Measurement of tracer concentration

Explain carefully the dispersed plug-flow model for representing departure from ideal plug flow. What are the requirements and limitations of the tracer response technique for determining Dispersion Number from measurements of tracer concentration at only one location in the system Discuss the advantages of using two locations for tracer concentration measurements. 

In situ measurement of the concentration of radioactive tracers in the different phases requires that the phases are separated and arranged according to density difference over the measurement cross section in a horizontal pipe. In general, the measurements are performed with two spectral gamma radiation detectors placed on top and bottom of the pipe respectively. 

Fig. 19-11. Example of tracer concentration measurements along a sampling arc.

FIGURE 10< 108 The procedure to measure the capture efficiency by the tracer gas method, aj The measurement of the reference concentration in the duct, when the tracer is released direcdy into the duct, fb) The measurement of the concentration in the duct, when the tracer is released from the source, / -= sampling point, 2 = pump, J = analyter, 4 - injection of tracer, 5 = tracer gas flow meter, 6 = tracer gas cylinder. 

Schulz, C. and Sick, V., Tracer-LIF diagnostics Quantitative measurement of fuel concentration, temperature and fuel/air ratio in practical combustion systems, Prog. Energy Combust. Sci., 31, 75, 2005. 

The accompanying table gives values of trace height at various times as read from a recorder for a pulse-tracer experiment (Thurier, 1977). In this experiment 1.5 cm3 of N2 was injected into a stream of He flowing steadily at 150 cm 3 s-1 through a stirred-tank reactor of volume 605 cm3. A thermal conductivity detector was used to compare the outlet stream (N2 + He) with the He feed stream, and the output fiom this as a trace on a recorder is a measure of the concentration of N2 in the outlet stream. 

With no tracer initially present, let a step function (in time) of tracer be introduced into the fluid entering a vessel. Then the concentrationtime curve for tracer in the fluid steam leaving the vessel, measured in terms of tracer concentration in the entering stream Co and with time in reduced units, is called the F curve. As shown in Fig. 5, the F curve rises from 0 to 1. 

Packed Beds. Data on liquid systems using a steady point source of tracer and measurement of a concentration profile have been obtained by Bernard and Wilhelm (B6), Jacques and Vermeulen (Jl), Latinen (L4), and Prausnitz (P9). Blackwell (B16) used the method of sampling from an annular region with the use of Eq. (62). Hartman et al. (H6) used a bed of ion-exchange resin through which a solution of one kind of ion flowed and another was steadily injected at a point source. After steady state conditions were attained, the flows were stopped and the total amount of injected ion determined. The radial dispersion coefficients can be determined from this information without having to measure detailed concentration profiles. 

The availability of an on-line radioisotope detector for CE is especially appealing for several reasons. First, state-of-the-art radiation detection technology offers extremely high sensitivity. Second, radioisotope detection affords unrivaled selectivity because only radiolabeled sample components yield a response at the detector. Third, the radiolabeled molecule possesses the same chemical properties as the un-labeled molecule, thereby permitting tracer studies. Fourth, radioisotope detection can be directly calibrated to provide a measurement of absolute concentration of the labeled species. Finally, a capillary electrophoresis system in which radioactivity detection is coupled with more conventional detectors adds extra versatility to this new separation technique. 

The first complete study of nickel metabolism in humans involved ingestion of 10 jug of 62Ni/kg body weight [262]. Isotope dilution (with 61Ni+) was used to measure 62Ni+ tracer concentrations in blood, urine, and feces. Potential complications due to contamination and dietary intake of nickel were eliminated by using the isotope tracer approach. 

Unsteady-state methods are experimentally easier to do, whereby a shot of tracer is added followed by the measurement downstream of the time variation of tracer concentration. This technique was used in bubble columns by Seher and Schumacher... 

Transport and dispersion was evaluated without any form of tuning by comparing a simulation of the ETEX-1 release to the official measurements of surface concentration. To facilitate comparisons with models evaluated during ATMES 11 (Atmospheric Transport Model Evaluation Study) an identical statistical methodology was employed (Mosca et al. 1998). Background values were subtracted so that only the pure tracer concentration was used. Measurements of zero concentration (concentrations below the background level) were included in time series to the extent that they lay between two non-zero measurements or within two before or two after a non-zero measurement. Hereby, spurious correlations between predicted and measured zero-values far away from the plume track are reduced. 

Variability in Absorption Estimates In this study, the occurrence of a negative absorption value for one subject and the absence of a significant vitamin C effect raise some questions about the accuracy of the method However, the expected changes in absorption due to dietary treatments may be masked by the analytical variations associated with absorption measurements and biological variabilities of iron absorption Analytical variations can be introduced at several stages of the analytical procedures incomplete fecal collection, inhomogeneous samples, iron contamination, incomplete colorimetric reaction, non-quantitative recovery after chemical ashing, and variations in isotopic measurements due to ion statistics, memory effects, instrument drift, etc Some of these are not as serious as others, for example, contamination with natural iron woiold not affect the estimate of tracer concentrations provided it occurs before the total iron content is measured ... 

Before granite cores were mounted to the sample holder they were kept in contact with 0.01 mol/L NaC104 solution, which is adjusted to a desired pH, for about one month. Before injecting the uranium tracer solutions, the solutions were adjusted to desired pHs (i.e., pH 5.5, 7.0 and 8.5) using NaOH or HCIO4 solutions. The tracer solution was then pumped to a narrow space between the granite core and the acryl cap at a flow rate of about 1.0 mL/min. All sorption experiments were performed at about 23 2 °C under an ambient condition. The solution samples for the measurement of uranium concentration were taken in akinetic process up to 2 weeks. 

The RTD is determined experimentally by injeeting an inert chemical, molecule, or atom, called a tracer, into the reactor at some time t = 0 and then measuring the tracer concentration, C, in the effluent stream as a function of time. In addition to being a nomeaetive species that is easily detectable, the tracer should have physical properties similar to those of the reacting mixture and be completely soluble in the mixture. It also should not adsorb on the walls or other smfaces in the reaetor. The latter requirements are needed so that the tracer s behavior will honestly refleet that of the material flowing... 

In determining the suitability of a particular reactor model and the parameter values from tracer tests, it is usually not necessary to calculate the RTD function E t). The required information can be acquired directly From measurements of effluent concentration in a tracer test. The theoretical prediction of the particular tracer test in the chosen model system is compared with the tracer measurements from the real reactor. The parameters in the model are chosen so as to obtain the closest possible agreement between the model and experiment. If the agreement is then sufficiently close, the model is deemed reasonable. If not, another model must be chosen. 

A quasi steady-state solution for the tracer distribution in a soilpolutnn has been developed for the inlet boundary concentration being a constant plus a Sinusoidal component. Then an unsteady state solution for tracer distribution a soil column was developed for the same inlet boundary condition as above. The unsteady-state tracer concentration distribution applies to the section of a soil column that still remembers the initial condition. The two solutions may be applicable to those planning experiments to measure parameters such as the dispersion coefficient from tracer tests. A sinusoidal loading of tracer at the inlet boundary may enable one to obtain repeated data traces at the column outlet as part of an extended experiment. Continued collection of tracer concentration vs. time data at the column outlet over a number of periods would enable one to collect data from repeated experiments, for each period of the sine wave would represent another experiment. This should enable one to obtain more replicates of data to improve statistical estimates of the dispersion coefficient than could be obtained by experimental methods that use a slug loading or a step change of concentration at the column inleL"... 

We next examine again the well-stirred reactor introduced in Chapter 4. Consider the following step-response experiment a clear fluid with flovvrate Q/ enters a well-stirred reactor of volume Vr as depicted in Figure 8.3. At time zero we start adding a small flow of a tracer to the feed stream and measure the tracer concentration in the effluent stream. 

Experimental data on exit-age or residence-time distributions most often take the form of discrete values of tracer concentration measured at successive time intervals after introduction of the tracer. Thus, the integrals involved can be replaced by summations in the analysis of actual data. We will illustrate the procedure for the analysis of a pulse-response experiment. Available are tracer concentrations in the effluent, C t) and corresponding times, and from these data we would like to determine the exit-age distribution, or E 0)d6, the distribution in terms of the residencetime variable 6. First determine E t) from C t) versus t by... 

You have designed the following tracer experiment to determine whether your continuous flow reactor for pilot-scale study is perfectly mixed, like an ideal CSTR. At time t = 0, you inject 100 g of an inert tracer into the 100 L reactor after establishing a feed flow rate of 25 L per min. Your technician measures the tracer concentration in the outlet stream for a few minutes and provides you with the following data ... 

Consider the simple two compartment system shown in Fig. 2. We suppose the inflow to compartment 1 is constant and the system is at a steady state. The experiment consists of injecting an impulse of D units of tracer into compartment 1 and measuring the tracer concentration in compartment 1. The solution for total tracer in compartment 1 is given by... 

To determine the RTD experimentally, widely used stimulus-response methods are applied. For this an inert tracer is introduced at the inlet of the reactor. The response of the system to the imposed inlet perturbation is obtained by measuring the tracer concentration at the reactor outlet as function of time. It is important that the tracer does not change the physical properties of the fluid, that is, viscosity and density of the fluid must remain constant. In addition, the tracer should not take part in the reaction process, not be adsorbed at parts of the reactor furthermore, it should be easy to measure even in low concentrations. 

Atpuigg < 0.0 It). The response at the reactor outlet to the pulse-like tracer injection is called the C-curve. The C-curve is experimentally determined by measuring the tracer concentration at the outlet following the inlet pulse. The measured values are referred to the total amount of tracer injected. 

Flow rates of solutions or suspensions are measured with tracers such as Br and Na. For gas flows, radioactive rare gases, Ar, Rr, and Xe are commonly used. Mixing processes are followed by measuring the tracer concentration. In mixing of solid particles, the best tracer is a fraction of the material itself made radioactive by neutron irradiation. 

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