喷动流化床最大喷动压降的多因素影响与关联

基于截面200 mm×20 mm,高1600 mm,锥角60°的矩形喷动流化床,以二组分混合颗粒、单一组分球形颗粒及非球形颗粒为物料进行最大喷动压降的实验研究.结果表明,最大喷动压降随静止床高、颗粒密度、颗粒球形度及二组分混合颗粒体系中沉积组分分率增加而增大,随流化气速增大而减小;增大颗粒粒径或喷口宽度,呈现先减小后增...     

Prediction of heat transfer to liquid-solid fluidized beds

Heat transfer coefficients to a liquid-solid fluidized bed in a cylindrical tube have been measured using water as liquid phase and three types of cylindrical steel particles, as well as glass, nickel, copper and lead spheres of different sizes as solid phase. The independent varaibles included heat flux, liquid velocity and particle physical properties. The experimental results as well as a data bank containing a large number of measured heat transfer coefficients for solid-liquid fluidization over a wide range of operational parameters have been compared with the predictions of most published correlations. A model for the prediction of heat transfer coefficients is proposed which predicts the present experimental data and the data of other investigators with good accuracy.     

Experimental studies and empirical models for the prediction of bed expansion in gas–solid tapered fluidized beds

Studies in the expansion behaviour of tapered fluidized bed systems are important for specifying the height of the bed. Data have been obtained on the expanded heights of tapered fluidized beds and bed expansion ratios for spherical and non-spherical particles have been calculated. Based on dimensional analysis, models have been developed as a function of geometry of tapered bed, static bed height, particle diameter, density of solid and gas and superficial velocity of the fluidizing medium. The data used to derive the models cover a wide range of operating conditions, with varying fluidization velocities. Effects of static bed height, particle diameter, density, tapered angle and superficial gas velocity over minimum fluidization velocity on bed expansion ratios have been investigated experimentally. A comparison has been made between the calculated values of bed expansion ratios using proposed models and the experimental data. It has been seen that calculated values by models agree well with the experimental values. Models have also been compared with literature data of conventional bed and found its applicability at higher gas velocities with good accuracy.     

Experimental and numerical study on a bubbling fluidized bed with wet particles

As liquid bridge between particles acts an important role in the particle system, it is of considerable significance to analyze the flow hydrodynamics of wet particles in fluidized beds, which will improve the reactor design and process optimization. Thus, experimental and numerical investigations on wet particles in a bubbling fluidized bed are conducted in current work. On experimental side, particle image velocimetry (PIV) technology is employed with a designed bubbling fluidized bed. The silicone oil is used in this work because it is nonvolatile and transparent. On numerical side, a modified discrete element method (DEM) numerical method is developed by compositing an additional liquid‐bridge module into the traditional soft‐sphere interaction model. Most of the physical parameters are chosen to correspond to the experimental settings. Good agreements of particle velocity are found between the DEM simulation and PIV measurement. The performance of different liquid contents and superficial gas velocities are examined. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1970–1985, 2016     

Distribution of solid particles in liquid fluidized beds

A method is proposed for calculating the steady-state distribution of mixtures of solid particles and of the axial bed porosity in liquid fluidized beds. The extent of bed stratification is assumed to be determined by the differences among the settling velocities of the fluidized particles and the random motion of these particles that is a result of fluidization. Predictions of particle distribution based on the present method were found to agree reasonably well with experimental data.     

液固循环流化床两相流动模型

引　言流化床换热器具有防、除垢和强化传热等优点 ,在化工、食品、海水淡化、废水处理等领域具有广阔的应用前景[1].目前 ,流化床换热器历经散式流化床、内循环流化床 ,已发展到外循环流化床换热器[2 ],它要求在较稀的颗粒浓度 (颗粒浓度小于 5% )、较高的流速 ( 1～ 3ｍ·ｓ- 1)下操作 .流化床换热器中液体流动及颗粒运动状态的研究对流化床换热器的设计和操作具有重要意义 ,但人们对循环流化床换热器中颗粒运动情况的研究还很缺乏 .考虑到循环流化床换热器中的每根换热管都可作为一个独立的循环流化床对待[3].本文试图建立一滑移速度模型…     

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.     

Factors Affecting Stable Operation of Counter‐current Multistage Fluidized Bed of Solid‐liquid System — Dimensions of Downspout

For the effective use of an ion exchange resin, a counter‐current multistage fluidized bed has been developed. An appropriate design of the downspout, which works to let particles flow down to next stage, is essential to obtain a stable resin flow. From an experimental study on the particle holdup and pressure drop in the multistage fluidized bed, a method to determine the downspout size has been developed. The relation between the liquid velocity within the downspout and the superficial liquid velocity has been derived from considering the pressure balance between the part of the fluidized bed and that of the downspout.     

Prediction of the Behavior of a Liquid‐Fluidized Bed of Inert Particles Used for Separation by Density

A liquid‐fluidized bed of inert particles was used to separate a pure object from a mixture. One (binary solid‐liquid‐fluidized bed) or two (tertiary solid‐liquid‐fluidized bed) types of objects with relatively large‐sized particles were immersed in an inert‐particle bed, and the bed behavior was observed for different liquid velocities. The void fraction and apparent density of the inert‐particle suspension were predicted by considering the effect of the change in object position for different liquid velocities. The prediction method, which considers the change in the minimum fluidization velocity, accurately expressed the changes in the void fraction and the apparent density of the bed with the position of the objects in the bed. Using this method, the liquid velocity required to separate a certain kind of object from a mixture can be predicted.     

Discrete particle simulations for flow of binary particle mixture in a bubbling fluidized bed with a transport energy weighted averaging scheme

Flow behavior of small and big particles with the same particle density in a bubbling fluidized bed is modeled by a combined approach of discrete particle method and computational fluid dynamics (CFD-DPM). The collision time of a collision pair is computed by a quartic equation in which the effect of acceleration due to the different diameters is considered. A transport energy weighted averaging approach is proposed to determine the local gas velocity at a particle. The fluidization behavior of binary mixture differing in size is experimentally and numerically studied in the gas bubbling fluidized bed. The distributions of mass fraction of small and big particles along the bed height are simulated, and the profiles of the mean particle diameters of binary mixture are determined. The numerical results are in agreement with experimental data. The distributions of granular temperature, stresses, and shear viscosities of small and big particles are compared.     

压力脉动法预测硅粉颗粒最小流化速度的实验

应用压力传感器研究了不同筛分粒径的硅粉的流化性质，证实流化床层的压力脉动标准方差σp随着表观气速的增加而线性增大, 根据σp=0的条件即可确定流化床的初始流化气速Umf. 此Umf与传统压降变化法得到的实验结果基本一致. 对测得的不同筛分粒级的硅粉的Umf进行拟合，得到了Umf与相应粒级平均粒径的关联式Umf=0.014e10(d–0.28)–0.012e–10(d–0.28)+0.065. 对双粒级复配混合颗粒体系的σp进行的实验研究发现，其σp介于相关单粒级体系的σp之间，并且粗颗粒组份的比例对σp的影响较大.     

Liquid-Solid mass transfer in a slurry bubble column and a gas-liquid-solid three-phase fluidized bed

Mass transfer coefficients between particles and liquids in a slurry bubble column and a three-phase fluidized bed containing small size particles were obtained with two mass transfer systems: (1) K⁺ –Na⁺ ion-exchange in cation-exchange resin bead beds, including anion-exchange resin beads as inert particles; (2) zinc dissolution by HCl in zinc-plated glass bead beds, and in beds of non-plated glass beads. Operating parameters were gas velocity, liquid velocity, particle diameter, and particle concentration. The dependence of mass transfer coefficients on these parameters is discussed from the viewpoint of the energy supplied into the systems. Correlations of the experimental data using dimensionless groups are compared to previous correlations.     

非球形混合颗粒的沉降特征长度

提出以Stokes径和Newton径作为计算非球形颗粒终端速度的特征长度,推导出联用双径计算终端速度的方程,并将混合颗粒的粒径分布计入双径值中.理论推导作了实验验证.     

磁流化床稳定性分析

应用Foscolo的颗粒床模型分析了磁流化床稳定性,得到了磁流化床的稳定判据;根据得到的判据分别对层流和湍流情况分析了磁场对磁流化床稳定性的影响。     

Particle attrition mechanism with a sonic gas jet injected into a fluidized bed

High velocity gas jets in fluidized beds provide substantial particle attrition: they are used industrially to control the particle size in fluid bed cokers and to grind products such as toner, pharmaceutical or pigment powders. One method to control the size of the particles in the bed is to use an attrition nozzle, which injects high velocity gas and grinds the particles together. An important aspect of particle attrition is the understanding and modeling of the particle breakage mechanisms. The objective of this study is to develop a model to describe particle attrition when a sonic velocity gas jet is injected into a fluidized bed, and to verify the results using experimental data. The model predicts the particle size distribution of ground particles, the particle breakage frequency, and the proportion of original particles in the bed which were not ground. It was found that the particle breakage frequency can be used to predict the attrition results in different bed sizes. A correlation was also developed, which uses the attrition nozzle operating conditions such as gas density and equivalent speed of sound to predict the mass of particles broken per unit time.     

Particle Image Velocimetry of a Swirling Fluidized Bed at Different Blade Angles

The particle motion within a stable swirling operation regime of a fluidized bed is investigated. Particle image velocimetry was used to elaborate the hydrodynamics of multi‐sized particles fluidized at different air distributor blade angles. The experimental results revealed that the superficial velocity is the most prominent aspect affecting the hydrodynamics of a swirling fluidized bed (SFB) followed by the bed weight, particle shape, and blade inclination angle. Particles of different sizes and shapes fluidized well in the SFB which emphasizes its superiority over contemporary techniques. The slug‐wavy regime in the SFB is promising and has considerable potential, especially for diffusion‐controlled reactions and processes in the industry. The particle velocity increased with air flow rate at shallow bed height but decreased with bed weight.     

Heat Transfer between Immersed Horizontal Tubes and Aerated Vibrated Fluidized Beds

Heat transfer coeffients between an immersed horizontal tube and an aerated vibrated fluldlzed bed are measured. There is a maximum value in the h-Г experlmental curve. The heat trander coefllcient increases with decreases in particle diameter in the fully fluidized region. The particle density has less effect on the heat transfer coetftclents. High smplltude and low frequency, or low amplitude and high frequency are favorable to heat transit. Exceedingly high gas veloclty is unfavorable to the surface-bed heat transfer. A model based on the ‘pocket‘ theory was proposed for predicting the surface-to-bed heat trausfer coefllclents in fully fluldlzed region. The predlctlons from the model were compared with observed data The reasonable fit suggests the adequacy of the model.     

RADIAL DISPERSION AND BUBBLE CHARACTERISTICS IN THREE-PHASE FLUIDIZED BEDS

The effects of gas and liquid velocities, liquid viscosity and particle size on the radial dispersion coefficient of liquid phase (D_r) and the bubble properties in three-phase fluidized beds have been determined. A new flow regime map based on the drift flux theory in three-phase fluidized beds has been proposed.

In three-phase fluidized beds, D, increases with increasing gas velocity in the bubble coalescing and in the slug flow regimes, but it decreases in the bubble disintegrating regime. The coefficient exhibits a maximum value in the bed of small particles with increasing liquid velocity at lower gas velocities. However, it increases with increasing liquid velocity at higher gas velocities. In two and three-phase fluidized beds of larger particles (6,8 mm), D_r exhibits a maximum value with an increase in liquid viscosity at lower gas velocities, but it increases at higher gas velocities. The mean bubble chord length and its rising velocity increase with increasing gas velocity and liquid viscosity. However, the bubble chord length decreases with an increase in liquid velocity and it exhibits a maximum value with increasing particle size in the bed. The radial dispersion coefficients in the bubble coalescing and disintegrating regimes of three-phase fluidized beds in terms of the Peclet number in the present and previous studies have been well represented by the correlations based on the concept of isotropic turbulence theory.     

Prediction of minimum bubbling velocity, fluidization index and range of particulate fluidization for gas-solid fluidization in cylindrical and non-cylindrical beds

A uniform fluidization exists between minimum fluidization velocity and minimum bubbling velocity. Experimental investigations have been carried out for determination of minimum bubbling velocity and fluidization index for non-spherical particles in cylindrical and non-cylindrical beds. In the present paper equations have been developed for the prediction of minimum bubbling velocity for gas-solid fluidization in cylindrical and non-cylindrical (viz. semi-cylindrical, hexagonal and square) beds for non-spherical particles fluidized by air at ambient conditions. A fairly good agreement has been obtained between calculated and experimental values. Based on the experimental data it is concluded that under similar operating conditions minimum bubbling velocity and the fluidization index are maximum in case of either semi-cylindrical conduit or hexagonal conduit for most of the operating conditions and minimum in case of square one. It is further observed that the range of uniform (particulate) fluidization is maximum in case of semi-cylindrical bed for identical operating conditions.     

A study of mass transfer in a packed bed reactor by electrochemical technique

Electrochemical method was used to study the mass transfer between the solid particles and the flowing liquid in a packed bed. From the limiting current of a single active particle immersed in inactive glass particles of the same size and shape, the mass transfer coefficients can be derived.Various size and shape of packing particles were used. The experimental results indicate that smaller packing particles have higher mass transfer coefficient. In the meantime, spherical packing particles have higher mass transfer coefficients than cylindrical particles of the same equivalent diameter. However they approach each other when liquid flowing velocity is increased.The wall-effect of the reactor on mass transfer was also observed.