Type of physical science: Classical physics
Field of study: Thermodynamics
The special conditions needed to create liquefied gases lead to the need for special containers to store these liquids. Without these containers, very-low-temperature work and even the convenient use of certain fuels would be impossible.
Overview
Liquefying a substance that is a gas under ordinary conditions calls for the creation of extraordinary conditions. If the liquid thereby created is to be maintained in the liquid state, it must be isolated in containers capable of sustaining those extraordinary conditions.
Molecules in a gas spend most of their time far away from one another. In a liquid, the molecules are more frequently near one another and, hence, are more strongly affected by the mutually attractive forces acting among them. The van der Waals force, usually the most important of the attractive forces, depends on the shape of the gas molecules. It is weakest in gases with small, spherical molecules. The monatomic, noble gases, and hydrogen were the last gases liquefied because of their small, spherical (or, in the case of hydrogen, nearly spherical) molecules.
Within certain limits, the attractive forces are stronger as the molecules get closer together. Therefore, to turn a gas into a liquid, methods must be devised to increase the time the molecules spend near one another. The most obvious way to do this is to compress the gas, decrease the volume the molecules have available to them, and push them closer together.
The container for storing a gas liquefied by compression must have the strength to maintain the necessary pressure on the gas. Gases that can be held in the liquid state by relatively low pressures are often stored in steel or aluminum bottles with walls sufficiently thick to withstand safely the pressures involved. Such containment suffers from several easily foreseen difficulties. Molecules that attract one another very weakly will need very high pressures to liquefy. The weight of a bottle with walls thick enough to contain such liquids would create serious handling problems. Furthermore, such pressures are inherently dangerous, creating a variety of safety problems. In practice, the risks and responsibilities are too high. Very few liquefied gases are stored at ordinary temperatures in pressurized containers.
A more subtle and satisfactory way of liquefying a gas is to cool it. With the exception of helium at very low temperatures, the behavior of molecules in a gas is dominated by random thermal motions of the molecules. The temperature of the gas measures the strength of this random motion. Hence, decreasing the temperature of the gas decreases the strength of the random motions. The gas liquefies when the strength of the random motions is weaker than that of the attractive forces between molecules.
Storage of these cold liquids requires maintaining low temperatures, which, in turn, requires preventing heat from reaching the liquid. There are three ways heat can be transported.
Convection is not very important for confined liquids, although it will distribute throughout the entire liquid any heat that affects part of the liquid by heat conduction or radiative transfer. Heat can be conducted to the liquid by any material in physical contact with it, especially parts of the container. Also, heat can be radiated into the liquid by light, especially infrared radiation from surrounding objects at temperatures higher than that of the liquid.
Keeping the liquid in the dark will not eliminate radiative transfer, because not only visible light but also infrared and ultraviolet light can transfer heat. What appears to the human eye as a dark room is actually full of black body radiation from the walls, floor, and ceiling of the room, not to mention the warm and highly radiative body of any observer in the room.
Fortunately, black body radiation is readily absorbed by most things that block visible light. It is also easily reflected by ordinary mirrors.
If one made a container for cryogenic liquids from opaque material or from glass coated on the outside with opaque material, the material would absorb light, including infrared light, falling on the container and prevent it from reaching the liquid. Unfortunately, absorbing the radiation, the opaque material would gain energy from the radiation, warm up, and reradiate the energy as more infrared light. Some of this reradiated light would go to the liquid, transferring heat energy to it. The warming of the absorber also risks conducting heat to the liquid. This design then has built-in heat leaks, which will lead to the slow heating of the liquid.
The best solution to the radiative heat transfer problem is to reflect the light away from the liquid. If the liquid is surrounded by mirrors facing outward, all incoming light will be reflected away from the liquid. Whatever light is not reflected is absorbed by the silvering material of the mirror and can reach the liquid as infrared radiation. Fortunately, mirrors can be made that reflect almost 100 percent of light incident on them.
Conduction is a greater problem. Any practical container must be made of some material, and every material conducts heat to some degree. It is impossible to eliminate totally the conduction of heat to the stored liquid, but it can be controlled. The rate of heat conduction is reduced as the thickness of the material it must pass through is increased and as the area through which it must pass is decreased. Also, the nature of the material matters. Copper and styrofoam of the same thickness and area conduct radically different amounts of heat (copper is a much better conductor). Thus, controlling heat conduction into the liquid requires the use of thick layers of poorly conducting materials for the container walls. If that makes for a weak container, one can strengthen the container with braces in as small an area as possible.
Applications
Containers for storing gases liquefied by pressure are quite familiar. The small fuel bottles for camp stoves and hand torches and the larger ones for outdoor barbecue grills hold propane at about 9 atmospheres of pressure. If one of the small bottles is shaken, the distinctive sound of liquid swishing inside is easily heard. The common butane lighter is another example.
Some of these lighters are made of clear plastic, which allows the user to see how much liquid remains in the lighter.
These examples illustrate the basic features of all such containers. Such a container must have sufficient wall strength to maintain the necessary pressure and a valve for access.
Butane and propane are hydrocarbons with carbon backbones, carbon atoms bonded together in a line with hydrogen atoms bonded on at whatever free sites are available. Butane is the longer molecule, with four carbons; propane has only three. Thus, propane will require more pressure to liquefy because it has a smaller molecule. Containers for propane must be stronger than those for butane. Butane can be stored in plastic containers; propane cannot. The valves on butane containers can also be lighter and simpler than those for propane.
Inexpensive but very effective containers made of low-conductivity materials such as styrofoam or cork are used at times for liquids as cold as liquid nitrogen. For very cold liquefied gases, however, more complicated containers are necessary.
The familiar thermos jug is an example of a container used for storing very cold liquefied gases. The thermos is made of glass or metal, and such containers have silvered or, in the case of metal, highly polished surfaces facing outward to reflect any incoming radiation away from the cold liquid. Generally called "dewars" (pronounced "doers") for their inventor, Sir James Dewar, these vessels are double-walled, with a vacuum between the walls. The vacuum prevents conductive losses through the walls of the vessel. Even liquid helium losses may be only 5 percent per week for well-designed dewars.
Metal is strong, so the cross-sectional area of the neck between the two walls can be small. Metals popular for these purposes--nickel-silver, monel, and inconel--are poor conductors compared to other metals, but they still conduct better than, and are more expensive than, glass.
Glass is a poor heat conductor and is less expensive, but is not strong. Also, glass responds poorly to sudden temperature changes and must be annealed very careful if made into a dewar vessel. Annealing consists of very slowly cooling the glass from the high temperatures at which the vessel is formed to room temperature. The slow cooling prevents the buildup of stresses in the glass. If the stresses are not eliminated, cooling from room temperature to cryogenic temperatures will increase the stress until the glass cracks. The air pressure outside then forces an implosion of the glass into the vacuum inside. Such highly undesirable events are not unknown, although great care is taken to avoid them. Pyrex glass is always used because its low thermal expansion reduces stresses.
In dewar vessels, the only place in which heat has good access to the stored liquid is through the mouth of the vessel. Heat can enter through the mouth itself or by conduction from the outer wall to the inner wall through the neck connecting the two. Both these problems are reduced as the size of the mouth is reduced because, in both cases, the area of heat access is reduced. Vessels used only for storage of cold liquids have mouths big enough only for storing and removing the liquid, which is accomplished by forcing the corresponding gas into the vessel through a tube and pushing the liquid out through a second tube. This design also prevents air from entering the container and freezing, jamming the container and rendering it useless. Vessels used for cooling equipment or experiment samples must have wider mouths and have correspondingly larger heat leaks.
Both conduction of heat and radiative transfer increase with an increased temperature difference between the liquid and the outside. Hence, extremely cold liquids such as helium and hydrogen, which boil at -269 and -253 degrees Celsius, respectively, are stored in double dewar vessels. The outer vessel contains a less cold (and less expensive) cold liquid, usually liquid nitrogen at -196 degrees Celsius. The inner vessel contains the helium or hydrogen. Heat leaking in from outside the double dewar system is controlled at the expense of the liquid nitrogen rather than being passed to the much colder and more valuable very-low-temperature fluids.
Storage vessels for very cold liquids may be pressurized. Metal double dewars are strong enough to sustain considerable pressure. The little heat that does leak in converts some of the liquid to gas. If not vented, this gas puts pressure on the remaining liquid, helping to keep it liquefied.
A special difficulty appears for liquid nitrogen. The hydrogen molecule has two possible magnetic states at room temperature; however, at the liquid temperature, all the molecules slowly convert to one state. The change evolves a fair amount of heat, which evaporates some liquid and can create high pressures. The problem is best solved by catalyzing the change, making much of it occur as the gas is liquefied, rather than by changing the design of the storage container.
Context
In the sixth century B.C., Thales of Miletus seemed to have been aware that heating water turned it into a gas. Enormous conceptual and technical difficulties stood in the way of imagining the reverse phenomenon: If a substance that is liquid at normal temperatures can be converted into a gas by heating it, can a substance that is a gas at normal temperatures be turned into a liquid by cooling it?
Two thousand years later, in the seventeenth century, Jan Baptista van Helmont was the first to be able to distinguish different gases from one another and from air. Then, Stephen Hales showed how to prepare relatively pure gas samples with his "pneumatic trough," as indicated in his book VEGETABLE STATICKS (1727). Henry Cavendish, Joseph Priestley, and Antoine-Laurent Lavoisier followed these beginnings with advances of their own so that, at the beginning of the nineteenth century, the study of gases was a lively area of research.
It is not clear where or with whom the idea of liquefying gases originated. It is inherent in the idea of latent heat of vaporization on which Joseph Black was lecturing as early as 1761.
Black was very clear on the point that withdrawing the latent heat from a gas would liquefy it. It is less clear which gases Black had in mind beyond steam. About the same time, Lavoisier was coming to the view that gases are in that state because they have combined with the "matter of fire." It is only a short step to the idea of liquefying gases by removing the heat. The technical problems of removing the heat and of storing any liquid thereby produced stood in the way of progress.
Because storage of liquids under pressure was technically easier, the earliest success in liquefying gases came by applying pressure to gases. By 1845, Michael Faraday was able to liquefy by pressure all the then-known gases but the six "permanent" gases: oxygen, hydrogen, nitrogen, carbon monoxide, acetylene, and nitric oxide.
Liquefaction by cooling had to await the discovery of the Joule-Thomson throttling process, in which a gas is pressurized and allowed to expand through a restricted opening. Under proper conditions, the gas can be cooled dramatically. In 1872, Dewar solved the storage problem by inventing the vacuum-jacketed vessels named for him.
With the two main technical difficulties removed, the six "permanent" gases and the newly discovered, very recalcitrant helium were all liquefied. Louis-Paul Cailletet and Raoul-Pierre Pictet simultaneously liquefied oxygen and nitrogen in 1877, and Dewar liquefied hydrogen in 1898. The last to be liquefied was helium, which was liquefied by Heike Kamerlingh Onnes in 1908.
The successful liquefaction and storage of these later gases, particularly nitrogen and helium, opened up the new field of low-temperature physics. With the liquids as coolants, the low-temperature behavior of other materials could finally be studied.
Principal terms
BLACK BODY RADIATION: radiation created by the random thermal motions of atoms; it rapidly strengthens as the temperature increases
CONVECTION: fluid motions caused by density differences between parts of the fluid at different temperatures
CRYOGENIC: having to do with very low temperatures
MONATOMIC: having molecules made of one atom
NOBLE GASES: helium, neon, argon, xenon, krypton, and radon
RADIATIVE TRANSFER: transport of heat by radiation that in this context consists of infrared and ultraviolet radiation and visible light
VAN DER WAALS FORCE: a force between molecules arising from instantaneous electrical polarization of molecules
Bibliography
MacDonald, David K. C. NEAR ZERO. Garden City, N.Y.: Doubleday, 1961. A brief and readable introduction to the world of low-temperature physics, this book surveys the technical problems of creating and maintaining low temperatures as well as the unusual problems that occur in that domain.
Mendelssohn, Kurt. THE QUEST FOR ABSOLUTE ZERO. 2d ed. New York: McGraw-Hill, 1966. More technical detail but still understandable for the layperson. The first four chapters are relevant for the topic of storage of liquefied gases.
Sloane, Thomas O'Conor. LIQUID AIR AND THE LIQUEFACTION OF GASES. 3d ed. New York: Munn, 1919. Although dated, it is a very good history of cryogenics. Chapters 6, 7, and 11 contain simple accounts of the scientific pioneers of working with liquefied gases.
Squire, C. F. LOW TEMPERATURE PHYSICS. New York: McGraw-Hill, 1953. A college-level account. The chapter on experimental methods is a good source of photographs and drawings of details of containment systems for liquefied gases.
White, Guy K. EXPERIMENTAL TECHNIQUES IN LOW TEMPERATURE PHYSICS. 2d ed. Oxford, England: Clarendon Press, 1987. Somewhat technical, but it contains a wealth of details. Chapter 2 is especially relevant.
The Behavior of Gases
Liquefaction of Gases
The Atomic Structure of Liquids
Essay by John A. Cramer
