Air sparging with flat sheet nanofiltration: a link between wall shear stresses and flux enhancement

Air sparging has shown to be very efficient for flux enhancement in different membrane processes and for different membrane geometries. This study focuses on the application of air sparging to nanofiltration flat sheet membranes and aims to understand which hydrodynamic parameter is responsible for flux enhancement. The twophase flow in a flat sheet cell was characterised: local wall shear stresses were measured using an electrochemical method. The flux enhancement during nanofiltration of clay suspensions was measured in the same cell and relationships could be established between this flux enhancement and some hydrodynamic parameters including the wall shear stresses.

Air sparging allows flux enhancement when nanofiltering clay suspensions (7 g.L–1) in a flat sheet cell: the higher the gas velocity, the higher the flux enhancement. Wall shear stresses could be determined in 9 points of the flat sheet module for several liquid and gas velocities. In the presence of a gas/liquid two-phase flow, the wall shear stresses can be characterised by their time-averaged value, amplitude and frequency. Some links could be established between the permeate flux in the presence of air sparging and some characteristics of wall shear stresses: timeaveraged value and amplitude of the wall shear stresses.