Showing papers on "Mass transfer published in 1993"

A dual-porosity model for simulating the preferential movement of water and solutes in structured porous media

Abstract: A one-dimensional dual-porosity model has been developed for the purpose of studying variably saturated water flow and solute transport in structured soils or fractured rocks. The model involves two overlaying continua at the macroscopic level: a macropore or fracture pore system and a less permeable matrix pore system. Water in both pore systems is assumed to be mobile. Variably saturated water flow in the matrix as well as in the fracture pore system is described with the Richards' equation, and solute transport is described with the convection-dispersion equation. Transfer of water and solutes between the two pore regions is simulated by means of first-order rate equations. The mass transfer term for solute transport includes both convective and diffusive components. The formulation leads to two coupled systems of nonlinear partial differential equations which were solved numerically using the Galerkin finite element method. Simulation results demonstrate the complicated nature of solute leaching in structured, unsaturated porous media during transient water flow. Sensitivity studies show the importance of having accurate estimates of the hydraulic conductivity near the surface of soil aggregates or rock matrix blocks. The proposed model is capable of simulating preferential flow situations using parameters which can be related to physical and chemical properties of the medium.

A water and heat management model for proton-exchange-membrane fuel cells

Abstract: Proper water and heat management are essential for obtaining high-power-density performance at high energy efficiency for proton-exchange-membrane fuel cells. A water and heat management model was developed and used to investigate the effectiveness of various humidification designs. The model accounts for water transport across the membrane by electro-osmosis and diffusion, heat transfer from the solid phase to the gas phase and latent heat associated with water evaporation and condensation in the flow channels. Results from the model showed that at high current (> 1A/cm[sup 2]) ohmic loss in the membrane accounts for a large fraction of the voltage loss in the cell and back diffusion of water from the cathode side of the membrane is insufficient to keep the membrane hydrated (i.e., conductive). Consequently, to minimize this ohmic loss the anode stream must be humidified, and when air is used instead of pure oxygen the cathode stream must also be humidified.

Predicting mass transfer in packed columns

Abstract: Countercurrent-flow columns are widely used in production processes in the chemical industry and their application in ecological engineering is of increasing importance. A theoretical model is presented here that allows mass transfer to be described in terms of packing geometry and physical properties which influence the gas-liquid or vapour-liquid systems in absorption, desorption and rectification columns. The relationships derived from the model can be applied to all countercurrent-flow columns, regardless of whether the packing has been dumped at random or arranged in a geometric pattern.

Analysis of Convection and Secondary Reaction Effects Within Porous Solid Fuels Undergoing Pyrolysis

Abstract: A mathematical model of transport phenomena (heat, momentum and mass transfer) and chemical processes (primary and secondary reactions) of the thermal degradation of wood is presented. Implicit finite difference equations for energy, momentum and chemical species mass balances are formulated according to an operator-splitting technique and are numerically solved. The progress of the pyrolysis process along a wooden slab, radiatively heated on one side, is characterized by the following main processes: 1( a virgin wood region, crossed by a slow flow of pyrolysis products, where temperature and pressure values decrease as the non-irradiated boundary is approached; 2( a primary pyrolysis region where, due to the relatively low temperatures, secondary reactions are not active and 3( a char layer where volatile products of primary pyrolysis mainly flow and, temperature being higher, undergo secondary reactions. For low medium permeabilities, a peak in the gas overpressure is observed, separating the v...

Biofilm formation: hydrodynamic effects on internal diffusion and structure

Abstract: Diffusion in microbial films produced by Pseudomonas fluorescens under turbulent flow conditions was studied using an inert substance (LiCl) Mass transfer coefficients in the biofilm were measured during formation of the biological deposits and for biofilms developed under different fluid velocities Mass transfer rates in the biofilm decreased with time, and more quickly in the case of biofilms subjected to high shear stresses The latter show lower final thicknesses and lower internal diffusivities The so‐called “active layer”;, if it exists, does not seem to have a fixed thickness (as proposed by some authors), since it will depend on the environmental conditions, particularly on fluid velocities

Heat Conduction and Mass Diffusion

Abstract: Steady state processes in one dimension steady state processes in several dimensions unsteady processes in one dimension multi-dimensional time-dependent processes other mass diffusion processes composite transport regions other applications numerical analysis.

Turbulence structure and mass transfer across a sheared air–water interface in wind-driven turbulence

Abstract: The mass transfer mechanism across a sheared air–water interface without bubble entrainment due to wave breaking was experimentally investigated in terms of the turbulence structure of the organized motions in the interfacial region in a wind-wave tank. The transfer velocity of the carbon dioxide (CO2) on the water side was measured through reaeration experiments of CO2, and the fluid velocities in the air and water flows were measured using both a hot-wire anemometer and a laser-Doppler velocimeter. The results show that the mass transfer across a sheared air–water interface is more intensively promoted in wind shear, compared to an unsheared interface. However, the effect of the wind shear on the mass transfer tends to saturate in the high-shear region in the present wind-wave tank, where the increasing rate of mass transfer velocity with the wind shear decreases rapidly. The effect of the wind shear on the mass transfer can be well explained on the basis of the turbulence structure near the air–water interface. That is, surface-renewal eddies are induced on the water side through the high wind shear on the air–water interface by the strong organized motion generated in the air flow above the interface, and the renewal eddies control the mass transfer across a sheared interface. The mass transfer velocity is correlated with the frequency of the appearance of the surface-renewal eddies, as it is in open-channel flows with unsheared interfaces, and it increases approximately in proportion to the root of the surface-renewal frequency. The surface-renewal frequency increases with increasing the wind shear, but for high shear the rate of increase slows. This results in the saturated effect of the wind shear on the mass transfer in the high-shear region in the present wind-wave tank. The mass transfer velocity can be well estimated by the surface-renewal eddy-cell model based on the concept of the time fraction when the surface renewal occurs.

Heat and mass transfer in consolidated reacting beds for thermochemical systems

Abstract: The method used to fill a chemical heat pump reactor with its porous reactive medium governs the heat and mass transfer in the reactor and hence its performance in terms of heat, power and capacity. The filling operation consists of mixing the salt, which takes place in the solid-gas reaction, and an inert graphite binder, and in confining this mixture to obtain a given apparent density. Depending on the binder properties and the mixing technique, the values (experimentally obtained) of the effective conductivities, λe, of the heat transfer coefficients, hsw, at the exchanger wall and of the permeabilities, k, cover a very large range: 0.2 W m −1 K −1 ⩽λ e ⩽40 W m −1 K −1 15 W m −2 K −1 K −1 ⩽h sw ⩽15,000 R m −2 K −1 , 0.03 × 10 −12 m 2 ⩽k⩽30 × 10 −12 m 2 . For some simple cases in which one limitation predominates over all the others, a simple relation has been set up between the reactor's power output per unit volume and a single filling parameter.

Diffusion Layer Theory for Turbulent Vapor Condensation With Noncondensable Gases

Abstract: In turbulent condensation with noncondensable gas, a thin noncondensable layer accumulated and generates a diffusional resistance to condensation and sensible heat transfer. By expressing the driving potential for mass transfer as a difference in saturation temperatures and using appropriate thermodynamic relationships, here an effective condensation thermal conductivity is derived. With this formulation, experimental results for vertical tubes and plates demonstrate that condensation obeys the heat and mass transfer analogy, when condensation and sensible heat tranfer are considered simultaneously

A distributed reactivity model for sorption by soils and sediments. 2. Multicomponent systems and competitive effects

Abstract: The uptake of individual hydrophobic organic compounds (HOCs) from aqueous solution by soils which exhibit heterogeneous reactivity is shown to be reduced in the presence of other HOCs. The observed behavior is consistent with expectations for competitive surface adsorption phenomena, being most marked at low solution-phase concentrations and coinciding with single-solute isothermal nonlinearity. Nonlinear sorption and competitive effects appear to relate to components or fractions of soil having different reactivity than that commonly attributed to soil organic matter. In particular, HOC uptake by the organic components of soils deriving from certain sedimentary rocks appears to occur by surface reactions rather than by partitioning to organic matrices. Ideal adsorbed solution theory (IAST) is used to predict mixed-solute competitive effects from single-solute sorption isotherms. The results are examined within the context of the distributed reactivity model (DRM), wherein overall sorption behavior by heterogeneous solids is explicitly attributed to different mechanisms. Competitive reductions in the sorption of HOCs in systems dominated by nonlinearly sorbing soil components and characterized by a two-compartment DRM are found to be modeled reasonably well with IAST. 22 refs., 12 figs., 3 tabs.

Solvent extraction in the process industries

Abstract: Volume 1: 1. Contacting Equipment 1.1. Aromatics extraction with the Kuhni column, an application requiring an extreme design flexibility. 2. Processes. 2.1. Separation and recovery of nickel and cobalt in ammoniacal systems - process development. 3. Rare Earths. 3.1. Some progresses in solvent extraction of rare earth in China. 4. Hydrodynamics and Mass Transfer. 4.1. The effect of viscosity on drop transport and breakup in pulsed and stirred liquid-liquid extraction columns. 5. Metals (Reagents and Modifiers). 5.1. Liquid-liquid extraction and molecular modelling. Volume 2: 6. Supercritical Fluid Extraction. 6.1. Supercritical fluid extraction using amine-entrainers. 7. Inorganic Materials. 7.1. The development of liquid-liquid extraction processes for the inorganic chemical industry using steady-state simulation. 8. Liquid Membranes. 8.1. Metal separation by supported liquid membranes: effect of the membrane characteristics. 9. Biotechnology. 9.1. Interfacial mass transfer and partition studies during whole broth extraction. 10. Organic Chemicals. 10.1. Comparative studies with phenol, furfural and N-methyl-2-pyrrolidone as solvents for lube oils extraction. 11. Control and Modelling. 11.1. Control of liquid-liquid extraction columns. Volume 3: 12. Metals 2 (Metals Extraction). 12.1. The solvent extraction of As, Sb and Bi from copper refining electrolytes using organophosphorus reagents. 13. Nuclear 1. 13.1. Solvent extraction processes used for the recycling of irradiated nuclear fuel. 14. Environmental. 14.1. AmMAR: concept for metal waste recovery. 15. Fundamentals. 15.1. Interfacial phenomena in hydrometallurgical solvent extraction systems. 16. Nuclear 2. 16.1. Distribution coefficient measurements of Tc, Np[IV] and Np[Vi] between nitric acid solutions containing U and Pu and 30per cent TBP/n-dodecane. Part contents.

Near-bed turbulence and hydrodynamic control of diffusional mass transfer at the sea floor

Abstract: A simple closure scheme for turbulent transport yields mean-velocity and dissolved-substance concentration profiles just above and solute fluxes across a sediment-water interface underlying uniform, steady flow. In the case of near-bed turbulence characterization, this approach retains both turbulent and viscous terms in expressions for important structural aspects of the near-bed regions of turbulent boundary layers. Moreover, a modified turbulent kinetic energy balance is used to define eddy viscosity in the viscousdominated region of flow very near the bed. In the case of characterization of mass distribution and diffusional transfer rate, the approach partitions important factors that control mass transfer rate: nearbed turbulent transport, interfacial-flux boundary condition, and reaction kinetics. Diffusive sublayers overlying smooth beds exposed to typical deep-sea conditions are predicted to be 1 mm thick, a result in agreement with the sublayer thickness observed in situ and the inferred thickness used in many hydrodynamic models. One application of the new closure scheme indicates that typical biogenic roughness in fine-sediment marine environments enhances solute exchange rates threefold over those expected for a smooth bed. This increase is due to enhanced turbulent transport in the vicinity of the rough bed and occurs in spite of viscous ponding of “dead water” among roughness elements. Diffusional transfer of dissolved substances across the sediment-water interface in marine environments is an important factor in early diagenesis of fine sediments accumulating over most of the earth’s surface. Boudreau and Guinasso (1982) explored many important aspects of this issue. In particular they advanced the notion, originally proposed by Levich (1942, 1962; see also Nernst 1904; Nernst and Merriam 1905; Langmuir 1912), that presence of a dzflusive sublayer can act as a region of impedance to scalar transport and thus act to retard some chemical reactions occurring at or just below the sea floor. This sublayer is the region next to an interface where molecular diffusion of mass or heat exceeds transport due to turbulent motions of the carrier fluid. Thus, an understanding of hydrodynamic control of diffusional mass transfer at the sea floor relies on the ability to characterize turbulent mass I Present address: Institute of Theoretical Geophysics, Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, U.K. Acknowledgments Ideas in this and earlier versions benefited from the comments of D. Archer, S. Emerson, P. Hill, P. Jumars, A. Nowell, J. Shimeta, and two anonymous referees. Special thanks are due B. P. Boudreau for his thorough review and helpful insight. This research was supported by ONR grant NO0

Estimating Diffusion Coefficients of Dense Fluids

Abstract: Gas injection (hydrocarbon, nitrogen, or carbon dioxide) into oil and condensate reservoirs may be attempted to recover more in situ hydrocarbons. In some cases, particularly in naturally fractured reservoirs, there has been a need for information on the rate and amount of mass transfer by diffusion. The most important property, to obtain such information, is the diffusion coefficient at reservoir conditions. A simple and generalized correlation in terms of viscosity and molar density is proposed to estimate diffusion coefficients for hydrocarbon systems. The correlation can be used for both gases and liquids up to a pressure of about 400 bar (6,000 psia). It has been shown that the proposed method may also be used to estimate effective diffusion coefficients in multicomponent systems with a reasonable degree of accuracy. Although the proposed correlation is based on experimental data in hydrocarbon systems, preliminary evaluations have shown that it is also satisfactory for non-hydrocarbon systems as well. The proposed equation predicts diffusion coefficients in gases with an absolute average deviation of 8% and in liquid systems with an absolute average deviation of 15%. The input parameters for the correlation are molecular weight, critical properties, and acentric factor of components in the system; mixturemore » molar density; low-pressure gas viscosity; and actual viscosity. The last three properties may be predicted from appropriate correlations.« less

Evaporation rate of water in a vessel

Abstract: The evaporation rate of water in a vessel was studied as a function of of temperature, humidity, air velocity, and diameter of the vessel. The atmospheric temperature was varied between 14 and 29 °C, the humidity between 24% and 75%, the air velocity between 0.09 and 7.9 m/s, and the diameter between 5.6 and 12.4 cm. The experimental results are compared with a theory that is based on the idea that the evaporation rate is equal to the diffusion rate of water vapor through the boundary layer covering the water surface in a vessel. In order to compare the experimental evaporation rate with the theory, the temperature of the water surface was carefully measured with a thin thermocouple. The agreement between the present experiments and the theory is generally good.

Heat transfer intensification in fixed bed adsorbers

Abstract: The low thermal conductivity of the solid sorbents is one of the major drawbacks of the sorption heat pumps development. A better thermal transfer in the adsorber fixed beds is required to obtain a decreased time of the processing cycles and thus a reduced adsorber size per unit of power. Small improvements in the parameters of thermal transfer were obtained with unconsolidated porous mixtures such as bimodal mixtures and metallic foams. New consolidated materials made of metallic foam and zeolite were developed. The measurements of the thermal conductivity and of the wall heat transfer coefficient show a great improvement in the thermal transfer quality. Resistances to mass transfer appear but they are consistent with the adsorption heat pump process. With this type of composite material it seems possible to reduce the adsorber size by a factor from 5 to 10.

Heat and mass transfer in impingement drying

Abstract: In industrial drying applications, efficient transfer of heat and mass between a drying medium and the material being dried is very critical for the overall economics of the operation. Impinging jets are therefore widely used for their enhanced tmnsport characteristics, especially for drying of continuous sheets of materials such as paper and textiles. In such applications, a thin sheet of material, as wide as 6m in cross machine direction, speeds at velocities as high as 90 km/hr under high velocity jets emerging from a confining surface parallel to the material surface. Many variables and effects need to k considered for proper design of such impinging jet systems: nozzle geometry and size, nozzle configuration, location of exhaust pons, nozzle-to-surface separation, jet-to-jet separation, cross flow, jet exit velocity and surface motion. For permeable materials, additional enhancement of heat and mass transfer that occur when some of the impinging gas is removed through the material makes this...

Effect of intraparticle convection on the chromatography of biomacromolecules.

Abstract: The effect of intraparticle convection in chromatographic columns packed with gigaporous particles (i.e., where dpore/dparticle > 10(-2)) on the band spreading of unretained biomacromolecules is investigated both experimentally and theoretically. A model is developed for the analysis of mass transfer in spherical particles of bidisperse pore structure when both convection and diffusion take place in the larger pores but only diffusion occurs in the smaller pores. The predictions of the model were experimentally verified. It is demonstrated that gigaporous particles have advantages over conventional porous particles (i.e., where dpore/dparticle < 10(-3)) for applications that do not require high resolving power, to bring about fast separation. This is because columns packed with gigaporous particles can be operated at high flow velocities without significant loss of efficiency due to the enhancement of mass transfer by intraparticle convection. The results of the model are used to examine the effectiveness of gigaporous column packings for rapid analytical chromatography and for the concentration and recovery of a dilute solute in a saturation-regeneration cycle utilizing frontal chromatography.

Mass transfer at the air/water interface: Mass accommodation coefficients of SO2, HNO3, NO2 and NH3

Abstract: We present here experimental determinations of mass accommodation coefficients β using a low pressure tube reactor in which monodispersed droplets, generated by a vibrating orifice, are brought into contact with known amounts of trace gases. The uptake of the gases and the accommodation coefficient are determined by chemical analysis of the aqueous phase. We report in this article measurements of βexp=(6.0±0.8)×10−2 at 298 K and with a total pressure of 38 Torr for SO2, (5.0±1.0)×10−2 at 297 K and total pressure of 52 Torr for HNO3, (1.5±0.6)×10−3 at 298 K and total pressure of 50 Torr for NO2, (2.4±1.0)×10−2 at 290 K and total pressure of 70 Torr for NH3. These values are corrected for mass transport limitations in the gas phase leading to β=(1.3±0.1)×10−1 (298 K) for SO2, (1.1±0.1)×10−1 (298 K) for HNO3, (9.7±0.9)×10−2 (290 K) for NH3, (1.5±0.8)×10−3 (298 K) for NO2 but this last value should not be considered as the true value of β for NO2 because of possible chemical interferences. Results are discussed in terms of experimental conditions which determine the presence of limitations on the mass transport rates of gaseous species into an aqueous phase, which permits the correction of the experimental values.