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Comparison of AGA Report No. 8 and GERG 2004 Equations for Gas Property Calculations

Mixture equations of state. SGERG-88, AGA8-DC92 (volumetric properties): compression factor, density (ISO 12213, GERG TM5, AGA report No.8) AGA8-DC92 (caloric properties): speed of sound, enthalpy (ISO 20765-1, AGA report No.10)GERG-2004: (ISO WD 20765-2, GERG TM15) compression factor

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Comparison of AGA Report No. 8 and GERG 2004 Equations for Gas Property Calculations

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    1. Comparison of AGA Report No. 8 and GERG 2004 Equations for Gas Property Calculations Eric W. Lemmon Thermophysical Properties Division National Institute of Standards and Technology Boulder, Colorado

    2. Mixture equations of state SGERG-88, AGA8-DC92 (volumetric properties): compression factor, density (ISO 12213, GERG TM5, AGA report No.8) AGA8-DC92 (caloric properties): speed of sound, enthalpy (ISO 20765-1, AGA report No.10) GERG-2004: (ISO WD 20765-2, GERG TM15) compression factor, caloric properties fundamental equation of state, vapor-liquid equilibrium, valid over the entire fluid region (single and 2-phase)

    3. AGA-8 mixture model AGA Report No. 8, K.E. Starling and J.L. Savidge, Compressibility Factors of Natural Gas and Other Related Hydrocarbon Gases Current industry standard Valid for gas phase only, cannot calculate phase equilibrium Temperature range: -130 ºC to 400 ºC (-200 ºF to 760 ºF) Pressures up to 280 MPa (40,000 psia) Detail characterization method for compressibility factor: “hybrid” type has features of virial EOS (power series in density) for low density calculations and exponential functions for high density calculations (similar to mBWR EOS)

    5. Why not just use pressure for the independent variable in our equation of state?

    7. Given density and temperature, all other properties can be calculated Iterative solutions required given input conditions of pressure and temperature; pressure and enthalpy; pressure and entropy; saturation temperature; vapor pressure; etc.

    8. Excess Helmholtz Energy Mixture Model Excess property model explicit in Helmholtz energy Independent parameters are density and temperature Generalized/Predictive High accuracy Requires accurate pure fluid equations of state Calculates all thermodynamic properties, including heat capacities, speed of sound, vapor-liquid equilibria, liquid-liquid equilibria, and critical lines

    11. Ranges of application for GERG-2004

    12. Overview of components

    13. Experimental data 21 components 70,000 experimental binary mixture data for correlation work (153) binary mixtures 22,000 experimental natural gas data and natural gas like multicomponent data for testing the new equation More than 95% of all experimental natural gas and natural gas like multicomponent mixture data listed in the “GERG Databank of High-Accuracy Compression Factor Measurements” (GERG TM4 and TM7) are described with deviations of less than ± 0.1% .

    14. Methane/Ethane VLE data

    15. When to use AGA-8 vs. GERG-2004 ISO 20765 has two parts Part 1 – AGA-8 in its Helmholtz form Part 2 – GERG-2004 The ISO document currently allows AGA-8 (part 1) or GERG-2004 (part 2) to be used. The GERG equation should be used when the uncertainties in AGA-8 increase, but the ISO document does not specify where this occurs. The uncertainties in AGA-8 increase at lower temperatures or lower methane concentrations (<80%). A new AGA-8 taskgroup has been formed to investigate the differences between the two models and document the region where differences are within 0.05 %, 0.1 %, and 0.2% in density.

    16. REFPROP - www.nist.gov/srd/nist23.htm 90 pure fluids Mixtures with up to 20 components All thermodynamic and transport properties, including heating value GERG-2004 is main mixture model, but can use AGA-8 or cubics as well Table and plot generation Version 8.0 only allows mixtures up to octane, version 8.1 will allow mixtures with nonane, decane, and hydrogen sulfide

    18. Methane

    19. Methane/Ethane: 99/1

    20. Methane/Ethane: 95/5

    21. Methane/Ethane/Propane: 92/6/2

    22. Methane/Ethane/Propane/Butane: 90/5/3/2

    23. C1/C2/C3/C4/C5: 85/8/4/2/1

    24. C1/C2/C3/C4/C5: 75/12/8/3/2

    25. C1/C2/C3/C4/C5/N2/CO2: 75/8/4/2/1/2/8

    26. C1/C2/C3/C4/C5/N2/CO2: 75/8/4/2/1/8/2

    27. C1-8/N2/CO2: 80/6/3/1/.6/.2/.1/.1/2/7

    28. C1/C2/C3/C4/C5/C6/C7/C8: 75/12/8/2/1/.8/.7/.5

    29. A mini version of REFPROP is freely available for calculating the differences in density, speed of sound, or isobaric heat capacities between AGA-8 and GERG-2004 (but other properties are not calculated in this free version). The program is available at: http://www.boulder.nist.gov/div838/theory/ refprop/REF-DEVS/REF-DEVS.HTM (upper/lower case is required in address) (current program is very slow, an update will be available soon…)

    30. Ken Hall’s Sample Compositions

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