Page 64 - Introduction to chemical reaction engineering and kinetics

46 Chapter 3: Experimental Methods in Kinetics: Measurement of Rate of Reaction

3.3 EXPERIMENTAL METHODS TO FOLLOW
THE EXTENT OF REACTION

For a simple system, it is only necessary to follow the extent (progress) of reaction by
means of one type of measurement. This may be the concentration of one species or
one other property dependent on concentration. The former would normally involve a
“chemical” method of analysis with intermittent sampling, and the latter a “physical”
method with an instrument that could continuously monitor the chosen characteristic
of the system. We first consider a-situ and in-situ measurements.

3.3.1 Ex-situ and In-situ Measurement Techniques

A large variety of tools, utilizing both chemical and physical methods, are available to
the experimentalist for rate measurements. Some can be classified as ex-situ techniques,
requiring the removal and analysis of an aliquot of the reacting mixture. Other, in-situ,
methods rely on instantaneous measurements of the state of the reacting system without
disturbance by sample collection.
Of the ex-situ techniques, chromatographic analysis, with a wide variety of columns
and detection schemes available, is probably the most popular and general method for
composition analysis. Others include more traditional wet chemical methods involv-
ing volumetric and gravimetric techniques. A large array of physical analytical meth-
ods (e.g., NMR, mass spectroscopy, neutron activation, and infrared spectroscopy) are
also available, and the experimenter’s choice depends on the specific system (and avail-
ability of the instrument). For ex-situ analysis, the reaction must be “quenched” as the
sample is taken so that no further reaction occurs during the analysis. Often, removal
from the reactor operating at a high temperature or containing a catalyst is sufficient;
however, additional and prompt intervention is sometimes necessary (e.g., immersion
in an ice bath or adjustment of pH).
In-situ methods allow the measurement to be made directly on the reacting system.
Many spectroscopic techniques, ranging from calorimetric measurements at one wave-
length to infrared spectroscopy, are capable (with appropriate windows) of “seeing”
into a reactor. System pressure (constant volume) is one of the simplest such measure-
ments of reaction progress for a gas-phase reaction in which there is a change in the
number of moles (Example l-l). For a reactor with known heat transfer, the reactor
temperature, along with thermal properties, also provides an in-situ diagnostic.
Figure 3.1 shows a typical laboratory flow reactor for the study of catalytic kinetics.
A gas chromatograph (GC, lower shelf) and a flow meter allow the complete analysis
of samples of product gas (analysis time is typically several minutes), and the determi-
nation of the molar flow rate of various species out of the reactor (R) contained in a
furnace. A mass spectrometer (MS, upper shelf) allows real-time analysis of the prod-
uct gas sampled just below the catalyst charge and can follow rapid changes in rate.
Automated versions of such reactor assemblies are commercially available.

3.3.2 Chemical Methods
The titration of an acid with a base, or vice versa, and the precipitation of an ion in an
insoluble compound are examples of chemical methods of analysis used to determine
the concentration of a species in a liquid sample removed from a reactor. Such methods
are often suitable for relatively slow reactions. This is because of the length of time
that may be required for the analysis; the mere collection of a sample does not stop
further reaction from taking place, and a method of “quenching” the reaction may be
required. For a BR, there is the associated difficulty of establishing the time t at which
the concentration is actually measured. This is not a problem for steady-state operation
of a flow reactor (CSTR or PFR).