Influence of Impeller Type on the Bubble Breakup Process in Stirred Tanks

Abstract

A phenomenological model based on the collision theory and on population balances has been used to study the effect of different impeller geometries on the breakup mechanisms governing bubble dispersion in a stirred tank. Five different types of impellers, including standard and nonstandard ones (the Rushton turbine, the propeller, two different pitched blade turbines, and a modified blade), located at different positions along the vertical axis were used to generate dispersions of air bubbles in water. Air flow rates from 0.6 to 2.8 × 10−6 m3/s were used. The dispersions generated and the breakup processes were recorded by means of a high speed video camera. The different behavior of the impellers, based on its physical effect on the bubbles as well as on the effect of the flow pattern developed in the tank on them, determine the breakup mechanisms responsible for the generation of bubbles dispersions. The Weber number and the energy applied in a region surrounding the impeller quantify the contribution of the different breakup mechanisms to the mean diameter of the dispersion. Good agreement was found between the experimental Sauter mean diameter of the bubbles and those predicted by the model. Consequently, the model provides also physical explanation for the main breakup mechanisms of each impeller based on the critical Weber number and the energy dissipated. The model also allows a good theoretical prediction of the mass transfer rate.