Chapter 57 - Hold-up and gas-liquid mass transfer performance of modified Rushton turbine impellers

Abstract

This chapter presents a comparison of six different blade profiles in disk turbines having the same diameter and blade projected dimensions, regarding gas hold-up and gas–liquid mass transfer performances. Gas volume fraction ɛv, and volumetric oxygen transfer coefficient kLα change in accordance with the energy dissipation rate achieved at the same rotational speed by the different radialflow impellers studied. However, at equal power and gassing rate, the different stirrers give the same ɛv and kLα values, as long as experimental errors are taken into account. Agitation is a very important factor in gas–liquid processes that need large gas handling capacity and effective gas dispersion, like fermentation and a variety of oxygenation and hydrogenation processes. Disk turbines are radial-flow impellers which are particularly suitable for gas–liquid dispersion through mechanical agitation. This is because the disk collects the gas underneath, forcing it into the high shear zone near the blades where bubble formation occurs, as well as because flow instabilities shown by open blade turbines are eliminated. Better mass transfer performance and gas handling capacity may be expected after actual Rushton turbines are retrofitted with streamlined blade impellers of the same size, because lesser power fall on gassing and retarded impeller flooding may be obtained comparatively to the standard fiat bladed impeller.