Hydrodynamic Sieving MF and UF Separations

Most practical membrane processes are continuous steady-state operations with a feed, permeate, and nonpermeate stream. Since membrane processes involve separation of a permeated component A from a second, rejected component B, a measure of separation efficiency is useful. Due to the diversity of applications, many different measures of separation efficiency are used in the various membrane subareas. Probably the easiest to use measure is the so-called separation factor, given in Equation 7.1, which shows the relative enrichment of component A vs. B due to the membrane process [4]  

Since the SF is a ratio of ratios, any measure of composition (mole fraction, mass fraction, concentration, etc.) can be used in Equation 7.1 as long as one consistently uses the same measure for both upstream and downstream phases in contact with the membrane. Locally within a module, the ratio of compositions leaving the downstream face of a membrane equals the ratio of the transmembrane fluxes of A vs. B. Local fluxes of each component are determined by relative transmembrane driving forces and resistances acting on each component. The ratio of the feed compositions in the denominator provides a measure of the ratio of the respective driving forces for the case of a negligible downstream pressure. This form normalizes the SF to provide a measure of efficiency that is ideally independent of the feed composition. 

For a given driving force, minimization of the membrane resistance requires the smallest possible effective membrane thickness, l. The ability to minimize l without introducing defects relies upon micromorphology control, and this topic impacts virtually all membrane applications. 

Microfiltration (MF) and ultrafiltration (UF) involve contacting the upstream face ofa porous membrane with a feed stream containing particles or macromolecules (B) suspended in a low molecular weight fluid (A). The pores are simply larger in MF membranes than for UF membranes. In either case, a transmembrane pressure difference motivates the suspending fluid (usually water) to pass through physically observable permanent pores in the membrane. The fluid flow drags suspended particles and macrosolutes to the surface of the membrane where they are rejected due to their excessive size relative to the membrane pores. This simple process 

For such micromolecularly selective processes, an additional partitioning phenomenon must also be considered in the flux expression to enable describing the 

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