Basic Researches On Relationship Between Flow Structure And "Transport-Reaction" In Bubbling Fluidized Beds

Bubbling fluidized beds are multi-scale systems in which there are particles in micro-scale and bubble and emulsion phase in meso-scale. The aggregation and dispersion of bubbles and particles have a great effect on the transport between gas and particles. Therefore, there are great significances to study the relationship between the meso-structure and transport. Nevertheless, the value of structural parameters should be obtained before building the relationship. Now, the EMMS model where adopt the energy minimization as the stable condition and the cluster diameter in emulsion phase are the main methods on the study of structure in bubbling fluidized beds. Furthermore, the conventional momentum and mass transfer coefficient do not consider the effect of meso-structure. In this work, according to the force balance, mass conservation and classical empirical correlations, the local structural parameters in bubbling fluidized beds are solved closely. Consequently, the relationships between meso-structure and momentum transfer, mass transfer and reaction are studied. The process is that based on the decomposition and integration of multi-scale method, the structural parameters model, structural drag model and structural mass transfer coefficient model are built to predict the flow behaviors by combining the two-flow model and to predict the mass transfer by using the carbon monoxide oxidation over Pt/Al2O3 catalyst and catalytic decomposition of ozone as the model reactions.Firstly, due to the complex structure, the bed is divided into three homogeneous dispersed phases. Then, the structural parameter model which describes the local structure in bubbling fluidized bed is obtained by the balance of force, mass conservation and classical empirical correlations, and the drag model based on the local structure is obtained by the momentum and mass conservation. In the study, the six structural parameters which describe the local structure of bubbling fluidized beds are solved by the structural parameter model. Comparing the structural parameters in the result, it could be found that the structural parameters describe the flow structure correctly and reflect the relationship among the flow structural parameters.Then, The structural parameter model and structural drag correlation are incorporated into the two-fluid model to simulate the hydrodynamics of Geldart A particles in two-and three-dimension bubbling fluidized beds. The axial and radial solid concentration profiles and particle velocity profiles are analyzed in simulation. The simulation results show that the solids rise in the center and subsequently descend near the wall. Meanwhile, there are reasonable agreements between simulated and experimental results. In the simulation of hydrodynamics of Geldart B particles, comparing the solid concentrations in the initialization fluidization and the complete fluidization, the bubble moving characteristic is captured explicit.Furthermore, the relationship between flow structure in bubbling fluidized bed and mass transfer is studied in the basis of the research on the local structure and momentum. At first, the average structural mass transfer coefficient, the structural mass transfer coefficients in emulsion phase and interphase are built according to the mass conservation. Then the mass transfer behaviors of ozone are also simulated by the structural mass transfer coefficients. Compared the ozone concentration in bubble phase with it in emulsion phase, it could be found that the mass exchange of gas between the bubble phase and the emulsion phase is the controlling step for the whole bubbling fluidized bed. Besides, the axial distribution of ozone could be predicted by the structural mass transfer model in the simulation.At last, the experiment on the catalyzed oxidization of carbon monoxide is implemented for measuring the controlled step and carbon monoxide concentration. In experiment, it could be found that when the initial concentration of carbon monoxide belong to the range of 1.0%-2.5%, the reaction of carbon monoxide oxidation over Pt/Al2O3 catalyst were dominated by mass transfer through experimental analysis. The reaction rate of carbon monoxide is R(cco,co3)= 120572exp(-42140/RT)co-1cO2 under the effects of reaction and internal diffusion. Besides, the mass transfer behaviors of carbon monoxide oxidation over a Pt/Al2O3 catalyst were simulated and compared with experimental data based on the mass transfer model. It showed that the simulation results were in good agreement with experimental data.