湍动流化床气固传质模型及其CFD模拟

发布时间：2019-01-02 07:36

【摘要】：湍动流化床反应器因其气固接触充分、热质传递效率高、处理量大等优点,已广泛应用于化学和石油工业等诸多领域。目前,湍动流化床的研究多集中在流动行为方面,传质行为的研究相对较少,传质行为的计算流体力学(Computational Fluid Dynamics,CFD)模拟更是鲜有报道。针对湍动流化床稀密两相均为半连续相的特性,本论文将其气固流动结构分为拟离散的气穴相和拟离散的聚团相,并使用Cc,Cd,Csc,Csd,Cf和Csf六个浓度参数描述湍动流化床的传质过程,从而根据质量守恒等原理建立基于结构的气固传质模型,最后通过CFD模拟验证气固传质模型的可靠性,并对湍动流化床的气固传质行为进行分析研究。提出将湍动流化床的流动结构分为拟均匀的稀密两相,根据两相间质量守恒建立基于湍动流化床非均匀结构的气固传质模型的新思路。首先,通过推导获得每一相的传质速率方程,将各相传质速率进行加和即可得总体传质速率,再结合平均传质速率定义,即可获得基于结构的湍动流化床传质系数的表达式,用于描述非均匀结构对湍动流化床气固传质的影响;其次,结合传质平衡原理、传质与反应的平衡关系等可得到一维和二维的组分输送方程,并实现传质方程所需六个浓度参数的封闭求解。气固流动参数由基于结构的湍动流化床曳力模型求解,传质模型的数值模拟由商业软件Fluent实现,以甲烷燃烧和臭氧分解实验结果对模拟结果进行校验,结果表明模拟结果与实验数据吻合较好,证明该传质模型具有较高的准确性。通过模拟研究还发现,虽然甲烷浓度在催化剂浓度较高的位置会因化学反应消耗而降低,但组分流动以及扩散的影响使甲烷浓度与催化剂浓度之间不存在绝对的大小对应关系。随着气速的增加,由于组分流入速率增加和停留时间变短,臭氧浓度有所增加,但因为气固传质得到强化,反应消耗的臭氧总量是增加的。此外,稀密两相间的组分交换过程是臭氧分解反应的控制步骤。本工作利用臭氧分解作为模型反应,通过以上的湍动流化床传质模型分析了传质模拟结果与相间传质系数求解关联式、气速的关系。结果表明,采用以聚团为基础的Foka关联式计算相间传质系数时,基于结构的湍动流化床传质模型的模拟结果最好。本文还进一步分析了颗粒含量、聚团当量直径对组分浓度的影响。
[Abstract]:Turbulent fluidized bed reactor has been widely used in many fields such as chemistry and petroleum industry because of its advantages of sufficient gas-solid contact high heat and mass transfer efficiency and large treatment capacity. At present, the study of turbulent fluidized bed is mostly focused on the flow behavior, but the study of mass transfer behavior is relatively few, and the computational fluid dynamics (Computational Fluid Dynamics,CFD) simulation of mass transfer behavior is rarely reported. In this paper, the gas-solid flow structure of turbulent fluidized bed is divided into quasi-discrete cavitation phase and quasi-discrete cluster phase, and Cc,Cd,Csc,Csd, is used to analyze the gas-solid flow structure of turbulent fluidized bed. The mass transfer process of turbulent fluidized bed is described by six concentration parameters of Cf and Csf. According to the principle of mass conservation, the gas-solid mass transfer model based on structure is established. Finally, the reliability of gas-solid mass transfer model is verified by CFD simulation. The gas-solid mass transfer behavior of turbulent fluidized bed was studied. The flow structure of turbulent fluidized bed is divided into quasi-uniform dense two-phase and a new idea of gas-solid mass transfer model based on non-uniform structure of turbulent fluidized bed is proposed according to the conservation of mass between two phases. Firstly, the mass transfer rate equation of each phase is derived, and the total mass transfer rate can be obtained by adding the mass transfer rate of each phase, and then the expression of mass transfer coefficient of turbulent fluidized bed based on structure can be obtained by combining the definition of average mass transfer rate. It is used to describe the effect of non-uniform structure on gas-solid mass transfer in turbulent fluidized bed. Secondly, according to the principle of mass transfer equilibrium and the equilibrium relation between mass transfer and reaction, the one-dimensional and two-dimensional component transport equations can be obtained, and the closed solution of six concentration parameters required for the mass transfer equation can be realized. The parameters of gas-solid flow are solved by a structure-based turbulent fluidized bed drag model. The numerical simulation of the mass transfer model is implemented by commercial software Fluent. The simulation results are verified by the experimental results of methane combustion and ozone decomposition. The results show that the simulation results are in good agreement with the experimental data, and it is proved that the mass transfer model has high accuracy. The simulation results also show that the concentration of methane decreases with the consumption of chemical reaction when the catalyst concentration is high, but there is no absolute relationship between the concentration of methane and the concentration of catalyst due to the effect of component flow and diffusion. With the increase of gas velocity, the ozone concentration increases due to the increase of component inflow rate and shorter residence time, but the total ozone depletion is increased due to the enhancement of gas-solid mass transfer. In addition, the component exchange process between two phases is the control step of ozone decomposition reaction. In this work, ozone decomposition is used as a model reaction, and the relationship between mass transfer simulation results and interphase mass transfer coefficient is analyzed by using the above turbulent fluidized bed mass transfer model. The results show that the structure-based turbulent fluidized bed mass transfer model has the best simulation results when the mass transfer coefficient between phases is calculated by using the Foka correlation formula based on clusters. The effects of particle content and cluster equivalent diameter on the component concentration were also analyzed.
【学位授予单位】：中国科学院大学(中国科学院过程工程研究所)
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TQ051.13

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