Page 345 - Elements of Chemical Reaction Engineering Ebook

31 6 Multiple Reactions Chap. 6

we normally need to use ODE solvers along with critical and creative thinking
skills to find the best answer. A number of problems at the end of this chapter
will allow you to practice these critical and creative thinking skills. These
problems offer opportunity to explore many different solution alternatives to
enhance selectivity and have fun doing it.
However, to carry CRE to the next level and to have a lot more fun solv-
ing multiple reaction problems, we will have to be patient a little longer. The
reason is that in this chapter we consider only isothermal multiple reactions,
and it is nonisothermal multiple reactions where things really get interesting.
Consequently, we will have to wait to carry out schemes to maximize the
desired product in nonisothermal multiple reactions until we study heat effects
in Chapters 8 and 9. After studying these chapters we will add a new dimen-
sion to multiple reactions, as we now' have another variable, temperature, that
we may or may not be able to use to affect selectivity and yield. One particu-
larly interesting problem (Ps-30) we will study is the production of styrene
from ethylbenzene in which two side reactions, one endothermic, and one exo-
thermic, must be taken into account. Here we may vary a whole slew of vari-
ables, such as entering temperature, diluent rate, and observe optima, in the
production of styrene. However, we will have to delay gratification of the sty-
rene study until we have mastered Chapter 8.

6.6 'The Attainable Region
A technique developed by Professors Glasser and Hildebrandt l2 allows one to
find the optimum reaction system for certain types of rate laws. The WW12
uses modified van de Vusse kinetics, that is,

kl
)B k3 ,C
A' k2
2A k4 > D
to illustrate what combination of reactors PFWCSTR should be used to obtain
the maximum amount of B. The combined mole balance and rate laws for
these liquid phase reactions can be written in terms of space-time as
de
-
'
- - - k,C, -+ k2CB - k,C:
van de Vusse dz
kinetics
5 = k,C, - k2CB - k3CB
d2

PFR
dCD - k4
---
dz 2
12Department of Chemical Engineering, Witswatersrand University, Johannes-
burg, South Africa. See also D. Glasser, D. Hildebrandt, and C. Crowe, ZEC Res., 26,
1803 ( 1987). http://www.engin.umich.edu/-cre/Chapters/ARpages~n~o/in~o.htm and
http://www.wits.ac.za/fac/engineerin~pr~ma~ARHomepag~~ame.
htm