Quantification of solid pressure in the concentration polarization (CP) layer of colloidal particles and its impact...

Here we describe the nature and implications of the "concentration polarization" (CP) layer that is formed during ultrafiltration of colloidal particles using a new approach in which the solid pressure, which arises from inter-particle interactions, and the inherent osmotic pressure are separately considered. The approach makes use of the particle transport mass balance between the convective and diffusive fluxes. The particle convection rate is hindered when inter-particle interactions take effect by reducing the particle velocities while the particle diffusion is solely controlled by the Brownian motion. An increase in solid pressure accounts for the reduction of the water potential caused by the relative motions of the particles and the surrounding water. A cell model is adopted to relate the local solid pressure with the local solid fraction and inter-particle interactions. The inter-particle interactions critically determine the form of particle accumulation (i.e. CP or gel/cake) on the membrane. The Shirato-Darcy equation is employed to relate the rate of increase in solid pressure, the relative liquid velocity and the solid fraction. Numerical integration approaches are employed to quantify the properties of the CP layer during both the development as well as the steady state phases (with steady state normally being achieved in a few minutes). The solid fractions are always no higher than those obtained when the inter-particle interactions are not considered. The decrease of the water potential caused by CP formation leads to the increase of both the solid pressure and the osmotic pressure. The dependence of the solid pressure on the solid fraction is usually stronger than that of the osmotic pressure. It is thus apparent that the solid pressure would be expected to dominate water potential reduction for solid fractions above a certain value though the solid pressure will be negligible when the solid fraction is relatively low.