On the local properties of compressible filter cakes

Doctoral thesis, 2012

Filtration as a means of separating solids from liquids is an important unit operation employed in a range of different industrial sectors, of which the forest products, mineral, chemical process and pharmaceutical industries are but a few. Accurate and applicable models for the filtration unit operation are imperative if industrial filtration equipment is to be designed correctly. Incompressible or near-incompressible filter cakes can currently be modelled satisfactorily, thereby facilitating scale-up and design. There is, however, a lack of good models for materials forming compressible filter cakes. Efforts attempting to model the cake build-up have often been based on average filtration properties; whilst providing certain information regarding the cake build-up, this approach is nevertheless inferior to that of using models based on local data. Models for filtration also often contain lumped parameters, e.g. the classical filtration equation describes the filtration properties of particles using the average specific filtration resistance parameter. An alternative would be to base their filtration properties on the interactions experienced during filtration. Important interactions that should be considered between particles include surface forces, friction and interlocking. This work investigates the properties of compressible filter cakes, and relates them to the properties of the particles. Three different materials are investigated: titanium dioxide, microcrystalline cellulose and green liquor dregs. Titanium dioxide is used to investigate the effects of electrostatic particle interactions on filtration behaviour. The focus placed on the behaviour of filter cakes formed from microcrystalline cellulose is on skin formation. Finally, the separation of green liquor, a process that is of great interest in the pulp and paper industry, is examined: green liquor dregs form hard-to-filter, compressible cakes. It could be concluded that the compressibility of the filter cake formed from titanium dioxide changed substantially when the ζ-potential was altered. Several published constitutive relationships for filtration are fitted; they yield very similar parameters that are in good accordance to the characterization of the material. A skin-forming behaviour was found for microcrystalline cellulose, which could be altered by affecting particle-particle and particle- filter medium interactions. Using sedimentation together with filtration, local solidosity was investigated for a larger range of solid compressible pressures. A constitutive relationship was able to describe this range successfully using the same parameters for both the sedimentation and filtration (cake build-up and expression) processes.