A 3-D analysis of gas-liquid mixing, mass transfer and bioreaction in a stirred bio-reactor

Gas-liquid stirred vessel two-phase mixing accompanied by bioreaction has been analysed using a 3-D networks-of-zones, in which non-axisymmetric phenomena can be included. The effect of the feed of liquid nutrient from a single dip-pipe can be incorporated so that previous 2-D limitations of axisymmetry are avoided(1). The turbulent swirl flow created by the impeller uses clock-wise and anti-clock-wise swirl coefficients, which can be estimated using image-reconstruction 3-D visual modelling The simulations can provide detailed predictions of the local gas hold-up distribution, the local mass transfer area, the partial segregation of both the dissolved oxygen and the nutrient and the extent of oxygen depletion of bubbles. The overall gas hold-up and mass transfer area are obviously summations of the local values and the local and overall reaction rates can be predicted as well as the local and overall oxygen absorption fluxes. Simulations are presented for a 3x[2x(10x10))x60] configuration of network s-of-zones for a 3 m 3 triple-impeller industrial pilot-plant bioreactor. The theoretical predictions are demonstrated using colour-augmented 3D contour maps and solid-body isosurface images created by AVS graphics. Severe non-uniformity of gas hold-up distribution and consequently spatially uneven oxygen mass transfer create significant partial segregation of both oxygen and nutrient. The simulated bioreactor is predicted to be far from perfectly mixed so that Tylosin producing micro-organisms will experience large variations in dissolved oxygen and nutrient concentrations as they circulate around the stirred fermenter.