New Insights into the Gas Dispersion and Mass Transfer in Shear-Thinning Fluids Inside an Aerated Coaxial Mixer via Analysis of Flow Hydrodynamics and Shear Environment

Abstract

The performance of an agitated bioreactor is hampered by an intense shear environment and low mass-transfer coefficient (kLa). The gas hold-up, kLa, and shear environment achieved by a coaxial mixer comprising a central impeller and an anchor impeller for various central impeller types, impeller speeds, pumping directions, and rotating modes were explored using tomography, dynamic pressure, and computational fluid dynamics methods. A novel method was introduced to explore the performance of aerated coaxial mixers in enhancing the mass transfer through analyzing the gas and liquid flow patterns. The results showed that the gas flow pattern created by the down-pumping pitched blade-anchor (PBD-anchor) mixer led to higher gas hold-up compared to that for the up-pumping pitched blade-anchor (PBU-anchor) mixer. However, the liquid flow pattern generated by the PBD-anchor mixer demonstrated poor mixing, resulting in a lower kLa compared to that for the PBU-anchor mixer. For all configurations, the mass-transfer efficiency attained in the co-rotating mode was higher than that in the counter-rotating mode. The shear environment generated by a coaxial mixer was comprehensively analyzed for the first time. Due to a lower shear environment, a bioreactor furnished by the co-rotating PBD-anchor is more suitable for shear-sensitive applications.