Separation and Fractionation of Macromolecular Solutions by Ultrafiltration

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Abstract
The present state-of-the-art of membrane ultrafiltration with reference to macromolecular fractionations is reviewed. Ultrafiltration is now a widely used technique, both in the laboratory and industrial applications, which stems from the development of asymmetric membranes followed by the recognition of the importance of fluid mechanical and mass transfer processes and their management through equipment design and fluid-flow practices. However, large-scale fractionation of macromolecular mixtures or solutions such as proteins has not yet been feasible. This inability is attributable to a number of factors, viz., concentration polarization and fouling processes which may also be coupled with limitations imposed by nonuniform pore size as well as protein–protein (solute) interactions, the latter being determined by the solution chemistry. It is now well recognized that boundary-layer and interfacial effects, in general, are extremely important in membrane applications, as evidenced by a number of manifestations. Several models have been put forward to explain the effects of concentration polarization, whereas membrane-fouling owing to solute-membrane interactions and membrane pore-obstruction or secondary membrane formation via macrosolute deposition, thus causing major changes in effective pore size distribution and therefore effecting inevitable changes in membrane characteristics, have hardly been considered in detail in ultrafiltration transport modeling. Nevertheless, the recognition of the importance of surface and colloid chemical phenomena in governing membrane performance has focused attention upon techniques of membrane modification and feed solution properties control as the key to ultrafiltration applications. These are particularly important for macro-molecular fractionations which depend upon a reasonably clear understanding of the mechanisms of the various processes and which emerge from a good deal of basic or fundamental research.