| Stirred reactor is widely used in agricultural fermentation industry because of its good operation performance,applicability and high mixing performance.Baffle is the key components of the stirred reactor and affects the mixing of materials.At present,straight baffle is widely used,but its mixing performance,gas retention capacity and power consumption cannot meet the requirements of some stirring systems for high efficiency,green and energy saving.For example,in the deep processing process of agricultural products of producing pullulan polysaccharide by fermentation of sucrose with Aureobasidium pullulans,the performance of straight baffle does not meet the requirements of the fermentation system for gas,mixing performance and energy conservation.Therefore,twisted-baffle is developed in this paper to improve the mixing performance and gas retention capacity of the stirred reactor,reduce the energy consumption of the reactor,and provide a baffle with excellent performance for the production of pullulan polysaccharide to meet people’s pursuit of high efficiency,green and energy saving of the reactor.In order to test the mixing performance,gas retention capacity and energy consumption characteristics of the twisted-baffle,the performance of the new baffle is studied in this paper with the straight baffle as the control group,using particle image velocity(PIV)and computational fluid dynamics(CFD)as the methods.The main research contents and conclusions are as follows:(1)The DES was used to simulate the single phase flow of the straight baffle reactor,and the flow field information of the stirred reactor was obtained.The results were compared with the data measured in the PIV experiment.The results showed that the predicted axial velocity,radial velocity and turbulent kinetic energy of DES were in good agreement with the results of the PIV experiment.DES could accurately simulate the flow situation of the stirred reactor.(2)DES was used to predict the flow field information of the reactor with twistedbaffles.Reactor with the straight baffle as the control group,DES was used as the test method,and the power,speed and maximum Lyapunov index were used as the evaluation indexes to analyze the performance characteristics of t reactor with twisted-baffles.The results showed that the power of the twisted-baffle reactor could be saved by 4.6% when the stirring speed was 120 r/min.In the lower part of the stirred reactor,the maximum Lyapunov exponents of the axial and radial velocities generated by the twisted-baffles are lower than those generated by the straight baffles,but in the upper part of the stirred reactor,the maximum Lyapunov exponents of the axial and radial velocities generated by the twisted-baffles are much higher than those generated by the straight baffles,especially the maximum Lyapunov exponents of the axial velocities are 58.2% higher,that is,the chaotic mixing effect of the lower part of the reactor with twisted-baffles is slightly worse than that of the reactor with straight baffles,The chaotic mixing effect of the upper part is better than that of the reactor with straight baffles.On the whole,the mixing performance of the reactor with twisted-baffles is improved.(3)First,the viscosity of CMC with different mass fractions was measured,and compared with the viscosity of the fermentation broth of pullulan polysaccharide produced by Aureobasidium pullulans measured earlier.The results showed that CMC solutions with mass fractions of 0.25%,0.5% and 1% could represent the fermentation broth of low viscosity,medium viscosity and high viscosity stages in the fermentation process.Secondly,the gas-liquid two-phase flow model of the stirred reactor was established by CFD,and the reliability and accuracy of the model were verified by gas holdup experiment.Then,a two-phase flow model was used to compare the gas holdup distribution and power consumption of the reactor with straight baffles and the reactor with twisted-baffles.It was found that the the reactor with twisted-baffles had lower energy consumption and better gas retention capacity,which could save about 5% of power and increase the gas holdup by 5.76%.Finally,orthogonal experiments were carried out to optimize the parameters in each fermentation stage with the torsional length,torsional angle of the twisted-baffle and the installation height of the agitator as control factors.Through variance analysis,range analysis and comprehensive evaluation analysis,it is calculated that the optimal combination scheme for the whole fermentation process is the torsional length of 90 mm,the torsional angle of 55°,and the installation height of the stirring impeller of 40 mm.The optimal reactor with twisted-baffles is actually assembled through this scheme.(4)The flow field in different planes of the optimal reactor with twisted-baffles in the low viscosity CMC solution system was studied by PIV technology,and compared with the straight baffle reactor.The study showed that the optimal reactor with twistedbaffles in the low viscosity CMC solution system was not conducive to fluid mixing at the bottom of the reactor,but could promote fluid mixing above the installation height of the agitator in the reactor.The effects of stirring speed,type of impeller and installation height of impeller on the flow field in the optimal twisted-baffle reactor were studied.The results showed that in order to promote the mixing of materials in the optimal twistedbaffle reactor,the installation height of agitator blade was 40 mm,which was the most appropriate or the most appropriate to increase the stirring speed. |
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