Physics:Acentric factor
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). The acentric factor is also said to be a measure of the non-sphericity (centricity) of molecules.[2]
Pitzer defined ω from the relationship
- [math]\displaystyle{ \omega = - \log_{10} (p^{\rm{sat}}_r) - 1, {\rm \ at \ } T_r = 0.7 }[/math]
where [math]\displaystyle{ p^{\rm{sat}}_r = \frac{p^{\rm{sat}}}{p_c} }[/math] is the reduced saturation vapor pressure and [math]\displaystyle{ T_r = \frac{T}{T_c} }[/math] is the reduced temperature.
For a series of fluids, as the acentric factor increases the vapor curve is "pulled" down, resulting in higher boiling points. For many monatomic fluids, [math]\displaystyle{ p_r^{\rm{sat}}{\rm \ at \ } T_r = 0.7 }[/math] is close to 0.1, which leads to [math]\displaystyle{ \omega \to 0 }[/math]. In many cases, [math]\displaystyle{ T_r = 0.7 }[/math] 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. [math]\displaystyle{ \omega }[/math] 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] |
| Acetone | 0.304[5] |
| Acetylene | 0.187 |
| Ammonia | 0.253 |
| Argon | 0.000 |
| Carbon Dioxide | 0.228 |
| Decane | 0.484 |
| Ethanol | 0.644[5] |
| Helium | -0.390 |
| Hydrogen | -0.220 |
| Krypton | 0.000 |
| Methanol | 0.556[5] |
| Neon | 0.000 |
| Nitrogen | 0.040 |
| Nitrous Oxide | 0.142 |
| Oxygen | 0.022 |
| Xenon | 0.000 |
See also
References
- ↑ Adewumi, Michael. "Acentric Factor and Corresponding States". Pennsylvania State University. https://www.e-education.psu.edu/png520/m8_p3.html.
- ↑ 2.0 2.1 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.
- ↑ 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. https://onlinelibrary.wiley.com/doi/pdf/10.1002/cjce.5450610617. Retrieved 10 August 2022.
- ↑ Yaws, Carl L. (2001). Matheson Gas Data Book. McGraw-Hill.
- ↑ 5.0 5.1 5.2 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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