Propane

Propane is a three-carbon alkane with the molecular formula C₃H₈. Under standard temperature and pressure it is a gas, but it can be readily compressed into a transportable liquid. A common by-product of natural gas processing and petroleum refining, propane forms a major component of liquefied petroleum gas (LPG), alongside compounds such as propene, butane, butene, butadiene and isobutylene. Owing to its clean combustion, favourable energy properties and ease of storage, propane is widely used as a domestic and industrial fuel, in low-emission transport systems and increasingly as an environmentally preferable refrigerant.

Historical Development

Propane was first synthesised in 1857 by the French chemist Marcellin Berthelot during his investigations into hydrogenation reactions. He produced propane by heating propylene dibromide with potassium iodide and water. In 1864 Edmund Ronalds identified propane dissolved in light crude oil from Pennsylvania.
Commercial interest increased in the early twentieth century. In 1910 Walter O. Snelling of the United States Bureau of Mines identified propane as a volatile fraction of gasoline, marking the start of the propane industry. Working with collaborators, Snelling developed methods for liquefying LPG components during gasoline refining. By 1911 he had produced relatively pure propane, and his processing method was patented in 1913.
Production grew rapidly from the 1920s, with substantial increases in marketed volumes and the introduction of key industry developments such as railway tank transport, odourisation for safety, and local filling plants. By the mid-twentieth century propane had become a common household fuel; by 1947, over half of homes in the United States used natural gas or propane for cooking. Industrial adoption also expanded, with thousands of propane-powered buses ordered during the 1950s. By the early twenty-first century the US propane industry was valued at between eight and ten billion dollars annually.

Etymology

The prefix prop- in propane and other three-carbon compounds derives from propionic acid, itself named from the Greek words protos (“first”) and pion (“fat”), denoting its position as the first in the series of fatty acids.

Physical and Chemical Properties

Propane is a colourless and odourless gas; for safety, odourants such as ethyl mercaptan are added to enable leak detection. It liquefies at −42 °C and solidifies at −187.7 °C. In the solid state it crystallises in the space group P2₁/n, its low melting point being associated with inefficient molecular packing.
In combustion, propane behaves similarly to other alkanes. With excess oxygen it burns completely to form carbon dioxide and water, releasing significant heat. In oxygen-limited conditions it forms carbon monoxide, soot or both. Complete combustion yields approximately 50 MJ per kilogram, and its carbon dioxide emissions per unit of heat released are low compared with most other fossil fuels.
The gas has a density of 1.808 kg m⁻³ at 25 °C—around 1.5 times that of air—while the liquid has a density of 0.493 g cm⁻³ at the same temperature. Liquid propane expands with temperature, a consideration essential for storage and safety. Propane also burns with a visible flame, owing partly to the presence of carbon–carbon bonds and unsaturated hydrocarbons within typical LPG mixtures.

Energy Content

The higher heating value (HHV) of gaseous propane, in which combustion products return to standard conditions, is approximately 50.3 MJ kg⁻¹. The lower heating value (LHV), where water remains in the vapour phase, is around 46.4 MJ kg⁻¹. These values make propane an energy-dense and efficient fuel for a range of applications.

Uses

Portable and domestic fuelPropane is widely used for barbecues, portable stoves, camping equipment and outdoor heating because it vaporises readily from its compressed liquid form even at low temperatures. Its low boiling point allows reliable use in cold climates, in contrast to fuels such as butane.
Transport fuelLPG, of which propane is a major component, powers buses, forklifts, automobiles, outboard motors and ice resurfacers. Its clean combustion offers reduced emissions compared with petrol or diesel.
Refrigeration and heat pumpsPropane (R290) has become increasingly popular as a refrigerant for heat pumps and stationary refrigeration systems. Compared with conventional hydrofluorocarbon refrigerants such as R410A, R32 and R134a, propane provides higher efficiency, improved high-temperature performance and minimal environmental impact. Its ozone depletion potential is negligible and its global warming potential is extremely low. However, its flammability requires specially designed equipment.
Off-grid refrigerationPropane fuels gas-absorption refrigerators, providing reliable cooling for locations without electrical supply, including recreational vehicles and remote dwellings.
Industrial and commercial applicationsPropane is used as a feedstock in petrochemical processes, for metal cutting and heating, and in agricultural operations such as crop drying.

Safety Considerations

Propane’s flammability and the density of its vapour, which is heavier than air, necessitate careful handling, storage and ventilation. Tanks must accommodate thermal expansion of the liquid, and odourants are required to aid detection of leaks. Because standard refrigeration and air-conditioning systems may not be designed for flammable refrigerants, substituting propane for non-flammable refrigerants in unsuitable equipment is restricted in many jurisdictions.