1、Designation: D 6968 03Standard Test Method forSimultaneous Measurement of Sulfur Compounds andMinor Hydrocarbons in Natural Gas and Gaseous Fuels byGas Chromatography and Atomic Emission Detection1This standard is issued under the fixed designation D 6968; the number immediately following the design
2、ation indicates 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 (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method is for the det
3、ermination of volatilesulfur-containing compounds and minor hydrocarbons in gas-eous fuels including components with higher molar mass thanthat of propane in a high methane gas, by gas chromatography(GC) and atomic emission detection (AED). Hydrocarbonsinclude individual aliphatic components from C4
4、to C6, aro-matic components and groups of hydrocarbons classifiedaccording to carbon numbers up to C12at least, such as C6-C7,C7-C8,C8-C9and C9-C10, etc. The detection range for sulfurand carbon containing compounds is approximately 20 to100 000 picograms (pg). This is roughly equivalent to 0.04 to2
5、00 mg/m3sulfur or carbon based upon the analysis of a 0.25mL sample.1.2 This test method describes a GC-AED method employ-ing a specific capillary GC column as an illustration for naturalgas and other gaseous fuel containing low percentages ofethane and propane. Alternative GC columns and instrument
6、parameters may be used in this analysis optimized for differenttypes of gaseous fuel, provided that appropriate separation ofthe compounds of interest can be achieved.1.3 This test method does not intend to identify all indi-vidual sulfur species. Unknown sulfur compounds are mea-sured as mono-sulfu
7、r containing compounds. Total sulfurcontent of a sample can be found by summing up sulfur contentpresent in all sulfur species.1.4 This method is not a Detailed Hydrocarbon Analysis(DHA) method and does not intend to identify all individualhydrocarbon species. Aliphatic hydrocarbon componentslighter
8、 than n-hexane, benzene, toluene, ethyl benzene, m,p-xylenes and o-xylene (BTEX) are generally separated andidentified individually. Higher molar mass hydrocarbons aredetermined as groups based on carbon number, excludingBTEX. The total carbon content of propane and higher molarmass components in a
9、sample can be found by summing upcarbon content present in all species containing carbon.1.5 The values stated in SI units are standard. The valuesstated in inch-pound units are for information only.1.6 This standard does not purport to address all of thesafety concerns, if any, associated with its
10、use. It is theresponsibility of the user 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 1265 Practice for Sampling Liquefied Petroleum Gas(Manual Method)D 1945
11、 Test Method for Analysis of Natural Gas by GasChromatographyD 3609 Practice for Calibration Techniques Using Perme-ation TubesD 4626 Practice for Calculation of Gas ChromatographicResponse FactorsD 5287 Test Method of Automatic Sampling of GaseousFuelsD 5504 Test Method for Determination of Sulfur
12、Com-pounds in Natural Gas and Gaseous Fuels by Gas Chro-matography and ChemiluminescenceD 5623 Test Method for Sulfur Compound in 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 b
13、y Gas Chro-matography and Flame Photometric DetectionE 840 Practice for Using Atomic Emission Detectors in GasChromatography2.2 Other References:1This test method is under the jurisdiction of ASTM Committee D03 on GaseousFuels and is the direct responsibility of Subcommittee D03.05 on Determination
14、ofSpecial Constituents of Gaseous Fuels.Current edition approved Oct. 1, 2003. Published November 2003.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards
15、 Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.ISO 19739 Natural GasDetermination of Sulfur Com-pounds by Gas chromatography3GPA 2199 DeterminationSpecific Sulfur Compounds4“Improved Measure
16、ment of Sulfur and Nitrogen Com-pounds in Refinery Liquids Using Gas ChromatographyAtomic Emission Detection,” Journal of Chromato-graphic Science, 36, No 9, September, 1998, p. 435.3. Terminology3.1 Abbreviations:3.1.1 A common abbreviation of hydrocarbon compounds isto designate the number of carb
17、on atoms in the compound. Aprefix is used to indicate the carbon chain form, while asubscript suffix denotes the number of carbon atoms (forexample, normal butane = n-C4; Iso-pentane = i-C5, aliphatichydrocarbons heavier than n-heptane but not heavier thann-octane = C7-C8).3.1.2 Sulfur compounds are
18、 commonly referred to by theirinitials (chemical or formula), for example, methyl mercaptan= MeSH, dimethyl sulfide = DMS; carbonyl sulfide = COS,di-t-butyl trisulfide = DtB-TS and tetrahydothiophene = THTor Thiophane.4. Summary of Test Method4.1 The sampling and analysis of gaseous sulfur compounds
19、is challenging due to the reactivity of these compounds.Samples should be collected and stored in containers that arenon-reactive to sulfur compounds, such as thin silica-linedstainless steel vessels and Tedlart bags with polypropylenefittings or the equivalent. Sample containers should be filledand
20、 purged at least three times to ensure representativesampling. Laboratory equipment must also be inert, wellconditioned and passivated with a gas containing the sulfurcompounds of interest to ensure reliable results. Frequentcalibration using stable standards is required. Samples shouldbe analyzed a
21、s quickly as possible not beyond the provenstorage time after collection to minimize sample deterioration.If the stability of analyzed sulfur components is experimentallyproven, the time between collection and analysis may belengthened.4.2 A 0.25 mL sample of the fuel gas is injected into a gaschrom
22、atograph where it is passed through a 30 meter, 0.32 mmI.D., thick film, methyl silicone liquid phase, open tubularpartitioning column, or a column capable of separating thesame target sulfur and hydrocarbon components. A wider bore(0.53 mm I.D.) column may be used for better compoundseparation and/
23、or for lower detection limits using a largerinjection volume.4.3 Atomic Emission DetectorsAll sulfur and carboncompounds can be detected by this technique. GC-AED hasrecently been developed for analysis of many elements, includ-ing sulfur and carbon. The AED uses a microwave inducedhelium plasma to
24、disassociate molecules and atomize/exciteelements at high temperature (5000C). The characteristicemission lines from specific excited atoms are detected by aPhoto Diode Array detector (PDA). Sulfur emission is mea-sured at 181 nm. Carbon emission (193 and 179 nm) can bemonitored simultaneously. The
25、amount of light emitted at eachwavelength is proportional to the concentration of sulfur orcarbon. Carbon and hydrogen emission can also be measured at498 and 486 nm, respectively, in a separate run using the sameGC procedure for additional elemental information. However,hydrogen response is not lin
26、ear and a quadratic calibrationcurve must be constructed for hydrogen measurement. GC-AED offers a very high degree of selectivity and a widedynamic range for detection of various types of compound.The AED, just like the Sulfur Chemiluminescence Detector(SCD) employed in Test Method D 5504 for sulfu
27、r analysis,has the advantage over other types of detector in that theelemental response is generally independent of the structure ofthe associated molecule containing the element of interest. Itoffers the potential of using a single standard to calibrate theinstrument for determination of all sulfur
28、 and hydrocarboncomponents, diminishing the need of multiple standards thatmay not be commercially available or that are prohibitivelyexpensive to prepare. The real-time simultaneous measurementof carbon and sulfur content by AED provides the elementalratio of carbon to sulfur for each sulfur compou
29、nd, which alongwith retention time can be used to confirm the identity of sulfurcompounds. The elemental ratio of carbon to hydrogen can beused to differentiate aromatic compounds from aliphatic com-pounds for identification and confirmation as well.4.4 Other DetectorsThis test method is written pri
30、marilyfor the atomic emission detector. The same GC method can beemployed with other detectors provided they have sufficientsensitivity and response to all sulfur and hydrocarbon com-pounds of interest in the required measurement range. AFID-SCD combination detector may satisfy these criteria.5. Sig
31、nificance and Use5.1 Gaseous fuels, such as natural gas, petroleum gases andbio-gases, contain varying amounts and types of sulfur com-pounds. They are generally odorous, corrosive to equipment,and can inhibit or destroy catalysts employed in gas process-ing. Their accurate measurement is essential
32、to gas processing,operation and utilization, and may be of regulatory interest.5.2 Small amounts (typically, 1 to 4 ppmv) of sulfurodorants are added to natural gas and other fuel gases for safetypurposes. Some sulfur odorants can be reactive, and may beoxidized, forming more stable sulfur compounds
33、 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 a technique to determine individualsulfur species in gaseous fuel and the total sulfur content bycalculation.5.4 Ga
34、s chromatography is commonly and extensively usedto determine all components in gaseous fuels including fixedgas and organic components (Test Methods D 1945 andD 1946). Major components measured are often used for thedetermination of gas property, such as heating value andrelative density. Higher mo
35、lar mass hydrocarbons are of3Available from International Organization for Standardization (ISO), 1 rue deVaremb, Case postale 56, CH-1211, Geneva 20, Switzerland.4Available from Gas Processors Association (GPA), 6526 E. 60th St., Tulsa, OK74145.D6968032interest even when present in small amounts be
36、cause theirlarger impact on heating value, hydrocarbon dew point and gasquality relating to gas operation, gas utilization and environ-mental impacts.6. Apparatus6.1 ChromatographAny gas chromatograph of standardmanufacture with hardware and software necessary for inter-facing to an atomic emission
37、detector and for the intendedapplication and performance.6.1.1 Sample Inlet SystemGas samples are introduced tothe GC using an automated or manually operated non-reactivestainless steel gas sampling valve heated continuously at atemperature significantly (10C) above the temperature atwhich the gas w
38、as sampled to avoid sample condensation anddiscrimination. Inert tubing made of non-permeable, 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 container to the gassampling valve and to th
39、e GC inlet system. Silica-coated 316stainless steel (s.s.) tubing is often employed. A fixed volume,0.25 mL, sampling loop made of the same non-reactivematerials is used to avoid possible decomposition or absorptionof reactive species. Other size fixed-volume sampling loopsmay be used for different
40、concentration ranges. An on-columnor a split/splitless injection system operated at the splitlessmode or at the split mode with a low split ratio may be usedwith capillary columns. One should avoid using a split linerwith a split ratio set to zero as a means of achieving splitlessinjection. A one-me
41、ter section of deactivated pre-columnattached to the front of the analytical column is recommended.The inlet system must be well conditioned and evaluatedfrequently for compatibility with trace quantities of reactivesulfur compounds, such as tert-butyl mercaptan.6.1.2 Digital Pressure TransmitterA c
42、alibrated s.s.pressure/vacuum transducer with a digital readout may beequipped to allow sampling at different pressures to generatecalibration curves.6.1.3 Column Temperature ProgrammerThe chromato-graph must be capable of linear programmed temperatureoperation over a range of 30 to 250C, in program
43、med ratesettings of 0.1 to 30C/min. The programming rate must besufficiently reproducible to obtain retention time repeatabilityof 0.05 min. (3 s) throughout the scope of this analysis.6.1.4 Carrier and Detector Gas ControlConstant flowcontrol of carrier and detector gases is critical for optimum an
44、dconsistent analytical performance. Control is best provided bythe use of pressure regulators and fixed flow restrictors. Thegas 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
45、-stant to 6 1 % at the required flow rates can also be used. Thesupply pressure of the gas delivered to the gas chromatographmust be at least 69 kPa (10 psig) greater than the regulated gasat the instrument to compensate for the system back pressure.In general, a supply pressure of 552 kPa (80 psig)
46、 is satisfac-tory.6.1.5 DetectorAn atomic emission detector calibrated inthe carbon and sulfur specific mode is used in this method.Other detectors capable of simultaneous measurement of sulfurand carbon as stated in 4.4 are not covered in this test method.The detector is set according to the manufa
47、cturers specifica-tions and tuned to the optimal sensitivity and selectivity for theapplication.6.1.5.1 When sulfur and hydrocarbon compounds are de-composed in the high temperature AED zone they quantita-tively produce excited state atomic sulfur and carbon species.A diode array detector detects th
48、e light emitted from thesespecies as they relax to ground states. Carbon containingcomponents are simultaneously detected at 179 and 193 nmwavelength for different sensitivity measurements extendingthe linear concentration range. Sulfur species are detected at181 nm with a high selectivity. The sele
49、ctivity is normallybetter than 33104, by mass of sulfur to mass of carbon. Thedetector response is linear with respect to sulfur and carbonconcentrations. The dynamic range of this linear relationship isbetter than 13104.6.2 ColumnA 30 m by 0.32 mm ID fused silica opentubular column containinga4mfilmthickness of bondedmethyl silicone liquid phase is used. The column shall provideadequate retention and resolution characteristics under theexperimental conditions described in 7.3. Other columns thatcan provide equivalent or desirable separation can be employedas well.
