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 consists of three main hydrocarbon groups: 1. Paraffins
These consist of straight or branched carbon rings saturated with hydrogen
atoms, the simplest of which is methane ( CH 4) the main ingredient of
natural gas. Others in this group include ethane ( C 2 H 6), and propane (
C 3 H 8).
With very few carbon atoms ( C 1to C 4) are light in density and are
gases under normal atmospheric pressure. Chemically paraffins are very
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.
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.
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.
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 - 400 o C 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 450 o C 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 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.
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.
A typical reaction:
C 16 H 34
C 8 H 18
C 8 H 16
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 - 500 o C )
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
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
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
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.
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).
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.
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.
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 -
380 o C . 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 ( H 2 S )
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.
Sulphur recovery plants
The hydrogen sulphide created from hydrotreating is a toxic gas that needs
further treatment. The usual process involves two steps:
the removal of the hydrogen sulphide gas from the hydrocarbon stream
the conversion of hydrogen sulphide to elemental sulphur, a
non-toxic and useful chemical.
Solvent extraction, using a solution of diethanolamine (DEA) dissolved in
water, is applied to separate the hydrogen sulphide gas from the process
stream. The hydrocarbon gas stream containing the hydrogen sulphide is
bubbled through a solution of diethanolamine solution (DEA) under high
pressure, such that the hydrogen sulphide gas dissolves in the DEA. The
DEA and hydrogen mixture is the heated at a low pressure and the dissolved
hydrogen sulphide is released as a concentrated gas stream which is sent
to another plant for conversion into sulphur. Conversion of the
concentrated hydrogen sulphide gas into sulphur occurs in two stages.
Combustion of part of the H 2 S stream in a furnace, producing
sulphur dioxide ( SO 2) water ( H 2 O ) and sulphur (S).
2H 2 S
2H 2 O
Reaction of the remainder of the H 2 S with the combustion
products in the presence of a catalyst. The H 2 S reacts with
the SO 2to form sulphur.
2H 2 S
2H 2 O
As the reaction products are cooled the sulphur drops out of the reaction
vessel in a molten state. Sulphur can be stored and shipped in either a
molten or solid state. Refineries and the environment
Air, water and land can all be affected by refinery operations. Refineries
are well aware of their responsibility to the community and employ a
variety of processes to safeguard the environment.
The processes described below are those used by the Shell refinery at
Geelong in Victoria, but all refineries employ similar techniques in
managing the environmental aspects of refining.
Preserving air quality around a refinery involves controlling the
Sulphur enters the refinery in crude oil feed. Gippsland and most other
Australian crude oils have a low sulphur content but other crude's may
contain up to 5 per cent sulphur. To deal with this refineries incorporate
a sulphur recovery unit which operates on the principles described above.
Many of the products used in a refinery produce hydrocarbon vapours. The
escape of vapours to atmosphere are prevented by various means. Floating
roofs are installed in tanks to prevent evaporation and so that there is
no space for vapour to gather in the tanks. Where floating roofs cannot be
used, the vapours from the tanks are collected in a vapour recovery system
and absorbed back into the product stream. In addition, pumps and valves
are routinely checked for vapour emissions and repaired if a leakage is
Smoke is formed when the burning mixture contains insufficient oxygen or
is not sufficiently mixed. Modern furnace control systems prevent this
from happening during normal operation.
Smells are the most difficult emission to control and the easiest to
detect. Refinery smells are generally associated with compounds containing
sulphur, where even tiny losses are sufficient to cause a noticeable
Aqueous effluent's consist of cooling water, surface water and process
The majority of the water discharged from the refinery has been used for
cooling the various process streams. The cooling water does not actually
come into contact with the process material and so has very little
contamination. The cooling water passes through large "interceptors" which
separate any oil from minute leaks etc., prior to discharge. The cooling
water system at Geelong Refinery is a once-through system with no
Rainwater falling on the refinery site must be treated before
discharge to ensure no oily material washed off process equipment leaves
the refinery. This is done first by passing the water through smaller
"plant oil catchers", which each treat rainwater from separate areas on
the site, and then all the streams pass to large "interceptors" similar to
those used for cooling water. The rainwater from the production areas is
further treated in a Dissolved Air Flotation (DAF) unit. This unit cleans
the water by using a flocculation agent to collect any remaining particles
or oil droplets and floating the resulting flock to the surface with
millions of tiny air bubbles. At the surface the flock is skimmed off and
the clean water discharged.
Process water has actually come into contact with the process
streams and so can contain significant contamination. This water is
treated in the "sour water treater" where the contaminants (mostly ammonia
and hydrogen sulphide) are removed and then recovered or destroyed in a
downstream plant. The process water, when treated in this way, can be
reused in parts of the refinery and discharged through the process area
rainwater treatment system and the DAF unit.
Any treated process water that is not reused is discharged as Trade Waste
to the sewerage system. This trade waste also includes the effluent from
the refinery sewage treatment plant and a portion of treated water from
the DAF unit.
As most refineries import and export many feed materials and products by
ship, the refinery and harbour authorities are prepared for spillage from
the ship or pier. In the event of such a spill, equipment is always on
standby at the refinery and it is supported by the facilities of the
Australian Marine Oil Spill Centre at Geelong, Victoria.
The refinery safeguards the land environment by ensuring the appropriate
disposal of all wastes.
Within the refinery, all hydrocarbon wastes are recycled through the
refinery slops system. This system consists of a network of collection
pipes and a series of dewatering tanks. The recovered hydrocarbon is
reprocessed through the distillation units.
Wastes that cannot be reprocessed are either recycled to manufacturers
(e.g. some spent catalysts can be reprocessed), disposed of in
EPA-approved facilities off-site, or chemically treated on-site to form
inert materials which can be disposed to land-fill within the refinery.
Waste movements within the refinery require a "Process liquid, Sludge and
Solid waste disposal permit". Wastes that go off-site must have an EPA
"Waste Transport Permit".