Gas hold-up in bubble columns: influence of alcohol addition versus operation at elevated pressures

Abstract

Measurements were carried out in a 0.15 m diameter bubble column with air-water system to which ethanol was added in concentrations ranging from 0.03 to 1 vol.%. Alcohol addition results in a significant increase in the gas hold-up, ε. This increase in e can be attributed to a delay in the point of transition from homogeneous to heterogeneous flow regime. The model of Krishna et al. (Chem. Eng. Sci. 54 (1999), 171) was found applicable, after accounting for the influence of alcohol addition on the regime transition parameters. Experimental data of Letzel et al. (Chem. Eng. Sci. 52 (1997), 3733), obtained in a 0.15 m diameter nitrogen-water bubble column, also showed that increased system pressures results in significantly increased gas hold-up. A careful comparison of the influence of alcohol addition and that of increased pressure leads to the conclusion that in the latter case, increased system pressures besides delaying the onset of the heterogeneous flow regime also promotes the break up of 'large' bubbles. One needs to modify the model to take account of the influence of increased pressure on the rise velocity of the swarm of 'large' bubbles.