An Overview of Chemical Reaction Analysis - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 113

A reaction system that contains both parallel and series reactions are called mixed reaction
system. For example, the three-reaction system:

C H OH / C H

4 þ

H O

2 5 2 2

C H OH / CH CHO þ H

2 5 3 2

C H

4 þ

CH CHO / C H

6 þ

H O

2 3 4 2

is a mixed reaction system. The first two reactions are parallel reactions, and the third reac-
tion is a series reaction of both the first and the second reactions.
Selectivity is defined as the fraction of a key reactant that is turned into the desired
product, i.e.

#moles of A converted to D
#moles of A reacted

S D = A ¼ (3.102)

Correspondingly, an instantaneous selectivity can be defined as

r A j forming D

d ð #moles of A converted to D Þ

s D = A ¼ ¼ (3.103)

d ð #moles of A reacted Þ r A

One of the goals in using catalysts is to increase the selectivity of a desired product.
Comparing Eqns (3.102), (3.69), and (3.70) , we can see the similarity between selectivity
and the less well-defined yield factor. However, selectivity is an especially useful concept
for multiple reactions.

3.11. COUPLED REACTIONS

Chemical and biochemical engineers often simplify reactions for ease of understanding in
applications. However, the integrity of stoichiometry is preserved when you add the omitted
species. Coupled reactions are dependent reactions, meaning a reaction written may not
occur if another reaction does not occur. Typical example is shown in Fig. 3.6 for the electrol-
ysis of NaCl in water to produce Cl 2 . This is an important reaction fromwhich Cl 2 and NaOH
are produced. The reaction occurring on the anode is given by

Cl / 2 Cl 2 þ 2 e (3.104)

which requires the reaction on the cathode to occur simultaneously:

H þ þ 2 2 e / H 2 (3.105)

such that the electron can be balanced. These two reactions are coupled. The ionization reac-
tions of NaCl and H 2 O are occurring in the aqueous solution, providing Cl and H þ . The
electrochemical reactions are perfect examples of coupled reactions.
In bioprocess engineering, approximation and simplification are commonly used. For
example, the approximate reaction (

x 3.4) is one that omitted species whose concentration