Cross-flow microfiltration of dilute macromolecular suspension

The operating condition effects on the cross-flow microfiltration performance of dilute macromolecular suspension were studied. Track-etched membranes of different pore sizes were used to filter blue dextran with a molecular weight of 2000 kDa. Increasing the membrane pore diameter results in higher filtration flux but decreases the dextran rejection coefficient. For filtration using 0.2-μm membrane, an increase in cross-flow velocity or filtration pressure leads to higher filtration flux, but the impact of cross-flow velocity is more significant. A membrane fouling model based on force analysis and membrane pore size reduction caused by dextran adsorption at pseudo-steady state is proposed. The effective membrane pore diameter and pseudo-steady filtration flux under various conditions can then be estimated. On the other hand, the pseudo-steady dextran rejection coefficient increases with increasing cross-flow velocity or decreasing filtration pressure when cross-flow velocity is lower than 0.3 m/s. This can be reasonably explained by the sweeping effect of tangential flow or using the modified gel-polarization model. However, a contrary trend occurs under the conditions with higher cross-flow velocity. This result is attributed to the dextran molecular deformation. The “coil-stretched” deformation of blue dextran molecules can be indicated using the Reynolds number in the permeating flow direction or Deborah number for macromolecular flow through porous media.