Search

Propane

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 petroleum refining, 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 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 1920s saw increased production of LP gas, with the first year of recorded production totaling 223,000 US gallons (840 m3) in 1922. In 1927, annual marketed LP gas production reached 1 million US gallons (3,800 m3), and by 1935, the annual sales of LP gas had reached 56 million US gallons (210,000 m3). 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. 

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 US gallons (26,000,000 m3) annually. In 2004 it was reported to be a growing $8-billion to $10-billion industry with over 15 billion US gallons (57,000,000 m3) of propane being used annually in the U.S. 

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

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, and they can store 80,000,000 barrels (13,000,000 m3) or more 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 railway 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

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 liter or 73.7 kg/GJ.

Energy content

The energy density of propane is 46.44 megajoules per kilogram (91,690 BTU per US gallon, 2220 kJ/mol, 50.34 kJ/g).

Weight

The density of compressed liquid propane at 25 °C is 0.493 g/cm3, which is equivalent to 4.11 pounds per gallon. Thus, compressed liquid propane weighs approximately 4.2 pounds per US liquid gallon, at 60 °F. Propane expands 1.5% per 10 °F.

Uses

The advantage of propane in cars 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 of propane is relatively high at 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. It must be noted that LPG in cars does contain a mixture of butane, where in domestic cylinders it doesn't.

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 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 by its Standard 1835|http://hd5propane.com/| for internal combustion engines. Not all products labelled "propane" conform to this standard. In Mexico, for example, the butane content is much higher.

Domestic and industrial fuel

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, as well as northern Australia and some parts of southern 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, 6.9 million American households use propane as their primary heating fuel. 

In North America, local delivery trucks called "bobtails", with an average tank size of 3,000 US gallons (11,000 L), 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 US gallons (38,000 L), 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.

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.

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. One particular demonstration was conducted by a professor at the University of New South Wales that intentionally tested the result of all the hydrocarbon refrigerant leaking from the air conditioning system into the passenger compartment of a vehicle and then provided with a source of ignition. The professor had stated that no fire or other dangerous event would occur. However, an explosion did in fact result. All four of the car's doors were bent outward, interior materials were burned and melted, and the professor and several observers in and near the car sustained burns to their face, ears, and hands, as well as lacerations from flying shards of broken window glass.

Motor 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.
Propane is also used as fuel for small engines, especially those used indoors or in areas with insufficient fresh air and ventilation to carry away the more toxic exhaust of an engine running on gasoline or diesel. More recently, there have been lawn care products like string trimmers, lawn mowers and leaf blowers intended for outdoor use but fueled by propane to reduce air pollution.

Other uses

  • 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 is commonly used as a propellant for Paintball and Airsoft guns, relying on the expansion of the gas to fire the projectile.

Propane risks and alternate gas fuels

Propane is denser 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.

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 temperature 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).

No comments:

Post a Comment

Linkwithin