Study On Particle Motion In Gas-solid Fluidized Bed Reactor And Computational Fluid Dynamics Simulation

Combined with the fossil energy structure of "poor oil,less gas and more coal" and the requirements of social,economic and ecological sustainable development,new coal chemical technology with high utilization and low environmental pollution should be developed vigorously in China.Fluidized bed reactors are widely used in Fischer-Tropsch synthesis and the conversion process of coal to light olefins via methanol,which are the key technologies of coal chemical industry.However,gas-solid fluidization involves complex two-phase flow,and its dynamics are difficult to detect.Besides,the existing fluid mechanics knowledge shows deficient with the expansion of the application scope,the large-scale of device,and the strengthening of process,etc.Therefore,a large amount of in-depth research is still needed to improve and optimize the design and operation of the fluidized bed reactor.In this paper,a large number of experiments were carried out to study the particle motion in turbulent fluidized bed(TFB),circulating fluidized bed(CFB)riser,and annular stripper.Computational fluid dynamics(CFD)model of gas-solid fluidized bed was established and gas-solid flow behavior was simulated by FLUENT software.Local solids holdup and particle velocity in a 150-mm-ID and 4.8m-high turbulent fluidized bed(TFB)were obtained by PC6M solid concentration analyzer and Laser Doppler velocimeter(LDV),respectively.The cross-sectional average solids holdup was higher in the lower section than that in the upper section,and the local solids holdup was lower in the center than that in the wall region of the riser at all axial location.The radial concentration gradient increased with the increase of superficial gas velocity or initial bed height.Particles had an upward acceleration process in the axial direction,and the higher the initial bed height,the longer the acceleration distance.The thickness of the annualr wall layer gradually narrowed along the axial height of the TFB.With the increase of superficial gas velocity and initial bed height or decrease of the particle size,the radial distribution of particle velocities with high in center and low in sidewall was steeper,and the annular wall layer was wider.Based on effects of operating conditions,particle properties,and measurement heights,a new correlation was developed for predicting the wall film thickness in the TFB.The error analysis showed that the predicted value was in good agreement with the measured value.A large set of measurements were performed in a circulating fluidized bed(CFB)riser to study the solids holdup and particle velocity distribution,using a PC6M solid concentration analyzer and a PV6D particle velocity analyzer.Fluidized solids included four kinds of particles,such as glass beads,white fused alumina,and SAPO-34 catalysts.The effects of superficial gas velocity,solids circulation rate,particle properties and measuring positions on solids holdup and particle velocity distribution were studied.The cross-sectional averaged solids holdup decreased along the riser height and the local solids holdup increased gradually towards the wall.Increasing solids circulation rate,particle size,and particle density or decreasing superficial gas velocity,the cross-sectional averaged solids holdup increased,the axial development became slower,and the radial solids concentration gradient increased.Based on the effects of superficial gas velocity,solids circulation rate,particle properties and measuring positions on solids holdup,two new correlations with wide application and high prediction accuracy were proposed to predict the cross-sectional averaged solids holdup and clarify the relationship between the local solids holdup and averaged solids holdup,respectively.Particle velocity was higher in the center region and lower in the wall region,exhibiting a liner or parabolic radial distribution.Meanwhile,along the axial direction,particle velocity in the center region increased first and then tends to be stable,and remained almost unchanged in the wall region,while the particle in the radial mid-region continued to accelerate slowly.With the increase of the superficial gas velocity and solids circulation rate,the particle velocity in the center region increased obviously,and the radial inhomogeneity increased.The particle velocity distribution in a large scale annular stripper with a riser located on the centerline were studied with the help of a Laser Doppler Velocimetry(LDV)system.The effects of superficial gas velocity,solids circulating rate,and superficial stripping gas velocity on the radial profiles of particle velocity were investigated.When the stripping gas velocity was small,the particles moved downward along the gravity in the stripper and particle descent velocity presented a ?-shaped radial distribution.Increasing the superficial gas velocity or the solid circulation rate,the particle downward movement in the middle region and inner wall region were promoted,respectively.Superficial stripping gas velocity had strong effect on particle velocity profiles near the stripper gas distributor.As the stripping gas velocity increased,the U-shaped radial profiles of particle velocity appeared at the bottom of the stripper.The prediction formulas with high accuracy were established to describe the relationship between the local paerticle velocity and the cross-sectional averaged particle velocity.A computational fluid dynamics(CFD)model for gas-solid two-phase flow was established based on a turbulent fluidized bed with a height of 4.8 m and an inner diameter of 0.15 m.Gas-solid flow behavior in turbulent fluidized bed(no solids circulation)was simulated by using a two-fluid model in FLUENT software.Based on the particle properties,the traditional Gidaspow drag model was modified to accurately calculate the gas-solid interphase force in a coarse grid.The effects of gas-solid drag model,particle size distribution,wall condition,and radial distribution function on the predictive ability of the model were investigated.The modified SGS drag model could predict the flow behavior in turbulent fluidized bed,and the smaller solid-wall specularity coefficient was more propitious to calculate the radial profiles of particle velocity accurately.