Time and pressure dependence of sieving coefficients during membrane plasma fractionation

The apparent sieving coefficient (S') of a membrane, defined as the ratio of permeate to feed concentration, is not an intrinsic property of the membrane, but depends upon operating conditions. The variation of albumin, IgG, and IgM sieving coefficients with time during dead-end filtration was investigated using human pooled plasma and two types of hollow fiber filters (cellulose acetate PF 100 and ethylene vinyl alcohol Eval 2A). For the same filtrate flux (3.6 10(-3) cm/min) the transmembrane pressure (TMP) rises faster with time for the Eval filter because of its smaller hydraulic permeability (0.26 10(-4) cm/min mmHg vs. 7.3 10(-4). The albumin S' of the PF 100 reaches 0.95 after 20 min of filtration but drops to 0.4 after 50 min, whereas the S' of the Eval filter, which never exceeds 0.6, remains at this level for at least 50 min. By varying the inlet flow rate the authors show that for a given membrane, the critical parameter that governs the S' is the TMP. These data suggest that it is essential to specify the TMP when quoting a value of S', and the most permeable membrane, with the highest maximum S', is not necessarily the one that maximizes the overall recovery over a long period.