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Alternative TitleExperimental Study and Numerical Simulation of Micromixing in a Double-Impeller Liquid-Solid Stirred Tank
Thesis Advisor程景才 ; 杨超
Degree Grantor中国科学院研究生院
Degree Discipline化学工程
Keyword微观混合   搅拌槽   固液两相   数值模拟   fm-pdf模型
Abstract在许多工业过程(比如矿物加工过程、高分子聚合、沉淀、结晶)中都伴随着快速液固反应,微观混合对这些复杂反应过程有重要的影响。微观混合效果不好往往会导致目标产物的收率及质量降低,进而增加分离成本。通过优化反应器的设计及操作,可以强化微观混合,从而提高效率,增加收益率,并且减少后续产品的分离成本。 以往对搅拌槽内微观混合的研究,侧重于单一液相体系。对于固液搅拌槽内微观混合的研究,国外也有少量报导,特别对于双层桨体系,但往往局限于宏观参数(比如固含率、颗粒等)的影响,国内关于固液两相搅拌反应器内微观混合的实验研究也极少。固体颗粒的存在对液相中微观混合的影响机理尚不明晰,深入探讨固液搅拌反应器内的微观混合过程,对化工单元设备的设计、开发和放大有重要指导意义。另外,通过CFD(计算流体力学)方法研究微观混合,可以大大降低成本和风险,也可以提供实验难以测得的反应器内微观混合一些重要信息。 本文在直径384 mm的椭圆底双层桨液固搅拌槽内,以碘化钾/碘酸钾平行竞争反应为工作体系,采用标准Rushton涡轮桨与下推式六叶斜叶桨的组合,研究不同转速、固含率、加料位置、颗粒大小以及不同能耗、桨组合、桨间距等条件下的离集指数XQ的变化;并将有限节点PDF(Probability Density Function)模型耦合到CFD中,对单相搅拌槽内的碘化钾/碘酸钾平行竞争反应过程进行了数值模拟,综合考察了宏观混合和微观混合对平行竞争反应的影响。主要取得了以下研究结果: (1) 与大颗粒体系相比,在固含率一定时,能耗增加使得粒径大小为450-600 μm的中等颗粒体系的离集指数值变化更加明显。当中等颗粒的浓度大于12.1 wt.%时,可以清楚地观察到“颗粒云”。当在颗粒云上方进料时,XQ增加显著,微观混合效果变差。 (2) 中等颗粒的浓度从0变化到20 wt.%,离集指数的值还是增加的,尽管不是很明显;当颗粒浓度小于5 wt.%时,颗粒对微观混合的影响几乎可以忽略;对含粒径为1-1.25 mm颗粒的体系,尽管颗粒浓度从1 wt.%变到9 wt.%及增加体系中输入能量,颗粒的存在对微观混合效果的影响不明显。 (3) 通过比较转速及能耗对离集指数的影响,可以看出RT+PBTD组合比RT+PRDTD组合效果好,更适合搅拌槽中的混合,因为前者不但达到相同的离集指数值时的功耗低而且湍动及悬浮能力较强。 (4) 在进料速率较大时(即宏观混合对微观混合有影响),多股进料优于单股进料,不仅能有效降低能耗,且较为显著地提高目标产物的产率,因为此时多股进料能充分利用混合能力。 (5) 有限节点PDF模型可以用于模拟连续搅拌槽中微观混合对碘化钾/碘酸钾平行竞争反应体选择性的影响。本文考察了不同转速时离集指数的变化,随着转速的增大,微观混合的效果变好,但是模拟结果与实验结果有一定的差异,自编的UDF程序还需进一步改善。 关键词:微观混合,搅拌槽,固液两相,数值模拟,FM-PDF模型
Other AbstractMicromixing may have an effect on fast and complicated reactions in many industrial processes including mineral processing, polymerization, precipitation, crystallization and biochemical processes. Poor micromixing may reduce the yield and quality of the desired products and then lead to higher purification costs. By optimizing the geometry of multiphase reactors and the operation, the effect of micromixing could be improved. Thus, the yield of the desired products will be enhanced and the purification cost will be cut down. So far, a large amount of work has been done to improve the micromixing in single-phase systems while relatively little work has been conducted in two-phase systems, especially with multiple impellers. Besides, the research in solid-liquid systems always limits to macro-parameters like particles and solid holdup, though there are some reports. However, the experimental study on micromixing in solid-liquid stirred tanks has rarely been found. The mechanism of the effect of particles on micromixing in solid-liquid systems, which are widely used in industry, is not very clear. Therefore, it is meaningful to explore micromixing in order to optimize the design of the reactors. However, with the development of computers, the CFD (computational fluid dynamics) approach can not only lower the cost and risk, but also help to provide important information on micromixing in the reactors, which is difficult to do experiments. The effect of solid particles on micromixing has been studied using the competitive iodide/iodate reaction system in stirred, multi-impeller, solid-liquid systems with a dished bottom, whose diameter is 384 mm. The influences of impeller speed, solid holdup, feed position, particle size, energy input, the combination of impellers and impeller clearance have been investigated. What’s more, the finite mode probability density function (FM-PDF) model was coupled with CFD and was embedded into Fluent, and the micromixing in a single-phase stirred tank was simulated. Both the effect of macromixing and micromixing on the selectivity of the parallel reactions has been considered. The main conclusions are as follows: (1) The change of the segregation index with the power input was more distinguishable only for the 450-600 μm particles as compared with the larger ones, at the same solid holdups. Also, for the smaller ones, cloud formation was clearly observed at a particle concentration of 12.1 wt.%. When feeding into the clear liquid above the cloud, the value of XQ increased surprisingly because of poor turbulence. (2) With the presence of the medium-sized particles, higher segregation index was observed whereas not very notable as the solid concentration increased from 0 to 20wt.%. The influence was found to be negligible when the particle concentration was less than 5wt.%. In the presence of the glass particles of 1-1.25 mm, when feeding at the tip of the pitched blade turbine, the influence on the selectivity was negligible, though the solid concentration was changed from 1 wt.% to 9 wt.% and the energy input was raised. (3) By comparison, it can be concluded that the combination of RT and PBTD was better and more suitable for mixing in the stirred tank than the combination of RT and PRDTD, because it had much lower just-suspended impeller speed and energy dissipation rate. (4) When the feeding rate was much faster, distributed feeding of reactants into more reactor volumes with high energy dissipation rate was beneficial to the overall selectivity. Multiple feeds could utilize more micromixing capacity to improve the selectivity of the desired products. (5) By using the finite mode PDF, the effect of micromixing on the iodide/iodate parallel reaction has been simulated in the continuous stirred tank. The influence of impeller speed on the selectivity has been investigated. Micromixing effect was raised with increasing impeller speed. However, the simulation results were not well coincident with the experimental data. Thus, the UDF code needs to be improved. Key words:Micromixing, Stirred Tank, Solid-Liquid System, Numerical Simulation, FM-PDF Model
Document Type学位论文
Recommended Citation
GB/T 7714
杨雷. 液固搅拌槽中微观混合的实验研究及数值模拟[D]. 中国科学院研究生院,2013.
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