Generalized Charts for the Enthalpy Departure

As in the case of the compressibility factor, it is often useful to have a visual idea of how generalized properties behave. Fig. 8.7 on page 324 is analogous to the compressibility factor charts from the previous chapter except that the formula for enthalpy is (H – H^ig) = (H – Hig)⁰ + ω(H – H^ig)¹. Note that one primary influence in determining the liquid enthalpy departure is the heat of vaporization. Also, the subcritical isotherms shift to liquid behavior at lower pressures when the saturation pressures are lower. The enthalpy departure function is somewhat simpler than the compressibility factor in that the isotherms do not cross one another. Note that the temperature used to make the departure dimensionless is T_c. A sample calculation for propane at 463.15 K and 2.5 MPa gives H^ig – H = [0.45 + 0.152(0.2)] (8.314) 369.8 = 1480 J/mole compared to 1489.2 from the Peng-Robinson equation.