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Apr-2015

Zeroing in on zero gases

In the global energy industry environmental legislation has become ever more stringent and fiscal monitoring for oil and gas trading has also become is increasingly important.

Stephen Harrison
Linde HiQ
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Article Summary
In turn, the accurate measurement of both product quality and emissions from processing has become essential - and the degree of accuracy required is increasing year by year. These developments demand precise calibration gas mixtures and ultra high purity zero gases as critical components of the measurement process. This is driving advances in the specialty gases manufacturing where multiple new products for monitoring and analysis are being made available to meet the emerging needs of scientists and instrumentation engineers in the hydrocarbon processing industry.

In refining, there are three main applications in which ultra-high precision gases have become indispensible.
The first relates to natural gas distribution, or custody transfer. At every point where natural gas changes ownership from one organisation to another through a pipeline, or crosses an international border, ultra-precise measurement is essential. This is because the exact heating value has to be established for correct invoicing and this is often also a taxable transaction when the transfer takes place across borders. Significant amounts of money are associated with these transactions, so even a small 1% measurement and billing error could translate into millions of euros per day of lost or gained revenue.

The second application involves environmental emissions monitoring, which is heavily legislated, demanding accurate and precise measurements to ensure a refinery or hydrocarbon processing facility is operating within its consent levels. High purity zero gases play a pivotal role in this measurement. Since such monitoring is a legal requirement, emissions measurements could have an impact on the hours the facility is permitted to operate over the course of the year and therefore can have a significant financial implication to the corporate balance sheet.

A spin-off this environmental application is emissions trading, which is a market-based approach used to control pollution by providing economic incentives for achieving reductions in the emissions of pollutants. Emissions trading is a common practice in the USA today, where facilities such as petrochemical plants are able to sell part of their emissions quota that has not been emitted within an environmental consent level. It is also an emerging practice in Europe. This means there is a direct financial consequence attached to accurate measurement. The smallest variation in accuracy could have immediate financial implications. Today greenhouse gases feature prominently in emissions trading schemes, notably carbon dioxide (CO2), which comprises the majority of traded greenhouse gas emissions, but also methane and nitrous oxide are of notable importance in this application.

In addition to achieving an extremely high degree of accuracy, results from analytical measurement must often be comparable regionally and internationally. This requirement is being enabled by an increasingly large number of international directives and standards that call for the metrological traceability of measuring results using certified reference materials as the basis for comparability.

Traceability is the verifying of results through calibration with measuring instruments of a known accuracy which are linked to acknowledged national measurement standards. In the world of physical metrology, such standard measures are the internationally recognised embodiment of the relevant SI units (International System of Units).

The calibration of high precision analytical instruments used in hydrocarbon processing, such as gas chromatography (GC) and Fourier transform infrared (FTIR) spectroscopy often requires two or three set points to create a calibration curve and a zero point setting to establish the baseline reading. To create this zero point setting, a zero grade gas is needed and it is these so-called zero gases that have now come under the spotlight

Trend in metrology
Various high precision analytical instruments are used to determine extremely small changes in hydrocarbon processes, the composition of natural gases and to monitor emissions. This calls for a carefully prepared calibration curve with low uncertainty which will require accurate calibration gas mixtures and a high purity zero gas. There is a definite trend in the realm of metrology calling attention to the importance of the zero gas. Permissible levels of impurities are falling and zero gases are being seen as part of the calibration curve. A lot of discussion is currently revolving around how to determine the minimum residual impurities after purification in zero gases.

For analytical instrumentation used in the petrochemical industry, high purity specialty gases are generally used in two ways. GC is the most common analytical procedure used in hydrocarbon processes, particularly for natural gas measurements. This instrumentation, whether the instrument is a GC-FID (GC with flame ionisation detector) or a GC-TCD (GC with thermal conductivity detector), requires a high purity carrier gas that is fit for purpose in terms of its purity. Furthermore, high purity gases are required to set the zero on most types of analytical instrument measuring gases. The increasing demands for analytical accuracy mean that only exceptionally low quantities of hydrocarbons or environmental pollutants such as NOx or CO2 are permitted in the carrier gas and the zero gas.

When it comes to monitoring emissions like CO2, the most common instrument used is a nondispersive infrared (NDIR) analyser that harnesses air or nitrogen as a zero gas for continuous monitoring. NDIR analysers frequently require a constant zero gas line in order to have a zero basis for their measurements.

FTIR spectroscopy can also be used at petrochemical facilities and, in terms of detection limits, particularly in the case of CO2, FTIR spectroscopy is more sensitive than NDIR. Furthermore, if a wide range of inorganic chemical species are being monitored, FTIR spectroscopy, which is also more sophisticated than NDIR, meets the evolving needs of the petrochemical industry as legislation is constantly updated. An FTIR will also require ultra-high accuracy zero gases to prepare the calibration curve and as a purge gas during normal operation.

The lowest detection limit and the highest precision for CO2 and nitrous oxide measurement can be achieved with a technology called cavity ring-down spectroscopy (CRDS), a highly sensitive optical spectroscopic technique which has been widely used to study gaseous samples that absorb light at specific wavelengths. It can accurately determine mole fractions down to the parts per trillion (ppt) level. This technology has become more common as the cost of these analysers has decreased in recent years. CRDS is able to detect less than 0.5 parts per billion (ppb) of nitrous oxide in a background matrix of nitrogen or air and these low detection limits cannot generally be achieved currently by any other type instrumentation. Zero gases are also needed to perform these precise measurements at these very low concentrations.

MACPoll
Together with national metrology institutes and associated instrumentation companies, Linde Gases, one of the largest specialty gases suppliers in the world, has played a collaborative role in a project called MACPoll (Metrology for Chemical Pollutants in Air). MACPoll was part of a European metrology research program that focused on research and development of zero gases. Even though it was EU focused, the project attracted international participants from as far afield as Japan, indicating the importance of its work on the global stage.
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