Clarification of liquids using filter aids Part I. Mechanisms of filtration

Abstract

Filter aids were characterized by an effective particle diameter and a pore diameter in the filter aid cake, calculated from the specific cake resistance using the Kozeny equation. The results agreed well with those from mercury porosimetry. The transition from surface filtration to depth filtration by filter-aid cakes was studied with uniform polystyrene particles as a model impurity. The critical ratio between pore diameter and impurity diameter was between 2 and 3. At conditions of non-surface filtration, an important concentration of impurity exists in the liquid flowing through the cake causing a danger of blocking. When pre-coat and body feed were used with polystyrene particles as an impurity, the blocking appeared to occur rather easily on top of the original pre-coat and on the filter medium. The type of pre-coat and the way in which it was formed were very important. In beer filtration, which is mainly a surface filtration, a small concentration of passing impurity was able to block the pre-coat layer, when the filter aid of the body feed was coarser than that of the pre-coat.