The effect of aspect ratio in counter-current gas-liquid bubble columns: Experimental results and gas holdup correlations

• Published in: International journal of multiphase flow 2017-09, Vol.94, p.53-78
• Main Authors: Besagni, Giorgio, Di Pasquali, Alessandro, Gallazzini, Lorenzo, Gottardi, Elia, Colombo, Luigi Pietro Maria, Inzoli, Fabio
• Format: Article
• Language: English
• Subjects: Bubble column; Counter-current mode; Electrolyte; Flow regime transitions; Gas holdup; Gas holdup correlation
• ISSN: 0301-9322; 1879-3533
• DOI: 10.1016/j.ijmultiphaseflow.2017.04.015

•The effect of the aspect ratio in a counter-current gas-liquid bubble column has been studied.•The aspect ratio, up to a critical value, decreases the gas holdup and destabilizes the homogeneous regime.•The critical value of the aspect ratio depends on the bubble column operation and liquid phase properties.•A new scheme of correlation for the prediction of the gas holdup in counter-current gas-liquid bubble columns has been proposed.•The correlation predicts fairly well the gas holdup measurements at the different aspect ratios. It is generally admitted that the gas holdup is independent of the column dimensions and gas sparger design if three criteria are satisfied: the diameter of the bubble column is larger than 0.15m, gas sparger openings are larger than 1–2mm and the aspect ratio is larger than 5. This paper contributes to the existing discussion; in particular, the effect of the aspect ratio (within the range 1–15) in a counter-current gas-liquid bubble column has been experimentally studied and a new gas holdup correlation to estimate the influence of aspect ratio, operation mode and working fluid on the gas holdup has been proposed. The bubble column, equipped with a spider gas sparger, is 5.3m in height, has an inner diameter of 0.24m; gas superficial velocities in the range of 0.004–0.23m/s have been considered, and, for the runs with water moving counter-currently to the gas phase, the liquid has been recirculated at a superficial velocity of −0.0846m/s. Filtered air has been used as the gaseous phase in all the experiments, while the liquid phase has included tap water and different aqueous solutions of sodium chloride as electrolyte. Gas holdup measurements have been used to investigate the flow regime transitions and the global bubble column hydrodynamics. The counter-current mode has turned out to increase the gas holdup and destabilize the homogeneous flow regime; the presence of electrolytes has resulted in increasing the gas holdup and stabilizing the homogeneous flow regime; the aspect ratio, up to a critical value, has turned out to decrease the gas holdup and destabilize the homogeneous flow regime. The critical value of the aspect ratio ranged between 5 and 10, depending on the bubble column operation (i.e., batch or counter-current modes) and liquid phase properties. Since no correlation has been found in the literature that can correctly predict the gas holdup under the investigated conditions, a new scheme of gas holdup correlati