Jet Fuel Stripper

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One of the important functions of a crude unit is to meet the flash­point specification for the jet fuel product. This is best done by varying the steam rate to the jet fuel stripper. However, the operators on this crude unit were adjusting the heavy virgin naphtha draw rate to meet the jet fuel flash spec.

Zip informed me that adjusting the steam to the jet fuel stripper did not influence its flash point. That is, the steam was ineffective in removing lighter hydrocarbons (i.e. naphtha) from the stripper feed. This was odd, because the stripper draw temperature was 435°F and the stripper bottoms temperature was 395°F. Typically, a 40°F AT indicates good stripping effi­ciency. However, when I inspected the stripper column, I saw that both the feed line and the stripper shell were entirely without insulation. The 40°F temperature reduction was primarily due to the ambient heat loss.

For the steam .to a stripper to be effective, the oil must enter the stripper close to its bubble point. If the feed is sub-cooled, the stripping steam cannot reduce the hydrocarbon partial pressure enough to promote vaporiza­tion of the oil. This renders the steam ineffective. The concept is easily understood if we remember that the heat of vaporization for stripping comes from the oil itself and not from the steam.

Pulling lighter jet fuel components into heavy virgin naphtha to meet the jet fuel flash spec not only reduces jet fuel production, it also downgrades the quality of the naphtha reformer feed.

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