Mixing characteristics in slurry stirred tank reactors with multiple impellers

Hydrodynamic parameters such as power consumption, gas holdup, critical impeller speed for solid suspension and mixing time were measured in slurry stirred tank reactors with multiple impellers. The experiments were mainly conducted in a stirred tank of 0.2 m i.d. with baffles. It contained two four-pitched blade downflow turbines for gas dispersion and one Pfaudler type impeller for solid suspension. As a part of scaling studies, additional experiments were also carried out in a larger stirred tank reactor (0.8 m i.d.) geometrically similar to the smaller one. Glass beads and polymeric particles were used as a solid phase. Solid concentration was in the range of 0-20% (V/V). Tap water and methanol were used as a liquid phase. The power consumption decreased due to an introduction of gas and the presence of solids caused a decrease in the extent of reduction in power consumption. A correlation for power consumption in aerated slurry systems was proposed. It was found that the presence of solids is responsible for a decrease in gas holdup. A new correlation for gas holdup in gas-liquid-solid three-phase stirred tank reactors was developed. It fit the present experimental data reasonably. The critical impeller speed for solid suspensions increased with increasing gas flow rate. However, its increase was rather smaller as compared with the predictions of the correlations available in the literature. We proposed a correlation of the critical impeller speed for solid suspension in the presence of gas. The mixing time complicatedly increased or decreased depending on gas flow rate, impeller speed, solids type and concentration.