Flux enhancement during Dean vortex microfiltration .8. Further diagnostics

Controlled centrifugal instabilities (called Dean vortices) resulting from flow around a curved channel have been used to reduce both concentration polarization and membrane fouling during microfiltration. These vortices enhance back-migration through convective flow away from the membrane-solution interface and allow for increased membrane permeation rates. As part of an effort to further understand the opportunities and limitations of using Dean vortex microfiltration, we report here on a series of experiments in which the performance of runs in the presence of vortices are compared to that in the absence of vortices. They include changing the following variables: transmembrane pressure, particle suspension concentration, ionic strength of the solution, and the reversibility of changing flow rate (from high rates in the presence of Dean vortices to low rates in the absence of such vortices and the reverse procedure) within a particular experiment, Unexpected results were observed for the ionic strength and flow reversal cases. The flux decreased with increasing ionic strength, passed through a minimum and then increased to a constant value. In the same experiment, the flux decreased when the flow rate was reversed from a high to low and back to a high value, until the flux was not affected by changing flow rate. We explain these interesting observations by contrasting the behavior of aggregated colloids in solution and within the deposited cake layer on the membrane surface. Pressure effects on the cake layer are also considered.