Gases, liquefaction of

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Photo by: John Keith

Liquefaction of gases is the process by which a gas is converted to a liquid. For example, oxygen normally occurs as a gas. However, by applying sufficient amounts of pressure and by reducing the temperature by a sufficient amount, oxygen can be converted to a liquid.

Liquefaction is an important process commercially because substances in the liquid state take up much less room than they do in their gaseous state. As an example, oxygen is often used in space vehicles to burn the fuel on which they operate. If the oxygen had to be carried in its gaseous form, a space vehicle would have to be thousands of times larger than anything that could possibly fly. In its liquid state, however, the oxygen can easily fit into a space vehicle's structure.

Liquefaction of a gas occurs when its molecules are pushed closer together. The molecules of any gas are relatively far apart from each other, while the molecules of a liquid are relatively close together. Gas molecules can be squeezed together by one of two methods: by increasing the pressure on the gas or by lowering the temperature of the gas.

Critical temperature and pressure

Two key properties of gases are important in developing methods for their liquefaction: critical temperature and critical pressure. The critical temperature of a gas is the temperature at or above which no amount of pressure, however great, will cause the gas to liquefy. The minimum pressure required to liquefy the gas at the critical temperature is called the critical pressure.

For example, the critical temperature for carbon dioxide is 88°F (31°C). That means that no amount of pressure applied to a sample of carbon dioxide gas at or above 88°F will cause the gas to liquefy. At or below that temperature, however, the gas can be liquefied provided sufficient pressure is applied. The corresponding critical pressure for carbon dioxide at 88°F is 72.9 atmospheres. In other words, the application of a pressure of 72.9 atmospheres on a sample of carbon dioxide gas at 88°F will cause the gas to liquefy. (An atmosphere is a unit of pressure equal to the pressure of the air at sea level, or approximately 14.7 pounds per square inch.)

A difference in critical temperatures among gases means that some gases are easier to liquefy than are others. The critical temperature of carbon dioxide is high enough so that it can be liquefied relatively easily at or near room temperature. By comparison, the critical temperature of nitrogen gas is −233°F (−147°C) and that of helium is −450°F (−268°C). Liquefying gases such as nitrogen and helium present much greater difficulties than does the liquefaction of carbon dioxide.

Methods of liquefaction

In general, gases can be liquefied by one of three general methods:(1) by compressing the gas at temperatures less than its critical temperature; (2) by making the gas do some kind of work against an external force, causing the gas to lose energy and change to the liquid state; and (3) by using the Joule-Thomson effect.

Compression. In the first approach, the application of pressure alone is sufficient to cause a gas to change to a liquid. For example, ammonia has a critical temperature of 271°F (133°C). This temperature is well above room temperature. Thus, it is relatively simple to convert ammonia gas to the liquid state simply by applying sufficient pressure. At its critical temperature, that pressure is 112.5 atmospheres.

Making a gas work against an external force. A simple example of the second method for liquefying gases is the steam engine. A series of steps must take place before a steam engine can operate. First, water is boiled and steam is produced. That steam is then sent into a cylinder. Inside the cylinder, the steam pushes on a piston. The piston, in turn, drives some kind of machinery, such as a railroad train engine.

As the steam pushes against the piston, it loses energy. Since the steam has less energy, its temperature drops. Eventually, the steam cools off enough for it to change back to water.

This example is not a perfect analogy for the liquefaction of gases. Steam is not really a gas but a vapor. A vapor is a substance that is normally a liquid at room temperature but that can be converted to a gas quite easily. The liquefaction of a true gas, therefore, requires two steps. First, the gas is cooled. Next, the cool gas is forced to do work against some external system. It might, for example, be driven through a small turbine. A turbine is a device consisting of blades attached to a central rod. As the cooled gas pushes against the turbine blades, it makes the rod rotate. At the same time, the gas loses energy, and its temperature drops even further. Eventually the gas loses enough energy for it to change to a liquid.

This process is similar to the principle on which refrigeration systems work. The coolant in a refrigerator is first converted from a gas to a liquid by one of the methods described above. The liquid formed then absorbs heat from the refrigerator box. The heat raises the temperature of the liquid, eventually changing it back to a gas.

There is an important difference between liquefaction and refrigeration, however. In the former process, the liquefied gas is constantly removed from the system for use in some other process. In the latter process, however, the liquefied gas is constantly recycled within the refrigeration system.

Using the Joule-Thomson effect. Gases also can be made to liquefy by applying a principle discovered by English physicists James Prescott Joule (1818–1889) and William Thomson (later known as Lord Kelvin; 1824–1907) in 1852. The Joule-Thomson effect depends on the relationship of volume, pressure, and temperature in a gas. Change any one of these three variables, and at least one of the other two (or both) will also change. Joule and Thomson found, for example, that allowing a gas to expand very rapidly causes its temperature to drop dramatically. Reducing the pressure on a gas accomplishes the same effect.

To cool a gas using the Joule-Thomson effect, the gas is first pumped into a container under high pressure. The container is fitted with a valve with a very small opening. When the valve is opened, the gas escapes from the container and expands quickly. At the same time, its temperature drops.

In some cases, the cooling that occurs during this process may not be sufficient to cause liquefaction of the gas. However, the process can be repeated more than once. Each time, more energy is removed from the gas, its temperature falls further, and it eventually changes to a liquid.

Practical applications

The most common practical applications of liquefied gases are the compact storage and transportation of combustible fuels used for heating, cooking, or powering motor vehicles. Two kinds of liquefied gases are widely used commercially for this reason: liquefied natural gas (LNG) and liquefied petroleum gas (LPG). LPG is a mixture of gases obtained from natural gas or petroleum that has been converted to the liquid state. The mixture is stored in strong containers that can withstand very high pressures.

Liquefied natural gas (LNG) is similar to LPG, except that it has had almost everything except methane removed. LNG and LPG have many similar uses.

In principle, all gases can be liquefied, so their compactness and ease of transportation make them popular for a number of other applications. For example, liquid oxygen and liquid hydrogen are used in rocket engines. Liquid oxygen and liquid acetylene can be used in welding operations. And a combination of liquid oxygen and liquid nitrogen can be used in Aqua-Lung™ devices (an underwater breathing apparatus).

Liquefaction of gases also is important in the field of research known as cryogenics (the branch of physics that deals with the production and effects of extremely low temperatures). Liquid helium is widely used for the study of behavior of matter at temperatures close to absolute zero, 0 K (−459°F; −273°C).

[ See also Cryogenics ; Gases, properties of ]