Development of a zone flow model for the confined impeller stirred tank (CIST) based on mean velocity and turbulence measurements

Abstract

The confined impeller stirred tank (CIST) is a test cell designed to scale down elevated local mixing conditions to the bench scale in a more uniform mixing field than the conventional stirred tank. In this study a zone flow model was developed to describe the flow in the vessel, based on mean and fluctuating velocities measured using a laser Doppler velocimeter (LDV). A set of 5 Rushton turbines were used for this first model with water as the test fluid. The impeller rotational speed was kept high enough to ensure fully turbulent flow in the entire vessel (Re≥20000). It is shown that all five impeller discharge streams have similar behaviours and that the jet leaving the impeller blades does not expand axially as much as the discharge flow in a single impeller stirred tank because the confined return flow reduces the jet width. The radial decay of mean velocity close to the centerline of the blades is proportional to 1/r, and the radial decay of energy dissipation is proportional to 1/r2. A single impeller stirred tank presents a much faster decay of dissipation proportional to 1/r4. Based on these results, and a number of other experimental measurements, the tank was divided into 5 volumes or zones. A zone flow model was developed to describe the flowrate and dissipation rate in each zone, giving the mean residence time and mixing energy for each zone as outputs. It is shown that the CIST has a more uniformly distributed energy dissipation (εmax/εave=8.3) than a single impeller stirred tank (εmax/εave>20).