An Overview of Chemical Reaction Analysis - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 91

separation of the desired product could make the entire process economically unfeasible. The
reaction kinetic principles learned here, in addition to the production of chemicals, can be
applied in areas such as living systems, waste treatment, and air and water pollution.
Reaction engineering is concerned with the rate at which reactions take place, together
with the mechanism and rate-limiting steps that control the reaction process. The sizing of
reactors to achieve production goals is an important element. How materials behave within
reactors, either chemically, physically, or biologically, are significant to the designer of a bio-
process, as is how the data from reactors should be recorded, processed, and interpreted.

3.3.1. Definition of the Rate of Reaction,

r

A
As we have discussed earlier, we say that a chemical reaction has taken place when a detect-
able number of molecules of one or more species has assumed a new form by a change in the
kind or number of atoms in the compound and/or by a change in structure or configuration
of these atoms. In this classical approach to chemical change, it is assumed that the total
mass is neither created nor destroyed when a chemical reaction occurs. The mass referred
to is the total collective mass of all the different species in the system. However, when
considering the individual species involved in a particular reaction, we do speak of the
rate of formation of a particular species. The rate of formation of a species, say species A,
is the number of A molecules being generated (from consuming other species) per unit
time per volume, that is,

Number of moles of species A formed
Time

r A ¼ (3.20)

Volume

As such, the reaction rate is defined and measured within a small volume (approaching zero)
and at an instant (or small interval) in time. Concentration is defined in the same manner:

Number of moles of species A
Volume

C A ¼ (3.21)

This would give the concentration C A units of moles per liter, mol/L (formerly, M as in
molarity) or kmol/m 3 .
The concentration can be defined in other units as well. One of the more common ones is

Mass of species A
Volume

C A ¼ (3.22)

which would give C A units of kg/m 3 . Therefore, units are important to engineers.
In biological, enzymatic, and heterogeneous reaction systems, the rate of reaction is
usually expressed in measures other than the volume of the culture (total volume of the reac-
tion mixture). For example,

Number of moles of species A formed
Time

r 0 A ¼ (3.23)

Mass of Catalyst

The catalyst in a biological system is usually of biomass or cells. The biomass concentration is
normally expressed in gram per liter, that is

m X
V

X ¼ (3.24)