Identification of sulphur, oxygen and nitrogen species in heavy oils by X-ray photoelectron spectroscopy

- XPS “cold” mode for the speciation of chemical environments of heteroelements.
- Database of sulphur, nitrogen and oxygen species contained in oils.
- Follow-up of the sulphur compounds at different steps of the hydrotreatment.

The removal of heteroelements issued from petroleum products is an important problematic in the developments of the refining processes, in particular for the refining of the unconventional oils for which it is necessary to better understand and quantify the chemical environments of those heteroelements. The aim of this work was to evaluate how the speciation of sulphur, nitrogen and oxygen in oils with the X-ray photoelectron spectroscopy in the “cold” mode could be the appropriate technique. The characterization of liquids and solids with low melting point, has been possible thanks to the use of the liquid nitrogen cooling system, for which the temperature can reach −135 °C on the sample holder in the analysis chamber.The S 2p high resolution spectra recorded for the sulphur organic compounds has allowed establishing a valuable database for sulphur environments where six classes can be distinguished. The methodology developed in this work has been applied to the analysis of a straight run atmospheric residue for which thiol and thiophenic/sulfide environment has been identified. The relative concentration of those two chemical classes has been evaluated. 70% of the whole sulphur is present in thiophenic/sulfide group whereas the rest is thiol chemical class. The analyses performed on hydrotreated effluents highlight larger reactivity of the thiol compounds. But, we are facing a quite high detection limit of the XPS technique estimated at 0.1 at.% not permitting the detection of sulphur in the highly desulphurised effluents. This is still a main limitation for deep hydrotreatment applications but this technique can be used to investigate visbreaking, deasphalting or combustion processes where the heteroelement removing is less severe. Developments on the XPS technique have to be continued in order to reduce its detection limit. The whole quantitative analysis must be considered carefully as hydrogen is not detected by XPS. However, the S/C, N/C and O/C atomic ratio of the reference samples are relatively consistent with combustion techniques.