Effect of sound on the fluidization of group B particles

The behaviour of several kinds of group B particles ranging from 100 μm to 600 μm was studied in a sound wave vibrated fluidized bed (SVFB). The fluidized bed consists of a transparent Plexiglas tube that is 54 mm i.d. × 1 m high. A speaker mounted at the top of the bed was supplied by a function generator with square waves and was used to generate the sound as the source of vibration of the fluidized bed. The influence of the particle size, density of particles and sphericity of particles on the minimum fluidization velocity, pressure fluctuations and bubble rise velocity in the SVFB was investigated. The minimum fluidization velocity decreased as the sound energy increased. When the sound energy was strong enough and greater than the critical power, the minimum fluidization velocity would approach the same value regardless of the degree of resonance (DOR) value if the particles were in spherical shape. For non-spherical shape particles the minimum fluidization velocity was the function of the DOR value if the power was greater than the critical power. For the middle particle size range, the standard deviation of pressure fluctuations in an SVFB became lower than the one without the effect of sound in high superficial gas velocity range, but the result was reverse for the low superficial velocity; for the large particle size range, the standard deviation of pressure fluctuations in an SVFB was larger than the one without the effect of sound. The sound could also reduce the bubble rise velocity in an SVFB.     

离散单元法及其在流态化领域的应用

经过三十余年的发展,离散单元法（discrete element method,DEM）已经发展成为一种广泛应用于过程工程领域中颗粒体系研究的数值方法,特别是将DEM与计算流体力学（computational fluid dynamics,CFD）相结合形成的CFD-DEM耦合方法,已经在流态化研究领域得到广泛应用。首先对DEM模型进行了综述,包括DEM模型的基本原理、颗粒形状模型、接触力模型、非接触力模型、流体作用力模型等;然后对CFD-DEM耦合方法及其在流态化领域的一些主要应用进行了介绍,包括在流化床、气力输送以及过程工程领域里的一些其他应用。最后对DEM模型以及CFD-DEM耦合方法的发展趋势进行了预测,希望能促进DEM方法的发展,并推动其在过程工程领域中的应用。     

Method to estimate uncertainty associated with parcel size in coarse discrete particle simulation

Coarse grained particle methods significantly reduce the computation cost of large‐scale fluidized bed simulation by lumping many real particles into a computation parcel. This research provides a method to estimate the errors associated with parcel size in large‐scale fluidized bed simulations. This uncertainty is first quantified in small scale domains by comparing results of discrete particle method with that employing coarse parcels of different sizes. Then, this uncertainty is correlated with parcel size and simulation domains consisting of a simple homogeneous cooling system and more complex bubbling and circulating fluidized beds. These correlations allow us to accurately estimate the uncertainty in large‐scale fluidized beds based solely on data obtained in smaller systems. The ability to estimate model‐related uncertainty in larger systems makes this method relevant for industrial applications. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2340–2350, 2018     

Numerical modeling of particle fluidization behavior in a rotating fluidized bed

In this study, numerical modeling of particle fluidization behaviors in a rotating fluidized bed (RFB) was conducted. The proposed numerical model was based on a DEM (Discrete Element Method)-CFD (Computational Fluid Dynamics) coupling model. Fluid motion was calculated two-dimensionally by solving the local averaged basic equations. Particle motion was calculated two-dimensionally by the DEM. Calculation of fluid motion by the CFD and particle motion by the DEM were simultaneously conducted in the present model. Geldart group B particles (diameter and particle density were 0.5 mm and 918 kg/m³, respectively) were used for both calculation and experiment. First of all, visualization of particle fluidization behaviors in a RFB was conducted. The calculated particle fluidization behaviors by our proposed numerical model, such as the formation, growth and eruption of bubble and particle circulation, showed good agreement with the actual fluidization behaviors, which were observed by a high-speed video camera. The estimated results of the minimum fluidization velocity (U_mf) and the bed pressure drop at fluidization condition (ΔP_f) by our proposed model and other available analytical models in literatures were also compared with the experimental results. It was found that our proposed model based on the DEM-CFD coupling model could predict the U_mf and ΔP_f with a high accuracy because our model precisely considered the local downward gravitational effect, while the other analytical models overpredicted the ΔP_f due to ignoring the gravitational effect.     

CFD–DEM modeling of gas fluidization of fine ellipsoidal particles

Particle characteristics are important factors affecting gas fluidization. In this work, the effects of both particle size and shape on fluidization in different flow regimes are studied using the combined computational fluid dynamic–discrete element method approach. The results are first analyzed in terms of flow patterns and fluidization parameters such as pressure drop, minimum fluidization, and bubbling velocities. The results show that with particle size decreasing, agglomerates can be formed for fine ellipsoidal particles. In particular, “chain phenomenon,” a special agglomerate phenomenon exists in expanded and fluidized beds for fine prolate particles, which is caused by the van der Waals force. The minimum fluidization velocity increases exponentially with the increase of particle size, and for a given size, it shows a “W” shape with aspect ratio. A correlation is established to describe the dependence of minimum fluidization velocity on particle size and shape. Ellipsoids have much higher minimum bubbling velocities and fluidization index than spheres. © 2015 American Institute of Chemical Engineers AIChE J, 62: 62–77, 2016     

High-velocity fluidization of solid particles in a liquid-solid circulating fluidized bed system

Particulate and aggregative fluidizations in a liquid-solid circulating fluidized bed system are characterized by chaotic time series analysis of local voidage and heat transfer fluctuations in terms of the correlation dimension and Kolmogorov entropy. Both correlation dimension and Komogorov entropy of the voidate fluctuation are found to decrease with decreasing bed voidage in both fluidization regimes, suggesting the suppression of chaotic motion of individual particles due to high solid concentration. The correlation dimension of the heat transfer fluctation in the aggregative fluidization regime is higher than that in the particulate fluidization regime. This reflects the complex convective motion of liquid and solid phases induced by the formation of the liquid streaks and aggregates of particles.     

黏性颗粒团聚机理及流化特性研究进展

黏性力的存在导致黏性颗粒在流化过程中易发生团聚,干扰正常的流态化。近年来,黏性颗粒流的研究重心逐渐转移到本征的团聚机理和流化特性。本工作综述了4种主要黏性力的力学模型、团聚判据及流态化实验与模拟研究进展,从力、运动及动力学的角度阐述了黏性力作用机制和黏性颗粒流化特性。分析表明,在颗粒尺度上,4种黏性力发展程度差异较大,黏性力动力学模型和团聚过程机理将成为未来研究的主要方向。在反应器尺度上,耦合黏性力模型的离散单元法模拟将继续作为重要的研究方法,其中,机理模型和计算能力是后续模拟中需要突破的重点。     

Critical fluidization velocities and maximum bed pressure drops of homogeneous binary mixture of irregular particles in gas-solid tapered fluidized beds

Recently, tapered fluidized bed has become more attractive because of the problems associated with conventional (cylindrical) beds like fluidization of widely distributed particles, entrainment of particles and limitation of fluidization velocity. There have been some investigations on hydrodynamics of uniform single size particles but there have been no detailed studies of homogeneous binary mixture of particles of different sizes and different particles in tapered beds. In the present work, an attempt has been made to study the hydrodynamic characteristics of homogeneous binary mixture of irregular particles in tapered beds having different tapered angles. Correlations have been developed for important characteristics, especially critical fluidization velocities and maximum bed pressure drops of homogeneous binary mixture of irregular particles in gas-solid tapered fluidized beds. Experimental values of critical fluidization velocities and maximum bed pressure drops have been compared with the developed correlations.     

A numerical study of fluidization behavior of Geldart A particles using a discrete particle model

This paper reports on a numerical study of fluidization behavior of Geldart A particles by use of a 2D soft-sphere discrete particle model (DPM). Some typical features, including the homogeneous expansion, gross particle circulation in the absence of bubbles, and fast bubbles, can be clearly displayed if the interparticle van der Waals forces are relatively weak. An anisotropy of the velocity fluctuation of particles is found in both the homogeneous fluidization regime and the bubbling regime. The homogeneous fluidization is shown to represent a transition phase resulting from the competition of three kinds of basic interactions: the fluid-particle interaction, the particle-particle collisions (and particle-wall collisions) and the interparticle van der Waals forces. In the bubbling regime, however, the effect of the interparticle van der Waals forces vanishes and the fluid-particle interaction becomes the dominant factor determining the fluidization behavior of Geldart A particles. This is also evidenced by the comparisons of the particulate pressure with other theoretical and experimental results.     

A hybrid approach to computing electrostatic forces in fluidized beds of charged particles

In particulate flow devices particles acquire electric charge through triboelectric charging, and resulting electrostatic forces can alter hydrodynamics. To capture this effect, the electrostatic force acting on individual particles in the device should be computed accurately. Electrostatic force is calculated using a hybrid approach consisting of: (1) long‐range contributions from an Eulerian electric field solved using the Poisson equation (2) short‐range contributions calculated using a truncated pairwise sum and (3) a correction to avoid double counting. Euler‐Lagrange simulation of flows incorporating this hybrid approach reveals that bed height oscillations in small fluidized beds of particles with monopolar charge decreases with increasing charge level, which is related to lateral segregation of particles. A ring‐like layer of particles, reported in experimental studies, forms at modestly high charge levels. Beds with equal amounts of positively and negatively charged particles are fluidized in a manner similar to uncharged particles. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2282–2295, 2016     

Correlations for critical fluidization velocity and maximum bed pressure drop for heterogeneous binary mixture of irregular particles in gas–solid tapered fluidized beds

The tapered fluidized bed is a remedial measure for certain drawbacks of the gas–solid system, by the fact that a velocity gradient exists along the axial direction of the bed with increase in cross-sectional area. To study the dynamic characteristics of heterogeneous binary mixture of irregular particles, several experiments have been carried out with varying tapered angles and composition of the mixtures with various particles. The tapered angle of the bed has been found to affect the characteristics of the bed. Models based on dimensional analysis have been proposed to predict the critical fluidization velocity and maximum bed pressure drop for gas–solid tapered fluidized beds. Experimental values of critical fluidization velocity and maximum bed pressure drop compare well with that predicted by the proposed models and the average absolute errors are well within 15%.     

颗粒旋转对鼓泡流化床内气固两相流动特性影响的数值模拟

运用考虑颗粒自旋转流动对颗粒碰撞能量交换和耗散影响的颗粒动理学方法,建立鼓泡流化床气固两相Euler-Euler双流体模型,数值模拟流化床内气体颗粒两相流动特性。分析表明,颗粒平动温度与旋转温度之比是法向和切向颗粒弹性恢复系数和摩擦系数的函数。与不考虑颗粒旋转效应计算结果相比,考虑颗粒旋转效应后床内较容易形成气泡,颗粒自旋转运动将导致床内非均匀结构更明显。并且床层平均空隙率和床层膨胀高度增加,床中心区域颗粒轴向速度提高,床内颗粒平动温度下降。考虑颗粒旋转效应后预测的颗粒轴向速度和颗粒脉动速度与文献实验结果基本吻合。考虑颗粒旋转效应后获得的气泡直径更接近于前人经验关联式。     

Two-area model for bubble distribution in a turbulent fluidized bed of fine particles

The flow behavior of gas and solid was investigated in FCC simulator of φ710×4000/φ870×11000mm.The axial and radial distributions were detected with matrix fiber-opticprobes.It was found that the distribution of bubble diameter in the turbulent region of the fluidizedbed of fine particles was different from the results reported for lab-scale experiments.Radially therewere three areas,i.e.,the central(r/R=0-0.4),the intermittent or stable(r/R=0.4-0.8)and thenear wall(r/R=0.8-1.0)areas respectively.It was noticed that bubbles were almost non-existing atthe near wall area.Hence,according to the coalescence and splitting theory of bubbles,a two-areamodel of bubble diameter distribution was proposed and a dimensionless parameter(γ_M)regarded asan index for'quality'of fluidization was deduced.     

Triboelectric charging of monodisperse particles in fluidized beds

To investigate the interplay between particle charging and hydrodynamics in fluidized beds, models for triboelectric charging and electrostatic forces were built into a computational fluid dynamics‐discrete element method model. Charge transfer was governed by the difference in effective work function between contacting materials as well as the electric field at the point of contact. Monodisperse particles were fluidized with an effective work function difference between the particles and the conducting walls. For smaller work function differences, hydrodynamics were not changed significantly as compared with an uncharged case. In these simulations, the average charge saturated at a value much lower than the value anticipated based on the work function difference, and a unimodal distribution of charges was observed. For larger work function differences, particles stuck to walls and bed height oscillations due to slugging were less pronounced. For these cases, a bimodal distribution of charges emerged due to effects from strong electric fields. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1872–1891, 2017     

A model for the pressure balance of a low density circulating fluidized bed

The pressure balance along the solid circulation loop of a circulating fluidized bed equipped with a solid flux regulating device has been modelled and the influence of the pressure balance on the riser behaviour has been predicted.The solid circulation loop has been divided into many sections, where the pressure drop was calculated independently: riser, cyclone, standpipe, control device and return duct. A new theoretical model, that is able to predict the pressure losses in the return path of the solid from the standpipe to the riser, has been built. A new correlation for cyclone pressure loss with very high solid loads has been found on the basis of experimental data.The pressure loss in the riser has been calculated by imposing the closure of the pressure balance, ΣΔP = 0. Once the riser pressure drop had been calculated, the holdup distribution along the riser was obtained by imposing a particular shape of the profile, according to the different fluid-dynamics regimes (fast fluidization or pneumatic transport). In the first case, an exponential decay was imposed and the bottom holdup was adjusted to fit the total pressure drop, in the second case, the height of the dense zone was instead varied.The experimental data was used to develop the sub-models for the various loop sections have been obtained in a 100 mm i.d. riser, 6 m high, CFB. The solid was made of Geldart B group alumina particles. The tests were carried out with a gas velocity that ranged between 2 and 4 m/s and a solid flux that ranged between 20 and 170 kg/m²s. A good agreement was found between the model and experimental data.     

Effect of mixing state of binary particles on bubble behavior in 2D fluidized bed

In this study, a thin 2D fluidized bed was used to investigate the effect of mixing state of the binary particles on bubble behavior through the analysis of images captured by a high-speed digital camera. Experimental results show that the mixing index increases gradually with increasing gas velocity and the binary particles are in different mixing states though they are in the steady fluidization state. The maximal bubble number is near the interface of the bed when the binary particles are in the segregation state, whereas the maximal bubble number is at the bottom when the binary particles are in the well mixing state. The small bubbles are position at the bottom and are adjacent to the bed wall, while the large bubbles are mainly located in the central regions of the bed. The average bubble diameter shows the different variation trends with the different mixing states of the binary particles. The correlations estimating bubble diameter according to the mixing state of the binary particles are developed, and the computing value agrees well with the experimental data.     

A correlation to the solution of the two-phase dispersion model

A correlation to the solution of the two-phase dispersion model has been developed for gas-solid fluidized bed reactors operating in the bubbling regime. An analytical solution was obtained for fractional gas conversion by using an exponential function to characterize the dense phase gas concentration profile. The coefficient of the exponential function was found to depend on gas axial dispersion and, in order to determine this parameter, a Peclet number correlation was developed. Model predicted gas conversions were in excellent agreement with experimental conversions for a variety of fluidized bed reaction data over a conversion range from 2.5 to 99%.     

Prediction of critical fluidization velocity and maximum bed pressure drop for binary mixture of regular particles in gas–solid tapered fluidized beds

The problems associated with conventional (cylindrical) fluidized beds, viz., fluidization of wider size range of particles, entrainment of particles and limitation of fluidization velocity could be overcome by using tapered fluidized beds. Limited work has been carried out to study the hydrodynamics of single materials with uniform size particles in tapered beds. In the present work, an attempt has been made to study the hydrodynamic characteristics of binary mixtures of homogeneous and heterogeneous regular particles (glass bead and sago) in tapered fluidized beds having different tapered angles. Correlations have been developed for critical fluidization velocity and maximum bed pressure drop for gas–solid tapered fluidized beds for binary mixtures of regular particles. Model predictions were compared with experimental data, which were in good agreement.