| Computational fluid dynamics (CFD) method has been used in the bio-process for reactor design and process optimization. Combination CFD method with bio-processes model to predict bio-processes and achieve process optimization is difficult. The flow field, mixing and mass transfer of multiphase flow are very complex. The hydrodynamic of the bioreactors are more complex due to the non-Newtonian broths. In this paper, stirred bioreactors with multiple impellers were employed to study the hydrodynamics in single-phase and gas-liquid flow with different media by experimental and CFD methods.It can be found that there are a pair of trailing vortices behind the blade of RT and HBT impellers, while only a trailing vortex behind the blade of WHd and WHu impellers by the angle-resolved large eddy PIV measurement techniques. The maximum turbulent kinetic energy of angle-resolved is about2times of that obtained by the time-averaged, and maximum turbulent energy dissipation is about3.5times of that obtained by time-averaged. When Cs=0.12, the power getted from volume integral of turbulent energy dissipation is equal to the power calculated by the measured torque. For3RT,22.1%of the total input energy is dissipated in the impeller region,45.2%in the stream region and32.7%in the remaining volume of the tank. For3WHu,39.2%of the total input energy is dissipated in the impeller region,23.3%in the stream region and37.5%in the remaining volume of the tank. Each impeller of the combination has own characteristic even the same type. εmax/εavg is the ratio of maximum turbulent energy dissipation to the average turbulent energy dissipation. The average εmax/εavg of RT impellers is18.6, the εmax/εavg of HBT impellers is26.9-34.1, the average εmax/εavg of WHd impellers is22.8, the εmax/εavg of WHu impellers is14.7-23.1.In the air-water system,3WHu produces53%higher mass transfer coefficient than HBT+2WHd, HBT+2WHu and3RT lie between them at gas superficial velocity. At high gas superficial velocity, however, all the tested configurations give almost similar mass transfer coefficient under equivalent power input. For3RT, highest hold-up is in the bottom impeller discharge stream and near the wall for the middle and top impellers. For the HBT+2WHd combination, there was no large variations of gas hold up in the bulk except region around the bottom impeller. For HBT+2WHu and3WHu, high gas hold-up was observed between the two up pumping impellers, and moderately low gas hold-up above the top impeller. Under gassed condition, each RT also generates two loops, but the lower loop is more near the wall than the upper loop. the mixing time increases with the gas flow rate increases in the agitator-dominated regime. Overall, mass transfer and mixing characteristics are quite good for the3WHu and HBT+2WHu combinations.CMC solutions with different concentrations which are preferred to mycelial fermentation broth were used for the study of bubble size and mass transfer characteristics in bioreactor. At same power input, the impeller combinations with up-pumping produce higher gas holdup than that of other combinations, gas holdup εG∝(PG/V)0.3VG0.62. At same power input, the rank of Sauter mean diameter is:3Whu> HBT+2WHu> HBT+2WHd>3RT, and the rank of interfacial area is:3RT> HBT+2WHd> HBT+2WHu>3Whu, this is due to more bubble coalescence for up-pumping impeller. Sauter mean diameter d32∝(PG/V)-0.12μα0.32εG0.14, while interfacial area α∝(PG/V)-0.14VG0.5μα-0.5. At low concentration of CMC solutions, impeller combinations with up-pumping give best mass transfer, the mass transfer coefficient kLα∝(PG/V)0.5VG0.45μα-0.78. Liquid phase mass transfer coefficient kL of WHu and HBT+2WHu is1.5-2times higher than that of3RT and HBT+2WHd, and kL∝(PG/V)0.11μα-0.24. Mass transfer depends on the flow fields gernerated by impellers and physical properties, because they determine the bubble dynamics and mass transfer performance.For highly viscosity xanthan gum solutions, in order to gain the same power input, the rotating speed of "small-diameter" impeller combinations increases as the concentration of xanthan gum increases, while it decrease for "large-diameter" impeller combinations.. For the "small-diameter" impeller combinations, the kLα value near the wall drop faster than other areas as the concentration of xanthan gum increases. While for the "large-diameter" impeller combinations, the kLα distribution is homogenous except the bottom area but with poor gas dispersion capability as concentration of xanthan gum increases. The rank of average kLα is:3RT> HBT+2WHu> HBT+2WHd> EG> HBT+2MIG> HBT+DHR. The obtained correlation shows that the kLα is heavily depend on specific power and viscosity, but less influenced by the gassing rate. The mixing time of small-diameter impeller combinations is greater than that of large-diameter combinations, the effect of viscosity on the mixing time is greater than that of power consumption on the mixing time.The fermentation of Aspergillus niger to produce glucoamylase with different impeller combinations, in order to keep same OUR, the final impeller speed is3WHu>3RT> HBT+2WHu, corresponds to the power input is3RT>3WHu> HBT+2WHu. Ultimately, the emzyme activity of3RT is the lowest, the emzyme activity of HBT+2WHu and3WHu are closer. High shear strain rate of3RT delays the formation of pellets and gains lower pellet concentration. Howerver, low shear strain rate of HBT+2WHu accelerates the formation of pellets and gains higher pellet concentration. The rheological properties of the fermentation broth show that the apparent viscosity in the bioreactor with HBT+2WHu is lower than that in the bioreactor with3RT. This also show that the formation of pellets do help to reduce the viscosity of the fermentation broth, and promote the mobility of the reactor, thereby increase the production efficiency of enzyme. Impellers with up-pumping have excellent mixing and low shear rate for the fermentation of filamentous fungi, and the kLα correlation for fermentation process have established for HBT+2WHu. These results provide useful clues and theoretical guidance for future process optimization.For the single-phase numerical simulation of3RT, the speeds obtained from Reynolds-averaged turbulence model are larger than the PIV data, but turbulent kinetic energy values are40-80%lower than the experimental values. The flow field getted by LES simulation are very similar with the PIV results, the values of turbulent energy dissipation are approximately15-30%lower than that of the experimental values in the stream region. The values of turbulent kinetic energy in other regions agree well with PIV results. CFD coupled PBM model was employed for numerical simulation of multi-phase flow. Simulated gas holdup is low using Ishii-Zuber drag force model. The modified Brucato drag force model can obtain satisfactory gas holdup. The simulation method of Hi Resolution convection difference scheme combining modified Brucato drag force model, enhancement factor4.5×10-6can predict well the gas holdup, interfacial area and kLα. the kLα value increases is due to the increase of the interfacial area results from the increase of drag force. The simulated kLα value is ower than the experimental value is mainly due to underestimation of the turbulent energy dissipation in the tank, followed by underestimation of the drag force. |
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