Experimental Study on Gas–Liquid–Liquid Hydrodynamics in an Air-Lift Solvent Extraction Mixer-Settler

Hydrodynamics and mixing characteristics for gas–liquid–liquid three-phase flow in an air-lift solvent extraction mixer settler are reported. The setup consists of a 0.5 L mix box (containing the mixing element) and a follow-on settler (2 L). The two immiscible liquid phases are 0.01 M HNO₃ (aqueous) and 30% tributyl phosphate in dodecane (organic). Air is used to disperse one phase (aqueous phase) into another in a batch operation. A high-speed imaging system is used to capture the state of dispersion. Systematic experiments are conducted to examine the effects of the air flow rate and organic-to-aqueous (O/A) ratio on important hydrodynamic parameters, namely, dispersion band thickness, drop size distribution, and mixing time in the aqueous phase. Within the range of operating parameters studied, the dispersion band thickness in the mix box increases with the air flow rate, reaches a maximum value, and then subsequently decreases. A higher O/A ratio leads to a decrease in the dispersion band thickness. Mixing time in the aqueous phase is dependent on dispersion band thickness, with higher mixing times corresponding to a high dispersion band thickness and vice versa. The drop size distribution is also found to be dependent on the O/A ratio and the air flow rate. A higher O/A ratio results in the formation of larger drops. Interestingly, the air flow rate exhibits a nonlinear effect on the drop diameter, initially decreasing and then increasing with an increase in the air flow rate.