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 in ...

''T''_c, and the

acentric factor The acentric factor is a conceptual number introduced by Kenneth Pitzer in 1955, proven to be useful in the description of fluids. It has become a standard for the phase characterization of single & pure components, along with other state descript ...

''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 In thermodynamics, the reduced properties of a fluid are a set of state variables scaled by the fluid's state properties at its critical point. These dimensionless thermodynamic coordinates, taken together with a substance's compressibility fact ...

) and

T_=\frac

(

reduced temperature In thermodynamics, the reduced properties of a fluid are a set of state variables scaled by the fluid's state properties at its critical point. These dimensionless thermodynamic coordinates, taken together with a substance's compressibility facto ...

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 is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms, ...

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

Dortmund Data Bank The Dortmund Data Bank (short DDB) is a factual data bank for thermodynamic and thermophysical data. Its main usage is the data supply for process simulation where experimental data are the basis for the design, analysis, synthesis, and optimizati ...

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