The main advantage of online monitoring is eliminating the sampling system. Without a sampling system, transportation lag time is eliminated, allowing good closed-loop control. Maintenance is also reduced, since many delicate sampling system components are eliminated. Finally, online monitoring detects the unaltered real process; with sampling, sample integrity can be impeded by filtering, condensation, and leakage.
Online monitoring requires an analyzer (usually a probe) that is clean and in good working order. Therefore, probe cleaners (see Tables 7.7.1 and 7.7.2) and placing the probe inside a sight-glass for convenient visual inspection (see Figure 7.7.2) are important. The capability of removing the probe from the pipe or tank without a process shutdown is also beneficial. This is accomplished using retractable probes (see Figure 7.7.8), which are periodically
(and automatically) withdrawn from the process for unattended cleaning and recalibration.
If the probe measures only the composition of the liquid phase, a periodically backflushed, porous filter cup (see Figure 7.7.6) can protect it from being coated with solids.
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