Vinyl chloride is the organochloride with the formula CH 2 :CHCl. It is also called vinyl chloride monomer, or VCM. This colourless compound is an important industrial chemical chiefly used to produce the polymer polyvinyl chloride (PVC). At ambient pressure and temperature, vinyl chloride is a gas with a sickly sweet odor. It is highly toxic, flammable and carcinogenic.
History
Vinyl chloride was first produced in 1835 by Justus von Liebig and his student Henri Victor Regnault. They obtained it by treating ethylene dichloride with a solution of potassium hydroxide in ethanol.
In 1912, Frans, 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. Whereas this method was widely used during the 1930s and 1940s, it has since been superseded by more economical processes based on ethylene, at least in the West.
Production
Vinyl chloride is produced on a substantial scale - approximately 31.1 million tons were produced in 2000. Two methods are employed, the hydrochlorination of acetylene and the dehydrochlorination of ethylene dichloride (1,2-dichloroethane).
Manufacturing details from ethylene dichloride
Direct chlorination
The production of vinyl chloride from ethylene dichloride (EDC) consists of a series of well-defined steps. Ethylene dichloride (EDC) is prepared by reacting ethylene and chlorine. In the presence of iron(III) chloride as a catalyst, these compounds react exothermically:
This process is very selective, resulting in high purity EDC and high yields. However any dissolved catalyst and moisture must be removed before EDC enters the VCM production process.
Thermal cracking
When heated to 500 °C at 15–30 atm (1.5 to 3 MPa) pressure, EDC vapor decomposes to produce vinyl chloride and anhydrous 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, EDC conversion is relatively low (50 to 60 percent). The furnace effluent is immediately quenched with cold EDC to stop 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.
Oxychlorination
Modern VCM plants use recycled HCl is to produce more EDC via oxychlorination, which entails the reaction of ethylene, oxygen and hydrogen chloride over a copper(II) chloride catalyst to produce EDC:
The reaction is highly exothermic.
Due to the relatively low cost of ethylene, compared to acetylene, most vinyl chloride has been produced via this technique since the late 1950s. This is despite the lower yields, lower product purity and higher costs for waste treatment. By-products of the oxychlorination reaction, may be recovered, as feedstocks for chlorinated solvents production. One useful byproduct of the oxychlorination is ethyl chloride, a topical anesthetic.
Waste treatment
For environmental reasons, the acidic aqueous wastestream is treated to remove organic compounds and neutralized before it can be sent to the plant's "outfall". An outfall is a monitored wastewater stream that must conform to the plant's standards. Some very hazardous wastes are generated in the recovery of the product vinyl chloride. These wastes require specialized procedures. These wastes are burned onsite in hazardous waste burners that again are subject to strict standards.
Production from acetylene
Acetylene reacts with anhydrous hydrogen chloride gas over a mercuric chloride catalyst to give vinyl chloride:
The reaction is exothermic and highly selective. Product purity and yields are generally very high.
This was the most common industrial route to VCM, before ethylene became widely distributed. When VCM producers shifted to using the thermal cracking of EDC described above, some used byproduct HCl in conjunction with a colocated acetylene-based unit. The hazards of storing and shipping acetylene meant that the VCM facility needed to be located very close to the acetylene generating facility.
Storage
Vinyl chloride is stored as a liquid. Often, the storage containers for the product VCM are high capacity spheres. The spheres have an inside sphere and an outside sphere. Several inches of empty space separate the inside sphere from the outside sphere. This void area between the spheres is purged with an inert gas such as nitrogen. As the nitrogen purge gas exits the void space it passes through an analyzer that is designed to detect if any vinyl chloride is leaking from the internal sphere. If vinyl chloride starts to leak from the internal sphere or if a fire is detected on the outside of the sphere then the contents of the sphere is automatically dumped into an emergency underground storage container.
Uses
Vinyl chloride is a chemical intermediate, not a final product. Due to the hazardous nature of vinyl chloride to human health there are no end products that use vinyl chloride in its monomer form. Polyvinyl chloride is very stable, storable, and less acutely hazardous than the monomer.
Vinyl chloride liquid is fed to polymerization reactors where it is converted from a monomer to a polymer PVC. The final product of the polymerization process is PVC in either a flake or pellet form. Literally, tens of billions of pounds of PVC are sold on the global market each year. From its flake or pellet form PVC is sold to companies that heat and mold the PVC into end products such as PVC pipe and bottles.
Until 1974, vinyl chloride was used in aerosol spray propellant. Prior to the removal of vinyl chloride from hair spray the accumulation of vinyl chloride vapor in hair salons may have exceeded the NOAEL (No Observable Adverse Effect Level) exposure guidelines.
Vinyl chloride was briefly used as an inhalational anaesthetic, in a similar vein to ethyl chloride, though its toxicity forced this practice to be abandoned.
Health effects
Vinyl chloride monomer
Almost all vinyl chloride monomer (VCM) is used to create polymers, primarily PVC (polyvinyl chloride).
Historically, workers in PVC plants were often exposed to high levels of VCM. Due to the carcinogenicity of VCM; many of those workers have contracted and died from cancer. The hepatotoxicity of VCM has long been established since the 1930’s when the PVC industry was just in its infant stages. In the very first study about the dangers of Vinyl Chloride (VC), published by Patty in 1930, it was disclosed that exposure of test animals to just a single short-term high dose of VC caused liver damage.. In 1949, a Russian publication by Tribukh discussed the finding that VC caused liver injury among workers. In 1954, Dr. Rex Wilson, Medical Director, and William McCormick, Industrial Hygienist and Toxicologist, both of B.F. Goodrich Chemical, published an article that stated that it was known VC caused liver injury for short-term exposures; but almost nothing was known about its long-term effects. They also stated that long-term animal toxicology studies should be performed to fill this void of information. It was noted in the study that if a chemical did not justify the cost of testing, and you knew what it could do to workers and the public, the chemical should not be made. Thereafter, in 1963, Lester and Greenberg published an article reporting their findings from research paid for in part by Allied Chemical. They too found liver damage in test animals from exposures below 500 parts per million (ppm). Then, in 1963, a Romanian researcher, Suciu, published his findings of liver disease in VC workers. In 1968, Mutchler and Kramer, two Dow researchers, reported their finding that exposures as low as 300 ppm caused liver damage in VC workers thus confirming earlier animal data in humans. In a 1969 presentation given in Japan, Dr. P. L. Viola, a European researcher working for the European VC industry, indicated, “every monomer used in V.C. manufacture is hazardous....various changes were found in bone and liver. Particularly, much more attention should be drawn to liver changes. The findings in rats at the concentration of 4 to 10 ppm are shown in pictures.” In light of the finding of liver damage in rats from just 4-10 ppm of VC exposure, Dr. Viola added that he “should like some precautions to be taken in the manufacturing plants polymerizing vinyl chloride, such as a reduction of the threshold limit value of monomer …” In 1970, Dr. Viola, reported that test animals exposed to 30,000 ppm of VC developed cancerous tumors. It should be noted that Viola began his research looking for the cause of liver and bone injuries found in VC workers. Dr. Viola’s findings in 1970 were a “red flag” to B.F. Goodrich and the industry. In 1972, Dr. Maltoni, another Italian researcher for the European VC industry, found liver tumors (including angiosarcoma) from VC exposures as low as 250 ppm for four hours a day.
In the late 1960’s the cancers that all of theses studies warned of finally manifested itself in workers. Dr. John Creech from B.F. Goodrich discovered angiosarcoma (a very ra
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