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. These challenges are made more diffi cult by the variety of regulations and specifi cations, and the implications they present for their refi ning process. Regulators across the globe are moving to even more restrictive regulations on sulfur content in a variety of fuels with many countries now requiring maximum sulfur concentration in automotive fuels of 10 to 15 parts per million (ppm).
These regulations have furthered the need for refi neries to maximize the precision of their sulfur analysis methodology. Desulfurization processes are expensive utilizing catalyst, hydrogen, and heat. By using a more precise sulfur measurement technique, refi ners can produce product closer to the specifi cation maximums, reducing giveaway and saving money. This savings is illustrated in Figure 1. In addition to production effi ciencies, refi ners 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 sulfur analysis available, refi neries, terminals, and test inspection certifi cation companies must take care to select a method that produces the least amount of variability in their measurements.
ASTM conducts Profi ciency 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 sulfur, using the samples provided. This paper will discuss the ASTM PTP sulfur results for Reformulated Gasoline (RFG) and Ultra Low Sulfur Diesel (ULSD) programs from 2015-2017 using the most common test methods for low sulfur 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 utilizes similar principles. Rather than using fi lters or traditional crystals that are fl at 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 sulfur signal and focus it onto the detector. This modifi ed 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, fl uorescent 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 sulfur or other interfering elements, is collected and subtracted from the sulfur signal to improve precision and LOD.
To isolate the sulfur signal and to reduce noise, WDXRF utilizes a fi lter and a collection crystal before the sulfur 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 specifi es 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 sulfur is oxidized to sulfur 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 fl uorescence that is emitted from the SO2
*), and the resulting * as it returns to the
stable state is detected by a photomultiplier tube. The resulting signal is a measure of the sulfur contained in the sample.
MWDXRF Diagram
Figure 1: Savings From Improved Measurement Precision
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