Laminar and turbulent shear-induced flocculation of fractal aggregates

A population balance model is used to model simultaneous coagulation and fragmentation in turbulent shear. The fractal-like aggregate structure is quantified using a mass fractal dimension, D-f = 2.05, derived from light-scattering measurements and is incorporated into the model using appropriate kinetic expressions. In addition, fluid viscous effects on aggregate collisions are taken into account using existing theories on viscous retardation. Flocculation experiments of a polystyrene particle/Al(OH)(3)/water system in a stirred tank are compared to the model results. Good agreement is found for both average-size evolution and steady-state size distribution using only one fitting parameter and assuming binary breakage. The average polystyrene-Al(OH)(3) aggregate size initially increases before reaching a constant steady-state value during coagulation-fragmentation in a stirred tank. Increasing the applied shear rate increases the coagulation and fragmentation rates, decreasing the steady-state average aggregate size, and the rime lag before steady state. The model developed in this work is applied to laminar shear data from the literature, showing excellent agreement.