“…Because the main essence of upgrading is to increase sulphur-free, nitrogen-free and metal-free distillates, as well as H/C ratio which typically stands at 1.5 [66], typical surface-based approaches for heavy oil upgrading involve carbon rejection (such as coking, thermal cracking, visbreaking), hydrogen addition (with slurry, fixed-bed, moving-bed, ebullated reactor), or their combination [5,67]. The traditional thermal conversion of heavy oil involves either pyrolysis, cracking (which may be catalytic or thermal-only), or hydrocracking to yield valuable distillates components [68]. The key reactions in upgrading have been detailed previously [6,69].…”