Modeling Aerated Fermenters with Computational Fluid Dynamics

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 200-dm3 laboratory stirred tank and xanthan fermentation is studied in a 70-m3 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 mix...