ABSTRACT
Membrane distillation (MD) and its extension membrane crystallization (MCr) are relatively new thermal membrane processes that have garnered significant interest as a potential low-cost, highefficiency, and energy-saving alternative to conventional separation processes. The driving force of both these processes is the partial pressure difference across the membrane. Their main advantages are the theoretically complete rejection of nonvolatile solutes, the relatively low operating pressure/temperature and membrane-
fouling problem. In the case of MCr, other benefits include the wellcontrolled nucleation and growth kinetics, the fast crystallization rates and reduced induction time, membrane surface promoting heterogeneous nucleation, and control of supersaturation level and rate. Despite the great potential of MD and MCr, they are still far from fulfilling all the expectations. To overcome the existing barriers, new and better membranes are necessary for further industrial exploitation of these technologies. Starting with the description of the basic principles of MD and MCr technologies, this chapter summarizes the membranes to be used in these operations together with the important and interesting recent developments in membrane and module fabrication. Future research directions of interest are also pointed out. 12.1 IntroductionMembranes and membrane technologies play a key role in all separation processes due to their capability to work as physical and selective support for the extraction of chemical species from one phase to the other. In the past few decades, numerous research papers have been reported on new membrane materials, aiming at improved separation characteristics (high permeability, high selectivity, high physical/chemical/thermal stability, etc.). Moreover, well-controlled structures combined with process engineering can optimize efficiency and productivity of the separation. The scientific efforts made membrane operations successful substitutes for conventional separation processes. Now membrane processes are extending their application in a wide range of industrial processes and emerging membrane processes, such as membrane distillation (MD) and membrane crystallization (MCr), which are more and more used, e.g., in semiconductors, air and water purification, pharmaceuticals, energy generation, biotechnology (Membrana GmbH, 2006; Li and Sirkar, 2005; Charcosset, 2006; Brunel et al., 2006; Drioli et al., 2006). These techniques belong to the class of membrane contactors in which a non-wetting membrane does not act as a conventional barrier or filter, but promotes mass and energy exchange between two opposite interfaces according to principles of phase equilibrium.
