Modeling Local Gas−Liquid Mass Transfer in Agitated Viscous Shear-Thinning Dispersions with CFD

• Published in: Industrial & engineering chemistry research 2007-10, Vol.46 (22), p.7289-7299
• Main Authors: Moilanen, Pasi, Laakkonen, Marko, Visuri, Olli, Aittamaa, Juhani
• Format: Article
• Language: English
• Subjects: Applied sciences; Chemical engineering; Exact sciences and technology; Heat and mass transfer. Packings, plates
• ISSN: 0888-5885; 1520-5045
• DOI: 10.1021/ie070566x

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.