DISPERSION IN TRICKLE-BED REACTORS WITH PULSING GAS-LIQUID DOWN-FLOW

Pulsing flow in trickle-bed reactors occurs at high gas and liquid input flow rates. It refers to the periodic passage of liquid rich slug and gas rich pulse down the column. A model based on the combination of the method of characteristics and Monte Carlo simulation of liquid phase dispersion in the slug and the pulse is developed to predict liquid dispersion in a pulsing column packed with non-porous, spherical particles. The predicted Peclet number as a function of Reynolds number shows good agreement with experimental data. In addition, it is found that the pulsing phenomenon does not contribute significantly to dispersion.     

DISPERSION IN TRICKLE-BED REACTORS WITH PULSING GAS-LIQUID DOWN-FLOW

Pulsing flow in trickle-bed reactors occurs at high gas and liquid input flow rates. It refers to the periodic passage of liquid rich slug and gas rich pulse down the column. A model based on the combination of the method of characteristics and Monte Carlo simulation of liquid phase dispersion in the slug and the pulse is developed to predict liquid dispersion in a pulsing column packed with non-porous, spherical particles. The predicted Peclet number as a function of Reynolds number shows good agreement with experimental data. In addition, it is found that the pulsing phenomenon does not contribute significantly to dispersion.     

Surfactant‐free microdispersion process of gas in organic solvents in microfluidic devices

The scaling of bubble/slug formation in organic solvents at microscale without surfactant was initially investigated by using T‐junction and symmetrically cross‐shaped microfluidic devices. Four unique organic solvents and three dispersion methods were used, forming different flow patterns and dispersion size. The flow pattern of uniform slug flow was investigated. Both the gas–liquid flow and dispersion size, which ranged from 400 to 1400 μm in length and 270 to 430 μm in diameter, depended on several factors including dispersion method, two‐phase flow rates, physical properties of the liquid phase, and structure of microchannels. A general equation L/w = k(Q_G/Q_L)^αCa^β was used to characterize the dispersion size with modification of Q_G/Q_L for different dispersion methods, considering the influences of breakup rate and transformation of the interface shape on the dispersion process. Three models were developed to predict the dispersion size for different dispersion methods, and calculated data were in good agreement with the experimental results. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

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.     

Film and slug behaviour in intermittent slug–annular microchannel flows

Whilst there are numerous experimental, theoretical and computational studies of Taylor flow in microchannels, the intermittent slug–annular regime has largely been neglected. In this paper time-resolved micro-PIV data are collected and used to study the flow characteristics of a gas–liquid system for flow regimes spanning Taylor to annular flow. The experimental work used a 1.73 mm diameter channel with water and nitrogen as the working fluids, for gas and liquid superficial velocity ranges of 0.35–8.65 m s^?1 (40<Re_G<1000) and 0.071–0.18 m s^?1 (120<Re_L<300), respectively. Time-averaged velocity profiles were obtained in the liquid film surrounding the gas bubbles (or the gas core in the pseudo-annular flow regime) and in the liquid slugs (which changed from regular slugs to annular rings as the gas superficial velocity was increased). These data showed that the velocity in the liquid film relaxed back to an equilibrium value following the passage of each liquid slug or annular ring. In contrast rather flat velocity profiles were observed in the liquid slug. Based on a simple representation of the flow structure, average gas holdups were estimated using independent experimental data obtained by the micro-PIV technique and by direct observation of the flow structure. A phenomenological model of intermittent slug flow, based on the representation of the flow structure as a train of slugs and bubbles moving over a liquid film, is used to interpret the experimental data. The modelling work highlights the different behaviour of the limiting cases of slug and annular flow, in terms of the gas–liquid interfacial shear and its influence on the pressure field.     

水平管内气液段塞流液塞长度发展规律的实验和模拟研究

In petroleum industry, the slug flow is a fre-quently encountered flow regime in multiphase flowpipeline. For pipeline designers, the liquid slug lengthdistribution is important for the proper design ofdownstream facilities, such as slug catcher and sepa-ration system. However, for its transient and unsteadynature, it is a great challenge for engineers to correctlypredict the flow parameters of slug flow, especiallythe maximum liquid slug length. The unit cell model for slug flow in horizontal…     

倾斜上升弹状流中Taylor气泡运动速度研究

用高速动态分析仪对倾斜上升管中气液两相弹状流中Ｔａｙｌｏｒ气泡的运动速度进行了研究。获得了无干扰流场下Ｔａｙｌｏｒ气泡运动参数的测量结果，并分析了混合物流速及管倾角对气泡头部位置的影响，以及由此而引起的气泡漂移速度及液体速度影响系数的变化情况，在理论分析的基础上，推荐了计算气泡运动速度的实验关联式。并且与可利用的结果进行了比较，两者符合较好。     

Horizontal air–water flow in a square cross‐section channel: Pseudo‐slug flow regime

New experimental data for air–water flow in a horizontal square cross‐section channel (H = 24.25 mm) is presented, including data on liquid hold‐up, gas and liquid velocities, and wave velocities and frequencies. For the majority of gas and liquid flow rates studied, the regime observed was pseudo‐slug. Using visualization studies it was possible to identify wavy‐stratified and pseudo‐slug flows. For the pseudo‐slug regime new correlations were obtained for liquid hold‐up, for gas and liquid velocities as a function of the ratio between gas and liquid mass flow rates, and for the frequency of roll‐waves as a function of gas and liquid mass flow rates.     

Characteristics of liquid slugs in gas–liquid Taylor flow in microchannels

The hydrodynamics of liquid slugs in gas–liquid Taylor flow in straight and meandering microchannels have been studied using micro Particle Image Velocimetry. The results confirm a recirculation motion in the liquid slug, which is symmetrical about the center line of the channel for the straight geometry and more complex and three-dimensional in the meandering channel. An attempt has also been made to quantify and characterize this recirculation motion in these short liquid slugs (L_s/w<1.5) by evaluating the recirculation rate, velocity and time. The recirculation velocity was found to increase linearly with the two-phase superficial velocity U_TP. The product of the liquid slug residence time and the recirculation rate is independent of U_TP under the studied flow conditions. These results suggest that the amount of heat or mass transferred between a given liquid slug and its surroundings is independent of the total flow rate and determined principally by the characteristics of the liquid slug.     

Intermittent two phase flow in horizontal pipes: Predictive models

A semi-mechanistic model for two phase gas-liquid slug flow proposed recently by Dukler and Hubbard has been modified and extended to apply to the entire intermittent flow regime. Flow predictions of the model proposed in this paper are compared with detailed experimental data recently obtained for an air-oil system. The model requires the use of empirical correlations for the slug velocity and the in situ liquid volume fraction in the slug. In addition, either the slug frequencies or length corresponding to the given design conditions must be known. However, calculated values of average pressure gradient and in situ liquid volume fraction are relatively insensitive to these latter parameters, and in fact, good results are obtained assuming a constant slug length. The paper includes a discussion of the limitations of the proposed model and the expected direction of further study required to extend its mechanistic aspects.     

垂直上升气液两相弹状流模型

基于等效弹单元思想,改进了预测垂直上升管中充分发展气液弹状流流动特性的模型。 模型中考虑了界面切应力对液膜运动的影响;并在液弹空隙度预测中引入临界气体夹带速度的概念,以此来描述弹状流中大气泡尾部的混合特性。本文提出的模型还考虑了管径对液弹空隙度的影响。弹状流模型的计算结果得到本文及其他作者实验数据的验证。     

Slug flow characteristics of gas–miscible liquids in a rectangular microchannel with cross and T-shaped junctions

The slug flow of an inert gas and two miscible liquids in microchannels has found its applications in the preparation of solid lipid nanoparticles (SLNs) by the liquid flow-focusing together with Taylor bubbles in microchannel systems, synthesis of metal nanoparticles or colloid silica in microreactors and enhancement of micro-mixing by interaction using gas bubbles in microfluidic devices. In this work, the flow characteristics of the slug flow generated by nitrogen gas and two miscible liquids (the aqueous surfactant solution and acetone or ethanol) flowing in a rectangular microchannel were investigated experimentally by using the high-speed optical imaging method. The microchannel system has a straight main channel for introducing one of the miscible liquids, a cross-junction for injecting of the other miscible liquid, and a T-junction for feeding the gas phase. The pressure drops were measured and images of Taylor bubbles and slug units at various velocities were obtained, from which other flow parameters were determined. Correlations for the velocity and length of Taylor bubbles, the bubble nose length, the bubble tail length, the liquid slug length, the maximum and minimum thicknesses of the liquid films around bubbles, as well as the pressure drop, were proposed. The calculated values of these parameters by using the correlations were compared with the experimental data. The results showed that the proposed correlations are in a good or reasonable agreement with experimental data and then expected to be available in the estimation of the slug flow parameters of the inert gas and two miscible liquids in rectangular microchannels.     

A simple mechanism of bubble and slug formation in Taylor flow in microchannels

A simple mechanism is proposed to explain and predict the bubble and slug lengths in Taylor (slug) flow in microchannels. The results obtained using the proposed approach are in good agreement with a correlation based on numerical experiments [Qian, D. and Lawal, A., 2006, Numerical study on gas and liquid slugs for Taylor flow in a T-junction microchannel. Chem Eng Sci, 80: 7609–7625] and available experimental data.     

利用一维波和段塞稳定性模型预测分层流向段塞流的转变

利用气液二相流一维波模型和段塞稳定性模型,对直径2.54 cm水平管内空气-水二相流出现段塞流时的各相临界表观速度和临界液层高度进行了理论预测。计算中发现,2种模型分别适用于不同的流速区域,在较低的气相流速下,一维波模型的预测结果比较理想,但是在较高的气速条件下不太适合,而利用段塞稳定性模型可以较好地获得高流速下分层流向段塞流的流型转变条件。因此,结合这2种模型对发生流型转变时的临界参数作了分析,并且应用于40 mm和50 mm水平管道的油气二相流实验。将理论计算的结果和实验测得的流型数据进行了对比,并且对影响流型的管径、流速等因素作了分析,结果表明计算得到的特征参数和实验数据比较吻合。     

Liquid holdup in concentric annuli during cocurrent gas‐liquid upflow

In the recent paper, an in‐depth investigation of liquid holdup during air‐water upflow through concentric annuli has been reported. The liquid holdup has been determined experimentally for the bubbly, slug and churn flow regimes. The drift flux model has been adopted for the theoretical estimation of holdup in the bubbly, dispersed bubbly and slug flow regimes. The pronounced effect of flow regime on this parameter as observed from experiments has been incorporated in the model by adopting different values of U₀, n and C₀. The asymmetry of the Taylor bubbles has been incorporated in the slug flow regime. The theoretical predictions exhibit a good agreement with the experimental data of the present work and that available in literature (Caetano et al., 1989b). The Hughmark's correlation is observed to correlate the churn flow data of the present work reasonably well.     

Vertical slug flow in laminar regime in the liquid and turbulent regime in the bubble wake—Comparison with fully turbulent and fully laminar regimes

An experimental and simulation study on free bubbling vertical slug flow in laminar regime in the main liquid and turbulent regime in the near-wake bubble region is reported. A non-intrusive image analysis technique and a previously developed slug flow simulator (SFS) were used. Two aqueous glycerol solutions (0.012-0.013 and 0.022 Pa s) were studied. A single bubble-to-bubble interaction curve was obtained. Strong interaction was found for bubbles flowing less than 3-4D apart, with slight interaction persisting for longer distances. The shape of the interaction curve bridges those for fully turbulent and fully laminar regimes. The experimental average bubble velocity in undisturbed conditions was shown not to follow the correlation-based predictions for laminar regime in the liquid. Alternative fitting coefficients are proposed. An entrance length of 50-80D (or 90-170D) was obtained for normal inlet slug length distributions centred on 2-5D (or 2-6D), for superficial gas and liquid velocities up to 0.40 and 0.30 m/s, respectively. More contrasting inlet slug length distributions were found not to converge within the length of the column (6.5 m). An overall comparison between the three regimes is presented.     

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.     

规整填料塔液相混合的研究

本文采用点源脉冲示踪的方法考察了装填２５０ Ｙ 型金属板波纹规整填料的填料塔中的轴向及径向返混。在规整填料塔的顶部注入 Ｋ Ｍｎ Ｏ４ 作为示踪剂,从塔的底部的不同径向位置取样。通过最优化方法计算出轴向返混系数 Ｄｚ 和径向返混系数 Ｄｒ,研究了液相和气相对规整填料的返混的影响,并就液相和气相对返混影响做了初步解释。实验结果表明：径向扩散系数和轴向扩散系数随气速和液体流速的增大而增大。得到轴向和径向混合系数的彼克列数（ Ｐｅｚ , Ｐｅｒ） 与液相和气相的表观雷诺数（ Ｒｅ１ , Ｒｅｇ） 的关联式。     

The effect of mass transfer on flow characteristics in vertical two-phase unstable flow

Studies carried out on the absorption of hexane from inert carrier gases into paraffin oil have shown that mass transfer can affect flow characteristics in unstable vertical two-phase flow. In the absence of mass transfer chain-flow patterns prevailed. These patterns were broken when mass transfer took place from the gas to the liquid phase. Bullet shaped slugs were then formed, the slugs pairing and coalescing as they rose up the column. The direction of mass transfer was critical no effect on flow pattern being observed when mass transfer took place from the liquid to the gas phase. Slug frequencies and slug sizes were found to be influenced by mass transfer and an effect of overall absorber length was observed. Individual slug sizes and separation distances were measured and varied widely even under constant flow conditions. Rise velocities were correlated using an equation of the form:where C₂ was a function of the fluid flow-rates and system physical-properties.     

Numerical study on gas and liquid slugs for Taylor flow in a T-junction microchannel

The rapid development of microfabrication techniques creates new opportunities for applications of microchannel reactor technology in chemical reaction engineering. The extremely large surface-to-volume ratio and the short transport path in microchannels enhance heat and mass transfer dramatically, and hence provide many potential opportunities in chemical process development and intensification. Multiphase reactions involving gas/liquid reactants with a solid as a catalyst are ubiquitous in chemical and pharmaceutical industries. The hydrodynamics of the flow affects the reactor performance significantly; therefore it plays a prominent role in reactor design. For gas/liquid two-phase flow in a microchannel, the Taylor slug flow regime is the most commonly encountered flow pattern. The present study deals with the numerical simulation of the Taylor flow in a microchannel, particularly on gas and liquid slugs. A T-junction empty microchannel with varying cross-sectional width (0.25, 0.5, 0.75, 1, 2 and 3 mm) served as the model micro-reactor, and a finite volume based commercial computational fluid dynamics (CFD) package, FLUENT, was adopted for the numerical simulation. The gas and liquid slug lengths at various operating and fluid conditions were obtained and found to be in good agreement with the literature data. Several correlations in the T-junction microchannel were developed based on the simulation results. The slug flows for other geometries and inlet conditions were also studied.