The essence of all liquefaction process is to cool the gas until it enters the two phase region. This cooling may be accomplished in several ways.
1. Cooling at constant pressure, as in a heat exchanger.
2. Cooling by expansion in an engine from which work is obtained.
3. Cooling by an expansion valve or throattling process.
The first method requires a heat sink at a lower temperature than that to which the gas to be cooled. An external refrigeration system is required if the gas temperature is to be reduced below normal surrounding temperature. Accordingly, the first method is normally used only as a means of precooling the gas.
The three method are illustrated in figure below this. The constan pressure path (1) approaches the two phase region (and liquefaction) most closely for a given drop in temperature. The isenthalpic expansion(3) will not result in liquefaction unless the initial state is at high enough pressure and low enough temperature for the constant enthalpy line to cut into two phase region.
