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Advanced sulphur analysis in hydrocarbons

Petra MAX™ delivers advanced D4294 sulphur analysis in addition to 12 elements from P to Zn including Ca, Fe, K, Ni, and V at sub-ppm levels for various applications:

Kyle Kuwitzky
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Article Summary
• Sulphur in diesel and gasoline using ASTM D4294 and ISO 8754
• Sulphur and heavy metals in crude using ASTM D4294 and ISO 8754
• Sulphur, Calcium, Phosphorus, and Zinc in unused lubricants using ASTM D6481
• Sulphur in bunker fuel and marine diesel using ISO 8217

In this whitepaper, we will discuss the following application studies:
• Application #1: S, Ni, V, and Fe Analysis of Crude Oil Using HDXRF®
• Application #2: Total Sulphur in Hydrocarbons from 17 ppm to 4.6 wt%

Petra MAX is a robust benchtop analyser that complies with ASTM D4294 and ISO 8754 for measuring sulphur in hydrocarbons. Petra MAX is powered by HDXRF, utilising XOS patented doubly curved crystal optics coupled with a high-performance silicon drift detector and an intense monochromatic excitation beam. This industry-leading technology reduces background noise and increases signal-to-noise output, enabling low detection limits and high precision without the need for consumable helium gas, a vacuum pump, or extensive sample preparation.

Advanced Analysis with HDXRF
Petra is powered by High Definition X-Ray Fluorescence (HDXRF) technology: an elemental analysis technique offering significantly enhanced detection performance over traditional EDXRF technology. This technique applies state-of-the-art monochromating and focusing optics, enabling dramatically higher signal-to-background ratio compared to traditional polychromatic X-Ray Fluorescence.

Figure 1 shows the basic configuration of HDXRF and its use of focused monochromatic excitation. In this system, the diffraction-based doubly curved crystal optics capture a wide angle of X-rays from the source and focus a narrow energy band (monochromatic) of X-rays to a small spot on a measurement cell. The monochromatic beam excites the sample and secondary, characteristic fluorescence X-rays are emitted. A detector processes those secondary X-rays and the instrument reports elemental composition of the sample.

Figure 2 compares the detector signal of polychromatic (competitor) with monochromatic (XOS) XRF to demonstrate how monochromatic excitation reduces background noise and improves signal definition that enables lower limits of detection and dramatically better precision. HDXRF is a direct measurement technique that requires no sample conversion, equating to no consumable gasses, little to no sample preparation, and results in just minutes.

Application Study #1: S, Ni, V, and Fe Analysis of Crude Oil Using HDXRF
Background: Test methods for measuring sulphur content, like ASTM D4294 and ISO 8754, have become critical for assessing the value of crude oil. The blending of crude oils from different sources has become more commonplace within the industry to meet specifications for the classification of sweet crude oil. The introduction of new crudes brings new challenges, like higher concentrations of metals such as nickel (Ni), vanadium (V), and iron (Fe).

Ni and V are known to rapidly deactivate process catalysts in the fluid catalytic cracker (FCC) and hydrotreaters. This occurs because nickel and vanadium are often contained in large porphyrin molecules, which are not able to penetrate into catalyst pores. As a result, the nickel and vanadium containing molecules end up depositing on the catalyst and plugging the pores, which blocks the active sites located within the catalyst material. There are many pretreat options for addressing Ni and V, however these systems require accurate understanding of the Ni and V content to appropriately treat these metals. In response, many refiners have incorporated Ni and V analysis into their routine crude assay, and pipelines have set specifications for Ni and V in their common stream sweet crude.

Fe is introduced into crude oil from corrosion byproducts during transportation and can lead to pump and exchanger fouling, and off-specification coke.

At most refinery and test labs, analysis of S, Ni, V, and Fe are performed separately. The sulphur analysis is performed using EDXRF and metal content is identified using ICP-OES. While EDXRF is capable of measuring Ni, V, and Fe in addition to S, the limits of detection do not meet the levels needed to control refinery processes. ICP-OES is able to provide the needed sub-ppm analysis however; sample preparation is complex and takes many hours to complete. It involves a series of heating, acid digestion, and ashing procedures that are quite labour intensive.
A rapid measurement technique for sulphur compliance and simultaneous analysis of Ni, V, and Fe is necessary to meet the needs of refiners, pipelines, terminals, and other petroleum test labs.

Application Study
In each study, 10 separate aliquots were prepared and analysed for 5 minutes each. Their individual measurement results and average are reported. Samples were prepared by transferring 6 mL to a 43 mm XRF sample cup and sealed with an Etnom film. Each refinery or pipeline location has their own specifications for specific elements like S, V, Ni and Fe. The desired level or limit for each heavy metal may vary depending on the detriment its presence causes to the equipment, process, or finished product. But in the case of V, Ni, and Fe, current methods can take a significant amount of time to prepare, including hours for ashing and analysis, and if outsourced, can be quite costly. Table A outlines common pipeline feed specifications.
Accuracy Study
To study the accuracy of Petra MAX, ten repeat measurements were performed on a commercially-available mineral oil reference material containing 10,000 ppm of S, and 10 ppm of V, Fe, and Ni. See Table 1 for results.

Precision Study
To study the precision of Petra MAX, 10 repeat measurements were performed on two different crude oil samples containing S, V, Fe and Ni. The results shown in Tables 2 and 3 demonstrate that Petra MAX delivers precise measurement results well below desired specifications, and therefore is a valuable tool for monitoring trends as well as identifying materials that simply do not meet specification.

Limit of Detection
By incorporating patented doubly curved crystal optics to monochromate and focus the excitation beam, the Petra MAX is able to achieve low limits of detection without the assistance of a vacuum pump or consumable helium.
In response to the increased blending of sweet crude oil with crude containing higher levels of metals and S, petroleum labs are beginning to see the need to measure other elements, in addition to S, as a part of their crude assay. This study demonstrates that Petra MAX delivers simultaneous trace metals and ASTM D4294 or ISO 8754 analysis of S, in a single measurement. Petra MAX performs this analysis without the difficult ashing sample preparation that makes the analysis of metals like V, Ni, and Fe in crude oil so difficult today.
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