ISO 23874:2006 pdf – Natural gas — Gas chromatographic requirements for hydrocarbon dewpoint calculation.
1 Scope This International Standard describes the performance requirements for analysis of treated natural gas of transmission or pipeline quality in sufficient detail so that the hydrocarbon dewpoint temperature can be calculated using an appropriate equation of state. It can be applied to gases that have maximum dewpoint temperatures (cricondentherms) between 0 °C and – 50 °C. The pressures at which these maximum dewpoint temperatures are calculated are in the range 2 MPa (20 bar) to 5 MPa (50 bar). Major components are measured using ISO 6974 (all parts) and the ranges of components that can be measured are as defined in ISO 6974-1. The procedure given in this International Standard covers the measurement of hydrocarbons in the range C 5 to C 12 . n-Pentane, which is quantitatively measured using ISO 6974 (all parts), is used as a bridge component and all C 6 and higher hydrocarbons are measured relative to n-pentane. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. ISO 6974-1, Natural gas — Determination of composition with defined uncertainty by gas chromatography — Part 1: Guidelines for tailored analysis ISO 6974-2, Natural gas — Determination of composition with defined uncertainty by gas chromatography — Part 2: Measuring-system characteristics and statistics for processing of data ISO 6974-3, Natural gas — Determination of composition with defined uncertainty by gas chromatography — Part 3: Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide and hydrocarbons up to C 8 using two packed columns
4 Materials 4.1 Certified-reference gas mixture for major components (CRM1), such as is required for ISO 6974 (all parts). Depending upon the working range and the accuracy required, more than one CRM can be needed. 4.2 Certified-reference gas mixture, for higher hydrocarbons (CRM2), containing as a minimum, n- pentane, n-hexane, benzene, cyclohexane, n-heptane, toluene, methylcyclohexane and n-octane. Ideally, the CRM2 should also contain n-nonane, n-decane, n-undecane and n-dodecane in methane. The mole fractions of components in CRM2 shall be chosen to be appropriate for the application, provided that the mixture is stable and free from the possibility of condensation in transport, storage and in use. 5 Apparatus 5.1 Measurement system for major components, comprised of a sample introduction and transfer unit, a separation unit, a detection unit, an integrator and a data reduction system, capable of meeting the analytical requirements described in 6.1.
3 Principle For hydrocarbon gas mixtures such as natural gas, the phase boundary or dewpoint line distinguishing single- phase gas from gas-liquid mixtures is a complex function of pressure, temperature and composition. For a given composition, the pressure at which the dewpoint temperature is at a maximum is intermediate between those found in transmission and distribution operations. The analysis shall be comprehensive for inert components and for hydrocarbons up to C 12 . It is not necessary to measure helium, hydrogen, water and sulfur compounds unless any of these is present at greater than 0,01 mole fraction. The analysis shall be performed in two parts. Major components (nitrogen, carbon dioxide and hydrocarbons from C 1 to C 5 ) shall be analysed according to ISO 6974 (all parts). Higher hydrocarbons (C 5 to C 12 ) shall be analysed to satisfy the requirements given in this International Standard. This allows the traceability of measurements according to ISO 6974 (all parts) to be extended to the higher hydrocarbons. It is not possible to identify all the measured higher hydrocarbons, nor is it possible to obtain a reference gas mixture that contains more than a few representatives of the higher hydrocarbons. The analytical data are, therefore, handled with a number of simplifying assumptions. ⎯ Unidentified components are allocated a carbon number or molar mass according to their positions in the chromatogram with respect to identified n-alkanes. ⎯ Alkanes of carbon number 7 and above are summed by carbon number and treated as fractions for input to the dewpoint calculation. ⎯ Average boiling points and densities of fractions are calculated from the individual boiling points and quantities of the components that comprise them; individual-component boiling points are calculated by interpolation between the bracketing n-alkanes  . ⎯ Sample components are quantified by comparison with n-pentane, which has been measured according to ISO 6974 (all parts), using relative response facto