ADIABATIC SORPTION OF BULK SINGLE ADSORBATE FROM AN INERT GAS-EFFECT OF GAS-SOLID MASS AND HEAT TRANSFER COEFFICIENTS

The effects of the gas-solid mass and heat transfer coefficients on the dynamics of adiabatic adsorption of a single adsorbate exhibiting Type I behavior were analyzed by numerically solving the conservation equations for a column. It was assumed that the adsorbate obeyed Langmuir equilibra and the local mass transfer rate could be described by the linear driving force model. The properties of the front zones were found to be controlled by mass transfer, while those of the rear zones were primarily determined by heat transfer. The separation between the two zones might vanish under certain conditions resulting in overlap of the two zones. The front zones were found to be constant pattern. The rear zones can be constant or proportionate pattern depending on the equilibrium properties and the feed condition of the adsorbate.     

ADIABATIC SORPTION OF DILUTE SINGLE ADSORBATE FROM AN INERT GAS—EFFECT OF GAS-SOLID MASS AND HEAT TRANSFER COEFFICIENTS

The effects of the gas-solid mass (k) and heat (h) transfer coefficients on the column dynamics for Type II adsorption system were analyzed by numerically solving the conservation equations for a single sorbate. Langmuir equilibrium and the linear driving force model for mass transfer were assumed

The transfer zone is larger in an adiabatic column than the corresponding zone in an isothermal column even for a small rise in column temperature. Initially proportionate pattern transfer zones are formed in both the adiabatic and the isothermal columns. As the column length increases these profiles approach to the constant pattern limit. Column adiabaticity slows down the rate of approach to constant pattern limit. On the other hand, higher adsorbate concentration and high k enhance this rate. The symmetry of the breakthrough curve may be distorted in an adiabatic column. The effects of k and h on the size of the mass transfer zone are interchangeable. Consequently, estimation of k from experimental breakthrough data by assuming thermal equilibrium between gas and solid phases may be erroneous.     

HEAT AND MASS TRANSFER DURING DRYING OF A LIQUID FILM FROM THE SURFACE OF A SINGLE INERT PARTICLE

ABSTRACT

This paper presents the results of theoretical and experimental studies on drying of aqueous suspensions of finely dispersed solids sprayed over the surface of an inert ceramic sphere. The effects of temperature and air velocity on kinetics of heat and mass transfer as well as peeling off the layer of a dry material from the sphere surface are described. The mathematical model of a drying process based on simplified ?gradientless? approach to transfer phenomena is proposed. The adequacy of the model developed for drying of the wet coat from a single sphere to the real drying process taking place in a bed of particulate carrier is confirmed by results of drying of several organic dyestuffs in an industrial spouted bed dryer with inert particles.     

MEASUREMENT OF COEFFICIENTS FOB SIMULTANEOUS HEAT AND MASS TRANSFER IN FOOD PRODUCTS

Two experimental devices were designed and built to determine four coefficients K_T K_M D_M D (or δ_T = D_T / D_M occurring in simultaneous heat and mass transfer equations, where,K _T and D_M are thermal conductivity and moisture diffcusivity respectively, D_T ( or δ _T is temperature gradient Induced moisture migration coefficient and K_M is moisture gradient Induced heat transfer coefficient. Three food materials, i.e. potato, bread dough and bread, were tested. From this study, it was found that the value of 5 was higher for low density food materials, such as bread, than for high density materials, such as potato. The coefficient & measures moisture migration contribution due to temperature gradient within the material. The average values of δ _T for potato, bread dough and bread were 0.0014, 0.0059 and 0.0127 per °C, respectively. The contribution of temperature gradient to the overall moisture migration is negligible In high density materials. However, this contribution may be important in the moisture migratlon analysls for low density materials. The moisture gradient induced heat transfer coefficient % as found to be negligible for the materials tested in this study     

MEASUREMENT OF COEFFICIENTS FOB SIMULTANEOUS HEAT AND MASS TRANSFER IN FOOD PRODUCTS

ABSTRACT

Two experimental devices were designed and built to determine four coefficients K_T K_M D_M D (or δ_T = D_T / D_M occurring in simultaneous heat and mass transfer equations, where,K _T and D_M are thermal conductivity and moisture diffcusivity respectively, D_T ( or δ _T is temperature gradient Induced moisture migration coefficient and K_M is moisture gradient Induced heat transfer coefficient. Three food materials, i.e. potato, bread dough and bread, were tested. From this study, it was found that the value of 5 was higher for low density food materials, such as bread, than for high density materials, such as potato. The coefficient & measures moisture migration contribution due to temperature gradient within the material. The average values of δ _T for potato, bread dough and bread were 0.0014, 0.0059 and 0.0127 per °C, respectively. The contribution of temperature gradient to the overall moisture migration is negligible In high density materials. However, this contribution may be important in the moisture migratlon analysls for low density materials. The moisture gradient induced heat transfer coefficient % as found to be negligible for the materials tested in this study     

浓度对传质系数的影响及多元物系传质

在筛板塔中对甲醇-乙醇-水三元物系进行精馏传质过程实验,考虑浓度对传质系数的影响建立了一个数学模型,对各组分在塔中的浓度分布进行模拟计算,计算结果能正确反映各组分在塔中浓度分布的变化规律.实验与计算结果都表明在某些浓度范围内乙醇呈现出比较复杂的变化规律.     

HEAT AND MASS TRANSFER IN THE ADSORBENT BED OF AN ADSORPTION HEAT PUMP

The heat and mass transfer equations governing an adsorbent bed in an adsorption heat pump and the mass balance equation for the adsorbent particles in the adsorbent bed were solved numerically to simulate the cycle of a basic adsorption heat pump, which includes isobaric adsorption, isosteric heating, isobaric desorption, and isosteric cooling processes. The finite difference method was used to solve the set of governing equations, which are highly nonlinear and coupled. The pressures of the evaporator and condenser were 2 and 20 kPa, respectively, and the regeneration temperature of the bed was 403 K. Changes in the temperature, adsorptive pressure, and adsorbate concentration in the adsorbent bed at different steps of the cycle were determined. The basic simulated cycle is presented in a Clausius-Clapeyron diagram, which illustrates the changes in average pressure and temperature of the adsorbent bed throughout the cycle. The results of the simulation indicated that the most time-consuming processes in the adsorption heat pump cycle were isobaric adsorption and isobaric desorption. The high thermal resistance of the bed slows down heat transfer, prolonging adsorption and desorption processes.     

CONVECTTVE HEAT AND MASS TRANSFER IN MOISTURE DESORPTION OF OAK WOOD BY INTRODUCING CHARACTERISTIC TRANSFER COEFFICIENTS

ABSTRACT

This paper presents the effect of forced convective heat and mass transfer coefficients on the predictive ability of a dynamic isothermal set of coupled partial differential equations. The analysis showed that the two coefficient two-dimensional model, resulted in a very good prediction of the experimental data.

The concept of experiment involves: six temperatures, six relative air humidities and three values of circulation velocity. Dimensions of oak (Quercus petraea) wood samples were 250×80×25mm. The experimental installation, designed and constructed for this study, enables the drying process of relatively small samples to be similar to the drying process of real timber.     

CONVECTTVE HEAT AND MASS TRANSFER IN MOISTURE DESORPTION OF OAK WOOD BY INTRODUCING CHARACTERISTIC TRANSFER COEFFICIENTS

This paper presents the effect of forced convective heat and mass transfer coefficients on the predictive ability of a dynamic isothermal set of coupled partial differential equations. The analysis showed that the two coefficient two-dimensional model, resulted in a very good prediction of the experimental data.

The concept of experiment involves: six temperatures, six relative air humidities and three values of circulation velocity. Dimensions of oak (Quercus petraea) wood samples were 250×80×25mm. The experimental installation, designed and constructed for this study, enables the drying process of relatively small samples to be similar to the drying process of real timber.     

CONTINUOUS PHASE MASS TRANSFER COEFFICIENTS FOR SINGLE DROPLETS MOVING IN AN INFINITE DROPLET CHAIN

Continuous phase mass transfer coefficients have been measured for a stream of drops falling freely in a stagnant liquid. Drop streams were produced by a dripping method and by a jet breakup method. Water and isobutanol, mutually saturated, were used as the dispersed and the continuous phases, respectively. Sodium hydroxide was used as solute and was transferred from the bulk solution of isobutanol to water drops which was initially free of solute. Resistance to the mass transfer was observed to be on the continuous phase side. The mass transfer coefficient and terminal velocity of drop streams were measured experimentally. The experimental results show that the mass transfer coefficient in the drop stream is affected by the shielding effect of the previous drops. An approximate analysis was conducted for the shielding effect of a drop stream on mass transfer coefficient caused by the superposing wakes produced by each drop in the stream. The resulting equation describes the effect of interdrop distance on mass transfer coefficient in the continuous phase side and the experimental data have been correlated as K,/U_t ^0.5 versus interdrop distance /. Good results were achieved and also can be applied to other liquid-liquid systems.     

EFFECT OF THE VARIABILITY OF HEAT AND MASS TRANSFER COEFFICIENTS ON CONVECTIVE AND CONVECTIVE-RADIATIVE DRYING OF POROUS MEDIA

A numerical investigation was conducted to study two-dimensional heat and mass transfer during convective drying of a clay brick. The established numerical code has allowed us to determine the effect of heat and mass transfer coefficients variability on state variables and on the drying kinetic.     

热泵吸附器中传热传质过程的研究

对吸附式热泵循环系统中的传热传质进行了理论分析和实验研究,建立了吸附器中热传导方程.计算求解值与实验结果相一致,说明了吸附器中的传热速率控制了热泵循环速度.据此,对吸附器中强化传热肋片进行了模拟分析,作为吸附器优化设计的理论依据.     

EFFECT OF FREE STREAM TURBULENCE ON HEAT AND MASS TRANSFER FROM A SPHERE

A model has been proposed for the momentum eddy diffusivity induced by free stream turbulence intensity. The eddy diffusivity model is applied to a sphere situated in a uniform crossflow in the presence of free stream turbulence. Numerical solution of the governing momentum and energy equations with the proposed eddy diffusivity model yielded results for the heat/mass transfer rates. The numerical predictions of the present work are compared with experimental data and the agreement between the two is seen to be very good.     

EFFECT OF FREE STREAM TURBULENCE ON HEAT AND MASS TRANSFER FROM A SPHERE

A model has been proposed for the momentum eddy diffusivity induced by free stream turbulence intensity. The eddy diffusivity model is applied to a sphere situated in a uniform crossflow in the presence of free stream turbulence. Numerical solution of the governing momentum and energy equations with the proposed eddy diffusivity model yielded results for the heat/mass transfer rates. The numerical predictions of the present work are compared with experimental data and the agreement between the two is seen to be very good.     

干燥过程热质传递的简化模型

对多孔介质体积平均理论进行简化,得到简化的热质传递干燥模型,用一个理论式表达模型中有效扩散系数。对香蕉片千燥的研究表明,模型的预测值与实验结果相     

液体沸腾的给热系数

关於液体沸腾给热系数的准数方程式,前人虽提出不少,但都不够完善,特别是各种盐溶液或多组份的液体混合物,其沸腾给热机理尚待研究。前人实验结果往往相差很大,甚至结论有互相矛盾的地方。 本文采用的研究设备是大容积内水平元管及水平元钣的加热装置。传热表面的材料有黄铜,紫铜,不锈钢,镀镍,镀铬表面等种,以比较不同传热面的影响。对同一材料的传热,又比较了清净表面,老化表面,和有污垢表面的影响。水平元管及水平元钣均用电热,元管的热负荷为1300～40000千卡/米~2·小时,元钣的热负荷范围为20000～900000千卡/米~2·小时。所用物料是清水,5～20%的食盐溶液。研究的结果是: （1）在低热负荷下,加热表面材料不同对沸腾给热是有影响的,主要是由於不同材料对液体的湿润能力不同,接触角大小不同的缘故。但这种影响只是数量上的,对沸腾机理并无影响,也即在下列关系中: a=c·q~n, （1）c值对各种表面来讲是不同的,但n仍然是一样的。 其中以黄铜的a为最高,镀铬的表面为最低。在高热负荷下,加热表面对a无显著的影响。 （2）对同一材料来讲,清净新鲜的表面a最大,老化的次之,有污垢的a最小,而且斜率也不同於前二者。 （3）加热面几何形状相同时,尺寸大小对a的影响在普通生产情     

氢气和一氧化碳在混二甲苯中的传质系数

在１Ｌ表面进气式高压机械搅拌釜中,采用气体间歇吸收技术,在不同的温度,气体压力下研究了Ｈ２和ＣＯ在混二 的体积传质系数,结果表明,Ｈ２和Ｃ类混二甲苯中的体积传质系数均随温度的升高而升高,随压力增加而增加,而且在桢的温度和压力下,ＣＯ在混二甲苯中的体积传质系数大于Ｈ２的体积传质系数,但数量级是相同的,同时随着搅拌速率的升高,体积传质系数也在增加。     

AN APPLICATION OF SIMULTANEOUS HEAT AND MASS TRANSFER IN A CYLINDER USING THE FINITE-DIFFERENCE METHOD

ABSTRACT

A study of simultaneous heat and mass transfer during drying an infinite cylinder shape material (twigs of ilex paraguayenais saint hilaire) was carried out. The finite-difference method was used to solve the drying model and a simultaneous heat and mass balance in each node was made. Models with different assumptions were tested and the external mass transfer coefficient was used as a parameter to fit the model to experimental data. The thickness of the node and the time step were selected considering the system stability.

Drying temperature, twig diameter and air velocity were selected as study variables. The models results were in good agreement with experimental measurements giving mass coefficient values between 1.97 10^?4and 9.55 10^?4 Kg/m² s.