Investigating a Stirred Bioreactor: Impact of Evolving Fermentation Broth Pseudoplastic Rheology on Mixing Mechanisms

M Constanza Sadino-Riquelme,Andrés Donoso-Bravo,David Jeison,Robert E Hayes,José Rivas

doi:10.3390/fermentation8030102

M Constanza Sadino-Riquelme, Andrés Donoso-Bravo + Show 3 more

Open Access

https://doi.org/10.3390/fermentation8030102

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Abstract

The culture medium in many fermentations is a non-Newtonian fluid. In bacterial alginate batch production, the broth becomes more pseudoplastic as the alginate concentration increases, which impairs the mixing process. This work characterizes the effect of the interaction between changing broth rheology and impeller mixing on a bioreactor fluid dynamics. Experimentally, a fermentation with evolving broth pseudoplastic rheology is reproduced. Three fermentation stages are mimicked using appropriate solutions of water and xanthan gum. Impeller torque measurements are reported. The weakening of the impellers’ interaction over the fermentation process is identified. To overcome the experimental limitations, CFD is applied to study the evolution of the fermentation fluid flow patterns, velocity field, dead zones, and vortical structures. Precessional vortex macro-instabilities are identified as being responsible for the unstable flow patterns identified at the earlier stages of the fermentation. A stable parallel flow pattern accounts for the weakest impellers’ interaction at the final stage. Overall, this work contributes with a complete workflow to adapt CFD models for characterization and aided design of stirred tanks with changing broth pseudoplastic rheology as well as an evolving flow regime.

Highlights

Many of the products and services in daily use are the result of the activity of microorganisms grown in bioreactors
Exceptions were found at alginate concentrations below 1 g/L when the culture medium behaves as a Newtonian fluid with a viscosity similar to water
The consistency coefficient shows a trend to increase with the alginate concentration but with very dispersed values

Summary

Introduction

Many of the products and services in daily use are the result of the activity of microorganisms grown in bioreactors. In many of those fermentation processes, the culture medium is a non-Newtonian fluid as a result of the content and nature of microorganisms and the presence of solids or polymeric substances. Some examples of such processes are fungi cultivation, wastewater treatment, and the production of extracellular biopolymers [1]. The alginate production requires aeration, the oxygen mass transfer—as well as the mass transfer of other metabolites—is impaired by the highly viscous culture medium, adversely affecting the production of the biopolymer [3]

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