Eulerian simulations for determination of the axial dispersion of liquid and gas phases in bubble columns operating in the churn-turbulent regime

Abstract

Fully three-dimensional (3D) transient simulations using computational fluid dynamics (CFD) have been carried out for bubble columns operating in the churn-turbulent flow regime. The bubble column is considered to be made up of three phases: (1) liquid, (2) "small" bubbles and (3) "large" bubbles and the Eulerian description is used for each of these phases. Interactions between both bubble populations and the liquid are taken into account in terms of momentum exchange, or drag, coefficients, which differ for the "small" and "large" bubbles. Water and Tellus oil, with a viscosity 75 times that of water, were used as liquid phase and air as gaseous phase. The transient tracer responses in the gas and liquid phases were monitored at three different stations in the column and the results analysed in terms of a one-dimensional axial dispersion model. The 3D simulation results for radial distribution of liquid velocity (V_L(r)), centre-line liquid velocity (V_L(0)), axial dispersion coefficients of the liquid (D_ax,L) and gas (D_ax,G) phases, for columns of 0.174, 0.38 and 0.63 m in diameter were compared with experimental data generated in our laboratories and also literature correlations. There is good agreement between the values of V_L(r),V_L(0) and D_ax,L from 3D simulations with measured experimental data. The axial dispersion coefficient of the small bubble population was almost the same as that of D_ax,L, whereas the dispersion of the large bubbles is significantly lower in magnitude. It is concluded that 3D transient Eulerian simulations are potent tools for investigating the gas and liquid residence time distributions and have potential use as scale-up tools.