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HDXRF vs ICP for nickel and vanadium in crude oil

Technology introduced over the last decade, such as horizontal drilling and hydraulic fracturing, has led to new sources of light tight oil (LTO).

Kyle Kuwitzky and Leslie Johnson
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Article Summary
LTO has grown in the US from essentially zero in 2010 to about 5 million barrels per day in 2017, exceeding the US production volume of non-tight oil. This trend is expected to continue with projections of 10 million barrels per day in the US by 2025, and significant supply in countries like Russia, China, Canada, Egypt, and Argentina. This is reshaping the landscape of available refining feedstock and challenges are arising across the industry. Refineries in the US Gulf Coast and across the world have invested significantly in processing units to handle much heavier crude oil. The new LTO contains significantly more naphtha than crude from conventional sources. Refiners are experiencing bottlenecks in the light ends distillation capacity and are having trouble keeping their conversion units, like the FCC, hydrocrackers, and cokers, full.

These changes are also having an impact on the quality of West Texas Intermediate (WTI) traded under the NYMEX Light Sweet Crude Oil (CL) futures contract delivered in Cushing, Oklahoma. The oil delivered is subject to specifications such as sulphur and API gravity, and oil blending near to the specification limit is common. Figure 1 plots the sulphur content of WTI delivered at Cushing. The sulphur content is consistently below the specified maximum of 0.42 wt% but never drops below 0.38 wt% as a result of oil blending.

However, this blending creates new processing challenges for refiners as oils from other sources can introduce changing levels of other contaminants. Figure 2 plots the vanadium content of WTI delivered at Cushing, and depicts a trend toward higher levels over time. This is a result of blending oils from different sources with the WTI prior to delivery in Cushing. These changes in other oil quality parameters due to blending have led to many issues for refiners processing the crude oil. In response, NYMEX has amended rule 200101 to add five additional quality specifications including nickel and vanadium for contracts with delivery in January 2019 and beyond. The maximum concentrations allowed under the amended rule are 8 parts per million in nickel, and 15 parts per million in vanadium.

Nickel and vanadium are naturally occurring in crude oil and become concentrated in the resids and heavier fractions of vacuum gas oils. They are known to rapidly deactivate cracking catalysts and can lead to off-specification coke, resulting in considerable costs to refiners. While refiners often look for opportunities to buy lower cost oils to improve profitability, understanding the content of contaminants like nickel and vanadium is important in order to adequately assess the impact on processing. Nickel and vanadium in crude oil can be tested using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) using ASTM test method D5708B. However, there are drawbacks to this technique. First, it requires a rigorous sample preparation process that involves strong acids, heating with hot plates, furnaces, and consumable gasses in a laboratory setting. Second, it is very time consuming: prep to analysis can take between 8 and 12 hours. Because of these drawbacks, ICP is not an efficient solution for analysis of nickel and vanadium in crude oil. In response, a faster, easier, and less expensive solution has been developed.

X-ray Fluorescence Spectroscopy (XRF) is an alternative technology to ICP and most commonly used for sulphur analysis in liquid hydrocarbons like crude oil, fuels and lubricants. Utilising standard methods like ASTM D4294 and ISO 8754, XRF is included in most crude oil specifications today. Petra MAX, a new XRF analyser, delivers ASTM D4294 sulphur compliance with simultaneous measurement of nickel, vanadium, iron, and nine other elements at sub-ppm levels.

Petra MAX is powered by High Definition X-ray Fluorescence (HDXRF) technology: an elemental analysis technique offering significantly enhanced detection performance over traditional XRF technology. This technique applies state-of-the-art monochromating and focusing optics, enabling higher signal-to-background ratio compared to traditional polychromatic XRF. HDXRF does not require sample conversion, equating to no consumable gasses, little to no sample preparation, and delivers results in minutes.

Figure 3 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 the secondary X-rays and the instrument reports elemental composition of the sample.

Figure 4 compares the detector signal of polychromatic (competitor) with monochromatic (XOS) XRF to demonstrate how monochromatic excitation reduces background noise and improves signal definition, delivering lower limits of detection and dramatically better precision.

HDXRF vs ICP study
A study was conducted to compare the sample preparation process and precision using Petra MAX, powered by HDXRF, and ICP to measure nickel and vanadium in crude oil. Four crude oil samples were obtained for the comparison study:

    A. Custom doped crude oil standard
 from VHG Labs

    B. Sour crude oil retain from Intertek

    C. Medium sour crude retain from

    D. Heavy sour crude retain from

Three independent laboratories analysed the sample set using ASTM D5708B (ICP) and Petra MAX (HDXRF). Each participant received two randomised sample sets packaged in blind duplicate for analysis. The resulting raw data sample means can be seen in Table 1.

ICP sample preparation notes
ICP performed by ASTM method D5708B requires extensive sample preparation when analysing crude oil. Crude oil is first transferred into a glass beaker and weighed. The sample is digested with sulphuric acid and heat, using both an infrared lamp and a hotplate. The sample is stirred continuously with a glass rod during digestion and carefully monitored to ensure that no sample is lost due to frothing and spattering. Once the foaming has finished, the heat is increased until the sample has been reduced to a carbonaceous ash. This process is completed inside a well-ventilated fume hood using gloves and a face shield for protection from strong oxidising fumes generated during digestion. This ash is then carefully transferred to a muffle furnace and all of the carbon is burned away. The remaining material is then reconstituted with nitric acid and returned to a steam bath. This beaker is then transferred to a hotplate where it remains until the liquid has fully evaporated. Once again, nitric acid is added and the sample is transferred to a volumetric flask and brought to a known volume with additional nitric acid. The sample is then ready for analysis. Users report that this process of sample preparation to results takes anywhere from 8 to 12 hours to complete.

HDXRF sample preparation notes
Samples prepared for analysis by Petra MAX are first shaken to homogenise the sample. Using a disposable pipette, 7 to 10 mL of sample is transferred to a disposable plastic sample cup designed for XRF analysis. An X-ray transparent film is then placed over the opening of the cup and affixed using a snap-on ring. Finally, the sample is placed in the instrument and ready for analysis. This process from sample preparation to results takes about 6 minutes.
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