# analysis of natural gas



## نبيل عواد الغباري (23 ديسمبر 2010)

Two methods used worldwide for analysis of natural gas are as follows. 
1) ASTM D 1945-98 (2001), ”Standard Test Method for Analysis of Natural Gas by Gas Chromatography” --- adopted by Egyptian gas suppler 
2) GPA* Standard 2261-1990, “Analysis for Natural Gas and Similar Gaseous Mixtures by Gas Chromatography” --- used by Japanese electric power companies
* Gas Processors Association., USA

The method 1) will be mainly described below, while the method 2) is mentioned in [ ] for reference and comparison. 

1. Summary of Test Method
 Components in a sample are physically separated by gas chromatography and are calculated by comparing either the peak height, or the peak area, or both, with the corresponding value obtained under identical operating conditions from a reference standard mixture of known composition. [the peak area only (from the viewpoint of wider detector response linearity)]

2. Apparatus
2.1 Detector: Thermal-conductivity type (TCD) or equivalent [TCD only], 
Its temperature control <0.3C [<0.2C]
2.2 Sample Inlet System: Volume <0.5ml (from detector response linearity), 
Sampler temperature- within 1C, Sampled volume repeatability <1% [<0.5%]
Do not use rubber or plastics not adsorb sample component. Keep the system small and the drier small to minimize the purging. 
2.3 Column Temperature Control: <0.3C [<0.2C] for isothermal operation.
 Temperature programming as feasible [Not mentioned]
2.4 Carrier Gas Control: <1% [<0.5%]
2.5 Columns (Refer to Table 1)
1) Adsorption column: for O2, N2, CH4 [for O2, N2] 
Molecular sieve 13X is recommended.
2) Partition column: for C2-C5, CO2, C6+; CH4 if separated [for CH4-C5, CO2, C6+; if it is difficult to separate CO2, use a porous polymer column for CO2] 
The separation time is within 40 min. [30min]
3) General:
o Other column packing materials that provide satisfactory separation may be used. 
o In multicolumn applications, it is preferred to use front-end backflash of the heavy ends.
o The chromatograms shown in ASTM standard are only illustrations of typical separations. The operating conditions, including columns, are also typical and are subject to optimization by competent personnel.
[GPA standard describes in detail how to prepare each column in Appendix B.]
2.6 Drier: must be provided in the sample entering system, ahead of the sample valve.
How to prepare it is described in Annex A2.2. 
2.7 Valves: required to permit switching, backflushing, or for simultaneous analysis.

3. Preparation of Apparatus
Linearity Check: As a minimum requirement, the linearity data must be obtained for any component that exceeds a concentration of 5mol%. --- for the check, pure compounds, manometer, and vacuum pump are needed.

4. Reference Standards
Moisture-free gas mixtures of known composition are required for comparison with the test sample. (not less 1/2 nor more 2 times the concentration of each component in the sample [not mentioned in GPA standard]) 

5. Procedure
5.1 Instrument Preparation 
After the instrument has stabilized, make check runs on the reference standard to establish instrument repeatability. Two consecutive checks must agree within 1% [0.5%] on each component,. The average of the two may be used as the reference standard.
5.2 Sample Introduction
In case of purging method: The amount of purging required must be established and verified for each instrument. The sample loop pressure should be near atmospheric. [GPA standard describes in detail how to purge in 6.4.3.]
5.3 Partition Column Run for Ethane and Heavier Hydrocarbons and Carbon Dioxide
 Enter the sample, and backflush heavy components when appropriate. Obtain a corresponding response on the reference standard.
5.4 Adsorption Column Run for Oxygen, Nitrogen, and Methane 
 Enter the sample, and obtain a response. Likewise Obtain a corresponding response on the reference standard.

6. Calculation
6.1 The expressed numerical value should not be presumed to be more accurate the certified value in the calibration standard.
6.2 External Standard Method 
1) Pentanes and Lighter Components
Measure the height of each component peak in the sample and reference standard.
 C = S x (A/B)
where: C = component concentration in the sample, mol%
 A = peak height of component in the sample, mm
 B = peak height of component in the standard, mm
 S = component concentration in the reference standard, mol%
2) Hexanes and Heavier Components
Measure the area of the hexanes portion and the heptanes and heavier portion of the reverse-flow peak. Also measure the area of both pentanes peaks.
 Corrected C6 area = 72/86 x measured C6 area
 Corrected C7+ area = 72/A x measured C7+ area
where: A = average molecular weight of C7+ fraction, but 98 is usually acceptable.
 Mol% of C6 = (Corrected C6 area) x (mol% of iC5+nC5) / (iC5+nC5 area) 
Mol% of C7+ = (Corrected C7+ area) x (mol% of iC5+nC5) / (iC5+nC5 area)
[Measure the area of the hexanes and heavier peak of the precut chromatogram. Also measure the total area of pentanes peaks.
 Corrected C6+ area = 72/A x measured C6+ area
where: A = average molecular weight of C6+ fraction, but 92 is usually acceptable.
 Mol% of C6+ = (Corrected C6+ area) x (mol% of iC5+nC5)/(iC5+nC5 area) ]
3) Normalize the mol percent values by multiplying 100/(sum of original values). The sum of the original values should not differ from 100.0% by more than 1.0%.

7. Precision
 Component, mol% Repeatability Reproducibility
 0 – 0.1 0.01 0.02
 0.1 – 1.0 0.04 0.07
 1.0 – 5.0 0.07 0.10
 5.0 – 10 0.08 0.12
 10 – 0.10 0.15
 [GPA standard statements on precision are for each component by relative percentage.]
Table 1 Columns and Operating Conditions for Natural Gas Analysis

The columns, and their operating conditions shown in ASTM (A) and GPA (G) are as follows. Their chromatographs are shown in each standard. 

(a) Adsorption Column – for O2, N2, methane (A): for O2, N2 (G)
13X molecular sieve 80/100 mesh-2 meter x ? – ?C – 0.25ml – He 30ml/min (A) 
5A molecular sieve, with pre-cut column – no chromatograph(A)
13X molecular sieve 40/60 mesh-10’ (3.05m) x 1/8” – 40C – ?ml – He25ml/min(G)

 (b) Partition Column – for ethane and heavier hydrocarbons, carbon dioxide (methane if possible) (A); methane and heavier hydrocarbons, carbon dioxide (G)
25% BMEE on Chromosorb P-7 meter x ? – 25C – 0.25ml – He40ml/min (A)
 Chromosorb PAW, Silicone 200/500-10 meter x ?– ?C – 0.25ml – He40ml/min (A)
DIDP-3 meter + DMS-6 meter x ? – 35C – 0.5ml – He75ml/min(A)
Silicone 200/500, 30%-30’(9.15m) x 1/8” + Silicone 200/500, 1%-1.5’ (0.45m) x 1/8” – 120C – 0.25ml – Inlet P.4atm (G)
Silicone 200/500 30’ (9.15m) x ? – 90C – 5ml – Inlet P.2atm (G)


(c) Porous Polymer Column – for carbon dioxide (if its analysis is impossible by (b).) (only GPA)
 Porapak Q 80/100-10’ (3.05m) x 1/8” – 100C – 0.5ml – Inlet P. 2atm(G)

(d) (For multi-column application)
 152 Squalane on Chromosorb PAW 80-100 mesh-3 meter + Porapak N 80-100 mesh-2 meter + Molecular Sieve 5-A, 80-100 mesh-2 meter – conditions not mentioned ​


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## مهندس المحبة (24 ديسمبر 2010)

مشكور على هذا الموضوع المفيد في المضمون وننتظر المزيد من مواضيعك المميزة ....


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## BEWITHME (25 فبراير 2011)

جزاك الله خيرا


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