The dominant technology for producing propylene is steam cracking. The same technology is applied to ethane to ethylene. These two conversions are the #2 and #1 processes in the chemical industry, as judged by their scale. In this process, propane undergoes dehydrogenation. The by-product is hydrogen: The yield of propene is about 85 m%. By-products are usually used as fuel for the propane dehydrogenation reaction. Steam cracking is one of the most energy-intensive industrial processes. The feedstock is naphtha or propane, especially in the Middle East, where there is an abundance of propane from oil/gas operations. Propene can be separated by fractional distillation from hydrocarbon mixtures obtained from cracking and other refining processes; refinery-grade propene is about 50 to 70%. In the United States shale gas is a major source of propane.
High severity fluid catalytic cracking uses traditional FCC technology under severe conditions in order to maximize the amount of propene and other light products. A high severity FCC unit is usually fed with gas oils and residues, and produces about 20–25 m% propene on feedstock together with greater volumes of motor gasoline and distillate byproducts.
Market and research
Propene production has remained static at around 35 million tonnes from 2000 to 2008, but it has been increasing in East Asia, most notably Singapore and China. Total world production of propene is currently about half that of ethylene. The use of engineered enzymes has been explored but is of no commercial value.
Uses
Propene is the second most important starting product in the petrochemical industry after ethylene. It is the raw material for a wide variety of products. Manufacturers of the plastic polypropylene account for nearly two thirds of all demand. Polypropylene end uses include films, fibers, containers, packaging, and caps and closures. Propene is also used for the production of important chemicals such as propylene oxide, acrylonitrile, cumene, butyraldehyde, and acrylic acid. In the year 2013 about 85 million tonnes of propene were processed worldwide. Propene and benzene are converted to acetone and phenol via the cumene process. Propene is also used to produce isopropanol, acrylonitrile, propylene oxide, and epichlorohydrin. The industrial production of acrylic acid involves the catalytic partial oxidation of propene. Propene is also an intermediate in the one-step propane selective oxidation to acrylic acid. In industry and workshops, propene is used as an alternative fuel to acetylene in Oxy-fuel welding and cutting, brazing and heating of metal for the purpose of bending. It has become a standard in BernzOmatic products and others in MAPP substitutes, now that true MAPP gas is no longer available.
Propene undergoes combustion reactions in a similar fashion to other alkenes. In the presence of sufficient or excess oxygen, propene burns to form water and carbon dioxide. When insufficient oxygen is present for complete combustion, incomplete combustion occurs allowing carbon monoxide and/or soot to be formed as well.
Environmental safety
Propene is a product of combustion from forest fires, cigarette smoke, and motor vehicle and aircraft exhaust. It is an impurity in some heating gases. Observed concentrations have been in the range of 0.1-4.8 parts per billion in rural air, 4-10.5 ppb in urban air, and 7-260 ppb in industrial air samples. In the United States and some European countries a threshold limit value of 500 parts per million was established for occupational exposure. It is considered a volatile organic compound and emissions are regulated by many governments, but it is not listed by the U.S. Environmental Protection Agency as a hazardous air pollutant under the Clean Air Act. With a relatively short half-life, it is not expected to bioaccumulate. Propene has low acute toxicity from inhalation. Inhalation of the gas can cause anesthetic effects and at very high concentrations, unconsciousness. However, the asphyxiation limit for humans is about 10 times higher than the lower flammability level.
Storage and handling
Since propene is volatile and flammable, precautions must be taken to avoid fire hazards in the handling of the gas. If propene is loaded to any equipment capable of causing ignition, such equipment should be shut down while loading, unloading, connecting or disconnecting. Propene is usually stored as liquid under pressure, although it is also possible to store it safely as gas at ambient temperature in approved containers.
Propene is detected in the interstellar medium through microwave spectroscopy. On September 30, 2013, NASA also announced that the Cassini orbiter spacecraft, part of the Cassini-Huygens mission, had discovered small amounts of naturally occurring propene in the atmosphere of Titan using spectroscopy.
