MODELING OF CONCENTRATION POLARIZATION AND DEPOLARIZATION WITH HIGH-FREQUENCY BACKPULSING

Rapid backpulsing to reduce membrane fouling during crossflow microfil tration and ultrafiltration is studied by solving the convection-diffu sion equation for concentration polarization and depolarization during cyclic operation with transmembrane pressure reversal. For a fixed du ration of reverse filtration, there is a critical duration of forward filtration which must not be exceeded if the formation of a cake or ge l layer on the membrane surface is to be avoided. The theory also pred icts an optimum duration of forward filtration which maximizes the net flux, since backpulsing at too high of frequency does not allow for a dequate permeate collection during forward filtration relative to that lost during reverse filtration, whereas backpulsing at too low of fre quency results in significant flux decline due to cake or gel buildup during each period of forward filtration. in general, short backpulse durations, low feed concentrations, high shear rates, and high forward transmembrane pressures give the highest net fluxes, whereas the magn itude of the reverse transmembrane pressure has a relatively small eff ect. Rapid backpulsing experiments with yeast suspended in deionized w ater were performed with a flat-sheet crossflow microfiltration module and cellulose acetate membranes with 0.07 mu m average pore diameter. The optimum forward filtration times were found to be 1.5, 3, and 5 s , respectively, for backpulse durations of 0.1, 0.2, and 0.3 s. Both t heory and experiment gave net fluxes with backpulsing of about 85% of the clean membrane flux (0.022 cm/s = 790 1/m(2) h), whereas the long- term flux in the absence of backpulsing is an order-of-magnitude lower (0.0026 cm/s = 94 1/m(2) h).