Modelling fermenters with CFD

Abstract

Abstract Agitated gas-liquid reactors are widely used in the biochemical industry. In aerobic fermenters the dissolution of oxygen to the fermentation broth is impportant for the efficient operation of the reactor. In order to make accurate designs for bioreactors the local reaction and mass transfer conditions need to be modelled in detail. To validate the simulation results, experimental information is needed, but it is difficult to acquire from industrial fermenters. An alternative is to validate phenomenological models against experiments with a simple model system. The validated models can then be used to simulate industrial scale fermenters. A mixture of a transparent xanthan gum and additives was used as a shear-thinning model system to study hydrodynamics in gassed 14 and 200 dm3 stirred laboratory vessels. Physical properties, bubble size distributions, mixing energy and gas hold-up were measured. The used mixing intensities (0.1-3 W/kg) and gas feeds (0.1-1 vvm) are in line with industrial operating conditions. A Eulerian Computational Fluid Dynamics (CFD) simulation of a 70 m3 industrial fermenter was made. The measured physical properties of 0.25 w-% aqueous xanthan solution and a bubble size of 2 mm were used. Gas-liquid mass transfer was modelled with two-film theory and simplified Maxwell-Stefan multicomponent diffusion. Xanthan gum bioreaction kinetics was included in the simulation. Local mass transfer and bioreaction were modelled, spreads of gaseous NH3 and an aqueous nutrient were also simulated. The developed reactor model allows the identification of potential problem areas in fermenters.