循环流化床流体特性研究

在 D2 0 0 mm× 2 560 mm自制循环流化床上利用生石灰进行了流体特性研究 .实验结果表明 :固体颗粒循环速率和表观气速是影响压力分布的两个重要因素 .降低气速、提高颗粒循环速率 ,可以提高床内颗粒浓度 ,增加颗粒停留时间 .最后经无因次分析和实验数据拟合 ,分别得到了循环流化床压降和颗粒浓度沿床轴向分布经验关联式 .     

循环流化床回流物料循环的特性

为了研究循环物料在流化床内回流并参与循环的过程,进一步揭示流化床内物料的循环特性,文中搭建了冷态循环流化床顶部回流试验台。利用激光多普勒粒子动态分析仪(PDA)对塔内颗粒回流时床内气固二相流场和颗粒浓度分布进行了测量。同时对物料回流量和床内表观气速等操作条件对回流过程的影响进行了考察。研究表明,回流颗粒在流化床内的返混可以分为3个区域:回流区、返混加速区和主流区,回流颗粒造成床内流场的不均匀性。回流长度主要受物料回流量和床内表观气速影响。     

摇摆流化床的气固流动特性

采用二维床及D类玻璃珠颗粒,在表观气速U_g=0.267~0.978 m/s、摇摆幅值Θ=5°~15°、摇摆周期T=8~20 s的实验条件下,对摇摆流化床内气固流动过程及气体通过流化床的时均总压降进行了研究,并通过与常规直立床和倾斜床进行对比,分析了床体摇摆对气固流动的影响。结果表明,在平均角速度ω_ave>2(°)/s的条件下,当初始装料量和表观气速相同时,气体通过摇摆流化床的时均总压降低于直立床,高于相同最大倾角时的倾斜床;惯性力所产生的压降在0.15 kPa以下,其对床层压降的影响较小,床体倾斜导致气体向边壁区域聚集是影响摇摆流化床内气固流动特性的主要因素,由此导致床内存在固定床和下行移动床状态的非流化区域,使得处于流化区域的颗粒量减少,同时还降低了流化床层在竖直方向的静压。非流化区域的存在还会造成流化区域的气速高于直立床表观气速,两者表观气速之比为1.04~1.49。     

气固并流式轴向移动床过滤器的压降特性

通过冷模实验，改变移动床表观气速、颗粒循环速率、入口气体含尘浓度等操作参数，研究了轴向移动床过滤器的压降特性和合适的操作条件，结合移动床内气固两相运动特点，修正了Ergun公式，在加尘条件下分析了床内滤饼对压降稳定性的影响。结果表明，在无尘负荷条件下(“纯”移动床操作)，颗粒的循环速率由0增至2.26 kg/(m2?s)时，设备的压降减小0.03 kPa。表观气速为0.126 m/s、入口气体含尘浓度为89.10 g/m3时，移动床内滤饼形成和破损呈动态平衡，过滤500 s后，压降可稳定在0.88 kPa，此时设备具有较高的除尘性能，粉尘捕集效率可达96%以上。     

大差异颗粒分级再生设备的性能研究

通过改变表观气速U、颗粒循环速率W、粉尘/捕集颗粒比R等操作参数,考察了大差异颗粒空气分级设备在设置内构件前后的压降和分级效率的变化。结果表明,自由床时,压降随表观气速的增大而增大,分离效率在U=0.27 m/s时达到最大值87%。捕集颗粒循环量对压降的影响较小,分级效率随W的增大而持续下降。粉尘/捕集颗粒比较低时,压降无变化,但增大至超过约翰逊网的阻塞限度后,操作压降呈指数型增长,分级效率迅速下降。设置内构件后,由于其起到了整流和分布作用,设备压降和分级效率的变化不如自由床时敏感,拓宽了可操作的粉尘/捕集颗粒比范围,但缩小了可操作的表观气速范围。将设备实际压降划分为约翰逊网压降、颗粒摩擦压降、气体出口压降三个部分,基于实验结果,给出了计算压降的模型。     

组合约束型提升管出口气固分布器的压降

在一套组合约束型提升管冷态实验装置上,通过实验研究了不同操作条件下提升管出口气固分布器的压降,并与常规气体分布器压降进行了对比。实验结果表明,在零床层及有床层的操作模式下,气固分布器压降均随提升管内表观气速和颗粒循环强度的增加而增大,在颗粒循环强度较低时,气固分布器压降曲线变化的斜率随着表观气速的增加而增大,在颗粒循环强度较高时,气固分布器压降曲线变化的斜率随着表观气速的增加而减小;随着开孔率及上部流化床层压降增加,气固分布器压降呈降低趋势,当流化床层压降达到一定程度后,分布器各孔方可实现有效布气,此后气固分布器压降趋于近似不变;在相同表观气速及开孔率下,气固分布器压降大于常规气体分布器压降。     

组合约束型提升管出口气固分布器的压降

在一套组合约束型提升管冷态实验装置上,通过实验研究了不同操作条件下提升管出口气固分布器的压降,并与常规气体分布器压降进行了对比。实验结果表明,在零床层及有床层的操作模式下,气固分布器压降均随提升管内表观气速和颗粒循环强度的增加而增大,在颗粒循环强度较低时,气固分布器压降曲线变化的斜率随着表观气速的增加而增大,在颗粒循环强度较高时,气固分布器压降曲线变化的斜率随着表观气速的增加而减小;随着开孔率及上部流化床层压降增加,气固分布器压降呈降低趋势,当流化床层压降达到一定程度后,分布器各孔方可实现有效布气,此后气固分布器压降趋于近似不变;在相同表观气速及开孔率下,气固分布器压降大于常规气体分布器压降。     

方形截面循环流化床出口效应的实验研究

在0.27 m(长)×0.27 m(宽)×10 m(高)方形截面循环流化床冷态实验台上,针对三个不同的表观气速(Ug=6.0 m/s,6.8 m/s,7.6 m/s),考察了半弯道光滑出口和T形突变出口两种不同结构对系统颗粒循环流率以及床内颗粒浓度分布的影响.实验得到的系统颗粒循环流率Gs=60 kg/m2s～170 kg/m2s.结果表明:两种出口结构下,系统颗粒最大循环流率都随主床表观气速的增加而增加;半弯道光滑出口结构下得到的最大循环流率大,其对应的最大循环流率分别为136kg/m2s,162 kg/m2s和170 kg/m2s;床内颗粒浓度为底部浓顶部稀的分布,T形出口的约束效应使出口附近产生了一个颗粒增浓的区域.     

气－固－固循环流化床中填料层内细颗粒的质量流率和平均动含率

在气－固－固循环流化床中，尺寸较大的固相被固定在床中形成一段填料层，较小的固相一般为细颗粒被气流携带穿过填料层。本文在此类流化床流体力学实验的基础上，推导了气－固两相并流流过填料层时压降的数学模型，回归了气－固之间的滑落速度与表观气速之间的关系。最后应用压降及滑落速度的经验关系式，联立求解得到通过填料层的细颗粒的质量流率以及填料层内细颗粒的平均动含率。     

方形截面循环悬浮床内边角效应的分析

边角效应是方形或矩形截面的循环悬浮床内气固流动结构的重要特点之一。对于烟气脱硫循环悬浮床,减小边壁颗粒层厚度,使床内颗粒径向分布趋向均匀,会有利于气固接触反应,提高脱硫效率。在方形截面循环悬浮床冷态试验台架上,以空气为流化介质,平均粒径40μm的玻璃珠为循环物料,采用双光路光纤浓度测量系统测定了不同轴向高度截面上的径向颗粒浓度分布,分析了床内构件、表观气速和循环流率对边角效应的影响。研究结果对选择操作条件,尤其是合适的循环流率,以及进一步探索改善床内气固流动结构的方向有参考价值。     

Solids flow characteristics in loop-seal of a circulating fluidized bed

The hydrodynamics of solids (FCC) recycle in a loop-seal (0.08 m) at the bottom of the downcomer (0.08 m-I.D.x4.0 m-high) in a circulating fluidized bed (0.1 m-I.D.x 5.3 m-high) have been determined. Solid flow rate through the loop-seal increases linearly with increasing aeration rate. At the same aeration rate, the maximum solid flow rate can be obtained at a loop-seal height-to-diameter ratio of 2.5. The effects of solid inventory, solid circulation rate and gas velocity on pressure balance around the CFB have been determined. At a given gas velocity and solid circulation rate, pressure drops across the downcomer and loop-seal increase linearly with increasing solids inventory in the bed. At a constant solid inventory, pressure drops across the riser and the downcomer increase with increasing solid circulation rate but decrease with increasing gas velocity in the riser. The obtained solid flow rate has been correlated with pressure drop across the loop-seal.     

Hydrodynamics and Flow Development in a 15.1 m Circulating Fluidized Bed Riser

Hydrodynamics and flow development are studied in a long riser circulating fluidized bed reactor (15.1 m). Optic fiber probes were used for the measurement of local solids distribution. Pressure drops were also measured with pressure transducers along the riser. The flow development in the riser center is almost instant with solids holdup remaining constant and low, and particle velocity remaining high along the riser. The particle flow is firstly developed from center, and then towards the wall. The riser height is an important factor for the design of circulating fluidized bed reactors. Increasing the solid circulating rate significantly slows down the flow development process, while increases in the superficial gas velocity accelerate it.     

Hydrodynamic modeling of a circulating fluidized bed

Hydrodynamics plays a crucial role in defining the performance of circulating fluidized beds (CFB). The numerical simulation of CFBs is very important in the prediction of its flow behavior. From this point of view, in the present study a dynamic two dimensional model is developed considering the hydrodynamic behavior of CFB. In the modeling, the CFB riser is analyzed in two regions: The bottom zone in turbulent fluidization regime is modeled in detail as two-phase flow which is subdivided into a solid-free bubble phase and a solid-laden emulsion phase. In the upper zone core-annulus solids flow structure is established. Simulation model takes into account the axial and radial distribution of voidage, velocity and pressure drop for gas and solid phase, and solids volume fraction and particle size distribution for solid phase. The model results are compared with and validated against atmospheric cold bed CFB units' experimental data given in the literature for axial and radial distribution of void fraction, solids volume fraction and particle velocity, total pressure drop along the bed height and radial solids flux. Ranges of experimental data used in comparisons are as follows: bed diameter from 0.05-0.418 m, bed height from 5-18 m, mean particle diameter from 67-520 μm, particle density from 1398 to 2620 kg/m³, mass fluxes from 21.3 to 300 kg/m²s and gas superficial velocities from 2.52-9.1 m/s.As a result of sensitivity analysis, the variation in mean particle diameter and superficial velocity, does affect the pressure especially in the core region and it does not affect considerably the pressure in the annulus region. Radial pressure profile is getting flatter in the core region as the mean particle diameter increases. Similar results can be obtained for lower superficial velocities. It has also been found that the contribution to the total pressure drop by gas and solids friction components is negligibly small when compared to the acceleration and solids hydrodynamic head components. At the bottom of the riser, in the core region the acceleration component of the pressure drop in total pressure drop changes from 0.65% to 0.28% from the riser center to the core-annulus interface, respectively; within the annulus region the acceleration component in total pressure drop changes from 0.22% to 0.11% radially from the core-annulus interface to the riser wall. On the other hand, the acceleration component weakens as it moves upwards in the riser decreasing to 1% in both regions at the top of the riser which is an important indicator of the fact that hydrodynamic head of solids is the most important factor in the total pressure drop.     

Hydrodynamics of gas–solid flow in the circulating fluidized bed reactor for dry flue gas desulfurization

Process design and scale-up require a fundamental understanding of the hydrodynamics of gas–solid flow in the circulating fluidized bed flue gas desulfurization (CFB-FGD) reactor although the CFB system has been widely used in flue gas desulfurization and flue gas cleaning processes. The hydrodynamics in the CFB-FGD reactor model was investigated by pressure measurements and specially designed sampling probe based on three dimensionless groups for practicable similarity of industrial CFB-FGD process. The results show that the pressure drop in the venturi section is predominant as high as 60% of the total pressure drop and the total pressure drop significantly increases with the increasing external solid circulating rates at the same superficial gas velocity. Moreover, the measurements of radial solid mass fluxes show that the flow pattern in the CFB-FGD reactor is a typical core–annulus flow and this flow structure prevails until the top of the reactor. Reflux ratios are used to quantitatively evaluate the internal solid reflux in the reactor and the values in the low section of the reactor are much higher than those in the upper section.     

提升管与气-固环流床层耦合反应器的流体力学特性及流动模型

针对催化汽油辅助反应器改质降烯烃技术,在一套提升管与气-固环流床层耦合反应器大型冷模实验装置上,研究了上部环流床层的流体力学特性。结果表明,在环流床层与提升管耦合操作的情况下,床层内颗粒环流存在两种推动力,分别为静压差推动力和颗粒喷射推动力;环隙与导流筒之间的整体平均固含率差随导流筒表观气速增加而增加,随颗粒外循环强度增加而降低;颗粒环流速度随导流筒表观气速和颗粒外循环强度增加而增加。通过对环流床层进行动量衡算,建立了提升管与环流床层耦合流动的数学模型,模型平均相对误差在15.95％以内。     

提升管-流化床耦合反应器颗粒约束返混区的流动特性及约束作用分析

针对催化汽油辅助反应器改质降烯烃工艺,在一套提升管-流化床耦合反应器大型冷态实验装置上,系统研究了提升管出口段的颗粒流动特性,通过定义约束指数R_i（R_i为颗粒约束返混区实际截面平均固含率与理论截面平均固含率之比）定量反映提升管出口分布器及流化床层的约束作用。结果表明,与常规提升管相比,耦合反应器提升管出口存在一个颗粒约束返混区,其长度主要受表观气速、颗粒循环强度及上部流化床内颗粒静床高度影响;由于出口设置了倒锥形分布器,使得颗粒约束返混区靠近提升管出口区域在表观气速较低和颗粒循环强度较大时,局部固含率最大值出现在量纲 1半径Φ＝0.7处;颗粒约束返混区的约束指数在靠近出口的过程中逐渐增大,气固流动受到分布器及上部流化床层的约束作用亦逐渐增强。     

循环型高通量输送床的建立与控制

密相输送床气化和双流化床气化是基于循环型流化床反应器发展起来的两种新型煤和生物质气化技术,根据这两种技术对流动的要求,提出了在循环流化床的下行床底部耦合一段移动床,为输送床内的流动提供足够高的驱动压力而提高颗粒循环量的技术思想。在根据该思想而建立的直径90 mm的输送床实验装置上的实验研究表明,利用所提出的床型构造可在表观气速9.6 m•s^-1下实现400 kg•m^-2•s^-1的颗粒循环量。输送床的一次风速和移动床松动风速是影响颗粒循环量和输送床内颗粒浓度的主要因素,但循环量随输送床一次风速的增大而增加的走势弱于普通循环流化床。移动床松动风速在小于颗粒最小流化速度的范围内轻微变动即可显著改变颗粒循环量和输送床内颗粒浓度。在保持输送床总气速不变的前提下,通过二次风可在40%的比例范围内调节颗粒循环量,且调节作用随二次风位置的增高而减弱。     

16m高气固提升管中的压力梯度与流动行为研究

在较宽操作条件范围对16m高提升管中气－固两相流（空气－FCC颗粒）的压力梯度进行了实验测试,进一步揭示了快速流态化和密相气力输送这两种流动形态的动力学特征及其与操作参数的关系。结果表明,在表观气速增大的过程中气固提升管中的轴向压力梯度并非总是不断趋于均匀分布;提升管高度对快速流态化到密相气力输送状态的过渡有重要影响,对于给定的表观气速,提升管高度增加将使过渡点所应的颗粒循环量和床层颗粒浓度都减小。本实验条件下所有过滤点对应的床层颗粒浓度较为一致,平均为0．0104,并由此得到过渡点操作参数Ug与Gs的关联式。本文研究表明,在以往工作基础上进一步研究提升管高度对流动行为的影响极有必要。     

Computational fluid dynamics of a circulating fluidized bed under various fluidization conditions

A computational fluid dynamics (CFD) model was developed to simulate the hydrodynamics of gas-solid flow in a circulating fluidized bed (CFB) riser at various fluidization conditions using the Eulerian-Granular multiphase model. The model was evaluated comprehensively by comparing its predictions with experimental results reported for a CFB riser operating at various solid mass fluxes and superficial gas velocities. The model was capable of predicting the main features of the complex gas-solids flow, including the cluster formation of the solid phase along the walls, for different operating conditions. The model also predicted the coexistence of up-flow in the lower regions and downward flow in the upper regions at the wall of the riser for high gas velocity and solid mass flux, as reported in the literature. The predicted solid volume fraction and axial particle velocity were in good agreement with the experimental data within the high density fast fluidization regime. However, the model showed some discrepancy in predicting the gas-solid flow behavior in the riser operating in dense suspension up-flow and low density fast fluidization regimes.