Liquid circulation and mixing in an inclined bubble column

Gas and liquid velocities were measured in an inclined rectangular column, at inclinations in the range 0?45° and superficial gas velocities of 0.002 to 0.12 cm/s. Liquid circulation was also characterized by measuring the time to mix a pulse of salt solution in the column. Inclining the column by up to 10° from the vertical induced a drastic change in behavior, from a bubble column with evenly distributed gas bubbles to a segregated system with rapid liquid circulation pumped by rising gas bubbles. The liquid velocities in the inclined column ranged from 10 to 50 times the superficial gas velocity. Further inclination of the column in the range of 10?45° had little effect on liquid velocity or mixing time. A compartmental model for the recirculating liquid flow matched the observed mixing times, but predicted oscillations in tracer concentration which were not detected in experiments.     

CFD modeling of flow,macro-mixing and axial dispersion in a bubble column

The flow pattern in a bubble column depends upon the column diameter, height, sparger design, superficial gas velocity and the nature of gas–liquid system. In this paper, the effect of some of these parameters have been simulated using Computational Fluid Dynamics (CFD). The relationship of these parameters with the interphase force terms has been discussed. A complete energy balance has been established. Using this methodology, the flow patterns reported by Hills (1974), Menzel et al. (1990), Yao et al. (1991) and Yu and Kim (1991) have been simulated. Excellent agreement has been shown between the CFD predictions and the experimental observations. The above model has been extended to homogenization of an inert tracer. In order to confirm this model, mixing experiments were carried out in a 200 mm i.d. bubble column. A radioactive tracer technique was used for the measurement of mixing time. Tc-99m (^99m Tc), in the form of sodium pertechnate salt, was used as the liquid phase tracer. Good agreement has been shown between the predicted and the experimental values of mixing time. The model was further extended for the estimation of axial dispersion coefficient (D_L). Excellent agreement between the simulated and the experimental values of the axial dispersion coefficient confirms the predictive capability of the CFD simulations for the mixing process.     

Mixing in a co‐current upflow bubble column reactors with and without internals

Liquid phase mixing is a phenomenon that results mainly due to convective and turbulent flow fields, which are generated by hydrodynamic interactions between the gas and liquid phases within a continuous co‐current upflow bubble column reactor. The extent of liquid phase mixing is usually quantified through the mixing time, or the axial dispersion coefficient. In the present work, the computational fluid dynamics (CFD) simulations for mixing and RTD in a continuous bubble column (with and without internals) are performed by using OpenFOAM 2.3.1. The superficial gas velocities were 0.014, 0.088, and 0.221 m/s and the superficial liquid velocities were 0.005 and 0.014 m/s. The simulations have been performed for three different configurations of the bubble column, that is, (a) an open bubble column, (b) a column with one vertical central rod of 36 mm diameter, (c) a column with the same central rod and four vertical additional rods of 12 mm diameter. The effects of superficial gas and liquid velocities and column internals were investigated on liquid phase mixing and the axial dispersion coefficient. Comparisons have been made between the experimental measurements and the CFD simulations.

     

INVITED REVIEW HYDRODYNAMIC AND MIXING MODELS FOR BUBBLE COLUMN REACTORS

The knowledge of the flow patterns of each phase of bubble column reactors is of considerable importance for the rational design and scale-up. The hydrodynamic models for the liquid phase have been reviewed. The models have been based on some form of pressure balance or energy balance. These two approaches have been compared and recommendations have been made regarding the range of applicability of the individual models. A comparison between the predicted and the experimental liquid velocities has been presented whenever possible. The range of variables has been pointed out for which further investigations are needed. The empirical and theoretical models for the mixing behavior of the different phases of bubble column reactors have also been reviewed.     

Liquid phase axial mixing in a bubble column with viscous non-newtonian liquids

This paper presents some new data for the liquid phase axial dispersion coefficient in a bubble column with highly viscous non-Newtonian liquids (μ_L > 0.03 Pa · s). The data were obtained in a 0.15 m diameter column operating in the slug flow regime, and the dispersion measurements were conducted using heat aas a tracer. The experimental results show that the dispersion coefficients increase with both gas and liquid velocities and quantitatively they are about three times higher than those obtained for the air-water system. The results are explained based on a known hydrodynamic model of vertical gas-liquid slug flow.     

喷嘴结构对射流鼓泡反应器混合和传质性能的影响

喷嘴结构对射流鼓泡反应器的混合和传质性能具有重要的影响。以空气-水作为模拟介质,使用双探头电导探针、电解质示踪法和动态溶氧法,对比研究了缩径式圆形喷嘴和旋扭三角形喷嘴对射流鼓泡反应器中气泡尺寸分布、平均气含率、液相混合时间和气液传质系数的影响规律。实验发现,随着气速或液体射流Reynolds数的增大,两种喷嘴对应的平均气含率、液相混合时间和气液传质系数具有相同的变化规律;与缩径式圆形喷嘴相比,采用旋扭三角形喷嘴的射流鼓泡反应器中气泡尺寸更小,平均气含率更高,宏观混合时间更短;当气体输入功占总输入功比例超过20%时,喷嘴结构对气液传质系数的影响较小,当气体输入功占总输入功比例小于20%时,旋扭三角形喷嘴的气液传质性能优于缩径式圆形喷嘴。研究结果可为工业射流鼓泡反应器喷嘴结构的优化提供理论指导。     

SIGNIFICANCE OF THE CENTRAL PLUME VELOCITY FOR THE CORRELATION OF LIQUID PHASE MIXING IN BUBBLE COLUMNS

The knowledge of the flow patterns of each phase of bubble column reactors is of considerable importance for the rational design and scale-up. The hydrodynamic models for the liquid phase have been reviewed. The models have been based on some form of pressure balance or energy balance. These two approaches have been compared and recommendations have been made regarding the range of applicability of the individual models. A comparison between the predicted and the experimental liquid velocities has been presented whenever possible. The range of variables has been pointed out for which further investigations are needed. The empirical and theoretical models for the mixing behavior of the different phases of bubble column reactors have also been reviewed.     

鼓泡床内气液两相流动的二阶矩模型与代数应力模型的模拟比较

A full second-order moment(FSM) model and an algebraic stress (ASM) two-phase turbulence model are proposed and applied to predict turbulent bubble-liquid flows in a 2D rectangular bubble column.Predication gives the bubble and liquid velocities,bubble volume fraction,bubble and liquid Reynolds stresses and bubble-liquid velocity correlation.For predicted two-phase velocities and bubble volume fraction the is only silight difference between these two models,and the simulation results using both two models are in good agreement with the particle image velocimetry(PIV) measurements.Although the predicted two-phase Reynolds stresses using the FSM are in somewhat better agreement with the PIV measurements than those predicted using the ASM,the Reynolds stresses predicted using both two models are in general agreement with the experiments.Therefore,it is suggested to use the ASM two-phase turbulence model in engineering application for saving the computation time.     

Single-phase flow model development for macroscopic liquid flow evaluation in gas–liquid reactors, by computational fluid dynamics

This paper proposes a single-phase flow model to simulate the flow induced in a liquid by the injection of gas dispersed in the form of a bubble curtain. It aims at predicting macroscopic liquid flow and mixing time. This single-phase flow model is developed as an alternative to two-phase flow models. The model is based on the assumption that the liquid flow is induced by a density imbalance between the bulk zone and the bubble curtain zone. The density in the bulk is set to the water density while the density in the bubble curtain corresponds to the air–water mixture density and is assessed by numerical simulations, thanks to an iterative procedure. Only the knowledge of the injected air flow rate and the bubble liquid relative velocity is required. The single-phase flow model is applied to assess the liquid flow and the mixing in open quarries having a complex geometry. The liquid velocities and the flow structure in the open quarries simulated with the single-phase flow model are in good agreement with those predicted by numerical simulations based on a two-phase flow model.     

基于气泡群相间作用力模型的加压鼓泡塔流体力学模拟

目前,多数文献报道了冷态加压湍动鼓泡塔内流动特征,并且通过实验数据回归相关经验关联式。然而,此类关联式适用范围有限,难以直接外推到工业鼓泡塔反应器条件。因此,在FLUENT平台上建立了基于气泡群相间作用力的、动态二维加压鼓泡塔计算流体力学模型。通过数值模拟考察了操作压力为0.5~2.0 MPa,表观气速为0.20~0.31 m·s^-1,内径0.3 m鼓泡塔内流场特性参数分布,并且与冷态实验数据进行比较。结果表明,采用修正后的气泡群曳力模型、径向力平衡模型以及壁面润滑力模型描述气泡群相间作用力,能够较为准确地反映平均气含率和气含率径向分布随操作压力和表观气速变化的规律。     

双层涡轮桨搅拌反应器内混合时间的大涡模拟

采用计算流体力学(CFD)方法对直径为0.476m双层涡轮桨搅拌反应器内的流动及混合进行了数值模拟,并实验测试了混合过程。利用大涡模拟(LES)及Smagorinsky-Lilly亚格子模型求解湍流流动与示踪剂传递过程,桨叶区域采用滑移网格技术。研究结果表明,大涡模拟得到的示踪剂响应曲线和混合时间与实验结果吻合良好,其预测精度明显优于基于雷诺平均(Reynolds-averaged Navier-Stokes,RANS)的标准k-ε模型的模拟结果。大涡模拟是研究搅拌反应器内非稳态及周期性湍流流动的有效方法。     

Axial dispersion in a rectangular bubble column

This paper presents the results of an experimental study on the gas holdup and the liquid phase axial dispersion coefficient in a narrow packed and unpacked rectangular bubble column. In both cases the gas and liquid flow rates were varied and the data were obtained by employing standard tracer technique. The gas holdup and the axial dispersion coefficient for both the packed and unpacked columns were found to be dependent on the gas and liquid flow rates. For given gas and liquid velocities and a given packing size in the case of the packed column, the rectangular column gave significantly higher dispersion coefficients than a cylindrical column of the equivalent cross sectional area. This result agrees very well with the one predicted by the velocity distribution model. The correlations for the Peclet number, the axial dispersion coefficient, and the fluid holdup for both the unpacked and packed bubble columns are presented.     

加温加压下CFD-PBM耦合模型空气-水两相流数值模拟研究

计算流体力学与群体平衡模型（CFD-PBM）结合可有效地模拟鼓泡塔内流体行为,较准确地预测流场、相含率以及局部气泡尺寸分布。以直径100 mm、高1.3 m的加温加压鼓泡塔为模拟对象,在系统压力为1 MPa、表观气速为0.08~0.24 m/s、温度为30~160℃条件下系统地考察了空气-水体系的表观气速、温度以及固含率对平均气含率、大小气泡气含率、气泡直径和气泡尺寸分布等参数的影响。结果表明,平均气含率的模拟结果和实验值在10%的误差范围内吻合较好;温度的变化主要影响了塔内气泡的聚并和破碎,并用聚并破碎的机理解释了温度对其流体行为的影响。     

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。     

Large Eddy Simulations of Mixing Time in a Stirred Tank

Large eddy simulations （LES） of mixing process in a stirred tank of 0.476m diameter with a 3-narrow blade hydrofoil CBY impeller were reported. The turbulent flow field and mixing time were calculated using LES with Smagorinsky-Lilly subgrid scale model. The impeller rotation was modeled using the sliding mesh technique. Better agreement of power demand and mixing time was obtained between the experimental and the LES prediction than that by the traditional Reynolds-averaged Navier-Stokes （RANS） approach. The curve of tracer response predicted by LES was in good agreement with the experimental. The results show that LES is a reliable tool to investigate the unsteady and quasi-periodic behavior of the turbulent flow in stirred tanks.     

Effect of interfacial forces and turbulence models on predicting flow pattern inside the bubble column

The numerical approaches have been used in many studies to predict the flow pattern inside the bubble column reactors because of the difficulties that are still found in designing and scaling-up the bubble columns. This review makes an effort to show suitable interfacial forces i.e., drag force, lift force, turbulent dispersion models and virtual mass and turbulence models such as standard k–ɛ model, Reynolds Stress Model, Large Eddy Simulation to predict flow pattern inside the bubble column using Eulerian–Eulerian. The effect of various interfacial forces and turbulence models on gas–liquid velocity and gas hold-up in bubble column is critically reviewed.     

湍流中气泡破碎建模与实验研究进展

综述了充分发展湍流中气泡破碎的机理和模型,将其机理归纳为湍流涡碰撞、黏性剪切、尾涡剪切脱落过程和界面不稳定性四类。对文献中气泡破碎速率和子气泡大小分布的预测模型进行了系统总结。分析讨论了现有气泡破碎模型的发展和局限性,并提出了未来的发展方向。同时,也综述了湍流中单气泡破碎的实验研究,依据产生湍流的方法归纳为四种情况：增大液体流速产生湍流,采用内构件产生湍流,搅拌产生湍流,以及圆锥反应器结合搅拌产生湍流。总结了现有气泡破碎实验的进展和局限,并进行了分析和展望。最后,通过将文献中气泡破碎速率模型预测值和实验数据进行对比,表明文献中多个破碎模型已经有了较好的预测能力。     

Hydrodynamics and mixing in highly viscous pseudo-plastic non-newtonian solutions in bubble columns

Gas hold-up structures in highly viscous pseudo-plastic carboxymethyl cellulose (CMC) solutions were measured in 0.10, 0.20, 0.38 and 1.0 m diameter batch bubble columns. A correlation for the gas hold-up based on a large number of data and covering a wide range of viscosities and column diameters is presented in the churn turbulent regime.

A flow transition chart for the churn turbulent and slug flow regimes is also presented for highly viscous pseudo-plastic solutions. Transient conductivity measurements were carried out in 0.20 and 0.38 m i.d. bubble columns with CMC solutions to measure mixing time. A “tanks in series with interstage recirculation” model was used to predict mixing times in the churn turbulent regime. Reasonably good agreement was found between the predicted and experimental values of mixing time.     

鼓泡床反应器内流动与传质行为的研究进展

总结了有关浆态鼓泡床反应器内流动、混合用气液传质特性的研究成果,详细地介绍了鼓泡床反应器内气含率、液速、液体轴向扩散系数、传质系数的测量方法,阐述了鼓泡床反应器性能的主要影响因素,如系统压力、温度、气体表观气速、液体性质及固含率等对流动、液相混合和传质特性的影响,并对鼓泡床反应器的应用前景进行了详述.