Membrane distillation for desalination and other separations

Membrane distillation is an emerging membrane technology used for desalination of seawater or brackish water, solution concentration, recovery of volatile compounds from aqueous solutions and other separation and purification processes. Membrane distillation differs from other membrane technologies in that the driving force for separation is the difference in vapor pressure of volatile compound across the membrane, rather than total pressure. The main advantage of membrane distillation over the conventional thermal distillation is that membrane distillation could occur at a much lower temperature than the conventional thermal distillation. The membranes used in membrane distillation are hydrophobic, which allow water vapor to pass through but not liquid solution. The vapor pressure gradient is created by heating the feed solution and cooling/purging the condensate in the permeate side. Therefore, membrane distillation enables separation to occur below the normal boiling point of the feed solution and could utilize low-grade heat from alternative energy sources. The objective of this review is to cover the basic principles and configurations of membrane distillation process, membrane physical characteristics, heat and mass transfer characteristics, and the effect of operating conditions. Also, major applications of this new technology in desalination, food industry and environmental protection, and latest patent developments and future trend in membrane distillation are presented.