Chapter 8 - Filtration

Abstract

This chapter discusses the filtration operation by which solid particles are separated from a liquid or gas, by forcing the mixture through a porous medium that retains the particles. There are two modes of filtration: surface filtration and depth filtration. In surface filtration, the fluid is forced through a porous medium (filter medium) that retains the particles on its surface. Particles larger than the openings of the porous surface are retained. The familiar laboratory operation of filtration through filter paper is an example of surface filtration. Filter media include paper, cloth, porous ceramic, sintered glass or metal, membranes, etc. In the case of depth filtration, the mixture flows through a thick layer of porous material of fibrous (glass wool, rock wool) or particulate (sand) structure. The solid particles of the mixture are retained as a result of collision with the fibers or particles of the filter medium or by adsorption. The passages available to flow are much larger than the retained particles. Retention is probabilistic and not absolute. This kind of filtration occurs in air filters, sand bed filters or the familiar oil filters in cars. The purpose of filtration may be the removal of undesirable solid particles from a liquid product (e.g., clarification of wine) or, on the contrary, recovery of a solid product from a solid/liquid mixture (separation of sugar crystals from mother liquor).