Diffusivities in the binary components system within MFI-type zeolite crystals

Diffusivities within crystals of highly siliceous MFI-type zeolite in a binary component system were measured over a temperature range of 448-498 K and under a total pressure of 13.3 Pa by a newly developed DRP method using paraffins (n-heptane, n-octane) and aromatics (ortho- and meta-xylenes). The diffusivities of the fast diffusing molecules were strongly affected by the co-existence of slow diffusing molecules. The diffusion of the former decreased as the number of slow molecules increased and the intrinsic diffusion rate of the slow molecules decreased. In contrast, the diffusivities of the slow molecules were not affected by the co-existence of fast molecules. To account for the difference in the diffusivities in the single and binary component systems, the random walk simulation was conducted. The difference in the diffusivities simulated agreed well with the results found experimentally. An empirical equation is proposed for the simple prediction of the reduction of the diffusivity of fast molecules in a binary components system by modifying the previously reported equation for the decrease in diffusivity caused by coke deposition. The estimated value using the empirical equation agreed well with the experimental data and the data calculated by the random walk simulation. (C) 2000 Elsevier Science B.V. All rights reserved.