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Atmospheric bottoms
Also known as: atmospheric resid, atmospheric residue, reduced crude, long residue, atmospheric reduced crude, topped crude
Atmospheric bottoms is the heaviest of the distillation cuts out of the atmospheric distillation tower. It consists of all of the components of crude oil that have boiling points above about 650F (343C). These cuts cannot be vaporized in the atmospheric distillation tower because they begin to crack or break down at the temperatures required to achieve vaporization. Consequently, they remain in liquid form during the atmospheric distillation process and literally come out of the "bottom" of the tower.
Typically, atmospheric bottoms is sent to a secondary distillation tower, the vacuum distillation unit. Here it is distilled under a vacuum, which allows the unit to operate at temperatures that do not result in cracking. This yields VGO and vacuum resid that can then be upgraded to yield light products.
In the absence of a vacuum distillation tower, atmospheric bottoms may be blended into residual fuel oil. However, this is typically a lower value use than upgrading.
Additionally, for crude oils that are light and sweet, it is possible to feed their atmospheric bottoms directly into an special version of the FCC called an RCC or resid catalytic cracker.
Author: Tim Fitzgibbon, Refining Industry Sr. Expert
