Session: 7.3 - Gas-Liquid flows

Paper Number: 169667

169667 - Using Xct to Quantify Air Entrainment at the Gas-Liquid Interface of a Stirred Tank Reactor 

Abstract: 

Air entrainment, or surface aeration, is a complex phenomenon commonly observed at the gas-liquid interface of shear-driven flows; it is commonly found in mixing systems, fermenters, and wave-like flows. For stirred tank reactors (STRs), where a fluid is mixed using a mechanical agitator, air entrainment is common when the shear between a gas and liquid region is sufficient to deform the surface and capture gas bubbles. The entrained air produces a multiphase flow that can disrupt the efficiency or yield of a mixing or fermentation process. In this study, air entrainment is quantified using X-ray Computed Tomography (XCT) to measure time-averaged local gas holdup (also known as void fraction) in an unsparged standard STR equipped with a Rushton-type disc impeller and baffles. Assuming quasi-steady-state conditions, 3D XCT images were acquired while varying the impeller speed and the distance between the impeller and the tank bottom. The results show gas holdup is significantly influenced by the clearance distance and impeller speed. Different operating conditions trigger different aeration mechanisms, such as vortex formation or liquid fall, that alter the gas holdup quantity in the system. In addition, the water surface was observed to deform and assume distinct shapes as conditions vary. Finally, the XCT images show how surface air entrainment in an STR can be quantified.

Presenting Author: Theodore (Ted) Heindel Iowa State University

Presenting Author Biography: