Hydrothermal desulfurization was evaluated for the upgrading of bitumen in supercritical water with alkali, based on the formation and decomposition of thiophene derivatives. In the alkaline hydrothermal treatment of bitumen at 430°C and 30 MPa with KOH, drastic reduction of sulfur content and visbreaking occur in the first several minutes, followed by relatively slow reactions. This study investigated the desulfurization at the latter stage. The results can be summarized as follows:
(1) In the alkaline hydrothermal treatment of bitumen, benzothiophenes (BTs) and dibenzothiophenes (DBTs) were formed first, and subsequent decomposition of BTs resulted in the overall desulfurization of bitumen.
(2) Alkali was consumed to form BTs and DBTs by decomposing bitumen.
(3) Amounts of each BT and DBT formed were influenced by the type of of bitumen, although the same BTs and DBTs were formed.
(4) Types of alkyl BTs detected represented about a half of the theoretically possible isomers, and the carbon number of the alkyl group(s) was mostly three or less.
(5) BTs were easily decomposed in comparison to DBTs and much smaller amounts of DBTs were formed, so that the desulfurization mainly progressed via BTs. The sulfur content of remaining DBTs in the upgraded oil was less than 0.3 wt%.