Lee–Kesler method

The Lee–Kesler method ^[1] allows the estimation of the saturated vapor pressure at a given temperature for all components for which the critical pressure P_c, the critical temperature T_c, and the acentric factor ω are known.

Equations

$\ln P_{r}=f^{{(0)}}+\omega \cdot f^{{(1)}}$

$f^{{(0)}}=5.92714-{\frac {6.09648}{T_{r}}}-1.28862\cdot \ln T_{r}+0.169347\cdot T_{r}^{6}$

$f^{{(1)}}=15.2518-{\frac {15.6875}{T_{r}}}-13.4721\cdot \ln T_{r}+0.43577\cdot T_{r}^{6}$

with

$P_{r}={\frac {P}{P_{c}}}$ (reduced pressure) and $T_{r}={\frac {T}{T_{c}}}$ (reduced temperature).

Typical errors

The prediction error can be up to 10% for polar components and small pressures and the calculated pressure is typically too low. For pressures above 1 bar, that means, above the normal boiling point, the typical errors are below 2%. ^[2]

Example calculation

For benzene with

• T_c = 562.12 K^[3]
• P_c = 4898 kPa^[3]
• T_b = 353.15 K^[4]
• ω = 0.2120^[5]

the following calculation for T=T_b results:

• T_r = 353.15 / 562.12 = 0.628247
• f⁽⁰⁾ = -3.167428
• f⁽¹⁾ = -3.429560
• P_r = exp( f⁽⁰⁾ + ω f⁽¹⁾ ) = 0.020354
• P = P_r * P_c = 99.69 kPa

The correct result would be P = 101.325 kPa, the normal (atmospheric) pressure. The deviation is -1.63 kPa or -1.61 %.

It is important to use the same absolute units for T and T_c as well as for P and P_c. The unit system used (K or R for T) is irrelevant because of the usage of the reduced values T_r and P_r.

References

1. Lee B.I., Kesler M.G., "A Generalized Thermodynamic Correlation Based on Three-Parameter Corresponding States", AIChE J., 21(3), 510-527, 1975
2. Reid R.C., Prausnitz J.M., Poling B.E., "The Properties of Gases & Liquids", 4. Auflage, McGraw-Hill, 1988
3. Brunner E., Thies M.C., Schneider G.M., J.Supercrit.Fluids, 39(2), 160-173, 2006
4. Silva L.M.C., Mattedi S., Gonzalez-Olmos R., Iglesias M., J.Chem.Thermodyn., 38(12), 1725-1736, 2006
5. Dortmund Data Bank

See also

• Vapour pressure of water
• Antoine equation
• Arden Buck equation
• Goff–Gratch equation