A study of permeation of n-butane through ZSM-5 membrane by using Monte Carlo and equilibrium/non-equilibrium molecular dynamics simulations

Molecular simulations have been performed on permeation and adsorption of n-butane in a ZSM-5 zeolite membrane. In all simulations, a flexible potential model is used for n-butane since the smallest size of the permeating molecule is 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 MC method are in good agreement with experimental adsorption data, being represented by the Langmuir adsorption isotherm model. Permeability of n-butane calculated from the muVT-NEMD method has a maximum against temperature, which agrees qualitatively with the experimental results. It is noted that the whole pores in the membrane are filled with molecules at room temperature. As compared with the local values of the Fick and the Maxwell-Stefan diffusion coefficients (D-F, D-MS) and the self-diffusion coefficients in the permeate direction (D-s,D-x), we confirm that the D-MS is almost independent of molecular loadings in the membrane, and that D-F > D-MS > D-s,D-x at high loadings. In addition, the three diffusion coefficients are almost identical at the zero loading. By providing a modification for the distribution coefficient (S) and diffusion coefficient (D) in the case of nonlinear isotherms, we have clearly demonstrated that the permeation at low temperatures is controlled by the diffusion process, where whole pores are filled with molecules, and then the controlling step changes to the adsorption process at higher temperature where the pores are characterized by medium loadings.