Predicting the performance of gas-sparged and non-gas-sparged ultrafiltration Original Research Article

Tubular and hollow fibre membrane ultrafiltration is commonly used for waste water treatment and potable water production often with the introduction of gas bubbles into the filtration feed to improve flux. For design purposes, accurate models of the process are required. The one dimensional (1D) flat plate boundary layer analysis remains a simple tool for mass transfer analysis of these processes. However, this analysis neglects effects of wall permeation or curvature of circular closed-conduits e.g. hollow fibre and tubular membranes. Another pitfall is use of bulk fluid properties for flux prediction. Consequently, flat plate analyses severely under-predict ultrafiltration flux in such membranes. In this work we analytically include curvature (edge) effects of circular conduits into flat plate analyses. Flux estimates incorporating both edge and suction effects were reliable and predicted flux increases from 47.2% to 131.2% over the classical analysis. It was also shown that use of bulk diffusion coefficients in flux estimates led to reduction of suction- and edge-inclusive flux by up to 61%. The improved 1D model derived here enabled more accurate, yet simple flux prediction. With regard to gas-sparged systems a mechanism of flux enhancement is proposed for hollow fibre membrane ultrafiltration. This mechanism is determined from previously published experimental and computational fluid dynamics studies of the capillary slug flow process, as well as from dimensional analysis of the process. A physicochemical model for flux prediction is designed around the postulated enhancement mechanism. Again the boundary layer analysis is adapted to include the effect of wall suction. The flux-prediction model enables estimation of upper and lower bounds which were correctly found to encapsulate experimental values.