Page 315 - Elements of Chemical Reaction Engineering 3rd Edition

Sec. 6.1 Maximizing the Desired Product in Parallel Reactions 287

Desired product:

i (3; 4ICA
[
-
rD = 0.0002 exp 36,000 - - (E6-1.1)
A---+D Unwanted product U:
A--+X
A+Y A >u
ru = {O.,ll8 exp[Z5,000 (A Cy (E6 1.2)
k)]}
-

Unwanted product Q:

{ [
rQ = 0.00452 exp 5000 - - - (%-I .3)
(3iO i)]} "
For C, = 1M How and under what conditions (e.g., reactor type, pressure, temperature, etc.)
ms, should the reactions above be carried out to minimize the concentrations of the
300 0.11 unwanted products U and Q?
340 1.w
380 5.50
420 21.65 Solu tion
Because pre exponential factors are comparable, but the activation energies of ~reac-
tions (E6-1.1) and (E6-1.2) are much greater than the activation energy of reaction
(E6-1.3), the rate of formation of Q will be negligible with respect to the rates of
formation of D and U at high temperatures:
300 420
rD
S, = - very large (E6 - 1.4)
'a
Now we need only to consider the relative rates of fiormation of D and U at high
temperatures: 1 I,rn[&- -4

rD - O.lle
s,, = - - I (E6-1.5)
TU C:
From Equation (E6-1.5) we observe that the amount of undesired product, U, can be
minimized by carrying out the reaction at low concentrations. Therefore, to maxi-
mize the conversion of A to D we would want to operate our reactor at high temlper-
atures (to minimize the formation of Q) and at low concentrations of A (to minimize
the fiormation of U). That is, carry out the reaction at
1. High temperatures.
2. Low concentrations of A, which may be accomplished by:
a. Adding inerts.
b. Using low pressures (if gas phase).
c. Using a CSTR or a recycle reactor.