REACTIVITY OF SULPHUR SPECIES IN BITUMEN PITCH AND RESIDUA DURING FLUID COKING AND HYDROCRACKING

Abstract

ABSTRACT X-ray photoelectron spectroscopy (XPS) was used to determine sulphur species and their distribution in fractions of fluid coking residue (CK) and hydro-cracking residue (HC) derived from an Athabasca bitumen pitch (ABP). The major sulphur species were sulphides/mercaptans and thiophenes in each case. No sulphoxides and sulphones were detected in any non-oxidised samples. Each residua was separated into between 9 and 15 front-cut samples and a single end-cut fraction by supercritical fluid extraction and fractionation (SFEF). This approach has been used to produce different molecular weight fractions, depending on the severity of separation conditions. In the fractions from the parent Athabasca bitumen pitch about 65–80 wt% of the total sulphur occurs as thiophenes with the remainder being sulphide species. The reactivity of the sulphur species in the parent pitch was determined under mild conditions using a standard technique for selective oxidation. In this procedure sulphides are preferentially oxidized to sulphoxides while thiophenic sulphur remains unchanged; under the same reaction conditions the degree of sulphide conversion generally declined with increasing molecular weight of the fractions. Commercial catalytic hydrocracking completely removed sulphides from the front-cut fractions of the parent pitch. However, the composition of the end-cut fraction from the residua produced by this process was virtually the same as that for the comparable fraction from fluid coking. This observation demonstrates that hydrocracking has little effect on intractable heavy ends. Also, hydrocracking removed only a relatively small proportion of thiophenic sulphur. On the other hand, while commercial fluid coking removed most of the sulphides from the parent pitch the residue was enriched with thiophene compared to the parent feed. This observation demonstrates the partitioning effect occurring during coking reactions.