Gases, liquefaction of

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.

Words to Know

Critical pressure: The minimum pressure required to liquefy a gas at its critical temperature.

Critical temperature: The temperature at or above which no amount of pressure, however great, will cause a gas to liquefy.

Cryogenics: The production and maintenance of low temperature conditions and the study of the behavior of matter under such conditions.

Liquefied natural gas (LNG): A mixture of gases obtained from natural gas or petroleum from which almost everything except methane has been removed before it is converted to the liquid state.

Liquefied petroleum gas (LPG): A mixture of gases obtained from natural gas or petroleum that has been converted to the liquid state.

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 ]

liquefaction Liquefaction is a condition in which a soil or sediment behaves as a fluid. This is the result of strong vibration and is normally related to a moderate to large earthquake. The process of liquefaction normally occurs only in sands and silty sands, although recent evidence suggests that gravels may also liquefy. The actual process of liquefaction is complex. Soils are composed of a mineral phase and a fluid phase. During an earthquake the soil mass may begin to compact. When this happens, the water in the pore spaces between grains exerts a higher pressure on the mineral particles. It has also been shown that the passage of seismic waves, which are a transient stress in their own right, also has an effect on these forces, referred to as pore-water pressures. As these pore-water pressures increase, the grains are forced apart, there is less frictional contact between sand grains, and the shear strength of the soil decreases. When the soil loses all its shear strength, liquefaction has occurred. This process is therefore important in soils that gain their strength from internal friction (i.e. grain-to-grain contact). Liquefaction does not occur in clay-rich sediments because the strength of clays generally comes from interparticle bonds.

The susceptibility of soils to liquefaction is controlled by a number of factors. These can be divided into two groups: the ground conditions, and the characteristics of the earthquake ground motions. The ground conditions that give rise to a high liquefaction susceptibility are low density, water saturation, and a silty sand particle-size distribution. The distribution of grain sizes is critical, and it is possible to identify a range of particle sizes at which liquefaction is likely to occur. Dense sands, with low water contents, are less likely to liquefy. The geological history of the soil is important, since river-deposited sands are more likely to liquefy than glacial deposits because of their differences in density.

In general, the larger the earthquake, the greater the possibility that liquefaction will occur. The duration of shaking is, however, a critical factor. A long duration of seismic shaking allows for the gradual increase in pore-water pressures as each stress pulse passes through the soil. The greater the stress pulse (measured as a ground acceleration), the greater increase in pore-water pressure. A long duration of shaking with a high ground acceleration is therefore more likely to cause liquefaction than a short duration of shaking with low ground acceleration. This would normally require an earthquake of magnitude greater than 5.

The engineering effects of liquefaction can be profound. The complete loss of bearing strength had serious consequences during the 1964 Niigata earthquake in Japan. The Kwangishicho apartment block in Niigata itself was tilted through an angle of over 40^o as a result of liquefaction around the foundations, although the buildings themselves did not suffer structural failure. Buried objects, such as septic tanks, have been found to float to the surface in liquefied sediments. Liquefaction can also result in a lateral spread, or flow slide, which can occur on slopes that are normally too low for landslides to occur. This effect was seen during the 1964 Alaska earthquake in which major landslides occurred at Turnagain Heights and Government Hill owing to liquefaction of underlying sand layers.

W. Murphy

liquefaction change of a substance from the solid or the gaseous state to the liquid state. Since the different states of matter correspond to different amounts of energy of the molecules making up the substance, energy in the form of heat must either be supplied to a substance or be removed from the substance in order to change its state. Thus, changing a solid to a liquid or a liquid to a gas requires the addition of heat, while changing a gas to a liquid or a liquid to a solid requires the removal of heat. In the liquefaction of gases, extreme cooling is not necessary, for if a gas is held in a confined space and is subjected to high pressure, heat is given off as it undergoes compression and it turns eventually to a liquid. Some cooling is, however, necessary; it was discovered by Thomas Andrews in 1869 that each gas has a definite temperature , called its critical temperature, above which it cannot be liquefied, no matter what pressure is exerted upon it. A gas must, therefore, be cooled below its critical temperature before it can be liquefied. When a gas is compressed its molecules are forced closer together and, their vibratory motion being reduced, heat is given off. As compression proceeds, the speed of the molecules and the distances between them continue to decrease, until eventually the substance undergoes change of state and becomes liquid. Although before the 19th cent. a number of scientists had experimented in liquefying gases, Davy and Faraday are usually credited with being the first to achieve success. The production of liquefied gases in large quantities (and consequently their use in refrigeration) was made possible by the work of Z. F. Wroblewski and K. S. Olszewski, two Polish scientists. The work of Sir James Dewar is also important, especially in the liquefaction of air and its change to a solid. Heike Kamerlingh Onnes first liquefied helium. The critical temperature of helium is -267.9°C, only a few degrees above absolute zero (-273.15°C). The processes for the liquefaction of gases as developed by Linde and others form the basis for those used in modern refrigeration . Liquefied gases are much used in low-temperature research; some, e.g., liquid oxygen, find use as rocket propellants. See liquid air ; low-temperature physics .

Liquefaction of Blood

A famous miracle claimed for the blood of St. Januarius, executed September 19, 309 C.E. In Lives of the Saints (1623), E. Kinesman stated: "The most stupendous miracle is that seen to this day in the church of St. Gennaro, in Naples, viz. the blood of St. Januarius, kept in two glass vials. When either vial, held in the right hand, is presented to the head of the saint, the congealed blood first melts, and then goes on apparently to boil." Scientists have pointed out that such a miracle may be accomplished scientifically by the use of ether or other chemicals.

However, the miracle has continued into modern times. On May 6, 1989, the blood liquified on schedule in Naples, and Cardinal Michele Giordano revealed that he had allowed scientists to study the relic secretly. The liquefaction traditionally occurs twice a year, on September 19, the day of the saint's death, and on the Saturday before the first Sunday in May. Cardinal Giordano, archbishop of Naples, stated that the "May Miracle" of 1989 occurred during the religious procession that precedes the usual ceremony for the liquefaction.

Sources:

Rogo, D. Scott. Miracles: A Parascientific Inquiry into Wondrous Phenomena. New York: Dial Press, 1982.

Thurston, Herbert. The Physical Phenomena of Mysticism. London: Burns Oates, 1952.

liquefaction The process of becoming or making a liquid by heating, cooling, or a change in pressure. In soils, the temporary transformation of material to a fluid state due to a sudden decrease in shearing resistance caused by a collapse of the structure associated with a temporary increase in pore fluid pressure.

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