Modelling of diffusion and reaction in porous catalysts using a random three-dimensional network model

Abstract

A new random pore network model has been applied to the problem of diffusion and reaction occurring simultaneously in porous catalysts. The model is not constrained to a regular lattice, nor is the interconnectivity of the pores restricted to a regular branching configuration since both these aspects of the model are inherently random. The diffusion and diffusion/reaction equations are solved numerically over the network for the case of a single, isothermal, irreversible, first-order reaction. Calculated tortuosities are found to vary with Thiele modulus as well as network connectivity and width of the pore-size distribution. The tortuosity under strong reaction control corresponds to the value obtained under diffusion-only conditions. This work suggests that tortuosities measured under diffusion-only conditions may not be generally applicable to conditions where both diffusion and reaction controls are important. For this model the tortuosity obtained under strong diffusion limitation is approximately 4 and increases with decreasing Thiele modulus. In regions of non-negligible diffusion control, values of the tortuosity are in the same range as the experimentally determined tortuosities. An analytical analysis of both random and regular grids yields values of the tortuosity, under conditions of strong diffusion control, which are in agreement with those obtained by numerical simulation. The random network model predicts tortuosities significantly different from those calculated using a regular grid network.