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.
A new simple method to validate CFD models in multiphase flows is presented. The experimental procedure used in this study is based on digital imaging and a simple algorithm to reduce the noise of an image. This method is inexpensive and easy to use and it can be applied to several liquid-solid systems where the liquid is transparent. The CFD modelling was done using a two-fluid Ä-ε model. Several drag correlations were tested in simulations with two impeller speeds and various particle sizes. The results were compared with experimental data. Comparison was also made with the just-suspended speed correlation by Zwietering.Une nouvelle méthode simple pour valider le modèle de DFN enécoulements de phases multiples est présentée. La méthode est fondée sur l'imagerie numérique et un algorithme simple visantà réduire le bruit d'une image. Cette méthode estéconomique et facileà utiliser; de plus, on peut l'utiliser avec plusieurs systèmes liquide-solide pour lesquels le liquide est transparent. La modélisation de la DFN aété accomplieà l'aide d'un modèle Ä-εà deux fluides. Plusieurs corrélations de résistance ontété misesà l'essai dans des simulations avec deux vitesses d'impulseur et diverses tailles de particules. Les résultats ontété comparés avec les données expérimentales. La comparaison aégalementété réalisée avec la corrélation de vitesse de suspension minimale par Zwietering.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.