Modelling of a low grade vertical cooling crystalliser using computational fluid dynamics

Results are presented from an axisymmetric finite element model of a low grade vertical continuous cooling crystalliser. The model predicts the distributions of axial, radial and tangential velocity, temperature, pressure, viscosity, and shear rate throughout the vessel. The effect of stirrer arms rotating at 0.25· rlmin has been included by using tangential volumetric forces applied between rows of cooling pipes. The simulations reveal that the flow and temperature distributions are highly non-uniform, with significant short circuiting of virtually uncooled massecuite. Distributions of crystal growth rate and kinetic growth coefficients are computed by solving a species transport equation, assuming the flow and temperature fields remain unchanged. The results indicate that crystal growth kinetics are controlled by diffusion in most of the crystalliser, primarily due to the low average shear rate (0.014 S-1). The large range of shear rates and temperatures encountered in the simulations exceed the range of validity of existing correlations for kinetic growth coefficients and other quantities relating to crystal growth, pointing to the need for more experimental work in this area.