The effect of surface diffusion on the catalytic selectivity of concurrent and consecutive reactions

Abstract

The effect which surface diffusion has on the selectivity of certain concurrent and consecutive catalytic reactions is investigated numerically by analysis of a simple mathematical model of the parallel chemical kinetic and physical transport processes occurring in the porous catalyst medium. Nonisothermal and interparticle mass transfer effects are considered in the analysis. For exothermic concurrent reactions selectivity is enhanced by surface diffusion when the preferred product is formed by the reaction with the greater activation energy. Convection of heat away from the catalyst particle, on the other hand, reduces the selectivity. Although the selectivity of exothermic consecutive reactions is also enhanced by surface diffusion for low values of the Thiele modulus, when the reaction occurs in a diffusion-limited region, surface diffusion further exacerbates the already restricted selectivity. Increased heat transfer also has a deleterious effect on the reaction selectivity. Diametrically opposed to the results obtained for exothermic reaction are the trends observed for endothermic reactions, the selectivities of which are explained in terms of the magnitude of the intra and interparticle mass and heat transport processes. The study concludes with a preliminary examination of the manner in which selectivity is affected when resistance to heat transfer is confined to a relatively stagnant boundary layer of fluid bathing the catalyst pellet. It is shown that, for certain ranges of parameters, multiple solutions exist.