The enthalpy of a system is a function of the internal energy, pressure and volume of the system, that is:
H = U + pV
where: H = enthalpy
U = internal energy
p = pressure
V = volume
Enthalpy is a state function, i.e., its value is the same for a given set of system conditions regardless of the path taken to reach the set of conditions. Enthalpy is evaluated relative to a reference state, usually the ideal gas state.
The liquid enthalpy is the enthalpy of the liquid portion of a system and is usually determined by expressions derived from an equation of state. PRO/II also has enthalpy values for saturated pure liquids as a functions of temperature only.
Available methods for calculating liquid enthalpy are listed below. See "Enthalpy" under "Basic Principles" in PRO/II Reference Manual, Volume 1, Chapter 2 for more information on enthalpies.
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