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流化床中不同粒径颗粒停留时间及其分布的调控研究
赵虎
Thesis Advisor朱庆山
2017-01
Degree Grantor中国科学院研究生院
Degree Name博士
Degree Discipline化学工程
Keyword流化床,平均停留时间,纵向挡板,宽筛分颗粒,氧化焙烧
Abstract

不同粒径颗粒在流化床中的停留时间及其分布对产物的转化率、选择性和收率有重要影响。在实际工业过程中要实现不同粒径颗粒的同步转化,则需要粗细颗粒平均停留时间与完全转化时间匹配。然而传统的流化床对不同粒径颗粒的停留时间差异的调控能力较差,往往会出现颗粒停留时间与转化时间不匹配的现象,严重影响产品质量,并导致过程能耗增加。因此,研究改善不同粒径颗粒停留时间和反应时间的匹配度具有重要意义。对此,本文设计了纵向挡板流化床和有侧出口的新型复合流化床,调控不同粒径颗粒停留时间差异。通过实验研究与计算机模拟相结合的方法,对其作用机理进行了深入分析,在此基础上,将其应用于钛铁矿流态化氧化焙烧过程,达到了不同粒径颗粒同步转化的目的。本论文的主要研究成果如下:1)系统考察了不同粒径颗粒在纵向挡板流化床中的停留时间及其分布规律。不同粒径颗粒在纵向挡板流化床内停留时间分布差异较小,特别是在混合进料时,无论是二元颗粒的混合物还是多元颗粒的混合物,不同粒径颗粒停留时间分布差异均很小。挡板高度对颗粒停留时间分布有明显影响,挡板高度越高,颗粒返混越大,颗粒停留时间分布越接近全混流,但对不同粒径颗粒平均停留时间差异影响较小。虽然气速也是影响颗粒平均停留时间的因素(气速与颗粒平均停留时间反相关),但在纵向挡板流化床内,气速对不同粒径颗粒平均停留时间差异的调控能力较差,不能满足不同粒径颗粒同步转化的要求。2)在新型复合流化床内,研究了气速和不同颗粒组成对不同粒径颗粒平均停留时间差异的影响。粗细颗粒的平均停留时间比率随着气速的增加先增大后减小,转变点在粗颗粒的终端速度附近。增大进料组成中粗颗粒的比例或者提高进料组成中粗颗粒的粒径,有利于增加粗细颗粒的平均停留时间差异。不同粒径颗粒停留时间存在差异的原因是颗粒分级,通过调节气速、增加粗颗粒浓度和粒径比率可以强化颗粒分级,达到增大不同粒径颗粒停留时间差异目的。实验结果表明,利用新型复合流化床调控不同粒径颗粒平均停留时间差异,可以满足化学反应控制和灰层扩散控制对不同粒径颗粒同步转化的需求。3)通过Fluent流体模拟软件对不同粒径颗粒在新型复合流化床中的流动进行了模拟。在相同床层高度时,粗颗粒的径向固含率明显大于细颗粒的,这是粗颗粒的平均停留时间大于细颗粒的直接原因。粗颗粒和细颗粒在浓相区的速度差异很小,使得粗细颗粒速度差的累积效应对不同粒径颗粒停留时间比率的影响较小。在侧出口位置不同粒径颗粒的速度方向不同,使得不同粒径颗粒发生分级,这是气速小于粗颗粒终端速度时,不同粒径颗粒停留时间差异增加的根本原因。4)采用新型复合流化床对不同粒径钛铁矿流态化氧化焙烧过程中的床内平均停留时间进行调控。结果表明,钛铁矿的氧化速率受界面化学反应控制和灰层扩散控制共同影响。通过分析氧化动力学数据和不同粒径颗粒停留时间比率的变化规律得出:粒径比为2.6的不同粒径钛铁矿,在温度为900 ℃,气速为0.34 m/s时,可以实现不同粒径钛铁矿的同步转化,很好地验证了冷态实验结果。;The residence time distribution (RTD) of the particles with different sizes have an important influence on the conversion, selectivity and yield of the products in the fluidized bed reactors. Different size particles require different residence times to achieve the complete conversion. However, it is difficult to match the residence times of the different size particles with their respective complete conversion times, especially when a wide size distribution (WSD) is encountered. Therefore, how to increase the matching degree between the optimum reaction time and the residence time for each solid species would be the key issue to the high quality of product for the fluidized bed reactor.In accordance with this problem, two new types of the fluidized beds with vertical baffles and side outlet were designed in this paper to modulate the mean residence time (MRT) and its difference between the particles with different sizes. The flow of the particles with different sizes was simulated to investigate the reasons of the MRT difference of particles with different sizes in the new fluidized bed with a side outlet. The synchronous of the oxidation of ilmenite particles with different sizes has been studied in the new fluidized bed with a side outlet. The principal results and major conclusions are as follows:1) The residence time and distributions of the particles with different sizes were systematically investigated in the vertical baffles fluidized bed. The RTD difference of the particles with different sizes is small in the fluidized bed of vertical baffles, especially for the mixtures of the binary or multiple with narrow size distributions. The height of the baffles has a significant effect on the RTD of the particles. The backmixing decreases with reducing the height of the baffles, but the height of the baffles has a limited effect on the MRT difference. The gas velocity has a great effect on the MRT of the particles, the MRT of particles decreases with increasing the gas velocity. But the gas velocity has little effect on the MRT difference for the particles with different sizes in the vertical baffles fluidized bed. Although the MRT difference can be modulated for the particles with different sizes, the results cannot meet the requirements of synchronous of particles with different sizes. Therefore, the fluidized bed with vertical baffles cannot be used to modulate the MRT of particles with different sizes.2) The MRT difference of particles with different sizes was studied by changing the gas velocities and feed compositions in a new type of fluidized bed with a side outlet. With increasing the gas velocity, the MRT ratio of the coarse and fine particles firstly increases and then decreases, and the transition point is near the terminal velocity of the coarse particles. It is beneficial to increase the MRT ratio of the coarse and fine particles by increasing the proportion of coarse particles or increasing the size of the coarse particles in the feed. The size grading, one of the reasons for causing MRT difference, can be strengthened by adjusting the gas velocity, increasing the concentration of coarse particles and size ratio. So the MRT difference of the particles with different sizes can be increased in the new fluidized bed to meet the requirement of the chemical reaction control and ash diffusion control reaction for the synchronous of particles with different sizes.3) The flow of the particles with different sizes was simulated in the new fluidized bed with a side outlet via Fluent. The radial solid holdup of the coarse particles is obviously larger than that of fine particles at the same height, which is the direct reason for the MRT of the coarse particles being larger than that of the fine particles. The velocity difference between the coarse particles and the fine particles in dense phase region is very small, so that the cumulative effect of the velocity difference between the coarse and fine particles has little effect on the MRT ratio of particles with different sizes. The particles with different diameters can be separated at the side outlet, where the coarse particles flow out from the side outlet more easily and the fine particles have a greater probability of flowing out from the top outlet. This is the reason for the MRT difference between the coarse and fine particles increasing with increasing the gas velocity under the terminal velocity of coarse particles.4) The synchronous of particles with different sizes was realized by modulating the MRT difference of the ilmenite particles with different sizes in the new fluidized bed. The oxidation rate of ilmenite is affected by the interaction of chemical reaction control and ash layer diffusion control. By analyzing the kinetic data of oxidation and the variation of MRT ratio of particles with different sizes, it is concluded that the synchronous of ilmenite with the size ratio of 2.6 can be achieved at a temperature of 900 ℃ in a gas velocity of 0.34 m/s. 

Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/26835
Collection中国科学院过程工程研究所
研究所(批量导入)
Recommended Citation
GB/T 7714
赵虎. 流化床中不同粒径颗粒停留时间及其分布的调控研究[D]. 中国科学院研究生院,2017.
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