Crude oil as produced in the oil field is a complex mixture of hydrocarbons
ranging from methane to asphalt, with varying proportions of
paraffins, naphthenes, and aromatics. The objective of crude distillation is
to fractionate crude oil into light-end hydrocarbons (Ci-C4), naphtha/
gasoline, kerosene, diesel, and atmospheric resid. Some of these broad
cuts can be marketed directly, while others require further processing in
refinery downstream units to make them saleable.
The first processing step in the refinery, after desalting the crude, is
separation of crude into a number of fractions by distillation. The distillation
is carried out at a pressure slightly above atmospheric. This is
necessary for the following considerations:
1. To raise the boiling point of the light-end carbons so that refinery
cooling water can be used to condense some of the C3 and C4 in the
overhead condenser.
2. To place the uncondensed gas under sufficient pressure to allow it
to flow to the next piece of processing equipment.
3. To allow for pressure drop in the column.
Crude oil is preheated in exchangers and finally vaporized in a fired
furnace until approximately the required overhead and sidestream products
are vaporized. The furnace effluent is flashed into the crude column
flash zone, where the vapor and liquid separate. The liquid leaving the
flash zone still contains some distillate components, which are recovered
by steam stripping. After steam stripping, the bottom product, also known
as reduced crude, is discharged from the tower. The bottom temperature
is limited to 700-7500F to prevent cracking.
The atmospheric resid is fed to a furnace, heated to 730-7700F
and next to a vacuum tower operated at a minimum practical vacuum
(80-110 mm Hg). The operating conditions are dictated by cracking andproduct quality required. The objectives of vacuum distillation is generally
to separate vacuum gas oil (VGO) from reduced crude. The VGO
may become feedstock for FCCU or hydrocracker units or used to make
lube base stocks. Depending on the end use, there may be one or more
sidestreams. The bottom stream from the vacuum distillation unit may be
used to produce bitumen or used for fuel oil production after mixing it
with small amounts of cutter stocks (in the diesel/kerosene range).
If the crude contains very high percentages of light-ends, a flash drum
or a prefractionator with an overhead condensing system is added ahead
of atmospheric tower. The prefractionator is designed to recover most of
the light-ends and a part of the light naphtha. The bottom stream from
prefractionator becomes feed to atmospheric tower.
