【作者】 陈雷；

【导师】 高正明； 包雨云；

【作者基本信息】 北京化工大学 ， 化学工程与技术， 2009， 博士

【摘要】 多相搅拌反应器广泛应用于石油、化工、生物、冶金、食品、制药及水处理等过程工业,由于反应器内往往进行大量的强放热反应,导致实际操作温度并非常温,而已有的工作主要集中于常温多相反应器性能的研究,缺少对热态多相搅拌反应器的研究。本研究室已对常温和热态多相搅拌反应器宏观性能进行了对比研究,结果表明：热态通气体系与常温体系具有较大区别,热态通气体系的相对功率消耗(通气功率与纯液相功率之比,RPD)要高于常温通气体系；热态通气体系的总体气含率明显小于常温通气体系。但已有研究结果尚未揭示温度对各种宏观特性影响的定量关系,无法满足该类反应器优化设计的要求；由于未进行反应器内介观及微观特性研究,因此与从机理上理解气液分散及固液悬浮特性的差距仍较远。为此,本文率先利用实验与CFD模拟相结合的手段从宏观和介观两个尺度上对热态多相搅拌反应器进行了深入研究,为该类反应器的优化设计提供参考。本文首先从宏观特性研究着手,在直径为0.476 m,液位与槽径比为1.8的椭圆底不锈钢搅拌槽内进行实验,采用三层组合桨,底桨为六叶半椭圆管涡轮桨(HEDT),中层桨和顶层桨均为上提操作的四宽叶翼形桨(WHU),定量研究了气液两相及气液固三相搅拌反应器内温度对包括通气搅拌功率、整体气含率以及临界离底悬浮转速在内的宏观流体力学性能的影响规律,结果表明：体系中RPD随温度的升高而增大,但随固含率的增加,温度对RPD的影响程度减弱；整体气含率随温度的升高而明显下降,下降幅度也随固含率增加而减小；并获得了包含温度定量影响的经验关联式,为该类反应器的工业设计提供有益参考。宏观特性的研究虽然可以为多相搅拌反应器的设计提供参考,但通过这种“黑箱”研究的方法无法评价搅拌槽内不同区域的气液分散状况,更无法深入了解搅拌槽内气液分散机理。为此,本文利用研制开发的可用于热态体系的双电导探针气泡测量及分析系统,研究了热态气液两相体系内的局部气含率分布、气泡尺寸分布、气液相界面积分布以及热态三相体系内的局部气含率分布等介观特性。研究结果表明：两相及三相体系内局部气含率随温度的升高而下降,这与整体气含率的研究结果一致；两相热态体系中气泡的全槽平均直径比常温体系大21%,这也是导致热态体系中气含率较低的主要原因。在研究反应器内介观特性的过程中发现,多相搅拌反应器内的局部气含率分布及气泡尺寸分布均与反应器内液相流型密切相关,为此,本文采用PIV测试技术对三层组合搅拌桨(HEDT+2WHU)产生的复杂流场进行详细的实验研究和分析,研究发现HEDT+2WHU组合桨使流体在半槽内形成4个循环,能量最高处位于底桨射流区内,底桨叶片后方存在上下两个尾涡,沿径向方向向槽壁运动,而中层桨和顶桨后方仅有一个尾涡,沿轴向方向向液面运动,高湍流动能区随尾涡一起运动,从而实现能量自桨叶向槽内主体流动区的传递。该研究结果为今后进一步深入研究多相体系内的流体力学性能奠定了基础。采用多层桨的多相流搅拌反应器内的流体流动状况十分复杂,虽然本文通过实验研究的方法获得了一些结果,但需要消耗大量的时间和精力,因此本文利用计算流体力学(CFD)的方法,将Euler-Euler双流体模型与综合考虑了气泡聚并与破碎的群体平衡模型(PBM)相结合,对多层桨气液搅拌槽内的局部气含率分布、气泡尺寸分布以及局部容积传质系数分布进行了数值模拟,模拟结果与实验值相吻合,该研究结果可以为工业设计提供更有效的指导。通过对本实验体系内局部气含率分布研究发现,在顶层桨上方区域的局部气含率远远高于搅拌槽内的其它区域,这对于一些要求气体在体系内分布均匀的工业过程不利,为了实现通过搅拌器结构设计来控制局部气含率分布的目的,本文利用实验及CFD相结合的方法研究了顶层桨尺寸变化对气液两相体系内通气搅拌功率、整体气含率以及局部气含率的影响,结果表明：当顶层桨直径为0.5T时,顶层桨上方局部气含率最大值可达50%,随顶层桨直径的减小,局部气含率极大值也减小,当直径为0.33 T时,局部气含率极值几乎消失。该研究有助于实现对多层桨气液搅拌反应器内气液分散状态的可控设计。更多还原

【Abstract】 Multi-phase stirred reactors are widely used in many process industries, such as chemical, petrochemical, biochemical, pharmaceutical and water treatment etc. A lot of exothermic reactions were carried out in such stirred tanks, which leads to the operation temperature higher than the ambient temperature. However, little attention has been paid on the systems operated at elevated temperature. Previous works have compared the global properties of cold and hot systems and radical differences were found. Agitator relative power demand (ratio of the gassed power to ungassed power, RPD) is greater at high temperatures while retained gas fractions are substantially reduced. Unfortunately, the quantitative relations between temperature and the global properties have not reported yet. Due to lack of the studies on the meso-and micro-characteristics of the multi-phase stirred tank, it is difficult to understand the mechanism of gas dispersion and solid suspension. Therefore, experimental study and CFD simulation were carried out in this paper on the macro-and meso-characteristics of multi-phase stirred reactor at different temperatures.Systematic studies were carried out in a triple-impeller stirred tank of 0.476 m diameter with dished base. The aspect ratio of the liquid is 1.8. The impeller combination consisted of a half elliptical disk turbine (HEDT) as the bottom and two up-pumping wide-blade hydrofoils (WHU) above HEDT. The effect of temperature on the global properties, including power consumption, total gas holdup and critical impeller speed for just complete off-bottom solid suspension, was studied quantitatively. Results show that RPD increases for the higher temperature, but the effect of temperature on RPD becomes weaker when more solids added in the system. The total gas holdup decreases apparently with the increasing of temperature, which also becomes less at higher solid concentration. Empirical correlations of the effect of temperature on the global properties were also presented in this paper.Though the global characteristics are necessary for the industrial design, it is difficult to evaluate the gas dispersion in different region of the tank, not mention to the mechanism of gas dispersion without the meso-characteristics. Therefore, the meso-characteristics, including the local void fraction, bubble size and gas-liquid interfacial area in the system at different temperatures, were measured by using dual electrical conductivity probe in this work. Results show that the voidage decreases evidently with the increasing of temperature, which is in good agreement with the results for overall gas holdup. The overall Sauter mean bubble size in the hot systems is about 21% larger than that in cold conditions, which accounts for the lower gas holdup in hot system.The distribution of local void fraction and bubble size has close relationship with the flow field produced by the multi-impeller in the stirred tank. The complicated flow pattern stirred by triple-impeller configuration (HEDT+2WHU) was investigated with PIV technique. Results show that 4 loops are formed in the half vessel and the region with high turbulent kinetic energy located at the discharging flow of the bottom impeller. A pair of trailing vortices formed behind each blade of the bottom impeller moved towards the wall. Single trailing vortex formed behind each blade of the middle and top impeller moved towards the liquid surface. Energy is transported to the bulk of the tank by the movement of vortices.Gas-liquid flow in the multi-impeller stirred tank is so complicated that it takes lots of time and effort to carry out the experimental research. Therefore, the computational fluid dynamics (CFD) method was used to study the flow and the gas dispersion. The Euler-Euler two-flow model, standardκ-εturbulent model and Multiple Frames of Reference (MFR) method were used in the simulation. A Population Balance Model (PBM) model has been implemented using the commercial CFD Code. Bubble breakup and coalescence have been modeled fundamentally using isotropic turbulence theory. The local gas holdup distribution, bubble size distribution and local volumetric mass transfer coefficient predicted by CFD are in agreement with the experimental results.The results of local void fraction distribution show that the voidage in the region above the top impeller is much higher than that in other regions, which is not benefit for some industrial process uniform gas dispersion requested. In order to optimize the void fraction distribution by the design of impeller configuration, experimental and CFD methods were used to investigate the influence of top impeller diameter on power consumption, total gas holdup and local void fraction. Results show that, there is an extreme maximum voidage of about 50% just above the top impeller while the ratio of the top impeller diameter to the tank diameter, Dtop/T, is up to 0.5. The maximum voidage decreases with the decreasing of Dtop/T and almost disappears when Dtop/T is as small as 0.33. The results are of importance to the optimum of industry aerated multi-impeller stirred reactors.更多还原