Constant Monitoring and on Line Analysis
The provision of a constant measure of an analyte in the process stream of a chemical plant presents a
unique set of problems. Ideally a specific analytical detector such as an ion-selective electrode is
inserted directly into the stream, to provide an electrical signal directly related to the amount of analyte
present. This ideal is rarely realized and it is more usual to divert some of the stream through a
sampling loop and to examine the material diverted. It is then possible to extend the scope and precision
of the analysis. Figure 12.10 shows schematically, a method for monitoring the acidity of a process
stream. The sample loop flows into a titration cell which contains a glass electrode to measure the pH,
with the signal from the electrode being fed to a reagent pump of accurately known stroke. A
continuous acid–base titration is carried out with the rate of supply of alkaline titrant (speed of the
pump) being controlled by the potential of the glass electrode. Thus, if the acid content of the process
stream goes up, so does the speed of the pump and vice versa. The rate at which the pump is operating
reflects the pH of the process stream and can be used to generate a control signal to the feedstock valves
so as to adjust the feedstock composition.
Figure 12.10
Schematic diagram of an analyser/controller for an acid-catalysed reaction.Non-destructive analysis is especially valuable in an on line situation. X-ray fluorescence has above all
become of major importance for the analysis of inorganic process streams. Cement production is an
example of the successful application of this technique. The X-ray analyser can be used for the
simultaneous assay of the various feedstocks (iron ore, clay and limestone) for Fe 2 O 3 , Al 2 O 3 , SiO 2 and
CaO. In turn the signals from the analyser are used to control the feedstock supplies to the blending mill
and to maintain an optimum product composition.
A number of valuable methods of analysis are essentially batch processes themselves and cannot be
operated on a continuous basis. Their use in an