The major requirement of MD membranes is that they not be wetted by the process liquids. To avoid liquid invasion of the pores, highly hydrophobic membranes with an appropriate pore size are used. The liquid surface tension also affects wetting. Organic solutes present in an aqueous solution reduce the surface tension to the point where spontaneous membrane wetting may occur. At this point, the surface tension is called the critical surface tension at which MD is no longer possible. Franken et al. found that the maximum allowable concentration of organic material in water cannot be calculated but has to be determined experimentally .
The second major consideration in membrane selection for this process is pore size and porosity. High porosities are of special interest since the area available for evaporation is directly related to flux. However, high porosities are usually associated with large pore sizes, which are undesirable as they increase the risk of membrane wetting.
In MD, the membrane is not involved in the transport phenomena on the basis of its selective properties. Volatile compounds are transferred across the membrane according to vapor—liquid equilibrium principia, whereas the microporous polymeric material acts as physical barrier between two phases and sustains the interfaces where heat and matter are simultaneously exchanged. Since the hydrophobic character of the membrane represents a crucial requirement, membranes have to be made from polymers with a low value of surface energy. Polymers such as polypropylene (PP), polytetra-fluoroethylene (PTFE), and polyvinylidene fluoride (PVDF) are commonly employed in the preparation of membranes for MD applications .
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