Page 272 - Pipeline Rules of Thumb Handbook

Gas—General 259

Facts about methane and its behavior

A study of the physical, transport and thermodynamic properties of methane
Dr. Robert O. Parker and Dr. William H. Kapfer, New York University, New York, NY

3
.
d
Interest in liquefied natural gas (LNG) requires that c = 10 17. lb ft (0 1630 g cc) 2
the physical, transport, and thermodynamic properties of
methane, the principal constituent of natural gas, be readily The main utility of critical constants to the engineer is
available. This article will present values most frequently used related to the fact that correlations of many of the physical,
in process calculations and in other studies associated with the transport, and thermodynamic properties of hydrocarbons
storage and processing of natural gas. In addition, sources of are best expressed in terms of reduced temperature, T R , and
more detailed information are listed. pressure, P R
Methane, CH 4 , is the hydrocarbon of lowest molecular
weight (16.042). The molecule is best visualized as a regular Ê T R = T , P R = P ˆ
tetrahedron with the carbon atom at the geometric center and Ë T c P c ¯
the hydrogen atoms at each of the four apices. Methane is an
end product of the anaerobic (without air) decay of plants, This is simply an application of the “principle of correspond-
that is, of the breakdown of certain very complicated mole- ing states” and is recommended only in the absence of
cules. As such, it is the major constituent (up to 97%) of sufficient reliable experimental data.
natural gas. It is the firedamp of the coal mines, and can be
seen as marsh gas bubbling to the surface of swamps. Equations of state
The physical, transport, and thermodynamic properties (pressure—volume—temperature behavior)
required are:
For ordinary conditions, low to moderate pressures and
Physical temperatures not too close to the normal boiling point, i.e.,
when inspection of a compressibility factor chart shows it to
1. Critical constants be nearly unity, the ideal gas law
2. Equation of state (PVT relationships)
3. Vapor pressure, including bp and fp PV = W RT (1)
4. Vapor and liquid density M

where P = pressure, psia
Transport
V = volume, ft 3
W = weight of gas, lb
1. Viscosity of liquid and gas
2. Thermal conductivity of liquid and gas M = molecular weight (here 16.042)
3
3. Diffusivity R = gas constant = 10.73psia (ft )/lb mole (°R)
T = temperature, °R
Thermodynamic is sufficient. For most other conditions Equation 1 is
modified by inclusion of the compressibility factor, Z, in the
1. Heat capacity right-hand member, viz:
2. Latent heat
3. Enthalpy W
4. Entropy PV = M ZRT (1a)
They are required for process calculations: material and The writers recommend compressibility factors of Nelson and
energy balances; heat transfer, mass-transfer and pressure loss Obert. If an analytical equation of state is preferred to Equa-
3
calculations. tion 1a the writers recommend the 8 constant equation of
Benedict, Webb and Rubin. 4
Critical Constants
2
)
2
a d
P = RTd + ( B RT - A o - C T d + ( bRT - ) 3
o
o
1
T c =-115 8 ∞ F - ( 82 1 ∞ C) cd (1 g d e ) -g d 2
.
.
3
2
+
+ ad +a 6 (2)
2
P c = 673 psia (45 8 atm) T 2

.