Ethylene
FEATURES
HAZARDOUS MATERIALS
Chemical Data Notebook Series:
It’s in the cryogenic liquefied form that ethylene, an important chemical in the manufacture of plastics, presents its toughest challenge to emergency responders. The extreme cold can destroy clothing and gear, and it presents the danger of frostbite even on the warmest days.
In addition, the liquid generates tremendous amounts of flammable gas at a very low temperature—its boiling point is -155° F. That means a small spill that finds an ignition source can produce a very large explosion and fire, and an accident involving a container of ethylene can produce a BLEVE (boiling-liquid, expanding-vapor explosion) if the container isn’t properly insulated and protected.
The chemical’s most common use is in polymerizing polyethylene, the highest-volume plastic in the world. The range of products made from different types of polyethylenes includes high-strength containers, toys, bottles, packaging film and sheets, and trash bags.
Ethylene (which is the common name for ethene) can also be used as the starting point for many other chemicals, including polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), polyvinylidene chloride (trademarked SARAN), polyacrylonitrile, and vinyl fluoride. One of ethylene’s more interesting uses is to ripen fruits and vegetables at their point of sale. This allows growers to pick and ship produce before it’s ripe, reducing the chances for bruising or rotting.
In its gaseous form, ethylene is colorless, with a slight ether-like odor. Its ignition temperature is 842° F, and its flammable range is from 2.75% to 34%. The ignition temperature is well within the range of all common ignition sources, and the flammable range is very wide. Thus, ethylene must be considered dangerous.
The vapor density at room temperature is 0.97, very close to the 1.0 density of air. However, when very cold gas is released from the cryogenic, liquefied form, its density will be increased to a point greater than that of air. In that case, the gas will hug the ground and flow along the low areas.
Ethylene is virtually insoluable in water and doesn’t react with it, but the extreme cold makes use of water in containing the chemical problematic.
Despite the presence of a double bond between the carbon atoms, which makes it a reactive molecule, ethylene is a relatively stable material. Its molecular structure is producing a molecular formula of C2H4. It’s the simplest of the series of unsaturated hydrocarbons known as alkenes, all of which contain one double bond.
H H
I I
c = c
I I
H H
Ethylene’s molecular weight is 28, and its freezing point is -272.4° F. The liquid has a specific gravity of 0.57 at its boiling point.
Ethylene is shipped in cylinders as compressed gas and in tank trucks and rail cars as a liquefied gas.
Double threat
The low temperature and high vapor-to-Iiquid ratio of the liquefied gas present major hazards to the responder. Contact with the liquid poses the possibility of severe frostbite. It also may destroy items of protective clothing by making them so brittle they literally disintegrate.
Synonyms
Identification Numbers and Ratings
United Nations/North America
UN/NA 1962 for the compressed gas UN/NA 1038 for the liquid
National Fire Protection Association 704 marking system rating
1-4-2
CAS
(Chemical Abstract Service)
74-85-1
RTECS
(Registry of Toxic Effects of Chemical Substances) National Institute for Occupational Safety and Health
KU5340000
STCC
(Standard Transportation Commodity Code)
Association of American Railroads, Bureau of Explosives
4905734 for the gas 4905735 for the liquid
CHRIS
(Chemical Hazard Response Information System)
U.S. Coast Guard ETL
U.S. Department of Transportation
Flammable gas
IMO
(International Maritime Organization)
2.1, flammable gas
Most rubber boots and gloves will prevent contact, but will be destroyed due to brittleness caused by the extreme cold. Face shields, goggles, and masks will protect the face and eyes from one splash, but the equipment will probably then be useless. Turnout gear may absorb the liquid, thus keeping the coldness next to the skin and becoming a potential torch if an ignition source is contacted. Positive-pressure, self-contained breathing apparatus will offer adequate respiratory protection from ethylene.
The vapor-to-liquid ratio of 487to-1 means that 487 cubic feet of gas will be produced for every cubic foot of liquid released. (In contrast, propane, a noncryogenic liquefied gas, produces 270 cubic feet of gas for each cubic foot of liquid.) This means that even a minor leak can lead to a major explosion, since the gas is so flammable. And its wide flammable range ensures that mixtures of the gas and air will almost never be too lean or too rich to burn.
Another hazard comes when ethylene is shipped as a compressed gas, in uninsulated containers. This could lead to the danger of container failure if the internal pressure is allowed to rise above the design strength of the container.
Ethylene is very reactive with all oxidizing agents and will explode in a chlorine atmosphere. It will also react with carbon tetrachloride, aluminum chloride, nitrogen dioxide, nitromethane, and sulfuric acid.
Once ignited
A fire involving ethylene must not be extinguished until it’s certain that the flow of gas can be stopped immediately after extinguishment.
If there’s been a spill of liquid, tremendous amounts of gas will be generated, especially if the pool of liquid (or, rather, the gas vapor from the liquid) is burning. Foam may slow down the evolution of gas by cooling down the fire, but it may also speed it up because of the differential in temperatures.
Applying water in a spray or fog may help protect exposures by absorbing heat. But be careful to prevent straight streams from reaching the pool, since, again, the relatively warm water will cause an increase in the evolution of gases.
If a cylinder or tank of the material is exposed to radiant heat or actual flame impingement, deliver large amounts of water to the container to cool it and prevent a catastrophic explosion. If the liquefied gas is in an insulated container, the danger is slightly reduced because less energy is reaching the inner tank. However, the possibility of an explosion still exists.
Keep water from reaching an emergency relief device on a container holding the liquid. If the vent has activated and gas is escaping, any water near the area will freeze instantly and, as a result, threaten to block the pressure relief device.
If any container is threatened by heat, evacuate at least a one-halfmile radius from the container site to protect from explosion, fire, and shrapnel. Larger containers such as rail tank cars may require a wider evacuation radius.
In a norifire situation, the emergency responder must still keep in mind ethylene’s flammability and its cryogenic nature. Evacuation downwind is generally the best course of action.
Whenever a liquefied gas is released from its container, the evolution of gas is rapid. This is especially true of cryogenic liquids like ethylene. Since the ground temperature is much higher than the -155° F boiling point of the liquid, the initial contact will produce a large quantity of the gas. However, the evolution will slow after a while, because everything around the spill will become extremely cold. This is why applying water would speed up evaporation—the water may be 200° F warmer than the liquid.
But the slowing of evaporation may lead to another hazard—percolation of the liquid into the soil. This means that even after the pool of liquid has been removed, a certain amount of ethylene will remain present—and hazardous— until it has been completely dispersed into the atmosphere.
If the liquid is producing vapors, you can use water spray to disperse them, just as long as you’re careful to keep the water away from the pool. Although foam may initially increase the production of vapors, high-expansion foam can be used to slow it overall. The foam may break down after a short time, but continued application could buy time for evacuation and allow the shipper or others to safely remove the product. Remember to be careful to control the increased volume of liquid as the foam breaks down.
Eliminate all ignition sources, and use spark-proof tools in any attempt to stop the leak. If liquid from the leak can be recycled back into the container, the pump you use must be explosion-proof and able to handle cryogenic liquids.
In many ways, the containment of cryogenic gases is similar to that of other liquids. Containment dikes may be built to form ponds, using soil, sand, or clay. Trenches may be dug to direct the flow to a safer area, such as a deep containment pit.
As always, the deep pit, with its smaller surface area, is preferable to the large containment pond, because the evolution of gases is directly proportional to the surface area of the liquid exposed to the atmosphere. In addition, the surface of a pit is more easily covered than a large pond, thus further slowing the evolution of gas. Keep in mind, however, that contamination of all surfaces is possible, and liquid may percolate into the ground faster from a pit than from a pond.
Don’t allow any ethylene to enter sewers or waterways. Ethylene gas will be generated in great quantities in closed spaces, and the enclosed sewers will thus allow the gas to spread throughout the system. Because of the high expansion ratio, the gas will then be pushed through the system, even up hills to higher elevations.
If the liquid enters a waterway, it will float on top of the water while it’s evaporating. Sections of small streams may be frozen, depending on both the size of the waterway and the ethylene released. The gas generated will be cold and may hug the surface of the water, flowing downstream with it. This may pose a great fire and explosion hazard all along the waterway. All downstream users must be notified and all intake pipes to industrial and commercial users must be closed.
Once the liquid is contained, it can be removed by pumping it from the containment area into secure vessels. All pumping equipment must be spark-proof, all motors must be explosion-proof, and all areas of the equipment in contact with the liquid must be able to withstand the extreme cold without failure.
Adsorption or absorption techniques may be used, but these could prove ineffective and dangerous; the coldness of the material will cause it to form a gel rather than actually being absorbed or adsorbed. If such methods are used, they must be initiated and supervised by personnel with experience in the safe handling of the liquid by these techniques. This will usually be the seller or buyer of the liquid.
Ethylene gas is essentially nontoxic. But it can cause unconsciousness or death by displacing oxygen and causing asphyxiation. Victims must be removed to fresh air immediately and kept warm and quiet. Artificial respiration should be administered if breathing becomes labored. Medical attention should be sought immediately.
For frostbite, administer large amounts of water—though not hot water—to wash the affected area without rubbing after all contaminated clothing has been carefully removed.
If eye contact has occurred, wash the eyes with large amounts of water for a minimum of 15 minutes, occasionally lifting the eyelids. Again, medical attention should be immediate.
Glossary
Cryogenic—Produced, used, or stored at temperatures that are – 150° F or less.
Hydrocarbon—Any compound or mixture of compounds containing only hydrogen and carbon in its molecules.
Saturated—Pertaining to hydrocarbons: compounds in which all bonds between carbon atoms are single bonds.
Unsaturated—Any hydrocarbon compound that contains at least one multiple bond between carbon atoms.
Percolation—The process by which liquids (or other materials) seep into the ground (or some other medium).