Lee–Kesler Method

The Lee–Kesler method

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

:$\ln P_ = f^ + \omega \cdot f^$

:$f^=5.92714 - \frac - 1.28862 \cdot \ln T_ + 0.169347 \cdot T_^6$

:$f^=15.2518 - \frac-13.4721 \cdot \ln T_ + 0.43577 \cdot T_^6$

with

:$P_=\frac$ ( reduced pressure) and $T_=\frac$ ( 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%.

Example calculation

For benzene with
* ''T''_c = 562.12 KBrunner E., Thies M.C., Schneider G.M., ''J.Supercrit. Fluids'', 39(2), 160–173, 2006
* ''P''_c = 4898 kPa
* ''T''_b = 353.15 K
* ''ω'' = 0.2120 Dortmund Data Bank

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.

See also

*Vapour pressure of water
*Antoine equation
*Tetens equation
*Arden Buck equation
*Goff–Gratch equation

References

{{DEFAULTSORT:Lee-Kesler Method
Thermodynamic models