1、Designation: D 7493 08Standard Test Method forOnline Measurement of Sulfur Compounds in Natural Gasand Gaseous Fuels by Gas Chromatograph andElectrochemical Detection1This standard is issued under the fixed designation D 7493; the number immediately following the designation indicates the year ofori
2、ginal 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 volatilesulfur-
3、containing compounds in gaseous fuels by gas chroma-tography (GC) and electrochemical (EC) detection. The testmethod is applicable to hydrogen sulfide, C1-C4 mercaptans,sulfides and tetrahydrothiophene (THT).1.1.1 Carbonyl sulfide (COS) is not covered in this testmethod.1.1.2 The detection range for
4、 sulfur compounds is approxi-mately from 0.1 to 100 PPMv or 0.1 to 100 mg/m3. Thedetection range may vary depending on the sample injectionvolume, chromatographic peak separation and the sensitivity ofspecific EC detector.1.2 This test method describes a GC-EC method employingpacked GC columns and a
5、 specific detector as an illustrationfor natural gas and other gaseous fuel containing mainly lighthydrocarbons. Alternative GC columns, detector designs andinstrument parameters may be used for the same analysis or fordifferent types of gaseous fuel, provided that appropriatechromatographic separat
6、ion and optimal detection of thesecompounds can be achieved.1.3 This test method does not intend to identify and measureall individual sulfur species, and is mainly employed formonitoring natural sulfur and sulfur odorant compounds com-monly found in fuel gases or employed as an odorous warningagent
7、 in fuel gases.1.4 The test method is normally employed for repetitiveon-line monitoring of sulfur components in fuel gases with asingle sulfur standard. The test method may be employed forlaboratory-quality measurement with more extensive calibra-tion. (See Test Methods D 5504, D 5623, D 6228, D 69
8、68,ISO 19739, ISO 6326-2, and GPA 2199.1.5 The test method can be used for measurement of alllisted sulfur compounds in air or other gases, provided that nocompound, which can interfere with the GC separation andelectrochemical detection, is present.1.6 This test method is written in conjunction wit
9、h PracticesD 5287, D 7165 and D 7166.1.7 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 thesafety concerns, if any, associated with its use. It is theresponsibility of the us
10、er of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D 3609 Practice for Calibration Techniques Using Perme-ation TubesD 4150 Terminology Relating to Gaseous FuelsD
11、 4626 Practice for Calculation of Gas ChromatographicResponse FactorsD 5287 Practice for Automatic Sampling of Gaseous FuelsD 5504 Test Method for Determination of Sulfur Com-pounds in Natural Gas and Gaseous Fuels by Gas Chro-matography and ChemiluminescenceD 5623 Test Method for Sulfur Compounds i
12、n Light Petro-leum Liquids by Gas Chromatography and Sulfur Selec-tive DetectionD 6228 Test Method for Determination of Sulfur Com-pounds in Natural Gas and Gaseous Fuels by Gas Chro-matography and Flame Photometric DetectionD 6968 Test Method for Simultaneous Measurement ofSulfur Compounds and Mino
13、r Hydrocarbons in NaturalGas and Gaseous Fuels by Gas Chromatography andAtomic Emission DetectionD 7165 Practice for Gas Chromatograph Based On-line/At-line Analysis for Sulfur Content of Gaseous FuelsD 7166 Practice for Total Sulfur Analyzer Based On-line/At-line for Sulfur Content of Gaseous Fuels
14、1This test method is under the jurisdiction ofASTM Committee D03 on GaseousFuels and is the direct responsibility of Subcommittee D03.12 on On-Line/At-LineAnalysis of Gaseous Fuels.Current edition approved Dec. 15, 2008. Published January 2009.2For referenced ASTM standards, visit the ASTM website,
15、www.astm.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.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.2.
16、2 ISO Standards:3ISO 19739 Natural Gas Determination of Sulfur Com-pounds by Gas chromatographyISO 6326-2 Gas Analysis Determination of Sulphur Com-pounds in Natural gas Part 2: Gas ChromatographicMethod Using an Electrochemical Detector for The De-termination of Odoriferous Sulphur Compounds2.3 GPA
17、 Standard4GPA2199 Determination - Determination of Specific SulfurCompounds by Capillary Gas Chromatography and SulfurChemiluminescence Detection3. Terminology3.1 Common terminology used in this method are cited inTerminology D 4150 3.2 Sulfur compounds are commonlyreferred by their initials (chemic
18、al or formula), for example,3.2 Abbriviations:hydrogen sulfide =H2Smethyl mercaptan = MeSH (MM)ethyl mercaptan = EtSH (EM)dimethyl sulfide = DMSi-Propyl mercaptan = IPMn-Propyl mercaptan = NPMt-Butyl mercaptan = TBMtetrahy-drothiophene= THT or Thiophane4. Summary of Test Method4.1 Gaseous fuel is di
19、rectly sampled on-line for analysis ofspecific and normally reactive sulfur compounds. Samples areintroduced to an inert GC instrument through an inert samplingsystem. Sulfur compounds are separated by a GC column andmeasured by an EC detector. The method requires frequentcalibration using stable st
20、andards. The test method conforms tothe standard practice of Practice D 7165.4.2 A fixed volume of the fuel gas (normally 0.25 mL) isinjected into an isothermal gas chromatograph where it ispassed through a 1.2 meter, 1.6 mm I.D., Chromosorb Wcolumn. A varying amount of sample and other GC columnswi
21、th or without column back-flush technique can be used forsensitive detection of sulfur with optimal separation.4.3 4.3 Specific GC-separated sulfur compounds are de-tected by an electrochemical detector utilizing chromic acidelectrolyte. Detectors with different physical designs are com-mercially av
22、ailable and may be employed.5. Significance and Use5.1 Gaseous fuels, such as natural gas, petroleum gases andbio-gases, contain varying amounts and types of sulfur com-pounds. These sulfur compounds are generally odorous, cor-rosive to equipment, and can inhibit or destroy catalystsemployed in gas
23、processing and end use, such as those used infuel cell. Their accurate on-line measurement is essential to gasprocessing, operation and utilization, and of regulatory interest.5.2 Small amounts (typically, total 4-6 PPMv) of sulfurodorants are added to natural gas and other fuel gases for safetypurp
24、oses. Some sulfur odorants can be reactive, and may beoxidized, forming more stable sulfur compounds having lowerodor thresholds. These gaseous fuels are analyzed for sulfurodorants to help in monitoring and to ensure appropriateodorant levels for public safety.5.3 This method offers an on-line tech
25、nique to continuouslyidentify and quantify individual target sulfur species in gaseousfuel with automatic calibration and validation.6. Apparatus6.1 ChromatographIndustrial gas chromatograph withisothermal oven and automatic injection valve and softwarenecessary for interfacing to a chromic acid ele
26、ctrochemicaldetector and for the intended application and performance. TheGC system must be inert, well conditioned and passivated witha gas containing the sulfur compounds of interest to ensurereliable results.6.1.1 Sample Inlet System The gas sample is introducedby sample loop injection. An automa
27、ted non-reactive gassampling valve is employed for fixed sample loop injection.The sample injection port must be heated continuously at atemperature significantly (10C) above the temperature atwhich the gas was sampled to avoid sample condensation anddiscrimination. Inert tubing made of non-permeabl
28、e, non-sorbing and non-reactive materials, as short as possible andheat traced at the same temperature, should be employed fortransferring the sample from a sample source to the gassampling valve and to the GC inlet system. Silica-coated 316stainless steel (s.s.) and non-permeable Teflon type tubing
29、 areoften employed. Different size fixed-volume sampling loops(0.25 to 10.0 mL) may be used for target concentration ranges,provided with adequate chromatographic separation. The samenon-reactive materials are used for the sampling loop to avoidpossible decomposition or absorption of reactive specie
30、s.When necessary, a precision glass syringe with a gas-tightTeflon-seated plunger is used to manually introduce sample orcalibration standards through a PTFE septum at the front of GCcolumns. The sampling and GC inlet system must be wellconditioned and evaluated frequently for compatibility withtrac
31、e quantities of reactive sulfur compounds, such as tert-butylmercaptan. A programmable and computer-controlled multi-stream sample selector can be used to sample fuel gases andcalibration gases.6.1.2 Column TemperatureThe gas chromatograph mustbe capable of maintaining an isothermal temperature, nor
32、mallyat 65 C, with temperature variation not exceeding 0.5 C6.1.3 Carrier and Detector Gas ControlConstant 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. Th
33、egas flow rate is measured by any appropriate means and therequired gas flow indicated by the use of a pressure gauge.Mass flow controllers, capable of maintaining gas flow con-stant to 6 1 % at the required flow rates can also be used. Thesupply pressure of the gas delivered to the gas chromatograp
34、h3Available 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 (GPA), 6526 E. 60th St., Tulsa, OK74145, http:/.D7493082must be at least 69 kPa (10 psig) grea
35、ter than the regulated gasat the instrument to compensate for the system back pressure.6.1.4 DetectorAn EC detector using chromic acid chem-istry (Fig. 1) is used in this method. The detector is setaccording to the manufacturers specifications for the applica-tion. One EC detector is normally employ
36、ed for measurement.A second EC detector may be employed for simultaneousdetection of late-eluting sulfur compound, such as THT, usinga shorter GC column and/or at different optimal separationconditions.6.1.4.1 The detector consists of a glass or methyl poly-methacrylate container. The electrodes, tw
37、o pieces of platinumgauze as grids arranged vertically in parallel, are welded in aborosilicate glass tube and separately connected to an amplifierfor 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 a
38、 vessel. A tube with the electrodes dips into thesolution such that the liquid is retained by capillary within thetube at a level approximately at midway between two grids.6.1.4.3 The gas flow from the GC column is dischargedthrough a narrow glass or Teflon tube (2 mm ID) immediatelyabove the upper
39、grid centre (normally 5 mm). Each sulfurcompound sequentially elutes and reacts with chromic acid.Possible reaction mechanisms are illustrated as Equation 1Eq 1and 2Eq 2. The redox reaction occurs on the electrode surfacecreating a potential difference between the two electrodes, thuscausing a curre
40、nt which is measured in a low resistancemeasuring circuit. For example, to t-butyl mercaptan is oxi-dized t-butyl sulfoxide and chromium oxide (Equation 2Eq 2)2 CrO31 2 R2SH2 RS 5O1Cr2O3(1)where:R = organic moieties, such as CxHy2 CrO31 2 C4H9SH2 C4H9SO 1 Cr2O31 H2O (2)6.2 ColumnA 1200 mm of 1.6 mm
41、ID glass or PTFEtubing packed with 150-180 um (80-100 mesh) Chromosorb Wsupport is used. The column shall provide adequate retentionand resolution characteristics under the experimental condi-tions described in 8.1. Other columns that can provide equiva-lent or desirable separation can be employed a
42、s well. Forexample, a 400 mm of 4 mm ID tubing packed with the samesupport has been used with different detector designs.AsecondGC column of the same ID and phase with a shorter length isoften employed for faster measurement of late-eluting sulfurcompounds, such as THT. Two GC columns can be plumbed
43、using a 10-port valve to direct light sulfur compounds and THTsequentially for complete detection using one or two ECdetectors. The elution of THT may also be accelerated byincreased carrier gas flow rate after the elution of TBM. Aback-flush column is not described in this method althoughback-flush
44、ing technique may be employed to remove highboiling and possible harmful gas components, which may haveundesirable effects on the analyzing GC column and theelectrochemical detector. The performance of GC columnsshall give adequate separation of target sulfur compounds fordesired accuracy and precis
45、ion.6.3 Data Acquisition6.3.1 The device and software must have the followingcapabilities:FIG. 1 Typical Electrochemical Detection CellD74930836.3.1.1 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 r
46、elativeretention 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 ED operation, such as gaspressure and flow control.7. Reagents and Materials7.1 Compressed Cylinder Gas StandardsGas standardsshou
47、ld be of high purity with certified stability and accuracy.Blended gaseous sulfur standards may be used if a means toensure accuracy and stability of the mixture is available. Gasstandards can be a source of error if their long term stabilityduring storage cannot be guaranteed.7.1.1 Sulfur Gas Stand
48、ardsSingle or multiple sulfur com-pounds in a compressed gas of high purity nitrogen, helium ormethane base gas may be used. Care must be exercised in theuse of compressed gas standards since they can introduceerrors in measurement due to lack of uniformity in theirmanufacture or instability in thei
49、r storage and use. Thenon-mandatory protocol for compressed gas standard cited inthe Appendix X1 of Test Method D 5504 can be used to ensurethe quality of standards and to establish traceability to a NISTor NMI standard reference material.7.1.2 The mix of a multiple sulfur standard should berecommended by a compressed gas standard manufacturer toassure the long term stability of sulfur components. Thestandard should by re-calibrated as needed.7.1.3 Compressed Gas Standard Delivery SystemPressureregulators, gas lines and fittings must be i
