The main two process of liquefaction of gases are-
1) By using Freezing Mixture
2) By Joule Thomson Effect
1) By using Freezing Mixture: Temperature of a gas can not be lowered very much by using Freezing mixture. Therefore, only a few gases like NH3, CO2, Cl2 etc could be liquefied by process temperature can be decreased to-20°C by using mixture of common salt and ice, instead of common salt using mixture of CaCl2 and ice temperature can be decreased to -54°C and temperature can be lowered into -110°C by using mixture of solid CO2 and Ether.
2) By Joule Thomson Effect: When a highly compressed gas is allowed to expand through a porous plug into a region of low pressure, so that no work is done against the external pressure, the temperature of gas falls down. This is called Joule Thomson effect of an ideal gas.
During the state maximum intermolecular attraction force is occurred. During expansion the molecules get far away from each other, as a result they need to work against the intermolecular attraction force. The energy for this purpose is taken from the gas itself, so its temperature falls.
German Scientist Lindy invented commercial process of liquefaction of gas taking Joule- Thomson effect as principal. According to the process of scientist Lindy, by compression and expansion of a gas repeatedly it temperature decreases gradually and reach critical temperature, then the gas turns into liquid. This process is at present used commercially to liquefy air.