On-line composition is usually measured by gas chromatographs. Other analysers include infra-red and ultra-violet analysers, mass spectrometers, refractive index analysers, etc. An example of an anlayser is shown in the Figure below.
Analysers have the advantage of directly measuring the product quality, but also have the drawbacks of high maintenance and slow dynamic response. They have a greater downtime than other instruments, and may be particularly troublesome when the stream analysed is fouling or contains impurities which interfere with the analyser internals.
are prone to large measurement lags which translate
into response delays in control system. The main
sources of lags are process lags, sample transfer lags (the dead time in the line
from the sampling point to the analyser) and analyser transfer lag (the time it
takes to transfer the components from the analyser sample valves to the detector).
With analyser controllers, it is essential to minimise sample transfer lags. Vapor samples are preferred, since they can travel faster. A liquid sample is often vapourised upon withdrawal if sampling lines are long. Heat tracing and insulation are usually required to keep the sample vapourised. To maintain high velocities, the sample withdrawn is often much larger than what the analyser requires, with its unused portion returned to the process.
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