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Abstract
Vibrating membrane systems have utility for feed streams with high viscosity and high solids concentrations such as milk protein concentrates (MPC) and yeast suspensions. Transverse vibration of high viscosity MPC60 and MPC66 solutions (12.93 cP and 21.06 cP at 10 ºC) at a constant permeate flux of 10 L/m^2 h and 5 L/m^2 h maintained high transmission of whey proteins (α-Lactalbumin and β-Lactoglobulin) while fully rejecting casein micelles. Substitution of a novel rotational vibration mechanism for the transverse mechanism resulted in less deposition of protein during the filtration of MPC60, which was attributed to the attenuation of large casein micelle aggregates due to centrifugal force generated by rotational oscillation.
Rotational vibration reduced fouling (dTMP/dt = 0.005 kPa/min) and consumed less power (0.048 W) compared to transverse vibration (dTMP/dt = 0.011 kPa/min at 0.077 W) during 20 h of filtration of 200 g/L yeast suspension under similar vibration conditions of 10.3 Hz and 36.7˚ (equivalent to 4 mm amplitude). In longer term experiments (26 h) the rotational vibration system maintained a stable TMP of approximately 13 kPa with high protein transmission of 84% at 15 L/m^2 h. Moreover, the use of intermittent shear enhancement and periodical relaxation in conjunction with vibration provided additional fouling control.
A numerical model, incorporating Computational Fluid Dynamics (CFD) to simulate shear stress distribution, was developed for cake formation on a hollow fibre membrane bundle subjected to rotational vibration. The model can effectively simulate the dynamic relationship between shear stress and cake formation with a limitation for the case at high membrane vibration frequency and high amplitude. Furthermore, it was determined that the shear stress on the fibres in the bundle was higher and more evenly distributed with an increase in spacing between fibres.
The experimental and numerical observations suggest that the use of a vibrating membrane may be a viable alternative when conventional fouling control strategies, such as cross-flow, are less effective and less economical in applications at high recovery and on feeds with elevated suspended solids and viscosity.