SHEAR-INDUCED FLOCCULATION OF COLLOIDAL PARTICLES IN STIRRED TANKS

Colloidal polystyrene and paramagnetic particles consisting of mixture s of polystyrene and magnetite are used to experimentally investigate flocculation kinetics in a stirred tank under turbulent shear flow. Th e effects of various parameters-agitation speed, solution pH, ionic st rength, particle size, and particle concentration-on the flocculation rate are investigated. A trajectory model applicable for shear-flow sy stems is formulated to describe particle flocculation in stirred tanks . The collision efficiency of particles is obtained from the limiting trajectory of one particle moving toward another and is a function of interparticle forces and flow properties. The collision frequency is d etermined as a function of particle size and energy dissipation. The f locculation frequency is then determined by multiplying the collision frequency by the collision efficiency and is incorporated into a popul ation balance model to predict the particle size evolution. Results su ggest that the flocculation rate is enhanced by increasing the agitati on speed, even though the collision efficiency is decreased at a highe r agitation speed. It is also found that the collision rate increases and the collision efficiency decreases as the particle size ratio is i ncreased. Results also suggest that the breakup rate of aggregates in a turbulent shear how could be significant and may need to be included in the population balance modeling to correctly predict the evolution of particle size distribution. (C) 1998 Academic Press.