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Sep-2019

Are you ready for the new ASTM D8252 Ni & V method using XRF?

To meet the increased demand for finished product while consistently improving overall profitability, refineries and terminals are dealing more and more with opportunity crudes.

Leslie Johnson
XOS
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Article Summary
This trend is expected to continue alongside heavy growth in overall refining capacity. According to Hydrocarbon Processing’s Industry Outlook: 2019 Update, there are a reported 415 CAPEX projects globally aimed at increasing overall refining capacity to process opportunity crudes1.

The inherent challenge in dealing with opportunity crudes is not just the increased sulphur content, but the presence of nickel and vanadium at higher concentrations than found in sweeter crude streams. Nickel and vanadium are problematic due to their impact on refining process efficiency. These elements rapidly deactivate the catalysts that are used in the catalytic cracker (FCC) and hydrotreater units.

How is the industry responding?
• First, NYMEX amended rule 200101 to add additional specifications for incoming contracts with a delivery date on or after January 2019. Among these specifications are nickel and vanadium concentration thresholds. These maximum concentrations are 8 ppm for nickel and 15 ppm for vanadium.

• Second, the American Society for Testing and Materials Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants approved a new test method for X-ray Fluorescence (XRF) analysis of nickel and vanadium in crude and residual oil to assist petroleum professionals in the laboratory environment in certifying their products: ASTM D8252.

Throughout this paper, we will review ASTM D8252 method compliance criteria, and compare data run on XOS Petra analysers with the method’s precision statements.

ASTM D8252 method highlights

ASTM D8252 allows for multiple XRF technologies to be used to provide immediate method compliance to those who have historically used XRF as a prescreen for Inductively Coupled Plasma Mass Spectroscopy (ICP) (ASTM D5708B). ASTM D8252 includes both wavelength dispersive XRF (WDXRF) and energy dispersive XRF (EDXRF) (Section 1.3), as well as polychromatic or monochromatic excitation (Sections 4.5.1 and 4.6.1)4. The Petra analysers used in the study below utilise EDXRF with monochromatic excitation, also known as HDXRF®—XOS’ branded technology, which allows for increased precision and superior performance over traditional EDXRF analysers. Please see the Technology Brief section at the end of this paper to learn more.

In Table 1 of this paper and the method, we can see the method range is from the pooled limit of quantitation (PLOQ), 1.9 ppm vanadium and 2.2 ppm nickel, to 50 ppm. The PLOQ is the “numerical limit at or above which the test results are considered to be quantitatively meaningful for commerce or regulatory activities”2. Therefore, D8252 aligns well with the NYMEX amended rule maximums. It should be noted that the PLOQs for this method are below the precision concentration ranges listed in the precision section of the ICP D5708 test method, which are 50-500 ppm and 10-100 ppm for vanadium and nickel, respectively. Note also that these ICP concentration ranges are above the NYMEX amended rule maximums.

Why XRF?
Historically, quantification of nickel and vanadium was performed by ICP analysis. ICP analysis can be demanding on the lab environment not only regarding time consumption, but also operator skill level. ICP analysis by ASTM D5708B requires rigorous sample preparation involving acid digestion and can take anywhere from 6 to 12 hours to complete a measurement. With an increasing demand for timely results alongside crude processing capacity, as well as the need to quickly onboard new and existing lab talent, refineries have looked to streamline their measurement processes to reduce time and lower the expertise threshold to meet their unique needs. Petra MAX analysis is well equipped to meet these needs as measurement takes as little as 5 minutes and sample preparation being quick to master. This measurement process is detailed in D8252 under Section 13 Procedure, and we will review this in more detail further into the paper.

Method precision
As with other ASTM methods, the precision statement in the method is a result of the Interlaboratory Study (ILS), which is detailed in Section 16.1: “The ILS was conducted in 2017 and included 15 laboratories analysing 22 samples in duplicate for nickel and 10 laboratories analysing 16 samples in duplicate for vanadium. The participating laboratories used a combination of best-fit regression calibration and matrix effects corrections based on best-fit regression, FP or theoretical alphas4.”

Calculated values from the resulting precision are detailed in Tables 2 and 3 of this paper. As a quick review, the difference between repeatability (r) and reproducibility (R) is as follows5:

•    REPEATABILITY (r) is defined as the difference between repetitive results obtained by the same operator in a given laboratory, applying the same test method with the same apparatus, under constant operating conditions, on identical test material and within short intervals of time, would in the long run and in the normal and correct operation of the test method, exceed the value calculated only once in 20 measurements (5% of the time). Or more simply put, repeatability is the maximum expected difference (at 95% confidence) between two measurement results run on the same material using the same apparatus, test method, and operator.

•    REPRODUCIBILITY (R) is the difference between two single independent results obtained by different operators, applying the same test method in different laboratories, using different apparatus on identical test material, would in the long run and in the normal and correct operation of the test method, exceed the value calculated only once in 20 measurements (5% of the time). Or, reproducibility is the maximum expected difference (at 95% confidence) between two measurements taken on the same material using the same test method by two different laboratories each using a different apparatus and operator.

HDXRF precision and ensuring compliance
To assess the precision of Petra MAX powered by HDXRF, two studies were performed:

• repeatability and accuracy study of Petra Max at 10 ppm using a known Ni and V in mineral oil check sample
reproducibility and accuracy of multiple Petra Max analysers using known and unknown Ni and V in crude oil samples

Table 4 contains repeatability data run for a mineral oil check sample—we ran this data to showcase not only repeatability but also the accuracy of Petra MAX analyser using a 10 ppm Ni and V in mineral oil sample. Ten separate aliquots were prepared using the sample preparation procedure outlined below in the reproducibility study, and each aliquot was measured for 300s each using the Petra Max Autosampler. The average of ten determinations of the 10 ppm sample was 9.98 ppm and 9.63 ppm for Ni and V respectively. The standard deviations were 0.07 ppm for Ni and 0.09 ppm for V. If a guideline of 2.8 times the standard deviation is used, repeatability for this Petra Max analyser can be estimated as 0.20 ppm for Ni and 0.25 ppm for V, which is well within D8252 calculated repeatability of 1.5 ppm.
In the reproducibility study, four crude oil samples were analysed:
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