Page 33 - Introduction to chemical reaction engineering and kinetics

1.5 Aspects of Chemical Reaction Engineering 15

system from equilibrium, in terms of the “distance” or affinity measured by the Gibbs-
energy driving force (AG)? Another type of question, which cannot be answered by
thermodynamic methods, is: If a given reacting system is not at equilibrium, at what
rate, with respect to time, is it approaching equilibrium? This is the domain of kinetics.
These questions point up the main differences between chemical kinetics and chem-
ical thermodynamics, as follows:

(1) Time is a variable in kinetics but not in thermodynamics; rates dealt with in the
latter are with respect to temperature, pressure, etc., but not with respect to time;
equilibrium is a time-independent state.
(2) We may be able to infer information about the mechanism of chemical change
from kinetics but not from thermodynamics; the rate of chemical change is de-
pendent on the path of reaction, as exemplified by the existence of catalysis;
thermodynamics, on the other hand, is not concerned with the path of chemi-
cal change, but only with “state” and change of state of a system.
(3) The AG of reaction is a measure of the affinity or tendency for reaction to occur,
but it tells us nothing about how fast reaction occurs; a very large, negative AG,
as for the reaction C + 0, + CO,, at ambient conditions, although favorable
for high equilibrium conversion, does not mean that the reaction is necessarily
fast, and in fact this reaction is very slow; we need not be concerned about the
disappearance of diamonds at ambient conditions.
(4) Chemical kinetics is concerned with the rate of reaction and factors affecting the
rate, and chemical thermodynamics is concerned with the position of equilibrium
and factors affecting equilibrium.

Nevertheless, equilibrium can be an important aspect of kinetics, because it imposes
limits on the extent of chemical change, and considerable use is made of thermodynam-
ics as we proceed.

1.4.6 Kinetics and Tkansport Processes

At the molecular or microscopic level (Figure l.l), chemical change involves only chem-
ical reaction. At the local and global macroscopic levels, other processes may be in-
volved in change of composition. These are diffusion and mass transfer of species as
a result of differences in chemical potential between points or regions, either within a
phase or between phases. The term “chemical engineering kinetics” includes all of these
processes, as may be required for the purpose of describing the overall rate of reaction.
Yet another process that may lead to change in composition at the global level is the
mixing of fluid elements as a consequence of irregularities of flow (nonideal flow) or
forced convection.
Still other rate processes occur that are not necessarily associated with change in com-
position: heat transfer and fluid flow. Consideration of heat transfer introduces contri-
butions to the energy of a system that are not associated with material flow, and helps
to determine T. Consideration of fluid flow for our purpose is mainly confined to the
need to take frictional pressure drop into account in reactor performance.
Further details for quantitative descriptions of these processes are introduced as re-
quired.

1.5 ASPECTS OF CHEMICAL REACTION ENGINEERING

1.51 Reactor Design and Analysis of Performance

Reactor design embodies many different facets and disciplines, the details of some of
which are outside our scope. In this book, we focus on process design as opposed to