DIAGNOSIS OF MEMBRANE FOULING USING A ROTATING ANNULAR FILTER .2. DILUTE PARTICLE SUSPENSIONS OF KNOWN PARTICLE-SIZE

In a continuing study of membrane fouling, we report on the behavior of dilute particle suspensions in a rotating annular filter with a nominal 0.45 mum surface modified polysulfone microfiltration membrane. Particle size (25.7, 11.9, and 2.02 mum), particle type (styrene/divinylbenzene and styrene/polyvinyltoluene), transmembrane pressure (10-100 kPa) and rotational speed (0, 1000, 2000, 3000 and 4000 rpm) were varied. The mean flux-transmembrane pressure data are analyzed using a recent narrow-gap solution to the Navier-Stokes equations for azimuthal flow in an annulus with Darcy's law for flow through a porous wall on the inner rotating cylinder [Belfort et al., J. Membrane Sci., 77 (1993) 1 ]. By accounting for centrifugal pressure, the data for all rotational speeds can be plotted with one universal correlation for each feed type. From these data, we suggest that particle intrusion into the pores of the membrane causing pore constriction and plugging is the dominant form of fouling for the systems studied here. Particle intrusion depends on membrane pore size distribution, density difference between the particles and the carrying fluid, particle size, and rotational speed. Also, intruded particles reduce volumetric flow through the blocked pores thus increasing the resistance to flow and reducing the mean permeation flux. Using a corrected version of the Meireles et al. [J. Membrane Sci., 56 (1991) 13 ] approach, changes in the mean membrane pore size distribution resulting from this intrusion are predicted. By minimizing the sum of the squares of the errors between calculated and measured flux-pressure slopes for two suspensions containing different particle diameters, we obtained best fit values for the mean radius r* and the variance sigma2 of the log-normal pore size distribution.