Aggregation kinetics of small particles in agitated vessels

Rapid coagulation by turbulence in stirred tanks was studied for particles and aggregates smaller than the Kolmogorov microscale. The coagulation kinetics are determined by the hoc structure and by the hydrodynamic and colloidal interactions between the colliding particles. The collision efficiency for doublet formation in the heterogeneous shear field of a stirred tank follows from particle trajectory analysis of solid particles in simple shear flow, provided the simple shear rate is made to correspond to the residence time weighted turbulent shear rate. Experimentally, the resulting aggregates proved to be fractal-like with their porosity increasing with aggregate size. Porosity within the aggregates results in penetration of the hoc surface by the fluid dow, giving rise to enhanced collision efficiencies compared to solid particles. The collision efficiencies between porous flocs may be estimated by a model that pictures a porous doc as consisting of an impermeable core and a completely permeable shell. With the collision efficiencies from this shell-core model the aggregate growth could be described adequately. Copyright (C) 1996 Published by Elsevier Science Ltd