ISO 20765-2 Natural gas. Calculation of thermodynamic properties. Single-phase properties (gas, liquid and dense fluid) for extended ranges of application

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ISO 20765-2 was prepared by Technical Committee ISO/TC 193, Natural Gas, Subcommittee SC 1, Analysis of Natural Gases.This International Standard specifies methods for the calculation of thermodynamic properties of natural gases,
manufactured fuel gases, and similar mixtures. It comprises three parts:
- Part 1: Gas phase properties for transmission and distribution applications
- Part 2: Single-phase properties (gas, liquid, and dense fluid) for extended ranges of application (this document)
- Part 3: Two - phase properties (vapor - liquid equilibria)
This part - Part 2 - has five normative annexes and two informative annexes.This part of ISO 20765 specifies a method of calculation for the volumetric and caloric properties of natural gases, manufactured fuel gases, and similar mixtures, at conditions where the mixture may be in either the homogeneous (single-phase) gas state, the homogeneous liquid state, or the homogeneous supercritical (dense-fluid) state.
NOTE Although the primary application of this document is to natural gases, manufactured fuel gases, and similar mixtures, the method presented is also applicable with high accuracy (i.e., to within experimental uncertainty) to each of the (pure) natural gas components and to numerous binary and multi-component mixtures related to or not related to natural gas.
For mixtures in the gas phase and for both volumetric properties (compression factor and density) and caloric properties (for example, enthalpy, heat capacity, Joule-Thomson coefficient, and speed of sound), the method is at least equal in accuracy to the method described in Part 1 of this International Standard, over the full ranges of pressure p, temperature T, and composition to which Part 1 applies. In some regions, the performance is significantly better; for example, in the temperature range 250 K to 275 K. The method described here maintains an uncertainty of < 0.1% for volumetric properties, and generally within 0.1% in speed of sound. Although the new equation accurately describes all volumetric and caloric properties in the homogeneous gas, liquid, and supercritical regions, and of vapor-liquid equilibrium states, its structure is more complex than that in Part 1.
NOTE All uncertainties in this document are expanded uncertainties given for a 95% confidence level (coverage factor k = 2).
The method described here is also applicable with no increase in uncertainty to wider ranges of temperature, pressure, and composition, for example, to natural gases with lower content of methane (down to 0.30 mole fraction), higher content of nitrogen (up to 0.55 mole fraction), carbon dioxide (up to 0.30 mole fraction), ethane (up to 0.25 mole fraction), and propane (up to 0.14 mole fraction), and to hydrogen-rich natural gases, to which the method of Part 1 is not applicable. The equations can be used for high CO₂ mixtures found in carbon dioxide sequestration applications.
The mixture model presented here is valid by design over the entire fluid region. In the liquid and dense-fluid regions the paucity of high quality test data does not in general allow definitive statements of uncertainty for all sorts of multi-component natural gas mixtures. For saturated liquid densities of LNG-type fluids in the temperature range from 100 K to 140 K, the uncertainty is < (0.1 – 0.3)%, which is in agreement with the estimated experimental uncertainty of available test data. The model represents experimental data for compressed liquid densities of various binary mixtures related to LNG to within deviations of ±(0.1 – 0.2)% at pressures up to 40 MPa, which is in agreement with the estimated experimental uncertainty as well. Due to the high accuracy of the equations developed for the binary subsystems, the mixture model can predict the thermodynamic properties for the liquid and dense-fluid regions with the best accuracy presently possible for multi-component natural gas fluids.