不规则的含铀矿粒和矿泥的流化特性

本工作系于室温下,用水流化35至约600目的12种窄粒级含铀矿粒和矿泥(平均粒径d_p=32—460μm,比重为2.58—2.63),在空隙率ε=0.530—0.957的范围内,测定了床层的膨胀特性.由于实验所得lgRe～1+lgε为非线性关系,作者采用双直线代表这一曲线,并相应地确定了各粒级矿粒和矿泥的第一和第二终端速度V_t1、V_t2,第一和第二空隙率指数n_1、n_2,两线折点空隙率ε_c,最低流化速度U_mf和最低流化点空隙率ε_mf.通过回归分析,在阿基米德数Ar=0.6—1700的范围内,求得了该不规则矿石的流化关系式.本文还对比球形和非球形物料,讨论了这种不规则矿石的流化特性和流化关系式.     

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.     

Microstructural aspects of the flow behaviour in a liquid—solids circulating fluidized bed

The local instantaneous and time‐average suspension densities were determined in a 76 mm diameter by 3 m tall liquid‐solids circulating fluidized bed riser using a fibre‐optic probe. Attempts were made to qualify the microflow structure through statistical analysis of the local bed voidage fluctuations obtained under different operating conditions for the first time. The results show that local microflow structure is uniform in the axial direction but non‐uniform in the radial direction with more flow fluctuation near the wall than in the core of the column for a given axial position. The standard deviation and intermittency index tend to increase with increasing solids circulating rates. Comparing with the gas—solids CFB, the liquid—solids CFB shows much more homogeneous flow structure in both the axial and radial microscopic flow behaviours. The microflow behaviours in the conventional liquid—solids fluidization, liquid—solids circulating fluidization and dilute‐phase liquid transport regimes are also characterized by examining the probability distribution and the intermittency index of the solids holdup.     

Investigation of fluidized bed behaviour using electrical capacitance tomography

The temporal and cross-sectional distributions of particles in a 127 mm diameter fluidized bed have been obtained using a new generation, high-speed electrical capacitance tomography. Two planes of eight electrodes were used and mounted at 160 and 660 mm from the gas distributor which was a 3 mm thick porous plastic plate (maximum pore size of 50-70 μ m). 3 mm diameter, nearly-spherical polyethylene granules made up the bed. Experiments at sampling frequencies of 200-2000 cross-sections per second and gas superficial velocities from just below the minimum fluidization to 83% above minimum fluidization velocities were used. The time series of the cross-sectional average void fractions have been examined both directly and in amplitude and frequency space. The last two used probability density functions and power spectral densities. The information gathered shows that the fluidized bed was operating in the slugging mode, which is not surprising given the size of the particles. It has been found that an increase in the excess gas velocity above the minimum fluidization velocity resulted in an increase in the mean void fraction, an increase in the length and velocity of the slug bubbles as well as the bed height, and a slight decrease in the slug frequency. The results are presented in a level of detail suitable for comparison with later numerical simulation.     

大差异双组分混合颗粒的最小流化特性

在一套f260 mm′2000 mm的有机玻璃实验装置中,对大差异双组分混合颗粒的最小流化特性进行了实验研究,得到了混合颗粒的流化曲线,由此给出了其起始流化速度、最小流化速度、临界分离速度、完全流化速度等特征速度. 实验结果表明,流化过程可分为4个阶段,即完全流化、大小颗粒分离、大颗粒静止小颗粒流化、固定床阶段,对应混合颗粒的3个状态：完全混合、部分混合部分分离、完全分离状态;混合颗粒的特征速度随小颗粒质量分率的增加而减小,且在小颗粒质量分率达到0.4~0.5后其减小的趋势减缓;混合颗粒的固定床阶段和完全流化阶段的床层空隙率及混合颗粒的体积收缩比在小颗粒质量分率为0.4时达到极值.     

Chaotic characteristics of local voidage fluctuation in a circulating fluidized bed

Local voidage fluctuations have been measured by using an optical transmittance probe at various axial and radial positions in a circulating fluidized bed riser with a 0.1 m i. d. and 10 m height. The chaotic time series analysis of the local voidage fluctuations has been adopted to characterize the nonlinear dynamics of the circulating fluidized bed riser. The variations of the correlation dimension and the Kolmogorov entropy of the voidage fluctuation were found to depend on the local time-average voidage. The axial and radial distributions of the correlation dimension and the Kolmogorov entropy were strongly affected by the solids flow structures (e.g. core-annulus flow) in various operating conditions. The correlation dimension of local voidage fluctuations increases along the riser, except the position near the distributor. Both, the correlation dimension and the Kolmogorov entropy of local voidage fluctuations near the wall, were found to be smaller than those at the center of the riser, independent of the solids circulation rate and the axial position.     

Distributor effects near the bottom region of turbulent fluidized beds

The distributor plate effects on the hydrodynamic characteristics of turbulent fluidized beds are investigated by obtaining measurements of pressure and radial voidage profiles in a column diameter of 0.29 m with Group A particles using bubble bubble-cap or perforated plate distributors. Distributor pressure drop measurements between the two distributors are compared with the theoretical estimations while the influence of the mass inventory is studied. Comparison is established for the transition velocity from bubbling to turbulent regime, U_c, deduced from the pressure fluctuations in the bed using gauge pressure measurements. The effect of the distributor on the flow structure near the bottom region of the bed is studied using differential and gauge pressure transducers located at different axial positions along the bed. The radial voidage profile in the bed is also measured using optical fiber probes, which provide local measurements of the voidage at different heights above the distributor. The distributor plate has a significant effect on the bed hydrodynamics. Owing to the jetting caused by the perforated plate distributor, earlier onset of the transition to the turbulent fluidization flow regime was observed. Moreover, increased carry over for the perforated plate compared with the bubble caps has been confirmed. The results have highlighted the influence of the distributor plate on the fluidized bed hydrodynamics which has consequences in terms of comparing experimental and simulation results between different distributor plates.     

Measurement and modeling of axial distribution of adsorbent particles in expanded bed: taking into account the particle density difference

Based on the previous work (Yun et al., Chem. Eng. Sci. 59 (2004) 449-457), after the analysis of adsorbent structure, bed expansion and in-bed fluid hydrodynamics, a model was developed to describe the axial variations of the mean particle size and local bed voidage for adsorbents with a broad size distribution and different densities in expanded bed. In the model, the different-size particles were assumed to have different densities while the equal-size particles have the same density. Particle densities were described by a function that considered two parts of each particle: the resin-core part consisting of resin and inert core-particulates, and the thin resin film part without inert core-particulates. With UpFront FastLine SP adsorbents as interest test particles, the particle size and local voidage changes with bed height were measured in a 20 mm I.D. column and then predicted by the model. It was shown that the mean particle size decreased, while the local bed voidage increased with increase of bed height. These trends are similar to those for particles with a broad normal size distribution and nearly uniform density. More obvious increase trends of voidage with increase of bed height exist in expanded beds with UpFront FastLine SP particles compared with those of Streamline particles. The present model gave a good prediction of bed expansion, the mean particle sizes and local voidages in most parts of bed compared with experimental data.     

液速陡变时液固流化床中颗粒浓度的变化模型

在液速陡变时,分别考察了铅直管液固流化床内粒径为225 μm和511 μm的玻璃微珠的体积分数分布随时间变化的规律,发现大小颗粒在不同的液速变化幅度下都呈现出同样的体积分数变化趋势.联立颗粒速度和颗粒的连续方程式模拟颗粒的体积分数变化过程,建立了一个相对简单的颗粒体积分数变化的数学模型.在模型中,用定常状态的空隙率方程...     

PVC类废塑料在冷态流化床中流化行为的研究

在Φ50 mm×800 mm圆柱体的冷态流化床反应器中,对PVC类废塑料、石英砂及其混合物的流化特性进行了研究。研究了PVC颗粒粒径与混合物料中PVC质量分数对混合物料的流化特性的影响规律,得到指导热态实验的关键参数。实验结果表明,PVC颗粒粒径与混合物料中PVC质量分数会影响混合物料的最小流化速度,也影响PVC颗粒与石英砂混合的均匀度。混合物料中PVC的质量分数越小,其最小流化速度就越小,混合物料也越容易实现充分混合;PVC颗粒为Geldart B类颗粒,但由于形状不规则,黏性力大,塌落特性明显,流化性能较差,显示出C类颗粒的流化特性,同时实际的最小流化速度要大于理论最小流化速度。PVC与石英砂混合物料冷态流化行为的研究结果为热态流化床降解PVC颗粒提供了基础数据和实践依据。     

Granular temperature in a liquid fluidized bed as revealed by diffusing wave spectroscopy

We report granular temperature and solid fraction fields for a thin rectangular bed (20×200 mm cross-section and 500 mm high) of glass particles (mean diameter of 165 μm and density of 2500 kg/m³) fluidized by water for superficial velocities ranging from 0.05U_t, which is approximately double the minimum fluidization velocity, to 0.49U_t, where U_t is the particle terminal velocity estimated by fitting the Richardson-Zaki correlation to the bed expansion data. At superficial velocities below 0.336U_t, the solid fraction and granular temperature are uniform throughout the bed. At higher superficial velocities, the solid fraction tends to decrease with height above the distributor, whilst the granular temperature first increases to a maximum before decaying towards the top of the bed. Correlation of the mean granular temperature with the mean solid fraction and the local granular temperature with the local solid fraction both suggest that the granular temperature in the liquid fluidized bed can be described solely in terms of the solid fraction. The granular temperature increases monotonically with solid fraction to a maximum at φ≈0.18 where it then decreases monotonically as φ approaches the close-packed limit.     

Parametric study of fine particle fluidization under mechanical vibration

Investigations into the effects of vibration on fluidization of fine particles (4.8-216 μm average in size) show that the fluidization quality of fine particles can be enhanced under mechanical vibration, leading to larger bed pressure drops at low superficial gas velocities and lower values of u_mf. The effectiveness of vibration on improving fluidization is strongly dependent on the properties (Geldart particle type, size-distribution and shape) of the primary particles used and the vibration parameters (frequency, amplitude and angle) applied. The possible roles of mechanical vibration in fine particle fluidization have been studied with respect to bed voidage, pressure drop, agglomeration, and tensile strength of particle bed. Vibration is found to significantly reduce both the average size and the segregation of agglomerates in the bed, thus improving the fluidization quality of cohesive particles. Also, vibration can dramatically reduce the tensile strength of the particle bed. Obviously, vibration is an effective means to overcome the interparticle forces of fine powders in fluidization and enhance their fluidization quality.     

The minimum fluidisation velocity,bed expansion and pressure-drop profile of binary particle mixtures

The total bed pressure drop, the pressure-drop profile, bed expansion and bed voidage have been measured for a variety of binary particle mixtures over a wide range of gas velocities.Apparent minimum fluidisation velosities have been defined for segregating systems, and the addition of dense particles of lower minimum fluidisation velocity can cause a decrease in apparent minimum fluidisation velocity of the mixture in a very similar fashion to the addition of finer particles to larger ones of the same density.The measured u_mf s are compared with presently derived simplified theoretical equations and with equations from the literature. It is clearly shown that because of the sensitivity of u_mf determination to voidage, such relationships cannot be used with confidence. However, the empirical equation of Cheung on average follows the shape of the experimental curves well, includig those for binary systems of different density, provided the bed is in a well-mixed condition.Bed pressure-drop profiles are related to the mixing/segregation state and to the amount of fluidisation of the bed and may offer a simple indirect method of determining these conditions in practice.     

Hydrodynamic characteristics of magnetically stabilized fluidized admixture beds of iron and copper particles

Hydrodynamic characteristics of fluidized beds of pure iron (1416 μm), copper (934 μ) and their admixture (25, 50 and 75 mass %) particles when exposed to a uniform magnetic field collinear with the gas flow are investigated. Bed pressure-drop data taken as a function of increasing and decreasing gas velocities (up to about 8 m/s) for different values of magnetic-field intensity over a wide range (0 to 17 272 A/m) are employed to determine the superficial minimum bubbling and fluidization gas velocities at ambient temperature and pressure. The minimum bubbling velocity is found to increase with an increase in the value of the magnetic-field intensity, as well as with the mass fraction of magnetizable particles in the bed. These data are correlated with an empirical relation, as well as with a semi-theoretical expression. The bed voidage data are also generated and analyzed, as also the bed quality fluidization in terms of interparticle magnetic forces. These hydrodynamic properties of magnetically stabilized fluidized-bed reactors are useful in their design and operation for a variety of chemical and biochemical applications.     

Distribution of large biomass particles in a sand‐biomass fluidized bed: Experiments and modeling

The axial distribution of large biomass particles in bubbling fluidized beds comprised of sand and biomass is investigated in this study. The global and local pressure drop profiles are analyzed in mixtures fluidized at superficial gas velocities ranging from 0.2 to 1 m/s. In addition, the radioactive particle tracking technique is used to track the trajectory of a tracer mimicking the behavior of biomass particles in systems consisting of 2, 8, and 16% of biomass mass ratio. The effects of superficial gas velocity and the mixture composition on the mixing/segregation of the bed components are explored by analyzing the circulatory motion of the active tracer. Contrary to low fluidization velocity (U = 0.36 m/s), biomass circulation and distribution are enhanced at U = 0.64 m/s with increasing the load of biomass particles. The axial profile of volume fraction of biomass along the bed is modeled on the basis of the experimental findings. © 2014 American Institute of Chemical Engineers AIChE J, 60: 869–880, 2014     

Characteristics of fluidization at high pressure

Experiments were conducted to investigate fluidization fundamentals at pressures up to 6485 kPa using nitrogen as the fluidizing gas. The particles under study were coal, char and Ballotini. Both a three-dimensional bed (10.16-cm-i.d.) and a two-dimensional bed (1.9x 10.16cm) were used in the experiments. The fundamentals of high pressure fluidization examined in this study include minimum fluidization velocity, bed voidage at minimum fluidization, bed expansion, and bubbling behavior. An empirical correlation was developed for determining minimum fluidization velocity. The effects of pressure upon bed voidage at minimum fluidization and expanded bed height were analyzed for several types of particles. High speed photographs were studied to describe bubbling behavior in a fluidized bed over a range of pressures.     

CFD simulation of gas-solid bubbling fluidized bed containing FCC particles

The hydrodynamics of a bubbling gas-solid fluidized bed of 57.4 μm FCC particles was simulated by using a state-of-the-art two-fluid model integrating the kinetic theory of granular flow for particulate phase stresses. The overestimation of the bed expansion was resolved by using a suitable scale factor in the drag model as suggested by McKeen and Pugsley (T.R. McKeen, T.S. Pugsley, Powder Technol., 129, 139 (2003)). This study showed that the method was appropriate in simulation of a gas-solid fluidized bed of Geldart A particles at high gas velocities (0.3 to 0.61 m/s). The reduction of computational time especially for simulation of large-scale systems was achieved. The time-averaged local voidage was compared with the experimental data and the trend of varying several parameters on the hydrodynamic of the bed was investigated. The simulation results showed both qualitative and quantitative agreement with the literature.     

宽粒体系液固流态化随机理论数学模型

用快速切割流化床研究了宽粒级液固流态化的轴向混合和分离,测定了粒度分布、平均粒径和空隙度沿床高的变化.证实了颗粒的运动有随机性.用马尔柯夫过程建立了一个随机游走模型,描述了流化床中宽粒体系的混合和分级特性.对钛铁矿和石英砂(粒径0.04～0.40mm)进行了检验.结果表明,体系的初始流态化速度、膨胀性质、粒度分布以及平均粒径和空隙度沿床高的变化的测定结果,同模型的预测值很好符合.     

Extending the EMMS/bubbling model to fluidization of binary particle mixture: Formulation and steady-state validation

The EMMS/bubbling model originally proposed for fluidization of monodisperse particles is extended to fluidization of binary particle mixture in this study. The dense and dilute phases are considered to comprise of two types of particles differing in size and/or density. Governing equations and the stability condition are then formulated and solved by using an optimization numerical scheme. The effects of bubble diameter are first investigated and a suitable bubble diameter correlation is chosen. Preliminary validation for steady state behavior shows the extended model can fairly capture the overall hydrodynamic behaviors in terms of volume fraction of bubbles and average bed voidage for both monodisperse and binary particle systems. This encourages us to integrate this model with CFD for more validations in the future.