ISO 6974-5:2000 pdf download
ISO 6974-5:2000 pdf download.Natural gas -Determination of
composition with defined uncertainty bygas chromatography 一-
Part 5: Determination of nitrogen, carbon dioxideand C1 to C5 and C6+ hydrocarbons for alaboratory and on-line process applicationusing three columns.
NOTE Columns 1 and 2 are boiling-point separation columns for the separation of propane, iso-butane, n-butane, neo-pentane, iso-pentane, n-pentane. They effectively are a single column, divided into short and long sections for rapid backflush of C6 components to the detector.
c) method of packing: packed by any suitable packing method providing uniform column packing and performance characteristics in accordance with 6.2. If purchased individually, as part of a system, or packed individually, their performance shall comply with the specification. When packed individually it is assumed that this will be according to a recognized technique.
NOTE The following packing method is suitable:
Close the column outlet with a sintered disc or glass wool plug. Connect a reservoir containing rather more packing than is needed to fill the column to the inlet and apply a pressure of 0,4 MPa of nitrogen to this reservoir. The flow of packing into the column is assisted by vibration. When the column is full, allow the pressure to decay slowly before disconnecting the reservoir.
d) conditioning: with freshly prepared columns, more stable baselines can be obtained by conditioning them overnight, with carrier gas flowing, at a temperature of 50 °C higher than that at which they are intended to operate. If this is necessary, but not easily achieved in the gas chromatograph in which the columns are to be used, they can be conditioned after being installed in another unit.
5.1.5 Thermal conductivity detector (TCD), with a time constant of not greater than 0,1 s, and internal volume appropriate for the column sizes and flowrate used.
5.1.6 Controller/peak-measurement system, having a wide range of sensitivity (0 V to 1 V), capable of measuring peaks on a sloping baseline and able to control automatic operation of the valves according to a sequence selected by the operator.
5.1.7 Auxiliary equipment, consisting of valves, tubing and any other accessories, to control the flow of sample gas to the chromatograph and for shutting off this flow for a defined period of time before injection.
6 Procedure
6.1 Gas chromatographic operating conditions
If the apparatus has been used for previous determinations, ensure that it is returned to the starting conditions before injecting a sample or calibration gas mixture.
Set the operating conditions for the apparatus (5.1) as follows.
a) Oven column: 70 °C to 105 CC, capable of being maintained to within ± 0,1 °iD
b) Carrier gas flowrate: dependent upon the column diameter. With the carrier gas flowing through all columns in the order column 1 to column 2 to column 3 [Figure 1 b)], set the flowrate so that the average linear velocity for nitrogen (total column length divided by hold-up time) is between 10 cm/s and 15 cm/s.
C) Valves: if the valves are not fitted in the column oven, set them to a temperature in the range of 70 °C to 105 °C (no less than the column temperature), isothermal and stable to ± 2 °C.
6.3.2 Chromatogram
A typical chromatogram is shown in Figure 2.
The order of elution of components should not change, but actual retention times shall be determined for each individual system.
Measure the areas or heights of component peaks from the detector with an integrator or data system. (Use areas for all major components, but it may be found that peak heights give better repeatability for some minor components, such as the pentanes, where peaks are both small and relatively wide.) Set the integration parameters so as to correctly allocate baselines, and so that there is no interference with peak measurement from valve-switching disturbances.
6.4 Determination
6.4.1 Sample valve purge
Purge the sample valve with the gas to be analysed, using at least 20 times the volume of the valve and associated pipework.
Stop the purge to enable the gas to reach the temperature of the valve and ambient pressure, then start the analytical cycle, injecting the sample and switching the valves as required.
If this volume of sample is not enough to purge the valve, contamination by air or by the previous sample will interfere with the determination. If either occur, then use a larger volume of sample for purging.
NOTE The sample loop should be purged with gas for a precise time period, at a defined rate, and the sample should then be allowed to equilibrate to ambient pressure before injection. In the absence of equipment which can confirm the latter, there should be a defined time between sample-valve shut-off and injection.
