Page 399 - Instrumentation Reference Book 3E

18 Chemical analysis:

gas analysis

C. K. LAIRD, edited by I.VERHAPPEN

18.1 Introduction ple is taken into the instrument either as a
continuous stream or in discrete aliquots and is
The ability to analyze one or more components of adjusted as necessary in the sampling unit to the
a gas mixture depends on the availability of suit- temperature, pressure, and flow-rate require-
able detectors which are responsive to the com- ments of the remainder of the system. Any treat-
ponents of interest in the mixture and which can ment of the sample, for example, separation of
be applied over the required concentration range. the sample into its components, removal of
Gas detectors are now available which exploit a interfering components, or reaction with an auxili-
wide variety of physical and chemical properties ary gas, is carried out in the processing unit and the
of the gases detected, and the devices resulting sample is passed to the detector. The signal from
from the application of these detection mechan- the detector is amplified if necessary and processed
isms show a corresponding variety in their select- to display or record the concentration of the com-
ivity and range of response. In a limited number ponents of interest in the sample.
of applications it may be possible to analyze a gas In many gas analyzers the time lag between
mixture merely by exposure of the sample to a sampling and analysis is reduced to a minimum
detector which is specific to the species of interest, by taking a continuous stream of sample at a
and thus obtain a direct measure of its concentra- relatively high flow rate, and arranging for only
tion. However, in the majority of cases no suffi- a small proportion to enter the analyzer, the
ciently selective detector is available, and the gas remainder being bypassed to waste or returned
sample requires some pretreatment, e.g., drying or to the process. Provision is also normally made to
removal of interfering components, to make it check the zero by passing a sample, free of the
suitable for the proposed detector. In these cases species to be analyzed, to the detector, and the
a gas analysis system must be used. instrument may also include facilities for calibra-
A block diagram of the components of a typ- tion by means of a ‘‘span” switch which feeds a
ical gas analyzer is given in Figure 18.1. The sam- sample of known concentration to the analyzer.
-
Sampling - Processing - -
7 unit L Detector L
L
Gas unit Gas sample
sample out
in

Power Signal
supply processing
i