Liquefied petroleum gas, LPG in short, is a member of oil products family. This colourless and volatile gas is manufactured from refinery gas or natural gas (including associated gas in oilfields) through pressurizing, cooling and liquefaction. LPG from refinery gas mainly consists of propane, propylene, butane, butylene, and a small amount of pentane, pentylene, as well as a slight amount of sulfide impurities. LPG from natural gas scarcely contains olefins.
1. Form and Properties
LPG mainly serves as a petrochemical feedstock for ethylene production through hydrocarbon cracking process or for syngas production through steam conversion. It can also serve as industrial fuel and domestic fuel as well as combustion engine fuel. The major quality control indexes include evaporation residue, sulfur content and sometimes olefin content. LPG is flammable and explodes when its concentration in the air reaches a certain range.
LPG mainly comprises of propane (C3H8) and butane (C4H10). Some LPG products may contain propylene (C3H6) and butylene (C4H8) as well. LPG can be usually manufactured from the petroleum gas in oil fields, gas fields, refineries and ethylene plants. Compared with other fuels, LPG boasts the following particular advantages.
1) Less pollution. LPG is a hydrocarbon composed of C3 and C4. It is free of dust and can be completely combusted. Its application in modern cities can largely reduce the pollution caused by coal and firewood, which served as major domestic fuels in the past.
2) Higher heat generation. Given the same weight, LPG can generate twice as much heat as coal does. Its heat generation volume can reach 45185-45980 kJ/m3.
3) Easy for transportation. LPG is a gas at atmospheric pressure and normal ambient temperatures, but it can be liquefied when moderate pressure is applied or when the temperature is sufficiently reduced. It can be transported by (railway) tankers on road and LPG vessels in water.
4) Stable pressure. Pipeline pressure to gas cookers remains invariable, which is convenient for LPG pipeline users.
5) Simple storage facilities and flexible supply. Compared with urban coal gas, LPG requires simpler storage facilities. It can be stored in tanks and can be filled in cylinders for customer use. LPG can also be supplied through gas distribution station and pipeline network and even can be filled in small bottles together with butane gas, serving as convenient hot pot fuel in restaurants. Due to the above advantages of LPG, it is widely used as industrial, commercial and domestic fuel. In addition, determined by its chemical compositions, LPG is a useful raw material for chemical industry and is widely applied in the production of various chemical products.
LPG is a byproduct produced during the process of crude catalytic cracking and thermal cracking in refineries. The major components of pyrolysis gas from catalytic cracking process include (%): hydrogen 5-6, methane 10, ethane 3-5, ethylene 3, propane 16-20, propylene 6-11, butane 42-46, butylene 5-6, hydrocarbon with more than 5 carbon atoms 5-12. The major components of pyrolysis gas from thermal cracking process include (%): hydrogen 12, methane 5-7, ethane 5-7, ethylene 16-8, propane 0.5, propylene 7-8, butane 0.2, butylene 4-5, hydrocarbon with more than 5 carbon atoms 2-3. It is easy to liquefy these hydrocarbons and to compress them to 1/250-1/33 of their original volume so that they can be stored in anti-high-pressure steal cylinders. When opening the valve of the LPG cylinders, the flammable gaseous hydrocarbons will go through the pipes into the burner. If lighted, LPG will form baby blue flame, giving out a large amount of heat during the combustion (with heat generation value of 92,100 kJ/m3-121,400 kJ/m3).
With the development of petrochemical industry, LPG draws more attention as a basic raw material for chemical industry and a new type of fuel. In chemical production, LPG can be turned into ethylene, propylene, butylene, butadiene, etc. through separation process in order to produce synthetic plastic, synthetic rubber, synthetic fibre and to produce pharmaceuticals, explosives, dyestuff, etc. Since it is of high heat value, free of smoke, dust and carbon residue and easy to handle, LPG has been widely applied in people's life as a fuel. Besides, LPG can also be applied in the areas including 1) non-ferrous metal smelting, 2) calcination in kilns, 3) automobile fuel and 4) domestic fuel.
4. Related knowledge
With the development of our petrochemical industry, many cities and towns have started to use LPG as a fuel.
LPG is convenient but it also has hidden dangers. Once the pipeline leaks or the valve is not tightly fastened, LPG will spread into the room. When its content reaches the explosion limit (1.7%-10%), LPG will explode if it meets any sparks or electric sparks. In order to help people find out LPG leaks in timely manner, the processing plants often mix a little smelly gaseous compound like mercaptan or thioether derivatives with LPG. When LPG leaks, it can be smelled instantly and corresponding emergency measures will be taken.