The Design Optimization Of Dual-impeller Stirred Tank Reactor Based On PIV And CFD

Due to excellent operational flexibility,strong applicability and high mixing efficiency,stirred tank reactors are widely used in agricultural fermentation,biopharmaceutical,chemical industries and other related industries.In the engineering field,a stirred tank with good mixing performance and energy-saving has always been the goal pursured by researchers.In recent years,optimization methods that integrate computational fluid dynamics(CFD),alternative models and intelligent optimization algorithms have shown significant advantages and contributions in industrial equipment modeling and optimization,which providing new research ideas for the design and optimization of stirred tank reactors.In addition,the development of nonlinear science and fluid chaotic mixing theory provides a reliable basis for quantitatively evaluating the chaotic state in the stirred tank reactor.Based on particle image velocity(PIV)and CFD,this paper conducts a design optimization study on the operating parameters and geometric configuration of a 5L dual-impeller stirred tank reactor.The main research results are as follows:(1)In a 5L dual-impeller stirred tank reactor,2D-PIV single-factor experiments were carried out to explore the influence of control factors: the installation height of the lower impeller,angle-resolved analysis and the type of the lower impeller on the test indicators:reactor flow field structure,velocity and turbulent kinetic energy distribution.The results showed that: when the installation height of the lower impeller varies from 35-75 mm,the flow pattern of the stirred tank changes from single loop to doble loop;The periodic and circumferential characteristics of the velocity and turbulent kinetic energy distribution in the cylindrical stirred reactor make the annular arrangement of the gas sparger superior to other structures;On the sampling line below the stirred tank,the horizontal range of25-65 mm covers most of the areas where the speed and turbulent kinetic energy change;When three types of radial flow impeller of RT,BBDT and CBDT were used as the lower impeller,the velocity and turbulent kinetic energy on the sampling line don’t change significantly.Therefore,the installation height of the lower impeller and the diameter of the gas sparger are selected as the control factors and the test level is determined.(2)Gas-liquid flow model of stirred tank reactors was established by CFD.The numerical model was verified by the experiement’s gas holdup which results showed good consistency.The verified CFD model combined with quadratic regression orthogonal design was used to investigate the influence of the three control factors of the installation height of the lower paddle,the diameter of the gas sparger and rotational velocity the on the power consumption,the overall gas holdup and the global mixing performance parameter S.The results of variance analysis showed that the significance of the installation height of the lower impeller and the rotational velocity on the three indicators is greater than the diameter of the gas sparger.In the process of optimizing the design of the dual-impeller gas-liquid stirred reactor,researchers should pay more attention to the design of the height of the lower impeller and the rotational velocity;Regression test showed that the P value of the regression equation is less than the critical value of 0.05,indicating that the quadratic regression model was suitable for describing the relationship between control factors and evaluation indicators;Then the regression equation were input into the Matlab multi-objective genetic algorithm toolbox to obtain the three-objective optimized Pareto front solution,and the optimal parameter optimization design scheme under low load/medium load/high load conditions was proposed.(3)In a standard full-baffle RT-RT stirred tank reactor,PIV was used to conduct experimental research on the velocity and turbulent energy field of a single-phase water system,and detached eddy simulation(DES)was used to establish a numerical model under the same operating conditions.Comparing the normalized axial velocity,radial velocity and turbulent kinetic energy of the experiment and simulation,the results showed that DES had a good simulation effect on the flow conditions of the stirred tank reactor,and the micro-characteristic analysis has good accuracy and reliability.(4)A new type of 45° twisted structure baffle was designed in the stirred tank reactor,and the mixing characteristics of the twised baffle and the standard baffle were compared under two stirring systems: the first was a single-phase flow system.The DES method was used to establish a numerical model of the flow field in the stirred tank reactor.The velocity and torque time series in the flow field are collected.The maximum Lyapunov exponent and power consumption were used as evaluation indicators.The results showed that the new baffle could provide better chaotic mixing performance in single-phase flow system without increasing power consumption;The second was a gas-liquid flow system,equipped with sodium carboxymethyl cellulose solutions of different viscosities,pseudoplastic fluids with different power law parameters in a stirred tank,and the gas holdup and mixing index were used as evaluation indicators.The results showed that the twisted baffle is not conducive to the retention of gas in the stirred tank,but it had a better global mixing index in low and medium viscosity systems.In summary,the application of the new twisted baffle needs to consider different mixing systems and mixing purposes to achieve the best mixing effect.