Cluster characteristics of Geldart group B particles in a pilot-scale CFB riser. II. Polydisperse systems

Experiments with a focus on the impact of polydispersity on clustering characteristics (namely, appearance probability, duration, and frequency) of Geldart Group B particles in a circulating fluidized bed (CFB) riser have been performed. Three mixtures are considered: (i) a density-difference binary mixture, with species of different material density (ρ_s) but similar particle sizes (d_ave), (ii) a size-difference binary mixture, with species of different d_ave but similar material density ρ_s, and (iii) a continuous particle size distribution (PSD). Local cluster information spanning the entire riser was obtained over a range of operating conditions using a fiber optic probe. Results show that cluster trends for the binary mixtures are similar to those reported in the companion work for monodisperse materials (Chew et al., this issue) on two counts. First, local riser position has a significant influence on all three cluster characteristics, while effects of operating condition and material type are secondary. Second, among the three cluster characteristics, the cluster appearance probability is most influenced by local position, and least affected by operating condition and material type. Furthermore, the density-difference binary mixture exhibits a distinctly lower cluster duration than either of its constituent components. In contrast, the size-difference binary mixture has a cluster duration profile that mimics one constituent component, while the frequency profile mimics the other. Comparing the two binary mixtures at any riser location, the density-difference binary mixture has lower cluster duration and higher frequency than the size-difference binary mixture regardless of local position. Finally, with respect to the continuous PSD, which was investigated under a wider range of operating conditions, the effect of operating condition is more apparent. This deviation may be due to an inherent behavioral difference between binary mixture and continuous PSD and/or to the wider range of operating conditions examined.     

The characteristics of cluster in a high density circulating fluidized bed

The cluster images in a high density circulating fluidized bed (HDCFB) measured by a one-dimensional optical fiber image analysis system are studied. The experimental results show that the characteristics of cluster in the riser vary greatly at different operating conditions. The radial cluster size, the cluster interception time and the probability of cluster appearance increase with bed density and decrease with gas velocity. Based on the analysis of the cluster image at various operating conditions, correlations of the radial cluster size, the cluster interception time and the probability of cluster appearance in a HDCFB are proposed. It is found that they are related to local solids concentration and gas velocity.     

Impact of material property and operating conditions on mass flux profiles of monodisperse and polydisperse Group B particles in a CFB riser

Experiments directed at understanding local mass flux behavior of Geldart Group B materials in the riser of a gas-solids circulating fluidized bed (CFB) have been carried out. Three monodisperse materials (with differences in particle size and/or material density), two binary mixtures (one with only a particle size difference between the species and the other with only a material density difference), and one continuous particle size distribution (PSD) have been investigated at four operating conditions. Results show that the riser axial position has the greatest influence on mass flux behavior, especially near the top of the riser, where profile shapes consistently have an inverted U-shape or V-shape. The material type (i.e., monodisperse materials of different particle sizes and/or particle densities or different types of polydispersity) and operating conditions effects are secondary but more apparent at the riser bottom. An interesting observation involving binary mixtures is that while the mass flux profiles of the density-difference binary mixture mimics that of one of its (monodisperse) constituent components, the size-difference binary mimics neither of its two monodisperse components.     

大型循环流化床流动结构分析

采用双光路光纤密度探头和激光多普勒测速仪测量了内径418mm,高18m的大型循环流化床提升管和下行床中的瞬态颗粒浓度信号和颗粒速度信号.对瞬态颗粒浓度和颗粒速度的概率密度分布分析表明,下行床中存在着和提升管中不同的微观流动结构,在提升管内流动结构存在明显的两相：即颗粒团相和空穴相,两相的固含率分别为接近1-ε_mf和0.01～0.02.而在下行床中,虽然在边壁也存在着颗粒的团聚行为,但不能形成稳定的、固含接近于起始流化状态固含值的颗粒团相.这种流动结构的区别揭示了下行床中气固顺重力场运动和提升管逆重力场运动在流动机制上的差异.     

Particle velocities and their residence time distribution in the riser of a CFB

The riser of a Circulating Fluidised Bed (CFB) is the key-component where gas-solid or gas-catalytic reactions occur. Both types of reactions require different conditions of operating velocities (U), solids circulation fluxes (G), overall hydrodynamics and residence times of solids and gas. The solids hydrodynamics and their residence time distribution in the riser are the focal points of this paper. The riser of a CFB can operate in different hydrodynamic regimes, each with a pronounced impact on the solids motion. These regimes are firstly reviewed to define their distinct characteristics as a function of the combined parameters, U and G.Experiments were carried out, using Positron Emission Particle Tracking of single radio-actively labelled tracer particles. Results on the particle velocity are assessed for operation in the different regimes. Design equations are proposed.The particle velocities and overall solids mixing are closely linked. The solid mixing has been previously studied by mostly tracer response techniques, and different approaches have been proposed. None of the previous approaches unambiguously fits the mixing patterns throughout the different operating regimes of the riser. The measured average particle velocity and the velocity distribution offer an alternative approach to determine the solids residence time distribution (RTD) for a given riser geometry. Findings are transformed into design equations.The overall approach is finally illustrated for a riser of known geometry and operating within the different hydrodynamic regimes.     

Experiments on particle cluster behaviors in a fast fluidized bed

A three-dimensional (3D) fast fluidized bed with the riser of 3.0 m in height and 0.1 m in inner diameter was established to experimentally study the cluster behaviors ofGeldart B particles.Five kinds of quartz sand particles (dp =0.100,0.139,0.177,0.250 and 0.375 mm and ρp =2480 kg·m-3) were respectively investigated,with the total mass of the bed material kept as 10 kg.The superficial gas velocity in the riser ranges from 2.486 to 5.594 m·s-1 and the solid mass flux alters from 30 to 70 kg· (m 2· s)-1.Cluster characteristics and evolutionary processes in the different positions of the riser were captured by the cluster visualization systems and analyzed by the self-developed binary image processing.The results found four typical cluster structures in the riser,i.e.,the macro stripe-shaped cluster,saddle-shaped cluster,U-shaped cluster and the micro cluster.The increasing superficial gas velocity and particle sizes result in the increasing average cluster size and the decreasing cluster time fraction,while the solid mass flux in the riser have the reverse influences on the cluster size and time fraction.Additionally,clusters in the upper region of the riser often have the larger size and time fraction than that in the lower region.All these effects of operating conditions on clusters become less obvious when particle size is less than 0.100 mm.     

Fluidization behavior in a circulating slugging fluidized bed reactor. Part I: Residence time and residence time distribution of polyethylene solids

Square nosed slugging fluidization behavior in a circulating fluidized bed riser using a polyethylene powder with a very wide particle size distribution was studied. In square nosed slugging fluidization the extent of mixing of particles of different size depends on the riser diameter, gas velocity, hold up and solids flux in the riser. Depending on the operating conditions the particle residence time distribution of a riser in the slugging fluidization regime can vary from that of a plug flow reactor to that of a well-mixed system.Higher gas velocities cause shorter particle residence times because of a significant decrease in the hold-up of particles in the riser at higher gas velocities. A higher solids flux also shortens the average residence time. Both influences have been quantified for a given polyethylene-air system.Residence time and residence time distribution were determined for different particle size and the influence of gas velocity, solids flux, hold up and riser diameter was studied. When comparing data from segregation and residence time experiments it is clear that segregation data can predict the spread in residence time as a function of overall residence time, particle size and gas velocity. The differential velocity between small and large particles found in the segregation experiments can predict the spread in residence time as found in the residence time distribution experiments with a powder with a broad particle size distribution. Raining of particles through the slugs was studied as a function of plug length, gas velocity and pulse length. It was found that raining is not the determining mechanism for segregation of particles.     

提升管反应器内聚丙烯颗粒浓度的轴向分布

在高14m,直径100mm×4mm的有机玻璃制成的冷模提升管内,模拟工业上聚丙烯实际生产操作条件,对空气-聚丙烯球形颗粒体系的床层颗粒浓度ρ的轴向分布及操作条件ρ的影响进行了考察。结果表明:提升管沿轴向自下而上可分为4个区域;沿提升管轴向呈上稀下浓的不均匀分布。     

内构件对于高密度提升管流体力学行为的影响

引　言近年发展的高密度提升管反应器 ,由于其较高的颗粒固含量 ,很容易达到很高的反应效率 .同时由于其具有的高气固通量、颗粒的循环操作方式和优良的传热性能 ,使得这一类反应器特别适宜于以中间产物为目的产品、要求高转化率和高选择性的强放热氧化 -还原类反应过程[1,2 ] .但大量实验表明 ,提升管特别是高密度提升管中空隙率、气体和颗粒速度沿径向的分布很不均匀 .这样将造成非常严重的颗粒和气体返混 ,固体颗粒停留时间分布变宽 ,反应程度参差不齐 ,造成反应器的处理能力偏低[1～ 4 ] .许多研究者采用在提升管中加设内构件的方法来改…     

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。     

耦合流化床提升管内固含率径向分布及沿轴向的发展

针对催化汽油辅助反应器改质降烯烃工艺,结合提升管与流化床的特点,建立了一套提升管与流化床耦合反应器大型冷态实验装置. 在不同操作条件下,采用PV-4A型光纤密度仪测定了提升管内固含率沿径向的分布规律. 结果表明,固含率径向分布整体上呈现中心小、边壁大的环-核结构分布特征;沿轴向向上,各径向位置上的固含率在颗粒加速区逐渐降低,在充分发展区趋于稳定,在颗粒约束返混区又有所升高;各径向位置上的固含率随表观气速增大或颗粒循环强度减小而减小,且均匀性变好;提升管上部流化床内颗粒静床高度只对颗粒约束返混区内固含率径向分布有影响,而对颗粒加速区和充分发展区的固含率径向分布影响较小;当表观气速较低或颗粒循环强度较大时,颗粒约束返混区上部局部固含率最大值出现在无因次半径f=r/R=0.7附近,此时局部无因次固含率es*=es/ 沿轴向在H>5.33 m时不再具有相似性;通过比较径向不均匀指数,得到轴向各区固含率径向分布趋于均匀的程度依次为：充分发展区>颗粒约束返混区>颗粒加速区. 利用实验数据回归出了局部固含率径向分布关联式,其平均相对误差在6%以内.     

Profiles of particle velocity and solids fraction in a high-density riser

Radial profiles of particle velocity and solids fraction in a high density circulating fluidized bed (HDCFB) at average cross-sectional solids fraction up to 0.21 were measured by an improved optical fiber laser doppler velocimeter and an optical fiber density sensor. The axial development of these radial profiles and the influence of operating conditions on the profiles were examined. The results showed that similar radial profiles of solids fraction exist in the HDCFB. The following Boltzman function can correlate well the solids fraction profile: (1 − )/(1 − ) = 2.2 − (2)/(1 + exp(10·r/R −7.665)). The radial profiles of particle velocity in HDCFB can also be described by the Boltzman function, that is, (V_p)/ (U_g) = (2.7)/(1 + exp(10·r/R −10·X_o) − 0.2). The Boltzman profiles of particle velocity in the high density operating regime was different with the parabolic shape operating in dilute phase regime. The local particle fluctuation velocity in the center of the riser increases with average solids concentration, while the fluctuation velocity decreases sharply as the radial position near the wall.     

提升管中气固两相流动行为的相似特性

在15.1 m高循环流化床实验装置上对提升管内的轴向压力梯度、局部颗粒浓度和颗粒速度进行了较系统的实验测试,研究了提升管中气固两相流充分发展段在不同操作条件下流动行为的相似特性.结果表明,在(G_s /ρ_p)^1.2/U^2.0_g相近的操作条件下,上行气固两相流充分发展段的颗粒浓度、下降颗粒时均速度、絮状物颗粒浓度和出现频率在空间的分布特征基本相同.对于同一提升管内的同一气固两相流系统,只要表观气速和颗粒循环速率按(G_s /ρ_p)^1.2/U^2.0_g同步变化,不同操作条件下的上行气固两相流在充分发展段就具有相似的宏观和微观流动行为.     

A Comparison of Solids Fluxes in a Pair of Downer and Riser Reactors

A direct comparison on solids flux was enabled by measurements obtained in a pair of riser and downer circulating fluidized bed reactors, of the same diameter, using suction probes. The operating conditions and the axial position were found to affect the solids flux in each reactor in a different manner. The solids flux in the riser were affected to a large degree by the gas velocity, in contrast with the downer where no visible effect was detected from changes in the gas velocity. The axial position has an effect on the shape of the solids flux profiles in the downer, but only small effects were observed in the riser. On the other hand, increases in overall solids flux leads to the increase of local solids flux in both the downer and the riser.     

流动方向对循环流化床中颗粒混合行为的影响

对循环流化床提升管及下行床两种不同气固流动方式对颗粒混合行为的影响进行了较为深入的对比分析，发现在影响循环流化床颗粒混合的众多因素（如操作条件、床层直径、颗粒性质及床层内构件等）中，气固流动方向是影响颗粒轴向混合的最主要因素．当气固流动为顺重力场时（下行床），颗粒的轴向混合很小，流型接近平推流；当气固流动为逆重力场的提升管时，轴向颗粒混合将成倍增大，颗粒流动远离平推流流动．分析表明，下行床中颗粒混合仅为单一的弥散颗粒扩散，而提升管中则存在着两种颗粒混合机制：弥散颗粒扩散及颗粒团扩散．弥散颗粒的扩散基本以平推流的形式通过循环流化床，提升管中大量的颗粒轴向返混归因于颗粒团的严重返混并由此形成了提升管中颗粒停留时间的双峰分布．     

Solids Holdup of High Flux Circulating Fluidized Bed at Elevated Pressure

Experimental studies on the solids holdup of a high‐flux circulating fluidized bed (HFCFB) at an operating pressure up to 0.5 MPa were carried out. The effects of operating pressure, solids mass flux and superficial gas velocity on the solids holdup distribution were systematically tested. It was found that the solids holdup at elevated pressure increases with increasing solids mass flux but decreases with increasing superficial gas velocity, which is similar to the trends at atmospheric condition. As a result, the condition of a high‐density circulating fluidized bed (HDCFB), i.e., solids holdup in everywhere of the riser is larger than 10 %) is easier to be achieved at elevated pressure than in a HDCFB operated at atmospheric pressure. In the current work, the condition of a full HDCFB with Geldart group B particles has been achieved successfully at 0.5 MPa.     

A new correlation for predicting solids concentration in the fully developed zone of circulating fluidized bed risers

The present work focuses on developing a new comprehensive correlation for better prediction of the solids concentration in the fully developed region of co-current upward gas-solid flow in circulating fluidized bed (CFB) risers. Systematic experiments were carried out in two risers (15.1 m and 10.5 m high with the same 0.1 m i.d.) with FCC and sand particles. The results obtained from about 200 sets of operating conditions show that the average solids concentration in the fully developed region is more than just a function of the corresponding terminal solids concentration, as most previous correlations are based on. Operating conditions, particle properties and riser diameters also have significant effects on the solids concentrations in the fully developed region of CFB risers. Based on our experimental data and those reported in the open literature from CFB risers up to 0.4 m in diameter and 27 m in height with superficial gas velocities and solids circulation rates up to 11.5 m/s and 685 kg/m²·s, a new empirical correlation for predicting the average solids concentrations in the fully developed region of CFB risers is proposed. The correlation works well for a wide range of operating conditions, particle properties and riser diameters.     

气固顺逆重力场流态化特征分析

气固并行顺重力场与逆重力场流动形成了迥然不同的流态化机制 :下行床中 ,局部颗粒的聚集会使局部颗粒及气体速度增大 ,而局部气体速度的增大又会破坏颗粒的聚集 ;提升管中因气固逆重力场流动 ,颗粒的聚集会使局部气体及颗粒速度降低 ,而这种降低又会加重颗粒的聚集。与提升管相比 ,下行床具有气固速度和颗粒含率径向分布均匀和气固停留时间短以及返混小等特点 ,其流型更接近平推流     

Study of gas–solids flow in a short CFB riser by statistical and chaotic deterministic analysis of optical fibre probe signals

In this work, a short CFB riser with a height of 2.42 m and an ID of 82 mm was operated under different dilute operating conditions to study the fluid dynamics of FCC catalyst particles (d_p = 80 µm, ρ_p = 902 kg/m³) in air. The electrical signals from the optical fibre probe were sampled at a frequency of 1000 Hz for a period of 30 s and were obtained at different positions along the radius and height of the riser. Data were analysed using both statistical methods (time average, standard deviation and frequency distribution) and chaos methods (construction of attractors; correlation dimension, D_ML; and Kolmogorov entropy, K_ML). Some results on solids holdup deduced from the electrical signal are also presented for the developed zone of the riser. It was verified that for very dilute conditions, increasing gas velocity produces more complex and less predictable fluctuations in solids concentration, while increasing solids flux generally reduces complexity and increases predictability. However, results for the most dilute condition used shows that for the radial position where solids holdup is higher (near the wall), the increase in solids concentration does not affect the mean free path of the particles, resulting in higher values of D_ML than in the dilute region (core). © 2012 Canadian Society for Chemical Engineering     

提升管内气固流动特性的离散元模拟

采用离散单元法模型对二维提升管内气固流动特性进行了数值模拟。利用标准k-ε模型模拟气相的湍流流动,考虑了颗粒间的van der Waals力和滚动摩擦的作用。通过对颗粒和气体流动行为的分析,得到了颗粒浓度、速度、温度及气体速度等的分布,研究了表观气速和颗粒循环速率对颗粒流动的影响。结果显示:颗粒在提升管内呈现边壁浓、中心稀的环核流动及上稀下浓的流动结构;气固两相都存在一定程度的返混现象;增加表观气速,使颗粒浓度降低、速度增大,颗粒分布更均匀;增加颗粒循环速率,使颗粒浓度增大,而颗粒速度对颗粒循环速率的变化不敏感,颗粒分布的不均匀性更强。模拟结果与文献中实验定性吻合。