Milk protein concentration by ultrafiltration with rotating disk modules

Ultrafiltration of UHT skimmed milk has been carried out using a smooth disk and a disk equipped with 6-mm-high vanes rotating at high angular velocity T near a circular 50 kD membrane. Permeate turbidity was less than 2 NTU confirming complete casein micelles retention. The addition of vanes to the disk increases the permeate flux by about 56% due to higher fluid core velocity resulting in higher shear rate at the membrane The permeate flux increases as T0.9 and remains independent of concentration until a volume reduction ratio (VRR) of 3 at 2000 rpm. The maximum theoretical VRR corresponding to zero flux was 9.6 for a disk with vanes.

Such tests are normally carried out by recycling the retentate only and not the permeate. But in order to eliminate the effect of initial fouling, the first 110 min of tests were carried out at initial concentration by recycling the permeate. In addition, during the first 5 min of filtration, the retentate valve was kept open, then shut progressively to reach the desired TMP (200 or 300 kPa) to minimize this initial fouling. After 110 min, the permeate recirculation was stopped and concentration was started until the permeate became equal to the dead volume in order to reach the maximum VRR. The variation of permeate flux with VRR is displayed using semilog coordinates in Fig. 4 for a smooth disk and a disk with vanes, a TMP of 300 kPa and a rotation speed of 2000 rpm. It is seen that the permeate flux for a smooth disk drops slowly below a VRR of 1.3 while, at higher concentrations, it obeys the classical concentration polarization law of the form The rotating disk filtration system permits reaching a volume reduction ratio of 9, higher than with conventional cross-flow UF systems, especially when the disk is equipped with vanes. For moderate concentration processes, another advantage is that the permeate flux hardly decays until a VRR of about 3 is reached. Permeate fluxes are higher than those reported with tubular membranes and continue to rise at 2500 rpm when the TMP exceeds 66 kPa. We have shown earlier [6] that the addition of vanes to the rotating disk decreases the specific energy consumed per m3 of permeate. However, the limitation of rotating disk systems is their higher equipment cost and their limited membrane area per module.