PH 0101 UNIT 4 LECTURE 81 LIQUEFACTION OF NITROGEN LIQUEFACTION OF OXYGEN LIQUEFACTION OF AIR ADIABATIC DEMAGNETISATION

PH 0101 UNIT 4 LECTURE 82 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 –18 o C. 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.

PH 0101 UNIT 4 LECTURE 83 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 CO 2 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.

PH 0101 UNIT 4 LECTURE 84 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.

PH 0101 UNIT 4 LECTURE 85 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.

PH 0101 UNIT 4 LECTURE 86 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.

PH 0101 UNIT 4 LECTURE 87 LIQUEFACTION OF OXYGEN

PH 0101 UNIT 4 LECTURE 88 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 143 o C, 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.

PH 0101 UNIT 4 LECTURE 89 WORKING Thus the temperature of B and T2 is reduced to –90 o C. 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 10 o C. 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.

PH 0101 UNIT 4 LECTURE 810 WORKING Now the temperature of C and T3 is reduced to –160 o C. 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 –160 o C 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.

PH 0101 UNIT 4 LECTURE 811 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.

PH 0101 UNIT 4 LECTURE 812 LIQUEFACTION OF NITROGEN The critical temperature of nitrogen is -147 o C. 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 -183 o C. It is allowed to boil under reduced pressure and a temperature of -218 o C is reached which cools high pressure nitrogen below its critical temperature and so liquid nitrogen is obtained.

PH 0101 UNIT 4 LECTURE 813 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

PH 0101 UNIT 4 LECTURE 814 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.

PH 0101 UNIT 4 LECTURE 815 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.

PH 0101 UNIT 4 LECTURE 816 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 CO 2 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 -20 o C

PH 0101 UNIT 4 LECTURE 817 CONSTRUCTION AND WORKING

PH 0101 UNIT 4 LECTURE 818 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 -70 o C. 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.

PH 0101 UNIT 4 LECTURE 819 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 -140 o C 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.

PH 0101 UNIT 4 LECTURE 820 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.

PH 0101 UNIT 4 LECTURE 821 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.

PH 0101 UNIT 4 LECTURE 822 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.

PH 0101 UNIT 4 LECTURE 823 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.

PH 0101 UNIT 4 LECTURE 824