Oct-2019

ASTM D2622 Compliance with D7039 Precision for Sulfur in Diesel

In August 2018, XOS published a paper that reviewed and discussed ASTM Proficiency Testing Program (PTP) diesel and gasoline sulphur data comparing reproducibility for three widely adopted methods for sulphur analysis: D7039 (Monochromatic Wavelength Dispersive X-ray Fluorescence or MWDXRF), D2622 (Wavelength Dispersive X-ray Fluorescence or WDXRF), and D5453 (Ultraviolet Fluorescence or UVF).

Joseph Iaia and Leslie Johnson
XOS

Article Summary

The PTP data concluded that between the critical concentrations of 5-15ppm, D7039 outperformed both D5453 and D2622.

A distinction made in this review was that XOS’ Sindie 2622 analysers have built-in software and hardware functionality that allows users to run measurements in D2622 or D7039 mode, allowing them to stay method compliant while utilising an MWDXRF analyser that offers better overall precision. In the following brief addendum, we will be reviewing data collected on an XOS analyser that utilises D7039 monochromatic excitation in both D7039 and D2622 measurement modes to fully understand the ability to couple D2622 method compliance with superior D7039 precision.

Application Study
To assess the comparison between measuring in D7039 and D2622 mode, 10 measurements of 10ppm sulphur in diesel were run on Sindie 2622 Gen 3.

For measuring in D7039 mode, 10 separate aliquots were run for 300 seconds with each aliquot representing a single measurement result. This data is showcased in Table 1. For D2622, 20 aliquots were prepared and measured in order to run duplicate determinations as per the method. One result is the average of two determinations (column 4 in Table 2).

Measurement time for each D2622 determination was 375 seconds, 75 seconds for the background measurement and 300 seconds for the sulphur measurement. The reason that the added background measurement time is only 20% of the total run time is because Sindie Gen 3 analysers have an enhanced optic and updated software that allows for a 38% reduction in total measurement time.

This equates to a time savings of 7.5 minutes for each sample (≤100 ppm) over the Sindie Gen 2 analyser.

Results
Looking at the data in Tables 1 and 2, there is similar performance across both measurement modes. There are minimal differences between the average, standard deviation, and relative standard deviation of each data set.
Additionally, the low RSD values for both data sets indicate very good precision at 10 ppm, with the RSD for the D2622 data being slightly lower due to the duplicate determinations.

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