Distillation of crude oil is the first step in the
refining of crude. It involves the separation of the crude oil into fractions
and these fractions are mixtures containing hydrocarbon compounds whose boiling
points lie within a specified range. Looking at it this way, because crude oil
is a mixture of hydrocarbons with different boiling temperatures, it can be
separated by distillation into groups of hydrocarbons that boil between
specific boiling points.
Atmospheric distillation and vacuum distillation are two
types of crude oil distillation performed in a refinery. Let’s look at how each
of the distillation is performed:
Atmospheric
Distillation of Crude Oil
This takes place in a distilling column at or near atmospheric pressure. The
crude oil is heated to about 350 - 400oC and the vapor and
liquid are piped into the distilling column. The liquid falls to the bottom and
the vapor rises, passing through a series of perforated trays (sieve trays).
Heavier hydrocarbons condense more quickly and settle on lower trays and
lighter hydrocarbons remain as a vapor longer and condense on higher trays.
Liquid fractions are drawn from the trays and removed. In this way the light
gases, methane, ethane, propane and butane pass out the top of the column,
petrol is formed in the top trays, kerosene and gas oils in the middle, and
fuel oils at the bottom. Residue drawn of the bottom may be burned as fuel,
processed into lubricating oils, waxes and bitumen or used as feedstock for
cracking units.
Vacuum
Distillation of Crude Oil
To
recover additional heavy distillates from crude oil residue, it may be piped to
a second distillation column where the process is repeated under vacuum,
called vacuum distillation. This
allows heavy hydrocarbons with boiling points of 450oC and
higher to be separated without them partly cracking into unwanted products such
as coke and gas. The heavy distillates recovered by vacuum distillation can be
converted into lubricating oils by a variety of processes. The most common of
these is called solvent extraction.
In one version of this process the heavy distillate is washed with a liquid
which does not dissolve in it but which dissolves (and so extracts) the
non-lubricating oil components out of it. Another version uses a liquid which
does not dissolve in it but which causes the non-lubricating oil components to
precipitate (as an extract) from it.
Other
processes exist which remove impurities by adsorption onto a highly porous
solid or which remove any waxes that may be present by causing them to crystallize
and precipitate out.
End-product of Crude Oil Distillation
End-product
of crude oil distillation may be grouped into light distillates, middle
distillates and heavy distillates. Now let’s look at each of them:
Light Distillates
- Liquid petroleum gas (LPG)
- Gasoline (also known as petrol)
- Kerosene
- Jet fuel
Middle Distillates
- Diesel fuels
- Residential heating fuel
- Other light fuel oils
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