SIMULATION OF DIFFUSION AND ADSORPTION IN ZEOLITES

We show that Poisson-distributed event times are required to correctly simulate diffusion and adsorption in zeolites. The main microscopic assumptions of the diffusion model are: the zeolite is a periodic array of sites, each of which may contain only one molecule; and transport is achieved by sorbate molecules randomly "jumping" from one site to an adjacent site. Three simulation schemes for moving the molecules are compared; two of the schemes exhibit significant deviations from expected behavior, showing that seemingly reasonable schemes can introduce subtle correlations that have observable consequences. The preferred third scheme implements Poisson-distributed event times and exhibits no significant deviations from expected behavior-even for transient conditions and high occupancies. The simulation technique is then extended to independent Poisson-distributed events of more than one type. Specifically, adsorption events are added to the simulation, resulting in Langmuirian adsorption and concentration independent intralattice diffusion, in agreement with experimental results for some zeolites. The simulation correctly reflects the microscopic assumptions, thus placing this type of simulation on a sound theoretical foundation. © 1991.