Numerical Simulation Of Solid-liquid Mixing And Flow Characteristics In Stirred Reactor Based On Solid Base-catalyzed Transesterification

As an environment-friendly renewable energy source,biodiesel is made through transesterification between renewable animal and vegetable oils,waste edible fat and oil with methanol or ethanol.It is a high-quality alternative to traditional fossil fuels.In the currently common process of producing biodiesel with solid base-catalyzed transesterification,the solid-phase catalyst and the liquid-phase reactants are mainly reacted in a solid-liquid agitated reactor.The mixed contact between the two directly affects the reaction rate and mass transfer effect.Therefore,it is of great research value and significance to understand the distribution and influence factors of the flow field and concentration field in the agitator in order to guide the design and optimization of the scale-up reactor.This research is financially supported by the National Natural Science Foundation of China(51876106),Primary Research&Development Plan of Shandong Province,China(2018GGX104027)and Young Scholars Program of Shandong University(YSPSDU,2015WLJH33).In this study,the solid-liquid agitator is used as the research object and using computational fluid dynamics(CFD)to simulate the agitator.The solid-liquid mixing and the complex flow characteristics are investigated,and the stimulation results was compared with experiment results to verify the simulated accuracy.Based on the similarity criteria,the agitated reactor was scaled up and optimized in structure,and the changes and influencing factors of the flow field,concentration field and power consumption in the agitator were analyzed through simulation.It provides a theoretical basis for the industrialization of solid base-catalyzed transesterification to produce biodiesel.The specific procedures are as follow:(1)The common 20L six-leaf Rushton paddle agitator was selected as the research object.Based on the Euler-Eulerian fluid model and through the multiple reference frame method(MRF),Fluent software was used to couple the particle dynamics theory(KTGF)to stimulate the solid-liquid two-phase flow,and the accuracy of the simulation was verified with the literature.The simulation shows that when the rotation speed is lower than 600rpm,the accumulation of particles at the bottom of the agitator is obvious,and the accumulation area gradually shrinks toward the center as the rotation speed increases to 1500rpm.The baffle plate forms a"double circulation" loop in the agitator,which enhances the turbulence of the fluid,but it will cause the accumulation of particles at the baffle plate,which hinder the uniform distribution of particles at the baffle plate.According to the simulation results,satisfying solid-liquid suspension can only be obtained when the rotation speed is higher than 900 rpm.(2)Using geometrically similar conditions to amplify the agitator and select blade tip speed as the amplification factor,the structural size parameters and the impeller speed of the 2000L-level agitator was obtained in accordance with the small agitator.Based on this,a modeling simulation of the amplificated agitator was carried out.The simulation showed that the influence of the amplified baffle on the solid phase concentration and turbulent kinetic energy distribution became weaker,and the higher the rotation speed,the weaker the influence.The amplificated agitator needs a higher blade tip speed to achieve the suspension effect before amplification.Increasing the rotation speed not only promotes the uniform suspension of the particles,but also causes a sharp increase in power consumption.When the rotation speed magnified 1.24 times and 1.08 times from 264rpm to 324 rpm and then to 390 rpm,the power consumption has expanded from 2317.13W to 4521.487W and then to 7789.28W,respectively increased 1.95 times and 1.74 times.Considering the influence of power consumption and particle concentration distribution,the most suitable rotation speed in the amplificated agitator are 324 rpm and 390 rpm.(3)In order to alleviate the problem of excessive power consumption of the amplificated agitator,the structure of it is optimized through using an open impeller and adjusting the height from the bottom.The optimized agitator is simulated and results showed that open impeller can enhance the material exchange between the upper and lower circulation circuits and is conducive to the uniform suspension of particles.But,lowering the height from the bottom to 0.2TL will result in the inability to form a "double circulation" loop with particles mainly suspending in the middle and lower parts of the agitator,heavily piling up at the bottom of the baffle.Taking power consumption and particle suspension conditions into consideration,the best design is an open-type agitator with a rotating speed of 324rpm and a height of 0.3TL from the bottom.