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

Acentric factor

The acentric factor \omega is a conceptual number introduced by Pitzer in 1955, proven to be very useful in the description of matter. It has become a standard for the phase characterization of single & pure components...

 ω are known.

Equations

with

(reduced pressure) und (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

Benzene

Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....

with

• T_c = 562.12 K
• P_c = 4898 kPa
• T_b = 353.15 K
• ω = 0.2120

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.