Factors contributing to the change in permeate quality upon temperature variation in nanofiltration

The selectivity of nanofiltration (NF) membranes is determined by membrane parameters as well as species mobilities (solute diffusivity and solvent viscosity). Changes in temperature affect each of these quantities, thereby altering membrane selectivity. To determine whether membrane parameters or mobilities primarily account for observed changes in permeate quality upon temperature increase, values of each property are either fitted from experimental data or calculated. Model validation with data from three feed compositions and two membranes reveals clear trends in temperature-dependent property-variation: pore size, net path length through membrane selective layer and negative membrane charge increase at higher temperature. An analytical approach is taken to explain the increase or decrease in permeate concentration due to each contributive factor, revealing the opposing effects of the two mobility factors. Modeling results further show that neither membrane parameter changes nor mobilities can alone explain selectivity changes with temperature. With increasing pressure, however, the net effect of membrane parameters increasingly overshadows that of the mobilities.