Propane

Propane
IUPAC name Propane
Identifiers
CAS number	74-98-6 Y
PubChem	6334
ChemSpider	6094
UN number	1978
ChEBI	32879
RTECS number	TX2275000
SMILES CCC
InChI InChI=1/C3H8/c1-3-2/h3H2,1-2H3
Properties
Molecular formula	C3H8
Molar mass	44.1 g mol−1
Appearance	Colorless gas
Density	1.83 kg/m3, gas 583 kg/m3, liquid[1]
Melting point	−187.6 °C (85.5 K)
Boiling point	−42.09 °C (231.1 K)
Solubility in water	0.04 g/L (0 °C)[1]
Hazards
EU classification	Highly flammable (F+)
R-phrases	R12
S-phrases	(S2), S9, S16
NFPA 704	4 1 0
Flash point	-104 °C (169.1 K)
Autoignition temperature	540 °C (813.1 K)
Explosive limits	2.37–9.5%
Related compounds
Related alkanes	Ethane Butane
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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

A mixture of propane and butane, used mainly as vehicle fuel, is commonly known as liquefied petroleum gas (LPG or LP gas). It may also contain small amounts of propylene and/or butylene. An odorant such as ethanethiol or thiophene is added so that people can easily smell the gas in case of a leak.

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. 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. Snelling's work with liquefied gas and that "...a steel bottle will carry enough [gas] to light an ordinary home for three weeks."^[2]

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.^[3] 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 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 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 U.S. homes had been equipped with either natural gas or propane for cooking.^[4]

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 it 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.^[5]

The "prop-" root found in "propane" and names of other compounds with three-carbon chains was derived from "propionic acid".^[6]

Sources

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. About 90% of U.S. propane is domestically produced.

The United States imports about 10% of the propane consumed each year, with about 70% of that 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, North American propane is stored in huge salt caverns located in Fort Saskatchewan, Alberta; Mont Belvieu, Texas and Conway, Kansas. These salt caverns were hollowed out in the 1940s,^[7] and they can store 80 million or more barrels of propane. When the propane is needed, most of it is shipped by pipelines to other areas of the Midwest, the North and the South, for use by customers. Propane is also shipped by barge and rail car to selected U.S. areas.

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

propane + oxygen → carbon dioxide + water

When not enough oxygen is present for complete combustion, incomplete combustion occurs when propane burns and forms water, carbon monoxide, carbon dioxide, and carbon.

2 C₃H₈ + 7 O₂ → 2 CO₂ + 2 CO + 2 C + 8 H₂O + heat

Propane + Oxygen → Carbon Dioxide + Carbon Monoxide + Carbon + Water

Unlike natural gas, propane is heavier than air (1.5 times as dense). 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 50 MJ/kg. The gross heat of combustion of one normal cubic meter of propane is around 91 megajoules^[8]

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 combustion, though not as clean as natural gas combustion. 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.

Greenhouse gas emissions factors for propane are 62.7 kg CO₂/ mBTU or 1.55 kg of CO₂ per litre or 73.7 kg/GJ.

Energy content

The energy density of propane is 46.44 megajoules per kilogram^[9] (91,690 BTU per gallon).

Weight per gallon

The density of propane at 25 degrees C is .493 grams per cubic centimeter. Converting gives 4.11 pounds per gallon. Thus, Propane weighs approximately 4.2 (+/- 0.1) pounds per US liquid gallon, at 60 degrees Fahrenheit. Propane expands 1.5% per 10 degrees F.

Uses

A 20lb propane cylinder.

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. In the United States 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 fueling facilities. A further saving for propane fuel vehicle operators, especially in fleets, is that pilferage is much more difficult than with gasoline or diesel fuels.

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 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 fuel in furnaces for heat, in cooking, as a energy source for water heaters, laundry dryers, barbecues, portable stoves, and motor vehicles. Propane remains a popular choice for barbecues and portable stoves because its 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.

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 extracted from natural gas, does not contain propylene. LPG, when refined from crude oil does contain propylene. Not all products labelled "propane" conform to this standard. In Mexico, for example, the butane content is much higher.

Domestic and industrial fuel

A local delivery truck, behind the pickup truck

Retail sale of propane in the US

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". 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.

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

In rural areas of North America and south California, as well as northern Australia and some parts of soutern India propane is used to heat livestock facilities, in grain dryers, and other heat-producing appliances. When used for heating or grain drying it is usually stored in a large, permanently placed cylinder which is recharged by a propane-delivery truck. As of 2000^[update], 6.9 million American households use propane as their primary heating fuel.^[10]

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 10,000 gallons, transport the propane from the pipeline or refinery to the local delivery plant. The bobtail and transport are 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.

Refrigeration

Propane is also instrumental in providing off-the-grid refrigeration, usually by means of a gas absorption refrigerator.

Blends of pure, dry "isopropane" (R-290a) (commercial term used to describe isobutane/propane mixtures) and isobutane (R-600a) have negligible ozone depletion potential and very low Global Warming Potential (having a value of 3.3 times the GWP of carbon dioxide) and can serve as a functional replacement for R-12, R-22, R-134a, and other chlorofluorocarbon or hydrofluorocarbon refrigerants in conventional stationary refrigeration and air conditioning systems.^[11]

In motor vehicles

Such substitution is widely prohibited or discouraged in motor vehicle air conditioning systems, on the grounds that using flammable hydrocarbons in systems originally designed to carry non-flammable refrigerant presents a significant risk of fire or explosion.^{[12][13][14][15][16][17][18][19]}

Vendors and advocates of hydrocarbon refrigerants argue against such bans on the grounds that there have been very few such incidents relative to the number of vehicle air conditioning systems filled with hydrocarbons.^[20][21] One particular test was conducted by a professor at the University of New South Wales that unintentionally tested the scenario of a sudden and complete refrigerant loss into the passenger compartment followed by subsequent ignition. He and several others in the car sustained burns to their face, ears, and hands, and several observers received lacerations from the burst glass of the front passenger window.^[22]

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 13 million vehicles worldwide use autogas.

Other

• Propane is used as a feedstock for the production of base petrochemicals in steam cracking.
• 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 risks and alternate gas fuels

A municipal propane tank in the US

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. The typical scenario is a leaking cylinder stored in a basement; the propane leak drifts across the floor to the pilot light on the furnace or water heater, and results in an explosion or fire. This property makes the use of propane generally unsuitable as a fuel for boats, and also unfavored as a cooking fuel.

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 it cannot be liquefied by compression at normal temperatures, as these are well above its critical temperature. It therefore requires very high pressure to be stored as a liquid, which poses the hazard that, in an accident, just as with any compressed gas tank (such as a CO₂ tank used for a soda concession) a CNG tank may burst with great force, or leak rapidly enough to become a self-propelled missile. Therefore, CNG is much less efficient to store, due to the large tank volume required. Another form of storing natural gas is as a low temperatur liquid in insulated containers as Liquefied Natural Gas (LNG). This form of storage is at low pressure and is around 3.5 times as efficient as storing it as CNG. Unlike propane, if a spill occurs LNG will evaporate and dissipate harmlessly because it is lighter than air. Propane is much more commonly used to fuel vehicles than is natural gas because the equipment required costs less. Propane requires just 1,220 kilopascals (177 psi) of pressure to keep it liquid at 37.8 °C (100 °F).^[23]

Retail cost

United States

As of November 2009^[update], the retail cost of propane was approximately US$2.20 per gallon, or roughly $24 per 1 million BTUs.^[24]

See also

References

External links

• Propane Education & Research Council (U.S.)
• World LP Gas Association (WLPGA)
• International Chemical Safety Card 0319
• NIOSH Pocket Guide to Chemical Hazards
• National Propane Gas Association (U.S.)
• Propane Gas Association of Canada
• European Chemicals Bureau
• UKLPG: Propane and Butane in the UK
• Propane Properties Explained Descriptive Breakdown of Propane Characteristics