ISO 20765-2 is a safety standard on natural gas. Natural gas is a naturally occurring hydrocarbon gas mixture consisting of methane and commonly includes varying amounts of other higher alkanes, and sometimes a small percentage of carbon dioxide, nitrogen, hydrogen sulfide, or helium. ISO 20765-2 specifies a to calculate 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. The method described in ISO 20765-2 is applicable with no increase in uncertainty to wider ranges of temperature, pressure, and composition for which the method of Part 1 is not applicable. For example, ISO 20765-2 is applicable 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. A practical usage is the calculation of properties of highly concentrated CO2 mixtures found in carbon dioxide sequestration applications.
The mixture model presented in ISO 20765-2 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 (–280 °F to –208 °F), the uncertainty is ≤ (0,1 – 0,3) %, which agrees with the estimated experimental uncertainty of available test data. The model represents experimental data for compressed liquid densities of various binary mixtures to within ± (0,1 – 0,2) % at pressures up to 40 MPa (5800 psia), which is also in agreement with the estimated experimental uncertainty. 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.
ISO 20765-2 on the calculation of thermodynamic properties is relevant to:
Natural gas is a fossil energy source that formed deep beneath the earth’s surface. ISO 20765-2 assists with the calculation of thermodynamic properties. 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. 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. The method presented in ISO 20765-2 uses reduced density, inverse reduced temperature, and molar composition as the input variables. Overall, ISO 20765-2 is useful as its purpose of the departure function is to further improve the accuracy of the mixture model in the description of thermodynamic properties.
EN ISO 20765-2:2018
ISO 20765-2:2015