Vinyl chloride is an organochloride with the formula H2C=CHCl. It is also called vinyl chloride monomer (VCM) or chloroethene. It is an important industrial chemical chiefly used to produce the polymer polyvinyl chloride (PVC). Vinyl chloride is a colourless flammable gas that has a sweet odor and is carcinogenic. Vinyl chloride monomer is among the top twenty largest petrochemicals (petroleum-derived chemicals) in world production. The United States remains the largest vinyl chloride manufacturing region because of its low-production-cost position in chlorine and ethylene raw materials. China is also a large manufacturer and one of the largest consumers of vinyl chloride. It can be formed in the environment when soil organisms break down chlorinated solvents. Vinyl chloride that is released by industries or formed by the breakdown of other chlorinated chemicals can enter the air and drinking water supplies. Vinyl chloride is a common contaminant found near landfills. Before the 1970s, vinyl chloride was used as an aerosol propellant and refrigerant.
Uses
Vinyl chloride, also called vinyl chloride monomer (VCM), is exclusively used as a precursor to PVC. Due to its toxic nature, vinyl chloride is not found in other products. Poly(vinyl chloride) (PVC) is very stable, storable and not toxic. Until 1974, vinyl chloride was used in aerosol spray propellant. Vinyl chloride was briefly used as an inhalational anaesthetic, in a similar vein to ethyl chloride, though its toxicity limited this use.
Production Globally, approximately 40 million tonnes of PVC resin are produced per year, resulting in approximately 10.2 million tonnes of vinyl chloride produced.
History Vinyl chloride was first synthesized in 1835 by Justus von Liebig and his student Henri Victor Regnault. They obtained it by treating 1,2-dichloroethane with a solution of potassium hydroxide in ethanol.
Acetylene-based routes In 1912, Fritz Klatte, a German chemist working for Griesheim-Elektron, patented a means to produce vinyl chloride from acetylene and hydrogen chloride using mercuric chloride as a catalyst. Acetylene reacts with hydrogen chloride over a mercuric chloride catalyst to give vinyl chloride:
C2H2 + HCl → CH2=CHCl The reaction is exothermic and highly selective. Product purity and yields are generally very high. This route to vinyl chloride was common before ethylene became widely distributed. When vinyl chloride producers shifted to using the thermal cracking of EDC described below, some used byproduct HCl in conjunction with a colocated acetylene-based unit. The hazards of storing and shipping acetylene meant that the vinyl chloride facility needed to be located very close to the acetylene generating facility. In view of mercury's toxicity, gold- and platinum-based catalysts have been proposed. The mercury-based technology is the main production method in China due to low price on coal from which acetylene is produced, with over 80% of national capacity as of 2018, even though the resulting PVC contains residues and is only suitable for low-end products like pipes.
Ethylene-based routes In the United States and Europe, mercury-catalyzed routes widely used in the 20th century have been superseded by more economical and greener processes based on ethylene. Ethylene is made by cracking ethane. Two steps are involved, chlorination and dehydrochlorination:
H2C=CH2 + Cl2 → H2ClC−CH2Cl H2ClC−CH2Cl → H2C=CHCl + HCl
Possible routes from ethane Numerous attempts have been made to convert ethane directly to vinyl chloride. Ethane, which is even more readily available than ethylene, is a potential precursor to vinyl chloride. The conversion of ethane to vinyl chloride has been demonstrated by various routes: High-temperature chlorination:
H3C−CH3 + 2 Cl2 → H2C=CHCl + 3 HCl High-temperature oxychlorination, which uses oxygen and hydrogen chloride in place of chlorine:
H3C−CH3 + O2 + HCl → H2C=CHCl + 2 H2O High-temperature oxidative chlorination: 4 H3C−CH3 + 3 O2 + 2 Cl2 → 4 H2C=CHCl + 6 H2O
Thermal decomposition of dichloroethane 1,2-Dichloroethane, ClCH2CH2Cl (also known as ethylene dichloride, EDC), can be prepared by halogenation of ethane or ethylene, inexpensive starting materials. EDC thermally converts into vinyl chloride and anhydrous HCl. This production method has become the major route to vinyl chloride since the late 1950s.
ClCH2−CH2Cl → CH2=CHCl + HCl The thermal cracking reaction is highly endothermic, and is generally carried out in a fired heater. Even though residence time and temperature are carefully controlled, it produces significant quantities of chlorinated hydrocarbon side products. In practice, the yield for EDC conversion is relatively low (50 to 60 percent). The furnace effluent is immediately quenched with cold EDC to minimize undesirable side reactions. The resulting vapor-liquid mixture then goes to a purification system. Some processes use an absorber-stripper system to separate HCl from the chlorinated hydrocarbons, while other processes use a refrigerated continuous distillation system.
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