纳米颗粒在振动流化床中的聚团流态化研究

【作者】 王辉；

【导师】 周涛；

【作者基本信息】 中南大学 ， 化学工艺， 2010， 硕士

【摘要】 纳米颗粒(属于Geldart C类颗粒,亦称黏性颗粒)在流化过程中易形成流化聚团、并最终以聚团形式流化,多数粘性颗粒的流化聚团具有较宽的粒径分布。目前,气固流态化领域中颗粒团聚机理及团聚物性质已成为研究热点。本文在传统流化床和振动流化床中,对纳米级SiO2、ZnO、TiO2三种颗粒进行了流态化实验,研究了其聚团流化性能和聚团尺寸变化规律。经研究发现,由于颗粒所特有的物理性质,三种纳米颗粒在传统流化床中低表观气速下易形成活塞和沟流。随表观气速的增加,床内鼓泡加剧,活塞与沟流消失,床中均出现分层现象,聚团偏析大,扬析严重,流化效果较差。振动能的引入可以有效的消除节涌、抑制沟流、降低最小流化速度、减小聚团平均尺寸和整个床层的偏析率,显著的改善了纳米颗粒的流化质量。同时,本实验系统的考察了操作条件对聚团尺寸的影响。实验结果表明：纳米颗粒在流化床中流化一段时间后聚团才形成比较稳定的硬聚团。初始床层越高,振动波传播受到的阻碍越大,浅床层(h0/D=0.8或1)有利于振动能的吸收,聚团流化性能好,但总体效果是高径比对聚团尺寸影响较小。振动参数对聚团尺寸的影响具有两面性,在恒定振幅下,试验初期随振动频率的增加聚团尺寸从300μm降低到100μ左右。当振动频率超过一临界值时,聚团尺寸随振动频率的增加反而增加。在A=3 mm时,SiO2、TiO2和ZnO的临界频率分别为10 Hz、15 Hz、15Hz。可能原因是额外的振动能不仅可以促进聚团的破碎,还可以增加颗粒间或聚团间的接触机会,在黏性力较大的情况下易形成大聚团。振动频率对聚团尺寸的影响要强于振幅。另外,较高气速可以使聚团大小减小。根据纳米颗粒聚团在振动流化床中碰撞能、有效振动能、流体剪切能和黏性能的平衡分析,建立了估算聚团大小的能量平衡模型。结果表明：随着振动强度的加大聚团尺寸的实验值和计算值均减小,在振动条件下该模型预测值与试验结果吻合较好,说明了模型的合理性。基于振动机理,探讨了振动床中聚团团聚和解聚的机理。本文研究为进一步开展有关研究和开发超细粉振动场流化床打下了良好的基础。更多还原

【Abstract】 Nano-particles, belonging to Geldart’s group C patricles or cohesive powders, are easy to aggregate into fluidized agglomerates during fluidization, which results in factually agglomerates of cohesive powders. Usually, the fluidized agglomerates are in wide size distribution. Measurements of agglomerate properties and study on the mechanism for particle agglomerating behavior have been one of the hot spots in the field of fluidization.The behavior and agglomerate sizes of different cohesive SiO2, TiO2 and ZnO nano-particles are investigated in the conventional fluidized bed (CFB) and vibro-fluidized bed (VFB). It was found that slugs and channels were formed firstly under low superficial gas velocity due to the specific physical and surficial properties of particles. With increasing the superficial gas velocity, slugs and channels vanished, whilst the bubbling in bed was intensified obviously, and there existed a severe degree of the size-segregation of the agglomerates throughout the whole bed, namely, the agglomerates sizes were small in the upper zone and large in the lower zone. Because of the severe entrainment, the fluidization quality was poor. The experimental results showed that slugging and channeling of bed disappeared, the minimum fluidization velocity and the measured agglomerate sizes were decreased, and the fluidization quality was significantly improved due to introduction of vibration energy. The effects of vibration parameters on three nano-particles agglomerate sizes are systematically investigated in VFB. The nano-particles agglomerates were formed during fluidization after a period of fluidization time. Vibration wave propagation was hampered by powers under higher original bed height of nano-particles, and shallow bed contributed to the absorption of vibration energy and better agglomerates fluidization quality which could be obtained at ho/D=0.8 or 1 for nanoparticles, while ratio of height to diameter had little effect on agglomerates sizes. Meanwhile, mechanical vibration had complex effect on agglomerate size. The average agglomerate size is changed from 300μm to around 100μm when the vibration is applied. The effect of vibration on the agglomeration in vibro-fluidized beds of nano-particles depends on the critical vibration frequency corresponding to a minimum agglomerate size, and the critical point thresholds linking to A=3 mm of SiO2, TiO2, ZnO were 10 Hz,15 Hz,15 Hz, respectively. It can help to suppress formation of agglomerates due to the additional vibration energy, and it can also prefer to the coalescence of the agglomerates due to the enhanced contacting probability between particles and/or agglomerates. However, the overall trend of agglomerate sizes decreased with increasing vibration intensity. In this context, the effect of the vibration frequency on agglomerate sizes is more obvious than that of the vibration amplitude. Both the experimental and theoretical results show that a higher gas velocity leads to a smaller agglomerate size.An energy balance model for the prediction of agglomerate sizes has also been established on the basis of energy balance of the agglomerate collision energy, the effective energy arising from vibration wave, energy generated by hydrodynamics shear and cohesive energy in VFB. The agglomerate sizes predicted is in good agreement with the experimental data for three nano-particles by the vibration excitation. Based on vibration mechanism, agglomeration and deagglomerates in VFB were also studied.更多还原