How Specialty Gases Differ from Industrial Gases
When it comes to compressed gases, there is often confusion over the difference between industrial gases (sometimes referred to as commodity or bulk gases) and specialty gases (sometimes referred to as cylinder gases, although industrial gases can also be supplied in cylinders). The Compressed Gas Association (CGA), who sets standards to which suppliers of all types of compressed gases conform, defines its mission as being 'dedicated to the development and promotion of safety standards and safe practices in the industrial gas industry.' In a broad sense, in that most compressed gases are used for some sort of industrial application, all could be considered to be industrial gases. So to define the true difference between industrial gases and specialty gases, one must look beyond the application to other factors such as complexity, level of purity and certainty of composition.
According to the CGA compressed gases are often grouped into five loosely defined families: atmospheric; fuel; refrigerant; poisonous; and those having no obvious ties to any of the other families. Assignment to these families is somewhat arbitrary and typically based on the origin, use or chemical structure of a gas. Specialty gases can belong to any of these five families. Essentially, they are industrial gases taken to a higher level. The dictionary describes one of the definitions of the word specialty as: an unusual, distinctive, or superior mark or quality. Specialty gases then, can be defined as high-quality gases for specific applications that are prepared using laboratory analysis and other preparation methods in order to quantify, minimize or eliminate unknown or undesirable characteristics within the gas. Regarding specialty gas mixtures, precise blending is also necessary to achieve very specific concentration values for the components contained within the mixture.
Specialty pure gases
Pure gases are considered to be specialty gases when they are used as support gases for laboratory instruments such as chromatographs, mass spectrometers and other various types of analyzers and detectors. Manufacturers of these types of highly sensitive instruments normally specify the purity level of pure gases to be used with their instruments. For example, high-purity, moisture-free helium is often used as a carrier gas in these instruments. When unwanted impurities are present, performance of a laboratory instrument may be compromised, or the instrument itself may be damaged. A good rule of thumb is, when purity (sometimes as high as 99.9999%) and/or quantification of trace impurities is an issue, a pure gas is considered to be a specialty pure.
Specialty pure gases are used in the manufacturing of semiconductors and other closely controlled applications as well. They may also be used to assess and monitor the integrity of a bulk pure gas. Carbon dioxide is a good example. Beverage-quality CO2, as used in the manufacture of soft drinks, can be classified as being more of a bulk-type gas because it is used in large quantities. However, because purity is a health concern, a specialty pure CO2, in which all trace impurities have been carefully quantified, is needed to calibrate instruments used to monitor the purity of the bulk CO2.
Specialty gas mixtures
Many specialty gases are actually gas mixtures that contain individual components. They are frequently used with various types of analyzers for process control and regulatory compliance. Some specialty mixtures are somewhat 'standard' and may contain only three or four components, such as nitric oxide and sulfur dioxide mixtures that are used by utility companies to calibrate Continuous Emissions Monitors (CEMs). Others may be quite complex, containing as many as 30 or more components. Usually, a specialty gas mixture is prepared using a Standard Reference Material (SRM) in order to validate accurate measurement of the mixture's components. This provides what is known as traceability to a known measurement standard from a recognized metrology institution such as the National Institute of Standards and Technology (NIST). Specialty mixtures typically have components measured in percentages, parts-per-million and parts-per-billion.
Laboratory analysis to quantify all components and impurities in a specialty mixture is nearly always critical. A formal document known as a Certificate of Accuracy or Certificate of Analysis is provided for each cylinder containing a specialty mixture, and also for some specialty pure gases. This certificate specifies the concentration values for all contents, as well as other important information such the method of blending, type of laboratory analysis and reference standard used to prepare the mixture and expiration date. Expiration date refers to the length of time the components of a mixture remain at their certified concentrations within the specified tolerances. Depending on the stability of the components, shelf life can vary from as little as six months to two years or more. Special cylinder preparation processes, such as Scott's Aculife cylinder inerting treatments, can be used to condition cylinder interior walls in order to extend a mixture's shelf life.
Specialty gases are typically not used in nearly as large a quantity as industrial gases and are supplied in steel or aluminum high-pressure cylinders containing up to 3000 pounds of pressure per square inch/gauge (psig). Hence, they are sometimes referred to as cylinder gases or bottled gases. The cylinder itself is typically not included in the price of the specialty gas it contains and must be returned to the gas supplier when the gas has been depleted. A nominal monthly cylinder rental is usually charged until the cylinder is returned. Many specialty gases are also available in small, portable and non-returnable cylinders such as Scott's SCOTTY Transportables. Other specialized containers include lecture bottles that are often used in laboratories and floating piston-type cylinders that are used to contain volatile liquid phase mixtures.
The cost of specialization
Due to blending technology, cylinder preparation, laboratory analysis and statistical quality control necessary to produce specialty gases, cost is much higher than for lower grade industrial gases. An A-size cylinder containing 218 cubic feet of a low grade of helium suitable for filling party balloons might cost little more than $50. The same cylinder containing 99.9999% pure research grade helium, with a total impurity of less than one part-per-million (1 ppm), would cost about $500. That's still a bargain considering 144 cubic feet of a three-component EPA Protocol mixture having an analytical accuracy of 1% may cost as much as $1,500. As with any other specialized product, the end cost of a particular specialty pure or gas mixture is largely determined by the degree of difficulty and complexity involved in its preparation.
Considerations when purchasing specialty gases
Purchasing specialty gases can be a daunting task. Because of today's bottom line-oriented business climate, one might consider selecting a specialty gas product based strictly on price. Be careful! While in some cases organizations such as the EPA may dictate minimum accuracy and manufacturing processes for certain gas mixtures, there are few industry-wide standards for specialty gas quality. Blending, analytical and cylinder preparation procedures vary between suppliers of specialty gases. Moreover, suppliers do not always use common nomenclature when describing their products. Even when product names are the same, the characteristics of the gases can be quite different. The best advice is to carefully evaluate your application needs before purchasing. Then talk with a specialty gas expert to be sure you fully understand how the characteristics of a particular pure gas or gas mixture will either meet or possibly compromise your application. Remember also that most specialty gases require the use of specialized delivery equipment that is constructed of materials that will protect gas purity and integrity.
About the Author
Bob Davis is the Environmental Marketing Manager with Scott Specialty Gases, the world's largest producer of EPA Protocol gases and a leading global manufacturer of specialty gases for all types of applocations. More information on the company and Scott's products can be found at http://www.scottgas.com.