Numerical Simulation Of Residence Time Distribution And Reaction In Gas-solid Bubbling Fluidized Beds

Gas-solid reactions have various industrial applications,such as metal-oxide reduction,ore calcination,coal/biomass pyrolysis,etc.Reaction kinetic analysis can reveal the influence of factors on the reaction rate and explore reaction mechanism,thus being of great importance in industrial operation.Since the gas-solid reactions mainly occur in the fluidized bed,the reaction kinetics test in the fluidized bed is closer to industrial production compared with the traditional thermogravimetric analysis.Research showed that backmixing becomes greater as the size of fluidized bed increases,which demonstrates less gas diffusion and more accurate reaction analysis in a micro fluidized bed.While the degree of gas back-mixing still needs to be analyzed quantitatively because of the influence of device structure,operating parameters and properties of particles.In this thesis,by calculating the residence time distribution(RTD)function of gas and combined with computational fluid dynamics(CFD)and the axial dispersion model,the degree of gas back-mixing in the micro fluidized bed is judged quantitatively so that to set right operating parameters to reduce back-mixing.Furthermore,the catalytic decomposition of methane is combined with CFD to reveal the reaction performance in the micro fluidized bed.The related studies are as follows:In Chapter 1,a literature review emphasizing the flow behaviors of bubbling fluidization,gas back-mixing and numerical simulations of micro fluidized beds is given.Then,the research route in this thesis is proposed.The Chapter 2 simulates the micro fluidized bed with two-fluid model(TFM)and Kinetic Theory of Granular Theory(KTGF).The grid-dependence and applicability of interphase drag models are first investigated,and the minimum bubbling velocity and average bed voidage obtained respectively from the simulation and experiments are compared to determine the suitable drag model.Then,the effects of a series of parameters including particle diameter,gas velocity,static bed height,and bed inner diameter on the gas back-mixing behaviors in the micro fluidized bed are analyzed,and compared to the prediction with the axial dispersion model to evaluate the deviation of flow in the micro fluidized bed from the ideal plug flow.Finally,a set of parameters for testing the gas-solid reactions in the micro fluidized bed with minimum gas back mixing are determined,which provide the useful fundamental data for testing in micro fluidized beds.The Chapter 3 combines the reaction kinetic models of catalytic decomposition of methane(CMD)and CFD to conduct the reactive simulation of a micro fluidized.The cracking rate of methane from the kinetic model is obtained and compared to the CFD simulation.Compared to the traditional bubbling fluidized bed,no evident bubbles can be detected in the micro fluidized bed,which is helpful to test the reaction data.Then the effects of the partial pressure of methane and reaction temperature on the reaction performance including the reaction rate of hydrogen,the concentration distribution of hydrogen and the coke content distribution are investigated through CFD simulations.Based on the above finding,the relation of coke content and the reaction time can be obtained under the relative homogeneous distribution of solids volume fraction,which can be expected to improve the expression of the reaction kinetics in the future reactor scaling-up reactor.The Chapter 4 concludes this study and presents the future work on improvement of the numerical methods for the simulation of gas-solid micro fluidized bed and proposing the intensification methods to further mitigate the gas back-mixing in the micro fluidized beds.