Gas void ratio and bubble diameter inside a deep airlift reactor

Void ratio (gas holdup) and bubble diameter measurements were made inside a 1.06-m diameter column containing bubbly flow at depths up to 24 m. Experiments were performed in order to identify differences in trends with column geometry and operating conditions from those found in smaller columns. Void ratio was found to increase as depth decreased regardless of the sparger type or column height. It was also found that larger columns exhibit a wider range of void ratios between the top and bottom than for smaller columns. A straightforward model was developed to predict the void ratio at heights greater than 2 m above the column bottom. The model incorporates the influences of hydrostatic pressure, superficial gas and liquid velocities, and a fitted bulk bubble-rise velocity while ignoring gas transfer with air as the gas of interest. The fitted slip velocities were found to compare well with literature measurements of single-bubble slip velocities. If the void ratio profiles are already known, the equation can also be used to estimate the bubble slip velocity, which is difficult to measure experimentally. Bubble diameter measurements were made using a submersible camera attached to a trolley. It was found that the Sauter mean bubble diameter does not change with gas flow rate and depth and can decrease substantially when not taking the few largest bubbles (outliers) into account. In contrast, differences were observed when comparing column types. With or without the outliers removed, the bubble column contained larger bubbles than the airlift reactor, which may justify conversion to an airlift reactor and clarify some important factors in the operation of commercial-scale columns.