Simulation of the isomerization of an ortho-diethylbenzene on zeolites using a continuum approach and different diffusion models

Abstract

The overall kinetics of the isomerization of the dialkylbenzenes on zeolites such as ZSM-5 are affected by the intrinsic kinetics and, in many cases, additionally by configurational diffusion inside the microporous zeolite. Simulation studies on the basis of a continuum model allow a deeper insight into this complex situation. It has been shown that both a reactant diffusion limitation and product shape selectivity (different diffusion rates of the products) may influence the overall kinetics (conversion/selectivity). Calculating concentration profiles inside the crystallite gives additional details about the interrelation of diffusion and reaction. Due to the strong interaction between the diffusing molecules themselves, as well as between diffusing molecule and zeolite surface, various diffusion models have been implemented. Depending on the diffusion model used, it could be shown that at high occupancies, differences occurred only in the calculated conversions, but not in the calculated selectivities. On the basis of the simulation results a recommendation is given concerning the determination of diffusion coefficients from measured overall kinetics. Furthermore, it was found that selectivity strongly depends on the chosen type of reactor, e.g. a plug flow reactor or an ideally mixed reactor. This is important for a comparison of data measured in different experimental set-ups.