Development And Application Of CFD-PBM Coupled Model For Large Scale Gas-solid Bubbling Fluidized Bed Reactor

Gas-solid bubbling fluidized bed(BFB)reactors are widely used in a variety of energy and chemical processes due to the excellent solid mixing,heat and mass transfer characteristics.However,there are still many problems in the design and application of the reactor due to the complexity of gas-solid hydrodynamics.Deep understanding of the heterogeneous characteristics in fluidized bed and its influence on multiphase transfer process is alwalys the scientific basis of fluidization engineering and the difficulty of its application and development.In recent years,computational fluid dynamics(CFD)method has developed rapidly in simulating multiphase flow in gas-solid fluidized bed,and has become an important assistant tool for the development,design and scale-up application of fluidized bed reactor.The coventional two-fluid model is based on the assumption of homogeneity at the level of computational cells,which means the grid size must be sufficiently small to accurately simulate the gas-solid drag force.When the two fluid model is used to simulate large-scale industrial BFB reactors,the number of grids will reach hundreds of millions,which is far beyond the existing computing capacity.Therefore,the development of heterogeneous drag model for coarse-grid simulation has become a hot topic in the study of multiphase flow CFD.However,the heterogeneous drag model reported in the literature mainly reflects the influence of bubbles on the drag force through the function of gas holdup.The numerical simulation results can not reproduce the bubble size distribution observed in the experiment,which has certain limitations.In this work,based on the application background of large-scale high temperature Fischer Tropsch reactor and the breakthrough of describing and simulating the heterogeneous structure of bubble in fluidized bed,a CFD model for simulating large-scale fluidized bed reactor is developed.The main research contents are as follows:A pseudo bubble-emulsion triple-phase drag(PBTD)model based on assumption of local heterogeneity was proposed,which could be applied to coarse grid simulations.The bubble size distribution and dynamic evolution in gas-solid bubbling fluidized bed were described by PBM,which was used to simulate bubble characteristics in gas-liquid two-phase flow.A CFD-PBM coupled model was established to combine the ability of CFD to simulate the gas-solid two-phase flow field and of PBM to calculate the local bubble size distribution.The pressure and viscosity of solid phase was modeld by the kinetic theory of granular flow(KTGF).The CFD-PBM coupled model was used to study the hydrodynamics in a lab-scale 2D fluidized bed reported in the literature.The simulation results of different bubble coalescence and break-up models were compared.Meanwhile,simulation results before and after the improvement of bubble coalescence and break-up models were compared.The fluidization characteristics,such as bed expansion height,average bubble holdup,instantaneous contour maps of bubble volume fraction distribution,average bubble size variation along bed height and bubble size distribution at different bed height,were investigated.By comparing the simulation results with the experimental data and the fine-grid simulation results of TFM-KTGF,the prediction ability of the improved model was confirmed.Using MATLAB programming language,the fine mesh TFM-KTGF simulation data was processed in detail,and the bubble coalescence and break-up behavior was studied.A new bubble tracking algorithm was developed.Rigorous statistical analysis of bubble coalescence and break-up motion was performed.The bubble coalescence and break-up frequency of different bed height and bubble size were calculated and evaluated,and the daughter bubble size distribution after breakup was obtained.The bubble break-up frequency calculated by the algorithm was compared with the correlation proposed in this work,and the accuracy of the correlation was confirmed.Two lab-scale 3D gas-solid bubbling fluidized beds were simulated to further evaluate the prediction ability of CFD-PBM coupled model.In case 1,fine-grid TFM-KTGF simulation and coarse-grid CFD-PBM simulation were carried out simultaneously.The results showed that the results of CFD-PBM coupled model under coarse-grid condition were in good agreement with those of TFM-KTGF model under fine-grid condition.In case 2,CFD-PBM coupled simulation and TFM-KTGF simulation were carried out with same coarse grid.Meanwhile,a CFD-without PBM simulation was carried out,which is using the PBTD model combined with bubble size correlation.The results showed that although the PBTD model could obtain good results under the coarse-grid condition,the time-averaged gas holdup and particle velocity obtained by CFD-PBM coupled model were more consistent with the experimental results.In addition,the CFD-PBM coupled model was used to predict the bubble size distribution which was difficult to be measured by experiments.The simulation results of the equilibrium bubble size and penetration height were in good agreement with those of correlations of literature.The PBTD model based on the assumption of local heterogeneity was applied to the simulation of a large high temperature Fischer Tropsch fluidized bed reactor with cooling coil.Due to the existence of internals,the radial bubble size distribution in the bed becomes narrower,and the bubble size tends to be more uniform.The bubble size distribution and radial change could be ignored.Therefore,the modified Mori-Wen model proposed by Ozawa et al.was used when dealing with the bubble size.A reaction model of high temperature Fischer Tropsch synthesis reactor based on PBTD drag model was established to predict the fluidization,mass transfer and reaction in the reactor.The simulation results were compared with the classical empirical correlations and the experimetnal measurement data.The mass transfer rate,reaction rate and the concentration distribution of various gas components were also investigated.The results showed that the production rate of[-CH2-]product was also higher when the mass transfer rate of H2 and CO was high.The gas component mass fraction at the outlet of the reactor is simulated,and good results are obtained which indicated that the PBTD model based on assumption of local heterogeneity developed in this work was suitable for simulating industrial scale reactors and was instructive for the development and operation of new reactor.