Tracer response techniques

2 DIAGNOSING AND CHARACTERIZING NONIDEAL FLOW 10.2.1 Tracer Response Techniques 

Suppose we were observing a vessel with fluid flowing through it at steady state. Suppose further that there was no change in density as the fluid flowed through the vessel. How would you go about trying to answer the question How long does each molecule of fluid spend in the reactor What experiment(s) could be carried out to answer the question  

To begin, you might recognize that every molecule may not spend exactly the same time in the vessel. If there were mixing in the direction of flow, some molecules that entered the vessel at a time, r = 0, might catch up to molecules that entered at an earlier time, say t = —8. Similarly, some of the molecules that entered at r = 0 might he overtaken by ones that entered at a later time, say r = In general, individual molecules will spend different amounts of time in the vessel. 

Suppose we inject a small amount of a matoial into the inlet stream, right at the boundary of the vessel. The material behaves exactly like the fluid that is flowing through the vessel, and is called a tracer. The tracer must be chosen so that its concentration in the efQu t from the vessel can he measured by a detector located right at the point that the fluid leaves the vessel. 

The use of a tracer to study the flow of a fluid through a vessel is called a tracer response technique. A known amount of tracer is injected in a known pattern (such as an instantaneous pulse), and the response of the tracer to the flow conditions that exist in the vessel is measured. The use of tracer response techniques is common in medicine, as well as in chemical engineering. 

---

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. 

The best way to characterize this behavior is by tracer-response techniques. This can identify such ailments as short circuiting, stagnant flow regions, shear forces which decrease droplet sizes and make separation me a difficult, and others. 

A LARGE number of oil-water separators of different designs were examined by means of a tracer response technique, as reported in Part 1 of this article in last week s Journal. 

A study of the residence time distribution (RTD) analysis of liquid phase has been performed. The liquid RTD is determined by means of the tracer response technique. An approximated 8-Dirac pnilse of tracer solution (NaCl) is injected into the reactors at a certain time (t = 0) and the outlet signal is detected by conductivity probe and recorded by the acquisition system. The tracer is injected as quickly as possible to obtain as closely as practical a 8-pulse of tracer at the inlet. 

In this section, we have seen that tracer response techniques can be a very powerful diagnostic tool that can help to uncover the reason(s) for unanticipated reactor... 

The interaal-age distribution function I(t) is not as important as E(t) and F(t) in the characterization of chemical reactors. However, it is quite important in medicine, where tracer response techniques are used for a variety of purposes such as measuring blood flow rates and characterizing the behavior of internal organs. The interaal-age distribution function is discussed here primarily for the purpose of completeness. 

Measurement of D/uL The correlations discussed previously cover a wide range of important conditions and configurations. Nevertheless, a reactor configuration may be encountered that has not been studied previously, and for which correlations are not available. In this event, D/uL must be measured using tracer response techniques. 

One disadvantage of the CIS model is that correlations do not exist that permit N to be predicted for given reactor configurations and flow conditions. For practical purposes, AT must be determined experimentally, via tracer response techniques, for each situation. 

The total volume of the V CSTRs N xV) should be the same as the total volume of the nonideal reactor that is being characterized. The total volume can be checked via tracer response techniques. If the mean residence time t in the vessel is measured, and if the vessel is closed, ... 

The number of CSTRs in series, N, must be determined via tracer response techniques. The number of CSTRs is related to the difference in the variance of the exit-age distribution between the reactor inlet and the reactor outlet. 

If the Dispersion model cannot be used, or is not physically appropriate, the external-age distribution function E f) must be measured using tracer response techniques. Once E i) is available, the macrofluid model can be used to establish bounds on reactor performance. Moreover, the shape of the external-age distribution function can suggest various compartment models, including the CSTRs-in-series model, which can be used to explore reactor performance. 

The external-age distribution E t) of a reactor can be measured using tracer response techniques. 

Ross (R2) measured liquid-phase holdup and residence-time distribution by a tracer-pulse technique. Experiments were carried out for cocurrent flow in model columns of 2- and 4-in. diameter with air and water as fluid media, as well as in pilot-scale and industrial-scale reactors of 2-in. and 6.5-ft diameters used for the catalytic hydrogenation of petroleum fractions. The columns were packed with commercial cylindrical catalyst pellets of -in. diameter and length. The liquid holdup was from 40 to 50% of total bed volume for nominal liquid velocities from 8 to 200 ft/hr in the model reactors, from 26 to 32% of volume for nominal liquid velocities from 6 to 10.5 ft/hr in the pilot unit, and from 20 to 27 % for nominal liquid velocities from 27.9 to 68.6 ft/hr in the industrial unit. In that work, a few sets of results of residence-time distribution experiments are reported in graphical form, as tracer-response curves. 

The theory necessary for understanding two-station tracer measuring techniques is outlined in Appendix 1. An arbitrary, but unimodal, impulse of tracer is created in a system inlet and the outlet response recorded, see Fig. 21 (Appendix 1). Then, the mean, Mj, of that which resides between the points at which inlet and outlet pulses are observed and recorded is equal to the difference in means of these two signals. Similarly, the variance, T2, and the skewness, T3 are equal to the differences in these respective moments between inlet and outlet. This enables the system transfer function to be defined in terms of a few low-order moments via eqns. (A.5) or (A.9) of Appendix 1, this in turn defining the system RTD. Recall that system moments and moments of the system RTD are one and the same. 

The experimental technique used for finding this desired distribution of residence times of fluid in the vessel is a stimulus-response technique using tracer material in the flowing fluid. The stimulus or input signal is simply tracer introduced in a known manner into the fluid stream enter-... 

A one-dimensional search optimization technique, such as the Fibonacci search, is employed to minimize Equation 8-113. A computer program (PROG81) was developed to estimate the equivalent number of ideal tanks N for the given effluent tracer response versus time data. Additionally, the program calculates the mean residence time, variance, dimensionless variance, dispersion number, and the Peclet number. 

Aside from permeability and absorbency complications, other universal concerns of helium leak detection are factors such as source operating pressure, spraying patterns (for tracer-probe technique), response time, clean-up time, and cold trap usage. Pump use and general helium leak detector maintenance operations are also fairly universal. 

The source operating pressure is the vacuum necessary to operate the leak detection device. This pressure is not specific, rather it is a pressure range within the leak detector which works. Optimistically, we want the helium leak detector, and the system to which it is connected, to have the greatest possible vacuum. This gives the tracer-probe technique the maximum sensitivity with the quickest response time. As an added benefit, when one is operating at a very high vacuum,... 

Cleanup time is almost always longer than response time because of the difficulty in desorption of helium from a vacuum system, compounded by helium permeation into porous materials. So, when using the tracer-probe technique do not overspray helium onto your system. The more helium that enters your system, the more that can permeate into porous materials and the longer the cleanup time. 

The RTD for a flowing fluid is normally obtained by the so-called stimulus-response technique. This technique involves the injection of a tracer at the inlet stream or at some point within a reactor and the observation of the corresponding response at the exit stream or at some other downstream point within the reactor. A suitable flow model can then be selected by matching the experimental RTD curve with that obtained from the mathematical model. This approach implies that a transient analysis of reactor and flow model behavior is necessary. 

The physical process of material dispersion is due to the hydrodynamic processes taking place in the flowthrough system and is therefore conveniently investigated by the stimulus response technique, which is based on introduction of a tracer into a flowing stream and on measurement of the dispersion of the tracer as caused by the transport process throughout the system. If the tracer is injected as a zone (stimulus), then the observed... 

To understand the behavior of the fluidized bed, one can determine the average residence time or the residence time distribution (RTD) from the tracer technique. For instance, RTD in fluidized bed dryer is usually carried out by means of the stimulus-response technique, in which an impulse of solids marked with some appropriate tracer is fed to the dryer and its time of elution and concentration measured at the exit of the dryer. The material of the tracer has to be such that it can be detected and does not react with the substrate material, and its form of application and response are well known (Levenspiel, 1972). 

The neutron activation technique mentioned in the preceding paragraph is only one of a range of nuclear methods used in the study of solids - methods which depend on the response of atomic nuclei to radiation or to the emission of radiation by the nuclei. Radioactive isotopes ( tracers ) of course have been used in research ever since von Hevesy s pioneering measurements of diffusion (Section 4.2.2). These techniques have become a field of study in their own right and a number of physics laboratories, as for instance the Second Physical Institute at the University of Gottingen, focus on the development of such techniques. This family of techniques, as applied to the study of condensed matter, is well surveyed in a specialised text... 

Development in recent years of fast-response instruments able to measure rapid fluctuations of the wind velocity (V ) and of fhe tracer concentration (c ), has made it possible to calculate the turbulent flux directly from the correlation expression in Equation (41), without having to resort to uncertain assumptions about eddy diffusivities. For example, Grelle and Lindroth (1996) used this eddy-correlation technique to calculate the vertical flux of CO2 above a foresf canopy in Sweden. Since the mean vertical velocity w) has to vanish above such a flat surface, the only contribution to the vertical flux of CO2 comes from the eddy-correlation term c w ). In order to capture the contributions from all important eddies, both the anemometer and the CO2 instrument must be able to resolve fluctuations on time scales down to about 0.1 s. 

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. 

---