Research On The Unified Model For Fast Fluidization Dynamics: Construction And Simulation

Fast fluidization is a high efficient technology for gas-solids contacting,heat and mass transfer, and reaction process. It has arrested more and moreattention in the chemical industry, metallurgy, energy and other fields,showing very good prospects. However, the master of gas-solid flowcharacteristics of fast fluidization, especially of high-density fluidized bed isnot enough. Most research work was done dispersed and isolated. They arelack of contacting. Therefore，it is necesory to develope a unified model forfast fluidization dynamics, in which various dynamic characteristics andrelated parameters should be fully descripted and predicted.According to the basic flow characteristics of fast fluidization, thedefinitions of type A choking and type C choking, an separate-phase-coexisted model was established on the thesis. Modified Yang‘s formulapredicting type A choking is used as the ’ constitutive equation ’. The modelpredicted successfully the continuous transition from type A to type Cchoking under the conditions of constant solid flow rate and reducedoperating gas velocity. Based on the force analysis for the upward dilutephase, the method for calculating the solids holdups of upper and bottomregions were obtained. Then, the model predicted the formation ofhigh-density circulating bed, showing that the solids holdups in the upper and the bottom regions were no longer changed with the increasing solid flowrate after high-density circulating bed formed.The established dynamic model was further improved through themechanistic analysis and model optimization. They are: i) the mesoscopicmechanistic ananysis to determine model parameter n and gas phase velocityeffective coefficient F(β); ii) the experimental re-correlation of friction factorfor modified Yang‘s formula; iii) the dimensionless reconstruction ofHarris‘s correlation for cluster size. The model results were well matchedwith hundreds experimental results available in the literature.The calculation method of―the maximum one-though solid carring forthe bottom bed‖Gs,thwas derived. The transition of axial solids holdupdistribution from―exponential function‖to―S distribution‖was explainedreasonably. The calculation method for―momentum balance height‖oftransition section and accelerating section was deducted. Then，the method topredict the axial distribution of bed solids holdup was completed. The modelpredictions of the minimum solid flow rate for fast fluidization were alsoperformed, which brought further the completeness of the model.Finally, the model was extended to the pressurized circulating fluidizedbed, and similar results were obtained. It provides useful reference forstructure design, operation and optimization of fast fluidized bed system.