The Detection of Volatile Organic Compounds (VOCs) – Part 2

29 Jul 2021

Volatile organic compounds (VOCs) are characterised by their tendency to evaporate easily at room temperature. In some sectors, VOCs pose a significant threat to health, for example in industries working with crude oil and its derivatives. Toxic VOCs – including benzene, butadiene, hexane, toluene, xylene and many others – are released during various stages of crude oil processing from extraction to refinery, and consequently the petroleum refining and petrochemical industry is recognised as a major source of VOC release into the environment. You can read more about the effects of VOCs on human health in our blog.

Occupational exposure limits for VOCs

The most frequently harmful form of VOC exposure is vapour inhalation. For the refining and petrochemical industry, personal gas monitors are used to protect workers from exposure to toxic VOCs, which include known carcinogens such as the aromatics benzene, toluene and styrene. Virtually all countries have established occupational exposure limits (OELs) for VOCs; these are designed to protect workers against the negative health effects of exposure to such hazardous substances. The OEL is the maximum concentration of an airborne contaminant to which an unprotected worker may be exposed over entire work shift. For example, in the United Kingdom, OELs are listed in EH40/2005 Workplace Exposure Limits. Read more about OEL limits for VOCs in our white paper.

For toxic gases, including VOCs, long term exposure is often measured via a time-weighted average, or TWA. That means the worker’s exposure to a gas is monitored across a given period, usually a work shift of 8 hours, to make sure the gas(es) remain(s) at or below the OEL throughout that time. Crowcon detectors have a proprietary TWA resume function, whereby accurate TWAs are recorded over an 8-hour/TWA period, even if detectors are turned off (during breaks) and on again. Read our blog on TWA resume to find out more. Furthermore, Crowcon detectors store TWA data in their logs, where it remains available for further analysis and to prove regulatory compliance. TWA alarms and near miss data can be exported into Crowcon Connect, a cloud-based portal that gives plant managers full visibility of, and easy access to, gas detection data, making it easy for them to ensure health and safety compliance, improve efficiency and raise levels of safety in the workplace.

VOC detection in personal detectors

As mentioned in our previous blog, many VOCs are toxic at low levels, while others are flammable at higher concentrations. VOCs are difficult to detect in ambient air, compared to inorganic gases such as NO2 & SO2. Additionally, more than 500 different compounds are defined as VOCs, and they can be emitted from many different sources. No perfect sensor technology exists to cover all aspects of measurement, so users must choose from a the available sensor technologies according to their requirements. The technologies that can measure VOC vapours include:
• colorimetric detector tubes
• passive (diffusion) badge dosimeters
• sorbent tube sampling systems
• pellistor sensors (also known as catalytic hotbead or Wheatstone bridge)
• photo-ionisation detection (PID)
• flame ionisation detection (FID)
• infrared spectrophotometry

Due to cost and size constraints, the most commonly-used forms of personal detector for VOCs are pellistor or PID based sensors. You can find out more about sensor technologies in our white paper. Both pellistor and PID sensor technologies are non-specific, so they can’t be used to distinguish one VOC/flammable risk from another. Consequently, pellistor sensors and photo-ionisation detectors can be considered complementary detection technologies for many applications where VOCs present a hazard.

Pellistors are commonly used to monitor combustible gases like methane, propane and others that are not detectable by PID. On the other hand, PID detects large VOC and hydrocarbon molecules that pellistor sensors may find it almost impossible to detect, certainly in the parts-per-million range required to alert to toxic levels. Thus, the best approach in many environments is a multi-sensor instrument for the detection of flammable and toxic gases with VOCs. Crowcon Gas-Pro unit is equipped to detect up to 5 different gases and comes with a built-in pump for confined space entry. Read our case study of a major upstream oil and gas company in the Middle East, which uses Crowcon’s Gas-Pro PID portable gas monitors to help protect employees from the risks of VOC.

VOC detection in ambient air

In addition to personal detection, industries are required to monitor ambient air quality around the factory perimeter. Owners of industrial sites close to residential areas are bound to monitor airborne harmful gases. Air quality monitoring in ambient air usually involves detection of greenhouse gases like NO2, SO2, CO2 and O3 in addition to VOCs. In some applications, malodourous gases like NH3, H2S must be detected and this can be done in conjunction with an odour unit (OU) read-out.
Crowcon offers air quality monitoring systems that include a sample pump and pre-treatment system, sensor array and data acquisition system. Crowcon’s sampling system uses a range of sensing technologies, including electrochemical sensors, metal oxide semiconductors, pellistor detectors and PID gas sensors, to detect a wide range of gases. This method of gas detection is well developed and provides fast time resolution – allowing for gases to be detected in a short period of time while reducing downtime. Design can be varied according to requirements. An online gas detection system allows for fast deployment on site and is also relatively inexpensive to purchase and operate when compared with gas chromatography or mass spectroscopy. You can read about Crowcon’s sampling solution for Shanghai’s largest wastewater treatment plant in our latest case study.

Gas-Pro PID

1. Emissions of volatile organic compounds from crude oil processing – Global emission inventory and environmental release (Science of The Total Environment, Volume 727, 20 July 2020, 138654)
2. Monitoring VOCs in Ambient Air – A new focus to meet policy needs (AWE International, Dec 2, 2019)
3. Odours in Sewerage—A Description of Emissions and of Technical Abatement Measures (MDPI Environments, 06-00089-v2)
4. Review of low-cost sensors for the ambient air monitoring of benzene and other volatile organic compounds (JRC Science Hub Report, JRC98368, EUR 27713)

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