Transport parameters of alcohol vapors through ion-exchange membranes

Abstract

According to the solution-diffusion model, the separation of vapors and liquid mixtures by nonporous membranes in sweeping gas pervaporation or vacuum pervaporation is due to the differences in the solubility and the diffusivity of the mixture components in the membrane material. In the case when ion-exchange membrane is used, the membrane ionic groups and counter-ions can play an important role in the mass transfer process. The permeation rate measurements which are based on the sweeping gas pervaporation concept were performed in order to determine the diffusivity of chosen aliphatic alcohol vapors (i.e. ethanol, i-propanol and t-butanol). The experiments were carried out using different sulfonic ion-exchange membranes: Nafion (perfluorinated polyethylene with pendant ether-linked side chains terminated with sulfonated groups), IonClad (irradiation grafted sulfonated styrene monomers onto polytetrafluoroethylene film) and PESS (membrane prepared by sulfonation of the interpenetrating polymer network system polyethylene–poly/styrene- co-divinylbenzene/) loaded with H +, Li + or K + ions as counterions. NAFION and IonClad membranes have the same backbone (i.e. polytetrafluoroethylene). The ionic strength of sulfonic groups is identical for IonClad and PESS membranes whereas the NAFION membrane get a much greater ionic strength. The diffusivity of ethanol was greatly affected by the presence of water in the membrane. The diffusivity characteristics in the sulfonated ion-exchange membranes indicated strong interactions between water and alcohol molecules. The increase of alcohols polarity increases their diffusion properties. The diffusion coefficient of t-butanol in investigated membranes increases when lithium is replaced by potassium.