Details

For a reversible process, the change in entropy from the initial state 1 to the final state 2 is zero, so . The Peng–Robinson equation of state is used to calculate the entropy of state :

,

where is entropy (J/[mol K]), the superscript represents an ideal gas, the subscript refers to the reference state, is the entropy departure function for a real gas, is the ideal gas entropy at the reference state and .

,

,

where , , and are heat capacity constants (); is temperature (K); is pressure (MPa); is the ideal gas constant (J/[mol K]), not to be confused with the subscript ; is the critical temperature (K); is a constant and , with as the acentric factor.

,

,

where and are constants, and is the critical pressure (MPa).

The compressibility factor is found by solving for the roots to:

.

Volume () is calculated using the compressibility factor: .

When in vapor-liquid equilibrium, the temperature is the saturation temperature. Antoine's equation is used to calculate the saturation temperature:

,

where , and are Antoine constants.