Effects of different olefinic active species on molecular growth of heavy aromatic fraction in deep processing of thermally cracked oil

Abstract

Although highly reactive olefinic species are abundant in thermally cracked oil, previous studies have predominantly focused on the macroscopic fouling effects of aliphatic olefins during transportation/storage. In contrast, the molecular-level mechanisms by which aromatic olefins promote coking during thermal conversion remained unclear. This work systematically investigated how aromatic olefins drive molecular growth and coke formation of heavy aromatic fractions. Thermal conversion experiments with vacuum residue (VR), resins (Re), and asphaltenes (Asp) mixed with olefin model compounds revealed that aromatic olefins accelerated coking compared to aliphatic olefins, with the coking-promoting effect intensifying as the number of aromatic rings in the molecule increased. At 410°C for 60 min, the addition of 5 wt% 9-vinylanthracene to VR increased the coke yield from 1.35 wt% to 5.72 wt%, versus 1.68 wt% with the addition of 5 wt% 1-octene. Similarly, the addition of 5 wt% 9-vinylanthracene elevated the coke yields for Re and Asp systems from 0.66 wt% and 41.28 wt% to 5.13 wt% and 50.84 wt%, respectively, compared to 1.08 wt% (Re) and 43.36 wt% (Asp) observed with the addition of 1-octene. Model compound system studies demonstrated that, aromatic olefins, unlike 1-octene, could further react with primary addition products formed by their initial co-addition with conventional aromatics, leading to the formation of secondary and high-order addition products. Notably, aromatic olefins exhibited increasing reactivity in continuous co-addition reactions as the number of aromatic rings increased. This was supported by 42.2 % high-order addition products detected in the toluene/9-vinylanthracene system versus 30.1 % in the toluene/styrene system. This study elucidated that aromatic olefin promoted molecular growth through continuous co-addition reactions, ultimately triggering phase separation and coke formation. The findings provided molecular-level insights into the adverse impacts of aromatic olefins on industrial processing of thermally cracked oil.