浅层流态化

随着流化床结构及局部性能研究的深入,流化床已不再被看作浑然一体了。研究发现,分布板附近的区域(分布板区)以及浓相床层以上的区域(稀相区)与床层主体(鼓泡区)有许多不同。特别是分布板区越来越受到研究者的重视。这不仅因为分布板是影响流化床均匀性和稳定性的重要构件,又是气泡的发源地,它的结构决定了气泡的最     

流态化反应器分布板附近的湍流运动

前言随着流化床结构及局部性能研究的深入,流化床已不再被看作浑然一体了。研究发现,分布板附近的区域(分布板区)以及浓相床层以上的区域,(稀相区)与床层主体(浓相区)有许多不同。特别是分布板区越来越受到研究者的重视。这是因为分布板是影响流化床均匀性和隐定性的重要构件,又是气泡的发源地,它的结构决定了气泡的最初尺寸及其分布。而且,分布板区还有许多其它区域无法比拟的优     

基于两相两区模型的气固流化床DEM模拟

气固流化床DEM模拟中,通常采用面积加权平均法计算局部空隙率,为了考虑网格中显著非均匀结构对局部空隙率的影响,提出一个计算局部空隙率的两相两区模型。该模型将网格中的非均匀结构虚拟划分为稀相和密相,将实际网格区域划分为稀区和密区,并采用时空关联性原理识别稀区和密区;模型还采用自适应方法计算网格中颗粒分布的非均匀度;将非均匀度作为非均匀结构的影响权重计算密区颗粒的局部空隙率。模拟了鼓泡流化床,模拟结果表明:与传统的DEM相比,基于两相两区模型的DEM能更好地模拟气泡形态,且能捕捉气泡冒出床层和气泡破裂的复杂现象。     

气固搅拌流化床内压力脉动特性的数值模拟

在传统气固流化床中引入搅拌桨,可减轻聚合物颗粒的黏附并强化流态化过程。采用计算流体力学(CFD)方法对搅拌流化床内的压力脉动特性进行数值模拟,考察流态化过程中的气泡行为。模拟过程采用多重参考坐标系方法解决搅拌桨区域的运动问题,由欧拉双流体模型和颗粒动力学方法模拟气固两相流。床层压力脉动的统计分析和功率谱分析表明,随着搅拌桨转速的增加,流化床内的压力脉动标准偏差和功率谱幅值变小,床层内的平均气泡尺寸减小,床层可由鼓泡流态化向散式流态化转变。     

空气重介质流化床流化特性及密度梯度分布研究

为实现物料的有效分选,以磁铁矿粉和玻璃微粉为混合加重质,研究了混合加重质的流化特性及空气重介质流化床床层密度梯度分布情况。结果表明:空气重介质流化床形成了均匀稳定的流化状态,当流化气速大于7.10 cm/s后,床层压降基本维持在510 Pa,床层密度基本不变,为1.71~1.74 g/cm3。当流化气速为7.95 cm/s时,流化床内气泡直径为15~25 mm,且分布均匀,流化床各层平均密度从上至下依次为1.72、1.74、1.74、1.74、1.73 g/cm3。流化床上部区域,超微细玻璃微粉被气流带到床层表面,使表面床层密度较小;流化床底部区域,气体分布相对均匀,并未形成大气泡,使该区域流化床床层平均密度偏小;而床层大部分区域床层平均密度均为1.74 g/cm3,比较稳定。因此,当流化气速为7.95 cm/s时,流化床内并未形成明显的分层和分级现象,说明加重质混合比较均匀,为空气重介质流化床分选物料创造良好条件。     

气固脉冲流化床的流体力学特性

在直径７０ｍｍ的流化床中,采用ＦＣＣ、空心玻璃珠、细沙等Ａ类和Ｂ类颗粒,在０￣１０Ｈｚ的频率范围内,测定了气固脉冲流化床的流体力学特性。采用时间继电器改变脉冲气流的频率和脉宽周期比,利用微压传感器记录床层压力变化,研究了瞬时床层压力、平均床层压力,最大床层压降、起始流化速度、床层高度等随操作条件的变化规律,并对脉冲流化床中的气泡现象进行了初步的观察和研究,发现在脉冲流化床中气泡的形成和发展受到了有     

旋流筛板式气固挡板流化床内压力脉动特性

在表观气速Ug=0.04~1.14 m/s时,采用旋流筛板构型的挡板式内构件,通过对比分析旋流筛板式气固挡板流化床与自由床内流动现象、压差脉动标准偏差和压力脉动标准偏差等参数,确定了旋流筛板式气固挡板流化床能有效破碎气泡的流动与操作条件。结果表明,构件下方区域颗粒随表观气速增加而不断转移至构件上方床层,造成构件下方区域密相床层高度持续降低,该区域出现3种流动状态并直接决定构件是否能破碎气泡。当Ug<0.44 m/s时,构件下方区域密相床层料位较高,形成下部为密相床层、上部为密相与大气泡交替通过构件的鼓泡床,此时构件具有抑制气泡生长并破碎气泡的作用,全床压差脉动及压力脉动标准偏差低于相同条件下的自由床;当0.44≤Ug<0.66 m/s时,密相床层料位较低,形成下部为密相床层、上部为单一稀相的湍动床,此时构件不再直接抑制气泡生长或破碎气泡,但构件下方密相床层的存在能降低构件下方及构件上方一定高度内床层的压力脉动强度;当Ug≥0.66 m/s后,密相床层完全消失,形成气体为连续相的稀相流化状态,构件不能破碎气泡、降低床层压力和压差脉动强度。     

振动流化床中流动结构的混沌分析

在内径90mm、静床高800mm的高床层流化床中,用动态压力传感器检测了不同气速条件下普通流化床和振动流化床中沿轴向的压力脉动信号,通过小波变换对信号除噪后,用混沌理论对信号进行了分析.通过关联维数和Kolmogorov熵定量表征振动流化床中的流动结构特征.结果表明:压力脉动信号的关联维数和Kolmogorov熵能够描述振动流化床中的流化状态;振动流化床中床层的流动结构存在两个区,在近分布板区域为射流区,床层主体部分为均匀流化区.     

图像法用于流化床颗粒混合特性的研究

利用基于相关的图像法速度测量技术,测量了小型二维气固流化床密相区颗粒运动速度分布.并提出了用颗粒速度不均匀指数来衡量流化床内颗粒的速度波动与混合.速度波动对于物料扩散与对流混合均十分重要.在流化床下部,颗粒运动速度往往低于床层表面,但速度不均匀指数则高于床层表面.在床层底部颗粒速度不均匀指数随流化床气泡产生而变化.床层内部混合以对流及扩散为主,而在床层表面,混合以对流为主.床层下部速度波动有利于颗粒在床内的扩散及对流混合.流化风量的增加可以增强颗粒的混合.针对这些结论,在流化床垃圾焚烧炉的设计中,对不同的组分选用不同的给料方式,可以促进物料在床内的混合.     

复合生物流化床流动特性研究

本文对具有导流简流化床与填料固定床相结合的复合生物流化床的床层流体力学特性、混合特性及传质特性进行了研究。复合床的流型可分为 PPBM,PFBM,PCBM,其中 PCBM 最适于流化操作,通过对不同固体颗粒的实验,对床层压降及摩擦因数、床层平均气含率、气泡下潜深度、液体的循环速率、床层混合特性及气-液传质系数进行了讨论和关联,并得出了相应的计算模型。     

MODEL FOR PREDICTING THE LATERAL DISPERSION COEFFICIENT OF SOLIDS IN GAS-SOLIDS FLUIDIZED BEDS

The lateral mixing of solids in a gas-solids fluidized bed is very complicated.It can be caused by:(a)bubble movement through the bed,(b)bubble burst at the bed surface,and(c)gross particle circulation in thebed.However,experiments show that the major factors effected the lateral mixing of solids are the bubblemovement through the bed and the bubble burst at the bed surface.Thus a model with two mixing re-gions,i.e.mixing in bubble rising region and mixing in bubble breaking region,was proposed.Based on thismodel,an equation for predicting the lateral dispersion coefficient of solids in gas-solids fluidized beds wasderived without any adjustable parameter.The calculated values by this equation are well comparable withthe observed data including the present work and the other investigations.     

双组分颗粒鼓泡流化床内气泡形状参数

在二维双组分鼓泡床实验装置上,采用高速摄像技术,对床内气泡的形状特性进行了研究,考察了不同形状气泡在床内的轴径向分布,探索了颗粒组成和操作气速对气泡形状的影响。结果表明:不同形状的气泡在鼓泡床内呈正态分布,球形度较好的气泡主要分布于床层底部和壁面附近,而细长的气泡则主要集中于床层中心区域。随着气体速率的增加,气泡的球形度和宽纵比降低,气泡形状趋于细长和不规则;随着重组分增加,气泡的球形度增大而宽纵比减小。双组分颗粒鼓泡流化床内气泡球形度的概率密度较单组分的分布更宽,而宽纵比的概率密度分布与添加的颗粒密度有关。     

FCC细颗粒湍流流化床流体力学性能研究

ＦＣＣ细颗粒湍流流化床流体力学性能研究李俊，张蕴璧，闫遂宁（西安石油学院，西安７１００６１）（西北大学）（洛阳石化公司）关键词：流化床，湍流，气泡，空隙率１前言湍流流化床有着广泛的工业背景。湍流流化床反应器存在着物质空间分布的不均匀现象，这种分布对湍...     

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.     

Observation of 3‐D Structure of Bubbles in a Fluidized Catalyst Bed

The interface area between the bubble and emulsion phases in a fluidized catalyst bed is one of the important parameters used to analyze and design the fluidized bed reactor. We used a fast‐scanning X‐ray CT system to observe the bubble shape and structure. We then obtained the transient 2‐dimensional cross sectional gas‐phase distribution in a fluidized catalyst bed. Using image‐processing techniques, pseudo 3‐dimensional images of the bubbles were reconstructed. The bubble structure was studied based on the 3‐dimensional images and the previously obtained results in a 2‐dimensional fluidized bed. It was found that the bubble shape was not spherical but complicated, and that the bubbles ascending in a fluidized catalyst bed consisted of some smaller bubbles.     

THE EFFECTS OF DISTRIBUTOR DESIGN ON FLUIDIZED-BED HYDRODYNAMIC BEHAVIOR

should be addressed. The distributor was investigated for the purpose of design and scale up of large fluidized-bed combustors. Four orifice plates with different configurations were used to study the effect of distributor design on bubble formation and solid mixing. Experiments were carried out on a three-dimensional fluidized bed of 27.94 cm diameter and a two-dimensional bed with dimensions of 30.48cm ×1.27 cm. Motion pictures were used to study bubble formation and coalescence. Pressure profiles inside the three-dimensional bed were measured for several distributors to study bubble flow patterns, and tracer particles were used to study mixing patterns at various superficial velocities and particle sizes. The results show that the distributor plate with two-size orifices causes a non-uniform gas bubble flow inside the bed. This non-uniform gas bubble flow is associated with variations in local bed density and local voidage. Horizontal or radial solid circulation is also caused by this non-uniform gas bubble flow. The local bed density and voidage variations and the radial solid circulation cause the bubbles to move toward the area above the smaller orifices as the bubbles rise up and coalesce. This reduces the wall effect, and the bed is very uniformly fluidized when the two-size orifice plate with small holes in the center is employed.     

三相循环流化床中气泡大小及其分布的实验研究

用光纤探头技术对三相循环流化床中的气泡大小及其分布进行了系统研究 ,实验测定了操作条件对气泡大小及其分布的影响规律 .实验结果表明 ,三相循环流化床中气泡的大小分布可用对数正态分布表征 ,在实验条件下气泡平均直径在床中心区域较小且沿半径方向由中心向边壁逐渐增大 ,并随表观气速的增大而减小 ,随固含率的增大而增大 ,表观液速对气泡平均直径的影响较小     

HYDRODYNAMICS OF FLUIDIZATION: EXPERIMENTAL AND THEORETICAL BUBBLE SIZES IN A TWO-DIMENSIONAL BED WITH A JET

During the past three years we have developed a tentative cold bed hydrodynamic model for an idealized fluidized bed gasifier. The computer program calculates the pressure, the void fraction and the velocities for a single size solid and for a gas. The fluid bed never reaches a steady slate, but continually oscillates, as bubbles form, rise through the bed and collapse on top of the bed.

In this paper we show that the hydrodynamic model can predict bubbles forming in the fluidized bed. Photographically determined bubble sizes agree with the predicted bubble sizes. They increase with height of bed and with jet velocity. The model produces bubble splitting which agrees with observations at higher jet velocities.     

Effect of a dispersed immiscible liquid phase on the hydrodynamics of a bubble column and ebullated bed

Secondary undesired reactions in ebullated bed resid hydroprocessors can generate an additional dispersed liquid phase, referred as mesophase, which is denser and more viscous than the continuous liquid phase and affects the operation and transport phenomena of the fluidized bed. This study investigates the effect of a dispersed immiscible liquid phase on the overall phase holdups, bubble properties, and fluidization behavior in a bubble column and ebullated bed. The experimental system consisted of biodiesel as the continuous liquid phase, glycerol as the dispersed liquid phase, 1.3 mm diameter glass beads, and nitrogen. The addition of dispersed glycerol reduced the gas holdups in the bubble column for the studied gas and liquid superficial velocities. Dynamic gas disengagement profiles reveal a rise in the large bubble population and reductions to the small and micro bubble holdups when increasing the glycerol concentration. Liquid–liquid–solid bed expansions at various liquid flowrates confirm particle agglomeration in the presence of a more viscous dispersed liquid phase. Overall phase holdups in a gas–liquid–liquid–solid ebullated bed were obtained while varying the gas and liquid flowrates as well as the glycerol concentration. A coalesced bubble flow regime was observed in the bed region without glycerol whereas the addition of glycerol resulted in the dispersed bubble flow regime due to particle clustering and a greater apparent particle size. The resulting bubble flow regime increased the bed and freeboard region gas holdups due to enhanced bubble break-up. Observations of the fluidized bed behavior following the addition of the dispersed glycerol are also discussed.     

Hydrodynamic behaviour of a draft tube gas-liquid-solid spouted bed

Experiments were conducted using various types of solid particles to investigate the hydrodynamic properties of a gas-liquid-solid spouted bed with a draft tube. The hydrodynamic properties under study include flow modes, pressure profile and pressure drop, bubble penetration depth, overall gas holdup, apparent liquid circulation rate and bubble size distribution. Three flow modes were classified: a packed bed mode, a fluidized bed mode and a circulated bed mode. It was found that the friction factor accounting for the friction loss in the bed varies linearly on a logarithmic scale with the Reynolds number defined based on the apparent liquid circulation rate. The bubble penetration depth in the annular region, overall gas holdup and apparent liquid circulation rate increase with an increase in gas or liquid velocity. At high gas flow conditions an optimal solids loading exists which yields a maximum apparent liquid circulation rate. A model was proposed to describe the liquid circulation behaviour in the draft tube three-phas spouted bed. The average bubble size in the draft tube region is higher than that in the annular region for both the dispersed bubble regime and the coalesced bubble regime in the draft tube region.