Part 2:Single-phase properties (gas, liquid, and dense fluid) for extended ranges of application
Gaz nature! — Calcul des pro priétés thermodynainiques — Partie 2: Propriétés des phases uniques (gaz, Iiquide,fluide dense) pour unegamme étendue d’applications.
4.1 Principle
The method is based on the concept that natural gas or any other type of mixture can be completely characterized in the calculation of its thermodynamic properties by component analysis. Such an analysis, together with the state variables of temperature and density, provides the necessary input data for the calculation of properties. In practice, the state variables available as input data are generally temperature and pressure, and it is thus necessary to first iteratively determine the density using the equations provided here.
These equations express the Helmholtz free energy of the mixture as a function of density, temperature, and composition, from which all other thermodynamic properties in the homogeneous (single-phase) gas, liquid, and supercritical (dense-fluid) regions may be obtained in terms of the Helmholtz free energy and its derivatives with respect to temperature and density. For example, pressure is proportional to the first derivative of the Helmholtz energy with respect to density (at constant temperature).
NOTE These equations are also applicable in the calculation of two-phase properties (vapour-liquid equilibria). Additional composition-dependent derivatives are required and are presented in Part 3 of this International Standard.
The method uses a detailed molar composition analysis in which all components present in amounts exceeding 0,000 05 mole fraction (50 ppm) are specified. For a typical natural gas, this might include alkane hydrocarbons up to about C7 or C8 together with nitrogen, carbon dioxide, and helium. Typically, isomers for alkanes C6 and higher may be lumped together by molar mass and treated collectively as the normal isomer.
For some fluids, additional components such as Cg, C10, water, and hydrogen sulfide may be present and need to be taken into consideration. For manufactured gases, hydrogen, carbon monoxide, and oxygen may also be present in the mixture.
More precisely, the method uses a 21-component analysis in which all of the major and most of the minor components of natural gas are included (see Clause 6). Any trace component present but not identified as one of the 21 specified components may be assigned appropriately to one of these 21 components (see Annexf).
4.2 The fundamental equation based on the Helmholtz free energy
4.2.1 Background
The GERG-2008 equation1!) was published by the Lehrstuhl für Thermodynamik at the Ruhr-Universität Bochum in Germany as a new wide-range equation of state for the volumetric and caloric properties of natural gases and other mixtures. It was originally published in 2007121 and later updated in 2008.111 The new equation improves upon the performance of the AGA-8 equationl3l for gas phase properties and in addition is applicable to the properties of the liquid phase, to the dense-fluid phase, to the vapourliquid phase boundary, and to properties for two-phase states. ISO 20765-2-2015 pdf free download.

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