Hold-up of fine particles in the packed dense bed of the multisolid pneumatic transport bed

The hold-up of fine particles in the packed bed of dense particles in the multisolid pneumatic transport bed was experimentally examined. In the experiments, three types of fine particles were used including FCC particles (65 μm), glass beads (105 μm) and fine sand (155 μm), while three types of dense particles were used including aluminum particles (5.50 mm and 6.96 mm) and iron oxide particles (11.08 mm). The hold-up of fine particles in the packed bed of dense particles was found to be higher by a factor as large as 6 compared with that observed in systems without dense particles. An interaction coefficient between fine particles and dense particles was defined based on the momentum balance equation of fine particles in the packed bed. This interaction coefficient, which is empirically correlated, was utilized to account for the hold-up of fine particles in the packed bed.     

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.     

磷矿颗粒流态化特性的实验研究

在高1 m、内径42mm的流化床中,对粒径54-600 μm、密度2 252-2 665 kg/m3的磷矿颗粒的流态化特性进行实验研究.实验结果表明:磷矿颗粒粒径和密度对磷矿颗粒流态化行为有较大影响,床层膨胀比随着磷矿颗粒粒径的增大而逐渐减小.当磷矿颗粒属于Geldart B类颗粒时,流化较好;而当颗粒平均粒径为82 ...     

气固流化床中颗粒的夹带和扬析特性

在内径0.5 m、总高6 m的流化床中对玻璃珠和白刚玉颗粒的夹带和扬析现象进行了实验研究,两种颗粒通过筛分配比得到不同粒径分布的实验物料,以0.25~0.76 m/s的速度分批进行流态化试验,考察了表观气速、床层物料粒径分布以及颗粒粒径对颗粒夹带和扬析速率的影响,得到了颗粒的扬析速率常数(Ki*)。实验结果表明,夹带量和扬析速率常数随着表观气速呈指数增加;扬析速率常数随着颗粒粒径减小先增加,达到一个临界粒径(dcrit)后,扬析速率常数会随着粒径减小趋于平缓或降低。提出了一组经验关联式,分别用于预测临界粒径两侧颗粒的扬析速率常数,关联式能很好地预测本工作实验条件下的数据,误差在30%以内,并且关联式能够对文献中的实验数据进行较好的预测,可以用于流化床装置的放大。     

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.     

Effect of cohesive powders on the elutriation of particles from a fluid bed

The elutriation of fine particles (Group C or A particles in Geldart's classification) from a fluid bed of mixed fine and coarse particles is investigated in a steady state. Al(OH)₃ and alumina and TiO₂ powder of 0.5- were used as fines. FCC, alumina, Al(OH)₃, limestone, silica sand, SiC particles of 44- were used as coarse particles. The paper investigates the effect on the elutriation rate constant of both fine powders and coarse particles (i) of the weight fraction of Geldart C powders in the bed, (ii) of the superficial gas velocity, and (iii) of the size of C powder and size of coarse particles in the bed.The elutriation rate constant of group C or group A particles is not only affected by the properties of the elutriated powders or particles and gas velocity, but also by both the weight fraction and size of C powder in the bed. This finding differs from the elutriation result of A or B particles from a fluidized bed.     

Onset of an innovative gasless fluidized bed—comparative study on the fluidization of fine powders in a rotating drum and a traditional fluidized bed

Geldart group C powders were found to be fluidized in rotating drums without requiring any external fluidizing gas. As a result, a rotating drum was proposed as a new gasless fluidized bed in contrast to a traditional fluidized bed, leading to a considerable amount of energy savings. In addition, the fluidization qualities of a series of Geldart group C powders were found to be further improved with the assistance of drum rotation because of the shearing movement among particles that eliminates channeling and cracks and possibly also breaks agglomerates. There is potential for the new gasless fluidized bed to replace some traditional fluidized beds where the fluidizing gas is not used as a reactant.In the gasless fluidized bed, a boundary layer of compacted powder adjacent to the drum wall was observed. The powder in this layer is carried up to the freeboard and then falls back to the powder bed, forming a powder circulation in the drum. The circulating powder leads to a circulation of internal gas in the drum, which essentially acts as fluidizing gas to realize the fluidization of Geldart C powders in the drum. In contrast to the fluidization of Geldart C powders, Geldart groups B and D powders show cascading and cataracting motions instead in the rotating drum due to their requirement of higher fluidization gas velocities. Geldart group A powders experience a transition of powder behavior between Geldart group B–D powders and C powders.     

Oxidation of sodium sulphide in the presence of fine activated carbon particles in a foam bed contactor

The oxidation of sodium sulphide in the presence of fine activated carbon particles (4.33 μm) has been studied at 75°C in a foam bed contactor. The existing single-stage model of a foam bed reactor has been modified to take into account the effect of heterogeneous catalyst particles and the absorption in the storage section. The variables studied are catalyst loading, initial sulphide concentration and the average liquid hold-up in the foam bed. It is seen that the rates of oxidation of sodium sulphide are considerably enhanced by an increase in the loading of activated carbon particles. The rate of conversion of sodium sulphide also increases with an increase in the average liquid hold-up in the foam. The modified model predicts these effects fairly well. The contribution of reaction in the storage section is found to be less than 2% of the overall rate of conversion in the contactor.     

Behaviour of Cohesive Powders in a Powder‐Particle Spouted Bed

Behaviour of cohesive powders in a powder‐particle spouted Bed (PPSB) has been investigated under several operating variables and solids properties. The elutriation rate constant based on the hold‐up of fine powders in the bed decreased with a decrease in the size of fine powders, and with an increase in the size of coarse particles under a constant superficial gas velocity. This finding is quite different from the elutriation phenomena of particles more than 100 µm in size. Moreover, the mean residence time of fine powders increased with a decrease in the superficial gas velocity and the size of fine powders, and decreased with a decrease in the size of coarse particles.     

气-固微型流化床压降特性及最小流化速度的实验研究

在内径3～20 mm的4个气-固微型流化床中,分别考察了A类和B类两种类型颗粒的流化特性,同时研究了床几何结构、操作条件、物相性质等各因素对其最小流化速度的影响.结果 表明,气-固微型流化床中的床层压降特性与颗粒类型密切相关,不同的流动状态下两种类型颗粒的流动特性存在显著地差异.在固定床阶段,与B类颗粒相比,A类颗粒与...     

气固搅拌流化床压力脉动的小波分析

在内径188 mm、静床高400 mm的搅拌流化床中,采用Geldart D类颗粒为实验物料,通过小波分析研究了不同气速和搅拌桨转速下搅拌流化床的压力脉动行为.实验发现,搅拌桨的转动作用促使在普通流化床中不易散式流态化的D类颗粒形成了散式流态化.随着气速的增加,第1尺度的小波能量特征值在某一个气速范围内发生急剧变化,进而提出了将该气速范围的下限和上限分别定义为临界鼓泡速度和充分鼓泡速度的判据.随搅拌转速的增加,散式流态化的气速操作范围线性增加.在鼓泡流态化状态下,气速是流化床气泡行为的主导因素,搅拌桨转速的增加对气泡产生的频率无明显影响但可使气泡的直径变小.     

Exit effect on hydrodynamics of the internal circulating fluidized bed riser

This is the first time an extensive investigation has been carried out regarding the effects of riser exit geometry on pressure drop and solid behaviour inside the Internal Circulating Fluidized Bed (ICFB) riser, using different riser exit geometries at several operating conditions.The Radioactive Particle-Tracking (RPT) technique was used for solid concentration measurements and solid residence time distribution at the exit zone. Experiments were conducted using Geldart B particles, in the gas superficial velocity range of 4 to 10 m/s. Axial solid hold-up, solid residence time distribution in the exit zone, and the reflux ratio factor k_m, (defined earlier by [E.H. Van der Meer, R.B. Thorpe, J.F. Davidson, Flow patterns in the square cross-section riser of a circulating fluidized bed and the effect of riser exit design, Chem. Eng. Sc. 55 (19) (2000) 4079-4099]), were the main criteria used to investigate the impact of gas-solid separator devices implemented at the ICFB riser exit.Solid residence time distribution results and axial solid hold-up profiles provided clear evidence that the separator device at the riser exit strongly influences the hydrodynamic structure of the ICFB riser. The V-shaped riser exit geometry was found to be the optimum of all the configurations studied.     

Slip velocity characteristics in the riser of circulating fluidised bed

Experiments were carried out in a conventional circulating fluidised bed to measure the axial pressure profile and total pressure drop, which covered a wide range of operating conditions. Material belonging to the Geldart A (fine material) as well as the Geldart B (course material) categories have been used in the present work. Slip velocity is determined from the total pressure drop and noticed that the slip velocity is much higher than the free fall velocity of single particle for Geldart A type material, while it is approximately equal to the free fall velocity of single particle for the Geldart B type materials. A model is developed for slip velocity taking into account all the hindrance effects: particle-particle, and particle-wall, and particle agglomeration. Predictions of the present model are validated with the data due to present study and the data reported in the literature.     

Why the two-fluid model fails to predict the bed expansion characteristics of Geldart A particles in gas-fluidized beds: A tentative answer

It is well known that two-fluid models (TFMs) can successfully predict the hydrodynamics of Geldart B and D particles. However, up to now, TFM have failed to accurately describe the hydrodynamics of Geldart A particles inside bubbling gas-fluidized beds: Researchers have reported that bed expansions are over-predicted by as much as 70%. In this work we show—for the first time—that TFM can predict the correct bed expansion, without any artificial modifications, provided that a sufficiently fine grid size and small time step is used. This suggests that the previously reported failure of TFM is mainly due to the lack of scale resolution, and that from a modeling point of view there is no fundamental difference between Geldart A particles and Geldart B and D particles.     

Two-phase theory and fine powders

The validity of the assumptions built into the two-phase theory has been re-examined for a range of fine powders. These long-standing but often criticised assumptions are that, in the dense phase of a bubbling fluidised bed, the voidage and velocity correspond to those at incipient fluidisation.Eleven different powders were used, and average particle size varied from 12 to 67 μm. Particle density ranged between 1300 and 5200 kg/m³. Dense phase velocity and voidage were measured by the bed collapse technique, and the systematic deviations from the above assumptions demonstrated.It has been shown that the minimum fluidisation point loses meaning at high fines levels; a transitional class on the Geldart [6] powder classification diagram, termed class AC, is postulated. This class is characterised by the absence of a meaningful incipient fluidisation point, and the absence of a contraction of the bed when bubbles first begin to pass through. The dense phase voidage has been found to increase more strongly than expected, and correlates strongly with fines level. Particle density has been found to have an insignificant effect on dense phase voidage in the range covered, and the gas velocity in the dense phase has also been found to increase and correlate strongly with increasing fines level. Correlations are presented for dense phase voidage and velocity.     

不同颗粒流化床层中挡板受力特性对比

颗粒性质对流化床内气固流动特性具有重要的影响,不同颗粒床层内气固流动特性的不同也将引起床层中内构件受力特性的变化。采用在测试挡板表面粘贴应变计的方法,系统对比测量了一个斜片挡板在FCC颗粒（Geldart A）和石英砂颗粒（Geldart B）两种流化床内受力特性的差异,并系统比较了操作参数变化时挡板在两种颗粒床层中受力特性变化规律的差异。结果表明,在相同的操作条件下,挡板在B类颗粒床层中受力载荷的均方根值大小约是A类颗粒床层中的2～3倍;除挡板安装在靠近分布器位置外,总体来讲,在两种颗粒的床层中,挡板所受载荷强度都随表观气速的增大而增大。但是,在两种颗粒床层中,挡板安装高度变化对挡板受力特性影响差异较大,在B类颗粒床层中所受载荷强度随着安装高度增大而增大,而在A类颗粒床层中所受载荷强度随安装高度增大呈现先减小后增大的趋势。此外,挡板倾角度θ在75°～90°之间变化时,挡板所受载荷强度在两种颗粒流化床中均随着挡板倾角增大呈现急剧下降的趋势,而当θ=0°～75°时,B类颗粒床层中挡板所受载荷强度随挡板倾角增大略有下降,而A类颗粒床层中挡板所受载荷强度变化并不十分明显。     

Effect of interparticle forces on the hydrodynamic behaviour of fluidized aerogels

Aerogel powders, particles of the group C according to the Geldart classification, are not particularly attractive as such for fluidized bed applications. The following study shows, however, that these fine solids form agglomerates above a minimum superficial velocity of 0.04 m/s and that the resulting new system fluidizes smoothly and homogeneously at room and at higher temperatures. A comprehensive model developed to represent the hydrodynamic behaviour of the system proposes that the agglomerate is a cluster of the original fixed bed materials and hence has the properties of the bulk matrix. Van der Waals forces on the elementary grains are used to predict the cluster size. This clustering process is finally analysed in the light of the classical fluidizability diagram.     

Modulating the mean residence time difference of wide-size particles in a fluidized bed

For non-catalytic gas-solid reaction, it is desirable to match the mean residence time(MRT) of particles and complete conversion time(t_c) in a fluidized bed. In this study, the MRT differences(MRT ratios) between the coarse particles and the fine particles were investigated in a continuous fluidized bed with a side exit by varying the superficial gas velocity, feed composition and particle size ratio. The results show that the MRT ratio increases firstly and then decreases with increasing the gas velocity. By controlling the gas velocity and the feed composition of coarse particles, the MRT ratio can be modulated from 1.8 to 10.5 at the gas velocity of 1.0 m·s~(-1) for the binary mixture with the size ratio of 2.2. The MRT ratio can reach to ~ 12 at the gas velocity of 1.2 m·s~(-1)for the particle size ratio of 3.3. The present study has endeavored to obtain fundamental data for an effective plant operation to meet the need of synchronously complete conversion of particles with different sizes during the film diffusion controlling reaction.     

Heat transfer in a pulsed bubbling fluidized bed

Surface-to-bed heat transfer and pressure measurements were carried out in a 0.17 m ID pulsed bubbling fluidized bed with glass bead and silica sand particles having mean diameters ranging from 37 μm to 700 μm to investigate the effects of flow pulsation on heat transfer and bed hydrodynamics. A solenoid valve was used to supply pulsed air to the bed at 1 to 10 Hz. The bed surface was found to oscillate with the frequency of pulsation, the oscillation's amplitude decreasing with frequency. The standard deviation of the bed pressure drop in the pulsed bed was found to be larger than that in the conventional bed due to the acceleration force imposed by pulsation. For both Geldart B and A particles, high frequency pulsation (7, 10 Hz) enhances the heat transfer compared to continuous flow, the enhancement diminishing with superficial gas velocity and particle size. For Geldart B particles, the effect of pulsation on heat transfer ceases around U_o/U_mf = 3.5, whereas 24% improvement in heat transfer coefficient was obtained for 60 μm glass bead particles (Group A) at superficial gas velocities as high as U_o/U_mf = 27. Furthermore, in the fixed bed (U_o/U_mf < 1) for Geldart B particles, 1 Hz pulsation was found to be very effective resulting in two- to three-fold increase in heat transfer coefficient compared to continuous flow at the same superficial gas velocity. The flow pulsation loses its effect on heat transfer with increasing static bed height, i.e., when H_bed/D > 0.85.     

气固流化床中声发射和流动模式关系

颗粒在气固流化床壁面区域（或局部空间区域）碰撞产生的声波能量反映了颗粒的碰撞速度和频率（活跃程度）,从中可以揭示流化床内颗粒的流动混合模式。通过在φ150mm流化床冷模装置中,对聚乙烯颗粒－空气体系进行流态化实验,运用声发射技术测得声能量沿气固流化床的轴向分布,继而获得了颗粒的流动模式,并发现其与颗粒粒径、表观气速和分布板形式密切相关。对于颗粒粒径为460 μm的聚乙烯颗粒,当表观气速在0.3～0.7 m·s^-1内,其对应的流动模式为带有滞留区的双循环流动模式。如果气速增大到0.8 m·s^-1以上时,流动模式将转化为无滞留区的单循环流动模式。而当颗粒平均粒径降为365μm,对应的双循环流动模式蜕化为单循环模式,壁面不存在滞留区。进一步发现,滞留区位置与静床高无关。研究同时发现,颗粒的流动模式和分布板形式密切相关,对于在多孔平板分布板下为单循环流动模式的小粒径颗粒,在锥帽式分布板下,则在稍高气速时表现为双循环流动模式。