Liquefaction of Gases


Liquefaction is the physical process or technique for the conversion of gaseous substances into their liquid state. When the pressure is applied to the gas molecules, they become very adjacent to each other, the temperature is reduced so there is not enough energy.

This removal of energy converts it into liquid. Nitrogen, methane and oxygen like gases use a low temperature for liquefaction and are stored at low pressures. The liquefaction can be done by increasing the pressure.

An increase in pressure will bring the molecules closer to each other which then combine to form a liquid. And secondly by decreasing the kinetic energy which will be done by lowering the temperature.

General Principle of Liquefaction

The conversion of a gas into a liquid needs high pressure and low-temperature level. High pressure brings the particles of gas near to each other. Low temperature deprives the molecules of kinetic energy and attractive forces begin dominating.

For every gas there exists a temperature level above which the gas cannot be liquefied, no matter just how much pressure is used.

Critical temperature (Tc)

The highest temperature level at which a substance can exist as a liquid is called its critical temperature (Tc).

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Crucial pressure (Pc)

There is a corresponding pressure that is needed to cause liquefaction at this crucial temperature (Tc). This is called crucial pressure (Pc).

The crucial temperature and the crucial pressure (Pc) of the substances are extremely essential for the workers handling the gases. These properties provide us the information about the condition under which gases liquefy. For example, O2 has a crucial temperature of 154.4 K (-118.75 ° C). It should be cooled below this temperature level before it can be liquefied by using high pressure. Ammonia is a polar gas. Its crucial temperature level is 405.6 K (132.44 ° C), so it can be liquefied by applying sufficient pressure close to room temperature.

Non- polar gases of low polarizability like Ar have a really low crucial temperature. The substances like H2O vapors and NH3 gas are amongst the polar gases and they have much better propensities to be liquefied CO2, cannot be liquefied above 31.1 oC, no matter just how much the pressure is applied. Anyways, if the temperature level of CO2 is kept below 31.1 oC, then lower pressure than crucial pressure is required to liquefy it. The value of the crucial temperature of a gas depends upon its size, shape, and intermolecular forces present in it.

When gas is determined at its crucial temperature and crucial pressure, then at that phase volume of 1 mole of gas is called crucial volume which is represented by Vc. The crucial volume of O2 is 74.42 cm3 mol-1, CO2, is 95.65 cm3 mol-1 and that of H2 is 64.51 cm3 mol-1.

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Methods of Liquefaction of Gases

There are various approaches to melt a gas. One of them is Linde’s method. It is based on the Joule-Thomson effect.

Joule Thomson Effect

Low temperature can be achieved by the Joule-Thomson effect, according to which when compressed gas is allowed to expand into an area of low pressure it gets cooled. The particles of the compressed gas are very near to each other and considerable attractive forces exist amongst them.

When gas is allowed to go through sudden expansion through the nozzle of a jet, then the molecules move apart. In this way, energy is required to get rid of the intermolecular attractions. This energy is taken from the gas itself, which is cooled.

Linde’s Technique of Liquefaction of Gases

Linde has utilized the Joule-Thomson effect as the basis for liquefaction. The device designed for this purpose is shown in the figure.


For the liquefaction of gas, it is compressed to about 200 atmospheres, and after that passed through a water-cooled pipe where the heat of compression is removed. It is then enabled to travel through a spiral pipe having a jet at the end. When the gas comes out of the jet the expansion takes place from 200 atm. to 1 atm. In this way, a considerable fall in temperature takes place.

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This cooled gas increases and cools the inbound compressed air. It returns to the compression pump. This procedure is repeated again and again. The liquid air is gathered at the bottom of the expansion chamber. All gases except H2 and He can be liquefied by the above procedure.

Claude’s process

In this method, compressed air is enabled to do mechanical work of expansion. This work is done at the expense of the kinetic energy of the gas and thus a fall of temperature level is noted. This concept is combined with the Joule-Thomson effect and utilized in Claude’s process of liquefaction of air.


Air is compressed to about 200 atmospheres and is passed through the pipe ABC. At C, a part of the air decreases the spiral towards the jet nozzle J and a part of the air is led into the cylinder D provided with an air-tight piston. Here the air moves the piston outwards and expands in volume as a result of which considerable cooling is produced.

The cooled air passes up the liquefying chamber during which procedure it cools the portion of the incoming compressed air. The precooled incoming compressed air then experiences Joule-Thomson expansion when passed through Jet nozzle J and gets cooled even more. The above process occurs consistently till the air is liquefied.