Lee-Kesler method
The Lee - Kesler method allows for any material the saturation vapor pressure Ps at the temperature T by means of the critical pressure Pc, the critical temperature Tc and the acentric factor ω of the substance according to the following equation to estimate:
With
(reduced pressure) and (reduced temperature)
The error may be up to 10 % of polar substances and low pressures, generally, the calculated pressure is too low. At pressures above 1 bar, ie above the boiling point of the error is less than 2%.
Example calculation
For benzene obtained with
- Tc = 562.12 K
- Tb = 353.15 K
- ω = 0.2120
Following calculation for T = Tb:
- Tr = 353.15 / 562.12 = 0.628247
- F ( 0) = -3.167428
- F ( 1) = -3.429560
- Pr = exp ( f (0) ω f (1) ) = 0.020354
It would be correct P = 101.325 kPa, normal pressure. The deviation is thus -1.63 kPa or -1.61 %.
It is important to ensure that T and T c and P and Pc are respectively used in the same units; which unit is, however, is by the use of the reduced size Tr and Pr irrelevant. A limitation of this statement is that an absolute temperature unit must be used, ie about Kelvin and Rankine, but not degrees Fahrenheit, degrees Celsius or degrees Reaumur.