Determination of Carbon Dioxide, Ethane And Nitrogen in Natural Gas by Gas Chromatography

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Plant Analytical Techniques GAS ANALYSIS: Determination of Carbon Dioxide, Ethane

And Nitrogen in Natural Gas by Gas Chromatography

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Gas Analysis: Determination of Carbon Dioxide, Ethane And Nitrogen in Natural Gas by Gas Chromatography

1 SCOPE AND FIELD OF APPLICATION

This document is a method for the determination of carbon dioxide, ethane and nitrogen in natural gas in the range 0-10% v/v.

2 PRINCIPLE

The gas sample will be injected automatically by a ten port valve onto the poraplot U column. The nitrogen will elute first and be switched to the mole sieve column. The mole sieve column will be isolated and the poraplot column will elute the carbon dioxide and ethane via a restrictor column to the detector. After the elution of the carbon dioxide and ethane the poraplot column will be back flushed. Then the nitrogen will be allowed to elute from the mole sieve column (see figure 1.)

A micro T.C.D. is used which depends on keeping one resistance filament as a reference and the second as an analyzer resistance filament.

3 Materials Required 3.1 Carrier Gas

3.1.1 Helium at not less than 100 kpa pressure.

3.2 Materials for the Preparation of Standards

3.2.1 Carbon Dioxide

3.2.2 Ethane

3.2.3 Nitrogen 3.2.4 Methane



METHANE IS EXPLOSIVE WHEN MIXED WITH AIR AT CONCENTRATION RANGING APPROXIMATELY FROM 4% TO 75% (V/V). CHECK FOR LEAKS

4 APPARATUS 4.1 General Description

4.1.1 Gas chromatographic system, the system comprises a gas chromatograph with a thermistor detector. A chrompack 9000 G.C. is suitable.

4.1.2 Characteristics of the Assembly

4.1.2.1 Column temperature, isothermal temperature control range

30°C - 50°C to within + 0.5 oC.

4.1.2.2 Detector temperature, should be within the range 80°C - 120°C to Within + 0.5 oC.

4.1.3 Gas Controls and Flow Measurement

4.1.3.1 Pressure regulators, supplied by manufacturers of the gas

chromatographic equipment.

4.1.3.2 Bubble flow meter, which can be used over a range of 0.1 to 50ml/MIN.

4.2 Injection Equipment

4.2.1 Injection system comprising a ten port gas sampling valve with a 50µl sample loop.

4.3 Columns

4.3.1 3 Columns are required.

4.3.1.1 Poraplot U Column (Quartz/Capillary) 10 m x 0.32mm t 25 m x 0.32 mm (Particle/Trap).



4.3.1.2 Mole sieve Column (Quartz Capillary). 4 m x 0.32mm X 25 m to 0.32mm deactivated fused silica.

4.3.1.3 Restriction Capillary Column. 10 m x 0.32mm deactivated fused silica.

4.3.2 Conditioning

4.3.2.1 The mole sieve column is conditioned by passing

helium through the column for 12 hours with the oven set at 17OOC.

4.3.3 Resolution

There shall be full resolution from peak height to base line between all peaks as shown on the typical chromatograph (see Figure 2).

4.4 Detector

The micro thermal conductivity detector is a dual channel device with a reference flow of carrier gas through the second channel. The detector measures the difference in thermal conductivity between two gas streams when a sample passes through one of the channels. Make-up gas is required.

4.5 Recorder

Having the following characteristics.

4.5.1 Suitable for a chart speed of 10 mm/min.

4.5.2 Single span 1mv measuring range. 5 Sample

5.1 The gas sample to be analyzed is collected in a Hilger watts cylinder.

5.2 Preparation of Test Portions



Approximately 100 ml of the sample shall be purged through the sample loop, before turning the sample into the chromatograph.

6 Procedure

6.1 Setting up the Apparatus

6.1.1 Oven and Columns

6.1.1.1 Oven Temperature - 40°C.

6.1.1.2 Helium Carrier to Detector 4.5 ml/min.

6.1.1.3 Detector Temperature - 100°C.

6.2 Calibration

6.2.1 Calibrate by external calibration.

6.2.2 Standard mixtures.

6.2.2.1 Purity of components. The purity of the gases used to make standard blends shall not be less than 99.5%

6.2.2.2.1 Calibration gas mixtures are produced using a

gas blending instrument, a Wosthoff Gas proportioning pump or a signal gas blender are suitable.

The composition of the calibration mixtures % v/v is:

Graph 1 Carbon Dioxide 1% Methane 99%

Carbon Dioxide 2% Methane 98%

Graph 2 Ethane 5% Methane 95% Ethane 10% Methane 90%

Graph 3 Nitrogen 1% Methane 99%

Nitrogen 2% Methane 98%

6.2.2.2 The calibration mixtures are passed directly into the sample loop (see 5.2).



6.2.3 Presentation of the calibration data. The calibration results are presented in the form of a graph for each component to be measured.

6.2.4 Working Standard

A sample of natural gas is taken.

Inject a portion of this gas into the gas chromatograph (see 5.2).

From the resulting chromatogram (see Figure 2) work out the carbon dioxide, ethane and nitrogen content, using the graphs (see 6.2.3).

6.3 Test

6.3.1 Preparation of the Test Portion

Collect natural gas sample as described in (5.1).

6.3.2 Introduction of the Test Portion

Purge 100 ml of sample through the 10 part valve (2), with the valve set in the purge position. Disconnect the sample cylinder, then turn the valve to the sample in position.

6.4 Examination of the Chromatogram

6.4.1 Typical Chromatogram

A typical chromatogram for the analysis is shown in Figure 2.

6.4.2 Identification

A list of the gases together with their typical retention times measured on the calibration mixture are used for identifying the peaks in the samples.



7 EXPRESSION OF RESULTS

7.1 Identification of sample components can be confirmed by comparison of retention distance data with that obtained for the working standard.

7.2 The components concentration, expressed as X v/v is given by the

expression.

C x HS Hc where C is the concentration of the components in the working standard (in % v/v); HS is the peak height of the component in the sample (in mm); HC is the peak height of the component in the standard (in mm).



8 REPEATABILITY







Date post:	02-Nov-2014
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