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Cryogenic liquefied gases are very cold liquids. The U.S. Department of Transportation (DOT) defines a cryogenic liquid, sometimes referred to as a "refrigerated liquid," as any liquid with a boiling point below -130 degrees Fahrenheit. Other sources list boiling points from -100 degrees F to -200 degrees 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.
Since they have boiling points of -130 degrees F or colder, all cryogenic liquids are above their boiling points at ambient temperatures. Liquid helium has a boiling point of -452 degrees F below zero; it is the coldest material known. It is also the only material on earth that never exists as a solid, only as a cryogenic liquid and as a gas.
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 containers for long periods, 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.
Photo by Robert Burke
Vertical fixed facility cryogenic tanks with heat exchangers.
The cryogenic liquefaction process begins when gases are placed into a large processing container. They are pressurized to 1,500 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 becomes hot as it is being filled. This is a result of the molecules colliding in the bottle. Once the pressure of 1,500 psi is reached, the material is cooled to 32 degrees F by using ice water. Once cooled, the pressure is once again increased, this time up to 2,000 psi, again accompanied by an increase in temperature. The material is then cooled to -40 degrees 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 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, 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. 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.
Other common materials that are made into cryogenics include flammable methane (LNG), hydrogen, oxidizers oxygen, fluorine and nitric oxide.
Helium (chemical symbol He). Helium, a gaseous non-metallic element from family eight on the Periodic Table, is a colorless, odorless and tasteless gas. It is non-flammable, non-toxic and non-reactive. Helium has a boiling point of -452 degrees F. It is slightly soluble in water.