| Circulating fluidized beds(CFBs)have many advantages such as higher mass/heat transfer,higher gas/solid throughput and continuous operation.CFBs have been widely used in various industries process,such as combustors,fluid catalytic cracking(FCC)and multizone circulating reactors(MZCR).Successful commercialization of CFBs relies on the quality of solids circulation in the CFB loop and the ability to scale up the reactors from lab to industrial scale.Unstable solids circulation within CFB loop has been studied by many researchers,unbalance pressure loop and high aeration rates in the downer are considered as the main reasons for the unstable state.Moreover,unstable solids circulation is also observed during the solids circulation establishment process,such as CFB boiler and MZCR.Flow pattern variation,pressure drop fluctuation and significant entrainment is revealed according to the operation experience,which make the risk of emergency shutdown and recycle gas heat exchanger blockage increased.However,the dynamic gas solid flow characteristic and the mechanism of unstable flow during the solids circulation establishment are still not clear,which deserves further investigation.Scale up effect on the hydrodynamic and hydrodynamic similarity rules have been investigated to guide the riser scale up design.However,the validation of different scaling rules according to different researchers is inconsistent,and there is no exact explanation for the discrepancy of the hydrodynamic similarity in the riser bottom.In addition,the particle group would change with the increase of the riser diameter according to full set scaling rules,leading to variation of cluster features.However,the effect of particle property is always ignored during the validation researches.On the other hand,the variation of chemical reaction during the scale up process has not been studied.Therefore,the evaluation of scaling rules with the consideration of cluster features and the scale up effect on reaction performance should be explored to give a more accurate conclusion about the validation of scaling rules.In the present study,unstable flow condition during solid circulation establishment in CFB is analyzed by visual observation,pressure drop and acoustic emission(AE)detection.Moreover,the mechanism of the unstable flow condition is revealed based on the computational fluid dynamics(CFD)simulation.The validation of scaling rules with the consideration of cluster effect is investigated by two-fluid models.The cluster features within different particle classes or operation conditions are approached with the energy minimization multiscale(EMMS)drag model.The validation of different scaling rules are investigated.The ozone decomposition in the riser is modeling based on the stable gas solid flow,the scale up effect on the chemical reaction in the riser are investigated.The whole work has been carried out on the following five aspects:1.Solids circulation establishment is realized in the CFB cold model and industrial unit,respectively.The gas solid flow characteristics during the solids circulation establishment are investigated by visual observation,the pressure drop in the riser and downer,and the particle loss by AE.The solids circulation establishment could be divided into three stages,dual fluidized stage(DFS),unstable transition stage(UTS)and circulation stage(CS).During UTS,the flow pattern in the downer alternation between fluidized bed and moving bed,leading to the pressure drop fluctuation and heavy particle loss.The effect of operation condition on the solids circulation establishment is also studied.Solids circulation could not be established with high inlet gas flow rate,while the smaller valve opening leads to light particle loss and longer establishment time range.In addition,the solids circulation establishment in industrial equipment is investigated by observations of the bed density in riser and downer bottom,the pressure drop and AE,and get the same result with cold model.2.Solids oscillation is put forward according to the unstable gas solid flow characteristics during solids circulation establishment.With a constant bed inventory,the oscillatory behavior is only initiated by the critical gas flow or valve opening.Two important features are noted from the experimental results.Firstly,the gas-solid flow pattern in the riser and the downer is in transition between various fluidization regimes.Secondly,the pressure drop of the riser and the downer fluctuates periodically alternating between Pd>Pr and Pd<Pr.The effect of operation condition and bed inventory on the oscillation circulation and the flow structure is studied by the time-domain and frequency-domain analysis of pressure signals.With a constant bed inventory and sufficient gas flow/valve opening,the oscillatory behavior is initiated.The oscillation period cycle of oscillation remains constant with the variation of valve opening,while the flow structures in the riser and downer change from bubbling fluidization into slugging fluidization with smaller valve opening.The effect of bed inventory on the oscillation process is obvious,the oscillation behavior changes form sine model to rectangular with the increase of bed inventory and thus slugging solids circulation is established.3.The oscillation circulation is simulated with Eulierian-Eulerian method,the mechanism of solids oscillation is revealed based on the transient flow information analysis.The results indicate that solids oscillation is triggered by the formation of slugging in the downer,which leads to a decrease of the downer pressure drop and a sharp increase of the riser bypass gas velocity.Besides,intense slugging fluidization is observed in the downer at larger valve opening or higher inlet gas velocity.A new structure CFB with unsymmetrical cone-shape downer is designed to eliminate the unstable condition,slugging fluidization in the downer has been changed into internal solids circulation in the new type downer.The effect of solids control valve structure and particle group on the oscillation circulation is investigated.Solids oscillation is still obvious in CFB with L-valve and loop seal,while the solids oscillation is disappeared in the CFB with Geldart A particles.4.The validation of scaling rules with the consideration of cluster effect is investigated by two-fluid models.Glicksman's full and simplified scaling rules are used to build two 1:3 scaled-up risers.The cluster features within different particle classes or operation conditions are approached with the EMMS drag model.The comparison of axial and radial solids concentration between three cases indicates that the simplified set scaled riser produces a poor fit throughout the length of the riser,and the full set scaled riser only produces a good similarity towards the riser top half region.The analysis of cluster characteristics of three cases illustrates that the accurate repetition of the clusters is essential for the hydrodynamic similarity.Simplified set scaled riser presents lower cluster characteristic values along the riser,and the full set presents poor similarity of cluster characteristics in the riser bottom region.5.Ozone decomposition in the riser is simulated with CFD in order to explore the scale up effect on the chemical reaction performance.The variation of gas solids contact efficiency is revealed by the comparison of cluster effect in each set up.The radial ozone uniformity becomes worse during the scale up process.The overall gas solids contact efficiency is improved during the scale up process with full set scaling rule.The effect of scale up on the gas solids contact efficiency in the core region is not obvious,while the effect of scale up on the gas solids contact efficiency in the wall region is related to the variation of cluster effect during scale up process. |
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