Fluids

GERG-2008 Equation of State for Natural Gas Mixtures

The current equation of state for calculating the properties of natural gas mixtures is the GERG-2008 equation (GERG is the European Gas Research Group). This equation is based on an excess Helmholtz energy approach using pure fluid equations of state (either those specified by GERG, or the current standards that have slightly higher accuracies) and a mixture model that specifies the excess contribution. The 2008 version is an extension of the 2004 version, containing the additional fluids nonane, decane, and hydrogen sulfide in addition to methane, nitrogen, carbon dioxide, ethane, propane, n-butane, isobutane, n-pentane, isopentane, n-hexane, n-heptane, n-octane, hydrogen, oxygen, carbon monoxide, water, helium, and argon. The 2008 edition also replaced the pure fluid equations of state for isopentane and carbon monoxide with published versions. These two models are fully described in the following publications:

Kunz, O. and Wagner, W., The GERG-2008 Wide-Range Equation of State for Natural Gases and Other Mixtures: An Expansion of GERG-2004, to be submitted to J. Chem. Eng. Data, 2012.

Kunz, O., Klimeck, R., Wagner, W., and Jaescke, M., The GERG-2004 Wide-Range Equation of State for Natural Gases and Other Mixtures: GERG Technical Monograph 15 and Fortschr.-Ber. VDI, Reihe 6, Nr. 557, VDI Verlag, Düsseldorf, 2007.

In the Refprop 9.1 program, the natural gas equation of state has been expanded to include ethylene, propylene, methanol, ethanol, toluene, benzene, cyclohexane, sulfur dioxide, ammonia, dodecane, acetone, and butylene. When selecting the option to use the full GERG-2008 equation of state (either through the GUI or by calling the “GERG2004” subroutine), the use of these additional fluids is still allowed. This is not the same as when the AGA-8 equation of state is selected, in which case only the original 21 fluids (same as those in the GERG-2008 model) are allowed.

HFO-1234yf, 1234ze(E), 1234ze(Z), 1233zd(E), and Refrigerant Mixtures

Equations of state are now available for these fluids. The fluid files are located below and should be placed in your Refprop\Fluids directory. The fluid files included in version 9.0 incorrectly used the NBP reference state.

• R1234yf.fld (uploaded June 6, 2012 with updated transport equations)
• R1234zeE.fld (uploaded June 6, 2012 with new CAS number and updated transport equations. This is for R1234ze(E)). NOTE The file name for R1234ze(E) has been renamed as R1234zeE.FLD to avoid confusion with R1234ze(Z). You should delete your old R1234ze.FLD file when downloading this version. The contents of the fluid file have not changed. «
• R1234zeZ.fld (uploaded January 12, 2015 with transport equations. This is for R1234ze(Z)).
• R1233zdE.fld (uploaded November 9, 2015)

New refrigerant predefined mixture files are available, such as that for R-448A. To add these to Refprop, unzip the file below in your Refprop\Mixtures directory.

• NEWMIX.ZIP (uploaded October 24, 2016 with 10 new mixtures. For mixtures with R1234ze(E), you will need to download the renamed file above for this fluid.) «

On November 10, 2015, we released a new mixture file (hmx.bnc) that now contains mixing parameters for every binary of the fluids R-32, R-125, R-134a, R-1234yf, and R-1234ze(E). With these, mixtures such as R-448A that contain all five of these no longer use the predictive methods to get the interaction parameters. If you would like this file, please contact us.

Humid Air

Although version 9.1 allows mixtures of nitrogen, argon, oxygen, and water as a consequence from the addition of the new natural gas mixture model, calculations for moist air have not been tested yet. It is likely that calculated values are reasonable, however, Refprop may not return results because the saturation routines may fail. The program calls the saturation routines to determine if the state is vapor, liquid, or two-phase. If you know that your state point is in the vapor phase, you can avoid the call to the saturation routines by using TPRHO instead of TPFLSH. The Fortran file FLSH_SUB.FOR gives additional information concerning the inputs to these routines. In the graphical interface, see the section below on forcing phase calculations. A moist air mixture could be made up starting with the composition of dry air used in Refprop: 0.7812 nitrogen, 0.0092 argon, and 0.2096 oxygen (on a mole basis). A small amount of water could be added to this composition and then normalized.

Mixture Models

The LJ6 mixture model has been removed in version 9.1 and replaced with KW0. This was necessary to implement a more efficient and stable algorithm. For most situations, this change will be completely transparent, and calculated values will not change. For those that have fitted mixture parameters to their own data (e.g., for proprietary mixtures), you can simply insert the lines from your old HMX.BNC file for a particular binary mixture into the new HMX.BNC file that comes with 9.1 (be sure that no other block exists for that mixture). When running SETUP, you may receive an error message about the LJ6 mixture model not found (ierr = –117); this should be ignored because the values are later internally converted to the KW0 model (this error message will be removed in future versions). Conversion of LJ6 to KW0 converts the LJ6 xeta parameter (for the reducing temperature) to the KW0 gammaT and betaT values. The numerical values of the parameters are different, but the calculated properties will be identical. For those wishing to fit mixing parameters to new experimental VLE data, KW0 should be used for the fitting process and only the gammaT value should be fitted.

Solids

The REFPROP program does not know the location of the solid-liquid interface for a mixture. For many of the pure fluids, melting line auxiliary equations are available and can be used to calculate liquid properties at the point where solids begin to form and can be used to keep the program from entering the solid phase. When melting lines are not available, the program uses the liquid phase density at the triple point as the maximum density, thus valid states between this density and the melting line will not be available. The location of the solid-liquid boundary can be calculated under the Calculate/Saturation Tables option. This option will also print out the vapor phase properties along the sublimation line if requested (and if an auxiliary equation is available).

Transport Properties for Nitrogen, Oxygen, Argon, and Air

The transport properties for nitrogen, argon, and oxygen in version 7.0 did not include the thermal conductivity enhancement for the critical region and did not represent the experimental data as well as possible. The transport equations for these fluids have been redone and now represent the data to within their experimental uncertainties. The publication below documents the new equations and shows all of the comparisons to data.

N2-Ar-O2 Transport equations documentation

Transport Properties for Pseudo-pure Fluids; Adding Pure Fluids to a Mixture Setup

The ability to load both a mixture and a pure fluid not associated with the mixture is now possible. For example, a natural gas mixture of methane, ethane, and propane could be loaded, along with R134a, which is not part of the mixture. By calling SETNC and PUREFLD, properties for either the mixture or the pure fluid can be made without ever calling SETUP more than once at the beginning. This is also useful for calculating transport properties when the pseudo-pure fluid equation of state is in use (for R-404A, R-407C, R-410A, and R-507A). Since transport properties are not available in the PPF files, the full mixture has to be loaded as well. The example program below gives all the details.

PPF-EX.FOR

Required Fluids for Distribution

There are several fluid files that Refprop accesses in order to run properly. These are nitrogen.fld, propane.fld, R134a.fld, and C12.fld (dodecane). These fluids are used as reference fluids in extended corresponding states methods employed in Refprop to predict transport properties for some instances. These fluids (and the hmx.bnc file) should be distributed in addition to those required in a particular application if the Refprop routines have been incorporated into a software package. Note that a licensing agreement must be purchased before distributing your software to others.