Silicone Membranes for Gas, Vapor and Liquid Phase Separations

Silicone Membranes for Gas, Vapor and Liquid Phase Separations 

Paola Bernardo , Gabriele Clarizia and Johannes Carolus Jansen 

Silicones are widely used for different membrane separations. The large diversity of membrane separation processes has resulted in a diiferent optimization of membrane materials and membrane structares for each process. Silicone rubber is the benchmark membrane material for organics-air separation as well as for organics-water pervaporation. The transport properties of silicone polymers, the membrane types as well as the potential and actual industrial applications are presented. Current research trends and recent progress in this field are addressed. Membrane systems based on silicone polymers are discussed, outlining their implications at diiferent levels in the process intensification logic. 

Keywords Siloxane polymers, membrane separation, composite membranes, gas and vapors transport 

An important application field for silicones, taking advantage of their unique chemical and physical properties, is that of membrane separation of gaseous and liquid mixtures. Their function is to provide a selective barrier for different molecular species, where selection takes place either on the basis of size or on the basis of physical interactions or both. 

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Polymers are the most used materials in membrane separation units. This is mainly due to their relatively easy processing by coating or phase inversion techniques and to a good reproducibihty in membrane preparation, which are coupled with reduced costs with respect to inorganic materials. Among the few polymers industrially used, silicones have a prominent role for different applications related to gas, vapor and liquid sepcffations. 

Equation (2.79) expresses the driving force in pervaporation in terms of the vapor pressure. The driving force could equally well have been expressed in terms of concentration differences, as in Equation (2.83). However, in practice, the vapor pressure expression provides much more useful results and clearly shows the connection between pervaporation and gas separation, Equation (2.60). Also, the gas phase coefficient, is much less dependent on temperature than P L. The reliability of Equation (2.79) has been amply demonstrated experimentally [17,18], Figure 2.13, for example, shows data for the pervaporation of water as a function of permeate pressure. As the permeate pressure (p,e) increases, the water flux falls, reaching zero flux when the permeate pressure is equal to the feed-liquid vapor pressure (pIsal) at the temperature of the experiment. The straight lines in Figure 2.13 indicate that the permeability coefficient d f ) of water in silicone rubber is constant, as expected in this and similar systems in which the membrane material is a rubbery polymer and the permeant swells the polymer only moderately. 

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