Hydrodynamics of gas-solid two-phase mixtures flowing upward through packed beds

The work reported here represents part of an effort to address the challenges related to a newly proposed process for hydrogen production through steam-methane reforming, in which a fine adsorbent carried by the gaseous reactants moves through a packed catalyst bed. Comprehensive experimental work was carried out on the hydrodynamic aspects of gas-solid two-phase mixtures flowing upwards through packed beds. The effects of column diameter, packed particle size, and suspended particle size on the pressure drop and solids hold-ups were investigated. It was observed that the pressure drop of gas-solid two-phase flows depended approximately linearly on the solids flux under the conditions of this work, and the dependence was affected by the suspended particle size, packed particle size, packed column diameter, and gas velocity. However, when the data were reprocessed in terms of the Euler number and the solid-to-gas mass flux ratio, they collapsed into a single line for a given packing condition, and the suspended particle size was found to impose little effect. An analysis was conducted on the pressure drop using a modified version of Metha-Hawley equation by taking into account the effects of suspended particles on the viscosity and density. A reasonably good agreement with experimental data was obtained. The experimental results of the solids hold-ups showed that the particle concentration in packed particle interstices was much higher than that at the entrance to the packed column. Effort was also made to relate the solids hold-ups to the operating parameters. It was found that the dynamic hold-up related fairly well to the solid-to-gas velocity ratio as well as the suspended-to-packed particle size ratio for a given packed column, whereas no clear relationship was obtained for the static solids hold-up. Based on the results of this study, recommendations for future work are given.     

Solids behaviour in a dilute gas-solid two-phase mixture flowing through monolith channels

This paper reports, for the first time, the solids behaviour in a dilute gas-solid two-phase mixture flowing through monolith channels. The non-intrusive positron emission particle tracking (PEPT) technique was used in the work, which allowed investigation of three-dimensional solids motion at the single suspended particle level. Processing of the PEPT data gave solids velocity and occupancy in the monolith channels. The results showed a non-uniform radial distribution of both the solids velocity and concentration. The highest axial solids velocity occurred in monolith channels located in the central part of the column, whereas the highest solids concentration took place at a position approximately 0.7 times the column radius. The axial distribution of the axial solids velocity showed an entrance region with a length of approximately 33 times the hydrodynamic diameter of a monolith channel under the conditions of this work. Analysis of the PEPT data also gave distributions of particle residence time and tortuosity in terms of solids motion. The distributions were approximately Gaussian-type with the tortuosity distribution more skewed toward the right hand side. The peak residence time and tortuosity decreased with increasing superficial gas velocity and the distributions were broadened at lower superficial gas velocities. The results of this work also provided a possible explanation to our previously observed early laminar-to-turbulent flow transition in monolith channels.     

Flow of a gas-solid two-phase mixture through a packed bed

This paper is concerned with an upward co-current flow of a gas-solid two-phase mixture through a packed bed, a system employed in a number of industrial processes. Experimental work was carried out by using glass balls for packed bed, and both glass beads and FCC as suspended particles. The effects of solids loading and gas velocity on the pressure drop as well as the static and dynamic solid hold-ups within packed bed were examined. Experimental results showed different behaviour of the FCC from glass beads. At a given gas velocity, pressure drop increased approximately linearly with solids loading with a slope for FCC much higher than that for glass beads. The static hold-up of glass beads was much lower than corresponding dynamic hold-up at a given gas velocity, and it did not seem to change much with solids loading under the conditions of this work. At a given gas velocity, the static hold-up of FCC, however, was found to be comparable with the corresponding dynamic hold-up. An analysis was conducted on the pressure drop using a modified version of the Ergun equation by taking into account the effects of suspended particles on the viscosity and density, as well as the gravitational force. It was found that the modified Ergun equation agreed well with the experimental results of both this work and those reported in the literature. Effort was also made to develop relationships for the dynamic hold-up and the interaction coefficient between the suspended and the packed particles, the so-called solid-phase friction factor in the literature. The dynamic hold-up was found to increase with an increase in the product of velocity ratio of the solid to gas phases and the square root of the diameter ratio of the suspended to packed particles, whereas the interaction coefficient increased in general with increasing Froude number but with significant scattering.     

Modelling of the behaviour of gas-solid two-phase mixtures flowing through packed beds

This work is concerned about gas-solid two-phase mixtures flowing upwards through packed beds. An Eulerian-based two-fluid model coupled with a newly proposed porosity distribution model is used to simulate the flow behaviour. The results are compared with recently published experimental results in terms of both hydrodynamics and solids motion. It is found that the use of the newly proposed porosity model not only gives better agreement with experimental porosity data, but also provides a much better prediction of the pressure drop than other porosity models could do. The results also show that the model predicts very well the dynamic hold-up of suspended particles, and captures the main features of the radial distributions of the suspended solids concentration and the axial solids velocity. A discrepancy occurs, however, at the wall region where the predicted axial solids velocity peak is sharper and higher than the measurements. The work also leads to a new relationship for the pressure drop of dilute gas-solid two-phase mixtures flowing through packed beds, which agrees very well with experiments.     

THE EFFECT OF DISTRIBUTORS ON TWO-PHASE AND THREE-PHASE FLOWS IN VERTICAL COLUMNS

Two-phase air-water and three-phase air-water-solids flows have been investigated in a 0.152 m internal diameter vertical column. Pressure drop across the distributor and average phase hold-up were measured for two-phase and three-phase flows over flow ranges of superficial gas velocity from 0 to 259 m/s and superficial liquid velocity from 0 to 0.0346 m/s. Gas hold-up/pressure-drop ratios were obtained as a function of superficial gas velocity and liquid flow rate. It was found that the bubble cap was better at gas distribution than that of the Koch static mixer in two-phase and three-phase vertical flows. Drift flux to gas hold-up and drift flux to gas flow rate correlations were obtained for two-phase and three-phase vertical flows. Three flow patterns, churn-turbulent, transition and ideal bubble flows, were observed for vertical upflow.     

矩形错流移动床稠密气固两相流的实验和模拟

在实验基础上,采用欧拉双流体模型对矩形错流移动床中稠密气固两相流动进行了模拟,分析了矩形错流移动床的空腔、贴壁、两相速度、气相停留时间、压降等. 结果表明,气固两相流动受端部效应影响. 随表观气速增大,贴壁(约0.19 m/s)和空腔(约0.31 m/s)现象出现. 空腔随时间发生变化,而贴壁为渐进型,气速与空腔和贴壁的形状有关. 气速分布、空腔和贴壁使压降沿轴向呈反C形分布. 进料影响区、主体流动区和下料影响区压降比约为6:10:9. 随表观气速增大,气相停留时间减小,空腔尺寸增大,贴壁厚度先增大后不变,压降增大;而颗粒循环强度的影响不大     

Solids behaviour in a gas-solid two-phase mixture flowing through a packed particle bed

This paper reports the solids behaviour in a dilute gas-solid two-phase mixture flowing through a packed bed. The positron emission particle tracking (PEPT) technique was used in the work, which allowed investigation of three-dimensional solids motion at the single suspended particle level. Processing of the data gave solids velocity, the residence time of suspended particles, bed tortuosity in terms of solids motion, as well as solids occupancy in the cross-section of the packed bed. The results suggest that the wall effect on the motion of suspended particles is limited to approximately one packed particle diameter under the conditions of this work. Both the average axial and radial velocities of suspended particles, normalised by the superficial gas velocity, change periodically with radial position, but the periodicity does not correspond exactly to the packed particle diameter. The peak and trough values of the average axial velocity of the suspended particles in the bulk region are, respectively, ∼25% and ∼15% of the superficial gas velocity under the conditions of this work and the superficial gas velocity shows little effect. The peak and trough values of the average radial velocity of the suspended particles in the bulk region are, respectively, +5% (positive) and -5% (negative) of the superficial gas velocity. The results of the residence time and tortuosity of the suspended particles show an approximately Gaussian distribution with the peak residence time and tortuosity increasing with decreasing superficial gas velocity. The occupancy data suggest that particles spend more time in an annular region close to the wall, indicating a non-uniform particle distribution across the packed bed cross-section.     

两相均流板对弯管中气固两相运动分布特征的影响

针对燃煤电厂烟气污染物脱除设备入口弯管内气固两相分布不均的问题提出一种新型两相均流板,采用CFD数值模拟首先对比分析了安装两相均流板与常规导流装置的均流效果,之后详细研究了新型两相均流板对弯管水平出口管道中气流速度、颗粒质量分布规律的影响以及板型及板间夹角对两相均流板均流效果的影响,并结合Design-Expert响应面法获得两相均流板的最优结构,将最优结构应用于工业性实验中,最后将计算结果与工业性实验数据进行了比较。结果表明:弯管内安装两相均流板较安装导流板/三角翼挡板气固两相均流效果更优,既可以使气固两相均匀分布又可以有效降低系统的压阻;随板间夹角的增大,管内的气流速度分布以及颗粒的质量浓度分布呈阶段性变化,而管内的压降则随板间夹角增大而增大;最佳两相均流板结构为夹角为75.49°的直板型两相均流板。模拟与实验结果吻合较好,利用提出的数值模拟方法可以详细真实地模拟计算大型及具有复杂内部结构的除尘器入口弯管中气固两相流动。     

Water electrolysis and pressure drop behaviour in a three-dimensional electrode

Water electrolysis from acidified solution was used as a model system to investigate the net contribution of hydrogen bubbles to the pressure drop increase in a three-dimensional electrode. A bed of silvered glass beads in both fixed and fluidized state was used, assuming an unchanging particle surface during the experiments. Pressure drop behaviour with time was measured for different experimental conditions and presented relative to the pressure drop determined for a bubble free bed. Parameters, such as current density, electrolyte velocity and particle size, greatly influence the relative pressure drop behaviour in the three-dimensional electrode. A sudden increase in the pressure drop occurs with the appearance of a gas phase in the bed, reaching a constant value (plateau) after a certain time; this plateau corresponds to steady state conditions. The pressure drop increases with increasing current density. This increase is in the range 40-150% relative to the bubble free electrolyte flow through the bed. Electrolyte flow-rate also strongly influences the pressure drop in the hydrogen evolving fixed bed electrode. It was observed that the relative pressure drop decreases with increasing electrolyte velocity. At higher flow rates, peaks occur on the pressure drop-time curves, indicating the existence of channeling inside the bed in which spouting occurs. The time to reach the pressure drop plateau decreases with increasing electrolyte velocity as do the time intervals corresponding to maximum pressure drop values. At the minimum fluidization velocity the peaks disappear and the relative pressure drop decreases with time, tending to approach a constant value. For hydrogen evolution in the fluidized bed, the pressure drop is lower than that measured in the absence of gas, and reason for this decrease being the gas hold-up in the bed.     

Effect of solid particles on gas hold-up in bubble columns

The gas hold-up in bubble columns containing fluidised plastic particles as solid phase was measured as a function of superficial gas velocity and solids concentration. The effects of particle size, density, wettability and concentration on gas hold-up and bubble coalescence were studied. It was found that the addition of non-wettable solids to the air/water mixture promotes bubble coalescence and, therefore, reduces the gas hold-up, while the addition of wettable solids suppresses bubble coalescence and increases the gas hold-up.     

颗粒负荷对小型旋风器性能影响的模拟分析

为探究颗粒负荷对小型旋风器内气固两相流动的影响,基于雷诺应力模型（RSM）和欧拉-欧拉方法的混合流模型（Mixture）进行气体-颗粒、颗粒-颗粒的相间耦合计算。采用粒径为0.5～5μm的颗粒组在40L/min、60L/min和80L/min的入口流量下模拟0~3kg/m³的5种不同颗粒浓度工况,通过对比旋风器内纯气相流场和颗粒负荷流场的不同,研究了颗粒的存在对流场的影响;探究了入口流量和浓度变化对旋风器内分离效率和压降特性的影响。基于模型有效性验证的数值模拟结果表明：较高颗粒浓度负荷使旋风器内的气相流场发生显著变化。随着入口流量的增大,旋风器的分离效率先增大后减小,压降呈非线性增大。随着颗粒浓度的增大,旋风器的分离效率逐渐增大,压降先减小后增大。     

Two-phase flow hydrodynamic study in micro-packed beds – Effect of bed geometry and particle size

Microscopic visualizations nearby the wall region of micro-fixed beds and hydrodynamic measurements during gas–liquid two-phase flows were carried out with an aim to investigate the effect of particle size and capillary tube shape on the bed pressure drop, flow regime transition, hysteresis and bed transient response to flow-rate step perturbations. Visualizations through inverted microscopy revealed that a decrease in particle size leads to early inception of a high interaction flow regime whereas changing capillary shape from circular to square had no effect on flow regime changeover. The effect of particle size on the wetting pattern hysteresis in square micro-packed beds was also investigated in both imbibition and drainage paths. It was found that wetting pattern hysteresis decreases with a decrease in particle size. Finally, the transient behavior of micro-fixed beds of circular and square geometries packed with particles of two different sizes were studied by monitoring the bed pressure drop variations upon step changes in liquid flow rate at iso-G conditions. Larger particle sizes and square geometry showed shorter transient times as compared to smaller particle sizes and circular geometry.     

补充风对水平管高压密相气力输送影响的模拟研究

针对水平管高压密相气力输送数理模型的缺陷与不足,引入Savage径向分布函数修正的颗粒动理学理论、基于Berzi摩擦压应力模型构建的摩擦应力模型以及修正的三段式曳力模型,在欧拉-欧拉方法的基础上建立了一个能同时兼顾水平管高压密相气力输送中稀相流、过渡流以及密相流输送特性的三维非稳态数理模型。并采用该数理模型考察了补充风对水平管高压密相气力输送的影响,模拟结果精准地预测了水平管压降及其随补充风的变化规律,而且其预测的水平管固相体积浓度分布与ECT图也是相吻合的,从而验证了数理模型的可靠性。模拟结果表明：随着补充风的增加,气固两相速度和湍动能以及颗粒拟温度增大,固相体积浓度减小。     

颗粒直径对吸附分离影响的数值模拟

基于Fluent的多孔介质模型,建立了变压吸附制氧发生器的立式填充床模型。采用用户自定义函数功能,以反映吸附传质、传热,并将多孔介质单相模型整合为更精确的气固两相耦合模型。在此基础上,模拟了吸附颗粒直径对气相压力、速度、床层压降以及氧气分离浓度、回收率等参数的影响情况。结果表明：床层压降随颗粒直径的增大而减小;床层对入口急流的抗穿透性能随颗粒直径的增大而减小;相同条件下,采用较小颗粒直径能够提高氧气分离浓度、回收率,原因在于小颗粒直径降低了床层内气体的流速,增加了吸附时间,促进了吸附的进行。     

Tomographic diagnosis of gas maldistribution in gas-solid fluidized beds

Phase distribution is one of the key hydrodynamic parameters useful for the design and performance assessment of fluidized bed dryers (FBDs). It has direct influence on the drying rate, thermal efficiency, residence time distribution and degree of mixing. The quality of fluidization strongly depends on the uniformity of distribution of the fluidizing gas and the physical properties of the material to be fluidized. In the present work, gamma ray tomography (GRT) study was carried out in the form of chordal solid hold-up, which was found to be greatly influenced by the gas distributor design. The performance of a gas distributor due to the prevalent practice of operating at lower values of distributor-to-bed pressure drop ratio was characterized in a 0.15 m diameter fluidized bed dryer over a broad range of superficial gas velocity. The effects of various parameters such as solids loading, particle size and particle density were analyzed with the help of the reconstructed solid hold-up profiles. The fluidization was studied in terms of maldistribution factor (χ), a value of 5% or less can be obtained by properly designing distributor for a given bed loading, particularly for batch fluidized bed dryers. An industrial size fluidized bed dryer of 1 m diameter was also examined tomographically to obtain quantitative information on the solid hold-up distribution within the bed.     

Hydrodynamics of a flowing gas-solids suspension in a tube at high angles of inclination

A study was made of pneumatic transport of particles of sand suspended in air in a 31.8 mm diameter Plexiglas tube inclined at either 72 or 90 degrees with respect to the horizontal plane. The solids holdup, pressure gradient and pressure drop fluctuations over a 4.55 m test section were measured as functions of the air velocity and the solids flux. Previously observed sudden changes in slope of plots of solids holdup versus solids flux at constant gas velocity in a vertical tube (Mok et al., 1989), which were identified as transition points between dilute and dense phases, were confirmed and extended to inclined tubes. Breaks were also found in plots of pressure drop fluctuations versus solids flux at constant gas velocity.     

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。     

Hydrodynamics of a spouted bed with an impermeable draft tube for binary particle systems

A spouted bed with an impermeable draft tube was employed to obtain fundamental data of binary mixtures of glass beads for both the operating conditions and the design factors. These data were compared with those for the coarser particle system only. From this view point, minimum spouting velocity, pressure drop, hold-up of solid particles within a draft tube, upward gas flow rate within the annulus and solids circulation rate were determined by changing the total gas flow rate and mass fraction of finer particles as operating parameters and by changing distance of entrainment zone and draft tube diameter as geometric parameters.     

电容层析成像在气固流化床测量中的应用

气固流化床广泛应用于化工、炼油、冶金、电力等工业生产领域。由于流化床中气固两相流动的复杂性,流化床测量方法的研究成为了一个重要的研究领域。电容层析成像（ECT）是20世纪80年代以来发展起来的一种非介入式多相流测量技术,已经在多相流测量领域发挥了重要作用。总结了国内外近年来应用ECT技术进行气固流化床测量的进展,特别是对气固流化床内固体相浓度、流化床流型识别、固体相速度等方面工作进行了总结,并简单介绍了ECT在流化床干燥中的应用,讨论了ECT和其他测量方法结合使用情况。最后对ECT在流化床测量中的局限性和发展方向进行了讨论。     

气固固循环流化反应器的流体力学行为

简单介绍了气固固三相反应设备的研究现状,描述了气固固循环流化反应器的操作特点,实验研究了气固固循环流化反应器的流体力学行为,如气固两相通过填料层的压降、细颗粒的固含率及细颗粒的质量流率等．