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Ind. Eng. Chem. Res. 1996, 35, 2531-2538
2531
Microkinetics of Water-Gas Shift over Sulfided Mo/Al2O3 Catalysts Carl R. F. Lund Chemical Engineering Department, SUNYsBuffalo, Buffalo, New York 14260
A microkinetic model was developed to explain the catalysis of the water-gas shift reaction by sulfided, alumina-supported molybdenum. In this model, the reaction takes place through a regenerative (reduction-oxidation) scheme wherein the catalyst surface is alternately oxidized by water and then reduced by carbon monoxide. The surface of the catalyst is equilibrated with gas-phase H2S under all reaction conditions studied. Coverages predicted by the model are consistent with the adsorption behavior of molybdenum sulfide catalysts. Simulations indicate that the effects of steam and H2S levels are closely related. A maximum in CO conversion with increasing steam level may only occur at certain H2S levels, and the ordering of CO conversion with increasing H2S levels may invert as the steam level is changed. There are two main classes of water-gas shift catalysts that are used commercially: high-temperature catalysts (typically oxides of iron and chromium) and low-temperature catalysts (typically copper and zinc based). It is also well established that supported, promoted molybdenum sulfides are active shift catalysts (Newsome, 1980). This paper describes a microkinetic model for the catalytic activity of sulfided Mo/Al2O3 catalysts for water-gas shift. Sulfur-tolerant watergas shift (wgs) catalysts are very similar to catalysts used for hydrodesulfurization, so that mechanistic knowledge about water-gas shift over these catalysts may provide some insight into hydrodesulfurization catalysis. The present results will also provide a basis for future comparison to sulfided CoMo/Al2O3 catalysts for wgs. There are few published studies of wgs using sulfided Mo/Al2O3 as a catalyst. Hou et al. (1983) found that the catalyst was inactive in the oxide form. ESCA analysis of sulfided catalysts revealed that the Mo(V) to Mo(IV) ratio changed as the ratio of H2S to H2 was varied. As the mole fraction of H2S present in the feed was increased, the catalytic activity also increased (over the range up to 5000 ppm H2S). This led the authors to suggest that one sulfur ligand could rapidly exchange for an oxygen ligand as in eq 1. The oxygen ligands so-generated were then proposed to participate in a redox cycle wherein CO removes the ligand and water replaces it (eqs 2 and 3). The observed rate passed S2–
S2– Mo5+
S2–
S–
+ H2O
Mo5+
+ H2S
(1)
+ CO2
(2)
+ H2
(3)
S2–
O–
+ CO
S2– Mo4+
O–
Mo5+
Mo4+
S2–
+ H2O
Mo5+
O–
through a maximum as the steam feed rate was increased. The authors noted that a simple LangmuirHinshelwood mechanism involving competitive adsorption of CO and H2O could explain this observation. They did not totally reconcile this suggestion with the mechanism of eqs 2 and 3. The apparent activation energy over the catalyst was 12.8 kcal mol-1. In another study (Laniecki and Zmierczak, 1991), sulfided, Mo-impregnated Y zeolite was prepared both
from ammonium heptamolybdate and from molybdenum hexacarbonyl precursors. The latter were more active, but this may largely be due to differences in the percentage of Mo exposed. In the more active carbonylbased catalyst, the Mo(V) state was not detected via XPS either in the catalyst as prepared or upon exposure to water. (Hou et al. (1983) observed it in their catalysts after sulfidation, i.e., even before exposure to reaction conditions.) This suggests that a mechanistic scheme like that above may not be operative over the zeolite catalysts. Alternatively, Mo(V) could still be catalytically important, but it might only form under reaction conditions or its concentration might be very small for these highly dispersed catalysts. Spillman (1988) also studied wgs kinetics over sulfided Mo/Al2O3 catalysts using a “spinning basket”, fully mixed reactor. Since this study has only been reported in the form of a Master’s thesis, it will be described here in more detail than usual. The Mo/Al2O3 was prepared by incipient wetness impregnation of Davidson γ-alumina (ground to 20-60 mesh) with an aqueous solution of ammonium heptamolybdate to produce a loading of 15 wt % (as MoO3). The impregnated catalyst was vacuum dried at 80 °C for 3 h and subsequently calcined at 500 °C in air for 6 h. In a typical experiment the catalyst was pretreated in 14.6% H2S, balance H2, while heating to 300 °C and then for an additional 2 h during which the temperature was held constant. The reactor was then purged with N2 for 1-h at the same temperature. The feed composition was next adjusted while bypassing the reactor, and the temperature was stabilized at the desired level. Temperatures of 250, 275, and 300 °C and pressures of 5 and 15 atm were employed. The gas hourly space velocity was 24 000 h-1 in each of the 72 experimental runs. During Spillman’s kinetic measurements the feed always contained H2S; its concentration was 1% by volume. The reactor would be run for 1-2 h at steady state, and then the feed composition was changed. The kinetics were described using rate expressions of the power-law type given in eq 4. In eq 4 Kwgs represents
[
r ) k0e-E/RTPCOaPH2ObPCO2c 1 -
PCO2PH2
]
KwgsPCOPH2O
(4)
the equilibrium constant for the overall reaction, and Pi represents the partial pressure of species i. The data at 5 atm were fit separately from those at 15 atm. Table 1 presents the values of the resulting power-law kinetic parameters.
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2532 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 Table 1. Power-Law Kinetic Model Parametersa catalyst (pressure)
k0
E
a
b
c
Mo/Al2O3 (5 atm) Mo/Al2O3 (15 atm)
6.3 6.0
5.27 5.95
0.7 0.8
0.14 0.29
0.0 -0.07
a Parameters are defined by eq 4, k has units of mol h-1 g-1 0 atm-(a+b+c), and E has units of kcal mol-1. All values are from Spillman (1988).
In the present study Spillman’s data have been reanalyzed using the microkinetic analysis method suggested by Dumesic et al. (Dumesic et al., 1987, 1993). The objectives of the study were, first, to determine whether the previously mentioned mechanistic suggestions remain valid at higher pressures (up to 15 atm vs. 1 atm) and lower temperatures (∼550 K vs. ∼725 K); second, to analyze the resulting mechanistic network; and, third, to establish a basis of comparison for future microkinetic analysis of sulfided CoMo/Al2O3 catalysts for wgs. Experimental Methods Model Formulation. The suggestions by Hou et al. (1983) form the basis of the mechanism investigated in the present study. The mechanism incorporates a regenerative (oxidation-reduction) process such as in eqs 2 and 3, but via adsorbed species. For simplicity, hereafter let * denote a vacant site which is envisioned to be like the Mo(IV) species in eq 2 or eq 3. It is assumed that adsorption takes place at the location of the missing ligand so that *-S is equivalent to the first species in eq 1 and *-O is equivalent to the second. The mechanism assumes that the catalyst is in a sulfided state, but the spectroscopic data mentioned previously suggest that the degree of surface sulfidation may vary with conditions. Sulfhydryl groups are very commonly observed on MoS2 (Maternova´, 1982; Maternova´, 1983; Miciukiewicz et al., 1987). They are formed by both the adsorption of H2S (Topsøe and Topsøe, 1993) and the adsorption of H2 (Ratnasamy and Fripiat, 1970; Massoth, 1975, 1978; Wright et al., 1980). Therefore, the sulfidation pathway given in reactions (5) and (6)
H2S + * + *-S a 2*-SH
(5)
2*-SH a 2*-S + H2
(6)
is included to account for variations in the degree of surface sulfidation as reaction conditions change. The associative adsorption of H2S followed by its surface dissociation could have been used in place of reaction (5), but this would have introduced additional unknown parameters. Furthermore, the molecular adsorption of H2S is expected to be weak, and Spillman’s data (used here in the kinetic modeling) were collected at a single H2S level, making it difficult to justify the added mechanistic complexity. It should be noted that reactions (5) and (6) require a pair site for the adsorption of H2S, limiting the applicability of the mechanism. In particular, if a surface initially consisted of 100% vacant sites, it could not become sulfided upon exposure to H2S according to these reactions because there would be none of the *-S sites that are necessary for reaction (5). Thus, as already noted, the mechanism investigated in this study assumes the catalyst is in a sulfided state. The oxidative pathway was constructed largely by analogy to the sulfidation pathway above. It is assumed that adsorption of H2O will be similar to that of H2S.
hydroxyl signal associated with the support. In one infrared study of unsupported MoS2 (Ratnasamy and Fripiat, 1970), the adsorption of water did not affect the frequency of the SH band, but adsorbed water or hydroxyls are not mentioned and the spectrum is not reported. In another study using a supported catalyst (Topsøe, 1980), it is noted that the hydroxyl absorption band disappears as the MoS2 loading increases to the point of monolayer coverage of the support. These spectroscopic studies thus suggest that if hydroxyl groups do exist on the sulfide (not the support), then they must be present in very low concentrations since they are not detectable by infrared spectroscopy. It will be seen that the results of the present study also indicate that surface hydroxyl groups are present in very low concentration under reaction conditions. Nonetheless, by analogy with the sulfidation steps above, the oxidative pathway is represented by reactions (7)-(10). In addition, reactions (11) and (12) are included to allow for sulfidation of an oxidized catalyst.
H2O + * + *-S a *-SH + *-OH
(7)
H2O + * + *-O a 2*-OH
(8)
2*-OH a 2*-O + H2
(9)
*-OH + *-SH a *-O + *-S + H2
(10)
H2S + * + *-O a *-SH + *-OH
(11)
*-S + *-OH a *-SH + *-O
(12)
The formulation of the reductive pathway was guided by the chemisorption behavior of the carbon oxides on MoS2. CO is known to adsorb on MoS2 with a relatively weak bond (Mauge´ and Lavalley, 1992). In fact, it has been suggested that CO adsorption can be used to titrate exposed MoS2 sites (Bachelier et al., 1981, 1984). The low-temperature chemisorption of oxygen is more commonly used for this titration even though it has been shown to be corrosive (oxidize into the bulk) at subambient temperatures (Tauster and Riley, 1981; Zmierczak et al., 1982; Kalthod, 1984; Muralidhar et al., 1984). The adsorption of NO is also commonly used for MoS2 titration (Topsøe and Topsøe, 1983; Miciukiewicz et al., 1987), but again there is some concern that this process may be corrosive (Prins et al., 1989). CO2, on the other hand, does not chemisorb on MoS2 (Zmierczak et al., 1987). Indeed, since CO2 does adsorb on Al2O3, one method for measuring the fraction of the support covered by the sulfide uses the difference between the uptake on the catalyst and the uptake on the support only. In constructing a reductive pathway, it is difficult to simultaneously satisfy the observations that CO adsorbs while CO2 does not adsorb. Ultimately, a LangmuirHinshelwood reductive pathway, reactions (13)-(15), was used in the model with the expectation that the concentration of adsorbed CO2 (and probably adsorbed CO, too) would be small. While formates have also been
CO + * a *-CO
(13)
*-CO + *-O a *-CO2 + *
(14)
*-CO2 a * + CO2
(15)
reported as intermediates in wgs over some catalysts
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Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 2533 Table 2. Estimated Preexponential Factors Used in the Microkinetic Modelinga reaction
Aforward (s-1)
Areverse (s-1)
H2S + * + *-S a 2*-SH 2*-SH a 2*-S + H2 H2O + * + *-S a *-SH + *-OH H2O + * + *-O a 2*-OH 2*-OH a 2*-O + H2 *-OH + *-SH a *-O + *-S + H2 H2S + * + *-O a *-SH + *-OH *-S + *-OH a *-SH + *-O CO + * a *-CO *-CO + *-O a *-CO2 + * *-CO2 a * + CO2
1.0 × 106 1.0 × 1013 1.0 × 106 1.0 × 106 1.0 × 1013 1.0 × 1013 1.0 × 106 1.0 × 1013 1.0 × 106 1.0 × 1013 1.0 × 1013
2.55 × 1012 5.17 × 108 5.44 × 1011 1.08 × 1012 1.30 × 108 2.59 × 108 5.08 × 1012 5.02 × 1012 2.16 × 1013 1.47 × 1014 2.35 × 105
a If ideal gas behavior is assumed, the preexponential factors for adsorption steps may be taken to have units of atm-1 s-1; with this assumption the fugacity in eq 18 should be replaced with the partial pressure in units of atm.
Figure 1. Reaction network used to model the water-gas shift reaction. Surface species are indicated by rounded rectangles. Locations where gaseous species enter the network are denoted by ovals, and equation numbers for reaction steps are given in parentheses.
these species were not added to the mechanism since there is no spectroscopic evidence for their existence on sulfides and they would introduce additional unknown parameters to the kinetic model. The mechanistic network is summarized in Figure 1. Microkinetic Analysis. The microkinetic analysis involved several steps (Dumesic et al., 1993). First, heats and entropies of formation of all species as if they existed in the gas phase were determined at a reference temperature of 500 K (Chase et al., 1985). For surface species the strengths of their bonds to the surface, Ei-s, were introduced as adjustable model parameters as given in eq 16. It was assumed in the present study
∆Hf(surf)-i ) ∆Hf(gas)-i - Ei-s
(16)
that adsorption caused the loss of all translational entropy so that the entropy of formation of surface species was given by eq 17. (The entropy values
∆Sf(surf)-i ) ∆Sf(gas)-i - Strans-i
[ (
)]
(2πmkBT)3/2v 5 ) ∆Sf(gas)-i - R + ln 2 h3
(17)
estimated in this way were only used to calculate the reverse preexponential factors once the forward preexponentials had been selected. This is done simply to ensure that the reaction rate predicted by the model will equal zero at equilibrium at the reference temperature.) Having heats and entropies of formation of all species (gas and surface), the heats and entropy changes for each reaction in the mechanism were calculated. For each step j in the reaction mechanism four kinetic parameters are needed: a preexponential factor and an activation energy for both the forward and reverse reactions. The forward preexponential factors, Aj-forward, were estimated on the basis of transition state theory (Zhdanov et al., 1988; Dumesic et al., 1993). If no assumptions are made regarding the ideality of the gas phase, the rate expression for adsorption steps can take the form of eq 18,where θv denotes the fraction of surface
dθi/dt ) kadsfiθv
(18)
i adsorbed on them, k is the rate coefficient, and fi is the fugacity of gas phase species i. This is the form for which the preexponential factors were estimated; in this form they have units of reciprocal seconds. The reverse preexponential factors, Aj-reverse, were then calculated by application of the equilibrium criterion (eq 19), evalu-
Aj-reverse ) Aj-forward exp(-∆S0j /R)
(19)
ating ∆S0j at the reference temperature of 500 K. In the present investigation the preexponentials were not varied from these initial guesses, which are listed in Table 2. Other adjustable model parameters, Rj and βj, were introduced to calculate the activation energies. The activation energy for step j in the exothermic direction, Ej-exothermic, was calculated using the Evans-Polanyi kinetic correlation (eq 20). The activation energy in the
Ej-exothermic ) Rj + βj∆Hj
(20)
endothermic direction was then calculated using eq 21.
Ej-endothermic ) Ej-exothermic - ∆Hj
(21)
One set of Evans-Polanyi parameters, Rox and βox, was used for all the steps in both the oxidative and sulfidation pathways, reactions (5)-(12). A second set of Evans-Polanyi parameters, Rred and βred, was used for the reductive pathway, reactions (13)-(15). The parameter β is typically near 0.5; here it was constrained to lie between 0.2 and 0.8 because values outside this range lead to unreasonable values for the kinetic parameters. In total there were 10 adjustable model parameters: Rred, Rox, βred, βox, and the surface bond strengths for S, SH, O, OH, CO, and CO2. Once the kinetic parameters were calculated for each mechanistic step, the reactor analysis was performed. The reactor was assumed to be perfectly mixed and to be operating at steady state. For any one kinetic experiment the set variables, i.e., the total inlet volumetric flow rate (sccm), νin; the inlet mole fractions, φin-i, of all gas-phase species i (i ) 1 through NG where NG is the number of gas-phase species); the mass of catalyst in the reactor (g), mcat; the site density of the catalyst (sites/g), Fsites; the temperature (K), T; and the pressure (atm), P, were known. (The site density was taken to be 1.34 × 1019 g-1 based upon low-temperature oxygen chemisorption on a catalyst prepared in the same manner as Spillman’s (Kalthod, 1984).) The outlet mole fractions, xi, of all gas-phase species i (i ) 1 through
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+
2534 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996
species (i ) NG + 1 through NS, where NS is the total number of species in the mechanismsincluding vacant sites); and the outlet total volumetric flow rate needed to be calculated. These response variables (the outlet flow rate was used in dimensionless form, xNS+1 ) νout/ νin) were calculated from mole balances on all but one gas-phase species (eq 22), mole balances on all but one surface species (eq 23), and a total mole balance (eq 24) and the requirements that the sum of the mole fractions by unity (eq 25) and the sum of the surface coverages be unity (eq 26).
of its value, so in such cases the value was changed to 1 kcal/mol.) This sensitivity analysis was performed for each model parameter that was adjustable for the purpose of fitting; it was also performed for every kinetic parameter (preexponential factors and activation energies). A sensitivity factor for a particular parameter j, Sj, was then calculated according to eq 29, where ψ0 is
mcatFsitesVstp 0 ) xixNS+1 - φin-i rnet-i Nνin
the value of the objective function with all parameters at their “best” values and ψj is its value when parameter j is changed as described above. Sj will approach 0 when parameter j has little impact upon the value of ψ, and it will approach unity when the quality of the fit is highly sensitive to parameter j. A “network analysis” was also performed on the mechanistic scheme. For this analysis representative reaction conditions were selected. The absolute forward rate and the absolute reverse rate of each step in the mechanistic scheme were calculated at these reaction conditions. If the net rate of any one step was less than 1% of the smaller of the forward and reverse absolute rates, that step was identified as being at quasiequilibrium. If the net rate of any one step was greater than 99% of the larger of the forward and reverse absolute rates, that step was identified as being irreversible in that direction. If both the forward and reverse absolute rates for any one step were less than the average of the net rates of all steps by 4 orders of magnitude or more, that step was identified as being kinetically insignificant. The absolute rates of the individual steps were also used to calculate the absolute rate of generation of each species and the absolute rate of consumption of each species. For intermediates in the mechanistic scheme the absolute rate of generation should equal the absolute rate of consumption (i.e., the net rate of generation is zero). For stable species the net rates of generation must be consistent with stoichiometry.
1eie NG - 1 (22)
0 ) rnet-i
NG + 1 e i e NS - 1
0 ) xNS+1 - 1 -
(23)
mcatFsitesVstp NG ( rnet-i) Nνin i)1
∑
(24)
NG
xi) - 1 ∑ i)1
(25)
0)(
NS
∑
0)(
xi) - 1
(26)
i)NG+1
The net rate with respect to species i, rnet-i, appearing in eqs 22-26 is defined in eq 27, where νij is the NS
NR
rnet-i )
∑ j)1
∏ ν 0
xi-νij) - kj2Porj2(
ij
ij
ij
i)1
i)1
(27)
stoichiometric coefficient of species i in reaction j (negative for reactants and positive for products), kj1 and kj2 are the forward and reverse rate coefficients, respectively, P is the total pressure, and orj1 and orj2 are the forward and reverse gas-phase reaction orders, respectively. Equations 22-26 were solved using a globally convergent Newton-Raphson method (Press et al., 1992). For each experimental run k, solving eqs 22-26 yielded the outlet CO mole fraction predicted by the model, yCOp-k. An objective function ψ (eq 28) was Ndata
ψ)
∑ (yCO k-1
m-k
- yCOp-k)2
(28)
defined as the sum over all Ndata experimental runs of the squares of errors between the measured outlet mole fractions of CO, yCOm-k, and the mole fraction predicted by the microkinetic model, yCOp-k. The model was fit to the data by varying the adjustable model parameters (surface bond strengths, R’s, and β’s) so as to minimize ψ. A downhill simplex method (Press et al., 1992) was used to find the minimum. All calculations were performed on a PowerMacintosh 7100 personal computer. When fitting was completed, a “sensitivity analysis” was conducted to determine which parameters were most critical to the quality of the fit. The sensitivity to a particular parameter was assessed by reducing that one parameter to 95% of its initial value and recomputing the value of ψ. (A bond strength or activation
Sj )
|
|
ψj - ψ0 ψj
(29)
Results Spillman originally fit power-law models to the data used herein; the value of the sum of the squares of the errors (eq 28) was 2.06 × 10-3. The value for the microkinetic model is nearly the same, 3.14 × 10-3. The fit using the microkinetic model is very good. Over a total of 72 data, the largest absolute difference between the predicted outlet CO mole fraction and the measured value was 0.022 (0.413 vs 0.435). The largest relative difference was 11.2% (0.0913 vs 0.0811). The values of the model parameters and their sensitivity factors (eq 29) resulting from the fitting process are given in Table 3. The parameters listed can be used to calculate the activation energies given in Table 4 along with their sensitivity factors. Even though the preexponential factors were not varied for the purpose of fitting the model to the data, sensitivity factors were, nonetheless, calculated for the preexponential factors. The largest sensitivity factor was 0.0292, indicating that the model is not highly sensitive to any of these parameters. This, in part, justifies not varying the preexponential factors during the fitting process. The fit of the model is most sensitive to the values of the O and OH surface bond strengths. In fact, all the model parameters associated with the oxidative pathway, except perhaps βox, have high sensitivity factors.
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Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 2535 Table 3. Model Parameter Values and Sensitivities Resulting from Fitting the Microkinetic Model to the Experimental Data parameter value
sensitivity
objective function, ψ 0.00314 17.52 Rox (kcal/mol) βox 0.800a Rred (kcal/mol) 14.88 βred 0.606 73.8 *-S bond strength (kcal/mol) 49.6 *-SH bond strength (kcal/mol) 126.1 *-O bond strength (kcal/mol) 71.6 *-OH bond strength (kcal/mol) 8.6 *-CO bond strength (kcal/mol) 3.2 *-CO2 bond strength (kcal/mol) a Value of the upper limit of the constrained range.
0.5898 0.2949 0.7020 0.0500 0.6154 0.8823 0.9925 0.9356 0.0007 0.0436
sensitivity factors for βred and the surface bond strengths of CO and CO2 are low. Discussion Network analyses were performed for a feed consisting of 33% CO, 67% H2O, and 1% H2S and using 2.0 g of catalyst and a feed rate of 1500 sccm. Temperatures of 250, 275, and 300 °C were analyzed at pressures of 5 and 15 atm for each temperature. In addition, analyses were conducted at 300 °C and 15 atm with the same feed and catalyst charge, but varying the feed rate between 1000 and 3000 sccm so as to sample a range of conversions. While there were systematic changes and differences over this range of conditions, the relative magnitude of surface coverages did not change, and the same reactions were found to be equilibrated or irreversible. Therefore, the reaction network will be discussed in terms of the simulation at 275 °C and 15 atm using 2.0 g of catalyst and 1500 sccm of a feed containing 33% CO, 67% H2O, and 1% H2S. The surface coverages are reported in Table 5. It was previously noted that for consistency with published findings the fractional coverage of OH, CO, and CO2 should be small. The table shows that this requirement has been satisfied by the model. Adsorbed oxygen covers the largest fraction of the surface, and most of the remainder is vacant. Adsorbed sulfur and sulfhydryl groups cover on the order of 1% of the surface according to the model. It should be noted, however, that the data used in fitting the model all had the same gas-phase H2S concentration. It would have been preferable to have data where the H2S level was varied. Table 6 provides the results of the network analysis for the same reaction conditions. It can be seen from the table that the net formation rate of each intermediate species is zero, as it must be. Also the net rates with respect to the reactants and products are consistent with the overall stoichiometry of the reaction. The analysis indicates that H2S is equilibrated with the surface at all the reaction conditions studied (reactions
(5), (11), and (12) are at quasi-equilibrium). In addition, the adsorption of water via reactions (7) and (8) is irreversible at all conditions studied. In most cases the surface reaction between the carbon oxides, reaction (14), is at a quasi-equilibrium; using the definition given earlier there are a few situations (lower pressure and lower temperature) where quasi-equilibrium is not realized; however, the reaction remains highly reversible in these situations. The mechanism used here is basically the same as that suggested by Hou et al. based upon experiments at ca. 723 K and 1 atm. The present results show that it can describe kinetics at significantly lower temperatures (∼550 K) and higher pressures (up to 15 atm). Hou et al. also observed, first, that the conversion of CO increased monotonically as the concentration of H2S increased and, second, that the CO conversion passed through a maximum as the concentration of H2O was increased. The microkinetic model was tested to see if it was able to predict such phenomena. Figure 2 shows a comparison between Hou et al.’s data and a microkinetic simulation using the same conditions. The simulation exhibits the proper trend, with conversion increasing at greater H2S mole fractions. The model underpredicts the experimental data by a factor of 2-3, but this really is not bad considering extrapolation involved. Specifically, the model has been extrapolated over 150 K, 5 atm, and 5000 ppm H2S. In addition, it was necessary to assume that the number of active sites per gram was the same as for Spillman’s catalyst. It should be noted that in this situation the model is making predictions about the effect of changing a variable (the H2S level) that was held constant during the fitting process. When a simulation was performed using the conditions (733 K, 1 atm, and 2500 ppm H2S) where Hou et al. saw a maximum in conversion while varying the steam concentration, the average conversion level was underpredicted by a factor of 2-4. Again, this agreement is not bad considering the range of extrapolation. However, the model predicted a monotonically increasing CO conversion and not a maximum as was observed experimentally. As a consequence, a number of simulations were conducted at a variety of conditions. Figure 3 presents the results of three simulations, all at 548 K and 5 atm, using 2.0 g of catalyst and 1500 sccm of a feed containing 2.5% CO. The plots show the effect of varying the inlet H2O mole fraction at three H2S levels. The figure also replots Hou et al.’s data as an insert. The results indicate that the appearance of a maximum in the conversion while varying H2O level is also dependent upon H2S level. The simulation using 5000 ppm H2S results in a curve with a shape quite reminiscent of Hou et al.; in contrast, by 2500 ppm the maximum has nearly disappeared, and it is not seen at 1000 ppm.
Table 4. Activation Energies Calculated from the Model Parameters H2S + * + *-S a 2*-SH 2*-SH a 2*-S + H2 H2O + * + *-S a *-SH + *-OH H2O + * + *-O a 2*-OH 2*-OH a 2*-O + H2 *-OH + *-SH a *-O + *-S + H2 H2S + * + *-O a *-SH + *-OH *-S + *-OH a *-SH + *-O CO + * a *-CO *-CO + *-O a *-CO2 + *
Eforward (kcal/mol)
sensitivity
Ereverse (kcal/mol)
sensitivity
3.23 20.7 7.93 17.5 11.2 18.3 12.8 7.93 9.65 16.4
3.49 × 10-2 6.22 × 10-1 1.98 × 10-1 2.41 × 10-2 1.49 × 10-2 8.25 × 10-3 1.14 × 10-2 7.83 × 10-3 6.84 × 10-1 8.61 × 10-1
21.1 4.703 19.9 17.5 19.1 14.3 18.7 19.9 18.3 12.5
6.41 × 10-1 7.59 × 10-2 2.95 × 10-3 1.66 × 10-4 8.73 × 10-3 3.74 × 10-3 6.89 × 10-3 8.91 × 10-2 8.58 × 10-1 7.49 × 10-1
-1
-1
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2536 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 Table 5. Predicted Surface Coveragesa species
fractional surface coverage
S SH O OH
1.18 × 10-3 1.54 × 10-2 7.27 × 10-1 7.93 × 10-5
species
fractional surface coverages
CO CO2 vacant
1.23 × 10-4 6.76 × 10-7 2.56 × 10-1
a Based upon a simulation at 275 °C and 15 atm using 2.0 g of catalyst and 1500 sccm of a feed containing 33% CO, 67% H2O, and 1% H2S.
Figure 3. Simulation showing the effect of steam level upon CO conversion at 548 K and 5 atm for three different H2S levels. The inlet CO mole fraction is 2.5%, and the balance is inert; the catalyst mass is 2.0 g, and the total flow is 1500 sccm. The inset replots results from Hou et al. (1983) at 733 K and 1 atm.
Figure 2. Effect of H2S level upon CO conversion as observed by Hou et al. (1983) at 723 K and 1 atm compared to the prediction of the microkinetic model at the same conditions.
Hou et al. attributed the maximum to competition for sites between steam and carbon monoxide. It should be noted that according to the model both steam and CO require a vacant site for adsorption; steam also requires an adjacent oxidized or sulfided site to allow its dissociation into hydroxyl/sulfhydryl groups. Since oxidized sites comprise most of the surface, it may be expected that virtually all vacant sites will have an adjacent oxidized site. Hence, steam and CO will indeed
compete for the same sites on the surface. Figure 4 shows the coverages calculated as a function of inlet steam level for the conditions of the 5000 ppm H2S simulation. Since a network analysis did not indicate any single step as rate determining, it is difficult to offer a precise kinetic explanation for this effect: it is the result of the interplay among all the mechanistic steps. The results in Figure 3 are also interesting in that they predict that the effect of increasing the H2S level upon catalytic activity can invert if the amount of steam is changed. The author is not aware of any reports to this effect in the literature. It is, however, similar to effects observed when promoters are added to sulfided CoMo/Al2O3 wgs catalysts (Spillman, 1988). When the promoter is added, the catalyst becomes more active under some conditions but less active under others. The microkinetic model that has been generated in this investigation will serve as a basis for studying the
Table 6. Rates of Generation and Consumption of Species with Percentages Attributable to the Mechanistic Stepsa percentage of rate via reaction species CO H2O CO2 H2 H2S
*-S *-SH *-O *-OH *-CO *-CO2 * a
type of rate abs. generation abs. consumption net formation abs. generation abs. consumption net formation abs. generation abs. consumption net formation abs. generation abs. consumption net formation abs. generation abs. consumption net formation abs. generation abs. consumption abs. generation abs. consumption abs. generation abs. consumption abs. generation abs. consumption abs. generation abs. consumption abs. generation abs. consumption abs. generation abs. consumption
rate
(s-1)
135.7 137.6 -1.97 0.0083 1.98 -1.97 2.47 0.494 1.97 15.8 13.8 1.97 2.57 2.57 0.0 671 671 673 673 903 903 649 649 392 392 257 257 397 397
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
100.0 100.0 91.53 91.55
8.47 8.45 100.0 100.0
82.04 82.10
14.15 14.09
3.81 3.81
91.53 91.55 0.35 0.35 0.70 0.70
8.47 8.45 3.86 3.38 3.36 3.84
0.00 0.27 0.27 0.00 0.28 0.00
0.00 0.02 0.05 0.00
0.49 0.43 0.60 0.69
0.09 0.08 0.08 0.09 0.07 0.06 0.08 0.09
0.03 0.03 0.02 0.02 0.03 0.03
95.70 95.92 95.56 95.34 71.24 71.08 98.96 99.18
28.17 28.39 35.10 34.60
0.59 0.59
0.00 0.46
0.00 0.04
0.05 0.05
34.16 34.65
64.90 65.40 99.81 99.04 64.57 64.08
0.19 0.96 0.62 0.12
Based upon a simulation at 275 °C and 15 atm using 2.0 g of catalyst and 1500 sccm of a feed containing 33% CO, 67% H2O, and 1%
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Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 2537
Figure 4. Predicted surface coverages for the simulation at 5000 ppm H2S shown in Figure 3.
effect of promoting sulfided Mo/Al2O3 wgs catalysts, first by adding Co and then by additionally introducing K or Pr. The approach that has been used may provide some interesting interpretations of the effects of promoters based upon changes in surface bond strengths.
Conclusions A simple form of the kinetic scheme illustrated in Figure 1 and given by eqs 5-15 was originally proposed to explain wgs kinetics over sulfided Mo/Al2O3 catalysts. Here it has been shown that such a mechanism can explain the kinetics at significantly lower temperatures and higher pressures than used in that original work. The result also extrapolates back to the conditions used in the study where the scheme was originally proposed with reasonable accuracy. It is quantitatively off by a factor of 2-4, but it captures the reported trends even though data from that original study were not used here in fitting the model. In capturing the essential kinetic behavior observed upon varying steam and H2S levels, the model further suggests that the interplay between the two may be more complicated than previously recognized. Under typical experimental conditions, the catalyst surface is equilibrated with gas-phase H2S and the adsorption of H2O is irreversible. Kinetics alone cannot establish the model as correct, of course, and it provides predictions of unreported kinetic behavior as well as surface coverages under reaction conditions that could be used in its further refinement. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the ACS, for partial support of this research. The author acknowledges Professor J. A. Dumesic (University of WisconsinsMadison) for numerous helpful comments and suggestions. Discussion with and comments by Professor S. W. Weller and his students Dr. N. Srivatsa, Dr. K. Keeler, and Mr. D. Spillman are also gratefully acknowledged.
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Received for review October 4, 1995 Accepted May 17, 1996X IE950608U
X Abstract published in Advance ACS Abstracts, July 1, 1996.
