Propane (C₃H₈)

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

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

Etymology

Its name was made from part of "propionic acid" (which also has 3 carbon atoms) and the -ane suffix.

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 carbondioxide.

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.

C₃H₈ + 3.5 O₂ → CO₂ + CO + C + 4 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 avapor 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

Propane is nontoxic; however, when abused as an inhalant it poses a mild asphyxiation risk through oxygen deprivation. Commercial products containhydrocarbons beyond propane, which may increase risk. Commonly stored under pressure at room temperature, propane and its mixtures expand and cool whenreleased 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 multiplebonds of propylene and butylene, create organic exhausts besides carbon dioxide and water vapor during typical combustion. These bonds also cause propane toburn 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.

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 avapor 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

Propane is nontoxic; however, when abused as an inhalant it poses a mild asphyxiation risk through oxygen deprivation. Commercial products containhydrocarbons beyond propane, which may increase risk. Commonly stored under pressure at room temperature, propane and its mixtures expand and cool whenreleased 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 multiplebonds of propylene and butylene, create organic exhausts besides carbon dioxide and water vapor during typical combustion. These bonds also cause propane toburn 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.

Uses

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 oftendubbed a "barbecue tank". Propane remains a popular choice for barbecues and portable stoves because its low boiling point of −42 °C (−43.6 °F) makes itvaporize as soon as it is released from its pressurized container. Therefore, no carburetor or other vaporizing device is required; a simple metering nozzlesuffices. Propane powers some locomotives, buses, forklifts, taxis and ice resurfacing machines and is used for heat and cooking in recreational vehicles andcampers. In many rural areas of North America, propane is used in furnaces, cooking stoves, water heaters, laundry dryers, and other heat-producingappliances. In this application, it is usually stored in a large, permanently-placed cylinder which is recharged by a propane-delivery truck. As of 2000, 6.9million 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 mostlybutane and propylene (5% maximum), plus odorants. This is the HD-5 standard, (Heavy Duty-5%maximum allowable propylene content) written for internalcombustion engines. LPG, when extracted from natural gas, does not contain propylene. LPG, when refined from crude oil does contain propylene. Not allproducts labelled "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-trailertrucks 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 generallyreferred 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, whereit is now sometimes called "cooking gas". North American barbecue grills powered by propane cannot be used overseas. The "propane" sold overseas is actuallya mixture of propane and butane. The warmer the country, the higher the butane content, commonly 50/50 and sometimes reaching 75% butane. Usage is calibratedto the different-sized nozzles found in non-U.S. grills. Americans who take their grills overseas — such as military personnel — can find U.S.-specificationpropane at AAFES military post exchanges.

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

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 Ozonedepletion potential and very low Global Warming Potential and can serve as a functional replacement for R-12, R-22, R-134a, and other chlorofluorocarbon orhydrofluorocarbon refrigerants in conventional stationary refrigeration and air conditioning systems.

In motor vehicles

Such substitution is widely prohibited or discouraged in motor vehicle air conditioning systems, on the grounds that using flammable hydrocarbons in systemsoriginally designed to carry non-flammable refrigerant presents a significant risk of fire or explosion.

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

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 isthe 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. About9 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 comparableto (though still less than) gasoline. Meanwhile it is noticeably cleaner (both in handling, and in combustion), results in less engine wear (due to carbondeposits) without diluting engine oil (often extending oil-change intervals), and until recently was a relative bargain in North America. Octane rating is anoticeably higher 110. 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 afunction 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 tocool. (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 pressurethat the process can no longer be supported. In addition, the lightweight, high-octane compounds vaporize before the heavier, low-octane ones. Thus theignition properties change as the tank empties. For these reasons, the liquid is often withdrawn using a dip tube.

Other

• 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.
• Propane is mixed with silicone to form a propellant (sold as green gas) which is used to power gas guns used in airsoft combat gaming.
• Liquid propane is commonly used in theme parks and in the movie industry as an inexpensive, high-energy fuel for explosions and other specialeffects.

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 ofexplosion 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 thefurnace or water heater, and results in an explosion or 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), largelymethane, is another gas used as fuel, but it cannot be liquified by compression at normal temperatures (which are well above the critical temperature ofmethane, and therefore requires very high pressure to store (which poses the hazard that in an accident, a CNG tank may burst with great force or leakrapidly enough to become a self-propelled missile). Therefore, CNG is much less efficient to store due to the large tank volume required. Thus propane ismuch more commonly used to fuel vehicles than natural gas and requires just 1,220 kilopascals (177 psi) of pressure to keep it liquid at 37.8 °C (100 °F).

Propane Tank Remainder Measurement

The fluid level in a propane tank can be measured with an internal magnetic "float". An external gauge can then sense the location of the float within thetank. This magnetic float-gauge system cannot accurately measure the total propane amount, since the gaseous portion of the propane within the tank isneglected from measurement.

The most accurate way to measure the propane left in a propane tank is to weigh it. Stamped into the side of the tank should be the letters TW followed by anumber. This number is the weight in pounds of tank when empty, or its tare weight. A typical 5 gallon propane tank may have a tare weight of 10 pounds. Ifthis tank were weighed at 20 lbs, it follows that there are 10 pounds of propane stored in the tank

A gallon of propane contains 91,690 BTUs. Multiplying this number by the number of gallons in the tank results in 152,205 BTUs of thermal energy (1.66 ×91,690 = 152,205).

The running time of a particular appliance can then be calculated if the BTU consumption of the appliance is known. This number, given in BTUs per hour, canusually be found on the appliance or from the manufacturer. Continuing this example, an appliance that consumes 12,000 BTUs per hour would provide 12.68hours of operation (152,205 ÷ 12,000 ≈ 12.68).

Sources

Propane is produced as a byproduct 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 to prevent condensation of these volatiles innatural gas pipelines. Additionally, oil refineries produce some propane as a by-product of production of cracking petroleum into gasoline or heating oil.

The supply of propane cannot be easily adjusted to account for 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% ofimported 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, Canada; Mont Belvieu, Texas and Conway,Kansas. These salt caverns were hollowed out in the 1940s and can store up to 80 million barrels of propane, or more. When the propane is needed, most of itis 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 toselected U.S. areas.