1、 Analysis for Natural Gas and Similar Gaseous Mixtures by Gas Chromatography Adopted as Tentative Standard, 1961 Revised and Adopted as a Standard, 1964 Revised 1972, 1986, 1989, 1990, 1995, 1999, 2000 and 2013 Gas Processors Association 6526 East 60th Street Tulsa, Oklahoma 74145 GPA Standard 2261-
2、13 DISCLAIMER GPA publications necessarily address problems of a general nature and may be used by anyone desiring to do so. Every effort has been made by GPA to assure accuracy and reliability of the information contained in its publications. With respect to particular circumstances, local, state,
3、and federal laws and regulations should be reviewed. It is not the intent of GPA to assume the duties of employers, manufacturers, or suppliers to warn and properly train employees, or others exposed, concerning health and safety risks or precautions. GPA makes no representation, warranty, or guaran
4、tee in connection with this publication and hereby expressly disclaims any liability or responsibility for loss or damage resulting from its use or for the violation of any federal, state, or municipal regulation with which this publication may conflict, or for any infringement of letters of patent
5、regarding apparatus, equipment, or method so covered. FOREWARD GPA 2261 provides the gas processing industry a method for determining the chemical composition of natural gas and similar gaseous mixtures using a Gas Chromatograph (GC). The precision statements contained in this standard are based on
6、the statistical analysis of round-robin laboratory data obtained by Section B. This standard was developed by the cooperative efforts of many individuals from industry under the sponsorship of GPA Section B, Analysis and Test Methods. Throughout this publication, the latest appropriate GPA Standards
7、 are referenced “Copyright2013 by Gas Processors Association. All rights reserved. No part of this Report may be reproduced without the written consent of the Gas Processors Association.” 1 1. SCOPE 1.1 This standard covers the determination of the chemical composition of natural gas and similar gas
8、eous mixtures within the ranges listed in Table 1, using a Gas Chromatograph (GC). The three columns represent the original Table 1, but separate the values to three distinct groups. The first group is concentrations lower than the data obtained from the round-robin project (RR-188). The second grou
9、p is concentrations used in the round-robin project (RR-188). The equations listed in the precision statement in this standard cover the range listed in the middle column, after outliers were removed. The third group is concentrations higher than the data obtained from the round-robin project (RR-18
10、8). The precision statement in this standard utilizes equations derived from a regression of the data in RR-188 and is detailed in GPA TP-31. The precision statement criterion applies only to values listed in Section 10, Table 6. 1.2 Components sometimes associated with natural gases, i.e., helium,
11、hydrogen sulfide, water, carbon monoxide, hydrogen and other compounds are excluded from the main body of the method. These components may be determined and made a part of the complete compositional data. Refer to Appendix A. Table I Ranges of Natural Gas Components Covered Component Lower Region Ro
12、und Robin Higher Region Nitrogen 0.01 - 0.1 0.1 - 30 30 Carbon Dioxide 0.01 - 0.1 0.1 - 30 30 Methane 0.01 - 40 40 - 100 N / A Ethane 0.01 - 0.1 0.1 - 10 10 Propane 0.01 - 0.1 0.1 - 10 10 Isobutane 0.01 - 0.25 0.25 - 4 4 n-Butane 0.01 - 0.25 0.25 - 4 4 Isopentane 0.01 - 0.12 0.12 - 1.5 1.5 n-Pentane
13、 0.01 - 0.12 0.12 - 1.5 1.5 * Hexanes Plus 0.01 - 0.1 0.1 - 1.5 1.5 * Heptanes Plus 0.01 - 0.1 0.1 - 1.5 1.5 *Data from round robin was only obtained for Hexanes Plus Table Note: Uncertainty in the Lower region can easily be ten times greater and in the higher region two to three times greater than
14、the center column. NOTE 1 Components not listed in Table 1 may be determined by procedures outlined in Appendix A or other applicable analytical procedures. Refer to Appendix A. 2. SUMMARY OF METHOD 2.1 Components to be determined in a gaseous sample are physically separated by gas chromatography an
15、d compared to calibration data obtained under identical operating conditions. A fixed volume of sample in the gaseous phase is isolated in a suitable inlet sample system and entered onto the column. 2.2 The full range analysis of a gaseous sample may require multiple runs to properly determine all c
16、omponents of interest. The primary run is on a partition column to determine air, methane, carbon dioxide, ethane and heavier hydrocarbons. When oxygen/argon content is critical in the unknown sample, or is suspected as a contaminant, a secondary run should be made to determine oxygen/argon and nitr
17、ogen in the air peak on the partition column. When carbon dioxide content in the unknown sample does not fall within the calibrated range on the partition column, a secondary run should be made to determine carbon dioxide content. When helium and/or hydrogen content are critical in the unknown sampl
18、e, a secondary run should be made to determine helium and/or hydrogen. 2.2.1 These analyses are independent and may be made in any order, or may be made separately to obtain less than the full range analysis. The configuration can consist of a single or multiple GCs to accomplish this. Refer to Appe
19、ndix A. 2.3 Response factors or response curves derived from calibration data are essential to accurately determine the composition of an unknown sample. The reference standard blend and the unknown samples must be run using identical GC operating conditions. 3. APPARATUS 3.1 Chromatograph - Any Gas
20、 Chromatograph may be used as long as the specifications for repeatability and reproducibility stated in Section 10 within the round-robin test component ranges listed in Table 1 are met or exceeded. The equipment described in this section has been proven to meet the above requirements; however othe
21、r configurations including portable and online may be acceptable. 3.1.1 Detector - The Thermal Conductivity Detector (TCD) has proven to be a reliable and universal detector for this method. 3.1.2 Sample Inlet System - A gas sampling valve capable of introducing sample volumes of up to 0.500 ml may
22、be used to introduce a fixed volume into the carrier gas stream at the head of the analyzing column. The Analysis for Natural Gas and Similar Gaseous Mixtures by Gas Chromatography 2 sample volume should be repeatable such that successive runs meet the precision requirements of Section 10. NOTE 2 Th
23、e sample size limitation of 0.500 ml or smaller is selected relative to linearity of detector response and efficiency of column separation. Larger samples may be used to determine low-quantity components in order to increase measurement accuracy. 3.1.3 Chromatographic Columns 3.1.3.1 Partition Colum
24、n - This column must separate nitrogen (air), carbon dioxide, and the hydrocarbons methane through n-Pentane. (or n-Hexane when a C7 plus analysis is performed). Silicone DC 200/500, 30% by weight on 80/100 mesh Chromosorb P, acid washed, packed into 30 x 1/8” SS tubing has proven to be satisfactory
25、 for this purpose. 3.1.3.2 Precut Column A backflush column similar to the partition column described in 3.1.3.1. This column must be of the same diameter and long enough to clearly separate the hexanes plus or heptanes plus fraction from the lighter components. Figure 1A shows an example chromatogr
26、am of a natural gas mixture using the precut column for grouping the hexanes and heavier (heptanes and heavier in Figure 1B). 3.1.3.3 Pressure Buffer Column - A lightly loaded column placed between the detector inlet and the column switching/sampling valve (Figure 2A, Column 3) may help to position
27、the hexanes and heavier peak to provide better resolution. This column is usually 1 wt% Silicone 200/500 between 12” and 40” long. (Figures 2A and 2B show a typical column switching/sampling valve arrangement). NOTE 3 The arrangements of columns, detectors and valves depicted in Figure 2A and 2B hav
28、e been determined to meet or exceed the performance criteria of this standard. (See Section 10, “Precision”.) 3.1.4 Temperature Control -The chromatographic columns and the detector should be maintained at temperatures consistent enough to provide repeatable peak retention times and compositional pr
29、ecision within the limits described in Section 10 during the reference standard and corresponding sample runs. 3.2 Carrier Gas - The contaminants in the carrier gas must be limited to levels that are known not to interfere with the analysis or cause maintenance problems with the GC. Refer to manufac
30、turer for recommendations regarding carrier gas quality 3.2.1 Pressure and Flow Control Devices - These devices should maintain flow rate consistent enough to provide repeatable peak retention times and compositional precision within the limits described in Section 10 during the reference standard a
31、nd corresponding sample runs. Two Stage regulators with stainless steel diaphragms have been shown to be satisfactory for this purpose. Figure 1A Chromatogram of early backflush of hexanes and heavier (C6+). Figure 1B Chromatogram of early backflush of heptanes and heavier (C7+). 3.3 Sample Conditio
32、ning Systems - GPA 2166 gives guidance for proper design and use of sample conditioning systems. The sample conditioning system should not cause the GC precision to fall outside the requirements in Section 10. NOTE 4 Valves and sample introduction system must be maintained at a temperature above the
33、 hydrocarbon dew point of the calibration blend and unknown samples. Supplemental heating may be required to accomplish this. Refer to GPA 2166 for guidance. 3.4 Integration System - The integration system should be configured to properly integrate all peaks of interest. Integration systems can not
34、correct for inadequate component separation. The integration system should not cause the GC precision to fall outside the requirements in Section 10. 3 Figure 2A Two Six port valves used for sample injection and precut backflush. Figure 2B One Ten port valve used for sample injection and precut back
35、flush. 4. NATURAL GAS QUALITY ASSURANCE 4.1 Determination of Linear Range - GPA 2198 describes procedures to establish the linear range of a GC system. This process is necessary to determine the proper calibration and analytical procedures for each instrument. 4.2 Fidelity Plot - GPA 2198 describes
36、the procedure to create a Fidelity Plot. The Fidelity Plot is a tool that can be used to monitor the validity of calibration standards and performance of GC systems. 4.3 Control Charts - GPA 2198 describes the use of Control Charts. Control Charts can be used to monitor each component in the calibra
37、tion blend and the GC performance over time. 4.4 Precision Test - Section 10 of this document establishes the precision requirements of this standard. 5. SAMPLE INTRODUCTION 5.1 Sample Introduction -The sample introduction must be performed in the same manner for calibration and subsequent unknown s
38、amples. It is acceptable to either perform a purged or evacuated introduction. Successive runs must be repeatable and not contain contamination from the previous injection. Refer to Appendix A for discussions on linearity, calibration and other related topics. 5.1.1 Purged Introduction - Determine t
39、he rate and duration of the purge. Perform alternate injections using a suitable reference blend and instrument carrier gas. Perform alternate injections of each material at various purge rates and purge durations. Note the rate and duration of each purge test and the component concentrations from e
40、ach run. Repeatability of each component must meet the criteria listed in Section 10, “Repeatability” on the sample runs for the purge rate to be acceptable. Results from the carrier gas blank run must not contain carryover (individual peaks) greater than 0.01 un-normalized mol % from the previous i
41、njection of sample for the duration to be sufficient. Once this has been established, this rate and duration should be used for all calibration and analytical runs. 5.1.2 Evacuated Introduction - Evacuate the sample entry system and observe the vacuum gage or manometer for pressure changes indicatin
42、g a leak. Leaks must be repaired before proceeding. Determine the pressure to be used for injections. Perform alternate injections of a suitable reference blend and carrier gas. Make replicate runs at the selected pressure. Repeatability of each component must meet the criteria listed in Section 10,
43、 “Repeatability”. Use this pressure for calibration and analytical runs. Results from the carrier gas blank run must not contain carryover (individual peaks) greater than 0.01 un-normalized mol % from the previous injection of sample. 5.1.3 Equilibration - All sample injections must be performed in
44、the same manner for known and unknown sample compositions. The sample introduction system must be allowed to equilibrate prior to operation of the gas sample valve. 5.2 Preparation and Introduction of Sample Samples must be properly conditioned prior to analysis. GPA 2166 gives guidance on proper he
45、ating of sample containers and sampling systems. Refer to GPA 2166. NOTE 5 To ensure representative samples are obtained in the field, refer to GPA Publication 2166. 5.2.1 Sample connections and tubing used in the sample entry system of the GC must be composed of material that does not cause sample
46、distortion. Stainless Steel and Nylon 11 have proven to perform in this manner. Rubber and other plastic tubing must not be used since these materials readily absorb hydrocarbons. 6. CALIBRATION PROCEDURE 6.1 Calibration 6.1.1 Response factors for the components of interest are determined in accorda
47、nce with the calculations discussed in Section 8. This can be accomplished by 4 various means. Either single level calibration(s) using one or more certified reference blend(s) or a multi-level calibration using at least three certified reference blends is acceptable. 6.1.2 Procedures discussed in S
48、ection 4 and the calibration type will determine the calibrated range. All components in the unknown samples should lie within the calibrated range for a specific GC. (See Section 10, “Precision”.) 6.1.3 Calibration should be verified on a set frequency. Verifications can utilize a single blend or m
49、ultiple blends. At least two runs should be made to verify repeatability. If the calculated concentrations deviate by more than the precision requirements for repeatability listed in Section 10, or the un-normalized total deviates by more than 1% from 100 %, instrument maintenance or recalibration may be necessary. First verify the calibration blend is valid, then verify the instrument is operating properly (repair as required), and then recalibrate if necessary. 6.1.4 Fidelity plots and Control Charts, as described in GPA 2198, are excellent tools to monitor instrumen
