Ind. Eng. Chem. Res. 1990,29, 1771-1776
1771
An Experimental Study of Optimal Active Catalyst Distribution in Pellets for Maximum Selectivity Hua Wu,+ Quan Yuan,* and Baolin Zhu Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, People’s Republic of China
The experimental results of improving selectivity by using nonuniformly active catalyst pellets are reported for a competitive reacting system, CO methanation over high metal-loaded Ni/A1203catalyst accompanied by deposition of carbon. The active metal Ni has been deposited in a narrow band to simulate the Dirac delta distribution. It is found that when the depth of the active band is greater than a certain value, the influence of deposition of carbon is negligible. The optimal active location is also identified where the active band leads not only to improvement of selectivity but also to the maximum effectiveness factor. The theoretical analysis indicates that the improvement of selectivity is essentially owed to the changes of both the value and the distribution of the H 2 / C 0 ratio within the pellets. Since the H 2 / C 0 ratio affects the selectivity of many CO hydrogenation systems, the obtained result is of direct relevance for the other CO hydrogenation systems.
1. Introduction Interest has been growing in the conversion of coal into synthesis gas and subsequently into different kinds of feedstocks for chemical- and energy-related industries. The H2/C0 ratio in the synthesis gas is one of the major operation factors of a reactor, affecting the product distribution. For the classical Fisher-Tropsch process, it is normally preferable for the synthesis gas to have a H2/C0 ratio of about 2 (Anderson, 1989). For the indirect Mobil M-Gasoline process, to produce methanol, a synthesis gas with a H2/C0 ratio of at least 2 is required (Chang, 1983). Tronconi et al. (1987) investigated the synthesis of alcohols over a Zn-Cr-K oxide catalyst; their results indicated that the optimal H2/C0 ratio for methanol is about 2 and for higher alcohols is slightly above 1. All the above figures represent that the synthesis gas from the second-generation gasifiers, which produce low H2/C0 ratio values (0.6-0.7), may not satisfy directly the requirement for the subsequent conversion process. It is therefore usually desirable for the catalyst to have water gas shift activity to some extent, but the shift step may not be economically rational. On the other hand, most of the results for the influence of the H2/C0 ratio on the product distribution in the literature were based upon the global experimental investigation of catalyst pellets. As mentioned by Carberry (1976), it is acceptable that the intraparticle mass-transfer resistance for the catalysts used in industry is generally not negligible; thus, if the reaction rate is not under kinetic control, the intraparticle mass-transfer limitation will result in the difference between the H2/C0 ratios in the bulk stream and at each point within the pellet; i.e., there will exist a distribution of the H2/C0 ratio within the pellet. It is intuitively understandable that, because of the influence of the H2/C0 ratio on selectivity, such a intraparticle distribution of the H2/C0 ratio will affect the product distribution. Thus, if some measures are found that can narrow the intraparticle distribution of the H2/C0 ratio, the selectivity of the CO hydrogenation will be certainly improved significantly. It has been shown both theoretically and experimentally that the nonuniform distribution of the active catalysts within the inert support can substantially improve the performance of the catalyst pellets, about which the literature has been reviewed by Dougherty and Verykios
* Author to whom correspondence should be addressed.
Present address: Dipartimento di Chimica Fisica Applicata, Politecnico di Milano, 20133 Milano, Italy. 0888-5885/90/2629-1771$02.50/0
(1987). The optimal catalyst activity distribution in pellets is given by the Dirac delta function, which was first found by Morbidelli et al. (1982) for a single isothermal reaction with bimolecular Langmuir-Hinshelwood kinetics and then extended by a few investigators (Morbidelli et al., 1984, 1985; Chemburkar et al., 1987; Vayenas and Pavlou, 1987a,b, 1988) and recently extended by Wu et al. (1990) to the general case of an arbitrary number of reactions with arbitrary kinetics, occurring in a nonisothermal pellet, with finite external heat- and mass-transfer resistances. Among these findings, the aspect of the Dirac delta distribution for the maximum effectiveness factor has received support from experimental studies (Lee and Varma, 1988; Masi et al., 1988; Wu et al., 1988; Chemburkar and Varma, 1990). The thin egg-shell catalysts widely used in industrial processes can be regarded as an example of practical application of the Dirac delta distribution. For the catalyst pellets with the narrow active band buried below the external surface, however, their applications have not received considerable attention in industrial practice except in the context of catalysts for automotive exhausts (Hegedus et al., 1979). In this paper, we fiit explore theoretically the possibility of practical application of the Dirac delta distribution on improving the selectivity of CO hydrogenation systems, which is based upon both the influence of the H2/C0 ratio on the selectivity and the analysis of the intraparticle distribution of the H2/C0 ratio, and we then take the CO methanation on high metal-loaded Ni/A1203catalyst as a test reaction to confirm the analytical results. The CO methanation over Ni/A1203catalyst is a competitive reacting system, involving the deposition of carbon in some form, which is referred to here as coking, when the H2/C0 ratio is low. Such a reacting system was first used by Rice and Hahn (1984); since the low Ni-loaded (1-2 wt %) catalyst was used, however, the effects of metal dispersion and metal-support interaction were dominated such that they failed to prove the influence of the nonuniform active catalyst distribution on the coking reaction. In the present work, however, the influence of the nonuniform active catalyst distribution on the coking reaction has been clearly identified. 2. Intraparticle Distribution of the H2/C0 Ratio 2.1. Basic Equations. Carbon monoxide hydrogena-
tion generally involves a complicated reaction network though; all products can be written from carbon monoxide and hydrogen in terms of a certain lumped stoichiometric relationship as follows: 0 1990 American Chemical Society
1772 Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990
CO
+ nHz
-
products
(1)
where the stoichiometric coefficient n depends upon the catalyst performance. For catalysts, such as cobalt, with a poor activity for the water gas shift reaction, the stoichiometric coefficient is generally limited to a value of about 2. For some catalysts (e.g., iron) with good water gas shift activity, it is possible for n to have a value of about 0.5. Consider the reaction system (1)occurring in a porous, symmetric, and nonuniformly distributed active catalyst pellet with negligible external mass- and heat-transfer resistances. The system is governed by the following steady-state mass and heat balance equations:
L[ull = @a(s)J’(ui,u,,T)
(3)
L[
(4)
with boundary conditions (BCs) dul/ds = du2/ds = dT/ds = 0, at s = 0 (5a) u 1 = u Z = T = 1 , at s = l
(5b)
where L [ ] is the Laplacian operator,
and
ui = Cco/Cco,s uz = C H ~ / C H * , ~T = t / t s s = r/R 4‘ = R2rs/[De3~oCco,s1 = n[De,COCCO,sl / [De,H2CH2,sl P = (-~m)De,CoCco,s/[tskel (6)
F(u,,u,,T) involves a lumped arbitrary kinetic relationship and, as one can see later, does not inherently have a direct influence on the analysis. a(s) is the activity distribution, which is defined as a(s) = k ( s ) / &
(7)
and satisfies the following relationship:
J1a(s) sn ds = 1
=
W U l l
2
nl 0
I
0.5
1
-s Figure 1. H2/C0 ratio distribution within the catalyst pellet, in the case of uniform catalyst activity distribution. @ = 0.031; y = 20.0; u = 3.15; D = 0.1;n = 0.
H2/C0 ratio, R or R,, changes also for different positions. Differentiating eq 11 with respect to u1 gives
From this equation, the following conclusions are readily obtained: If D < 1,dRc/dul C 0, indicating that R, increases with the decrease of u,; since the concentration of carbon monoxide decreases as its position moves inward, it means that the hydrogen to carbon monoxide ratio increases as its position moves toward the center of the pellet. If D > 1, on the contrary, the Hz/CO ratio decreases as its position moves inward. For D = 1, the ratio remains unchanged throughout the pellet and is equal to the value at the external surface, R,,+. Figure 1 shows some numerically calculated curves of the H2/C0 ratio within the pellet, where the activity distribution is uniform and the following expression has been used for the function F(u1,u2,T)in eqs 2-4:
(8)
2.2. Hz/CO Ratio within the Pellet. The combination of eq 2 with eq 3 yields UU2l
0
(2)
L[u,J = D#2a(s)J’(ul,uz,T) = PP’a(s)J’(ui ,uz,T )
’
(9)
which, when integrated twice with BCs (5a) and (5b), leads to ~2 = 1 - D ( l - ~ 1 ) (10) which provides the relationship between the concentration of carbon monoxide and hydrogen within the catalyst pellet. By dividing both sides of eq 10 with ul,we obtain the ratio of the two reactants within the pellets:
or
Since the concentration of carbon monoxide, u l , changes for different positions within the pellet, from eq 11, the
It can be seen that the difference between the H2/C0 ratios within the pellet and at the extemal surface is severe, increasing with the increase of the Thiele modulus. If the selectivity of the system depends primarily upon the Hz/CO ratio, such a distribution of the Hz/CO ratio is certainly undesirable. The above results indicate that the ratio of the concentration of reactants within the pellet is different from that at the external surface not only because the effective diffusivities of the reactants differ from each other but, even if their effective diffusivities are the same, from the expression of D , it is still possible to give rise to the difference of their ratio within the pellet as long as one can force D # 1. For the Dirac delta distribution, since all the active catalysts have been concentrated in a specific narrow band within the pellet, this apparently leads to all the active catalysts being exposed to one H2/C0 ratio. It is now clear that if the selectivity of CO hydrogenation to certain products requires a specific Hz/CO ratio greater than that in the bulk, it can be satisfied by using the Dirac delta
Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990 1773 Table I. Properties of the Ni/A120SCatalyst pellet size (diameter X thickness), mm thickness of the active band, mm Ni load based on the active band, wt % based on pellet, wt 70 app density, g/cm3 effective diffusivitiesO for CO,cmz/s cm2/s for H2, effective thermal conductivity,b cal/ (cms-K)
Table 11. Ni Distribution within the Pellets Used for Our ExDerimentsO profile function no.
20 x 7.7 1.7 25. 5.5 1.03
a,(s) =
1
1.36 X 5.30 X 1.90 x 10-4
3
a3(s) = 1!.55 a4(s) =
4
Go,*
D = 0.267n-
= 0.267n/R,,
(14)
CHZ.8
Therefore, for catalysts with a poor water gas shift activity, since n = 2, the synthesis gas produced by the secondgeneration gasifiers with the original H,/CO ratio of 0.6-0.7 satisfies D 1; consequently, it is possible to get a desired H2/C0 ratio higher than that in the bulk by depositing all the active catalysts in a specific narrow band within the pellet. It is worth noting that since the original H,/CO ratio is smaller than n, the H2/C0 ratio in the bulk will decrease as the CO conversion increases. Then from eq 14, after the CO conversion increases to a certain value, the opposite situation, D > 1, may occur. Thus, for catalysts with a poor water gas shift activity, the way of moving the active band inward to get a greater H2/C0 ratio can work only for CO conversion lower than a certain value. For catalysts with a good water gas shift activity, since the stoichiometric coefficient may have roughly the same value as the original H 2 / C 0 ratio, it is therefore possible to get a desired selectivity by depositing all the active catalysts in a specific narrow band within the pellet for any value of the CO conversion.
3. Experimental Section 3.1. Catalyst Preparation. The Ni/A1,03 catalyst pellets, in which the active metal Ni was deposited in a narrow band, were prepared by wet-pressing and are reported in our previous paper (Wu et al., 19881, where it can be seen from Figure 4 in that paper that the pellet was prepared such that it looked like a catalyst pellet that was nested in an alumina mold. The main catalyst characteristics are listed in Table I. The effective size of all the pellets is taken as 20 X 7.7 mm (diameter X thickness); then, the dimensionless thickness of the active band is 0.22. Six kinds of Ni/A120, catalyst pellets with the active band at different positions within the pellet were prepared. The
lo 1:
a2(s) = 4.55
2
OCarrier, Ar; pressure, 1 MPa. *Evaluated from the Prater number (@),which was obtained from the maximum experimental effectiveness factor in the absence of deactivation.
distribution and suitably moving the active band toward the center of the pellet when D < 1, and it cannot be done when D > 1. For the later case, since the H2/C0 ratio decreases as its location moves inward, the active band for the Dirac delta distribution is probably at the external surface, although such a choice still does not satisfy the requirement for the selectivity. The opposite conclusions can be obtained if a H2/C0 ratio smaller than that in the bulk is required for the selectivity. The above description may be referred to as the physical explanation for one of the reasons that the Dirac delta distribution is optimal for the selectivity in the present situation. Let us now consider the possible value of D within the pellet for the synthesis gas produced by the second-generation gasifiers. Assuming that the diffusion of reactants within the porous pellet is dominated by the mechanism of Knudson diffusion, from the definition of D, we have
0.84 < s I 1 0.62 I s I 0.84 0 6 s < 0.62 0.77 < s 6 1 0.55 6 s I 0.77 0 6 s < 0.55 0.69 < s 6 1 0.47 6 s I 0.69 0 I s < 0.47 0.61 < s 5 1 0.39 I s 6 0.61 0 6 s < 0.39 0.45 < s 5 1 0.23 I s I 0.45 0 6 s < 0.23 0.22 < s 6 1 0 6 s 6 0.22
4.55
1:
4.55
5
a5(s) = ( i . 5 5
6
a6(s)
{
= :,55
Measured by cutting the pellets after all runs.
Table 111. Feed Compositions no. co, % I 2.85
I1
3.65
Hz,%
Ar, %
15.31 8.42
81.84 87.9
profiles of the active element Ni for these pellets are listed in Table 11. 3.2. Apparatus and Procedure. The CO methanation was carried out in an internal cycle reactor at t = 483-633 K and P i= 1 MPa. The impeller speed in the reactor was adjusted so that the behavior of an ideal CSTR can be assumed. The schematic diagram of the apparatus and the reduction procedure of the catalysts were shown elsewhere (Wu et al., 1988). Since the CO methanation is highly exothermic, to protect the catalyst from sintering, gas mixtures with low CO concentrations (
