Mass transfer in a laminar-flow parallel plate electrolytic cell with simultaneous development of velocity and concentration boundary layers

Ionic mass transfer in a parallel plate electrochemical cell having a flow entrance representative of practical industrial cells has been studied under laminar flow condition using the ferri- and ferrocyanide redox couple. The working electrode was segmented so that local mass transfer coefficient could be measured. The mass transfer coefficients, measured for the entrance region of the cell, which is defined as the region where both hydrodynamic and concentration boundary layers are developing simultaneously, are correlated and compared with results from other studies. For the electrode section beyond the entrance region, where the flow is fully developed but the concentration boundary layer continues to grow, a correction term is added to the electrode dimension parameter in the Graetz number. In this way the Leveque equation can still be used to correlate the local mass transfer coefficient. The value of the correction term is calculated by matching the downstream mass transfer boundary layer thickness to that of a Leveque-type cell. This scheme improves substantially the convergence of the experimental data correlation and results in good agreement with the Leveque equation.     

平板降膜溶液除湿再生过程实验研究及模型验证

在平板降膜溶液除湿/再生实验平台上,以LiCl水溶液作为除湿溶液,实验研究了空气、溶液的入口参数对空气出口参数的影响,并根据实验数据得到了耦合传热传质系数的关联式,为NTU-Le模型提供了重要的数据支持,同时将实验数据与NTU-Le传热传质模型计算所得的数据进行比较,来验证NTU-Le传热传质模型在平板降膜溶液除湿/再生过程中的适用性和准确度。结果显示：实验数据和模型计算值之间的偏差均在10%以内,表示NTU-Le模型适用于平板降膜溶液除湿/再生过程。     

错流降膜液体干燥剂除湿/再生传热传质数学模型及分析

分析了错流降膜液体干燥剂除湿及再生传热传质过程 ,建立了基于实际除湿系统的描述再生和除湿过程的数学模型 ,考虑到除湿过程中产生的热效应 ,以氯化钙溶液为除湿剂时 ,对气侧和液侧的传热传质系数进行了理论和数值求解 .计算结果表明 ,传热传质系数与气流流动状态、除湿剂的热物理性质等因素有关     

Modeling of Rotary Desiccant Wheels

Rotary desiccant wheels are widely used in dehumidification and energy recovery applications. In this work, we have developed a 2D, steady state model of a rotary desiccant wheel. Mass and energy balance equations for the air streams and the desiccant wheels were developed. The hydraulic diameter and surface area for heat and mass transfer were calculated based on knowledge of the flute geometry. Appropriate correlations for the Sherwood number and Nusselt number were used to estimate heat and mass transfer coefficients. The model is capable of predicting steady state behavior of desiccant wheels having at the most three sections (process, purge, and regeneration). The mathematical model was validated using a real desiccant wheel, and the calculation results are in reasonable agreement with the experimental data. Based on this model, the temperature and humidity profiles in the wheel during both the dehumidification and the regeneration processes are analyzed. The simulated results were used to gain an insight into the operation of desiccant wheels. The model and the presented results will be useful for optimizing dehumidification and energy recovery applications.     

顺流型溶液与空气热质交换性能

在顺流型平板降膜热质交换测试装置上对氯化锂水溶液与湿空气除湿、再生性能进行了实验研究,结合NTU-Le模型,着眼于湿空气与溶液耦合热质交换特性,获取了顺流条件下不同空气流量、溶液流量、溶液温度工况的耦合传热传质系数随运行参数的变化情况。研究结果为相关过程模型验证提供可靠数据,同时也为相关设备性能分析与计算提供重要基础数据。     

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.     

INFLUENCES OF INTERFACIAL CONVECTION AND PHYSICAL PROPERTIES OF DESICCANT SOLUTIONS ON THE MASS TRANSFER PERFORMANCE OF ABSORPTION SYSTEMS

In order to investigate the mass transfer phenomenon of the absorption of water vapor in desiccant solutions, a system of absorption cell was designed. The gradient in surface tension can result from the contact process between the desiccant solution and water vapor. This induced interfacial disturbance will affect the mass transfer performance. The experimental results demonstrate that the theoretical mass transfer coefficients predicted by boundary layer theory are less than those of the experimental values. In addition, the mass transfer performance of the absorption system can be enhanced by adding ethanol to the absorbent solutions. In order to describe the level of mass transfer performance affected by the interfacial convection resulting from the addition of ethanol, the enhancement factor was defined in this study. The enhancement factor is from 1.26 to 1.56 for adding ethanol to DEG (diethylene glycol) solution, and from 1.36 to 1.61 for adding ethanol to TEG (triethylene glycol) solution.     

压缩空气溶液除湿中不同除湿剂除湿性能比较

在压缩空气溶液除湿实验平台上,分别以LiBr和LiCl水溶液作为除湿剂,实验研究了两种溶液在压缩空气溶液除湿系统中的除湿性能。以溶液表面水蒸气分压力作为比较基准,压缩空气出口含湿量和除湿量作为除湿性能的评价指标,对二者的除湿能力进行比较分析。同时基于压缩空气溶液除湿器传热传质模型,结合实验数据,研究了LiBr、LiCl溶液与压缩空气间的传质系数大小以及变化规律。结果表明：在相同的处理工况下,采用LiCl溶液对压缩空气进行除湿能得到更低的空气出口含湿量和更高的除湿量,LiCl溶液除湿过程的传质系数也高于LiBr溶液,即在压缩空气溶液除湿系统中LiCl溶液具有更优的除湿能力和传质性能。     

The influence of molecular diffusion on mass transfer between turbulent liquids

The influence of molecular diffusion on liquid—liquid mass transfer in a stirred transfer cell has been found by measuring the rates transfer of helium and iso-butane from water to toluene and dekalin. These solutes have very different diffusion coefficients, their presence does not alter the physical properties of the liquids and, because their equilibrium distributions strongly favour the organic phases, the water phase mass transfer coefficient could be determined and was found to depend on the square root of the diffusion coefficient.The results are compared with the predictions of a model for liquid—liquid mass transfer under turbulent conditions, based on the approach of an eddy to the interface being restrained by interfacial tension and gravitational forces and taking into account eddy pressure fluctuations in both phases. This model provides a correlation for these results, as well as water phase mass transfer coefficients for the transfer of iso-butane from water to n-octanol, and previous stirred transfer cell results.     

基于热质传递解耦特性的溶液除湿过程传热传质系数(Ⅰ)模型与Le-hD分离测量法

溶液除湿过程是溶液除湿空调系统中的一个非常重要的耦合传热传质过程。本文对填料塔结构的溶液除湿器建立了一种NTU-Le模型,并基于此模型得到了溶液除湿过程传热传质特性——Lewis数对空气出口含湿量基本无影响,提出一种溶液除湿耦合热质传递过程的热质传递解耦方法——Le -h_D分离测量法,来测定溶液除湿过程的耦合传热传质系数。     

基于热质传递解耦特性的溶液除湿过程传热传质系数(Ⅱ)实验与Le-hD分离测量法应用

对采用规整波纹填料结构的溶液除湿器除湿过程进行了实验研究,空气与溶液流型组织形式为叉流,基于Le-hD分离测量法得到空气入口流量、温度、含湿量以及溶液入口质量分数、温度对耦合传质系数的影响,并采用数据回归的方法对传质系数与Lewis数进行拟合,得到该类结构除湿器除湿过程的传质系数与Lewis数的关联式,并进行了74组稳态实验对该关联式进行误差分析与验证,结果表明根据关联式计算得到的进出口参数变化与实验进出口参数变化之间相对误差很小,进出口空气温度变化、含湿量变化误差分别仅在6%、10%以内,进出口溶液温度变化相对误差不超过12%,表明Le-hD分离测量法的准确性和可接受性。     

基于热质传递解耦特性的溶液除湿过程传热传质系数(Ⅱ)实验与Le-hD分离测量法应用

对采用规整波纹填料结构的溶液除湿器除湿过程进行了实验研究,空气与溶液流型组织形式为叉流,基于Le-h_D分离测量法得到空气入口流量、温度、含湿量以及溶液入口质量分数、温度对耦合传质系数的影响,并采用数据回归的方法对传质系数与Lewis数进行拟合,得到该类结构除湿器除湿过程的传质系数与Lewis数的关联式,并进行了74组稳态实验对该关联式进行误差分析与验证,结果表明根据关联式计算得到的进出口参数变化与实验进出口参数变化之间相对误差很小,进出口空气温度变化、含湿量变化误差分别仅在6％、10％以内,进出口溶液温度变化相对误差不超过12％,表明Le-h_D分离测量法的准确性和可接受性。     

Liquid—liquid mass transfer in systems agitated by submerged jets

Mass transfer across a plane interface has been investigated for liquid—liquid systems, with the liquid phases agitated by vertically opposed, submerged jets, one in each phase. Mass transfer coefficients were determined for the systems ethyl acetate—water and butanol—water, and for the transfer of dissolved helium between water and toluene, and toluene between toluene and water. The latter two solutes provided a wide range of molecular diffusion coefficients. When turbulent eddies penetrated to the interface the mass transfer coefficients were found to be proportional to D^0.5, and could be represented by the Levich-Davies mass transfer model for mass transfer between turbulent liquids. The characteristic turbulence velocities in this model were related to the velocities of the liquids from the jet nozzles, and to the equipment dimensions (e.g. the distances of the jets from the interface, the radius of the vessel and the diameters of the jet nozzles) by an expression based on the hydrodynamic behaviour of jets.For the low interfacial tension system butanol—water, a flat disc had to be placed in the interface at the region of jet impingement, to prevent disruption of the interface, but, even so, only a limited range of jet flow rates could be used. Only at the highest flow rates were turbulent conditions obtained, and most of the experimental mass transfer coefficients for this system were between the values predicted by the Levich-Davies model and the Levich “three-zone” model for boundary layer flow.     

A new model for mass transfer in direct contact membrane distillation

The mass transfer process in direct contact membrane distillation (DCMD) for three kinds of membranes was measured. Water fluxes at different temperatures and the membrane distillation coefficients (MDC) for each membrane were obtained directly from experimental data. The fact that the MDC values of membranes with larger pore size increase with temperature indicates that Poiseuille flow plays an important role in the process of mass transfer through the membrane. Based on this conclusion, a three-parameter model, named the Knudsen diffusion-molecular diffusion-Poiseuille flow transition (KMPT) model, was developed to predict MDC and water flux for membrane distillation. The parameters of the KMPT model for each membrane employed in this study, by which MDC at various temperatures can be determined, were evaluated by a nonlinear regression. The values of MDC and water fluxes for each membrane predicted by KMPT model agree well with that obtained directly from the experiment results. A large contribution of Poiseuille flow to mass transfer was observed and can be attributed to the distribution of large pores in the membranes. The KMPT model also provides a method for estimation of the effect pore size using the ratio of the MDCs; the ratio of the Poiseuille flow to molecular diffusion MDC provides the best estimation.     

Statistical evaluation of a liquid desiccant dehumidification system using RSM and theoretical study based on the effectiveness NTU model

In the present study the performance of an air dehumidifier using lithium bromide (LiBr) as a desiccant was investigated. Response surface methodology (RSM) was used to assess individual and interactive effects of the six main factors (velocity, temperature and humidity of air, flow rate, temperature and concentration of desiccant) on dehumidification mass rate. A reduced quadratic statistical model was derived to predict dehumidification mass rate. The maximum dehumidification mass rate was obtained 0.154 g/s under the optimal conditions of an air velocity of 4.1 m/s, desiccant flow rate of 0.035 kg/s, air humidity ratio of 0.0185 kg/kg, desiccant concentration of 0.48 kg/kg, air temperature of 29.5 °C, and desiccant temperature of 21.8 °C. The effectiveness number of transfer unit (NTU) model was employed to describe the coupled heat and mass transfer. The results of the model and the experimental data show good agreement. Dimensionless mass and heat transfer coefficients correlations are proposed; the average absolute differences between the predicted values and the experimental findings for Sh and Nu numbers were calculated as 2.14% and 5.27%, with the discrepancies mainly within ±9% and ±13%, respectively.     

Coupled diffusion of water and ethanol in a polyimide membrane

In this work, the sorplion and steady state permeation of water‐ethanol mixture in an aromatic polyimidc membrane was investigated with the objective of revealing the coupling cherts of water and ethanol in their course of diffusion. The solubility of the watcr‐ethanol mixture in the membrane was determined by means of swelling‐distillation techniques. The sorption behavior was interpreted in terms of the thermodynamic equilibrium equations. The Maxwell‐Stefan mass transfer model was employed as a framework for modeling diffusion. Model equations derived therefrom enable us to cplculate coupled fluxes of water and ethanol from the experimental data. The coupling effects in the diffusion of water and ethanol were revealed by comparing the calculated coupled flux with non‐coupling flux, which was calculated by integration of the Fickian first law using independent diffusion coefficients. Furthermore, the implication of the sole adjustable parameter included in the derived model equations was discussed.     

Mass Transfer Enhancement in Double‐Pass,Parallel‐Plate Mass Exchangers under Asymmetric Wall Fluxes

A device involving mass exchangers with a permeable membrane inserted to divide the open duct into two subchannels for conducting double‐pass laminar countercurrent operations under asymmetric wall mass fluxes, which results in a substantial improvement of the mass transfer, has been developed and investigated theoretically. The resultant partial differential equations for such a double‐pass forced‐convection mass transfer problem are referred to as conjugated Graetz problems and solved analytically in this work by using an eigenfunction expansion in terms of power series for the homogeneous part and an asymptotic solution for the inhomogeneous part. The theoretical predictions of mass transfer efficiency improvement in double‐pass, parallel‐plate mass exchangers are obtained by suitably adjusting the permeable membrane location. They are represented graphically and compared with the results from an open duct of a single‐pass operation (without a permeable membrane inserted). The increment of power consumption is also discussed.     

基于热不平衡考虑的溶液除湿/再生传热传质系数(Ⅰ)模型与Le-hD测量法

溶液除湿装置是溶液除湿空调系统中一个重要单元,其内部传热传质规律对系统性能起到决定性影响.在溶液绝热除湿模型基础上,提出考虑热不平衡的除湿模型用以计算溶液除湿过程的传热传质系数.模拟计算发现考虑热不平衡模型计算的溶液和空气出口参数与实验值能更好吻合,从而说明该模型能更准确计算除湿过程传热传质系数.当热容率比C*≥1.0...