What are the General Principles of Liquefaction 2022
The conversions of gases into liquids require high pressure and low temperature.
High pressure brings the molecules close to each other.
Low temperature decreases the K.E of molecules. Thus, attractive forces are developed among them, and gas is converted into liquid.
The highest temperature at which a substance can exist as a liquid is called critical temperature. Tc denotes it.
The minimum pressure required to liquefy the gas at the critical temperature is called the Critical Pressure It is denoted by Pc.
The volume occupied by one mole of gas at Tc and Pc is called Critical Volume. Vc denotes it.
Example: For CO₂ Tc = 31.1 ° C
Pc = 72.9 atm,
Vc = 95.65 cm³ mol – ¹
Importance of critical temperature
● Factory workers must know the critical conditions (Te, Pc) for the liquefaction of gases on an industrial scale.
● A gas can be liquefied only below its critical temperature.
Examples: Tc for O₂ is 154.4 K ( -118.75 ° C). Similarly, Te for CO₂ is 31.1 ° C. So, these gases can be liquefied only below their Tc. Hence, these must be cooled below their Te, and pressure is applied to liquefy them.
Effect of Polarizability on critical temperature
● Nonpolar gases have low polarizability and have a very low Tc e.g Tc of Ar is 150.9 K (-122.26°C).
● Polar gases have high polarizability and comparatively high Tc e.g Tc of NH3 is 405.6 K (132.44°C).
● Thus, polar gases are easily converted into liquids.
The value of Tc for gas depends upon its size, shape, and Intermolecular forces. The critical volume of O₂ is 74.42 cm³ mol-¹, of CO₂ is 95.65 cm³ mol-¹ and that of H₂ Is 64.51 cm³mol-¹.
Methods for Liquefaction of Gases
Various methods are used for the liquefaction of gases. These methods are generally based upon the Joule Thomson effect.
Joule Thomson Effect
Joule – Thomson effect When compressed gas is allowed to expand into a region of low pressure, it gets cooled. This is called Joule – Thomson – Effects
Reason: In a compressed gas, molecules are very close and have attractive forces. When gas is expanded suddenly through the nozzle of a jet molecules move away from each other. This process requires energy to overcome attractive forces. This energy is taken from the gas itself, hence it is cooled.
● It is based upon the Joule Thomson effect.
● Lind liquefied air by this process.
● The compressed air (about 200 atm) is passed through water cooled pipe where the heat of compression is removed.
● This compressed air is then passed through a spiral tube having a jet at the end. When the gas comes out of the jet into the low-pressure area (1 atm), it expands and is cooled due to Joule – Thomson effect.
● The cooled air moves up, cools the incoming gas of the Jet, and then again enters into the compression pump, to be compressed again.
● By repeating compression and expansion, again and again, the air is liquefied.
● All gases except Hydrogen, and Helium can be liquefied by this process.