Chemical Data Notebook Series #18:
The key word in describing the liquefied gas butadiene is reactive. Butadiene is a monomer, so it can react with itself to form a polymer called polybutadiene, a synthetic rubber. It can also react with other chemicals, usually in violent fashion. Furthermore, butadiene is subject to boiling-liquid, expanding-vapor explosions, or BLEVEs, can cause frostbite on touch, and is suspected of being a human carcinogen.
Butadiene, as a gas, is colorless and flammable, with a faint aromatic odor. It’s easily liquefiable, and is usually shipped and stored as a liquid for economic reasons. Butadiene is used in the polymerization of rubber and plastics, and as a feedstock for making other chemicals.
It has a boiling point of 24.1° F and a freezing point of -164° F. Like other hydrocarbons, it’s insoluble in water. Being a gas, it has no flash point, although some references list – 105° F as the flash point of the liquefied gas. Butadiene’s ignition temperature is 780° F, well within the range of all common ignition sources, and its flammable range is from 2 to 12 percent. Its molecular weight is 54, its vapor density is 1.865, and the specific gravity of the liquid is 0.62. Butadiene’s chemical formula is C
Butadiene is extremely flammable. As a gas, it’s always ready to burn if it has the proper air mixture and ignition source. Accumulations of butadiene gas within the flammable range will ignite explosively. Its vapor density of 1.865 means that as the gas is released, it will sink to the ground and flow along low spots (remember, gases are fluids) until it finds an ignition source. The ignited butadiene will then flash back to the source of the vapors.
Because it’s an easily liquefiable gas, butadiene will release tremendous quantities of gas whenever it’s released from its container. Its boiling point of 24.1° F means that whenever the liquid is exposed to air, tremendous quantities of gas will be generated.
It’s usually shipped in pressurized containers. So anytime flame impingement increases the pressure high enough to approach the container’s design strength, a catastrophic failure will occur, releasing all the contents, mostly as gas in a fireball. The container will often be hurled like a rocket, trailing burning butadiene behind it. This, of course, is a BLEVE. All flammable gases are subject to BLEVE, and butadiene is no exception.
As a monomer, butadiene can also explode because of a polymerization reaction, a very special chemical reaction whereby molecules of the product can react with each other to form a new product, an ultralong chain molecule called a polymer. This reaction is exothermic. When carried out in a polymerization reaction under control of a chemist, chemical engineer, or other process control worker, the evolved heat is transferred as it’s generated. However, should the process get out of control, there’s a very real danger of explosion.
1,3-Butadiene Buta-1,3-diene Divinyl
Identification Numbers and Ratings
(United Nations/North America)
(Chemical Abstract Service)
(Registry of Toxic Effects of Chemical Substances)
(Standard Transportation Commodity Code) Association of American Railroads,
Bureau of Explosives
4905703 (inhibited, impure, for further refining) 4905704 (inhibited, made from petroleum)
4905705 (inhibited, made from alcohol)
(Chemical Hazards Response Information System)
(International Maritime Organization)
Flammable gas, 2.1
National Fire Protection Association 704 rating
During transportation, this hazard is reduced bv mixing an inhibitor into the butadiene. However, if the container is somehow subjected to heat, the inhibitor may be forced out of the monomer, and there will again be the danger of an explosion. This violent, uncontrolled reaction is known as runaway polymerization. There’s some dispute over whether particular explosions have been BLEVEs or runaway polymerization, but the results are the same: Anyone within the danger zone will be killed.
Among the other chemicals and materials butadiene reacts violently with are acid anhydrides, alkylene oxides, alcohols, amines, ammonia, caustics, copper (and its alloys), crotonaldehyde, ethers, halogens (and compounds containing halogens), phenols, and strong oxidizers. It doesn’t react with water, but it’s stable only when inhibited.
Whenever an ignition source is present, butadiene will ignite explosively if it’s in the flammable range (and, according to Murphy’s law, it will be). Whenever a container of butadiene is heated, the pressure inside the container will climb, which is true of any gas or liquid. Since butadiene is stored as a liquid above its boiling point, the situation can become critical very rapidly.
Whether the heat is being radiated from another fire or direct flame impingement is occurring, the pressure inside the tank can rise very quickly to the level that triggers the safety relief valve. (On tank cars and tank trucks, the valve is usually spring-loaded and has a rated strength of 250 pounds per square inch.) Once the pressure rises to the valve’s rated strength or above, the gas will be vented to the outside. This continues until the pressure drops to below the rated strength, when the valve reseats itself and reseals the tank. This venting usually occurs intermittently until the heat source produces enough energy to continuously heat the liquefied gas, raising the internal pressure faster than the pressure-relief device can vent it. This will produce continuous venting.
If the temperature at the valve is 780° F or more, the gas will ignite. If an accident has occured that’s bent the vent so that the flame impinges on the tank, or if flame from some other source is impinging anywhere above the liquid level, a BLEVE is imminent as the metal weakens from the external heat and internal pressure.
It’s mandatory that all exposed containers of butadiene be kept cool with water, if it can be done safely. Some references call for a 2,500-foot radius as a safety zone, but even this might not be enough! Water should be discharged from unmanned monitors and care should be taken not to extinguish the flame from a burning vent. If the fire is extinguished, the collection of unburned gas may explode. The vapors at the vent should be reignited if it can be done safely.
An old rule of many fire departments was that one should never approach an exposed tank from the ends. The new rule should be: Never approach an exposed tank. Remember that once the tank has begun to vent, the inhibitor is probably in the process of being driven out of the butadiene. Heat and pressure are the necessary ingredients to start polymerization, and both are present in a fire-exposed tank of butadiene.
If a release has occurred and the liquid is in a burning pool, your chances of extinguishment diminish as the size of the pool increases. Dry chemical, carbon dioxide, foam, and water spray are possible extinguishing agents on small quantities. But the application of water to the liquid will accelerate the generation of gas.
If extinguishment of a pool fire is necessary, foam may be the only effective agent. Water spray or fog will cool down the fire, but may not extinguish it. Reignition of the gas usually occurs, many times caused by a piece of metal that the fire has heated to above the ignition temperature. Be very careful of extinguished pool fires in easily liquefiable gases.
An alternate decision is to let it continue to burn if there’s no danger to life, property, or the environment. Consumption of the fuel is a legitimate method of extinguishment and is used—when safe to do so—if the situation is too dangerous to attempt extinguishment by other means; where it may be too difficult to extinguish; or where neither water nor foam is available. In the case of butadiene, it will almost always be the choice, since extinguishment of a pool fire would permit large quantities of hot gas to continue to be generated from the pool.
If a tank, tank car, or tank truck is venting or leaking gas and the gas has been ignited, extinguishment of this flame should never be attempted unless the flow of gas can be stopped immediately after extinguishment.
If the container is being impinged by flame, a BLEVE is possible, even if the flame is impinging on the liquid space. If the flame is big enough and hot enough, the liquid inside may be heated to a point above its critical temperature (306° F), at which point there will be an instantaneous conversion inside the tank of liquid to gas, causing a pressure rise so rapid that the safety relief device has no chance to lower it.
The resulting explosion and fireball will be very much larger than the “normal” BLEVE, where flame impingement on the vapor space causes the tank to tear, producing an opening in the tank that causes an instantaneous drop in pressure as the material escapes. The normal BLEVE “catches” the liquid in a state not allowed by nature, that of existing as a liquid above its boiling point. The drop in pressure causes a conversion of some of the liquid to gas, further tearing of the tank, and possible “rocketing” of the tank.
Also remember the possibility of the runaway polymerization, which will usually occur within a container.
If the butadiene hasn’t yet ignited, always approach the site of a leak or spill from upwind, eliminate all possible ignition sources (especially downwind), and be ready to start evacuation and withdrawal. The application of foam might slow the generation of gas from a pool of liquid, but it might also accelerate it, since it’s warmer than the liquefied gas.
Any equipment used in mitigation of the incident must be nonsparking and compatible with butadiene. Any metal containing copper will cause violent reactions, so bronze and brass tools can’t be used if they’ll come in contact with the liquid.
The butadiene must not be allowed to enter a waterway. With a specific gravity of 0.62, the liquid will float on the water, boiling away as it flows. If the liquid enters a sewer, the gas generated will fill the pipe and flow downhill, at the same time being forced uphill ts own pressure, until all the liquid has evaporated. At any catch basin or manhole, there will be gas within the flammable range. The resulting underground explosion may be city wide.
Aeration—A process by which air is bubbled through a liquid.
Air stripping—A process by which a material is pumped through an air chamber or sprayed into the open air at high pressure in order to remove contaminants.
Alloy—A physical mixture of metals or plastics, rather than a chemical combination.
Caustics—Corrosive bases, such as lithium hydroxide, sodium hydroxide, and potassium hydroxide. May include hydroxides of calcium, beryllium, and magnesium.
Critical temperature—The temperature above which it’s impossible for a gas to be liquefied or to remain a liquid.
Exothermic—Heat-liberating. Contrast with endothermic, which describes a reaction in which heat is absorbed.
Halogens—The elements fluorine, chlorine, bromine, and iodine. A halogenated compound is usually a hydrocarbon compound with one or more of the halogens substituted for one or more hydrogens.
Inhibitor—A substance added to a monomer to prevent the polymerization reaction from occurring. Also known as a stabilizer. In the reactor, an initiator (or starter) is used to overcome the inhibitor and start the reaction.
Reactor—A pressure vessel (resembling a pressure cooker) that can be cooled or heated as necessary.
Sparging—A process by which air or other gas is bubbled through a liquid, solid, or gas to remove a contaminant.
If liquid butadiene does enter a waterway, all downstream users must be notified immediately, since they may draw liquid butadiene into an industrial operation and cause an explosion.
Air stripping, sparging, and aeration are techniques that will remove the product from a waterway. However, care must be taken to prevent an accumulation of the gases thus removed, and their downwind movement must not endanger life or property.
In all large spills, evacuation of populated areas downwind must be considered immediately. Foam may cover the pool effectively, but the foam itself might be shortlived. Continuous reapplication may be necessary. The use of water spray or fog immediately downwind of the spill can disperse gas generated by the spill. However, any water that contacts the liquid will speed evolution of gas. Care must also be taken to prevent liquid butadiene from floating away from the spill on a stream of water used to disperse gas.
In some cases, the spill may be contained by building a dike of soil, sand, clay, or other absorbent materials. This will prevent the spread of the liquid while allowing the application of foam. If this technique is used, a trench or ditch may be dug to lead the liquid to a safer place of containment, and a containment pit may be dug. In all cases, there will be percolation of the liquid into the soil. This absorbed liquid will continue to release gas, but at a somewhat slower rate than the pooled liquid. All contaminated soil must be checked by the proper environmental authorities to determine its disposition. Contamination may be prevented bv use of impervious film or sheeting in the pit, pond, or trenches.
Whenever possible, the liquid may be salvaged by pumping it into a secure container. This should be done only by the proper personnel with the proper equipment. This is usually the seller, the shipper, or a professional clean-up team. Emergency personnel shouldn’t attempt salvage.
There are many proponents of a “controlled burn” of flammable liquids and gases that have been accidentally and dangerously released. Their argument is that if the vapors or gases are burning, there will be no violent explosion of accumulated, unburned gases. That’s true, but the deliberate ignition of a leaking, flammable gas is a very serious and dangerous technique. It should be considered only if no exposures will be threatened and no life endangered by the deliberate burn, or if, without the burn, the threat to human life and property is very grave. It should be carried out in a way that prevents an explosion upon ignition.
There are several less visible hazards of butadiene. Long-term exposure studies with animals indicate that it may cause cancer. Acute exposures may be asphyxiating if the concentration is high, but in low concentrations for short periods, butadiene is considered to have low toxicity; its short-term exposure limit (STEL) is 1,250 parts per million for 15 minutes and its threshold-limit value/time-weighted average (TLV/TWA) is 1,000 ppm. Butadiene also has a toxic combustion product: carbon monoxide.
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A victim who has breathed high concentrations of butadiene must be removed to fresh air as quickly as possible and kept warm and quiet. Artificial respiration should be administered if breathing becomes difficult. Medical attention should be sought immediately.
If the liquid contacts skin, it can cause frostbite. If skin contact has occurred, all contaminated clothing should be removed and the affected area should be flushed with large amounts of water that’s tepid or cool—never warm or hot.
If the product has gotten into a person’s eyes, they should be flushed with water for at least 15 minutes, with the eyelids lifted often. Medical attention is mandatory.
For responders, protective clothing must be of the type that will prevent contact with the skin, plus face shields and splashproof goggles to prevent contact with the eves. Low temperature is the only contact hazard that liquid butadiene poses, and total encapsulating suits probably won’t be necessary. Rubber clothing or any other impervious material that protects against the cold will suffice.
Although gas masks with chin canisters may offer protection against low concentrations of butadiene, it’s always safer to use positive-pressure, self-contained breathing apparatus with a full facepiece when encountering situations in which the concentrations of the gas could increase suddenly. With butadiene’s expansive nature, that will be nearly every incident.