AGGREGATION KINETICS OF SMALL PARTICLES IN AGITATED VESSELS

Rapid coagulation by turbulence in stirred tanks was studied for parti cles and aggregates smaller than the Kolmogorov microscale. The coagul ation kinetics are determined by the hoc structure and by the hydrodyn amic and colloidal interactions between the colliding particles. The c ollision efficiency for doublet formation in the heterogeneous shear f ield of a stirred tank follows from particle trajectory analysis of so lid 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 w ith 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 par ticles. The collision efficiencies between porous flocs may be estimat ed by a model that pictures a porous doc as consisting of an impermeab le core and a completely permeable shell. With the collision efficienc ies from this shell-core model the aggregate growth could be described adequately. Copyright (C) 1996 Published by Elsevier Science Ltd