旋转填充床内传质效应及微观混合的CFD模拟研究

发布时间：2018-02-19 21:28

本文关键词： CFD 三维模型破碎微观混合 RPB　出处：《北京化工大学》2016年博士论文　论文类型：学位论文

【摘要】：旋转填充床反应器(Rotating Packed Bed,RPB)具有极强的过程强化性能和微观混合性能,由于受到其内部复杂填料结构及观测手段的限制,关于RPB的流体力学特征和混合行为的基础研究相对较少。本文通过对RPB反应器内部填料进行合理简化,建立了三维RPB模型,分别在Design Modeler和Meshing-ICEM内创建几何模型和进行网格划分,采用流体计算软件ANSYSY (FLUENT)对液相在RPB内的流体流动破碎过程和混合过程进行模拟研究。应用Realizable k-ε模型、VOF模型以及Sliding Meshes模型来计算反应器内流体流动和气-液两相边界。三维模拟结果可以清晰地显示液体在整个反应器空间内的分布及存在形态(孔流、液膜流动和液滴)。液体被旋转填充床内填料切割的过程,既有分散的过程,也有聚并的过程。填料转速对液相颗粒大小及存在形态具有明显影响,而进口速度对旋转填充床空腔内的液相颗粒大小影响不大。转速、进液速度的增加和气液接触角的降低有利于提高液体的比表面积,进而有利于传质。液体停留时间随着转速和气液接触角的升高而降低。与二维模拟相比,三维模拟对液滴尺寸、液体速度和停留时间的描述更接近实际情况,模拟的液体尺寸和液体速度值更接近实验值。由于VOF模型受网格大小制约,对于捕捉小尺寸液滴特别是小于最小网格数尺寸的液滴具有一定的缺陷,受计算机计算能力的限制,模拟得到的RPB内子液滴尺寸分布具有一定的误差。本文利用实验和模拟结合的方法,通过实验获得液滴破碎信息,即得到不同尺寸液滴在丝网破碎的可能性、生成子液滴尺寸、撞击位置对破碎的影响、聚集子液滴尺寸信息,把破碎信息编写入Population Balance(PBM)方程中,得到了 RPB内更合理的子液滴尺寸分布图。这种方法同时可减小网格数降低计算时间,对于大尺寸反应器的模拟计算具有借鉴作用。另外,基于RPB内流体流动的CFD模型,在稳定流场内添加液-液化学反应,模拟研究RPB的微观混合效率。考察了氢离子转化率、产物(H3BO3)、副产物(I2和I3-)以及离集指数在填料区径向上的浓度分布。模拟结果表明:液-液反应过程和微观混合过程主要是在距进口 10毫米以内的区域(端效应区)内进行;各组分在RPB内的分布、反应进程及微观混合细节都能通过模拟结果得到清晰地显示;转速的增加能明显强化RPB内的微观混合效应,这与前人实验结果趋势一致,且离集指数在相同填料径向位置上与实验值相差不大。本文运用CFD理论基础建立了的简化后的RPB模型,模拟研究了 RPB内流体的流动、破碎特征以及微观混合效率,这对RPB反应器的操作、放大以及优化都能够提供一定的指导和理论依据。
[Abstract]:Rotating Packed BedRPBs have strong process-strengthening and micro-mixing properties, due to the limitation of complex packing structure and observation methods. There are few basic studies on the hydrodynamic characteristics and mixing behavior of RPB. In this paper, the three-dimensional RPB model is established by reasonably simplifying the inner packing of RPB reactor, and the geometric model and meshing are created in Design Modeler and Meshing-ICEM, respectively. The fluid flow breakage and mixing process of liquid phase in RPB were simulated by ANSYSY fluent. The Realizable k- 蔚 model and Sliding Meshes model were used to calculate the fluid flow and gas-liquid two-phase boundary in the reactor. Boundary. Three-dimensional simulation results can clearly show the distribution and form of liquid in the whole reactor space (pore flow, Liquid film flow and droplet flow. The process of liquid cutting in a rotating packed bed involves both dispersion and aggregation. The filling speed has a significant effect on the size and morphology of liquid particles. However, the inlet velocity has little effect on the size of liquid particles in the cavity of rotating packed bed. The increase of rotational speed, the increase of liquid velocity and the decrease of gas-liquid contact angle can improve the specific surface area of liquid. The liquid residence time decreases with the increase of rotational speed and gas-liquid contact angle. Compared with the two-dimensional simulation, the three-dimensional simulation describes the droplet size, liquid velocity and residence time more closely to the actual situation. The simulated liquid size and liquid velocity value are closer to the experimental values. Because the VOF model is restricted by the mesh size, it has some defects in capturing small size droplets, especially those smaller than the minimum mesh size, and is limited by the computer computing ability. There is a certain error in the size distribution of the sub-droplet in the simulated RPB. In this paper, the breakage information of the droplet is obtained by using the method of combining experiment with simulation, that is, the possibility of the droplet breaking in the wire mesh with different sizes is obtained, and the size of the sub-droplet is generated. The effect of impact position on the breakage, the size information of the aggregator droplet is compiled into the Population balance Population equation, and a more reasonable distribution map of the sub-droplet size in the RPB is obtained. This method can also reduce the mesh number and reduce the calculation time. In addition, based on the CFD model of fluid flow in RPB, the micro-mixing efficiency of RPB was simulated by adding liquid-liquid chemical reaction in the steady flow field, and the conversion of hydrogen ions was investigated. The concentration distribution of H3BO3, by-products I2 and I3-) and the separation index in the radial packing region. The simulation results show that the liquid-liquid reaction process and the microscopic mixing process are mainly carried out in the region (end effect zone) less than 10 mm from the inlet. The distribution of each component in RPB, reaction process and microscopic mixing details can be clearly displayed by simulation results, and the increase of rotational speed can obviously strengthen the microscopic mixing effect in RPB, which is consistent with the previous experimental results. In this paper, the simplified RPB model based on CFD theory is used to simulate the flow, fracture characteristics and microscopic mixing efficiency of the fluid in RPB. This can provide some guidance and theoretical basis for the operation, amplification and optimization of RPB reactor.
【学位授予单位】：北京化工大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TQ021.4

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