Membrane separations, multistage systems

Often in practice, several membranes for gas separation are cascaded with recycle streams to effect a multistage system for increasing the separation factor to an effective level, particularly when the individual gases in the mixture have similar permeabilities. 

Of significance is that multistage membrane separations for binary systems can be treated graphically, in a fashion similar to the classical McCabe-Thiele method for binary distillation. This is developed and illustrated in Chapter 4 and affords a convenient means for evaluating separation possibilities, in determining the effect of permeability, reflux... 

Multistep and multistage membrane systems of the type shown in Figure 21.5 have been used to treat a number of vapor-gas streams. However, hybrid processes in which membrane separation is combined with another separation process, are attractive because they may enable each unit operation to operate in its preferred range, improving overall process efficiency. The most important of these hybrid processes is the combination of condensation under pressure with membrane separation, illustrated in Figure 21.6. In this process, a vapor-permeable membrane unit is combined with a vapor condensation-flash unit (Baker and Wijmans, 1994 Baker et al., 1998 Wijmans, 1993/1992). 

In general, high selectivities can be obtained in liquid membrane systems. However, one disadvantage of this technique is that the enantiomer ratio in the permeate decreases rapidly when the feed stream is depleted in one enantiomer. Racemization of the feed would be an approach to tackle this problem or, alternatively, using a system containing the two opposite selectors, so that the feed stream remains virtually racemic [21]. Another potential drawback of supported enantioselective liquid membranes is the application on an industrial scale. Often a complex multistage process is required in order to achieve the desired purity of the product. This leads to a relatively complicated flow scheme and expensive process equipment for large-scale separations. 

The transport of cobalt(II), copper(II), nickel(II), and zinc(II) from aqueous sulfate solutions by novel di(p-alkylphenyl)phosphoric acid carriers in bulk and emulsion liquid membrane transport processes is reported by Walkowiak and Gega in Chapter 13. To probe the mechanism of the liquid membrane transport processes, interfacial tension measurements are conducted. A multistage emulsion liquid membrane system for separation of the transition metal cation mixtures is developed. 

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