Vacuum Distillation

Vacuum distillation involves taking the residue produced in the crude distillation unit and further separating this fraction into lighter fractions or cuts. As the atmospheric (or crude distillation unit) residue is "heavy", the separation of this stream into various fractions would normally require more energy and higher temperatures. In order to reduce the amount of energy input required, this process is effected under vacuum, ie. less than atmospheric pressure. In modern designs, the vacuum used is sometimes as low as a few mm H₂O. The vacuum acts so as to reduce the boiling point of the various fractions.

Because of the temperatures involved, there is an opportunity to recover some of the heat with other refinery process streams. This leads to enhanced efficiency in the overall refinery operation.

Again, vaporisiation of the inlet atmospheric residue stream is important to pre-condition the feed. This is normally effected by a fired heater, which is located directly in front of the distillation column. A vacuum distillation column typically produces several fractions:

• inerts/gas
• vacuum gas oil (light, medium and heavy)
• residue