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Resetting FCC Operating Limits to Process Domestic Crudes

The 2014 AFPM Cat Cracking Conference in Houston branched out into challenges refiners are facing with processing higher ratios of North American crudes, as in paraffinic shale crudes blended with asphaltenic heavy Canadian crudes. To be sure, these technical issues will probably resurface at future refinery venues going forward. What are some of these issues?

Well, it is a dichotomy of sorts, as we keep hearing about all the good qualities of shale crudes and light tight oils (LTOs [from shale source rock]) defined by low sulfur, high API gravity, etc., then why all the problems? First of all, these are new feedstocks and refiners need a reasonable learning curve to introduce this new chemistry into process units, many of which were purposed for processing heavy crudes from Canada, Venezuela and other waterborne imports.

With the introduction of shale crudes into the market, a higher percentage of asphaltenes from the heavy crude fraction (e.g., Canadian based bitumen) are precipitating out, leading to slurry based fouling requiring higher volumes of dispersants and stabilizers. Higher volumes of antimony are found in the slurry, which is why slurry fouling control programs are becoming more urgent, as the antimony is a significant FCC catalyst deactivator. In addition, gas production from LTO processing overwhelms the FCC wet gas compressor (WGC) and associated gas plant. To mitigate this, FCC operators attempt to reduce the FCC riser temperature to about 925 °F, channeling unvaporized feed through the riser and leading to impending coking problems, all because the riser temperature was lowered to avoid having the WGC bump up against upper operating limit constraints.

Lack of understanding the WGC and riser’s minimum feed vaporization “setting” can bring the entire FCC unit down. Sometimes, a mini-turnaround is needed to focus on these crucial settings, while also using this downtime to investigate what could interrupt a targeted FCC four-year run length. These issues, and others that will be discussed in ensuing blogs, will only increase in frequency. Already, 33% of US crudes are now light sweet shale crudes, and 35% of those shale crudes are actually LTOs — and we thought only three years ago that the US was evolving into the world’s largest heavy crude refining market!

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Posted by: The Refining Community

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