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Sep-2013

What makes speciality gases so special?

The complexities that go into the manufacture and handling of specialty 
gases for the petrochemical sector, which differentiate them from other types of industrial gases.

Stephen Harrison and Steen Sorensen
Linde Gases Division
Viewed : 1417
Article Summary
Advanced industrial processes in the petrochemical sector continue to demand speciality  gases with ever higher levels of purity and more precise accuracies. An increasing number of specifications for these gases now go down to parts per billion (ppb) and sometimes even to parts per trillion (ppt). Gas supply companies have to ensure that the speciality  gas supplied is the one that best corresponds to the application it will be used for in this industry.

The characteristics of these exotic, non-standard speciality  gases, in comparison with standard industrial and medical gases, divide the gas supply market into specific segments. Within the pure speciality  grades, purity can reach up to 99.99999% (7.0). Higher purity means fewer and lower levels of the impurities that cause problems with high-tech production processes or instrumentation and analytical measurement.

This high level of purity, compared to the same gas at an industrial or medical purity, is one of the differentiators that make a speciality  gas “special”. Oxygen, for example, is a common medical gas and its purity must be suitable for people to breathe. Oxygen is also used in industrial applications, for instance, when it is mixed with acetylene to create a flame to weld and cut metal. In this application, the purity level of the oxygen need only be sufficient to create a flame. Oxygen is also used in speciality gas applications, such as laboratory instrumentation, but the purity of oxygen required in this laboratory gas application is much higher.

Another characteristic of speciality  gases in comparison to medical or industrial gases is the complexity of the product. While a gas mixture for a welding application could comprise a mixture of argon and carbon dioxide to weld steel, and a gas mixture used in a medical application could harness a mixture of nitrous oxide and oxygen for anaesthesia, speciality gas mixtures are far more complex. Instead of two or three different chemicals in the mix, there could be a combination of 20 or 30 chemicals. In addition, instead of blending these chemicals to a tolerance of, say, plus or minus 5%, the end user might require the component to be blended at an accuracy of plus or minus 1%.

Refinery and petrochemical processing calibration mixtures can range from a simple 2.5% methane/air mixture, to much more complex mixtures of 20 or more hydrocarbon components. Most calibration mixtures are stored outside the analyser stations - and with winter temperatures often dropping to minus 15 degrees Celsius or more, gas mixtures must be specially formulated to avoid the condensation of some heavy mixture components. Additionally, environmental mixtures need to comply with requirements for Continuous Emission Monitoring Systems (CEMS).

With any gas mixture used for calibration purposes, the most important requirement is that it must accurately and repeatedly report values of the relevant instrument being calibrated. Calibration should be precise and must be proved to be so. Many gas mixtures used to conduct environmental monitoring are required by national environmental authorities to be accredited. Accreditation is therefore an important factor in the production of speciality  gases, proving to petrochemical plant operators that the gas mixture has been prepared to the required quality.

Speciality  gas plants and filling stations should achieve certification as producers under ISO 9001, often with selected facilities independently accredited to programmes such as ISO 17025:2005 as testing and/or calibrating laboratories and ISO Guide 34 that provides the highest level of quality assurance. Companies such as Linde Gases can confidently state that methods used to certify its accredited calibration gas mixtures are accurate, consistent, documented and validated.

Scale of supply is another major differentiator between speciality  gases and industrial and medical gases. The quantities in which speciality  gases are requested by end users are frequently much smaller than the other gases. How much of the gas will be used and how many customers will want this particular product also influences scale of supply.

Some of the most common industrial gases are supplied to customers through pipelines in quantities such as thousands of tons per day, or in bulk format by 20-30 ton road tankers, where the liquefied gas is supplied to customer facilities and vaporised on site to yield the gas required. This is a cost effective way to buy high quantities of standard industrial or medical gases. In contrast, speciality  gases are typically supplied in cylinders containing about 10 cubic metres of the gas or in small portable cylinders that only contain one cubic metre of the product.

There are often more than 1,000 gas cylinders of different types on a large petrochemical site at any given time, while an ASU (air separation unit) supplies tonnage scale nitrogen or oxygen requirements. Bulk storage tanks also contain liquefied gases for intermediate volume supply requirements.

Speciality gas production
In many cases speciality  gases and mixtures are unique “one-off” products developed for a specific customer application and they require a great deal more product engineering compared to the standard industrial or medical gas products. For this reason, they are not always “off the shelf” items and can even take several weeks to produce in the most complex of cases.

Where a raw material of the required purity cannot be sourced, the gas producer must buy the highest purity available and introduce additional purification processes in-house to achieve an end product of a sufficiently high purity.

Finally, the level of quality control associated with speciality  gas production is far higher than with industrial and medical gases. Sophisticated laboratory instrumentation is used to analyse and verify the constitution of many products and customers are then provided with a certificate declaring the analytical results.

As many as 200 different multi-component gas mixtures are often required at a large refinery complex. These are all “made-to-order” and therefore require fairly lengthy production, certification and delivery lead times in comparison to the standard “off-the-shelf” industrial gases range. To facilitate timely repeat ordering of these complex gas mixtures, plant instrument tag numbers can be used to allow personnel to reference exactly where the mixtures are deployed in the different sections of the plant.
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