Study on molecular structure and association behaviour of heavy subfractions of vacuum residue by an improved separation method

Vacuum residue (VR) is the most complex component of crude oil. Due to the special structure of heavy subfractions, physical aggregation and chemical coking reactions easily occur through molecular force, which affects the normal processing. Therefore, in-depth study and analysis of their composition, structure and association behaviours are particularly important. In view of the shortcomings of the traditional separation method in terms of separation accuracy, mainly including the purification of asphaltenes and the poor separation of resins. In this paper, a reasonably improved separation method is adopted, and the multi-stage asphaltene extraction and the multi-stage silica gel coupling separation are carried out innovatively, which achieves a high yield of 99% while ensuring the separation accuracy. The samples were characterized by elemental analysis (EA), gel permeation chromatography (GPC), Fourier-transform infrared spectroscopy (FT-IR), hydrogen nuclear magnetic resonance spectroscopy (¹H-NMR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscope (SEM) to study their structural characteristics and association behaviours. The results show that the main forms of heteroatoms in asphaltene and resin surface are C-O-C, C-OH, pyridine, pyrrole, and thiophene, and the content of these substances is higher in asphaltene. Compared with resins, asphaltenes contain a more peri-condensed aromatic structure and shorter alkyl substituent side chains. By studying the hydrogen bond and acid–base interaction, it is found that asphaltene and resin mainly contain OH-OH, OH-π, and OH-ether O, of which the content of OH-OH is the highest. Asphaltene and resin have more neutral and basic substances. These hydrogen bonds and acid–base interactions caused by heteroatoms are the main forces for the association of the heavy subfractions of the VR.