Investigation of local gas holdup and bubble dynamics using four-point optical probe technique in a split-cylinder airlift reactor

A four-point optical fiber probe technique was employed in a split-cylinder airlift reactor with an air-water system. Effect of superficial gas velocities- 0.3, 1.0, 2.0, and 2.8 cm/s- was studied on the local gas holdup and bubble dynamic properties namely, bubble passage frequency, interfacial area, bubble chord length, and bubble rise velocity in both the riser and downcomer sections of the reactor. The bubble chord length and bubble velocity in the riser and downcomer followed log-normal distribution and normal distribution respectively. The gas holdup and interfacial area in the riser increased by 900 and 800% for a superficial gas velocity increase from 0.3 to 2.8 cm/s, while those at the top of the downcomer by 500, and 400% for a superficial gas velocity increase from 1.0 to 2.8 cm/s, respectively. At each superficial gas velocity, the bubble passage frequency, gas holdup, and interfacial area did not vary significantly along the axis of the riser. An axial variation in bubble passage frequency, gas holdup, and interfacial area was observed in the downcomer. A correlation for the variation of the gas holdup in the riser was developed based on superficial gas and liquid circulation velocities. The correlation for the gas holdup in the downcomer was developed to account for the axial variation observed.