Numerical Simulation Of Different Impeller Combination And The Effect On Citric Acid And Transglutaminase Fermentation Processes

Agitation is one of the main factors affecting fermentation process and the type ofimpeller decides the agitation efficiency in a large extent. Different impeller has differentcharacteristic, producing different flow pattern, velocity and shear rate. If we can combinedifferent impellers to take their individual advantages, agitation and fermentation efficiencycould be improved.In this study, we analyzed the characteristic of different impellers and designed differentimpeller combinations. Four kinds of impeller combinations were simulated with commercialsoftware FLUENT (Version2.3, Fluent Inc., USA). The flow pattern, shearing distribution,velocity distribution, mixing time were compared to obtain an optimal impeller combination.Thereafter, different impeller combinations were applied to citric acid fermentation andtransglutaminase (TG) fermentation experiments to verify the simulation results. Main resultsare as follows:(1) The characteristic of four impellers were analyzed. Physical model of these fourimpellers and their combinations DT+DT, CM6+DT，DTBT6，CM6+BT6were built withGambit software. Velocity vector of four impeller combinations showed that CM6+BT6canproduce better-distributed mixing effect. Contour plot of velocity showed that CM6+BT6combination can produce more homogeneous velocity. The turbulent kinetic energy ofdifferent impeller combinations showed that CM6+BT6can provide enough kinetic energyand keep lower turbulent dissipation at the same time. Histograms of different impellercombinations showed that CM6+BT6can produce higher velocity. Analysis of the radical andaxial velocity distributions of different combinations showed that the axial velocity ofCM6+DT and CM6+BT6combinations were higher than DT+DT and DT+BT6, and thedistribution of radical velocity were more reasonable than the later two.(2) We measured the mixing time of four impeller combinations by experiment. Themixing time was smaller in the order of DT+DT, CM6+DT, DT+BT6, CM6+BT6. When theagitation speed increased, the mixing time was shortened, and the difference between differentcombinations was decreased. Compared with CFD simulation results, the errors between themwere small, and the results are reliable.(3) The four impeller combinations were applied to the citric acid and TG fermentationprocess. Fermentation results of CM6+BT6were better than the results of other impellerscombinations. The maximal citric acid content was15.2%,24.6%higher than the controlgroup (DT+DT). The maximal TG activity and biomass of Streptomyces hygroscopicusreached4.7U/mL and42.9g/L respectively, which were53%and40.9%higher than thecontrol group (DT+DT).In summary, both experiment and simulation results showed that CM6+BT6was the bestimpeller combination. The results in this study will be useful for further studies on impellercombination.