Propane is a three-carbon alkane with the molecular formula C3H8, normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and crude oil petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central heating. Propane is one of a group of liquefied petroleum gases (LP gases). The others include butane, propylene, butadiene, butylene, isobutylene and mixtures thereof.
Propane containing too much propene (also called propylene) is not suited for most vehicle fuels. HD-5 is a specification which establishes a maximum concentration of 5% propene in propane. All propane fuels include an odorant, almost always ethanethiol, so that people can easily smell the gas in case of a leak. Propane as HD-5 was originally intended for use as vehicle fuel. HD-5 is currently being used in all propane applications.
Propane is nontoxic. However, when abused as an inhalant it poses a mild asphyxiation risk through oxygen deprivation. Commercial products contain hydrocarbons beyond propane, which may increase risk. Commonly stored under pressure at room temperature, propane and its mixtures expand and cool when released and may cause mild frostbite.
Propane combustion is much cleaner than gasoline or Diesel combustion, though not as clean as natural gas combustion.
Propane was first identified as a volatile component in gasoline by Walter O. Snelling of the U.S.A. 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. It was during this time that 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. Snelling had produced relatively pure propane by 1911, and on March 25, 1913, his method of processing and producing LP gases was issued a patent. A separate method of producing LP gas through compression was created by Frank Peterson and patented in 1912.
The 1920s saw increased production of LP gas, with the first year of recorded production totaling 223,000 US gallons in 1922. In 1927, annual marketed LP gas production reached 1 million gallons, and by 1935, the annual sales of LP gas had reached 56 million gallons. Major industry developments in the 1930s 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 homes in the U.S.A. had been equipped with either natural gas or propane for cooking.
Propane is produced as a by-product of two other processes, natural gas processing and petroleum refining. The processing of natural gas involves removal of butane, propane, and large amounts of ethane from the raw gas, in order to prevent condensation of these volatiles in natural gas pipelines. Additionally, oil refineries produce some propane as a by-product of cracking petroleum into gasoline or heating oil. The supply of propane cannot easily be adjusted to meet increased demand, because of the by-product nature of propane production. This resulted in many historical price hikes including one in January/February 2014. According to one report, about 90% of U.S. propane is domestically produced with about 10% imported and 70% of the imported portion coming from Canada via pipeline and rail. The remaining 30% of imported propane comes to the United States from other sources via ocean transport.
After it is produced, propane is stored in huge salt caverns in North America. Examples of these are Fort Saskatchewan, Alberta; Mont Belvieu, Texas and Conway, Kansas. These salt caverns were hollowed out in the 1940s and they can store 80,000,000 barrels or more of propane. When the propane is needed, much of it is shipped by pipelines to other areas of the U.S.A.. Propane is also shipped by truck, ship, barge, and railway to many U.S. areas.
Properties and reactions of propane
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. When not enough oxygen is present for complete combustion, incomplete combustion occurs when propane burns and forms water, carbon monoxide, and carbon dioxide. Unlike natural gas, propane is heavier than air. 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 combusted properly propane produces about 50 MJ/kg of heat. The gross heat of combustion of one normal cubic meter of propane is around 91 megajoules.
The density of liquid propane at 25 °C (77 °F) is 0.493 g/cm3, which is equivalent to 4.11 pounds per U.S. liquid gallon or 493 kg/m3. Propane expands at 1.5% per 10 °F. Thus, liquid propane has a density of approximately 4.2 pounds per gallon (504 kg/m3) at 60 °F (15.6°C).
Propane is a popular choice for barbecues and portable stoves because the low boiling point of −42 °C (−44 °F) makes it vaporize as soon as it is released from its pressurized container. Therefore, no carburetor or other vaporizing device is required; a simple metering nozzle suffices. Propane powers some locomotives, buses, forklifts, taxis and ice resurfacing machines and is used for heat and cooking in recreational vehicles and campers. Since it can be transported easily, it is a popular fuel for home heat and backup electrical generation in sparsely populated areas that do not have natural gas pipelines.
Storage and transportation
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 vaporization 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 addition, the lightweight, high-octane compounds vaporize before the heavier, low-octane ones. Thus, the ignition properties change as the cylinder empties. For these reasons, the liquid is often withdrawn using a dip tube.
Commercially available “propane” fuel, or LPG, is not pure. Typically in the U.S.A. and Canada, it is primarily propane (at least 90%), with the rest mostly ethane, propylene, butane, and odorants including ethyl mercaptan. This is the HD-5 standard, (Heavy Duty-5% maximum allowable propylene content, and no more than 5% butanes and ethane) defined by the American Society for Testing and Materials (ASTM) by its Standard 1835 for internal combustion engines. Not all products labeled “propane” conform to this standard. In Mexico, for example, where much of the bottled liquefied gas sold is, in fact, butane, gas labeled “propane” may actually consist of 60% propane and 40% butane.
In North America, local delivery trucks with an average cylinder size of 3,000 gallons fill up large cylinders that are permanently installed on the property, or other service trucks exchange empty cylinders of propane with filled cylinders. Large tractor-trailer trucks, with an average cylinder size of 10,000 gallons transport propane from the pipeline or refinery to the local bulk plant.
Propane is also being used increasingly for vehicle fuels. In the U.S., over 190,000 on-road vehicles use propane, and over 450,000 forklifts use it for power. It is the third most popular vehicle fuel in America, behind gasoline and Diesel fuel. In other parts of the world, propane used in vehicles is known as autogas. In 2007, approximately 13 million vehicles worldwide use autogas.
The advantage of propane in cars is its liquid state at a moderate pressure. This allows fast refill times and affordable fuel cylinder construction. Price ranges typically just over half that of gasoline but a propane gallon contains ~ 37% less energy per gallon so the price difference is not significant on a energy-equivalent basis.
Propane is noticeably cleaner than gasoline (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 of propane is relatively high at 110. In the U.S.A., the propane fueling infrastructure is the most developed of all alternative vehicle fuels. Many converted vehicles have provisions for topping off from “barbecue bottles”. Purpose-built vehicles are often in commercially owned fleets, and have private-access fueling facilities.