Residence Time as a Concept

FIGURE 4.1 A reactor system at three different points in time. 

The above illustration reflects the same reactor system at three different points in time. At the time it leaves the reactor, the particle under consideration has reached its maximum age that is equal to its residence time, ti, which we will denote hereafter as t. 

If the reactor is a plug flow one, all particles in the same, arbitrarily chosen cross-section will be of the same age. All volume elements would have thus reached the same age once they leave the reactor. This is why they also have the same residence time. As a consequence, in an ideal PFR, there is no distribution of residence times of any kind. 

In reactors with some degree of backmixing, however, this cannot be achieved, and there is always a distribution of the residence times of the volume elements. The largest possible distribution of residence times is found in CSTRs, in which the residence times of individual volume elements are spread throughout the time frame, from zero to infinity. 

Increased rotation frequency gives larger differences in the residence times in the volume elements 

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