Definitions

mercuric chloride

mercuric chloride

mercuric chloride or mercury (II) chloride, chemical compound, HgCl2, a white powder of colorless rhombohedral crystals, somewhat soluble in water. It is also called bichloride of mercury or corrosive sublimate. It is extremely poisonous. Raw egg white may be given as an antidote, since mercuric chloride reacts with egg albumin to form a nearly insoluble precipitate; medical treatment should be sought immediately. Mercuric chloride is sometimes used in dilute solution as an antiseptic for inanimate objects and as a fungicide. It is also used in preparing other mercury compounds; it reacts with mercury metal to form mercurous chloride. Mercuric chloride is prepared by reacting mercury with chlorine gas or by subliming a mixture of mercuric sulfate and sodium chloride (common salt).

Vinyl chloride is the organic compound with the formula CH2CHCl. This colourless compound is an important industrial chemical chiefly used to produce the polymer, polyvinyl chloride (PVC). At room temperature, vinyl chloride is a gas with a sickly sweet odor that is easily condensed. It is highly toxic.

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 Whereas this method was widely used during the 1930s and 1940s, it has since been superseded by more economical processes, at least in the West.

Production

Vinyl chloride is produced on a substantial scale - between 12 and 15M tons were produced in 1984. Two methods are employed, the hydrochlorination of acetylene and the dehydrochlorination of dichloroethylene.

Manufacturing details from ethylene dichloride

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:
CH2=CH2 + Cl2 → ClCH2CH2Cl
When heated to 500 °C at 15–30 atm (1.5 to 3 MPa) pressure, EDC decomposes to produce vinyl chloride and HCl.
ClCH2CH2Cl → CH2=CHCl + HCl
A propylene refrigerant can be used to chill the outlet stream prior to a series of distillation towers. The last distillation tower has pure HCl going from the top and product vinyl chloride coming out of the bottom. The recycled HCl is used to produce more EDC. The recycling process involves a copper(II) chloride-catalyzed oxychlorination of ethylene. Oxychlorination entails the combined action of oxygen and hydrogen chloride to produce chlorine in situ:
CH2=CH2 + 2 HCl + ½ O2 → ClCH2CH2Cl + H2O
Due to the economical advantages of this recycling as well as the low cost of ethylene, most vinyl chloride has been produced via this technique since the late 1950s. By-products of the oxychlorination reaction, are recovered, being called a "byproduct". One useful byproduct of the oxychlorination is ethyl chloride, a topical anesthetic. 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, produced by the hydrolysis of calcium carbide, is treated with hydrogen chloride to give vinyl chloride:
C2H2 + HCl → CH2=CHCl
The method is not widely practiced in the west owing to the cost of the acetylene and the associated environmental impact of its production.

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

In the late 1960s, Dr. John Creech and Dr. Maurice Johnson were the first to clearly link and recognize the carcinogenicity of vinyl chloride monomer to humans when workers in the polyvinyl chloride polymerization section of a B.F. Goodrich plant near Louisville, Kentucky, were diagnosed with liver angiosarcoma, a rare disease.[16] 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 that 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.

A 1997 U.S. Centers for Disease Control and Prevention (CDC) report concluded that the development and acceptance by the PVC industry of a closed loop polymerization process in the late 1970s "almost completely eliminated worker exposures" and that "new cases of hepatic angiosarcoma in vinyl chloride polymerization workers have been virtually eliminated."[17]

According to the EPA, "vinyl chloride emissions from polyvinyl chloride (PVC), ethylene dichloride (EDC), and vinyl chloride monomer (VCM) plants cause or contribute to air pollution that may reasonably be anticipated to result in an increase in mortality or an increase in serious irreversible, or incapacitating reversible illness. Vinyl chloride is a known human carcinogen that causes a rare cancer of the liver."[18] EPA's 2001 updated Toxicological Profile and Summary Health Assessment for VCM in its Integrated Risk Information System (IRIS) database lowers EPA's previous risk factor estimate by a factor of 20 and concludes that "because of the consistent evidence for liver cancer in all the studies...and the weaker association for other sites, it is concluded that the liver is the most sensitive site, and protection against liver cancer will protect against possible cancer induction in other tissues."[19]

A 1998 front-page series in the Houston Chronicle claimed the vinyl industry has manipulated vinyl chloride studies to avoid liability for worker exposure and to hide extensive and severe chemical spills into local communities.[20] Retesting of community residents in 2001 by the U.S. Agency for Toxic Substances and Disease Registry (ATSDR) found dioxin levels similar to those in a comparison community in Louisiana and to the U.S. population.[21] Cancer rates in the community were similar to Louisiana and US averages.[22]

Dioxins

The environmentalist group Greenpeace has advocated the global phase-out of PVC because they claim dioxin is produced as a byproduct of vinyl chloride manufacture and from incineration of waste PVC in domestic garbage. The European Industry, however, asserts[citation needed] that it has improved production processes to minimize dioxin emissions. Dioxins are a global health threat because they persist in the environment and can travel long distances. At very low levels, near those to which the general population is exposed, dioxins have been linked[citation needed] to immune system suppression, reproductive disorders, a variety of cancers, and endometriosis. According to a 1994 report by the British firm, ICI Chemicals & Polymers Ltd., "It has been known since the publication of a paper in 1989 that these oxychlorination reactions [used to make vinyl chloride and some chlorinated solvents] generate polychlorinated dibenzodioxins (PCDDs) and dibenzofurans (PCDFs). The reactions include all of the ingredients and conditions necessary to form PCDD/PCDFs.... It is difficult to see how any of these conditions could be modified so as to prevent PCDD/PCDF formation without seriously impairing the reaction for which the process is designed." In other words, dioxins are an undesirable byproduct of polymerizing PVC and eliminating the production of dioxins while maintaining the polymerization reaction may be difficult. Dioxins created by vinyl chloride production are released by on-site incinerators, flares, boilers, wastewater treatment systems and even in trace quantities in vinyl resins.[23] The US EPA estimate of dioxin releases from the PVC industry was 13 grams TEQ in 1995, or less than 0.5% of the total dioxin emissions in the US; by 2002, PVC industry dioxin emissions had been further reduced by 23%.[24]

The largest well-quantified source of dioxin in the US EPA inventory of dioxin sources is barrel burning of household waste.[25] Studies of household waste burning indicate consistent increases in dioxin generation with increasing PVC concentrations.[26] According to the EPA dioxin inventory, landfill fires are likely to represent an even larger source of dioxin to the environment. A survey of international studies consistently identifies high dioxin concentrations in areas affected by open waste burning and a study that looked at the homologue pattern found the sample with the highest dioxin concentration was "typical for the pyrolysis of PVC". Other EU studies indicate that PVC likely "accounts for the overwhelming majority of chlorine that is available for dioxin formation during landfill fires."[27]

The next largest sources of dioxin in the EPA inventory are medical and municipal waste incinerators.[28] Studies have shown a clear correlation between dioxin formation and chloride content and indicate that PVC is a significant contributor to the formation of both dioxin and PCB in incinerators.[29]

In February 2007, the Technical and Scientific Advisory Committee of the US Green Building Council (USGBC) released its report on a PVC avoidance related materials credit for the LEED Green Building Rating system. The report concludes that "no single material shows up as the best across all the human health and environmental impact categories, nor as the worst" but that the "risk of dioxin emissions puts PVC consistently among the worst materials for human health impacts." [30]

References

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