Page 86 - Handbook of Natural Gas Transmission and Processing Principles and Practices

Pressure-explicit EoS for mixtures are models expressing the pressure P of a phase as a function of
its temperature T, molar volume v, and composition (vector of component mole fractions z ).
Fugacity or fugacity coefficient expressions (which are involved in the 2-phase equilibrium
condition) can be derived from such a model (Smith et al., 2005; Michelsen and Mollerup, 2007;
Sandler, 2017) and expressed with respect to variables .
For instance, cubic EoS, detailed thereafter, are EoS models usable for natural gases. In their
general form, they can be written as:

(2.10)

where are universal constants (i.e., component-independent constants), and are EoS-
mixture parameters described in Section 2.3, R is the gas constant, is the mole number of species i

in a phase, and is the total amount of matter.

2.2.5.4. Expression of the Equilibrium Condition When Using Pressure-Explicit EoS
to Model Fluid Properties
Let us consider a system in 2-phase equilibrium. The molar volume and composition of the liquid
phase are denoted as and (vector of component mole fractions); for the gas phase, they are
denoted as and .
When using a pressure-explicit EoS to model a fluid mixture, the VLE condition given by Eqs. 2.8
and 2.9 can be written as:

(2.11)
or more simply as

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