# Acentric Factor (chemicals.acentric)¶

This module contains a lookup function, a definition function, and correlations for a chemical’s acentric factor, normally given the variable $$\omega$$.

A similar variable called the stiel polar factor can be calculated from its definition as well.

For reporting bugs, adding feature requests, or submitting pull requests, please use the GitHub issue tracker.

## Lookup Functions¶

chemicals.acentric.omega(CASRN, method=None)[source]

Retrieve a chemical’s acentric factor, omega.

Automatically select a method to use if no method is provided; returns None if the data is not available.

$\omega \equiv -\log_{10}\left[\lim_{T/T_c=0.7}(P^{sat}/P_c)\right]-1.0$
Parameters
CASRNstr

CASRN [-]

Returns
omegafloat

Acentric factor of compound

Other Parameters
methodstring, optional

The method name to use. Accepted methods are ‘PSRK’, ‘PD’, or ‘YAWS’. All valid values are also held in the variable omega_all_methods.

Notes

A total of four sources are available for this function. They are:

• ‘PSRK’, a compillation of experimental and estimated data published in the Appendix of [2], the fourth revision of the PSRK model.

• ‘PD’, an older compillation of data published in (Passut & Danner, 1973) [3].

• ‘YAWS’, a large compillation of data from a variety of sources; no data points are sourced in the work of [4].

• ‘ACENTRIC_DEFINITION’, the precalculated results using the VaporPressure object of Thermo and the critical properties of chemicals.

References

1

Pitzer, K. S., D. Z. Lippmann, R. F. Curl, C. M. Huggins, and D. E. Petersen: The Volumetric and Thermodynamic Properties of Fluids. II. Compressibility Factor, Vapor Pressure and Entropy of Vaporization. J. Am. Chem. Soc., 77: 3433 (1955).

2

Horstmann, Sven, Anna Jabłoniec, Jörg Krafczyk, Kai Fischer, and Jürgen Gmehling. “PSRK Group Contribution Equation of State: Comprehensive Revision and Extension IV, Including Critical Constants and Α-Function Parameters for 1000 Components.” Fluid Phase Equilibria 227, no. 2 (January 25, 2005): 157-64. doi:10.1016/j.fluid.2004.11.002.

3

Passut, Charles A., and Ronald P. Danner. “Acentric Factor. A Valuable Correlating Parameter for the Properties of Hydrocarbons.” Industrial & Engineering Chemistry Process Design and Development 12, no. 3 (July 1, 1973): 365-68. doi:10.1021/i260047a026.

4

Yaws, Carl L. Thermophysical Properties of Chemicals and Hydrocarbons, Second Edition. Amsterdam Boston: Gulf Professional Publishing, 2014.

Examples

>>> omega(CASRN='64-17-5')
0.635

chemicals.acentric.omega_methods(CASRN)[source]

Return all methods available for obtaining omega for the desired chemical.

Parameters
CASRNstr

CASRN, [-]

Returns
methodslist[str]

Methods which can be used to obtain omega with the given inputs.

chemicals.acentric.omega_all_methods = ('PSRK', 'PD', 'YAWS', 'ACENTRIC_DEFINITION')

Tuple of method name keys. See the omega for the actual references

## Definitions¶

chemicals.acentric.omega_definition(Psat, Pc)[source]

Returns the acentric factor of a fluid according to its fundamental definition using the vapor pressure at a reduced temperature of 0.7Tc.

$\omega \equiv -\log_{10}\left[\lim_{T/T_c=0.7}(P^{sat}/P_c)\right]-1.0$
Parameters
Psatfloat

Vapor pressure of the fluid at a reduced temperature of 0.7 [Pa]

Pcfloat

Critical pressure of the fluid [Pa]

Returns
omegafloat

Acentric factor of the fluid [-]

References

1

Poling, Bruce E. The Properties of Gases and Liquids. 5th edition. New York: McGraw-Hill Professional, 2000.

Examples

Water

>>> omega_definition(999542, 22048320.0)
0.3435744558761711

chemicals.acentric.Stiel_polar_factor(Psat, Pc, omega)[source]

This function handles the calculation of a chemical’s Stiel Polar factor, directly through the definition of Stiel-polar factor. Requires the vapor pressure Psat at a reduced temperature of 0.6, the critical pressure Pc, and the acentric factor omega.

$x = \log_{10} P_r|_{T_r=0.6} + 1.70 \omega + 1.552$
Parameters
Psatfloat

Vapor pressure of fluid at a reduced temperature of 0.6 [Pa]

Pcfloat

Critical pressure of fluid [Pa]

omegafloat

Acentric factor of the fluid [-]

Returns
factorfloat

Stiel polar factor of compound, [-]

Notes

A few points have also been published in [2], which may be used for comparison. Currently this is only used for a surface tension correlation.

References

1

Halm, Roland L., and Leonard I. Stiel. “A Fourth Parameter for the Vapor Pressure and Entropy of Vaporization of Polar Fluids.” AIChE Journal 13, no. 2 (1967): 351-355. doi:10.1002/aic.690130228.

2

D, Kukoljac Miloš, and Grozdanić Dušan K. “New Values of the Polarity Factor.” Journal of the Serbian Chemical Society 65, no. 12 (January 1, 2000). http://www.shd.org.rs/JSCS/Vol65/No12-Pdf/JSCS12-07.pdf

Examples

Calculating the factor for water:

>>> Stiel_polar_factor(Psat=169745, Pc=22048321.0, omega=0.344)
0.02322146744772713


## Correlations¶

chemicals.acentric.LK_omega(Tb, Tc, Pc)[source]

Estimates the acentric factor of a fluid using a correlation in [1].

$\omega = \frac{\ln P_{br}^{sat} - 5.92714 + 6.09648/T_{br} + 1.28862 \ln T_{br} -0.169347T_{br}^6} {15.2518 - 15.6875/T_{br} - 13.4721 \ln T_{br} + 0.43577 T_{br}^6}$
Parameters
Tbfloat

Boiling temperature of the fluid [K]

Tcfloat

Critical temperature of the fluid [K]

Pcfloat

Critical pressure of the fluid [Pa]

Returns
omegafloat

Acentric factor of the fluid [-]

Notes

The units of the above equation are atmosphere and Kelvin; values are converted internally.

References

1

Lee, Byung Ik, and Michael G. Kesler. “A Generalized Thermodynamic Correlation Based on Three-Parameter Corresponding States.” AIChE Journal 21, no. 3 (1975): 510-527. doi:10.1002/aic.690210313.

Examples

Isopropylbenzene, from Reid (1987).

>>> LK_omega(425.6, 631.1, 32.1E5)
0.32544249926397856