Experimental And Numerical Investigation On Hydrodynamics Of Micro-fluidized Bed

Micro-Fluidized Bed(MFB)has the advantages of fast heating,less particle inventory,easy operating,better safety under high temperature and pressure conditions,et al.Such characteristics make MFB potentially more suitable for the applications of high-throughput screening of catalyst,fluid-solid reaction analysis and so on.And thus in recent years MFB is gaining increasing interests from researchers.A typical industrial application of MFB is the so-called Micro Fluidized Bed Reaction Analyzer(MFBRA)which was for the first time developed in Institute of Process Engineering,Chinese Academy of Sciences.Limited by the experimental measurement methods,currently researches on MFB mainly focus on the study of minimum fluidization/bubbling velocity,pressure drop and bed expansion behavior.To intensify the understanding of the hydrodynamics of MFB and to guide the future development and optimization of MFBRA,this work is devoted to studying the size and boundary wall effect of MFB on the flow structure,solid distribution and gas back-mixing by conducting numerical and experimental tests.To evaluate the suitability of Two-Fluid Model(TFM)for MFB systems,comparisons between TFM simulation results and discrete particle simulation results were first conducted.It was found that the bed expansion behavior predicted by TFM simulations were in well agreement with those given by discrete particle simulations.And in consistence with discrete particle simulation results,the predicted minimum turbulent velocity by TFM decreased with the decrease of bed size.Such results demonstrated that TFM could be applied to MFB systems,and the experimentally observed delay of fluidization in MFB might be attributed to the effect of distributor.Based on the validation of TFM simulations,the effects of particle-wall boundary conditions(specularity coefficient,particle-wall restitution coefficient)and the bed size on the predicted flow structure were evaluated.The simulation results shows that,though specularity coefficient and particle-wall restitution coefficient has ignorable influence on bed expansion behavior,they could notably affect the radial distribution profile of particle velocity.The particle velocity increased with the increase of particle-wall restitution coefficient.Increasing the bed size would lead to the higher bed expansion and larger minimum turbulent velocity.The gas back-mixing behavior in MFB with size of 11 mm,15mm and 22 mm were then investigated both experimentally and numerically.The comparison with the experimental data shows that the simulation results were in reasonably agreement with experimental data.Based on this,the influence of superficial gas velocity,bed height and bed size on gas backing mixing characteristics were thoroughly studied.Both the experimental and numerical results shows that gas back-mixing was strengthened with the increase of superficial gas velocity,bed height and bed size.Such results suggests that,to minimize the gas back-mixing in MFBRA,the size of MFB and particle inventory and gas superficial velocity should not be too large.