Ultrafiltration is a membrane separation process in which solvent is removed from solutions containing high-molecular-weight solutes such as proteins. The principle is similar to that of reverse...

Ultrafiltration is a membrane separation process in which solvent is removed from solutions containing high-molecular-weight solutes such as proteins. The principle is similar to that of reverse osmosis, in that pressure is applied to the solution on the feed side of a supported membrane and solvent passes through the membrane. The high-molecular-weight solutes cannot pass through the membrane. One difference from ordinary reverse osmosis is that the osmotic pressure caused by the solutes, even at high concentrations, is usually felt to be negligible because of the high solute molecular weight. Another difference is that the solutes may have only a limited solubility, so that a layer of precipitated solutes, or gel, can readily form adjacent to the membrane surface on the feed side. The performance of ultrafiltration devices has been successfully interpreted in terms of rate limitations on the solvent flux due to the resistances to solvent flow from both the membrane itself and varying thicknesses of gel or precipitated solutes on the surface of the membrane. The thickness of this geI layer and its consequent resistance to solvent permeation represent a steady-state balance between the rate at which the solute is brought to the membrane surface by convection with the permeating solvent, on the one hand, and the rate of mass transfer of solute back into the bulk feed solution, on the other: see for example, Porter, (1972). Porter (1972) presents water fluxes (in cubic centimeters of water per minute and per square centimeter of membrane area) observed in a stirred ultrafiltration device of the sort shown in Fig. 1 when the feeds were aqueous solutions containing varying concentrations of bovine serum albumin: these data are shown in Fig. 2. In the case of 0.9% saline solution (equivalent to the albumin solutions with zero concentration of albumin) the membrane was sufficiently "open" for nothing to be retained: the salt passes freely through the membrane with the water.