Specialty Gases for VOC Analyses

It is generally recognized that some volatile organic compounds (VOCs) can have severe, negative influences on human health. Certain VOCs can constrain normal function of the central nervous system, causing headaches, fatigue, drowsiness, and discomfort, and a number of VOCs have even been proven to be carcinogenic. Legislation around VOC emissions is becoming increasingly stringent, with severe financial penalties often the outcome for non-compliance. Detection and analysis of industrial VOCs now demands continuous innovation to assist companies in complying with tightening regulations. This article looks at the role of specialty gases in monitoring, detection, and analysis of such emissions.

The number of processing plants coming on stream worldwide continues to increase, bringing the release of volatile organic compounds (VOCs) into the atmosphere under intense scrutiny by environmental authorities. For example, the United States Environmental Protection Agency (US EPA) regulates the emissions of VOCs to prevent ground-level ozone formation, a key constituent of photochemical smog.

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The risks associated with industrial VOCs are aggravated by the fact that hazardous concentrations are usually very low and the health issues they can cause can be accumulative and slow to develop. It has been reported that asthma and other respiratory diseases are on the rise, affecting both young and old, and VOCs have been implicated. VOCs can cause sensory stimulation, tissue inflammation, anaphylaxis, and nerve toxic reactions; some can also impact the normal function of the central nervous system, causing headaches, fatigue, drowsiness, and discomfort. Research indicates that alcohols, aromatic hydrocarbons, and aldehydes have the potential to stimulate mucous membranes and upper respiratory tracts. Furthermore, a number of VOCs are proven carcinogens or potential carcinogens, such as benzene, trichloroethylene, and formaldehyde. More disturbing is that the concentrations of VOCs can be greater inside a building than in the ambient air outside because of emissions from paint on the walls, carpets and furniture. Many researchers around the world have presented papers on Indoor Air Quality or IAQ. The Japanese even have a more descriptive name for this, Sick Building Syndrome.¹

By measuring ambient air, processing plants are able to determine if any of their raw materials, process intermediates, or end-products are in the air and, if so, determine the emission source. In the petrochemical industry, this applies to a broad spectrum of components.

Solvents are a major source of man-made VOCs that when exposed to higher temperatures, such as in a production process exhaust stream, evaporate and enter the atmosphere where they create a foul smell. There are several different technologies to reduce or remove solvents from exhaust streams and reuse depending on the recovery value and concentration of the solvents.

One of the most effective ways to recover solvent vapours is to condense and capture them using liquid nitrogen as a cooling media in a process called low temperature or cryogenic condensation. When liquid nitrogen is used to cool the condenser, VOC emissions are rapidly reduced to low levels by trapping the VOCs at extremely low temperatures. They can then be reintroduced to the industrial process.

Increasing regulatory requirements have created more rigorous demands in measurement and, with new compounds to evaluate, laboratories performing environmental analysis of air quality are continually faced with new challenges. The growing demand for accurate identification and quantification of VOCs across both ambient and indoor environments has initiated requests from chemical analysts around the world for low-level multi-component VOC calibration gas mixture standards.

Designing a Gaseous Calibration Standard

The process of designing a new gaseous calibration standard begins by determining the safety issues associated with working with the pure compound, as well as the new calibration standard, to establish appropriate safety procedures for personnel working in the development laboratory.

The next step is to review the component's vapour pressure to evaluate if it will allow for the manufacture of a gaseous standard in a cylinder under full pressure. If the vapour pressure is too low to allow full pressure at the requested concentration, the developers must determine a combination of concentration and pressure that will allow the production of the standard.

Another key step is to determine the availability of the compound in order to confirm that it is commercially available in a relatively pure form. Occasionally, if a specific compound is not available commercially, specialty gases companies may ask the client to supply the compound to allow them to potentially manufacture a gaseous standard.

Assuming that the compound can be obtained and that all of the health and safety issues are acceptable, the next step is to address the package for the proposed standard. There are multiple cylinder materials and treatments, as well as multiple cylinder valve choices. Cylinders can be constructed of aluminium, steel, stainless steel, and more exotic materials. Common materials used for specialty gas valves include stainless steel and brass. In addition to the traditional materials, there are now new coating processes that can assist in developing a cylinder and valve package that will provide stability.

Once the several different methods of cylinder preparation are added to the equation there is a potentially large number of cylinder, cylinder preparation, and valve combinations that need to be considered and investigated to provide a package that will ensure the stability of the gas standard.

Calibration Gas Mixture Standard Preparation

When sourcing VOC raw materials, specialty gases companies generally procure the highest purity commercially available product. Once the material is received, it is assayed to confirm the purity and, if any impurities exist, they are identified and quantified.

Almost all VOC calibration standards manufactured are produced in a balance of nitrogen. The process starts with high-quality liquid nitrogen from the supply tank. It is vaporized, pressurized, and passed through various stages of purification equipment before it is used in a standard. The resultant nitrogen is typically 99.99999% pure and free of chemicals that will react with the VOCs in the mixture or interfere with the analysis, but it will normally contain some level of another inert gas, such as argon.

Most hydrocarbon processors purchase only a small volume of a VOC standard because the quantities needed for their work are small. The standard is typically supplied in a small cylinder with an internal volume of 0.9 litres that, at full pressure, contains about 104 litres of gas, or a little less than five moles. Therefore, if the requested concentration of the VOC were 1 ppm, a total of 5 × 10^-6 moles of the VOC would be required.

Because it would be difficult to produce an accurate standard in this very small amount, the manufacturing process begins using a larger 30-litre water volume cylinder, containing approximately 4000 litres of gas at full pressure, or about 170 moles. After the VOC standard is produced in the larger cylinder, the mixture can be decanted to a number of smaller cylinders for final analysis and shipment to the client.

Example Study: China

Emissions and pollution control issues are a priority the world over, both in developed and developing economies, and VOC emission control is high on the agenda.

Authorities are also becoming far stricter than ever before on calibration standards used for environmental emission measuring. In China, for example, the general rule is that calibration standards must align with GBW, the official Chinese quality standard. GBW accreditation is a requirement for gas production facilities wishing to supply specialty calibration gas mixtures to both domestic and foreign-owned companies operating in China. The country does not generally allow the import and use of any calibration gas mixture not conforming to GBW standards, although there are some notable exceptions relating to gas mixtures beyond its technology thresholds for local manufacture within the country, such as the more complex VOC calibration gas mixtures.

The Future

VOC monitoring today is all about emissions control, keeping within legislation, and preventing the loss of valuable VOCs, but in the not-too-distant future this arena could expand to include emissions trading and that, if and when it happens, will have a direct financial impact on petrochemical companies.

As soon as VOCs become molecules included in emissions trading programmes, the focus will shift dramatically beyond just keeping emissions within legal limits, to embrace trading good environmental behaviour for money — or for the right to keep your plant open.

Reference

1. Michael C. Baechler, Donald L. Hadley, Thomas J. Marseille, and Robert D. Stenner, Sick Building Syndrome: Sources, Health Effects, Mitigation, (Noyes Data Corporation, Park Ridge, New Jersey, USA, 1991).

Stephen Harrison is Linde's Global Head of Specialty Gases & Specialty Equipment and is based in Munich, Germany. He is a British Chartered Engineer (MIChemE) with a career in industrial gases spanning over 20 years, 10 of which have been focused in the area of specialty gases. He has worked in an international capacity for both Linde Gases and previously BOC. He holds a master's degree in chemical engineering from Imperial College, London.

E-mail: hiq@linde-gas.com

Website: http://hiq.linde-gas.com/

This article is from The Column. The full issue can be found here:http://images2.advanstar.com/PixelMags/lctc/digitaledition/July24-2014-uk.html#2