Compartment model approach: Mixing in large scale aerated reactors with multiple impellers

Bioprocesses are operations especially sensitive to local concentration gra dients of substrate or pH. These phenomena occur in large scale fermentatio ns as a consequence of imperfect mixing. Therefore, prediction of mixing is of major importance for reactor design and process scale-up. A compartment model to describe mixing in a gas-liquid/multi-impeller system was develop ed and verified on a 30 m(3) large scale fermenter equipped with four Rusht on turbines. A fluorescence pulse-response technique was employed to quanti fy mixing at different stirrer speeds and aeration rates in the regime of L imited recirculation. The influence of aeration on mixing was minor: at fir st negative (reduced mechanical power input), then positive (increased powe r transfer from gas). These trends were described qualitatively and quantit atively by the developed compartment model. Comparison between simulations and experiments shows that pulse-response curves and actual level of mixing time were predicted well. Stirrer speed, aeration rate, gas hold-up, power input and reactor geometry were utilized to obtain model parameter values. They are based on the general knowledge of hydrodynamics in this kind of v essels. That makes the model useful for (bio)process design and for further extension with microbial kinetics.