Liquefaction of gases

 liquefaction of gases

  • For liquefaction of gases having fairly high critical temperature e.g. ammonia, chlorine, sulphur dioxide and carbon dioxide, the application of a suitable pressure alone is sufficient.
  • For liquefaction of permanent gases which have very low critical temperature e.g. hydrogen, oxygen, nitrogen, helium etc. application of pressure alone will not bring liquefaction but a combination of cooling, pressure and even expansion is to be applied.
    i.e., these gases can be liquefied if they are first cool below their respective critical temperatures and then compressed.
  • It is necessary to cool a gas below its critical temperature before it can be liquefied.
  • As the value of \(T_c\) of permanent gases are very low.
     to attain these low temperatures, two general principles are used :
    1. The Linde’s Method based on Joule Thomson effect to cause cooling
    2. The Claude’s method based on cooling due to work done by the gas in adiabatic expansion.       
Faraday’s Method
Liquefaction of gases by Faraday's Method

• In 1823, Faraday used V-shaped tube sealed at both ends.
At the one end, the gas was prepared and at the other end, the gas was liquefied by the use of freezing mixture.
• Faraday used freezing mixture of ice with various salts for external cooling of gases.

Note:-
•The melting of ice and dissolution of salts both are endothermic process.
•The gases liquefied by this method had their critical temperature above or just below the ordinary temperature.
•The gases like ammonia, chlorine, sulphur dioxide, hydrogen sulphide, hydrogen chloride and carbon dioxide could be easily liquefied by using freezing mixtures.
•While permanent gases like nitrogen, hydrogen, helium, oxygen etc. could not be liquefied under these conditions because these gases has much low critical temperature.

Linde’s Process

In this method, cooling is caused by Joule Thomson effect.
According to Joule Thomson effect, when a gas under high pressure is allowed to expand adiabatically through a fine hole into a region of low pressure, it is accompanied by cooling (i.e., the gas suffers a fall in temperature).
As the intermolecular forces of attractive exist between the gas molecules which hold the molecules together.
As the gas expand, the molecules fall apart from one another.
Therefore, work has to be done in order to overcome the cohesive or attractive forces to separate the molecules.
The work is done by the system at the expense of the kinetic energy of gaseous molecules.
As a result, the kinetic energy decreases.
As we know, K.E∝ Temperature 
Cooling(fall in temperature) results.

Note that, In this case no external work has been done by the gas in expansion.

It was observed from experiment that gases show cooling during the Joule-Thomson expansion only if they are below a certain temperature known as the Inversion temperature \(T_i\) .

The inversion temperature is the characteristic of each gas.

The inversion temperature is related to the van der Walls constants a and b by the following expression:
\(T_i\,=\,\frac{2a}{Rb}\)

Note:
• At inversion temperature, if a gas under pressure passes through a porous plug and expands adiabatically into a region of low pressure, then there is neither fall nor rise in temperature.
• At inversion temperature \(\Rightarrow\) no Joule Thomson effect.
• Above Inversion temperature \(\Rightarrow\) small rise of temperature
• Below Inversion temperature \(\Rightarrow\) small fall of temperature

Most of the gases get cooled in the Joule-Thomson expansion because the inversion temperature lies within the range of ordinary temperatures in that gases.
At ordinary temperatures, H and He show rise in temperature(warmed up) instead of cooling in Joule Thomson expansion.
 they have very low inversion temperature (-80°C & -240°C respectively)
But if these gases are first cooled below their respective inversion temperatures, then these gases also show cooling on expansion in accordance with the Joule-Thomson effect.

Procedure :

Liquefaction of gases by Linde's Process

The air to be liquefied is compressed to a pressure of about 200 atm by means of a compressor P
During compression, heat is produced which is removed by passing the gas through water cooled coil C refrigerated through (H20 or liquid NH3).The water vapour present in the air condenses and is separated.
The dried air is passed through a copper spiral coil S , from which it is allowed to expand suddenly through jet J to almost atmospheric pressure.
The pressure of the gas falls to about 50 atm and the temperature falls.
The cooled air rises up in the chamber D and cools further the incoming compressed air.
This somewhat cooler gas is again sent to compressor C and the process is repeated several times till the temperature is low enough so that the gas is converted into liquid.
The liquid can be collected from the tap at the bottom of the chamber.

Claude’s Process

Claude’s method is more efficient than Linde’s method for the liquefaction of gases.
In this method, a combination of Joule-Thomson effect and adiabatic cooling is used.
The gas is made to expand against a pressure instead of being allowed to expand freely.
The external work is done by the gas at the expense of its kinetic energy which decreases.Hence there is a fall of temperature (cooling results).
The work thus gained is used to operate the compressors.
Procedure:-

  • The air free from CO2 and water vapours(purified air) is first compressed to about 200 atm pressure by the compressor.
    The compressed air/gas is cooled and then passed through the pipe ABC viz., divided into two parts at C.
  • The compressed gas partly enters tube D and then enters into a cylinder fitted with piston P.
  • The gas enters the cylinder and pushes the piston and is made to do some external work against the piston.
    As a result, the K.E. of air falls and hence cooling occurs.
    Some part of the air is expanded through the spiral coil S into jet J and gets cooled by Joule-Thomson effect.
    The cooled air is sent back into the compressor and the process is continuously repeated till the temperature is low enough so that the air converted into liquid.
    The liquid air can be collected from the tap at the bottom of the chamber.
Cooling by Adiabatic Demagnetisation

cooling by adiabatic demagnetization

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