Harman Patil (Editor)

Lee–Kesler method

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The Lee–Kesler method allows the estimation of the saturated vapor pressure at a given temperature for all components for which the critical pressure Pc, the critical temperature Tc, and the acentric factor ω are known.

Contents

Equations

ln ⁡ P r = f ( 0 ) + ω ⋅ f ( 1 )

f ( 0 ) = 5.92714 − 6.09648 T r − 1.28862 ⋅ ln ⁡ T r + 0.169347 ⋅ T r 6

f ( 1 ) = 15.2518 − 15.6875 T r − 13.4721 ⋅ ln ⁡ T r + 0.43577 ⋅ T r 6

with

P r = P P c (reduced pressure) and T r = 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%.

Example calculation

For benzene with

  • Tc = 562.12 K
  • Pc = 4898 kPa
  • Tb = 353.15 K
  • ω = 0.2120

    • the following calculation for T=Tb results:

  • Tr = 353.15 / 562.12 = 0.628247
  • f(0) = -3.167428
  • f(1) = -3.429560
  • Pr = exp( f(0) + ω f(1) ) = 0.020354
  • P = Pr * Pc = 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 Tc as well as for P and Pc. The unit system used (K or R for T) is irrelevant because of the usage of the reduced values Tr and Pr.

    References

    Lee–Kesler method Wikipedia
    (Text) CC BY-SA