Fundamental insights into the variety of factors that influence water/alcohol membrane permeation selectivity

Abstract

The primary objective of this article is to develop a fundamental understanding of water/methanol and water/ethanol mixture permeation across microporous membrane constructs. Configurational-Bias Monte Carlo (CBMC) simulations were undertaken for water/methanol and water/ethanol mixture adsorption in all-silica zeolites (CHA, DDR, FAU, LTA) and ZIF-8. Additionally, Molecular Dynamics (MD) simulations were used to determine the intra-crystalline diffusivities in water/alcohol mixtures. The combination of CBMC and MD simulations allow the calculation of membrane permeation selectivities. This study provides insights into the influence of the structural properties of the microporous layer on the permeation selectivity. Another key result that emerges is that the water/alcohol permeation selectivity becomes increasingly in favor of water as the feed mixtures becomes richer in alcohol. The reason for the dependence can be traced to strong hydrogen bonding between water and alcohol molecular pairs resulting in cluster formation and enhanced water ingress; these results provide a rationalization of a number of membrane permeation studies in the literature. A corollary to the reported results is that the use of the Ideal Adsorbed Solution Theory (IAST) is unable to provide a quantitative description of mixture adsorption equilibrium and thermodynamic non-idealities need to be accounted for in modelling membrane permeation of water/alcohol mixtures.