气固微型流化床反应分析仪的理想流型判据分析

Analysis of criteria for ideal flow patterns in gas-solid micro fluidized bed reaction analyzer

微型流化床因具有气体返混小和可操作性强等优点,在反应动力学测量等领域备受关注。获得流型随操作参数的变化规律,才能实现微型流化床理想流型的有效调控。采用双流体模型耦合考虑结构的相间曳力(又称多尺度CFD)模拟了一系列A类和B类颗粒在不同操作条件下的流化行为,结合颗粒浓度和气体停留时间分布(RTD)特征考察气速、床径和初始床高对气体返混行为的影响。研究表明,当微型流化床在初始鼓泡到湍动流态化之间操作时,才逼近颗粒全混流和气体近似平推流的运动状态。进一步分析气体RTD曲线形状特征(如拖尾、多峰等),提出采用斜度S<0.6作为微型流化床平推流判据,弥补了原判据[满足方差σ_(t)^(2)<0.25且峰高E(t)h>1]的不足,为微型反应分析仪的流型调控奠定了基础。

Micro fluidized bed(MFB)has received much attention in the reaction kinetics measurement due to its unique features like small gas back-mixing and good operability.Only by obtaining the change pattern of flow pattern with operating parameters we can achieve effective control of the ideal flow pattern of the micro fluidized bed.In this study,the combination of two-fluid model(TFM)and the structure-dependent drag model(namely multiscale CFD)is used to simulate a series of fluidization behaviors of Class A particles and Class B particles in MFBs.Then the effects of gas velocity Ug,bed diameter Dt and initial bed height Hs on the gas back-mixing behaviors are investigated by analyzing the characteristics of the solid concentration distribution and the gas residence time distribution(RTD).It is shown that the ideal flow pattern only appears when the MFB is operated between incipient bubbling fluidization and incipient turbulent fluidization.The RTD curves are further analyzed and a new criterion,that is,the skewness S<0.6,which takes into account the features of RTD curve(e.g.,trailing,multiple peaks),is proposed,thus avoiding the misjudgment by using the original criterion[the varianceσ_(t)^(2)<0.25 and peak height E(t)h>1].In the future,the effects of material properties and extreme operating conditions will be further investigated to establish an operating standard for MFB used in reaction kinetics measurement.