What
comes out of an oil field?
Major
Crude oil. (Texas tea, Black gold)
Minor
Gas (Natural gas, Methane).
Condensate and natural gas liquids (only sometimes found in an oil
field)
Crude Oil
Crude oil comes in a variety of colours and thicknesses. It may
be black, it may be brown, it may be greenish. It may be relatively
thin, it may be relatively thick.
The 'thickness', or density, of crude is measured against a scale
developed by the American Petroleum Institute. The denser and heavier
the oil, the lower on the 'API' scale it is.
Regardless of what it looks like, the raw crude oil as it is pumped out
of the ground is not much
use. To be able to
use crude oil, the different components that make it up need to be
seperated out. Crude oil run through various processes in oil
refineries will seperate out various proportions of the constituents of
that particular crude. But when crude is refined, it yields differing
amounts
of the various component parts ('fractions'), depending on how the
crude was formed, and what kind of organic material it originated from.
Crude from one field may have a different 'yeild profile' to crude from
a field somewhere else.
Several thousand different chemicals can be identified in the various
crude oils.
The huge pressure and high temperature in the deep underground oil
reservoir means that some chemical compounds which would be gases at
normal temperatures and pressures above the ground remain liquid under
the ground, and form a part of the liquid crude. When the crude is
pumped up from the depths, the pressure is released and temperature
drops. The liquified gases in the crude are then released.
This is
normally methane, and is not usually associated with any other volatile
compounds which liquify at normal above-ground temperature and
pressure. The gas usually associated with oil wells is therefore called
'dry gas'. Historically, the gas that bubbles out as the crude comes to
the
surface has simply been diverted and burnt ('flared off') from the top
of a tall pipe at the well head.
Light crude
Depending on the field, some crude oils are naturally 'runny' and
light. They are easy to refine, and are highly sought after. These are
lower density oils.
Sweet crude
Crude oil with very little sulfur in it. Excess sulfur has to be
removed from crude at the refinery, a process that costs extra money.
Heavy crude
Others are very thick, viscous, and heavy. Heavier oils are often found
relatively close to the surface. Any lighter more volatile components
that might have been formed have vaporised and found their way to the
surface and disappeared. They are made up of large molecules such as
hexadecane (16 carbon & 34 hydrogen atoms, or C
16H
34)
and octadecane (18 carbon & 38 hydrogen atoms or C
18H
38)
They have to be refined in
specialised refineries especially built to handle them. These large
molecules are split or 'cracked' into smaller molecules. Hexadecane -
in effect fuel oil - can be cracked down to a mix of octane, hexane,
and a little ethylene. Octane and hexane are components of gasoline.
This is done by heating.
Sour crude
Some crudes are naturally high in sulphur. If there is more than 2.5%
sulfur present, they are called 'sour'
crudes.
Condensate
See below
Natural gas liquids
See below
What comes out of a gas field?
Major
Gas (Natural gas, Methane).
Minor
Condensate and natural gas liquids (frequently associated with a gas
field)
Gas
The pressure and high temperatures in the deep underground gas
reservoir means that some low boiling point hydrocarbon compounds
(which would be liquids at
normal temperatures and pressures above the ground) become gases under
the ground.
These 'gasified liquids' form a part of the flow of gas
when it is piped up from the reservoir. When the gas is flows up from
the depths, the
pressure is released and temperature
drops. The 'gasified liquids' in the cooling gas stream then condense
(just as steam condenses back to water as it cools). These liquid
condensates and natural gas liquids are quite usual in gas fields. The
gas from gas fields is therefore usually 'wet gas'.
Natural Gas
Almost pure methane (CH
4), this gas remains a gas at normal
temperatures and pressures. It will not change to the liquid state
unless cooled to -161.5
oC, or subjected to very high
pressures (or a bit of both) . This is why cylinders used to hold
compressed natural gas in CNG powered vehicles must be made very
thick-walled. They have to withstand the high pressures needed to put
useful amounts of natural gas into the tank.
In a similar way, when methane is transported on ships, it is made
liquid in a highly specialised facility, and has to be
held in special thick walled vessels capable of holding high pressures.
The gas is also kept very cold, so that the amount of pressure needed
is
reduced.
Natural gas liquids
These are those hydrocarbons in the natural gas (methane) stream
that are either liquid at normal temperatures and pressures, or can be
relatively easily turned into a liquid with application of moderate
pressure. These liquids can be separated from the gas (methane) either
at the oil or gas fields adjacent facilities, or elsewhere at a
specialist natural gas processing plant.
These liquids are usually made up of both lighter hydrocarbons such as
propane,
and heavier hydrocarbons, such as pentane.
The heavier hydrocarbons that are liquid at
normal temperatures are often called 'lease
condensate'. The largest
molecule, pentane, liquifies just below 36
oC.
These smaller molecules such as ethane, ethylene, propane,
butane, butylene, isobutane, and isobutylene can be retrieved from the
natural gas liquids at the specialist natural gas processing plants.
The smallest molecule, propane, can be turned into a liquid at -42
oC.
These smaller hydrocarbons are mixed together and pressurised to form
'liquified petroleum gases', or LPG. This gas
can be held in relatively thin walled bottles, so is sold worldwide for
both domestic cooking as 'bottled gas' and as a transport fuel.
The term 'Natural
gas liquids' includes
both
the liquids captured at natural gas plants,
and the lease condensate.
Condensate
Condensate refers to a specific portion of the Natural Gas Liquids. It
is more usually referred to as 'lease condensate'.
If condensate is not removed from the natural gas stream before it is
retailed, the condensates tends to
return to a liquid state in the gas pipeline. Special technology is
needed to remove the condensates.
'Lease' condensate
This is a mixture of mainly pentane and
those heavier hydrocarbons that condense out to a liquid at normal
pressure and temperature which is recovered from the natural gas in
the lease operators separation facilities near by the gas field.
It excludes the lighter molecules in the gas stream, such as propane
and butane. These are recoverd at specialist natural gas processing
plants (see below).
Natural Gas Plant Liquids
Smaller 'condensate' hydrocarbons that are gases at normal temperature
and pressure remaining in the natural gas stream after the liquids have
been removed by the lease operator. These 'gaseous' condensates are
mainly small molecules such as butane and propane. They are recovered
at
the natural gas processing plants. Some heavier molecules, such as
isopentane, may be recoverd. Plant operators refer to these liquids as
'natural gasoline'.
Products made by the Natural Gas Plants using these liquids include
ethane, liquefied petroleum gases (propane,
butane, propane-butane mixes and ethane-propane mixes), isopentane,
and small amounts of finished
gasoline, special naphthas, jet fuel, kerosene, and distillate fuel oil.
Liquid hydrocarbons with more hydrogen atoms, such as hexane and
heptane, can be recovered from the fractionisation of the gas stream.
They can be used to 'make' gasoline, but have to be blended with other
liquid
hydrocarbons from distillation of crude oil in order to be useful.
Natural gas liquids cannot be counted as 'oil equivalent' by
themselves - they depend on crude oil to become useful.
LNG - Liquified Natural Gas
Natural gas (methane) can be transported in its normal gaseous state
inside a
pipeline. But transcontinental gas pipelines are very expensive. As a
result there is increasing demand for natural gas that has been
compressed and cooled (to minus 160 degrees celsius) until it is liquid
- at which point it is around 1/600th of its original volume - then
transported by ship to a
facility that can turn the liquid back into a gas in a controlled
manner. The gas can then be distributed via pipelines in the usual way.
What comes out of a refinery?
Break-downs
of the crude oil that went in.
Crude is heated through a defined temperature range that causes the
liquids in the desired fraction to boil off. For 'straight run'
gasoline, for example, as the crude is heated at the bottom of a tall
'still' the low boiling point liquids that make up gasoline all
vaporise. These 'gasoline vapors' reach the top of the still where
temperatures are not more than 200
oC, and the vapors of the
low boiling point fraction of the crude are drawn off and condensed .
It is hotter lower down the column, and higher boiling point liquids
are first vaporised, then drawn off and condensed at this lower point.
These tall column stills are known as 'fractionating towers'.
The distilled liquid fractions may then be further refined by removing
impurities with various chemicals.
The less desirable higher boiling point liquids that make up 'kerosene'
are usually further broken down using catalysts to more desirable
liquids
suitable for gasoline and for making aviation gas. The gasoline liquids
from this process are blended with 'straight run' gasoline. A variety
of other chemical techniques, such as isomerisation and
dehydrogenation, are used to improve some refinery liquids for use in
gasoline.
The light gas oils, or fuel oils, can either be further split for
manufacture of more gasoline, or retained and refined for when demand
for furnace oil is high (winter), or when extra quantities of deisel
fuel are needed. Refineries have some flexibility in changing the
product mix between more or less proportions of gasoline versus fuel
oils, but it can't be done instantly.
The type of irreducible residues that remain from distillation depends
on the makeup of the crude. It may be tars, or 'asphalt'
Some crudes - heavy crudes - have no appreciable gasoline components.
They have to be subjected to heating cycles to crack down into useful
hydrocarbon liquids to be able to make gasoline. This requires more
heating than light crudes. Heavy crudes therefore cost more to process.
As a result, heavy crudes are less expensive to buy as the cost of
dealing with them is significantly higher and there is less profit in
them. In addition, they require specially configured refinery
processes requiring more capital to build. Once built, refineries
handling heavy crudes make more gross profit (at least) than refineries
handling light crude - but only so long as heavu crudes sell cheaper.
As more refineries are converted to handle heavy crudes, demand will go
up, and the price advatage reduce.
Heavy crude takes more energy to process than light crude. Overall,
there is roughly 14%-18% more energy needed to refine a barrel of heavy
crude than to refine a barrel of light crude.
The gasoline-burning national car fleet of the USA means that USA
refineries try to maximise
gasoline production, whereas the increasingly diesel-powered European
car fleet means European refineries try to maximise light gas oil and
diesel. European refineries typically break a barrel of crude oil down
into about 25% gasoline, 50%
light gas oil/diesel where USA refineries typically break a barrel of
crude down into around 50% gasoline and 25%
diesel/heating oil.
Gasoline
Crude oil (UK) or Petroleum (USA) is heated to extract the components
with the lowest boiling point. The smaller molecule hydrocarbons that
are usually a liquid at ambient temperatures are (smallest molecules to
largest) pentane, hexane, heptane,octane, decane, and dodecane. Pentane
is a liquid until the temperature hits 36
oC, when it
volatises into a gas. Dodecane is a liquid until temperatures hit just
over 215
oC. The other liquids turn into a gas at
temperatures between these ranges. When these six liquid hydrocarbons
are put into a mixture together, the mixture is called 'gasoline'. Some
of the lighter liquids are chemically 'reformed' to make them more
suitable as a car fuel.
The heptane component is a straight chain hydrocarbon molecule tending
to combust very quickly in high compression engines, causing
pre-ignition or 'knocking'. A chemically 're-formed' branch-chained
form of octane (C
8H
18),
'iso octane', is considered the ideal fuel, as it combusts at a slower
pace, giving better compression. Iso-octane is given a nominal rating
of 100, as the perfectly combusting fuel for modern high compression
car engines. Heptane, on the other hand, is given an 'octane rating' of
zero.
Blends of gasoline are measured against a standard comprised of a
defined proportionate blend of iso-octane and heptane (the 'Research
Octane'). The closer to
100 the 'octane rating'* of the gasoline blend is, the better the
explosion profile in the engine and the less tendency to
knocking.
Many modern engines have tended to reduce the importance of the octane
rating as electronics automatically adjust the timing and fuel:air mix
for optimal combustion regardless of the octane rating (to a point).
A barrel of crude ultimately yields about 45% gasoline product.
*
Most of the world uses the 'Research
Octane Number' (RON) to measure octane. The USA uses an average of RON
plus the 'Motor
Octane Number'. Thus a 91octane fuel is the equivalent of 87 in USA.
Kerosene
The two heaviest and least volatile components of gasoline, decane and
dodecane, when mixed together are known as 'kerosene'. A specially
modified blend of kerosine (avgas) is used in jet engines.
A barrel of crude ultimately yields about 4.5% kerosine.
Fuel oils or light gas oil
The liquids with the largest molecules in a crude are hexadecane and
octadecane. Heavy crudes have little else. These molecules are heated
up and further split apart (with the aid of a catalyst) to make them
usable. Hexadecane (C
16H
34),
for example, can be split into various proportions of octane (C
8H
18),
hexane (C
6H
12) and a small amount of ethylene (C
2H
4).
The octane and hexane liquids are used as components of gasoline.
Light gas oil is also further refined into grades for home heating fuel
oil, and
highly refined grades for diesel.
A barrel of crude ultimately yields about 36.5% fuel oil.
Lubricating oils
The fraction of the crude that has very many carbon atoms is used as
liquid lubricant.
A barrel of crude ultimately yields about 2% lubricating oils.
Grease
The fraction of the crude that has even more carbon atoms is used
semi-solid grease. High paraffin crudes are best for grease production.
Paraffin wax
The heaviest molecules in the crude are solids at normal temperatures.
Paraffin wax is used for candles.
Averaged, a barrel of crude ultimately yields about 11.5% grease,
paraffin wax,
tars, ethylene and other miscellaneous products.
The cultural divide
In America, the fuel we put in our cars tank is 'gasoline', always
shortened to 'gas'. The tank is called a 'gas tank'.
In the UK and former colonies, the fuel we put in our cars tank is
'petroleum', always shortened to 'petrol'. The tank is called a 'petrol
tank'. (
But the
American term 'gas; has infiltrated the language and is now used
interchangably for 'petrol', and almost as frequently).
In USA, the oil that comes out of the ground is called 'crude', or it
might be called 'petroleum'. (Petroleum is literally correct, as it
comes from the latin '
petra',
stone, and '
oleum', oil).
No-one in USA would think of putting 'petroleum' in their car, because
it would be tantamount to filling up with crude oil!
In the UK, many cars have been converted to dual fuel - they can run on
gasoline or on compressed natural gas. If a driver needs 'gas', they
may need gasoline ('petrol') or they may need more natural gas.....
In the UK, the black liquid that comes out of the ground is either
called 'crude', or simply 'oil'. Outside the oil industry, crude is
never
called petroleum in the UK. The idea that you could get petroleum - the
stuff you
run your car on - straight out of the ground would seem like a bizarre
joke.
©
Copyright
2006 Sustainable Living Organisation
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