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ROBERT BURKE
Published: December 1996

Also See: 1996 North American Emergency Response Guidebook

Cryogenic liquefied gases are very cold liquids. The U.S. Department of Transportation definition of a cryogenic liquid, sometimes referred to as "refrigerated liquids", is any liquid with a boiling point below -130° F. Other sources list boiling points from -100° F to -200° F. If cryogenic liquids are shipped above 41 psi and have no other hazard, they are considered a compressed gas and would be placarded as a non-flammable compressed gas. Cryogenics may carry other placards such as flammable gas, poison gas, or oxidizer. If cryogenics do not have any other placardable hazard, they are not considered a hazardous material by the DOT. Materials listed under hazard Class 2, that are shipped as liquefied gases, such as cryogenics, exhibit other hazards not indicated by the placard. Cryogenics, or refrigerated liquids, as they are sometimes called have boiling points of -130° F below zero or greater. Therefore, all cryogenic liquids are above their boiling points at ambient temperatures. Liquid helium has a boiling point of -452° F below zero; it is the coldest material known. It is also the only material on earth that doesn't ever exist as a solid, only as a cryogenic liquid and as a gas.

Cryogenic Boiling Points Helium -452° F Hydrogen -423° F Neon -411° F Nitrogen -321° F Air -318° F CO -312° F Fluorine -307° F Argon -303° F Oxygen -297° F Methane -257° F Krypton -244° F Nitric Ox -241° F NF3 -200° F Xenon -162° F

Unlike propane and other liquefied gases, gases that are liquefied into cryogenics are liquefied through a process of alternating pressurization, cooling, and ultimate decompression. Therefore, they do not require pressure to keep them in the liquid state. However, if they will be in a container for a long period of time they are pressurized to keep them liquefied as long as possible. Non-pressurized cryogenics are kept cold by the temperature of the liquid and the insulation around the tanks. The cryogenic liquefaction process begins when gases are placed into a large processing container. They are pressurized to 1500 psi. The process of pressurizing a gas causes an increase in heat. The molecules within a container move faster causing more collisions with each other and the walls of the container. As the molecules collide heat is generated. For example, the top of a Self-Contained Breathing Apparatus (SCBA) bottle or an oxygen bottle become hot as they are being filled. This is a result of the molecules colliding in the bottles. Once the pressure of 1500 psi is reached, the material is cooled to 32° F by using ice water. Once cooled, the pressure is once again increased, this time up to 2000 psi, again accompanied by an increase in temperature. The material is then cooled to -40° F with liquid ammonia. Once the material is cooled, all of the pressure is released at once and the resulting heat decrease turns the gases into cryogenic liquids. During the decompression process the heat present within the container decreases as the pressure rapidly decreases.

Many of the gases found on the periodic table of chemical elements are extracted from the air and turned into cryogenic liquids. These include; neon, argon, krypton, xenon, oxygen and nitrogen. All of the gases except oxygen are considered inert, that is to say, they are non-toxic, non-flammable, and non-reactive. To extract these materials from the air, the air is first turned into a cryogenic liquid by the process mentioned previously. Then the liquid air is processed through a type of distillation tower where each component gas is extracted off as it reaches its own boiling point. Once extracted the gases are then liquefied by the same process of alternate pressurization and decompression.

Some other common materials that are made into cryogenics include flammable methane (LNG), hydrogen, and oxidizers oxygen, fluorine and nitric oxide.

Helium, He, is a gaseous non-metallic element from the noble gas family, family eight on the periodic table. Helium is a colorless, odorless and tasteless gas. It is non-flammable, non-toxic, and non-reactive. Helium has a boiling point of -452° F. It is slightly soluble in water. Even though it is an inert gas, helium can still displace oxygen and cause asphyxiation. The vapor density is 0.1785, which is lighter than air. Helium is derived from natural gas by liquefaction of all other components. Helium has a UN 4-digit identification number of 1046 as a compressed gas and 1963 as a cryogenic liquid.

DEWAR Portable Insulated Big Thermos Bottle Gages and Valves On Top

Neon, Ne, is a gaseous non-metallic element from the noble gas family. It is a colorless, odorless and tasteless gas. Neon is present in the earth's atmosphere at 0.0012% of normal air. It is non-flammable, non-toxic, and non-reactive. It does not form chemical compounds with any other chemicals. It is however, an asphyxiant gas and will displace oxygen in the air. The boiling point of neon is -410° F. It is slightly soluble in water. Neon has a vapor density of 0.6964 which is lighter than air. The UN 4-digit identification number is 1065 when compressed and 1913 as a cryogenic liquid. Its primary uses are in luminescent electric tubes and photoelectric bulbs. It is also used in high-voltage indicators, lasers (liquid) and cryogenic research.

Argon, Ar, is a gaseous non-metallic element of family eight. It is present in the earth's atmosphere to 0.94% by volume. It is a colorless, odorless, and tasteless gas. It does not combine with any other chemicals to form any compounds. The boiling point is -302° F. It is slightly soluble in water. The vapor density is 1.38 so it is heavier than air. The UN 4-digit identification number is 1006 as a compressed gas and 1951 as a cryogenic liquid. It is used as an inert shield in arc welding electric and specialized light bulbs, (neon, fluorescent, and sodium vapor), in Geiger-counter tubes, and lasers.

Krypton, Kr, is a gaseous non-metallic element of family eight. It is present in the earth's atmosphere to 0.000108% by volume. It is a colorless, odorless gas. It is non-flammable, non-toxic and non-reactive. It is however, an asphyxiant gas and can displace oxygen in the air. At cryogenic temperatures krypton exists as a white, crystalline substance with a melting point of 116° K. The boiling point of krypton is -243° F. Krypton is known to combine with fluorine at liquid nitrogen temperature by means of electric discharges or ionizing radiation to make KrF2 or KrF4. These materials decompose at room temperature. Krypton is slightly water soluble. The vapor density is 2.818 which is heavier than air. The UN 4-digit identification number is 1056, as a compressed gas, and 1970 as a cryogenic liquid. It is used in incandescent bulbs, fluorescent light tubes, lasers and high-speed photography.

Xenon, Xe, is a gaseous, non-metallic element from family eight. It is a colorless, odorless gas or liquid. It is a gas at standard temperatures and pressures. It is non-flammable, and non-toxic, but is an asphyxiant and will displace oxygen in the air. The boiling point is -162° F. The vapor density is 05.987 which is heavier than air. It is chemically unreactive however it is not completely inert. The 4-digit UN identification number is 2036 for the compressed gas and 2591 for the cryogenic liquid. Xenon is used in luminescent tubes, flash lamps in photography, lasers and as an anesthesia.

Xenon compounds. Xenon combines with fluorine through a process of mixing the gases, heating in a nickel vessel to 400° C, and cooling. The resulting compound is xenon tetra fluoride, XeF4. The resulting product is composed of large colorless crystals. Compounds of xenon difluoride, XeF2, and hexafluoride, XeF6, can also be formed in a similar manner. The hexafluoride compound melts to a yellow liquid at 122° F and boils at 168° F. Xenon and fluorine compounds will also combine with oxygen to form oxytetrafluoride, XeOF4, which is a volatile liquid at room temperature. These compounds with fluorine must be protected from moisture to prevent the formation of xenon trioxide, Xe03, which is a dangerous explosive when dried out. The solution of xenon trioxide is a stable weak acid that is a strong oxidizing agent.

Cryogenic Expansion Ratios Fluorine 981 to 1 Oxygen 862 to 1 Argon 841 to 1 Hydrogen 840 to 1 Helium 754 to 1 Nitrogen 697 to 1 LNG 637 to 1

Cryogenic Liquids have very large expansion ratios, some as much as 900 or more to 1; they can form massive vapor clouds. These vapor clouds can obscure vision of the source of the leak making the location of the source difficult to find. Vapor clouds from cryogenic liquids can travel great distances and require evacuation of the public. The visible cloud is not the total extent of the hazard. The warmer air on the outer edge of the vapor cloud causes the gas to become invisible. It is then possible to be in an oxygen enriched atmosphere, flammable atmosphere or in a gas that can cause asphyxiation and the gases will not be visible. Because of the large liquid to gas expansion ratios, the other hazards of cryogenic liquids are magnified many times. If the cryogenic liquid is flammable or toxic these hazards are intensified because of the potential of large gas cloud production from a very small amount of liquid. As the size of the leak increases, so does the size of the vapor cloud. This means that 1 gallon of a cryogenic liquid can produce as much as 900 gallons of a gas. Because of the large expansion ratios cryogenic liquids can turn into gases and displace oxygen in the air which can harm responders by simple asphyxiation. Simple asphyxiation is not poisoning, but rather just not enough oxygen to breathe. The atmospheric contains about 21% oxygen. When the oxygen in the lungs, and ultimately in the blood is reduced, unoxygenated blood reaches the brain and the brain shuts down. It may only be a few seconds between the first breath and collapse. Response personnel should always put SCBA on prior to entering a confined space or other area where asphyxiant gases may be present.

Being very cold, cryogenic liquids can cause frostbite and solidification of body parts. When the parts thaw out, the tissue is irreparably damaged. Touching uninsulated piping and valves of cryogenic liquid containers can cause skin to stick to the metal, much like when kids put their tongues on an ice cube tray or fence post and it gets stuck. There isn't any protective clothing or equipment that can be worn to protect the body from the effects of contact with a cryogenic liquid. Anything that the cryogenic liquid contacts will become solidified and brittle. Gloves can provide protection from skin contacting piping and valves. Body parts that have come in contact with cryogenics should be treated like frostbite. Parts should be flushed with water that is cool to luke warm to limit additional tissue damage.

Cryogenic liquids are shipped and stored in special containers. On the highway the MC 338 tanker is used to transport cryogenic liquids. The tank is usually not pressurized but is heavily insulated to keep the materials cold. There is a heat exchanger underneath the belly of the tank truck to facilitate the off loading of product as a gas. Railcars are also specially designed to keep the cryogenic liquids cold inside the containers to minimize the boiling off of the gas. Fixed storage containers of cryogenic liquids are usually very tall, small diameter tanks. These are insulated and resemble large vacuum bottles that keep the liquid cold. These containers are also under pressure to keep the material liquefied. There is a heat exchanger near the tanks. This heat exchanger is a series of metal tubes with fins around the outside. The liquids are passed through the tubes and the liquid is warmed above its boiling point and turns into a gas. The gas is then used in the facility for whatever purpose it was intended. Valves on fixed tanks have very long stems because the moisture around the valves is frozen due to the cold of the cryogenics. As this moisture is frozen, ice forms on the piping and around the valves. If the valve stem were not extra long, the valve would not be accessible because of the ice.

Tall Thin Tanks Heat Exchanger Next to Buildings

Other gases, such as hydrogen, are liquefied, sometimes made into cryogenics and placarded as flammable gases. Liquid oxygen is placarded as an oxidizer or non-flammable compressed gas. Oxygen though non-toxic, is very reactive with hydrocarbon based materials and is an oxidizer. Liquid Oxygen in contact with an asphalt surface such as a parking lot or highway can create a contact explosive. Dropping an object, driving over the area or even walking on the area can cause an explosion to occur.

Nitrogen, N2 is a gaseous non-metallic element, that is the 2nd most produced industrial chemical with 68.04 billion pounds in 1995. It is a colorless, odorless, tasteless gas that makes up 78% of the air that is breathed. The boiling point of nitrogen is -320° F. It is slightly soluble in water. Nitrogen does not burn and is non-toxic. It may, however, displace oxygen and be an asphyxiant gas. The vapor density of nitrogen is 0.96737 which makes it slightly lighter than air. The UN 4-digit identification number is 1066 as a compressed gas and 1977 as a cryogenic liquid. As a cryogenic liquid, the NFPA 704 designation is Health-3, Flammability-0, and Reactivity-0. Nitrogen is used in the production of ammonia, cyanides, and explosives, as an inert purging agent and a component in fertilizers. It is usually shipped in insulated containers, insulated MC 338 tank trucks, and tank cars.

Oxygen, O2, like nitrogen, is an non-metallic elemental gas, which is the 3rd largest volume industrial chemical with 53.48 billion pounds produced in 1995. Oxygen makes up approximately 21% of the air breathed. The boiling point of oxygen is -297° F. It is non-flammable, but supports combustion. Oxygen can explode when exposed to heat or organic materials. The vapor density of oxygen is 1.105 which makes it slightly heavier than air. Oxygen is incompatible with: oils, grease, hydrogen, flammable liquids, solids and gases. The UN 4 digit identification number for oxygen is 1072 as a compressed gas and 1073 as a cryogenic liquid. The NFPA 704 designation for liquid oxygen is Health-3, Flammability-0, and Reactivity-0. Liquid oxygen is shipped in Dewier flasks and MC 338 tank trucks. It may also be encountered in cryogenic rail cars.

Courtesy Robert Burke

Carbon dioxide, CO2 is the 22nd highest volume industrial chemical with 10.89 billion pounds produced in 1995. Carbon dioxide is a colorless, odorless gas. It can also be a solid, (dry ice), which will undergo sublimation and turn back into carbon dioxide gas, or a cryogenic liquid. It is miscible with water. It is not flammable or toxic but can be an asphyxiant gas and displace oxygen. In 1993 two workers were killed on board a cargo ship when a carbon dioxide fire extinguishing system discharged. The oxygen in the area was displaced by the carbon dioxide and the men were asphyxiated. Carbon dioxide has a vapor density of 1.53 which is heavier than air. It may be shipped as a cryogenic or liquefied compressed gas. Frozen (solid) carbon dioxide gas is known as dry ice. It has a UN 4-digit number of 2187 as a cryogenic and 1013 as a compressed gas. The NFPA 704 designation is Health-3, Flammability-0, and Reactivity-0. It is used primarily in carbonated beverages and fire extinguishing systems.

When responding to fires involving liquefied gas tanks, firefighters often apply water to cool the tanks. Cryogenic liquids are already colder than water at any temperature and water will act as a super heated material causing the cryogenic to heat up and vaporize faster. This will cause pressure to build up inside the tank and the tank may fail violently. Care should be taken when applying water to "cool" cryogenic containers.

Related:

• 1996 North American Emergency Response Guidebook
  
• HazMat Studies Archives
  
• The Street Chemist