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.

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