气液界面Marangoni效应对传质系数的影响

在气液相际传质过程中,界面Marangoni湍动会对传质过程产生重要的影响,为此,建立了一套气液接触传质设备,以使得通过N2与异丙醇稀溶液逆流接触将液体中使表面张力降低的溶质解吸出来,从而引发Marangoni湍动,提高传质速率。发生Marangoni对流时,液相的传质系数比只依靠扩散传质而不考虑Marangoni效应时大,因此引出增强因子F这一概念,通过计算F的值即可判断Marangoni效应对传质速率影响的程度。提出了一个包括Marangoni准数的计算传质系数关联式,其计算结果与实验结果相符．     

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.     

Equilibrium facilitated mass transfer in plate type reactive membrane separation devices

The introduction of mobile carrier species which react reversibly with the solute species in liquid membrane mass transfer separation devices can give enhanced mass transfer rate. For one-dimensional laminar boundary layer flow of a Newtonian fluid past a single-membrane separator with equilibrium facilitated transport inside the membrane, the mass transfer rate in the separator was analyzed numerically. For a Damköhler number equal to infinity, the effect of maximum facilitation factor, dimensionless equilibrium constant, dimensionless velocity and similarity transformation parameter on the performance of separators was investigated. The dependence of the membrane-fluid interfacial concentration and equilibrium facilitation factor on dimensionless velocity and similarity transformation parameter was discussed.     

Effect of interfacial velocity and interfacial tension gradient on momentum,heat and mass transfer

The effect of interfacial velocity and interfacial tension gradient on momentum, heat and mass transfer was studied theoretically. After reviewing current knowledge regarding interfacial velocity effects, the interfacial tension gradient effect is discussed as a numerical solution of the laminar boundary layer equations. The enhancement and suppression of interfacial mobility, and the rates of interphase momentum, heat and mass transfer were related to the Marangoni number. A diagram showing the effect of interfacial tension gradient on momentum, heat and mass transfer is presented.     

Transient rise velocity and mass transfer of a single drop with interfacial instabilities—Numerical investigations

Full 3D-simulations of transient interfacial mass transfer accompanied by Marangoni convection at a single spherical droplet in a quiescent liquid were performed in a moving reference coordinate system. The flow and concentration field are solved simultaneously, coupled via the additional Marangoni stress generated by concentration gradients at the interface. Fluid dynamics and mass transfer are investigated in the Marangoni convection dominated toluene/acetone/water system. The numerical results are qualitatively and quantitatively compared with own experimental results. The simulation results reveal that mass transfer is always enhanced—compared to calculations where no Marangoni convection appears—independently from the initial solute concentration. The enhancement factor of mass transfer ranges between 2 and 3.     

Sherwood number in flow through parallel porous plates (Microchannel) due to pressure and electroosmotic flow

An expression for Sherwood number is developed from first principles for combined pressure‐driven and electroosmotic flow in a porous rectangular microchannel. This quantifies the mass transfer of an electrically neutral solute in the microchannel and is useful for designing microfluidic devices and porous media flows. The convective‐diffusive species balance equation, coupled with the velocity field, is solved within the mass transfer boundary layer utilizing similarity method. From the simulations, it is observed that the Sherwood number increases as the electric double layer near the channel wall becomes more compact (as manifested through a decrease in the Debye length), and it reaches a constant value around the scaled Debye length of 40. The Sherwood number becomes constant at higher Debye lengths as electrokinetic effects become negligible. A detailed analysis of dependence of Reynolds number, dimensionless permeation velocity, ratio of driving force and scaled Debye length on Sherwood number is presented. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1693–1703, 2012     

Experimental investigation of Marangoni effect in 1-hexanol/water system

In this work, the interfacial phenomena of a single hanging drop have been observed and captured by a Schlieren optical system. The extraction fractions at different hanging times were determined. For the system without surfactant, the Marangoni effect induced by interphase mass transfer of a solute displays regular convection patterns. The addition of surfactants changed the mode of interfacial instability significantly but in different ways: SDS enhanced the mass transfer and Triton X-100 reduced the extraction fraction.     

伴有Marangoni效应的传质动力学

结合Marangoni对流的流体动力学条件，通过建立的半经验模型研究了伴有Marangoni效应的传质动力学，阐述了Marangoni效应增强传质的机理，得到了传质Sherwood数与Marangoni数之间的连续指数关联，从而得以解释不同实验过程中得到的不同Sherwood数与Marangoni数之间的关系.研究表明，由Marangoni效应而增强的传质系数与界面Marangoni湍动的表现形式有关.     

Investigation of the mass transfer across the interface of two concurrent laminal flowing liquids in a horizontal cylindrical channel—I Experimental results

The concentrations of the solute (pyridine) were measured by means of modified liquid scintillation techniques to within 0.66,μm of the interface as well as in the bulk of the organic (toluene) phase during the solute transfer from the toluene to the water phase at different temperatures, flow rates and solute concentrations. These measurements allow the comparison of the mass transfer resistance of the diffusive transfers with the interfacial resistance. In extremely pure systems the interfacial resistance is negligible in comparison with the resistances of the diffusive transfers. In the presence of surface active agents the interfacial resistance strongly increases and exceeds those of the diffusive transfers. The solute transfer from the water to the toluene phase because of the Marangoni instabilities could only be considered qualitatively.The concentrations of the solute (pyridine) to within 0.66 μm of the interface and in the bulk of the organic phase (toluene) during the solute transfer from the organic to water phase in a horizontal cylindrical flow channel with concurrent flow of the phases at different temperatures, flow rates and solute concentrations were measured by means of a modified liquid scintillation technique. These measurements allow the comparison of the mass transfer resistances of the diffusive transfers to and from the interface with the interfacial resistance. In extremely pure systems, the solute concentration at the interface quickly drops to very low values with increasing distance from the channel entrance, which indicates that the mass transfer resistance of the interface is negligible in comparison with the resistances of the diffusive transfers in the laminar flow region. In the presence of surface active agents (LPC) the solute concentration to within 0.66 μm of the interface dramatically increases and exceeds the solute concentration in the bulk of the toluene phase. This indicates a strong increase of the mass transfer interfacial resistance. The high solute concentration at the interface, exceeding its bulk concentration, is caused by the enrichment of the solute in the interface, which was measured by the interfacial tension depression in the presence of the solute. Because the solute transfer from the water to the toluene phase causes interfacial (Marangoni) instabilities, which increase the geometrical interfacial area due to uncontrolled eruptions and formations of microemulsions, the solute concentration could not be measured to within 0.66 μm of the interface in the toluene phase during the water to toluene solute transfer. Only qualitative statements could be made with regard to the area of the interface. Further measurements are necessary in order to determine quantitative relationships with regard to the mass transfer interfacial resistance as a function of different process parameters     

Sherwood number in porous microtube due to combined pressure and electroosmotically driven flow

Mass transfer of a neutral solute in a porous microtube is quantified in this study. An analytical expression of the Sherwood number is developed from first principles for combined flow of pressure driven and electroosmotic flow. Similarity solution method is adopted for solution of convective-diffusive species balance equation with coupled velocity profile, within the mass transfer boundary layer. It is observed that the Sherwood number increases with decrease in the Debye length (as the electric double layer becomes more compact) and it becomes constant beyond scaled Debye length of 60. Effects of the Reynolds number, dimensionless suction velocity, ratio of driving force and scaled Debye length have been investigated in detail. The analysis is useful for efficient design of microfluidic devices and flow through porous media.     

界面湍动对气液传质的影响

气液传质过程中经常伴有界面湍动。界面湍动由传质的不均匀性引起,反过来又极大地促进传质。介绍了界面湍动的产生机理和形成条件,对4种不同Ra和Ma准数的情况分别进行了分析。讨论了界面湍动强度对气液传质系数的影响关系。传质系数与Man成正比,其中n随着Marangoni对流胞类型的不同而在1/3和1之间变化。在气液系统中,液相与气相阻力比越大,由界面湍动引起的传质的增强效应越显著。     

电泳萃取技术处理专利兰A醇溶液

:电泳萃取是电泳与萃取技术耦合形成的一种新型分离技术。本文应用该技术以水为萃取剂从醇溶液中回收专利兰 A。在自行设计的连续电泳萃取池内实验研究了电场强度 ,两相流速等因素对于萃取率及传质通量的影响。实验结果表明 ,电场强度的增大可以有效地提高溶质的萃取率 ,传质通量也随之增大 ;有机相流速的增大 ,将使萃取率下降 ,传质通量则可能存在极大值 ;水相流速的变化对萃取率及传质通量的影响均不显著且规律性不强 ,电泳萃取从醇溶液中萃取专利兰 A的萃取率可达 95 %以上 ,是一种十分有效的分离染料的方法     

CO2在纳米流体解吸过程中的微对流现象

建立了一套水平气液界面传质模拟装置,利用粒子成像测速仪（PIV）对CO2从乙醇和乙醇纳米流体中解吸过程液相流场进行了观测和分析。传质发生时,在近界面处均观察到了由Marangoni效应造成的湍动。通过对平均速度的分析,发现在乙醇纳米流体中发生的湍动强度和湍动范围较乙醇溶液中大。纳米流体中的Marangoni效应加剧了纳米粒子的布朗运动,引发了纳米流体中的微对流,从而将界面处的湍动传递至液相主体中,导致液相主体的漩涡增多、流体混合加剧,促进了气液传质。     

MASS TRANSFER INTO LAMINAR GAS STREAMS IN WETTED-WALL COLUMNS WITH COUNTERCURRENT GAS-LIQUID FLOW

The effect of the gas and liquid flow rates on the mass transfer rate in laminar gas streams in wetted-wall columns with countercurrent gas-liquid flow was studied. An approximate analytical solution was obtained for the average gas-phase Sherwood number as a function of the gas-phase Graetz number and the dimensionless interfacial gas velocity. Experiments were carried out on the absorption of ammonia into aqueous sulfuric acid solution and of methanol vapor into water, using two columns of different lengths. The agreement between the experimental and the predicted effects of both gas and liquid flow rates on the gas-phase mass transfer rate was found to be fairly good.     

MASS TRANSFER INTO LAMINAR GAS STREAMS IN WETTED-WALL COLUMNS WITH COUNTERCURRENT GAS-LIQUID FLOW

The effect of the gas and liquid flow rates on the mass transfer rate in laminar gas streams in wetted-wall columns with countercurrent gas-liquid flow was studied. An approximate analytical solution was obtained for the average gas-phase Sherwood number as a function of the gas-phase Graetz number and the dimensionless interfacial gas velocity. Experiments were carried out on the absorption of ammonia into aqueous sulfuric acid solution and of methanol vapor into water, using two columns of different lengths. The agreement between the experimental and the predicted effects of both gas and liquid flow rates on the gas-phase mass transfer rate was found to be fairly good.     

Experimental investigation and numerical simulation of Marangoni effect induced by mass transfer during drop formation

Marangoni effect induced by interphase mass transfer plays an important role in liquid–liquid extraction and reaction processes. The interaction of Marangoni effect and interphase mass transfer during drop formation at different injection rates and different initial solute concentrations was investigated by experimental and numerical simulation. The extraction fraction was measured and the corresponding correlation was proposed. The level‐set method coupled with mass‐transfer equation is for the first time used to simulate the mass‐transfer induced Marangoni effect during drop formation. The simulated drop volume, shape, and extraction fraction are in good accordance with experimental data. Through the numerical simulation, it is found that the mass transfer in the first mass‐transfer period is the most efficient during drop formation when Marangoni convection occurs. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4424–4439, 2013     

气液界面对流传质的观测与定量分析

采用光学纹影系统对乙醇和水双组分解吸传质过程的对流结构的界面湍动进行了定性观察和定量分析。建立了一套水平非稳态气液传质设备,试验观测了乙醇和水体系中液相组分向气相传质过程的Marangoni界面对流结构。还通过对传统纹影方法的改进,对乙醇解吸传质过程的浓度(本文用质量分数表示)梯度场进行了定量测量。定量分析表明乙醇和水系统解吸过程中,引发界面湍动的原因是局部较大的表面张力梯度。定量分析的结果很好的解释了伴随Marangoni效应的传质过程的混乱的对流结构,为进一步对界面湍动现象的分析提供了帮助。     

Dynamic interfacial tension minima in finite systems

A physical model is presented to explain the dramatic interfacial tension minima that may sometimes occur during the equilibration of a surface-active solute between two liquid phases. The effects of bulk phase mass transfer resistances, interfacial sorption kinetics, and finite phase volumes are included. Adsorption and desorption barriers are treated both by linear and Langmuir kinetics. Dynamic interfacial tension minima are predicted whenever the solute mass transfer resistance is higher and/or a large desorption barrier exists in the phase towards which transfer is occurring. Experimental dynamic tension minima for acidic organic phases contacting an alkaline aqueous phase can be rationalized by the existence of desorption barriers. The relevance of such tension minima to interfacial turbulence, spontaneous emulsification, and enhanced oil recovery is discussed.     

Simulation of interfacial Marangoni convection in gas-liquid mass transfer by lattice Boltzmann method

Interfacial Marangoni convection has significant effect on gas-liquid and/or liquid-liquid mass transfer processes. In this paper, an approach based on lattice Boltzmann method is established and two perturbation models, fixed perturbation model and self-renewable interface model, are proposed for the simulation of interfacial Marangoni convection in gas-liquid mass transfer process. The simulation results show that the concentration contours are well consistent with the typical roll cell convection patterns obtained experimentally in previous studies.     

Numerical simulation of the Marangoni effect on transient mass transfer from single moving deformable drops

A level set approach was adopted in numerical simulation of interphase mass transfer from a deformable drop moving in a continuous immiscible liquid, and the simulation results on Marangoni effect were presented with respect to three experimental runs in the methyl isobutyl ketone–acetic acid–water system. Experiments showed that when the solute concentration was sufficiently high, the Marangoni effect would occur with the interphase mass transfer enhanced. Numerical results indicated that the mass‐transfer coefficient with Marangoni effect was larger than that without Marangoni effect and stronger Marangoni effect made the drop deform more easily. The predictions were qualitatively in accord with the experimental data. Numerical simulation revealed well the transient flow structure of Marangoni effect. © 2011 American Institute of Chemical Engineers AIChE J, 2011