Analysis of asphaltene structure and its effects on the coking behavior in the process of hydrothermal cracking

C₆-asphaltene and deasphalted oil were obtained by using the Ta-he atmosphere residue (THAR) as the feedstock and n-hexane as the solvent. The deasphalted oil was then mixed with C₆-asphaltene to prepare the residue with different asphaltene contents for autoclave hydrothermal cracking experiments. The molecular structure parameters, functional groups, crystal structure and surface morphology of C₆-asphaltene were systematically analyzed by elemental analysis, ¹H -NMR and ¹³C-NMR, GPC molecular weight, FT-IR, XRD and SEM. The results show that the aromatics in C₆-asphaltene exhibit a highly branched degree and the side chains consist of mainly methyl, ethyl and propyl groups; the aromaticity (f_A) of C₆-asphaltene reaches 0.57. The aromatic ring systems have the peri-and cata-condensed structures, with high condensation degree and large aromatic slice sheets. The influence of C₆-asphaltene content on its hydrothermal cracking behavior was further investigated. The result illustrate that the conversion of the residue oil is gradually increased with the increase of the asphaltene content; however, when the asphaltene content exceeds 5.12%, the increment of conversion is at the cost of the rapid coke formation. In addition, the yield of light oil increases at first and then levels off with the increase of the content of aliphatic carbon (f_C³) in long chains, whereas the carbon residue and aromaticity (f_A) show a good linear relationship with the yield of coke.