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Precision Comparison Between ASTM Test Methods of D7039, D2622, and D5453 for desulfurization of fuels
Environmental concerns cause significant changes in engine fuel specifications. Obviously, the motor fuel quality will continue to be modified to improve both combustion quality and post-combustion gas clean-up performance. To minimize the air pollution, the regulations are updated to a more severe level and bring some new restrictions.
Nowadays, many countries determined the limitations of maximum sulfur concentration in automotive fuels from 10 to 15 parts per million (ppm). Moreover, during the refining process, these compounds inactivate the catalysts used in oil refining and consequentially cause problems such as corrosion in pumps, transmission pipelines, and refinery equipment.
Desulfurization methods
The chemical nature of the sulfur has direct bearing on its removal. Desulfurization of compounds that contain aliphatic sulfur, i.e. thiols and sulfides, is not as complex as desulfurization of compounds that contain aromatic sulfur, such as thiophenics.
Hydrodesulfurization (HDS) in combination with carbon rejection technologies, such as coking and fluid catalytic cracking (FCC), are the main industrial technologies employed for the desulfurization of heavy oil. Despite the fact that these technologies are entirely capable of desulfurizing heavy oil, their carbon footprints are substantial.
All of these technologies involve high temperature processing which increases the refining cost as heavier and more sulfur-rich crude oils are being processed.
Hydrodesulfurization method
Hydrodesulfurization is the most applicable method in the petroleum industry to decrease the sulfur content of crude oil. Typically, HDS is performed by co-feeding oil and H2 gas to a fixed-bed reactor packed with a suitable HDS catalyst. The standard HDS catalysts are NiMo/ Al2O3 and CoMo/Al2O3, but there are many more types available. During HDS, the sulfur in the organosulfur compounds is converted to H2S.
Determination and analysis methods of sulfur content in crude oil
In order to lower the cost of desulfurization, it is necessary to select and utilize a precise method for determination and analysis the sulfur content in crude oil. Several standard methods have been employed to approach the expected result. Within the several standard methodologies, ASTM D7039, D2622 and D5453 methods are well known for determination of sulfur content.
ASTM Method D7039 (Monochromatic Wavelength Dispersive X-Ray Fluorescence)
Monochromatic Wavelength Dispersive X-ray Fluorescence (MWDXRF) is a subset of WDXRF that uses the same principles of this technique. Instead of 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 utilized to collect the sulfur 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 contains a high-intensity X-rays sources to excite elements of interest within a sample. Upon exposure, fluorescent X-rays are emitted from the sample at energy levels that are unique and specified to each element.
In addition, 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 reduce noise, WDXRF utilizes a filter and a collection crystal before the sulfur signal reaches the detector.
WDXRF is also different with MWDXRF in that it doesn’t specify excitation type (i.e. monochromatic OR polychromatic excitation), while MWDXRF specifies monochromatic excitation.
ASTM Method D5453 (Ultraviolet Fluorescence)
In Ultraviolet Fluorescence (UVF) technology, a hydrocarbon sample is either injected into a high temperature (1000 °C) combustion furnace directly 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.
Then the produced water 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 sulfur contained in the sample.
Conclusion
On the basis of a comparison between the result of the three methods mentioned above from 2019 to 2021 which is contributed to sulfur reproducibility, the ASTM Proficiency Testing Programs (PTP) illustrated via graph 1 and Ultra Low Sulfur Diesel (ULSD) programs demonstrated in graph 2 indicated that D7039 offer users the most reliability when evaluating sulfur in fuel, while offering significant advantages in measurement time and ease of use.
The graphs and demonstrate that ASTM method D7039 precision is better than or equivalent (within 10%) to method D5453 100% of the time, and method D2622 88% of the time when evaluating samples with an average sample concentration of 2 – 6 ppm.
Furthermore, D7039 precision is better than or equivalent (within 10%) to D5453 75% of the time, and D2622 88% of the time when evaluating samples with a mean sample concentration of 4 – 7 ppm. Additionally, the tables and graphs illustrate that test method D7039 contains most of the lower reproducibility values of sulfur.
It is important and useful to note that while the D7039 method had the best total reproducibility, it is probable to employ an XOS instrument that complies with D2622 methods while obtaining the level of performance of D7039 technology. As mentioned above, graphs show statistical result based on the concentration and efficiency in short and reasonable time during recent previous years.
As illustrated in graph 1, utilizing the ASTM RFG PTP to estimate Sulfur Reproducibility precisely resulted in higher efficiency with less wasted time from 2019 to 2021. Also, the graph 2 demonstrates that ASTM ULSD PTP possesses less sulfur reproducibility during the past two years within the time cycle date.
Graph 1: 2019-2021 ASTM RFG PTP Sulfur Reproducibility
(Sorted by Decreasing Sample Average Value* (ppm))
Graph 2: 2019-2021 ASTM ULSD PTP Sulfur Reproducibility
(Sorted by Decreasing Sample Average Value (ppm))