Modelling Investigation On Fluid Flow Behaviors In The Gas-Liquid-Solid Three-Phase Moving Bed

The three-phase moving bed reactor is a new type of multiphase reactors which solve the coking and deactivation of the catalyst in the gas-liquid-solid three-phase reaction.The continuous flow of catalyst particles can realize the replacement and regeneration of the catalyst online.The catalyst activity is always maintained at a high level,which improves product output and quality.However,the current research progress of this reactor is slow,and the related literature is rare.In order to promote the development of this reactor and provide help for its scale-up design.Based on the existing moving bed and trickle bed theories,we have selected a dense particle model suitable for the experimental system was determined and the corresponding model parameters were optimized.The simulation models of liquid-solid forces in the low liquid flow zone and the high liquid flow zone were established and defined respectively.Finally,the CFD simulation method of gas-liquid-solid three-phase co-current moving bed was developed based on the Euler multiphase flow model.The flow behavior of gas-liquid-solid phases in a three-phase moving bed was studied by using this CFD model.The main research contents and conclusions are as follows:(1)The gas-solid flow in a gas-solid concurrent moving bed was simulated.The predictive abilities of the commonly used frictional pressure model(Based,Johnson)and radial distribution function model(Lun,Syamal O’Brien)were evaluated by comparing them with experimental results.The simulation results show that the solid volume fraction using the Johnson model is lower than that using the Based model,and the solid mass flow rate using the Syamlal O’Brien model is much higher than that using the Lun model.Because the three models can not accurately describe the solid volume distribution in the outlet area,the simulated values of solid mass flow rate are higher than the experimental data.The relative deviations of the average pressure drop can be reduced substantially after the Ergun coefficient correction was applied.The relative deviations of the Based-Lun,Johnson-Lun,and Johnson-SO models were reduced from 68.6%,73.3%,78.2%to 13.2%,29.7%and 42.3%,respectively.However,axial distribution of solid mass flow rate using the Based-Lun model is not conserved.After further optimization of Johnson-Lun model,the simulated value of the model is in good agreement with the experimental value,and the relative error is less than 10%.Therefore,the Johnson-Lun model is selected in the subsequent simulation after optimization.(2)The inhibitory effect of liquid bridge force on particle flow in the low liquid flow area and the promoting effect of liquid-solid drag force on particle flow in the high liquid flow area were qualitatively revealed through the experimental data.Therefore,simulate the liquid-solid interaction in different zones by using CFD.In the low liquid flow region,based on the Mikami model and the Cohesive force model,a continuous model of liquid bridge force was established by fitting model constants with experimental data.In the high liquid flow area,the liquid-solid drag force is defined by means of equations suggested by attou.The simulation results show that the simulated values of liquid holdup and discharge rate are consistent with the experimental data,and the overall relative deviations are 15.5%and 12.6%(low flow)and 1.7%and 7.2%(high flow),respectively.In addition,the movement of particles will affect the liquid distribution.In the trickle bed,the liquid is concentrated in the central tube region,while in the moving bed,the liquid is mainly distributed in the annular region between the wall and the central tube;At the low liquid flow rate,the relative deviation of liquid volume of moving bed and trickle bed decreases with the increase of liquid flow rate,however,the relative deviation increases at the high liquid flow rate.(3)The CFD simulation method of the gas-liquid-solid three-phase moving bed is established based on the Euler-Euler multiphase flow model.The solid model adopts the Johnson-Lun model,the gas-solid drag model adopts the modified Ergun coefficient,and the interaction between liquid and solid adopts the liquid bridge force model and the attou model.The liquid holdup,solid discharge rate and pressure drop simulated values are consistent with the experimental data,the overall relative deviations are 10.8%,8.4%and 12.2%,respectively.Further study shows that the movement of particles can reduce the flow velocity of the gas phase,but does not affect the radial distribution of the gas phase.However,particle movement will significantly affect the radial distribution of the liquid phase.Because the particles move,the migration to the center is enhanced in the low liquid flow rate region and weakened at the high liquid flow rate.The dominant force of the gas phase is different under different conditions,resulting in different gas flow behaviors.With the increase of the relative velocity of gas and liquid,the pressure gradient force gradually dominates,which makes the radial variation trend of gas velocity and liquid velocity change from the same to the opposite.