Method for separating liquid compositions on the basis of molecular weight

A process for separating a blood plasma into a high molecular weight stream and a low molecular weight permeate stream utilizing a separation module having separation means comprising a plurality of thin channels or hollow fibers having walls through which ultrafiltration is effected, said separation module having substantially an aspect ratio between about 50 and 5,000 and, defined by Equation 1 ##EQU6## said process comprising the steps of introducing said blood plasma into an inlet portion of said separation module at substantially a shear rate defined by Equation 2, ##EQU7## removing said high molecular weight stream and said low molecular weight stream from said separation module,

• recirculating at least a portion of said high molecular weight stream to an inlet portion of said separation module thereby to form a recirculation stream, and
• controlling the ratio of the recirculation stream flow rate to the permeate stream flow rate to between about 5 and 100 and so that the ratio of the transmembrane pressure at the outlet to said separation means to the transmembrane pressure at the inlet of said separation means to between about 0.0 and 0.85 wherein the total membrane area in said module is defined by Equation 3 ##EQU8##