Flux prediction of black liquor in cross flow ultrafiltration using low and high rejecting membranes

Experimental studies were carried out to observe the flux decline behaviour and rejection characteristics of kraft black liquor in a rectangular cross flow ultrafiltration cell. A theoretical model was developed to predict flux and real rejection for osmotic pressure controlled ultrafiltration. Predicted values were compared with the experimental results. Standard macromolecules (PEG-6000 and dextran-18 600) were also used for comparison. Growth of the concentration boundary layer, variation of the concentration profile across the length of the channel and estimation of the membrane surface concentration at different cross flow velocities were studied through the developed model. The membrane used for the standard solutes gave rejection values for black liquor in the range of 18 to 40%. This range was improved by using high rejecting membrane (85-92%). Successive filtration through both the membranes were also carried out (using the permeate of low rejecting membrane as feed to the high rejecting) and gave a rejection range of 91-95%. These experimental values were also compared with the model predictive results. The results obtained from the low rejecting membrane showed excellent agreement; however, discrepancies were observed and explained for the results from high rejecting membranes, either direct or successively used.