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

GBH Enterprises, Ltd.

Plant Analytical Techniques
GAS ANALYSIS:

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

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Gas Analysis:

1

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

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%
Carbon Dioxide 2%

Methane 99%
Methane 98%

Graph 2

Ethane 5%
Ethane 10%

Methane 95%
Methane 90%

Graph 3

Nitrogen 1%
Nitrogen 2%

Methane 99%
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


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

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