Shear-induced aggregation-fragmentation: Mixing and aggregate morphology effects

This work extensively reviews shear-induced simultaneous aggregation and fragmentation research. The kinetic effects of aggregate structure, fluid flow field, and solids concentration are identified as the dissertation theme. The evolution of the floc size distribution in a coagulating and fragmenting suspension of spherical particles is examined using a population balance technique. Initially, coagulation rapidly depletes the primary particles, forming a second large floc mode. This mode develops and the primary particles disappear until coagulation and fragmentation balance each other. The time before steady state is reached is a unique function of the opposing coagulation and fragmentation rates.; Three different flocculation/mixing impellers produce a rapid initial aggregate growth and an increasingly irregular aggregate surface structure prior to steady state. More frequent intermittent exposure to high shear rates produces smaller particles as the flow field shifts from radial to axial flow. The effect of fluid flow field on aggregate size is modeled with a multiple compartment description. Assuming a constant averaged shear rate is shown to over- or under-predict the steady state average aggregate size depending on the flow conditions. Raising applied shear rates fragments aggregates, forming small, compact structures. Returning the shear rate to its original value forms a smaller and more dense steady state aggregate structure than the original steady state.; The shear-induced rotation rate of fractal aggregates deviates from spherical behavior only at low shear rates {dollar}rm (gamma = 1 ssp{lcub}-1{rcub}).{dollar} Assuming a shear-induced aggregate collision rate based on maximum aggregate radius and a lacunarity of unity is an overestimation. Simulated aggregates, lacking the restructuring history of realistic aggregates, have a lower lacunarity than experimental aggregates. A flocculation model incorporating the effects of aggregate structure and hydrodynamic forces describes suspensions undergoing laminar or turbulent shear and is in excellent agreement with experimental data.; Turbulent shear-induced aggregation at high solids fractions is evaluated using a new light scattering technique. The suspended aggregates undergo simultaneous, mixing-limited aggregation and fragmentation but more rapidly than dilute suspensions. Enhanced aggregation kinetics at increased solids fractions {dollar}({lcub}le{rcub}1%){dollar} are predicted by existing population balance techniques.