Solid circulation rate in a circulating fluidized bed in the presence of fine powders

Fine powders (Geldart's group C) are added to a circulating fluidized bed (CFB) of coarse particles (Geldart's group A) and the solid circulation rate (SCR) is investigated with addition of fine powders of different sizes and different fractions (different hold-ups) to the bed. Experiments were carried out in a CFB of 2 m in height and 0.052 m in diameter, using FCC catalyst particles of as the coarse particles and cohesive aluminum hydroxide powders of 0.5- as the fine powders. The effects of hold-up of fine powders in the bed, fine powders size, and superficial gas velocity on the SCR were investigated.The SCR strongly depended on the hold-up of fine powders of 0.5- in size and noticeably decreased with increasing the hold-up of fine powders under constant gas velocity. This dependency disappeared when the size of fine powders was larger than . Thus, depending on the size of fine powders added to the CFB, two distinct regions for the changes of SCR could be clearly identified.     

Density-dependent separation of dry fine coal in a vibrated fluidized bed

The main purpose of coal separation is to reduce ash, sulfur, mercury and other mineral contaminants in the coal to increase the calorific value and benefit the environment. Dry coal beneficiation has obvious advantages over the wet process although the latter is currently the predominant method in use throughout the world. A vibrated fluidized bed was constructed for separating dry fine coal particles from unwanted gangue particles. An experimental investigation of vibrational energy transmission, and the interaction between vibration and gas flow, was performed. The motivation for these experiments was a theoretical development of the principles involved in forming a dense-media vibrated fluidized bed (DMVFB). The mechanism of bubble breaking by vibration is discussed. A formula for calculating the critical vibration frequency at which bubbles can be efficiently broken and bubble formation restrained is proposed. The experimental results demonstrate that the density of a dense-media vibrated fluidized bed is uniform, with a maximum relative error of 1.68% under optimal technological and operating conditions. The < 6 mm fine coal was efficiently separated with a probable error E value of 0.07 t/m³. A lower limit of separation of 0.5 mm was achieved. The DMVFB separation efficiency is higher than wet jig with E value of 0.11 t/m³.     

Spouting of fine powder from vertically vibrated bed

Experimental and numerical studies of spouting of powder from vertically vibrated bed are performed. The powder flows out vigorously through a side orifice of the vertically vibrated vessel in which the powder is contained, provided that the vibration acceleration is greater than the gravitational one. The mechanism of the efflux and the relationship between the efflux rate and the vibration condition are examined. The interstitial air pressure in the bed is measured and is compared to the numerical analysis considering the relative motion of the powder bed to the vibrated vessel and percolation air flow. The outflow behavior of powder is observed by using a high-speed video camera. The experimental and the numerical results show that the powder spouts out intermittently with periodic air outflow, which is generated in response to the change of the gap between the bed and the vessel base. Furthermore, it is found that the efflux rate of powder is in proportion to the generated air pressure and in inverse proportion to the vibration frequency.     

Dry beneficiation of fine coal deploying multistage separation processes in a vibrated gas-fluidized bed

In this study, multistage separation processes were introduced into fine coal beneficiation in a vibrated gas-fluidized bed in order to improve the separation performance. The changes of coal properties, operational parameters, and density segregation characteristics during multistage separation were systematically studied. The results showed that with an increase in the separation stage, the range of density was narrowed down and the required input energy decreased. The results showed that with the same yield, the ash content of clean coal decreased from 12.53% for single separation stage to 8.91% for three separation stages, indicating a significant improvement on separation accuracy.     

Synthesis of fine powders of KTP-group compounds of stoichiometric composition

Precipitation of crystalline monophase nanopowders (50-100 nm in diameter) of stoichiometric binary orthophosphates of titanyl and alkaline metal in aqueous solutions has been studied. It has been shown that, in the formation of those powders, titanyl hydrophosphate can be used as a precursor. Rising temperature and alkaline ion concentration in the solution decrease the size of the powder grains, while elongating the exposure of the synthesized solid phase in the suspension increases grain sizes. An effective technique to synthesize fine-dispersed powders of monophase stoichiometric binary orthophosphates of titanyl and alkaline metal has been developed.     

Preparation of barium hafnate fine powders by oxalate method

Barium hafnate (BaHfO₃) precursor was prepared by mixed barium nitrate and hafnium (IV) chloride with ammonium oxalate aqueous solution. Barium hafnate powders were obtained by decomposition of the precipitate precursor at 800 °C for 2 h. As a dispersant, polyethylene glycol (PEG6000) was used in the precipitation reaction and the influence of dispersant dosage on particle size was studied. X-ray diffraction analysis (XRD) showed the powders was pure cubic BaHfO₃ without impurities. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) demonstrated that the size distribution was uniform, and the average grain size was about 50 nm.     

Mass transfer in shallow aerated vibrated powder beds

Mass transfer rates are reported for granular beds (depths = 24 and 1 mm) aerated by upward flow of nitrogen both in absence of vibration and also subjected to vibration at intensities affording maximum accelerations = two and four times gravity. Nitrogen flows were both below and above minimum gas-fluidizing velocities in absence of vibration.At both 24- and 1-mm bed depths, vibration at intensity four times gravity can enhance mass transfer by a factor between about 1.4 and 2. For mass transfer, bed depth is an important variable, with or without vibration. Without vibration, mass transfer at 1-mm depth can be ∼ 10 to 15 times greater than transfer in a 24-mm bed. A correlation is available that deals successfully with influence of bed depth in our data, both with and without vibration.Data reported herein at 1-mm depth bear upon performance of a chemical microreactor with cross-flow of gaseous reactants through particles in the coherent-expanded (CE) vibrated-bed state. The data suggest that a mass-transfer limitation need not ordinarily be taken into account in analysis of microreactor kinetic data for a chemical reaction for which a fluid bed reactor is an appropriate commercial choice.     

Mathematical modelling of temperature induced moisture migration in bulk powders

This paper reviews the importance of temperature gradient induced moisture diffusion in powder beds. Redistribution of moisture by this mechanism is important for a wide variety of products, especially those which are crystalline in nature. Moisture transfer in these products can result in very high local water activity and lead to microbial spoilage and caking problems. A conceptual model was developed for heat and moisture transfer due to the application of thermal gradients across a one-dimensional powder bed. The validity of the key assumptions in the model formulation were demonstrated and a mathematical model was derived and solved numerically. The model was validated against experimental data collected using crystalline lactose powders as a working example.     

Neutron adsorption performance of Dy2TiO5 materials obtained from powders synthesized by the molten salt method

Dy₂TiO₅ powders were synthesized by molten salt and solid-state methods. The influences of molten medium on phase compositions and microstructures were analyzed. The addition of molten salt lowered significantly the synthesis temperature and resulted in uniform powders. Green bodies compacted from the prepared powders were pressureless sintered at 1600 °C. Sinterability, mechanical properties and neutron absorption performance of the sintered pellets were studied. Results showed that molten salt synthesis resulted in materials with higher fracture toughness and bending strength, excellent hardness and neutron adsorption performance compared to the solid-state process. The neutron absorption rate reached 86.6% for 8 cm thick pellets.     

Effect of inclination on gas-fluidized beds of fine cohesive powders

Results are presented of an experimental investigation on how bed inclination affects the fluidization and sedimentation behavior of fine cohesive particles. In contrast with the expected Geldart C behavior, and due to self-agglomeration, these fine particles are uniformly fluidized by gas in a vertically oriented bed, displaying a fluid-like regime and expanding smoothly as the gas velocity is increased. When the gas flow supply to the bed is suddenly stopped, the initial sedimentation velocity of the vertically oriented bed is similar to the fluidizing gas velocity as corresponds to uniform fluidization. The main effect of inclination is to induce fluidization heterogeneity. The local gas velocity increases in the adjacent region to the upper wall at the expense of the region adjacent to the lower wall. This situation anticipates the onset of local bubbling in the region adjacent to the upper wall. Meanwhile the region adjacent to the lower wall remains in a solid-like state and does not reach the fluid-like state until values of the gas flow are applied much higher than those needed in a vertical fluidized bed. As a consequence, the expansion and fluidization uniformity of the tilted bed are hindered. If the gas supply to the inclined bed is suddenly stopped, and because of induced heterogeneity, sedimentation takes place at a decreased rate as compared with sedimentation velocity in the uniformly fluidized vertical bed.     

Confirmation of Faraday's explanation of bunkering in vibrated granular beds

Faraday, like Chladni earlier, saw a shallow layer of a fine powder collect in a circular heap (a ‘minibunker’) at an antinode of a vibrating plate. He saw ‘Faraday circulation’: powder motion in a shallow surface layer of the heap, downward from its center to its edge; inward motion centerward at the edge; by inference, motion upward toward the middle of the heap's interior. What he saw led him to postulate formation of a partial vacuum beneath the powder heap, creating (1) an external wind blowing the powder toward an antinode and (2) a flow of air inward from the heap's edge, driving powder toward its center. Although some experimentalists have recently advanced alternative explanations of vibrated-bed heaping (‘bunkering’), we are able to confirm the essentials of Faraday's thought. At suitable amplitude and frequency, vertical sinusoidal vibration of a fine-powder bed causes it, in each vibration cycle, to experience a free-flight interval during which pressure gradients in its interior drive powder centerward. A center-high bunker forms, displaying Faraday circulation. When bed-floor collision terminates flight, pressure gradients reverse direction; but passage of a compaction front has locked particles against further movement. Before a next flight interval, an increase in porosity will reverse the compaction that accompanied heap-floor collision. In particles 177-μm and larger, we see the compaction front cinematographically; in 88-μm particles, we infer it from floor-pressure data. Although, given sufficient time, a bed of large particles (e.g., 707 μm) will form a bunker, it displays the wall-friction-driven circulation elucidated by Muchowski, not the Faraday pattern.     

Numerical simulation of cohesive particle motion in vibrated fluidized bed

The effect of vibration on the cohesive particle motion in the bed was examined by using discrete element method(DEM). The bed of dried fine particles was treated as the objective of calculation. The van der Waals force was used as the cohesive force because the van der Waals force was considered to be the main cohesive force in this case. Since the actual calculation time was too long, the fine cohesive particle was difficult to be treated. So a relatively large particle (1.0 mm in diameter) was used in calculation and the van der Waals force was assumed that the ratio of gravity force to van der Waals force of particle used in this calculation was equal to that of a fine particle (6.0 μm in diameter), to express the effect of van der Waals force significantly. The calculation results were compared with that case of cohesionless particle.In the case with vibration, the cohesive particle motion in the bed is observed, though no fluidization state appears in the case without vibration, and there is no bubble in the bed even the fluidization state. In the case of cohesive particle, the collision energy between particle and wall caused by vibration gap propagates from the bottom to top of bed, and the particle moves vigorously at the top of bed in the case with vibration. As the vibration gap increases, the effect of vibration on the cohesive particle motion becomes larger, i.e., the low vibration frequency at the same vibration strength or the large vibration strength at the same vibration frequency promotes the fluidization of the bed.     

Experimental investigations of granular temperature in vertical vibrated beds

Granular temperature is a key property in granular flows, however, this topic has not been fully understood due to many research difficulties. The studies about granular temperature in vibrated beds are relatively few. This study presents experimental results to investigate the role of granular temperatures in vertical vibrated beds. The image processing technology and particle tracking method were employed in a series of experiments in vibrated granular beds, dry or with small amount of liquids. We briefly review one of our earlier published experimental works dealing with granular temperature in dry granular beds influenced by vibrational conditions. The granular temperature increases with the increasing vibrational acceleration and vibrational velocity. The other new experimental results of granular temperatures in a wet vibrated bed are also presented and compared with dry systems. The granular temperature decreases with increasing viscosity of the added liquid and increasing liquid amount.     

Prediction of minimum fluidization velocity for vibrated fluidized bed

The prediction of minimum fluidization velocity for vibrated fluidized bed was performed. The Geldart group A and C particles were used as the fluidizing particles. The method based on Ergun equation was used to predict the minimum fluidization velocity. The calculated results were compared with the experimental data.The calculated results of minimum fluidization velocity are in good agreement with experimental data for Geldart group A particles. For group C particles, the difference between the calculated results and experimental data is large because of the formation of agglomerates. In this case, the determination of agglomerate diameter is considered to be necessary to predict the minimum fluidization velocity.     

Mixing in vibrated granular beds with the effect of electrostatic force

In this study, the DEM (Discrete Elementary Method) is used to simulate the behavior of granular mixing in vibrated beds. First, the velocity fields are simulated by the DEM model to examine the convective currents in a three-dimensional vibrated granular bed. Then, in order to characterize the effect of electrostatic force on the granular flow, the electrostatic number Es is defined as the ratio of the electrostatic force to the particle weight. Also, to quantify the quality of mixing, the mixing degree M is used by the well-known Lacey index. The top–bottom and the side–side initial loading patterns of two groups of glass bead with different colors are employed to investigate the influence of the convective currents on the granular mixing. To simplify the electrostatic effect, these two groups of glass beads are given opposite charges and the charged strength is assumed to be constant. The simulation results demonstrate that the granular temperatures increase linearly with the increasing Es number. Meanwhile, the mixing rate constants, calculated from the time evolutions of mixing degree, increase with the increasing Es number in power law relations. The role of granular temperature in the granular mixing is also discussed in this paper.     

Synthesis of fine and high purity Cr2AlC powders by novel method

Cr₂AlC powders using Cr/Al/C and Cr/Al/Cr₃C₂ systems as raw materials were successfully synthesised by a microwave hybrid heating method for the first time. The mixtures of Cr, Al and graphite or Cr₃C₂ with different molar ratios were used to investigate the formation of Cr₂AlC phase. For Cr/Al/C with the molar ratios of 2:(1.0–1.2):1 system, Cr₂AlC with a small amount of Cr₇C₃ and Cr₂Al was synthesised at 1000°C for 3 min, and the average particle size was ?1?μm. For Cr/Al/Cr₃C₂ with the molar ratio of 1:2:1 system, high purity Cr₂AlC powders was synthesised at 1000°C for 3 min, and the average particle size was ?1?μm. The synthesis of high purity Cr₂AlC powders for short time was attributed to the combination of the hybrid heating effect and the introduction of Cr₃C₂ as carbon source. Microwave hybrid heating is a promising method for the preparation of various other MAX phases.     

Circulation of particles in a vibrated bed with an inner tube

The particle motion in a vibrated bed with an inner tube was simulated by the discrete element method (DEM). A height difference is observed in the vibrated particle bed between the interior and annulus of the inner tube. The bed height difference is strongly affected by the ratio of the cross section at the interior to that at the annulus of the inner tube. When the inner tube is immersed in the particle bed, the bed height difference causes the circulation of particles in the bed. The direction and velocity of particle circulation can be controlled by changing the inner tube diameter and the circulation velocity is also controlled with vibration conditions.     

Mixing in a vibrated granular bed: Diffusive and convective effects

In this study, Discrete Elementary Method (DEM) is employed to simulate the motion and mixing behavior of granular materials in a three-dimensional vibrated bed, which is energized by vertical sinusoidal oscillations under different vibrating conditions. With frictional sidewalls, the convection flow is a very important phenomenon in the vibrated granular bed. The influence of vibrating conditions, including vibration acceleration and frequency, on the formation of symmetric convection flow is investigated in this study. In order to characterize the convective flow and the diffusive motion of granular materials, the dimensionless convection flow rate, J_conv, and the vertical self-diffusion coefficient, D_yy, are defined, respectively. Péclet number, Pe, is employed to characterize the ratio of the convective flow to the diffusive motion in vertical direction. The role of Pe in the formation of symmetric convection flow is discussed in detail. Moreover, the top-bottom initial loading pattern of two groups of glass beads with different colors is employed to investigate mixing behavior of granular materials. The well-known Lacey index is employed as the mixing degree, M, to quantify the mixing quality. The mixing rate is calculated from a least-square fit using the time evolution of M. The simulation results demonstrate that the mixing rates increase with increasing J_conv and D_yy in exponential relations.     

SiAlON粉体合成的研究进展

概述了SiAlON材料的组成、结构特点、理化性能和应用领域 ,全面阐述了SiAlON粉体的合成方法以及由天然原料合成SiAlON粉体的研究进展     

Segregation of cohesive powders in a vibrated granular bed

The effects of small amounts of added liquid on the segregation behavior of a granular system under vertical vibration by DEM simulation are investigated in this study. The cohesive forces of grains are incorporated into DEM simulations via a simplified dynamic liquid bridge force model. The simulation results show that capillary forces in addition to viscous forces have an important effect on the segregation phenomenon. The segregation rate of larger intruder rises to the top of the bed is found to depend on the liquid content. The segregation rate is sharply increased when a small amount of liquid is added to granular system. A transition to the reduction of segregation rate occurs at a critical liquid content. It has shown that this transition can be interpreted as the increase of attractive force between grains due to viscous force. The viscous forces make the particles stick more tightly to each other and retard the movement of particles, thus reducing the segregation rate. The segregation rate is also related to the convection motion of the granular system. The presence of convection enhances the segregation rate of wet granular materials.