Comparative study on the coking characteristics of two low-asphaltene heavy oils and their main fractions

Heavy oil coking is an important process for in situ combustion. The coking characteristics of two low-asphaltene heavy oils from Fengcheng (FC) and Hongqian (HQ) oilfields, which are different in viscosity, density, and composition, were studied in a fixed bed reaction system in pyrolysis and oxidizing atmospheres. FC heavy oil showed a 35% higher coke yield than HQ heavy oil, and the coke oxidation activity was worse, whereas the coke elemental compositions and heat values were similar for both cokes. Kinetic analysis of coke oxidation showed that both cokes from FC and HQ oils had similar activation energies, 124 kJ/mol for cokes from oil pyrolysis and 138 kJ/mol for cokes from oil oxidation, but their active site numbers are different. The coking characteristics of two oils' main fractions (saturates, aromatics, and resins) were studied and compared. In the inert atmosphere, the main coking fraction was resins with a coke yield of 17%, whereas all three fractions produced coke in the oxidizing atmosphere. The coke yields of saturates, aromatics and resins were around 7%, 25% and 57% in oxidizing atmosphere, respectively. There was a small difference in the coke yields of the same fraction from the two heavy oils. Saturates and aromatics from the FC heavy oil showed a 20-30% higher coke yield than the same fractions from HQ oil because of the different contents of low-boiling-point components. After removing the influence of low-boiling-point components, the coke yield of the heavy oil could be predicted by a weighted average method based on the main fraction contents and their coke yields. Upon comparing the coke produced by different fractions, the coke from saturates showed better oxidation activity than that from aromatics and resins. The oxidation activities of coke produced by FC aromatics, HQ aromatics, and HQ resins were similar, but that of coke produced by the FC resins was worse. FC oil coke was less reactive than HQ oil coke probably due to the high resin content of FC heavy oil and the low oxidation activity of the coke from its resins.