Undulated insert for boosting desalination efficiency in membrane systems

Abstract

Circular feed spacers are used to enhance flow mixing in membrane modules and thereby improve their efficiency. However, this involves increased fouling when the spacers are mounted in a zigzag arrangement or high power consumption when submerged. In view of this, a two-dimensional computational model of fluid flow and mass transfer is used to investigate the impact of using an undulated insert as a turbulence promoter in a membrane application. Thus, this study focuses on the effect of varying the amplitude and wavelength of the insert on desalination performances of a narrow channel bounded by two semi-permeable membranes. According to computational fluid dynamics (CFD) simulations, under the considered operating conditions, the increase of the insert amplitude from 0.2 to 0.7 mm could reduce the salt accumulation on the membrane walls by\u2009~\u20092.7%, improve the permeate flux by\u2009~\u20091%, but drastically increase the axial pressure drop by\u2009~\u20091148%. Increasing the insert wavelength from 3 to 24 mm could promote salt accumulation on the membrane walls by\u2009~\u20094.8%, reduce the permeate flux by\u2009~\u20091.4%, but minimize the axial pressure drop by\u2009~\u200996%. Plots of the Sherwood number versus the Power number highlighted that the optimal insert must have a low amplitude and a short wavelength. Some of the insert geometries tested in this study generated better performances than commercial feed spacers, confirming the benefits of undulated inserts and suggesting their use in membrane separation processes.