EFFECTS OF NATURAL-CONVECTION INSTABILITY ON MEMBRANE PERFORMANCE IN DEAD-END AND CROSS-FLOW ULTRAFILTRATION

The effects of natural convection instability on ultrafiltration perfo rmance have been tested experimentally in empty (without spacer) and s pacer-filled channel cells. In dead-end operation, the permeate fluxes at the gravitationally unstable orientation of the empty cell are enh anced up to 3.5 times for dextran solution and 5.5 times for BSA solut ion compared with the results at the stable orientation. In cross-flow operation, flux improvement by natural convection instability occurs when the cross-flow velocity is below the critical value of around 0.1 -0.2 m/s (Re = 35-90). A general criterion for determining whether nat ural convection effects dominate is identified considering the mass tr ansfer between the membrane surface and the bulk. The criterion is tha t when Gr/Re-2 > ca. 3 (for the empty cell) or Gr/Re-2 > ca. 500 (for the spacer-filled cell) natural convection instability is of importanc e. A mass transfer correlation for the mixed convection membrane syste m is presented.