KINETICS OF CONCENTRATION-POLARIZATION FORMATION IN CROSS-FLOW FILTRATION OF PLASMA FROM BLOOD - EXPERIMENTAL RESULTS

The formation and disappearance of the concentration-polarization layer in crossflow microfiltration of blood has been investigated by submitting a membrane to step changes in the transmembrane pressure, while recording the time variation of pressure and permeate flux. When the pressure is suddenly raised from an unpolarized regime to a level corresponding to complete concentration-polarization, the permeate flux reaches a peak in 0.4-0.6 sec, which surpasses the equilibrium level by 60 to 80% depending upon the pressure, returning to the concentration-polarization equilibrium level in 3-4 sec. This data shows that the membrane retains its original permeability only during the first 0.5 sec of the pressure change and that concentration-polarization takes about 3-4 sec to build-up. This formation time decreases with increasing pressure. When the pressure returns to its initial level, the concentration-polarization disappears instantaneously and the process can be repeated at a frequently up to 0. 7 Hz. However, when a stepped pressure increase is applied in the concentration-polarization regime, with initial pressure above 150 mmHg, the permeate flux hardly changes, which confirms that once concentration-polarization is established, the system membrane-polarization layer ceases to behave as a porous medium even with increased resistance. These data can explain the permeate flux increase observed when pressure and flow pulsations at 1 Hz are superimposed on the retentate.