Plasma filtration in Couette flow membrane devices

This report investigates the effects of transmembrane pressure (ptm), rotation speed, and membrane characteristics on the performance of a rotating membrane device for separating plasma from whole blood. This device consists of 58 cm2 polymeric membrane rotating at high speed (3,000-4,000 r/min) inside a concentric cylinder and produces a Couette type flow in a gap 0.9 mm thick. Blood enters tangentially at the top, exits tangentially at bottom, and plasma is collected by an axial duct. It is found that this device yields maximum filtration velocities (plasma filtration flux per unit area) in the range of 0.5-0.8 cm/min, which are 10-20 times larger than those obtained by using hollow fiber plasma filters. This high performance is not due to the effect of centrifugal forces on red blood cells but rather to the very high shear rates, on the order of 20,000 sec-1, generated in the gap by toroidal flow instabilities (Taylor vortices). When a 0.8 micron pore size Nuclepore (polycarbonate) membrane is used, the filtration velocity presents a narrow peak of 0.6 cm/min at ptm = 10 mm Hg at 3,000 r/min and decreases to 0.35 cm/min at larger ptm. With a 0.5-micron pore nylon membrane the filtration speed increases continuously with ptm and reaches a plateau of 0.5 cm/min.