What is vacuum bottom?

from the base of the atmospheric distillation tower, the fractionated atmospheric residuum or heavier ends from the first stage refining process are conveyed to the vacuum tower. Due to the high viscosity of this residual fluid a charge pump is required to boost this byproduct to the vacuum tower. At this next refinery unit the medium is once again pre-heated prior to its introduction into the base of this vessel. The hot process stream is exposed to a hard vacuum. The sub-atmospheric pressure promotes vaporization of the heavier materials at temperatures below cracking conditions. Greater amounts of light ends are removed, including light and middle fractions of fuel oils, gas oils and a residuum called Vacuum Bottoms. The resulting vacuum residue, now even more viscous than the original residuum feedstock, again requires boosting to either the facility coking unit or asphalt plant, where the medium is cooked to produce coke for use in steel and aluminum production.

 

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