COMPUTER-SIMULATIONS OF CATALYST DEACTIVATION .1. MODEL FORMULATION AND VALIDATION

Catalyst deactivation due to site coverage and pore blockage is a percolation process, and is characterized by a transition point (percolation threshold) at which a previously connected sample-spanning cluster of open (unplugged) pores becomes disconnected, so that macroscopic transport and reaction are no longer possible. In a percolation process the length scale over which the system is macroscopically homogeneous (i.e. the length scale over which macroscopic continuum equations of transport and reaction are applicable), is of fundamental importance. Most previous models have not taken this fact into account and, as a result, may not be valid over certain macroscopic length scales. We have developed a percolation model of catalyst deactivation which is formally correct both at microscopic and macroscopic length scales. The porous catalyst is represented by a network of interconnected pores. The effect of finite molecular size of the reactants, and the reslting hindered diffusion, is taken into account. When applied to catalytic pore plugging during hydrodemetallation, the predictions are in quantitative agreement with the experimental data.