Irreducible Mass-Transport Limitations during a Heterogeneously Catalyzed Gas-Phase Chain Reaction:  Oxidative Coupling of Methane

Abstract

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.

Kinetics of a Gas-Phase Chain Reaction Catalyzed by a Solid: The Oxidative Coupling of Methane over Li/MgO-Based Catalysts

“…Furthermore, the formation of carbonates could also influence the kinetics of active-site regeneration and/or oxygen acti- C2H4; ], CO; 2, CO2. Calculated using the heterogeneous reactor model equations of Couwenberg et al (1996) and the heterogeneous reactions of Table 2 with corresponding kinetic parameters of Table 3 coupled to the gas-phase reaction network of Chen et al (1994b). Experimental conditions: see Table 1.…”

“…The continuity equations in the intraparticle phase describe internal diffusion with simultaneous reactions on the catalyst surface and gas-phase reactions in the pores of the catalyst. Simulations with this model (Couwenberg et al, 1996) show that the concentration of the methyl radicals in the center of the catalyst pellet is approximately 5 times higher than the interstitial concentration, even when no significant concentration gradient occurs for methane and oxygen.…”

“…Therefore, during the simulation of the kinetic experiments these limitations have to be taken into account by use of a heterogeneous reactor model. Couwenberg et al (1996) developed a one-dimensional heterogeneous model distinguishing between the interstitial and intraparticle phase. Usually a film model is applied to describe mass transfer from the interstitial phase to the intraparticle phase (Froment and Bischoff, 1990).…”

“…Hereafter, the flow of helium is stopped. For the conditions applied during the line-out and the conversion and selectivity reached at the end of the procedure the reader is referred to Couwenberg (1996). Although the line-out temperature applied for the Sn/Li/MgO catalyst is 100 K lower than that for Li/MgO, the space time yield after line-out is considerably higher.…”

“…Several routes for direct CO x formation on the catalyst surface were proposed in the literature. McCarty (1992) Lines: calculated using the heterogeneous reactor model equations of Couwenberg et al (1996) and the heterogeneous reactions of Table 2 with corresponding kinetic parameters of Table 3 coupled to the gas-phase reaction network of Chen et al (1994b Aparicio et al (1991) mentioned the reversible adsorption of a methyl radical on O * , leading to a methoxy species. This methoxy species subsequently leads to carbon dioxide through a series of steps which are potentially infinitely fast.…”