Energy losses due to large bubbles beneath spargers in circulating bubble columns

Abstract

A large stationary air bubble hanging beneath an air sparger in a downward liquid flow can exist at flow rates substantially greater than flow rates which would wash away large free (slug/Taylor) bubbles. The existence of the large stationary bubble in a circulating bubble column is detrimental because the driving force for circulation is reduced, potentially leading to the column stalling. Therefore, it is important successfully to estimate the energy losses associated with this bubble, and its effects on the circulation velocity in such columns. This large bubble was studied experimentally using a vertical downcomer of internal diameter 0.105m. The energy losses associated with this bubble were determined from the pressure profiles measured along the bubble. The loss of total head due to the bubble was roughly 0.7×length of the bubble. A theoretical model that depends on more than just the bubble length was subsequently developed and found to agree within 15% with the experimental loss. A model of a simple U-tube circulating bubble column, which predicts the circulation velocity, was developed by balancing the net weight difference between the downcomer and riser with the losses in the flow circuit. The energy losses associated with the large bubble have been taken into account; these are the same order of magnitude as the total frictional losses in the whole column. This model was tested against the experimental measurements obtained in a 10.2m circulating bubble column of internal diameter 0.24m, operated under different combinations of riser and downcomer air flow rates. It has been shown that the presence of this bubble significantly affects the circulation velocity, and the model gives good agreement to experimental measurements, with predictions improving with the length of the bubble.