Enhancement of turbulent liquid-liquid dispersions in a round-bottom stirred tank equipped by a flexible impeller

Abstract

Stirred tanks have been widely used for liquid-liquid mixing in the chemical industry. In the present work, the influences of flexible impeller on the hydrodynamic characteristics and droplet size distribution of oil in water dispersions were investigated using experimental and numerical approaches. The one-way Fluid Structural Interaction (FSI) analysis was carried out to model the geometry of the flexible impeller used in CFD simulations. Computational fluid dynamics (CFD) was coupled with the population balance model (PBM) to simulate the liquid-liquid dispersion process in the stirred tank. The numerical model was validated by the experimental results. Compared with the rigid impeller, the flexible impeller generates a more uniform distribution of turbulent kinetic energy and turbulent dissipate rate in the stirred tank. Compared with the case of rigid impeller, the region indicating the trailing vortices behind the impeller blades was significantly reduced by the flexible impeller. It was found the flexible impeller with a smaller elastic modulus can result in narrower droplet size distributions in turbulent dispersion process. Its peak value of the number fraction frequency was in the range of 244-308μm while that was 308-388μm for the case of rigid impeller. It was found that most of the dispersed phase droplets occur in the turbulent inertial subrange regime. Related correlations between the Sauter mean diameter and local turbulent field were also obtained. The shape factor accounting for the effects of the impeller shape on the Sauter mean diameter was found to be dependent on the material property of the impeller blades, such as the Poisson's ratio and elastic modulus.