ASTM D1945-14 Standard Test Method for Analysis of Natural Gas by Gas Chromatography

In the standard "ASTM D1945-14 Standard test method for analysis of natural gas by gas chromatography" published by the American Society for Testing and Materials (ASTM), given in the standard A test method is described for determining the chemical composition of natural gases and similar gas mixtures within the composition range shown in the table. This test method can be abbreviated for analysis of lean natural gases containing negligible amounts of hexane and higher hydrocarbons or for the determination of one or more components as required.

Components in a representative sample are physically separated by gas chromatography (GC) and compared with calibration data obtained under the same operating conditions from a reference standard mixture of known composition. Multiple heavy end components of a sample can be grouped into irregular peaks by reversing the direction of passage of the carrier gas through the column to group the heavy ends as C5 and heavier, C6 and heavier, or C7 and heavier. The composition of the sample is calculated by comparing the peak heights or peak areas, or both, with the corresponding values ​​obtained with the reference standard.

This test method is important for providing data for calculating physical properties of the sample, such as calorific value and relative density, or for monitoring the concentrations of one or more components in a mixture.

The detector used in these tests should be of the thermal conductivity type or equivalent in sensitivity and resolution. The thermal conductivity detector should be sensitive enough to produce a signal of at least 0,25 mV for 1 mole of n-butane in a 0,5 mL sample.

In these tests, strip chart recorders or electronic integrators or both are used as recording instruments to view the separated components. Although a strip chart recorder is not required when using electronic integration, it is highly desirable for evaluating instrument performance.

The recorder should be a strip chart recorder with a full-range scale of 5 mV or less. The width of the chart should be at least 150 mm. A maximum pen response time of 2 s and a minimum chart speed of 10 mm/min are required. Faster speeds of up to 100 mm/min are preferred if the chromatogram must be interpreted using manual methods to obtain areas.

For electronic or computing integrators, proof of separation and response equivalent to that for a recording device is required for displays other than a chart recorder.

One piece of equipment used in these tests is the attenuator. If the chromatogram is to be interpreted using manual methods, an attenuator should be used with the detector output signal to maintain maximum peaks within the recorder plot range. The attenuator should be accurate to within 0,5 percent between attenuator range steps.

Before starting the tests, the device must be prepared. For this purpose, a linearity check must be performed first. To determine the linearity of the response for the thermal conductivity detector, the following procedure must be completed:

• The major component of interest (methane for natural gas) is loaded into the chromatograph via a fixed-size sample loop with partial pressure increments of 13 to 100 kPa, or 13 kPa at the prevailing atmospheric pressure.
• The integrated peak responses for the field generated at each pressure increase are plotted against their partial pressures.
• The plotted results should give a straight line. A perfectly linear response displays a straight line at a 45 degree angle using logarithmic values.
• Any squiggly line indicates that the constant volume sample loop is too large. A smaller loop size should replace the constant volume loop and the above procedure should be repeated.
• Linearity over the range of interest must be known for each component. It is useful to construct a table noting the response factor drift with varying concentration.
• It should be noted that nitrogen, methane, and ethane gases exhibit less than 1 percent compressibility at atmospheric pressure. Other natural gas components exhibit significant compressibility at pressures below atmospheric pressure.
• Most components with vapor pressures less than 100 kPa cannot be used as pure gases for linearity studies because they do not exhibit sufficient vapor pressure to give a vacuum gauge reading of up to 100 kPa. For these components, a mixture containing nitrogen or methane can be used to produce a partial pressure that can extend the total pressure to 100 kPa. Using the table in the standard for vapor pressures at 38 degrees, the maximum pressure at which a given component can be mixed with nitrogen is calculated.

When starting the tests, the appropriate columns are first run as required for the desired study. The operating conditions are set and the chromatograph is allowed to stabilize. For hexanes and higher, the sample loop is heated. Most modern chromatographs are equipped with temperature-controlled valve ovens. When valve ovens are not available, it is recommended to mount the gas sampling valve in the chromatograph oven and run at column temperature.

After the instrument has apparently stabilized, control runs are made on a reference standard to determine instrument repeatability. Two consecutive controls should fall within the repeatability limits for the mole percent amount of each component present.

The average of two consecutive controls or the last control that falls within the repeatability limits of the previous control on each component may be used as the reference standard for all subsequent runs until there is a change in instrument operating conditions.

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