Residence time distribution functions external

Residence time distribution functions were developed by Danckwerts [3] and are defined as external or internal RTD functions. The external RTD function /(f) is defined such that f(t)dt is the fraction of fluid exiting the system with a residence timebetween t and t + dt and the internal RTD function g(t) is defined such that g(t)dt is the fraction of the fluid in the system with a residence time between t and t + dt. 

After studying this chapter the reader will be able to describe the cumulative F(t), external age E(t), and internal age I(t) residence-time distribution functions and to recognize these functions for PFR, CSTR, and laminar flow reactions. The reader will also be able to apply these functions to calculate the conversion and concentrations exiting a reactor using the segregation model and the maximum mixedness model for both single and multiple reactions. 

The exit-age distribution function is also known as the external-age distribution function. It is sometimes simply called the residence time distribution function. However, this can cause some confusion. As we shall see shortly, the exit-age distribution function is not the only function that is used to characterize the distribution of residence times. 

One method of characterising the residence time distribution is by means of the E-curve or external-age distribution function. This defines the fraction of material in the reactor exit which has spent time between t and t -i- dt in the reactor. The response to a pulse input of tracer in the inlet flow to the reactor gives rise to an outlet response in the form of an E-curve. This is shown below in Fig. 3.20. 

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