Crude oil is the starting material for a large number of consumer products our modern world is depending on. The most common and well-known compounds are gasoline and kerosene as fuels for vehicles and planes. But also asphalt is made of crude oil as well as chemical reagents to make plastics and pharmaceuticals. For 2016, the IEA Oil Market Report forecasts a worldwide average demand per day of about 96 million barrels each day regardless of the oil price.
All of the oil we use today was formed millions of years ago when large quantities of dead organisms were buried underneath sedimentary rock and were then subjected to heat and pressure. The resulting crude oil is, therefore, a complex mixture of various hydrocarbons with water, salts and other impurities (such as sulfur). The necessary separation of the components, called refining, is done by distillation (making use of the difference in their boiling points) and other processes, including catalytic cracking.
Desalting of crude oil is crucial
Crude oil usually contains inorganic chloride salts which can react with the oil to form organic chlorides. Both organic and inorganic chlorine compounds can cause considerable corrosion of the refining installation, pipelines, and tanks and can lead to catalyst poisoning. Therefore desalting is one of the first separation processes that take place at the front end of a refinery. Desalting removes the majority of the salts by the addition of water, which dissolves the salts and other water-soluble impurities from the crude oil feedstock. The mixture is then separated; the water goes to an industrial wastewater treatment facility and the desalted crude oil is refined into various petroleum products.
Even very low chlorine concentrations in crude oil are detrimental, no matter whether they come from inorganic salts or organic compounds. Obviously, a fast and reliable determination of the chlorine concentration in crude oil and related matrices is beneficial for any refinery. Traditionally wet-chemical methods have been employed but they take time and are sensitive to errors. Chlorine analysis by X-ray fluorescence (XRF), on the other hand, is easy and straightforward and does not involve tedious sample preparation such as dilution. Even trace levels of chlorine can precisely be quantified by XRF.
The determination of low concentrations of chlorine is, however, complicated by sulfur, which is present in crude oil in concentrations of mass%. Sulfur and chlorine are neighbors in the periodic table of the elements and have very similar excitation conditions. Therefore chlorine peaks from low concentrations tend to disappear among the much higher sulfur peaks. This makes determination of trace levels (mg/kg) of chlorine in the presence of high amounts of sulfur (mass%) a challenging task for any XRF spectrometer.
Tuning an X-ray fluorescence spectrometer for crude oil analysis
To meet this challenge, three essential components of an XRF spectrometer need to be tailored: the X-ray tube should produce a low spectral baseline while maintaining optimal excitation of S and Cl. The X-ray detector has to be able to process high sulfur count rates and at the same time resolve possible overlaps between sulfur and chlorine peaks. Last but not least the software should be able to take other elements into account and correctly apply the required matrix corrections for differences in total composition.
With such a customized XRF spectrometer the determination of very low levels of chlorine in the presence of high sulfur concentrations is fast, easy and accurate. Quantification of 2 mg/kg chlorine next to 1-5 m% sulfur is not a problem anymore. And what is even better is the XRF-inherent possibility to determine all other relevant elements at the same time!
Anyone thinking this quantification limit is only possible on a floor standing high power XRF should read about the advances in benchtop XRF allowing similar quantification limits in the presence of high sulfur.