1、Designation: D7493 14 (Reapproved 2018)Standard Test Method forOnline Measurement of Sulfur Compounds in Natural Gasand Gaseous Fuels by Gas Chromatograph andElectrochemical Detection1This standard is issued under the fixed designation D7493; the number immediately following the designation indicate
2、s the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method is for on-line measurement of
3、 volatilesulfur-containing compounds in gaseous fuels by gas chroma-tography (GC) and electrochemical (EC) detection. This testmethod is applicable to hydrogen sulfide, C1 to C4 mercaptans,sulfides and tetrahydrothiophene (THT).1.1.1 Carbonyl sulfide (COS) is not covered in this testmethod.1.1.2 The
4、 detection range for sulfur compounds is approxi-mately from 0.1 to 100 ppmv (mL/m3) or 0.1 to 100 mg/m3.The detection range may vary depending on the sampleinjection volume, chromatographic peak separation and thesensitivity to the specific EC detector.1.2 This test method describes a GC-EC method
5、employingpacked GC columns and a specific detector for natural gas andother gaseous fuel composed of mainly light (C4 and smaller)hydrocarbons. Alternative GC columns, detector designs andinstrument parameters may be used, provided that chromato-graphic separation, quality control and measurement ob
6、jectivesneeded to comply with user, or regulator needs or both, areachieved.1.3 This test method does not intend to identify and measureall individual sulfur species, and is mainly employed formonitoring naturally occurring reduced sulfur compoundscommonly found in natural gas and fuel gases or empl
7、oyed asan odorant in these gases.1.4 The test method is typically employed in repetitive orcontinuous on-line monitoring of sulfur components in naturalgas and fuel gases using a single sulfur calibration standard.Need for a multipoint calibration curve or quality controlprocedures can be satisfied
8、by making use of proceduresdelineated in Test Methods D5504, D5623, D6228, D6968,ISO 19739, or GPA 2199.1.5 The test method can be used for measurement of allsulfur compounds listed in Table 1 in air or other gaseousmatrices, provided that no compounds that can interfere withthe GC separation and el
9、ectrochemical detection are present.1.6 This test method is written as a companion to PracticesD5287, D7165 and D7166.1.7 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.8 This standard does not purport to address all of thesa
10、fety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety, health, and environmental practices and deter-mine the applicability of regulatory limitations prior to use.1.9 This international standard was developed in accor-da
11、nce with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM St
12、andards:2D3609 Practice for Calibration Techniques Using Perme-ation TubesD4150 Terminology Relating to Gaseous FuelsD4626 Practice for Calculation of Gas ChromatographicResponse FactorsD5287 Practice for Automatic Sampling of Gaseous FuelsD5504 Test Method for Determination of Sulfur Compoundsin Na
13、tural Gas and Gaseous Fuels by Gas Chromatogra-phy and ChemiluminescenceD5623 Test Method for Sulfur Compounds in Light Petro-leum Liquids by Gas Chromatography and Sulfur Selec-tive Detection1This test method is under the jurisdiction ofASTM Committee D03 on GaseousFuels and is the direct responsib
14、ility of Subcommittee D03.12 on On-Line/At-LineAnalysis of Gaseous Fuels.Current edition approved July 1, 2018. Published July 2018. Originally approvedin 2008. Last previous edition approved in 2014 as D7493-14. DOI: 10.1520/D7493-14R18.2For referenced ASTM standards, visit the ASTM website, www.as
15、tm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis inter
16、national standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Com
17、mittee.1D6228 Test Method for Determination of Sulfur Compoundsin Natural Gas and Gaseous Fuels by Gas Chromatogra-phy and Flame Photometric DetectionD6968 Test Method for Simultaneous Measurement of Sul-fur Compounds and Minor Hydrocarbons in Natural Gasand Gaseous Fuels by Gas Chromatography and A
18、tomicEmission DetectionD7165 Practice for Gas Chromatograph Based On-line/At-line Analysis for Sulfur Content of Gaseous FuelsD7166 Practice for Total Sulfur Analyzer Based On-line/At-line for Sulfur Content of Gaseous Fuels2.2 ISO Standards:3ISO 19739 Natural gas Determination of sulfur com-pounds
19、by gas chromatography2.3 GPA Standard4GPA 2199 Determination - Determination of Specific SulfurCompounds by Capillary Gas Chromatography and SulfurChemiluminescence Detection3. Terminology3.1 Common terminology used in this method are cited inTerminology D4150. Sulfur compounds are commonly re-ferre
20、d by their initials (chemical or formula), for example,3.2 Abbreviations:hydrogen sulfide =H2Smethyl mercaptan = MeSH (MM)ethyl mercaptan = EtSH (EM)dimethyl sulfide = DMSi-Propyl mercaptan = IPMn-Propyl mercaptan = NPMt-Butyl mercaptan = TBMtetrahydrothiophene = THT or Thiophane4. Summary of Test M
21、ethod4.1 Gaseous fuel is directly sampled on-line for analysis ofsulfur compounds. Samples are introduced to the GC instru-ment through a sampling system. Sulfur compounds areseparated by a GC column and measured by an EC detector.The method requires periodic calibration using certified stan-dards.
22、The test method conforms to the practices stated inPractice D7165.4.2 A fixed volume of the sample (normally 0.25 mL) isinjected into a gas chromatograph operating isothermallywhere components are separated using two chromatographiccolumns.4.3 GC-separated sulfur compounds are determined usingan ele
23、ctrochemical detector utilizing a chromic acid electro-lyte.5. Significance and Use5.1 Gaseous fuels, such as natural gas, petroleum gases andbio-gases, contain sulfur compounds that are naturally occur-ring or that are added as odorants for safety purposes. Thesesulfur compounds are odorous, corros
24、ive to equipment, and caninhibit or destroy catalysts employed in gas processing andother end uses. Their accurate continuous measurement isimportant to gas processing, operation and utilization, and isfrequently of regulatory interest.5.2 Small amounts (typically, total of 4 to 6 ppmv) of sulfurodo
25、rants are added to natural gas and other fuel gases for safetypurposes. Some sulfur odorants are reactive, and may beoxidized to form more stable sulfur compounds having lowerodor thresholds which adversely impact the potential safety ofthe gas delivery systems and gas users. Gaseous fuels areanalyz
26、ed for sulfur compounds and odorant levels to assist inpipeline integrity surveillance and to ensure appropriate odor-ant levels for public safety.5.3 This method offers an on-line technique to continuouslyidentify and quantify individual target sulfur species in gaseousfuel with automatic calibrati
27、on and validation.6. Apparatus6.1 ChromatographIndustrial gas chromatograph with anisothermal oven, automatic injection valve, and software nec-essary for interfacing to a chromic acid electrochemical detec-tor and designed for the intended application. The GC systemmust be inert, well-conditioned a
28、nd passivated with a gascontaining the sulfur compounds of interest to ensure reliableresults.3Available from International Organization for Standardization (ISO), 1, ch. dela Voie-Creuse, Case postale 56, CH-1211, Geneva 20, Switzerland, http:/www.iso.ch.4Available from Gas Processors Association (
29、GPA), 6526 E. 60th St., Tulsa, OK74145, http:/.TABLE 1 Typical Retention Times of Sulfur Components of Different GC-ECD RunsGC-EC instrument GC-EC #1 GC-EC #2 GC-EC #3GC-Column andparameters18 in. ID 70 cm L,N2, 12 mL/min, 65 C1.6 mm ID 1200 mm L,N2, 100 mL/min, 20 C4 mm ID 400 mm L,N2, 100 mL/min,
30、20 CDetector Size 520 mm 520 mm 3025 mmSulfur Compound RT (sec.) RT (sec.) RT (sec.)Hydrogen sulfide, H2S30 30 30Methyl mercaptan (MeSH) 70 66 60Ethyl mercaptan (EtSH) 105 150 80Dimethyl sulfide (DMS) 120 200 80i-Propyl mercaptan (IPM) 160 240 160t-Butyl mercaptan (TBM) 220 342 240n-Propyl mercaptan
31、 (NPM) 265 426 290i-Butyl mercaptan (IBM) 440 . 560n-Butyl mercaptan (NBM) 585 . .Thiophane (THT) 900A720A2100AThe shorter GC column is employed for simultaneous accelerated analysis of THT.D7493 14 (2018)26.1.1 Sample Inlet SystemThe gas sample is introduced tothe GC by sample loop injection. An au
32、tomated non-reactivegas sampling valve is employed for a fixed sample loopinjection. The sample injection port must be heated continu-ously at a temperature significantly (10 C) above the tem-perature at which the gas was sampled to avoid samplecondensation and discrimination. Inert tubing made of n
33、on-permeable, non-sorbing and non-reactive materials, as short aspossible and heat traced at the same temperature, should beemployed for transferring the sample from a sample source tothe gas sampling valve and to the GC inlet system. Silica-coated 316 stainless steel (s.s.) and non-permeable polyte
34、tra-fluoroethylene (PTFE) type tubing are often employed. Differ-ent size fixed-volume sample loops (0.25 to 10.0 mL) may beused to target multiple concentration ranges for components ina gas, provided chromatographic separation and quality controlobjectives are obtained. The same non-reactive mater
35、ials areused for the sample loop to avoid possible decomposition orabsorption of reactive species. The sampling and GC inletsystem must be well conditioned and evaluated frequently forcompatibility with trace quantities of reactive sulfurcompounds, such as tert-butyl mercaptan.Aprogrammable andcompu
36、ter-controlled multi-stream sample selector can be usedto sample fuel gases and calibration gases.6.1.2 Column TemperatureThe gas chromatograph mustbe capable of maintaining an isothermal temperature, normallyat 65 C, with temperature variation not exceeding 60.5 C.6.1.3 Carrier and Detector Gas Con
37、trolConstant flowcontrol of carrier and detector gases is necessary for optimumand consistent analytical performance. Control is best providedby the use of pressure regulators and fixed flow restrictors. Thegas flow rate is measured using a gas flow meter eithervolumetrically or based upon mass flow
38、 rates. Mass flowcontrollers, capable of maintaining gas flow constant to 61%at the required flow rates should be used. The supply pressureof the gas delivered to the gas chromatograph must be at least69 kPa (10 psi) greater than the regulated gas at the instrumentto compensate for system back press
39、ure.6.1.4 DetectorAn EC detector, whose operation is basedupon the reduction/oxidation reaction between reduced sulfurcompounds and a solution of chromic acid (Fig. 1), is used inthis method. The detector is set according to the manufacturersspecifications for this particular application. One EC det
40、ector isnormally employed for measurement. A second column isemployed for detection of late-eluting sulfur compounds, suchas THT.6.1.4.1 The detector consists of a glass or methyl polymeth-acrylate container. The electrodes, two pieces of platinumgauze grids, are arranged vertically in parallel, and
41、 are weldedin a borosilicate glass tube. These grids are isolated from eachother and other conductive materials and connected to anamplifier for data acquisition.6.1.4.2 The electrolyte, a solution of chromium (VI) oxidein distilled or deionized water (100 g/L or 0.66 mole/L), iscontained in an acid
42、-resistant vessel. A tube fitted with theelectrodes is dipped into the solution such that the liquid isretained by capillary action within the tube at a level approxi-mately midway between two grids.6.1.4.3 The gas flow from the GC column is dischargedthrough a narrow glass or PTFE tube (2 mm ID) im
43、mediatelyFIG. 1 Typical Electrochemical Detection CellD7493 14 (2018)3above the upper grid center (normally 5 mm). Each sulfurcompound sequentially elutes and reacts with chromic acid.Possible reaction mechanisms are illustrated as Eq 1 and Eq 2.The redox reaction occurs on the electrode surface, cr
44、eating apotential difference between the two electrodes, thus causing acurrent to be measured (using a low resistance measuringcircuit). For example, t-butyl mercaptan is oxidized to t-butylsulfoxide and chromium oxide (Eq 2).2 CrO312 R 2 SH2 RS 5 O1Cr2O3(1)where:R = organic moieties, such as CxHy2
45、CrO312 C4H92 SH2 C4H92 SO1Cr2O31H2O (2)6.2 Column1200 mm of 1.6 mm ID glass or PTFE tubingpacked with 150 to 180 um (80 to 100 mesh) Chromosorb Wsupport has been successfully used in performance of this testmethod. However, other columns that provide adequate reten-tion and resolution characteristic
46、s under the experimentalconditions as described in 8.1 can be used. A second GCcolumn of the same ID and phase, but of a shorter length, canbe employed for faster measurement of late-eluting sulfurcompounds such as THT. In this case, two columns areconnected to the GC injection system using a 10-por
47、t valve todirect sample flow through the appropriate column and thenonto the EC detector; thus, allowing measurement of lowmolecular weight sulfur gases and high molecular weightsulfur gases such as THT from a single sample injection. Theelution of high molecular weight sulfur gases such as THT maya
48、lso be accelerated by increased carrier gas flow rate after theelution of TBM. When samples may contain high boiling orinstrument damaging substances, a backflush column may beemployed to remove these materials before they reach thechromatographic column and EC detector. The performance ofGC columns
49、 shall give adequate separation of target sulfurcompounds for the particular application.6.3 Data Acquisition6.3.1 The device and software must have the followingcapabilities:6.3.1.1 Graphic presentation of the chromatogram.6.3.1.2 Digital display of chromatographic peak areas.6.3.1.3 Identification of peaks by retention time or relativeretention time, or both.6.3.1.4 Calculation and use of response factors.6.3.1.5 External standard calculation and data presentation.6.3.1.6 Instrument control for electrochemical detectoroperation, such
