ETHYLENE

Ethylene (IUPAC name: ethene ) is a gaseous organic compound with the formula C2H4. It is the simplest alkene (older name: olefin from its oil-forming property). Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon . Ethylene is widely used in industry and is also a plant hormone.[3] Ethylene is the most produced organic compound in the world; global production of ethylene exceeded 107 million tonnes in 2005.[4] To meet the ever increasing demand for ethylene, sharp increases in production facilities are added globally, particularly in the Persian Gulf countries and in China.[5] Structure and properties Orbital description of bonding between ethylene and a transition metal. This hydrocarbon has four hydrogen atoms bound to a pair of carbon atoms that are connected by a double bond. All six atoms that comprise ethylene are coplanar. The H-C- H angle is 119°, close to the 120° for ideal sp² hybridized carbon. The molecule is also relatively rigid: rotation about the C-C bond is a high energy process that requires breaking the π-bond. The π-bond in the ethylene molecule is responsible for its useful reactivity. The double bond is a region of high electron density , thus it is susceptible to attack by electrophiles. Many reactions of ethylene are catalyzed by transition metals, which bind transiently to the ethylene using both the π and π* orbitals. Being a simple molecule, ethylene is spectroscopically simple. Its UV-vis spectrum is still used as a test of theoretical methods. [6] Uses Major industrial reactions of ethylene include in order of scale: 1) polymerization , 2) oxidation , 3) halogenation and hydrohalogenation , 4) alkylation , 5) hydration , 6) oligomerization, and 7) hydroformylation . In the United States and Europe, approximately 90% of ethylene is used to produce three chemical compounds— ethylene oxide , ethylene dichloride, and ethylbenzene —and a variety of kinds of polyethylene .[7] Main industrial uses of ethylene. Clockwise from the upper right: its conversions to ethylene oxide , precursor to ethylene glycol, to ethylbenzene , precursor to styrene , to various kinds of polyethylene , to ethylene dichloride, precursor to vinyl chloride. Polymerization See also: Ziegler-Natta catalyst and Polyethylene Polyethylenes of various types consume more than half of world ethylene supply. Polyethylene, also called polythene, is the world's most widely-used plastic, being primarily used to make films used in packaging, carrier bags and trash liners. Linear alpha- olefins, produced by oligomerization (formation of short polymers) are used as precursors, detergents, plasticisers, synthetic lubricants, additives, and also as co-monomers in the production of polyethylenes. [7] Oxidation Ethylene is oxidized to produce ethylene oxide , a key raw material in the production of surfactants and detergents by ethoxylation . Ethylene oxide also hydrolyzed to produce ethylene glycol , widely used as an automotive antifreeze as well as higher molecular weight glycols and glycol ethers. Main article: Wacker process Ethylene undergoes oxidation by palladium to give acetaldehyde . This conversion remains a major industrial process (10M kg/y). [8] The process proceeds via the initial complexation of ethylene to a Pd(II) center. Halogenation and hydrohalogenation Major intermediates from the halogenation and hydrohalogenation of ethylene include ethylene dichloride, ethyl chloride and ethylene dibromide . The addition of chlorine entails "oxychlorination," i.e. chlorine itself is not used. Some products derived from this group are polyvinyl chloride , trichloroethylene , perchloroethylene , methyl chloroform, polyvinylidiene chloride and copolymers , and ethyl bromide .[9] Alkylation Major chemical intermediates from the alkylation with ethylene is ethylbenzene , precursor to styrene . Styrene is used principally in polystyrene for packaging and insulation, as well as in styrene-butadiene rubber for tires and footwear. On a smaller scale, ethyl toluene, ethylanilines, 1,4-hexadiene, and aluminium alkyls. Products of these intermediates include polystyrene , unsaturated polyesters and ethylene- propylene terpolymers .[9] Oxo reaction The hydroformylation (oxo reaction) of ethylene results in propionaldehyde , a precursor to propionic acid and n-propyl alcohol.[9] Hydration Ethylene can be hydrated to give ethanol, but this method is rarely used industrially. Niche uses An example of a niche use is as an anesthetic agent (in an 85% ethylene/15% oxygen ratio).[10] It can also be used to hasten fruit ripening, as well as a welding gas. [7][11] Production In 2006, global ethylene production was 109 million tonnes.[12] By 2010 ethylene was produced by at least 117 companies in 55 countries.[5] Ethylene is produced in the petrochemical industry by steam cracking . In this process, gaseous or light liquid hydrocarbons are heated to 750–950 °C, inducing numerous free radical reactions followed by immediate quench to stop these reactions. This process converts large hydrocarbons into smaller ones and introduces unsaturation. Ethylene is separated from the resulting complex mixture by repeated compression and distillation. In a related process used in oil refineries, high molecular weight hydrocarbons are cracked over zeolite catalysts. Heavier feedstocks, such as naphtha and gas oils require at least two "quench towers" downstream of the cracking furnaces to recirculate pyrolysis-derived gasoline and process water. When cracking a mixture of ethane and propane, only one water quench tower is required. [9] The areas of an ethylene plant are: 1. steam cracking furnaces: 2. primary and secondary heat recovery with quench; 3. a dilution steam recycle system between the furnaces and the quench system; 4. primary compression of the cracked gas (3 stages of compression); 5. hydrogen sulfide and carbon dioxide removal (acid gas removal); 6. secondary compression (1 or 2 stages); 7. drying of the cracked gas; 8. cryogenic treatment; 9. all of the cold cracked gas stream goes to the demethanizer tower. The overhead stream from the demethanizer tower consists of all the hydrogen and methane that was in the cracked gas stream. Cryogenically (−250 °F (−157 °C)) treating this overhead stream separates hydrogen from methane. Methane recovery is critical to the economical operation of an ethylene plant. 0. the bottom stream from the demethanizer tower goes to the deethanizer tower. The overhead stream from the deethanizer tower consists of all the C2,'s that were in the cracked gas stream. The C2 stream contains acetylene, which is explosive above 200 kPa (29 psi).[13] If the partial pressure of acetylene is expected to exceed these values, the C 2 stream is partially hydrogenated. The C2's then proceed to a C2 splitter. The product ethylene is taken from the overhead of the tower and the ethane coming from the bottom of the splitter is recycled to the furnaces to be cracked again; 1. the bottom stream from the de-ethanizer tower goes to the depropanizer tower. The overhead stream from the depropanizer tower consists of all the C 3's that were in the cracked gas stream. Before feeding the C3's to the C3 splitter, the stream is hydrogenated to convert the methylacetylene and propadiene (allene) mix. This stream is then sent to the C3 splitter. The overhead stream from the C3 splitter is product propylene and the bottom stream is propane which is sent back to the furnaces for cracking or used as fuel. 2. The bottom stream from the depropanizer tower is fed to the debutanizer tower. The overhead stream from the debutanizer is all of the C 4's that were in the cracked gas stream. The bottom stream from the debutanizer (light pyrolysis gasoline) consists of everything in the cracked gas stream that is C 5 or heavier. [9] Since ethylene production is energy intensive, much effort has been dedicated to recovering heat from the gas leaving the furnaces. Most of the energy recovered from the cracked gas is used to make high pressure (1200 psig) steam. This steam is in turn used to drive the turbines for compressing cracked gas, the propylene refrigeration compressor, and the ethylene refrigeration compressor. An ethylene plant, once running, does not need to import steam to drive its steam turbines. A typical world scale ethylene plant (about 1.5 billion pounds of ethylene per year) uses a 45,000 horsepower (34,000 kW) cracked gas compressor, a 30,000 hp (22,000 kW) propylene compressor, and a 15,000 hp (11,000 kW) ethylene compressor.