A study of agglomeration of coal-ash in fluidized beds

Experimental data on agglomeration of coal-ash particles in a fluidized bed have been presented. It has been observed that above the “initial sintering temperature”, the ash particles are defluidized at velocities above their minimum fluidization velocity. The expression for minimum fluidization velocity has been modified by including a force-term due to the phenomenon of sintering in addition to those due to drag, gravity and kinetic energy.     

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.     

Hydrodynamics of gas–solid fluidization in tapered beds

Gas–solid fluidization has a wide range of industrial applications like catalytic reactions, combustion, gasification, etc. In a number of these applications, there is particle size reduction during the operation leading to severe entrainment and limitation of operating velocity. The various problems associated with particles of different sizes or changing particles sizes could be overcome by adopting tapered beds in fluidization operation. In the present investigation, the fluidization phenomenon in tapered beds has been critically assessed through experimental investigations using particles of different sizes and materials and wide range of apex angles of the vessels. The effect of particle size and apex angle on the fluidization behaviour is clearly brought out which has not been reported so far in literature. The importance of compressive force existing in tapered beds is highlighted. In addition, correlations for all hydrodynamic characteristics, viz. critical fluidization velocity, minimum velocity for full fluidization, maximum velocity for defluidization, peak pressure drop, fluctuation ratio, compressive force, and hysteresis have been developed some of which are proposed for the first time.     

Fluidization of dried wastewater sludge

Experimental measurements were conducted of the minimum fluidization velocities and the entrainment (terminal) velocities for beds of dry, digested wastewater sludge particles. With the use of air, experiments were performed in an 80 mm i.d. glass column with carefully sieved, very narrow fractions of particles in a wide range of 0.65-5.3 mm. Physical examinations of the dried sludge particles revealed their irregular, mostly isometric shapes, their porous character, and their fibrous nature. New correlations, which cover the transitional flow region, have been developed to estimate the conditions of incipient fluidization and those of entrainment for such peculiar materials. Their explicit forms also enable the direct computation of the particle size corresponding to a chosen terminal velocity and the straightforward calculation of the terminal velocity of a given, irregularly shaped particle.     

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.     

Measurement of particle concentration in a Wurster fluidized bed by electrical capacitance tomography sensors

It is essential to measure and monitor the particle flow characteristics in a Wurster fluidized bed to understand and optimize the coating processes. In this article, two electrical capacitance tomography (ECT) sensors are used to measure the particle concentration in different regions in a Wurster fluidized bed for the “cold” particle flows. One ECT sensor has a 12‐4 internal‐external electrodes and another has eight electrodes. The 12‐4‐electrode ECT sensor is used to measure the particle concentration in the annular fluidization region (outside of the Wurster tube) and the eight‐electrode ECT sensor is used to measure the particle flow in the central region (inside the Wurster tube). The effect of particle type, particle moisture, fluidization velocity, and geometrical parameters on the Wurster fluidization process is studied based on the two ECT measurements. The radial particle concentration profiles in the annular fluidization and central flow regions with different operation parameters are given. Fast Fourier Transform analysis of the particle concentration in the Wurster tube is performed with different superficial air velocities. The optimum operating ranges of the Wurster fluidization process for different particles are given. In the end of the article, computational fluids dynamics simulation results are given and used to compare with the measurement results by ECT for a typical Wurster fluidized bed. © 2014 American Institute of Chemical Engineers AIChE J 60: 4051–4064, 2014     

Fluidization characteristics of oil palm frond particles in agitated bed

Fluidization characteristics of crushed oil palm fronds were studied. The elongated shape of the particles and their fibrous nature created entanglement between the particles and caused the bed to form crack and plug flow when aerated in ordinary fluidized bed. Fluidization of the fibres became feasible with the aid of mechanical agitation. Agitation helped to loosen the entanglement of the fibres which prevents air to pass through the bed of particles, as a result, fluidization state could be attained. Experiments were carried out in a column with height of 72 cm and ID of 14.4 cm. Superficial air velocities used ranged from 0.1 to 1.1 m/s, bed heights ranged from 4 to 8.5 cm, agitation speeds ranged from 300 to 500 rpm and particle initial moisture contents from 0.5 to 2.4 g water/g dry solids. Analysis of the fluidization characteristics showed that minimum fluidization velocity was independent with bed height and agitation speed. However, investigation on the effect of particle initial moisture content showed that minimum fluidization velocity increased with particle moisture content. A new empirical correlation to predict minimum fluidization velocity has been derived which gives good agreement with experimental data in this study and the data from other study in the literature.     

Simulation and experiment of segregating/mixing of rice husk–sand mixture in a bubbling fluidized bed

The fluidization behavior of rice husk–sand mixture in the gas bubbling fluidized bed is experimentally and theoretically studied. The relevancy of the pressure drop profile of rice husk–sand mixture to the definition of its minimum fluidization velocity is discussed, and the minimum fluidization velocity of rice husk–sand binary mixture is determined. The distributions of mass fraction of rice husk particles along the bed height are measured, and the profiles of the mean particle diameter of mixture are determined. A multi-fluid gas–solid flow model is presented where equations are derived from the kinetic theory of granular flow. Separate transport equations are constructed for each of the particle classes, allowing for the interaction between particle classes, as well as the momentum and energy are exchanged between the respective classes and the carrier gas. The distributions of the mass fraction of rice husk particles and the mean particle diameter of binary mixture are predicted. The numerical results are analyzed, and compared with experimental data.     

稻草及热解半焦颗粒流化特性

研究了稻草及不同温度热解半焦颗粒在内径100 mm、高1000 mm的有机玻璃流化床中的流化特性. 结果表明,稻草颗粒无法单独流化,而其热解半焦颗粒可单独流化;半焦颗粒的最小流化速度随粒径增大而增大,与床层高度无关,筛分粒径为0.45~0.9, 0.18~0.45, 0.125~0.18 mm的半焦颗粒的最小流化速度分别为0.19, 0.16, 0.14 m/s;300~550℃温度范围内稻草热解半焦颗粒的流化特性无明显区别;半焦与稻草颗粒混合流化时,稻草颗粒不大于20%(w)时床层有较好的流化质量,混合颗粒的最小流化速度都随混合颗粒中稻草含量增大而增大.     

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.     

Effective particle diameters for simulating fluidization of non‐spherical particles: CFD‐DEM models vs. MRI measurements

Computational fluid dynamics—discrete element method (CFD‐DEM) simulations were conducted and compared with magnetic resonance imaging (MRI) measurements (Boyce, Rice, and Ozel et al., Phys Rev Fluids. 2016;1(7):074201) of gas and particle motion in a three‐dimensional cylindrical bubbling fluidized bed. Experimental particles had a kidney‐bean‐like shape, while particles were simulated as being spherical; to account for non‐sphericity, “effective” diameters were introduced to calculate drag and void fraction, such that the void fraction at minimum fluidization (ε_mf) and the minimum fluidization velocity (U_mf) in the simulations matched experimental values. With the use of effective diameters, similar bubbling patterns were seen in experiments and simulations, and the simulation predictions matched measurements of average gas and particle velocity in bubbling and emulsion regions low in the bed. Simulations which did not employ effective diameters were found to produce vastly different bubbling patterns when different drag laws were used. Both MRI results and CFD‐DEM simulations agreed with classic analytical theory for gas flow and bubble motion in bubbling fluidized beds. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2555–2568, 2017     

The use of peat granules in a fluidized bed bioreactor

This study describes the particle characteristics and fluidized hydrodynamics of peat granules. Peat granules, moistened with water, are a potential packing material in a gas–solid fluidized bed bioreactor used for treating air pollution. Information on the fluidization of wet peat granules is lacking. In order to advance this new type of bioreactor and to scale up its design for industrial use, fluidization studies of suitable packing material are required. Using abiotic experiments, three sizes of peat granules have been fluidized with air and fluidization characteristics were observed at different superficial gas velocities. Relative to other biomass particles, peat granules have a high particle density and sphericity, which contributes to favourable fluidization behaviour, without gas channelling. Fluidization experiments demonstrate that as the mean size of peat particles increased, minimum fluidization velocity increased. Increasing the moisture content of the peat granules resulted in a transition from bubbling bed fluidization to poor fluidization behaviour. Other types of moist biomass particles such as sawdust are difficult to fluidize and typically exhibit Geldart group C behaviour. In contrast, it was observed that wet peat granules could be fluidized in a bubbling bed regime, typical of group B particles.     

CFD study of the minimum bubbling velocity of Geldart A particles in gas-fluidized beds

The minimum bubbling velocity, which demarcates the homogeneous and heterogeneous fluidization regimes, plays a pivotal role in gas fluidization of Geldart A particles. We systematically study the effect of gas and particle properties on the minimum bubbling velocity of Geldart A particles in gas-fluidized beds using both Eulerian–Eulerian and Eulerian–Lagrangian models. We find that the minimum bubbling velocities as obtained from the simulations are in reasonable agreement with the well-known experimental correlation of Abrahamsen and Geldart (Powder Technology, 1980, 26: 35–46). To our best knowledge, this is the first time that the minimum bubbling velocity is correctly predicted by Eulerian–Eulerian models, without using an artificial ad-hoc modification of the gas–solid interaction force. Furthermore, we have performed a systematic investigation into the effect of the specific method that is used for determining minimum bubbling velocity. Our simulations show that the minimum bubbling velocity that would be obtained from the simulated bed contraction is larger than the one obtained from visual observation, which in its turn exceeds the one obtained from sudden change of standard deviation of pressure drop. We find that the abrupt change of the granular temperature with increasing superficial gas velocity may be a more suitable indicator for identifying the onset of heterogeneous fluidization.     

Fluidization behavior in a circulating slugging fluidized bed reactor. Part I: Residence time and residence time distribution of polyethylene solids

Square nosed slugging fluidization behavior in a circulating fluidized bed riser using a polyethylene powder with a very wide particle size distribution was studied. In square nosed slugging fluidization the extent of mixing of particles of different size depends on the riser diameter, gas velocity, hold up and solids flux in the riser. Depending on the operating conditions the particle residence time distribution of a riser in the slugging fluidization regime can vary from that of a plug flow reactor to that of a well-mixed system.Higher gas velocities cause shorter particle residence times because of a significant decrease in the hold-up of particles in the riser at higher gas velocities. A higher solids flux also shortens the average residence time. Both influences have been quantified for a given polyethylene-air system.Residence time and residence time distribution were determined for different particle size and the influence of gas velocity, solids flux, hold up and riser diameter was studied. When comparing data from segregation and residence time experiments it is clear that segregation data can predict the spread in residence time as a function of overall residence time, particle size and gas velocity. The differential velocity between small and large particles found in the segregation experiments can predict the spread in residence time as found in the residence time distribution experiments with a powder with a broad particle size distribution. Raining of particles through the slugs was studied as a function of plug length, gas velocity and pulse length. It was found that raining is not the determining mechanism for segregation of particles.     

几种典型铁矿粉的流化特性

利用内径150 mm的D型有机玻璃流化床模型,对澳矿、巴西矿、北方矿和钒钛矿典型铁矿粉的流化特性进行了实验研究,获得了其流化特性曲线、初始流化速度和最大床层压降,并将初始流化速度的实测值和理论计算值进行了比较分析. 结果表明,矿粉粒度是影响其流化特性的最主要因素,粒径越大,床层所需要的初始流化速度越大,实测值和理论估算值基本相符;粒度小于0.125 mm钒钛矿流动性较差,在流化过程中易出现沟流现象;粒度范围较宽的矿粉,完全流态化时,细矿粉随气流夹带逸出明显;在粒度相同的情况下,几种不同的铁矿粉的开始流化速度接近,而床层压降有较大差异,巴西矿的床层压降明显大于其他三种铁矿粉. 最大床层压降的最小值均出现在粒度为0.25~0.425 mm,为铁矿粉流态化还原过程中较适宜的粒度范围.     

The effect of cohesive forces on the fluidization of aeratable powders

The effects of cohesive forces of van der Waals type in the fluidization/defluidization of aeratable type A powders in the Geldart classification are numerically investigated. The effects of friction and particle‐size distribution (PSD) on some design‐significant parameters, such as minimum fluidization and bubbling velocities, are also investigated. For these types of particles, cohesive forces are observed as necessary to fully exhibit the role friction plays in commonly observed phenomena, such as pressure overshoot and hysteresis around minimum fluidization. This study also shows that a full‐experimental PSD consisting of a dozen particle sizes may be sufficiently represented by a few particle diameters. Reducing the number of particle types may benefit the continuum approach, which is based on the kinetic theory of granular flow, by reducing computational expense, while still maintaining the accuracy of the predictions. Published 2013 American Institute of Chemical Engineers AIChE J 60: 473–484, 2014     

Discrete particle simulation of gas fluidization of ellipsoidal particles

Fluidization is widely used in industries and has been extensively studied, either experimentally or theoretically, in the past decades. In recent years, a coupled simulation approach of discrete element method (DEM) and computational fluid dynamics (CFD) has been successfully developed to study the gas–solid flow and heat transfer in fluidization at a particle scale. However, to date, such studies mainly deal with spherical particles. The effect of particle shape on fluidization is recognized but not properly quantified. In this paper, the CFD–DEM approach is extended to consider the fluidization of ellipsoidal particles. In the simulation, particles used are either oblate or prolate, with aspect ratios varying from very flat (aspect ratio=0.25) to elongated (aspect ratio=3.5), representing cylinder-type and disk-type shaped particles, respectively. The commonly used correlations to determine the fluid drag force acting on a non-spherical particle are compared first. Then the model is verified in terms of solid flow patterns. The effect of aspect ratio on the flow pattern, the relationship between pressure drop and gas superficial velocity, and microscopic parameters such as coordination number, particle orientation and force structure are investigated. It is shown that particle shape affects bed permeability and the minimum fluidization velocity significantly. The coordination number generally increases with aspect ratio deviating from 1.0. The analysis of particle orientations shows that the bed structures for ellipsoids are not random as that for spheres. Oblate particles prefer facing upward or downward while prolate particles prefer horizontal orientation. Spheres have the largest particle–particle contact force and fluid drag force under the comparable conditions. With aspect ratio deviating from 1.0, particle–particle interaction and fluid drag become relatively weak. The proposed model shows a promising method in examining the effect of particle shape on different flow behaviour in gas fluidization.     

Computational investigation of the mechanisms of particle separation and “fish‐hook” phenomenon in hydrocyclones

The motion of solid particles and the “fish‐hook” phenomenon in an industrial classifying hydrocyclone of body diameter 355 mm is studied by a computational fluid dynamics model. In the model, the turbulent flow of gas and liquid is modeled using the Reynolds Stress Model, and the interface between the liquid and air core is modeled using the volume of fluid multiphase model. The outcomes are then applied in the simulation of particle flow described by the stochastic Lagrangian model. The results are analyzed in terms of velocity and force field in the cyclone. It is shown that the pressure gradient force plays an important role in particle separation, and it balances the centrifugal force on particles in the radial direction in hydrocyclones. As particle size decreases, the effect of drag force whose direction varies increases sharply. As a result, particles have an apparent fluctuating velocity. Some particles pass the locus of zero vertical velocity (LZVV) and join the upward flow and have a certain moving orbit. The moving orbit of particles in the upward flow becomes wider as their size decreases. When the size is below a critical value, the moving orbit is even beyond the LZVV. Some fine particles would recircuit between the downward and upward flows, resulting in a relatively high separation efficiency and the “fish‐hook” effect. Numerical experiments were also extended to study the effects of cyclone size and liquid viscosity. The results suggest that the mechanisms identified are valid, although they are quantitatively different. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

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。     

Fluidization dynamic characteristics of carbon nanotube particles in a tapered fluidized bed

In this work, a tapered fluidized bed (TFB) without a distributor for fluidizing carbon nanotube (CNT) was applied for improving the dead zone, blockage, and fracture of distributor, which occurred in actual production. Experiments were performed under different superficial gas velocities, static bed heights, CNT agglomerate size, and positions of pressure probe. To obtain multi-perspective and multi-scale understanding of fluidization dynamics of gas–CNT flow in the TFB without a distributor, the standard deviation, skewness, kurtosis, wavelet decompositions and homogeneous index analysis methods were adopted. Some noticeable phenomena were observed. Particle movements including inter-particle, gas–particle and particle–wall dominate dynamic characteristics. The amplitudes of pressure fluctuations of coarse agglomerated multi-walled CNT were more sensitive to the gas velocity than that of fine agglomerated multi-walled CNT. The sensitively of energy contribution of the meso- and macro-structures was that the sensitivity of the measured position was less than the sensitivity of the energy contribution by the changes of particle size, and the sensitivity of the energy contribution by the changes of particle size was less than the energy contribution by the changes of gas velocity. The fluidization quality of coarse agglomerated multi-walled CNT was better than that of fine agglomerated multi-walled CNT, which was verified by the skewness and wavelet analysis.