CONCENTRATION POLARIZATION FORMATION IN ULTRAFILTRATION OF BLOOD AND PLASMA

This paper describes a technique for investigating the kinetics of concentration polarization formation in ultrafiltration of bovine blood, plasma and albumin (BSA) solutions using polysulfone hollow fiber membranes of 30000 Da cut-off. The membrane is submitted to step pressure changes while monitoring the variation of instantaneous permeate flux. When the pressure is suddenly raised from an unpolarized regime to a high polarization one, the permeate flux reaches a peak in 4 to 7 sec and decays back to its equilibrium level in 2 min, for blood and plasma, and in less than 1 min, for albumin solution. With blood, the apparent filtration resistance (neglecting osmotic pressure) reaches its maximum of 6X10(13) m-l when 200 ml of blood has been filtered (on a 0.65 m(2) membrane). The red cell contribution to this resistance is about 20%. The filtration resistance is higher with plasma than with albumin, at the same protein concentration. In the initial buildup of concentration polarization the resistance increases linearly with albumin mass carried towards the membrane, with a rate independent of concentration. Nevertheless, the final resistance at equilibrium increases with increasing albumin concentration. The time for complete reversal of concentration polarization is estimated by repeating pressure increments, while increasing the time interval between two increments, until the permeate peak following the pressure step reaches its maximum. This time is found to be of the order of 12 sec for BSA solutions.