Effects of column diameter and liquid height on gas holdup in air-water bubble columns

Experiments on the total gas holdup, αG, in air-water cylindrical bubble columns were carried out to investigate effects of the column diameter, DH, and the initial liquid height, H0, on αG. Ranges of DH and H0 were 160 ⩽ DH ⩽ 2000 mm and 400 ⩽ H0 ⩽ 4000 mm, respectively. The superficial gas velocity, JG, was varied from 0.025 to 0.35 m/s. The characteristics of gas holdup showed that all the flows in the present experiments were pure heterogeneous. The following conclusions were obtained for αG in air-water bubble columns: (1) the effects of DH and H0 on αG are negligible when scaling up from small to large bubble columns, provided that αG in the small columns are obtained for DH ⩾ 200 mm and H0 ≳ 2200 mm. The height-to-diameter ratio is useless in evaluation of the critical height, above which αG does not depend on H0, (2) for the above ranges of DH and H0, Akita-Yoshida's and Koide's correlations can give good evaluations of αG for a wide range of JG by tuning the model constants, (3) for DH < 200 mm, the decrease in DH increases the population of large bubbles, which results in the decrease in αG, and (4) for H0 ≲ 2200 mm and DH ⩾ 200 mm, αG at a constant JG decreases with increasing H0 and approaches an asymptotic value, and the Froude number using JG and H0 as the characteristic scales well correlates αG in this regime.