Properties of coal tar heavy fraction and its relevance to coking in hydrocracking

The hydrogenation experiment on the heavy fraction (>350 ℃) of medium/low temperature coal tar, prepared by vacuum distilling, was conducted in an autoclave, and the asphaltene was separated from heavy fraction with n-heptane solvent. The properties such as structure parameters, functional group and microstructure were analyzed by ¹H-NMR, XRD, FT-IR, SEM, ultimate analysis and average molecular weight determination (VPO). Moreover, the relevance of coking to C₇-asphaltene properties was discussed. The results show that C₇-asphaltene units are mainly composed of polycyclic aromatics linked in side with a few and short aliphatic chains (mainly n-alkane less than 3 C atoms). The asphaltene units have small average molecular weights and there is no aromatic sheet stacking structure existing. The propensity to aggregate for asphaltene units was weakened because O atoms in asphaltene are mainly distributed in epoxyalkane or ethers (C-O-C) and they are different from the petroleum asphaltene whose O atoms exist in the peripheral phenolic and alcoholic hydroxyl. So, the intermolecular hydrogen bond is more difficult to build. Furthermore, short chains are not easy to break and fewer polycyclic aromatic radicals are generated in the hydrocracking process, and the asphaltene has a lower coking potential.