Search Results (2)

For the aeration of highly viscous non-Newtonian fluids, prior studies have demonstrated the improved efficacy of dual coaxial mixing bioreactors fitted with two central impellers and a close clearance anchor. Evaluating the effectiveness of these bioreactors involves considering various mixing characteristics, with a specific emphasis on shear rate distribution. The study of shear rate distribution is critical due to its significant impact on the mixing performance, gas dispersion, and homogeneity in aerated mixing systems comprising shear-thinning fluids. Although yield-pseudoplastic fluids are commonly employed in various industries, there is a research gap when it comes to evaluating shear rate distribution in aerated mixing bioreactors that utilize this fluid type. This study aims to investigate shear rate distribution in an aerated double coaxial bioreactor that handles a 1 wt% xanthan gum solution, known as a Herschel–Bulkley fluid. To achieve this goal, we employed an experimentally validated computational fluid dynamics (CFD) model to assess the effect of different mixing configurations, including down-pumping and co-rotating (Down-Co), up-pumping and co-rotating (Up-Co), down-pumping and counter-rotating (Down-Counter), and up-pumping and counter-rotating (Up-Counter) modes, on the shear rate distribution within the coaxial mixing bioreactor. Our findings revealed that the Up-Co system led to a more uniform local shear distribution and improved mixing performance. Full article

Uniform gas dispersion and shear distribution in highly viscous non-Newtonian fluids are challenging due to the complex rheological behavior exhibited by this type of medium. In addition, most large-scale bioreactors used in biochemical processes such as wastewater treatment and fermentation demand higher aspect ratios (i.e., fluid height to tank diameter ratio) than 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 area 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 of more than one is hard to attain. It has been reported that a well-designed mixing configuration contributes to maintaining a consistent fluid viscosity, resulting in improved mixing performance and consistent final product quality. Recent studies have demonstrated the superior performance of double coaxial bioreactors furnished with two central impellers and one anchor for uniform shear distribution and gas dispersion in pseudoplastic fluids. Despite the widespread use of yield-pseudoplastic fluids in various industries, a knowledge gap was identified for analyzing the shear distribution within the double coaxial mixers containing pseudoplastic fluids possessing yield stress. This study examined the effect of four coaxial mixing configurations, including down-pumping and co-rotating, up-pumping and co-rotating modes, down-pumping and counter-rotating, and up-pumping and up-pumping and up-pumping and counter-rotating modes, on the local shear rate distribution. In this regard, computational fluid dynamics (CFD) was employed for the evaluation of the local shear distribution within the coaxial bioreactor. Full article