Membrane distillation
Membrane distillation (MD) is a thermally driven process, in which a water vapour partial pressure difference between both sides of a porous hydrophobic membrane is created by means of a temperature difference. This leads to water evaporation on the hot side of the membrane, transportation of the vapour through the porous membrane and condensation on the colder side of it. Only vapour molecules are capable of passing through the membrane, due to its hydrophobic nature the membrane cannot be wetted (up to a certain limiting pressure, which depends on the membrane and fluid characteristics) as a result vapour/liquid interfaces are to be formed on both sides of the membrane. The goal of this technology is its lower operating temperatures and pressure, solutions having temperatures much lower than its boiling point under pressures near atmosphere can be used [5], allowing it to use low-grade waste and/or alternative energy sources, such as solar energy. To maintain the necessary partial vapour pressure difference that drives the process, there are several configurations that could be applied (depicted in Fig.1).
Direct contact membrane distillation (DCMD) in which both the permeating flux and the coolant are in contact with the membrane; vacuum membrane distillation (VMD) in which the vapour is vacuumed out of the module and condensed if needed, in a separate device; air gap membrane distillation (AGMD) in which an air gap separates the condensation surface and the membrane and sweeping gas membrane distillation (SGMD), in which the vapour is carried out of the module by a stripping gas and condensed outside. Amongst them, DCMD and AGMD, because they do not need an external condenser to recover the distillate, are best suited for applications where water is the permeating flux [6] and therefore for desalination. In DCMD the vapour diffusion path is limited to the thickness of the membrane, thereby reducing mass and heat transfer resistances; AGMD has an additional gap interposed between the membrane and the condensation surface giving rise to higher heat and mass transfer resistances. Between these two, DCMD due to its configuration shows the highest heat looses (by heat conduction across the membrane matrix), although its distillate production is greater than that of the AGMD. It should be pointed out that experiences with AGMD have the lowest permeate flux amongst all the configurations but on the other hand, the air gap interposed between the membrane and the cooling plate reduces the heat losses by conduction and the temperature polarization [5], one of the
biggest problems to be solved in MD, and also allows larger temperature differences across the membrane, which can compensate partially the mass transfer resistance. VMD and SGMD have higher permeate fluxes and lower (almost negligible in the case of VMD) conduction heat losses, but as energy efficiency is a key factor in MD, and in both configurations it is difficult to achieve heat recovering, and taking into account what said before about not being suitable for water applications, these configurations have received little attention in comparison with DCMD and AGMD.
