As quality and environmental demands on the petroleum industry become stricter, key elemental contaminants require ever-lower levels of quantification. Examples include stricter regulations on sulfur (S) in vehicle fuels, and reduced trace metals such as nickel (Ni) and vanadium (V) that poison catalysts and induce corrosion. Preventing corrosion is essential to decreasing expensive maintenance and replacement costs for the large industrial engines found in marine vessels, industrial boilers, power facilities and similar operations. Such engines typically consume diesel fuel oils (also called gas oil) which may retain natural contaminants such as vanadium and nickel from crude oil even after refining. These elemental contaminants remain and concentrate in the heaviest distillate fractions associated with heavy fuels; refining process contaminants also concentrate in these streams. When traces of vanadium and nickel in fuel oils react with other contaminants in aggressive engine environments they form vanadium oxides and other corrosive compounds. At high temperatures, in oxygen-rich combustion engine environments, the concentration and interaction of these abrasive and corrosive elemental contaminants can become damaging, reducing equipment service life. Various industrial diesel oil specifications typically restrict traces of vanadium and nickel to 100 ppm or less, in some cases setting limits at sub-ppm levels, for example, vanadium less than 0.5 ppm and the sum of vanadium plus nickel plus other metals less than 1.0 ppm. Determining metal content can indicate the type and severity of wear occurring within the unit. International Standard ISO 14597:1997 specifies wavelength-dispersive X-ray fluorescence (WDXRF) as a preferred technique for analyzing trace metal contaminants such as nickel and vanadium in petroleum products. It may also be applied to semi-solid and solid petroleum products that are either liquefied by moderate heating or completely soluble in the specified organic solvent mixture. The method is applicable to products having vanadium contents in the range 5 mg/kg to 1 000 mg/kg, and nickel contents in the range 5 mg/kg to 100 mg/kg, although precision data have only been determined up to 100 mg/kg for vanadium and 60 mg/kg for nickel; higher contents may be determined by appropriate dilution. The proven ability of WDXRF to produce highly reliable and repeatable results is increasingly solicited for such analyses in the petroleum industry due to its ease of sample preparation, multi-element analysis and high sensitivity and precision. Its main advantages for such analyses are:
- Excellent repeatability
- Excellent resolution, especially for light elements (Na to Ca)
- High dynamic range (sub-ppm to 100 %)
- Flexibility in terms of measurement of analytical lines, background positions and internal references
- Little or no sample preparation in most cases (samples can be directly measured without dilution)
- Multi-element and multi-matrix capability.
WDXRF was put to the test on a series of four petroleum standards to analyze sub-ppm traces of vanadium and nickel in diesel fuel oils. See the complete study including tables and charts illustrating the analytical conditions used, and the limits of detection achieved.