Flow Pattern, Mixing, Gas Hold-Up and Mass Transfer Coefficient of Triple-Impeller Configurations in Stirred Tank Bioreactors

Global and local gas liquid characteristics (gas hold-up, volumetric mass transfer coefficient), and flow field, mixing time of the liquid phase are investigated for various triple-impeller configurations. Four types of impellers (Rushton turbine (RT), hollow blade turbine (HBT), wide-blade hydrofoil impeller pumping down (WHd), and pumping up (WHu)) were used to form four combinations (3RT, HBT+2WHd, HBT+2WHu, and 3WHu). The results show that the axial impellers combination (3WHu) provides more effective homogenization performance than the impellers with combined radial and axial flow (HBT+2WHu, HBT+2WHd), while the radial impellers combination (3RT) is the worst. When the gas superficial velocity is 1.625 mm s(-1), 3WHu produces a 53% higher mass transfer coefficient than HBT+2VVHd, HBT+2WHu, and 3RT lie between them. When the gas superficial velocity reaches up to 8.124 mm s(-1) however, all of the tested configurations give almost similar mass transfer coefficients under equivalent power input. For 3RT, the highest hold-up is in the bottom impeller discharge stream and near the wall for the middle and top impellers. For the HBT+2WHd combination, there was no large variations of gas hold up in the bulk except the region around the bottom impeller. For HBT+2WHu and 3WHu, high gas hold-up was observed between the two up pumping impellers, and moderately low gas hold-up above the top impeller. There are three zones of higher interfacial area for the HBT+2WHu and 3RT combination, two zones of higher interfacial area for 3WHu, and only one zone of higher interfacial area for HBT+2WHd combination. At low gas velocity, the flow pattern generated by each impeller combination results in different gas bubble trajectory and different bubble breakup and coalescence kinetics, which in turn influences both local and average gas holdup directly, and also affects the local specific interfacial area, that is, influences the mass transfer coefficient indirectly. At higher gas velocity, the power drop also contributes to change of the gas hold-up and mass transfer at the same specific power consumption.