细粉下料过程的气固流体动力学作用分析

细颗粒粉体下料时受气固流体力学作用在料仓出口附近形成逆压力梯度,使得粉体下料流率实验值远低于理论预测值。而且该压力梯度力直接测量较困难,对模型修正和发展提出了挑战。以玻璃微珠、流化催化裂化（FCC）催化剂颗粒、褐煤和聚氯乙烯（PVC）颗粒为实验材料,首先开展粉体静力学与动力学测试,借助休止角（AOR）、豪斯纳比（HR）和卡尔流动指数（CFI）多个粉体流动性判据综合分析不同粉体的流动特性;在分析粉体料仓出口附近气固流动特征的基础上,结合Jenike流动与不流动判据,将作用在细颗粒粉体上的逆压力梯度力引入到拱应力平衡方程;进一步,提出了利用迭代算法获得逆压力梯度力的方法,实现了对逆压力梯度力与粉体料仓下料流率的预测。建立的粉体下料流率模型考虑了气固流体动力学作用对粉体下料流动的影响,有效改善了传统模型对细粉体流率预测偏高的问题,模型预测偏差从60%以上降低至±20%。     

复杂流道结构料仓的下料流率预测

在实验室搭建的有机玻璃料仓下料平台上,分别以自由流动粉体玻璃微珠和黏附性粉体煤粉和聚氯乙烯为实验介质,针对无改流体(No-In)、封闭改流体(Con-In)和开放改流体(Ucon-In)三种情况所形成的不同流道结构,开展了粉体料仓下料及其流率建模研究,定量分析了改流体对粉体下料流率的促进作用,对比给出了玻璃微珠、煤粉和聚氯乙烯在不同流道结构料仓内的下料特性。研究表明,改流体的引入有利于提高料仓下料流率,Con-In促进流动效果最明显,对于流动性弱的煤粉,下料流率提升幅度达到最大的58%。基于剪切摩擦区的概念,提出流率校正因子F对最小能量理论方程进行了修正,将理想的料仓下料模型拓展至实际下料过程。进一步,对于Con-In,根据流道结构特征结合对粉体的受力分析,修正了模型中的锥角项;对于Ucon-In,基于粉体下料流动竞争机制,提出分阶段下料模式并关联了内层和夹层的下料流率,最终建立了复杂流道结构料仓的下料流率预测模型。该模型综合考虑了粉体物性、下料流型和流道结构的影响,可有效预测自由流动粉体和黏附性粉体流经传统料仓(No-In)和改流体料仓(包括Con-In和Ucon-In)的粉体下料流率,且预测偏差<10%。     

CPFD在细颗粒料仓下料中的应用

借助计算颗粒流体力学(CPFD)的数值模拟方法,研究了细颗粒玻璃微珠在不同结构料仓内的下料特性,获得了料仓出口直径和半锥角对颗粒下料流动的影响。在实验室可视化下料平台开展了验证实验,模拟结果与实验结果吻合较好。模拟结果表明:下料流率与料仓出口直径2.5次方呈正比;料仓半锥角增大,下料流型从质量流过渡至漏斗流。CPFD模拟给出了料仓下料过程的细节信息,并获得了料仓结构对颗粒流动形态转变的临界面相对高度的影响。     

细颗粒在锥形和平底料仓中的重力卸料特性

对细颗粒在料仓内的重力卸料特性进行了实验研究,分别考察了颗粒粒径和料仓构型对卸料流率的影响,采用高速摄像仪对料仓出口处颗粒的卸料形态进行捕捉,并分析其周期性。结果表明,随着颗粒粒径的增大,颗粒卸料流率受逆向压力梯度和"空环效应"两种因素共同影响,先增大后基本保持恒定。另外,锥形和平底料仓中细颗粒的卸料形态,均随粒径增大由不稳定的间歇卸料向稳定的连续卸料过渡。在间歇卸料形态下,由于间歇卸料周期较长,锥形料仓的卸料流率要小于平底料仓;在连续卸料形态下,受料仓锥角影响,锥形料仓的卸料流率要大于平底料仓。最后,通过修正逆向压力梯度项,分别建立了适用于锥形和平底料仓的细颗粒重力卸料流率预测模型,卸料流率的预测值和实验值吻合很好。     

不同载气供料对煤粉料仓下料的影响

煤粉料仓下料是气流床粉煤气化工艺中粉体供料单元的重要组成部分,其安全稳定运行具有重大的经济意义。在煤粉循环供料系统上,研究了以空气和CO2为载气的气力输送供料对料仓内煤粉重力下料和通气下料的影响。研究表明,料仓内粉体的堆积或流化状态对后续粉体的流动行为影响明显。对于重力下料,CO2供料后的重力下料过程中煤粉床层空隙率较高,下料流率较大;对于通气下料,空气供料后煤粉难以被CO2气体流化,从而对应较低的下料流率。     

不同载气供料对煤粉料仓下料的影响

煤粉料仓下料是气流床粉煤气化工艺中粉体供料单元的重要组成部分,其安全稳定运行具有重大的经济意义。在煤粉循环供料系统上,研究了以空气和CO₂为载气的气力输送供料对料仓内煤粉重力下料和通气下料的影响。研究表明,料仓内粉体的堆积或流化状态对后续粉体的流动行为影响明显。对于重力下料,CO₂供料后的重力下料过程中煤粉床层空隙率较高,下料流率较大;对于通气下料,空气供料后煤粉难以被CO₂气体流化,从而对应较低的下料流率。     

煤粉料仓通气下料流动行为

在可视化有机玻璃料仓系统上对煤粉下料过程及其特性进行了实验研究。研究表明,煤粉介质粒径小、比表面积大,属于黏附性非自由流动粉体,下料流动困难;料仓通气技术可以显著改善煤粉流动性从而促进料仓下料,但过量通气反而会抑制煤粉下料。通过分析下料过程中料仓壁面的压力信号,揭示了料仓压力分布特性,分析了超侧压现象并定义了压力转换面。实验发现,通气可以在一定程度内减小超侧压幅度,扩大煤粉流化疏松区域,提升转换面高度。此外,研究还发现通气下料过程中存在气体穿透现象,穿透临界高度随通气量的增加而提高,并且穿透后煤粉下料流率降低。     

煤粉料仓通气下料流动行为

在可视化有机玻璃料仓系统上对煤粉下料过程及其特性进行了实验研究。研究表明,煤粉介质粒径小、比表面积大,属于黏附性非自由流动粉体,下料流动困难;料仓通气技术可以显著改善煤粉流动性从而促进料仓下料,但过量通气反而会抑制煤粉下料。通过分析下料过程中料仓壁面的压力信号,揭示了料仓压力分布特性,分析了超侧压现象并定义了压力转换面。实验发现,通气可以在一定程度内减小超侧压幅度,扩大煤粉流化疏松区域,提升转换面高度。此外,研究还发现通气下料过程中存在气体穿透现象,穿透临界高度随通气量的增加而提高,并且穿透后煤粉下料流率降低。     

料仓的卸料流动

对粉体在料仓中卸料流动的一些研究成果,即料仓卸料流动及其影响因素,进行简述,并分别对粗颗粒粉体和细颗粒粉体卸料流动的理论和实验研究结果进行了介绍和评述。     

料仓的卸料流动

对粉体在料仓中卸料流动的一些研究成果，即料仓卸料流动及其影响因素，进行简述，并分别对组颗粒粉体和细颗粒粉体卸料流动的理论和实验研究结果进行了介绍和评述。     

Gravitational discharge of fine dry powders with asperities from a conical hopper

The effects of particle properties, especially the surface roughness and particle type, on the gravity discharge rate and flow behavior of fine dry powders from a conical hopper are studied in detail. The van der Waals force is considered to dominate the discharge of small particles, while the empty annulus effect dominates the discharge of large particles. To predict the van der Waals force between two rough spherical particles, a model based on Rumpf theory is adopted. The effect of surface roughness can be reflected by Bond number Bo_g which is correlated with discharge rate. By modifying the powder bed porosity and Beverloo constant, the discharge rates of fine dry powders can be well predicted by an empirical correlation. Finally, not only the ratio of hopper outlet size to particle size D₀/d_p but also the Bond number Bo_g is found to be an important indicator to determine the powder flowability. © 2017 American Institute of Chemical Engineers AIChE J, 64: 427–436, 2018     

气－固－固循环流化床填料层中压降及细颗粒含率的轴向分布

在气－固－固循环流化床中，尺寸较大的固相（通常作为催化剂）被固定在床中形成一段填料层，较小的固相一般为细颗粒（通常用作吸附剂或热载体）被气流携带穿过填料层。本文从讨论此类流化床中气－固两相并流流过填料层时压降的数学模型入手，应用实验测得的填料段的压降，细颗粒的平均动含率及细颗粒的循环流率等实验结果，回归了数学模型中的有关参数。在此基础上，应用这一数学模型对细粉的截面平均动含率在填料层轴向的分布行为进行了研究     

The effect of interstitial gas on milling

In a research program on the influence of interstitial gas on the handling of fine powders, particle diameter less than 100 μm, the effect on milling is also investigated.The influence of the interstitial gas is exhibited through the drag force, due to velocity differences, which the gas exerts on the solid particles of the powder. These forces strongly influences the behavior of the powder.Our investigations of milling showed that the milling parameters, i.e. the specific rates of breakage and the breakage parameter, were dependent on the powder flow behavior. Two extremes were the regime of free-flowing powder, where the rate of breakage was high and the grinding of the individual particles was rather ineffective, and the regime in which the powder did not flow at all, where the rate of breakage was low, but where the grinding of the single particles was rather fine.     

Experimental study on fluidization of fine powders in rotating drums with various wall friction and baffled rotating drums

Fine powders were found to be fluidized in a rotating drum by internal cycling gas by the drum rotation. It is essentially a fluidized bed without requiring any external fluidizing gas. Such a rotating drum can be regarded as a new gasless fluidized bed for fine powders in contrast to a traditional fluidized bed, possibly leading to a considerable amount of energy savings. In addition, the fluidization quality of fine powders was 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. Five regimes were identified in the rotating drum including slipping, avalanching-sliding, aerated, fluidization and re-compacted regimes. It was also found that drum wall friction plays an important role to fluidize fine powders because the friction carries particles to the freeboard, leading to gas cycling that fluidizes the powders. As well, three types of specially designed baffles were utilized to promote powder fluidization in rotating drums. These baffles effectively bring an early onset of all the regimes in rotating drums by reducing powder-wall slipping, carrying particles and bringing additional gas to the powders.     

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.     

细颗粒振动流态化行为的二维床观察

本文利用一种新型复合振动充化装置，使在一般操作条件下无法实现正常流化的细颗粒床层处于良好的流化状态。通过观察发现，振动的引入可以破坏沟流的存在而使细颗粒流化；其振动流化有三种存在形式，提出了振动流化的物理模型和流化相同，可以解释细颗流化状态与振动条件及其物性的关系。     

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.     

Characterization of powder flow: Static and dynamic testing

Many characterization techniques are available to determine the flow properties of powders; however, it is debated which method(s) are the most appropriate. In this study, sample fine powders with a medium particle size between 22 and 31 µm were characterized using a variety of techniques that tested powders under different stress states, ranging from static to dynamic. It was found that characterization techniques that were more dynamic such as fluidized bed expansion were best suited for predicting the fluidization performance while characterization techniques that were more static such as cohesion were better for predicting agglomeration. It was also found that results from static and dynamic characterization do not necessarily agree, where fine powders that showed good fluidization performance also displayed increased agglomeration, and vice versa. This suggests that flow properties are dependent upon the stress state and that no single technique is suitable for the full characterization of a powder. In other words, both static and dynamic characterization techniques must be employed to completely understand the flow properties of a powder and predict how it will behave under different process conditions.     

水平气固两相高浓度输送流动机理

本文通过水平气固两相分层流动模型，分析了高浓度、高混合比、连续低速气力输送流动机理研究表明，稳定分层流动输送时，悬浮颗粒易集中于滑动床表面；层问颗粒剪切力是滑动床移动的上要驱动力；滑动床高度大于管道半径时将出现不稳定输送