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Local mass transfer in viscous gas−liquid systems is investigated, using phenomenological models. A rigorous model incorporating local mass, momentum, turbulence, and population balances for bubbles is developed. Local hydrodynamics and gas−liquid mass transfer are investigated in a viscosity range of 0.001−47.9 Pa·s, specific mixing power range of 0.8−1.4 W/kg(liquid), and gassing rate of 0.7 vvm. The simulation results are verified against mixing time, gas holdup, and mass transfer rate experiments from a 0.2 m3 Rushton agitated vessel. The experimental system is aqueous xanthan, which exhibits viscous pseudoplastic behavior typical to many fermentation broths. The model predicts successfully the cavern formation, gas-slug creation, poor mixing at peripheral areas, and heterogeneous mass transfer, which the vessel averaged models are unable to do. Still, there is room for improvement in the basics of computational fluid dynamics CFD (turbulence and liquid flow) when modeling viscous gas−liquid reactors. Population balances for bubbles are needed to describe viscous gas−liquid dispersion accurately in agitated vessels, since the majority of the mass transfer area is located in small bubbles, whereas most of the gas volume is in the larger ones. In our simulations, over 50% of the mass transfer took place in less than 10% of the reactor volume. The order of magnitude drop of volumetric mass transfer coefficient (k L a) with increasing viscosity is predicted correctly. However, the simulated k L a decreases too rapidly at low (0.25 wt %) xanthan concentrations. The developed model allows qualitative investigation of local conditions in the vessel, thus giving new possibilities for reactor design, operation, scale-up, and troubleshooting.

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Various aspects of viscous gas-liquid modeling are discussed. A detailed model is developed for aerated fermenters. Rigorous gas-liquid mass transfer, xanthan bioreaction kinetics, and non-Newtonian hydrodynamics are combined with computational fluid dynamics (CFD). Gas-liquid hydrodynamics is investigated in a 200dm 3 laboratory stirred tank and xanthan fermentation is studied in a 70-m 3 agitated reactor. Sub-models needed by the CFD simulation are verified against the hydrodynamic and oxygen transfer experiments. Multicomponent gas-liquid mass transfer is modeled based on the Maxwell-Stefan diffusion. A bubble swarm drag correction is developed for viscous shear-thinning fluids with a single bubble size. The laboratory stirred tank simulations predict the cavern and gas slug formation. The "snapshot" fermenter simulations show significant inhomogeneity of gas-liquid mass transfer rate, dissolved oxygen concentration (DO), and apparent viscosity of liquid in the bioreactor. Impeller flooding and poor mixing at the bottom of the fermenter were identified. The developed model can be used for the scaleup and detailed design of aerobic bioreactors.

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