Experimental Tracer Techniques

Tracer techniques have long been employed in an attempt to describe nonideal reactor behavior by quantifying residence time information and data. As noted in an earlier section, some of the elements of a fluid remain in a reactor longer than other elements, with the result that the conversion may vary from element to element depending on its residence time. 

The two most common tracers can be classified as radioactive and colored. The two most common methods of injection are pulse input and step input, with the former favored in practice. Details on the pulse and step method follow. 

TABLE 14.3 Analytical Measurement Equipment for Tracer Experiments 

Gas chromatography Adsorption of a compound on a carrier material (gas phase) 

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All isotopes of element 85, astatine, are intensely radioactive with very short half-lives (p. 795). As a consequence weighable amounts of the element or its compounds cannot be prepared and no bulk properties are known. The chemistry of the element must, of necessity, be studied by tracer techniques on extremely dilute solutions, and this introduces the risk of experimental errors and the consequent possibility of erroneous... 

Methane emission from ruminants can be estimated by using the ERUCT technique (Emissions from Ruminants Using a Calibrated Tracer). The tracer can either be isotopic or nonisotopic. Isotopic tracer techniques generally require simple experimental designs and relatively straightforward calculations [31]. Isotopic methods involve the use of (3H-)CH4 or (14C-)CH4 and ruminally cannulated animals. 

Obtaining Eft), t, and of from experimental tracer data involves determining areas under curves defined continuously or by discrete data. The most sophisticated approach involves die use of E-Z Solve or equivalent software to estimate parameters by nonlinear regression. In this case, standard techniques are required to transform experimental concentration versus time data into Eft) or F(t) data the subsequent parameter estimation is based on nonlinear regression of these data using known expressions for Eft) and F t) (developed in Section 19.4). In the least sophisticated approach, discrete data, generated directly from experiment or obtained from a continuous response curve, are... 

The vast majority of studies assessing the quantity of extracellularly released organic compounds from primary producers have relied on different experimental protocols based on the 14C-tracer technique developed to measure photosynthesis in marine waters (Steeman-Nielsen, 1952). Using this technique, the extracellular DOM release by phytoplankton has been studied in both pure cultures and natural water samples. From pure culture... 

A tracer technique was employed in an effort to gain further information about the reaction mechanism (10). One alpha hydrogen atom of n-octyl alcohol was replaced with an atom of deuterium. The steps of the two reaction mechanisms under discussion are presented in Fig. 2. If the exchange between hydrogen and deuterium is small, the presence of significant quantities of deuterium in the gaseous product may be explained only by mechanism (1). The exact experimental details of this investigation will now be discussed. 

Fig. 9.12 Experimental verification of the RTD function in extruder by radioactive tracer techniques with a 44.2-mm-diameter, 24 1 L/D extruder, liquid polyester resin, and a radioactive manganese dioxide tracer Asterisk, Experiment 1 , Experiment 2 smooth curve indicates theoretical prediction. [Reprinted by permission from D. Wolf and D. H. White, Experimental Study of the Residence Time Distribution in Plasticating Screw Extruders, AIChE J., 22, 122-131 (1976).]...

Gontier, G., Gerino, M., Stora, G, and Melquiond, J. (1991) A new tracer technique for in situ experimental study of bioturbation processes. In Radionuclide in the Study of Marine Processes (Kershaw, P.J., and Woodhead, D.S., eds.), pp. 198-196, Elsevier Science, New York. 

Experimental studies of aging usually employ radioactive-tracer techniques for observation of exchange of ions between precipitate and solution, the adsorption of suitable dyes on the surface of a precipitate to measure the extent of the specific surface, microscopic or x-ray observation of the precipitate, or a combination of these. 

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]. 

Graybiel AM, Hartwieg EA (1974) Some afferent connections of the oculomotor complex in the cat an experimental study with tracer techniques. Brain Res., 81, 543-551. 

It is obvious that tritium-labeled polar impurities were formed in the stored solvent. On the basis of these results it was decided to conduct all subsequent adsorption experiments with solutions prepared not more than 2 days before. The authors are aware that experimenters in this field appreciate the general requirement for solvent purity in adsorption experiments. However, it was thought that these data should be presented to emphasize the special importance of solvent purity when studying coadsorption of solvent and solute by tracer techniques and to point out the relatively short time required for impurities to develop. 

With the TEX-PEP technique experiments on the diffusion and adsorption of mixture of n-hexane/2-methylpentane in large silicalite-1 crystals have been performed. By modeling the experimental tracer exchange curves values of intracrystalline diffusion coefficient and adsorption constant were obtained. Slight preference for the adsorption of /t-hexane was found. Diffusivity of -hexane sharply decreases with increasing fraction of its isomer, since the last one occupies channel intersections thus blocking zeolite network. 

One of the earliest applications of radioactive tracer techniques to surface investigations is that of Paneth and Vorwerk/ who used the Pb isotope, thorium B, to determine the surface area of powdered PbS04 by exchange between the surface Pb ions of the solid and the radioactive Pb in solution. Since then a vast number of studies of catalytic systems involving the use of radioisotopes have been reported. As the earlier literature has been extensively reviewed by Campbell and Thomson, the present Report is mainly concerned with the literature published between 1970 and mid-1982, particular emphasis being placed on new developments in experimental methods. 

It is not possible to relate quantitatively the mixing times obtained using tracer techniques with the mixing times obtained using visual observation. Indeed, it is usually not possible to compare even the mixing times obtained in different experimental studies where the same technique has been used, because the necessary background information, e.g. size of concentration probe, is often not available. 

Any charge transport by protons must directly correspond to an adequate transport of protonic mass. A straightforward way to determine the true proton conductivity, therefore, is to record directly the proton selfdiffusion coefficient. The traditional way to do this is through tracer techniques. They may be very accurate but require difficult experimental work and measurement periods of the order of days or weeks for a single component and temperature. A further disadvantage is the inherent system perturbation by isotope substitution, which is particularly critical for the substitutions H/ H and... 

Hence the axial dispersion is quantified by fitting the above equations to experimental tracer concentration-time profile. Some of the experimental techniques applied to characterize an MSR are listed in Table 15.2. The following points should be borne in mind in coimection with the RTD for an MSR ... 

The above statement that the radiations from tracers are not affected by the chemical state of the sample means that the radiation carries no chemical information on the tracer. All that the radiations inform about is the location, distribution, and amount of the tracer. Another disadvantage of the radioactive tracer technique is that it requires an experimental facility with the license for the use of a nuclide in the necessary amount. 

A comprehensive listing of various tracers and experimental tracer methods is provided by Wen and Fan [1975], including flow visualization techniques. 

In view of the drawbacks of foregoing models in applying Sct or using experimental correlation obtained by the inert tracer technique, some dependable models have been recently developed to overcome such insufficiency as shown in subsequent sections. 

The volume average of Du calculated is compared with experimental data reported by Cai and Chen [15] for the same tray column under different vapor rates (F factor) as shown in Fig. 4.9. Although the experimental measurement is performed by using inert tracer technique and the comparison is only approximate, it demonstrates that the prediction of Du is feasible by using the method of CMT without doing tedious experimental work. 

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