The Lee–Kesler method allows the estimation of the

saturated vapor pressure Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phase ...

at a given temperature for all components for which the

critical pressure In thermodynamics, a critical point (or critical state) is the end point of a phase equilibrium curve. The most prominent example is the liquid–vapor critical point, the end point of the pressure–temperature curve that designates conditions ...

''P''_c, the

critical temperature Critical or Critically may refer to: *Critical, or critical but stable, medical states **Critical, or intensive care medicine * Critical juncture, a discontinuous change studied in the social sciences. * Critical Software, a company specializing i ...

''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

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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

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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.

References

{{DEFAULTSORT:Lee-Kesler Method Thermodynamic models

Equations

Typical errors

Example calculation

References

See also