Tracer technique, diffusion

Dispersion in packed tubes with wall effects was part of the CFD study by Magnico (2003), for N — 5.96 and N — 7.8, so the author was able to focus on mass transfer mechanisms near the tube wall. After establishing a steady-state flow, a Lagrangian approach was used in which particles were followed along the trajectories, with molecular diffusion suppressed, to single out the connection between flow and radial mass transport. The results showed the ratio of longitudinal to transverse dispersion coefficients to be smaller than in the literature, which may have been connected to the wall effects. The flow structure near the wall was probed by the tracer technique, and it was observed that there was a boundary layer near the wall of width about Jp/4 (at Ret — 7) in which there was no radial velocity component, so that mass transfer across the layer... 

Dual Tracer Technique. The dual tracer measurement technique utilizes two gas tracers with diffusion coefficients that are substantially different, such as He and SFe. This technique can also be utilized with one volatile (gas) tracer and one nonvolatile tracer. We will derive the relevant equations to determine hquid film coefficient from the diffusion equation for both cases, beginning with the two gas tracers. 

The methods described so far for studying self-diffusion are essentially based on an observation of the diffusion paths, i.e. on the application of Einstein s relation (eq 3). Alternatively, molecular self-diffusion may also be studied on the basis of the Fick s laws by using iso-topically labeled molecules. As in the case of transport diffusion, the diffusivities are determined by comparing the measured curves of tracer exchange between the porous medium and the surroundings with the corresponding theoretical expressions. As a basic assumption of the isotopic tracer technique for studying self-diffusion, the isotopic forms are expected to have... 

A peeling method, in conjunction with a radioactive tracer technique, was used to obtain actual profiles of DOP concentration developed through the matrix of a PVC disc dipped into a limited volume of olive oil. The plasticiser concentration distributions were reproduced to a good approximation using a known model, adjusted by neglecting the (small) amounts of oil which diffused into the polymer. 17 refs. 

Water dynamics is slowed down by the electric field of the cation, as revealed by diffusion coefficient reduced by a factor of two, compared with pure SPC/E water [132]. A reduction of D of water in ionic solutions is also observed experimentally, with values, determined with the tracer technique, ranging from 1.22 10-5 cm /s for Li+ to 0.52 and 0.53 10 5 cm /s for Fe3+ and Al3+, respectively [206]. 

One application of radioactive tracer techniques of considerable importance in solid state structural problems is the emanation method due to Hahn (21). The method has been employed widely and has been developed recently as a sensitive, quantitative method by Gregory (22), who has employed it for the investigation of structural changes and for the extraction of diffusion data. The procedure in outline is as follows A convenient amount of a radioactive element which decays to an isotope of radon (e.g., radiothorium) is ccprecipitated... 

Thus, by measuring oil and water self-diffusion coefficients, it was quite easy to establish whether oil or water or none of them are confined to discrete domains, i.e. to droplets . In the first work on microemulsion structure by self-diffusion [46], using both tracer techniques and NMR spin-echo measurements, it was clearly shown that microemulsions can indeed be bicontinuous over wide ranges of composition, which is manifested by both... 

The diffusion constant and activation energy can be measured directly by radioactive tracer techniques in which the initial distribution of radioactive ions is followed as a function of time and distance. Values of the diffusion constant and activation energies thus determined are then compared with values from ionic conductivities. 

Diffusion measurements fall into two broad classes. Under macroscopic equilibrium, i.e. if the overall concentration within the sample remains constant, molecular diffusion can only be studied by following the diffusion path of the individual molecules ( microscopic measurement by quasielastic neutron scattering (QENS) [48,183,184], nuclear magnetic relaxation and line-shape analysis, PFG NMR) or by introducing differently labelled (but otherwise identical) molecules into the sample and monitoring their equilibration over the sample ( macroscopic measurements by tracer techniques) [185,186]. The process of molecular movement studied under such conditions is called self-diffusion. 

Another reassuring result is the agreement between the D, values found by NMR or tracer techniques or determined at small momentum transfers by QNS on solids or liquids. From this well established macroscopic value, one can hope that preliminary QNS results obtained at higher Q values will be confirmed and extended. Indeed they are expected to lead either to the determination of characteristic jump-lengths and residence times in solids or to the finding of an abnormal proton diffusivity in solutions. This is a domain where QNS can bring a unique and important contribution to the understanding of the conduction mechanism. 

The diffusion of zinc in n- and p-type aluminium antimonide was measured at 660 to 860C by using a radio-tracer technique. At high donor concentrations, the diffusion constant was found to decrease. The diffusion was described by ... 

The dependence of the diffusivity upon the Hg vapour pressure and the temperature was determined by using tracer techniques. It was found that the solubility in Hgo.8Cdo.2Te was governed by neutral atoms, and that the diffusivity was... 

The diffusion of Pb2+ ions in single-crystal KCl was studied at 200 to 475C. A radioactive tracer technique was used which involved diffusion from a deposited... 

The diffusion of carbon in single and polycrystalline ZrC was measured at 1350 to 2150C, using radioactive tracer techniques. Volume and short-circuit enhanced tracer diffusion coefficients were represented by ... 

The O self-diffusion coefficients were determined for polycrystalline microspheres by using an isotope exchange technique with 1 0 as the tracer. The diffusion coefficients for 1099 to 1644C could be described by ... 

Hevesy started as an electrochemist, since the topic of his dissertation was the production of alkali metals by melt electrolysis [12], He published several electrochemical papers on the electrolytic production of metals [13] but also on several other electrochemical topics, e.g., on the electrocapillarity [14], Nevertheless, we do not consider him as an electrochemist since he became famous as a discoverer of a new element, hafnium, and especially as a leading person in the area of radioactivity. He studied the electrochemistry of radioactive elements (Figs. 12.8,12.9, and 12.10) and used the electrochemical techniques successfully also in radiochemistry, ionic diffusion in electrolytes, and metals [15-19]. In 1913, he carried out the first radioactive tracer experiment with Friedrich Adolf Paneth in Vienna [20]. The use of tracer technique opened up new vistas also in electrochemistry. It provided a reliable method to investigate kinetics and equilibrium of electrode processes, e.g., adsorption dissolution, deposition, and underpotential deposition of metals. 

Over the last decades, the application of computational fluid dynamics (CFD) to study the velocity and temperature profiles in packed column has been frequently reported [1-5]. However, for the prediction of concentration profile, the method commonly employed is by guessing an empirical turbulent Schmidt number Sc, or by using experimentally determined turbulent mass diffusivity D, obtained by using the inert tracer technique under the condition of no mass transfer [6, 7]. Nevertheless, the use of such empirical methods of computation, as pointed out in Chap. 3, is unreliable and not always possible. To overcome these drawbacks, the development of rigorous mathematical model is the best choice. 

Fig. 5.7 Comparisons of experimental data, the simulation by two-equation model and by conventional model for Run T17 (dash dot lines one-dimensional model with mass diffusivity obtained by inert tracer technique, solid curve two-equation model, and circle and square experimental data), a CO2 loading, b Enhancement factor (Reprinted from Ref. [8], Copyright 2006, with permission from Elsevier.)...

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