Nonporous membrane contactors

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

In general porous membranes are used in membrane contactors in which the membrane primarily acts m a barrier between the phases (It is also possible that nonporous membranes are used in this case, e.g. silicone rubber membranes. This will be considered at the end of this section, see figure VI - 59). Now two concepts are possible where the pores are either filled with the gas phase or with the liquid phase. 

The liquid-liquid membrane contactor is characterised by two liquid streams separated by a porous or nonporous membrane. In case of a porous membrane the feed phase may either wet or not wet the membrane. Firstly we will consider the case where the feed is an organic solvent from which a solute has to be removed while the permeate phase is an aqueous phase. If now a hydrofobic porous membrane is used the membrane will be wet and the pores will be filled. At the permeate side an aqueous stream is pumped now which does not wet the membrane and is not miscible with the organic solvent. An interface will formed at the permeate side (figure VI - 58a) and the actual liquid-liquid extraction will occur at this interface. If the feed is an aqueous stream and the membrane is hydrofobic then the feed will not wet the membrane. The (hydrofobic) organic solvent is now used at the permeate side and this will wet the membrane which implies that now the interface is forrried at the feed/membrane side (figure VI - 58b). Figure VT - 58 also... 

Bessarabov DG, Jacobs EP, Sanderson RD, Beckman IN. Use of nonporous polymeric flat-sheet gas-separation membranes in a membrane-liquid contactor Experimental studies. JMembrSci 1996 113 275-284. 

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