BS EN ISO 6974-5:2001 – Natural gas — Determination of composition with defined uncertainty by gas chromatography — Part 5: Determination of nitrogen, carbon dioxide and C1 to C5 and C6+ hydrocarbons for a laboratory and on-line process application using three columns

This part of ISO 6974 describes a gas chromatographic method for the quantitative determination of natural gas constituents using a three-column system. This method is applicable to natural gases of limited range, on-line and automatically calibrating on a regular basis for gas samples not containing any hydrocarbon condensate and/or water. It is applicable to the analysis of gases containing constituents within the mole fraction ranges given in Table 1 . These ranges do not represent the limits of detection, but the limits within which the stated precision of the method applies. Although one or more components in a sample may not be detected present, the method can still be applicable. This part of ISO 6974 is only applicable if used in conjunction with parts 1 and 2 of ISO 6974.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of this part of ISO 6974. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. However, parties to agreements based on this part of ISO 6974 are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. For undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC maintain registers of currently valid International Standards.
ISO 61 42, Gas analysis — Preparation of calibration gas mixtures — Gravimetric method.
ISO 6974-1 :2000, Natural gas — Determination of composition with defined uncertainty by gas chromatography — Part 1: Guidelines for tailored analysis.
ISO 6974-2:— 1 ) , Natural gas — Determination of composition with defined uncertainty by gas chromatography — Part 2: Measuring-system characteristics and statistics for data treatment.
ISO 7504:1 984, Gas analysis — Vocabulary.
3 Principle
Determination of nitrogen, carbon dioxide and hydrocarbons from C 1 to C 6+ by gas chromatography using a three- column switching/backflush arrangement, configured as shown in Figure 1 . The three chromatographic columns are connected by two six-port valves for handling sample injection and backflushing operations (or alternatively a single ten-port valve is used) to a thermal conductivity detector (TCD) which is used for quantification.
A single sample is injected first onto a boiling-point separation column, divided into short and long sections. The C 6 and heavier hydrocarbons are initially retained on the short section of this column. The long section of this column retains C 3 to C 5 hydrocarbons. The lighter components (nitrogen, methane, carbon dioxide and ethane) pass rapidly and unresolved through the boiling-point separation column onto a porous polymer-bead column, suitable for their retention and separation. Following an accelerated backflush of the short column situated closest to the detector, the heavier C 6+ hydrocarbons (determined as a recombined “pseudo component” rather than by the summation of individual component measurements) elute first and are quantified as a single peak. Next, from the longer section of this column situated farther from the detector, the C 3 to C 5 hydrocarbons are separated then quantified by TCD. Finally, by redirecting carrier gas onto the porous polymer-bead column, the lighter components, i.e. nitrogen, carbon dioxide, methane and ethane, are separated then quantified by the detector. A six-port valve either connects this column to the carrier-gas flow or by-passes it during measurement of C 3 to C 5 components.
BS EN ISO 6974-5:2001 – Natural gas — Determination of composition with defined uncertainty by gas chromatography — Part 5: Determination of nitrogen, carbon dioxide and C1 to C5 and C6+ hydrocarbons for a laboratory and on-line process application using three columns