Bengt Löfstedt reviews the ‘what, where and how’ of gas monitoring at refineries
Gas concentration monitoring at refineries can be of importance for safety reasons, but it’s also essential to control production quality, to minimise losses and thereby production costs, and to meet environmental objectives, potentially with reduced pollution and improved health as results.
Which gases to monitor depends on the production processes in play, and where the monitoring is to take place. For process control purposes, a range of hydrocarbons such as methane (CH4) and benzene (C6H6) can be of interest, but also for example carbon monoxide (CO), carbon dioxide (CO2), water (H2O), and hydrogen sulphide (H2S). The concentrations can range between 0 and 100%.
Tail gas monitoring, i.e. the emissions of air pollutants at the end of the production processes, might cover the same gases that are of interest for process control. However, the tail gas often undergoes some combustion process, for example in an afterburner, and other types of gases might also be monitored. This can include for example sulphur dioxide (SO2), nitric oxide (NO) and nitrogen dioxide (NO2). The pollutants at this stage are usually measured in parts-per-million (ppm) ranges. Gas monitoring is sometimes combined with flow monitors to yield the total emissions of pollutants in units of weight-per-time. The driving force behind emissions monitoring is often requirements from legislators and environmental authorities, requiring proofs of emissions limits being observed.
A third application area for gas monitoring is surveillance of the ambient air close to the production facility. When done for personal protection, it is often done with wearable monitors of one or a few hazardous gases, such as CO or H2S. If the concentrations approach dangerous levels, often in the ppm range, the monitor can issue an alarm and the wearer can leave the area before being affected by the gas.
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. This type of monitoring reveals the long-term exposure to air pollution, often in levels of parts-per-billion (ppb). The types of pollutants to monitor are often the same as those found in the production process, including SO2, NO2 (NOx), benzene, and H2S, but other pollutants of concern such as ozone (O3) and particulate 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. This 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.
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. The 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 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. The 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. The 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
