Atmospheric resid is further fractionated in a Vacuum Distillation tower. Products that exist as a liquid at atmospheric pressure will boil at a lower temperature when pressure is significantly reduced. Absolute operating pressure in a Vacuum Tower can be reduced to 20 mm of mercury or less (atmospheric pressure is 760 mm Hg). In addition, superheated steam is injected with the feed and in the tower bottom to reduce hydrocarbon partial pressure to 10 mm of mercury or less.
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Vaccum Distillation Units
Atmospheric resid is heated to approximately 750˚F (400˚C) in a fired heater and fed to the Vacuum Distillation tower where it is fractionated into light gas oil, heavy gas oil and vacuum resid.
Typical products and their true boiling points (TBP) from crude oil distillation (i.e., both atmospheric and vacuum tower products) are:
| Initial TBP - ˚F (˚C) | Final TBP - ˚F (˚C) | |
|---|---|---|
Light Naphtha |
80 (27) |
200 (95) |
Heavy Naphtha |
200 (95) | 380 (195) |
Kerosene |
355 (180) |
500 (260) |
Light Gas Oil |
470 (245) | 650 (345) |
Heavy Gas Oil |
630 (330) |
800 (425) |
Vacuum Gas Oil |
775 (410) | 1000 (540) |
Vacuum Resid |
1000 (540) |
|