Molecular Simulation Of Alcohol/Water Mixtures' Adsorption And Diffusion In Zeolite Molecular Sieve Membrane

Zeolite membranes have broad application prospects in the areas of Pervaporation, gas separation and membrane reactor because of their unique physical and chemical properties, and their pore size in the similar range to the solvent molecular size. 4A and MFI-type zeolite membranes have an excellent performance in the separation of alcohol/water mixtures. It is important to study the relation between the microstructure of systems of solvent/zeolite membranes with the properties of thermodynamics and mass transfer, which can provide the theoretical background for understanding the absorption and diffusion of the solvent component in the zeolite membranes. In this paper, grand canonical Monte Carlo (GCMC), molecular dynamics (MD) were used to study the adsorption and diffusion behaviors of methanol/water and ethanol/water mixtures in the 4A and MFI zeolite membranes, respectively.By adjusting the atomic charge of methanol, ethanol and water molecules, the adsorption isotherms, adsorption heats and the self-diffusion coefficients of each component at different loadings were calculated with the aid of COMPASS force field, and compared with the experimental observation reported in literature. At the same time, the effect of time-steps and rigid, partial rigid, flexible zeolite models on the simulation results was studied. The parameters and methods for the simulation were thus determined. And then the adsorption and diffusion behaviors of methanol/water and ethanol/water mixtures in zeolite 4A were investigated, including the adsorption isotherm of each component, the self- diffusion coefficients, distribution profiles, typical trajectories of water and hydrogen bonding at various feed alcohol concentrations. A good separation performance of 4A zeolite for the methanol/water and ethanol/water was explained at the molecular level.Then, the models for the simulation were constructed according to the practical separation process. MD method was used to study the permeation of the methanol/water and ethanol/water mixtures through MFI zeolite membrane. First, the permeation of a single component was investigated, and it is confirmed that the MFI zeolite membrane is hydrophobic. And then the permeation of each component in alcohol/water mixtures at different alcohols concentrations from the feed side to the permeate side (under vacuum) was studied, including distribution profiles, hydrogen bonding, etc. Concentration polarization phenomena in the process of membrane separation was intuitively observed. At last, the self-diffusion coefficients of each component were compared with those experimentally determined.