Gas holdup distributions in large-diameter bubble columns measured by computed tomography

Abstract

Using the computed tomography (CT) and computer automated radioactive particle tracking (CARPT) facilities at the Chemical Reaction Engineering Laboratory (CREL), time-averaged gas holdup distributions and liquid recirculation velocities were measured in a 44 cm diameter bubble column for air–water and air–drakeoil systems at 2, 5, and 10 cm/s superficial gas velocities, which cover bubbly, transition and churn-turbulent flow regimes, respectively. Gas holdup was found to increase only slightly with the increase in axial distance from the distributor, but increased significantly with the increase in superficial gas velocity, as expected. A lower gas holdup was observed in the air–drakeoil system than in the air–water system. This could be predominantly attributed to the formation of large bubbles in the former case due to the higher viscosity of drakeoil (approximately 0.03 Pas (=30 cp)). At high superficial gas velocities, the time-averaged cross-sectional gas holdup distributions were almost symmetric for both air–water and air–drakeoil systems. However, at 2 cm/s superficial gas velocity, an asymmetry in the holdup distribution was observed, which manifested itself in an asymmetric liquid recirculation pattern. At all gas velocities, the radial gas holdup distribution for the air–water system was steeper than that for the air–drakeoil system, yielding steeper radial liquid velocity profiles. Comparison of the gas holdup obtained in the 44 cm diameter column and that obtained in a 10 cm diameter column is discussed.