Page 520 - Elements of Chemical Reaction Engineering Ebook

490 Steady-State Nonisotherrnal Reactor Design Chap. 8

Another possible way to operate such a reactor is to use multiple-stage
operation with progressively higher coolant temperatures. Because pressure
Analyzing the drop over the reactor is small (-0.7 atm), neglecting the pressure drop does not
effects Of pressure affect the exit conversion significantly (Figure 8-17). The effect is more signifi-
drop
cant at lower reactor inlet temperatures because the rate of reaction is apprecia-
ble over a longer portion of the reactor bed. At higher inlet temperatures the
conversion is limited by the approach to equilibrium, and hence the pressure
drop has a negligible effect.

0 1200 . 1275 2 I350 5 ~ 0 0
0
Inlet temperature (%I
Figure 8-17 Fractional conversion as a function of reactor inlet temperature.

8.6 Multiple Steady States
In this section we consider the steady-state operation of a CSTR in which a
first-order reaction is taking place. We begin by recalling the hydrolysis of pro-
pylene oxide, Example 8-4.
If one were to examine Figure E8-4.2, one would observe that if a parame-
ter were changed slightly, there might be more than one intersection of the
energy and mole balance curves. When more than one intersection occurs, there
is more than one set of conditions that satisfy both the energy balance and mole
balance, and consequently there will be multiple steady states at which the reac-
tor may operate.
Neglecting shaft work in the CSTR, and setting Aep = 0, [Le., for
Atp = O,AH",,(T) = AH",,(T,) = AH",,] the energy balance [Equation (8-51)]
can be rearranged in the form

Substituting the mole balance on a CSTR,
FAOX = -rAV (2-12)