Crossflow Microfiltration of particle Suspensions - The influence of hydrodynamics and physico-chemical interactions

Abstract: Crossflow microfiltration is a technique to separate suspended particles (or cells or droplets) from liquids by use of a membrane that is permeable for the liquid but impermeable for the particles. The permeate flow drives particles to the membrane surface, where they may be deposited and form a fouling layer. To limit the growth of this fouling layer a flow is applied tangentially to the membrane (crossflow). This crossflow causes particle transport away from the membrane surface and prevents the particles from depositing on the membrane. Other factors, such as the particle charge or the membrane charge may also influence the particle transport near the membrane surface, and in that way influence the fouling behaviour. It was investigated which mechanisms are important for the particle transport in crossflow microfil- tration. Important mechanisms, such as Brownian diffusion, shear-induced diffusion, turbulent diffusion, and particle-particle interactions, were described in detail, and based on these mechanisms models were developed to calculate the permeate flux. Model predictions were compared with experimental fluxes for the microfiltration of silica particle suspensions, obtained at well defined hydrodynamic and physico-chemical conditions, and a good agreement was observed. It was found that shear-induced diffusion was the main mechanism of particle transport governing the flux in the crossflow microfiltration of particle suspensions, but particle charges can increase fluxes considerably. Not only the amount of fouling, but also the reversibility of fouling depends on hydrodynamic and physico-chemical parameters. Cake layers formed of stronger repelling particles showed increased reversibility. A novel method was developed to determine electri- cal properties of membrane surfaces, based on water flux measurements at various pH and salt concentration, combined with the theory of the electroviscous effect. This method was found to be an effective means of characterising membranes. However, it was found that the electrical proper- ties of the membranes do not have a significant influence on the permeate flux in the filtration of micron-sized particle suspensions.