Characterizing Impeller Performance with Examples of Process Results

Impellers in agitated vessels are often described in terms such as high flow, high efficiency, high shear etc. These terms are qualitative and are not helpful when carrying out an agitator design / sizing calculation. The impellers are essentially pumps, they are machines that move fluid inside the vessel, and their performance characteristics can be defined in the same way as a pump; in terms of their power input, the flow and head generated and their efficiency.

Impellers are also required to generate shear, either to disperse a second liquid, gas or solids phase in order to generate surface area for mass transfer or to promote coagulation / flocculation of fine particles suspended in the liquid phase.

The region of highest shear in a stirred tank is found in the trailing vortex at the tip of the impeller blades. The properties of the vortex must be taken into account in order to correctly calculate a local shear rate. In this presentation a method for quantifying the flow and shear characteristics of impellers will be presented demonstrating how this method may be applied to shear driven processes. The calculation method will be verified by comparing with results from droplet break-up and flocculation experiments.

The main conclusion of this work is that the Rushton turbine, traditionally considered “high shear”, actually generates larger droplets and flocs than a Hydrofoil, traditionally considered to be “low shear” at the same power input. Understanding this is important when selecting the appropriate impellers for processes where high or low-shear mixing are requirements.