超细粉在声场导向管喷流床中的聚团尺寸预测模型

超细粉的流化性能与聚团尺寸密切相关。通过分析超细粉聚团在声场导向管喷流床中的形成过程,提出了高速射流的剪切作用和聚团间的碰撞作用是决定聚团尺寸的主要原因。在此基础上,结合聚团在射流剪切过程和聚团间碰撞过程中的力平衡分析,建立了声场导向管喷流床中聚团尺寸分布的预测模型;并运用这一模型成功预测了不同射流气速下,超细TiO_2颗粒在声场导向管喷流床中的聚团平均直径和聚团尺寸分布。     

超细粉在声场导向管喷流床中的聚团尺寸预测模型

超细粉的流化性能与聚团尺寸密切相关。通过分析超细粉聚团在声场导向管喷流床中的形成过程,提出了高速射流的剪切作用和聚团间的碰撞作用是决定聚团尺寸的主要原因。在此基础上,结合聚团在射流剪切过程和聚团间碰撞过程中的力平衡分析,建立了声场导向管喷流床中聚团尺寸分布的预测模型;并运用这一模型成功预测了不同射流气速下,超细TiO₂颗粒在声场导向管喷流床中的聚团平均直径和聚团尺寸分布。     

粒径及声场对SiO2超细颗粒流化行为的影响

采用内径为56 mm的玻璃管流化床,考察了平均粒径分别为5~10 nm(1#), 0.5 mm(2#)及10 mm(3#)的SiO2超细颗粒在无声场及声场存在下的流化行为. 无声场时,1#和2#颗粒可在较高的气速下形成稳定聚团,单位质量颗粒团间作用力与原生颗粒相比显著下降,因而可实现稳定的聚团流化,3#颗粒因颗粒间粘性力较大,无法实现稳定流化. 40~60 Hz的声场对3种超细颗粒的流化行为均可起到一定的改善作用,在此频率范围外,声场的作用不明显. 提高声压级,可以使1#和2#颗粒团发生一定程度的破碎,聚团尺寸减小,最小流化速度降低. 在实验范围内,添加声场无法使3#颗粒实现稳定流化.     

纳米级SiO2颗粒流化床的塌落行为

在直径为 35mm的小型流化床中考察了纳米级SiO2 颗粒 (商品牌号R972 )的床层塌落行为 .该种经过表面改性的颗粒可以通过自团聚形成稳定、均匀的聚团散式流化 .实验证明 ,这种颗粒的床层塌落行为明显不同于普通的GeldartA类与C类颗粒 ,其塌落过程可分为恒速沉降阶段和减速压实阶段 .通过对塌落速率的分析 ,提出了聚团散式流化的床层塌落机制     

纳米级SiO2颗粒流化床的塌落行为

在直径为 35mm的小型流化床中考察了纳米级SiO₂ 颗粒 (商品牌号R972 )的床层塌落行为 .该种经过表面改性的颗粒可以通过自团聚形成稳定、均匀的聚团散式流化 .实验证明 ,这种颗粒的床层塌落行为明显不同于普通的GeldartA类与C类颗粒 ,其塌落过程可分为恒速沉降阶段和减速压实阶段 .通过对塌落速率的分析 ,提出了聚团散式流化的床层塌落机制     

粘附性颗粒流态化研究进展

主要阐述了粘附性颗粒流态化特性（初始流化速度、床膨胀、塌落）及聚团流化过程,提出了粘附性颗粒流态化的机理是聚团流化．并对近年来国内外在预测聚团大小、床层结构模型、改善粘附性颗粒流化性能方面的研究进展进行了总结．     

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。     

超细粉在内循环流化床中的流态化特性

在内径120 mm的半圆柱形内循环流化床中,以平均粒径387 nm的Ti O2为原料,考察了单独通入流化气、射流气和同时通入流化气和射流气三种流化方式下超细粉的流化特性以及射流气速对超细粉聚团尺寸的影响。结果表明:同时通入流化气和射流气时,流化气能促进粉体循环,消除环隙死区;高速射流能有效破碎聚团,显著减小聚团尺寸,从而使超细粉在环隙区与导流管之间形成稳定循环,小聚团在环隙区实现平稳流态化。随着射流气速的增大,聚团尺寸减小,粒度分布变窄,在射流气速分别为60,90,120,150 m/s的条件下,聚团平均直径分别为194,158,147,135μm。     

无团聚Yb2O3—ZrO2陶瓷超细粉的制备

本实验用溶胶－凝胶法制备了平均颗粒尺寸30nm左右的Yb2O3-ZrO2超细粉，获得了粉体颗粒小、分布均匀，颗粒形貌较理想，无硬团聚的超细粉，并探讨了高分子表面活性剂对粉末性能的影响。     

在具有锥形料腿的循环流化床中流化CaCO3超细颗粒

根据粘附性颗粒在流化过程中形成的聚团具有较宽粒径分布并因此导致大聚团在流化床中沉积和死床的问题,提出了循环流化床的锥形回料系统设计. 该回料系统包括两部分：锥形料腿和带辅助进气的V型阀. 实验证明,锥形料腿通过提供变化的表观流化气速,克服了流化聚团沉积死床等现象;而V型阀的辅助进气,对于保证V型阀顺利输送粘附性颗粒具有关键性作用. 借助这种回料系统,实现了高粘附性超细CaCO3颗粒在循环流化床的稳定快速流化. 从提升管内部拍摄的照片显示,尽管提升管采用较高的流化气体速度,但超细CaCO3颗粒仍然是以聚团的形式被流化. 对在提升管不同高度采集的聚团分析表明,处于快速流化状态的CaCO3聚团的直径远小于传统流化床中聚团的直径,并且在提升管高度方向聚团直径没有较大的变化. 同时实验还显示,提升管轴向空隙率呈S型分布,而径向则体现环-核结构,具有典型的快速床特征.     

Review on the nanoparticle fluidization science and technology

Gas fluidization has an ability to turn static particles to fluid-like dense flow, which allows greatly improved heat transfer among porous powders and highly efficient solid processing to become reality. As the rising star of current scientific research, some nanoparticles can also be fluidized in the form of agglomerates, with sizes ranging from tens to hundreds of microns. Herein, we have reviewed the recent progress on nanomaterial agglomeration and their fluidization behavior, the assisted techniques to enhance the fluidization of nanomaterials, including some mechanical measures, external fields and improved gas injections, as well as their effects on solid fluidization and mixing behaviors. Most of these techniques are effective in breaking large agglomerates and promoting particulate fluidization, meanwhile, the solid mixing is intensified under assisted fluidization. The applications of nanofluidization in nanostructured material production and sustainable chemical industry are further presented. In summary, the fluidization science of multidimensional, multicomponent and multifunctional particles, theirmulti-phase characterization, and the guideline of fluidized bed coupled process are prerequisites for the sustainable development of fluidized bed based materials, energy and chemical industry.     

On the fluidization of cohesive powders: Differences and similarities between micro- and nano-sized particle gas–solid fluidization

The fluidization of cohesive powders has been extensively researched over the years. When looking at literature on the fluidization of cohesive particles, one will often find papers concerned with only micro- or only nano-sized powders. It is, however, unclear whether they should be treated differently at all. In this paper, we look at differences and similarities between cohesive powders across the size range of several nanometres to 10s of micrometres. Classification of fluidization behaviour based on particle size was found to be troublesome since cohesive powders form agglomerates and using the properties of these agglomerates introduces new problems. When looking at inter-particle forces, it is found that van der Waals forces dominate across the entire size range that is considered. Furthermore, when looking into agglomeration and modelling thereof, it was found that there is a fundamental difference between the size ranges in the way they agglomerate. Where the transition between the types of agglomeration is located is, however, unknown. Finally, how models are made and agglomerate sizes are measured is currently insufficient to accurately predict or measure their sizes consistently.     

Processing and Sintering of Ultrafine MgO-ZrO2 and (MgO, Y2O3)-ZrO2 Powders

Chemical coprecipitation was used to produce ultrafine and easily sinterable MgO-stabilized and (MgO, Y₂O₃) stabilized ZrO₂ powders. The sintering behavior is very sensitive to post-precipitation washing because "hard" agglomerates form when the precipitated gels are washed with water, whereas "soft" agglomerates form when they are washed with ethanol. The soft agglomerates pack uniformly, resulting in homogeneous shrinkage of powder compacts to near-theoretical density. The hard agglomerates result in compacts which have regions of localized densification and a signifiint fraction of residual porosity.     

细颗粒的团聚流态化及其判定

本文在考查了不同细颗粒的流态化过程及物料的气动特性与粘附力关系的基础上，提出了当量流态化的概念；实验发现聚团密度的减小是改善细颗粒流化性能的一种有效途径；根据气动情况下不同的成团结果，把细颗粒的聚团流态化分成了三类：沟流；似A类聚团流态化；似B／D类聚团流态化；同时，结合实验结果，给出了不同细颗粒聚团流态化类型的定量判据     

Analysis of agglomerated packings

Results of computer simulation of the packing of particles in compacts from agglomerated powders are presented. The effect of the characteristics of agglomerated powders, such as the number of particles in the agglomerates, the size distribution of agglomerates, and the volume share of the fine fraction (individual particles) on the factors that determine the sinterability of compacts, i.e., the density, the mean number of contacts per particle, and the mean size and the mean coordination number of the pores, is investigated. It is established that compared to compacts from individual particles the presence of agglomerates sharply worsens the packing characteristics. The worst effect is due to agglomerates containing less that 30 – 40 particles. The packing characteristics can be improved by using powders with a wide size distribution of agglomerates or by adding unagglomerated particles. It is interesting that computer models of powder compacts can be used for predicting the strength properties of the materials sintered from these powders. Data on the influence of the packing characteristics on the mean strength and the Weibull modulus are presented.Translated from Ogneupory, No. 4, pp. 14–17, April, 1995.     

Fluidization enhancement of agglomerates of metal oxide nanopowders by microjets

The quality of gas–solid fluidization of agglomerates of nanoparticles has been greatly enhanced by adding a secondary flow in the form of a high‐velocity jet produced by one or more micronozzles pointing vertically downward toward the distributor. The micronozzles produced a jet with sufficient velocity (hundreds of meters per second), turbulence, and shear to break‐up large nanoagglomerates, prevent channeling, curtail bubbling, and promote liquid‐like fluidization. For example, Aerosil R974, an agglomerate particulate fluidization (APF) type nanopowder, expanded up to 50 times its original bed height, and difficult to fluidize agglomerate bubbling fluidization (ABF) type nanopowders, such as Aeroxide TiO2 P25, were converted to APF type behavior, showing large bed expansions and homogeneous fluidization without bubbles. Microjet‐assisted nanofluidization was also found to improve solids motion and prevent powder packing in an internal, is easily scaled‐up, and can mix and blend different species of nanoparticles on the nanoscale. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Effects of acoustic vibration on nano and sub-micron powders fluidization

Fluidization of nano and sub-micron powders with and without acoustic vibration was investigated. The effects of sound pressure level and frequency were studied. Loudspeakers located under the distributor plate were used as the sound source to disintegrate larger agglomerates concentrated at the bottom of the bed. Nanoparticles showed fluid-like behavior similar to Geldart's A group and application of sound vibration improved their fluidization quality. Submicron particles were hard to fluidize and their fluidization quality was partially improved by sound excitation. Bed compaction, caused by rearranging of the agglomerates, was observed for submicron particles at low gas velocities while the bed was fixed. Nanoparticles did not experience any bed compaction. Sound vibration led to a decrease in minimum fluidization velocity and an increase in bed pressure drop and bed expansion for both types of particles. The fluidization quality of both particles increased at low frequencies, while the reverse was observed at higher frequencies. Fluidization of these particles was improved by increasing sound pressure level. There was a critical sound pressure level of 110 dB, below which the effect of sound vibration was insignificant. A novel technique was employed to find the apparent minimum fluidization velocity from pressure drop signals.     

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.