Vacuum versus sweeping gas operation for binary mixtures separation by dense membrane processes

When a gaseous phase takes place at the downstream side of a membrane separation process, two distinct options can be schematically operated in order to induce a driving force from that side: apply a lower pressure than the upstream one (thanks to a vacuum pump for instance), or exert a diluting effect by an inert gas sweep (under higher total downstream pressure conditions). The pros and cons of these two alternatives, particularly in terms of overall energy consumption, are often questioned, for instance for pervaporation applications where the first one (vacuum) is most often chosen but has never been systematically compared to the second one (sweeping gas). A simplified analysis of the above question is presented in this work. Based on a theoretical framework, which has been recently proposed, an easy to handle analytical solution to this problem is obtained. This expression enables the energy consumption under similar separation performances to be computed, providing that the permeabilities of the two compounds of the feed mixture remain constant. More specifically, it is shown that the energy consumption versus total downstream pressure curve shows invariably an asymmetrical bell shape with a maximum located at a fixed position. Operating conditions leading to minimal work of pumping will always be located at one of the two limits of the curve, that is to say either for low vacuum or high flowrate gas sweeping conditions. Nevertheless, taking into account the overall work of separation (i.e. the work needed for pure compounds recovery) clearly plays in favor of the vacuum operation.