Liquefaction Of Gases

Type of physical science: Classical physics

Field of study: Thermodynamics

Gases can be liquefied by compressing or by cooling them using one of a number of techniques. The liquids thus produced are interesting in their own right and have many uses in science, industry, and medicine.

Overview

Most people are aware that a substance can exist in solid, liquid, or gaseous form, and that heating a solid is likely to liquefy the solid first and then turn it into a gas. It is equally true that cooling a gas will liquefy it. In certain respects, liquids and gases are simpler than solids because they have no microscopic organization. Hence, the liquefaction of a gas is an easier change to understand than is, for example, the change from a liquid to a solid.

The most fundamental difference between a liquid and a gas is that the average distance between molecules is greater in a gas. The greater thermal motion of molecules in the gas makes the molecules spend most of their time far away from one another. In a liquid of the same substance, the molecules are more frequently near one another and hence are more strongly affected by the mutually attractive forces acting between them. The longer average time of attraction in the liquid pulls the molecules together, reducing their average separation and making the liquid more compact and tightly bound together than the gas.

There are several types of attractive forces between molecules. A universal and often dominant force, the van der Waals force (named for Johannes Diderik van der Waals) arises because the electrons in molecules and atoms are more mobile than the much heavier protons. If the electrons happen to be instantaneously concentrated on one side of the molecule, they make that side seem electrically negative, while the other side appears electrically positive. During that instant, the molecule is polarized. Its negative end will repel electrons in nearby molecules, and the neighboring molecules also become instantaneously polarized. Because the process necessarily produces negative ends near positive ends, the van der Waals force basically pulls molecules together.

The van der Waals force depends on the shape of the gas molecules. Highly symmetrical molecules are hardest to polarize, so the van der Waals force is weak in gases made of such molecules. The noble gases, whose molecules are single, spherical atoms, and hydrogen were the last gases liquefied because of their small, spherical (or, in the case of hydrogen, near spherical) molecules and correspondingly weak intermolecular attractions.

To turn a gas into a liquid, ways must be found either to decrease the distances between molecules or to weaken the thermal motions. The most obvious way to decrease the distance is to compress the gas, decreasing the volume the molecules have available to them and pushing them closer together. Alternatively, cooling the gas weakens the thermal motions. Excepting 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.

Pressure techniques for liquefaction developed early because of their relative simplicity. A piston and a way of pushing the piston in are almost all that is needed to liquefy many gases. Nevertheless, applying pressure to a gas does more than compress the gas. It also heats the gas (the diesel engine depends on just such heating for the ignition of its fuel mixture).

Air hockey provides a helpful example. The puck rebounds faster if the deflector is pushed toward it rather than moved sideways. The motion of the deflector adds energy to the motion of the puck. In the same way, the motion of the piston into the gas adds energy to the motion of molecules striking its face. The additional energy shows up as heat. Using compression requires a way of disposing of the heat generated in the process. As the rate of producing liquid is increased, the rate of getting rid of heat must also be increased. Fortunately, the heat can be dumped to the surrounding through heat exchangers, but the rate at which this can be done will limit the rate of liquid production. Bigger and better heat exchangers and better coolants are the solution.

No matter how rapidly an apparatus can dissipate heat, some gases do not liquefy well under pressure. Hydrogen, helium, and other "problem" gases will not liquefy under pressure.

Thomas Andrews was the first to point out that no gas can be liquefied by the application of pressure unless the process takes place below a critical temperature, which is characteristic of the gas involved. Above the critical temperature, it is not really possible to distinguish the gas phase of the substance from the liquid phase. The problematic gases have critical temperatures below room temperature and must be precooled or refrigerated in some way to get them below the critical temperature before pressure liquefaction will work. If one has the refrigerator to precool the gas, why not use it to liquefy the gas and bypass compression entirely?

There are three basic refrigeration techniques for gases. Cooling by doing work is the reverse of heating by compression. The gas pushes a piston back as it expands, losing thermal energy in doing work on the piston. The expansion has the undesirable effect of making the gas less compact and liquidlike. Nevertheless, the cool gas can be used to precool more compressed gas prior to expansion. Cooled gas and incoming compressed gas pass through but do not mix in a heat exchanger where heat from the incoming gas flows to the cooled gas. The precooled incoming gas starts expansion at a lower temperature than the gas of the previous cycle and then cools to a lower temperature than the gas used to precool it. The gas in turn precools another cycle of compressed gas, and the process continues until the temperature becomes low enough for liquid to form.

A second process uses a cold fluid to cool the fluid to be liquefied. For example, liquid nitrogen can be used to liquefy other gases. More commonly, this process uses a cooling fluid cycled between its liquid and vapor stages by compression and expansion. This is the common refrigeration cycle of refrigerators and air conditioners using Freon gas or other coolants. The coolant changes from a liquid to a gas, absorbing heat from the gas to be cooled. The vaporized coolant is then compressed, cooled by exchanging heat with the surroundings, and liquefied to be reused as a coolant.

The third process, throttling, was discovered in the middle of the nineteenth century.

Called the Joule-Thomson effect, it is the cooling of a gas as it expands through a porous plug or a very narrow orifice. An almost paradoxical feature of the process is that it can also heat the gas.

Each type of gas has a specific temperature, its inversion temperature, above which it heats as it expands and below which it cools as it expands. For nitrogen, the inversion temperature is about 625 Kelvins, so room-temperature nitrogen is easily cooled by throttling. Hydrogen has a low inversion temperature of 202 Kelvins, but throttling in hydrogen becomes useful only below about 90 Kelvins.

The throttling process is used to liquefy helium, but because of the 34-Kelvin inversion temperature of helium, a lengthy precooling period with liquid nitrogen and a cascade of compressors and expansion coolers is needed before the Joule-Thomson effect is initiated at approximately 25 Kelvins. Once some liquid has accumulated, evaporation of the liquid keeps the container and surroundings at the necessary low temperature for more liquid to accumulate.

Applications

At first, liquefaction of gases was a problem in pure science demanding skilled performance from experimental physicists. Michael Faraday exhausted the list of liquids created by pressure. Liquefaction by cooling began with the independent liquefaction of oxygen and nitrogen by Louis-Paul Cailletet and Raoul-Pierre Pictet in 1877. Cailletet used cycles of cooling by doing work, sometimes supplemented by precooling with liquid ethylene and methane, to obtain his results. Pictet used cascaded refrigerators with sulfur dioxide and then carbon dioxide as the successive working fluids. James Dewar then liquefied hydrogen in 1898, using throttling for the first time in conjunction with cooling. In 1908, Heike Kamerlingh Onnes used Dewar's procedure first to liquefy helium.

Hydrogen surprised scientists by having an odd and problematic behavior. After liquefaction, hydrogen spontaneously heats up and some of it evaporates, creating pressure problems for the containment system. It was some time before this behavior was understood.

Each hydrogen molecule has two possible magnetic states at room temperature, but only one of these states is stable at the temperature of the liquid. Hydrogen liquefied from room temperature contains molecules in both states at first, but those in the unstable, higher-energy state slowly convert to the more stable and less energetic state. As they convert, they release the energy difference between the two states. This energy can go only into internal energy, which will appear as heating of the liquid. The liquefaction of hydrogen thus provides another example of the fact that pure research always leads to more questions and new science.

The first efficient process for liquefying helium was the combination of cooling by cycles of compression and expansion and the throttling process using liquid nitrogen as a precoolant and helium itself as the lubricant for the expansion pistons. Pyotr Leonidovich Kapitsa, working in England, perfected the helium process in 1934, and Samuel C. Collins of the Massachusetts Institute of Technology so improved the design in the 1940's that the A.D. Little Company in the United States eventually began manufacturing "Collins" liquefiers, hundreds of which have now been produced.

Scientifically, the liquefaction of helium also brought surprises. Superconductivity appears in a number of metals at liquid helium temperatures, as Kamerlingh Onnes himself discovered while in the process of making the obvious use of the new liquid, studying the properties of materials at very low temperatures. Liquid helium itself was then found to be a superfluid, capable of nonviscous flow and of other unusual behavior such as flowing up the side of containers. Both these phenomena opened windows on new science, requiring extremely ingenious explanations based on quantum mechanics.

Liquefaction of gases eventually became an applied area of science, and many different liquefied gases became readily available commercially. Some, such as propane and butane, are used as fuels. Others are used in industrial, military, and research areas, where the liquid form may be more a matter of convenience than of necessity. The liquid propellant rocket engines of the space age use oxygen in the liquid state to burn the propellants. Some liquefied gases, such as oxygen and nitrous oxide, have medical uses. Medical physics uses involve liquid nitrogen and liquid helium for superconducting sensing devices such as magnetic imaging and SQUID (superconducting quantum interference device) magnetometers.

Liquid helium and liquid nitrogen are the mainstay coolants of low-temperature physics. The superconducting magnets of the powerful particle accelerators would burn up without truly vast quantities of these coolants. Liquid nitrogen is the coolant of preference for such physics demonstrations as driving nails with frozen bananas. It has even seen such uses as freezing warts to remove them and freeze branding (rather than heat branding) of cattle. Unlike heat branding, freeze branding does not damage the hide, so the whole hide can later be made into leather when the animal is slaughtered.

Context

Most gases are not difficult to liquefy. Yet, now that liquefied gases are in widespread use, it may seem surprising that liquefaction of gases is so recent a technology. The ability to liquefy many gases, however, goes back no further than the middle of the nineteenth century, when Michael Faraday was able to liquefy all but the six "permanent" gases known at that time.

One of the reasons that this ability to liquefy gases was slow to develop was that the idea could not occur to someone who did not know that gases exist. The recognition of the existence of different gases was slow in coming. Air had been recognized (and confused with breath) for a long time, but it was not until the seventeenth century that anyone realized that other gases existed. Indeed, the word "gas" derives from the seventeenth century work of Jan Baptista van Helmont, who used a Greek word for "chaos" as a generic name for the new substances he was distinguishing from air. Van Helmont was able to see that methane, carbon monoxide, sulfur dioxide, and a few other gases are not the same as air. He did not give these gases their modern names but instead used Latin names of his own invention.

Recognition that many distinctly different gases exist was a prerequisite to attempts at liquefying the gases, but there were other obstacles, too. Liquefaction techniques had to be developed as well as storage containers for the liquids. A far more fundamental problem was the conceptual one: Someone had to see liquefaction as possible before it could be attempted.

The behavior of water was regarded as unique. Water was the only known liquid until Arab alchemists distilled alcohol. Other liquids were considered to be water with other materials dissolved in it. As the realization grew that water was not the only liquid and air was not the only gas, it finally became possible to imagine turning gases into liquids. Thus, at the end of the eighteenth century, Joseph Black in Scotland and Antoine-Laurent Lavoisier in France were both talking of gases as substances to which heat had been added. It is difficult to determine who first saw that removing the heat might liquefy the substance. Nevertheless, advances in liquefying gases occurred rapidly in the first half of the nineteenth century, culminating in Faraday's work.

Faraday liquefied gases by pressure. For the six "permanent" gases (oxygen, nitrogen, hydrogen, acetylene, carbon monoxide, and nitric oxide), pressure of the order that scientists of that time could produce was inadequate (and very dangerous with acetylene). A new technique was needed for the six gases and also for helium, once it was discovered.

Joule-Thomson throttling, the cooling of a gas as it expands through a narrow channel, was discovered around the time Faraday's work with pressure liquefaction was ending. Throttling and cooling a gas by letting a gas do work against a piston became the basis for liquefaction by cooling. Refrigeration techniques worked out by Cailletet and Pictet, who independently liquefied oxygen and nitrogen in 1877, came into use at the end of the nineteenth century. The liquids thereby obtained could not be accumulated in large quantities until Dewar invented the vacuum-insulated vessel named for him. Dewar liquefied hydrogen in 1898, incorporating throttling into his process. Once dewars became available, low-temperature studies of materials became possible. True low-temperature physics was born in 1908 when Heike Kamerlingh Onnes liquefied helium at the very low temperature of 4 Kelvins.

Principal terms

CRITICAL TEMPERATURE: the temperature above which a gas cannot be liquefied by compression and the liquid phase cannot be distinguished from the vapor phase

HEAT EXCHANGER: any device that keeps substances physically separate while allowing them to exchange heat

INVERSION TEMPERATURE: the temperature above which a gas heats when throttled and below which it cools when throttled

JOULE-THOMSON EFFECT: the throttling process

THERMAL MOTION: the random motion of molecules and atoms which constitutes heat (internal) energy

THROTTLING PROCESS: allowing a gas to expand on passage through a porous plug or narrow channel

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 things that occur in that domain.

Mackinnon, Lachlan. EXPERIMENTAL PHYSICS AT LOW TEMPERATURES. Detroit: Wayne State University Press, 1966. The first chapter, "Introduction and Techniques," has a clear discussion of liquefaction procedures and contains numerous valuable diagrams.

Mendelssohn, Kurt. THE QUEST FOR ABSOLUTE ZERO. 2d ed. New York: McGraw-Hill, 1966. Somewhat technical but still readable.

Sloane, Thomas O'Conor. LIQUID AIR, AND THE LIQUEFACTION OF GASES. 3d ed. New York: Munn, 1919. Although dated, it is a good history of cryogenics. Chapters 6, 7, and 9 should be reviewed for simple accounts of the scientific pioneers of working with liquefied gases.

White, Guy K. EXPERIMENTAL TECHNIQUES IN LOW TEMPERATURE PHYSICS. 3d ed. Oxford, England: Clarendon Press, 1987. Although technical, this book contains a wealth of valuable details. Chapter 2 is especially relevant.

Zemansky, Mark Waldo. TEMPERATURES VERY LOW AND VERY HIGH. Princeton, N.J.: D. Van Nostrand, 1964. Chapter 3 on the approach to absolute zero contains a relevant discussion and many helpful diagrams.

The Behavior of Gases

The Storage of Liquefied Gases

The Atomic Structure of Liquids