Ultrafiltration

Abstract

“Ultrafiltration” is a process of separation whereby a solution containing a solute of molecular dimensions significantly greater than that of the solvent is depleted of solute by being forced under an hydraulic pressure gradient to flow through a suitable membrane. “Reverse osmosis”, ultrafiltration, and ordinary filtration differ superficially only in the size scale of the particles which are separated; differentiation between the three is in large measure arbitrary. It is, however, convenient to reserve the term “reverse osmosis” for membrane separations involving solutes whose molecular dimensions are within one order of magnitude of those of the solvent, and to use “ultrafiltration” to describe separations involving solutes of molecular dimensions greater than ten solvent molecular diameters, and below the limit of resolution of the optical microscope (ca. 0.5 μ). “Ultrafiltration” thus encompasses all membrane moderated, pressure activated separations involving solutions of modest molecular weight (ca. 500 and up) solutes, macromolecules, and colloids. At present, ultrafiltration processes are largely confined to aqueous media, and most of what follows relates to aqueous systems. There are, however, no fundamental reasons why ultrafiltration cannot be performed with non-aqueous solvents (utilizing, of course, solvent resistant membranes); as a matter of fact, there are numerous commercially important petroleum and petrochemical purifications which can and ultimately will be performed by ultrafiltration with suitably constituted membranes.