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液液和气液液搅拌槽内混合过程的数值模拟与实验研究
Alternative TitleNumerical Simulation and Experimental Studies on Mixing Processes in Liquid-Liquid and Gas-Liquid-Liquid Stirred Reactors
程荡
Subtype博士
Thesis Advisor杨超
2013-11
Degree Grantor中国科学院研究生院
Degree Discipline化学工程
Keyword多相搅拌槽 宏观混合 微观混合 数值模拟 实验研究 快速沉淀反应
Abstract液液及气液液搅拌槽广泛应用在化工、冶金和环境工程等工业过程中。强烈的液滴(气泡)聚并破碎和可能的相反转现象等,使得该类反应器的多相流动和混合过程极其复杂。如果搅拌槽反应器内涉及非常复杂的化学反应,尤其是快速反应,其产率和产品质量与反应器内的多相混合状态密切相关。目前,国内外从实验和数值模拟两方面对液液及气液液搅拌槽内混合过程的研究还极少,对其内部的混合机理及规律还缺乏认识。因此,深入研究液液及气液液搅拌槽内的混合特性对于相关多相反应器的操控、设计、优化和放大具有重要的意义。基于此,本文从实验和数值模拟两方面对液液及气液液搅拌槽的混合特性进行了研究,取得了以下成果:(1)基于Eulerian-Eulerian观点的“两流体”模型,采用k-ε-Ap两相湍流模型对体系的湍流特性进行描述,结合改进的“内外迭代法”,对Rushton桨搅拌槽内液液两相搅拌混合体系的流体动力学特性进行了三维全流场数值模拟。研究了三个不同曳力系数模型对相含率分布的影响,以及不同均一滴径对流场的影响。对分散相完全分散的临界转速进行了预测,提出了二种数值判据以确定临界转速,当选择合适的滴径时,两个数值判据是有效的。(2)数值模拟研究了Rushton桨液液搅拌槽内连续相中的宏观混合过程。流场与示踪剂输运方程解耦求解,两相湍流计算采用各向同性的k-ε-Ap模型和各向异性的EASM模型。结果表明,EASM模型与标准k–ε模型都能预测出分散相相含率和粘度对混合的影响;在示踪剂响应曲线和混合时间的预测上,EASM模型的预测结果好于标准k–ε模型。(3)在直径为240 mm的平底圆柱有机玻璃槽内,以空气和惰性油相为分散相,自来水为连续相,采用电导率方法对气液液搅拌槽内连续相中的宏观混合过程进行实验研究。考察了示踪剂加入点位置、电导率监测位置、桨型、转速、桨位置、油相体积分数及气含率等对混合时间和功耗的影响,并拟合实验数据得到气液液体系混合时间关联式。研究发现,低气含率抑制连续相宏观混合,高气含率促进宏观混合。与气相影响相反,油相在低相含率时增强宏观混合强度,在高相含率时减弱宏观混合强度。气液液体系功耗随油相相含率和气流率增加而减小。(4)在直径为240 mm的平底圆柱有机玻璃槽内,以空气和煤油为分散相,硫酸钡水溶液为连续相,采用硫酸钠与氯化钡沉淀反应来表征气液液复杂体系中的微观混合效应,对气液液搅拌槽内的微观混合过程进行实验研究。考察了加料点位置、桨型、煤油相含率、气流率、转速和表面活性剂等对沉淀产物的粒度分布及形貌等的影响。结果发现,气液液体系中平均粒径随油相体积分数先增大后减小;随气流率增大,平均粒径先增大后减小;惰性相的加入不影响连续相中沉淀颗粒的形貌。(5)基于Eulerian-Eulerian多流体的观点,将单相有限节点的概率密度函数模型扩展为两相形式,来描述液液两相体系中微观混合对硫酸钡沉淀反应的影响。同时实验研究了液液搅拌槽内微观混合对硫酸钡沉淀反应的影响。研究发现,SP-FM-PDF(Single-phase finite-mode probability density function)模型不适合于两相体系。本文推导的TP-FM-PDF(Two-phase finite-mode probability density function)模型能较好地预测出油相相含率对平均粒径的影响,且预测值与实验值吻合较好。SP-FM-PDF模型和TP-FM-PDF模型都成功预测出了转速对平均粒径的影响,TP-FM-PDF预测值更接近实验值。(6)采用PLIF(Planar laser induced fluorescence)方法结合折射率匹配技术测量了水(分散相)—硅油(连续相)体系中分散相的宏观混合时间。研究发现,轴流桨的分散相混合时间比径流桨大;径流桨(RDT和HCDT)高度从T/6上升到T/3,混合时间减小;分散相混合时间随分散相增多而减小。
Other AbstractLiquid-liquid and gas-liquid-liquid stirred reactors are extensively used in industrial processes, such as chemical industry, metallurgy and environmental engineering. The intense drop breakup and coalescence as well as possible phase inversion make the multiphase flows and mixing processes in such reactors are extremely complicated. The productivity and product quality of fast chemical reactions in liquid-liquid and gas-liquid-liquid stirred reactors are usually dependent on the macro- and micro-mixing efficiencies. To the best of our knowledge, there is very little information on the mixing processes in liquid-liquid and gas-liquid-liquid stirred systems. In-depth study on their mixing characteristics is expected to shed new light on the control, design, optimization and scale-up of such complex stirred reactors. Accordingly, their mixing processes are experimentally and numerically studied in this work. The important findings are concluded as follows.(1) Based on the Eulerian-Eulerian two-fluid approach with the two-phase k-ε-Ap turbulence model, the three-dimensional flow of immiscible liquid-liquid dispersion in a baffled stirred reactor with a Rushton impeller is numerically simulated. An improved inner-outer interative procedure is adopted to deal with the impeller rotation. Different drag formulations are examined, and the effect of droplet size on both the dispersed phase holdup distribution and the velocity field is analyzed. Two numerical criteria are proposed to predict the critical impeller speed. The computational model produces fairly good results only when a suitable droplet diameter is assumed.(2) Numerical simulations of turbulent immiscible liquid-liquid mixing processes in a cylindrical stirred reactor driven by a Rushton turbine are carried out. The hydrodynamic equations and the tracer transport equation are resolved in an uncoupled way. An isotropic standard k-ε turbulence model and an anisotropic two-phase explicit algebraic stress model (EASM) are used for flow field simulations. Quantitative comparisons of the homogenization curve and mixing time predicted by the EASM are conducted with reported experimental data and other predictions by the standard k-ε model. The comparisons show that the EASM predictions are in satisfactory agreement with experimental data and better than the k-ε model ones. (3) The gas-liquid-liquid macro-mixing in a stirred reactor of standard geometry with a diameter of 240 mm has been investigated. Tap water is used as the continuous phase, immiscible oil and air as the dispersed ones. The mixing time of the continuous phase and the power consumption have been determined by means of the electrical conductivity and shaft-torque techniques, respectively. The effects of probe/tracer injection position, agitation speed, type of impeller, clearance of impeller off tank bottom, oil volume fraction, gas holdup and physical properties of the dispersed liquids on the macro-mixing time and power consumption of the gas-liquid-liquid system have been studied. Gas aeration damps the macro-mixing process of the continuous phase at low aeration rates, but enhances the mixing efficiency at higher gas flow rates. Contrary to the gas effect, the dispersed oil phase increases the macro-mixing intensity at low holdups and decreases the macro-mixing intensity of the continuous phase at higher holdups. The power consumption decreases with the increasing dispersed oil phase volume fraction both with and without aeration and also falls with the increase of gas flow rate. A new correlation for the liquid phase mixing time in gas-liquid-liquid three-phase stirred reactors is proposed based on a widely-used single-phase model. (4) The micro-mixing characteristics of a gas-liquid-liquid stirred reactor of standard geometry operated in a single-feed semi-batch mode have been investigated with the fast precipitation process using barium chloride aqueous solution as the continuous phase, kerosene and air as
Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/15502
Collection研究所(批量导入)
Recommended Citation
GB/T 7714
程荡. 液液和气液液搅拌槽内混合过程的数值模拟与实验研究[D]. 中国科学院研究生院,2013.
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