Tracer injection methods

Liquid-phase mixing in three-phase fluidized beds can be described using the dispersion model. A two-dimensional model considers both radial and axial dispersions. Both axial and radial dispersion coefficients are strong functions of operating conditions such as liquid and gas velocities and properties of liquid and solid phases. Evaluations of liquid-phase dispersion coefficients are based on a tracer injection method and subsequent analysis of the mean and the variance of the system response curves. 

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. 

G. L. Stegemeier and G. E. Perry. Method utilizing spot tracer injection and production induced transport for measurement of residual oil saturation. Patent US 5168927, 1992. 

When velocity profiles are present across the cross-sections of the tracer injection and sampling points, then great care must be taken over the appropriate tracer injection and collection methods if the true system RTD is to be recovered (see, for instance, ref. 4 and also refs. 12 and 26). Our attention is, for the moment, restricted to situations where flat velocity profiles are present. 

The second method is an indirect method, based on the liquid s average residence time evaluated with the tracer injection technique. From the first moment of the RTD curve the total external liquid hold-up can be calculated. 

The above example demonstrates that treatment of the basic data by different numerical methods can produce distinctly different results. The discrepancy between the results in this case is, in part, due to the inadequacy of the data provided the data points are too few in number and their precision is poor. A lesson to be drawn from this example is that tracer experiments set up with the intention of measuring dispersion coefficients accurately need to be very carefully designed. As an alternative to the pulse injection method considered here, it is possible to introduce the tracer as a continuous sinusoidal concentration wave (Fig. 2.2c), the amplitude and frequency of which can be adjusted. Also there is a variety of different ways of numerically treating the data from either pulse or sinusoidal injection so that more weight is given to the most accurate and reliable of the data points. There has been extensive research to determine the best experimental method to adopt in particular circumstances 7 " . 

For many cases in which the RTD cannot be calculated theoretically, experimental techniques have been developed to measure it. Such techniques are used by introducing a tracer material into the system and recording its concentration at the exit.9 These methods are discussed in great detail in the literature. In general, a step change in tracer concentration results directly in the F(t) function, and an impulse type of tracer injection results directly in the/(f) function. 

Vergnes107 has outlined a simple method for the determination of the Peclet number when the tracer injection is sharp enough to be considered as a true Dirac 6 impulse and the response equation is described by the solution of the dispersion model for a system open at both ends. The method does not require knowledge of the tracer quantity added, the sensitivity of the recorder, or the chart speed. 

A variation of the indicator dilution method (see preceding problem) is used to measure total blood volume. A known amount of a tracer is injected into the bloodstream and disperses uniformly throughout the circulatory system. A blood sample is then withdrawn, the tracer concentration in the sample is measured, and the measured concentration [which equals (tracer injected)/(total blood volume) if no tracer is lost through blood vessel walls] is used to determine the total blood volume. 

Tracer Impuls Method In this method a certain amount of tracer is injected (very short time interval) into the system and the concentration is measured at the oudet of the column. The concentration followed in time can directly be converted to the E i) curve as described above. 

Figure 3.38 Typical chromatograms obtained in the determination of equilibrium isotherms by pulse chromatographic methods, (a) Injection on a concentration plateau, and response of a selective detector for a tracer, (b) Response of a nonselective detector for the tracer injection made in (a), (c) Individual profiles of the labeled tracer (gray line) and the unlabeled component (black line).

Figure 4.27 Illustration of the binary step and pulse method. Plateau concentrations 45mg/mL for component 1, 50 mg/mL for component 2. Amoimt of tracer injected 0.3 mg fp = 0.143 min mL/min. L = 25cm N= 2500 fcgi = 6.17 /Cq2 = 12.3 bi = 0.0267 62 = 0.05 to = 60 s. (a) Elution of an isotopic tracer of component 1 in a mobile phase containing components 1 and 2 chromatograms shown by detectors selective for components 1, 2, and the tracer, (b) Same as (a), but chromatograms shown by a nonselective detector (Total) and by a detector selective for 1, but having the same response for 1 and its isotopic tracer, (c) Same as (a), but with injection of an isotopic tracer of component 2. (d) Same as (b), but with injection of an isotopic tracer of 2. (e) Selective chromatograms obtained upon injection of a mixture of isotopic tracers of 1 and 2. (f) Chromatograms obtained with a nonselective detector (Total), and with detectors selective for 1 or 2.

The method of dilution, with its two variants integration method and injection method with constant flow. Here an appropriate radiotracer is injected in the process, and samples are collected in a point enough downstream to assure a complete mixture. The injection of the tracer and its concentration, both in the injected solution and in the diluted samples, are measured. From these data, the flow can be easily calculated, a. Integration Method the tracer (activity A) is injected during a short interval of time. In a section, situated downstream, we measure in a point the function C(t) representing its passage the flow is given by ... 

Injection method with constant flow the tracer is injected in a section at constant flow, q, and at a concentration Cq. In a section situated downstream we determine, after reaching a constant regime of concentration, the concentration C of the tracer. If Q is the flow to be measured, an equation of balance of the tracer gives ... 

Figure 3 Example of spray column with measuring points MS 1 to 6 Separate sampling of both phases for determination of concentration profiles, ES-OS ports for hold-up measurement by hydrostatic method, tracer injection, FA,FB windows for photographic determination of drop size.

Figure 7 Determination of backmixing (eddy diffusivity) by steady state method (A), showing devices for tracer injection, sampling electrodes and arrangement of sampling points. Unsteady state arrangement is shown in B.

Besides the dispersion model, the tanks in series model is the other onedimensional model widely used to represent non-ideal flow. Here the fluid is thought to flow through a series of equal-size ideal stirred tanks, and the parameter in this model is the number of tanks in the cascade (iV q). The RTD curves and moments of this model are easy to obtain, since problems of proper boundary conditions and method of tracer injection and measurement do not intrude. need not be an integer for curve-fitting purposes. It is strictly empirical, and no theoretical justification, such as Taylor diffusion, or theoretical estimates of the model parameter, are generally possible. This model starts from the mass balance equation for a series of i stirred vessels with 1 < i < iV, and AT, the number of vessels in the series (or the number of equivalent stages,... 

A non-intrusive method for RTD characterization has been claimed [17], which has been proven already for visualization of velocity fields in microchannels [18]. Aphoto-activated fluorescent dye dissolved in an aqueous solution is introduced continuously into a flow. A defined section of the inlet channel is exposed to a UV pulse to activate the tracer, which turns fluorescent Due to this inside start of the pulse experiment, artifacts from peripheral equipment can be eliminated. The method generates almost ideal input signals, which simplifies the numerical treatment of experimental data. The new approach was found to be superior to various traditional injection methods. The ideal shape of the stimulus signal was demonstrated for an analytically well-defined straight channel and compared with a signal derived from deconvolution of non-ideal input signals [19]. 

Typically, there are two ways to inject tracers, steady tracer injection and unsteady tracer injection. It has been verified that both methods lead to the same results (Deckwer et al., 1974). For the steady injection method, a tracer is injected at the exit or some other convenient point, and the axial concentration profile is measured upward of the liquid bulk flow. The dispersion coefficients are then evaluated from this profile. With the unsteady injection method, a variable flow of tracer is injected, usually at the contactor inlet, and samples are normally taken at the exit. Electrolyte, dye, and heat are normally applied as the tracer for both methods, and each of them yields identical dispersion coefficients. Based on the assumptions that the velocities and holdups of individual phases are uniform in the radial and axial directions, and the axial and radial dispersion coefficients, E and E, are constant throughout the fluidized bed, the two-dimensional unsteady-state dispersion model is expressed by... 

In the step injection method, the reactor receives a step input of a fixed and constant tracer concentration for a reactor with a constant volumetric flow rate. The concentration of the tracer is kept constant until the concentration at the outlet is equal to that at the feed the rest may then be discontinued. A typical concentration-time curve for this type of injection is presented in Figure 14.14. An analytical description of the outlet concentration-time relationship is usually difficult to represent. However this injection method is usually easier to carry out. 

The method is based on the international standard ISO 4053/IV. A small amount of the radioactive tracer is injected instantaneously into the flare gas flow through e.g. a valve, representing the only physical interference with the process. Radiation detectors are mounted outside the pipe and the variation of tracer concentration with time is recorded as the tracer moves with the gas stream and passes by the detectors. A control, supply and data registration unit including PC is used for on site data treatment... 

The method covers the full dynamic range of linear velocities from a few centimetres/second to over 100 meters/seconds with one and the same instrumental set-up. Only the amount of tracer used per injection is varied. 

For this kind of case, a modification of the dilution method is being developed. Instead of using an external fixed-geometry measurement chamber, a suitable part of the process, e.g. a stretch of pipe, is used. A radiation detector is mounted on the outside of the pipe, and a tracer emitting sufficiently hard gamma radiation is used. As sufficient mixing can be achieved by injecting upstream the separator the radiation level found will be strictly proportional to the concentration and thus inversely proportional to the true flow rate. 

The method implies injection of a mixture of 3 radioactive tracers each being distributed into one of the 3 phases. The tracers must show such differences in the emitting y-radiation energy spectra that they can be simultaneously detected by on line y-spectrometry. Candidate tracers are Br-82 as bromobenzene for oil, Na-24 or La-140 for water, and Kr-85 for gas. The tracers are injected simultaneously at a constant rate into the flow in the pressurised pipe, and the concentration is detected as series of instantaneous measurements taken downstream as illustrated in figure 2. 

After having proved the principles a dynamic test facility has been constructed. In this facility it is possible to inject 3 tracers in a flownng liquid consisting of air, oil and water. By changing the relative amounts of the different components it is possible to explore the phase diagram and asses the limits for the measurement principle. Experiments have confirmed the accuracy in parameter estimation to be below 10%, which is considered quite satisfactorily for practical applications. The method will be tested on site at an offshore installation this summer. 

The technique just described requires the porous medium to be sealed in a cell, so It cannot be used with pellets of irregular shape or granular material. For such materials an alternative technique Introduced by Eberly [64] is attractive. In Eberly s method the porous pellets or granules are packed into a tube through which the carrier gas flows steadily. A sharp pulse of tracer gas is then injected at the entry to the tube, and Its transit time through the tube and spreading at the exit are observed. A "chromatographic" system of this sort is very attractive to the experimenter,... 

Tracer Type. A discrete quantity of a foreign substance is injected momentarily into the flow stream and the time interval for this substance to reach a detection point, or pass between detection points, is measured. From this time, the average velocity can be computed. Among the tracers that have historically been used are salt, anhydrous ammonia, nitrous oxide, dyes, and radioactive isotopes. The most common appHcation area for tracer methods is in gas pipelines where tracers are used to check existing metered sections and to spot-check unmetered sections. 

Tracer methods involving chemical markers injected into the contents of the tank may be used. Instmmentation capable of picking up the chemical marker can then determine the presence of a leak caused by seepage of the tracer into the ground. This, like the hydrocarbon sensing method, is genericaUy referred to as soil vapor monitoring. This method suffers the same weaknesses that have to do with undertank soil permeabUities. 

The time that a molecule spends in a reactive system will affect its probability of reacting and the measurement, interpretation, and modeling of residence time distributions are important aspects of chemical reaction engineering. Part of the inspiration for residence time theory came from the black box analysis techniques used by electrical engineers to study circuits. These are stimulus-response or input-output methods where a system is disturbed and its response to the disturbance is measured. The measured response, when properly interpreted, is used to predict the response of the system to other inputs. For residence time measurements, an inert tracer is injected at the inlet to the reactor, and the tracer concentration is measured at the outlet. The injection is carried out in a standardized way to allow easy interpretation of the results, which can then be used to make predictions. Predictions include the dynamic response of the system to arbitrary tracer inputs. More important, however, are the predictions of the steady-state yield of reactions in continuous-flow systems. All this can be done without opening the black box. 

To establish the well drainage boundaries and fluid flow patterns within the TFSA-waterflood pilot, an interwell chemical tracer study was conducted. Sodium thiocyanate was selected as the tracer on the basis of its low adsorption characteristics on reservoir rocks (36-38), its low and constant background concentration (0.9 mg/kg) in produced fluids and its ease and accuracy of analysis(39). On July 8, 1986, 500 lb (227 kg) of sodium thiocyanate dissolved in 500 gal (1.89 m3> of injection brine (76700 mg/kg of thiocyanate ion) were injected into Well TU-120. For the next five months, samples of produced fluids were obtained three times per week from each production well. The thiocyanate concentration in the produced brine samples were analyzed in duplicate by the standard ferric nitrate method(39) and in all cases, the precision of the thiocyanate determinations were within 0.3 mg/kg. The concentration of the ion in the produced brine returned to background levels when the sampling and analysis was concluded. 

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