Page 72 - Handbook of Natural Gas Transmission and Processing Principles and Practices

2.2. Fundamentals of Natural Gas Phase Behavior
2.2.1. Single-Component Systems

The behavior of a single-phase unary system can be described by any two intensive variables
(temperature T, pressure P, molar volume v, molar enthalpy h, and so forth) characterizing the
studied phase. Temperature and pressure are certainly the easiest process variables to be measured,
explaining why the representation of the information related to the phase behavior of a pure
component is frequently reported in a pressure–temperature P-T phase diagram. The 2-phase
equilibrium condition applied to a pure component i states that its temperature, pressure, and
molar Gibbs energy g (i.e., its chemical potential) remain the same in each phase:
i

(2.1)
As a direct consequence, the two intensive variables T and P are subject to one constraint
equation (the phase-equilibrium condition) so that in a pressure ( )–temperature ( ) plane, the
locus of the 2-phase equilibrium can be described by a curve P(T).
This result is in agreement with Gibbs's phase rule claiming that the variance (also called number
of degrees of freedom), denoted dof, of a system containing one component is

(2.2)
where denotes the number of coexisting phases within the a one-component system.
Consequently, for a 2-phase equilibrium, the variance is equal to one. The 2-phase system being
monovariant, the choice of the temperature fixes the equilibrium pressure.
As a consequence, in a P–T plane, the representation of 2-phase equilibrium loci (liquid +

vapor or liquid + solid or vapor + solid) requires spaces of dimension 1, i.e., simple curves.
Still according to Gibbs' phase rule, single-phase pure species have a variance equal to 2, whereas
3-phase pure species show a variance equal to zero. Consequently, the graphical representations of
such spaces in the P-T plane are area and points as illustrated in Fig. 2.1.
The critical point is defined as the terminal point of the vaporization curve in the P-T plane. At
this point, all the intensive properties (density, molar enthalpy …) of the liquid and vapor
coexisting phases become strictly identical.

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