固定床鼓泡反应器中微气泡的形成演化规律

发布时间：2018-06-12 15:36

本文选题：固定床鼓泡反应器 + 微气泡　；参考：《浙江大学》2017年硕士论文

【摘要】：固定床鼓泡反应器是一种特殊的鼓泡反应器,其中固体颗粒固定不动,气液两相并流向上,液相是连续相,气相是分散相,气相以气泡的形式分散于液相中。气泡尺寸对相间传质至关重要,传统固定床鼓泡反应器中气体分布器产生的气泡尺寸都在毫米级以上,特别是在低气液比条件下不利于气液相间传质。本文提出了一种新型的固定床鼓泡反应器,使用文丘里型气液分布器产生微米级的气泡,利用微气泡比表面积大、上升速度慢和溶解速度快的特点来强化传质。围绕固定床鼓泡反应器中微气泡的形成演化规律这一关搖科学问题,通过研究单个文丘里气泡发生器和文丘里型气液分布器的微气泡发生性能,阐明了微气泡的形成机制和调控方法;通过研究气泡在液相床层和规整填料中的运动规律以及聚并破碎行为,揭示微气泡的演化机制。主要工作和研究结果如下:1.综合使用高速摄像机测量气泡尺寸分布和CFD模拟,研究了单个文丘里气泡发生器和文丘里型气液分布器的微气泡发生性能,提出了文丘里气泡发生器和文丘里型气液分布器的设计原则,获得了优选的结构参数和操作参数。首先,研究了气速、液速等操作条件和喉管直径等结构参数对气泡Sauter平均直径的影响规律,建立了气泡Sauter平均直径与液相雷诺数、气相雷诺数的经验关联式,发现了低气液比条件下通气量小于抽吸量导致的气泡Sauter平均直径随时间周期性波动的现象。其次,在验证CFD模型准确性的基础上,探讨了操作条件对文丘里气泡发生器内速度分布和压力分布的影响规律,发现提高液速或降低气速使得压力轴向分布变陡,液体对气体的剪切破碎作用增强,有利于生成微气泡。模拟还发现,在低气液比条件下,扩大段内液体出现偏流现象,与实验结果一致,且随着气液比的增大,偏流现象逐渐消失。最后,考察了床型、文丘里气泡发生器间距、气液分布器倾斜角度对文丘里型气液分布器出口气泡尺寸的影响。研究发现,增大管间距有利于减少气泡间的聚并,推荐的管间距不小于78mm,且文丘里型气液分布器具有较好的抗塔板倾斜性能。2.在对液相床层中不同尺寸的气泡进行受力分析的基础上,综合使用高速相机和粒子轨迹测速法(PTV),实验揭示了液相床层中不同尺寸气泡的运动规律以及不同液相高度床层内气泡的运动规律和分布规律。研究发现,气泡在液相床层中的运动轨迹按照气泡尺寸可以分为三类:对于直径小于1 mm的微米级气泡,其运动轨迹为一条直线;对于直径为1.0～5.5mm的气泡,其运动轨迹是一条周期性振荡的S型或者"之"字型曲线;对于直径大于5.5mm的气泡,其运动轨迹是一条偶尔出现摆动的曲线。文丘里气泡发生器产生的气泡以气泡束的形式进入液相床层,且气泡束的宽度随着床层高度、气量和液量的增加而增大。在靠近文丘里气泡发生器的区域,气泡运动主要受液体的运动状态控制,不同尺寸的气泡表现出相似的运动行为;在远离文丘里气泡发生器的区域,气泡行为主要由气泡自身控制。3.利用高速相机研究发现了气泡在规整填料床层内具有的3种主要聚并机制和4种主要破碎机制,以及微气泡和常规气泡在规整填料床层内行为的差异性。其中,气泡在规整填料床层聚并机制主要有:降速聚并、压缩聚并、粘壁聚并,破碎机制主要有:剪切破碎、撞击破碎、加速破碎、拉伸破碎。而且,随着填料间距的增大,气泡表现出的聚并机制和破碎机制种类逐渐增多。在间距为3mm的规整填料床层内,与填料间距尺寸相当的常规气泡表现出聚并、破碎、形变和自由运动等多种行为,而微气泡以聚并和自由运动为主,很难发生破碎和形变。此外,催化剂类型和尺寸对气泡的破碎聚并行为具有重要影响。与空床时相比,装填催化剂以后,床层内气泡Sauter平均直径显著增大,床层对微气泡具有明显的聚并作用。随着球形颗粒粒径的增大,气泡Sauter平均直径先增大后减小,当球形颗粒粒径为3 mm时床层内形成的气泡Sauter平均直径最大。选择较大粒径的球形颗粒有助于保持微鼓泡状态。
[Abstract]:A fixed bed bubbling reactor is a special bubbling reactor in which solid particles are fixed, gas-liquid two phase and flow up, liquid phase is continuous phase, gas phase is dispersed phase and gas phase is dispersed in liquid phase in the form of bubbles. Bubble size is vital to mass transfer in the gas distributor in a traditional fixed bed bubble reactor. The size is above the millimeter level, especially under the condition of low gas and liquid ratio. This paper presents a new type of fixed bed bubbling reactor, which uses the Venturi type gas distributor to produce micrometer scale bubbles. The mass transfer is strengthened by the characteristics of microbubbles, which are larger than the surface area, slow rising speed and fast dissolution rate. The formation and evolution of microbubbles in a fixed bed bubbling reactor is a scientific problem. The formation mechanism and regulation methods of microbubbles are clarified by studying the microbubbles in the single Venturi bubble generator and the Venturi gas distributor, and the motion laws of bubbles in the liquid bed layer and the regular packing are studied. The evolution mechanism of microbubbles is revealed and the evolution mechanism of microbubbles is revealed. The main work and research results are as follows: 1. the bubble size distribution and CFD simulation are used to measure the bubble size distribution and the performance of the microbubbles in the single Venturi bubble generator and the Venturi gas distributor. The Venturi bubble generator and Venturi type gas are proposed. The optimum structure parameters and operating parameters are obtained. First, the influence of the structure parameters, such as gas velocity, liquid speed, and other structural parameters on the average diameter of the bubble Sauter, is studied. The empirical correlation of the average diameter of the bubble Sauter, the Reynolds number of the liquid phase and the Reynolds number of the gas phase is established, and the low gas ratio is found. On the basis of verifying the accuracy of the CFD model, the effect of operating conditions on the velocity distribution and pressure distribution in the Venturi bubble generator is discussed, and the axial distribution of the pressure distribution in the Venturi bubble generator is discussed. The shear breaking effect of the liquid on the gas is enhanced and it is beneficial to the formation of microbubbles. It is also found that the phenomenon of partial flow occurs in the expanded section under the condition of the ratio of low gas to liquid, which is in accordance with the experimental results, and the phenomenon of partial flow gradually disappears with the increase of the gas and liquid ratio. Finally, the bed type, the spacing of Venturi bubble generator, and the gas and liquid distributor are examined. The influence of the tilt angle on the outlet bubble size of the Venturi gas distributor is found. It is found that the increase of the tube spacing is beneficial to reducing the coalescence between the bubbles and the recommended tube spacing is not less than 78mm, and the Venturi gas distributor has a good anti tray tilt performance.2. in the basis of the force analysis of different sizes of bubbles in the liquid bed layer. On the base of the high speed camera and particle trajectory velocimetry (PTV), the motion law of different sizes of bubbles in the liquid bed and the law of movement and distribution of bubbles in the high bed layer of liquid phase are revealed. It is found that the movement track of bubbles in the liquid bed can be divided into three types according to the size of the bubbles: for the small diameter, the size of the bubbles can be divided into small diameter. The trajectory of a micro bubble at 1 mm is a straight line; for a bubble with a diameter of 1 to 5.5mm, its trajectory is a periodic oscillating S or "" type curve; for a bubble with a diameter greater than 5.5mm, its trajectory is an occasional swing curve. The bubbles generated by the Venturi bubble generator are a bubble beam. In the form of the liquid bed layer, the width of the bubble beam increases with the height of the bed, the amount of gas and the amount of liquid. In the area near the Venturi bubble generator, the bubble movement is mainly controlled by the motion state of the liquid, and the bubbles of different sizes show similar motion behavior; in the area far away from the Venturi bubble generator, the bubble row 3 main mechanisms of coalescence and 4 main breakup mechanisms in a structured packing bed, and the differences in behavior of microbubbles and conventional bubbles in a structured packing bed, were found to be mainly controlled by a high speed camera.3. using a high speed camera. The mechanisms of polycondensation and adhesion are mainly: shear breaking, impact crushing, accelerated breaking, and tensile breaking. Moreover, as the spacing of packing increases, the coalescence mechanism and breaking mechanism of bubbles increase gradually. In the regular packing bed with spacing of 3mm, the conventional bubbles with the same spacing between the packing show and break. In addition, the type and size of the catalyst have an important effect on the fragmentation and aggregation of the bubbles. Compared with the empty bed, the average diameter of the bubble Sauter in the bed layer is significantly increased and the bed layer is microbubbles compared with the empty bed. With the increase of the particle size of spherical particles, the average diameter of the bubble Sauter increases first and then decreases. When the spherical particle size is 3 mm, the average diameter of the bubble Sauter in the bed layer is the largest.
【学位授予单位】：浙江大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TQ051.14

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