1、Designation: D7940 14Standard Practice forAnalysis of Liquefied Natural Gas (LNG) by Fiber-CoupledRaman Spectroscopy1This standard is issued under the fixed designation D7940; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the yea
2、r 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 standard practice is for both on-line and laboratoryinstrument-based determination of composition for liquef
3、iednatural gas (LNG) using Raman spectroscopy. The basicmethodology can also be applied to other light hydrocarbonmixtures in either liquid or gaseous states, if the needs of theapplication are met, although the rest of this practice refersspecifically to liquids. From the composition, gas propertie
4、ssuch as heating value and the Wobbe index may be calculated.The components commonly determined according to this testmethod are CH4,C2H6,C3H8, i-C4H10, n-C4H10,iC5H12,n-C5H12, neo-C5H12,N2,O2. The applicable range of thisstandard is 200 ppmv to 100 mol %. Components heavier thanC5 are not measured
5、as part of this practice.NOTE 1Raman spectroscopy does not directly quantify the componentpercentages of noble gases, however, inerts can be calculated indirectly bysubtracting the sum of the other species from 100 %.1.2 The values stated in SI units are to be regarded asstandard. No other units of
6、measurement are included in thisstandard.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its 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
7、 limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D3588 Practice for Calculating Heat Value, CompressibilityFactor, and Relative Density of Gaseous FuelsD4150 Terminology Relating to Gaseous FuelsE691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a
8、Test MethodD1945 Test Method for Analysis of Natural Gas by GasChromatographyD1946 Practice for Analysis of Reformed Gas by GasChromatographyD7833 Test Method for Determination of Hydrocarbons andNon-Hydrocarbon Gases in Gaseous Mixtures by GasChromatography2.2 BS EN Standards:3BS EN 60079-28 Explos
9、ive Atmospheres. Protection ofEquipment and Transmission Systems using Optical Ra-diationBS EN 60825-1 Safety of Laser Products Part 1: EquipmentClassification, Requirements and Users Guide2.3 ISO Standards:4ISO 6974-5 Natural GasDetermination of Compositionwith Defined Uncertainty by Gas Chromatogr
10、aphy, Part 5:Determination of nitrogen, carbon dioxide and C1 to C5and C6+ hydrocarbons for a laboratory and on-line pro-cess application using three columns3. Terminology3.1 Definitions: Refer to D4150 for definitions related togaseous fuels.3.2 Definitions of Terms Specific to This Standard:3.2.1
11、Accumulations, nwhile the exposure time is opti-mized to control the amount of light entering the camera for asingle exposure, multiple exposures can be co-added to im-prove signal-to-noise. The number of exposures co-added arereferred to as accumulations.3.2.2 charge-coupled device, nsilicon based
12、two dimen-sional light sensor characterized by possessing a grid ofpotential energy wells where light-generated free electronscollect and then are read out sequentially.3.2.3 charge-coupled device (CCD) binning, vprocess ofcombining “bins” or pixel wells on the CCD.3.2.4 Exposure Time, nthe CCD conv
13、erts photons toelectrons over time for a measurement. The exposure time1This 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 June 1, 2014. Published
14、 July 2014. Originally approvedin 2014. DOI: 10.1520/D7940-14.2For 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 Document Summary page onthe ASTM website
15、.3Available from British Standards Institution (BSI), 389 Chiswick High Rd.,London W4 4AL, U.K., http:/.4Available from International Organization for Standardization (ISO), 1, ch. dela Voie-Creuse, CP 56, CH-1211 Geneva 20, Switzerland, http:/www.iso.org.Copyright ASTM International, 100 Barr Harbo
16、r Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1indicates the amount of time allocated for capturing photons.The number of electrons is counted at the end of the allottedexposure time via a binning process.3.2.5 incident light, i, nmonochromatic light illuminatedinto sample.3.2
17、.6 Raman Scattering Effect, nan energy transfer pro-cess between photons and molecules. In this photon-moleculeinteraction, scattered light has a different wavelength comparedto the incident wavelength. The Raman wavelength shiftspectrum is unique for each molecule because the shift isdependent upon
18、 the molecular bonding structures.3.2.7 Raman spectroscopy, na type of molecular vibrationspectroscopy in which a laser is used to excite virtual energystates in the molecules being illuminated, which then decay,producing new photons that are the sum and differencebetween the laser and the vibration
19、al frequencies. These newphotons are then collected and analyzed to determine thevibrational spectrum of the molecules.3.2.8 Raman spectrum, nplot of intensity against Ramanwavelength shift.3.2.9 scattered light, sscattered light as a result of theRaman scattering effect3.2.10 signal strength, na me
20、asure of the amount ofRaman-scattered photons reaching the CCD. Usually somescaled combination of raw areas of peaks from compoundsexpected to be present in LNG.3.2.11 wavenumber, nabbreviated as cm-1; method ofspecifying the wavelength of optical radiation. Raman bandsare constant wavenumber shifts
21、 from the excitation sourceindependent of the wavelength of that source.3.3 Acronyms:3.3.1 GCGas chromatograph3.3.2 GHVGross Heating Value3.3.3 LNGLiquefied natural gas3.3.4 LPGLiquefied petroleum gas3.3.5 NISTNational Institute of Standards and Technology4. Summary of Practice4.1 Measurement of the
22、 volume fractions of individualmolecular species contained in a liquid stream of interest suchas LNG is accomplished by obtaining and analyzing Ramanspectra (Fig. 1). Monochromatic light from a laser is directeddown a fiber-optic cable through a sample-compatible probeoptic and into the liquid to be
23、 measured. Monochromaticphotons interact with the molecules of the liquid via the Ramaneffect to produce new photons whose wavelengths have beenshifted in proportion to vibration frequencies of the molecules.These new, shifted photons are collected through the sameoptics that delivered the original
24、monochromatic light and aredirected down a separate fiber that is connected to a detectionmodule. The detection module contains a spectrograph, whichdirects photons of different wavelengths to different pixels ona CCD detector. The CCD pixels integrate the photons fallingon them into a digital signa
25、l, whose value is proportional to thenumber of photons. Thus, a spectrum is produced, representinga histogram charting the number of photons detected at eachwavelength, corresponding to the number of molecules withparticular vibration frequencies. The spectra can be mathemati-cally processed to yiel
26、d the molecular composition of theliquid. Using standards such as Practice D3588, the energycontent, and indices such as the Wobbe index, can be calcu-lated from the composition information.5. Significance and Use5.1 The composition of liquefied gas fuels (LNG, LPG) isimportant for custody transfer
27、and production. Compositionaldetermination is used to calculate the heating value, and it isimportant to ensure regulatory compliance. CompositionalFIG. 1 LNG Raman SpectraD7940 142determination is also used to optimize the efficiency of lique-fied hydrocarbon gas production and ensure the quality o
28、f theprocessed fluids.5.2 Alternatives to compositional measurement using Ra-man spectroscopy are described in Test Method D1945, Prac-tice D1946, and Test Method D7833.5.3 The advantage of this standard over existing standardsmentioned in 5.2 above, is that Raman spectroscopy candetermine compositi
29、on by directly measuring the liquefiednatural gas. Unlike chromatography, no vaporization step isnecessary. Since incorrect operation of on-line vaporizers canlead to poor precision and accuracy, elimination of the vapor-ization step offers a significant improvement in the analysis ofLNG.6. Interfer
30、ences6.1 Cosmic rays may be detected by the CCD, thus inter-fering with the Raman spectrum. Typically, a data collectionprocess and algorithm is used to eliminate this consideration.Spectra are taken in pairs and mathematically compared toeach other to determine if a cosmic ray has excited specificp
31、ixels, which can then be excised from the data set6.2 Thermally generated electrons add to the true scatterspectrum. This effect is minimized by detector design andmanufacturing as well as cooling, and the remaining thermalbackground is subtracted.6.3 The standard is intended for sample locations wh
32、ere thesample phase is entirely liquid. Mixed-phase (gas and liquid)or the presence of a significant amount of bubbles due toinsufficient insulation or cavitation may impact the precisionand accuracy.7. Apparatus7.1 Analyzer Base Unit (Fig. 2)The base unit contains theelectrically powered system com
33、ponents including the laserwith associated safety devices, detection module, controlelectronics, data communication equipment, human/machineinterface, and environmental control equipment. This is typi-cally mounted inside an enclosure suitable to the installationsite.7.1.1 LaserGenerates monochromat
34、ic light for the cre-ation of a Raman signal within the sample. To be effective, thelaser shall have a narrow-enough line width with a stable-enough power output and wavelength so as not to compromisethe generation and analysis of the Raman spectra. The wave-length of the laser must match the design
35、 specifications of thedetector module such that the Raman photons produced are inthe wavelength range that the detector module can detect. Inaddition, the laser photons and Raman photons must be of sucha wavelength that they can be transmitted through a fiber-opticcable with minimal attenuation. Gen
36、erally a laser of wave-length 785 nm has been found to work well, but other lasers inthe range of 500 to 800 nm may also be possible, providing thedetector is chosen appropriately. The laser shall also havefeatures that make it compatible with both explosive atmo-sphere safety (see EN60079-28) as we
37、ll as eye safety (see EN60825-1). This will generally include a remote-capable power-interlocking system, a redundant power-monitoring system,and a visible operation indicator light system. Typical perfor-mance values meet these criteria:7.1.1.1 Power stability 65 % long term over operatingtemperatu
38、re range (0 to 45 C),7.1.1.2 Line width 1cm-1, andFIG. 2 Analyzer Base UnitD7940 1437.1.1.3 Wavelength stability 0.005 nm short term (severalminutes) and 0.05 nm long term (years).7.1.2 Detection Module7.1.2.1 SpectrographOptical device for separating theRaman signal photons by wavelength and imagin
39、g them onto adetector. To provide sufficient separation between molecularvibration frequencies and allow the detection of Raman pho-tons that carry this information, the spectrograph shall combinehigh spatial and spectral resolution; high optical throughput;and stability over time, temperature, and
40、environmentalchanges. The spectrograph shall also provide sufficient freespectral range to capture all the vibration frequencies ofinterest. Spectrographs using holographic transmission grat-ings and refractive imaging optics are ideally suited to thistask. The spectrograph also shall include a notc
41、h or edge filterto block excess un-shifted laser light without significantlyaffecting signal photons. The following typical performancevalues meet these criteria:(1) Spectral range 150 to 3500 cm-1(2) Spectral resolution7cm-1(3) Spectral thermal stability 0.1 cm-1/C band shift,(4) Optical throughput
42、Numerical aperture (NA) of spec-trograph matched to fiber NA (typically f/1.8), and(5) Notch or edge filter8 optical density at laser wave-length and 80 % transmission beyond 200 cm-1.7.1.2.2 CCDThe CCD detector is a silicon-chip-basedtwo-dimensional array of light-sensitive pixels having thecharact
43、eristics of high-quantum efficiency, high linearity, ad-equate dynamic range, with very low background noise,coupled with a sufficient number of pixels to support systemresolution requirements. The low-noise characteristic is due toa combination of the design of the chip, cooling of the chip toreduc
44、e thermally generated signals, and low-read noise elec-tronics. The chip shall be contained in a hermetic vacuumDewar to prevent ambient contamination from interfering withspectral accuracy. Typical performance values:(1) Quantum effciency40 % at spectral range center;(2) Spectral rangeAt least 4 %
45、quantum efficiency from400 to 1050 nm;(3) Dynamic range16-bit digital, with 50 000 electronquantum well capacity;(4) Read noise10 electrons; and(5) Dark count 0.5 electrons/pixel/second, typicallyrequires cooling to at least 30 C.7.1.2.3 Spectrum StandardLight source of known spec-trum used to stand
46、ardize/calibrate the detection module andprovide the correct mapping of scattered wavelength to physi-cal CCD pixel coordinates/location. Typically, an atomic emis-sion source such as a neon light is used for this purpose.7.1.2.4 Raman Shift StandardA physical sample havingknown Raman shift characte
47、ristics used to determine theoperating wavelength of laser module and correct for anyshort-term or accumulated error. The position of the spectralband(s) of this material can be used to calculate the operationalwavelength of the laser:i51Sv 11sD(1)where iis the incident wavelength (the operational w
48、ave-length of the laser) in cm, sis the measured wavelength ofthe Raman band in cm, and x is the accepted standard posi-tion of the Raman band of the reference material in wave-numbers.7.1.3 Fiber-Optic CableTypically contains two individualoptical fibers, one of which carries laser light to the sam
49、ple,with the other returning the Raman signal back to the detectionmodule. For Raman systems using lasers with the theoreticalability to exceed the limits set forth in EN 60079-28, there shallbe some form of breakage detection or armoring associatedwith the cable. A typical approach is to incorporate into thecable a pair of electrical conductors that carry an intrinsicallysafe level of current that is integral to the laser power interlock.Interruption of this current by cable damage shuts down thelaser.7.1
