CAS OpenIR  > 研究所(批量导入)
Thesis Advisor陈家镛
Degree Grantor中国科学院研究生院
Place of Conferral北京
Degree Discipline化学工艺
Abstract本文以滴流床反应器为对象,从理论和实验两个方面研究了滴流床内气-液两相并流流动的流体力学,目的在于探讨滴流反应器内可能存在的各种相互作用对流动状况的影响。本文通过系统的实验研究,获得必要的实验数据,来验证理论模型的正确性,也丰富了对滴流床内流体力学特性的认识,尤其是积累了有关小尺寸三叶形填料的润湿程度的实验数据。同时建立了滴流床反应器内气液两相并流流动的数学模型,为滴流床反应器的优化设计和优化控制打下基础,并为三相反应器的研究由准数关联式水平向数学模型阶段的推进作了探索性研究。本文将滴流床反应器内填料看成是以规则的体心立方形式堆积,研究了床层内滞留的液体的形式与体积,从而得到计算静持液量的数学模型。利用化学反应法,本文系统地研究了滴流床反应器内填料的润湿程度。采用在球形和三叶形填料表面上镀镍的方法,堵住了三叶形填料的内孔,避免了填料内孔的扩散问题。本文将多相流体力学和计算流体力学的方法引入滴流床反应器内的流体力学现象的研究。经过分析研究,本文选择了均匀小球的简单立方堆积形式作为本研究的填料填充方式。从流体力学的基本原理出发,对气液相、液固相间的相互作用进行了充分的考察,建立了滴流形式下的二维流体力学模型,以计算床层内的总持液量、填料表面被液体完全润湿所需的最小液体表现质量流速、气液两相流压降等滴流床反应器的宏观参数。为了精确求解建立的数学模型,本文对求解过程中的正交贴体座标和网格的产生、模型微分方程的离散化及代数方程组的求解三个步骤进行了分析,设计出了一种数值计算稳定、精度高和收敛速度较快的数值计算方法。为了解决研究域的边界不规则这一问题,本文利用了数值座标变换技术。为了解决研究域的边界不规则这一问题,本文利用了数值座标变换技术,由强限制法和弱限制法出发,发展了一种生成平面正交贴体座标的方法,实现了直角座标向正交贴体座标的转换,并在这种座标系上产生正交网格。采用幂函数差分格式对模型的微分方程进行离散化,这种格式具有保证解收敛的作用。采用逐行迭代法求解代数方程组,这种方法的计算速度较快,在行的两端点上的边界信息可以立即传递到域的内部;通过两上座标方向上交替采用逐行迭代的方向,就可以把所有边界上的信息迅速传递到中间区域。根据流体力学模型计算出的总持液量数据与文献提出的有关计算球形填料上总持液量的经验关联式得出的计算值进行了比较,结果发现由本文的模型计算出的总持液量值与大部分关联式的结果比较接近;将此模型计算的总持液量结果与实验数据进行比较,符合程序非常令人满意(< ± 10%)。在进行比较的同时还发现文献中各个关联式间的结果相差较大,给选择合适的关联式进行设计带来了困难,这也说明建立普遍适用的流体力学模型的重要性。本文还由模型计算出填料外表面被液体完全润湿所需的最小液体表现质量流速为4kg/m~2·s,这与文献的实际数据及本文的实验数据吻合得较好。这说明本文建立的模型是比较符合实际的,经过完善,会成为适用性较广的理论模型。
Other AbstractIn this dissertation, the hydrodynamics of gas-liquid cocurrent flow through a trickle bed rector was investigated by both experimental and theoretical approaches so as to enhance the understanding of the effects of various interactions between coexisting three phases on the hydrodynamic performance of the reactor. Systematic experimental work were carried out on hydrodynamic characteristics of the trickle bed, in particular, with small size three-lobed catalyst as packings. The accumulated experimental data were later used to verify the theoretical model developed in this work. An interactive hydrodynamic model of gas-liquid two-phase flow through the packing in the trickle bed was established to serve the foundation for further optimization of the design and operational control of trickle bed reactor. The partial success of the present work on mathematical modeling of multiphase chemical reactors indicates the step can be take on modeling from empirical and semi-empirical approaches to more theoretically sound basis. With the assumption that the spherical particles were packed in normal orthorhombic mode in the trickle bed reactor, the shape and volume of liquid packets stagnated in the bed were analyzed in order to obtain the mathematical model for calculating the static liquid holdup. When the gas-liquid feed to the reactor was simultaneously stopped, the liquid would drain from the column. After a maximum period of 30 min, the drainage essentially ceased, but there was still part of liquid stagnated in the packings and generally the amount of liquid sustained at contacting points between the packing particles in the form of liquid pendular rings. An axial symmetric pendular ring at the contacting point with its tangent plane perpendicular to the gravity was analyzed with hydrostatics. It was found that infinite number of pendular rings with a spectrum of shape and volume could exist, all of which satisfied the condition of hydrostatic equilibrium. However, the experimental results suggested that the static liquid holdup obtained for the same packing with the same system was a definite value and can be repeated experimentally. Therefore, possibly only one potential liquid ring existed in reality. In order to find the most probable liquid ring existing in practice, the thermodynamic theory of equilibrium state was applied to set up the criterion for the stability of liquid pendular rings. With this criterion, the shape and volume of thermodynamically pendular rings. With this criterion, the shape and volume of thermodynamically stable liquid ring can be singled out from infinite number of possible liquid rings. Furthermore, the dependence of shape and volume of the unique liquid ring on the angle between the tangent plane and the direction of gravity was analysed and the theoretical model of calculating the static liquid holdup in the trickle bed was obtained. The results from theoretical prediction were compared with published experimental data of spheres packed in trickle bed reactors and satisfactory agreement was evidenced. In this work, the total liquid holdup was measured by tracer pulse experiments and dynamic liquid holdup by volume method. Their difference gave the static liquid holdup. These experimental results of static liquid holdup were found to be in rather good accordance with the results given by the theoretical model developed. The wetting efficiency of packing in a trickle bed reactor was studied systematically by the chemical reaction method. A thin layer of nickel was chemical plated on the packing spheres and three-lobed extrudates. The internal pores of three-lobed extrudates were blocked by plating so that the influence of internal diffusion was excluded. The rates of reaction of the nickel catalyzed decomposition of hydrogen peroxide were determined in a 0.7 wt.% H_2O_2 aqueous solution and in a mixture containing 0.7 wt.% H_2O_2, 90 wt.% ethanol and 9.3 wt.% water with packing so f 1.5mm spheres and three-lobed extrudates respectively. H_2O_2 decomposition was irreversible and first order with respect to H_2O_2 concentration over a quite wide concentration range. The wetting efficiency of packing in trickle bed reactors was calculated as the ratio of the observed apparent reaction rate constant as measured in a test section of the trickle bed reactor to the reaction rate constant determined for the same packing in a stirred tank reactor under sufficient intensity of agitation. It was found that there existed a critical superficial liquid mass velocity, above which complete wetting of packing was observed and this critical value was 5 kg/m~2·s for both air-water and air-ethanol water solution systems under the experimental conditions studied (such as reactor geometry and properties of packings). By shutting off the inlet and outlet streams simultaneously, the volume of liquid that would drain from the reactor was measured as the dynamic liquid holdup. The total liquid holdup was obtained with tracer pulse experiments. These measurements were carried out for four kinds of liquid or solution and on three kinds of packings, and the bed pressure drops were also measured by differential pressure transformer. The experimental data of dynamic liquid holdup and pressure drop across the bed obtained on three-lobed extrudates were correlated based on the correlations proposed by Turpin and Specchia. The correlations of the dynamic liquid holdup and bed pressure drop were resulted with the average relative error between the experimental data and the predicted values being ±10.9%, ±6.5% respectively. The static liquid holdup was about 0.05 on spherical packings and 0.11 on three-lobed exudate packings respectively. The hydrodynamic investigation in this work contributed to the data on small-size packings in trickle bed reactors and also enhanced the knowledge on extent of wetting of packings in trickle bed reactors. The new methodology of introduction of multiphase hydrodynamics and computational fluid mechanics into chemical reaction engineering was adopted to explore the hydrodynamic phenomena in trickle bed reactors. A simple cubic array of spheres was chosen as the arrangement of packing for modeling of a trickle bed. Started from basic principles of hydrodynamics, the interaction between gas and liquid or fluid and solid was amply examined and a two-dimensional hydrodynamic model in trickling flow regime was established to evaluate the macroscopic physical quantities, such as total liquid holdup, gas-liquid two-phase flow pressure drop and critical superficial liquid mass velocity for complete wetting. In order to resolve the mathematical model accurately, the procedures for generation of orthogonal body-fitted curvilinear coordinates and grid network, discretization of differential equations and solution of the linear algebraic equations were analyzed, and a numerical method which had the advantage of stability of computation, high precision and fast convergency was designed. In order to resolve the problem of irregular domain of liquid flow on spherical surface, a method for generating orthogonal body-fitted curvilinear coordinates was developed from the strong constraint and weak constraint methods given in the literature of computational mathematics, such that the Cartesian coordinates were transformed to orthogonal body-fitted curvilinear coordinates and meanwhile orthogonal grids were generated in this coordinate system. The power-law difference scheme was used to the discretion of differential equations in this work. This kind of scheme had the advantage of numerical stability and convergency. The linear algebraic equations were solved by line-by-line iterative method. The convergence of line-by-line method was fast, because the boundary conditions at the ends of the line were transmitted at once to the interior of the domain, no matter how many grid points were in the line. By alternating the directions of line-by-line iteration, the information can be brought to the interior of domain from all the boundaries. By comparing the total liquid holdup calculated from the hydrodynamic model with the results of empirical correlations given in the literature on spheric pickings, it was found that the results of total liquid holdup predicted by hydrodynamic model were close to most of the experimental results given in literature, and also agreed very satisfactorily with the present experimental results. However, difference among correlations published in the literature were quite remarkable. This would given difficulty for choosing proper available correlations for design, scale-up and operation optimization of trickle bed reactors. It also demonstrates the importance of establishing mathematical model with better applicability and universality. In this work the critical superficial liquid mass velocity obtained from the hydrodynamic model developed was 4 kg/m~2·s. It was in satisfactory agreement with the results published in the literature and obtained by the presented experimental work. All the results demonstrated that the hydrodynamic model established in this work was very promising and quite worthwhile to be further developed into a powerful investigation tool in the research and development of trickle bed reactors.
Document Type学位论文
Recommended Citation
GB/T 7714
熊天英. 滴流床反应器流体力学性质的研究[D]. 北京. 中国科学院研究生院,1991.
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