Scale Effects on the Hydrodynamics of Bubble Columns Operating in the Homogeneous Flow Regime

Abstract

Measurements of gas holdup were made with the air-water system in bubble columns of 0.1, 0.15 and 0.38 m diameter, equipped with identical distribution devices. For operation with superficial gas velocity in the range 0-0.04 m/s, the total gas holdup was found to decrease with increasing column diameter. Of all the literature correlations for the gas holdup, only the Zehner correlation anticipates this decrease in the gas holdup with increasing column diameter. The reason for this scale dependence is because the strength of the liquid circulations increases with increasing scale. Such circulations accelerate the bubbles travelling upwards in the central core. Computational fluid dynamics (CFD) simulations were carried out using the Eulerian description for both the gas and the liquid phases in order to verify the scale dependence of the hydrodynamics. Interactions between the bubbles and the liquid are taken into account in terms of a momentum exchange, or drag, coefficient. The drag coefficient is determined from the Mendelson correlation for bubble rise velocity. The turbulence in the liquid phase is described using the k-e model. The simulation results verify the trends predicted by the Zehner (1989) correlation. It is concluded that Eulerian simulations are useful tools for scaling up bubble columns.