Compressibility factors ~ MECHTECH GURU

Compressibility factors

Compressibility factors

The deviation from ideal behavior of a gas is expressed in terms of the compressibility factor Z, which is defined as the ratio of the actual volume to the volume predicted by the ideal gas law.

Z = Actual volume/volume predicted by ideal gas law

 = v/RT/P

 = Pv/RT

 For an ideal gas Pv = RT 

and hence Z = 1 at all temperatures and pressures.

The experimental P-v-T data is used to prepare the compressibility chart.

                              Reduced pressure, PR = P/Pc,

                              Reduced temperature, TR = T/Tc

                              Reduced volume, vR = v/vc

Where Pc, Tc and vc denote the critical pressure, temperature and volume respectively.

These equations state that the reduced property for a given  state is the  value  of this property in this state divided by the value of this same property by at the critical point.

The striking fact is that when such Z versus Pr diagrams are prepared for  a  number  of different substances, all of them very nearly coincide, especially when the substances have simple, essentially spherical molecules.

We need to know only critical temperature and critical pressure to use this basic generalized chart.

In general it can be noted that idealized gas behavior for very low pressures as compared to critical) regardless of temperature. Furthermore, at high temperatures (greater than twice Tc), the ideal-gas model can be assumed to good accuracy to pressures as high as 4-5 times Pc.


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