Production of low temperatures (liquefaction of gases)

Comparison between adiabatic and Joule-Thomson expansion

(i)                 No heat exchanges with the surroundings are allowed in both the process.

(ii)               Joule-Thomson expansion where u+PV  remains constant, is an isenthalpic process, while adiabatic expansion process where entropy remains constant, is an isentropic process.

(iii)             In adiabatic expansion the work done is useful in the sense that it is utilized in overcoming some external forces. In both the cases, however, the required energy is furnished by the kinetic energy of the molecules.

(iv)              Joule-Thomson expansion may produce either a cooling or a heating effect, whereas adiabatic expansion can produce only a cooling effect.

(v)                Joule-Thomson cooling or heating cannot be observed with a perfect gas. But cooling is always resulted by expanding a perfect gas adiabatically.

(vi)              Intermolecular forces being necessarily small, Joule-Thomson cooling is less than that obtainable with adiabatic expansion under similar conditions.

(vii)            An adiabatic expansion is a reversible process, but a throttling process is an irreversible one.

Historical development of liquefaction of gases

Principle of combined cooling and compression was the earliest method of liquefaction of gases applied by Faraday (1823). The apparatus used for the liquefaction of chlorine consists of a bent glass tube, one end of which containing the substance from which chlorine can be liberated, is heated while the other end is immersed in a freezing mixture. Chlorine gas evolving by application of heat liquefies at the cold end when the pressure is sufficiently high. Faraday also succeeded to liquefy ammonia, sulphur dioxide, nitrous oxide, carbon dioxide and cyanogens by this process. But gases like oxygen, nitrogen, hydrogen, carbon monoxide and methane could not be liquefied by this method, even when extremely high pressure as high as 3000 atmospheres were applied. Faraday termed them permanent gases.

The discovery of the critical temperature by Andrews (1863) during his experiments with carbon dioxide clearly established the fact that for every gas there is a temperature, (critical temperature) below which it can be liquefied by application of a suitable pressure, but above this temperature it cannot be liquefied by application of pressure, however high. Prior to Andrews’ discovery the idea of this critical temperature was first suggested by Cagnaird de la Tour in a general way.

Methods employed in the liquefaction of gases

The various methods employed to liquefy gases may be classified under the following three heads.

(1)   Cascade process, which utilizes a series of liquids with successive lower boiling points to reach the low temperature in stages.

(2)   Regenerative Joule-Thomson process based on the Joule-Thomson effect and regenerative cooling.

(3)   Adiabatic expansion process based on the cooling produced when a gas expands adiabatically doing external work.

Liquefaction of helium

The inversion temperature of helium is -238°C (35°A). So to obtain the Joule-Thomson cooling it must be precooled below that temperature. Thus the problem of liquefaction of helium was more intricate than that of hydrogen. In 1908, Kamerlingh Onnes succeeded in liquefying helium by applying a similar method as that used by Dewar in liquefying hydrogen. Instead of using liquid air to precool the gas, in this case liquid hydrogen was used. The process and the plant are similar to that described in connection with the liquefaction of hydrogen. The critical temperature of helium is 5°A (-268°C) and liquid helium boils normally at 4.2°A (-268.8°C).

Properties of liquid helium

Liquid helium shows different behaviors on either side of a transition temperature of 2.186°A called the lambda point. Above this temperature it behaves in a normal fashion and is known as helium I. Below this it behaves as a super fluid and is known as helium II. Helium I contracts but helium II expands on cooling. Helium II possesses a very low viscosity but abnormally high thermal conductivity being 200 times that of copper.

Solidification of helium

Dr. Keeson, in 1926, was able to produce solid helium, by subjecting the gas to a very high pressure of 130 atmospheres, cooling it in liquid helium and then allowing it to be expand. Later experiments showed that helium at 4.2°A solidified under 140 atmospheres and at 3.2°A under 86 atmospheres and at 1.1°A under 23 atmospheres. The solidification point of helium has not yet been definitely established. But it is known to be within 1°A. So helium can hardly be distinguished visually from the liquid helium and hence must have almost the same refractive index as the liquid.

Properties of material at very low temperatures

(1)   The atomic heat of all substances tends to zero of temperature is approached.

(2)   The magnetic susceptibility of paramagnetic salts varies inversely as the absolute temperature in the region of very low temperatures.

(3)   The electrical resistance of metals in most cases vanishes or becomes negligible at very low temperatures. This is known as super-conduction (or supra-conduction). A current once induced in a super-conductor continues undiminished for days together, so long as the temperature is kept low.

(4)   The rate of chemical action is considerably reduced.

(5)   Bacteria and seeds are found to retain their germinative activity unimpaired even after exposure to liquid air temperature, though a moderately high temperature kills them.

(6)   Some materials like cotton, wool, leather, etc. begin to fluoresce after exposure to liquid air temperature.

(7)   Lead becomes plastic at -190°C, while India rubber and glass become brittle.

Uses of liquid air and other liquefied gases

(1)   Production of high vacuum:

High vacuum can be obtained by using liquefied gases. For examples, if a vessel filled with a less volatile gas than air, such as sulfurous acid or water vapor is surrounded by liquid air, all the gas inside becomes solidified and thus high vacuum is produced. If air be present in the vessel, liquid hydrogen may be employed to condense it. If charcoal be put inside the vessel, it will absorb the gas and help producing high vacuum.

       

(2)   Analytical uses of air:

Liquid air is of great use in drying and purifying gases. Water vapor and the less volatile impurities are easily removed by surrounding the gas in question with the liquid air and for this purpose it is now used as a common laboratory reagent.

(3)   Preparation of oxygen and rare gases from liquid air by fractional distillation:

The boiling point of nitrogen is -195.8°C and that of oxygen is -182.9°C. Hence, if liquid air forms an easy method of obtaining these constituents. Air also contains small traces of rare gases, such as helium, neon, argon, krypton and xenon. These are also obtained from liquid air by fractional distillation.

(4)   Calorimetric application:

In order to find the specific heat of substances at low temperatures calorimeters of liquid air, liquid oxygen or liquid hydrogen may be employed just like an ice calorimeter.

(5)   Use of liquefied gases in scientific research:

Properties of materials at low temperatures have been studied and found different from those at ordinary temperatures. Bacteria and seeds retain their activity unimpaired even after exposure to liquid air temperature though a moderately high temperature is fatal.

(6)   Industrial uses of liquefied gases:

Liquid oxygen for industrial uses is also obtained from liquid air. Liquid air and liquid oxygen are stored for use in submarines and hydroplanes for respiration. Liquid oxygen and hydrogen have recently been used as components of fuel for rockets. Liquid oxygen mixed with charcoal is employed for preparing explosives.