Showing papers on "Mass transfer coefficient published in 1993"

Mass transfer from nonaqueous phase organic liquids in water-saturated porous media

Abstract: Results of dissolution experiments with trapped nonaqueous phase liquids (NAPLs) are modeled by a mass transfer analysis. The model represents the NAPL as isolated spheres that shrink with dissolution and uses a mass transfer coefficient correlation reported in the literature for dissolving spherical solids. The model accounts for the reduced permeability of a region of residual NAPL relative to the permeability of the surrounding clean media that causes the flowing water to partially bypass the residual NAPL. The dissolution experiments with toluene alone and a benzene-toluene mixture were conducted in a water-saturated column of homogeneous glass beads over a range of Darcy velocities from 0.5 to 10 m d(-1). The model could represent the observed effluent concentrations as the NAPL underwent complete dissolution. The changing pressure drop across the column was predicted following an initial period of NAPL reconfiguration. The fitted NAPL sphere diameters of 0.15 to 0.40 cm are consistent with the size of NAPL ganglia observed by others and are the smallest at the largest flow velocity.

Transport studies for the sorption of copper ions by chitosan

Abstract: The ability of chitosan, prepared from waste prawn shell, to sorb copper (II) ions from aqueous solutions has been studied. Equilibrium studies show that chitosan has a maximum sorption capacity for copper ions of about 40mg/g chitosan. The mass transport characteristics have been investigated by monitoring the kinetics in an agitated batch adsorber. An external mass transfer coefficient and an intraparticle diffusion rate parameter have been determined for a number of system variables including agitation, initial copper ion concentration, chitosan mass, chitosan particle size and solution temperature.

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

Fundamental Mass Transfer Model for Indoor Air Emissions from Surface Coatings

Abstract: The paper discusses the work of researchers at the U.S. EPA's Air and Energy Engineering Research Laboratory (Indoor Air Branch) who are evaluating mass transfer models based on fundamental principles to determine their effectiveness in predicting emissions from indoor architectural coatings. As a first step, a simple model based on Fick's Law of Diffusion has been developed. In the model, the mass transfer rate is assumed to be controlled by the boundary layer mass transfer coefficient, the saturation vapor pressure of the material being emitted, and the mass of volatile material remaining in the source at any point in time. Both static and dynamic chamber tests were conducted to obtain model validation data. Further validation experiments were conducted in a test house. Results of these tests are presented.

The effects of gas phase convection on mass transfer in spin coating

Abstract: The thickness uniformity of photoresist films deposited by spin coating critically influences the resolution of photolithography. This thickness uniformity depends on uniform evaporation from the film during drying. Simple scaling arguments demonstrate that, if the mass transfer coefficient at the surface of the wafer does not vary with radial position, then the dry coated resist film thickness will be independent of radial position. A model is presented for the compressible, laminar, steady‐state, axisymmetric air flow in a spin coating apparatus for 6‐in.‐diam wafers. Flow fields computed by a finite‐element–Newton method are used to evaluate the radial profile of the mass transfer coefficient at the surface of the rotating wafer, and to calculate the trajectories of particles that are generated as photoresist is flung from the edge of the spinning wafer. At a spin speed of 2000 revolutions/min and exhaust flow rate of 100 l/min through the coater, the calculations predict that the mass transfer coeffic...

A semi-empirical transient method for modelling frost formation on a flat plate

Abstract: This paper models the frost formation process employing a semi-empirical transient model for a flat plate under forced convection conditions. This continuous differential model employs the Euler's method to solve for the temperature and thickness of the frost layer using existing correlations for the convective heat transfer coefficient and the Lewis analogy to compute both a mass transfer coefficient and an enthalpy transfer coefficient. The model is also based on empirical correlations for predicting the frost thermal conductivity and density. Model results are compared with existing experimental data and with numerical data of other investigators and are found to agree very well in the applicable temperature and humidity range of the frost density and conductivity correlations.

Prediction of evaporation heat transfer coefficient and pressure drop of refrigerant mixtures in horizontal tubes

Abstract: A study on the prediction of heat transfer coefficient and pressure drop of refrigerant mixtures is reported. Heat transfer coefficients and pressure drops of prospective mixtures to replace R12 and R22 are predicted on the same cooling capacity basis assuming evaporation in horizontal tubes. Results indicate that nucleate boiling is suppressed at qualities greater than 20% for all mixtures, and evaporation becomes the main heat transfer mechanism. For the same capacity, some mixtures containing R32 and R152a show 8–10% increase in heat transfer coefficients. Some mixtures with large volatility difference exhibit as much as 55% reduction compared to R12 and R22, caused by mass transfer resistance and property degradation due to mixing (32%) and reduced mass flow rates (23%). Other mixtures with moderate volatility difference exhibit 20–30% degradation due mainly to reduced mass flow rates. The overall impact of heat transfer degradation, however, is insignificant if major heat transfer resistance exists in the heat transfer fluid side (air system). If the resistance in the heat transfer fluid side is of the same order of magnitude as that on the refrigerant side (water system), considerable reduction in overall heat transfer coefficient of up to 20% is expected. A study of the effect of uncertainties in transport properties on heat transfer shows that transport properties of liquid affect heat transfer more than other properties. Uncertainty of 10% in transport properties causes a change of less than 6% in heat transfer prediction.

Intensification of mass transfer in aqueous two-phase systems.

Abstract: A novel technique which intensifies conventional aqueous two-phase extraction by conversion of dispersed phase into colloidal gas aphrons (CGAs) has been developed for extraction of an enzyme. In the present work, amyloglucosidase (1,4-α-D-glucan glucohydrolase) was extracted using a polyethylene glycol-sodium sulfate-water system. The lighter phase, i.e., polyethylene glycol (PEG) rich phase, was converted into CGAs which were then dispersed into a salt rich phase. The effect of type of surfactant and its concentration, dispersed phase velocity, phase composition, and type of sparger on the dispersed phase mass transfer coefficient was investigated. The results suggests 9–16 times higher values of mass transfer coefficient compared to spray column. The multiorifice sparger at concentrations of 0.33 g/L of cetyl trimethyl ammonium chloride yielded best results. © 1993 John Wiley & Sons, Inc.

Absorption: Fundamentals & Applications

Abstract: Theory of Absorption. Gas-liquid Equilibria. Expressions of concentrations and properties of fluids. Model of the interfacial region. Methods of calculation of the generalized Stefan-Maxwell dfffusion coefficients. Equations of change for multicomponent Systems. Introduction. Rate of homogeneous chemical reactions. Differential mass balance equations. Other balance and constitutive equations. One-dimensional diffusion in a gaslayer. Mass Transferin Turbulent Flow. Turbulent flow and restricted applicability of equations of change. Models of mass transfer in turbulent flow. Definitions of mass transfer coefficients. General remarks on transfer models. The effect of solid particles on the rate of physical absorption. The effect of surface tension on mass transfer rate. Simultaneous Mass Transfer with Chemical Reaction. Introduction. Interphase mass transfer with chemical reaction. Final remarks. Desorption. Mechanism of desorption. Simultaneous physical absorption and desorption of gases. Super saturation effect. Balance Equations of the Equipment for Multicomponent and Binary Systems. Introduction. Mass balances. Energy balances. Final remarks. Calculation of Absorbers. Introduction to the Design of Absorbers. The aims of absorption. Basic characteristics of absorbers. Mass balance of an absorber in physical processes (binarydilute, isothermal systems). Mass balance of the absorber for pure gases. Calculation of desorbers. Models of fluid flow in absorbers. Methodology of absorber calculations. Packed Columns. Introduction. Cocurrent phase flow. Balance equations of packed columns for multicomponent systems. Simulation of industrial packed columns. Methods of column calculation using the concept of the theoretical plate. Bubble Columns. Introduction. Hydrodynamics of bubble columns. Appendix. Solution of algebraic equation systems. Systems of ordinary differential equations.

Surface mass transfer coefficients for wood

Abstract: Experimentally determined mass transfer coefficients for wood surfaces are often much lower than theoretically and experimentally determined coefficients for other surfaces. An investigation of this is made by comparing mass transfer coefficients for water and saturated non-wood surfaces, with coefficients measured for wood surfaces. It is found that measurements on wood surfaces are usually evaluated by assuming that Fick's law describes diffusion in wood. As this is not always true, it is proposed that the mass transfer coefficients measured on wood surfaces are low because the internal mass transfer in wood is not well understood. New measurements are presented which support this conclusiou.

Effect of mass transfer on the current distribution in monopolar and bipolar electrochemical reactors with a gas-evolving electrode

Abstract: The current distribution in an electrochemical reactor with vertical parallel-plate electrodes was experimentally determined. The research was performed with monopolar and bipolar electrodes. The reactor has a gas-evolving electrode and at the counter electrode an electrochemical reaction with combined diffusion and charge-transfer kinetic control, takes place. Therefore the kinetics at the counter electrode are influenced by the bubble-induced convection and by the forced convection of the electrolyte. These reactors are found in many electrochemical processes, for example, electrowinning of metals and electrosynthesis. The test reactions were hydrogen evolution at the cathode and the anodic oxidation of sulphite to sulphate from basic solutions. The current distribution shows a minimum at a distance of approximately six times the equivalent diameter of the reactor from the inlet region. This minimum is a consequence of the interaction between forced convection and the bubble-induced convection, which shifts the mass transfer coefficient of the anodic reaction along the reactor. The effects of the total current, the volumetric electrolyte flow rate and the metal phase resistance on the current distribution are also analysed. The experimental current distribution data are compared with theoretical expectations and good agreement is found.

Internal mass transfer in hollow fiber supported liquid membranes

Abstract: The study of mass transfer in hollow fiber supported liquid membranes is justified by a large number of separation processes. The analysis starts from the definition of an overall permeability coefficient which is a lumped parameter of a particular system and process conditions which gathers both mass transfer and operation parameters. By applying the film theory the contribution of the interfacial mass-transfer coefficient due to the inner boundary layer and the contribution of the supported liquid membrane permeability coefficient can be separated and analyzed. The study yields overall permeability coefficients that can be compared with those expected from hollow fiber design equations suggested earlier. The second approach considers the continuity mass conservation equation and the associated boundary conditions for the solute in the inner fluid. The analysis by means of the fundamental equations separates the effects of the operation variables such as the hydrodynamic conditions and length and diameter of the fibers from the mass-transfer properties of the system, described by the wall Sherwood number. The scope of the present work is to compare both methods of describing a hollow fiber supported liquid membrane module, analyzing the influence of the internal mass transfer on the design of such systems. Inmore » the experimental system under consideration, the simultaneous separation-concentration of phenol from aqueous solutions with hollow fiber supported liquid membrane modules is performed. The influence of the flow rate of the inner aqueous phase on the phenol separation rate has been studied.« less

Mass transfer in packed columns: The cylinder model

Abstract: In order to predict mass transfer in packed columns, it is necessary to know the interfacial area. The well-known and often cited equations (Onda, Kolev, Zech, etc.) do not yield precise values for the mass transfer coefficient β L a e or the interfacial area a e , especially for modern packing elements. A new model will be presented which takes into account the structure of packing (cylinder model) as well as the structure of the liquid hold-up. This model allows the separation of liquid flow through packings into its main constituents, i.e. rivulets and drops (freely falling). The determination of the structure of liquid flow allows the prediction of effective interfacial areas as well as of volumetric mass transfer coefficients for irrigated large packing elements manufactured from plastics, metals or ceramic. The applicability to novel future packings is one of the advantages of the new model

Reactive liquid‐liquid test system Zn/Di (2‐ethyl hexyl) phosphoric acid/n‐Dodecane. Equilibrium and kinetics

Abstract: This contribution reviews the models for equilibrium and kinetics of the system Zn/D2EHPA/n-dodecane (and other diluents), published during the past decade, as well as values of the appropriate model parameters, i.e. of equilibrium and reaction rate constants. The reviewed kinetic models comprise those for kinetic, diffusional and mixed mass transfer regimes. It is shown that, under certain conditions, the rate equation within the kinetic regime has the same mathematical structure as the equations for the overall mass transfer rate within the diffusional and mixed mass transfer regimes

Mass transfer and phase holdup characteristics in three-phase fluidized beds

Abstract: The effects of liquid surface tension (42.6 ∼ 72,4 mN/m) and viscosity (1 ∼214mPa • sn), liquid (0.01 ∼0.12m/s) and gas (0.01 ∼0.20m/s) velocities and particle sizes (1 — 8 mm) on phase holdup and mass transfer coefficient ( kLa) have been determined in a 0.142 m-I.D. × 2.0 m-high Plexiglas column. The gas phase holdup increases with liquid velocity, and the rate of increase in gas phase holdup sharply increases with gas velocity in the bed of surfactant solutions. In the beds of 1.0 and 1.7 mm glass beads, the bed contraction occurs whereas in the beds of 2.3 mm glass beads the bed contraction does not occur with an aqueous soltuion of ethanol (σ = 50.4 mN/m). The value of kLa increases with decreasing surface tension (σ ) but it decreases exponentially with increasing liquid viscosity in continuous bubble columns and three-phase fluidized beds. In three-phase fluidized beds with surfactant solutions, kLa increases with gas and liquid velocities and particle size. In three-phase fluidized beds of viscous...

Separation of ethanol from aqueous solutions by supercritical carbon dioxide — Comparison between simulations and experiments

Abstract: In order to evaluate the mass transfer coefficient between liquid and supercritical phases in counter-current flow, we have studied the extraction of ethanol from an aqueous solution by supercritical carbon dioxide, as the equilibrium data are relatively well known. Extensive mass transfer results at a pressure of 10 MPa and a temperature of 313 K are obtained in our pilot plant equipped with a 1.4 m high, 54 mm i.d. column packed with stainless steel BX 64 Sulzer packing. From these results, we calculate the extraction efficiency in terms of height equivalent to a theoretical stage (HETS: 0.5 to 1 m) and the overall mass transfer coefficient (kGa = 4 × 10−3 to 12 × 10−3 sect-1) on the basis of (1) a two-film mass transfer description, and (2) an equation-of-state derived from the Redlich-Kwong-Soave equation proposed by Schwartzenruber and Renon.1

Mass transfer model building in drying

Abstract: The objective of much experimentation in drying is to build or discover a suitable model for a drying system. In this study, an attempt is made to formulate an approach to the problem of iteratively improving models, guided by the inefficiences of an initial tested model. This model involved both moisture diffusion coefficient and external mass transfer coefficient A statistical analysis was applied to the residual populations created by fitting a model to the experimental data, so as to exhibit possible model inadequacies. Inspection of the tendencies produced. suggested what modifications were necessary for improvement. The cycle was repeated until an adequate model was found.