Mixing Process Transfer and Scale Up For Stirring Tanks by Using CFD Simulations: Two Practical MSD Animal Health Case Studies MSc. Dipl. Phys. Eng. Cosmin Vatră1 BSc. Richard Smits2 1
Aseptconn A.G. – Dietikon, Switzerland
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MSD Animal Health – Boxmeer, The Netherlands
Mixing Process Transfer and Scale Up For Stirring Tanks by Using CFD Simulations Mixing processes in stirring tanks are involving nonlinear phenomenon due to which the transfer or scale-up from one specific geometry (tank and one or more mixers) to another is not possible based on simple proportionality criteria [1]. Comparison between the mixing with two different tank and mixer geometries should consider very complex criteria, and the data obtained with Computational Fluid Dynamics (CFD) simulations are the perfect tool . We are presenting hereby the analysis of the transfer of two mixing processes from tanks with paddle type mixers with slow movement, to ones with NovAseptic® mixers having pitched impellers. The first criteria when you transfer or scale-up a mixing process from one geometrical configuration to another, is to check the turbulence regime and try to maintain the same average energy dissipation rate per mass unit. This is a good approach to assure the same dissolving capacity for powders and gases. The second criterion is to check and compare the fluid streams, and internal flows. In case the tanks contain whole cells or microorganism, the third criterion is to check the impact of the new geometry on the cells or microorganisms. If the tanks have fermenter or bioreactor functionalities, the criteria become even more complex and should take into account the volumetric oxygen mass transfer coefficient (kLa) and the gas holdup, for which will be important the average value and the distribution through the whole liquid. Hereby, we are presenting some examples from a project done for MSD Animal Health, a subsidiary of Merck & Co., Inc., Rahway, NJ, USA. The stirring tanks were used in a production phase to mix some liquid ingredients and cells also being present. The tanks do not have spargers for supplying the cells with air or oxygen. Therefore, the kLa and gas holdup were not investigated, while the impact on the cells was considered, as it has some importance in the process.
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