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Precision comparison between ASTM test methods D7039, D2622, and D5453

For many years, professionals in the petroleum industry have faced challenges regarding compliance and quality of product.

Joseph Iaia and Leslie Johnson
Article Summary
These challenges are made more difficult by the variety of regulations and specifications, and the implications they present for their refining process. Regulators across the globe are moving to even more restrictive regulations on sulphur content in a variety of fuels with many countries now requiring maximum sulphur concentration in automotive fuels of 10 to 15 parts per million (ppm).

These regulations have furthered the need for refineries to maximise the precision of their sulphur analysis methodology. Desulphurisation processes are expensive utilising catalyst, hydrogen, and heat. By using a more precise sulphur measurement technique, refiners can produce product closer to the specification maximums, reducing giveaway and saving money. This savings is illustrated in Figure 1.  In addition to production efficiencies, refiners can avoid inaccurate reporting which can lead to regulatory missteps and contract disputes by using a test method with better precision.

With several different methodology options for sulphur analysis available, refineries, terminals, and test inspection certification companies must take care to select a method that produces the least amount of variability in their measurements.

ASTM conducts Proficiency Testing Programs (PTP) several times per year. In each PTP study, ASTM sends samples of hydrocarbon products or feedstocks to various participant sites. Each participating laboratory performs analyses following ASTM methods for various test parameters, including sulphur, using the samples provided. This paper will discuss the ASTM PTP sulphur results for Reformulated Gasoline (RFG) and Ultra Low Sulphur Diesel (ULSD) programs from 2015-2017 using  the most common test methods for low sulphur automotive fuels: D7039, D2622, and D5453. First, an understanding of the test methods is critical to interpreting the data presented.

ASTM Method D7039 (Monochromatic Wavelength Dispersive X-Ray Fluorescence)
Monochromatic Wavelength Dispersive X-ray Fluorescence (MWDXRF) is a subset of WDXRF that utilises similar principles. Rather than using filters or traditional crystals that are flat or singly curved, MWDXRF incorporates doubly curved crystal (DCC) optics to provide a focused, monochromatic excitation X-ray beam to excite the sample. A second DCC optic is used to collect the sulphur signal and focus it onto the detector. This modified methodology delivers a signal-to-background ratio that is 10-times more precise than traditional WDXRF, which improves method precision and Limit of Detection (LOD).

ASTM Method D2622 (Wavelength Dispersive X-ray Fluorescence)
Wavelength Dispersive X-ray Fluorescence (WDXRF) is a type of X-ray Fluorescence, or XRF, which uses high-intensity X-rays to excite elements of interest within a sample. Upon exposure, fluorescent X-rays are emitted from the sample at energy levels that are unique to each element. Additionally, the background signal, an energy region not characteristic of sulphur or other interfering elements, is collected and subtracted from the sulphur signal to improve precision and LOD. To isolate the sulphur signal and to reduce noise, WDXRF utilises a filter and a collection crystal before the sulphur signal reaches the detector. WDXRF also differs from MWDXRF in that it doesn’t specify excitation type (i.e. monochromatic OR polychromatic excitation), whereas MWDXRF specifies monochromatic excitation.

ASTM Method D5453 (Ultraviolet Fluorescence)
In Ultraviolet Fluorescence (UVF) technology, a hydrocarbon sample is either directly injected into a high temperature (1000°C) combustion furnace or placed in a sample boat that is cooled and then injected into the combustion furnace. The sample is combusted in the tube, and sulphur is oxidised to sulphur dioxide (SO2) in the oxygen-rich atmosphere.

Water produced during the sample combustion is removed  by a membrane dryer and the sample combustion gasses are exposed to ultraviolet (UV) light. SO2 is excited (SO2*), and the resulting fluorescence that is emitted from the SO2* as it returns to the stable state is detected by a photomultiplier tube. The resulting signal is a measure of the sulphur contained in the sample.

Precision & ILS results
Hundreds of participants are involved in the monthly ULSD PTP program, which exclusively looks at sulphur. The monthly RFG PTP boasts over a hundred participants running a variety of test methods for differing RFG parameters. The data shown represents sulphur data collected throughout the study from January 2015 to December 2017.

Understanding the Data (Mean Concentration and Reproducibility)
Both graphs and tables shown below track average sample concentration and reproducibility (R). Reproducibility is the difference between two single and independent results obtained by different operators applying the same test method in different laboratories using different apparatus on identical test material. A lower reproducibility value correlates to a better level of precision which can minimise risks from inaccurate reporting such as regulatory fines and contract disputes.

The data presented is filtered to show all samples whose average concentration ranged between 5 and 15 ppm. These values were chosen based on the most common regulatory requirements for sulphur content in automotive fuel in Europe, United States, China, and others around the world. It is critical for an analyser to have low reproducibility values (better precision) when measuring these types of samples. When interpreting the data, keep in mind:

Graphs 1 & 2
• Both graphs are sorted by decreasing sample mean.
• Each column cluster in the graphs represents reproducibility for one sample measured by multiple laboratories each using D7039, D2622, or D5453.
• Within each column cluster, each colour-coded bar corresponds to reproducibility for one test method. D7039 is in orange, D2622 is in grey, and D5453 is in blue.
• The numerical value of each method/bar is graphed on the left axis. (Remember - lower R values are indicative of better precision.)
• For many test methods, precision is often dependent on concentration. For context, the monthly average sulphur concentration is graphed as a red dot and its value is shown on the right axis of the graphs.

Tables 1 & 2
• Both tables are sorted by decreasing sample mean.
• Both tables are colour-coded to indicate relative monthly performance; green represents the best method reproducibility, yellow represents the second best reproducibility, and red represents the poorest reproducibility.

The average R value across the 3 years of study data is the key performance indicator shown in both graphs and tables. A summary of the reproducibility of the RFG and ULSD PTP samples for 2015-2017 showed that ASTM D7039, using MWDXRF, had:
• The best precision for RFG 100% of the time compared to D2622
• The best precision for RFG 91% of the time compared to D5453
• The best precision for ULSD 89% of the time compared to D2622
• The best precision for ULSD 67% of the time compared to D5453

It is important to note that while the D7039 method had the  best reproducibility in the PTP data, it is possible to utilise an instrument that complies with D2622 methods while obtaining the level of performance of D7039 technology. Those looking to meet D2622 compliance with D7039 precision can use XOS’ Sindie 2622 analyser which uses the same monochromatic excitation of D7039 analysers while still meeting the D2622 methodology.

In both Tables 1 and 2, test method D7039 contains most of the lower R values (marked as green) which indicates better PTP precision.

When measuring for critical elements such as sulphur, a highly precise testing method is vital. Low precision methods can lead to products being off spec which can costs refineries millions of dollars in fines, or product downgrading. Reducing variability  in sulphur analysis is critical to reducing sulphur giveaway, and from the data shown, MWDXRF methods offer the highest level   of precision and reliability.
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