Molecular Characterization of Polar Heteroatom Species in Venezuela Orinoco Petroleum Vacuum Residue and Its Supercritical Fluid Extraction Subfractions

Abstract

A Venezuela Orinoco petroleum vacuum residue (VR) was subjected to supercritical fluid extraction fractionation (SFEF) and separated into 13 extractable fractions and an unextractable end-cut. Detailed molecular composition of polar heteroatom species in the SFEF subfractions were determined by electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The SFEF subfractions were also subjected to high-temperature gas chromatography (GC) for their simulated distillation analysis, gel permeation chromatography (GPC) for their molecular distributions, and open column liquid chromatography for their saturates, aromatics, resins, and asphaltenes (SARA) compositions. In ESI FT-ICR analysis, the mass spectra showed that the mass range and maximum peak of the SFEF subfraction increased as the SFEF subfraction became heavier. Multifunctional group compounds, such as N1S1, N1S2, N1O1, and N2, show high relative abundance in heavier subfractions. The double bond equivalence (DBE) values and carbon numbers of all class species increased steadily as the SFEF subfraction became heavier. This indicated that the molecules in various SFEF subfractions are separated by their aromaticity and molecular weight. The SFEF end-cut could not be thoroughly characterized by ESI because of its low intensity, while basic species detected by positive-ion ESI were suppressed by a strong response of metal porphyrin species. Results from GPC and SARA compositional analysis show that the end-cut enriches most of the asphaltene in feedstock and has the highest apparent molecular size.