Page 65 - PVT Property Correlations

Dry Gases Chapter | 3 43

Estimation of Reduced Pressure and Temperature
Some PVT properties are correlated with reduced pressure and temperature.
The most critical is the z-factor. When defined for a gas mixture, reduced
pressure and temperature are usually referred to as pseudoreduced pressure
and pseudoreduced temperature. They are defined by the following
equations:
p
p pr 5 ð3:19Þ
p pc
T
T pr 5 ð3:20Þ
T pc

Estimation of Gas Deviation Factor

Gas deviation factor (z-factor) is used in the calculation of almost all PVT
properties of gas. There are more than 20 known (and different) correlations
for calculating z-factor. All of them compute z-factor from known pseudore-
duced pressure and pseudoreduced temperature.

Estimation of Other PVT Properties of Gas

Several gas properties can be directly calculated if the z-factor is known at
specific pressure and temperature. These properties include gas formation
volume factor, density, and compressibility of the gas. Viscosity of the gas,
however, is computed from correlations. Several correlations for gas viscos-
ity exist.

Sequence of Calculations
The main input that allows engineers to calculate all dry gas PVT properties
is either the gas-specific gravity or the gas composition. In either case, impu-
rities (nonhydrocarbon) mole fractions must be known for greater accuracy.
The first step is to calculate the pseudocritical pressure and temperature
using one of the correlations. The second step is to calculate the pseudore-
duced pressure and temperature at the specified system pressure and temper-
ature. Then z-factor is calculated from any of the available correlations.
After that, other PVT properties are calculated using the z-factor values.
Fig. 3.7 is a flow diagram that describes the sequence of calculations for
computing dry gas PVT properties.