Question
-
Is there a route for post-treating gasoline with minimal octane loss?
Mar-2021
Answers
-
Steven Zink, Honeywell UOP, Steven.Zink@honeywell.com
Assuming FCC pre-treat capacity installation is not an option, sufficiently low octane deltas on post-treatment of FCC gasoline can be achieved via sequential application of several key molecular management operations. Light mercaptan sulphur is catalytically sweetened out of the light-cut, highest octane olefin fraction, which is recovered as a distillate product in a gasoline fractionator, with a recovery rate that is subject to the gasoline pool sulphur specification. The hydrotreating catalyst processing the mid-cut should perform with a very high ratio of desulphurisation rate vs olefin hydrogenation rate, in one or two hydrodesulphurisation stages, depending on the feed sulphur content and the maximum tolerable octane loss. At 10 ppm or less sulphur specification (US Tier 3), this fraction will require polishing, due to olefin recombination with the hydrogen sulphide produced in situ. Catalytic polishing of the deeply desulphurised mid-cut, carried out at relatively higher temperatures where recombination is less thermodynamically favourable, should be leveraged as much as possible to retain more olefins. The hydrogen partial pressure should be kept only just high enough to achieve the desired extent of desulphurisation, to limit octane loss by olefin hydrogenation.
To limit the loss of olefins to recombination, the hydrogen sulphide partial pressure should be kept as low as possible via application of a well-maintained amine scrubber to the recycle gas. The heavy gasoline fraction contains the lowest concentration of olefins (more aromatic than olefinic) and the highest sulphur concentration. Such sulphur is also the slowest to convert, relative to the light-cut and mid-cut sulphur. Considering that the few olefins in the heavy fraction contribute the least to gasoline octane, the heavy fraction is best routed to a conventional naphtha hydrotreater, to enable greater production of high-octane reformate gasoline.
Mar-2021
SPONSORS