Critical flux enhancement in gas assisted microfiltration

The use of gas as a means of enhancing the critical flux of non-circular multi-channelled (star-shaped) ceramic membranes is considered. The effects of various superficial gas and liquid velocities and the use of different nozzles have been investigated and for the range of conditions used it is seen that two-phase flow can produce a critical flux of up to 1.7 times greater than the single-phase flow. Increasing superficial gas and liquid velocities increase the critical flux. The best results are found in the slug flow regime. These are attributed to the increase in mean velocities of the fluid, the increase in shear stress number, the secondary flow and wakes created by the slugs and most importantly, the ability of gas slugs to increase the fraction of the filtration area exposed to crossflow by reducing the extent of the stagnated regions within the points of the star. At lower velocities, as the nozzle size increases, a maximum critical flux is obtained and is attributed to the effects of bubble sizes. However, when higher velocities are employed, the effects of nozzles are not apparent as the increased turbulence reduces the size of the bubbles.