Separation of Macromolecules by Dynamic Ultrafiltration

Ultrafiltration is mainly used to concentrate macromolecules and removing salts and smaller molecules through the membrane. Sharp separation is rarely seen which is partly due to the coupling of solute and water transport and the concentration polarization at the membrane surface. In case of real fractionation of macromolecules a decoupling of the solute transport from the water transfer together with a minimization of the concentration polarization of the larger molecules have to take place. Using hollow fiber membranes under high-frequency backflushing the concentration polarization can be minimized due to the non-steady state operation. The build-up of the polarized highly concentrated layer at the membrane surface takes typically 10-30 seconds why it is possible to obtain a dynamic layer with a substantially reduced surface concentration thereby increasing the selectivity of the membrane.The paper describes the modeling of the dynamics of the concentration polarization and how it influences the membrane selectivity and productivity. The modeling is further supported by experiments fractionating dextrans and proteins on a hollow fiber system using backflushing intervals from 1 to 30 seconds and backflushing times from 0,1 to 5 seconds. Introduction The modeling of ultrafiltration is often done by either the gel model or by the osmotic pressure model. The gel model can often describe the permeate flux as a function of pressure and concentration reasonably well. However, the assumption of a constant concentration of the macromolecules at the membrane surface equal to the gel concentration and with a varying thickness depending of the pressure would be expected to result in a constant retention of the macromolecules independent of the flux rate which are rarely seen. The osmotic pressure model on the other hand can explain such variations and it has further been shown, that it can also explain the limiting flux with increasing pressure, due to an exponential increase in the osmotic pressure of the macromolecules at the concentrations, which are observed at the membrane surface. The background for description of the concentration polarization in ultrafiltration are usually the film model which assumes a steady-state concentration profile between the bulk solution and the membrane surface. Since the concentration at the membrane surface might well be one to two orders of magnitude higher than in the bulk solution, a certain amount of permeate is needed before steady-state is obtained. In the present work some earlier findings (1) of the relation between the measured osmotic pressure of dextrans and their concentration is used to model the dynamics of the concentration polarization build-up at the membrane surface and how this influences the permeate flux and observed retention, assuming the true membrane retention to be constant and the permeate flux to depend on the water permeability with the difference in pressure and osmotic pressure gradient to be the driving force. This is illustrated in Fig.1 together with the experimental relation for the osmotic pressure. Osmotic pressure model ( ) ( ) ( ) 3 2 2 1 m m m c A c A c A ⋅ + ⋅ + ⋅ = ∆π ( ) π ∆ − ∆ ⋅ = P L J p v       = − − k J C C C C v