GPA STD 2177-2013 Analysis of Natural Gas Liquid Mixtures Containing Nitrogen and Carbon Dioxide by Gas Chromatography.pdf

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1、 Analysis of Natural Gas Liquid Mixtures Containing Nitrogen and Carbon Dioxide by Gas Chromatography Adopted as Tentative Standard. 1977 Revised and Adopted as a Standard. 1984 Revised 1989, 1995, 2003, 2012, 2013 Gas Processors Association 6526 East 60th Street Tulsa, Oklahoma 74145 GPA Standard 2

2、177-13 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, sta

3、te, 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 gu

4、arantee 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 pat

5、ent regarding apparatus, equipment, or method so covered. FOREWARD GPA 2177 provides the gas processing industry a method for determining the compositional analysis of demethanized liquid hydrocarbon streams containing nitrogen, air and carbon dioxide. Purity products such as an ethane/propane mix a

6、nd other products that meet the compositional ranges listed in the standard may also utilize this method. The precision statements contained in this standard are based on the statistical analysis of round-robin laboratory data obtained by GPA Section B. The detail of how this data yielded the equati

7、ons covered in the precisions statements can be found in GPA RR188. 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 are refere

8、nced. “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 method is intended for the analysis of demethanized liquid hydrocarbon streams containing nitrogen/air

9、 and carbon dioxide, and purity products such as an ethane/propane mix that fall within the compositional ranges listed in Table I. This method is limited to mixtures containing less than 5 mole percent of heptanes and heavier fractions (20 mole percent of hexanes and heavier fractions). Table I Com

10、ponents and Recommended Compositional Ranges Components Concentration Range (mole %) Nitrogen 0.01 5.0 Carbon Dioxide 0.01 5.0 Methane 0.01 5.0 Ethane 0.01 95.0 Propane 0.01 100.0 Iso-Butane 0.01 100.0 N-Butane and 2,2-Dimethylpropane 0.01 100.0 Iso-Pentane 0.01 15.0 N-Pentane 0.01 15.0 2,2-Dimethyl

11、butane, 2,2-Dimethylbutane, and 2-Methylpentane, 3-Methylpentane, Cyclopentane, N-Hexane 0.01 15.0 Heptanes & Heavier 0.01 5.0 1.2 The heptanes and heavier fraction if present in the sample is analyzed by either (1) use of a pre-cut column to elute heptanes and heavier first as a single peak, or (2)

12、 reverse flow of carrier gas after n-hexane, and peak grouping of the heptanes plus fraction. For purity mixes without heptanes and heavier, no reverse of carrier flow is required. NOTE 1 This method can be used for a hexanes plus analysis by adjusting the timing of the backflush event to occur foll

13、owing the elution of n-pentane. NOTE 2 CAUTION: In the case of unknown samples with a relatively large C6+or C7+fraction and where precise results are important, it is desirable to determine the molecular weight (or other pertinent physical properties) of these fractions. Since this method makes no

14、provision for determining physical Properties, the physical properties needed must be determined by extended analysis or agreed to by the contracting parties. The extended analysis will be made according to GPA Publication 2186. NOTE 3 For samples containing more than 5 mole percent of heptanes and

15、heavier fractions (20 mole percent of hexanes and heavier fractions), analysis will be made according to GPA Publication 2103. 2. SUMMARY OF METHOD 2.1 Components to be determined in a demethanized hydrocarbon liquid mixture are physically separated by gas chromatography and compared to calibration

16、data. A fixed volume of sample in the liquid phase is isolated in a suitable sample inlet system and flashed onto the chromatographic column. NOTE 4 For lab chromatographs, the comparison to calibration data is performed under identical operating conditions. For on-line chromatographs, the sample ma

17、y not necessarily be at the same conditions as the calibrations data so care should be taken to ensure that the pressure at the sample injection point is sufficiently above the vapor pressure of the product. 2.2 A precut column is backflushed when n-hexane has eluted into the main analytical column,

18、 so that the heptanes+ are eluted as a single peak at the beginning of the chromatogram. Two advantages of this method are: (1) Better precision in measuring the C7+portion of the sample, and (2) Reduction in analysis time over the single column approach by approximately 40%. See Figure 1. 2.2.1 As

19、an alternate method, the components nitrogen/air through n-hexane are individually separated with the carrier flow in the forward direction. The numerous heavy end components are grouped into an irregular shaped peak by reversing direction of carrier gas through the column via a switching valve imme

20、diately following the elution of normal hexane. (See Figure 2.) Samples which contain no heptanes+ fraction are analyzed until the final component has eluted with no reverse of carrier flow. 2.3 The chromatogram is interpreted by comparing the areas of component peaks obtained from the unknown sampl

21、e with corresponding areas obtained from a run of a selected reference standard. Any component in the unknown suspected to be outside the linearity range of the detector, with reference to the known amount of that component in the reference standard, must be determined by a response curve. (See Sect

22、ions 5.1 & 5.1.1 for further explanation of instrument linearity check procedures.) 3. APPARATUS 3.1 Any gas chromatograph may be used which meet the criteria listed in Section 8, Precision. The following equipment specifications have been found useful in designing an instrument to meet those criter

23、ia. 3.1.1 Environment - The gas chromatograph shall be housed in a climate controlled environment to ensure stability. Analysis of Natural Gas Liquid Mixtures Containing Nitrogen and Carbon Dioxide by Gas Chromatography 2 3.1.2 Detector - The detector shall be a thermal conductivity type. It must be

24、 sufficiently sensitive to produce a detectable peak for each component of interest at 100 ppm molar volume. 3.1.3 Sample Inlet System. Liquid - A liquid sampling valve must be provided, capable of entrapping a fixed volume of sample at a pressure at least 200 psi (1379 kPa) above the vapor pressure

25、 of the sample at valve temperature, and flashing this fixed volume into the carrier gas stream ahead of the analyzing column. A metering valve downstream of the liquid sampling valve is required to maintain the sample as a single phase liquid through the entire sample inlet system. (See Valve C in

26、Figure 3.) The fixed sample volume should not be of such volume as to impact instrument linearity and should be repeatable so that successive runs agree within 1% on each component peak area. The liquid sampling valve should be mounted exterior of any type heated Figure 1 Chromatogram of a Demethani

27、zed Hydrocarbon Liquid Mixture. (Frontal Carrier Gas Flow Through N-Hexane, with Precut of Heptanes Plus). Figure 2 Chromatogram of Demethanized Hydrocarbon Liquid Mixture. (Frontal Carrier Gas Flow through N-Hexane with backflush of Heptanes Plus). 3 compartment and thus be operated at laboratory a

28、mbient conditions. A sample filter is an optional device to protect the liquid sampling valve from scoring due to the presence of foreign contaminants such as metal shavings, dirt, etc., in an NGL sample. The filter should be of a small total volume, of an inline type design and contain a replaceabl

29、e/disposable element. NOTE 5 CAUTION: A filter may introduce error if not handled properly. The filter should be clean and free of any residual product from previous samples so that a build up of heavy end hydrocarbon components does not result. (This may be accomplished by a heating/cooling process

30、 or inert gas purge, etc.) The filter element should be 15 micron size or larger so that during the purging process NGL is not flashed, preventing fractionation and bubble formation. 3.1.4 Chromatographic Columns 3.1.4.1 Column No. 1 (Analytical Column) - A partition column must be provided capable

31、of separating nitrogen/air, carbon dioxide and the hydrocarbons methane through normal hexane. (See Figures 1 and 2.) Separation of carbon dioxide must be sufficient so that a sample containing 0.01 mole percent carbon dioxide will produce a detectable peak on the chromatogram. (The Silicone 200/500

32、 column, containing a 27-30 weight percent liquid phase load, has proven satisfactory for this type of analysis.) (See Paragraph B-2, Appendix B, GPA Publication 2261 for further explanation and construction of column.) 3.1.4.2 Column No. 2 (Precut Column) A partition column similar to column No. 1,

33、 it must be of the same diameter as column No. 1. The column must be of an appropriate length to clearly separate the heptanes plus fraction from the hexanes and lighter components. 3.1.5 Temperature Control The chromatographic column(s) and the detector shall be maintained at temperatures stable en

34、ough to achieve the precision requirements in Section 8. 3.1.6 Carrier Gas System Components of the carrier gas system must provide the ability to control the pressure and/or flow stable enough to achieve the precision requirements in Section 8. It is desirable to use a carrier gas which contains mi

35、nimal quantities of critical impurities (i.e. oxygen, water, and total hydrocarbons). 3.2 Integration Systems Integration software packages offer numerous options regarding peak interpretation. Users are advised to optimize integration prior to calibration and subsequent unknown sample runs. After e

36、ach analysis is completed the integration should be verified. 3. Sample Containers 3.3.1 Floating Piston Cylinder - A strongly preferred and recommended device suitable for securing, containing and transferring samples into a liquid sample valve and which preserves the integrity of the sample. (For

37、proper operation refer to GPA 2174.) These cylinders must have an acceptable mixing device. 3.3.2 Double Valve Displacement Cylinder - An alternate device, used in the absence of a Floating Piston Cylinder, suitable for securing, containing and transferring samples into a liquid sample valve. (For p

38、roper operation refer to ASTM D-1265.) NOTE 6 CAUTION: This container is ONLY acceptable when the displacement liquid does not appreciably affect the composition of the sample of interest. Specifically, components such as CO2 or aromatic hydrocarbons are partially soluble in many displacement liquid

39、s and thus may compromise the final analysis. This caution is of the utmost importance and should be investigated prior to utilizing this technique. Figure 3 Pre-Cut Valve Configuration. 4 4. ANALYSIS PROCEDURE 4.1 General - In the routine analysis of samples described in the scope of this procedure

40、, it is possible to obtain all components of interest from a single run. Response factors, on a selected reference standard, are used to convert peak areas of the unknown sample to liquid volume percent (mole percent or weight percent are acceptable). 4.2 Introduction of Sample The procedures for co

41、nnecting and introducing a sample either via floating piston cylinder or a double valve displacement cylinder are defined in GPA 2198. 4.3 Precut (or Backflush) Valve Switching 4.3.1 The use of a pre-cut column to group the C7+ fraction at the beginning of the chromatogram as a single peak is the pr

42、eferred method. (An acceptable valve configuration for the pre-cut method is illustrated in Fig. 3.) The valve position is switched when normal hexane and lighter components have traveled through Column 2 and are in Column 1. At this point, heptanes and heavier components are retained in Column 2. W

43、hen the valve is reversed, the heptanes plus fraction will elute from Column 2 first. Baseline must be clearly and distinctly established before elution of the C7 + peak so an accurate measurement of this peak can be obtained. After the elution of n-hexane the run should be terminated and the valve

44、returned to the initial position. 4.3.2 An alternative to the precut method is, after the elution of n-hexane, the carrier gas flow is reversed by means of a backflush valve operated manually or automatically. (An acceptable backflush valve configuration is shown in Fig. 4.) Reversing carrier flow c

45、auses severe bias, deviations (See Fig. 2). When using electronic digital integrators, exercise care to ensure integration does not occur until baseline is adequately re-established. The resulting irregular shaped C7 + peak is eluted over a period of time equivalent to time on forward flow minus the

46、 retention time for the air peak. Only after baseline is re-established should the run be terminated and carrier now returned to original direction. 5. CALIBRATION RUN 5.1 In conjunction with a calibration on any specific chromatograph, the linear range of the components of interest MUST be determin

47、ed. The linearity should be established for any new chromatograph and re-established whenever the instrument has undergone a major change (i.e. replaced detectors, increased sample size, switched column size, or dramatically modified run parameters). 5.1.1 This method utilizes gravimetrically prepar

48、ed standards of a higher concentration than is contained in the unknown. A set of response factors is first determined for all components via a blend mix. (See Section 5.3.) A second (or third) gravimetrically determined standard (either purity or blend) may then be run, using the originally obtaine

49、d response factors, which contain a concentration of individual components exceeding the expected amounts in the unknowns. If both (or all three) runs match their respective standards within the precision Guidelines allowed in Section 8, then the instrument may be considered linear within that range. NOTE 7 This method omits the need of a gas sample valve on the chromatographic instrument. However, several accurate primary NGL standards are required and the exact point at which nonlinearity occurs is not determined. An alternative and