FCCU Optimization for Tier 3 Gasoline

Ascent was asked to evaluate options to meet Tier 3 10 ppm sulfur in gasoline specifications, review the FCCU Gasoline Hydrotreater (Cat Gasoline Hydrotreater, CGHT) unit, and identify modifications to meet Tier 3 specifications

A client needed to meet the Tier 3 10 ppm sulfur in gasoline specification. They recognized that high sulfur levels in the final hydrotreated CGHT product was the main obstacle to achieving the Tier 3 specification. Prior engineering studies had focused on the high sulfur CGHT product and recommended expensive new reactors to mitigate. When Ascent was brought on board, the client’s focus was on how to revamp the CGHT or change its operation to meet Tier 3. Ascent was asked to help evaluate various options to achieve the desired gasoline sulfur level

The sulfur content in the CGHT feed (sour naphtha from the FCCU) was less than design, but the product sulfur content was higher than design. This was at least partly due to operating in low severity Octane Preservation mode, which results in less octane loss than in high severity mode. High severity operation does improve sulfur removal, and was a feasible option, but it had a large economic penalty due to increased octane loss. The client was proposing to change the catalyst in the polishing reactor, but commercial operating data did not give confidence that this would achieve the desired product sulfur.

Ascent expanded the project focus to a holistic review of the entire complex system, which lead to much more cost effective solutions across the refinery. The refinery had an FCCU Feed Hydrotreater (Cat Feed Hydrotreater, CFH) to improve the FCCU feed quality and to remove sulfur. Unfortunately, the capacity of this unit was less than the capacity of the FCCU and so it did not treat 100% of the FCCU feed. Ascent convinced the project team that reviewing the CFH was crucial to finding the best Tier 3 overall refinery strategy. Ascent recognized that hydrotreating more FCCU feed would reduce the sulfur in the CGHT sour naphtha feed, and with a large enough reduction, the CGHT product would meet the Tier 3 specification without changing CGHT catalyst or increasing the CGHT reaction severity.

The project scope did not initially include review of the CFH for several reasons. The belief was that the CFH was already operating at its maximum charge rate and could do no more, although the actual unit bottleneck was not identified. The unit was believed by the plant to have been designed with minimal design margins, but innovative review by Ascent identified significant debottlenecking potential with zero to moderate capital investment. There was also resistance to using the CFH as the Tier 3 compliance mechanism because of the need for the unit to take periodic turnarounds, but Ascent was able to prove that these concerns could be mitigated.

Further investigation determined the CFH charge rate limit had been set by a safeguarding study. A scoping study by Ascent indicated that the unit could be significantly debottlenecked, confirmed by a test run that proved that revised operating conditions could increase the unit capacity by more than 20% for little to no capital investment. Moderate capital investment could increase capacity by 50%. The increased CFH capacity not only helped meet Tier 3 gasoline specifications, but improved FCCU yields, maximized hydrogen uptake and volume swell, and reduced the required CGHT severity and octane loss. This was far more attractive than the expensive CGHT revamp options, which cost more and provided only compliance without yield benefits and came with a penalty in octane loss.

Part of Ascent’s work was to evaluate the CGHT in detail and identify no or low cost options to improve its performance. Unit simulation and detailed equipment evaluations identified several relatively minor equipment design issues which needed to be corrected for the unit to operate up to its potential. Ascent also recommended some changes in operation which would help improve the CGHT’s performance at little or no cost.

Another key Tier 3 compliance option identified by Ascent was the option to undercut FCCU gasoline. Ascent proved through rigorous analysis, simulation, and hydraulic modeling that the LCO product circuit was hydraulically limited. This hydraulic limitation prevented undercutting FCCU naphtha. Ascent proposed much more drastic FCCU gasoline undercutting, greatly reducing the CGHT feed sulfur and also greatly reducing the quantity of the most difficult to treat heavy sulfurs. During the majority of the year, this additional distillate production was economically attractive in addition to aiding Tier 3 gasoline sulfur compliance.

Bottom Line:
Ascent’s cost-conscious approach allowed for identification of low capital cost options to expand the CFH capacity by 20-50% while reducing CGHT feed sulfur to meet Tier 3 product specifications. Not only that, Ascent’s recommended strategy with the CFH expansion and FCCU gasoline undercutting was a profitable project due to increased hydrogen uptake, increased volume swell, improved FCCU yields, and maximization of distillate over gasoline.

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