微通道内单乙醇胺水溶液吸收CO2/N2混合气的传质特性

采用高速摄像仪对400 μm×400 μm T形微通道内单乙醇胺（MEA）水溶液吸收混合气中CO₂过程的气液两相流及传质特性进行了实验研究,微通道内的压力降采用压力传感器进行测量。考察了弹状流型下气液两相流量及MEA浓度对压力降、比表面积和传质性能的影响。结果表明,当MEA浓度不变,气液两相流量增大时,压力降、比表面积、传质系数、体积传质系数和增强因子均增大,并逐渐趋于恒定。当气液流量不变,MEA浓度增大时,压力降、传质系数、体积传质系数和增强因子增大,但比表面积减小。实验条件下,压力降范围为2.00~5.23 kPa,化学吸收过程的传质系数范围为7.74×10^-4~2.97×10^-3 m·s^-1。对于伴有快速化学反应的传质过程,以Sherwood数、Reynolds数、Schmidt数及增强因子为变量建立了体积传质系数的预测关联式,平均偏差为5.09%,具有良好的预测性能。     

Mass Transfer and Liquid‐Film Formation in a Spray Tower for SO2 Removal in Sodium Hydroxide Solution

Spray towers allow for controlling air pollution in which a liquid is sprayed in small droplets to produce a large interfacial area for mass transfer between a gas and a liquid phase. An experimental study of a spray tower for removing SO₂ is described. The experiments were carried out under different operating conditions by varying the gas velocity, liquid flow rate, and SO₂ concentration. SO₂ removal efficiency, volumetric mass transfer coefficient, and liquid‐film formation as a result of the collision of droplets against the tower wall are investigated. Removal efficiency and volumetric mass transfer coefficient are analyzed as a function of gas velocity, liquid flow rate, and SO₂ concentration, while liquid‐film formation is evaluated as a function of tower height. The results indicate high removal efficiency. Correlations to predict the volumetric mass transfer coefficient are also proposed.     

Coupled two-phase mass transfer to spherical drops — Trace gas absorption and oxidation by a single drop

Mass transfer across gas and liquid boundary layers into the core of drops with liquid phase first order chemical reaction has been analyzed for spherical drops in the Reynolds number range of 50 < Re_g < 400. The realistic and computationally efficient simulation of this gas absorption system is applicable in a variety of engineering fields including gas-liquid mass transfer in drops and sprays. The present paper deals with the fluid mechanics and mass transfer with chemical reaction of a single drop. In computer experiments good predictive agreement has been achieved with measured data. The theoretical results were generalized to show the influence of three major system parameters: Peclet number Pe_g or Pe_l Damköhler number Da and the distribution coefficient at the gas-liquid interface, M, on mass transfer and to demonstrate the importance of coupled gas- and liquid-phase resistances to gas absorption under practical conditions.     

Modeling of absorption of NO<Superscript>2</Superscript> with chemical reaction in a falling raindrop

A model has been proposed for the scavenging of NO₂ in a falling raindrop. After absorption, aqueous NO, undergoes a second order reaction to form various ions such as NO ₂ ^- , NO ₃ ^- and H⁺. The model is based on the unsteady state convective diffusion equation, which was solved for given boundary conditions by using implicit alternate direction (ADI) method. The circulation of fluid inside and outside the raindrop has been taken into account to realistically describe the flow field in the numerical domain. The model predictions indicate that the pH of a raindrop is a direct function of the drop size and bulk concentration of NO,. The model predicted a pH of about 4.9 for a 100-micron raindrop falling through a 20-ppb ambient concentration of NO₂. For the same ambient concentration of NO², a 10-micron raindrop would have a pH of about 4.75. The predictions also suggested that for all practical purposes the gas phase resistance may be taken as the rate-controlling step. The predicted values of gas-side mass transfer coefficient compared well with the estimated values using standard mass transfer correlations.     

Solid–liquid mass transfer in a gas sparging contactor equipped with a tube of circular fins

The solid–liquid mass transfer rate at a stack of circular fin surfaces in a gas sparging contactor was investigated. A diffusion-controlled dissolution technique of copper in an acidified chromate solution was employed. Variables studied included the number of actively exposed fins ranging from 5 to 20, pertinent physical properties of the solution, and air superficial velocity. Experimental data showed that the rate of the diffusion-controlled mass transfer increases with increasing superficial air velocity and decreases with increasing chromate solution acid concentration. Moreover, at relatively low superficial air velocity, increasing the number of actively exposed fins results into a continuous increase in the mass transfer coefficient. At relatively higher superficial air velocity, however, the mass transfer coefficient decreases in the 5–10 range of actively exposed fins and then reverts to increase in the 15–20 range. An empirical correlation relating the mass transfer j factor to Re_g, Fr, and a dimensionless height defined as the ratio of the height of actively exposed fins to the column equivalent diameter was developed based on the data generated in this study, with ±6.45% average deviation.     

Effect of Drag–Reducing Polymers on the Rate of Liquid-Solid Mass Transfer in Fixed Beds of Spheres under Forced Convection Conditions

The effect of drag–reducing polymers on the rate of liquid – solid mass transfer in a packed bed reactor under forced convection conditions was studied by measuring the rate of diffusion–controlled dissolution of copper spheres in acidified chromate solutions. The variables investigated were superficial liquid velocity, sphere diameter, bed height, and polymer concentration. The mass transfer coefficient was found to increase with increasing superficial liquid velocity. Increasing both sphere diameter and bed height were found to decrease the mass transfer coefficient. Polymer addition was found to decrease the rate of mass transfer by an amount ranging from 29.2 to 56.9% depending on superficial liquid velocity and polymer concentration. Mass transfer data were correlated in absence and in the presence of drag–reducing polymer, using the following equations, respectively: J_d = 3.71Re^–0.54 and, J_d = 2.5 Re^–0.61where J_d is mass transfer J-factor and Re is the Reynolds number.     

Flux and Characteristic Parameters in Mediated Transport through Liquid Membranes. II. A Model for Co-transport and Experimental Study of Kl Transport through a Bulk Liquid Membrane

Abstract

In this article we establish a steady-state theoretical model for cotransport through liquid membranes. Integration of the flux equation gives the concentration in the receiving phase as a linear function of time under certain conditions. From this, an expression for the relative permeability of the carrier-permeant complex with respect to the carrier is obtained; this permeability depends on the equilibrium constant of the interphases reaction and on another parameter related to the initial concentration of permeant in extramembrane phases. An experimental study of the variation of permeant concentration in the receiving phase allows determination of several characteristic transport parameters.     

New liquid membrane technology for simultaneous extraction and stripping of copper(II) from wastewater

In this paper, a new liquid membrane technique, hollow fiber renewal liquid membrane (HFRLM), is presented, which is based on the surface renewal theory, and integrates the advantages of fiber membrane extraction, liquid film permeation and other liquid membrane processes. The results from the system of CuSO₄+D2EHPA in kerosene+HCl show that the HFRLM process is very stable. The liquid membrane is renewed constantly during the process, the direct contact of organic droplets and aqueous phase provides large mass transfer area. These effects can significantly reduce the mass transfer resistance in the lumen side. Then the mixture of feed phase and organic phase flowing through the lumen side gives a higher mass transfer rate than that of stripping phase and organic phase, because the aqueous layer diffusion of feed phase is the rate-controlling step. The overall mass transfer coefficient increases with increasing flow rates and D2EHPA concentration in the organic phase, and with decreasing initial copper concentration in the feed phase. The overall mass transfer coefficient also increases with increasing pH in the feed phase, and reaches a maximum value at pH of 4.44, then decreases. Also, there is a favorable w/o volume ratio of 20:1 to 30:1 for this process. Compared with hollow fiber supported liquid membrane and hollow fiber membrane extraction processes, HFRLM process has a high mass transfer rate. Mathematical model for the HFRLM process based on the surface renewal theory is developed. The calculated results are in good agreement with experimental results under the conditions studied.     

Solid‐Liquid Mass Transfer at Gas Sparged Tube Bundles

The solid‐liquid mass transfer characteristics of an in‐line tube bank immersed in a gas‐liquid bubble column were investigated by measuring the rate of diffusion‐controlled dissolution of copper surface in acidified dichromate solution. Variables studied were the number of rows in the tube bank, physical properties of the solution, and nitrogen flow rate. The mass transfer coefficient was found to increase with increasing nitrogen flow rate. Increasing the number of rows in the tube bank was found to decrease the mass transfer coefficient. The data were correlated for the following conditions: 0.0021 < Fr.Re < 0.1603, 1 < N_r < 5 and 850 < Sc < 1370 by the equation J = 0.15 ( Fr.Re )^–0.15. Comparison was made between the present mass transfer data and previous heat and mass transfer studies conducted in packed and empty bubble columns.     

MASS TRANSFER COEFFICIENTS FOR REACTIVE FLUIDS FLOWING IN CONDUITS WITH SEMI-PERMEABLE WALLS

The advancing front theory is an approximate solution for mass transfer into a reactive fluid when the reaction can be assumed to be very fast. The theory has had considerable use in predicting mass transfer characteristics for reactive fluids flowing in conduits. In this paper, the mass transfer coefficient, in the form of the local, fluid-side Sherwood number, is derived for reactive flow in conduits with semi-permeable walls. The local, fluid-side Sherwood number is given as a function of the Graetz number, the wall Sherwood number, and a dimensionless reaction strength parameter. The limiting cases of both the constant wall concentration boundary condition (Sh_w?∞) and the constant wall flux boundary condition (Sh_w?∞)are investigated. Comparisons of the results with the classical Graetz and Leveque theories give conclusions about the accuracy of the advancing front theory for the worst possible case.     

中空纤维支撑液膜萃取Cu(II)的传递性能

The transport of Cu(II) from aqueous solutions containing buffer media through hollow fiber supported liquid membrane (HFSLM) using di(2-ethylhexyl) phosphoric acid (D2EHPA) dissolved in kerosene as membrane phase and hydrochloric acid as striping phase was investigated. A set of factors were studied, including tube side velocity, shell side velocity, pH of the feed phase, Cu(II) concentration in the feed phase, buffer media concentration and D2EHPA concentration in the membrane phase. Experimental results indicate that the mass transfer coefficient increases with increasing both carrier concentration in the organic phase and flow rates on the tube side and shell side, and decreases with increasing initial Cu(II) concentration in the feed phase. With increasing pH value and acetate concentration in the feed phase, the mass transfer coefficient reaches a maximum value then decreases. The optimal operating conditions are obtained at pH value of 4.44 and 0.1 mol&;#8226;L－1 acetic ion concentration in feed phase, and carrier volume fraction of around 10% in kerosene as organic phase. A mathematical model of the transport mechanism through HFSLM is devel-oped. The modeled results agree well with the experimental ones.     

木家具表面挥发性污染物散发传质特性

木家具表面挥发性污染物的散发是一类污染环境的复杂传质行为。为准确把握传质特性,首先建立了一套更完善的新式完全解析模型,综合考虑了两大传质机理(扩散、对流)、气候箱中污染物阻力、背景浓度和进气浓度,同时适用于无换气或换气条件下的模拟研究;例如,预测气候箱或木家具中污染物浓度变化,评价传质速率或散发量,以及估算室内浓度达标时限。然后通过对模型的分析提出了一套简便快捷的新式实验法,利用木家具在换气箱中散发的逐时浓度,一并求取目标污染物的3个重要传质参数:可散发量ρ₀、扩散系数δ和分配系数β。实验部分使用该换气箱法测算了某密度板家具中甲醛和总挥发性有机化合物的ρ₀、δ和β,与文献中基于密封箱的数据处理方法所得结果的变异系数小于5%,并有效避免了出现负压等密封箱采样可能导致的潜在问题。将传质参数代入数值算法预测了密封箱和换气箱中木家具散发挥发性污染物的浓度变化,与实验测量的数据吻合良好。该模型和方法能够准确预测或测定木家具表面挥发性污染物传质特性。     

微通道内离子液体/乙醇混合溶液吸收CO2的传质特性

采用高速摄像仪对微通道内离子液体/乙醇混合溶液吸收CO₂的传质行为进行了实验研究。考察了弹状流型下气液两相流量比和离子液体浓度对液侧体积传质系数k_La和液侧传质系数k_L的影响。当离子液体浓度不变时,k_La、k_L均随气液流量比的升高而增大并逐渐趋于恒定。当液相流量不变时,对于不同浓度的离子液体溶液,液侧体积传质系数k_La和液侧传质系数k_L随气液流量比的变化曲线出现了交叉点。在交叉点之前,k_La和k_L均随着离子液体浓度的增大而减小;在交叉点之后,k_La和k_L均随着离子液体浓度的增大而增大。提出了用于预测液侧体积传质系数k_La的新的量纲1经验关联式,预测效果良好。     

Gas backmixing in the dense region of a circulating fluidized bed

The gas backmixing characteristics in a circulating fluidized bed (0.1 m-IDx5.3-m high) have been determined. The gas backmixing coefficient (D_ba) from the axial dispersion model in a low velocity fluidization region increases with increasing gas velocity. The effect of gas velocity onD _ba in the bubbling bed is more pronounced compared to that in the Circulating Fluidized Bed (CFB). In the dense region of a CFB, the two-phase model is proposed to calculate D_bc from the two-phase model and mass transfer coefficient (k) between the crowd phase and dispersed phase. The gas backmixing coefficient and the mass transfer coefficient between the two phases increase with increasing the ratio of average particle to gas velocities (U_p/U_g).     

A qualitative computational study of mass transfer in upward bubble train flow through square and rectangular mini-channels

In this paper we present a new method for numerical simulation of conjugate mass transfer of a dilute species with resistance in both phases and an arbitrary equilibrium distribution coefficient. The method is based on the volume-of-fluid technique and accounts for the concentration jump at the interface by transforming the discontinuous physical concentration field into a continuous numerical one. The method is validated by several test problems and is used to investigate the mass transfer in upward bubble train flow within square and rectangular channels. Computations are performed for a single flow unit cell and a channel hydraulic diameter of 2 mm. The simulations consider the transfer of a dilute species from the dispersed gas into the continuous liquid phase. Optionally, the mass transfer is accompanied by a first-order homogeneous chemical reaction in the liquid phase or a first-order heterogeneous reaction at the channel walls. The results of this numerical study are qualitative in nature. First, because periodic boundary conditions in axial direction are not only used for the velocity field but also for the concentration field and second, because the species diffusivity in the liquid phase is arbitrarily increased so that the liquid phase Schmidt number is 0.8 and the thickness of the concentration and momentum boundary layer is similar. Two different equilibrium distribution coefficients are considered, one where the mass transfer is from high to low concentration, and one where it is vice versa. The numerical study focuses on the influence of the unit cell length, liquid slug length and channel aspect ratio on mass transfer. It is found that for the exposure times investigated the liquid film between the bubble and the wall is saturated and the mass transfer occurs by the major part through the bubble front and rear so that short unit cells are more efficient for mass transfer. Similar observations are made for the homogeneous reaction and for the heterogeneous reaction when the reaction is slow. In case of a fast heterogeneous reaction and when the main resistance to mass transfer is in the gas phase, it appears that for square channels long unit cells are more efficient, while large aspect ratio rectangular channels are more efficient than square channels, suggesting that for these conditions they might be more appropriate for use in monolithic catalysts.     

Influence of Pressure on the Hydrodynamics and Mass Transfer Parameters of an Agitated Bubble Reactor

The present study deals with the pressure effects on the hydrodynamic flow and mass transfer within an agitated bubble reactor operated at pressures between 10⁵ and 100 × 10⁵ Pa. In order to clarify the flow behavior within the reactor, liquid phase residence time distributions (RTD) for different operating pressures and gas velocities ranging between 0.005 m/s and 0.03 m/s are determined experimentally by the tracer method for which a KCl solution is used as a tracer. The result of the analysis of the liquid‐phase RTD curves justifies the tank‐in‐series model flow for the operating pressure range. Good agreement is obtained between theoretical and experimental results assuming the reactor is operating as perfectly mixed. Two parameters characterizing the mass transfer are identified and investigated in respect to pressure: the gas‐liquid interfacial area and volumetric liquid‐side mass transfer coefficient. The chemical absorption method is used. For a given gas mass flow rate, the interfacial area as well as the volumetric liquid mass transfer coefficient decrease with increasing operating pressure. However, for a given pressure, a and k_La increase with increasing gas mass flow rates. The mass transfer coefficient k_L is independent of pressure.     

Mass transfer during the removal of dissolved heavy metals from wastewater flows in fluidized beds of ion exchange resins

A fluidized bed of cation exchange resin was used for the removal of cupric ions from a simulated wastewater effluent consisting of copper sulphate solution. Variables investigated were: superficial liquid velocity, particle diameter, bed height and copper sulphate concentration. These variables were studied with respect to their effect on the solid-liquid mass transfer coefficient. The coefficient was found to increase with increasing superficial liquid velocity. Increasing particle diameter, bed height and copper sulphate concentration were found to reduce the mass transfer coefficient. The experimental data can be correlated by the equation valid for the following conditions: 0.23 < Re₁ < 2.27; 0.52 < ? < 0.87; 0.0127 < d_p/d < 0.0417 and 0.0095 < d_p/L < 0.125.     

Physical and reactive extraction of salicylic acid—II. Investigations on fixed,freely suspended and pulsed droplets

By the use of the modified liquid scintillation technique, new information of high accuracy and high time resolution was gained on physical and reactive extraction on droplets. The influence of the size and hydrodynamics of droplets on the physical and reactive extraction of salicyclic acid is investigated on fixed, freely suspended single droplets in a steady as well as in a pulsated flow. At pH 0.8, the physical extraction parameters are determined. With increasing pH, the enhancement factor increases at first due to the spontaneous dissociation of acid, at higher pHs due to gradual elimination of the mass transfer resistance in the continuous phase. At pH 12, this resistance is completely eliminated. This allows us to determine the fractional mass transfer coefficients in the droplet phase k_d and in the continuous phase k_c. The influence of the concentration of a secondary amine, LA-2—which forms a complex with the acid—on the fractional mass transfer coefficient k_d is investigated also.     

Mass transfer to single spheres settling at their terminal velocities

Mass transfer to single spherical cation exchange particles in the size range 0.04–0.14 mm has been accurately measured. The particles were allowed to fall at their terminal velocities in a stationary dilute alkaline solution. The liquid film controlled mass transfer coefficient has been correlated in the range 6<Re_o<95, where Re_o is the terminal Reynolds number. The experimental results also confirm the validity of two numerical solut of the mass transfer equation in boundary layer flow.