A COMPARATIVE-ANALYSIS OF FLUX LIMIT MODELS FOR ULTRAFILTRATION MEMBRANES

Here the permeate flux versus concentration characteristics of 0.1% w/ w to 8% w/w aqueous solutions of dextran T-500, with a mean molecular weight of 500 000 dalton, are studied when they are tangentially filte red at 298 K through a new polyamide asymmetric membrane, with an appl ied pressure of 700 kPa, while the recirculation velocity in the reten tate loop is kept constant in a range from 0.08 to 0.49 m/s. In such c onditions, all these solutions are totally retained. The mass transfer coefficient is calculated, within the frame of the film theory for th e concentration-polarization phenomenon, by studying the permeate flux as a function of concentration. It allows the evaluation of the membr ane surface concentration as well. This concentration is also calculat ed by taking into account the osmotic limit theory. These two results for c(m) are tested by calculating the expected permeate fluxes in ter ms of the resistance model and comparing with the experimental ones. I t can be concluded that the osmotic limit model reproduces the experim ental permeate fluxes better, and in a wider range, than the gelation model.