Page 96 - Elements of Chemical Reaction Engineering 3rd Edition

I

Rate Laws 3

and Stoichiometry

Kinetics is nature’s way of preventing everything
from happening all at once.
-S. E. LeBlanc

We have shown that in order to calculate the time necessary to achieve a given
conversion X in a batch system, or to calculate the reactor volume needed to
achieve a conversion X 111 a flow system, we need to know the reaction rate as
a function of conversion. In this chapter we show how this functional depen-
dence is obtained. First there is a brief discussion of chemical kinetics, empha-
sizing definitions, which illustrates how the reaction rate depends on the
concentrations of the reacting species. This discussion is followed by instruc-
tions on how to convert the reaction rate law from the concentration depen-
dence to a dependence on conversion. Once this dependence is achieved, we
can design a number of isothermal reaction systems.

3.1 Basic Definitions

A homogeneous reaction is one that involves only one phase. A heterogeneous
reaction involves more than one phase, and reaction usually occurs at,or very
near the interface between the phases. An irreversible reaction is one that pro-
ceeds in only one direction and continues in that directiw until the reactants
Types of reaction& are exhausted. A reversible reaction, on the other hand, can proceed in either
direction, depending on the concentrations of reactants and products relative to
the corresponding equilibrium concentrations. An irreversible reaction behaves
as if no equilibrium condition exists. Stric y speaking, no chemical reaction is
completely irreversible, but in very many 1 actions the equilibrium point lies
r
so far to the right that they are treated as irreversible reactions.

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