(77d) Emulsification and Dispersion in Agitated Liquid-Liquid Systems: Linking Hydrodynamics to Drop Size

Emulsions and dispersions are ubiquitous in the chemical process industries and form the basis of many food, pharmaceutical, personal care and other consumer products. An important step in equipment selection is prediction of drop size for a variety of devices ranging from stirred tanks and static mixers to high shear rotor-stator mills and high-pressure homogenizers. While these devices are of disparate geometry, their analysis can be put on common ground by connecting drop behaviour to local hydrodynamics. Mechanistic models for drop break up and particle fracture are based on the local, scale dependent fluid stress. As a result, having accurate estimates of the extent and intensity of the dispersion zone allows tuning of models for drop size, scaling of drop size data from one device to another of different geometry but with similar local hydrodynamics, and development of physical models for dispersion in the absence of size data.

Scaling arguments will be presented for laminar flow where local shear rate governs drop behaviour, and for turbulent flow where the forces acting on a drop transitions from inertial to viscous as the drop size decreases and local turbulent energy dissipation rate (power per mass) governs drop dynamics. The role of surfactants will be considered. Mechanistic correlations for drop size distribution will be presented and related to local hydrodynamics. The efficacy of different approaches to estimate local deformation rates will be discussed. Detailed flow field results, both experimental and via CFD, will be presented and related to drop or particle dispersion. Population Balance Models (PBM) will be considered with respect to development of breakage kernels and validation of modern CFD-PBM simulations. Examples will be taken from the author’s work as well as that of others. Limitations and future directions will be discussed. The presenter was Dr. Leng’s co-author on the NAMF Handbook chapter titled “Immiscible Liquid-Liquid Systems”. There will be recurring reference to Doug’s contributions as well as his influence on the current practice and research directions.