Fundamentals of membrane distillation

The study of nonisothermal flux of air through porous media dates back to 1873, whereas the existence of a nonisothermal liquid transport through membranes was first described in 1907. This phenomenon termed thermo-osmosis (TO) did not involve any liquid/vapor phase transition and was carried out through both dense and porous hydrophilic membranes. About 50. years later, when porous hydrophobic membranes were used and the nonisothermal vapor transport was studied through dry pores, the phenomenon was known as membrane distillation (MD). This nonisothermal membrane separation process is applied mostly in desalination and for the treatment of different types of wastewaters including brines for water production. It was known 50. years ago but has only recently made its way toward industrial applications. It still needs to be improved further in various key aspects. Compared with TO, much more interest is being devoted to MD. The total number of published articles on MD, as of December 31, 2013, is more than seven times greater than that of TO. A significantly increased interest in membrane distillation (MD) technology has been observed during the past 13. years for both its experimental and theoretical aspects, including MD membrane engineering. More than 58% of the research studies were performed using the direct contact membrane distillation (DCMD) configuration. A sharp increase of investigations on fabrication and modification of membranes for MD has been seen during the past 10. years. However, this amounts to only 16.8% of the total published studies on MD. This chapter provides a comprehensive historical perspective of thermo-osmosis (TO) and MD, important key characteristics of MD, membranes used in MD and possible MD technological configurations, different transport mechanisms through MD membranes and developed theoretical models, different fields of applications of MD, and future trends related to interesting and promising research fields in MD.