Particulate separation using inertial lift forces

Abstract

A cross-flow membrane filtration device is described in which particles are retained not by the use of small pore size, but rather by the creation of a lift force which overcomes the drag of the filtrate and prevents the particles from reaching the membrane surface. The lift force is produced by the velocity gradient near the wall, in the same way as the inertial lift forces found in steady channel flow. In this work, oscillatory flow over the membrane surface was used to produce high shear rates in a flat channel containing suspensions of polystyrene spheres and discs several hundred microns in diameter. It was found that two species of particle could be separated by size because of the different balance between lift and drag forces, with none of the larger species being in the permeate. The same membrane would transmit both sizes under low shear conditions. Current theories are reviewed and found to be inapplicable to high frequency oscillatory flow and so a new semi-empirical expression is developed for the lift velocity and used to correlate the results. Reference is also made to the relevance of these results to the analysis of particle behaviour in conventional membrane microfiltration.