CFD Modelling of Bioprocesses: Integrating Mechanical Mixing, Aeration and Dynamic Rheology

  • Author / Creator
    Sadino Riquelme, Maria Constanza
  • Bioprocesses currently have a huge importance in worldwide sustainable development. However, the design of bioreactors based on experimental and empirical knowledge poses a challenge for the industrial biotechnology. Thus, CFD has gained attention as a design tool.
    Although CFD modelling has previously been applied to study bioprocesses where mechanical mixing and aerations are key, the fact that the broth rheology changes over time has been mostly ignored. Additionally, many inaccurate practices have been used for the CFD models adaptation. In consequence, the obtained results have a limited scope and the level of confidence in such works is severely reduced at best.
    The goal of this thesis is to adapt and validate a CFD model to predict the effect of the dynamic interaction between stirring, aeration and changing fluid rheology for bioreactors. The methodology consists of an experimental approach as well as a modelling approach. The microbial alginate production is selected as case study.
    Experimentally, the bioprocess is reproduced in a stirred and aerated batch reactor. The process kinetics is characterized and the importance of reporting statistical data for an unbiased analysis of the parameters uncertainty and process reproducibility is shown. The density and rheological parameters of the broth are estimated at different stages of the fermentation, confirming that its rheology evolves from Newtonian to pseudoplastic.
    Batch and continuous abiotic systems are implemented, using non-Newtonian and Newtonian fluids, to mimic the fluid dynamics of the fermentation at different stages. It is concluded that, under unaerated conditions, the impeller interaction decreases as the viscosity increases; while, when aeration is included, the formation and breakaway of air cavities modifies the fluid dynamics. Additionally, it is proved that the probes affect the impeller torque. The similarity between the torque curves of the continuous systems and the fermentation supports the idea that their underlying mixing mechanisms are similar. Therefore, the CFD modelling of the batch abiotic systems would help to understand the evolving fluid dynamics of the microbial alginate batch production.
    Regarding modelling accuracy, the CFD models are proved to be able to capture the effect of the probes on the fluid dynamics of the stirred tank. Therefore, these tank’s internal elements should not be neglected. The simplification of the liquid level as a flat and fixed surface should not be applied either. The headspace should be implemented, especially for a process with aeration or when unaerated conditions can lead to a surface vortex. Regarding the numerical accuracy, the sliding mesh approach should be used instead of multiple reference frames. The SST k-omega, k-kl-omega and laminar models are shown to work for modelling a stirred tank with a flow in the turbulent, transitional and laminar regime, respectively. Thus, different single-phase CFD models are successfully validated for a stirred tank without aeration, to be able to simulate a changing fluid rheology as well as an evolving flow regime. When including the aeration, it is concluded that the mixture model is not able to predict the interface shape as well as the Eulerian model. However, only the mixture model shows to be numerically stable.
    The CFD models are applied to study the evolution of the fermentation fluid flow patterns, velocity field, dead zones and vortical structures. Precessional vortices are identified as responsible for the unstable flow patterns identified at the earlier stages of the fermentation. A stable parallel flow pattern accounts for the higher mixing times and dead zones at the final stage. Under the applied operating condition, the aeration affects the meso and macromixing mechanisms.
    Overall, this work presents a standardized framework for the modelling of mixing tanks and contributes with a detailed analysis of the effect of a fermentation broth with a changing rheology on the fluid dynamics of a stirred bioprocess, under aerated and unaerated conditions.

  • Subjects / Keywords
  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.