A HOMOGENEOUS MODEL FOR MASS TRANSFER ENHANCEMENT IN GAS-LIQUID-LIQUID SYSTEMS

A modified homogeneous mass transfer model based on penetration theory is proposed for the mass transfer enhancement in gas-liquid-liquid systems. The present model assumes the existence of a shuttle mechanism between the organic dispersed phase and the continuous aqueous phase and considers the effect of diffusivity along with the solubility ratio and the dispersed phase holdup. The concept of effective diffusivity for gas in liquid-liquid emulsions has been used to consider the effect of diffusivity on mass transfer enhancement. The proposed model predicts the experimentally obtained enhancement factor with reasonable accuracy and numerical simplicity. Fresh experiments were also conducted to further validate the proposed model.     

Prediction of mass transfer coefficients in an asymmetric rotating disk contactor using effective diffusivity

Mass transfer characteristics have been investigated in a 113 mm diameter asymmetric rotating disk contactor of the pilot plant scale for two different liquid–liquid systems. The effects of operating parameters including rotor speed and continuous and dispersed phase velocities on the volumetric overall mass transfer coefficients are investigated. The results show that the mass transfer performance is strongly dependent on agitation rate and interfacial tension, but only slightly dependent on phase flow rates. In this study, effective diffusivity is used instead of molecular diffusivity in the Gr?ber equation for estimation of dispersed phase overall mass transfer coefficient.The enhancement factor is determined experimentally and there from an empirical expression is derived for prediction of the enhancement factor as a function of Reynolds number. The predicted results compared to the experimental data show that the proposed correlation can efficiently predict the overall mass transfer coefficients in asymmetric rotating disk contactors.     

Prediction of mass transfer coefficients in a pulsed disc and doughnut extraction column

A study of the mass transfer performance for a pulsed disc and doughnut extraction column has been presented for a range of operating conditions. The mass transfer performance has been investigated for both directions of mass transfer. This study has examined the mass transfer coefficients which has incorporated the effects of back‐mixing in the continuous phase. The effect of operating variables including pulsation intensity and dispersed and continuous phase velocities on volumetric overall mass transfer coefficient has been investigated. The experiments showed that mixer‐settler, transition and emulsion regimes exist in the column depending on the pulse characteristics. In the present work, effective diffusivity is substituted for molecular diffusivity in the Gröber equation for estimation of overall mass transfer coefficients. The enhancement factor is determined experimentally and there from a single empirical correlation is derived for prediction of enhancement factor in terms of Reynolds number, holdup and Eötvös number for all operating regimes and each mass transfer direction. The experimental results are in very good agreement with the values calculated by the proposed equation. © 2011 Canadian Society for Chemical Engineering     

The Effect of Size and Concentration of Nanoparticles on the Mass Transfer Coefficients in Irregular Packed Liquid–Liquid Extraction Columns

This investigation explored the effects of nanofluids on mass transfer enhancement using an irregularly packed liquid–liquid extraction column and the chemical systems of water–acetic acid–toluene. SiO₂ nanoparticles with sizes of 10, 30, or 80 nm are dispersed in toluene–acetic acid to produce nanofluids with different volume fractions of 0, 0.01, 0.05, and 0.1 vol.%. The effects of nanoparticle size and concentration on dispersed phase mass transfer coefficient were discussed based on the experimental data. This is for the first time that the effect of nanoparticle size is studied in liquid–liquid extraction systems. It was found that the mass transfer enhancement was more significant in nanofluids with smaller particles. It was also observed that mass transfer coefficient is larger in nanofluids compared to that in dispersed phase without nanoparticles, with a peak enhancement at a nanoparticle volume fraction of 0.05 vol.% for 10-nm particles and 0.01 vol.% for 30- and 80-nm particles. The maximum mass transfer coefficient enhancement was approximately 42% at 0.05% concentration of nanoparticles using smaller particles (10 nm). Finally, a novel correlation for prediction of effective diffusivity in the presence of nanoparticles has been proposed, which is a function of nanoparticle size and its concentration. The main advantage of this approach is that the principal effect of these two parameters is considered in correlation without which the experimental data could not be fitted with an acceptable accuracy.     

第三分散相液滴增强气体吸收的一维非稳态非均相传质模型

A model for one-dimensional unsteady heterogeneous mass transfer was developed based on Danckwerts’ surface renewal theory in order to describe the mass transfer enhancement of absorption process for a slightly soluble gas in a gas-liquid-liquid system. The model accounts for the mass transfer resistance within the dispersed phase and the effect of emulsion viscosity on mass transfer. An analytical solution for enhancement factor was obtained by Laplace domain transformation. The absorption rates of carbon dioxide in the dodecane-in-water and castor oil-in-water systems were measured in a thermostatic reactor, and the enhancement factors were calculated at different volume fractions of dispersed phase and stirrer speeds. The model predictions agree well with the experi-mental data.     

Performance improving with temperature in liquid–liquid extraction using cumene–isobutyric acid–water chemical system

The performance of single drops was investigated in liquid–liquid extraction while temperature was changed within the range of 15–40 °C. The recommended system of cumene–isobutyric acid–water with mass transfer resistance mainly in aqueous phase was used. An average enhancement of 75.6% in the rate of transfer was revealed. The extraction efficiency is the most influencing term due to molecular diffusivity enhancement. For modeling, a simple correlation was proposed for the effective diffusivity in Newman's equation, while continuous phase mass transfer coefficient was directly included. Using this model, relative deviation of the overall mass transfer coefficient was within only ±5.6%.     

Enhancement of oxygen mass transfer rate in the presence of nanosized particles

Absorption of gases into a liquid is essentially important for two- or three-phase reactions, because the diffusion of a sparingly soluble gas, like oxygen, across a gas-liquid interface generally limits the reaction rates. Using a third, dispersed phase, the mass transfer rate could be significantly increased. The question arises how the absorption rate can be described in the presence of very fine, nanometer size particles or droplets. Its mathematical model should take into account the specific properties of the nanoparticles, e.g. the Brownian motion of particles, its effect on the diffusion of the bulk phase molecules, the mass transfer rate into the nanoparticles, its dependency on the particle size, etc. The mass transfer rate of oxygen, in the presence of nanometer size, organic droplets, has been investigated both experimentally, using organic submicron n-hexadecane droplets, and theoretically. The effect of the Brownian motion of the nanoparticles as well as its effect on the diffusivity in the nanofluid has been discussed. Accordingly, the enhanced diffusion coefficient, due to the convective motion of the continuous liquid phase induced by the moving particles, has been predicted and its effect on the mass transfer enhancement has been calculated using both homogeneous and heterogeneous mathematical models. The predicted data were compared to the measured ones.     

水溶液中甲苯分散相增强难溶气体吸收

利用难溶气体的物理吸收过程,在具有恒定气液界面面积的吸收装置中研究了常压和室温条件下分散液相(甲苯)对气液传质的增强作用。通过测定气相(丙烷和氢气)压力随时间的变化,计算出液侧传质系数和传质增强因子。试验发现,当传质组分在分散液相与连续液相内的分配系数较大,或在两相间的相对扩散系数较大,且分散相形成的液滴较小时,加入分散液相可显著增强气液传质;增强因子随分散液相含率的增大而增大,但增大幅度逐渐减小;当气体在纯水中的传质系数增大时,同等条件下分散相对气液传质的增强作用减小。     

A general enhancement factor model of the physical absorption of gases in multiphase systems

A general mass transfer enhancement factor model (GEFM) has been developed based on the enhancement mechanism proposed in this paper taking both shuttle effect and hydrodynamic effect into account to calculate the overall enhancement factor due to the presence of dispersed particles. GEFM can describe the phenomena of the enhancement factor increasing with partition coefficient (m), surface covering fraction (α) and dispersed phase volume fraction (φ) increase, leveling off at higher φ, changing of enhancement factor (E) at different conditions. Moreover, GEFM can give the enhancement factor not only of multiphase system relative to the pure liquid phase (even at quite high φ), but also in the same system under different conditions (e.g. stirrer speed, viscosity). The enhancement factors predicted from GEFM are consistent with the experimental observations well.     

Modeling the axial distribution of mass transfer coefficient in an agitated-pulsed extraction column

The dispersed phase holdup and drop size in solvent extraction columns vary along the column height and this affects the mass transfer coefficient and interfacial area. In this article, mass transfer study was performed experimentally using a 25 mm diameter agitated pulsed column. The axial distribution of mass transfer coefficient was determined by coupling population balance equation and axial dispersion model by taking the longitudinal variation in hydrodynamic performance into consideration. Feasibility of different mass transfer models in predicting concentration profiles was evaluated and a novel correlation based on effective diffusivity was developed. The results showed that both overall and volumetric mass transfer coefficients have significant change along the column height and greatly depends on the agitation speed and pulsation intensity. Increasing dispersed phase velocity also augments the overall mass transfer coefficient. The maximum number of transfer unit was measured to be 10 m⁻¹ at agitation speed of 1000 rpm.     

加入分散有机相强化气体吸收的传质过程(英文)

Mass transfer enhancement of gas absorption by adding a dispersed organic phase has been studied in this work. Various dispersed organic phases (heptanol, octanol, isoamyl alcohol, heptane, octane, and isooctane) were tested respectively in the experiment. According to the theoretical model and experimental data, the overall volumetric mass transfer coefficient and enhancement factor were obtained under different dispersed organic phase volume fraction and stirring speed. The experimental results indicate that gas-liquid mass transfer is enhanced at different level by adding a dispersed organic phase. The best performance of enhancement were achieved with the dispersed organic phase volumetric fraction of 5% and under an intermediate stirring speed of 670 r·min-1. Among the organic phases tested in the experiment, alcohols show better performance, which gave 20% higher enhance-ment of overall volumetric mass transfer coefficient than adding alkanes.     

A dispersion model of enhanced mass diffusion in nanofluids

An explanation for the enhancement of mass diffusion in nanofluids is presented using arguments based on dispersion in diluted fixed beds. Starting from the generalized Langevin equation, it is shown that the velocity field established around a Brownian nanoparticle is similar to the velocity field predicted by Brinkman equations leading to the analogy between dispersion in diluted fixed beds and dispersion in nanofluids. The proposed model predicts the order of magnitude of mass diffusion enhancement we observed recently (10-fold enhancement of rhodamine 6G mass diffusivity under the optimum conditions for a suspension of 10-nm alumina nanoparticles in deionized water). Contrarily to other Brownian motion-based models of diffusion in nanofluids, the proposed model samples the whole Maxwell–Boltzmann distribution of particle velocities rather than taking the non-representative root mean square velocity. The model also shows a strong dependence on the mass transfer Péclet number and, consequently, justifies the order of magnitude differences between the mass diffusivity and thermal conductivity enhancements reported in the literature.     

Use of axial dispersion model for determination of Sherwood number and mass transfer coefficients in a perforated rotating disc contactor

The mass transfer process in a perforated rotating disk contactor (PRDC) using a toluene-acetone-water system was investigated.The volumetric overall mass transfer coefficients are calculated in a PRDC column.Both mass transfer directions are considered in experiments.The influences of operating variables containing agitation rate,dispersed and continuous phase flow rates and mass transfer in the extraction column are studied.According to obtained results,mass transfer is significantly dependent on agitation rate,while the dispersed and continuous phase flow rates have a minor effect on mass transfer in the extraction column.Furthermore,a novel empirical correlation is developed for prediction of overall continuous phase Sherwood number based on dispersed phase holdup,Reynolds number and mass transfer direction.There has been great agreement between experimental data and predicted values using a proposed correlation for all operating conditions.     

细颗粒增强气液传质机理及模型研究

分散细颗粒(包括分散固体颗粒和分散液滴)可以明显增强气液传质。简要叙述了目前提出的三种颗粒增强传质机理：传输作用,边界层混合作用和阻止气泡聚并作用,讨论了每种机理可能适用的体系。评述了近年来发展的有代表性的经验模型和基于不同机理的理论模型,并指出今后模型的建立应以湍流理论为桥梁,综合考虑多种传质机理。     

ZSM-5型分子筛强化水吸收CO_2

在难溶气体吸收体系中加入第3分散相粒子是强化传质过程的一个重要手段,文中采用恒温搅拌釜对ZSM-5型分子筛/水浆料强化CO2的吸收过程进行了实验研究,考察了硅铝摩尔比(120,150,360)、固含率和气相CO2体积分数对增强因子的影响。结果表明:随着分子筛硅铝摩尔比的增加,其疏水性增强,增强因子随之增大;随着分子筛固含率的增加,气液界面处颗粒的覆盖率增大,增强因子迅速增大,当气液界面处颗粒的覆盖率逐渐接近最大覆盖率时,则增强因子逐渐趋于恒定值;随着气相CO2体积分数的增加,溶质在溶液中的扩散作用逐渐增强,而分子筛颗粒输运作用逐渐减弱,增强因子减小。针对分散相微粒增强难溶气体的吸收过程,提出了一个三维非均相传质模型,计算结果和实验数据吻合良好。     

气液液三相体系的二维非均相传质模型与求解

在柱坐标系中建立了描述分散第二液相增强气液传质的轴对称二维非均相传质模型,采用有限差分法计算了传质组分在液膜区内的轴向和径向浓度分布以及传质增强因子,计算方法快捷、有效,并且能很好地吻合现有文献中的实验数据.     

Effective diffusivity in a structured packed column: Experimental and Sherwood number correlating study

Mass transfer coefficients along a structured packed column were experimentally determined to obtain a new correlation for dispersed phase Sherwood number based on molecular diffusivity. Then in a comparative investigation, the correlation was re-established based on effective diffusivity. The applied chemical systems were toluene/acetic acid/water (T/A/W) and butyl acetate/acetic acid/water (B/A/W). The effects of droplet size and packing height on experimental Sherwood number were also discussed. It was shown that local Sherwood number could be increased up to 188% with increasing the droplet size from 6 to 9 mm in fixed dispersed phase flow rate. It was also observed that when height of packing increased from 10 to 40 cm, local Sherwood number decreased by almost 48% for constant dispersed phase flow rate. The results have shown that the proposed correlation based on effective diffusivity can estimate the experimental drop Sherwood number with high accuracy (error of less than 5%). Moreover, current research shows that replacing molecular with effective diffusivity in some theoretical models can correct their estimation.     

多项体系气体吸收的三维非均相传质模型

A three-dimensional heterogeneous mass transfer model was proposed to investigate the enhancement of dispersed particles on gas absorption. The strategy to calculate local and overall enhancement factors is proposed. Instead of a global grid, the composite overlapping grid is adopted, which simplifies the setup and solution of the three-dimensional model equations. It is found that dispersed particle hold-up, particle size, liquid-solid partition coefficient of transported component, characteristic contact time, and the shortest distance between particles and gas-liquid interface have major influence on absorption enhancement factor. The particle-particle interaction on gas absorption and the lateral diffusion of transported component in liquid film were studied with the multi-particle simulation. The proposed model predicted the experimental data from the literature reasonably well.     

Use of axial dispersion and plug flow models for determination of axial mixing and mass transfer coefficient in an l‐shaped pulsed packed extraction column

In this study, the volumetric overall mass transfer and phases axial mixing coefficients have been investigated in a pilot plant of an L‐shaped pulsed packed extraction column by using two liquid systems of toluene/acetone/water and n‐butyl/acetone/water. The mass transfer performance has been evaluated using two methods of axial dispersion and a plug flow model. The effect of the operational variables and physical properties, including the dispersed and continuous phases flow rates, pulsation intensity, and interfacial tension, on mass transfer and phases axial mixing coefficients have been considered. It has been found that the pulsation intensity and the continuous phase flow rate seriously affect the mass transfer coefficient, however, the dispersed phase flow rate has a weaker effect. Also, the axial mixing of a phase is strongly affected by the pulsation intensity and the flow rate of the phase itself and it is not affected by the second phase flow rate. Finally, new correlations are proposed to accurately predict the mass transfer and axial mixing coefficients.     

正十二烷-水乳液吸收丙烷的传质增强作用(英文)

A one-dimensional unsteady heterogeneous parallel mass transfer (ODUHPMT) model was developed for the absorption enhancement of volatile organic compounds (VOC) by the dispersed droplets. An analytical solution for enhancement factor was obtained based on surface renewal theory and the Laplace domain transformation. The absorption rate of propane into water at dif-ferent stirring speeds with the added micro dodecane droplets was investigated experimentally in a thermostatic stirred tank. The mass transfer flux across the gas-liquid interface and the enhancement factor were measured. The results showed that the dodecane has an obvious enhancement effect on propane absorption into water, the maximum enhancement factor reached 11. The enhance-ment factor increased with increasing dodecane volume fraction and decreased with increasing stirring speed. The experiment data agreed well with the model predictions and showed high prediction accuracy of ODUHPMT model.