Computational fluid dynamics simulation improves the design and characterization of a plug‐flow‐type scale‐down reactor for microbial cultivation processes

Abstract

The scale-up of bioprocesses remains one of the major obstacles in the biotechnology industry. Scale-down bioreactors have been identified as valuable tools to investigate the heterogeneities observed in large-scale tanks at the laboratory scale. Additionally, computational fluid dynamics (CFD) simulations can be used to gain information about fluid flow in tanks used for production. Here, we present the rational design and comprehensive characterization of a scale-down setup, in which a flexible and modular plug-flow reactor was connected to a stirred-tank bioreactor. With the help of CFD using the realizable k-ε model, the mixing time difference between a 20 and 4000 L bioreactor was evaluated and used as scale-down criterion. CFD simulations using a shear stress transport (SST) k-ω turbulence model were used to characterize the plugflow reactor in more detail, and the model was verified using experiments. Additionally, the model was used to simulate conditions where experiments technically could not be performed due to sensor limitations. Nevertheless, verification is difficult in this case as well. This was the first time a scale-down setup was tested on high-cell-density Escherichia coli cultivations to produce industrially relevant antigen-binding fragments (Fab). Biomass yield was reduced by 11% and specific product yield was reduced by 20% during the scale-down cultivations. Additionally, the intracellular Fab fraction was increased by using the setup. The flexibility of the introduced scale-down setup in combination with CFD simulations makes it a valuable tool for investigating scale effects at the laboratory scale. More information about the large scale is still necessary to further refine the setup and to speed up bioprocess scale-up in the future.

Scale‐related process heterogeneities change properties of high‐cell‐density fermentation broths demonstrated with Escherichia coli B and K‐12 strains

“…The scale-up of bioprocesses, with the particular challenge of heterogeneities and gradients in production scale, can affect the production organism and can have a negative impact on USP and DSP. With the comparison of our scale-down setup, 37,42 simulating a production-scale reactor (>1000 L), with standard laboratory-scale cultivations as reference, we investigated the influence of scale-related process heterogeneities on E. coli fermentation broth properties at fermentation end.…”

“…Upstream-relevant parameters of the cultivations used are already described in previous publications. 37,42 However, to give a summary, CDM concentration, extracellular DNA concentration and Fab amount in the fermentation broths used for this study can be found in the supporting information (Table S1).…”

“…To investigate the influence of process heterogeneities, similar to the conditions for large‐scale production processes (>1000 L), on the fermentation broth properties, the scale‐down setup described by Mayer et al 37 . was used.…”

“…In the work reported here, we used the custom‐built scale‐down setup described by Mayer et al ., 37 consisting of a STR connected to a PFR. This setup was used to investigate the change of fermentation broth properties, due to process heterogeneities, similar to the conditions of a production‐scale bioreactor of several thousand liters.…”

“…Mayer et al described details about the processes, sampling, CDM and product analysis. 37,42 The fermentation broths characterized in this work were analyzed immediately after cultivation if not mentioned otherwise. The broth was harvested and stirred at room temperature while performing the different analytical methods.…”