Application of gas/liquid two-phase flows during crossflow microfiltration of skimmed milk under constant flux conditions

This work constitutes a first approach to determine the critical zone of stability during gas-sparged crossflow microfiltration () of skimmed ultra-high temperature (UHT) and reconstituted milks for the separation of casein micelles from soluble proteins. Conditions for stable operation were investigated with and without air sparging by imposing, at a constant wall shear stress, different levels of permeate flux while monitoring the variation in the transmembrane pressure (TMP). The determination of the critical fluxes allowed to assume a common domain of stability for single- and two-phase flows conditions, thus confirming the relevance of the wall shear stress value during microfiltration of skimmed milk whichever way it is generated (standard crossflow filtration or unsteady gas/liquid flow). Whatever the filtration conditions (single-phase flows/two-phase flows), during the phase of increasing flux, a significant decrease in soluble protein transmission was observed: for reconstituted milk, under two-phase flow conditions, the transmission decreased from 80% to 60% for α-lactalbumin (α-LA) and from 50% to 30% for β-lactoglobulin (β-LG). This was due to the sharp increase in TMP when the flux was close to the limiting flux. During the phase of decreasing J, separation performance was strongly altered: for the same J, the TMP was significantly higher and lower soluble protein transmissions were observed, especially for the β-LG. These results showed the transition to an irreversible fouling, which led to a more tightly packed, thus less porous, cake structure. Unsteady filtration conditions, as well as standard ones, failed to disrupt it.