Page 319 - Elements of Chemical Reaction Engineering 3rd Edition

Sec. 6.2 Maximizing the Desired Product in Series Reactions 291

ratio of the realction rate of a given product to the reaction rate of the key reac-
htantaneous yield tant A. This is sometimes referred to as the instantaneous yield.2

rD
Yield based on rates YD = - (6-13)
-rA

In the case of reaction yield based on molar flow rates, the overall yield,
L
YD, is defined as the ratio of moles of product formed at the end of the reac-
tion to the number of moles of the key reactant, A, that have been consumed.
For a batch system:

Overall yield (16- 14)

For a flow system:

I
Overall yield (6-15)

As a consequence of the different definitions for selectivity and yield,
when reading literature dealing with multiple reactions, check carefully to
$ ascertain the definition intended by the autl_lor. From an economic standpoint it
is the overul2 sedectivities, S, and yields, Y, that are important in determinlii
profits. Howeveir, the rate-based selectivities give insights in choosing reactor-
and reaction schemes that will help maximize the profit. However, many times
there is a conflict between selectivity and conversion (yield) because you want
to make a lot of your desired product (D) and at the same time minimize the
undesired product (U). However, in many instances the greater conversion you
achieve, not only do you make more D, you also forrn more U.

6.2 Maximizing the Desired Product
in Series Reactions

In Section 6.1 we saw that the undesired product could be minimized by
adjusting the reaction conditions (e.g., concentration) and by choosing the
proper reactor. For series of consecutive reactions, the most important variable
is time: space-time for a flow reactor and real-time for a batch reactor. To illus-
trate the importance of the time factor, we consider the sequence

J. J. Carberry, in Applied Kinetics and Chemical Reaction Engineering, R. L. Gorring
and V. W. Weekman, eds. (Washington, D.C.: American Chemical Society, 1967),
p. 89.