出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/04/30 20:08:52」(JST)
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The classical Carnot heat engine
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Branches
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Laws
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Systems
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System properties
Note: Conjugate variables in italics
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Material properties
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Equations
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Potentials
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Scientists
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Real gases – as opposed to a perfect or ideal gas – exhibit properties that cannot be explained entirely using the ideal gas law. To understand the behaviour of real gases, the following must be taken into account:
For most applications, such a detailed analysis is unnecessary, and the ideal gas approximation can be used with reasonable accuracy. On the other hand, real-gas models have to be used near the condensation point of gases, near critical points, at very high pressures, to explain the Joule–Thomson effect and in other less usual cases.
Isotherms of real gas
Dark blue curves – isotherms below the critical temperature. Green sections – metastable states.
The section to the left of point F – normal liquid.
Point F – boiling point.
Line FG – equilibrium of liquid and gaseous phases.
Section FA – superheated liquid.
Section F′A – stretched liquid (p<0).
Section AC – analytic continuation of isotherm, physically impossible.
Section CG – supercooled vapor.
Point G – dew point.
The plot to the right of point G – normal gas.
Areas FAB and GCB are equal.
Red curve – Critical isotherm.
Point K – critical point.
Real gases are often modeled by taking into account their molar weight and molar volume
Where P is the pressure, T is the temperature, R the ideal gas constant, and Vm the molar volume. a and b are parameters that are determined empirically for each gas, but are sometimes estimated from their critical temperature (Tc) and critical pressure (Pc) using these relations:
The Redlich–Kwong equation is another two-parameter equation that is used to model real gases. It is almost always more accurate than the van der Waals equation, and often more accurate than some equations with more than two parameters. The equation is
where a and b two empirical parameters that are not the same parameters as in the van der Waals equation. These parameters can be determined:
The Berthelot equation (named after D. Berthelot[1] is very rarely used,
but the modified version is somewhat more accurate
This model (named after C. Dieterici[2]) fell out of usage in recent years
.
The Clausius equation (named after Rudolf Clausius) is a very simple three-parameter equation used to model gases.
where
where Vc is critical volume.
The Virial equation derives from a perturbative treatment of statistical mechanics.
or alternatively
where A, B, C, A′, B′, and C′ are temperature dependent constants.
Peng–Robinson equation of state (named after D.-Y. Peng and D. B. Robinson[3]) has the interesting property being useful in modeling some liquids as well as real gases.
The Wohl equation (named after A. Wohl[4]) is formulated in terms of critical values, making it useful when real gas constants are not available.
where
.
[5]
This equation is based on five experimentally determined constants. It is expressed as
where
This equation is known to be reasonably accurate for densities up to about 0.8 ρcr, where ρcr is the density of the substance at its critical point. The constants appearing in the above equation are available in following table when P is in KPa, v is in , T is in K and R=8.314[6]
| Gas | A0 | a | B0 | b | c |
|---|---|---|---|---|---|
| Air | 131.8441 | 0.01931 | 0.04611 | -0.001101 | 4.34×10^4 |
| Argon, Ar | 130.7802 | 0.02328 | 0.03931 | 0.0 | 5.99×10^4 |
| Carbon Dioxide, CO2 | 507.2836 | 0.07132 | 0.10476 | 0.07235 | 6.60×10^5 |
| Helium, He | 2.1886 | 0.05984 | 0.01400 | 0.0 | 40 |
| Hydrogen, H2 | 20.0117 | -0.00506 | 0.02096 | -0.04359 | 504 |
| Nitrogen, N2 | 136.2315 | 0.02617 | 0.05046 | -0.00691 | 4.20×10^4 |
| Oxygen, O2 | 151.0857 | 0.02562 | 0.04624 | 0.004208 | 4.80×10^4 |
The BWR equation, sometimes referred to as the BWRS equation,
where d is the molar density and where a, b, c, A, B, C, α, and γ are empirical constants. Note that the γ constant is a derivative of constant α and therefore almost identical to 1.
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| リンク元 | 「実在気体」「nonideal gas」 |
| 関連記事 | 「real」「GAS」「REAL」「gas」「g」 |
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