Investigating Air Entrainment Phenomenon at the Gas-Liquid Interface in Stirred Tank Reactors Using XCT

X-ray computed tomography (XCT) is a powerful noninvasive measurement technique that allows the visualization and quantification of multiphase flows in various systems, including plunging liquid jets, bubble columns, and fluidized beds. It can produce time-average three-dimensional maps of gas holdup (also called void fraction) within a multiphase flow. In this study, XCT was used to examine the interfacial phenomenon of air entrainment in a stirred tank reactor (STR), a mechanical unsparged mixing system equipped with baffles and a Rushton-type impeller. Air entrainment in STRs occurs when shear at the gas-liquid interface is sufficient to distort the interface and trap gas bubbles, which can affect the efficiency and yield of the mixing operation. The findings show three-dimensional XCT images of time-average local gas holdup and gas dispersion at various impeller submergence levels. Such maps relate to different air entrainment mechanisms under different operating conditions. Furthermore, the gas-liquid interface adopted different deformation patterns as the clearance distance and impeller speed were varied.