화학공학소재연구정보센터
Separation and Purification Technology, Vol.62, No.1, 212-221, 2008
Effect of vibration frequency and membrane shear rate on nanofiltration of diluted milk, using a vibratory dynamic filtration system
The effect of membrane shear rate during nanofiltration of skim milk diluted with two volumes of water representing dairy effluents, using a vibrating VSEP module with a Desal 5 DK membrane, was investigated. This shear rate (gamma(max)) was varied by performing tests at 60.75, 60.2, 60.0 and 59.8 Hz. In long-term industrial use, it is necessary to limit the vibration amplitude d of the membrane rim to about 2-2.3 cm, against d= 3.0 cm at the maximum and resonant frequency of 60.75 Hz. At a pressure (TMP) of 4000 kPa, 45 degrees C and initial concentration, permeate fluxes decreased from 220 L h(-1) m(-2) at 60.75 Hz (with a shear rate of 1.35 x 10(5) s(-1)) to 95 L h(-1) m(-2) at 59.8 Hz (0.64 x 10(5) s(-1), d= 1.5 cm). The permeate carbon oxygen demand (COD) due to lactose increased from 20mgO(2) L-1 at 60.75 Hz to 60 mgO(2) L-1 at 59.8 Hz, remaining much smaller than in the feed (36,000 mgO(2) L-1). Permeate conductivity increased from 500 mu S cm(-1) at 60.75 Hz to 800 mu S cm(-1) at 59.8 Hz, for a feed conductivity of 2000 mu S cm(-1). During concentration tests, performed at 4 frequencies, the permeate flux J at various volume reduction ratios of 1, 3 and 7 was given by a single equation J = 3.0 x 10(-6) gamma(1.56)(max), showing that the flux is mainly controlled by shear rate whether this shear rate is lowered by reducing the frequency or increasing the concentration. Tests performed with a real dairy effluent gave a variation of permeate flux and conductivity with TMP similar to the model, up to 3000 kPa. Above 3000 kPa, the real effluent flux dropped due to higher membrane fouling. (c) 2008 Elsevier B.V. All rights reserved.