Introduction:

Liquefaction of gases is the method via which substances in their gaseous state are transformed or changed to the liquid state. Whenever pressure on a gas is increased, its molecules closer altogether, and its temperature is reduced, which eliminates adequate energy to make it change from the gaseous to the liquid state.

Liquefaction is the significant method commercially as substances in the liquid state take up much less room than they do in their gaseous state. As an illustration, oxygen is frequently employed in space vehicles to burn the fuel on which they operate. If the oxygen had to be taken in its gaseous form, a space vehicle would have to be thousands of times bigger than anything that could possibly fly. In its liquid state, though, the oxygen can simply fit to a space vehicle's structure.

Liquefaction of a gas takes place whenever its molecules are pushed closer altogether. The molecules of any gas are relatively far apart from one other; whereas the molecules of a liquid are relatively close altogether. Gas molecules can be squeezed altogether by one of two methods: by raising the pressure on the gas or by lowering the temperature of the gas.

History regarding Liquefaction of gases:

Pioneer work on the liquefaction of gases was taken out by the English scientist named Michael Faraday (1791-1867) in the early 1820s. Faraday was capable to liquefy gases by high critical temperatures like chlorine, hydrogen bromide, hydrogen sulphide and carbon dioxide via the application of pressure alone. This was not till a half century later, though, that researchers found ways to liquefy gases having lower critical temperatures, like oxygen, nitrogen and carbon monoxide. The French physicist Louis Paul Cailletet (1832-1913) and the Swiss chemist Raoul Pierre Pictet (1846-1929) invented devices by using the nozzle and porous plug process for liquefying such gases. It was not until the end of the 19th century that the two gases having the lowest critical temperatures, hydrogen (-399.5°F [-239.7°C; 33.3K]) and helium (-449.9°F [-267.7°C; 5.3K]) were liquefied via the work of the Scottish scientist James Dewar (1842-1923) and the Dutch physicist Heike Kamerlingh Onnes (1853-1926), correspondingly.

Explanation of Liquefaction of gases:

There are huge empty spaces (or voids) separating the small molecules of gases from one another. Each and every molecule enjoys an almost independent existence. Molecules are in the state of continuous fast motion and encompass negligible attractive forces among them due to broad separation. This is mainly so, whenever temperature is high and pressure is low. Whenever the temperature of the gas is lowered, both the volume of the gas and the kinetic energy of the molecules reduce. The molecular motion becomes slow and molecules become sluggish. The progressive reduction of temperature brings the molecules closer and closer as they are not able to oppose the attractive force which begins operating between them. At last, at adequately low temperature, the voids among the molecules become less than 10-5cm and the gas changes to the liquid state.

This method of liquefaction via bringing gas molecules closer can as well be accomplished by increasing the pressure of the gas: this as well reduces the volume of the gas. For illustration, sulphur dioxide can be liquefied at 265 K if pressure is 760 mm of Hg. This can as well be liquefied at 293K if the pressure is raised to 2470 mm of Hg.

Therefore, liquefaction of gases can be accomplished by either decrease of temperature or through increase of pressure.

Uses of liquefied gases:

Liquefied and gases compressed beneath a high pressure are of great significance in industries.

1) Liquid sulphur dioxide and liquid ammonia are employed as refrigerants.

2) Liquid carbon dioxide finds utilization in soda fountains.

3) Liquid chlorine is utilized for bleaching and disinfectant aims.

4) Liquid air is a significant source of oxygen in the rockets and jet-propelled planes and bombs.

5) Compressed oxygen is employed for the welding uses.

6) Compressed helium is employed in the airships.

Liquefaction of Gases - Critical Temperature and Pressure:

Two significant properties of gases are significant in developing processes 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, though great, will cause the gas to liquefy. The minimum pressure needed to liquefy the gas at the critical temperature is termed as the critical pressure.

For illustration, the critical temperature for carbon-dioxide is 304K (87.8°F [31°C]). That signifies that no amount of pressure exerted to a sample of carbon-dioxide gas at or above 304K (87.8°F [31°C]) will cause the gas to liquefy. At or beneath that temperature, though, the gas can be liquefied provided adequate pressure is applied. The corresponding critical pressure for carbon-dioxide at 304K (87.8°F [31°C]) is around 72.9 atmospheres. In another words, the application of a pressure of 72.9 atmospheres of pressure on a sample of carbon-dioxide gas at 304K (87.8°F [31°C]) will cause the gas to liquefy.

The differences in critical temperatures between gases signify that certain gases are simpler to liquefy than are others. The critical temperature of carbon-dioxide is high adequate in such a way that it can be liquefied relatively easily at or close to room temperature. On contrast, the critical temperature of nitrogen gas is around 126K (-232.6°F [-147°C]) and that of helium is 5.3K (-449.9°F [-267.7°C]). Liquefying gases like nitrogen and helium apparently present much greater complexities than does the liquefaction of carbon-dioxide.

Methods of Liquefaction:

In common, gases can be liquefied through one of three general processes: (a) By compressing the gas at temperatures below than its critical temperature; (2) by making the gas do some type of work against the external force, causing the gas to lose energy and convert to the liquid state; and (c) by making gas do work against its own internal forces, as well causing it to lose energy and to liquefy.

In the primary approach, the application of pressure alone is adequate to cause a gas to change to a liquid. For illustration, ammonia consists of a critical temperature of around 406K (271.4°F [133°C]). This temperature is fine above room temperature; therefore it is relatively simple to transform ammonia gas to the liquid state simply through applying enough pressure. At its critical temperature, that pressure is 112.5 atmospheres; however the cooler the gas is to start with, the less pressure is required to make it condense.

The modern processes of cooling the gas to or beneath their Tc and thus of liquefaction of gases are completed by Linde's and Claude's method.

1) Linde's method: This method is mainly based on the Joule-Thomson effect that states that 'Whenever a gas is allowed to expand adiabatically from an area of high pressure to an area of very low pressure, it is accompanied through cooling'.

2) Claude's method: This method is mainly based on the principle that whenever a gas expands adiabatically against an external pressure (that is, as a piston in an engine), it does some external work. As work is done via the molecules at the cost of their kinetic energy, the temperature of the gas descends causing cooling.

3) By adiabatic demagnetization.

Joule-Thomson effect:

Whenever a real gas is allowed to expand adiabatically via a porous plug or a fine hole to an area of low pressure, it is accomplished through cooling (apart from for hydrogen and helium that get warmed up).

Cooling occurs as some work is done to overcome the intermolecular forces of attraction. As an outcome, the internal energy reduces and so does the temperature.

Ideal gases don't show any cooling or heating as there are no intermolecular forces of attraction that is, they don't exhibit Joule-Thomson effect.

Throughout Joule-Thomson effect, the enthalpy of system remains constant.

Joule-Thomson coefficient:

μ = (∂T/∂P) H

For cooling, μ = positive (as dT and dP will be negative)

For heating μ = negative (as dT = + ve and dP = -ve).

For no heating or cooling μ = 0 (as dT = 0).

Liquefaction of Gases - Making a Gas work against External Force:

A simple illustration of the second process for liquefying gases is the steam engine. The principle on which a steam engine functions is that water is boiled and the steam formed is introduced to a cylinder. Within the cylinder, the steam pushes on the piston that drives some type of machinery. Since the steam pushes against the piston, it loses energy. That loss of energy is reflected in the lowering of temperature of the steam. The lowered temperature might be adequate to cause the steam to change back to water.

In practice, the liquefaction of a gas by this process occurs in two steps. At first, the gas is cooled and then it is forced to do work against some external system. For illustration, it might be driven via a small turbine, where it causes a set of blades to rotate. The energy loss resultant from driving the turbine might then be adequate to cause the gas to convert to a liquid.

The process illustrated so far is identical to the principle on which refrigeration systems work. The coolant in a refrigerator is first transformed or changed from a gas to a liquid by one of the methods illustrated above. It then absorbs heat from the refrigerator box, transforming back into a gas in the process. The difference between refrigeration and liquefaction, though, is that in the former method, the liquefied gas is continuously eliminated from the system for use in some other method, whereas in the latter method, the liquefied gas is continuously recycled in the refrigeration system.

Liquefaction of Gases - Making a Gas work against Internal Forces:

In certain ways, the simplest process or method for liquefying a gas is simply to take benefit of the forces that operate between its own molecules. It can be done by forcing the gas to pass via a small nozzle or a porous plug. The change that occurs in the gas throughout this method based on its original temperature. If that temperature is less than certain fixed value, termed as the inversion temperature, then the gas will for all time be cooled as it passes via the nozzle or plug.

In several cases, the cooling that takes place during this method might not be adequate to cause the liquefaction of gas. Though, the method can be repeated more than once. Each time, more energy is eliminated from the gas, its temperature drops further, and, ultimately, it converts to a liquid. This type of cascade effect can, however be used by either of the last two processes of gas liquefaction.

Practical Applications of Liquefaction of Gases:

The most significant benefit of liquefying gases is that they can then be stored and transported in much more packed form than in the gaseous state. Two types of liquefied gases are broadly employed commercially for this cause, liquefied natural gas (LNG) and liquefied petroleum gas (LPG). LPG is the mixture of gases obtained from the natural gas or petroleum that has been transformed to the liquid state. The mixture is stored in strong containers which can withstand at very high pressures. LPG is employed as a fuel in motor homes, boats and homes which don't have access to the other forms of fuel.

Liquefied natural gas is identical to LPG; apart from that it has had almost everything apart from methane eliminated. LPG and LNG have lots of similar uses.

In principle, any gas can be liquefied, thus their compactness and simplicity of transportation has made them well-liked for a number of other applications. For illustration: liquid oxygen and liquid hydrogen are employed in the rocket engines. Liquid oxygen and liquid acetylene can be employed for welding purposes. And a combination of liquid oxygen and liquid nitrogen can be employed in aqualung devices.

Liquefaction of gases is as well significant in the field of research termed as cryogenics. Liquid helium is broadly employed for the study of behavior of matter at temperatures close to the absolute zero - 0K (-459°F [-273°C]).

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