Experiment And Simulation Of Particle Residence Time Distribution Based On The Modified Structure-based Drag Model In Gas-solid Bubbling Fluidized Bed

Gas-solid bubbling fluidized bed(BFB)has been widely used in mineral processing because of its high gas-solid contacting efficiency and fast transfer rate between phases.The size distribution of the mineral particles is usually wide,and the greater the particle size is,the longer the reaction time will be.However,the difference of the residence time of coarse and fine particles in traditional fluidized bed is too small to meet the production requirements.Therefore,it is important to adjust the residence time of coarse and fine particles in the fluidized bed and match it with the reaction time.The residence time distribution(RTD)in the classical fluidized bed tends to be full mixed flow.Therefore,the particle fraction near the mean residence time(MRT)is small.Most of the previous researches have taken MRT as the control target when regulating the residence time of wide particle size,which seems insufficient.Therefore,it is necessary to obtain the complete RTD curve.With the development of computational fluid dynamics(CFD),to obtain RTD by simulation is time-saving and at low cost.Therefore,it is important to establish a complete model of calculating RTD for industrial production.The drag force model plays a key role in predicting the RTD in the fluidized bed.The traditional drag model overestimates the gas-solid interaction force in the bed due to the assumption of the homogeneous flow structure,which leads to the deviation when calculating the flow field and RTD.In this study,RTD with single or binary particle in different fluidized beds was simulated by using the structure-based drag model based on the heterogeneous flow structure.In addition,the fluidized bed with longitudinal baffle was designed,and the RTD curve of the binary particles was measured,and the RTD of the coarse and fine particles was studied.The results of this paper are as follows:(1)The RTD of single particle size in bubbling fluidized bed is simulated.Compared with experimental data,it is found that the RTD calculated by structure-based drag model agrees better with theoretical value than that of traditional non-uniform drag model.The calculated RTD has a long tail and multi peaks at the tail,which indicates that the bed has a certain circulation flow.The poor contact effect between gas and solid leads to a lower value of t50,and the t90 value is higher due to the presence of particle back mixing or dead zone in the fluidized bed.Therefore,the fluidization structure should be closer to the plug flow to improve the efficiency of the reactor by setting baffle or transforming the fluidized bed into a multi-stage fluidized bed.(2)The fluidization structure and RTD of multi chamber fluidized bed are simulated by CFD.The comparison between the calculation results and experimental data and the calculation of the recovery mass of tracer show the rationality of the structure-based drag model and RTD calculation model.The method of dividing the fluidized bed into two chambers can reduce the back mixing,improve the RTD and make it tend to plug flow.With the increase of fluidizing gas velocity and bed height,the distribution of residence time is extended,and the variance of residence time increases accordingly.The calculated radial distribution of solid content fluctuates more with the increase of h/H,which leads to the non-uniformity of fluidization structure and RTD distribution in the bed.(3)The influence of bed size and bubble size correlation on the distribution of particle residence time and gas-solid fluid dynamics in fluidized bed was studied by using the structure-based drag model.It is found that Darton bubble correlation is the best choice for Geldart B particles at low gas velocity.When scaling up a BFB,the RTD calculated by the structure-based drag model is reduced in comparison with the experimental value which is related to the increasing of the bed size and the excessive estimation of the bubble size.On the other hand,the over-prediction of particle velocity in the bed due to the friction on the front and back wall is not being considered in the 2D simulation result in the deviation between the numerical simulation and the experimental data.Therefore,3D simulation should be carried out first when the calculation accuracy is required to be higher in order to ensure the accuracy of the RTD simulation results.(4)The RTD characteristics of binary particles in BFB were studied by 3D simulation.The dynamic behaviors of fluidization in the fluidized bed are simulated by using the structure-based drag model.The results show that the drag model is applicable to both systems,which further verifies the correctness and applicability of our drag model.The Hd increases with the decrease of gas velocity or the increase of particle size.The increase of feed rate can make the solid flow model closer to the plug flow,while the increase of gas velocity and bed height can make a wider residence time distribution.For the binary particles in fluidized bed,with the increase of dilution,the particle diffusion gets larger and larger,which results in the calculation value of MRT of binary mixture smaller than that of single system.The reason why the MRT of coarse particles is long is that it always gathers at a relatively small vertical velocity at the bottom of the bed,which also conforms to the hydrodynamic behavior of a typical fluidized bed.(5)Through the RTD tracing experiment of binary particles,it is found that the gas velocity and solid flow rate have little effect on the regulation of coarse and fine particles RTD in the non internal component fluidized bed.After adding vertical baffle,the effect of gas velocity and solid flow rate on the regulation of RTD of coarse and fine particles increased significantly,which was manifested in the increase of peak height and the decrease of tail drag of RTD curve.The form of baffle plays an important role in regulating the coarse and fine particles RTD,the baffle with side opening can separate the coarse and fine particles significantly,and the residence time difference of coarse and fine particles can be improved by adjusting the gas velocity and feed rate.In the last chapter,the main conclusions and innovations of this paper are summarized,and expectations for future research are put forward on the basis of the existing work.