MASS TRANSFER IN GAS-SPARGED POROUS ELECTRODES

The electrochemical limiting current method was used to measure mass transfer coefficients with cocurrent upward gas-liquid flow in packed bed electrodes. Liquid-solid and overall gas-solid coefficients were determined from limiting current data in the presence of inert or reactive gas flows with the use of a plug flow model. The presence of inert gas flow increased mass transfer coefficients over those in single-phase flow by a factor of up to 1.7. As a result of boundary layer penetration by gas bubbles, sparging with reactive gas increased mass transfer rates by a factor of up to 3.5.     

MASS TRANSFER IN GAS-SPARGED POROUS ELECTRODES

The electrochemical limiting current method was used to measure mass transfer coefficients with cocurrent upward gas-liquid flow in packed bed electrodes. Liquid-solid and overall gas-solid coefficients were determined from limiting current data in the presence of inert or reactive gas flows with the use of a plug flow model. The presence of inert gas flow increased mass transfer coefficients over those in single-phase flow by a factor of up to 1.7. As a result of boundary layer penetration by gas bubbles, sparging with reactive gas increased mass transfer rates by a factor of up to 3.5.     

THE EFFECTS OF THE SOLUBILITY LAW ON MASS TRANSFER IN ANNULAR LIQUID JETS

Underpressurized, annular liquid jets have been studied under steady and transient conditions in order to assess the effects of Henry's and Sievert's solubility laws on mass transfer. It is shown that, in general, Henry's solubility law predicts a higher, steady state mass absorption rate than Sievert's law. The rate at which the under-pressurized, annular liquid jet tends towards an equilibrium, non-pressurized configuration is higher for Henry's than for Sievert's solubility law. Both this rate and the steady state mass absorption rate increase as the Weber number, the ratio of the solubility at the inner interface to that at the outer one, the product of the gas constant times the temperature and the solubility of the gases surrounding the jet, the ratio of the product of the pressure of the gases surrounding the annular jet times the annular jet's mean radius at the nozzle exit to the liquid surface tension, and the nozzle exit angle are increased, and as the Froude and mass Peclet numbers, the annular jet's thickness-to-mean radius ratio at the nozzle exit and the initial pressure of the gases enclosed by the annular jet are decreased.     

MASS TRANSFER EFFECTS ON THE ETCH RATE OF GAAS IN FLOW SYSTEMS

In industrial wet etching reactors, the fluid contacts the substrate surface as a spray of flowing stream, thus introducing mass-transfer resistances to the reaction rate. The etching of gallium arsenide in H₂O₂-NH₄OH-H₂O solutions was studied using an open-channel flow reactor to simulate the industrial conditions. The etch rate was always lower than that obtained under kinetic control, and the dependence of etch rate on H₂O₂ concentration shifted closer to first order. From the calculation of the ratio of rate constant to mass-transfer coefficient, the reaction-rate and mass-transfer resistances were both significant in this system. When the mass-transfer coefficient was calculated from equations for flow past a flat plate, the prediction of etch rate was good, particularly when the starting length for velocity boundary layer development ahead of concentration boundary layer development was taken into account. Another approach for the calculation of mass-transfer coefficient, based on the assumptions for flow between parallel plates, best represented the relative insensitivity of etch rate to fluid velocity.     

MASS TRANSFER EFFECTS ON THE ETCH RATE OF GAAS IN FLOW SYSTEMS

In industrial wet etching reactors, the fluid contacts the substrate surface as a spray of flowing stream, thus introducing mass-transfer resistances to the reaction rate. The etching of gallium arsenide in H₂O₂-NH₄OH-H₂O solutions was studied using an open-channel flow reactor to simulate the industrial conditions. The etch rate was always lower than that obtained under kinetic control, and the dependence of etch rate on H₂O₂ concentration shifted closer to first order. From the calculation of the ratio of rate constant to mass-transfer coefficient, the reaction-rate and mass-transfer resistances were both significant in this system. When the mass-transfer coefficient was calculated from equations for flow past a flat plate, the prediction of etch rate was good, particularly when the starting length for velocity boundary layer development ahead of concentration boundary layer development was taken into account. Another approach for the calculation of mass-transfer coefficient, based on the assumptions for flow between parallel plates, best represented the relative insensitivity of etch rate to fluid velocity.     

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.     

INFLUENCE OF MIXING INTENSITY ON THE MASS TRANSFER COEFFICIENT ACROSS LIQUID-LIQUID INTERFACES

The influence of mixing intensity on the liquid-liquid mass transfer have been investigated using a binary system (water/n-butanol) in a modified stirred cell type contactor which overcomes the shortcomings associated with the previous contactors. It is observed that the mass transfer coefficient is affected by the degree of mixing intensity in both phases. The dependence of the aqueous and organic mass transfer coefficients on the mixing rate is about the same when both phases are agitated at the same speed and these coefficients become asymptotic values for the coefficients obtained at non identical agitation rate in both phases.     

RADIATION AND NON-DARCY EFFECTS ON CONVECTION IN POROUS MEDIA

Combined conduction, convection and radiation heat transfer in a gray fluid-saturated sparsely packed porous medium is examined analytically for marginal convection using linear stability analysis. The effects of boundary and inertia which were absent in the usual Darcy model are considered. The Milne-Eddington approximation is employed to determine the solutions valid for transparent and opaque media which absorbs and emits thermal radiation. It is shown that the nature of the bounding surfaces and radiation significantly influence the critical Rayleigh and wave numbers. The mechanism for suppressing or augmenting convection is discussed in detail. The results obtained using Galerkin technique are compared with the existing results of Darcy model and of non-radiating systems and agreement is found.     

HEAT AND MASS TRANSFER IN STAGNATION-POINT FLOW OF A POLAR FLUID TOWARDS A STRETCHING SURFACE IN POROUS MEDIA IN THE PRESENCE OF SORET,DUFOUR AND CHEMICAL REACTION EFFECTS

This work is focused on the numerical solution of steady boundary-layer stagnation-point flow of a polar fluid towards a stretching surface embedded in porous media in the presence of the effects of Soret and Dufour numbers and first-order homogeneous chemical reaction. The governing boundary-layer equations of the problem are formulated and transformed into a self-similar form. The obtained equations are solved numerically by an efficient, iterative, tri-diagonal, implicit finite-difference method. Both assisting and opposing flow conditions are considered. Comparisons of the present numerical results with previously published work under limiting cases are performed and found to be in excellent agreement. Representative results for the fluid velocity, angular velocity, temperature, and solute concentration profiles as well as the local heat and mass transfer rates for various values of the physical parameters are displayed in both graphical and tabular forms.     

COMPUTER SIMULATION OF MASS TRANSFER IN AGGREGATIVE FLUIDIZED BEDS

A mathematical model for simulation of unsteady-state mass transfer in an aggregative fluidized bed of porous particles is proposed. A quasi-steady state approach is being used to solve the model. A computer program is written for solving the model and the rate of drying of air in a fluidized bed of silica-gel particles is calculated as an example. To evaluate the accuracy of the proposed model, a pilot-scale fluidized bed is erected and the experimental results are compared to the calculated values. A good agreement is observed between them.     

HYDRODYNAMICS AND MASS TRANSFER IN BUBBLE COLUMNS: EFFECT OF SOLIDS

The hydrodynamics and mass transfer in a large diameter bubble column (D_c 0.305 m), specifically, the effects of gas velocity and the presence of solids on the gas holdup structure, gas-liquid interfacial area, and volumetric mass transfer coefficients in viscous as well as low viscosity solutions are studied. The sulfite oxidation technique was employed to measure the gas-liquid interfacial area. Volumetric mass transfer coefficients were measured using a chemical method (sulfite oxidation) as well as physical absorption of oxygen from air, and the overall gas holdups were measured using the hydrostatic head technique. The effect of solids on the gas holdup structure was examined using the dynamic gas disengagement method. With the addition of polystyrene particles, the gas-liquid interfacial area decreased for low viscosity systems, whereas it increased for viscous systems. This was shown to be due to the effect of solids on bubble coalescence. The wettability characteristics of solid surfaces in the presence of different liquids have been suggested as the reason for the effect of solids on coalescence. Oil shale slurries presented a special case because of the mineral dissolution effect.     

非饱和含湿多孔介质微波冷冻干燥过程传热传质分析

基于升华冷凝模型,对非饱和含湿多孔介质微波冷冻干燥过程作了数值计算．结果表明,干燥过程中不饱和含冰区内的冰饱和度有较大变化．通过与不考虑升华冷凝区相比较,表明升华冷凝区的存在不可忽略．     

HYDRODYNAMICS AND MASS TRANSFER IN A SPOUTED BED DRYER

ABSTRACT

The disposal of sludge generated by water treatment plants poses major financial and environmental problems. Drying of the sludge is an essential pan of any disposal process which may include incinerating, landfilling or upgrading. In the present study, experiments were carried out to investigate the drying of sludge in a spouted bed and to characterize the hydrodynamics and mass transfer mechanisms. The effect of bed moisture content on the minimum spouting velocity (U_ms) was examined for sludge granules. U_ms was found to increase with increasing the moisture content of the sludge particles. The gas phase mass transfer coefficient in the dryer was determined using porous calcined alumina particles and the results were compared with predictions by correlations available in the literature. The existing correlations gave poor predictions for the mass transfer coefficient. A new correlation for the mass transfer coefficient in a spouted bed dryer was developed based on the experimental results.     

MASS TRANSFER WITH HETEROGENEOUS CHEMICAL REACTION OF THE FIRST ORDER IN FALKNER-SKAN FLOW OF A POWER-LAW FLUID

Mass transfer with a heterogeneous chemical reaction of the first order in a Falkner-Skan flow of non-Newtonian power-law fluid is investigated. This problem is solved by the method of Laplace transform following the approach suggested by Apelblat (1980). The solution is obtained in a closed analytical form.     

MASS TRANSFER WITH HETEROGENEOUS CHEMICAL REACTION OF THE FIRST ORDER IN FALKNER-SKAN FLOW OF A POWER-LAW FLUID

ABSTRACT

Mass transfer with a heterogeneous chemical reaction of the first order in a Falkner-Skan flow of non-Newtonian power-law fluid is investigated. This problem is solved by the method of Laplace transform following the approach suggested by Apelblat (1980 Apelblat , A. ( 1980 ). Chem. Eng. J. , 19 , 19 – 37 . [CSA] [CROSSREF]  [Google Scholar]). The solution is obtained in a closed analytical form.     

ANALYTICAL/NUMERICAL PREDICTION OF THE TOTAL MASS TRANSFER NEAR THE ENTRANCE TO THE MASS TRANSFER SECTION OF ROUND TUBES

The underlying idea behind the classical Lévêque problem was to provide a simple asymptotic solution for the incipient development of the mass (heat) boundary layer flow in a round tube. Inspired by physical concepts, Lévêque assumed that the hydrodynamic boundary layer was confined to a thin annular region near the tube wall so that the fluid velocity varied linearly with y = R ? r, y being the distance measured from the wall. The present article addresses the Lévêque problem from a mathematical perspective, adhering to the original concentration conservation equation in cylindrical coordinates continually without making any hydrodynamic assumptions a priori. The approximate solution procedure to be pursued here combines the transversal method of lines (TMOL) with the Fröbenius power series method. The quality of the local and global mass transfer results obtained with the new TMOL/Fröbenius methodology surpasses the quality of the counterpart results determined by the traditional Lévêque approach.     

THERMAL CONVECTIVE INSTABILITY IN TRANSLUCENT POROUS MEDIA WITH RADIATIVE HEAT TRANSFER

The onset of thermal convection in a translucent porous layer is considered. Attention is focused on the effect of radiative heat transfer on the critical Rayleigh-Darcy number and the convection cell shape. If we consider the contribution of radiative heat transfer, the basic temperature profile is non-linear and the thermal convective instability is influenced by the ratio of conduction to radiation heat flux, the temperatures at the boundary surfaces, and radiative parameters such as wall emissivity, scattering albedo and extinction coefficient as well as the usual Rayleigh-Darcy number. Effects of these parameters on the onset of convective instability are investigated with the help of linear stability theory employing the Darcy's law and the radiative transport equation simplified by the P₁ approximation. The increased effective thermal conductivity due lo the radiation inhibits the onset of convection and causes increased critical Rayleigh-Darcy number and decreased convection cell size. The results of the present work may be exploited to find out the optimal diameter of aerogel pellets and the air pressure in the double pane window filled with the translucent silica aerogel granules to suppress natural convection.     

CFD MODELLING OF NATURAL CONVECTION HEAT AND MASS TRANSFER IN HYGROSCOPIC POROUS MEDIA

Abstract

This paper presents the derivation of a model to predict heat and mass transfer in a system consisting of a turbulently flowing fluid overlying a saturated hygroscopic porous medium. Comparisons with experimental and numerical simulations have been carried out to check the accuracy of components of the model. Finally, a case study using silica gel as a representative hygroscopic porous medium is presented as an application of the model. It is shown that moisture is convected from the warm interior of a bulk of porous medium to the relatively cool periphery. This result has profound practical implications when the hygroscopic medium is stored agricultural produce as the region of high moisture content may become moldy.     

CFD MODELLING OF NATURAL CONVECTION HEAT AND MASS TRANSFER IN HYGROSCOPIC POROUS MEDIA

This paper presents the derivation of a model to predict heat and mass transfer in a system consisting of a turbulently flowing fluid overlying a saturated hygroscopic porous medium. Comparisons with experimental and numerical simulations have been carried out to check the accuracy of components of the model. Finally, a case study using silica gel as a representative hygroscopic porous medium is presented as an application of the model. It is shown that moisture is convected from the warm interior of a bulk of porous medium to the relatively cool periphery. This result has profound practical implications when the hygroscopic medium is stored agricultural produce as the region of high moisture content may become moldy.