Abstract: The conversion laws from feedstock to product on molecular level were studied based on the operation data from a commercial FCC unit, and the results of the apparent conversion of different hydrocarbons with aromatics formation and conversion were given. The results showed that comparing the compositions between feedstock and product, the paraffin hydrocarbon was increased by 31.47 percentage points, the naphthenic hydrocarbon was decreased by 23.21 percentage points, and the aromatic hydrocarbon was decreased by 17.47 percentage points. The mole ratio of product to feedstock was 5.92. The general apparent conversion was 80.77%, and the apparent conversions of paraffin, naphthenic and aromatic hydrocarbons were 91.99%, 96.62% and 66.10%, respectively. In cycloalkanes, the apparent conversions of monocyclic, dicyclic and tricyclic plus naphthenic hydrocarbons were 94.60%, 97.17% and 97.72%, respectively. In aromatics, the apparent conversions of monocyclic, dicyclic and tricyclic plus aromatic hydrocarbons were 86.60%, 19.64% and 68.56%, respectively. The proportions of monocyclic, dicyclic and tricyclic plus aromatic hydrocarbons in products by cracking the aromatic hydrocarbons with the corresponding aromatic ring number in feedstock were 13.4%, 38.8% and 86.1%, respectively, and the rests came from the aromatization or thickening of the feedstock or intermediate products.

Key words: heavy oil, catalytic cracking, hydrocarbons, molecular level, aromatics