Changes in asphaltenes during thermal cracking of residual oils

This study attempts to understand how asphaltene molecules change under thermal stress. Asphaltene samples were precipitated from three crude oils and their atmospheric residues, vacuum residues, and pitch, using n-heptane. The pitch samples were obtained from the thermal cracking of vacuum residues at three operating temperatures (400, 415, and 430 °C) and three reaction times (30, 50, and 60 min). Asphaltene samples were characterized using gel permeation chromatography (GPC), ultraviolet spectroscopy (UV), metal content (Ni and V) analyses, and elemental analysis of C, H, N, and S. The experimental observations suggest that thermal cracking conditions influence the structure and the properties of asphaltene. An increase in cracking severity resulted in a notable decrease in the molecular size of asphaltene; a significant increase in aromaticity; a reduction in the H/S ratio, which indicates the presence of sulfur as a disulfide (–C–S–C–) and not as thiophenol or HS groups; and the concentration of metals in the core of smaller polyaromatic asphaltene molecules.