Vinyl chloride

Vinyl chloride monomer (VCM), also known as chloroethene in IUPAC nomenclature, is an important industrial chemical chiefly used to produce its polymer, polyvinyl chloride (PVC). At room temperature, VCM is a toxic, colorless gas with a sickly sweet odor.

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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. While this method was widely used during the 1930s and 1940s, it has since been superseded by more economical processes

Production

A vinyl chloride monomer manufacturing plant generally consists of seven (7) different plant areas, 1) the Ethylene Dichloride (EDC) production area; 2) the Reactors that convert EDC to VCM; 3) the Distillation towers that separate and purify the VCM product; 4) the Oxychlorination process area; 5) the By-product Recovery area; 6) the Environmental Protection area (Waste Treatment and Hazardous Waste storage); 7) the VCM and By-product storage .

STEP 1 - Create the raw material for the VCM reaction, ethylene dichloride (EDC), by reacting ethylene with chlorine

Ethylene Dichloride (EDC) is manufactured on an industrial scale starting with the basic building blocks of ethylene and chlorine.^[1] In the presence of iron(III) chloride as a catalyst, these compounds react exothermically to produce ethylene dichloride according to the chemical equation

CH₂=CH₂ + Cl₂ → ClCH₂CH₂Cl

STEP 2 - React EDC to form VCM and hydrogen chloride (HCl)

One common method of producing VCM is a reaction that is conducted in a bath of boiling EDC. When then heated to 500 °C at 15–30 atm (1.5 to 3 MPa) pressure, the EDC decomposes to produce VCM and HCl.

ClCH₂CH₂Cl → CH₂=CHCl + HCl

The above reaction shows that one mole of EDC reacts to form one mole of VCM and one mole of HCl.

STEP 3 - Cool the reactor effluent and purify products and by-products

Propylene refrigerant can be used to chill the reactor outlet stream. Once the stream is liquified it passes through a series of distillation towers. The last distillation tower has pure HCl going from the top and product VCM coming out of the bottom.

STEP 4 - React recycled HCl to make more EDC which is sent back to STEP 2

CH₂=CH₂ + 2 HCl + ½ O₂ → ClCH₂CH₂Cl + H₂O

HCl is an undesirable result of the reaction to produce VCM. In industrial practice, after the HCl is separated from the VCM, the HCl is mixed with oxygen and reacted with additional ethylene on a copper(II) chloride catalyst. This reaction converts the HCl back to the raw material for making VCM, ethylene dichloride (EDC). This allows the vast quantities of HCl produced in the VCM production reaction to be recycled to produce more VCM. This recycle reaction is known as Oxychlorination.

The HCl, consumed in Step 4, exactly balances that produced in Step 2. The resulting balanced process neither requires HCl as an input nor produces it as a waste. Due to the economical advantages of this process, most VCM has been produced via this technique since the late 1950s.

STEP 5 - By-product Recovery

Of course, the desired Oxychlorination reaction, shown above, is not the only reaction that occurs. Other competing reactions produce other chlorinated organic compounds. An undesired compound that can be purified and sold is called a "byproduct". One salable byproduct of the Oxychlorination process is ethyl chloride., which can be used as a topical anesthetic. Competing reactions also occur in Step 2 and by-products from Step 2 are recovered in the distillation processes of Step 3.

STEP 6 - Environmental Protection

Also produced, as can be seen in the Oxychlorination reaction above is water. The water product is acidic because of the presence of HCl and must be 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 permissible or "EPA Permitted" chemical waste analysis. The EPA, both State and Federal, regulates the allowed pH and specific amount of chemicals that are allowed to be sent back to the environment through the outfall.

Some very hazardous wastes are generated in the recovery of the product VCM. These wastes and any process materials that come in contact these wastes are stored in specialized drums in specialized containment areas. These wastes are burned onsite in hazardous waste burners that must meet strict standards from the State and Federal Environmental Protection Agencies.

STEP 7 - Product and By-product Storage

Once the vinyl chloride monomer (VCM) is produced it 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 VCM is leaking from the internal sphere. If VCM 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

VCM is a chemical intermediate. It is not a final product. Due to the hazardous nature of VCM to human health there are no end products that use vinyl chloride in its monomer form. Once VCM has been polymerized it is very stable and non-hazardous and can be used for a great number of end products.

VCM 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 is 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, VCM was used in aerosol spray propellant. Prior to the removal of VCM from hair spray the accumulation of vinyl chloride vapor in hair salons may have exceeded the NOAEL (NO Adverse Effect Level) exposure guidelines.

VCM 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 depresses the central nervous system, and inhaling its vapors produces symptoms similar to alcohol intoxication. These include headache, dizziness, and loss of coordination, and in severe cases may progress to hallucination, unconsciousness, and death by respiratory failure.

In the late 1960s, Dr. John Creech and Dr. Maurice Johnson were the first to clearly link and recognize the carcinogenicity of VCM to humans when workers in the PVC section of a B.F. Goodrich plant near Louisville, Kentucky, were diagnosed with liver angiosarcoma, a rare disease.^[2] Since that time, studies of PVC workers in Australia, Italy, Germany, and the UK have all associated certain types of occupational cancers with exposure to vinyl chloride. The link between angiosarcoma of the liver and long-term exposure to vinyl chloride is the only one which has been confirmed by the International Agency for Research on Cancer. All the cases of angiosarcoma developed from exposure to vinyl chloride monomer, were in workers who were exposed to very high VCM levels, routinely, for many years.

References

1. ^ Allen, D. T.. Chapter 4 - Industrial Ecology. Green Engineering. EPA.
2. ^ Creech and Johnson (Mar 1974). "Angiosarcoma of liver in the manufacture of polyvinyl chloride.". Journal of occupational medicine. : official publication of the Industrial Medical Association. 16 (3): 150-1.