Page 24 - Introduction to chemical reaction engineering and kinetics

6 Chapter 1: Introduction

A common laboratory device is a batch reactor, a nonflow type of reactor. As such, it
is a closed vessel, and may be rigid (i.e., of constant volume) as well. Sample-taking or
continuous monitoring may be used; an alternative to the former is to divide the react-
ing system into several portions (aliquots), and then to analyze the aliquots at different
times. Regardless of which of these sampling methods is used, the rate is determined in-
directly from the property measured as a function of time. In Chapter 3, various ways of
converting these direct measurements of a property into measures of rate are discussed
in connection with the development of the rate law.

To illustrate a method that can be used for continuous monitoring of the composition of
a reacting system, consider a gas-phase reaction carried out in a constant-volume batch
reactor at a given temperature. If there is a change in moles of gas as reaction takes place,
the measured total pressure of the system changes continuously with elapsed time. For
example, suppose the reaction is A + B + C, where A, B, and C are all gases. In such a
case, the rate of reaction, ?-A,is related to the rate of decrease in the partial pressure of A,
PA, which is a measure of the concentration of A. However, it is the total pressure (P) that
is measured, and it is then necessary to relate P to PA. This requires use of an appropriate
equation of state. For example, if the reacting system canbe assumed to be a mixture of
ideal gases, and if only A is present initially at pressure pAo, PA = 2pA, - P at any instant.
Thus, the reaction can be followed noninvasively by monitoring P with respect to time (t).
However, ?-A must be obtained indirectly as a function of P (i.e., of PA) by determining, in
effect, the slope of the P (or p&t relation, or by using an integrated form resulting from
this (Chapter 3).

Other properties may be used in place of pressure for various kinds of systems: for
example, color, electrical conductivity, IR spectroscopy, and NMR.
Other methods involve the use of continuous-flow reactors, and in certain cases, the
rate is measured directly rather than indirectly. One advantage of a flow method is
that a steady-state can usually be established, and this is an advantage for relatively
fast reactions, and for continuous monitoring of properties. A disadvantage is that it
may require relatively large quantities of materials. Furthermore, the flow rate must be
accurately measured, and the flow pattern properly characterized.
One such laboratory flow reactor for a gas-phase reaction catalyzed by a solid (par-
ticles indicated) is shown schematically in Figure 1.2. In this device, the flowing gas
mixture (inlet and outlet indicated) is well mixed by internal recirculation by the rotat-
ing impeller, so that, everywhere the gas contacting the exterior catalyst surface is at the
same composition and temperature. In this way, a “point” rate of reaction is obtained.
Experimental methods for the measurement of reaction rate are discussed further in
Chapter 3, and are implicitly introduced in many problems at the ends of other chapters.
By these means, we emphasize that chemical kinetics is an experimental science, and
we attempt to develop the ability to devise appropriate methods for particular cases.

1.4.4 Kinetics and Chemical Reaction Stoichiometry
All chemical change is subject to the law of conservation of mass, including the con-
servation of the chemical elements making up the species involved, which is called
chemical stoichiometry (from Greek relating to measurement (-metry) of an element
(stoichion)). For each element in a closed reacting system, there is a conservation equa-