Determination of gas-liquid mass-transfer and solids-suspension parameters in mechanically-agitated three-phase slurry reactors

Abstract

This paper describes methods for determining gas-liquid mass-transfer and off-bottom solids-suspension parameters, and the interactions between these parameters, for mechanically-agitated three-phase slurry reactors. A steady-state sodium sulphite feeding method was used to obtain estimates of the overall volumetic mass-transfer coefficient ( k La ). In this work, the effectiveness of performing a gas-phase oxygen mass balance (in place of a sulphite balance on the liquid phase) to determine the sulphite oxidation rate was demonstrated. Mass-transfer data for a high-solidity ratio axial-flow impeller were generated over a range of air flowrates and impeller speeds. The ‘just-off-the-bottom suspension’ criterion (JOBS) is a useful and widely-used criterion for solids suspension in slurry reactors. This criterion was used to obtain solids-suspension data for a finely-ground, fast-settling mineral slurry with a wide particle-size distribution, in both two- and three-phase systems. The effect of increasing gas flowrates on the ability to maintain solids in suspension was demonstrated. The effectivenessand limitations of these methods to characterise three-phase systems was illustrated using bacterial sulphide oxidation as the reacting system. The results obtained in this work emphasise the importance of incorporating the effects of physical gas-liquid-solid interactions in the design of mechanically-agitated three-phase slurry reactors.