SIMULTANEOUS HEAT AND MASS TRANSFER IN LAMINAR GAS STREAMS IN WETTED-WALL COLUMNS

In the previous papers (Hikita and Ishimi,1976b; 1978, Hikita et al., 1979), the results of theoretical analysis of the mass transfer rate in laminar gas streams in wetted-wall columns have been reported. The similarity of the forms of the basic differential equations for mass and heat transfer makes it possible to apply the results of the mass transfer analysis to the heat transfer analysis. The present study was undertaken to confirm the above analogy between mass and heat transfer experimentally, and to investigate the effect of the gas and liquid flow rates on the heat transfer rate in laminar gas streams in wetted-wall columns.     

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.     

Simultaneous heat and mass transfer in turbulent gas streams in wetted-wall columns

The effect of the gas and liquid flow rates on the gas-phase mass and heat transfer rates in turbulent gas streams in wetted-wall columns with cocurrent and countercurrent gas-liquid flow was studied. The experiments were carried out on the adiabatic vaporization of water into a turbulent air stream under both conditions of rippling and no rippling on the surface of the falling liquid film. The experimental results showed that the effect of the gas and liquid flow rates on the mass and heat transfer rates are in good agreement with the theoretical predictions and that the effect of the rippling on the mass and heat transfer rates can be neglected.     

改良型湿壁塔液膜传质特性与端末效应的研究

本文采用疋田晴夫等提出的改良型湿壁塔作为建立研究传质的基础实验装置.测定了液膜厚度沿壁的分布,实验结果表明膜厚的实验值与理论值吻合较好;又选用CO_2-H_2O系统对塔的液膜传质特性进行了研究,并提出了液膜传质系数的无因次准数关联式;实验结果与渗透理论计算值比较,获得了良好的一致性.研究还表明通过简单地操作控制即能消除由端末效应引起的偏差.因此改良型湿壁塔是进行气液传质研究的良好装置.     

An estimation of the film-penetration model parameters

It it shown in this paper that the process of evaporation of liquids into a turbulent gas flow in a wetted-wall column can be represented by the film-penetration model. The numerical values of the two parameters S and L of this model, estimated from mass transfer data, appear to be well defined functions of flow conditions. The surface renewal mechanism used in the considered mass transfer model was shown to be consistent with the momentum transfer concept.The results may be used to predict mass transfer processes with chemical reactions in similar flow systems, by applying the theoretical developments using the film-penetration theory [7–10].The same procedure may be used to estimate the film-penetration parameters in other flow systems, such as the flow of liquid films in a wetted-wall column, or other complicated system such as stirred tanks, bubble columns, packed columns etc.     

Penetration theory solution for gas absorption and chemical reaction in cocurrent and countercurrent flow wetted-wall columns

Penetration theory solutions are provided for gas absorption with or without a (pseudo) first-order irreversible chemical reaction in cocurrent and countercurrent flow wetted-wall columns, taking into account a constant gas-film resistance as well as the axial decrease in gas composition due to absorption, while assuming non-rippling laminar flow for the liquid phase and plug flow for the gas phase. Limiting solutions are also obtained for situations when either the gas phase resistance or the axial variation of gas composition is negligible. The results are suitable for ϕ ≥ 3 and can be used for both Newtonian and non-Newtonian liquids.     

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     

Surfactant effects on gas absorption in a coke-oven gas treatment process

The effect of organic impurities in industrial stripped coal water (SCW) on the absorption of CO₂ was measured experimentally. Removal of these impurities via activated carbon showed a marked improvement in interphase mass transfer of a vertical wetted-wall column absorber. However, this benefit was not found in a stirred-cell absorber, in which a different flow pattern from that in wetted-wall column absorber is expected. An ad hoc systematic study on the effects of three deliberately added surfactants on gas absorption by pure water in three different absorbers with different flow patterns was thereafter conducted. The experimental results reveal that absorption deterioration also prevails only in a vertical, wetted-wall column absorber and the reduction in liquid phase mass transfer by the addition of surfactant can be satisfactorily correlated with surface pressure of solutions. This indicates that the effect of the industrial impurities in SCW on gas absorption may successfully be simulated under the same flow pattern by a surfactant solution with the same surface pressure. A possible modification of the existing coke-oven gas (COG) treatment process for the benefit of absorption enhancement was finally proposed.     

EXPERIMENTAL STUDY FOR HEAT AND MASS TRANSFER IN WAVY FILM ABSORPTION WITH THE PRESENCE OF NON-ABSORBABLE GASES

Experimental measurements for the heat and mass transfer in laminar wavy film absorption with the presence of non-absorbables are presented. Water vapor was absorbed into a falling film of aqueous lithium chloride solution on a vertical tube with the air present in the gas phase acting as the non-absorbable whose concentration was accurately detected by a mass spectrometer gas analyzer. Although the depressing effect of the air on the absorption rate was much more significant in the low air concentration range, the absorption reduction rate with increasing air concentration was found to be lower than that reported previously. The reduction was found to be continuous even up to 30% of air content. Besides the non-absorbables, the effects of other operational parameters such as solution flow rate, inlet solution temperature, inlet solution concentration, absorber wall temperature, absorber water vapor pressure, and effective contact length on the absorption rate were also tested. These effects were shown to follow trends similar to those predicted by a numerical solution for smooth film absorption. The heat and mass transfer coefficients were correlated by using Reynolds number, Prandtl number, Schmidt number, capillary-buoyancy coefficient and air concentration.     

HEAT TRANSFER CHARACTERISTICS AT EVAPORATING WATER SURFACE IN LAMINAR STREAM OF SUPERHEATED STEAM

The heat transfer characteristics at a horizontal evaporating water surface exposed to a laminar stream of superheated stenm, which is a radiation participating real gas, have been investigated and compared to those of a hot air flow by means of a real-time laser holographic inter ferometry. Water surface temperatures and temperature profiles within the laminar boundary layer were measured precisely under adiabatic evaporation conditions. Temperature gradients at the water surface were determined and local Nusselt numbers were estimated. Those experimental results were compared to the analytical results obtained by solving the laminar boundary equations. It can be concluded here that both temperature profiles within the laminar boundary layer and heat transfer characteristics at the evaporating water surface in superheated steam stream are quite different from those in hot air flow due to an influence of gas radiation from superheated steam.     

2,4-Xylidine Degradation with Ozonation: Mass Transfer and Reaction Kinetics

The kinetics of the degradation of 2,4-xylidine by ozonation as well as the ozone mass transfer in a wetted-wall column were investigated. A laboratory-scale ozone contactor was designed, and a steady-state wetted wall reactor model was developed. The model was based on countercurrent-connected and perfectly mixed mass transfer stages. It was possible to describe the evolution of the pollutant and ozone concentrations along the reactor length coordinate in various conditions. The model was used for the evaluation of the ozone mass transfer coefficient, reaction rate kinetics, and stoichiometric coefficient from experimental data. The ozone mass transfer coefficient for the wetted-wall column was estimated from the experiments in the absence of chemical reactions. When the estimated parameters were applied, the ozonation model of the wetted-wall column showed good agreement between the fitted and experimental data.     

HEAT TRANSFER CHARACTERISTICS AT EVAPORATING WATER SURFACE IN LAMINAR STREAM OF SUPERHEATED STEAM

ABSTRACT

The heat transfer characteristics at a horizontal evaporating water surface exposed to a laminar stream of superheated stenm, which is a radiation participating real gas, have been investigated and compared to those of a hot air flow by means of a real-time laser holographic inter ferometry. Water surface temperatures and temperature profiles within the laminar boundary layer were measured precisely under adiabatic evaporation conditions. Temperature gradients at the water surface were determined and local Nusselt numbers were estimated. Those experimental results were compared to the analytical results obtained by solving the laminar boundary equations. It can be concluded here that both temperature profiles within the laminar boundary layer and heat transfer characteristics at the evaporating water surface in superheated steam stream are quite different from those in hot air flow due to an influence of gas radiation from superheated steam.     

化学吸收法测定多级离心雾化旋转填料床有效相界面积及体积传质系数

对多级离心雾化旋转填料床的有效相界表面积及体积传质系数,用化学吸收法做了实验测量。实验结果表明：多级离心雾化旋转填料床不仅具有超低气阻,而且可产生良好的喷雾扩展相界面积的作用,体积传质系数比传统填料塔提高一个数量级,接近气液逆流型旋转填料床。多级离心雾化旋转填料床开拓了新的气液传热传质途径。     

Asymmetric,coupled transport of heat and mass in a duct

Mutually dependent heat and mass transfer for laminar gas flow in a parallel-plate channel is investigated analytically. Sublimation occurs at one of the channel walls, while the other wall is nonsubliming. Both walls are thermally insulated from the external environment. The analysis of the problem requires simultaneous treatment of the energy and mass diffusion equations, with coupling provided by the boundary conditions. Mathematical expressions are derived and numerical results are presented for various quantities that characterize the heat and mass transfer processes. Comparisons are made with the case of symmetric transfer (i.e., both walls actively participating). In general, unsymmetric transfer is found to slow down the axial development of the temperature and mass fraction fields.     

CONVECTIVE HEAT TRANSFER CHARACTERISTICS OF GAS IMPINGEMENT ON SMALL HEATED DEVICE IN LIQUID COOLANT BATH

Heat transfer characteristics of a small heated device have been investigated in a liquid bath with gas jetimpingement as function of gas flow rate,coolant temperature,liquid phsicochemical properties,heat flux,heat source size,ambient pressure and the distance between jet and heated wall.The experimental results show that the agitation of liquid caused by gas jet bubbles increases greatly therate of heat transfer,and the evaporation of coolant near the wall,which was due to the concentration differencebetween gas-liquid interface and bulk gas phase,gives additional enhancement of heat transfer.The rate ofevaporation related to the bubble growth was mathematically formulated.By using the simultaneous heat and mass transfer model,the convective heat transfer coefficient and masstransfer coefficient can be deduced from the experimental results.In addition,the local heat transfer coefficient and the distribution of evaporation heat flux on the smallheated surface are investigated mathematically and experimentally.     

Simultaneous heat and mass transfer for multicomponent distillation in a wetted-wall column

A simultaneous heat and mass transfer model was applied to multicomponent distillation in a wetted-wall column. Experiments were carried out, using the benzene-toluene-ethylbenzene ternary system, in a wetted-wall column of 2.2 cm i.d. and 100 cm long equipped with special probes designed for simultaneous liquid sampling and temperature measurements. The experimental results show that the bulk liquid phase was saturated, indicating no resistance to mass transfer in the liquid film. Confirmation of the liquid phase saturation was made through a comparison of the experimentally measured liquid temperature with the calculated bubble point temperature. The average deviation between the measured and calculated temperatures was found to be 0.26°C.Individual mass transfer rates were evaluated locally by measuring compositions and temperatures as functions of column height and were compared to theoretical predictions using the exact film model solution of the Maxwell-Stefan multicomponent equations developed by Krishna and Standart. The comparison shows good agreement with average deviations of 15.76% for benzene, 23.09% for toluene and 23.23% for ethylbenzene.     

加压湿壁塔内的气液传热传质

本文研究了加压条件下空气水系统在湿壁塔内同向流动时传递行为。分析中考虑了界面处气相混和物非理想行为及剪切应力的影响,忽略传质对气体运动影响。提出了计算加压条件下气侧局部传热传质系数关系式,并对两相加压下传递行为进行系统地分析。分析结果表明,采用常压计算方法进行加压传热传质计算,将产生较大的偏差。     

CONJUGATED HEAT TRANSFER IN COCURRENT FLOW MULTI-STREAM HEAT EXCHANGERS

Considering the cross-sectional velocity profile to be piecewise-constant in each stream of a multi-stream heat exchanger for cocurrent thermally developing flow, this study analytically solves the related conjugated Graetz problem by using an integral transform method. Further, it obtains an analytical solution in an explicit form to the fluid temperatures that vary two-dimensionally. A numerical example is provided for the case of a triple-tube heat exchanger. The numerical results demonstrate the effects of the thermal conductivity ratios of the fluids and the Péclet number ratios on the temperature distribution in the streams. In addition, in order to show the importance of considering streamwise variations in the overall heat transfer coefficients when designing laminar flow heat exchangers, the amount of exchanged heat calculated by the presented analytical solution is compared with that predicted on the basis of bulk temperatures and constant overall heat transfer coefficients.     

Direct heat and mass transfer in structured packings

Direct heat transfer is an important method in the exchange of heat between two countercurrent process streams within a column. The process can be simulated using either the theoretical stage or the rate based concept. With both concepts, a reliable heat transfer coefficient is needed. Additionally, the rate of the heat transfer coefficient is influenced by the simultaneous mass transfer.

A number of application-dependent methods to estimate the heat transfer coefficient have been developed, mainly for random packings. It is the purpose of this paper to extend this work to structured packings.

A number of experiments with air/water have been performed in a column of 300 mm inner diameter with Mellapak 250.Y, 250.X and 125.X at ambient conditions. A second group of measurements were done using an oil/air system where only sensible heat was transferred.

Based on these experimental results a method was developed to predict the heat transfer coefficient for structured packings. The method is applied to examples of industrial importance, like a gas quench, a gas saturator and a pump-around zone in an atmospheric tower.