Transient rise velocity and mass transfer of a single drop with interfacial instabilities - experimental investigations

An experimental study of transient drop rise velocities and mass transfer rates was carried out in the system toluene/acetone/water which is known to show interfacial instabilities. The rise velocity of toluene drops was studied without added solute (acetone) in the diameter range 1-3 mm and with added solute for 2 mm drops. The initial concentration of the transferred solute was varied from 0 to 30 g/L. The transient drop rise velocities were used to quantify the Marangoni effect since the drag coefficient depends on the strength of the Marangoni convection patterns caused by interfacial tension gradients. In addition, mass transfer measurements were carried out in order to determine the modification of the mass transfer rate due to Marangoni convection. Velocity and mass transfer measurements were then correlated via the contact time. Results reveal the existence of a range in which a critical value for the solute concentration can be defined for Marangoni convection.     

Drop rise velocities and fluid dynamic behavior in standard test systems for liquid/liquid extraction—experimental and numerical investigations

The fluid dynamic behavior of single organic droplets rising in a quiescent ambient liquid is investigated experimentally and numerically. Three standard test systems for liquid/liquid extraction recommended by the European Federation of Chemical Engineering (EFCE) but without the respective transfer component have been chosen for the investigations: toluene/water, n-butyl acetate/water, and n-butanol/water, representing systems with high, medium, and low interfacial tension. Simulations are performed using the academic code Navier, which features a sharp interface representation and a variational formulation of the curvature. The validity of both correlations and the results obtained by dynamic numerical simulation is discussed in terms of terminal rise velocity, aspect ratio, and the onset of shape oscillation. The numerical results show excellent agreement with experiments in all three test systems, and with simulations in the n-butanol/water system published recently by Bertakis et al. (2010). The presented numerical method is applicable for a wide range of interfacial tension, whereas the investigated correlations lose accuracy with decreasing interfacial tension.     

Sedimentation behavior of droplets for the reactive extraction of zinc with D2EHPA

The sedimentation characteristics of the reactive standard test system zinc + D2EHPA are investigated in this work. Experiments with single droplets rising in a stagnant continuous phase have been carried out. The concentration of D2EHPA, zinc, and sulfuric acid as well as the diameter of the droplets are varied. The velocity of droplets is observed to be transient for several seconds. High mass‐transfer rates increase the velocity of single droplets for the case of reactive extraction whereas for physical systems contrary behavior is observed. Therefore, droplets seem to behave principally different in reactive and physical extraction. This is explained by the interfacially active properties of D2EHPA. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Systematic Analysis of Single Droplet Coalescence

Coalescence behavior in liquid‐liquid dispersions is controlled by various parameters. Thus, data from different research groups can differ significantly. Dynamic coalescence processes were analyzed systematically with the EFCE standard test system toluene/water in two different research laboratories. After comparability was proven with standardized batch settling tests and drop rise velocity measurements, the influence of drop size ratio and varying NaCl and NaOH concentrations on coalescence probability was investigated. Since the ions led to a coalescence inhibition, no clear impact of drop size ratio could be observed. The analysis of drop contact showed longer contact times for increasing equivalent drop diameter. This promotes coalescence and corresponds to film drainage model.     

Distillation tray efficiency and interfacial area

Three pure liquids, viz. water, ethanol and toluene were evaporated in air on a 10 cm dia. sieve plate to ascertain the effect of liquid properties on the mass transfer in the gas phase.

It was found that two extreme forms of dispersion exist on the tray, viz. a liquid dispersion (droplets) at high air velocity and low liquid hold-up, and an air dispersion (bubbles) at lower air velocity and higher liquid hold-up.

The mass transfer outside the jetting droplets is found to increase with increasing air velocity and with decreasing surface tension.

The mass transfer of the cellular foam is found to decrease as the air velocity increases, and is correlated with an internal circulation Re-number and a Fo-number for the contact time.

The difference in bubbling mechanism between bubbles rising from a single orifice and those rising from closely spaced orifices is discussed.     

宽Re范围内气泡和液滴曳力系数的关联模型

流粒(气泡或液滴)的曳力系数CD和上升/终端速度因有助于准确预测反应器内相含率分布、液相速度分布、流粒停留时间和传质速率而具有重要意义。但现有用于估算流粒CD的关联式大多分段且只在低雷诺数Re区间内有效,并难以同时准确预测不同实验体系和操作条件下的实验结果。针对这些不足,基于实验测量和理论分析,本工作提出了一个能够在整个Re范围内合理描述实验观测到的流粒CD演变规律的新数学关联式,并利用单组实验数据和编写的求解多维无约束线性优化问题的计算机程序确定了模型参数,且将新流粒CD关联式与现有经验模型、实验结果或三维数值模拟结果进行了对比。结果表明,新流粒CD关联式展现出优异的预测能力,能对不同实验体系(空气?水、空气?甘油?水、空气?甘油、甲苯?水、正丁醇?水、正辛醇?水等)、不同操作条件(气泡：0.1相似文献   

Flow characteristics of large oscillating drops in liquid-liquid systems

A study was made of the flow characteristics of large oscillating drops of pure liquid-liquid systems, using a thermostatically-controlled, rising drop column, 50 mm in diameter and 1000 mm in length. Mirrors in the jacket enabled front and side views of drops to be photographed simultaneously. Single drops in the size range 5–10 mm were investigated with both mutually-saturated phases and when the solute was being transferred from the dispersed phase. The systems studied were (1) toluene and acetone (dispersed)-water (continuous), and (2) n-heptane and acetone (dispersed)-water (continuous). Acetone concentrations were varied up to 3.75 kmol/m³. The oscillations of a travelling drop were asymmetrical; therefore, the amplitude cannot be expressed accurately in terms of only two axes. The area change of the drop compared to that of a sphere of equal volume ‘ε’, was shown to represent the amplitude accurately. The periods of droplet oscillation were uniform for the mutually saturated systems of constant physical and flow properties but changed when mass transfer was taking place. The interfacial tension exerted a marked effect on the amplitude, which also depended upon the oscillation frequency. The amplitude changed with droplet size in a similar manner to the terminal velocity, i.e. it increased with increasing size until it reached a maximum, subsequently decreasing less rapidly. The drag coefficient increased with increasing rate of mass transfer from the drop. Correlation of the results and the area eccentricity ‘ε’ by dimensional analysis embracing all possible parameters and physical properties affecting drop oscillation, resulted in the correlation ε = 0.22 Sr^0.42 We^?0.53 M^0.13 with a mean deviation of ± 14%. This will facilitate more accurate prediction of the interfacial area for mass transfer calculations, relating to equipment containing droplets in the oscillating regime.     

A study of mass transfer from single large oscillating drops

A study was made of mass trasfer rates from single large oscillating drops of pure liquid-liquid systems, in the size range of 5 to 10 mm. A thrermostatically-controlled, 50 mm in diameter, 1000 mm long, rising drop column was used, in which mirrors in the jacket enabled front and side views of drops to be photographed simultaneously. The systems studied were (1) toluene and acetone (dispersed)-water (continuous), and (2) n-heptane and acetone (dispersed)-water (continuous). High concentrations of acetone (up to 3.75 kmol/m³) were used to examine the effect of different parameters on the mass transfer rate, frequency and amplitude of oscillation in countercurrent operation. Previous theories and empirical correlations [2–6, 12, 13, 15] for the prediction of overall mass transfer coefficients showed large deviations from measured values. These may have aarisen because the models do not represent droplet oscillation accurately, and/or apply only to oscillations of small droplets. Fair agreement was obtained for small oscillating droplets as low solute concentrations. The oscillations of a travelling drop were asymmetrical; the period of oscillation was uniform for mutually-saturated systems but changed when mass transfer was taking place. The periods were longer than those predicted by the Lamb [7] and Shroeder and Kintner [37] correlations. Terminal velocities predicted from literature correlations [32, 34] did not give reasonable agreement with experimental data when there was mass transfer of solute. The drag coefficient increased with increasing mass transfer rate from the drop. Correlation of the results and the dispersed phase mass trasfer coefficients by dimensional analysis resulted in the correlation

1 List of symbols at the end of the paper.

with a mean deviation of ±23%, by insertion of experimental oscillation frequency data. This will facilitate more accurate prediction of the dispersed phase mass transfer coefficients relating to equipment containing droplets in the oscillating regime, e.g. pulsed columns or agitated tanks.     

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。     

Salt Effects on Single Aqueous Drops Falling through an Immiscible Organic Liquid

Drop settling (or rising) is crucial to the breaking of liquid-liquid dispersions. Most of the empirical correlations for terminal velocity of drops are formulated based on experimental data for systems of dispersed organic phase and continuous aqueous phase using pure and nearly pure liquids. Because the presence of the various third components in one or both of the phases mayaffect the settling of the drops, the terminal velocity and the onset of oscillation of aqueous drops containing various concentrations of electrolytes falling through n-dodecane were studied in this work. The applicability of existing empirical correlations to these reversed systems of continuous organic phase and dispersed aqueous phase was also examined.

Dissolved electrolytes have the effect of causing the terminal velocities and their peak velocities at the onset of drop oscillation to become faster and the peak diameters to become smaller when their concentrations are high enough to cause sufficient changes of physical properties. Existing correlations for drop terminal velocity in pure systems are shown to be adequate in the presence of electrolytes. For high interfacial tension systems, the Hu-Kinter correlation predicts the terminal velocity of organic drops in water and aqueous drops in organics equally well, independent of the presence of electrolytes.

The criteria of Hu-Kintner can be used to estimate the peak velocities and peak diameters at the onset of oscillation within a deviation of 3.7%.     

水包油型乳化液油滴的管内节流破碎行为与机理

实验研究了水包油型乳化液油滴在管内节流元件处的破碎行为,分析了破碎机理. 结果表明,液滴破碎主要发生在节流元件内壁及下游附近,其概率是施于液滴上湍流应力与液滴表面能之比的递增函数,是流体韦伯数及节流元件两侧最大压差的递增函数;在湍流状态(Re>4000)下,液滴充分振荡且受到较大的水流惯性力和速度梯度剪切力,更易破碎;由苏丹红IV染色的正庚烷体系界面张力由非染色时的47 mN/m降到23.6 mN/m,黏性力对液滴破碎的影响程度下降,受流速、压差等影响的惯性力起决定性作用,液滴破碎程度更大;流速决定流体对分散相油滴的湍流剪切破碎力,流速增大则油滴粒径破裂程度加大,而流速取决于流量和节流比;注入染色正庚烷油相体积增大(0.5~5 mL),削弱了节流元件的液滴破碎作用,两相流体系倾向于形成更大直径的液滴,中位径一般为20~35 mm.     

Electrically stressed water drops in oil

The deformation of water droplets in an electric field has been studied in hydrocarbon liquids. A water drop will elongate in an electric field due to the electrostatic pressure, and becomes unstable when a critical field limit is reached. By applying different voltage waveforms it is possible to measure both transient, time varying and static effects. An experimental method has been developed to study the rheological properties of the water droplet. The experimental results in surfactant-free model oil are in good agreement with classic Taylor theory. In diluted crude oil a correlation was found between the drop behavior in an electric field and the interface elasticity modulus, measured by dilational interfacial rheology. The static drop deformation was reduced with increasing elastic modulus due to adsorption of polar surfactants to the water/oil interface. Drop surface oscillations will be damped by the viscosity of the bulk liquid. The dynamic drop behavior was studied in viscosity standards prepared from different concentrations of polystyrene in toluene. The drop oscillations were modeled as damped oscillator, and the influence of viscosity on the eigen-frequency and damping coefficient was studied for different drop sizes. The oscillations can also explain the premature break-up of water droplets sometimes observed in the experiments.     

Prilling of aluminum sulfate hydrates

A prilling technique was used to produce droplets of aluminum sulfate hydrates in the size range of 2–2.5 mm. Production of spherical particles of aluminum sulfate hydrates by using a prilling technique has never been studied, nor carried out. The effects of the orifice geometry and hydrostatic head on drop formation were measured. The flow rates of hydrated aluminum sulfate melt through bores of 0.08 and 0.11 cm were measured for liquid heads of 10–100 cm. It was found that the break‐up of a liquid jet produced particles in the size range 2.0–2.5 mm with a 0.8 mm orifice. The modified Meister and Scheele correlation, which is the first application on a high viscous fluid in air system, was found to be satisfactory for estimating the particle size obtained by the jet break‐up mechanism. © 2000 Society of Chemical Industry     

Diffusion of Large Nitrogen Bubbles into Tap Water Studied by the Rotary Chamber Technique

The rotary chamber technique allows extended observation times of rising bubbles up into the 10‐mm range. After establishing the technique in combination with optical sizing in previous publications, this work completes the case of nitrogen bubbles in tap water. The rise velocity and the mass transfer are measured for five degrees of water saturation. A one‐parameter model equation involving the Archimedes number is offered for calculation. The limit of the analytical boundary layer model is depicted.     

低质量流速下四头内螺纹管内汽-水两相流摩擦压降特性试验研究

在压力为11.3-21.5MPa、质量流速为250～1 200kg/(m2·s)和工质干度为0～1的工况范围内,对结构为Φ28.6mm×5.8mm的四头内螺纹管中汽水两相流体在水平绝热条件下的摩擦压降特性进行了试验研究.研究结果表明,内螺纹管内两相流体摩擦压降大于同等条件下光管;两相摩擦倍率随工质干度的增大,在高干度时...     

Salt Effects on Single Aqueous Drops Falling through an Immiscible Organic Liquid

Drop settling (or rising) is crucial to the breaking of liquid-liquid dispersions. Most of the empirical correlations for terminal velocity of drops are formulated based on experimental data for systems of dispersed organic phase and continuous aqueous phase using pure and nearly pure liquids. Because the presence of the various third components in one or both of the phases mayaffect the settling of the drops, the terminal velocity and the onset of oscillation of aqueous drops containing various concentrations of electrolytes falling through n-dodecane were studied in this work. The applicability of existing empirical correlations to these reversed systems of continuous organic phase and dispersed aqueous phase was also examined. Dissolved electrolytes have the effect of causing the terminal velocities and their peak velocities at the onset of drop oscillation to become faster and the peak diameters to become smaller when their concentrations are high enough to cause sufficient changes of physical properties. Existing correlations for drop terminal velocity in pure systems are shown to be adequate in the presence of electrolytes. For high interfacial tension systems, the Hu-Kinter correlation predicts the terminal velocity of organic drops in water and aqueous drops in organics equally well, independent of the presence of electrolytes. The criteria of Hu-Kintner can be used to estimate the peak velocities and peak diameters at the onset of oscillation within a deviation of 3.7%.     

Dynamics of Emerging Water Droplet Subjected to Sidewall with Different Wettabilities in a Fuel Cell Cathode Channel

On‐site experiments on low temperature fuel cells revealed that water droplets tend to emerge into the gas channel at the corner. This motivates the present investigation on the effects of the location where water emerges from and the wettability of sidewall on the dynamic behaviour of liquid water in gas channel by numerical simulations that employ the volume‐of‐fluid method. A microchannel with a square cross‐section of 0.25 mm in width and a pore of 0.05 mm in diameter is adopted. The simulation results for different pore locations and wettabilities of sidewall show that the behaviour of water droplet only depends on the wettability of bottom wall when it emerges from the centreline without attaching to the sidewall and gains the highest pressure drop. When the emergence location shifts towards the corner, the water droplet unavoidably attaches to the sidewall and thus the wettability of sidewall is found to have significant effects on its dynamics: hydrophobic sidewall results in droplet detachment and fast removal with the highest pressure drop and the minimal water saturation, whereas hydrophilic sidewall leads to a water film and accumulation with the lowest pressure drop and the minimal water coverage on the bottom wall.     

Selective Separation of Toluene from n‐Heptane via Emulsion Liquid Membranes Containing Substituted Cyclodextrins as Carrier

Abstract

The selective separation of toluene from n‐heptane is investigated using unsubstituted α‐cyclodextrin (αCD), β‐cyclodextrin (βCD) and also with two substituted CDs as a carrier in oil/water/oil‐type emulsion liquid membranes. The separation factor for toluene to n‐heptane is evaluated from the extraction of an equimolar mixture of toluene and n‐heptane. βCD shows the highest selectivity followed by αCD, hydroxypropyl‐αCD and hydroxypropyl‐βCD. The separation performances, represented by the permeation rate and separation factor, are analyzed systematically by varying the operating parameters: contact time, concentration of carriers, volume fraction of the membrane phase, and the relative amount of solvent. In this paper the effects of carriers and interfacial tension on dispersed phase drop size, internal droplets size, and size distribution are also systematically investigated.