Molecular simulations were made of the permeation and adsorption of n-butane in a ZSM-5 zeolite membrane. In all of the simulations, a flexible potential model was used for n-butane since the smallest size of the permeating molecule was almost the same as the pore size of the ZSM-5 crystal. Equilibrium densities of n-butane in the ZSM-5 membrane calculated from the Monte Carlo method were in good agreement with experimental adsorption data, being represented by the Langmuir adsorption isotherm model. The permeability of n-butane calculated using the μVT-NEMD method exhibited a maximum versus temperature; which agreed qualitatively with experimental results. It was noted that the whole of the pores in the membrane were filled with molecules at room temperature. As compared with the local values of the Fick and Maxwell-Stefan diffusion coefficients (DF, DMS) and the self-diffusion coefficients in the permeate direction (Ds,x), it was confirmed that DMS was almost independent of molecular loadings in the membrane, and that DF > DMS > Ds,x at high loadings. In addition, the three diffusion coefficients were almost identical at zero loading. By providing a modification for the distribution coefficient and diffusion coefficient in the case of non-linear isotherms, it was clearly demonstrated that permeation at low temperatures was controlled by the diffusion process, where entire pores were filled with molecules, and the controlling step then changed to adsorption at higher temperature where the pores were characterized by medium loadings.

A Study of Permeation of N-Butane through ZSM-5 Membrane by using Monte Carlo and Equilibrium/Non-Equilibrium Molecular Dynamics Simulations. S.I.Furukawa, T.Nitta: Journal of Chemical Engineering of Japan, 2003, 36[3], 313-21