Rigid ceramic filters

The word ‘filtration’ refers to separation of particles from a fluid by passage through some form of porous medium. A distinction can be drawn between two main types of filtration behaviour: ‘depth’ filtration and ‘surface’ (or ‘barrier’ or ‘cake’) filtration (Figure 6.1). In depth filtration, collection of particles from the gas occurs throughout the filter medium, which usually consists of fibres or grains (see Section 6.2). In surface filtration, the medium acts as a barrier to the solids so that a dust ‘cake’ is built up on the up-stream surface, with no penetration into the medium itself. In practice, filtration behaviour depends on the properties of both the dust and the medium: not only the relative size of the pores in the medium but also on the surface properties (such as adhesion) of both dust and medium. In general, gas cleaning using rigid ceramic media approximates to surface filtration. Perfect surface filtration is rare and with a new (or ‘virgin’) surface filter it is usual for there to be a short period of penetration into the surface layers of the medium before cake deformation begins. During this short period, the filtration efficiency may be slightly lower than in steady operation.