Rubbery vapor-permeable membranes

Rubbery membranes provide better selective purge capability. The final reason that mbbery, vapor-permeable membranes are usually preferred is illustrated in Figure 21.2, which shows flow schemes for two types of membrane systems used... 

In the separation of vapor/gas mixtures, rubbery polymers, such as silicone rubber, can be used to permeate the more condensable vapor components, or glassy polymers can be used to permeate the smaller gases. Although glassy, gas-permeable membranes have been proposed for a few applications, most installed plants use... 

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. 

To effectively use ionomer membranes for dehydration applications it is necessary to understand water transport in these polymers. Molecular diffusion in swollen polymers does not follow the classical Fickian behavior. Fickian behavior is observed for diffusion of gases at low pressure through rubbery polymers at temperatures well above Tg. Under these conditions permeability is independent of gas pressure. Glassy polymers show pressure dependent permeabilities. These effects disappear at higher pressures and can be explained by dual mode theory. Similarly, permeabilities of vapors such as water in hydrophobic or mildly hydrophilic membranes are independent of water vapor pressure. 

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