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Vacuum distillation
Also known as: vac tower, vacuum unit, vacuum flasher, VDU
The vacuum tower separates the atmospheric resid generated by the atmospheric distillation tower into its components by distilling the resid under a vacuum.
The vacuum is critical for separating the VGO from vacuum resid to provide feed separately for the cracking units (FCC and hydrocracking) and deep conversion units (coker, visbreaker, asphalt). The heavier the crude processed, the larger the vacuum unit that will be required to separate the bottom of the barrel.
How it works
Atmospheric tower bottoms are injected into the vacuum tower under a pressure at about 1/20th of atmospheric pressure (typically 25 to 40 mmHg or lower). Under these low pressures, the atmospheric resid will vaporize at temperatures below those where it starts to crack. This allows separation of very heavy components without cracking.
Inputs
The feed to the vacuum is either atmospheric bottoms from the distillation tower or purchased reduced crude.
Products
There are two key distillation cuts that result from the vacuum distillation step:
- VGO or vacuum gasoil - Typically sent to the FCC or hydrocracker to be upgraded into light products. Usually separated further into light and heavy VGOs.
- Vacuum resid - Literally the bottom of the barrel. Typically blended into residual fuel oil or upgraded through a coker or visbreaker. This is also used to make asphalt, but this ordinarily requires cutting deeper to yield a heavier, more asphaltene-rich stream than is ideal for coker or visbreaker feed.
