Six ways to optimize Tier 3 gasoline

This Q&A appeared in the journal PTQ Catalysis March 15, 2021:

Q: Is there a route for post-treating gasoline with minimal octane loss?

A: George Hoekstra, President, Hoekstra Trading LLC, george.hoekstra@hoekstratrading.com

Here are six handles to improve octane/sulfur performance in your gasoline desulfurizer:

1) Feed properties: controllable variables are feed source (FCC naphtha and other naphtha streams), sulphur species distribution, olefinicity, iso/normal ratios for paraffins, iso/normal ratio for olefins, FCC naphtha cut points, and heavy tail composition. In one of our commercial field tests, detailed analysis of feed and products revealed a way to reduce octane loss from 8 RON to 5 RON when making 10 ppm sulphur gasoline. The change was implemented to realize the benefit in normal Tier 3 operation.

2) Splitter cut point: thiophenic sulphur should all be driven down into the HDS reactor while maximising C6-olefins in the overhead. Careful optimisation of this can save 2 RON octane versus ‘blind’ operation.

3) HDS reactor conditions: reaction rates of sulphur, olefins, and aromatics are affected differently by changes in reactor operating variables and can by optimised using modern analytical methods and our model.

4) HDS catalysts: we have tested competitive FCC gasoline post-treating catalysts from Axens, Albemarle, Haldor Topsoe, and a fourth supplier, side-by-side in a multi-client catalyst testing program, and ranked them on activity and octane/sulphur selectivity. Results are available to anyone in Hoekstra Research Report 6.

5) FCC catalyst technology: FCC catalyst and additives directly affect the distribution of naphtha sulphur, olefins, and aromatic species. By looking at the FCC and its post-treater as an integrated unit, it is possible to increase the reactivity of rate-limiting sulphur compounds, increase the octane of the FCC naphtha, and reduce feed sensitivity to octane loss on hydrogenation. This can have a bigger effect than selection of the post-treating catalyst.

6) Capital investments: one of our refineries performed a field test in 2017 showing unacceptably high octane loss when making 10 ppm sulphur gasoline. They did a $20 million revamp to reduce octane loss by 2.5 AKI with $27 million per year benefit. This small capital investment also enabled them to become a supplier of Tier 3 sulphur credits. Refiners considering investment in new units should consider improved, 21st century gasoline desulphurisation technologies.

These optimisation steps are enabled by use of modern analytical methods, an industry-best octane model, and a process model that tracks the critical reactions affecting octane/sulphur performance, all of which were developed in a three-year, multi-client research programme and are available to anyone at negligible cost by contacting george.hoekstra@hoekstratrading.com, +1 630 330-8159.

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