Auslegung geschüttelter Bioreaktoren für hochviskose und hydromechanisch empfindliche Fermentationssysteme

  • Design of shaken bioreactors for fermentation systems with elevated viscosity and hydromechanical sensibility

Peter, Cyril Patrick; Büchs, Jochen (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2007)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2006

Abstract

iotechnological processes are increasing their share on the chemical industry. Their application ranges from ton-scale bulk chemicals to microgram-scale pharmaceutical specialties. Biotechnological processes often make processes more economic and sustainable or even are the only method of producing certain chemicals. In the development of biotechnological processes, screening for improved strains or media is a key issue. Important reactor types at this stage are shaken reactors, such as Erlenmeyers or microtiterplates. Purposive research, however, must ensure suitable and reproducible experimental conditions. The design of non-limiting operating conditions for shaken reactors has so far been subject to some literature reports describing few operations such as mass transfer and volumetric power consumption. Special challenges are posed by processes showing elevated viscosity: Those may lead to limitations in momentum-, mass-, or heat transfer. In shaken reactors, however, a particular phenomenon has been reported. The out-of-phase phenomenon has shown a potential to unconsciously lead screening projects into a totally unwanted direction. Furthermore, hydromechanically sensitive processes, such as those employing oils as a second immiscible phase, or mammalian, plant or filamentous cells have shown considerable difference when transferred from shake flasks to stirred tank reactors. Consequently, this work aims at the description of shaken reactors such as baffled and unbaffled shake flasks as well as 48-well microtiter plates from a chemical engineering point of view. The particular focus is on processes with elevated viscosity and increased hydromechanical sensibility. As a result, the literature correlation for the fundamental relationship of the dimensionless power number as function of the Reynolds number has been confirmed on a basis of 6894 data points covering extreme operating conditions. Specifically, it was found that the volumetric power consumption in shake flasks is independent of the shaking diameter as long as the fluid movement is in-phase. In small flasks with a nominal volume up to 250mL an elevated viscosity, the risk for the out-of-phase state is increasing with decreasing flask size. To enable the design of operating conditions for process with pseudo-plastic flow behavior, a dimensionless correlation was found for the effective shear rate in unbaffled shake flasks. Hydromechanical stress was investigated using drop size measurements in some liquid/liquid two-phase systems. It was found, that drop size distributions in shake flask and stirred reactors are self similar. There is any influence of neither the shaking diameter nor the filling volume on hydromechanical stress. Its value in shake flasks is about ten times lower than in stirred tanks at the same volumetric power consumption. This gives a quantitative explanation for the common observation that pellets in stirred tanks are smaller than in shake flasks. An equation was developed for calculating the hydromechanical stress in unbaffled and baffled shake flasks and a critical Reynolds number for turbulent flow is proposed to 60,000. Baffled flasks only exhibit better dispersing characteristics than unbaffled flasks when compared under the same operating conditions. When compared at the same volumetric power consumption, however, both reactor types are equivalent. Unbaffled and baffled shake flasks show similar power characteristics as stirred tanks: Baffled reactors display higher power numbers and are indeed independent of the Reynoldsnumber. The oxygen supply in shake flasks is found to remain nearly constant as long as the fluid movement is in-phase. Addition of oil to the fermentation broth showed a maximum increase of the oxygen transfer rate of 15% under conditions investigated. 48-well microtiterplates have been found to also be subject to the out-of-phase phenomenon, even on a quantitative basis. Furthermore, a correlation for the effective shear rate was found for this type of bioreactor. No significant influence of the viscosity on the oxygen transfer was found. The results presented in this work provide an extension of the chemical engineering description of shaken bioreactors. Processes with hydromechanical sensibility or highly viscous properties may now be operated under suitably and reproducibly designed operating conditions to provide a basis for a systematic research of the process itself.

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