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Propylene

physical science Maturity 11-13 Next article →

Propylene, also known as propene, is an unsaturated organic compound. It has the chemical formula C3H6. This molecule is the second simplest member of the alkene class of hydrocarbons. It exists as a colorless gas with a faint petroleum-like odor. Propylene is a vital building block in the petrochemical industry. In fact, it is the second most important starting product after ethylene.

Polypropylène
Polypropylène

Scientists use several different methods to produce propylene. The most dominant technology is called steam cracking. In this process, propane is used as the feedstock, which is the starting material. Cracking the propane yields a mixture of several different compounds. These include ethylene, methane, hydrogen gas, and other related substances. The yield of propylene from this specific method is about 15 percent. Another principal feedstock used is naphtha, especially in Asia and the Middle East.

Cumene-process-overview-2D-skeletal
Cumene-process-overview-2D-skeletal

Other advanced technologies allow for more specific production goals. One method is olefin conversion technology, often called the Phillips triolefin process. This technology uses rhenium and molybdenum catalysts to interconvert propylene with ethylene and 2-butenes. This process is based on an olefin metathesis reaction. It is very efficient and can achieve propylene yields of about 90 wt%. Another method is the Methanol-to-Olefins process. This converts synthesis gas into methanol, and then into ethylene or propene. This process produces water as a by-product.

Cumene-process-overview-2D-skeletal
Cumene-process-overview-2D-skeletal

High severity fluid catalytic cracking, or FCC, is another industrial method. This process uses traditional FCC technology under much more severe conditions. It uses higher temperatures and higher steam injection rates. It also uses higher catalyst-to-oil ratios. This method is used to maximize the amount of propene and other light products. It typically uses gas oils and residues as the feedstock. This process produces about 20–25 percent propylene by mass. However, these high-temperature processes are expensive and have a high carbon footprint.

Cumene-process-overview-2D-skeletal
Cumene-process-overview-2D-skeletal

Propylene has a long history of scientific discovery. John Williams Reynolds discovered the gas in 1850. He was a student of the scientist A. W. von Hoffmann. Reynolds found that propylene was the only gaseous product of the thermal decomposition of amyl alcohol when it reacted with bromine and chlorine. Since that discovery, production has grown significantly. In 2013, about 85 million tonnes of propylene were processed worldwide. Total world production is currently about half the amount of ethylene produced.

Cumene-process-overview-2D-skeletal
Cumene-process-overview-2D-skeletal

Most of the propylene produced globally is used to make polypropylene. This is a very important commodity thermoplastic. Manufacturers consume nearly two thirds of all global propylene production to make this plastic. Polypropylene is used for many items like films, fibers, and containers. It is also used for packaging and small parts like caps and closures.

Polypropylène
Polypropylène

Beyond plastics, propylene is used to create chemicals like propylene oxide and acrylonitrile. It is also used in the cumene process to make acetone and phenol. In workshops, it can be used as an alternative fuel for oxy-fuel welding and metal heating.

Cumene-process-overview-2D-skeletal
Cumene-process-overview-2D-skeletal

Propylene is also found in nature and outer space. It is a product of combustion from forest fires and aircraft exhaust. It can be found in urban and industrial air samples. In 2013, NASA scientists announced a major discovery regarding the moon Titan. Using data from the Cassini orbiter, they detected naturally occurring propene in Titan's atmosphere. This discovery solved a 32-year-old mystery by filling a gap in the predicted hydrocarbons on the moon.

Cumene-process-overview-2D-skeletal
Cumene-process-overview-2D-skeletal

While useful, propylene must be handled with extreme care. It is a highly flammable gas with an explosion risk. It is usually stored as a liquid under pressure to keep it stable. It is also considered a volatile organic compound, so many governments regulate its emissions. However, it has a relatively short half-life and is not expected to bioaccumulate. It has low acute toxicity from inhalation and is not considered to be carcinogenic. The main danger is its ability to displace oxygen, which can act as an asphyxiant.

Cumene-process-overview-2D-skeletal
Cumene-process-overview-2D-skeletal
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