PRODUCTION • PROCESSING • HANDLING


Layout of an in-situ continuous emissions monitoring system


Emitter


Receiver Fibre optic cable


Stack


can leave the area before being affected by the gas.


concern such as ozone (O3


Air quality monitoring (AQM) for the general benefit of the staff at the facility or the inhabitants of the neighbourhood is often based on permanently installed AQM stations. Tis type of monitoring reveals the long-term exposure to air pollution, often in levels of parts-per-billion (ppb). Te types of pollutants to monitor are often the same as those found in the production process, including SO2 benzene, and H2


S, but other pollutants of ) and particulate


, NO2


matters can also be monitored, while at it. At best, AQM shows pollution levels well below limits set by the legislators. AQM can be initiated by local authorities wishing to monitor and protect the public health, but it can just as well be on initiative from the facility, to (hopefully) show that the air pollution levels are limited and under control. An AQM station used for general, long-term monitoring can double as an alarm system for accidental releases of air pollutants from the refinery. Tis allows countermeasures to be taken, at best long before any staff member or neighbour is affected by the release. Further, an AQM station can also be used to substantiate diffuse emissions occurring from leakages in e.g. pipes and valves. In combination with monitoring of wind speed and wind direction, pollutants can be back-tracked to specific source locations, revealing unknown or excess leakages, and ensuring that proper actions can be taken to stop or reduce the emissions.


(NOx),


Analyser cabinet So, how are the gas concentrations


monitored? It depends on gas type and concentration range. However, in most cases, the measurement devices use the optical properties of the gaseous molecules, looking at absorption light. Te more absorption of gas-specific wavelengths, the higher concentration of that gas. Two types of instruments exist: sampling, which uses pumps, tubing and often pre-treatment of small gas volumes before the absorption is measured in an internal cell, and in-situ (“at place”) where the absorption is measured by sending a light beam through the actual gas monitored (“open-path”). Open-path monitors have several


advantages over sampling instruments, in particular for permanent AQM


Monitoring the ambient air close to the production facility can help finding sources of emissions


applications. A sampling instrument captures the gas in a single inlet point. If a plume of an emitted gas does not pass that point, the emission will not show. In contrast, the monitoring results from an open-path instrument are average concentrations along the light beam, often several hundred metres long. A series of light beams can form an optical fence around the facility, capturing the plume no matter of its direction. A single open-path system can often use several beams of light and monitor multiple pollutants. In contrast, sampling instruments are often designed to measure concentrations of just a single pollutant, resulting in rapidly increasing costs also if just a few types of gases are to be monitored in a few measurement points. In addition, a sampling system often requires more frequent maintenance and more consumables, compared to an open-path system. Te latter might come with a somewhat higher initial price tag, but in the long run, the total cost of ownership is more favourable for open-path systems. Te low maintenance requirements also make open-path systems less prone to handling errors, giving more uptime and reliable monitoring results. In the end, the choice of instrument depends on what to monitor, and where to monitor it. A good supplier with good references will provide guidance to the best monitoring solution for the specific application


Bengt Löfstedt is with Opsis www.opsis.se


www.engineerlive.com 33


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