Ideal Gas Law
Relationships that describe the pressure-volume-temperature (PVT) behavior of gases are called equations of state. The simplest equation of state is called the ideal gas law and is given by
where
p = absolute pressure
V = total volume that the gas occupies
n = moles of gas
T = absolute temperature
R′ = gas constant
When R′ = 10.73, p must be in pounds per square inch absolute (psia), V in cubic feet (ft3), n in pound-moles (lb-mols), and T in degrees Rankine (°R). The ideal gas law was developed from Boyle’s and Charles’s laws, which were formed from experimental observations.
The petroleum industry works with a set of standard conditions—usually 14.7 psia and 60°F. When a volume of gas is reported at these conditions, it is given the units of SCF (standard cubic feet). As mentioned in sometimes the letter M will appear in the units (e.g., MCF or M SCF). This refers to 1000 standard cubic feet. The volume that 1 lb-mol occupies at standard conditions is 379.4 SCF. A quantity of a pure gas can be expressed as the number of cubic feet at a specified temperature and pressure, the number of moles, the number of pounds, or the number of molecules. For practical measurement, the weighing of gases is difficult, so gases are metered by volume at measured temperatures and pressures, from which the pounds or moles may be calculated. Example 2.1 illustrates the calculations of the contents of a tank of gas in each of three units.
Example 2.1 Calculating the Contents of a Tank of Ethane in Moles, Pounds, and SCF
Given
A 500-ft3 tank of ethane at 100 psia and 100°F.
Solution
Assuming ideal gas behavior,
Pounds = 8.32 × 30.07 = 250.2
At 14.7 psia and 60°F,
SCF = 8.32 × 379.4 = 3157
Here is an alternate solution using Eq. (2.4):
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