Page 85 - Handbook of Natural Gas Transmission and Processing Principles and Practices

importance for PT flash calculations, introduced thereafter.

2.2.5.2. The 2-Phase Equilibrium Condition
The 2-phase equilibrium condition is a set of three equations. It imposes the equality of the
temperature, pressure, and chemical potential of each component in the two equilibrium phases. It
is the central equation for 2-phase equilibrium calculations. For a p-component system in VLE, this
condition can be written as:

(2.8)

where is the chemical potential of species i in a given phase defined as the molar partial
Gibbs energy of species i.
It is, however, well acknowledged that the chemical potential cannot be derived in an absolute
manner from EoS or activity-coefficient models classically used to perform VLE calculations.
Consequently the chemical-potential uniformity equation is converted into equations involving
quantities derivable from these models, such as the fugacity or the activity:

(2.9)

where x and y are the equilibrium mole fractions of i in the liquid and gas phases, respectively.
i
i
As a final remark, note that the 2-phase equilibrium condition only involves intensive phase
variables.
2.2.5.3. Models for Calculating Vapor–Liquid Equilibria in Natural Gas Systems
Two classes of models could be used potentially to perform phase-equilibrium calculations on
natural gases: (1) pressure-explicit EoS and (2) activity-coefficient models.
Pressure-explicit EoS are models capable of describing subcritical states, vapor–liquid critical
phenomena, and supercritical states. They are thus usable from low to high temperatures and from
low to high pressures.
On the contrary, activity-coefficient models do not have the capacity to describe vapor–liquid
critical states and supercritical states, and they are thus limited to low-temperature and low-
pressure domains.
Natural gases contain generally compounds that are supercritical at temperature and pressure
conditions of geological reservoirs or in gas-processing plants. Consequently, only pressure-explicit
EoS can be considered for their thermodynamic modeling.

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