Understanding membrane selectivity in pervaporation of water‐rich water:ethanol mixtures

Abstract

In pervaporation, membrane selectivity is defined as the ratio of the ratios of two components of a liquid:liquid mixture in the permeate and in the feed. In a thermodynamically-controlled system, this value will be the same as the difference between vapour and liquid composition in the absence of a membrane. We demonstrate this to be the case for water:ethanol pervaporation at high water:ethanol ratios both for an unmodified polydimethylsiloxane membrane and for a membrane modified by grafting a layer of hydrophilic polyacrylamide. We observe transient kinetic deviations toward greater selectivity on addition of salts which push the thermodynamic vapour pressure equilibrium towards ethanol, and to a more significant degree with grafting of polyacrylamide to the hydrophobic membrane, suggesting that these modifications retard the flow of water through the membrane. The physical plausibility of the chemical potential gradient used in interpretation of pervaporation data by the solutiondiffusion model is critiqued.

Hydrophobic Gas Transfer Membranes for Wastewater Treatment and Resource Recovery

“…Selectivity is defined as the ratio of extracted target compounds to overall recovery. 181 The constant fluctuations in the composition of waste streams pose distinctive challenges to resource recovery from wastewater, compared to other separation processes. In particular, recovering a specific compound from a complex mixture presents a uniquely difficult proposition.…”

Issue Highlights

“…For water:ethanol pervaporation at high water:ethanol ratios, we demonstrate that this value is the same as the difference expected between liquid and vapour composition in the absence of a membrane, both for an unmodified polydimethylsiloxane membrane and for a membrane modified by grafting hydrophilic polyacrylamide. We observe transient kinetic deviations toward greater selectivity on the addition of salts, which are significantly enhanced on the grafting of polyacrylamide to the hydrophobic membrane …”