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

•Effects of column diameter and liquid height on gas holdup are clarified.•Gas holdups at various column diameters and liquid heights are obtained.•A guideline for developing gas holdup correlations is obtained. 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⩽2000mm and 400⩽H0⩽4000mm, respectively. The superficial gas velocity, JG, was varied from 0.025 to 0.35m/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⩾200mm and H0≳2200mm. 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