Influence of feed concentration on the accuracy of permeate flux decline prediction in ultrafiltration

A theoretical model was used for permeate flux decline prediction. Ultrafiltration experiments were performed varying TMP and feed concentration. The feed solution consisted of an aqueous solution of polyethylene glycol with a molecular weight of 35,000 Da. Monotubular Al2O3–TiO2 ceramic membranes with a molecular weight cut off (MWCO) of 5 kDa (Tami Industries, France) were used in the experiments. Experimental data and model predictions were compared. The influence of feed concentration in the accuracy of permeate flux decline prediction was studied. The model predicted very accurately the reduction in the influence of transmembrane pressure (TMP) on permeate flux as the feed concentration increased. The best predictions were obtained for the highest feed concentration tested. For this feed concentration the influence of TMP on experimental and predicted permeate flux was very low.

• • • • The main conclusions of this work are As membrane resistance is high and the values of the gel layer concentration estimated are low, membrane resistance and an instantaneous pore blocking and/or adsorption phenomenon control ultrafiltration of PEG 35,000 for the experimental conditions tested. As a result, for most of the experimental conditions tested, gel layer formation was not enhanced. Therefore, permeate flux varied slightly with time. According to the values of the critical pressure obtained, a gel layer forms over the membrane surface. The concentration of the gel layer is low and it is far from the solubility limit of PEG. Consequently, the gel layer resistance is also low and it scarcely contributes to permeate flux decline. The gel layer thickness is small mainly due to a reduced TMP excess available for its formation. A reduced membrane length also contributes to a rapid achievement of steady-state permeate flux. For the experimental conditions tested, the limiting permeate flux as well as the maximum gel layer thickness is only accomplished for TMPs of 0.4 and 0.5 MPa. The variation of permeate flux with TMP decreases with an increase in the feed concentration. At high feed concentrations this variation is negligible. For low TMPs (0.2 and 0.3 MPa) the influence of feed concentration on permeate fluxes is lower than in the case of high TMPs (0.4 and 0.5 MPa).