Page 20 - Introduction to chemical reaction engineering and kinetics

2 Chapter 1: Introduction

position of matter occurs by chemical reaction. The chemical reaction is normally the
most important change, and the device is designed to accomplish that change. A reactor
is usually the “heart” of an overall chemical or biochemical process. Most industrial
chemical processes are operated for the purpose of producing chemical products such
as ammonia and petrochemicals. Reactors are also involved in energy production, as
in engines (internal-combustion, jet, rocket, etc.) and in certain electrochemical cells
(lead-acid, fuel). In animate objects (e.g., the human body), both are involved. The
rational design of this last is rather beyond our capabilities but, otherwise, in general,
design includes determining the type, size, configuration, cost, and operating conditions
of the device.
A legitimate objective of CRE is to enable us to predict, in the sense of rational
design, the performance of a reactor created in response to specified requirements and
in accordance with a certain body of information. Although great strides have been
taken in the past few decades toward fulfilling this objective, in many cases the best
guide is to base it, to some extent, on the performance of “the last one built.”

1.3 KINETICS AND CHEMICAL REACTION ENGINEERING

In chemical kinetics, the chemical reactor used to carry out the reaction is a tool for
determining something about the reacting system: rate of reaction, and dependence
of rate on various factors, such as concentration of species i (cJ and temperature (T).
In chemical reaction engineering (CRE), the information obtained from kinetics is a
means to determine something about the reactor: size, flow and thermal configuration,
product distribution, etc. Kinetics, however, does not provide all the information re-
quired for this purpose, and other rate processes are involved in this most difficult of
all chemical engineering design problems: fluid mechanics and mixing, heat transfer,
and diffusion and mass transfer. These are all constrained by mass (stoichiometric) and
energy balances, and by chemical equilibrium in certain cases.
We may consider three levels of system size to compare further the nature of kinetics
and of CRE. In order of increasing scale, these levels are as follows:
(1) Microscopic or molecular-a collection of reacting molecules sufficiently large to
constitute a point in space, characterized, at any given instant, by a single value
for each of ci, T, pressure (P), and density (p); for a fluid, the term “element of
fluid” is used to describe the collection;
(2) Local macroscopic-for example, one solid particle reacting with a fluid, in which
there may be gradients of ci, T, etc. within the particle; and
(3) Global macroscopic-for example, a collection or bed of solid particles reacting
with a fluid, in which, in addition to local gradients within each particle, there
may be global gradients throughout a containing vessel, from particle to particle
and from point to point within the fluid.
These levels are illustrated in Figure 1.1. Levels (1) and (2) are domains of kinetics
in the sense that attention is focused on reaction (rate, mechanism, etc.), perhaps in
conjunction with other rate processes, subject to stoichiometric and equilibrium con-
straints. At the other extreme, level (3) is the domain of CRE, because, in general, it is
at this level that sufficient information about overall behavior is required to make deci-
sions about reactors for, say, commercial production. Notwithstanding these comments,
it is possible under certain ideal conditions at level (3) to make the required decisions
based on information available only at level (l), or at levels (1) and (2) combined. The
concepts relating to these ideal conditions are introduced in Chapter 2, and are used in
subsequent chapters dealing with CRE.