Numerical predictions of flow behavior and cluster size of particles in riser with particle rotation model and cluster-based approach

Flow behavior of particles in a circulating fluidized bed (CFB) riser is numerically simulated using a two-fluid model incorporating with the kinetic theory for particle rotation and friction stress models. The particle rotations resulting from slightly friction particle-particle collisions was considered by introducing an effective coefficient of restitution based on the kinetic theory for granular flow derived by Jenkins and Zhang [2002. Kinetic theory for identical, frictional, nearly elastic spheres. Physics of Fluids 14, 1228-1235]. The normal friction stress model proposed by Johnson et al. [1990. Frictional-collisional equations of motion for particles flows and their application to chutes. Journal of Fluid Mechanics 210, 501-535] and a modified frictional shear viscosity model proposed by Syamlal et al. [1993. MFIX Documentation and Theory Guide, DOE/METC94/1004, NTIS/DE94000087] were used as the particle frictional stress model. The drag force between gas and particle phases was modified with cluster-based approach (CBA). The flow behavior of particles and the cluster size in a riser of Wei et al. [1998. Profiles of particle velocity and solids fraction in a high-density riser. Powder Technology 100, 183-189] and Issangya et al. [2000. Further measurements of flow dynamics in a high-density circulating fluidized bed riser. Powder Technology 111, 104-113] experiments are predicted. Effects of the rotation and friction stress models on the computed results are analyzed. It is concluded that particle rotations reduce the cluster size and alter the particle flows and distributions through more particle fluctuation energy dissipations. Effects of frictional stress on flow behavior and cluster size are not significant because the particle phase in the CFB riser is not dense enough to take into account for the particle-particle contact interactions.     

A hydrodynamic model of a circulating fluidised bed with low‐density particles

In this study, a two‐dimensional mathematical model was developed considering the hydrodynamic behaviour of a circulating fluidised bed biomass gasifier (CFBBG), which is also applicable for other low‐density particles. In the modelling, the CFB riser was divided into two regions: a dense region at the bottom and a dilute region at the top of the riser. Kunii and Levenspiel's [Kunii and Levenspiel, Powder Technol. 61, 193‐206 (1990)] model was adopted to express the vertical solids distribution with some other assumptions. Radial distributions of bed voidage were taken into account in the upper zone by using Zhang et al.'s [Zhang et al., Chem. Eng. Sci. 46(12), 3045‐3052 (1991)] correlation. For model validation purposes, a cold model CFB was employed, in which sawdust was transported with air as the fluidising agent. The column is 10 m in height and 280 mm in diameter, and is equipped with pressure transducers to measure axial pressure profile and with a reflective optical fibre probe to measure local solids holdup. A satisfactory agreement between the model predictions and experimental data was found. © 2011 Canadian Society for Chemical Engineering     

60米高循环流化床内物料浓度分布的冷态试验

利用电厂循环流化床锅炉现有的结构和设备, 搭建提升管高度60m、内径400mm的超高循环流化床冷态实验台, 重点研究了流化风速和颗粒密度对提升管内轴向和径向空隙率分布的影响。实验结果表明:空隙率分布形式与流化风速和物料密度密切相关, 对于一定的床料高度, 在底部密相区一直有床料堆积的情况下, 随着流化风速的增加, 提升管底部密相区空隙率增大, 上部稀相区的空隙率减小并且其在径向的分布变得更加不均匀;在一定的流化风速下, 密度较小的物料将更多的被带入上部稀相区, 上部稀相区的空隙率减小, 其在径向分布将变得更加不均匀。     

Identification and characteristics of different flow regimes in a circulating fluidized bed

A series of experiments was conducted in a 0.3-m diameter circulating fluidized bed (CFB) cold model to evaluate the operating flow regimes and their transitions. A single unambiguous experimental method was developed to identify the transitions between CFB operating regimes. Experiments were conducted at riser gas velocities ranging from dense phase turbulent, through fast fluidization (S-shape riser pressure profile), and up to dilute-phase flow regimes. A transient method was applied to a low density, Geldart Type B, cork bed material. Two distinct transition velocities were found by analyzing the time required to empty out all solids from the riser of the CFB after cutting off solids flow. The lowest transition velocity marked the transition between the dense-phase turbulent and the fast fluidization flow regimes, while a higher or second transition represented the transition between the fast fluidization and the dilute-phase flow regimes. Based on the experimental results, the axial pressures and its fluctuations along the riser exhibited markedly distinct profiles in each of the three different operating flow regime regions as defined by these two transport velocities.     

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.     

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。     

Experimental validation of the gas-solid flow in the CFB riser

In this paper, an experimental study is performed to investigate the flow structure in a circulating fluidized bed (CFB). The typical core-annulus structure and small amount of back-mixing of particles near the wall of the riser were observed. The axial solid concentration distributions contain a dilute region towards the up-middle zone and a dense region near the bottom and the top exit zones. Furthermore, the solid concentration decreases with the increase of the superficial gas velocity, and increases with the increment of the circulation rate at the same height position. The total pressure drop of the main bed represents a linear relationship with the solid flux rate. In the dense phase zone, the solid concentration increases linearly with the augmentation of the solid flux, however, the change of the solid concentration is slight, even unchangeable at the up zones. In addition, based on the Energy-Minimization Multi-Scale (EMMS) method, a revised drag force model is proposed, which is coupled in the Eulerian two-fluid model for simulating the flow structure in the riser. Numerical results are consistent with the experimental data, which indicate the revised drag force model is very successful in simulating flow structure of the dense gas-solid two-phase flow.     

Mean and fluctuating gas phase velocities inside a circulating fluidized bed

Gas phase velocities is an area in circulating fluidized beds (CFB) that has traditionally received little attention. The dynamics and motion of particles or clusters inside the bed has been the main focus of research. This is because particles dominate the fluid mechanics and heat transfer inside a CFB. However, gas phase motions also effect particle motion. Gas eddies or fluctuations can play an important role in transporting particles to and from the wall. They also help in providing a uniform temperature throughout the bed by promoting mixing. This paper deals with how particles effect the mean and fluctuating gas velocities throughout the cross-section of a riser.Gas velocities were measured inside a cold scale model CFB using a shielded hot wire anemometer. At the centerline, typical mean gas velocities were measured which were approximately twice the superficial gas velocity. These high velocities are likely caused by the negligible net gas upflow in the annulus region. The presence of many dense, downward flowing clusters in the annulus makes this a reasonable assumption.Previous work on gas phase turbulence in two phase flows has typically used either laser measurement techniques in very small diameter risers or in larger risers with very low particle concentration. The general results have shown that smaller particles, on the same order of magnitude as those typically used in CFB and FCC reactors, tend to damp out the gas phase fluctuations. This implies that gas phase motion behaves close to a laminar fashion. This present research measures gas phase fluctuations with typical particle concentrations inside a CFB (∼1-5%). The results indicate that at larger particle concentrations where clusters are formed, the gas phase fluctuations increase dramatically. This suggests that length scales based on cluster size, as opposed to particle size, should be used in estimating the increased levels of gas fluctuations caused by the solid phase. Hence, models which ignore the effect of clusters on the gas or which treat the gas phase as laminar like flow, yield a misleading picture of the flow dynamics inside a CFB riser.     

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。     

CFB提升管等效反应器网络模型

对气固循环流化床（CFB）提升管内的非均相流动行为进行了计算流体力学（CFD）模拟。基于CFD时均流场数据与信息,搭建了用于描述提升管内非理想流动过程的等效反应器网络（ERN）模型。在ERN模型建立过程中提出了反应器网络的6个拓扑结构参数和一个等效判据,并系统地分析了等效反应器网络结构六参数的性质与确定方法,从而形成了一套CFB提升管流动模型建模方法。     

Cluster characteristics of Geldart Group B particles in a pilot-scale CFB riser. I. Monodisperse systems

Experiments in a circulating fluidized bed (CFB) riser with Geldart Group B particles have been carried out with an emphasis on cluster characterization. In this paper, the focus is on monodisperse materials; results for polydisperse materials are contained in the companion paper (Chew et al., this issue). A fiber optic probe was used for detection of solids, with the resulting solids cluster identification accomplished using wavelet decomposition. Three cluster characteristics were evaluated – appearance probability, duration, and frequency – as a function of riser position, operating condition, and type of solids (i.e., variations in particle size and/or material density). Results indicate that the riser position has the most dominant influence on cluster characteristics with the cluster appearance probability appearing to be largely a function of riser position only, and insensitive to changes in operating condition or material type. The cluster duration and frequency are influenced by the particle material and operating condition, though only from mid-height of the riser upwards. The results presented provide a comprehensive picture of factors affecting cluster trends.     

Developments in the understanding of gas–solid contact efficiency in the circulating fluidized bed riser reactor:A review

In the last several decades, circulating fluidized bed reactors have been studied in many aspects including hydrodynamics, heat and mass transfer and gas–solid two phase contacting. However, despite the abundance of review papers on hydrodynamics, there is no summary paper on gas–solid contact efficiency to date, especially on high density circulating fluidized beds(CFBs). This paper gives an introduction to, and a review of the measurement of contact efficiency in circulating fluidized bed riser. Firstly, the popular testing method of contact efficiency including the method of heating transfer experiment and hot model reaction are discussed, then previous published papers are reviewed based on the discussed methods. Some key results of the experimental work are described and discussed. Gas–solid contact efficiency is affected by the operating conditions as well as the particle size distribution. The result of the contact efficiency shows that the CFB riser is far away from an ideal plug flow reactor due to the characteristics of hydrodynamics in the riser. Lacunae in the available literature have been delineated and recommendations have been made for further work.     

Hydrodynamic characteristics of fine particles in the riser and standpipe of a circulating fluidized bed

The hydrodynamic properties in the riser and standpipe. and the cyclone efficiency have been determined in a circulating fluidized bed (CFB) unit consisting of a riser (0.05 m-IDX3.8 m high), a standpipe (0.068 m-IDX2.5 m high) as a primary cyclone/bubbling fluidized bed, and a secondary cyclone. Silica gel powder (mean diameter = 46 μm) was used as the bed material. The effects of gas velocity in the riser and initial solid loading on the solid circulation rate, and the solid holdups in the riser and standpipe have been determined. The effects of gas velocity in the standpipe on the efficiencies of primary and secondary cyclones have been also determined as functions of solid circulation rate and solid entrainment rate. The solid circulation rate increases with increases in the gas velocity in the riser and in the initial solid loading. The efficiencies of primary and secondary cyclones increase with an increase in the gas velocity in the riser. However, the efficiency of primary cyclone decreases and that of secondary cyclone increases slightly, with an increase in the gas velocity in the standpipe.     

Flow phenomena in the exit zone of a circulating fluidized bed

An experimental investigation of the gas and solids flow in a pilot-scale circulating fluidized bed (CFB) cold model with two different abrupt exit configurations (L-shape and extended top) has been carried out. Measurements of axial pressure profiles, high-speed video images of the flow phenomena at the wall as well as local optical probe measurements inside the exit zone are presented. Contrary to published results obtained in bench-scale CFB risers the axial profiles of the apparent solids volume concentration obtained by pressure measurements showed no indication of an increased solids hold-up in the vicinity of the exit, which confirms the conclusion by Pugsley et al. (Can. J. Chem. Eng. 75 (1997) 1001) that this is a scale effect. The local probe measurements showed the well-known core–annulus flow structure prevailing until the riser top. In the vicinity of the exit this flow structure is superimposed by a strong horizontal velocity component directed to the exit duct. In comparison to the conventional L-shaped abrupt exit the extended top does not increase the solids inventory in the riser.     

A model for the circulating fluidized bed

A model for the hydrodynamics of circulating fluidized beds is proposed. The model combines existing entrainment and bed expansion correlations with a system pressure balance. Starting from a knowledge of the powder size distribution, particle density, gas velocity, equipment geometry and solids inventory, the variation of voidage within the riser is predicted, together with distribution of solids between the riser and supply hopper (or ‘slow’ bed). Qualitative trends with gas velocity and solid circulation flux show good agreement with experiment, but better quantitative agreement must await the development of an improved correlation for entrainment.     

丁烷氧化制顺酐循环流化床反应系统的温度匹配(I)反应系统模型

以VPO催化剂上正丁烷选择氧化制顺酐为例 ,对以变价金属复合氧化物为催化剂、按照氧化 -还原机理进行的一大类烃类选择氧化反应催化循环的温度匹配问题进行了初步模拟 .结果表明 ,使烃类选择氧化 (催化剂被还原 )和催化剂氧化再生分开进行 ,特别是根据催化剂氧化再生动力学的要求 ,适当调节再生过程的温度 ,既能明显改善反应系统的性能 ,又可以大幅度降低催化剂固体颗粒的循环量 ,取得节能降耗的效果 .本文介绍模型的建立和求解方法     

循环流化床燃烧数学模型及试验研究

利用循环流化床内气－固两相流动基础方面的研究成果, 提出床内气固浓－淡流动模型, 建立适用不同结构参数的循环流化床燃烧模型, 考虑了床内气体、固体颗粒的返混、循环过程以及煤燃烧、污染气体的生成和分解、颗粒磨损等过程． 在循环流化床燃烧试验台上进行实验研究, 模型仿真结果和实验数据吻合良好, 表明气固两相浓－淡流动模型所建立的循环流动床燃烧系统模型可以正确地模拟循环流化床的燃烧过程．     

Numerical simulation of flow behavior of particles and clusters in riser using two granular temperatures

Cluster in CFB riser significantly affects performance of circulating fluidized beds. To model hydrodynamic behavior in CFB risers, three phase flows were assumed in the riser, the gas phase, the dispersed particle phase, and the clusters phase. The gas-solid multi-fluid model is extended to give the macroscopic averaged equations with constitutive equations for both particle phases from kinetic theory of granular flow. The clusters and the dispersed particles have their own fluctuating energy or two individual granular temperatures. Interactions between the cluster and its surrounding dispersed particles were obtained from kinetic theory of granular flow. Drag force for gas to dispersed particles and the clusters are empirically determined. The momentum exchange between dispersed particles and clusters is modeled using the concept of molecular dynamics. Cluster properties are predicted with the cluster-based approach. The distributions of volume fractions and velocities of gas, dispersed particles and clusters are predicted. Computed solid mass fluxes and volume fractions agree with Manyele et al. [S.V. Manyele, J.H. Parssinen, J.X. Zhu, Characterizing particle aggregates in a high-density and high-flux CFB riser, Chemical Engineering Journal, 88 (2002) 151-161.] and Knowlton [T.M. Knowlton, Modelling benchmark exercise. Workshop at the Eighth Engineering Foundation Conference on Fluidization, Tours, France, 1995.] experimental data.