Development of cryogenic LNG reference liquids Dr Paul Holland EffecTech Group

Transcription

1 Development of cryogenic LNG reference liquids Dr Paul Holland EffecTech Group

2 Introduction Importance of LNG composition measurements Issues when measuring cryogenic LNG Cryostat design Cryostat operation Results Overcoming the cryostat / GC / vaporiser paradox Conclusions

3 LNG composition E = V LNG *D LNG *GCV LNG

4 LNG composition E = V LNG *D LNG *GCV LNG ISO 6578

5 LNG composition E = V LNG *D LNG *GCV LNG ISO 6578 ISO 6976

6 LNG composition E = V LNG *D LNG *GCV LNG ISO 6578 ISO 6976 Both standards require composition data

7 GCs have been the instrument of choice

8 pressure bar Typical LNG Phase Diagram sample GC temperature C dewline bubble line critical point

9 pressure bar Typical LNG Phase Diagram sample Heat 0 GC temperature C dewline bubble line critical point

10 Fractionation & Enrichment We all know what happens when we heat a mixture of hydrocarbons. The most volatile components vaporise first leaving an enriched sample behind. Not representative of the whole mixture. Careful vaporisation is needed.

11 Vaporiser standards ISO 8943 describes procedures for both continuous and discontinuous sampling of LNG by vaporising it to make it amenable to GC analysis LNG vaporisers are required to achieve a phase transition (liquid to gas) without fractionation of components causing changes in composition EN describes a procedure for suitability testing of LNG sampling systems. It uses a means of supercritical gasification of the LNG to provide a reference sample for comparison

12 Direct measurement Raman spectroscopy is capable of measuring LNG composition directly, using a probe immersed in the liquid All analytical methods for natural gas / LNG need to be calibrated or validated. GC methods use certified gas mixtures of appropriate composition in cylinders. This is not practicable for LNGs The purpose of this work is to demonstrate that traceable LNG-type mixtures can be prepared and used to validate direct measurement methods

13 LNG reference liquid production - concept Condensation of a highly accurate gas mixture Primary Reference Gas Mixture (PRGM) Liquid nitrogen cooled cryostat Capable of maintaining steady T & P over defined range of T & P Means of sampling and vaporising the LNG to verify the liquid composition Analytical system with necessary precision Vacuum system with capability ~ 10-8 mbar

14 LNG reference liquid production - design PRGM Reference Gas Li N 2 cooled cryostat Verification by Gas Chromatography Accumulator for homogenisation of re-gasified LNG g

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16 Process Place reference gas on balance & connect. Evacuate whole system up to cylinder valve. Cool cryostat to desired temperature. Introduce reference gas at fixed pressure (1 bar). Monitor loss of mass from cylinder. Once desired mass condensed, homogenise liquid. Calibrate chromatograph using same reference gas. Sample LNG via vaporiser & accumulator. Measure vaporised LNG with chromatograph.

17 Cryostat Temperature Stability (18 hrs) Stability ± 100mK Range K Sub-cooling >10 K

18 Pressure (psi) Cryostat Pressure Stability Range 0 45 psi Cryostat Pressure (psi) Time (Hours)

19 Paradox The project was conceived to develop liquid reference standards as there was some concern over vaporiser issues. Liquid standards being validated using a GC & vaporiser Liquid standards Vaporiser

20 Paradox The project was conceived to develop liquid reference standards as there was some concern over vaporiser issues. Liquid standards being validated using a GC & vaporiser Liquid standards Vaporiser Validate our vaporisation system against EN to break the paradox

21 Vaporiser suitability criteria Values for mass CV, gas density and LNG density taken from Table 1 of EN Class Physical property Typical value Maximal random error A B CV in kj/kg Gas density kg/m *10-4 LNG density kg/m Continuous sampling Maximal systematic error Not significant CV in kj/kg Gas density kg/m * *10-4 LNG density kg/m

22 Results CV differences between reference gases and reference liquids CV CV Difference Mixture # KJ/kg MJ/m3 KJ/kg MJ/m

23 Results GHV differences between reference gases and reference liquids mixture # 6 data mix #6R3 gravimetric (reference) values measured values corrected reference values D R3 xi U(xi) yi U(yi) % RSD xi c U(xi c ) nitrogen methane ethane propane iso-butane n-butane GCV (15/15) Gas Density LNG Density 93K

24 Nitrogen The cryostat temperature of 93K is above the boiling point of nitrogen (77K). It would therefore be expected that nitrogen will favour the gas phase. Cryostat was designed to minimise nitrogen loss to headspace. The corrected reference values use the measured nitrogen value (as long as En ratio consistent with PRGM value) and the gravimetric hydrocarbon data normalised to 100%

25 E n Number Agreement between measured and reference values is quantified through the E n number, which divides the difference in values by the combined uncertainty E n meas Ref U 2 meas U 2 Ref An E n value of 1 or less shows that the measured and reference values are in statistical agreement

26 Results GHV differences between reference gases and reference liquids mixture # 6 data mix #6R3 difference En-number Difference meas. - corrected Limits (% gravcorrected meascorrected D R3 MJ/m 3 KJ/kg kg/m 3 EN relative) nitrogen 0.000% methane % ethane 0.023% propane 0.032% iso-butane % n-butane % GCV (15/15) % Gas Density LNG Density 93K

27 Conclusions Comparing the gravimetric with the corrected composition data gives E n numbers throughout of less than 1 Comparing measured and corrected composition data gives satisfactorily low E n values for all components Comparison of the calculated properties shows differences which are well within the limit values required by EN 12838

28 Future Work with Cryostat Further validation of Raman spectroscopy Studies on the physical properties of LNG Transportable cryostat for on-site validation / calibration Other research where empirical data is not available