LPG is a mixture of propane and butane containing also a small portion of propylene and butylene. It is a natural derivative and a by-product. When natural gas is extracted from the oil field, 90% of the gas is methane which is generally named as "natural gas", and the remaining 10% is “LPG”. Gas processing is the source of approximately 60% of LPG produced worldwide. In crude oil refining, LPG is the first product produced on the way to making the heavier fuels such as diesel, jet fuel oil and gasoline, and it is the source for the other roughly 40% of LPG global supplies.
LPG itself is the liquid form of the inflammable propane or butane. It is a kind of dangerous goods, and as such, ethanethiol which is an irritating odorant is always added to enable easy detection of any LPG leakage for safety assurance.
In the course of burning LPG, most of the carbon molecules will be transformed into energy with very little residual particles, thus not causing any air pollution. LPG is traditionally utilized as a clean energy for household use. The application of LPG has been in the recent years expanded to the manufacturing of products that require heat processing (such as ceramic, glass), and it also serves as the fuel of automobiles substituting gasoline or diesel.
Under normal pressure and temperature, propane and butane are in gas form. Propane and butane transform from gas into liquid respectively at -42C and -4C or by adding pressure of 0.71 Mpa and 0.14 Mpa at a temperature of 20C. In order to store or transport propane and butane safely and economically, the gases have to be liquefied and stored or transported in containers or carriers either sustainable to high pressure (liquefied by pressurization) or equipped with devices to maintain low enough temperature (liquefied by refrigeration).