Analysis of Local Shear Rate Distribution in a Double Coaxial Bioreactor containing Biopolymer Solutions using Computational Fluid Dynamics

Uniform gas dispersion and shear distribution in highly viscous non-Newtonian fluids is a challenging task due to intricate rheological behavior exhibited by this type of media. In addition, most of the large-scale bioreactors in biochemical processes such as wastewater treatment, fermentation, and pharmaceutical industries demand higher aspect ratios (i.e., fluid height to tank diameter ratio) compared to laboratory-scale bioreactors. This, in turn, underlines uneven gas and shear distribution throughout the bioreactor, especially those comprising yield-pseudoplastic fluids. For this type of fluid, there are two distinct zones within the bioreactor, a higher-shear zone with a lower apparent viscosity around the impeller and a lower-shear zone with a higher apparent viscosity away from the impeller. Due to the viscosity gradient, homogeneous gas dispersion within a single impeller aerated bioreactor with an aspect ratio more than one is hard to attain. It was established that a well-designed mixing configuration contributes to maintaining a uniform fluid viscosity, resulting in improved mixing performance and homogenous product quality. Recent studies have reported the superior performance of double coaxial bioreactors furnished with two central impellers and one anchor in terms of uniform shear distribution and gas dispersion for pseudoplastic fluids. Despite the widespread use of yield-pseudoplastic fluids in a variety of industries, however, a knowledge gap was identified for analyzing the shear distribution within the double coaxial mixers containing pseudoplastic fluids possessing yield stress. The objective of this study was to examine the effect of four coaxial mixing configurations including down-pumping and co-rotating mode, up-pumping and co-rotating mode, down-pumping and counter-rotating mode, and up-pumping and counter-rotating modes on the local shear rate distribution. In this regard, computational fluid dynamics (CFD) was employed for quantitative and qualitative evaluations of the local shear distribution within the coaxial bioreactor.