Transport phenomena in ultrafiltration: membrane selectivity and boundary layer phenomena

Retention characteristics of UF membranes are de— lineated through comparison of the combined viscous flow and frictional model with the Ferry—Faxen equation. The molecular weight cut—off concept is discussed using calculations of how sharp cut-off curves one can expect of UF membranes with uniform pore sizes and how heteroporosity will affect such curves. Experimental retention—flux curves for different macromolecules have been measured. From these data the molecular weight cutoff curves at varying pressure levels can be determined and by comparing with the model an average pore radius can be calculated. Permeate flux is often more or less pressure independent, which has been explained by a gel layer formation at the membrane surface. However, recently it has been shown that for many types of macromolecules it is rather an osmotic pressure of the concentrated boundary layer, which is responsible for the special flux-pressure relationship.