A particle's eye view of crystallizer fluid mechanics

Abstract

It has been recognized for some time that flow and mixing in industrial crystallizers has an effect on the kinetics of growth, nucleation and agglomeration and consequently on the crystal size distribution. Yet, a common assumption in population balance modelling is that the fluid mechanical environment experienced by growing crystals is uniform. In practice, however, industrial crystallizers provide extremely varied flow conditions, with local velocities, shear rates and energy dissipation rates varying by orders of magnitude throughout the vessel. A computational fluid dynamics simulation of the flow in a stirred tank was used to illustrate that, in a Lagrangian sense, the particles experienced regions with very different local micromixing characteristics, mean velocities, slip velocities, shear rates and turbulence levels; the sampling of these regions depended only slightly on the particle size and, for the flow considered here, the Eulerian and particle Lagrangian statistics were similar. However, the distribution of slip velocities experienced by the crystals was strongly dependent on the particle microscale and macroscale Stokes numbers. The consequent effects for the estimation of the average growth, nucleation and agglomeration kinetics used in population balances were also considered.