A new equation of state for the thermodynamic properties of natural gases, similar gases, and other mixtures, the GERG-2008 equation of state, is presented in this work. This equation is an expanded version of the GERG-2004 equation. GERG-2008 is explicit in the Helmholtz free energy as a function of density, temperature, and composition. The equation is based on 21 natural gas components: methane, nitrogen, carbon dioxide, ethane, propane, n-butane, isobutane, n-pentane, isopentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane, hydrogen, oxygen, carbon monoxide, water, hydrogen sulfide, helium, and argon. Over the entire composition range, GERG-2008 covers the gas phase, liquid phase, supercritical region, and vapor–liquid equilibrium states for mixtures of these components. The normal range of validity of GERG-2008 includes temperatures from (90 to 450) K and pressures up to 35 MPa where the most accurate experimental data of the thermal and caloric properties are represented to within their accura...

The GERG-2008 Wide-Range Equation of State for Natural Gases and Other Mixtures: An Expansion of GERG-2004

In a preceding project, functional forms for “short” Helmholtz energy equations of state for typical nonpolar and weakly polar fluids and for typical polar fluids were developed using simultaneous optimization. In this work, the coefficients of these short forms for the equations of state have been fitted for the fluids acetone, carbon monoxide, carbonyl sulfide, decane, hydrogen sulfide, 2-methylbutane (isopentane), 2,2-dimethylpropane (neopentane), 2-methylpentane (isohexane), krypton, nitrous oxide, nonane, sulfur dioxide, toluene, xenon, hexafluoroethane (R-116), 1,1-dichloro-1-fluoroethane (R-141b), 1-chloro-1,1-difluoroethane (R-142b), octafluoropropane (R-218), 1,1,1,3,3-pentafluoropropane (R-245fa), and fluoromethane (R-41). The 12 coefficients of the equations of state were fitted to substance specific data sets. The results show that simultaneously optimized functional forms can be applied to other fluids out of the same class of fluids for which they were optimized without significant loss of a...

Short Fundamental Equations of State for 20 Industrial Fluids

VLE predictions with the Peng–Robinson equation of state and temperature dependent kij calculated through a group contribution method

The parameters of the anisotropic united atoms potential for linear alkanes proposed by Toxvaerd [S. Toxvaerd, J. Chem. Phys. 107, 5197 (1997)] have been optimized on the basis of selected equilibrium properties (vapor pressures, vaporization enthalpies, and liquid densities) of ethane, n-pentane, and n-dodecane. The optimized parameters for the CH2 and CH3 groups form a regular sequence with those of methane and the force centers are found between the carbon and hydrogen atoms, as expected. The resulting potential, called AUA4, has been compared with Toxvaerd’s potential (AUA3) by using several molecular simulation methods (Gibbs ensemble Monte Carlo, thermodynamic integration, and molecular dynamics). An investigation performed at temperatures ranging from 140 to 700 K and with various chain lengths up to 20 carbon atoms has shown AUA4 to provide systematic improvements of vapor pressures, vaporization enthalpies, and liquid densities for pure n-alkanes. Significant improvements have been also noticed o...

Optimization of the anisotropic united atoms intermolecular potential for n-alkanes

In 2004, we started to develop a group contribution method aimed at estimating the temperature dependent binary interaction parameters (kij(T)) for the widely used Peng–Robinson equation of state (EOS). Because our model relies on the Peng–Robinson EOS as published by Peng and Robinson in 1978 and because the addition of a group contribution method to estimate the kij makes it predictive, this model was called PPR78 (predictive 1978, Peng Robinson EOS). In our previous papers eleven groups were defined: CH3, CH2, CH, C, CH4 (methane), C2H6 (ethane), CHaro, Caro, Cfused aromatic rings, CH2,cyclic and CHcyclic Ccyclic. It was thus possible to estimate the kij for any mixture containing alkanes, aromatics and naphthenes at any temperature. In this study, the PPR78 model is extended to systems containing carbon dioxide. To do so, the group CO2 was added. The results obtained in this study are in many cases accurate.

Predicting the phase equilibria of CO2+hydrocarbon systems with the PPR78 model (PR EOS and kij calculated through a group contribution method)

As a continuation of a program to provide data on binary systems of interest to the petroleum and natural gas industry, the behavior of the n-pentane-carbon dioxide system was studied. The equilibrium ratios for each component were calculated at each temperature from the phase composition data. The equilibrium-phase densities were calculated from the measured phase composition and refractive index by use of the Lorentz-Lorenz molar refractivity relationship. The experimental measurements of the equilibrium-phase compositions and refractive indices, together with the calculated phase densities and vapor-liquid equilibrium ratios for both components are given in tabular form. As suspected previously, a considerable discrepancy was found between the measured phase compositions and the earlier data of Poettman and Katz. This is exemplified in the graphical form which shows the 2 sets of data for the 100/sup 0/F isotherm.

Equilibrium-phase properties of n-pentane-carbon dioxide system

Current methods for predicting the phase behavior and other thermodynamic properties of multicomponent systems of industrial interest usually require interaction parameters for each of the possible binary pairs which can be formed from the components in the mixture. As part of a general program to provide data of this type for systems of interest to the petroleum and natural gas industry, the behavior is studied of the i-butane-carbon dioxide binary because measurements on this system had not previously been reported in the literature. The study includes an experimental determination of the compositions and the refractive indices of the coexisting equilibrium phases at temperatures of 100/sup 0/, 160/sup 0/, 220/sup 0/, and 250/sup 0/F and at pressures from the vapor pressure of i-butane to the critical region of the system. Tabular data provide experimental equilibrium-phase properties for an i-butane-carbon dioxide system.

Equilibrium-phase properties of isobutane-carbon dioxide system

Virtually all modern design procedures that involve the determination of the thermodynamic properties of multicomponent fluid systems make use of binary interaction parameters for each possible binary pair that can be made up from the various molecular species in the multicomponent system. In most instances, the binary interaction parameters for paraffin hydrocarbon-paraffin hydrocarbon pairs are negligible, but the interaction parameters for paraffin hydrocarbon-nonparaffin hydrocarbon pairs or for nonhydrocarbon pairs are normally not negligible and have an appreciable effect on predicted behavior of the multicomponent system. The object of the study was to obtain experimental data on the equilibrium phase properties of binary mixtures of nitrogen and n-pentane over a sufficiently wide range of pressure, temperature, and composition so that the binary interaction and other parameters for the system could be calculated. (12 refs.)

George J. Besserer

Papers

Equilibrium-phase properties of n-pentane-carbon dioxide system

Equilibrium-phase properties of isobutane-carbon dioxide system

The equilibrium phase properties of the nitrogen-isopentane system.

Refractive indexes of ethane, carbon dioxide, and isobutane

Equilibrium-phase properties of isopentane-carbon dioxide system