Experimental investigation of a rotating fluidized bed in a static geometry

Rotating fluidized beds in a static geometry are based on the new concept of injecting the fluidization gas tangentially in the fluidization chamber, via multiple gas inlet slots in its cylindrical outer wall. The tangential injection of the fluidization gas fluidizes the particles tangentially and induces a rotating motion, generating a centrifugal field. Radial fluidization of the particle bed is created by introducing a radially inwards motion of the fluidization gas, towards a centrally positioned chimney. Correctly balancing the centrifugal force and the radial gas-solid drag force requires an optimization of the fluidization chamber design for each given type of particles. Solids feeding and removal can be continuous, via one of the end plates of the fluidization chamber.The fluidization behavior of both large diameter, low density polymer particles and small diameter, higher density salt particles is investigated at different solids loadings in a 24 cm diameter, 13.5 cm long non-optimized fluidization chamber. Scale-up to a 36 cm diameter fluidization chamber is illustrated.Provided that the solids loading is sufficiently high, a stable rotating fluidized bed in a static geometry is obtained. This requires to minimize the solids losses via the chimney. With the polymer particles, a dense and uniform bed is observed, whereas with the salt particles a less dense and less uniform bubbling bed is observed. Solids losses via the chimney are much more pronounced with the salt than with the polymer particles.Slugging and channeling occur at too low solids loadings. The hydrostatic gas phase pressure profiles along the outer cylindrical wall of the fluidization chamber are a good indicator of the particle bed uniformity and of channeling and slugging. The fluidization gas flow rate has only a minor effect on the occurrence of channeling and slugging, the solids loading in the fluidization chamber being the determining factor for obtaining a stable and uniform rotating fluidized bed in a static geometry.     

INVESTIGATION ON THE INCIPIENT FLUIDIZATION AND HEAT TRANSFER IN A CENTRIFUGAL FLUIDIZED BED DRYER

Based on the continuum theory, a physical model of gas-solid two phase flow in a centrifugal fluidized bed has been proposed. A set of governing equations to describe the fluidization state are obtained and solved numerically after some simplifying. The quantitative experimental study on the characteristics of the incipient fluidization in the centrifugal fluidized bed is performed to examine the proposed model. Gas-solid two phase heat transfer in CFB during a drying process is also conducted. The influences of bed thickness, particle diameter, physical properties of particle, rotating speed of the bed and the gas superficial velocity on heat transfer characteristics are examined. A correlation that can be used to calculate the heat transfer coefficients in the drying process in CFB is obtained.     

INVESTIGATION ON THE INCIPIENT FLUIDIZATION AND HEAT TRANSFER IN A CENTRIFUGAL FLUIDIZED BED DRYER

Abstract

Based on the continuum theory, a physical model of gas-solid two phase flow in a centrifugal fluidized bed has been proposed. A set of governing equations to describe the fluidization state are obtained and solved numerically after some simplifying. The quantitative experimental study on the characteristics of the incipient fluidization in the centrifugal fluidized bed is performed to examine the proposed model. Gas-solid two phase heat transfer in CFB during a drying process is also conducted. The influences of bed thickness, particle diameter, physical properties of particle, rotating speed of the bed and the gas superficial velocity on heat transfer characteristics are examined. A correlation that can be used to calculate the heat transfer coefficients in the drying process in CFB is obtained.     

离心流化床中气固传热特性的实验研究

本文使用玻璃细珠、黄砂和氧化铝球在不同操作条件下进行了离心流化床气固两相间传热特性的实验研究．测定了气体入口、出口和床层温度随时间的变化．分析了床层厚度、粒径、物料性质、床体转速和气体流速等因素对气固传热特性的影响．首次获得了离心流化床气固两相传热的准则方程．     

Gas–solid fluidized beds in vortex chambers

This review deals with gas–solid fluidized beds in vortex chambers. High-G fluidization can be achieved in a static geometry and allows significant process intensification. Thin, dense and more uniform particle beds can be obtained at high gas–solid slip velocities, intensifying interfacial transfer of mass, heat and momentum and reducing the gas–solid contact time. Existing fluidized bed processes can be carried out more efficiently and novel processing routes can be developed, e.g., involving cohesive particles or a dispersed liquid phase in relatively high concentrations.The first section of the review discusses the unique hydrodynamic characteristics of gas–solid fluidized beds in vortex chambers. The flow pattern, flexibility in the operating conditions and stability conditions are explained.The design of vortex chambers is dealt with in the second section and is critical for processing both larger and fine particles. The influence of the gas and solids in- and outlet design is focused on and insight is gained from recent theoretical, experimental and CFD studies.In the third section (potential) applications are discussed and process intensification and novel processing routes demonstrated. The fourth and last section presents extensions of the concept. Multi-zone operation and the integration of other technologies in vortex chambers are considered.     

离心流化床中强制对流换热的实验研究

对离心流化床干燥器中气体与被干燥颗粒物料之间的强制对流换热进行了实验研究,获得了各主要运行参数对气固换热系数的影响规律,并利用场协同原理分析了对流换热强化的机理. 实验证明,在一定转速范围内,在气流速度方向和热流方向（温度梯度方向）一致时,换热的准则关联式具有Nu＝CRePr的形式. 获得了满足Nu～RePr呈直线关系的Pe(Pe=RePr)数变化范围和临界Pe数,当Pe数大于临界值后,离心流化床中对流换热强度随Pe数增加而增大的趋势会明显减缓并偏离线性区.     

Rotating fluidized bed with a static geometry: Guidelines for design and operating conditions

Computational fluid dynamics (CFD) simulations of the hydrodynamic behavior of rotating fluidized beds in static geometry (RFB-SG) are carried out for gas–solid flows. The rotating motion of the reactor bed is induced by the tangential injection of the gas along the circumference of the fluidization chamber. Steep gradients in the gas velocity fields both in radial and tangential direction generate turbulence. The radial and tangential drag forces fluidize the particle bed in both radial and tangential direction.An Eulerian two-fluid model is used. Gas phase turbulence is accounted for by a k–ε model adapted for rotational flows. The RFB-SG simulations provide guidelines for a design and operation with a high efficiency in gas–solid momentum transfer, excellent gas–solid separation and limited solids losses. Hydrodynamic variables like the centrifugal force, the injection pressure, the radial and tangential slip velocities, solids hold-up are calculated for both polymer particles (300 μm, 950 kg/m³, Geldart Group B) and glass beads (70 μm, 2500 kg/m³, Geldart Group A) to allow for a comparison among different fluidization chamber designs. Unstable bed behavior, like slugging and channeling, is also numerically predicted.     

Wärme- und Stoffübertragung in der Wirbelschicht

Heat and mass transfer in fluidized beds . Methods of calculation are presented which enable reasonably accurate prediction of heat and mass transfer coefficients in fluidized beds. Some well established equations already given in the literature could be used for estimating the range of existence of the fluidized bed and for calculating maximum fluidization velocities, bed expansion, and particle-to-fluid heat and mass transfer. However, a new model had to be developed for the wall-to-bed heat transfer from solid surfaces immersed in fluidized beds. This model makes use of some basic ideas adopted from the kinetic theory of gases in order to describe the mechanism of energy transfer through the moving particles. Predictions with this new model are in good agreement with most of the experimental observations, particularly regarding the effects of particle diameter, temperature, pressure, physical properties of gas and particles, and gas velocity.     

离心流化床动力学模拟

在前人研究的基础上,进一步地考虑了离心流化床在初始流化后气速增大引起的床层膨胀,通过引入气速与空隙率变化函数关系建立了离心力场气固流态化的动力学模型,得出了空隙率和床层膨胀关系式,从而得出床层压降的计算公式。理论预测的针对不同转速、不同颗粒的床层压降与临界速度的关系与实验数据吻合良好,能更准确地预测实验结果。     

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。     

气固两相流强化传热研究进展

在气流中加入颗粒,形成气固两相流。根据气流速度的不同,气固两相流分为鼓泡流态化、快速流态化、气力输送等形式。不同的流动形态,两相流内颗粒浓度及颗粒的运动规律不同,其传热特点也存在差异。通过回顾几种多相流流态的传热特点,总结了多相流与传热面换热的影响因素、气固两相流的传热机理与模型。气固两相流中颗粒浓度、颗粒运动对其传热起决定性作用,而操作参数（气流速度、床层压力、床层温度等）则主要通过改变颗粒浓度和颗粒运动影响传热。此外,通过气固两相流强化传热的应用实例--气固两相流与填充床的热交换,分析了颗粒在对流换热中所起的作用,并进一步提出了今后研究方向和难点所在。     

气-固微型流化床压降特性及最小流化速度的实验研究

在内径3～20 mm的4个气-固微型流化床中,分别考察了A类和B类两种类型颗粒的流化特性,同时研究了床几何结构、操作条件、物相性质等各因素对其最小流化速度的影响.结果 表明,气-固微型流化床中的床层压降特性与颗粒类型密切相关,不同的流动状态下两种类型颗粒的流动特性存在显著地差异.在固定床阶段,与B类颗粒相比,A类颗粒与...     

Non-intrusive monitoring of bubbles in a gas-solid fluidized bed using vibration signature analysis

A reliable method was developed to study bubble behavior by analysis of vibration signals in fluidized beds. The advantage of this method is that the vibration probe is in indirect contact with the process. Accelerometers were used to record vibration signals generated by particle flow through the fluidized bed at various superficial gas velocities and particle sizes. Measurement of vibration signals, sampled at 25 kHz for 30 s, enabled investigation of changes in flow structure related to flow regime transitions. To study bubble behavior under different conditions, different particle sizes were used in the experiments. The measurements were extensively analyzed using wavelet and fast Fourier transforms. Results indicate that the vibration frequency generated by bubbles is between 1000 and 3000 Hz. The vibration analysis was effectively used to detect minimum fluidization and transition from bubbling to slugging in gas-solid fluidized beds.     

Experimental studies and empirical models for the prediction of bed expansion in gas–solid tapered fluidized beds

Studies in the expansion behaviour of tapered fluidized bed systems are important for specifying the height of the bed. Data have been obtained on the expanded heights of tapered fluidized beds and bed expansion ratios for spherical and non-spherical particles have been calculated. Based on dimensional analysis, models have been developed as a function of geometry of tapered bed, static bed height, particle diameter, density of solid and gas and superficial velocity of the fluidizing medium. The data used to derive the models cover a wide range of operating conditions, with varying fluidization velocities. Effects of static bed height, particle diameter, density, tapered angle and superficial gas velocity over minimum fluidization velocity on bed expansion ratios have been investigated experimentally. A comparison has been made between the calculated values of bed expansion ratios using proposed models and the experimental data. It has been seen that calculated values by models agree well with the experimental values. Models have also been compared with literature data of conventional bed and found its applicability at higher gas velocities with good accuracy.     

Fundamental particle fluidization behavior and handling of nano-particles in a rotating fluidized bed

The fluidization behavior of the three kinds of nano-particles (TiO₂, SiO₂, Al₂O₃) was analyzed in a rotating fluidized bed (RFB). Bed pressure drop, minimum fluidization velocity, bed expansion, entrainment and particle mixing characteristics under various centrifugal accelerations were experimentally investigated. The effects of centrifugal acceleration on agglomerate size and density were analyzed based on a Richardson-Zaki approach coupled with a fractal model.The bed pressure drop behavior showed almost similar to that of A or B-particles of Geldart's classification. Dimensionless particle bed height became smaller when the centrifugal acceleration was larger. Size of agglomerate decreased and its density increased with an increase in centrifugal acceleration. The agglomerate size in the RFB showed smaller than that in other types of fluidized bed system such as vibration and magnetic field as well as in a conventional fluidized bed, and the agglomerate density became larger. Particle entrainment became smaller in the case of the higher centrifugal acceleration. These results confirmed that the RFB can reduce the size of a nano-particle agglomerate and fluidize nano-particles at high gas velocity without any significant entrainment. The RFB is thus expected as more effective gas-solid fluidization system for handling of a large amount of nano-particles than other types of fluidized bed.     

Numerical and experimental investigation of a fluidized bed chamber hydrodynamics with heat transfer

The hydrodynamics and heat transfer of a gas-solid fluidized bed chamber was investigated by computational fluid dynamic (CFD) techniques. A multifluid Eulerian model incorporating the kinetic theory for solid particles was applied to simulate the unsteady state behavior of this chamber. For momentum exchange coefficients, Syamlal-O’Brien drag functions were used. A suitable numerical method that employed finite volume method was applied to discretize the equations. The simulation results also indicated that small bubbles were produced at the bottom of the bed. These bubbles collided with each other as they moved upwards forming larger bubbles. Also, the solid particle temperature effect on heat transfer and hydrodynamics was studied. Simulation results were compared with the experimental data in order to validate the CFD model. Pressure drops and mean gas temperature predicted by the simulations at different positions in the chamber were in good agreement with experimental measurements at gas velocities higher than the minimum fluidization velocity. Furthermore, this comparison showed that the model could predict hydrodynamics and heat transfer behaviors of gas solid fluidized bed reasonably well.     

柱形流化床传热特性的数值模拟

对Shedid等搭建的圆柱体流化床采用欧拉?欧拉法进行三维数值模拟,考察了颗粒球形度、表观进气速度和床料初始堆积高度对流化床内垂直加热壁面与流动床料之间对流传热特性的影响,采用有效导热系数分别计算气相和固相的对流传热系数。结果表明,随表观进气速度增大,流化床内颗粒物料湍流运动加剧,加热壁面平均温度和流体平均温度下降,壁面流体间传热平均温度差减小,壁面流体间对流传热系数增大;随初始床料高度增加,流化床内颗粒与加热壁面的接触面积增大,导致固相平均对流传热系数增大。     

CFD simulation of gas solid flow in FCC strippers

In this paper, the hydrodynamic characteristics in bubbling fluidized beds (FCC Strippers) were simulated by using computational fluid dynamics (CFD) code (Fluent 6.2.16). The modified Gidaspow drag model based on the effective mean diameter of the particle clusters predicted the expected bubbling fluidization behavior and bed expansion. Compared with the bed densities of in the empty-cylinder stripper, bed densities in the V-baffled stripper were at the superficial gas velocity of 0.10-0.20 m/s. The overall trend of the time-averaged bed density at various superficial gas velocities were in agreement with the experimental data. The results illustrated that internal baffles had an important effect on the fluidization hydrodynamics. Internal baffles improved break-up and redistribution of bubbles and intensified the gas-solid contact. The simulation results also indicated that appropriate modification of the internal configuration eliminated the dead flow region in the strippers, and enhanced the gas-solid mixing remarkably, showing benefit for the mass and heat transfer in the fluidized bed.     

Flow characteristics in packed and fluidized rotating beds

Free particles in a rotating tapered cylindrical container are, as a consequence of the circular motion, forced radially outwards towards the circumference to form a packed annular bed. If the container wall is porous and a fluid is allowed to flow radially inward, the bed material may become fluidized. In the resultant centrifugal fluidized bed, radial accelerations many times one g can be generated, permitting much larger fluid flow rates during fluidization than are possible with a conventional fluidized bed. Entrainment of material can also be greatly reduced in such a geometry.The flow in both packed and fluidized beds in a rotating system is analyzed. Expressions are found for the shape of the bed as well as the distribution of particles in packed beds containing mixtures of materials having different size distributions and densities. Based on these results, equations predicting the pressure drop and radial flow distribution are deduced. The condition and location where initial fluidization of the packed bed occurs are predicted. Similarly, conditions in the fluidized state are analyzed, and it is shown that the tangential velocity distribution in the bed under these conditions is directly proportional to the bed radius.Experiments confirming the validity of the analytic models are conducted over a range of operating conditions for different bed materials.     

鼓泡流化床内高灰煤气化过程的颗粒尺度特性研究

基于多相流体质点网格方法(MP-PIC)对高灰煤在三维鼓泡流化床气化过程进行了数值模拟研究。在欧拉-拉格朗日框架下将气相和固相分别视作连续介质和离散相处理。首先,将模拟得到的出口处气体组分结果与实验数据进行对比,实验数据与模拟结果具有良好的一致性。其次,研究了煤颗粒在气化炉内的温度、传热系数、速度和停留时间,从颗粒尺度揭示了鼓泡流化床气化炉内的颗粒分布特性和气固流动特征。结果表明:在气化炉入口附近煤颗粒与床层温差最大,传热系数最大;由于流化床内强非线性的气固流动,床中煤温度和传热系数的空间分布不均匀;煤颗粒和床料的瞬时速度具有稳定的波动幅度,其中垂直方向速度波动最明显,且煤颗粒的瞬时速度比床料的瞬时速度略大;由于颗粒间的剧烈碰撞,延长了煤颗粒停留时间。此外,对鼓泡流化床中煤气化过程颗粒尺度的研究,有助于深入了解固体颗粒的流动行为以及气固相相互作用特性,对鼓泡流化床反应器的设计优化具有重要意义。