Optimizing FCC - Pre-Treat Unit Operation to Reach Tier 3 Gasoline Sulfur Targets

摘要

The United States Environmental Protection Agency is considering implementation of more stringent Tier 3 ultra low sulfur gasoline (ULSG) sulfur specifications that would likely require all gasoline to contain ≤10 wppm sulfur. While the anticipated Tier 3 sulfur limit would seem to be a small step from the current Tier 2 limit of ≤30 wppm, the impacts on refiners, especially those without FCC naphtha post-treat capabilities, could be significant. Likely consequences for the refiner of having to meet the lower FCCU feed sulfur limits are that FCC-PT (FCC feed pre-treater) unit cycle lengths will be decreased significantly and operating costs for these units will be increased. This paper examines the likely impacts of the anticipated Tier 3 ULSG specs on refinery FCC-PT unit operations and the consequences for refineries. Units operating in different pressure regimes are simulated to show practical examples of the expected impacts for different types of FCC-PT units. These case studies assume 100% hydrotreatment of FCC feed and are for illustration purposes only. Actual impacts of the Tier 3 specifications will be highly dependent upon specific unit feed compositions, unit capabilities and constraints, and FCC-PT operation strategies. For units operating in constant hydrodesulfurization (HDS) mode, implementation of the Tier 3 motor gasoline (mogas) specs will likely cause FCC-PT unit cycle length to be reduced by 20-40%, even if very high activity CoMo or NiCoMo VGO HDS catalysts are used. For high pressure FCC-PT units operating in maximum aromatics saturation mode, the decrease in cycle length could be 50% or higher. Economic assessments of the impacts will be highly individualized. The refiner will need to draw the process envelope broadly to account for both the FCC-PT and FCC units' expected performance, as the better quality FCCU feed will significantly impact that unit's performance in terms of yields and qualities. Refiners with FCC naphtha post-treat will need to include that unit in the process envelope also, as the lower sulfur content of the Tier 3 FCC naphtha could help them keep octane losses similar to those incurred meeting Tier 2 gasoline specs. Finally, refiners who have the flexibility to consider alternative modes of FCC-PT operation, such as mild hydrocracking (MHC), may opt to evaluate alternative operating strategies and the projected impacts on overall yields and qualities of both intermediate and finished product streams.