Modern life would hardly be possible
without products derived from crude oil.
These include fuels and oils for most
forms of transport,
lubricating oils and tar and bitumen
for road surfacing.
Crude oil is also the starting material
for
a huge variety of organic, carbon-based,
chemicals and products,
such as plastics and synthetic fibres.
Most oil refineries are on the coast
and have good road and rail links. On
this site
two crude distillers together process
34,000 tonnes of crude oil per day.
There is also a catalytic cracker
and a platformer for converting straight
chain molecules
to branched chain. Onsite is a range of
storage tanks for the different products.
Crude oil is brought in by pipeline to
storage tanks.
Many of the products from this refinery
are used by the chemical industry in the surrounding area. This is crude oil.
It's a mixture of different hydrocarbons.
The first step in refining is to separate the oils into fractions of different boiling ranges
by fractional distillation.
The crude oil is first heated by gas or
oil fired burners,
this vapourises most of the oil. The vapour
is fed into the bottom of the
fractionating tower, a column
containing over 40 perforated trays. The
bottom of the tower
is kept at 360 degrees Celsius and the
top
at about 100 degrees Celsius. The vapours rise up the tower
and condense to liquid when they arrive
at a tray that is sufficiently cool.
The perforations in the trays allow
vapours
to rise up the tower and liquids to drip
down.
Liquids called fractions
are piped off at different levels in the
tower. The higher up the tower,
the lower the boiling point and the
shorter the hydrocarbon chains.
Gases from the top of the tower
are used as fuel gases, both for sale and
in the refinery.
The thick liquid residue from the base
of the column may be distilled again
under reduced pressure to give further
separation.
There is more demand for the shorter
chain fractions from the initial, or primary,
distillation
than for the longer ones. Longer chain
fractions are therefore cracked,
the process of breaking the molecules to
give shorter chain fractions.
10,000 tonnes of petroleum fractions
are cracked each day at this plant. In
catalytic, or cat, cracking
the long chain fraction is mixed with a
powdered catalyst of silicon dioxide
and aluminium oxide and the mixture
enters the reactor
at about five hundred degrees Celsius
for five seconds.
Here the cracking takes place. The
catalyst
becomes coated with carbon and is
recycled. The catalyst passes to a
regenerator
where the carbon is burnt off in air. The
products of cracking
include alkenes. These are hydrocarbons with
reactive double bonds.
They're vital starting materials for
chemical processes,
such as polymerisation. Valuable
short chain alkanes for petrol
are also produced.
After fractional distillation and cracking
fractions may undergo further treatment,
including desulfurisation to remove
sulfur,
which would produce sulfur dioxide when
the fuels are burnt,
platforming, in which
straight chain alkanes are converted to
branched chain alkanes,
by heating over a platinum catalyst. The
branched chains
produce better quality petrol. Fractions
are also blended,
mixed to give products of the required
properties
for use as fuels and lubricants. The
products are
transported off-site by road, rail, sea and
air.
