Diffusion limitation effects in the washcoat of a catalytic monolith reactor

In monolith reactors with catalytic washcoat, intraphase diffusion effects have often been underestimated. This paper presents numerical simulations of diffusion/reaction in the washcoat of a catalytic monolith reactor. The problem is solved in two dimensions for the washcoat geometry obtained in a 1-mm square monolith channel, as revealed by scanning electron microscopy. For the oxidation of carbon monoxide, propane or methane diffusion limitation can be substantial at typical operating conditions. The two-dimensional solution is compared to a one-dimensional approximation. The latter approximation for the real washcoat is valid provided that the characteristic length is selected carefully. This value is different from the characteristic length of the two-dimensional washcoat. The characteristic length of a two-dimensional geometry is not sufficient in itself to describe all washcoat geometries. It is predicted that the application of a diffusion barrier of 4-6 mu m of inactive washcoat may be used to lower the rate of reaction in the inlet region of the monolith reactor.