Propane

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 + 5O2 → 3CO2 + 4H2O + heat

When not enough oxygen is present for complete combustion, propane burns to form water and carbon monoxide.
 * 2C3H8 + 7O2 → 6CO + 8H2O + 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.

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 may cause mild frostbite during rapid expansion.

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.

Uses


Propane is used as fuel in cooking on many barbecues, portable stoves, combustion-based potato guns, and in motor vehicles. The ubiquitous 4.73-gallon (20 Lb.) steel container is often dubbed a "barbecue bottle". 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, plus odorants. This is the HD5 standard, written for vehicle fuels; note 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" fill up large tanks that are permanently installed on the property (sometimes called pigs), or other service trucks exchange empty cylinders of propane with filled cylinders. The bobtail 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 is the fastest growing fuel source in the Third World, especially in China and India. Its use frees up the huge rural populations from time-consuming ancient chores such as wood gathering and allows them more time to pursue other activities, such as increased farming or educational opportunities. Hence it is sometimes referred to as "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 &mdash; such as military personnel &mdash; can find U.S.-specification propane at AAFES military post exchanges.

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. Compressed Natural Gas (CNG) is another gas used as fuel but is lighter than air and thus less risky. Propane is bought and stored in a liquid form (LPG) and thus more fuel energy can usually be stored in the same space than CNG.

Refrigeration
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 CO 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 room temperature and 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 noticeably higher, 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 vapour space above the liquid. The vapour 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 can also be used as a supplement or "power adder" to diesel fueled engines. A typical diesel engine burns only about 75% of the fuel injected into each cylinder. Injecting propane at approximately a 1:4 ratio into the diesels intake raises combustion efficiency to over 95%. The higher octane rating of propane slows down the combustion event, allowing more complete burning of the diesel fuel and consumption of propane in the process. The result is more power across the engines RPM range and an increase in fuel economy.

Other

 * It is also 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.
 * It is used as fuel in hot air balloons.
 * It is used as a propellant along with silicone oil (for lubrication) in airsoft guns. Propane is sold for airsoft guns labeled as "green gas" which is just propane with the ethanethiol removed. The Tippmann C-3 is a propane-powered paintball marker, as opposed to the usual (non-combustion) CO2 or nitrogen.
 * It is used in some combustion based potato guns.
 * It is used in semiconductor manufacture to deposit silicon carbide
 * It can be used as a fuel for some types of jet engine, in particular in valved and valveless pulse jet systems.

History
Propane was first identified as a volatile component in gasoline by Dr. Walter O. Snelling of the U.S. Bureau of Mines in 1910. Car owners had complained of disappearing fuel, between the time they had filled up and the time they arrived home.