What thing(s) indicate the need to increase CDU stripping steam?
Answers
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13/08/2020
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A:
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Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
For a side stripper it would normally be linked to a spec like flash point or copper strip corrosion (H2S). For the bottom of the tower, it is linked to yields, but seeing the difference in yields that results from increasing stripping steam is not always easy to see. Generally it is recommended to run the maximum steam possible up to the recommended best practice of 10-12lb steam/bbl of atmos tower bottoms. Running maximum steam is esepcially important when the vacuum tower is a constraint. The increased stripping helps reduce vacuum tower feed rate, which lowers furnace duty and the vapor traffic in the column.
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12/08/2020
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A:
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Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
It is not clear from the question whether you are referring to steam in the base of the atmospheric tower or the side strippers. I’ll take the side-strippers first:
The biggest impact of steam in the side-strippers is on the flash-point of the product. If the flash-point is off-spec, increasing the stripping steam is often a good response. If the product is going to a hydrotreater (you posted the question in the Hydroprocessing section, so I’ll assume so), there is a second opportunity to correct the flash-point in the stripper tower at the back end of the hydrotreater. There is a significant energy advantage to correcting the flash-point at the side stripper, but there may be reasons why the flash is allowed to drift at the crude unit and corrected in the hydrotreater (hydraulic constraints in the crude tower, overhead dew-point, possible cut-point advantage etc).
Regarding the stripping steam in the base of the tower, its function is to remove distillate material from the residue to prevent it reaching the vacuum tower. If the vacuum tower is overloaded with distillate material (excessive vacuum tower overhead slop oil / hotwell oil, hydraulic constraints in the upper section of the vacuum tower, too much diesel range material in the VGO etc.), increasing the base steam in the atmospheric tower may help. However, this will increase the heavy diesel / gasoil side-draw end-point, push the tower closer towards a vapour hydraulic constraint and move the overhead system closer to dew-point.
I hope that helps.
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