Page 36 - Handbook of Natural Gas Transmission and Processing Principles and Practices

Lower heating/calorific value, MJ/kg 38–50
Stoichiometric lower heating value, MJ/kg 2.75
Methane concentration, volume % 80–99
Ethane concentration, volume % 2.7–4.6
Nitrogen concentration, volume % 0.1–15
Carbon dioxide concentration, volume % 1–5
Sulfur concentration, weight% ppm <5
Specific CO 2 formation, g/MJ 38–50
In the following section we discuss important gas properties including specific gravity,
compressibility factor, formation volume factor, density, isothermal compressibility, and viscosity.

1.6.1.1. Gas Specific Gravity
Specific gravity of a natural gas is defined as the ratio of gas density to the density of air, both
defined at the same pressure and temperature. These densities are usually defined at standard
conditions (14.7 psia and 60°F). Therefore, specific gravity of gas is defined as:

(1.1)

where M is the molecular weight of gas and M is the molecular weight of air that is equal to 29.
air
Once we can calculate the value of molecular weight of a mixture, we can calculate the specific
gravity of the mixture. For a gas mixture, we can calculate the molecular weight as:

(1.2)

where M is the molecular weight of component i, y is the mole fraction of component i, and n is the
i
i
total number of components.
Various gas properties, including the molecular weights for pure components, are given in
Table 1.2.

1.6.1.2. Gas Compressibility Factor
The volume of a real gas is usually less than what the volume of an ideal gas would be, and hence a
real gas is said to be super-compressible. The ratio of the real volume to the ideal volume, which is a
measure of the amount the gas deviates from perfect behavior, is called the super compressibility
factor, sometimes shortened to the compressibility factor. It is also called the gas deviation factor,
and given the symbol “Z”. The gas deviation factor is by definition the ratio of the volume actually
occupied by a gas at a given pressure and temperature to the volume it would occupy if it behaved
ideally.
The real gas equation of state is then written as:

(1.3)
where P is the pressure, V is the volume, T is the absolute temperature, Z is the compressibility
factor, n is the number of kilomoles of the gas, and R is the gas constant.
The gas deviation factor, Z, is close to 1 at low pressures and high temperatures which means the

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