PREDICTION OF PRESSURE DROP IN A VENTURI SCRUBBER: EFFECT OF LIQUID FILM AND RE-ENTRAINMENT

A mathematical model has been developed for prediction of pressure drop in a Venturi scrubber. This model includes the effect of the amount of liquid film and re-entrainment of liquid droplets from liquid film. The result of the present model is compared with experimental data of Viswanathan et al. (1985) as well as with the other models (Viswanathan et al., 1985; Boll, 1973). Results of this study indicate that at high liquid to gas ratios prediction of pressure drop can be improved by considering re-entrainment and liquid film effects. Also the effects of gas velocity and liquid to gas ratio were investigated on the rate of droplet re-entrainment and pressure drop.     

Prediction of liquid entrainment in evaporatorsBerechnung des entrainments in verdampferanlagen

This report presents three correlations for the prediction of liquid entrainment in evaporators without droplet separators. The results are compared with experimental data obtained from such apparatus.     

气液过滤过程中液滴二次夹带现象分析

在天然气长距离输送过程中,天然气夹带的液滴严重影响压缩机组的安全可靠运行。利用所建立的滤芯过滤性能检测装置,以癸二酸二辛酯为实验介质,分析了天然气净化用滤芯气液过滤过程中液滴二次夹带现象的特征,比较了滤材润湿性和过滤速度等参数的影响。结果表明:当滤芯仅由聚结层组成时,滤芯过滤过程中存在液滴二次夹带现象,二次夹带将导致下游气体中液滴数量增多,且有较大液滴出现,稳态阶段滤芯累积效率在大粒径处下降;液滴在滤材表面的润湿性对二次夹带现象具有重要影响,可润湿型滤材表面更易出现液滴二次夹带现象,在相同过滤速度下,若需提高过滤效率,宜选用不可润湿型滤材制作工业滤芯;在0.1～0.3 m·s^-1过滤速度范围内,提高过滤速度可减少液滴二次夹带现象的发生,与不可润湿型滤芯相比,可润湿型滤芯过滤效果的改善更为明显;滤芯增加排液层可有效消除液滴二次夹带现象。     

折流板式除沫器性能分析及研究进展

针对生产实践中遇到液滴夹带影响设备及工艺的情况介绍了高效率、低压降、体积小的折流板式除沫器的应用背景.分析了工业上常用的折流板式除沫器类型、转折角度、叶片间距和级数等设计参数,临界分离粒径dcr和临界气流速度等特性参数对分离效率、压降和处理量等主要性能指标的影响情况.对折流板式除沫器国内外研究进展、存在的问题及发展方向进行了综述,指出通过建立数学模型结合理论分析,推导出除沫器效率理论计算式显得十分重要.对除沫器的力学性能进行分析,得出一些改进措施可使除沫器整体性能得到优化;采用高速摄影等先进实验设备对折板除沫器壁面液滴的运动及之后的沉积过程进行研究仍属较新的方向.     

A correlation giving improved description of the capacity and efficiency of vane-type gas–liquid separators

Vane-type demisters are attractive droplet separators because they combine robustness and sufficient separation efficiency with a low-pressure drop, a high capacity and some fouling resistance. Experience shows that their optimal operation is close to the maximum capacity point and even that at this point the separation efficiency is better at higher pressures. This article proposes a generalized expression this point, as a function of gas and liquid flow and properties, drawing on knowledge about other, related, unit operations, namely cyclone separators and distillation towers. The generalized expression relates the dimensionless velocity with the Archimedes number and a flow parameter. The expression builds on earlier work on a criterion for the inception of droplet entrainment in two-phase film flow, pioneered by Ishii and Grolmes, and is supported by considerable experimental work published over the last 50 years.     

改造气液分离设备提高分离效率

针对目前气液分离设备普遍存在的弊端,结合气液流动的特点,改进雾沫分离器的结构形式,气体输送管道水平段增设盲肠分离器、气液分离器等,提高气液分离效率.     

Influence of the gas—liquid separator design on hydrodynamic and mass transfer performance of split-channel airlift reactors

Three geometric configurations of gas–liquid separators were used in split-channel airlift reactors (0·1 m³ liquid volume; riser-to-downcomer cross-sectional area ratio = 0·7; aspect ratio = 3.6) to test the effect of geometry on hydrodynamic performance and oxygen transfer behaviour. For otherwise fixed conditions, the design of gas–liquid separators affected the induced liquid circulation rate, the depth of penetration of the bubble layer in the downcomer, the gas holdup in the downcomer, the mixing time and the overall volumetric gas–liquid oxygen transfer coefficient. The gas holdup in the riser was only marginally affected by the design of the separator. The impact of the various separator designs on hydrodynamic behaviour could be explained as emanating from a combination of the gas–liquid separating ability of the design and its hydraulic resistance.     

Void fraction in vertical gas-liquid slug flow: Influence of liquid slug content

In this study we develop a model for computing the mean void fraction and the liquid slug void fraction in vertical upward gas-liquid intermittent flow. A new model for the rate of gas entrained from the Taylor bubble to the liquid slug is formulated. It uses the work done by the pressure force at the rear of the Taylor bubble. Then an iterative approach is employed for equating the gas entrainment flux and the gas flux obtained via conservation equations. Model predictions are compared with experimental data. The developed iterative method is found to provide reasonable quantitative predictions of the entrainment flux and of the void fraction at low and moderate liquid slug void fraction conditions. However, with an increased liquid slug void fraction experimental data indicate that the flow in the liquid slug transits to churn-heterogeneous bubbly flow thus gas entrainment flux tends to zero. Considering this effect in the iterative model significantly improved the predictions for large liquid slug void fraction conditions.     

Drop Entrainment in a Rotary Spraying Apparatus

Drop entrainment is studied in a rotary tray apparatus with spraying and multiple recycling of the liquid in the contact elements. The main contribution to the amount of the entrained liquid is from the fine drops carried away by the flowing gas (vapor) from the cloud of the sprayed liquid. Equations are suggested for calculating the liquid entrainment rate.     

Design Criteria for Oilfield Separators Improved by Computational Fluid Dynamics

Oilfield separator data ranging from light‐oil conditions to heavy‐oil conditions were incorporated into suitable two‐phase and three‐phase computational fluid dynamics (CFD) models to provide improved design criteria for separator design methods. The CFD simulation results revealed that the most important affecting parameters are vapor density and oil viscosity. In contrast with the classic design methods, noticeable residence times were required for liquid droplets to penetrate through the fluid interfaces. Moreover, it was indicated that the Abraham equation should be used instead of the Stokes' law in the liquid‐liquid separation calculations. The velocity constraints caused by re‐entrainment in horizontal separators were also studied and led to novel correlations.     

轴流导叶式气液旋流分离器的试验研究

轴流导叶式气液旋流分离器与其他离心式气液分离器比较,其特点是阻力损失小。经试验证明,在低含液浓度下该分离器对气液两相具有较高的分离效果,且采用合理的溢流管和筒体结构形式可以减少短路流和二次流夹带,提高其分离效率。     

Multi-objective optimization of drainage-plates in wave-plate mist eliminators using experiment and data-driven modelling for lower water loss and energy requirement

Wave-plate mist eliminators are widely employed as gas–liquid separation devices to prevent the liquid escaping from thermal power plants or other cooling towers. In this study, the wave-plate mist eliminator with drainage plates was numerically analyzed and the effects between geometrical variables on two objectives, namely, pressure drop (ΔP) and separation efficiency (η), were revealed. Plate spacing, width, and length, as well as the relative position of the drainage plate, were thoroughly investigated. A combined strategy was developed for multi-objective optimization of the wave-plate mist eliminator by integrating computational fluid dynamics (CFD) simulation, response surface methodology (RSM), non-dominated sorting genetic algorithm-II (NSGA-II), and a technique for order of preference by similarity to ideal solution (TOPSIS) method. The results demonstrated that the relative position of drainage plates has a greater impact on the overall performance, whereas the width of drainage plates has the minimum effect. With the implementation of NSGA-II and the TOPSIS method, an optimal solution for the design of the mist eliminator was obtained. After comparing with the baseline case, the optimized case presents promising characteristics with high separation efficiency (enhanced by 3.6%~9.06%) and a low energy consumption coefficient (reduced by 72.30% at η = 45%).     

Dispersion of liquid–liquid phase by a jet-induced gas–liquid–liquid mixing column developed for separation of organic pollutants

This article presents the gas and liquid entrainment and its dispersion in a gas–liquid–liquid mixing column. The variations in phase entrainment is observed with the change in the paraffin liquid and kerosene volume fraction from 5% to 35% due to the increase in the flow resistance with increase in the effective viscosity of the liquid–liquid mixture. The degree of dispersion is enunciated based on the axial dispersion model and the flow resistance of the phases in the column. A correlation is proposed to interpret the entrainment of phase as a function of operating variables within the range of experimental conditions.     

横掠气流作用下波形板壁降膜破裂分析

以边界层理论为基础,基于独立假设建立了波形板通道内降液膜在横向气流驱动下沿屈折角横向偏移的二维边界层模型。通过量纲分析和理想层流假设进行模型简化及边界层方程求解,建立了横向切应力驱动下波形板壁降膜破裂的力平衡模型并给出破膜速度的临界准则,模型包括波形板通道几何参数、气液两相物性及流动参数等多种因素的影响,与本文实验结果和已有理论模型进行对比,所建模型能够更准确地预测液膜发生破裂的临界条件。进一步的量纲分析表明惯性离心力、气流剪切力、壁面黏性力和表面张力共同决定着液膜的运动、变形直至破裂,由它们之间的平衡关系可以确定液膜的稳定与否。     

同心圈式超重力旋转床液泛模型

同心圈式超重力旋转床是一种新型超重力旋转床。液泛是超重力旋转床流体力学的重要特征。同心圈式超重力旋转床液体分布器和转子内缘之间的环形空间内的液滴被气体夹带,液滴受到离心力和气体曳力的作用,通过建立微分方程可获得液滴径向速度为零时的液滴运动径向距离。当该径向距离小于环形空间的径向距离,此时产生雾沫夹带液泛。由此建立同心圈式超重力旋转床雾沫夹带液泛模型。实验以空气和水为物系,测定了转子直径为1000 mm、高度为100 mm的同心圈式超重力旋转床在不同转速和表观液速下气体进口和出口之间的气相压降随表观气速的变化。气相压降随表观气速的增大先缓慢增大后快速增大。用表观气速对气相压降求导和目测旋转床中心气体出口处出现大量液体被气体夹带来确定液泛点气速。通过液泛点气速求得雾沫夹带液泛模型的系数k,并对该系数k进行关联。该雾沫夹带液泛模型的计算值和实验值吻合很好,平均偏差为3.1%。该模型优于Sherwood液泛模型,对同心圈式超重力旋转床的工业应用提供了必要的设计依据。     

螺旋片导流式气液分离器的数值模拟与试验研究

The gas/liquid spiral separator, a key component in the compressed air system, was used to remove liquid and oil from gas stream by centrifugal and gravitational forces. To optimize the design of the separator, the relationship between the performance and structural parameters of separators is studied. Computational fluid dynamics （CFD） method is employed to simulate the flow fields and calculate the pressure drop and separation efficiency of air-liquid spiral separators with different structural parameters. The RSM （Reynolds stress model） turbulence model is used to analyze the highly swirling flow fields while the stochastic trajectory model is used to simulate the traces of liquid droplets in the flow field. A simplified calculation formula of pressure drop in spiral structures is obtained by modifying Darcy＇s equation and verified by experiment.     

Solids entrainment into gas, liquid, and gas-liquid spray jets in fluidized beds

The injection of liquid into a fluidized bed is a crucial step in many processes such as fluid coking, fluid catalytic cracking, or gas-phase polymerization, whose performance greatly depends on good and rapid contact between the injected liquid and the fluidized particles. The liquid spray, created by two-phase (gas-liquid) nozzles, forms a jet, i.e. a gas-rich cavity within the fluidized bed. Past studies have shown that good liquid-solid contact requires a large entrainment rate of particles into the jet, followed by intensive mixing of liquid droplets and entrained particles within the jet. The objective of this study is the experimental measurement of solids entrainment into spray jets. The specific application of interest is the enhancement of solids entrainment under conditions relevant to the fluid coking process.A novel and accurate experimental technique has been developed to measure the solids entrainment from a fluidized bed into two-phase gas-liquid jets, gas jets and liquid jets. The effects of operating conditions of the nozzle (sonic versus subsonic) and of the fluidized bed on the solids entrainment have been investigated. The differences between the mechanisms of solids entrainment for two-phase gas-liquid, gas and liquid jets have been analyzed.This experimental tool has been applied to the design and testing of a mixing chamber consisting of a cylindrical tube placed at a certain distance downstream of the nozzle tip, resulting in a confined, turbulent jet with enhanced liquid-solid mixing properties.     

Creeping Film Models for Rotating Channels

A theoretical analysis is presented for the development of a liquid film at the walls of vertically placed centrifugal separators. The liquid film arises at the walls of these separators due to the action of the centrifugal force on the entrained liquid phases. The analysis applies to situations, for which the liquid load is small, i.e., the film thickness is much smaller than the height of the channel. This implies that the film can be regarded as a laminar boundary‐layer type flow with negligible inertia effects. Solutions are presented for situations in which the liquid droplets flow co‐currently and counter‐currently with the gas flow.     

Einsatz von Lamellen-Tropfenabscheidern in Trennkolonnen

Droplet separators, or mist eliminators, are a very important internal part of fluid separation columns. They remove liquid droplets or mist from the gas stream and are installed at the top of the column with vertical gas or vapor flow. They reduce emissions to the atmosphere or to subsequent process steps. Droplet separators even can increase the separation efficiency of fluid separation processes, like absorption, desorption, distillation, evaporation etc. In this work the approach to develop a new vane type droplet separator for separation columns based on experimental investigation and CFD analysis is presented. The new droplet separator has a higher separation efficiency at the same pressure drop compared to state-of-the-art standard droplet separators.     

Entwicklungstendenzen bei der Partikelabtrennung aus Gasen mit filternden Abscheidern

Cleanable surface filters are used as highly efficient separation devices for removing particles from gases in three major areas of application, viz. reduction of emissions, product recovery, and process gas purification. In order to utilize their excellent separating properties in processes where unfavorable operating conditions mean that competing kinds of separators have hitherto been used, filtering separators are undergoing constant development. This paper presents and discusses future-oriented developments from current work and completed plant in the three areas of ‘modeling/computer simulation’, ‘filter medium development’, and ‘crude gas conditioning’.