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 H2
O. The vacuum acts
so as to reduce the boiling point of the various fractions.
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
- vacuum gas oil (light, medium and heavy)