Separation of Vapor-Phase Alcohol/Water Mixtures via Fractional Condensation Using a Pilot-Scale Dephlegmator:  Enhancement of the Pervaporation Process Separation Factor

In pervaporation, a liquid mixture contacts a membrane surface that preferentially permeates one of the liquid components as a vapor. Our approach to improving pervaporation performance is to replace the one-stage condenser traditionally used to condense the permeate with a fractionating condenser called a dephlegmator. For example, pervaporation of 5 wt % aqueous ethanol yields a vapor containing 35 wt % ethanol. The separation factor for the process is 10. Condensation of this vapor in a dephlegmator yields a vapor product stream containing 90% of the permeating ethanol at a concentration of 85 wt % ethanol. The net result of the combined pervaporation−dephlegmation process is to transform the 5 wt % ethanol feed into an 85 wt % ethanol condensed product. For the overall process, the separation factor increases 11-fold to 108. Pilot-scale parametric experiments were performed with a plate−fin heat exchanger operated as a dephlegmator. The process was modeled with commercial process simulation software; good agreement between the model and the pilot results was obtained.