CONCENTRATION POLARIZATIONS KINETICS IN U NSTEADY PLASMA FILTRATION OF BLOOD

In plasma separation by membrane, the blood flows tangentially along a microporous membrane while the plasma crosses the membrane under the action of a transmembrane pressure gradient. As in other cross flow fi ltrations of suspensions, the permeate flux increases linearly at low transmembrane pressure and reaches a plateau at high transmembrane pre ssure due to concentration polarization and membrane fouling. Concentr ation polarization consists in the formation of a high particle concen tration boundary layer near the membrane which is assumed to occur rap idly, in less than one minute. In the case of plasma filtration from b lood, this layer consists of cells which are rejected by the membrane and it forms a reversible secondary membrane which becomes the limitin g filtration mechanism. In this condition, the permeate flux becomes a pproximately independent of transmembrane pressure and is controlled m ostly by the shear rate. In order to study the kinetics and time scale s of establishment of these phenomena, we have investigated the transi ent response of microporous membrane under stepped charges in transmem brane pressure. When the pressure is suddenly raised from the unpolari zed regime to a level corresponding to complete concentration polariza tion, the permeate flux reaches a peak in 0.4-0.6 s, which supasses th e equilibrium level by 60 to 80% depending upon the pressure, returnin g to the concentration polarization equilibrium level in 3 to 4 s. The se data show that the membrane retains its original permeability only during the first 0.5 s of the pressure change and that concentration p olarization takes about 3 to 4 s to build-up. This formation time decr eases with increasing pressure. When the pressure returns to its initi al level, the concentration polarization disappears instantaneously an d the process can be repeated at a frequency up to 0.7 Hz. However, wh en a stepped pressure increase is applied in the concentration polariz ation 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 expl ain the permeate increase observed when pressure and flow pulsations a t 1 Hz are superimposed on the retentate.