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Compressibility factor values are usually obtained by calculation from equations of state (EOS), such as the virial equation which take compound-specific empirical constants as input. For a gas that is a mixture of two or more pure gases (air or natural gas, for example), the gas composition must be known before compressibility can be calculated.
In thermodynamics and fluid mechanics, the compressibility (also known as the coefficient of compressibility [1] or, if the temperature is held constant, the isothermal compressibility [2]) is a measure of the instantaneous relative volume change of a fluid or solid as a response to a pressure (or mean stress) change.
Figure 7: Generalized compressibility chart for a van der Waals gas. Real gases are characterized by their difference from ideal by writing p v = Z R T {\displaystyle pv=ZRT} . Here Z {\displaystyle Z} , called the compressibility factor, is expressed either as Z ( p , T ) {\displaystyle Z(p,T)} or Z ( ρ , T ) {\displaystyle Z(\rho ,T)} .
The largest and the lowest solution are the gas and liquid reduced volume. In this situation, the Maxwell construction is sometimes used to model the pressure as a function of molar volume. The compressibility factor = / is often used to characterize non-ideal behavior. For the van der Waals equation in reduced form, this becomes
In gas dynamics we are interested in the local relations between pressure, density and temperature, rather than considering a fixed quantity of gas. By considering the density ρ = M / V {\displaystyle \rho =M/V} as the inverse of the volume for a unit mass, we can take ρ = 1 / V {\displaystyle \rho =1/V} in these relations.
The virial expansion is a model of thermodynamic equations of state.It expresses the pressure P of a gas in local equilibrium as a power series of the density.This equation may be represented in terms of the compressibility factor, Z, as = + + + This equation was first proposed by Kamerlingh Onnes. [1]
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Isothermal curves depicting the non-ideality of a real gas. The changes in volume (depicted by Z, compressibility factor) which occur as the pressure is varied. The compressibility factor Z, is equal to the ratio Z = PV/nRT. An ideal gas, with compressibility factor Z = 1, is described by the horizontal line where the y-axis is equal to 1.