2008/9 Schools Wikipedia Selection. Related subjects: Chemical compounds

skeletal structure of a propane molecule
displayed structure of a propane molecule
3D model of a propane molecule
IUPAC name Propane
CAS number [74-98-6]
RTECS number TX2275000
Molecular formula C3H8
Molar mass 44.096 g/mol
Appearance Colorless gas
Density 1.83 kg/m3, gas
0.5077 kg/L, liquid
Melting point

−187.6 °C (85.5 K)

Boiling point

−42.09 °C (231.1 K)

Solubility in water 0.1 g/cm3 (37.8 °C)
MSDS External MSDS
EU classification Highly flammable (F+)
NFPA 704
R-phrases R12
S-phrases (S2), S9, S16
Explosive limits 2.37–9.5%
Related compounds
Related alkanes Ethane
Supplementary data page
Structure and
n, εr, etc.
Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)
Infobox disclaimer and references

Propane is a three-carbon alkane, normally a gas, but compressible to a liquid that is transportable. It is derived from other petroleum products during oil or natural gas processing. It is commonly used as a fuel for engines, barbecues, and home heating systems.

When sold as fuel, it is commonly known as liquified petroleum gas (LPG or LP-gas) which can be a mixture of propane along with small amounts of propylene, butane, and butylene. The odorant ethanethiol is also added so that people can easily smell the gas in case of a leak.

Properties and reactions

Propane undergoes combustion reactions in a similar fashion to other alkanes. In the presence of excess oxygen, propane burns to form water and carbon dioxide.

C3H8 + 5 O2 → 3 CO2 + 4 H2O + heat

When not enough oxygen is present for complete combustion, propane burns to form water and carbon monoxide.

2 C3H8 + 7 O2 → 6CO + 8 H2O + heat

Unlike natural gas, propane is heavier than air (1.5 times denser). In its raw state, propane sinks and pools at the floor. Liquid propane will flash to a vapor at atmospheric pressure and appears white due to moisture condensing from the air.

When properly combusted, propane produces about 2,500 BTU per cubic foot of gas (91,600 BTU per liquid gallon). The gross heat of combustion of one normal cubic meter of propane is around 50  megajoules (≈13.8  kWh) or 101 MJ/m3 in SI units.

Propane is nontoxic; however, when abused as an inhalant it poses a mild asphyxiation risk through oxygen deprivation. It must also be noted that commercial product contains hydrocarbons beyond propane, which may increase risk. Propane and its mixtures, under normal pressures experienced while being stored in a tank, may cause mild frostbite during rapid expansion. Propane in its liquid form remains a cool -42 degrees Celsius with an ambient exterior temperature of 20 degrees Celsius.

Propane combustion is much cleaner than gasoline, though not as clean as natural gas. The presence of C-C bonds, plus the multiple bonds of propylene and butylene, create organic exhausts besides carbon dioxide and water vapor during typical combustion. These bonds also cause propane to burn with a visible flame.


Retail sale of propane cylinders.
Retail sale of propane cylinders.

Propane is used as fuel in cooking on many barbecues, portable stoves and in motor vehicles. The ubiquitous 4.73-gallon (20 Lb.) steel container is often dubbed a "barbecue tank". Propane powers some locomotives, buses, forklifts, and taxis and is used for heat and cooking in recreational vehicles and campers. In many rural areas of North America, propane is used in furnaces, cooking stoves, water heaters, laundry dryers, and other heat-producing appliances. As of 2000, 6.9 million American households use propane as their primary heating fuel.

Commercially-available "propane" fuel, or LPG, is not pure. Typically in the USA and Canada, it is primarily propane (at least 90%), with the rest mostly butane and propylene (5% maximum), plus odorants. This is the HD-5 standard, (Heavy Duty-5%maximum allowable propylene content) written for internal combustion engines. LPG, when cracked from methane (natural gas) does not contain propylene, only when refined from crude oil. It should be noted that not all products labeled "propane" conform to this standard. In Mexico, for example, the butane content is much higher.

Domestic and industrial fuel

In North America, local delivery trucks called "bobtails", with an average tank size of 3,000 gallons, fill up large tanks (sometimes called pigs) that are permanently installed on the property, or other service trucks exchange empty cylinders of propane with filled cylinders. Large tractor-trailer trucks called "cargo-liners", with an average tank size of 18,000 gallons, transport the propane from the pipeline/refinery to the local delivery plant. The bobtail and transport is not unique to the North American market, though the practice is not as common elsewhere, and the vehicles are generally referred to as tankers. In many countries, propane is delivered to consumers via small or medium-sized individual tanks.

Propane use is growing rapidly in non-industrialized areas of the world. Propane is replacing wood and other traditional fuel sources in such places, where it is now sometimes called "cooking gas".

As an aside, North American barbecue grills powered by propane cannot be used overseas. The "propane" sold overseas is actually a mixture of propane and butane. The warmer the country, the higher the butane content, commonly 50/50 and sometimes reaching 75% butane. Usage is calibrated to the different-sized nozzles found in non-U.S. grills. Americans who take their grills overseas — such as military personnel — can find U.S.-specification propane at AAFES military post exchanges.

Retail sale of propane in Monmouth, Oregon
Retail sale of propane in Monmouth, Oregon

North American industries using propane include glass makers, brick kilns, poultry farms, and other industries that need portable heat.

Propane risks and alternate gas fuels

Propane is heavier than air. If a leak in a propane fuel system occurs, the gas will have a tendency to sink into any enclosed area and thus poses a risk of explosion and fire.

Propane is bought and stored in a liquid form (LPG) and thus fuel energy can be stored in a relatively small space. Compressed Natural Gas (CNG), largely methane, is another gas used as fuel but requires very high pressure to liquefy (which is dangerous) and therefore is much less efficient to store due to the large tank volume required. Thus propane is much more commonly used to fuel vehicles than natural gas and only requires about 5 atmospehres of pressure to keep it liquid at room temperature.


Propane is also instrumental in providing off-the-grid refrigeration, also called gas absorption refrigerators. Made popular by the Servel company, propane-powered refrigerators are highly efficient, do not require electricity, and have no moving parts. Refrigerators built in the 1930s are still in regular use, with little or no maintenance. However, certain Servel refrigerators are subject to a recall for carbon monoxide poisoning.

In highly purified form, propane (R-290) can serve as a direct replacement in mechanical refrigeration systems designed to use R-12, R-22 or R-134a chloro- or fluorocarbon based refrigerants. Today, the Unilever Ice Cream company and others are exploring the use of environmentally friendly propane as a refrigerant. As an added benefit, users are finding that refrigerators converted to use propane are 9-15% more energy efficient.

Vehicle fuel

Propane is also being used increasingly for vehicle fuels. In the U.S., 190,000 on-road vehicles use propane, and 450,000 forklifts use it for power. It is the third most popular vehicle fuel in America, behind gasoline and diesel. In other parts of the world, propane used in vehicles is known as autogas. About 9 million vehicles worldwide use autogas.

The advantage of propane is its liquid state at a moderate pressure. This allows fast refill times, affordable fuel tank construction, and ranges comparable to (though still less than) gasoline. Meanwhile it is noticeably cleaner (both in handling, and in combustion), results in less engine wear (due to carbon deposits) without diluting engine oil (often extending oil-change intervals), and until recently was a relative bargain in North America. Octane rating is a noticeably higher 110, which could result in more power, though exploiting this extra "octane" requires significant engine modification. However, public filling stations are still rare. Many converted vehicles have provisions for topping off from "barbecue bottles." Purpose-built vehicles are often in commercially-owned fleets, and have private fueling facilities.

Propane is generally stored and transported in steel cylinders as a liquid with a vapor space above the liquid. The vapor pressure in the cylinder is a function of temperature. When gaseous propane is drawn at a high rate the latent heat of vaporisation required to create the gas will cause the bottle to cool. (This is why water often condenses on the sides of the bottle and then freezes). In extreme cases this may cause such a large reduction in pressure that the process can no longer be supported. In addition, the lightweight, high- octane compounds vaporize before the heavier, low-octane ones. Thus the ignition properties change as the tank empties. For these reasons, the liquid is often withdrawn using a dip tube.


  • Propane is used as a feedstock for the production of base petrochemicals in steam cracking.
  • Propane is used in some flamethrowers, as the fuel, or as the pressurizing gas.
  • Some propane becomes a feedstock for propyl alcohol, a common solvent.
  • Propane is the primary fuel for hot air balloons.
  • It is used in semiconductor manufacture to deposit silicon carbide
  • Liquid Propane is commonly used in theme parks and in the movie industry as an inexpensive, high energy fuel for explosions and other special effects.


Propane was first identified as a volatile component in gasoline by Dr. Walter O. Snelling of the U.S. Bureau of Mines in 1910. The volatility of these lighter hydrocarbons caused them to be known as "wild" because of the high vapor pressures of unrefined gasoline. On March 31, the New York Times reported on Dr. Walter's work with liquefied gas and that "...a steel bottle will carry enough [gas] to light an ordinary home for three weeks."

It was during this time that Dr. Snelling, in cooperation with Frank P. Peterson, Chester Kerr and Arthur Kerr created ways to liquefy the LP Gases during the refining of natural gasoline. Together they established American Gasol Co., the first commercial marketer of propane. Dr. Snelling had produced relatively pure propane by 1911 and on March 25, 1913 his method of processing and producing LP Gases was issued patent #1,056,845. A separate method of producing LP Gas through compression was created by Frank Peterson and patented in 1912.

The 1920's saw increased production of LP Gas with the first year of recorded production totaling 223,000 gallons in 1922. In 1927, annual marketed LP Gas production reached one million gallons and by 1935, the annual sales of LP Gas had reached 56 million gallons. Major industry developments in the 1930's included the introduction of railroad tank car transport, gas odorization and the construction of local bottle filling plants. The year 1945 marked the first year that annual LP Gas sales reached a billion gallons. By 1947, 62% of all U.S. homes had been equipped with either natural gas or propane for cooking.

In 1950 1,000 propane-fueled buses were ordered by the Chicago Transit Authority and by 1958 sales in the U.S. had reached 7 billion gallons annually. In 2004 was reported to be a growing $8 billion to $10 billion industry with over 15 billion gallons of propane being used annually in the U.S.

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