振动流化床中纳米颗粒的流态化行为

【作者】 杨静思；

【导师】 周涛；

【作者基本信息】 中南大学 ， 化学工程， 2008， 硕士

【摘要】 在传统流化床中考察了黏性纳米SiO₂、ZnO、TiO₂颗粒的流化性能,由于颗粒所特有的物理性质与表面性质,研究发现三种纳米颗粒在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,扬析严重,流化效果较差。三种颗粒在振动流化床中均出现平衡压降高于床层重力产生的压降的现象。振动有效地减小了聚团尺寸,提高了床层的气含率,明显改善了流化效果。恒定振幅（3mm）下,振动频率的增加可显著降低纳米颗粒的最小流化速度。与无振动时相比,频率为40Hz时纳米SiO₂的最小流化速度可降低50%;而恒定频率（40Hz）下,振幅的变化对最小流化速度的影响并不是很大。不同高径比下SiO₂物料流态化实验表明,初始颗粒床层高度对颗粒流化性能有较大影响。当床径为一定值（40mm）时,初始床层越高,振动波传播受到的阻碍越大,振动效果越不明显。本实验中,h_o/D=1时,纳米SiO₂的流化效果最好。振动和气速协同作用可以有效地抑制气泡的形成与合并。低气速、高振动频率或高气速、低振动频率都可以获得好的流化效果。与振动频率、振幅和气速相比,流化次数对黏性颗粒的流态化影响相对较微弱,由于在二次流化后形成相对较为稳定的聚团,故颗粒床流化性能基本上不再随流化次数的增加而发生较大改变。基于Ergun方程和Richardson-Zaki方程定义了两种聚团直径——最小流化速度聚团直径和终端速度聚团直径。通过计算,对黏性纳米聚团的减小进行了证实,并据此对聚团的合并与破碎进行了分析。计算结果与实验值一致。更多还原

【Abstract】 The behavior of different cohesive nano-particles was investigated in the conventional fluidized bed. It was found that slugs and channels were formed firstly under low superficial gas velocity due to the specific physical and surficial properties of particle. With increasing the superficial gas velocity, slugs and channels vanished, whilst the bubbling in bed was intensified obviously, and there existed a distribution of agglomerate sizes, namely, the agglomerates sizes were small in the upper zone and large in the lower zone. Because of the severe entrainment, the fluidization ability was poor.Vibrations imposed on gas-fluidized beds of nano-particles caused increase in equilibrium pressure drop, and improved fluidization quality obviously with effectively reduction of agglomerate size as well as looser bed layer. At certain amplitude （3mm） of vibrations applied, the minimum fluidization velocity decreased significantly with increasing vibration frequency. Comparing to the fluidization without vibration, the minimum fluidization velocity of SiO₂ under vibration in this study could be reduced by 50%; while the minimum fluidization velocity was nearly independent of vibration amplitude at almost constant frequency of about 40Hz. Experimental results proved that a proper original bed height of nano-particles with a constant bed diameter （40mm） contributed to better fluidization quality which could be obtained at h₀/D=1 for SiO₂ nano-particles. Further, combinations of high gas velocity and low vibration frequency as well as high vibration frequency and low gas velocity assured two domains with favorable conditions to perform stable fluidization. It could be concluded that the effect of fluidization times on fluidization behavior was relatively small comparing with the other three parameters （vibration frequency, amplitude, gas velocity）. The agglomerates sizes were relatively stable after the second fluidization.Combining the Ergun and the Richardson-Zaki equations, two types of agglomerate size, namely agglomerate size at the minimum fluidization velocity and agglomerate size at the terminal fluidization velocity, have been calculated to confirm the reduction of agglomerate size. The probability of agglomerate coalescence and breakup has beenanalyzed. The calculated values were in agreement with the experimentalresults.更多还原