Transfer Diffusion Coefficient Research Articles

Studies on the Cd(II) removal from water by adsorption

The feasibility of using hematite for the removal of Cd(II) from aqueous solutions was investigated employing an adsorption technique. The adsorption was found to depend on Cd(II) concentration, pH, particle size of hematite, agitation rate and temperature of the system. The maximum removal was found to be 98% for the cadmium concentration of 44.88 μmol l −1 at a temperature of 20°C and pH 9.2 with 40 g l −1 of hematite of particle size <200 μm with agitation 125 rpm rate after 2 h. The applicability of the Langmuir isotherm was tested for the Cd(II)-hematite system at optimum conditions. The process follows first order kinetics. The mass transfer coefficient, diffusion coefficient and thermodynamic parameters have been determined to elucidate the mechanism of uptake. The uptake of cadmium is partially diffusion controlled and partially due to an electrostatic effect along with specific adsorption involving Cd ++ and CdOH + which seem to play an effective role. The data has been subjected to multiple regression analysis and a computer model has been developed to predict the removal of Cd(II) from water under certain boundary conditions. The present technique has been found to be cost-effective.

Multigrid anisotropic diffusion

A multigrid anisotropic diffusion algorithm for image processing is presented. The multigrid implementation provides an efficient hierarchical relaxation method that facilitates the application of anisotropic diffusion to time-critical processes. Through a multigrid V-cycle, the anisotropic diffusion equations are successively transferred to coarser grids and used in a coarse-to-fine error correction scheme. When a coarse grid with a trivial solution is reached, the coarse grid estimates of the residual error can be propagated to the original grid and used to refine the solution. The main benefits of the multigrid approach are rapid intraregion smoothing and reduction of artifacts due to the elimination of low-frequency error. The theory of multigrid anisotropic diffusion is developed. Then, the intergrid transfer functions, relaxation techniques, diffusion coefficients, and boundary conditions are discussed. The analysis includes the examination of the storage requirements, the computational cost, and the solution quality. Finally, experimental results are reported that demonstrate the effectiveness of the multigrid approach.

Supercritical carbon dioxide extraction of essential oils: Modeling and simulation

Extraction of essential oils using supercritical CO 2 was studied. A mathematical two-phase model was developed to simulate extraction yield at different operating conditions. Unsteady-state mass balance for solute in solid and in supercritical phases led to two partial differential equations that were solved numerically using a linear equilibrium relationship. The model has three parameters including mass transfer, axial dispersion and intraparticle diffusion coefficients. The last one was used as the model tuning parameter and the others were predicted applying existing experimental correlations. The model is able to show the influence of different process parameters such as particle size and flow rate on the essential oil recovery.

A better understanding of the properties of alginate solutions and gels by quantitative magnetic resonance imaging (MRI)

A new, fully automated magnetic resonance imaging (MRI) procedure has been used to measure for the first time all the MRI parameters of water in both sodium alginate solutions and calcium alginate gels at concentrations of 1%, 2%, 3%, 4% (w/w). Spin–spin ( T 2) and spin–lattice ( T 1) relaxation times, magnetisation transfer processes (MT) and diffusion coefficients ( D) were measured to evaluate their potential use for studying gelling mechanisms and for determining gel characteristics. Four alginate samples, with different extents of O-acetylation were also studied and the use of MRI to discriminate between them was evaluated.

Mass transfer model for osmotic dehydration of fruits and vegetables—I. Development of the simulation model

A mathematical model based on mass transfer in plant tissues was developed with the aim of finding a simple predictive model which was easy to use and yet had a broad application scope in the osmotic dehydration of foods. The model takes into account the capacity of each constituent to diffuse within tissue by using a diffusion coefficient and a transmembrane mass transfer coefficient. The model also depends on the mass ratio of osmotic solution to product, on the initial chemical composition of the product and solution, and on the product shape. The application of the model to apple dehydration with polyethyleneglycol (PEG) solutions allowed us to quantitatively simulate the time evolution of cellular and extracellular volumes which was observed previously in histological sections under the microscope. By using experimental data obtained during potato dehydration in mixed solutions of sucrose and salt, mass transfer and apparent diffusion coefficients were fitted in order to evaluate product behavior under different process conditions. A good quantitative agreement was obtained between the experimental and calculated results. The model presented in this paper allows the main variables of osmotic dehydration to be estimated using a low number of fitting parameters, and interprets, through a simple algorithm, the complex phenomena of the main mass transfer mechanisms in plant tissues.

Successive electron-transfers in low ionic strength solutions. Migrational flux coupling by homogeneous electron transfer reactions

Analysis of successive heterogeneous electron transfer reactions requires consideration of the coupling of molecular fluxes by homogeneous electron transfer reactions. We demonstrate that bimolecular electron transfer between members of the redox system that differ by +2 in oxidation state (i.e., reproportionation) can strongly influence the magnitude of transport-limited currents when ion migration contributes to molecular transport. A theoretical description of the problem is developed in the limit of infinitely fast homogeneous electron transfer and identical diffusion coefficients. An analytic solution is presented that allows calculation of voltammetric limiting currents at microelectrodes for any stepwise n-electron transfer as a function of the ratio of redox and supporting electrolyte concentrations. The results demonstrate that migrational flux coupling of homogeneous electron transfer reactions must be included in the reaction mechanism to obtain even qualitatively accurate predictions of the current. The theory is found to be in good agreement with values of voltammetric currents measured at an inlaid 11.2-μm radius Pt microdisk for the two- and three-electron reductions of Ru(bpy)32+ in acetonitrile+toluene solutions.

Color impedance and electrochemical impedance studies of WO3 thin films: H+ and Li+ transport

Abstract Ac potential-modulated optical transmittance (color impedance) and electrochemical impedance measurements were employed in an in situ investigation of the electrochromic behavior of evaporated amorphous tungsten trioxide (a-WO3) films in non-aqueous (LiClO4 + propylene carbonate), aqueous acid (H2SO4) and mixed non-aqueous-aqueous acid (PC + HClO4) electrolytes. The optical and electrochemical responses were essentially the same with the 0.8 μm thick films that were examined, indicating that all of the injected charge was involved in the faradaic process. The behavior of the WO3 films in LiClO4 + PC and H2SO4 electrolytes was relatively simple compared to that of the PC + HClO4 electrolyte, in which the charge transfer resistances and diffusion coefficients were found to vary considerably with potential, probably due to a shift from proton insertion at more positive potentials to lithium ion insertion at more negative potentials. In all of the electrolytes, evidence was seen for two types of diffusion processes, a slower one, most likely due to diffusion within small clusters of WO6 octahedra, and a faster one, due to diffusion in nanometer-scale spaces between these clusters.

A study on the development of the high efficiency condensing heat exchanger

A computer simulation program of a high efficiency condensing heat exchanger is developed. The flue gas flows outside bare tube bundles both in strong cross flow and in weak counter flow and the cooling water inside the tubes. Condensing heat exchangers achieve high efficiency by reducing flue-gas temperatures to a level at which most of the water vapor in the flue gas is condensed and the latent heat associated with phase change of the water is recovered. The computer model has been verified by comparison with measured data. To verify the model, heat transfer coefficient was adjusted, along with the mass transfer diffusion coefficient and pressure drop coefficient, to achieve agreement between predicted and measured data. The efficiencies of heat exchanger increase 2.3 ~ 8.1% by condensations of 6.3 ~ 62.6% of the water vapor in the flue gas.

Effect of geometry on the effective moisture transfer diffusion coefficient

The effective moisture transfer diffusion coefficient ( D eff ) of ready-to-eat breakfast cereals (RTE) is determined either from an individual particle or a bulk of material in the literature. The D eff is assumed to be the same in both cases which can be dependent on the material thickness. In this study, the effect of bed depth of a bulk of two types of RTE cereal on D eff was investigated at several bed depths at 75% RH. Both linear and nonlinear solutions of Fick's 2nd Law were applied to determine D eff from the weight vs time data. The magnitude of D eff increased with an increase in bed depth. Since the captured air between the particles in the bulk volume contributed significantly to the moisture transfer in the system, the overall D eff of the system increased as the total volume of the captured air increased at higher bed depths. The relation was shown to have a square of the thickness dependence.

Investigation of the Redox Couple Benzophenone/Benzophenone Anion Radical in Acetonitrile and N,N‐Dimethylformamide by Electrochemical and Spectroelectrochemical Methods

AbstractDiffusion layer imaging (DLI), time‐resolved spectroelectrochemical methods and electrochemical impedance spectroscopy (EIS) were applied to determine the influence of the solvent and the supporting electrolyte on reaction parameters for the benzophenone/benzophenone radical anion redox couple (BP/BP*−) formed at a Hg‐covered Au wire in aprotic solvents.Results are reported for the solvents acetonitrile (AN) and N,N‐dimethylformamide (DMF) using two different tetraalkylammonium salts as supporting electrolytes. Heterogeneous rate constants for the electron transfer reaction and diffusion coefficients of the neutral BP and the radical anion BP*− obtained with the different methods are discussed and compared.

Diffusion of cell-associated water in ripening barley seeds.

Diffusion rate and restricted diffusion of cell-associated water in ripening barley seeds were examined by NMR microscopy using the pulse gradient spin-echo and the pulse gradient stimulated-echo methods. Changes in the mobility of cell-associated water and properties of cell membranes during ontogeny seed were assessed. Diffusion coefficients for bulk water transfer were high (greater than 0.9 x 10(-5) cm2/s) throughout the growth stages. The highest diffusion coefficient observed was comparable to the self-diffusion coefficient of pure water. Water compartment sizes and the permeability of the cell membranes in the seed were determined by Meerwall and Ferguson's modification of the model of Tanner or by the method of Callaghan et al. The endosperm consisted of large cells and vascular bundle small cells with permeable membranes.

Estimation of the mass transfer coefficients and diffusion coefficients of nickel and vanadium in Kuwait crude oils: Prediction of long term leaching under simple experimental conditions

Abstract One of the major concerns following the Iraqi invasion is the possibility of ground water contamination caused due to massive oil spillage. Nickel and vanadium are the major metals which exist in trace amounts in crude oils. Leaching of nickel and vanadium was studied on a laboratory scale using four different crude oils, Kuwait export (KE), Burgan (B), Ratawi (R) and Weathered (W). Nitric, sulphuric and acetic acids (pH 4 and 6) were the leaching medium. The rate of leaching was found to be significantly dependent on the type of crude and the pH value of the leaching medium. The values for the mass transfer coefficient of nickel and vanadium are in the range 0.3 ‐ 3.18×10‐3 cm3min‐1 and 0.04 ‐ 6.4×10‐3 cm3 min‐1. The diffusion coefficient values for nickel and vanadium are in the range 0.17–22.2×10‐9 cm2 day‐1 and 0.06–45.4×10‐7 cm2 day‐1. An overall study of these values suggest that nitric acid is the most effective leachant for both metals in the four crude oils. Time for 10% leaching of nick...

Electrochemical behavior of formetanate and chlordimeform pesticides

AbstractThe electrochemical behavior of formetanate and chlordimeform is studied by using dc polarography, cyclic voltammetry, ac polarography, and differential pulse polarography (DPP) in universal buffers of pH 2.0 to 12.0, including 1 M HCIO4 and 1 M HCl. The number of electrons is determined by millicoulometry. Kinetic parameters such as transfer coefficients, diffusion coefficients, and heterogeneous forward rate constant values are evaluated using these techniques. The results obtained suggest the nature and mechanism of the electrode process. Differential pulse polarography is used for the estimation of formetanate in formulation.

Studies on the electrode kinetics of polypyrrole in aqueous solution by a.c. impedance measurement

Electrode kinetics of PPy(Cl −) film in acidic and neutral solutions has been studied by the a.c. impedance method. The values of the charge transfer resistance and diffusion coefficient of counter-ions in the PPy film depend on the pH value of the solution in contact and on the potential applied. The exchange current density i o of the PPy film electrode is 1.15 mA cm −2 and the apparent diffusion coefficient D app of the counter-anion is 1.8 × 10 −9 cm 2s −1, in a pH 3, 0.5 M KCl solution at 0.1 V(SCE). Both i o and DP app decrease with decrease of applied potential. In a neutral solution, two redox processes can be distinguished in the a.c. impedance plots.

Reactive Deposition of Cobalt Electrodes: VII . Mechanistic Study in the Presence of Dissolved Oxygen—Calculation of Kinetic Parameters

The effect of the colloid layer on the mechanism and kinetics of cobalt deposition in the presence of dissolved oxygen has been quantitatively investigated. In the presence of oxygen, the reduction of Co2+ ions is a single two‐electron process, and the deposition of cobalt is through a three‐dimensional progressive nucleation and crystal growth mechanism under the diffusion/interfacial electron transfer control, as in the case of absence of oxygen. However, in the presence of oxygen, the kinetics of cobalt deposition are greatly influenced by the dynamic states, i.e., the forming and breaking processes of the colloid layer at the electrode surface, and are related to the reduction of Co2 ions in the presence and absence of a colloid layer. The porous structure of the cobalt electrode formed in the presence of oxygen is directly dependent on the characteristics of localized nucleation and limited crystal growth processes. In the presence of the colloid layer at the electrode surface, the exchange current density and Tafel slope for the reduction of Co2+ions were found to be and 95 mV/decade, respectively. When the inhibiting effect of the colloid layer is eliminated, of and Tafel slope of 30 mV/decade were obtained, which are very close to the values found in the absence of oxygen reported in literature. The electron transfer rate and diffusion coefficient of Co2+ ions are both decreased due to the existence of the colloid layer in the vicinity of the electrode surface.

Membrane extraction of silver by di(2-ethylhexyl) dithiophosphoric acid

A membrane-based process for silver recovery from waste solutions was derived. Silver diffused from the feed solution through the supported liquid membrane containing di(2-ethylhexyl)dithiophosphoric acid (DTPA) as a carrier into the receiver solution, which contain thiourea at a low pH. Silver transfer rate and diffusion coefficient in the membrane were found to be up to 1.5 × 10 -9 mol/cm 2-sec and 2 × 10 -6 cm 2/sec, respectively. The membranes used demonstrated an ability to work in contact with waste solutions from silver reclamation eluants until silver had been transferred even against a very high concentration gradient. The dependencies of silver transfer rate on such factors as pH in the receiver and feed phases, DTPA and thiourea concentrations, were investigated.

Laserspektroskopische Bestimmung thermophysikalischer Eigenschaften transparenter Fluide

AbstractDetermination of thermophysical properties of transparent fluids by laser spectroscopy. This review deals with the determination of thermophysical properties of transparent fluids by laser‐spectrometric methods. On the basis of the phenomena of laser‐Rayleigh and laser‐Brillouin scattering the paper demonstrates that the transfer coefficients thermal conductivity, diffusion coefficient, and viscosity, as well as the velocity of sound, sound attenuation, specific heats, and compressibility are fundamentally derivable at thermodynamic equilibrium over a wide range of temperature and pressure. Typical measurements show that the methods used for measuring some of the quantities have already reached standards maturity while others are still in their initial stages.

Electrochemical behavior of nitrofurantoin and assay of its formulations

AbstractThe electrochemical behavior of nitrofurantoin was studied by employing do polarography, cyclic voltammetry, and differential pulse polarography in universal buffers of pH 2.0–12.0. Kinetic parameters such as transfer coefficients, diffusion coefficients, and heterogeneous forward rate constant values were evaluated using these techniques. Differential pulse polarography was employed for the estimation of this drug in several pharmaceutical formulations.

Voltammetric behavior and measurement of nimorazole

AbstractThe electrochemical reduction behavior of nimorazole has been studied by employing dc polarography, differential pulse polarography, cyclic voltammetry, and chronoamperometry in the supporting electrolytes of pH ranging from 2.00 to 12.50. Parameters such as transfer coefficients, diffusion coefficients, and heterogeneous forward rate constant values are evaluated using these techniques. The analytical measurement of the drug is also discussed.

The protection of iron against corrosion with polymer films prepared by cathodic polymerization of halogenated xylenes

Polymer films were prepared by cathodic polymerization of halogenated xylenes at an iron electrode in a non-aqueous electrolyte solution. The ability of the films to protect against corrosion was examined in an aerated 0.5 M Na2SO4 solution by polarization measurements of an iron rotating disk electrode uncoated and coated with the polymers. The best protection of the electrode was obtained with a polymer film prepared by cyclic sweeps of potential in an oxygen-saturated acetonitrile solution of α,α′-dibromo-p-xylene. Mechanisms of the corrosion protection were investigated on the basis of the Koutecky-Levich equation. Diffusion of oxygen in the aqueous bulk solution was not a rate-determining step in the corrosion process of the iron electrode coated with the polymer film. Rate constants of anodic and cathodic charge transfer reactions and diffusion coefficients of oxygen and ferrous ion through the film, obtained by computer simulation of the polarization curves, provide evidence for the protective mechanisms.