Dynamics of gas dispersion in a rising bubble plume in presence of surfactant

Abstract

Understanding dispersion behaviour of bubbles emitting from a sparger is a critical element of mineral flotation process. This aspect was investigated in the present study involving a bubble plume in a semi-batch rectangular column in the presence of an anionic surfactant. First, high-speed imaging was used to visualise the bubble plume behaviour at different air flow rates (0.1 – 0.5 L/min). An image processing code was developed to determine the mean bubble diameter which indicated a decrease in the mean bubble diameter from ∼ 0.60 mm to 0.51 mm with increasing gas flow rates. A transient 3D Eulerian-Eulerian multiphase CFD model with a bubble population balance sub-model was also developed to quantify the gas holdup and turbulence energy dissipation rate distribution in this system utilising the experimentally measured mean bubble size. Experimentally, it was observed that symmetry of the bubble plume was disrupted at higher gas flow rates leading to larger dispersion of gas bubbles towards the top of the column. This observation was explained by the CFD model which predicted asymmetric transverse velocity profiles that increased in the axial direction. The model also predicted increasing gas holdup in the system (∼0.02 to 0.11) with increasing gas flow rates. The corresponding turbulence energy dissipation rate increased from ∼ 0.014 to 0.076 m²/s³ with maximum turbulent energy dissipation rate occurring near the gas distributor zone. Also, a transition from a bubbly to a distinct foam zone was noted at the free surface in the higher gas flow rate cases which was explained by the turbulence energy dissipation rate in the system.