搅拌反应器内气液两相流的CFD研究进展

搅拌式气液反应器因其操作灵活、适用性强等优点,在过程工业中应用广泛.综述了采用计算流体力学CFD技术对搅拌反应器内气液两相流动行为的数值模拟研究.Euler-Euler双流体模型作为主要方法用于描述气液两相流动,在其基础上耦合相对简单的气泡数密度函数模型或复杂的群体平衡模型,可较为准确地预测搅拌反应器内气泡尺寸和局部气含率及其分布规律.CFD模拟结果可用以分析和评价不同搅拌桨叶、搅拌桨组合和气体分布器的气液分散性能,对气液反应器的结构优化和过程强化提供了有效手段.     

三段进气环流反应器气泡参数与传质特性研究

多级环流反应器通常在段间安装多孔分布板或其它内构件,增加了流体流动的阻力,导致环流液速降低。为此,今在多级环流反应器研究的基础上,设计了一种新型三段进气、多级环流反应器,利用新通入气体的抽提力和喷射力,缓解了段间的流动压降,并加速气泡的破碎;重点考察了进气量在各段中的分配比例对分段进气多级环流反应器流体力学的影响。应用多通道电导探针气泡特征参数测量仪和溶解氧分析仪分别测量气泡参数(气含率、气泡速度、气泡直径)及体积传质系数。实验结果表明:三段同时进气时,气泡直径明显减小,气液接触比表面积增加显著;体积传质系数远高于多级环流反应器,最大值约为多级环流反应器的2.3倍。根据修正双膜理论,计算得到了三段进气时体积传质系数的经验关联式,较好地反映出体积传质系数的变化规律。     

基于介尺度PBM模型的生物反应器放大模拟及实验研究

对于通气搅拌式工业生物反应器的放大设计而言,精确预测气泡尺寸和体积传质系数非常重要,因此需要建立合适的气泡聚并和破碎模型,以保证反应器的高效操作。以5 L通气搅拌式生物反应器为对象,以气泡尺寸和体积传质系数的实验数据为基准,模拟并考察了两种聚并模型和四种破碎模型对生物反应器内流体流动行为以及传质能力的影响。结果表明,基于介尺度理论的修正聚并模型与考虑黏流剪切的破碎模型组合,所得模拟结果与实验数据吻合最好,这为大型生物反应器的桨型优化提供了模型基础。因为工业化生物发酵通常是在大型生物反应器中进行,搅拌桨型对生物反应器效能至关重要,故本研究在选定最优气泡聚并破碎模型的基础上,通过叶轮末端剪切力相等的放大原则将5 L通气搅拌式工业生物反应器放大到400 m³,同时考察了六斜叶圆盘搅拌桨、非对称式抛物线搅拌桨、布鲁马金式搅拌桨以及六直叶圆盘搅拌桨等桨型组合对气泡破碎能力和气体分散效果的影响,并通过综合对比气含率、体积传质系数等参数,得到400 m³通气搅拌式生物反应器的最优桨型组合。     

气液反应体系相界面传质强化研究

气液反应界面传质的强化是当今高效和节能反应器研究的重要课题,弄清反应器内的气液传质机理是对气液反应器进行数学描述的关键。反应过程的能效和物效与体系中的传质系数k_G,k_L,k_S以及相界面面积a等参数直接相关,这些参数受气泡尺寸、分布、表观气速和气含率等因素的制约。就确定的体系和反应条件而言,这些因素会因反应器的结构尤其是搅拌和混合方式的变化而异。文章从理论上分析了影响气液界面传质的各因素,建立了较为详细的理论模型。理论计算结果表明:气泡大小是影响气液界面传质和最终反应速率的重要流体力学参数,微米级气泡对反应过程的强化作用明显。能量耗散率是决定体系气泡大小的深层原因,强化气液反应器设计时应重点考虑。     

吹气发泡法制备闭孔泡沫铝发泡过程的三维数值模拟

吹气发泡法制备闭孔泡沫铝的过程实质上是搅拌流场中复杂的两相流动过程,应用计算流体力学方法分析由倾斜轴倾斜叶片引起的发泡熔池内气液两相强旋湍流流动过程. 在双流体模型基础上引入多重参考系法描述搅拌两相流场,通过分析相间相互作用及湍流模型进行封闭. 解气泡数密度函数的输运方程来分析气泡聚合和破碎引起的气泡尺寸变化. 应用体积积分的方法,计算平均及局部气含率及气泡直径. 考察了桨叶转速及气体流率对气泡直径及其分布的影响. 结果显示,气含率随桨叶转速和气体流率增加而增大;气泡直径随气体流率增加而增大,随桨叶转速增大而减小.     

适用欧拉-拉格朗日方法模拟气液泡状流的气泡破碎模型

欧拉-拉格朗日方法已被广泛应用于模拟鼓泡塔等气-液反应器内的流型、气泡尺寸（或气含率）及其分布。文献中该方法主要基于临界Weber数观点来描述气泡破碎行为,且破碎后的子气泡尺寸由随机数确定。但现有实验和理论研究表明,临界Weber数约束不能体现气体密度等物性参数和泡内气体重分布对气泡破碎行为的影响。针对这些不足,提出了适用欧拉-拉格朗日框架且考虑泡内气体重分布贡献的气泡破碎机理模型,并利用开源软件OpenFOAM开发了基于新破碎模型的求解器。新模型预测结果能较好地吻合实验测量的时均轴向液速、气泡尺寸及其分布等实验数据。特别地,考虑泡内气体重分布现象的破碎机理模型成功预测了实验观测的气泡尺寸双峰分布特征。     

下行床反应器内催化裂化过程的CFD模拟

耦合湍流气粒多相流模型和催化裂化集总动力学模型,建立了描述下行床内多相流动和催化裂化过程的反应器数学模型,并利用计算流体力学单元模拟软件CFX4.3对下行床内的催化裂化过程进行了数值模拟及分析.模型能预测出在工业应用中反应器内最受关注的诸多参数,如固含率、相间滑移速度、压降、气固相的加速区以及各组分浓度的分布情况.预测结果表明,气相反应的进行将导致反应器内的气粒流动行为发生较大变化,充分考虑反应与流动行为的耦合十分重要;而反应器床径的增大将导致转化率和各产物收率的下降.     

拟泡-乳曳力模型在大型高温费托流化床反应器中的CFD模拟研究

以带冷却盘管的大型高温费托流化床反应器为研究对象,开展三维计算流体力学模拟研究。传统双流体模型基于局部平均的假设,认为单位控制体内气固两相均匀分布,网格尺寸必须足够小才能正确揭示局部非均匀结构的所有细节。采用双流体模型模拟大型工业化流化床装置时,将导致网格数量过于庞大,远超现有计算能力。为提高计算效率的同时不损失模拟精度,提出了基于局部非均匀假设、适用于粗网格的拟泡-乳三相非均匀曳力(PBTD)模型。该模型将流化床分为乳化相气体、乳化相颗粒以及气泡三相,分别建立守恒方程,体现气泡的非均匀特性对气固曳力的影响。乳化相内气固曳力以及气泡相与乳化相内颗粒的曳力分开考虑。采用PBTD模型耦合传质和反应模型,建立基于局部非均匀假设的高温费托合成反应器三维流动-传递-反应模型,包括各相守恒控制方程、气泡尺寸模型、相间物质和动量交换模型、高温费托合成反应动力学模型以及初始和边界条件,预测反应器内的流场和组分浓度分布。研究结果表明:在粗网格条件下,非均匀曳力模型可以预测床层内相含率的分布情况,预测的床层膨胀高度与经验公式计算值接近,偏差为1.2%。反应器出口气体组分的质量分数与试验测量值相近,偏差在1.5%~16.0%。模拟结果证实,基于非均匀假设的PBTD模型适用于模拟工业规模的鼓泡流化床反应器,对其设计开发和工业运行具有指导价值。     

低高径比气升式环流反应器数值模拟分析

利用商用计算流体力学软件Fluent,利用Euler-Euler双流体模型,重点针对好氧反应的特点,对一种具有低高径比(H/D=1.67)的环流气升式反应器内的气液两相流动及混合性能进行研究,描述出反应器内气含率和环流液速等参数的详细分布,分析模拟结果,气液速度分布和气含率分布等与实际情况基本吻合,从而证实了计算结果的有效性,为工业实际应用提供一定参考。     

用于四氯化钛生产的复合式气力输送反应器的一维模拟—I. 数学模型

复合式气力输送反应器是一种将提升管和多颗粒气力输送床串联使用的新型反应器,其中多颗粒气力输送床是由循环床叠加在密相湍床上形成的,适合于所需单程反应时间较长且需颗粒间、气固间作用力较大-防粘结、提高传质效率等的气固反应体系. 本工作用一维流化床反应器模型研究了用于四氯化钛生产的复合式气力输送反应器的反应性质,其中气体成分在泡相和乳相间的扩散速率通过气泡聚并模型进行计算并用粒子数平衡模型(Population balance)模拟颗粒沿轴向的粒径分布变化. 该模型可以同时获得反应温度、床内颗粒组成、气体成分、颗粒扬析量、反应效率等对生产非常重要的指标参数.     

气液喷射器的结构设计与性能分析

喷射式环流反应器是一种高效的多相反应器,由于其具有良好的传热、传质和混合特性,目前已被广泛应用于生物、化学、制冷和环境保护等工程领域。气液喷射器作为喷射式环流反应器的核心部件之一,其结构尺寸对环流反应器的传质特性和适用环境都具有显著影响。为考察结构尺寸对喷射器性能的影响,本文根据其工作原理,设计了一台模试气液喷射器,并通过冷模试验对其进行性能测试研究。试验结果表明：气液喷射器的引气能力主要取决于其混合喉管与喷嘴出口截面比f₃/f₁以及喷射器进出口压力降Δp_p/Δp_c,而环流反应器的气含率仅与喷射器的液相射流量和气体引射量有关。相同液相流量条件下,喷射器的最大引射空气量随截面比f₃/f₁的增大而增大,反应器内的平均气含率随之增加;提高液相射流与引射气体的速度差能够加强两股流体间的剪切作用,使气泡更易发生破碎。当喷射器的气液比大于2.6时,反应器内的混合流体可达到乳化状态。     

Multiphase catalytic reactors: a perspective on current knowledge and future trends

Conventional and emerging processes that require the application of multiphase reactors are reviewed with an emphasis on catalytic processes. In the past, catalyst discovery and development preceded and drove the selection and development of an appropriate multiphase reactor type. This sequential approach is increasingly being replaced by a parallel approach to catalyst and reactor selection. Either approach requires quantitative models for the flow patterns, phase contacting, and transport in various multiphase reactor types. This review focuses on these physical parameters for various multiphase reactors. First, fixed-bed reactors are reviewed for gas-phase catalyzed processes with an emphasis on unsteady state operation. Fixed-bed reactors with two-phase flow are treated next. The similarities and differences are outlined between trickle beds with cocurrent gas-liquid downflow, trickle-beds with countercurrent gas-liquid flow, and packed-bubble columns where gas and liquid are contacted in cocurrent upflow. The advantages of cyclic operation are also outlined. This is followed by a discussion on conventional reactors with mobile catalysts, such as slurry bubble columns, ebullated beds, and agitated reactors. Several unconventional reactor types are reviewed also, such as monoliths for two-phase flow processing, membrane reactors, reactors with circulating solids, rotating packed beds, catalytic distillation, and moving-bed chromatographic reactors.

Numerous references are cited throughout the review, and the state-of-the-art is also summarized. Measurements and experimental characterization methods for multiphase systems as well as the role of computational fluid dynamics are not covered in a comprehensive manner due to other recent reviews in these areas. While it is evident that numerous studies have been conducted to elucidate the behavior of multiphase reactors, a key conclusion is that the current level of understanding can be improved further by the increased use of fundamentals.     

Modellbildung für Mehrphasenströmungen in Reaktoren

Modelling multiphase flow in reactors. This paper is intended to enhance the knowledge of two phase reactors. Their performance depends not only on reaction kinetics but essentially on fluid-dynamically fixed quantities such as volume fractions, dispersion coefficients in the fluid and in the gas phase, as well as the volumetric mass transfer coefficient. These are basically dependent on the slip velocity of the phases and on the internal circulation speed of the fluid. The circulation either results from the fluid passing through the vessel (as in a bubble column) or it has to be effected separately in order to avoid separation. Suspension of solids or aeration of liquids in loop and stirred reactors can serve as examples. Dimensionless groups that characterize the main material and geometric properties can be derived using the slip velocity of the largest stable bubble and a certain circulation parameter. The power of this method will be shown by the characterization of bubble columns and gas-liquid mixers. Simple laws comparable with those applying to bubble columns emerge when approaching the flooding point of a disc turbine in a tank. Power numbers, gas volume fractions, and volumetric mass transfer coefficients can be bundled over the whole spectrum of the non-flooded states, when their characteristic quantities are divided by the quantities relating to the flooding point.     

液体中气—液射流特性的研究

在气-液两相喷射式装置中气体或/和液体是经由单个(或多个)喷咀射入塔内的。本研究用频闪摄影技术对气、液向上共同喷射时液体中的气-液射流特性进行了实验研究,并建立了气泡直径分布函教及射流特征尺寸的关联式。同时,对射流两相区内两相的流动用一两相模型进行模拟,考察了喷咀直径、初始条件、液体压头和气泡直径对器内两相流动的影响。     

多相流搅拌反应器研究进展

综述了近五年冷态气液固三相搅拌反应器的研究成果,阐明近期研究热点已转为热态多相流搅拌反应器的流体力学行为。概述了热态气液两相体系中通气功率、混合时间、局部及总体气含率的研究结果。指出今后研究重点应为热态的固液悬浮、气液传质过程及其机理,同时说明用计算流体力学（CFD）对气液固三相搅拌反应器的数值模拟仍属起步阶段。     

CIRCULATION IN LARGE SCALE JET BUBBLE COLUMNS

A hydrodynamic study in a gas-liquid jet bubble column was undertaken in a column with a 122 cm diameter cylindrical section and a conical bottom section approximately 180cm in height. Due to the jetting action in the cone, the circulation patterns are different from those in cylindrical bubble columns. In order to examine this difference in flow pattern, circulation velocity measurements were undertaken (for V₅ = 0.39-2.73 cm/s and V, = 0-0.044 cm/s) using the Pavlov tube technique. These measurements should be helpful in understanding other design parameters (such as mixing, phase distribution, transport coefficients, etc.). Pressure drops were also measured at nine axial taps and using these values, sectional average gas holdup values were calculated. The study was centered on the conical section; the cylindrical bubble column was assumed to be the limiting case for the interpretation of the data. Using the experimental observations, the height of the jet effective region was approximated by two different methods. A simple empirical correlation to find the centerline velocity is proposed.     

基于段塞流的通用气液两相流模型的建立与验证

气液两相混输流动中,由于段塞流与其他流型均有联系,从段塞流出发建立模型可以使不同流型下的计算模型得到统一。基于段塞流建立气液两相流动量方程和连续性方程,完善模型对气泡和液滴夹带的处理。建立模型的闭合关系式,对关键参数(壁面及气液相间水力摩阻系数、液塞平移速度、平均液塞长度)的计算闭合关系式进行优选,得到适用于模型的闭合关系式,同时,对液滴和气泡的夹带给出夹带条件及相关参数计算式,最终建立基于段塞流的通用气液两相流模型。使用3组不同来源的实验数据验证了模型计算压降和持液率的准确性,实验数据分别来自中国石油大学(华东)、大庆油田实验基地的气液两相流实验以及国外研究人员的实验研究,包含各个气液流型。模型具有较高的计算精度,优于未经关系式完善和优化的原始模型,大部分压降及持液率参数的计算误差在±15%以内。     

新型洗涤冷却室内气液两相的分布特性

采用双头电导探针法对新型洗涤冷却室环隙鼓泡床内气液两相的局部气含率、气泡直径等分布规律进行了实验研究,并用Fluent商业软件对床层内气含率分布进行了模拟计算,模拟结果与实验结果吻合较好。研究结果表明：新型洗涤冷却室内部构件对环隙鼓泡床内气液两相的分布特性影响显著,气相分布更加均匀,液面波动更趋平稳,有效地减少了气体带水问题,相比国外技术具有更好的操作弹性。     

CFD modeling of slurry bubble column reactors for Fisher-Tropsch synthesis

Industrial bubble column reactors for Fischer-Tropsch (FT) synthesis include complex hydrodynamic, chemical and thermal interaction of three material phases: a population of gas bubbles of different sizes, a liquid phase and solid catalyst particles suspended in the liquid. In this paper, a CFD model of FT reactors has been developed, including variable gas bubble size, effects of the catalyst present in the liquid phase and chemical reactions, with the objective of predicting quantitative reactor performance information useful for design purposes. The model is based on a Eulerian multifluid formulation and includes two phases: liquid-catalyst slurry and syngas bubbles. The bubble size distribution is predicted using a Population Balance (PB) model. Experimentally observed strong influence of the catalyst particles concentration on the bubble size distribution is taken into account by including a catalyst particle induced modification of the turbulent dissipation rate in the liquid. A simple scaling modification to the dissipation rate is proposed to model this influence in the PB model. Additional mass conservation equations are introduced for chemical species associated with the gas and liquid phases. Heterogeneous and homogeneous reaction rates representing simplified FT synthesis are taken from the literature and incorporated in the model.Hydrodynamic effects have been validated against experimental results for laboratory scale bubble columns, including the influence of catalyst particles. Good agreement was observed on bubble size distribution and gas holdup for bubble columns operating in the bubble and churn turbulence regimes. Finally, the complete model including chemical species transport was applied to an industrial scale bubble column. Resulting hydrocarbon production rates were compared to predictions made by previously published one-dimensional semi-empirical models. As confirmed by the comparisons with available data, the modeling methodology proposed in this work represents the physics of FT reactors consistently, since the influence of chemical reactions, catalyst particles, bubble coalescence and breakup on the key bubble-fluid drag force and interfacial area effects are accounted for. However, heat transfer effects have not yet been considered. Inclusion of heat transfer should be the final step in the creation of a comprehensive FT CFD simulation methodology. A significant conclusion from the modeling results is that a highly localized FT reaction rate appears next to the gas injection region when the syngas flow rate is low. As the FT reaction is exothermal, it may lead to a highly concentrated heat release in the liquid. From the design perspective, the introduction of appropriate heat removal devices may be required.