Gas Back-mixing Characteristics In Micro Fluidized Bed

Micro fluidized bed(MFB)has received a lot of attention because of many traits different from traditional fluidized beds and potential specific applications.This work systematically studied the gas back-mixing characteristics in micro fluidized bed(MFB)through experiments,simulation and verification by application.First,it revealed the fact that the gas flow in MFBs can maximally approach the plug flow and then defined what is"micro fluidized bed" for the first time.Computational Fluid Dynamics(CFD)simulation was in turn performed to verity the experimental results.At last,gas-solid reaction research was carried out in the micro fluidized bed reaction analyzer(MFBRA)using Geldart A and Geldart B particles as bed materials to offer the effective application examples of reaction analysis using MFB under optimized conditions leading to gas plug flow.1.Characteristics of gas back-mixing in micro fluidized bed system.The influence on gas residence time distribution(RTD)in MFB system was investigated with respect to the inner diameters D of MFB,static bed height Hs of FCC partciles and superficial gas velocity Ug.Here,the MFB system mainly includes gas mixing part,micro fluidized bed and detecting part.The results show that the impact of operating parameters on gas back-mixing in MFB system becomes greater as Hs and Ug increase.This causes great deviation between experimental RTD of the system.Analyzing the applicability of ADM for gas flow in MFB system clearly identified the range of operation leading to limited gas back-mixin,and this in turn evidences the plausible judgement of MFB as a vessel to allow gas plug flow.2.Characteristics of gas back-mixing in micro fluidized bed.Here the influence of entrance and exit was removed from the gas RTD measured above.The variance of gas RTD in micro fluidized beds with different inner diameters was quantitatively investigated by combining the mathematical method of deconvolution and axial dispersion model(ADM).The resulting RTD for MFB was studied with respect to different particles,static particles height Hs and gas velocity Ug.We for the first time found that the RTD of gas is subject to an unique quantitative correlation between the E(t)h and ?t2,the peak and variance of RTD for MFB,respectively.For the bed fluidized with Geldart B particles,the?t2 of RTD is below 0.25,and the height of E(t)peak E(t)h is larger than 1.0.Also,E(t)h exponentially increases as ?t2 decreases to 0.At this point,Pea,g is greater than 20 but the gas flow in the tested MFB is highly approaching to a plug flow.For Geldart A particles,the ?t2 of RTD is between 0.25 and 5.0,and the height of E(t)peak is larger than 0.25 but smaller than 1.0.Its Pea,g is only higher than 10 when Hs= 20 mm.Thus,the gas flow in the MFB has certain,although limited gas dispersion.These results provide solid basis to analyze the gas flow characteristics and back-mixing extent in MFBs,and make also our definition for the first time of what is the distinctive MFB.3.CFD modeling of gas back-mixing in micro fluidized bed.Two-fluid model was used to investigate the RTD of gas in MFB.The influences of modeling parameter settings on CFD simulation results were systematically investigated and discussed by comparing the simulation and experimental data.The gas RTD obtained from CFD simulation are similar with that from experiments under different but typical operating conditions.This verifies the feasibility of using deconvolution method and the reliability of using the axial dispersion model.Meanwhile,the quantitative correlation between the peak height E(t)h and the variance ?te is also validated through calculation from the axial dispersion model.Using the steady tracer response method in our CFD simulation aims revealed that the amount of gas back-mixing in micro fluidized bed is hardly over 2%.4.Research analyses in MFBRA using Geldart B particles.The isothermal kinetics of active coke combustion using silica sand as fluidized particles was analyzed in MFBRA.The influence of operating conditions on kinetic parameters was investigated.When the gas velocity was too low or the fluidized particles were too small,the external diffusion affected the reaction,leading to the smaller reaction activation energy.On the contrary,the reactant and fluidized particles can not fully mix at very high velocity and for very large fluidized particles.This also caused the lower heat and mass transfer efficiency and thus the smaller reaction activation energy.The maximal activation energy appeared at larger conversion.The reaction activation energy increases with decreasing inner diameter of the reactor,which verifies the relationship between gas back-mixing and reactor diameter.The best operating conditions aiming at reaction analysis can be determined:gas velocity is 3 times Umf of fluidized particles,in which the heat and mass transfer efficiency is guaranteed and meanwhile the influence of external diffusion on reaction can be greatly suppressed.5.Research analysis in MFBRA using Geldart A particles.Under its optimized operating conditions,MFBRA was adopted to study the reaction kinetics of catalytic decomposition of methane(CMD)using ?-Al2O3 as fluidized particles.The kinetic behavior of CMD obtained from three different methods was compared:MFBRA method,gas switching method in micro fluidized bed and gas switching method in fixed bed.The hydrogen production rate appeared much higher when catalyst is injected into an MFB reactor by pulse,in comparison with the other two methods taking the gas-switching method.The carbon nanotubes(CNTs)produced in an MFB are more dispersed than that in fixed bed due to the efficient mixing of solid particles and reactants.In addition,the injection method is similar with the continuous feeding of reactants in an industrial process.Therefore,the micro fluidized bed reaction analyzer(MFBRA)is proven to be an alternative effective tool for determining the kinetics of methane decomposition and offering the high reference values for CMD in industrial fluidized beds.