Tracer Response Curves for Ideal Reactors Qualitative Discussion

2 Tracer Response Curves for Ideal Reactors (Qualitative Discussion) 

Consider a vessel at steady state with a constant-density fluid flowing through it At r = 0, a sharp pulse of tracer is injected into the stream entering the vessel. Let s use what we know about the two ideal reactors to construct their tracer response curves, at least qualitatively. We will quantify these curves later in this chapter, after the necessary mathematical tools have been developed. 

In an ideal PFR, elements of fluid pass through the reactor in single file. There is no fluid mixing in the direction of flow. Each element of fluid spends exactly the same time in the vessel. Therefore, every molecule of an ideal tracer will spend exactly that time in the vessel. The detector at the vessel exit will sense the entire quantity of injected tracer at the same time. The tracer response curve will resemble the one shown below. 

The time that the tracer spends in an ideal PFR is t = t = V/ u. This is easy to see if the reactor has a constant cross section A in the direction of flow. In this case, the velocity of the fluid in the direction of flow is vjA at every point in the reactor. If the length of the vessel in the direction of flow is L, the time required to traverse the vessel is Lj v/A) = V/v. 

Consider a radial-flow reactor (see Problem 3.1) that behaves as an ideal PFR. Show that the time required for a sharp pulse of tracer to emerge from the reactor is V/u. 

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