Wednesday, 03 August 2011 05:47

Hydrocarbons, Aliphatic Unsaturated

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The unsaturated hydrocarbons are commercially important as starting materials for the manufacture of numerous chemicals and polymers, such as plastics, rubbers and resins. The vast production of the petrochemicals industry is based on the reactivity of these substances.

1-Pentene is a blending agent for high-octane motor fuel, and isoprene is used in the manufacture of synthetic natural rubber and butyl rubber. Propylene is also used in synthetic rubber manufacture and in the polymerized form as polypropylene plastic. Isobutylene is an antioxidant in the food and food-packaging industries. 1-Hexene is used in the synthesis of flavours, perfumes and dyes. Ethylene, cis-2-butene and trans-2-butene are solvents, and propadiene is a component of fuel gas for metalworking.

The principal industrial use of ethylene is as a building block for chemical raw materials which, in turn, are used to manufacture a large variety of substances and products. Ethylene is used also in oxyethylene welding and cutting of metals, and in mustard gas. It acts as a refrigerant, an inhalation anaesthetic, and as a plant growth accelerator and fruit ripener. However, the amounts used for these purposes are minor in comparison with the quantities used in the manufacture of other chemicals. One of the major chemicals derived from ethylene is polyethylene, which is made by catalytic polymerization of ethylene and is used for the manufacture of a variety of moulded plastic products. Ethylene oxide is produced by catalytic oxidation and in turn is used to make ethylene glycol and ethanolamines. Most of the industrial ethyl alcohol is produced by the hydration of ethylene. Chlorination yields vinyl chloride monomer or 1,2-dichloroethane. When reacted with benzene, styrene monomer is obtained. Acetaldehyde is also made by oxidation of ethylene.


Health hazards

Like their saturated counterparts, the lower unsaturated aliphatic hydrocarbons, or olefins, are simple asphyxiants, but as the molecular weight increases the narcotic and irritant properties become more pronounced than those of their saturated analogues. Ethylene, propylene and amylene have, for example, been used as surgical anaesthetics, but they require large concentrations (60%) and for that reason are administered with oxygen. The diolefins are more narcotic than the mono-olefins and are also more irritating to the mucous membranes and the eyes.

1,3-Butadiene. Physico-chemical hazards associated with butadiene result from its high flammability and extreme reactivity. Since a flammable mixture of 2 to 11.5% butadiene in air is easily reached, it constitutes a dangerous fire and explosion hazard when exposed to heat, sparks, flame or oxidizers. On exposure to air or oxygen, butadiene readily forms peroxides, which may undergo spontaneous combustion.

Despite the fact that over the years, the experience of workers with occupational exposure to butadiene, and laboratory experiments on humans and animals, had appeared to indicate that its toxicity is of a low order, epidemiological studies have shown that 1,3-butadiene is a probable human carcinogen (Group 2A rating by the International Agency for Research on Cancer (IARC)). Exposure to very high levels of gas may result in primary irritant and anaesthetic effects. Human subjects could tolerate concentrations up to 8,000 ppm for 8 hours with no ill effects other than slight irritation of the eyes, nose and throat. It was found that dermatitis (including frostbite due to cold injury) may result from exposure to liquid butadiene and its evaporating gas. Inhalation of excessive levels—which might produce anaesthesia, respiratory paralysis and death—can occur from spills and leaks from pressure vessels, valves and pumps in areas with inadequate ventilation. Butadiene is discussed in more detail in the Rubber industry chapter in this volume.

Similarly isoprene, which had not been associated with toxicity except at very high concentrations, is now considered a possible human carcinogen (Group 2B) by IARC.

Ethylene. The major hazard of ethylene is that of fire or explosion. Ethylene spontaneously explodes in sunlight with chlorine and can react vigorously with carbon tetrachloride, nitrogen dioxide, aluminium chloride and oxidizing substances in general. Ethylene-air mixtures will burn when exposed to any source of ignition such as static, friction or electrical sparks, open flames or excess heat. When confined, certain mixtures will explode violently from these sources of ignition. Ethylene is often handled and transported in liquefied form under pressure. Skin contact with the liquid can cause a “freezing burn”. There is little opportunity of exposure to ethylene during its manufacture because the process takes place in a closed system. Exposures may occur as a result of leaks, spills or other accidents that lead to release of the gas into the air. Empty tanks and vessels that have contained ethylene are another potential source of exposure.

In air, ethylene acts primarily as an asphyxiant. Concentrations of ethylene required to produce any marked physiological effect will reduce the oxygen content to such a low level that life cannot be supported. For example, air containing 50% of ethylene will contain only about 10% oxygen.

Loss of consciousness results when the air contains about 11% of oxygen. Death occurs quickly when the oxygen content falls to 8% or less. There is no evidence to indicate that prolonged exposure to low concentrations of ethylene can result in chronic effects. Prolonged exposure to high concentrations may cause permanent effects because of oxygen deprivation.

Ethylene has a very low order of systemic toxicity. When used as a surgical anaesthetic, it is always administered with oxygen. In such cases, its action is that of a simple anaesthetic having a rapid action and an equally rapid recovery. Prolonged inhalation of about 85% in oxygen is slightly toxic, resulting in a slow fall in the blood pressure; at about 94% in oxygen, ethylene is acutely fatal.

Safety and Health Measures

For those chemicals with which no carcinogenicity or similar toxic effects have been observed, adequate ventilation should be maintained to prevent exposure of workers to a concentration above the recommended safe limits. Workers should be instructed that smarting of the eyes, respiratory irritation, headache and vertigo may indicate that the concentration in the atmosphere is unsafe. Cylinders of butadiene should be stored upright in a cool, dry, well-ventilated location away from sources of heat, open flames and sparks.

The storage area should be segregated from supplies of oxygen, chlorine, other oxidizing chemicals and gases, and combustible materials. Since butadiene is heavier than air and any leaking gas will tend to collect in the depressions, storage in pits and basements should be avoided. Containers of butadiene should be clearly labelled and coded appropriately as an explosive gas. Cylinders should be suitably constructed to withstand pressure and minimize leaks, and should be handled so as to avoid shock. A safety relief valve is usually incorporated in the cylinder valve. A cylinder should not be subjected to temperatures above 55 °C. Leaks are best detected by painting the suspected area with a soap solution, so that any escaping gas will form visible bubbles; under no circumstances should a match or flame be used to check for leaks.

For possible or probable carcinogens, all appropriate handling precautions required for carcinogens should be instituted.

Both in its manufacture and usage, butadiene should be handled in a properly designed, closed system. Antioxidants and inhibitors (such as tert-butylcatechol at about 0.02 weight per cent) are commonly added to prevent the formation of dangerous polymers and peroxides. Butadiene fires are difficult and dangerous to extinguish. Small fires may be extinguished by carbon dioxide or dry chemical fire extinguishers. Water may be sprayed over large fires and adjacent areas. Wherever possible, a fire should be controlled by shutting off all sources of fuel. No specific preplacement or periodic examinations are needed for employees working with butadiene.

The lower members of the series (ethylene, propylene and butylene) are gases at room temperature and highly flammable or explosive when mixed with air or oxygen. The other members are volatile, flammable liquids capable of giving rise to explosive concentrations of vapour in air at normal working temperatures. When exposed to air, the diolefins can form organic peroxides which, upon concentration or heating, can detonate violently. Most commercially produced diolefins are generally inhibited against peroxide formation.

All sources of ignition should be avoided. All electrical installations and equipment should be explosion-proof. Good ventilation should be provided in all rooms or areas where ethylene is handled. Entry into confined spaces that have contained ethylene should not be permitted until gas tests indicate that they are safe and entry permits have been signed by an authorized person.

Persons who may be exposed to ethylene should be carefully instructed about and trained in its safe and proper handling methods. Emphasis should be given to the fire hazard, the “freezing burns” due to contact with the liquid material, use of protective equipment, and emergency measures.

Hydrocarbons, aliphatic unsaturated, tables

Table 1 - Chemical information.

Table 2 - Health hazards.

Table 3 - Physical and chemical hazards.

Table 4 - Physical and chemical properties.



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