Page 412 - Instrumentation Reference Book 3E

Process chromatography 395

AC lOOV

Transformer I

18-8 Stainless

Voltage measured at the end of 4KSl's resistance
(Output)
Figure 18.1 6 Measuring circuit for semiconductor
sensor.

out automatically the analytical procedure for
chromatographic separation, detection, and meas-
urement of predeterniined constituents of gas-
eous mixtures. Samples are taken from process
Figure 18.15 Semiconductorsensor streams and are presented, in a controlled manner
and under known conditions, to the gas chroma-
tograph. Successive analyses may be made, on a
Semiconductor detectors are mainly used as
low-cost devices for detection of flammable gases. regular timed basis, on aliquots of sample taken
from a single stream, or by use of suitable stream
A suitable power-supply and measuring circuit is switching valves, a single process chromatograph
shown in Figure 18.16. The main defect of the may carry out automatic sequential analyses on
devices at present is their lack of selectivity.
process streams originating from several different
parts of the plant.
.3.10 Properties and applications of gas The main components of a typical process
detectors chromatograph system are shown in Figure
18.17. These components are: a supply of carrier
The properties and applications of the most com- gas to transport the sample through the column
monly used gas detectors are summarized in and detector, a valve for introduction of known
Table 18.2.
quantities of sample, a chromatographic column
to separate the sample into its components, a
.4 Process c h ro rn a t o g ra p h y detector and associated amplifier to sense and
measure the components of the sample in the
On-line or process gas chromatographs are carrier-gas stream, a programmer to actuate the
instruments which incorporate facilities to carry operations required during the analytical