Page 51 - PVT Property Correlations

Dry Gases Chapter | 3 31

and
m
ρ 5 ð3:5Þ
V
Then,
RT
p 5 ρ ð3:6Þ
M
Solving for density,
pM
ρ 5 ð3:7Þ
RT
Substituting in the specific gravity definition, we have
ρ pM g =RT
γ 5 ρ g a 5 pM a =RT 5 M g 5 M g ð3:8Þ
g
29
M a
The molecular weight of the gas (also called apparent molecular weight)
is calculated from the following equation:
nc
X
M g 5 y i 3 M i ð3:9Þ
i51
where y i is the mole fraction of the components forming the gas mixture,
and nc is the number of components. The mole fractions of the gas are mea-
sured in the compositional measurement procedure.

Gas Deviation Factor
Also called gas compressibility factor, z-factor, or supercompressibility, gas
deviation factor is defined as the ratio between the actual (or real) volume of
the gas and the ideal volume of the gas. It is given by the following
equation:

V real
z 5 ð3:10Þ
V ideal
Gas deviation factor is a measure of how the gas behavior deviates from
the ideal behavior (given by the ideal gas law). It is determined in the labora-
tory by comparing the volume occupied by the gas under pressure and tem-
perature with the volume of the gas calculated from the ideal gas law under
the same pressure and temperature.
Gas deviation factor has been correlated with parameters that are readily
available so it can be computed in absence of laboratory data. It approaches
unity at pressures and temperatures close to standard pressure and tempera-
ture. The gas deviation factor is a property essential to determine for any