Mass Transfer in Multiphase Mechanically Agitated Systems

Abstract

In this chapter, the results of the experimental studies concerning the volumetric mass transfer coefficient kLa obtained for mechanically agitated gas – liquid and gas – solid – liquid systems are discussed. Mechanically agitated gas – liquid and gas –solid – liquid systems are widely used in many processes, for example oxidation, fermentation or wastewater treatment. In such cases, oxygen mass transfer between gas and liquid phases in the presence of solid particles can be described and analyzed by means of the volumetric mass transfer coefficient kLa. In the gas – liquid and gas – solid – liquid systems, the kLa coefficient value is affected by many factors such as geometrical parameters of the vessel, type of the impeller, operating parameters of the process (impeller speed, aeration rate), properties of the continuous phase (density, viscosity, surface tension, etc.) and also by the type, size and loading of solid particles. To improve the efficiency of the processes conducted in gas – liquid and gas – solid – liquid three – phase systems two or more impellers on the common shaft are often used (Kielbus—Rąpala & Karcz, 2009). Multiple – impeller stirred vessels due to the advantages such as increased gas hold – up, higher residence time of gas bubbles, superior liquid flow characteristics and lower power consumption per impeller are becoming more important comparing with single – impeller systems (Gogate et al., 2000). As the number of energy dissipation points increased with an increase in the impellers number on the same shaft, there is likely to be an enhancement in the gas hold – up due to gas redistribution, which results into higher values of volumetric gas – liquid mass transfer coefficient (Gogate et al., 2000). Correct design of the vessel equipped with several agitators, therefore, the choice of the adequate configuration of the impellers for a given process depends on many parameters. That, which of the parameters will be the most important depends on the kind of process, which will be realized in the system. For the less oxygen demanding processes the designer attention is focused on the mixing intensity much more than on the volumetric mass transfer coefficient. When the most important thing is to achieve high mass transfer efficiency of the process, the agitated vessel should be such designed that the configuration of the agitators used ensure to get high both mixing intensity and the mass transfer coefficient values (Kielbus-Rąpala & Karcz, 2010).