A hydrodynamic comparison between rotating disk and vibratory dynamic filtration systems

The purpose of this work is to compare the effects of various hydrodynamic parameters (transmembrane pressure, shear rate, fluid viscosity and solute concentration) on the permeate flux provided by two different dynamic filtration systems using same membrane material and same fluids. Tested systems were two rotating disk modules designed in our laboratory and a VSEP pilot with a circular vibrating membrane. Tests fluids consisted of baker's yeast microfiltration (MF) at 0.2 mum and of UHT skim milk ultrafiltration (UF) at 50 kDa. The characteristic shear rate was taken to be the maximum one (gamma(m)) at the membrane outer rim in each module. It was varied by changing the rotation speed of the disk or by changing the vibration frequency of the VSEP. The highest permeate fluxes were obtained with rotating disks equipped with vanes because they generated the largest shear rates. But in MF, when the disk speed was adjusted to produce the same maximum shear rate as in the VSEP, permeate fluxes variations with time in both modules were identical at the same TMP and yeast concentration. Flux variations with TMP were also very close in this case. In concentration tests by MF at constant speed or frequency, permeate fluxes (J) in L h(-1) m(-2) provided by the two rotating disks (with and without vanes) and the VSEP were well correlated by a single equation, J = 4.3 x 10-6gamma(m)(1.46), where the shear rate was varied by the concentration change. Permeate fluxes were also similar in UF of milk for the VSEP and rotating disk when shear rates were matched. The variation of permeate flux with shear rate at constant concentration for the two rotating disks systems with and without vanes was correlated by a single equation J = 0.136gamma(m)(0.594), while the corresponding equation for the VSEP was J = 0.110gamma(m)(0.587), Our data suggest that, in these devices, the flux is mainly governed by the maximum shear rate and not by details of internal flow and can be increased to very high levels by increasing rotation speed or vibration amplitude or by equipping the disk with large vanes. This information can be used for scaling up industrial systems. (C) 2004 Elsevier B.V. All rights reserved.