A heterogeneous reactor model was developed describing kinetic experiments on the heterogeneously catalyzed oxidative coupling of methane in a laboratory fixed-bed reactor. The catalyst produces radicals which react further through gas-phase reactions in the pores of the catalyst and in the interstitial phase. The reactor model accounts for the irreducible mass-transport limitations for the reactive radicals, which occur even at conditions where no mass-transport limitations occur for the molecules, both reactants and reaction products. The effects of these irreducible mass-transport limitations on the conversion and selectivity of the process were investigated and were found to be essential for an adequate description of experimental data.

A kinetic model was developed for the heterogeneously catalyzed oxidative coupling of methane for temperatures between 923 and 1023 K, inlet methane-to-oxygen ratios of 2 to 12, and oxygen conversions between 10 and 100%. This model features 10 catalytic reactions coupled to 39 gas-phase chain reactions and accounts for irreducible mass-transport limitations for reactive intermediates. The observed conversions and selectivities are adequately described up to 200 kPa total pressure. The observed strong increase of the conversions between 200 and 1000 kPa is described qualitatively. The catalyst not only produces radicals but also acts as an important radical quencher. Regeneration of the active sites through water desorption was found to be a kinetically significant step in the catalytic sequence producing methyl radicals. At atmospheric pressure approximately 90% of the methane and oxygen is converted on the surface of Sn/Li/MgO, the balance being converted through branched-chain reactions in the pores of the catalyst and in the interstitial phase. For Li/MgO only one-third of the methane is converted on the catalyst surface. For Sn/Li/MgO the catalytic oxidation of methyl radicals, rather than gas-phase reactions, limits the selectivity toward C2 products.

The oxidative coupling of methane with cofeeding of ethane was investigated experimentally both in the absence and in the presence of a Sn/Li/MgO catalyst. Cofeeding ethane in the absence of catalyst results in a higher total radical concentration, explaining the strong increase of the observed feed conversions. The hydrogen-peroxy radical-concentration increase is more pronounced than the corresponding methyl radical concentration increase, resulting in a lower selectivity. The combined effect of feed conversion and selectivity is beneficial for inlet ethane-to-methane ratios lower than 4 mol%. Ethane cofeeding results in a slight increase of the oxygen conversion in the presence of a Sn/Li/ MgO catalyst. This can be accounted for by a mechanism in which both the hydrogen abstraction from the hydrocarbon and the regeneration of the active sites are kinetically significant. The corresponding decrease of methane conversion results from competition between methane and the more reactive ethane for these sites. The addition of ethane does not result in a beneficial effect on conversions to ethane or C2.

Abstrset-A multitubular catalytic fixed bed reactor for the oxidative coupling of methane is designed. A heterogeneous two-dimensional reactor model taking into account axial dispersion of mass and enthalpy is applied. A reaction network consisting of 36 gas phase radical reactions and 10 catalytic reactions, based upon experiments with an Sn/Li/MgO catalyst at atmospheric pressure, temperatures of 923%1,023 K and methane-to-oxygen ratios of 2-12, is used. Intrapellet concentration profiles of molecules and, even more so, of radicals are shown to affect the selectivity towards ethane and ethylene. The importance of homogeneous reactions in the void space between the catalyst pellets is highlighted.The reactor dimensions are determined by heat removal c&siderations.

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