pletely consumed. The time the bubble spends in the reactor is termed the bub-
ble residence time. What was important in the analysis of this reactor was not
the average residence time of the bubbles but rather the residence time of each
bubble (i.e., the residence time distribution). The total reaction rate was found
by summing over all the bubbles in the reactor. For this sum, the distribution
of residence times of the bubbles leaving the reactor was required. An under-
standing of residence-time distributions (RTDs) and their effects on chemical
reactor performance is thus one of the necessities of the technically competent
reactor analyst.
System 2 A packed-bed reactor is shown in Figure 13-2. When a reactor is
packed with catalyst, the reacting fluid usually does not flow through the reac-
tor uniformly. Rather, there may be sections in the packed bed that offer little
resistance to flow, and as a result a major portion of the fluid may channel
through this pathway. Consequently, the molecules following this pathway do
not spend as much time in the reactor as those flowing through the regions of
high resistance to flow. We see that there is a distribution of times that mole-
cules spend in the reactor in contact with the catalyst.