CAS OpenIR
流化床中宽筛分颗粒停留时间调控研究
张立博
Thesis Advisor李洪钟
2018-07
Degree Grantor中国科学院研究生院
Degree Name博士
Degree Discipline化学工程
Keyword平均停留时间差别,宽筛分颗粒,挡板,流化床
Abstract

流化床反应器常被用于固相加工过程中处理宽筛分物料,如矿物焙烧、铁矿直接还原等等。宽筛分物料中大小不同粒径的颗粒所需完全转化时间不同,然而传统流化床中各粒级颗粒的停留时间与其所需的完全反应时间往往难以匹配。因此,对宽筛分颗粒中各粒级颗粒停留时间进行调控,使其与完全转化时间相匹配,实现宽筛分颗粒在流化床反应器中同步转化,对提高转化的选择性和效率具有重大意义。针对宽筛分颗粒平均停留时间(MRT)调控研究,选用平均粒径为1:2:4的三种窄分布玻璃珠(同密度)按一定比例混合作为宽筛分物料,在连续进出料流化床中进行研究。探寻流化床中宽筛分颗粒MRT差别影响因素和机理,并以此进行调控优化以实现不同控制机制下各粒级颗粒停留时间与其所需转化时间相匹配。在此基础之上,综合各调控因素建立可预测宽筛分颗粒各粒级颗粒MRT数学模型,为宽筛分颗粒各粒级MRT调控研究提供指导。本论文取得的主要创新性结果如下:(1) 详细研究了粒径分布、进料速率和气速等因素对宽筛分颗粒中各粒级颗粒MRT差别的影响,研究发现各粒级颗粒MRT差别随气速增加而增加,但几乎不受进料速率和粒径分布的影响。但单一的气速调控,无法实现粒径相差四倍的颗粒MRT与各粒级所需完全转化时间相匹配。此外,研究发现各粒级颗粒MRT差别源于气速对各粒级颗粒排出方式的影响不同,高表观气速下粗细颗粒在床层上方“稀相区”中产生分离,细颗粒比粗颗粒更容易因气体夹带而排出流化床,从而增加了粗细颗粒MRT差别。(2) 根据“稀相区”粗细颗粒分离增加粗细颗粒MRT差别机理,提出通过增置水平内构件在流化床中产生多个“稀相区”调控粗细颗粒MRT差别研究思路。系统考察了多孔挡板数目、位置、结构参数(开孔率和开孔孔径)以及其他类型内构件对各粒级颗粒MRT差别影响规律,发现多孔挡板调控效果最为显著,增加挡板数目、降低挡板开孔率或孔径均可增加各粒级颗粒间MRT差别。另外,研究显示每一床层中宽筛分颗粒并未出现明显分级,但从下往上的各层中粗颗粒浓度逐渐减小而细颗粒浓度逐渐增加。(3) 借鉴气液精馏塔理论,提出对多挡板床中多粒级颗粒浓度分布模型化思路,结合床层物料藏量预测,建立了可预测多挡板流化床中各粒级颗粒MRT的数学模型。模型研究发现,与以往仅能预测颗粒整体MRT的模型相比,所建模型可以准确的预测宽筛分颗粒中各粒级颗粒MRT。此外,所建模型可预测各控制条件下宽筛分颗粒MRT理想调控时所需最优气速、挡板数目以及挡板结构参数。(4) 提出利用CFD模拟方法研究流化床放大过程中宽筛分颗粒MRT差别变化。先采用欧拉—欧拉多流体模型与多组分曳力模型耦合准确模拟了多挡板流化床中多颗粒体系气固流化行为。后续利用床层平均粒径函数描述各粒级颗粒浓度变化,从而预测各粒级颗粒MRT差别程度。研究发现流化床放大过程中(床径:100 mm、190 mm和400 mm)床层平均粒径轴向变化有所减小,但变化范围并不显著,意味着应用该规则放大研究时宽筛分颗粒MRT差别程度随流化床床径增加略有减小。;Fluidized beds have been extensively applied for multi-size solids processing, such as mineral roasting, iron ore direct reduction, etc. For the wide size distribution (WSD) particles, different size particles require different residence times for a complete conversion. However, in conventional fluidized beds, it is difficult to match the residence times of the different size particles with their respective complete conversion times. It is therefore of great significance to adjust the residence times for particles with different sizes to match their complete conversion times to achieve synchronously complete conversion in a fluidized bed, which would greatly improve the conversion efficiency.To study the residence time of WSD particles, three different size glass beads with a size ratio (1:2:4) were mixed to simulated the WSD particles. The residence time of these particles was studied in a continuous fluidized bed. The influence of operation parameters on the mean residence time (MRT) difference of each size particles for the WSD particles was analyzed, and ways to adjust the MRT difference were also explored. Furthermore, a mathematics model was developed to predict the MRT of each size particles. The main results and major conclusions are as follows:(1) The effect of the feed rate, particle size distributions and gas velocity on MRT of WSD particles were studied. The MRT difference for each size particles increases with increasing the gas velocity. However, the feed rate and particle size distributions have minor influence on the MRT difference for different size particles. For the WSD particles with a maximum diameter ratio of 4, the MRT difference cannot reach 4 by only changing the gas velocity. It was found that the MRT difference is caused by the discharge mode difference of coarse and fine particles at high gas velocities, where a dilute zone formed above the bed surface, due to the fact that the most of ejected fine particles can be discharged from the bed while the ejected coarse particles often return back to the bed.(2) According to the mechanism of MRT difference, it was proposed to create multiple dilute zone in a fluidized bed through horizontal internals to enlarge the MRT difference of the coarse and fine particles. Consequently, the effect of baffle numbers, baffle position, baffle structural parameters (free area; hole diameter), and baffle types on the MRT difference for each size particles were studied. Compared with other type of baffles, the perforated baffle can significantly increase MRT difference for each size particles. Moreover, perforated baffles with a low free area or a small hole diameter are more effective to adjust the MRT difference. It was also found that no obvious segregation was found within each layer, while it showed that the concentration of the coarse particles decreases with increasing the number of the layer from the bottom. (3) Referring to the theory of gas-liquid distillation, the elutriation degree and relative elutriation degree were proposed for determining the particles concentration distribution on each layer. Combining with the hold-up prediction, a mathematic model was developed to determine the particle concentration in each layer, and to determine the MRT of each size particle. It showed that the model could well predict the MRTs of each size particles on the multi-baffle fluidized bed. Compared to previous studies about the prediction of the MRT for the particles using the average size, the proposed model could accurately predicted the MRT of each size particles. Besides, the model could predict the operation effects on the MRT difference, such as the optimum gas velocity, baffle number and baffle structural parameters.(4) It was attempted to study the MRT adjustment and the scale-up effect through CFD simulation for WSD particles. The Euler - Euler multi-fluid model with a structural drag model was used to simulate the fluidization of WSD particles in multi-baffle fluidized beds. Then the function of the average size was used to estimate the particles concentration in each layer, which can be converted to the MRT difference for each size particles. It found that the axial difference of the average size decreased slightly in scale-up of the fluidized beds (bed diameter, 100 mm, 190 mm and 400 mm), indicating that the MRT difference of WSD particles varies slightly with increasing the bed size

Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/26905
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
张立博. 流化床中宽筛分颗粒停留时间调控研究[D]. 中国科学院研究生院,2018.
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