Page 65 - Introduction to chemical reaction engineering and kinetics

3.3 Experimental Methods to Follow the Extent of Reaction 47

Figure 3.1 Example of a laboratory catalytic flow reactor

An alternative mode of operation for the use of a BR is to divide the reacting system
into a number of portions, allowing each portion to react in a separate vessel (BR), and
analysing the various portions at a series of increasing times to simulate the passage of
time in a single BR. It may be more convenient to stop the reaction at a particular time
in a single portion, as a sample, followed by analysis, than to remove a sample from a
complete batch, followed by quenching and analysis.

3.3.3 Physical Methods
As chemical reaction proceeds in a system, physical properties of the system change
because of the change in chemical composition. If an appropriate property changes in
a measurable way that can be related to composition, then the rate of change of the
property is a measure of the rate of reaction. The relation between the physical prop-
erty and composition may be known beforehand by a simple or approximate model, or
it may have to be established by a calibration procedure. An advantage of a physical
method is that it may be possible to monitor continuously the system property using an
instrument without disturbing the system by taking samples.
Examples of physical-property changes that can be used for this purpose are as fol-
lows:
(1) Change of pressure in a gas-phase reaction involving change of total moles of
gas in a constant-volume BR (see Example l-l); in this case, the total pressure
(P) is measured and must be related to concentration of a particular species. The
instrument used is a pressure gauge of some type.
(2) Change of volume in a liquid-phase reaction; the density of a reacting system
may change very slightly, and the effect can be translated into a volume change
magnified considerably by means of a capillary tube mounted on the reactor,
which, for other purposes, is a constant-volume reactor (the change in volume is
a very small percentage of the total volume). The reactor so constructed is called