Vacuum Distillation

Vacuum distillation of the atmospheric residue yields additional and valuable distillates, which could otherwise be thermally destroyed if further distillation was attempted at atmospheric pressure and above.
The atmospheric bottom, also known as reduced oil, is sent to the vacuum unit where it is further separated into vacuum gas oil and vacuum residues.
Vacuum distillation improves the separation of gas oil distillates from the reduced oil at temperatures less than those at which thermal cracking would normally take place.

The basic idea on which vacuum distillation operates is that, at low pressure, the boiling points of any material are reduced, allowing various hydrocarbon components in the reduced crude oil to vaporize or boil at a lower temperature. Vacuum distillation of the heavier product avoids thermal cracking and hence product loss and equipment fouling.
Vacuum distillation can also be referred as "low temperature distillation".
In distilling the crude oil, it is important not to subject the crude oil to temperatures above 370 to 380 °C because the high molecular weight components in the crude oil will undergo thermal cracking and form petroleum coke at temperatures above that. Formation of coke would result in plugging the tubes in the furnace that heats the feed stream to the crude oil distillation column. Plugging would also occur in the piping from the furnace to the distillation column as well as in the column itself.
The constraint imposed by limiting the column inlet crude oil to a temperature of less than 370 to 380 °C yields a residual oil from the bottom of the atmospheric distillation column consisting entirely of hydrocarbons that boil above 370 to 380 °C.
To further distill the residual oil from the atmospheric distillation column, the distillation must be performed at absolute pressures as low as 10 to 40 mmHg (also referred to as Torr) so as to limit the operating temperature to less than 370 to 380 °C.
The absolute pressure of 10 to 40 mmHg in the vacuum column is most often achieved by using multiple stages of steam jet ejectors.
A typical vacuum distillation unit is shown in Figure below.
Hot RCO either from the steam-heated storage tank or from the bottom of the atmospheric distillation column is pumped through a series of preheaters (heat exchanger train) followed by heating in a pipe-still heater to raise the temperature to 360°C–370°C. This hot stream is then flashed in a multiplated distillation column where vacuum (below atmospheric pressure) is maintained by steam ejectors by medium pressure superheated steam as the motive fluid that entrains the top hydrocarbon vapors, which are condensed by water coolers.
Usually, three numbers of ejectors are used; the first stage sends the uncondensed vapor to the second stage followed by condensation. The uncondensed vapor then enters the third stage followed by condensation and the uncondensed vapor from the third stage is vented out through
a flare or stack.
A vacuum of 30–40 mm of mercury is maintained at the top of the column and 100–120 mm at the bottom. Condensates from these ejectors are collected in a drum, known as a hot well.
The oily layer is then sent to an oil–water separator vessel from which oil is drawn as vacuum gas oil (VGO), part of which is sent to the column as the reflux and the rest to storage. Condensates from the hot well and the separator drum are drained to the sewer or water collection system. A few plates below the top plate of the column, an additional VGO is drawn and sent to the diesel/gas oil pool.
SO is the next vacuum distillate, which is drawn a few plates below the gas oil draw plate. Similarly, the other vacuum distillates drawn from the plates below are light oil (LO), intermediate oil (IO), and heavy oil (HO) in this sequence.
LO is sent to the vis-breaking unit for the production of low viscous fuel oils, whereas SO, IO, and HO distillates are further stripped in a side stripper by steam to remove the lighter components to adjust the flash points.