Physical Chemistry I CHM-170

LIQUEFACTION OF GASES

 

Liquefaction is the reverse of the process of vaporisation. High pressure and low temperature (less than the critical temperature) are required for liquefaction of gases. The various methods employed for the liquefaction of gases depend on the technique used to attain low temperature. Two such methods are described below.

Linde's Method

 Linde used Joule Thomson effect as the basis for the liquefaction of gases. When a compressed gas is allowed to expand into a region of low pressure, it produces intense cooling, and this phenomenon is called Joule-Thomson effect. In a compressed gas the molecules are very close to each other and attractions between them are appreciable. But when this compressed gas is allowed to escape through a jet into region of low pressure, the molecules move apart. In doing so, energy is needed to overcome the intermolecular attractions. The energy for it is taken from the gas itself which is thereby cooled.

Linde liquefied air by employing this method. The compressed air above 200. atmosphere is passed through a water-cooled pipe where the heat of compression is removed. This cooled and compressed air is then passed through a spiral pipe, with a jet at the end through which the air comes out. The free expansion of air at the jet results in a considerable fall in temperature. The cooled air which is now at about 1 atmosphere pressure passes up cooling the incoming compressed gas of the spiral tube and returns to the compression pump. By repeating the process of compression and expansion, a temperature low enough to liquefy air is reached. The liquefied air collects at the bottom of the expansion chamber.

 

 

The Linde's apparatus for liquefaction of air.

 At room temperature all gases except H₂ and He are found to be cooled by this Joule Thomson expansion. At ordinary temperature, H₂ and He show a negative Joule Thomson effect, since these gases possess a very low Inversion temperature which is defined as the temperature characteristic of a gas below which only the gas cools when allowed to expand. If expansion takes place below the inversion temperature, a fall in temperature takes place while expansion above the inversion temperature produces a rise in temperature. For most of the gases, the inversion temperatures are around the ordinary room temperature hence they get cooled by Joule - Thomson expansion. The inversiontemperatures of H₂ and He are-80°C and-240°C respectively. Below these temperatures, these gases can also be cooled by the Joule Thomson expansion. The inversion temperature may be achieved by adiabatic expansion pr

 Claude's Method

This method for liquefaction of gases is more efficient than that of Linde. Here two effects are utilized for the cooling of gases. One is adiabatic expansion and the other is Joule-Thomson effect. In this method, also the cooling is produced by free expansion of compressed gas. But in addition, the gas is made to do mechanical work by driving an engine. The energy for it comes from the gas itself and as a result it, cools. Thus in Claude's method the gas is cooled not only by overcoming the intermolecular forces but also by performance of work. The apparatus employed by Claude is shown in Fig. 2.19. Compressed gas is passed through a pipe which bifurcates at A. A part of the compressed gas goes into the expansion cylinder where it expands and does work by driving the piston back and as a result it cools. The cooled gas then enters the liquefying chamber and cools the incoming compressed gas which when releases at the jet suffers a further cooling and liquefies. The unliquefied gas goes back to the compressor and the whole process is repeated several times. All known gases can be liquefied by this method.