REFINING OF PETROLEUM
Petroleum is a complex mixture of
organic liquids called crude oil and natural gas, which occurs naturally in the
ground and was formed millions of years ago. Crude oil varies from oilfield to
oilfield in colour and composition, from a pale yellow low viscosity liquid to
heavy black 'treacle' consistencies.
Crude oil and natural gas are extracted from the
ground, on land or under the oceans, by sinking an oil well and are then
transported by pipeline and/or ship to refineries where their components are
processed into refined products. Crude oil and natural gas are of little use in
their raw state; their value lies in what is created from them: fuels,
lubricating oils, waxes, asphalt, petrochemicals and pipeline quality natural
gas.
An oil refinery is an organised and coordinated
arrangement of manufacturing processes designed to produce physical and
chemical changes in crude oil to convert it into everyday products like petrol,
diesel, lubricating oil, fuel oil and bitumen.
As crude oil comes from the well it contains a mixture
of hydrocarbon compounds and relatively small quantities of other materials
such as oxygen, nitrogen, sulphur, salt and water. In the refinery, most of
these non - hydrocarbon substances are removed and the oil is broken down into
its various components, and blended into useful products.
Natural gas from the well, while principally methane,
contains quantities of other hydrocarbons - ethane, propane, butane, pentane
and also carbon dioxide and water. These components are separated from the
methane at a gas fractionation plant.
Petroleum hydrocarbon structures
Petroleum consists of three main hydrocarbon groups:
Paraffins
These consist of straight or
branched carbon rings saturated with hydrogen atoms, the simplest of which is
methane (CH4) the main ingredient of natural gas. Others in this
group include ethane (C2H6), and propane (C3H8).
Hydrocarbons
With very few carbon atoms (C1
to C4) are light in density and are gases under normal atmospheric
pressure. Chemically paraffins are very stable compounds.
Naphthenes
Naphthenes consist of carbon rings,
sometimes with side chains, saturated with hydrogen atoms. Naphthenes are
chemically stable, they occur naturally in crude oil and have properties
similar to paraffins.
Aromatics
aromatic hydrocarbons are compounds
that contain a ring of six carbon atoms with alternating double and single
bonds and six attached hydrogen atoms. This type of structure is known as a
benzene ring. They occur naturally in crude oil, and can also be created by the
refining process.
The more carbon atoms a hydrocarbon molecule has, the
"heavier" it is (the higher is its molecular weight) and the higher
is its the boiling point.
Small quantities of a crude oil may be composed of
compounds containing oxygen, nitrogen, sulphur and metals. Sulphur content
ranges from traces to more than 5 per cent. If a crude oil contains appreciable
quantities of sulphur it is called a sour crude; if it contains little or no
sulphur it is called a sweet crude.
The refining process
Every refinery begins with the separation of crude oil
into different fractions by distillation.
The fractions are further treated to convert them into
mixtures of more useful saleable products by various methods such as cracking,
reforming, alkylation, polymerisation and isomerisation. These mixtures of new
compounds are then separated using methods such as fractionation and solvent
extraction. Impurities are removed by various methods, e.g. dehydration,
desalting, sulphur removal and hydrotreating.
Refinery processes have developed in response to
changing market demands for certain products. With the advent of the internal
combustion engine the main task of refineries became the production of petrol.
The quantities of petrol available from distillation alone was insufficient to
satisfy consumer demand. Refineries began to look for ways to produce more and
better quality petrol. Two types of processes have been developed:
- breaking
down large, heavy hydrocarbon molecules
- reshaping
or rebuilding hydrocarbon molecules.
Distillation (Fractionation)
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 two specified boiling points. Two types of
distillation are performed: atmospheric and vacuum.
Atmospheric distillation takes place in a distilling column
at or near atmospheric pressure. The crude oil is heated to 350 - 400oC
and the vapour and liquid are piped into the distilling column. The liquid
falls to the bottom and the vapour 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 vapour 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.
To recover additional heavy distillates from this
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
crystallise and precipitate out.
Reforming
Reforming is a process which uses heat, pressure and a
catalyst (usually containing platinum) to bring about chemical reactions which
upgrade naphthas into high octane petrol and petrochemical feedstock. The
naphthas are hydrocarbon mixtures containing many paraffins and naphthenes. In
Australia, this naphtha feedstock comes from the crudes oil distillation or
catalytic cracking processes, but overseas it also comes from thermal cracking
and hydrocracking processes. Reforming converts a portion of these compounds to
isoparaffins and aromatics, which are used to blend higher octane petrol.
- paraffins
are converted to isoparaffins
- paraffins
are converted to naphthenes
- naphthenes
are converted to aromatics
e.g.
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catalyst
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heptane
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->
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toluene
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+
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hydrogen
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C7H16
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->
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C7H8
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+
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4H2
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catalyst
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cyclohexane
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->
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benzene
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+
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hydrogen
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C6H12
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->
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C6H6
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+
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3H2
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Cracking
Cracking processes break down heavier hydrocarbon
molecules (high boiling point oils) into lighter products such as petrol and
diesel. These processes include catalytic cracking, thermal cracking and
hydrocracking.
e.g.
A typical reaction:
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catalyst
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C16H34
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->
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C8H18
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+
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C8H16
|
|
Catalytic cracking is used to convert heavy hydrocarbon fractions
obtained by vacuum distillation into a mixture of more useful products such as
petrol and light fuel oil. In this process, the feedstock undergoes a chemical
breakdown, under controlled heat (450 - 500oC) and pressure, in the
presence of a catalyst - a substance which promotes the reaction without itself
being chemically changed. Small pellets of silica - alumina or silica -
magnesia have proved to be the most effective catalysts.
The cracking reaction yields petrol, LPG, unsaturated
olefin compounds, cracked gas oils, a liquid residue called cycle oil, light
gases and a solid coke residue. Cycle oil is recycled to cause further
breakdown and the coke, which forms a layer on the catalyst, is removed by
burning. The other products are passed through a fractionator to be separated
and separately processed.
Fluid catalytic cracking uses a catalyst in the form of a
very fine powder which flows like a liquid when agitated by steam, air or
vapour. Feedstock entering the process immediately meets a stream of very hot
catalyst and vaporises. The resulting vapours keep the catalyst fluidised as it
passes into the reactor, where the cracking takes place and where it is
fluidised by the hydrocarbon vapour. The catalyst next passes to a steam
stripping section where most of the volatile hydrocarbons are removed. It then
passes to a regenerator vessel where it is fluidised by a mixture of air and
the products of combustion which are produced as the coke on the catalyst is
burnt off. The catalyst then flows back to the reactor. The catalyst thus
undergoes a continuous circulation between the reactor, stripper and
regenerator sections.
The catalyst is usually a mixture of aluminium oxide
and silica. Most recently, the introduction of synthetic zeolite catalysts has
allowed much shorter reaction times and improved yields and octane numbers of
the cracked gasolines.
Thermal cracking uses heat to break down the residue from vacuum
distillation. The lighter elements produced from this process can be made into
distillate fuels and petrol. Cracked gases are converted to petrol blending
components by alkylation or polymerisation. Naphtha is upgraded to high quality
petrol by reforming. Gas oil can be used as diesel fuel or can be converted to
petrol by hydrocracking. The heavy residue is converted into residual oil or
coke which is used in the manufacture of electrodes, graphite and carbides.
This process is the oldest technology and is not used
in Australia.
Hydrocracking can increase the yield of petrol components, as well
as being used to produce light distillates. It produces no residues, only light
oils. Hydrocracking is catalytic cracking in the presence of hydrogen. The
extra hydrogen saturates, or hydrogenates, the chemical bonds of the cracked
hydrocarbons and creates isomers with the desired characteristics.
Hydrocracking is also a treating process, because the hydrogen combines with
contaminants such as sulphur and nitrogen, allowing them to be removed.
Gas oil feed is mixed with hydrogen, heated, and sent
to a reactor vessel with a fixed bed catalyst, where cracking and hydrogenation
take place. Products are sent to a fractionator to be separated. The hydrogen
is recycled. Residue from this reaction is mixed again with hydrogen, reheated,
and sent to a second reactor for further cracking under higher temperatures and
pressures.
In addition to cracked naphtha for making petrol,
hydrocracking yields light gases useful for refinery fuel, or alkylation as
well as components for high quality fuel oils, lube oils and petrochemical
feedstocks.
Following the cracking processes it is necessary to
build or rearrange some of the lighter hydrocarbon molecules into high quality
petrol or jet fuel blending components or into petrochemicals. The former can
be achieved by several chemical process such as alkylation and isomerisation.
Alkylation
Olefins such as propylene and butylene are produced by
catalytic and thermal cracking. Alkylation refers to the chemical bonding of
these light molecules with isobutane to form larger branched-chain molecules
(isoparaffins) that make high octane petrol.
Olefins and isobutane are mixed with an acid catalyst
and cooled. They react to form alkylate, plus some normal butane, isobutane and
propane. The resulting liquid is neutralised and separated in a series of
distillation columns. Isobutane is recycled as feed and butane and propane sold
as liquid petroleum gas (LPG).
e.g.
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catalyst
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isobutane
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+
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butylene
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->
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isooctane
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C4H10
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+
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C4H8
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->
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C8H18
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|
Isomerisation
Isomerisation refers to chemical rearrangement of
straight-chain hydrocarbons (paraffins), so that they contain branches attached
to the main chain (isoparaffins). This is done for two reasons:
- they create
extra isobutane feed for alkylation
- they
improve the octane of straight run pentanes and hexanes and hence make
them into better petrol blending components.
Isomerisation is achieved by mixing normal butane with
a little hydrogen and chloride and allowed to react in the presence of a
catalyst to form isobutane, plus a small amount of normal butane and some
lighter gases. Products are separated in a fractionator. The lighter gases are
used as refinery fuel and the butane recycled as feed.
Pentanes and hexanes are the lighter components of
petrol. Isomerisation can be used to improve petrol quality by converting these
hydrocarbons to higher octane isomers. The process is the same as for butane
isomerisation.
Polymerisation
Under pressure and temperature, over an acidic
catalyst, light unsaturated hydrocarbon molecules react and combine with each
other to form larger hydrocarbon molecules. Such process can be used to react
butenes (olefin molecules with four carbon atoms) with iso-butane (branched
paraffin molecules, or isoparaffins, with four carbon atoms) to obtain a high
octane olefinic petrol blending component called polymer gasoline.
Hydrotreating and sulphur plants
A number of contaminants are found in crude oil. As
the fractions travel through the refinery processing units, these impurities
can damage the equipment, the catalysts and the quality of the products. There
are also legal limits on the contents of some impurities, like sulphur, in
products.
Hydrotreating is one way of removing many of the
contaminants from many of the intermediate or final products. In the
hydrotreating process, the entering feedstock is mixed with hydrogen and heated
to 300 - 380oC. The oil combined with the hydrogen then enters a
reactor loaded with a catalyst which promotes several reactions:
- hydrogen
combines with sulphur to form hydrogen sulphide (H2S)
- nitrogen
compounds are converted to ammonia
- any
metals contained in the oil are deposited on the catalyst
- some of
the olefins, aromatics or naphthenes become saturated with hydrogen to
become paraffins and some cracking takes place, causing the creation of
some methane, ethane, propane and butanes.
