Page 105 - Introduction to chemical reaction engineering and kinetics

Chapter 5

Complex Systems

In previous chapters, we deal with “simple” systems in which the stoichiometry and
kinetics can each be represented by a single equation. In this chapter we deal with
“complex” systems, which require more than one equation, and this introduces the ad-
ditional features of product distribution and reaction network. Product distribution is
not uniquely determined by a single stoichiometric equation, but depends on the reac-
tor type, as well as on the relative rates of two or more simultaneous processes, which
form a reaction network. From the point of view of kinetics, we must follow the course
of reaction with respect to more than one species in order to determine values of more
than one rate constant. We continue to consider only systems in which reaction oc-
curs in a single phase. This includes some catalytic reactions, which, for our purpose
in this chapter, may be treated as “pseudohomogeneous.” Some development is done
with those famous fictitious species A, B, C, etc. to illustrate some features as simply as
possible, but real systems are introduced to explore details of product distribution and
reaction networks involving more than one reaction step.
We first outline various types of complexities with examples, and then describe meth-
ods of expressing product distribution. Each of the types is described separately in
further detail with emphasis on determining kinetics parameters and on some main
features. Finally, some aspects of reaction networks involving combinations of types of
complexities and their construction from experimental data are considered.

5.1 TYPES AND EXAMPLES OF COMPLEX SYSTEMS

Reaction complexities include reversible or opposing reactions, reactions occurring in
parallel, and reactions occurring in series. The description of a reacting system in terms
of steps representing these complexities is called a reaction network. The steps involve
only species that can be measured experimentally.

51.1 Reversible (Opposing) Reactions

Examples of reversible reacting systems, the reaction networks of which involve oppos-
ing reactions, are:
(1) Isomerization of butane (4

n-C4Hr0 e i-C4H,,

(2) Oxidation of SO,

so, + lo *so,
2 2
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