(665c) Systematic Evaluations of Pervaporation Performance and Property Calculations for Water Treatment

Pervaporation is a membrane process that operates on a solution-diffusion mechanism, where a substance is drawn in the vapor phase across a dense, non-porous membrane via either a vacuum or a sweeping gas and then condensed and collected. The driving force for pervaporation is the difference in vapor pressures between the feed and permeate sides of the membrane. Currently, pervaporation is primarily employed for separations of organic substances and water, with some applications in biotechnology. There is also interest in adapting pervaporation for desalination processes, as opposed to membrane distillation (MD) or reverse osmosis (RO). Both MD and RO processes use porous membranes, which can allow undesirable species to pass through the membrane. The non-porous nature of pervaporation membranes lends itself to longer membrane life and excellent selectivity, and also obviates problems of wetting and non-selectivity found in MD processes. Wetting occurs when the solution enters the pores of the membranes and destroys the ability of the membrane to separate the materials. One primary parameter of interest in evaluating the performance of a pervaporation system is the amount of water, referred to as flux, that can be collected on the permeate side of the system. Currently, flux is calculated by determining the mass of water collected by the condensing step in the pervaporation process. The flux is then used to calculate permeance in the membrane, which represents the flux normalized by driving force. Permeance is an intrinsic property of any given membrane, and should in theory remain constant under varying experimental conditions. However, we hypothesize that more water is passed through the membrane than is collected; meaning our current method of flux, and therefore permeance, calculation is inaccurate due to significant amounts of water potentially being lost as vapor. To investigate this hypothesis, we performed a rigorous study of current condensing technologies in order to determine the amount of water captured as permeate in comparison to the amount of water lost to vapor. By determining the amount of water lost, we were able to recommend reformulations of flux calculations for more accurate membrane characterization.