CFD-PBM Investigation on Droplet Size Distribution in a Liquid-Liquid Stirred Tank: Effect of Impeller Type

The influence of impeller type on the liquid-liquid dispersion system was investigated by computational fluid dynamics coupled with the population balance model (PBM). Rushton turbine (RT), pitched-blade turbine (PBT), and parabolic blade turbine (PB) produce different flow fields and turbulence distribution. The droplet size distribution (DSD), breakage frequency, and coalescence rate were predicted with the Alopaeus breakage model and the CT coalescence model. The simulated velocity field and DSD were validated with particle image velocimetry and image analysis, respectively. The analysis of the interaction between eddy and droplet shows that the turbulent inertial regime was the dominant mechanism of droplet breakage in the impeller region for radial-type impellers (RT and PB), while the turbulent viscous regime for an axial impeller (PBT). RT and PB can produce smaller droplets than PBT, about 16.4 and 10.7%, respectively. The average turbulent dissipation rates of RT and PB are about 7 and 10 times that of PBT in the impeller region, which explains the better performance of these two radial impellers in terms of droplet breakup.