Scale effects on the hydrodynamics of bubble columns operating in the heterogeneous flow regime

Abstract

CFD simulations were carried out in the Eulerian framework using transient threedimensional strategy in order to describe the influence of column diameter on the hydrodynamics and dispersion characteristics of bubble columns operating in the heterogeneous flow regime. All simulations were carried out with air as the gas phase and water as the liquid phase. A bi-modal distribution of bubble sizes, 'small' and 'large', was assumed for the gas phase. Interactions between the bubbles and the liquid are taken into account by means of a momentum exchange, or drag, coefficient. For small bubbles, the drag coefficient was estimated from experimental data on single bubble terminal velocity. For large bubbles, the drag coefficient is estimated from the experimental data on the large bubble swarm velocity in the limiting case where the superficial gas velocity U approaches the regime transition velocity U_trans. The turbulence in the liquid phase is described using the k-ε model. For a bubble column operating at U = 0.15 ms⁻¹, simulations were carried out for columns of 1, 2, 4, 6 and 10m in diameter to determine gas holdup, the liquid circulation velocity, and the axial dispersion coefficient of the liquid phase, D_ax,L, and that of the large and small bubble phases. The results demonstrate the strong increase of liquid circulations, and D_ax,L with increasing column diameter. The dispersion of the small bubbles has the same order of magnitude as that of D_ax,L.