UNSTEADY-STATE FLUX BEHAVIOR IN RELATION TO THE PRESENCE OF A GEL LAYER

The unsteady-state flux behaviour has been studied for silica and dextran in a stirred ultrafiltration cell. Under the experimental conditions dextran and silica show a clearly different flux behaviour. During the filtration of dextran only a polarization layer is build up. For silica also a gel layer formation occurs. As a result the time to reach steady-state flux is less than a minute for dextran, whereas the formation of the silica gel layer takes more than one hour. The osmotic pressure model provides a good description of the flux for the experiments with dextran. If mass transfer coefficients are used which are higher than those electrochemically measured the transient flux for silica can be rather well predicted by the gel-polarization model. The use of flux measurements under unsteady-state conditions as an indication for the presence of a gel layer is discussed.