Experimental evaluation of a uniform transmembrane pressure crossflow microfiltration unit for the concentration of micellar casein from skim milk

A uniform transmembrane pressure (UTMP) crossflow microfiltration (CFMF) system maintains a low but uniform transmembrane pressure (ΔPTM) with high crossflow velocity (CFV), which reduces fouling and cake build-up, and improves the utilization of available filtration area. A CFMF system, with a 0.2 μm nominal pore size ceramic filter, filtration area 0.184 m2, was operated in both UTMP and non-UTMP modes. The two modes were compared for their effectiveness in maintaining a steady flux during the separation of casein micelles from skim milk up to a concentration factor (CF) 10 at 50°C. Experiments were performed at an average CFV of 7.2 m s−1 and ΔPTM from 89 to 380 kPa. Up to CF 4 the non-UTMP mode maintained a slightly better flux and process time than the UTMP mode, but reached the minimally acceptable flux (below 0.005 kg m−2 s−1) at CF 6. Depending upon the ΔPTM maintained, the UTMP mode approached the minimal flux at CF 7 or 10 depending upon the combination of ΔPTM and CFV used. Cake resistance (Rcm) was modified to include the effect of an increase in retentate viscosity with concentration. Rcm increased for the non-UTMP mode and decreased for the UTMP mode with a decrease in the ratio of permeation flux/wall shear stress (Jp/τw) (which occurred as the retentate gets concentrated). This indicated that the cake formed during the non-UTMP mode of operation was more compact and durable (harder to erode) than in the UTMP mode. A central composite rotatable design estimated the optimal operating region at a CFV of 7.1 m s−1 and ΔPTM of 241±10 kPa to achieve maximum flux and a high concentration.