Influence of impeller design on oxygen transfer in a stirred water tank with square cross-section

Abstract

This paper presents an experimental study on the mass transfer performance in a gas-liquid stirred vessel. Three impellers – circular with flat blades, rectangular with slightly curved blades, and trapezoidal with flat perforated blades – were tested for two aspect ratios of the water column inside a tank. The study aimed at identifying the type of impeller and operating conditions that maximize the aeration efficiency of vortex aerators and at verifying whether the usage of impellers with perforated blades could decrease the energy consumption while maintaining a good aeration efficiency. The results revealed the existence of two critical impeller speeds. At the first critical speed, the operating regime changes from subcritical or non-aerated to supercritical or aerated. The vortex specific interfacial area increases almost linearly up to the second critical speed and then tends to stabilize. The impeller with perforated blades did not confirm the expectations, its efficiency being very poor. The rectangular impeller was the most efficient in the subcritical regime. The circular impeller performed better in the supercritical regime. These two impellers were found to operate roughly in the same range as other conventional impellers for an aspect ratio of 1. This finding, coupled with the geometrical simplicity, recommends the circular and rectangular impellers for usage in tank aerators and, possibly, in bioreactors.