MD is a thermally driven process, in which water vapour transport occurs through a non wetted porous hydrophobic membrane. The term MD comes from the similarity between conventional distillation process and its membrane variant as both technologies are based on the vapour-liquid equilibrium for separation and both of them require the latent heat of evaporation for the phase change from liquid to vapour which is achieved by heating the feed solution. The driving force for MD process is given by the vapour pressure gradient which is generated by a temperature difference across the membrane. As the driving force is not a pure thermal driving force, membrane distillation can be held at a much lower temperature than conventional thermal distillation. The hydrophobic nature of the membrane prevents penetration of the pores by aqueous solutions due to surface tensions, unless a transmembrane pressure higher than the membrane liquid entry pressure (LEP) is applied. Therefore, liquid/vapour interfaces are formed at the entrances of each pore. The water transport through the membrane can be summarized in three steps:
Membrane distillation is a relatively new membrane separation process which might overcome some limitations of the more traditional membrane technologies. In particular high solute concentrations can be reached and ultra pure water can be produced in a single step. The possibility of an industrial development of this technology is related to the growing commercial availability of membranes of potential interest. When micro porous hydrophobic membrane separates two aqueous solutions at different temperatures, selective mass transfer across the membrane occurs: this process takes place at atmospheric pressure and at temperatures which may be much lower than the boiling point of the solutions. The hydrophobicity of the membrane prevents the transport of the liquid phase across the pores of the partition while water vapor can be transported across them from the warm side, condensing at the cold surface. The driving force is the vapor pressure difference at the two solution membranes interfaces. Because the process can take place at normal pressure and low temperature, Membrane distillation could be used to solve various wastewater problems, to separate and recover chemicals, and also the high osmotic pressure aqueous solution of substances sensitive to high temperatures. The possibility of using solar, wave or geothermal energy, or existing low temperature gradients typically available in industrial processing plants is particularly attractive. The fundamental simplicity of traditional distillation is compromised by various factors such as the need for complete removal of all non condensable gases. The use of vacuum pumps, high pressure vessels, deaeration devices, etc. are required for removing the effects of the non condensable gases, with a significant energy consumption. A number of distillation processes have been proposed with the aim of eliminating the need for creating a vacuum. Hydrophobic micro porous membranes allow easy passage of water vapor, but completely block the flow of liquid water. Surface tension of the water prevents its passage through the pores of the hydrophobic material. If feed water is in contact with one of the surfaces of the membrane, the gap distance between the evaporation and condensation surfaces could be reduced to the thickness of the membrane, thus preventing contamination by the feed water. Early work was presented by Findley (1967) and Findley et al. (1969) where transport in vapor phase across porous partitions was studied. The membrane materials used in these works were paper hot cup, glass fibers, aluminum foil and similar. The efficiency of the process was related to the stability of membrane materials. In the 1980s new microporous hydrophobic membranes became commercially available and membrane desalination (MD) again attracted significant attention.
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