Numerical and experimental evaluation of droplet breakage of O/W emulsions in rotor-stator mixers

Abstract

Emulsification of a high viscosity oil in water mixture was studied in a rotor-stator mixer. Experimental characterization of drop sizes and high-speed visualizations of drop breakup phenomena were performed, combined with computational fluid dynamics analysis of the flow field and population balance modeling. Single-phase flow simulations were carried out with a sliding mesh CFD model and combined with analysis of streamlines, turbulent eddy dissipation fields and high-speed visualizations of drop breakage. From frames taken over few seconds, with the oil-water system initially fully segregated, three drop breakage mechanisms were identified, supported by the CFD analysis: (i) a combination of the large scale circular motion in the tank and the fluid jet emanating from the stator holes; (ii) droplets that are pulled into the rotor – stator system from the bottom and flow through the gap between the stator and rotor, undergoing extremely high shear stresses, being expelled through the stator slots in several fragments, and iii) droplets that remain caught at the recirculation zones in the stator holes, increasing exposure time to high shear during the rotor blade passage. Finally, an optimization algorithm was coupled to a population balance model and the measured drop size distributions were reproduced in good agreement with experimental data, including the bimodal behavior attributed to the generation of satellite droplets.