The acentric factor ω is a conceptual number introduced by Kenneth Pitzer in 1955, proven to be useful in the description of fluids.[1] It has become a standard for the phase characterization of single & pure components, along with other state description parameters such as molecular weight, critical temperature, critical pressure, and critical volume (or critical compressibility).

Pitzer defined ω from the relationship

where is the reduced saturation vapor pressure and is the reduced temperature.

The acentric factor is said to be a measure of the non-sphericity (centricity) of molecules.[2] As it increases, the vapor curve is "pulled" down, resulting in higher boiling points. For many monatomic fluids, is close to 0.1, which leads to . In many cases, lies above the boiling temperature of liquids at atmosphere pressure.

Values of ω can be determined for any fluid from accurate experimental vapor pressure data. The definition of ω gives values which are close to zero for the noble gases argon, krypton, and xenon. is also very close to zero for molecules which are nearly spherical.[2] Values of ω ≤ -1 correspond to vapor pressures above the critical pressure, and are non-physical.

The acentric factor can be predicted analytically from some equations of state. For example, it can be easily shown from the above definition that a van der Waals fluid has an acentric factor of about −0.302024, which if applied to a real system would indicate a small, ultra-spherical molecule.[3]

Values of some common gases

Molecule Acentric Factor[4]
Acetone0.304[5]
Acetylene0.187
Ammonia0.253
Argon0.000
Carbon Dioxide0.228
Decane0.484
Ethanol0.644[5]
Helium-0.390
Hydrogen-0.220
Krypton0.000
Methanol0.556[5]
Neon0.000
Nitrogen0.040
Nitrous Oxide0.142
Oxygen0.022
Xenon0.000

See also

References

  1. Adewumi, Michael. "Acentric Factor and Corresponding States". Pennsylvania State University. Retrieved 2013-11-06.
  2. 1 2 Saville, G. (2006). "ACENTRIC FACTOR". A-to-Z Guide to Thermodynamics, Heat and Mass Transfer, and Fluids Engineering. doi:10.1615/AtoZ.a.acentric_factor.
  3. Shamsundar, N.; Lienhard, J.H. (December 1983). "Saturation and metastable properties of the van der waals fluid". Canadian Journal of Chemical Engineering. 61 (6): 876–880. doi:10.1002/cjce.5450610617. Retrieved 10 August 2022.
  4. Yaws, Carl L. (2001). Matheson Gas Data Book. McGraw-Hill.
  5. 1 2 3 Reid, R.C.; Prausnitz, J.M.; Poling, B.E. (1987). The Properties of Gases and Liquids (4th ed.). McGraw-Hill. ISBN 0070517991.


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