NOMENCLATURE

A combined population balance (PB) and CFD model was developed to model the behaviour of a continuous stirred tank gibbsite precipitator. The model is 2-phase Eulerian-Eulerian with the population balance particle size groups modelled with a series of solid-phase scalar transport equations. The population balance kernels describe the birth, death, and growth rate of the crystals depending on the local rates of nucleation, agglomeration and crystal growth. The crystallisation time scales are orders of magnitude longer than the flow timescales. Although this tends to homogenise the crystal size and supersaturation throughout the tank, it also makes the numerical problem stiff. The precipitator under consideration was poorly mixed, with particles settling to a high concentration in the lower part of the tank. A number of cases with different feed rates and impellor speeds were investigated. It was demonstrated that they lead to different final crystal size distributions depending on residence time, supersaturation and solids concentration