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Reply To: Recycle ratio of Delayed Coker unit

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Mike Kimbrell

Recycle ratio directly controls the HCGO 95% point with higher recycle resulting in lower distillation temperatures. A low recycle Coker will have limited mass transfer equipment in the wash zone so the fractionation efficiency will be low between the coke drum overhead vapors and the HCGO. As the recycle ratio increases the contact efficiency increases and entrainment issues are less important.

A low recycle Coker with a recycle ratio of 1.05 will have a HCGO 95% point of approximately 1050 F (565 C). At a recycle ratio of 1.20, the HCGO 95% point will be approximately 900 F (480 C). There is variability around these values of roughly 100 F (50 C) depending on the unit. I would estimate a Coker operating at a 1.10 recycle ratio to have a HCGO 95% point of 975 F (525 C). The 95% point of the HCGO can be as low as 775 F (415 C) when the Coker was operated at a recycle ratio of 1.45.

It is common to monitor HCGO quality by monitoring density, color or MCR (micro carbon residue). These are relatively quick tests that can provide trends in the HCGO quality. Distillation and asphaltenes content are other tests that are done less frequently. Entrainment of asphaltenes into the HCGO can cause rapid catalyst deactivation in any downstream hydroprocessing unit. Additionally, these asphaltenes can cause the HCGO filter to blind and appear to be plugged and they do not backwash off very effectively. If the HCGO is routed directly to a FCC unit, entrainment of asphaltenes can result in a lower conversion in that unit.

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