In extractive or ex situ systems, process material is transferred from the sample point to an external analyzer. Because the analyzer (e.g., process gas chromatograph) is installed away from the process, maintenance is more manageable than for inline devices (see Figure 3.12.2).
Like inline analyzers, extractive samplers can be located at several different points in a process. However, calibration and reference streams can also be routed to the analyzer with the process samples, something not possible with inline systems. If more detailed analyses on specific materials are needed later, users can divert the sample to a collection vessel.
The disadvantages of extractive systems are that they can be bulky and slow and generate a lot of waste. For
example, an analyzer sampling six different streams requires an enclosure at the process to house the related machinery. Depending on the distance that the sample must travel to the analyzer, times ranging from 20 to 60 sec are common. Filtration, dilution, or concentration may also be required when the sample arrives at the analyzer. When heated samplers and transfer lines are required to keep a sample at a particular temperature, installation and maintenance costs can also be significant.
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