Znd. Eng. Chem. Res. 1994,33, 14-20
14
Effect of COz on Methane Oxidative Coupling Kinetics Kevin J. Smith' Department of Chemical Engineering, University of British Columbia, Vancouver, British Columbia, V6T 124, Canada
Jan Galuszka Energy Research Laboratories, CANMET, Ottawa, Ontario, K1A OGI,Canada
The kinetics of methane oxidative coupling have been investigated over a Li/Pb/Ca catalyst in the presence of C02 in the feed gas. The reduction in CH4 conversion and increase in C2 selectivity, observed when C02 is added to the feed gas, are accounted for by modifying rate equations for the heterogeneous reactions. The modification assumes that the number of active sites on the catalyst is determined by the equilibrium established between active Li202 and inactive Li2CO3. An EleyRideal type rate equation best describes a range of experimental data obtained at 700 "C, including data acquired with C02 added t o the feed. Furthermore, the observed beneficial effect of adding auxiliary 0 2 to the midpoint of the catalyst in a fixed bed reactor is predicted by the model. Accordingly, the auxiliary 02 is thought to regenerate the catalyst thereby increasing CHI conversion. Introduction Methane oxidative coupling (OCM) to ethane and ethylene has been the subject of much research in recent years. Most studies have aimed at the development of more active and selective catalysts (Keller and Bhasin, 1982;Pitchai and Klier, 1986; Scurrell, 1987;Amenomiya et al., 1990;Poirier et al., 1991). Fewer studies have dealt with the reaction kinetics of methane oxidative coupling, although it is now well established that, at high temperature (>700 "C) and with undiluted CH4/02 feed gas, homogeneous gas phase reactions are important together with the reactions that occur on the catalyst surface (Amenomiyaet al., 1990;van Kasteren et al., 1990;Mackie, 1991). Various studies have presented detailed accounts of the homogeneous gas phase reactions (Geerts et al., 1990; Mackie, 1991). The kinetics of the oxidative coupling reaction have been described by empirical power laws for both the rate of C2 formation (ethane plus ethylene) and for the rate of COXformation (carbon monoxide plus carbon dioxide). Recently, reported values of the exponents of the power law kinetics have been summarized for different catalysts (Amenomiyaet al., 1990). For example,with PbO catalysts the exponents (apparent reaction order) for the reaction CH4 CZare in the range 0.8-1.1 and 0-1.4 for methane and oxygen, respectively. For the reaction CH4 COX, the exponents for methane and oxygen are 0.4 and 0.71.8, respectively. Other kinetic models described in the literature consider the surface reactions and are based on or are consistent with some mechanistic proposal. On P b catalysts, for example, Asami et al. (1987) assumed that the rate-determining step (RDS) for C2 production was the formation of CH3 radical by reaction of CH4 with PbO and that the COXformation proceeded via the reaction between adsorbed oxygen and gas phase CH4. On the basis of the pseudo-steady-state hypothesis, two rate equations were derived that satisfactorily described their data. More recently, Lehman and Baerns (1992)reported a kinetics study of methane oxidative couplingon a NaOH/ CaO catalyst over a wide range of temperatures and 02 and CHI partial pressures. The C02 production rate data were best described by a power law that suggested that CH4 and 0 2 were dissociatively adsorbed on the catalyst
-
-
* Address correspondence to this author.
surface prior to formation of C02. The C2 production rate data were described by a Langmuir-Hinshelwood type kinetic model consistent with a mechanism in which the RDS was the reaction between weakly adsorbed CH4 and dissociatively or associatively adsorbed oxygen. Comparison of their kinetic models with other previously published models showed significant improvement although the general trends of their data were satisfactorily described by all models studied, based on an F-test of the ratio of variances of the models. Few of the kinetic equations described in the literature take account of the effect of product gases such as C02 on the heterogeneous rate of reaction. Also C02 addition to the feed stream of OCM has been studied and various effects have been reported. The first group to report some relevant data (Korf et al., 1987, 1989; Roos et al., 1990) concluded that C02 acted as a poison for the reaction of CH4 with 0 2 because of competitive adsorption on the sites where 0 2 and possibly also CH4 were adsorbed. On the basis of this model they derived kinetic expressions for the rate of CHI consumption as a function of 02,CH4 and C02 partial pressures. For the Li/MgO catalyst they claimed (Korf et al., 1987,1990)that active sites are formed by the decomposition of Li2CO3 to Liz02 (or Li+O-) in the presence of oxygen. Addition of C02 to the feed also deactivated their catalyst, according to the reactions:
-
Li202 Li20 + (1/2)0,
-
(1)
Li20 + CO, Li,CO, (2) A similar set of equilibrium reactions between Na2CO3, NazO, and Na202 have been proposed to explain the performance of NazCOs/lanthanide oxide catalysts (Tong et al., 1990) and other Na-based catalysts (Campbell and Lunsford, 1988). The overallreaction equilibrium between the active site (for example Li202) and the deactivated catalyst site can be written as Li202+ CO,
-
Li,CO,
+ (1/2)02
(3) from which it is concluded that the extent of deactivation will depend on the partial pressures of C02 and 0 2 . Recently Suzuki et al. (1990) reported that the use of C02 as a reactive diluent for OCM increased the C2 yield and selectivity but lowered CH4 conversion for some MgOand Sm2Os-based catalysts, whereas for CaO and SrO
0888-588519412633-0014$04.50/0 0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994 15 catalysts, the presence of large amounts of Cop was detrimental. Formation of stable surface carbonate was claimed to be responsible for catalyst deactivation. Partial adsorption was thought to be responsible for inhibition of the deep oxidation of methane and Cp products. Lunsford et al. (1992,1993) presented a similar view. They stated that the enhanced C2 selectivity on Li/MgO could be attributed to the poisoning effect that Cop had on the secondary reactions between alkyl radicals and the surface modified by the presence of Cop. They also noted a negative effect of Cop on CH4 conversion.Aika et al. (1990) claimed Cp yield improvement over PbO/MgO and alkaline earth metal doped CaO catalysts through involvement of Cop as a methane oxidant. There is general agreement, therefore, that Cop added to the feed stream or produced during OCM accumulates on the catalyst surface, eliminating some active sites. The extent of the overall effect seems to be dependent on the catalyst and process conditions, which may explain some differences among the reported results. However, there seems to be agreement that Cop addition has a negative effect on CHI conversion. In our recent study (Zhou et al., 1993) of threecomponent Pb-based catalysts, it was shown that Cop addition to the feed gas resulted in deactivation of a Li/ Pb/Ca catalyst, reflected in lower Cp yield and CHI conversion. The degree of Cp selectivity improvement was not sufficient to offset the detrimental effect of C02 on CH4 conversion. Furthermore addition of oxygen to the midpoint of the same Li/Pb/Ca catalyst in a fixed bed reactor resulted in an increase in CH4 conversion without a reduction in CZselectivity (Smith and Galuszka, 1992). This observation was explained (Smith et al., 1991) in terms of 02 regenerating the deactivated catalyst surface, presumably by carbonate conversion to an active oxide (the reverse of reaction 3), thereby increasing the CH4 conversion. Since the kinetics of a heterogeneous catalytic reaction depend directly on the number of active sites, the methane oxidative coupling kinetic equations should consider the loss in active sites by Cop deactivation accordingto reaction 3. For plug flow reactors operated in the integral mode, such a correction is essential since the number of active catalyst sites will not be constant throughout the reactor but will depend on the COz and 0 2 partial pressures at each location within the reactor. Kinetic equations described in the literature implicitly assume a constant number of active sites throughout the catalyst bed. In the present work, the kinetics of the methane oxidative coupling reaction over a Li/Pb/Ca catalyst have been investigated. The objective was to develop a kinetic model that accounts for the effect of Cop on CHI conversion and Cp selectivity observed for this catalyst and reported elsewhere (Zhou et al., 1993). The model is based on an estimate of the number of active sites assuming an equilibrium between lithium carbonate and oxide surface species. PbO did not form stable carbonates at the operating conditions, and although calcium carbonates were stable, their contribution to the overall effect of Cop was relatively minor for Li/Pb/Ca catalysts as has been discussed elsewhere (Zhou et al., 1993). Futhermore, the kinetic model has been used to predict the effect of auxiliary oxygen addition within the catalyst bed in an attempt to clarify the experimental observations of Smith et al. (1991) and Smith and Galuszka (1992) in which a beneficial effect of the auxiliary oxygen was reported.
Experimental Section The Li/Pb/Ca catalyst was prepared by adding dropwise an appropriate amount of Pb(N03)p to Ca(0H)p (Fisher, Certified). The Li promoter was added as LiOH to the resulting paste which was subsequently extruded, dried at 120 "C for 2 h in air, and calcined at 800 "C for 1h. After coolingin a desiccator, the 1/16-in.extrudates were crushed and sieved to -20/+40 mesh particles. Air contact was minimized following calcination by rapid cooling in a desiccator and storing the sized material in an airtight container. This procedure limited the amount of Ca(0H)p and CaC03 formed by moisture and Cop contact with the CaO. The resulting Li/Pb/Ca catalyst had a surface area of 0.6 m2/g and a composition of Lip0:PbO:CaO = 6.7: 17.1:76.4 w t %. The catalysts were tested in a 1/2-in.-diameter fixed bed quartz microreactor placed in a tubular furnace. The reactor was operated isothermally and the temperature was controlled using an ERO Electronics Model 548 temperature controller with a thermocouple placed in the middle of the catalyst bed used as the measuring element. The reactor was loaded with 3 g of catalyst prior to cofeeding CH4/0p in the desired ratio to the reactor. The CH4/0p feed gas was diluted in He and contained approximately 90% by volume He. To investigate the influence of Cop on the catalyst performance, Cop was included in the He/CH4/0p feed gas, the Cop replacing an equivalent amount of He. Product and unreacted feed gases were analyzed using two gas chromatographs, fitted with a flame ionization (FID) and thermal conductivity (TCD) detector, respectively. The auxiliary 02 addition experiments were performed by adding an auxiliary feed stream of Op/He or He at the middle of the catalyst bed through a quartz dip tube (3mm diameter). The auxiliary OdHe addition experiments have been compared to experiments where the Op/He auxiliary stream was replaced by inert He at the same auxiliary feed gas flow rate. This ensures the same superficial gas velocity through the catalyst bed in the absence or presence of 0 2 in the auxiliary feed stream. Additional experimental details have been given elsewhere (Zhou et al., 1993; Smith et al., 1991). The catalysts were tested at atmospheric pressure in the CHdOdHe feed gas or the CHdOdCOdHe feed gas, at reaction temperatures of 700 "C. The contribution of gas phase reactions to the measured performance data is expected to be minor at these conditions. The feed gas + Poz)/Ptotal)lis dilution ratio, Pr, of 0.1 [Pr = (PcH, significantly lower than the ratio of about 0.3 identified by previous investigators as the minimum required for gas phase reactions to be of significance ( > 5 % methane conversion) (Kalenik and Wolf, 1990). However, it should be noted that contrary to the blank runs of Lehman and Baerns (1992) and despite our own measurements made with the reactor filled with inert Si-A1 that gave CH4 conversions less than 1%at 700 "C (Smith, 1990), we do not consider the low conversions obtained from these tests as a valid criterion for excluding the contribution of gas phase reactions to the catalytic reaction. As observed by Amenomiya et al. (1990), OCM catalysts are needed to produce CH3 radicals which then react in the gas phase. Therefore the evaluation of CH4 conversions in only thermally stimulated OCM, carried out in an empty reactor or one filled with inert, is not an adequate test to judge the extent of gas phase reactions in the presence of the OCM catalysts. The more appropriate assumption pertinent to the experimental conditions reported here is that the surface of the catalyst governs the rate-determining
16 Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994
step. Accordingly, the model assumes that the whole kinetics are governed only by heterogeneous reactions. Inclusion of the homogeneous reactions that may become significant at higher temperatures and in a less dilute feed gas will follow directly from the published kinetics of the homogeneous reactions (Geerts et al., 1990; Mackie, 1991; Santamaria et al., 1991).
Model Development In the present study, kinetic equations for the heterogeneous oxidative coupling reactions have been modified to account for the interaction of COZwith the catalyst. The modification is based on the assumption that the surface of the catalyst contains a constant number of potentially active sites (A,) and that at any given moment some of these sites (Ad) are deactivated by COZadsorbed on the surface. The remaining part of the surface consists of active sites (A,). Therefore at any given moment [A,] = [A,] + [Ad] (4) and the surface coverage by active sites can be expressed as follows: 6, = [A,] / [A,] (5) Assuming peroxide to be an active site, the process of deactivation can be written similarly to the equilibrium reaction 3: A, + CO2(g) A d + (1/2)02(g) (6) The overall equilibrium of eq 6 is assumed to be given by the parameter K4 (a pseudoequilibrium constant) according to the equation (7) Hence the surface coverage by active sites is given by 6, = 1/(1+ K4Pco,iPo21i2)
Table 1. Kinetic Equations for Methane Oxidative Coupling Modified for the Effect of COr model
cz
rate of formation c02
ref
gas phase or oxidized to COZ,the fraction x will be relatively small. Since only a fraction 7 of the active sites is in the transient state of reactivation, the surface coverage by these sites is equal to Ot = 7[A,I/[AJ
= 76,
(10)
Similarly, for the surface coverage by CH3 species, OCH, = x6a
(11)
and for the fraction available for methane interaction, ,6 = t e a (12) The rate of carbon oxide formation is assumed to be determined by the reaction between the adsorbed CH3 species and the surface mobile oxygen species being in a transient state of reactivation following CH4 splitting. Hence, rCO,
= k18cH!t
= k,XT6:
(13)
If for the purpose of this derivation we assume that the adsorption is of the Freundlich type, eq 13 can be written in the form
(8)
The derivation of the surface coverage by active sites is based on the purely chemical process represented by reaction 3 and eq 6. In practice, however, OCM catalysts are usually multicomponent systems with complex adsorption/desorption characteristics for COZand 0 2 . Both COZ and 0 2 can be retained on the surface in forms different from that identified in reaction 3. From the point of view of the kinetic model, however, reaction 3 and eq 6 suffice for the description of the COz/Oz surface interactions. Temperature is also an important factor influencing the abundance of different surface forms of COZand 0 2 . A recent study by Galuszka (1993) of the Li/MgO catalyst showed that the system reaches an equilibrium between the amount of CO2 retained on the surface, the amount of 02 utilized, and the extent of oxidative coupling of methane to C2 products. At low temperatures, CO2 accumulates on the surface to the point of complete deactivation of the catalytically active sites, leading the system to selfextinction. Therefore, low concentration of COZ resulting from operation at differential conversion levels does not preclude its inhibitory effect as proposed by Lehman and Baerns (1992). The active sites of the catalyst (A,) will include a fraction T that has split a methane molecule and is being reactivated, a fraction x that has adsorbed CH3 species, and a fraction 5 that is vacant and able to interact with methane. Therefore,
r+x+t=l (9) Since methyl species are either readily desorbed to the
and
If CH4 and 0 2 are dissociatively chemisorbed as proposed by Lehman and Baerns (19921, then a = 0.5 and b = 0.5. If the rate of Cz formation is proportional to the coverage by the surface oxygen species and CH4 adsorbs on the fraction of sites available for CHI interaction, then
rcz = k'6,0t
= k'[rO:
(16)
and
assuming the adsorption is of the Freundlich type. If the CZformation results from an Eley-Rideal type reaction between gas phase CH4 and adsorbed oxygen as has been proposed (Shimada and Galuszka, 1992), and the oxygen is associatively adsorbed according to Langmuir-Hinshelwood kinetics, then (18) and
In a similar manner the model equations of Table 1 were derived. Models 1, 2, and 3 assume a Langmuir-
Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994 17 Table 2. Kinetic Data without COz in the Feed (Temperature = 700 O C ; Catalyst Mass = 3 g; Pressure = 1 atm) feed (pmol/s) CHI 0 2 He 6.83 2.24 65.81 10.96 2.23 60.05 2.21 0.93 33.25 6.45 3.22 97.41 3.31 1.63 49.79 4.81 2.33 64.99 4.85 2.32 64.95 4.95 4.93 63.96 2.53 4.83 64.39 9.35 4.98 60.32 4.99 2.40 65.19 4.52 2.24 67.60 2.31 2.26 69.79 1.09 2.21 69.27 5.21 2.60 66.55 5.21 5.21 63.95
product (pmol/s) CZ+ COX 0.35 0.82 0.53 0.93 0.14 0.48 0.29 0.83 0.18 0.62 0.25 0.76 0.32 0.71 0.37 0.81 0.19 0.55 0.67 1.27 0.27 0.81 0.23 0.72 0.12 0.49 0.04 0.32 0.28 0.79 0.28 0.63
CH~ conv ( % ) 22 18
34 22 30 26 28 31 37 28 27 26 32 37 26 23
c2+ sei(%)
46 53 37 41 37 40 47 48 41 51 40 39 33 20 41 47
Hinshelwood adsorption for oxygen with terms D1 and 0 3 in the denominator reflecting associative and dissociative oxygen adsorption, respectively. For the empirical power law model it was assumed that an Eley-Rideal type mechanism controls the C Zformation. Thus the denominator for rcz is not squared. The term Dz in the denominator of the rate equations presented in Table 1 accounts for the COZdeactivation. Note that with D Z= 1the kinetic equations simplify to the form that does not account for the effect of COz. Although many other forms of the rate expressions were examined, most were unable to describe the experimental data and are not discussed here. The model fitting procedure assumed an integral, fixed bed, plug flow reactor operated isothermally since the experimental data included results at high conversions. The parameters of the kinetic equations were estimated by a nonlinear regression routine based on Marquardt's compromise (Marquardt, 1963) that combines steepest descent and linearization algorithms. The objective function minimized in the regression routine was chosen as
where XCH~, is the methane conversion and S C ~is, the ~ C2 selectivity. Superscripts M and P correspond to the measured and predicted values, respectively. The methane conversion and Cp selectivity were calculated by simultaneously integrating the equations
dnco,ld W = rcoz where W is the catalyst charged to the reactor and ncOz and ncz are the molar flows of COZand C Zrespectively. ncOz and ncz were related to the CH4 conversion and C Z selectivity through the stoichiometry of the reactions. A Runge-Kutta-Gill procedure was used to numerically integrate eqs 21 and 22. Results and Discussion The data of the present study were obtained at 700 "C for a range of feed gas flow rates andcompositions. Table 2 summarizes the data without COZadded to the feed gas (Smith, 1990)and Table 3, the data with COZadded to the feed gas (Zhou et al., 1993). All the data reported were
Table 3. Kinetic Data with C 0 2 in the Feed (Temperature = 700 O C ; Catalyst Mass = 3 g: Pressure = 1 atm) feed (pmol/s) CHI 0 2 C02 He 5.21 5.21 1.49 62.46 5.21 2.60 2.45 64.10 5.21 5.21 3.72 60.23
product (rmolls) CH( cz+ C2+ COX conv(%) sel(%) 0.28 2.00 21 52 0.20 2.83 15 51 0.15 3.90 9 63
Figure 1. Product composition as a function of time on stream at 700 OC. Total feed flow rate = 98 cm3 (STP) min-l; 3 g of catalyst. (A) Feed gas CHJOdCOdHe = 6.7/3.3/3.3/86.7 % .(B)Feed gas CHJ 0 d H e = 6.7/3.3/90%. ( 0 )CH4; (m) C02; (+) C2.
obtained following an initial stabilization period of about 200 min. As shown in Figure 1,in the presence or absence of C02, the catalyst stabilizes within 200 min and the data reported herein are average values of measurements taken from 200 to 500 min time on stream. From Figure 1it may also be concluded that in the presence of COZthe catalyst reached stable operation faster than in the absence of COz. The data of Table 2 without COZadded to the feed were fitted to the equations of Table 1with Dz = 1,i.e., without any correction for the effect of COZ. The regression coefficients (R2)for the models range from 69 to 87 % with the empirical power law (model 4) giving the best fit to the data based on the R2 values (see Table 4). The goodness of fit for this model is shown in Figure 2. The model proposed by Lehman and Baerns (model 1)and the EleyRideal type equation (model 3) also gave reasonable fits to the data. The calculated F-statistic was above the critical value for each model, and it is concluded that all the models are statistically significant. The results of various power law correlations published in the literature for P b catalysts and fitted to the data of Table 2 are summarized in Table 5. The power law correlation of the present work, shown in Figure 2, gave a significantly better fit to the data than any of those using previously reported exponents for P b catalysts. Presumably this is due to the presence of Li in the three-component catalyst of the present work. The power law models also had calculated F-values greater than the critical value. The kinetic models of Tables 4 and 5 are unable to predict the deactivating effect of COz when it is added to the feed, as demonstrated for model 1by the data of Table
18 Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994 Table 4. Estimated Parameter Values for Kinetic Models in the Absence of C02 model 1
2
parameters
R2 (%)
kl = 89.4 pmol. g-1 s-1 atm-2 k2 = 15.5 pmol. g1 s-1 atm-l K3 = 9.4 atm-1 kl = 8.1 pmol.
80
residual standard calcd SSQ" error F-statistic
557
4.4
38.7
4
4
CHI 5.21 5.21 5.21 5.21
g-1 s-1 atm-1 K3 = 0.26 atm4,5 k l = 96.1 pmol. g-l s-l atm-2 kz = 15.4 pmol. g-1 s-l atm-1 K3= 26.9atm-1 kl = 5.1 pmol. gl s-l atm-1.3 k~ = 3.5 pmol. g-1 s-1 atmq.6 a = 0.9 b = 0.4 c = 0.3 d = 0.3
69
875
5.5
21.5
model 1
79
581
4.5
36.4 2
87
369
6.8
29.0
3
n 4
50
(I
~
10
CHdconv(%) meas wed 26 22 15 22 21 32 9 32
Cpsel(%) meas wed 41 43 51 43 52 43 63 43
parameters R2(%) k l = 90.3 pmol. g-1 s-1 atm-2 k2 = 18.2 pmol. g-1 s-1 atm-1 K3 = 3.7 atm-l K4 = 5.3 atmq,5 80 k1 = 9.3 pmol.
residual standard calcd SSQ" error F-statistic
1066
5.6
34.0
45
2978
9.6
7.0
85
830
4.9
48.2
88
662
4.6
32.5
gl s-l atm-l.5
70 1
0
He 66.5 64.1 62.5 60.2
kz = 22.1 pmol.
SSQ = sum of squares.
2o 10 0
feed (pmolls) O2 Con 2.60 0 2.60 2.45 5.21 1.49 5.21 3.72
Table 7. Estimated Parameter Values for Kinetic Models in the Presence of CO2
gl s-l atm-1.5
kz = 15.4 pmol. 3
Table 6. Effect of Cot in the Feed Predicted by Model 1 with 4= 1
20
30
40
50
60
70
Measured %
Figure 2. CH4 conversions (0) and Cz selectivities ( 0 )measured versus values calculated by the empirical power law of Table 4 at 700 "C, without COz in the feed.
6. Thus it is concluded that the effect of adding CO2 to
the feed is more than to simply reduce CHI and 0 2 partial pressures. Accordingly, the data of both Tables 2 and 3 were fitted to the kinetic modelsofTable to take account of the effect of C02 based on eq 6. The results of this regression analpis are presented in Table 7. All the models are statistically significant based on the calculated F-statistic. The goodness of fit for the two models with the lowest standard error (models 3 and 4) is shown in Figures
g-1 s-1 atm-l K3 = 0.1 atm4,5 K4 = 7.4 atm4,6 kl = 88.7 pmol. g-1 s-1 atm-2 kz = 20.7 pmol. g-1 s-1 atm-1 K3 = 13.0 atm-1 K4 = 7.2 atmq.5 kl = 5.4 pmol. gl 8-1 atm-1.3 kz = 5.8 pmol. g-1 s-1 atm-0.6 K4 = 6.3 atm-O.5 a = 0.9 b = 0.4 c = 0.3 d = 0.3
SSQ = sum of squares.
3 and 4, respectively. Of the models derived from mechanistic proposals, the regression analysis shows that the Eley-Rideal model (model 3) had the highest value of the F-statistic. The ability of model 3 to predict the effect of C02 added to the feed is shown in Table 8. The empirical power law does marginally better than model 3 based on the magnitude of the standard errors of the models, but this model has many more fitted parameters, some of which are not statistically significant. Hence the calculated F-statistic is lower thanfor models 1and 3. We therefore conclude that methane oxidative coupling kinetics Over a Li/Pb/Ca catalyst is best described by the Eley-Rideal type kinetics modified to account for the deactivating effect of co2. Accordingly, the extent of deactivation by CO2 is determined by the equilibrium of reaction 6 and from the point of view of the kinetic models the equilibrium is described by the magnitude of the model Parmeter K4. The assumed deactivation mechanism implies that the
Table 5. Power Law Fits to Kinetic Data in the Absence of C02 in the Feed rc, = klPChaPo,bpmol.gl.s-l rco, = k&H,CPo,d pmo1.gl.s-1 eq exponents estd paramso a b c d ref ki kz residual SSQ R2(%) standard error 0.8 1.1 0.4 1.5 Hinsenet al. (1985) 3.4 31.6 2096 25 8.4 1.1 1.4 0.8 1.0 Carreiroet al. (1988) 268 214 1829 35 7.8 0.9 0 0.4 0.7 Asami et al. (1987) 1.2 23.8 1386 50 6.8 1.0 0 0.5 0.5 Lehman and Baerns (1992) 1.5 14.0 1026 63 5.8 0.9 0.4 0.3 0.3b this work 5.1 3.5 369 87 6.8 Partial pressures in atm. a , b, c, and d also estimated parameters.
calcd F-statistic 5.0 8.1
15.0 25.5 29.0
Ind. Eng. Chem. Res., Vol. 33,No. 1, 1994 19 70 60
/ I
I
O'
8
50
50
20
2o
8
t
0 0
10
0 0
1 0 2 0 3 0 4 0 5 0 6 0 7 0 Measured % Figure 3. CHI conversions (0) and Cz selectivities ( 0 )measured at 700 OC versus values calculated by the Eley-Rideal model 3 of Table 7, with CO2 in the feed. 70 60
2
50
ae 8 4 0
-m
3
430 0
20 10
0 10
30 40 50 60 70 Measured % Figure 4. C& conversions (0) and C2 selectivities ( 0 )measured at 700 OC versus values calculated by the power law model 4 of Table 7, with CO2 in the feed. 0
20
Table 8. Effect of COt in the Feed Predicted by Model 3 feed (pmolls) CH4 conv (%) Cz sel ( % ) CHI 5.21 5.21 5.21 5.21
02 2.60 2.60 5.21 5.21
CO2 0 2.45 1.49 3.72
He 66.5 64.1 62.5 60.2
meas 26 15 21 9
pred 21 11 23 16
meas 41 51 52 63
pred 44 57 52 58
Table 9. Effect of Oxygen Addition at the Midpoint of the Catalyst Bed (Temperature = 700 O C ; Pressure = 1 atm; Catalyst Mass = 3 g; Primary Feed = 50 mL/min; CH4/02/ He = 13/6.7/80.3 mol % ) auxiliarv flow (mL/min) 50 50 a
(%)
0 6.7
meas 29 37
pred" 28 36
meas 42 45
pred" 49 49
Predicted by model 3 and the parameters given in Table 7.
number of active sites is determined by the COdO2 partial pressure ratio in the reactor and that increasing the 0 2 partial pressure will increase the number of active sites. Experimental evidence for this effect is presented in Table 9. Auxiliary OdHe, added to the midpoint of the Li/Pb/ Ca catalyst bed, increases the CH4 conversion without loss in C2 selectivity compared to the case of addition of inert He at the same flow rate (Smith et al., 1991). Table
0.001 0.01
0.1
1
10
100 1000 10000
K ~ atm0.5 ,
Figure 5. Influence of model parameter K4 on predicted CHI conversion and C2 selectivity with and without auxiliary 02 added at the midpoint of the catalyst bed. Open symbols, CHI conversion; closed symbols, CZselectivity. (O,.) with 02; (0, a) without 0 2 .
9 also presents the effect of auxiliary 0 2 addition as predicted by model 3 of the present work. The measured and predicted values are in reasonable agreement and show the same trends in CHI conversion and C2 selectivity upon addition of the auxiliary oxygen. Thus the kinetic equations provide a reasonable description of the effect of C02 on Li/Pb/Ca catalysts and are able to predict the observed increase in CH4 conversion, without loss in C2 selectivity, upon addition of 0 2 to the midpoint of the catalyst bed. This increase is not only due to methane oxidative coupling kinetics but is also due to an increase in the number of active sites as a result of regeneration by the auxiliary 0 2 . The present kinetic analysis of the effect of C02 is based on the complex interactions between C02 and the oxide catalyst surface. These interactions have been approximated by the chemical equilibrium between deactivated (carbonate) sites and active (peroxide) sites and quantified by the parameter K4. As was observed previously, this purely chemical process may be altered by the nature of the catalyst and operating conditions. Nevertheless, it is still informative to examine the apparent effect of K4 on conversions and selectivities. Large values of K4 are representative of catalysts that retain C02 on the surface. Small values indicate that the C02 inhibitory effect on the reaction rates is negligible. Figure 5 shows the predicted effect of K4 on conversion and selectivity, with and without the auxiliary 0 2 addition, assuming all other parameters are unchanged. The prediction shows that, in the case of high values of K4, addition of 7 % 0 2 in the auxiliary feed has little effect on conversion and decreases selectivity marginally. At low values of K4, addition of 02 significantly increases conversionand marginally increases selectivity. In practical terms, in the former case the surface is mostly covered by C02 (carbonate) and only a small fraction is available to interact with methane. Consequently the added 0 2 is not able to increase the population of active sites either through the reverse of reaction 6 or by direct reactivation. In the latter case the surface is carbon dioxide free and added oxygen can be effectively utilized to increase methane conversion. I t is evident, therefore, that the beneficial effect of the auxiliary 0 2 addition reported by Smith et al. (1991) may not be the same for all catalysts, consistent with our own experimental observations on various P b catalysts (Smith, 1990).
20 Ind. Eng. Chem. Res., Vol. 33,No. 1, 1994
Conclusions Rate equations for methane oxidative coupling, based on the mechanistic proposals of previous investigators (Korf et al., 1987, 1990; Tong et al., 1990; Lehmann and Baerns, 1992; Shimada and Galuszka, 1992) and the assumption that COz deactivates peroxide catalytic sites via carbonate formation, have been derived. The modified models are able to describe the observed increase in C2 selectivity and decrease in CH4 conversion when COZis added to the feed gas of a Li/Pb/Ca catalyst, in addition to the selectivity and conversion trends measured over a wide range of operating conditions. The best model based on the calculatedF-statistic assumed an Eley-Rideal type reaction mechanism. Furthermore, this model predicts the beneficial effect of auxiliary 02addition at the midpoint of the catalyst in a fixed bed reactor. Simulating the effect of different catalysts by changing the model parameter K4 suggests that on different catalysts the effect of 0 2 addition will depend on the active oxide and deactivated carbonate equilibrium established at the synthesis conditions. Acknowledgment Support for this work from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. Experimental data reported herein were obtained as a result of research sponsored jointly by the Alberta Research Council and CANMET under Supply and ServicesContract No. 23440-7-9175/01-SQ,supported by the Federal Panel on Energy Research and Development (PERD). The assistance of T. Painter and B. Zhou in obtaining some of the experimental data is gratefully acknowledged. Literature Cited Amenomiya, Y.; Birsa, V. I.; Goledzinowski,M.; Galuszka, J.; Sanger, A. R. Conversion of Methane by Oxidative Coupling. Catal. Rev.-Sci. Eng. 1990, 32 (3), 163-227. Asami, K.; Shikada, T.; Fujimoto, K.; Tominaga, H. Oxidative Coupling of Methane Over PbO Catalyst: Kinetic Study and Reaction Mechanism. Znd. Eng. Chem. Res. 1987,26,234&2353. Campbell, K. D.; Lunaford, J. H. Contribution of Gas Phase Radical Coudina in the Catalytic Oxidation of Methane. J.Phys. Chem. 1988,92; 5792-5796. Galuszka. J. Role of CO, in ImDedine Low TemDerature Oxidative Coupling of Methane.-Prese;ted ail3th North American Meeting of the Catalysis Society, Pittsburgh, May 24,1993; paper PC40. Geerts, J. W. M. H.; Chen, Q.;van Kasteren, J. M. N.; van der Wiele, K. Thermodynamics and Kinetic Modeling of the Homogeneous Gas Phase Reactions. Catal. Today 1990,6, 519-526. Kalenik, Z.; Wolf, E. E. Comments on the Effect of Gas Phase Reactions on Oxidative Coupling of Methane. J.Catal. 1990,124, 566-569. Keller, G. E.; Bhasin, M. M. Synthesis of Ethylene via Oxidative Coupling of Methane. J. Catal. 1982, 73, 9-19. Korf, S. J.; Roos, J. A.; de Bruijn, N. A.; van Ommen, J. G.; Ross, J. R. H. Influence of COz on Oxidative Coupling of Methane over a Lithium Promoted MgO Catalyst. J.Chem. Soc., Chem. Commun. 1987,1433-1434.
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Received for review May 27, 1993 Revised manuscript received September 16, 1993 Accepted September 27, 19930 Abstract published in Advance ACS Abstracts, December 1, 1993. @
