Filtration Theory: Formation and Structure of Compressible Filter Cakes

Abstract

Recent analyses of cake filtration have been aimed at providing store detailed descriptions of the fluid motion through the cake due to the hydraulic pressure gradient. This gradient causes an inter-facial momentum transfer in the form of viscous drag at the particle-fluid interfaces. If the shape (or the physical strength) of the solids is such that the packing arrangement in the bed can sustain this drag force without further movement, then the cake is regarded as incompressible. However, some particle rearrangement generally occurs to yield a compressible cake. During compression the porosity decreases with tine at any given distance from the filter cloth, and simultaneously a porosity distribution is obtained throughout the depth of the cake. This distribution often ranges from a minimum at the cake/cloth interface to a maximum at the growing cake surface, but instances of a minimum porosity some distance from the filter cloth have been reported ^1–3 when characteristics of the solid/liquid system are such that the cake collapses after deposition of a critical amount of solids. The compressive action itself causes the interstitial flow rate of liquid to increase towards the medium.