旋转填充床流体流动可视化与传质模型研究

发布时间：2018-08-30 09:43

【摘要】：化学工业是我国国民经济的支柱产业,为我国社会经济发展和国防建设提供了大量的化学品和基础材料,同时也产生废水、废气等有害污染物质,严重制约着我国化学工业的可持续发展。化工生产过程清洁、节能降耗一直是科学家和工程师们的目标,作为化工过程强化装备的旋转填充床的研究与工业应用有助于实现这一目标。旋转填充床的多孔填料可以将液体切割成细小的液滴和液膜,气液接触面积急剧增大,具有优异的质量传递和微观分子混合性能。端效应区是旋转填充床填料内缘处传质和混合最剧烈的区域,根据端效应区的特征,旋转填充床的传质被划分为端效应区、填料主体区和空腔区,简称“三区”。然而,由于旋转填充床中流体运动速度较快不易观测,导致其各区域内流体流动形态及尺寸的研究还不系统。对旋转填充床传质有重要影响的空腔区及端效应区的传质研究还有待加强,这些流体流动和传质等基础研究的滞后均阻碍了旋转填充床的进一步结构优化和工业应用。本文首先采用高速摄像技术对空腔区流体流动进行观测,并对空腔区传质表面积进行模型化研究。通过概率计算划分了端效应区和填料主体区,建立了包含旋转填充床的端效应区、填料主体区和空腔区为传质过程的液相体积传质系数模型,即三区液相传质模型。并结合气相传质和化学反应的影响,将三区传质模型用于单乙醇胺(MEA)吸收CO_2过程的预测,可为旋转填充床的设计和应用提供理论指导。主要研究结果如下:1.利用高速摄像技术对旋转填充床空腔区内的流体流动状态进行研究,系统考察了转速、液体体积流量、填料外半径、液体粘度和表面张力对空腔区流体流型、液滴平均直径、液滴大小、尺寸分布及液滴平均速度的影响规律。结果表明:空腔区存在两种典型的流型(液滴流和液线流)以及两种液体断裂方式(液线-液滴和液膜-液线-液滴),同时获得不同操作条件下的液体流型转变判据;液滴平均直径随着转速、填料外半径和液体粘度的增加而减小,随着液体表面张力的增大而略微增大,液体初始速度对液滴平均直径的影响不大;液滴大小分布符合R-R分布,分布指数m范围为4.47到9.43之间。液滴平均合速度和径向速度主要随转速和填料外半径的增大而增大,受液体初始速度、液体粘度和表面张力的影响不大。通过量纲分析得到了液滴平均直径和速度的关联式,预测值与实验值误差分别在±20%和±10%以内。2.基于空腔区流体流动的可视化研究,发现空腔区的传质贡献分为三部分,分别为母代液滴、器壁液膜和子代液滴,对上述三部分进行模型化,建立空腔区总传质表面积模型。从模型结果可以看出,相比母代液滴的传质,器壁液膜和子代液滴的传质表面积对空腔区总传质表面积贡献最大。采用NaOH-CO_2实验体系考察了转速、液体初始速度和填料外半径对空腔区传质表面积的影响,并与模型预测值进行对比。结果表明,空腔区传质表面积预测值和实验值误差在±20%以内,说明模型可较好的描述空腔区的传质表面积。3.利用概率计算的方法得到端效应区厚度,结合端效应区厚度、以及前人对旋转填充床填料区流体流动状态和传质理论研究的结果,建立了包含端效应区、填料主体区和空腔区的液相体积传质系数模型,即三区液相传质模型,并通过NaOH-CO_2化学吸收实验验证模型值与实验值误差在±15%以内,说明模型可以很好的预测旋转填充床的液相传质过程,为旋转填充床的设计提供理论支撑。4.基于三区液相传质模型,同时考虑气相传质和化学反应对传质的影响,将三区模型用于MEA-CO_2吸收过程的预测,结果表明,大部分实验值与模型预测值吻合良好,误差在±20%以内,CO_2吸收率随着转子转速的增加先增大后略微减小,随着液体体积流量和MEA浓度的增加而增大,随着气体体积流量的增加而减小。
[Abstract]:Chemical industry is the pillar industry of China's national economy. It provides a large number of chemicals and basic materials for China's social and economic development and national defense construction. At the same time, it produces harmful pollutants such as waste water and exhaust gas, which seriously restricts the sustainable development of China's chemical industry. The goal of the engineers is that the research and industrial application of rotating packed beds as chemical process intensification equipment will help to achieve this goal. According to the characteristics of the end-effect zone, the mass transfer in the rotating packed bed is divided into the end-effect zone, the main filler zone and the cavity zone, which are called "three zones". However, the velocity of fluid movement in the rotating packed bed is not easy to be observed, resulting in the flow pattern and flow pattern in each zone. The study of mass transfer in cavity region and end effect region, which have important influence on mass transfer in rotating packed bed, needs to be strengthened. The lag of basic research on fluid flow and mass transfer hinders further structural optimization and industrial application of rotating packed bed. The end-effect zone and the main packing zone are divided by probability calculation. The liquid volume mass transfer coefficient model including the end-effect zone of the rotating packed bed and the main packing zone and the cavity zone is established, that is, the three-zone liquid mass transfer model. The three-zone mass transfer model was used to predict the absorption of CO_2 by monoethanolamine (MEA), which can provide theoretical guidance for the design and application of rotating packed bed. The effects of volume flow, packing radius, liquid viscosity and surface tension on the flow pattern, droplet average diameter, droplet size, size distribution and droplet average velocity in the cavity region were investigated. The average diameter of droplets decreases with the increase of rotational speed, packing radius and liquid viscosity, and increases slightly with the increase of liquid surface tension. The initial liquid velocity has little effect on the average diameter of droplets; the size distribution of droplets conforms to R-R distribution, and the distribution index M. The average combined velocity and radial velocity of droplets increase with the increase of rotational speed and packing radius, but have little effect on the initial velocity, liquid viscosity and surface tension. (2) Based on the visualization of fluid flow in the cavity region, it is found that the mass transfer contribution in the cavity region is divided into three parts, namely, the parent droplet, the wall film and the offspring droplet. The influence of rotational speed, initial liquid velocity and packing radius on the mass transfer surface area in the cavity region was investigated by NaOH-CO_2 experimental system, and compared with the predicted value of the model. The open model can be used to describe the mass transfer surface area in the cavity region. 3. The thickness of the end effect region is calculated by probability method. Combined with the thickness of the end effect region and the results of previous theoretical studies on fluid flow and mass transfer in the packing region of rotating packed bed, the liquid volume mass transfer including the end effect region, the main packing region and the cavity region is established. Coefficient model, namely three-zone liquid-phase mass transfer model, was used to verify the error between the model value and the experimental value by NaOH-CO_2 chemical absorption experiment. It shows that the model can predict the liquid-phase mass transfer process of rotating packed bed and provide theoretical support for the design of rotating packed bed. 4. Based on the three-zone liquid-phase mass transfer model, the gas-phase mass transfer is considered simultaneously. The three-zone model was used to predict the absorption process of MEA-CO_2. The results show that most of the experimental values are in good agreement with the predicted values. The errors are within (+20%). The CO_2 absorption rate increases first and then decreases slightly with the increase of rotor speed, and increases with the increase of liquid volume flow rate and concentration of MEA. Volume volume flow decreases.
【学位授予单位】：北京化工大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TQ051.1

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