Saturday, February 4, 2023

Liquefaction of gases and its Methods, Applications, Examples, Principal, Linde-Claude, Co2, Hydrogen

Liquefaction of gases is a very important term, if we compared the gases form and its liquid form. In liquid form, gases have a lot of uses, and we drive our daily routine applications with the help of liquid gases.

In this article, I am going to discuss the topic, Liquefaction of gases and various terms involved in it, such as,

  1. What is the Liquefaction of gases definition?
  2. What are the Methods of liquefaction of gases?
  3. Principle of Liquefaction of gases.
  4. Conditions for Liquefaction of gases.
  5. Application of Liquefaction of gases.
  6. Examples of Liquefaction of gases.
  7. Co2 liquefaction process.
  8. Liquefaction of helium gas, hydrogen, oxygen, nitrogen, air, etc.
  9. Cascade process liquefaction of gases.
  10. Linde process for the Liquefaction of gases.
  11. Claude’s method of liquefaction of gases
  12. Some other gas constants, and terms with definitions.

What is the Liquefaction of gases definition?

Liquefaction of gases means the process into which the gas substances are converted from gases to a liquid state.

Liquefaction of gases is carried out by various methods or processes, and with the help of low temperatures, and by cryogenics techniques. 

What is Cryogenics: It is the branch of science which deals with the effects, production, of very low temperatures.

In this, the researchers study the temperature range near to Zero Kelvin (0K), absolute zero temperature, and study the change of chemical and physical properties of a gas, after phase change (gas to liquid).

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In thermodynamics, the thermodynamics constant, the pressure, volume, and temperature play a major role in the process of liquefication of gases or any such other processes.

Hence, please take a look at the definitions of these constants, with different terms such as Isothermal, Isobaric, Isochoric, adiabatic, etc, before going ahead.

Methods of liquefaction of gases?

The work on liquefaction of gases was starting in the early 1800 by Pioneer, and then many other researchers try different methods and techniques to liquefy the gases.

All the researchers, such as Michael Faraday, Louis Paul Cailletet, Thomas Andrew, William Hampson, Carl Von Linde, Raoul Pierre Pictet, James Dewar, Heike Kamerlingh Onnes, etc.

All these worked on liquefaction of gases with very low-temperature techniques, and by the application of different pressure values.

There are many methods of attaining low temperatures that are required for the liquefaction of gases, such as,

By Coldwater, freezing mixture method,

Joule Thomson effect (Joule-Thomson Porous plug experiment)

Compression of gases below its critical temperature,

By the application of pressure, such as adiabatic expansion, evaporation of liquids, etc.

In these all methods, sometimes gases are to do work against internal energy or forces and sometimes worked against the external forces applied by some means.

Examples of Liquefaction of Gases

Liquid Sulfur Dioxide gas (SO2), Liquid Chlorine gas (Cl2), Ammonia (NH3) in Liquid form, Liquid form of (CO2), air, Oxygen (O2), Hydrogen (H2), Helium (He), etc, etc. All these gases as well as the air can be liquefied by various processes, and methods or the systems that you will learn ahead in this article.

Processes of Liquefaction

Some of the liquefication processes or the methods are,

  1. The Hampson-Linde cycle (The Linde’s Process),
  2. The Cascade method or the Pictet process,
  3. Claude’s Process

Principles of Liquefaction of gases

The principles in these methods or processes, that are

Principle1 In which when a gas is compressed by a sufficient amount of pressure below its critical temperature, as a result liquefaction starts.

Principle2 When we reduce the pressure, and the gas or the liquid is allowed to evaporate, then due to evaporations, it causes cooling.

Principle3 On the basis of the Joule Thomson effect (Porous plug experiment).

Conditions for Liquefaction of gases

The required conditions for liquefaction in terms of Pressure, volume, and temperature are Critical temperature, as well as high or low pressure, inversions temperature, the Isothermal process (which is used in Thomas Andrew’s experiment), and the adiabatic process, etc.

Let us discuss in short, about conditions given above. First, let us discuss Thomas Andrew’s experiment on Co2 gas.

Liquefaction of carbon dioxide, CO2 liquefaction

Liquefaction of gases and its methods
Thomas Andrew’s Experiment on CO2  gas –

Thomas Andrew (1862) performed an experiment on Carbon dioxide at different temperatures. He investigates the change of state of matter from gaseous to liquid form. In this experiment on Carbon dioxide gas, Thomas Andrew investigates the relation among, temperature, pressure, and volume.

He performed this experiment at different constant temperatures, with a change in pressure and, change in volume.

As the temperatures taken by Andrew are constant, then, we know that when the temperature remains constant then the process is an Isothermal process.

Hence the compression in this experiment is an Isothermal compress.

Let’s take the experiment in brief, As we know, Thomas Andrew performs his experiment at different constant temperatures.

Let us take the 21.5°C temperature range, and explain the process of liquefaction of CO2.

At Point A the CO2 is at a gaseous state, Increasing pressure will result in a decrease in the volume of CO2 gas.

At point B the CO2 gas starts liquefied and at point C the CO2 gas is completely condensed or converted into liquid CO2.

At point D, the gas is in a liquid state.

From point C to D the volume change is V2 to V3 and the pressure change is P2 to P3.

If the compression is continued this will result in a sharp rise in pressure, and almost no change in volume because the liquid is incompressible.

At a decreased temperature the CO2 gas curve shows deviation from the ideal gas behavior.

As the temperature increases, the dome shape graph or area became more and more narrow.

But at 30.98°C, the gas curve shows considerable deviations from the ideal gas behavior.

At 30.98°C, the CO2 gas starts liquefied, and more compression results in a sharp rise in the pressure graph.

Also, 30.98°C is the Critical temperature of CO2 gas.

Inside the dom shape area, an equilibrium state between two states of matter is seen.

Thomas Andrew concluded that below Critical temperature gas can be liquefied only by the application of pressure alone.

But, above the critical temperature, the gas cannot be liquefied, however, high pressure may be applied.

Let’s discuss these Critical constants in brief.

What is Critical temperature?

Critical Temperature (Tc): The critical temperature is the temperature at which a gas changes into a liquid.


It is the highest temperature at which gas appears in the form of liquid.


The Critical temperature is the temperature below which gas can be liquified and above which gas cannot be liquified, however, high pressure may be applied.

Also, the Critical temperature is correlated with Intermolecular forces of attraction.

Strong is the Intermolecular force of attraction More is the Critical temperature value and easier is the liquefaction of the gas.

For example,

Ammonia (NH3), has a Critical temperature value (Tc) = 132.4°C    OR      405.5 K (Kelvin)

Ammonia has strong Intermolecular forces of attraction, hence can be easily liquified by applying sufficient pressure.

The critical temperature of some gases are given below

Gases Names Critical Temperature (Tc) in °C
Sulfur Dioxide (SO2) 157.3 °C
Dichlorine or Chlorine gas (Cl2) 144.2 °C
Ammonia (NH3) 132.4 °C
Hydrogen chloride (HCl) 51.7 °C
Carbon Dioxide (CO2) 30.98 °C
Methane Gas (CH4) −82.4 °C
Oxygen (O2) −118.8 °C
Nitrogen (N2) −146 °C
Hydrogen (H2) −240 °C
Helium (He) −267.8 °C

The Intermolecular force of attraction order from strong to weak (Given Below)


In the Above order, the SO2 has the highest Critical temperature value.

He has the lowest Critical temperature value.

Hence SO2 can be liquefied easily as compared to He.

What is the difference between a vapor and a gas?

When gas is above its Critical temperature, it is a gas.

When gas is below its Critical temperature, it is termed as a vapor.

What is Critical Pressure?

Critical Pressure (Pc):  The minimum pressure required to liquefy a gas at Critical temperature is called Critical Pressure.

For example, at 30.98 °C Carbon dioxide gas converts into liquid at a pressure of 73 atm.

Here 30.98 °C is the Critical temperature of CO2 and 73 atm is the critical pressure of CO2.

What is Critical volume?

Critical Volume (Vc): The volume of one mole of gas at Critical temperature and Critical pressure is called the Critical Volume.

Critical Constants in terms of  Van der Waals Constants. 

The Critical Temperature (Tc)

Liquefaction of gases and its methods

The Critical Pressure (Pc)

Liquefaction of gases and its methods

The Critical Volume (Vc)

Liquefaction of gases and its methods

Here, In the above expressions the R = Gas Constant, and a, b are the Van der Waals Constant.

Liquefaction of gases and its methods

Liquefaction of gases by the Freezing mixture method. 

If we mixed some suitable salt with the Ice, then a freezing mixture formed. The temperature of this freezing mixture is lower than the temperature of Ice (0°C).

If we continued this process the temperature decreasing rapidly until the solution becomes completely saturated.

Now the solution is in equilibrium with the ice and the salt.

If we pass a gas inside this mixture by some applications, the outcome gas will be cool and further used for liquefaction of gases.


We can say that by this method, we have a low temperature, and we can use this to liquefied a gas further. Some gases need pre-cooling before starting the liquefaction process. For example, Helium (He), Hydrogen (H).

Liquefaction of gases when work is done by the gas by its internal energy.

Whenever the gas is allowed to work against its internal forces by some means, such as Adiabatic expansion, Joule Thomas effect, etc.

Then, as the work done by the gas is against internal forces, gas molecules lose energy and resulting in reduced gas temperature.

During Adiabatic expansion, by the use of internal energy against external pressure, a loss of internal energy by the gas molecules, resulting in the reduced temperature of the gas. This reduced temperature is further used to liquefy the gas. 

As we know that an adiabatic process is a process, where the net quantity of heat contained in a system remains constant, which means neither the heat enters the system nor leaves the system.

What is the Joule-Thomson effect?

According to the Joule Thomson effect when a gas is allowed to pass through a porous plug (or through a small nozzle) from a high-pressure region to low pressure region, it undergoes a change in temperature.

At normal temperatures, most of the gases show a cooling effect, except Hydrogen and Helium. At normal temperature, H2 and He show a heating effect.

Also, there is a term named inversion temperature, which affects the Joule Thomson effect.

What is the inversion temperature?

It is that certain temperature (or some fixed value of temperature) below which when a gas expands it always shows a decrease in temperature of the gas, and above which, increase in temperature of the gas.


Inversion temperature is the temperature at which the Joule Thomson effect is Zero.

It means at a temperature of inversion, neither cooling nor heating of the gas.

The inversion temperature of Hydrogen (H2) and Helium (He)?

Inversion temperature of Hydrogen (H2) = (around) −73°C

Inversion temperature of Helium (He) = (around) −249°C

If we pass either Hydrogen (H2) or Helium (He) through a porous plug experiment above their inversion temperature, then instead of cooling, heating will be produced.

That’s why these gases are required pre-cooling before starting their liquefaction process.

The Inversion temperature of different gases is different.

Let us discuss liquefaction processes in brief.

The Pictet or Cascade Process of liquefaction of Oxygen gas. 

This process was first used by Pictet in 1878. He successfully obtained a small quantity of Liquid Oxygen with the help of pressure applied, and with other liquefied gases.

Cascade system or Process: A process is called the Cascade process, When a single stage is not enough to produce the desired result, therefore the process takes place in a number of stages in a sequence.

Cascade system for Liquefaction of Oxygen Gas


Cascade Liquefier or Apparatus for Liquefaction of Oxygen Gas.

Liquefaction of gases and its methods, applications, examples, Principal, Linde process, co2, helium, oxygen, Hydrogen, critical temp, pressure, volume, complete
A Cascade Liquefier –

As you can see in the above figure that, before getting liquid oxygen many stages of liquefaction are used. That’s why we called it a cascade system or a Cascade liquefier, which is used to liquefy Oxygen or air.

As you know this process is first used by Pictet after sometime K Onnes (Kamerlingh Onnes) used this apparatus.

About the Apparatus 

  1. In this apparatus, three compressors C1, C2, C3 are used to fulfill the requirement of sufficient pressure. Also, the C1, C2, and C3 have a suction side which is used during the process.
  2. Three condensers R1, R2, R3 are used, into which three refrigerants cold water, Methyl chloride, and ethylene are used to get the desired result.
  3. The Liquid oxygen is collected in the last, into a Dewar flask.

Principles This apparatus work on two principles.

  1. The first, Principle, compression of gases below its critical temperature resulting in a change to liquid.
  2. The second is, producing cooling by the principle of evaporation of liquids.

How does it work?

First, the gaseous methyl chloride (CH3Cl) is pumped by the compressor C1 into the spiral tube. The refrigerant in condenser R1 surrounding this tube starts liquefying the methyl chloride.

This is because the critical temperature of methyl chloride is 143°C, which is more than room temperature as well.

Now the liquid methyl chloride comes in Condensor R2 through the tube. Here one portion of condenser R2 is connected with the suction side of compressor C1.

Here due to the evaporation of liquid methyl chloride in reduced pressure, more cooling as a result produced, and the temperature of condenser R2 decreases more.

The evaporated methyl chloride return back to the compressor C1 through the suction side of the compressor.

Now the gaseous ethylene (C2H4) is pumped by the compressor C2 into the next spiral tube.

Here the refrigerant, liquid methyl chloride which is achieved in the previous stage, surrounding the tube which contains gaseous ethylene, starts to convert this gas into liquid ethylene.

This is because the critical temperature of ethylene is around 9.2°C.

Now, this liquid ethylene comes in Condensor R3, and one portion of R3 condenser is connected with the suction side of compressor C2.

Here evaporation of liquid ethylene takes place in reduced pressure like in the previous stage, and the evaporated ethylene return back to the compressor C2 through the suction side of the compressor.

Therefore, due to the evaporation process more cooling is produced into the condenser R3, which is more than the cooling that we achieved in Condenser R2.

This cooling has a temperature of around −160°C.

Now, the oxygen (which is in gaseous form) is pumped by the compressor C3 into the next spiral tube.

Here, due to the very low temperature inside the Condenser R3 the oxygen gas into the spiral tube starts converting into liquid and later collected into a Dewar flask.

This is because the critical temperature of oxygen gas is around −118°C.

Here, likewise the previous stages, the evaporated oxygen return back to the compressor C3 through the suction side of the compressor.

If we continue this cascade system, we can liquefy air and other gases like Nitrogen, etc.

Note: But by this system, we cannot liquefy the gases that have very low critical temperatures, such as Hydrogen (Tc around −240 °C) and Helium (Tc around −267.8 °C).

Let us talk about the other two very important processes of liquefaction of gases,

  1. The Hampson-Linde cycle or commonly named as the Linde’s Process of Liquefaction.
  2. Claude’s Process.

Linde’s Method of liquefaction of gases.

The Hampson-Linde cycle or the Linde’s liquefaction process is used by coupled with regenerative cooling and the Joule Thomson effect.

By this method, we can easily liquefy air, and many other gases too.

Liquefaction of gases and its methods, applications, examples, Principal, Linde process, co2, helium, oxygen, Hydrogen, critical temp, pressure, volume, complete
Linde’s Method of Liquefaction of Gases –

The above figure is Linde’s method for Liquefaction of Air and some other gases too.

By this figure, you can understand that liquefaction of air or those gases that have a low value of critical temperatures is hard, as compared to those that have high critical temperature values.

About this apparatus

  1. In this method, two compressors C1 at (25 atm pressure) and C2 (200 atm pressure) are used.
  2. Heat exchangers R1 and R2 are used into which cold water and a freezing mixture is used as a refrigerant.
  3. A Liquid solution of KOH (Potassium Hydroxide), that is required to get pure air.
  4. Two chambers E1 and E2, and P1 and P2 are the two small nozzles.
  5. At last, the liquid air is collected into a Dewar flask.


Linde’s process of liquefaction is work on the principle of the Joule Thomson effect coupled with regenerative cooling.

Linde’s process Working

This method is quite different as we compared to the previous one, the Cascade method.

First, the air is pumped at a pressure of 25 atm into the spiral tube. The air gets cooled after passing through the R1 heat exchangers. Here the gas becomes cool because of cool water inside the R1 heat exchangers. This cooled air then passes through a liquid solution of Potassium hydroxide (KOH).

The reason for the use of the KOH solution is that air contains many gases and water vapors too. To separate air from water vapors this solution is used, and also this solution absorbs CO2 gas from the air (The Critical temperature of water = 374°C). After this, the air further moves in the second compressor C2.

In the C2 compressor, the air is pumped at a pressure of 200 atm into the next spiral tube. Now the gas becomes cool again, after passing through the second heat exchanger R2. Here the gas-cooled because of the Freezing mixture inside the R2 heat exchangers.

Now the temperature of this air decreases to around −20°C. Then this pre-cooled air is allowed to expand through nozzle P1 in a chamber E1 and suffers the Joule Thomson effect. Due to this effect, more cooling is produced into the chamber E1, and pressure reduces to about 50 atm.

This cooled air then returns back to the compressor C2 and where it is again pumped at a pressure of 200 atm into the spiral tube. This air again suffers the Joule Thomson effect, and more cooling is produced in chamber E1.

Repeating some cycles of this process, more and more cooling is produced in chamber E1. After getting sufficient temperature, the cooled air is allowed to expand through nozzle P2 in chamber E2 and again suffers the Joule Thomson effect, and pressure reduces to about 1 atm.

Now the temperature decreases to around −188°C in chamber E2 and the air gets liquefied. This liquefied air is collected into the Dewar flask.

Also, in chamber E2 the un-liquefied air is returned back to the compressor C1, this further cooled the air, and where it is again pumped at a pressure of 25 atm into the spiral tube. 

This is the overall Linde’s process for liquefaction of air.

Claude’s method of liquefaction of gases

Claude’s process works on the same principle as Linde’s process. Hence cooling of the air, or if we say liquefaction of gases is carried out by the help of the Joule Thomson effect.

But, the only difference between Linde Claude’s process of liquefaction of air, or other gases is that in Claude’s process there is an isentropic expansion.

That’s why Claude’s process is more efficient than Linde’s process.

The principle used in Claude’s Process

Claude’s method works on two principles.

First, the Joule Thomson effect.

The second is a mechanical expansion (By, the use of an expansion turbine).

What is an expansion turbine or the turboexpander?

The expansion turbine or the turboexpander is an axial-flow or centrifugal turbine, through which a high pressurized gas is allowed to expand to produce work. This work is used to rotate a shaft, which is often connected with a compressor or generator.

Due to the turbo-expander, the outcoming gas has a very low temperature as compared to the temperature of input gas. This is because, in this process, the work is done by the gas, and due to this the gas loses its kinetic energy and resulting in a decrease in temperature of the gas.

Working of Claude’s process

As you know Claude’s process is modified Linde’s process, Therefore, like Linde’s process, the gas which is at 200 atm pressure is pumped into the spiral tube, the gas then moved further. In Claude’s process, this gas is divided into two sections. In the first section, the gas is allowed to expand through the expansion turbine (turbo-expander). In the second section, the gas is allowed to suffers the Joule Thomson effect.

Therefore, more cooling is produced inside the chamber. One is by turbo-expander, and the second is by the Joule Thomson effect. The overall process is repeated until the gas gets liquefied completely, and during each cycle of repetition, the un-liquefied gas is returned back to the Compressors.

The name of Claude’s process is based on the name of a French engineer and inventor Georges Claude. He was born on 24 Sept 1870 in Paris ( France) and died on 23 May 1960 in Saint-cloud (France).

The very low critical temperature of H2, and He

Now I will discuss the very low values of critical temperatures for gases like Neon, Hydrogen, and Helium gas.

The Critical temperature (TC) values of these gases are

Neon (Ne)       = −228.7°C

Hydrogen (H2) = −240°C

Helium (He)    = −267.8°C

For liquefying these, we need a very low-temperature range. The hydrogen and helium must be kept below their inversion temperature while suffers the Joule Thomson effect.

The principle used in Hydrogen and Helium’s liquefaction.

Liquefaction of Hydrogen and helium works on the principle of the Joule-Thomson effect coupled with regenerative cooling. 

In the liquefaction of Hydrogen, liquid air is used as a refrigerant, and in the liquefaction of Helium, Liquid hydrogen is used as a refrigerant.

By the use of previous processes, we can get liquefied Hydrogen and helium too. The Douch physicist Heike Kamerlingh Onnes was the first who liquefied Helium.

Applications of liquefaction of gases

Liquefied gases and liquefaction processes are used in a variety of works in many fields such as medical, industrial, scientific, etc.

  1. Preservations of Biosamples, for example, freezing the semen where liquid nitrogen is used, liquid oxygen that is available in hospitals, used to give a better breath to the patients who have breathing problems. The combination of liquid oxygen and liquid nitrogen can be used in aqualung devices.
  2. Liquefaction of gases is used in refrigeration systems. Liquid ammonia is used in ice plants for cooling (Based on the principle of evaporation of the liquids).
  3. Liquefied gas is used for commercial purposes such as home fuel, LPG (Liquid petroleum gas).
  4. The liquid Oxygen and the Hydrogen that is produced by the process of liquefaction of gases are used in the Rocket propellant.
  5. For easy storage of gases, and as well as easy transportation of gases. For example, In air conditioning systems, where stored liquid gas is used as a refrigerant, like R-290, R-600A.
  6. Liquid acetylene and liquid oxygen can be used for welding purposes in the Industrial area.

Question For You?

According to the above-given Liquefaction methods can you tell me, which method of air liquefaction is more efficient? Give your answer in the Comment Box.

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Some of the Info taken from