| What is Crude
Oil? |
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All
crude oils are mixtures of hydrocarbons. Hydrocarbons are chains of
carbon and hydrogen atoms in different lengths. Each carbon atom in
the chain is bonded to two hydrogen atoms, except for the end ones
which have three. A very short chain of hydrocarbons appears as a
gas, longer ones are liquids, and as the chains get longer the
substances get more solid, until very long chains appear as a solid
e.g. wax. To turn crude oil into something we can use it must be
refined, in a place that is called a refinery. The refining process
changes the oil from something we cannot use, unless refined, into
something we can. |
| The first stage in converting the crude oil in to a
useable oil product is a process called fractional distillation, using a distillation
tower. Fractional Distillation separates the oil down into groups of hydrocarbons of
similar lengths, called fractions, using heat. As different fractions have different
boiling points, they are separated out from the crude oil and stored. |
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| How
does a distillation tower work? |
|
| The crude oil is heated to a temperature at which
most of it will boil. The hot liquid (hot crude oil) passes into the distillation tower,
where the volatile molecules turn into vapours and rise up the tower. |
| As the top of the
tower is cooler than the bottom the vapours gradually cool and condense as they rise. The
tower contains a series of shallow collection trays which collect the liquid as it
condenses. |
| Devices called bubble caps force the rising vapours to pass through the liquid
on the trays. The vapours condense to liquid when they arrive at a tray that is
sufficiently cool. When a liquid condenses on a tray it is piped off, and
thus forms a fraction. |
| Short chains have lower boiling points, so the higher the collection tray the
shorter the chain lengths that collect there. |
| The
different fractions of the crude oil have different uses. The
lightest fractions that are drawn off from near the top of the tower
are used in making petrol (also known as spirit). From the middle of
the tower we collect middle distillates, such as kerosene for home
heating or gas oil which is used to run generators and engines. Any
fuels which are used for transportation are also treated separately
to reduce their sulphur content, so reducing the risk of polluting
the environment when they are burnt in engines. |
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| At
the bottom of the distillation tower a thick liquid collects that
has not been fractional distilled by this tower. This thick liquid is
then reheated and subjected to distillation in another tower at less than atmospheric pressure. |
| The boiling points of molecules are reduced by the effect of vacuum, so that relatively
high boiling point products can be recovered from the thick liquid. This distillation
tower is called a vacuum tower and thus allows the production of fractions that cannot be
produced in the normal distillation tower. The vacuum tower produces fractions that are
made into products such as lubricants and waxes. In the bottom of the vacuum tower an even thicker
non-volatile residue is produced. This can be turned into bitumen, for road building, or
burnt to power big industrial plant such as power stations. |
| Can fractions be further manipulated? |
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| Some products are in greater demand than others,
and because chemical engineers wish to maximise the amount of certain
products that are
made a further process has been developed to make the most of a fraction. This process is
called "Cracking" and breaks long-chain fractions into shorter chains. Cracking is achieved
by using high temperature, and pressure, in conjunction with a catalyst to break, or
crack, the chains. The shorter chains produced can then be sold as a different product.
For example a fraction used to made Kerosene can be cracked into petrol, so changing the
fraction into a product that is more in demand, and that sells at a higher price. |
| The final process that a refinery can undertake is
called reforming. Using techniques similar to those used in cracking, ie heat, pressure
and different catalysts, allows the chemical engineers to physically alter the chains of
hydrocarbons. Depending on the techniques used a straight chain of hydrocarbons could be
reformed into a bunched chain of hydrocarbons. This bunched chain could have a different,
more valuable use. A good example is octane, a C8 hydrocarbon, which can be reformed into
iso-octane a product that burns more efficiently in car engines and thus gives better
performance, or more miles per gallon. |
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The cracking and reforming processes effectively remove some
of the hydrogen from the hydrocarbon chains. This leaves molecules that are unsaturated,
because they have less than the maximum amount of hydrogen. These unsaturated molecules
have carbon-carbon double bonds and are called Alkenes. Alkenes are chemically far more
reactive than the saturated hydrocarbons, which are called Alkanes. (Alkanes have few
chemical reactions and are normally burnt as fuels). As the Alkenes have extra chemical reactivity of alkenes
they can be converted into other useful products making them the starting material, called
a feedstock, for the petrochemical industry. In addition reforming also produces another
group of products called Aromatic compounds, which are unsaturated compounds with rings
rather than chains of carbon atoms. These also have special chemical properties and thus
are useful as feed stocks for the petrochemical industry. |
| Nearly all refineries operate 24 hours a day,
seven days a week. Refinery managers control all the processes (fractional distillation,
cracking and reforming, and de-sulphurisation) to ensure that the refinery is producing as
much of the products that are in demand, and as little of the products that are not.
This keeps the refinery running at maximum profitability and efficiency. |