Industrial: Refining: Vaccum Distillation

The residue from the atmospheric tower contains gas oils that are not recovered. The Crude Unit heater outlet temperature is limited due to the concerns over thermal cracking. Thus, the residue from the bottoms of the Atmospheric Distillation unit can be piped to another distillation column operated under vacuum (0.4 PSI), called a Vacuum Distillation unit (VDU), to recover additional heavy distillates. This allows the heavy hydrocarbons (boiling points >450^oC) to be separated without the threat of partial thermal cracking into less desirable products such as coke. Since the boiling points of hydrocarbons are a function of pressure, the VDU can vaporize these components at a temperature lower than that at which thermal cracking will occur.

VDU designs typically fall into one of two categories: dry design and wet design. Dry design VDUs do not introduce steam into the tower, and are more applicable to gas oil and fuels production. Wet design VDUs inject steam to the vacuum heater and the vacuum tower bottoms stripper. This allows this design to get the same degree of vaporization as the dry design, but at lower temperatures. The wet design is more suited to the production of gas oil, lube oils, asphalt, or waxes.

Vacuum columns have larger diameters and simpler internals when compared to the atmospheric columns. The vacuum is produced through the use of steam ejectors or vacuum pumps pulling a vacuum on the top of the distillation column.

Typical products from VDU, depending on the design, include an overhead stream, called light vacuum gas oil (LVGO). It is often used as a lube base stock, a heavy fuel oil, or can be fed to another conversion unit. Heavy vacuum gas oil (HVGO) is a side draw off the tower, and can be used to make asphalt, or can be fed to a Visbreaker or Coker unit for further processing.