Periodic pressure fluctuations in fluidized beds

In shallow gas fluidized beds harmonic oscillations of the pressure around its equilibrium value can be observed. Three aspects of these vibrations have been analysed: the frequency, the critical bed height and the damping. The frequency decreases with the inverse of the square root of the bed height for values below the critical height.For bed heights larger than the critical height the fluctuations cease to be harmonic, the bed breaks up and voids are formed leading to the formation of bubbles or slugs. The critical bed height can be calculated from the frequency and the wave velocity. The maximum value of the critical bed height is a few hundred particle diameters, thus most beds will fluidize heterogeneously. Damping of the oscillations is governed by the ratio of the fluid- to solids-density; the lower this values the higher the damping. The damping is liquid fluidized beds is such that oscillations are prevented.     

布风方式对流化床混合特性的影响

通过将离散单元法同计算流体力学相结合,对流化床内物料混合过程进行了研究。给出了水平布风板均匀布风、倾斜布风板非均匀布风2种情况下的示踪颗粒场历变过程。模拟结果表明:瞬时颗粒场组图能够较为直观表征床内混合现象;其中,在均匀布风情况下,床内气泡横向运动受到限制,颗粒整体横向运动能力较弱,混合方式以扩散混合为主;而对于非均匀布风流化床,床内存在较大的横向颗粒浓度梯度,对流混和起主要作用,且混合速度较为迅速。     

Distribution of the gas flow in fluidized beds with a slugging behaviour

Capacitance probe measurements of the visible bubble flow rate have been made in a pressurized fluidized bed burning coal. The bed, of 0.3 × 0.3 m cross-section, was operated at pressures between 1.0 and 2.0 MPa and at temperatures between 750 and 900°C. The fluidizing velocity was 0.95 m/s and the mean particle diameter was 0.9 mm. Based on the experimental results, a model of the gas distribution between the bubble phase and the particulate phase in fluidized beds with a slugging behaviour was developed. The model accounts for the lack of bubble flow obtained if the two-phase theory is employed. In order to verify the model, simultaneous measurements of the visible bubble flow rate and of the gas flow rate through the bubbles were carried out in a bed of similar geometry but operating at ambient conditions. In this bed the fludizing velocity was varied between 1.6 and 2.7 m/s and the mean particle diameter was 1.0 mm. The through-flow of gas was measured with the aid of pressure probes. Evaluation of the experimental results using the model showed that this gas through-flow in the bubble phase subsequently increases the superficial gas velocity in the particulate phase between the vertically aligned bubbles (slugs), and that this gas velocity in excess of the incipient fluidization velocity is responsible for the large deviation from the two-phase theory. The associated increase of the particulate phase voidage was calculated via the Ergun equation.     

Studies on Liquid—Gas and Three‐Phase Fluidized Beds with Pulsating Air Flows

The effects of air‐flow pulsation and water and air flowrates on the hydrodynamics of liquid—gas and three‐phase fluidized beds containing 3‐mm glass beads have been studied in a 90‐mm i.d. column. Under steady‐flow conditions, both types of bed contained a relatively large number of small bubbles. With a pulsing air flow, however, a smaller number of much larger bubbles or slugs were formed. This was attributed to different mechanisms of bubble formation at the distributor. Variations in phase holdup were explained in terms of the effects of the operating parameters on the bubble characteristics.     

不等粒径流化床的软球模拟

基于颗粒轨道模型,提出了粒径分别服从均匀分布与正态分布的软球方法,其中流体运动用Navier-Stokes方程描述,颗粒运动服从牛顿第二定理.模拟了不等粒径流化床中的气泡和节涌现象,并分别研究了表观气速、颗粒刚度系数、粒径分布不同时,固相颗粒的速度分布规律.其研究结果表明：分布板结构和表观气速对气泡行为有一定影响,随表观气速的增加,气泡形成、上升、破裂的速度加快;并且刚度系数越大,颗粒轴向速度随时间衰减越快;宽粒径分布的颗粒轴向速度大于窄粒径分布的颗粒轴向速度.     

Mass transfer from a slug in a fluidised bed at elevated pressures

The rate of mass transfer between bullet-shaped bubbles (slugs) and the dense phase of gas fluidised beds of fine particles (glass ballotini 64 μm and 101 μm average diameter) has been measured at pressures between 1 and 5 bar.The experimental values were compared with the theoretical predictions of Hovmand and Davidson[1] and Guedes de Carvalho and Davidson[1]; most values fall within 40% of the theoretical predictions, but the effect of gas diffusivity on mass transfer is not corroborated by experiment except for one set of values. Reduction of mass transfer by saturation of the wake is also considered and an approximate quantitative prediction of this effect is derived that is thought to be applicable to slugs in beds of fine particles.     

EXPERIMENTAL OBSERVATIONS OF FLUIDIZED BEDS AT ELEVATED PRESSURES

The object of the work described here was to elucidate the effects of operation under pressure on the physical behaviour of gas fluidized beds. Extensive measurements of various bubble properties such as size, shape and rise velocity in beds of coarse powders (mean particle diameters of 184 μm and 450μm) operated at pressures of up to 81 bar were made by photographing the images created by irradiation of the bed with X-rays, and analysing the bubble silhouettes thereby obtained. Most of the results presented here are averages of some 200 individual measurements.

Experimental evidence to support the following picture of the effect of pressurization on the behaviour of freely bubbling gas fluidized beds is presented. Both bubble interaction (tendency to coalesce) and the incidence of bubble splitting increase with increasing pressure; the two are intimately connected. The nett results are a decrease in bubble size with increasing pressure over most of the pressure range and an increase in the tendency for bubbles to distribute non-uniformly in a radial direction. This latter tendency probably causes gross solids circulation in the bed, and this in turn leads to higher bubble rise velocities than those observed for single bubbles under similar conditions. The splitting mechanism accounting for the decrease in bubble size was found to be intrusion of the wake into the bubble void by the flow of gas through the wake region of a leading bubble during pair coalescence.

An updated review of other published work relating to the subject of experimental observations of the effects of pressure on gas fluidized beds is included in the form of a table.     

D类颗粒节涌流态化的实验和数值模拟

应用双流体气固两相流模型,对均一粒径分布的Geldart D类颗粒的节涌流态化过程进行了模拟. 研究了流化过程中流化状态转变、床层膨胀比以及压力脉动功率谱,从Froude准数和颗粒动能的角度分析了模拟结果中的流化特性,并与实验数据进行了对比. 模拟结果准确预测了Geldart D类颗粒的节涌流态化特性,对床层膨胀高度和功率谱的预测与实验基本吻合. Froude准数和颗粒动能随时间的周期性脉动较好地反映了流化床内节涌流态化的气泡行为.     

Flow structure of the solids in gas-solid fluidized beds

Existence of clusters in dense fluidized beds was investigated by analyzing the time-position data of a tracer obtained in several radioactive particle tracking experiments. It was found that in the case of sand particles, more gas passes through the bed as bubbles with increasing the superficial gas velocity and in the case of FCC powder, flow of the gas through the bed as bubbles does not increase in the turbulent fluidization regime. Cluster diameters were estimated from their velocities and found that descending clusters are generally larger than ascending ones and the size of both increases with increasing the superficial gas velocity. Bubble velocities evaluated in this work are in good agreement with the correlations in the bubbling regime of the fluidization available in the literature.     

The onset of slugging in gas—solid fluidized beds with large particles

The onset of slugging in gas—solid fluidized beds with relatively large particles was studied by means of pressure transducers coupled with the on-line cross-correlation technique. The rise velocity of bubbles was employed as the criterion for the onset of slugging. Different sizes and types of particles and different column diameters were used to obtain data on the bubble rise velocity. A semiempirical model has been derived to predict the superficial gas velocity at the onset of slugging. The model appears to predict the present and available experimental results reasonably well.     

Discrete particle simulations of an electric-field enhanced fluidized bed

Reducing the size of gas bubbles can significantly improve the performance of gas-solid fluidized reactors. However, such a control of bubbles is difficult to realize without measures that either use a lot of energy or deteriorate the fluidization behavior. In this paper, we present the results of discrete particle simulations of an electric-field enhanced fluidized bed, and compare these results to experimental data.The simulations show a significant effect on the size of bubbles, both with horizontal and vertical electric-fields applied. When the field strength is increased to values higher than those used in the experiments, the particles are found to form strings in the direction of the electric field. At very high field strengths, defluidization is observed, consistent with the experiments.Through the analysis of the bubble behavior, it is concluded that moderate strength electric fields distribute gas more evenly at the bottom of the bed. As the bubbles rise through the bed, the coalescence rate is lower because of the guiding paths, or resistance, the particles form due to the field. This results in a smaller average bubble size in the higher region of the bed. The simulations presented here show how and why the electric fields reduce bubble size in electric-field enhanced fluidized beds.     

The stability of slugs in fluidised beds of fine particles: A theory based on particle pick-up from the wake

A theory for slug stability in fluidised beds of fine particles is developed based on the idea of gas circulation inside bubbles.Data on the intensity of circulation inside gas slugs in liquids are reported and an analogy is drawn with slugs in fluidised beds; this information is coupled with data on gas pick-up velocity for flow over granular beds, to yield a simple criterion for slug stability. A test of this criterion is provided by comparison with data on the stability of freely slugging beds.An X-ray study of slugs in fluidised beds at various pressures is also described that gives further support to the predictions of the theory.     

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...     

Numerical study on microscopic mixing characteristics in fluidized beds via DEM

In this paper, discrete element method (DEM), combined with computational fluid dynamics (CFD), is used to investigate the micro-mixing process in fluidized beds (FBs) of uniform particles. With the aid of snapshots and adoption of Lacey and Ashton indexes, mixing evolvement for two cases, fluidized bed using horizontal distributor with even gas supply and fluidized bed using inclined distributor with uneven gas supply, is discussed in detail. Results indicate that the Ashton index appears to be more effective in assessing the mixing dynamics in this work. Further analyses illustrate that in the case of horizontal distributor incorporated with even gas supply, diffusive mixing pattern is predominant, since bubbles lateral motion is reduced in such a bed; whereas, there is a faster convective mixing process in a fluidized bed using inclined distributor with uneven gas feed, followed by shear mixing. Generally, localized air supply induces the density gradient of particle distribution in the bed, which is the basic agent of convective particle stream. The analyses are confirmed by the comparison of solid flux during the simulations of the two cases. In addition, the mixing mechanism and the mixing time scale agree well with published experimental results.     

微小流化床流化特性分析

在内径4.3, 5.5, 10.5, 15.5, 20.5和25.5 mm的6个气固微小流化床中,考察了石英砂和不同粒径的催化裂化催化剂的流化特性. 研究了流化床尺寸、颗粒及流化介质物性对微小流化床床层压降及最小流化速度的影响. 结果表明,不同颗粒及流化介质的微小流化床床层压降实验值均小于计算值. 传统的压降关联式不能直接用于微小流化床. 其最小流化速度随床径减小呈指数增大,在高径比1:1~3:1范围内,最小流化速度随料高增大近似呈线性增大,其增大速度随床径增大而变缓. 基于实验数据得出了微小流化床最小流化速度的关联式.     

气固搅拌流化床中压力脉动特性

气固搅拌流化床反应器可用于黏结性聚合物颗粒的流态化过程,流化床中通气湍动与搅拌的相互作用关系仍不明确。通过压力脉动的统计分析、功率谱分析和小波分析,考察了搅拌桨型式和搅拌转速对流态化特性的影响规律。实验发现,搅拌转速和搅拌桨型式对床层压力影响较小,但对压力脉动影响显著。搅拌流化床中搅拌与通气湍动对流态化共同作用,双层锚式桨、框式桨等小桨叶面积的搅拌桨在较高转速条件下能强化流态化过程,与普通流化床相比具有更小的气泡尺寸和压力脉动,搅拌可抑制气泡聚并、破碎气泡,维持床层均匀流态化;而新型具有大桨叶面积的自清洁桨的搅拌作用强烈,在较高的转速下易形成桨叶前方的颗粒堆积和桨叶后方的气体短路等非正常流化现象,适宜于中等转速的操作条件。     

Convective solids transport in large diameter gas fluidized beds

It is demonstrated that the convective solids transport occurring in large diameter gas fluidized beds can be predicted quantitatively on the basis of measured properties of the bubble phase. Based on the fundamental findings of Rowe and co-workers [5], who have shown the solids mixing in gas fluidized beds for particle diameters greater than 100 μm to be caused solely by the action of rising bubbles, an equation has been derive from extensive measurements of the bubble development in a 1 m diam. fluidized bed of quartz sand which relates the convective solids mass flow due to solids transport in the bubble wakes to easily determinable parameters. The predictions of this relationship are found to bein good agreement with direct measurements of the convective solids transport carried out by Schmalfeld [21] on a pilot scale in a semicylindrical bed of 0.8 m diam.     

Hydrodynamics of gas-solid flow around immersed tubes in bubbling fluidized beds

A numerical study was conducted based on the gas-solid two-fluid model using the body-fitted coordinate system to analyze the behavior of particles and bubbles flow in bubbling fluidized beds without and with immersed tubes. The kinetic theory of granular flow was implemented in the model. The images of simulated instantaneous particle concentration and velocity gave the process of the formation, coalescence and eruption of bubbles. The effects of the tube pitch and superficial gas velocity on the fluidization in a bubbling fluidized bed were investigated. Calculated bubble frequencies without and with immersed tubes were in agreement with previous experimental and simulation findings. The wavelet multi-resolution analysis was used to analyze the simulated data of instantaneous particle concentration. From the random-like particle concentration fluctuations, the fluctuating components due to particle flow and bubble motion can be extracted based on the wavelet multi-resolution analysis over a time-frequency plane.     

A new spouting regime in beds of coarse particles deeper than the maximum spoutable height

A new spouting regime for coarse particles in beds deeper than the maximum spoutable height has been identified and its location on a phase diagram is presented for an air spouted bed of 5 mm glass spheres. The periodic formation and breakup of single bubbles originating at the top of the spout at lower gas velocities than required to observe the new regime has been studied using high speed photographs. An analogy is drawn between these observations and the breakup of bubbles in a turbulently fluidized bed.     

An Eulerian modeling approach of wood gasification in a bubbling fluidized bed reactor using char as bed material

We present an Eulerian multiphase approach for modeling the gasification of wood in fluidized beds. The kinetic theory of granular material is used to evaluate constitutive properties of the dispersed solid phase. Comprehensive models for wood pyrolysis, char gasification and homogeneous gas phase reactions are taken into account. The dispersed solid phase within the reactor is modeled as three continuous phases, i.e., one phase representing wood and two char phases with different diameters. In contrast to most other studies we investigate a fluidized bed which consists of wood and char particles without additional inert particles such as limestone or olivine. 2D simulation results for a lab-scale bubbling fluidized bed reactor are presented and compared with experimental data for product gas and tar concentrations and temperature. We investigate the influence of two different classes of parameters on product gas concentrations and temperature: (i) operating conditions such as initial bed height, wood feeding rate, and reactor throughput and (ii) model parameters like thermal boundary conditions, primary pyrolysis kinetics, and secondary pyrolysis model. Two different pyrolysis models are implemented and are compared against each other. The numerical results indicate (i) a relatively low influence of the investigated operating conditions on the main product gas components, (ii) a high sensitivity of main product gas components CO and CO₂ on the thermal boundary condition, and (iii) a very strong influence of operating conditions and model parameters on the tar content in the product gas.