Volumetric Gas−Liquid Mass Transfer Coefficient in Aerated Stirred Tank Reactors with Dense Floating Solid Particles

Abstract

The effect of floating solid particles on volumetric gas−liquid mass transfer coefficients was investigated using a stirred tank of 0.2 m inside diameter (i.d.) with dual four-flat blade disk turbines for solid concentrations up to 50 vol %. Two kinds of floating solids, synthetic adsorbent particles (particle size of 375 μm and wet density of 793 kg m−3) and polypropylene particles (particle size of 480 μm and wet density of 854 kg m−3) were used. At very low impeller speeds the floating particles formed a stagnant layer on the liquid surface, and none of the solids were dispersed. As the impeller speed was increased, particles were partially dispersed, and the stagnant layer volume was reduced. With further increasing impeller speed, all solids were ultimately dispersed throughout the tank. The gas hold-up and volumetric gas−liquid mass transfer coefficient in a baffled or unbaffled stirred tank with dense floating solids particles are measured. The power consumption and gas hold-up decreased with an increase in floating solids concentration. It was found that the presence of floating solid particles significantly reduced the mass transfer coefficient at a given impeller speed. The presence of floating solids increased the apparent viscosity of the slurry and hence probably resulted in increased bubble coalescence tendency enhancing the formation of larger bubbles. They might lead to a decrease in gas hold-up or specific surface area and subsequently induce a significant decrease in the volumetric gas−liquid mass transfer coefficient. The experimental data for gas hold-up and volumetric gas−liquid mass transfer coefficient are satisfactorily correlated to power consumption, superficial gas velocity, and solid volume fraction.