The driving force as key element to evaluate the pervaporation performance of multicomponent mixtures

Pervaporation presents an efficient alternative to other high energy demanding technologies, such as distillation, in the separation of challenging mixtures in the chemical industry (e.g., azeotropic mixtures). In this work, the transesterification reaction between butanol and methyl acetate to produce methanol and butyl acetate is considered as case of study of an equilibrium-limited reaction. The objective is to emphasize the importance of a prior evaluation of the driving force of each compound in order to determine when pervaporation is an appropriate technique to perform the separation of multicomponent mixtures. Three commercial membranes from PolyAn GmbH (Germany) and two membranes from Sulzer Chemtech (Switzerland), selected from a previous study, are used for the separation of equimolar mixtures of methanol-methyl acetate-butanol-butyl acetate, at 30, 40 and 50 °C (±2 °C). Results show that all the studied membranes present a preference for the permeance of butanol, reaching permeances of butanol until 140,000 GPU. However, methanol and methyl acetate are the compounds with a higher driving force due to their higher volatility, leading to higher concentration in the permeate and preferential separation instead of butanol. Thus, the study of permeances/permeabilities and selectivities is required but not sufficient to evaluate the real performance of pervaporation. The driving force has to be evaluated, and membranes that enhance the effect of the driving force by permeating the species with the highest driving force should be preferably selected. Then, pervaporation is used under conditions of maximum performance.