Chapter Four - Pervaporation

Abstract

This chapter describes the fundamentals and applications of pervaporation as an advanced, energy-intensive separation system. Pervaporation has been developed in the 1980s and has by now matured as a state-of-the-art technology, particularly in some application areas. This is related to the availability of suitable membrane materials and their ability for differential transport, leading to separation between individual compounds in a feed mixture. An overview is given of currently available membranes, polymeric as well as ceramic and hybrid, in three types of separations: (1) hydrophilic separations, which mainly refer to dehydration, (2) hydrophobic separations, i.e., the removal of water from an organic mixture, and (3) organophilic separations in which an organic compound is separated from an organic mixture. Transport through pervaporation membranes is governed by diffusion and is described by relatively simple solution-diffusion models, or more advanced models based on the Maxwell–Stefan equations. Solution-diffusion models make some essential simplifications such as the neglecting of mutual interactions between permeating compounds and interactions with the membrane itself, which yields an approximation that is of use for practical use. The use of this type of models is explained on the basis of transport equations. In addition, the fundamentals and uses of Maxwell–Stefan equations for modelling of pervaporation are clarified, including a practical approach to their application for pervaporation. Details of applications of hydrophilic, hydrophobic, and organophilic pervaporation are further described in separate sections, with focus on available membrane materials (including blended and modified structures) and their performance as reported in the scientific literature. It is shown that a large number of potential membrane materials have been studied, although few have been scaled-up so far. Nevertheless, the general trend in the applications described in this chapter is clear. One specific application, the purification of (bio)ethanol starting from dilute solutions, is described in more detail in view of its importance for large-scale applications. Ethanol purification is an interesting application for several reaons: (1) the potential of significant energy savings in comparison with the reference technology, distillation, (2) the combination of a hydrophobic application (removal of ethanol from water) and a hydrophilic application (upgrading of the produced ethanol), and (3) the challenge in developing a membrane material combining a high permeability with a high separation factor for ethanol and water. An outlook for the potential of pervaporation for purification of (bio)ethanol is given. Integration of pervaporation with systems for reaction or separation is further described based on the potential of available membranes for separation. Hybrid separation systems often combine pervaporation for azeotropic compositions with distillation for a wider concentration range. Combined reaction/separation systems are found in pervaporation membrane reactors, in which one component is separated from a reaction medium (either the product or a by-product) by an integrated pervaporation membrane. These integrated systems are described from the point of view of the role of pervaporation.