DIFFUSION LIMITATION EFFECTS IN THE WASHCOAT OF A CATALYTIC MONOLITH REACTOR

In monolith reactors with catalytic washcoat, intraphase diffusion eff ects have often been underestimated. This paper presents numerical sim ulations of diffusion/reaction in the washcoat of a catalytic monolith reactor. The problem is solved in two dimensions for the washcoat geo metry obtained in a 1-mm square monolith channel, as revealed by scann ing electron microscopy. For the oxidation of carbon monoxide, propane or methane diffusion limitation can be substantial at typical operati ng conditions. The two-dimensional solution is compared to a one-dimen sional approximation. The latter approximation for the real washcoat i s valid provided that the characteristic length is selected carefully. This value is different from the characteristic length of the two-dim ensional washcoat. The characteristic length of a two-dimensional geom etry 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 th e inlet region of the monolith reactor.