Gas-liquid mass transfer in unbaffled dual-impeller mixers

The gas-liquid mass transfer performance of different unbaffled dual-impeller mixers was investigated experimentally using low- and high-viscosity Newtonian and non-Newtonian liquids. The tested configurations were composed of two Rushton turbines located at two different height levels in the vessel in centered or off-centered positions. Various mixers were compared based on their respective mass transfer performance measured by means of two dissolved oxygen probes located at different levels in the vessel. k_la correlations and an axial homogeneity criterion were established for the different configurations and liquids used. It was concluded that the dual shaft mixer consisting of two off-centered shafts appears to be a very interesting and flexible configuration to deal with Newtonian liquids in a large range of viscosities. However, concerning the gas-liquid dispersion in non-Newtonian liquids, it is shown that off-centered shaft configurations have to be avoided.