搅拌流化床中粘结性颗粒流态化特性研究

发布时间：2018-05-28 11:47

本文选题：气固流化床 + 搅拌　；参考：《浙江大学》2015年硕士论文

【摘要】：气固流化床反应器广泛应用于聚烯烃工业,高粘结性固体颗粒较难流态化,极易发生聚合物粘釜、颗粒聚团结块甚至死床等不正常操作现象,影响流化床装置的正常运行。为了解决粘结性颗粒稳定流态化的问题,论文在普通流化床中引入搅拌构件,设计和开发具有自清洁特征的搅拌流化床反应器。围绕这一目标,开展了三方面的研究工作：(1)搅拌流化床中D类颗粒的流化特性研究；(2)搅拌流化床中粘结性颗粒的流态化实验研究；(3)粘结性颗粒流态化的数值模拟研究。取得了以下研究成果：(1)获得了搅拌对Geldart D类颗粒流态化行为的影响规律。搅拌可以抑制和破碎气泡,使得搅拌流化床与普通流化床相比具有较小的气泡尺寸和相对较低的压力脉动；搅拌与气流湍动协同作用,大桨叶面积的自清洁桨搅拌作用强烈,适宜中等转速的操作条件,而较高的转速易形成桨叶前方的颗粒堆积和桨叶后方的气体短路等不利现象,双层锚式桨、框式桨等小桨叶面积的搅拌桨需要较高的转速以强化流态化过程。(2)开发了自清洁搅拌流化床反应器,实现了粘结性固体颗粒的稳定流态化。研究了石蜡颗粒在普通流化床、双层锚式搅拌流化床和自清洁搅拌流化床中流态化与粘结行为,搅拌桨叶与内构件的啮合剪切作用可以有效破碎粘结性颗粒的聚团,维持床层的聚团流态化；采用床层压降、压力脉动标准偏差和S吸引子多种指标监测流态化过程,S值能及时准确地反映颗粒的粘结程度与粘结发展趋势。(3)获得了颗粒间相互作用对气固流动行为的影响规律。基于计算流体力学(CFD)模拟发现气固流动强烈依赖于颗粒碰撞的恢复系数,当恢复系数较大时,颗粒趋于理想碰撞,床层发生均匀流态化,与转速无关；搅拌转速较低时,流化质量改善不明显,转速较高时,气泡尺寸减小,床层可由均匀流态化向均匀流态化转变。基于二维床实验和离散单元法(DEM)模拟发现,粘结性颗粒首先粘附在一起形成粘结核,颗粒粘结性较强时,越来越多的颗粒粘附于粘结核上,形成单边或双边架桥结构,很短的时间就会发生死床；若颗粒的粘结性不强,床层中气流作用与颗粒的粘附作用相当,则以聚团的形式流态化。
[Abstract]:Gas-solid fluidized bed reactor is widely used in polyolefin industry. It is difficult to fluidize solid particles with high adhesion, and it is easy to occur abnormal operation phenomena such as polymer cladding kettle, particle agglomeration and even dead bed, which affects the normal operation of fluidized bed plant. In order to solve the problem of stable fluidization of cohesive particles, a stirred bed reactor with self-cleaning characteristics was designed and developed by introducing a mixing component into a common fluidized bed. Around this goal, three aspects of research work were carried out: 1) fluidization characteristics of D particles in agitated fluidized bed; (2) fluidization experiment of cohesive particles in agitated fluidized bed; (3) numerical simulation study on fluidization of cohesive particles. The effect of stirring on fluidization behavior of Geldart D particles was obtained. The agitation can restrain and break the bubble, make the agitated fluidized bed have smaller bubble size and lower pressure pulsation compared with the conventional fluidized bed. It is suitable for the operation condition of medium speed, but the higher speed is easy to form the unfavorable phenomena, such as the accumulation of particles in front of the blade and the gas short circuit behind the blade, etc. A self-cleaning agitated fluidized bed reactor was developed for the agitator with small blade area such as frame propeller, which needs higher rotating speed to strengthen the fluidization process. The stable fluidization of cohesive solid particles was realized. The fluidization and bonding behavior of paraffin particles in conventional fluidized bed, two-layer anchor agitated fluidized bed and self-cleaning agitated fluidized bed were studied. Maintain the agglomeration fluidization of the bed, adopt the pressure drop of the bed, The pressure fluctuation standard deviation and S attractor can accurately reflect the bonding degree of particles and the development trend of bonding in time. The influence of particle interaction on gas-solid flow behavior is obtained. Based on computational fluid dynamics (CFD) simulation, it is found that the gas-solid flow strongly depends on the recovery coefficient of particle collision. When the recovery coefficient is large, the particle tends to collide perfectly, and the bed has a uniform fluidization, which is independent of the rotational speed, and when the stirring speed is low, The fluidization quality is not improved obviously, and the bubble size decreases when the rotating speed is high, and the bed can be changed from uniform fluidization to uniform fluidization. Based on the two-dimensional bed experiment and discrete element method (DEM) simulation, it is found that the cohesive particles first adhere to each other to form a cohesive core, and when the particle adhesion is strong, more and more particles adhere to the bonding nucleus, forming a unilateral or bilateral bridging structure. The dead bed will occur in a very short time, and if the adhesion of particles is not strong and the air flow in the bed is equivalent to the adhesion of particles, the fluidization will be in the form of agglomeration.
【学位授予单位】：浙江大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TQ021.1

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