Dynamic Perturbations: An Alternative for Improving Viscous Mixing in Stirred Tanks
Philippe Tanguy, Gabriel Ascanio, Edmundo Brito-De La Fuente, Mario Alvarez
URPEI - Ecole Polytechnique
Canada

Keywords: Chaotic, Non-Newtonian, Mixing


Viscous mixing in polymerization reactors is usually carried at low to moderate Reynolds numbers. Under these conditions, the onset of caverns around the impeller can lead to long or infinite mixing times and as a consequence, the process becomes inefficient. The caverns can be eliminated if the impeller speed is increased, however a high energy input is required to achieve total homogenization of the fluid and in some cases this can be a drawback when mixing is carried out in a shear sensitive media. An alternative approach to avoid or eliminate the formation of caverns consists of perturbing the flow within the vessel.
Mixing of Newtonian and non-Newtonian fluids has been experimentally investigated by using different unconventional configurations based on off-centered impellers and time-varying speeds and rotation directions. A color-discoloration technique based on a fast acid-base reaction has been used for determining the mixing times in the vessel. Newtonaian and mild to pronounced shear-thinning fluids have been tested.
A series of mixing indicators determined for standard and two non-standard configurations will be compared. The two non-standard scenarios will consist of: a) two off-centered co- and counter-raotating, and b) a single impeller rotating alternatively clockwise and counter-clockwise at different speeds. It will be shown that mixing can be notoriously improved if the flow is continuously perturbed, irrespective of the fluid rheological behavior.