temperature n the monolth channel (1 mm x 1mm). The contour plots represent the dagonal face of the smulated channel secton reachng from the nner corner of the catalytc walls to the channel axs; the dagonal coordnate has been enlarged for vsual clarty, the total length s 1.1 cm wth the frst mllmeter beng noncatalytc (no wall s shown). The lower rght fgure shows the temperature profles n the channel wall, the nlet, and the front symmetry face at the catalyst entrance.

8 The smulaton also reveals that chemcal reactons n the gas phase are not sgnfcant at atmospherc pressure but become mportant above 1 bar. If the monolth s coated wth an alumna washcoat, the dameters of the pores are on the order of mcrometers. Molecular dffuson nsde the pores can lmt the overall reacton rate. In Fgure 3, we exemplary show computed concentraton and coverage profles nsde the washcoat at one mllmeter behnd the catalyst entrance. Here most of the oxygen s already consumed, and also the oxygen concentraton nsde the washcoat decreases rapdly due to catalytc reacton wth methane. Deeper nsde the pores, oxygen s vanshed and water reacts wth methane to form syngas and carbon doxde. Ths behavor explans why the product water shows ts hghest concentraton at the washcoat nlet. Concentraton [mol m -3 ] O CO 2 CH 4 CO O Dstance [ µ m] Surface coverage O(s) H(s) CO(s) C(s) OH(s) Dstance [ µ m] O(s) Fg. 3. Speces concentratons (left) and surface coverage (rght) nsde the washcoat. Washcoat parameter: thcness = 1 µm, porosty =.51, tortousty = 3, γ = 1 5 m -1. REFERENCES 1. D. A. Hcman and L. D. Schmdt, AIChE J. 39 (1993) M. Huff and L. D. Schmdt, J. Catal. 155 (1995) R. E. Hayes and S. T. Kolaczows, Introducton to Catalytc Combuston, Gordon and Breach Scence Publ., Amsterdam, L. L. Raja, R. J. Kee, O. Deutschmann, J. Warnatz, L. D. Schmdt, Catal. Today 59 (2) O. Deutschmann and L. D. Schmdt, AIChE J. 44 (1998) D. K. Zerle, M. D. Allendorf, M. Wolf, O. Deutschmann, J. Catal. 196 (2) C. R. H. de Smet, M.H.J.M. de Croon, R.J. Berger, G. B. Marn, J. C. Schouten, Appl. Catal. A 187(1) (1999) D. Papadas, L. Edsberg, P. Björnbom, Catalyss Today 6 (2) S. Tummala, L. D. Schmdt, O. Deutschmann, publcaton n preparaton. 1. Fluent Verson. 4.4, Fluent Inc., Lebanon, New Hampshre, O. Deutschmann, DETCHEM: Computer pacage for detaled chemstry n CFD codes, dmann/detchem.html, D. Chatterjee, O. Deutschmann, J. Warnatz, Faraday Dscussons, accepted for publcaton.