Possible effects of air sparging for nanofiltration of salted solutions

摘要

Among the different ways to improve the flux in ultrafiltration or microfiltration, air spraging recently appeared as an interesting process for different applications. Air spraging consists in injecting air in the concentrate compartment during filtrationg time. A gas/liquid two-phase flow is then obtained at the membrane surface. This study focused on the use of air sparging in nanofiltration membranes. Experiments were performed with a lab-scale flat sheet module. Previous results showed that air spraging allows flux enhancement in the case of liquid/liquid suspensions and liquid/solid suspensions, for which the main limiting phenomenon is particle deposit. The objective here was to characterise the effects of gas spraging for a salt (CaCl_2)/water solution when mass transfer is limited by concentration polarisation or osmotic effects. At high concentrations, air sparging has no effect on the permeate flux nor on CaCl_2 retention factor. This was explained by the fact that, for CaCl_2 solutions until 10 g·L~(-1), and even for a low liquid velocity, the transfer limitation is due to osmotic effects and no polarisation concentration occurs. In that case, no benefice of turbulence increase can expected.