Ind. Eng. Chem. Res. 1990,29, 2-10
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KINETICS AND CATALYSIS ~~~~~~~
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Oxidative Coupling of Methane in the Gas Phase. Kinetic Simulation and Experimental Verification Horst Zanthoff and Manfred Baerns* Ruhr-University Bochum, POB 10 21 48,0-4630Bochum, FRG
The conversion of methane and oxygen was simulated using kinetic data available from the literature. Such temperatures and partial pressures of oxygen and methane were applied, which result experimentally in the formation of mainly ethane and ethylene and minor amounts of C3 hydrocarbons as coupling products and of carbon oxides as total oxidation products: 1.84 I PcH,/bar I8, 0.18 5 Po,/bar 5 0.8,878 IT/K I1071. There was good agreement between simulated and experimental data on product selectivities above 1000 K. The selectivity to C2+ hydrocarbons increases with temperature and CH4/02 ratio, while that to the carbon oxides decreases correspondingly. The dependence of C2+selectivity on pressure is only minor; however, the C2H4/C2H6ratio decreases with increasing pressure. Maximum Cz+ selectivities are on the order of 50%.
Introduction When methane reacts with oxygen in the presence of a suitable catalyst, ethane, ethylene, and small amounts of C3 and C4hydrocarbons are formed along with the undesired carbon oxides (Hinsen and Baerns, 1983; Hinsen et al., 1984; Ito et al., 1985; Sofranko et al., 1987; Lee and Oyama, 1988); in some instances, also traces of higher hydrocarbons including aromatic species were observed (Sofrankoet al., 1987). The selectivity to Cz+hydrocarbons ( n 1 2) is affected by the reaction conditions applied and the catalysts used. Reaction temperatures are usually varied between approximately 900 and 1100 K; the ratio of methane to oxygen partial pressure is generally 3-10 at atmospheric pressure. Multivalent metal oxides, e.g., PbO and MnO, alkali and alkaline-earth metal compounds, and rare-earth compounds have been reported as suitable catalysts. Cz+selectivities achieved under these conditions amounted to 50-9070 ; the corresponding degrees of methane conversion were about 35-10%. The reaction between methane and oxygen is certainly initiated on the catalytic surface, but there is still some debate whether the methyl species produced by dissociative adsorption of methane continue to react on the surface or whether they are completely desorbed as radicals into the gas phase before further reaction goes on (Asami et al., 1987a,b; Lunsford et al., 1987). More recent experiments conducted under elevated pressures (Ptot,l= 2.3-9.5 bar; CH4/O2/N2= 10:1:1.5; 800 < T/K < 1100) showed that appreciable amounts of Cz+ hydrocarbons can be also obtained in the absence of a catalyst; in the Results and Discussion section of this paper, reference is made to these results in more detail. Under atmospheric pressure, no homogeneous gas-phase reaction occurs in the absence of a catalyst to any appreciable extent at the usual reaction times applied for the catalytic conversion of methane. Against this background, it was the intention of the present study to simulate the homogeneous gas-phase reaction between methane and oxygen on the basis of kinetic data for the various possible elementary reaction steps which are mainly known from 0888-5885/90/2629-0002$02.50/0
combustion research. For this purpose, a kinetic scheme was derived which allowed the calculation of the time dependence of the concentrations of the various compounds and, hence, the determination of the selectivity of the reaction as a function of the degree of conversion. These relationships are then compared with experimental data obtained within the range of the above-mentioned experimental conditions. From the relative importance of the various reaction steps, it might be anticipated that also the catalytic process could benefit by designing better catalysts that favor the desirable reaction steps as compared to the undesired ones.
Simulation Procedure From the literature, it is known that, in principle, several hundred elementary reaction steps including approximately 40 species may occur in the conversion of methane and oxygen. To reduce these numbers to values that can be handled by the available computer program for simulation, it was required to select the important steps and species by taking into account the existing experimental evidence. Therefore, a slightly simplified kinetic reaction scheme (164 reaction steps and 28 species) was set up which was the basis for subsequent simulation. Since there is a certain deviation among the kinetic data (rate constants and energies of activation) known from experiments in pyrolysis, shock tubes, photolysis, and flames, as reported in the literature for some elementary reactions, a selection procedure outlined later was applied. The various parts of the simulation procedure are explained in the following section. Elementary Reaction Steps. The gas-phase reaction between oxygen and methane is based on a radical chain mechanism. The large number of possible reaction steps can be classified in the following way: addition reactions
+ B' A' + B
A'
--
1990 American Chemical Society
AB
AB'
Ind. Eng. Chem. Res., Vol. 29, No. 1, 1990 3
+ + + + + + - + -- ++
substitution reactions AX
AX'
+B
A'
B-A
BX'
BX'
AX B o - A' A AX' B' disproportionation reactions
BX BX
AB A' B' AB' A B' In general, only a few of the possible reactions are significant under defined reaction conditions. The importance of a reaction depends on its rate constant and the concentration of the species involved. The reaction scheme is discussed and evaluated below for the most important reactions and species. Only reactions with more than 5% relative influence (see later) are mentioned. (Annotation: For species reacting in a similar way, the reaction equations were generalized; the various reacting species are designated by "X". If there is no need for a reaction partner, the X is set in parentheses.) (a) Activation of Methane. Methane may be activated as follows: CH4 (+ X) e 'CH3 + 'H(X) where X = 'H02, 'OH, and 'H. Since the activation energies of the starting reactions CH4 + O2 'CH, 'HOz
-
CH4
-+
+ 'CH3 + 'H
amount to about 190-250 and 350-450 kJ/mol, respectively, which are high compared to reactions of methane with 'H, 'OH, HO,', or '0, where the activation energy amounts to about 10-75 kJ/mol, these reactions do not play an important role apart from the first microseconds of reaction. The influence of reactions involving O-containing compounds (02,'HOz, 'OH, '0) should decrease with an increasing methane to oxygen ratio, because the oxygen concentration (and therefore the concentrations of the radicals 'HOP, 'OH, and '0) decreases. (b) Nonselective Oxidation of CH,'. CH3' can be converted to formaldehyde or methanol and finally to COX by two possible pathways depending on the temperature. At temperatures above 900K, methoxy species are formed (Frenklach and Bornside, 1984; Westbrook, 1979) CH3' + (X)O, CH30*+ O(X) CH3'
+02
-
-+
CH30' (+ X)
CHzO
+ 'OH
CH20
+ H(X)
where X = H and an inert molecule. The oxidation of formaldehyde via CHO' yields CO, which is further oxidized to CO,: CHzO X CHO' HX
+
- + - + - co, + x
where X = 02,'HOP, 'OH, 'H, and 'CH, CHO' (+ X) CO 'H(X) where X = Oz and an inert molecule co + O(X) where X = 'H and 'OH. From low-temperature experiments (
