High shear skim milk ultrafiltration using rotating disk filtration systems

(T)he ultrafiltration (UF) of UHT (sterilized at ultrahigh temperature) skim milk with two prototypes rotating disk systems of 15.5 and 26 cm W. using the same 50 kDa PES membrane is investigated. The larger stainless steel unit had a maximum rotation speed of 1,500 rpm. The smaller nylon unit had a maximum speed of 3,000 rpm. Disks equipped with radial vanes of various heights were used in the tests. The large module yielded a stabilized permeate flux of 276 L h(-1) m(-2) at 45degreesC and a transmembrane pressure (TMP) of 1, 000 kPa when fitted with a 2-mm vanes disk rotating at 1, 500 rpm, confirming the reduction of concentration polarization by high shear rate. The mean shear rate on membrane gamma(m) was shown to be proportional to (komega)(1.8) , where omega was the disk angular velocity and k was a coefficient depending upon vanes height. Permeate fluxes observed with the two modules at various speeds and with different disks were well correlated by a single function of mean shear rate, given in Lh(-1)m(-2) by J = 0.301 gamma(m)(0.552). In concentration tests with a 6-mm vanes disk, the permeate flux remained constant until a volume reduction ratio (VRR) of 3, then decayed to a theoretical VRR maximum of 9.2 at zero flux. It can thus be concluded that rotating disk filtration devices are very efficient in UF as high shear reduces concentration polarization. The specific power consumed per m(3) of permeate was found to be less for the smooth disk than for a disk with vanes.