Membrane distillation (MD) is a thermally driven separation process that employs a hydrophobic membrane as a barrier for
IntroductionMembrane distillation (MD) is a thermally driven separation technique using microporous hydrophobic membranes and performing on the principles of vapor-liquid equilibrium under different configurations. In this process, only volatile compounds (mainly water) of the feed stream evaporate at the membrane pore entrance, cross the membrane pores in vapor phase to finally be either condensed or removed as a vapor from a membrane module. The hydrophobic nature of the membrane prevents the pores from wetting by capillary forces. MD is known as a promising technology for many applications such as desalting seawater, brackish water, highly saline water [1,2], and removing organic compounds and heavy metals from aqueous solutions [3,4]. MD has also been used to manage waste water such as radioactive waste waters, oily waste waters [5], where the product could be safely discharged to the environment or the waste streams could be reused in an appropriate industrial activity. In biotechnology and food processing applications, MD has also been found as a promising tool, for instance, for removing ethanol and other metabolites from fermentation broths [6], for gentle concentration of valuable compounds in fruit juices [7], and in herb extract such as Ginseng [8].MD has many attractive features as compared to conventional separation processes. Low operating temperatures (~30-70°C) is one of them since the feed is not necessarily heated up to the boiling point like in thermal distillation. Thus, MD may advantageously utilize alternative energy sources, such as solar energy, geothermal energy, waste heats from power plant, etc. [9]. Compared with pressure driven membrane filtration processes such as nanofiltration or reverse osmosis, lower operating pressure translates to lower equipment costs and increased process safety. It is worth highlighting that membrane fouling in MD seems to be less of a problem for many applications than that in pressure-driven filtration processes [10].MD is, however, attended by some drawbacks. Compared to reverse osmosis, MD is known to have a lower permeate flux, and the susceptibility of permeate flux to processing conditions, particularly to temperature and concentration, is considerably high. Also, the trapped air within the membrane pores
