Fluidization Characteristics of a Fine Magnetite Powder Fluidized Bed for Density-Based Dry Separation of Coal

Gas-solid fluidized bed separation technique is very beneficial for saving water resources and for the clean utilization of coal resource. The hydrodynamics of 0.15–0.06 mm fine Geldart B magnetite powder were experimentally and numerically studied to decrease the lower size limit. The results show that the static bed height should be controlled near 300 mm (e.g., 300–350 mm). The bubble size, amount, and frequency of the fine particle bed are smaller than those of the bed containing 0.3–0.15 mm large Geldart B particles, thus leading to a higher bed activity. The pressure drop and density of the fine particle bed are uniform and stable, which indicates a good fluidization quality. Furthermore, simulated results are consistent with experimental data, which indicates the correctness and effectiveness of the simulations. The superficial gas velocity should be adjusted to not more than 1.8U _mf for the fine particle bed. Additionally, wide size range magnetite powder, which contains 94.23 wt% < 0.3 mm particles with a 0.3–0.06 mm particles content of 91.38 wt%, was used in an industrial scale modularized demonstration system for 50-6 mm coal density separation. The ash content of feed coal was reduced from 55.35% to 14.67% with a probable error, E, value of 0.06 g/cm^3.     

混合物入选体积量对振动流化床分离特性的影响

利用计算机在线测量床层密度及分析颗粒在床层中的分布,考察入选体积量对流化床中床层密度及颗粒分离效率的影响。研究表明,当入选体积量的分选上限为２０％时,流化床才能对颗粒混合物进行有效的分选,并且在振动条件下比在非振动条件下的分选效率略高一些,这些研究结果对有振动重介质流化床分离细粒焦渣混合物具有一定的指导意义。     

Experimental and numerical simulation studies of the fluidization characteristics of a separating gas-solid fluidized bed

Gas-solid fluidized bed separation expands the choices of highly efficient dry coal beneficiation methods. The hydrodynamics of 0.3-0.15 mm large Geldart B magnetite powder were studied using a combination of experimental and numerical methods to optimize the design of the solid medium used in the fluidized bed. The results show that the Syamlal-O'Brien drag model is suitable for simulating the bed and it is verified that simulated and experimental results are consistent with each other. If the static bed height is no more than 300 mm then the bed height has minimal effect on the fluidization characteristics. As the superficial gas velocity increases the bed activity is improved. However, at the same time the uniformity and stability of the bed drop. Therefore, the gas velocity should be adjusted to no more than 2.0U_mf. The density of the Geldart B bed is uniform and stable, which indicates a relatively high fluidization quality. Furthermore, compounded medium solids consisting of < 0.3 mm magnetite powder with a 0.3-0.15 mm particle content of 65.25% and < 1 mm fine coal were used in a pilot gas-solid fluidized bed of 5-10 ton/h capacity. The pilot bed was used to separate 50-6 mm coal. This test resulted in the coal ash content being reduced from 23.74% to 11.79% with a probable error, E, of 0.07 g/cm³ and a recovery efficiency of 98.26%. This indicates that the bed has good separating performance. Nevertheless, to increase the applicability of the separating bed a further study emphasizing a decrease in the lower size limit of the magnetite powder should be performed.     

Separation characteristics for fine coal of the magnetically fluidized bed

Dry beneficiation technology with an air dense medium fluidized bed is briefly introduced. Systems for the dry separation of fine coal consisting of a magnetically fluidized bed (MFB) as separation medium are described. The magnetic stabilization produces homogeneity and stability of the fluidized bed and thus enhances the separation efficiency by preventing remixing of the feedstock to be separated. Experiments have been conducted in a 100-mm inner diameter cylinder and results show that a uniform and stable MFB can be formed and fine coal (<6 mm size) can be separated efficiently.     

Experimental study and simulation of vibrated fluidized bed drying

The paper addresses numerical simulation for the case of convective drying of seeds (fine-grained materials) in a vibrated fluidized bed, analyzing agreement between the numerical results and the results of corresponding experimental investigation. In the simulation model of unsteady simultaneous one-dimensional heat and mass transfer between gas phase and dried material during drying process it is assumed that the gas-solid interface is at thermodynamic equilibrium, while the drying rate (evaporated moisture flux) of the specific product is calculated by applying the concept of a “drying coefficient”. Mixing of the particles in the case of vibrated fluidized bed is taken into account by means of the diffusion term in the differential equations, using an effective particle diffusion coefficient. Model validation was done on the basis of the experimental data obtained with narrow fraction of poppy seeds characterized by mean equivalent particle diameter (d_S,d = 0.75 mm), re-wetted with required (calculated) amount of water up to the initial moisture content (X₀ = 0.54) for all experiments. Comparison of the drying kinetics, both experimental and numerical, has shown that higher gas (drying agent) temperatures, as well as velocities (flow-rates), induce faster drying. This effect is more pronounced for deeper beds, because of the larger amount of wet material to be dried using the same drying agent capacity. Bed temperature differences along the bed height, being significant inside the packed bed, are almost negligible in the vibrated fluidized bed, for the same drying conditions, due to mixing of particles. Residence time is shorter in the case of a vibrated fluidized bed drying compared to a packed bed drying.     

Effect of vibration energy on fluidization and 1–6 mm coal separation in a vibrated dense medium fluidized bed

In this study, a uniform and stable vibrated dense medium fluidized bed (VDMFB) with Geldart B magnetite powder was formed, where it was considered suitable for 1–6 mm coal separation. The formation mechanisms of the best separation environment including the fluidization phenomena under different vibration parameters, the transfer process of the vibration energy, the pressure drop fluctuation and particle oscillation velocity distribution at different bed heights, as well as the corresponding effect on the uniformity and stability of the bed density were investigated. Furthermore, the separation experiment was carried out, and the results exhibited an efficient dry separation performance in the VDMFB.     

细颗粒振动流态化研究进展

概括了细颗粒在振动流化床中的流态化实验研究和理论模型,着重介绍了振动场对细颗粒流化行为的影响,综述了振动场中粘性颗粒的运动,有利流化的振动参数以及聚团尺寸测量和计算的研究现状。     

带内置水平管的大颗粒振动流化床流体力学研究

以大颗粒MAP（磷酸-铵）为试验物料对带内置水平管的大颗粒振动流化床流体力学进行了试验研究,结果表明,振动的引入对床层临界压降有明显的降低作用;同样床层条件下振动频率越大床层临界压降越低,振幅的改变对床层压降影响不大;低气速下振动使床层孔隙率降低而导致压降高于普通流化床;在带内置水平管的大颗粒振动流化床中,振动对大粒径颗粒的影响小于小粒径颗粒;由试验拟合出经验公式,与试验数据吻合良好。     

Development of a tribolelectric procedure for the measurement of mixing and drying in a vibrated fluidized bed

Fluidization of fine, pharmaceutical powders makes them easier to dry, coat and mix. Fine powders, however, are difficult to fluidize well with gas flow only. Vibration can often help achieve smooth fluidization at a lower gas flow. The objective of the present study was, thus, to develop reliable and quick experimental methods to characterize mixing and drying in vibrated fluidized beds of fine powders.Effective mixing is critical in many industrial applications and, in gas-solid fluidized beds, requires gas velocities greater than the minimum bubbling velocity (U_mb). There are a number of techniques available for determining U_mb. However, they often are impossible or impractical to use in an industrial application. A new measurement technique involving the use of triboelectric probes was developed. Signal characteristics obtained from sophisticated signal analysis were used to identify the minimum bubbling velocities. These predictions corresponded well with the values obtained from more traditional laboratory methods such as the bed pressure gradient.In a fluidized bed, particles hitting a metal probe will generate a small triboelectric current. Triboelectric probes are able to detect rapid changes in particle surface properties. Surface properties of the particles were modified by wetting the particles in a low shear mixer. This change was detected by triboelectric probes at various locations inserted throughout the bed. The water adsorbed on the particles will begin to evaporate when exposed to the gas stream and the surface properties of the particles will gradually return to their original dry state. The triboelectric probes were able to monitor this drying process. The effects of vibration amplitude on the mixing and drying rate of the bed were also determined.     

Pyrite enrichment in vibrated fluidized bed based on density segregation

It is of significance for the environmental protection, the shortage of water, and the economic benefits to recovering the pyrite from the gangue with high sulphur content (GHSC) by the vibrated fluidized bed. In this study, characteristics such as the sulphur form of pyrite, size distribution, and dissociation degree of sample were analysed. Based on the above analysis, a vibrated fluidized bed was first used for the pyrite enrichment from GHSC. The effect of air velocity, vibration intensity, and bed height on the separation performance was analysed by separation experiments. The results show that the separation efficiency increased firstly and then decreased with increasing gas velocity, vibration intensity, and bed height. The total sulphur contents (TSCs) of ?6 + 3, ?3 + 1, and ?1 + 0.5 mm size fraction materials were up to 37.94%, 35.43%, and 30.61%, respectively, and the enrichment grades were 1.72, 1.90, and 2.29 times, respectively, under the optimal conditions, indicating that using a vibrated fluidized bed was efficient for the pyrite enrichment.     

Devolatilization of coals of northeastern India in inert atmosphere and in air under fluidized bed conditions

Devolatilization of five coals having volatile matter in the range of 31 to 41% was studied in argon and in air under fluidized bed conditions. The diameter of the coal particles varied between 4 and 9.5 mm. The variation of devolatilization time with particle diameter was expressed by the correlation, t_v = Ad_vⁿ. The superficial gas velocity was found to have a significant effect on the rate of devolatilization. The devolatilization rate increased with the increase in the oxygen concentration in the fluidizing gas. The correlations developed in this study fitted the mass versus time profiles of the coal particles satisfactorily. The same correlations were found to be appropriate for predicting devolatilization of a batch of coal particles. The correlations developed in the present study will be useful for the design of fluidized bed combustors.     

Numerical simulation of particle motion in vibrated fluidized bed

The particle motion in vibrated fluidized bed was examined by using numerical simulation. A two-dimensional fluidized bed was used, and discrete element method (DEM) was used as for calculating the particle motion in vibrated fluidized bed. Geldart group B particle, which is regarded to have no effect of agglomerate, was used as the fluidizing particle. The vibration directions (vertical and horizontal) and vibration parameters (frequency and amplitude of vibration) were changed.In the case with vertical vibration, large bubbles caused by vibration gap (defined as the gap between particle bed and wall caused by vibration) appear in bed. When the vibration strength is high and the vibration frequency is low, it is difficult to fludize the particle bed in the case with horizontal vibration because the vibration gap acts like the channel. When the vibration frequency increases at the same vibration strength, the effect of vibration on the particle motion becomes small. The effect of vibration on the pressure loss in particle bed is large in the case with horizontal vibration.     

Simulation and experimental studies on fluidization properties in a pressurized jetting fluidized bed

The influence of pressure on the bubble size and average bed voidage has been investigated experimentally and computationally in a circular three-dimensional cold-flow model of pressurized jetting fluidized bed of 0.2 m i.d. and 0.6 m in height with a central jet and a conical distributor, which roughly stands for the ash-agglomerating fluidized bed coal gasifier. The pressurized average bed voidage and bubble size in the jetting fluidized bed were investigated by using electrical capacitance tomography (ECT) technique. The time-averaged cross-sectional solids concentration distribution in the fluidized bed was recorded. The influence of pressure on the size of bubble and the average bed voidage in a pressurized fluidized bed was studied. Both experimental and theoretical results clearly indicate that there is, at the lower pressure, a small initial increase in bubble size decided by voidage and then a decrease with a further increase in pressure, which proves the conclusion of Cai et.al. [P. Cai, M. Schiavetti, G. De Michele, G.C. Grazzini, M. Miccio, Quantitative estimation of bubble size in PFBC, Powder Technology 80 (1994) 99-109]. At higher pressure, bubbles become smaller and smaller because of splitting. The average bed voidage increases gradually with the pressure at the same gas velocity. However, there is a disagreement between the experimental results and simulation results in the average bed voidage at the higher gas velocity, especially at the higher pressure. It suggests that the increase in density of gas with pressure may result in the drag increase and the drag model needs to be improved and revised at higher pressure.     

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.     

Elutriation and attrition of char form a large fluidized bed

Previous correlations for elutriation of particle fines did not fit the present data obtained in a large fluidized bed using char/sand mixture as the bed material. A new form of correlation (after Zenz and Weil [1]) for the elutriation rate constants, E_i, has been developed which gives a good agreement with the experimental data: E/?_gU_g = 9.43 × 10^?4 (U²_g/gd_p)^1.65.The attrition rate of production of fines of char particles in char/sand fluidized bed has been found to increase with U_g—U_mf as an exponential function: k_a = ae^{b(U_g—U_mf)}. The dependence of attrition rate constant on d_p appears small, if it is present at all. Attrition would appear to be occuring only in the bubble wakes.A small char layer (without sand) was found on the char/sand bed surface when it was fluidized. The elutriation rate was found not to be significantly influenced by this thin char layer. This is a unique confirmation of the theory of entrainment that the majority of ejected particles originate from the wakes of bubbles which have grown as they rise through the bed.     

振动流化床中水平换热管与粗钛矿传热特性的研究

以粗粒级原钛矿为实验物料,测定了振动流化床与水平加热管间的传热系数,研究了粗钛矿在流化状态下的传热特性,并对影响传热过程的因素进行了分析,得出了适宜的操作条件。在操作气速0.1ms-1、振幅2mm及振动频率1100min-1的条件下,振动流化床水平换热管的传热系数达到最大值。实验结果为粗钛矿的高效节能干燥提供了重要实验依据。     

流化床燃烧/气化器内流动特性

以灰熔聚流化床粉煤气化工艺为背景,概括了射流床内床层动力学、射流特性以及密相中的气泡性质的研究进展。旨在为该工艺过程的工业化放大和实际操作提供必要的参考和有益的信息。     

EFFECT OF DRYING ON POWDER COATING EFFICIENCY AND AGGLOMERATION IN VIBRO-FLUIDIZED BED

Glass beads of 43 μm were coated in a vibro-fluidized bed by atomizing a fine silica powder together with polyvinyl alcohol aqueous solution. The coating efficiency and weight fraction of the agglomerated particles were measured under various experimental conditions, and their dependencies on the frequency and the direction of vibration were investigated. The coating efficiency and the degree of the agglomeration among core particles correlated well with an index R. The index R was introduced to evaluate quantitatively the drying conditions in a fluidized bed.

Application of vertical vibration on the fluidized bed lowered the coating efficiency somewhat, while it prevented agglomeration. From the experimental results it was confirmed that coating with high quality and high efficiency, where few agglomerates were produced and silica powder was utilized efficiently, was possible in a vibro-fluidized bed with adequate vibration frequency and orientation of the vibration vector.     

脉冲气流对烯烃流化床气泡特性的影响

流化床具有良好的传热传质效率,因此广泛应用于煤粉燃烧、气力输送等场合。但是流化床在工作过程中会产生一定的恶性流动,脉冲的振动能量可以有效地提高流化床的传热系数,减少恶性流动现象发生。本文设计、搭建了烯烃流化床实验平台,通入混合脉冲气流。通过改变脉冲气流的相关参数,对静电信号进行功率谱密度函数分析。并基于静电信号估算气泡尺寸,从而获得流化床内气泡行为的变化。实验结果表明,脉冲气流的加入对气泡尺寸有一定的影响。随着脉冲频率的增加,气泡尺寸呈现先减小后增大的趋势,在脉冲频率为0.5Hz左右时气泡尺寸最小。脉冲气流加入后使得气泡尺寸减小,提高了颗粒流化效果,因此烯烃流化床内流化结束后的团聚颗粒质量明显降低。     

Voidage distribution around bubbles in a fluidized bed: Influence on throughflow

In this work, a new method for measuring void fraction distribution around endogenous bubbles in a 2D fluidized bed is presented. The technique is based on illuminating a transparent-wall 2-dimensional bed with diffuse light from the rear and recording the distribution of light that penetrates the bed. The recording is made with a high speed video-camera, which gives frames with grey level corresponding to the light penetration and from which the voidage distribution around the bubbles can be determined. In this way, voidage distribution in the region very close to the bubble contour (r/R_b ? 1.2) is obtained, which was not possible in previous studies due to limitations in spatial resolution. A correlation is proposed for the voidage at the contour of the bubble, with the voidage depending on the radial position and the polar angle ε(r, θ).In addition, the effect of the voidage distribution on the throughflow crossing the bubbles was studied and an increase of 20% was determined for the average bubble geometry of the more than 100 bubbles analysed.