Model for Calculation of Agglomerate Sizes of Nanoparticles in a Vibro‐fluidized Bed

The behavior of SiO₂ nanoparticles and the effects of operating conditions on nanoparticle agglomerate sizes have been investigated under conditions created in a vibro‐fluidized bed (VFB). The experimental results reveal that the vibrations imposed in the bed can suppress slugging and/or channeling, in contrast to conventional fluidization with upflow only. The vibrations imposed in the particle bed affect both the minimum fluidization velocity and the agglomerate size, both of which decrease with increases in the energy introduced to the bed by the vibrations. The effect of vibrations on the agglomeration in vibro‐fluidized beds of nanoparticles depends on the critical vibration frequency corresponding to a minimum agglomerate size. Both the amplitude and the frequency of the applied vibrations have significant effects on the agglomerate size. The experimental results and the consequent analysis reveal that increasing levels of vibrations in the bed yields finer agglomerates. The Richardson‐Zaki scaling law combined with Stokes law permits the prediction of agglomerate sizes and the extent of initial bed voidage. The average agglomerate sizes predicted are in good agreement with those determined experimentally.     

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。     

Experimental and theoretical study on the agglomeration arising from fluidization of cohesive particles—effects of mechanical vibration

A novel technique that can prevent the disruption of agglomerates when sampling the agglomerates from a fluidized bed has been developed and has been applied to the investigation of the agglomeration behaviour of cohesive particles during fluidization with and without mechanical vibration. A new model for the prediction of agglomerate size has also been established on the basis of the energy balance between the agglomerate collision energy, the energy due to cohesive forces and the energy generated by vibration. The accuracy of the model is tested by comparing the theoretical results with the experimental data obtained both in the present work and in the literature. Effects of gas velocity and mechanical vibration on agglomeration for two cohesive (Geldart group C) powders in fluidization are examined experimentally and theoretically. The experimental results prove that mechanical vibration can significantly reduce both the average size and the degree of the size-segregation of the agglomerates throughout the whole bed. However, the experiments also reveal that the mean agglomerate size decreases initially with the vibration intensity, but increases gradually as the vibration intensity exceeds a critical value. This suggests that the vibration cannot only facilitate breaking the agglomerates due to the increased agglomerate collision energy but can also favour the growth of the agglomerates due to the enhanced contacting probability between particles and/or agglomerates. Both the experimental and theoretical results show that a higher gas velocity leads to a smaller agglomerate size.     

振动流化床中流动结构的混沌分析

在内径90mm、静床高800mm的高床层流化床中,用动态压力传感器检测了不同气速条件下普通流化床和振动流化床中沿轴向的压力脉动信号,通过小波变换对信号除噪后,用混沌理论对信号进行了分析.通过关联维数和Kolmogorov熵定量表征振动流化床中的流动结构特征.结果表明:压力脉动信号的关联维数和Kolmogorov熵能够描述振动流化床中的流化状态;振动流化床中床层的流动结构存在两个区,在近分布板区域为射流区,床层主体部分为均匀流化区.     

Fluidization of nanoparticles: A simple equation for estimating the size of agglomerates

In this work, we revise data published in the last decade on the size of agglomerates in gas-fluidized beds of nanoparticles. Experimental measurements reviewed are based on non-invasive techniques, mainly consisting of laser-based planar visualization of agglomerates in the splash zone and indirect derivation from the fit of bed expansion, settling, and/or minimum fluidization velocity data to empirical correlations. Special attention is focused on the effect of fluidization aids such as vibration, magnetic assistance, sound excitation or centrifugation. Independent measurements performed by diverse authors or by the same authors using different techniques are confronted. Empirical models proposed to predict agglomerate size are also reviewed. Most of these models are difficult to apply in practice because they rely on parameters that need to be measured in the fluidization experiment or assumed. We propose a simple equation to estimate agglomerate size derived from the balance between the local shear force on the particle attached at the outer layer of the agglomerate and the interparticle adhesion force. In general, the results predicted by this equation are in satisfactory agreement with the reviewed experimental data.     

Fundamental particle fluidization behavior and handling of nano-particles in a rotating fluidized bed

The fluidization behavior of the three kinds of nano-particles (TiO₂, SiO₂, Al₂O₃) was analyzed in a rotating fluidized bed (RFB). Bed pressure drop, minimum fluidization velocity, bed expansion, entrainment and particle mixing characteristics under various centrifugal accelerations were experimentally investigated. The effects of centrifugal acceleration on agglomerate size and density were analyzed based on a Richardson-Zaki approach coupled with a fractal model.The bed pressure drop behavior showed almost similar to that of A or B-particles of Geldart's classification. Dimensionless particle bed height became smaller when the centrifugal acceleration was larger. Size of agglomerate decreased and its density increased with an increase in centrifugal acceleration. The agglomerate size in the RFB showed smaller than that in other types of fluidized bed system such as vibration and magnetic field as well as in a conventional fluidized bed, and the agglomerate density became larger. Particle entrainment became smaller in the case of the higher centrifugal acceleration. These results confirmed that the RFB can reduce the size of a nano-particle agglomerate and fluidize nano-particles at high gas velocity without any significant entrainment. The RFB is thus expected as more effective gas-solid fluidization system for handling of a large amount of nano-particles than other types of fluidized bed.     

振动流化床中浅床层时床层与分布板之间的弹性作用

采用A类颗粒作为实验粉体,通过测量床层对分布板的作用力,分析了振动流化床中分布板与床层的作用机理,提出了振动流化床中在浅床层共振时分布板与床层的弹性作用模型. 实验表明,分布板与床层之间是连续的弹性作用,周期性的机械振动通过分布板与床层之间的弹性作用以波动形式在床层中传播. 该模型计算结果与实验测定结果符合的较好.     

振动流化床中磷石膏颗粒流态化性能研究

在振动流化床中进行了磷石膏颗粒流态化性能的冷模实验研究。结果表明,磷石膏颗粒在普通流化床中流化性能很差,难以实现正常的流化态,不适合在普通流化床中进行流态化分解反应。而振动的引入极大地改善了磷石膏颗粒的流态化性能。并考察了振幅和进料高径比对磷石膏颗粒的床层压降、膨胀比、最小流化速度的影响。为热模实验条件的选择提供了理论基础。     

粘性SiC颗粒聚团流态化特性

对不同粒径的°SiC粘性颗粒的流态化实验表明,颗粒粒径对流化性能有较大影响,颗粒粒径越小,颗粒间粘附力越大,其流化性能越差;提出了粘性颗粒自然聚团数Ae_n和流态化聚团数Ae_f,用来表征颗粒的流化性能;指出了应开展粘性颗粒聚团流态化的研究。     

Application of discrete element method simulation for studying fluidization of nanoparticle agglomerates

The usefulness of discrete element method simulation for studying fluidization of nanoparticle agglomerates is explored. Nanoparticle agglomerates were simulated by using solid particles of equivalent sizes and densities. Validity of the present simulation was assessed through comparisons of simulation results and experimental observations of bed expansion, characteristic fluidization behaviour, and dense‐bed settling. The simulation was then used to investigate initial bed expansion and bed uniformity under particulate fluidization conditions. The role of inter‐agglomerate interparticle force in fluidization of nanoparticle agglomerates was examined. A stability analysis originally developed for addressing the transition from particulate to bubbling fluidization for conventional particles was used for predicting the start of bubbling in fluidized beds of nanoparticle agglomerates.     

Nanofluidization as affected by vibration and electrostatic fields

In this paper we investigate the behavior of a fluidized bed of silica nanoparticles under the influence of externally applied vibrations and an electrostatic field. We have observed that the application of these fields separately has opposite effects on bed expansion. On one hand, vertical vibrations enhance bed expansion as the vibration intensity is increased up to a critical value. On the other hand, an electrostatic field applied in the horizontal direction, hinders bed expansion. In previous research papers, it has been suggested that the size of nanoparticle agglomerates could be affected either by vibration or by the action of the electric field. Using the modified Richardson-Zaki method to analyze our experimental data we find that vertical vibration tends to decrease the average agglomerate size in agreement with previous research. However, in this work we look further into the physical mechanisms which affect the response of the fluidized bed. Our results suggest that both vibration and the electric field produce a significant perturbation to the flow of agglomerates within the fluidized bed. Vibration transmits a vertical motion to the agglomerates that enhances bed expansion until the vibration velocity becomes of the order of the expected rising velocity of macroscopic bubbles. At this critical point, bubble growth is stimulated by vibration. A horizontal electrostatic field produces a drift of the charged agglomerates toward the walls that gives rise to fluidization heterogeneity and bed collapse. When both fields are simultaneous applied, these opposed effects can be practically compensated.     

A model for estimating agglomerate sizes of non-magnetic nanoparticles in magnetic fluidized beds

The behavior of SiO₂, TiO₂ and ZnO non-magnetic nanoparticles and the effects of processing parameters on agglomerate sizes were investigated systematically in a magnetic fluidized bed (MFB) by adding coarse magnets. A mathematical model is developed based on energy balance among the agglomerate collision energy, magnetic energy, energy generated by turbulent shear and cohesive energy to predict the agglomerate sizes. The results showed that slugging of the bed disappeared and the measured agglomerate sizes decreased, so that the fluidization quality of non-magnetic nanoparticles was significantly improved by adding coarse magnets due to introduction of magnetic field. The average agglomerate sizes predicted by this model are in agreement with the experimental data.     

旋流振动流化床干燥性能的研究

文章以聚四氟乙烯粒子和蔗糖为物料研究了旋流振动流化床的流体力学及干燥特性,并与振动流化床的各种特性进行了对比。实验在一个干燥室直径为240 mm的小型圆筒振动流化床中进行,采用斜孔分布板使床内产生旋流。实验结果表明:在空气分布板开孔率相差不多的情况下,物料流化以前,旋流振动流化床的床层压降要大于普通振动流化床,一旦物料达到正常流化状态以后,2种振动流化床的床层压降相差不多;旋流振动流化床可以降低物料的临界含湿质量分数。     

Aggregation behavior of nanoparticles in fluidized beds

The fluidization behavior of fumed silica, zirconia, and iron oxide nanopowders was studied at atmospheric and reduced pressures. Using a high-speed laser imaging system, the characteristics of fluidized aggregates of nanoparticles were studied in real time. The effect of different particle interactions such as London-van der Waals, liquid bridging and electrostatic on different fluidization parameters was studied at atmospheric pressure. The reduction of interparticle forces resulted in a reduced aggregate size and minimum fluidization velocity (U_mf) and an increased bed expansion. Nanoparticles were also fluidized at reduced pressure (∼ 16 Pa) with vibration to study the effect of low pressure on the minimum fluidization velocity. Aggregate properties (size, density) instead of primary nanoparticle properties were found to govern the minimum fluidization velocity and expansion of the fluidized bed. An important consideration is the relative strength of intra-aggregate interparticle forces (forces within the aggregate holding nanoparticles together) to inter-aggregate interparticle forces (forces between aggregates). This relative strength may be inferred from the sphericity of the aggregates during fluidization.     

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 ...     

声场流化床流动特性研究进展

声场流化床是将声场引入普通流化床,采用颗粒为床层介质的流固相处理系统。声波可以有效降低颗粒聚团尺寸,显著改善超细颗粒的流化质量。本文介绍了声场流化床的基本原理以及近年来在基础研究和应用方面取得的进展及成果,综述了声场流化床在流体力学特性、颗粒特性、声场参数、流态化模型、颗粒团聚以及流化质量机理等方面的研究,并对声场流化床目前存在的问题及发展趋势提出了一些建议。     

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.     

微细粉体在振动流化床中团聚行为的研究

本文研究了微细粉体在振动流化床中的团聚行为和振动参数对团聚物尺寸的影响。观察了床层流化现象,并发展了团聚物的测量方法。提出了Ｃ类粉体振动流化的团聚机理。     

振动流化床中双组分颗粒流化特性的研究

本文研究了内径为１４８ｍｍ振动圆柱床中等密度和不等密度的双组分颗粒流化特性,考察了不同振动强度对双组分颗粒的床层空隙率、最小流化速度及相图的影响,给出了床层空隙率和最小流化速度的计算式,此计算值与实验值基本相符,且对振动流化床的实际操作和工程设计起到一定的指导作用。     

AGGLOMERATION BEHAVIOR IN A BUBBLING FLUIDIZED BED AT HIGH TEMPERATURE

Minimum fluidization velocity and agglomeration behavior were investigated at high temperature in an 80?×?30?mm two-dimensional quartz fluidized bed and in an 82?mm i.d. circular fluidized bed. Bed materials tested were two sizes of glass beads as well as three sizes of fluidized bed combustor (FBC) ash. The minimum fluidization velocity decreased with increasing bed temperature, whereas the minimum sintering fluidization velocity increased with the bed temperature. The sintering of glass beads belongs to visco plastic sintering, the first type. FBC ash agglomerate has higher amounts of SiO₂, Al₂O₃, Na₂O, K₂O, and SiO₂ than in the original ash, indicating that low melting eutectics were formed and that the liquid phase in a silicate system was formed. The agglomeration of FBC ash belongs to the second type, an excessive quantity of liquid being formed by melting or chemical reaction.