PH0101 UNIT 4 LECTURE 8
LIQUEFACTION OF NITROGEN
LIQUEFACTION OF OXYGEN
LIQUEFACTION OF AIR
ADIABATIC DEMAGNETISATION
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CRYOGENICS
Cryogenics is a branch of Physics that deals with
the production and effects of very low temperatures.
In the early history of thermometry, ice was
considered to be the coldest and its temperature was
taken as the lowest temperature.
It was Fahrenheit, who first experimentally
demonstrated that a mixture of ice with common salt
gives a lower temperature of the order of –18oC.
Later, temperatures lower than this temperature could
be attained.
The general principle of production of low
temperature is to remove the heat content from a body.
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LIQUEFACTION OF GASES
For a long time it was thought that air remains in the gaseous state at
all temperatures.
But Andrew’s experiments on CO2 led to the discovery of critical
temperature.
The critical temperature is the temperature below which a gas can be
liquefied by mere application of pressure.
But it cannot be liquefied above the critical temperature, however,
larger may be the applied pressure.
Below the critical temperature, the gas is termed as vapour and above
the critical temperature it is called a gas.
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LIQUEFACTION OF GASES
So, the liquefaction of gases is linked with the
production of low temperatures.
The substances which are gaseous at ordinary
temperatures can be converted into liquid state if
sufficiently cooled and simultaneously subjected to a high
pressure.
There are various methods of liquefaction of gases.
In this section, let us see three methods of liquefaction
of gases, in detail.
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CASCADE PROCESS
The cascade process can be used to produce very low
temperatures.
The basic principle is that when a liquid evaporates at reduced
pressure, it cools.
Evaporation causes cooling, because when a liquid evaporates
it takes up the latent heat either from the liquid itself or from the
surrounding vessel.
Oxygen and Nitrogen can be liquefied by cascade process. In
this case a series of liquids with successively lower boiling point
is employed, so that the desired low temperature is attained.
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LIQUEFACTION OF OXYGEN
In 1878, Pictet liquefied oxygen by cascade process.
Later, H.K.Onnes modified the apparatus
The apparatus consists of compression pumps P1, P2 and
P3. T1, T2 and T3 are the three tubes which are
surrounded by outer jackets A, B and C. ‘D’ is a Dewar’s
flask which collects liquid oxygen.
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LIQUEFACTION OF OXYGEN
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WORKING
Methyl chloride from compressor P1 is pumped into the tube T1 ,
which is surrounded by a jacket A.
Cold water at room temperature is circulated through A. Since the
critical temperature of methyl chloride is 143oC, it is liquefied by
applying suitable pressure.
The liquid methyl chloride trickles into the jacket ‘B’ which is
connected to the suction side of the compressor P1 .
The liquid boils under low pressure and evaporates by absorbing
necessary latent heat from itself and produces cooling.
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WORKING
Thus the temperature of B and T2 is reduced to –90oC.
Methyl chloride vapour is pumped back into P1 and the process
is continued further.
Ethylene from compressor P2 is pumped into the tube T2,
which is surrounded by the jacket B.
While it is passing through T2 which is at –90oC, it is
liquefied because its critical temperature is 10oC.
The liquid ethylene trickles into the jacket C which surrounds
the tube T3.
Evaporation of ethylene under reduced pressure takes place
by absorbing latent heat from itself.
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WORKING
Now the temperature of C and T3 is reduced to –160oC.
The ethylene vapour is sucked and pumped back into P2 and
the process is continued.
Now, oxygen from compressor P3 is pumped into the tube T3 at
high pressure.
Liquid ethylene at –160oC in the jacket C surrounds the tube T3.
Since this temperature is well below the critical temperature of
oxygen (-118oC ) and the pressure is high, oxygen gets liquefied.
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WORKING
The liquid oxygen is collected in the Dewar’s
flask.
Oxygen in the form of gas in the Dewar’s flask
is circulated back to the pump P3 and the
process is repeated.
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LIQUEFACTION OF NITROGEN
The critical temperature of nitrogen is -147oC.
To cool it below this temperature, the same cascade process
is adopted with the addition of a fourth unit (stage) containing
liquid oxygen.
Liquid oxygen has a normal boiling point of -183oC.
It is allowed to boil under reduced pressure and a temperature
of -218oC is reached which cools high pressure nitrogen below
its critical temperature and so liquid nitrogen is obtained.
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LINDE’S PROCESS – LIQUEFACTION OF AIR
Linde in 1896 liquefied air using Joule –
Thomson effect (or Joule – Kelvin effect) and
regenerative cooling technique.
Before going into detail about this process, it is
essential to understand the Joule - Thomson effect
and regenerative cooling technique
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JOULE THOMSON EFFECT
If a gas is allowed to expand through a fine
nozzle or a porous plug, so that it issues from
a region at a higher pressure to a region at a
lower pressure there will be a fall in
temperature of the gas provided the initial
temperature of the gas should be sufficiently
low.
This phenomenon is called Joule – Thomson
effect.
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REGENERATIVE COOLING
The principle of regenerative cooling
consists in cooling the incoming gas by the
gas which has already undergone cooling
due to Joule – Thomson effect.
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CONSTRUCTION AND WORKING
The pump P1 compresses air to a pressure of about 25
atmosphere and is passed through a tube surrounded by a
jacket through which cold water is circulated.
This compressed air is passed through KOH solution to
remove CO2 and water vapour.
This air, free from CO2 and water vapour is compressed to
a pressure of 200 atmospheres by the pump P2.
This air passes through a spiral tube surrounded by a
jacket containing a freezing mixture and the temperature is
reduced to -20oC
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CONSTRUCTION AND WORKING
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CONSTRUCTION AND WORKING
This cooled air at high pressure is allowed to come out of the
nozzle N1.
At N1, Joule – Thomson effect takes place and the incoming air is
cooled to -70oC.
This cooled air is circulated back into the pump P2 and is
compressed.
It passes through the nozzle N1 and is further cooled.
Then it is allowed to pass through the nozzle N2 from high
pressure to low pressure, and is further cooled.
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CONSTRUCTION AND WORKING
Then it is allowed to pass through the nozzle N2 from high
pressure to low pressure, and is further cooled.
As the process continues, after a few cycles, air gets cooled to
a sufficiently low temperature well below its critical temperature
of -140oC and after coming out of the nozzle N2, gets liquefied
and is collected in the Dewar’s Flask. The unliquefied air is again
circulated back to the pump P1 and the process is repeated.
The whole apparatus is packed with cotton wool to avoid any
conduction or radiation.
By applying the principle of Joule – Thomson effect and
regenerative cooling, Hydrogen and Helium can also be liquefied.
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ADIABATIC DEMAGNETIZATION PROCESS
This process is used to reduce the temperature of paramagnet
it salts nearer to ‘0’ K.
We know that the molecular dipole magnetic moments of a
paramagnetic specimen are randomly oriented at thermal
equilibrium.
In this state there is maximum disorderliness of the system
and its entropy is maximum.
By the application of an external field, all the magnetic
dipoles are aligned themselves in a common direction and
hence there is an orderliness of the system.
So, the entropy of the system decreases and there is a
rejection of energy.
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ADIABATIC DEMAGNETIZATION PROCESS
The heat rejected by the specimen when it is magnetised is taken
away by the surroundings and the original thermal equilibrium is
restored.
Therefore the thermal motion of the molecules is unaffected.
If the specimen is now thermally insulated from its surroundings
and the external magnetic field is switched off, (i.e. adiabatically
demagnetised) the magnetic dipoles again get random orientation
in order to reach equilibrium which is a state of maximum
disorder.
Therefore the entropy of the system increases.
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ADIABATIC DEMAGNETIZATION PROCESS
When the entropy increases due to disorderly
orientation of magnetic dipoles, there should be a
corresponding decrease in entropy of disorderly
thermal motion because the total energy in entropy
during an adiabatic process should be zero.
Thus there is a reduction in thermal energy of the
molecules and therefore the temperature of the
specimen falls.
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ADIABATIC DEMAGNETIZATION PROCESS
Usually gadolinium sulphate, which is a paramagnetic
salt is used. It is placed in a tube which is immersed in
liquid helium bath of about 1K and magnetised by the
application of a strong magnetic field.
By insulating the tube from the surrounding bath and
evacuating the tube, the specimen is adiabatically
demagnetised.
Now, the temperature of the specimen is very much
reduced. Temperatures of the order of 0.002K can be
attained by this process.
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