Some Fluid Dynamic Characteristics in the Scale-up of Rushton Turbine-Agitated Tanks

Turbine-agitated tanks are used in chemical and biochemical applications to increase mixing which, in turn, affects the yield, productivity and product quality. The purpose of this work was to study the hydrodynamic influence on the scale-up of turbine-agitated tanks, commonly used in fermentation. Local turbulent flow parameters, the turbulent kinetic energy and the local energy dissipation rate, were related to the global scale-up parameters: the impeller tip speed, impeller diameter and power per unit mass, and to the local flow parameter, the calculated convective velocity.

A system for turbulent velocity measurements was designed using constant-temperature anemometry as the measuring method. A split-film probe was used which enabled velocity measurements to be made in two directions simultaneously. Measurements were performed in three tanks of different geometries and sizes (0.75 m3, 12 m3 and 30 m3). Different operating conditions, in regard of power input per unit mass, were used, comparable both to commercial fermentation and bench-scale units.

Several estimates and hypothesis for the convective velocity were evaluated. Different methods for calculating the local energy dissipation rate were compared for measurements performed both in the impeller region and the bulk region of the tanks.

It was shown that there was a correlation between the local turbulent parameters, turbulent kinetic energy and energy dissipation rate, and the local flow parameter, the convective velocity Uconv, independent of the tank size, the tank geometry or position in the tank, which confirms the similarity of the flows at high turbulent Reynolds numbers. The similarity of the flow conditions at high turbulent Reynolds numbers was also evidenced by scaling of energy spectra in the space domain for tanks of different geometries, sizes and at different positions in the tanks. Correlations are given for the scaling of local flow parameters, the turbulent kinetic energy and the energy dissipation rate, in the impeller region of the tank, using global scale-up parameters. Long-term characteristics have been detected in the mean flow and in the turbulent flow in the impeller and bulk zones. Increasing the power supplied to a process affects the impeller region more than the bulk region, and therefore increases the turbulent mixing in the process. This was shown by the higher values of the slopes of turbulent parameters compared with global parameters.