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TU Berlin

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Philipp Waldherr, M.Sc.



Physikalische Chemie


Experimental and numerical analysis of mechanical stress on filamentous microorganisms in stirred tank reactors

Bachelor's / Master's Theses

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Filamentous microorganisms are commercially exploited as cell factories in several sectors including food, pharma and detergent industry. Biotechnological processes are acknowledged as complex and still a high potential for product yield enhancement exists.

Cultivations are mostly carried out in stirred tank reactors. Mechanical stress on the microorganisms induced by the stirrer has a significant impact on product performance as the particle agglomerates are sensitive to shear and elongation forces. This work aims at characterizing different stirrers in terms of their suitability for the cultivation of shear sensitive microorganisms and developing low-shear stirrers.

CFD analysis

Previous studies suggest that the mechanical stress of a stirrer is determinded by both the maximal stress and the frequency particles pass through zones of high shear rates. To record the shear stress a particle experiences over time, a numerical method is developed. Massless Parcels are tracked according to the Particle-Tracking-Method (cf. Fig 1a). Fig. 1b shows the residence time at a certain shear rate as a function of the respective shear rate. Rushton turbines show a significant lower mechanical stress compared to a wave-ribbon impeller.

Figure 1 Particle tracks and experienced shear rates for a wave-ribbon impeller (a) and comparison of the mechanical stress induced by a wave-ribbon-impeller and a rushton turbine (b)

Experimental analysis

Exemplarily investigations are carried out using the filamentous fungus Aspergillus niger. Morphology of the fungus can be modified by the addition of salts to the cultivation medium resulting in pelleted or mycelial growth. Both growth types are part of the investigations. Experimental analysis focuses on the influence of mechanical stress on the productivity of the microorganisms.

Figure 2: Pellets of Aspergillus niger (l) and mycelial growth A. niger (r)

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