Computing Thermal Properties of Natural Gas by Utilizing AGA8 Equation of State (original) (raw)
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Journal of Petroleum Science and Engineering, 2006
In this contribution, 10 equations of state (EoSs) are used to predict the thermo-physical properties of natural gas mixtures. One of the EoSs is proposed in this work. This EoS is obtained by matching the critical fugacity coefficient of the EoS to the critical fugacity coefficient of methane. Special attention is given to the supercritical behavior of methane as it is the major component of natural gas mixtures and almost always supercritical at reservoir and surface conditions. As a result, the proposed EoS accurately predicts the supercritical fugacity of methane for wide ranges of temperature and pressure. Using the van der Waals mixing rules with zero binary interaction parameters, the proposed EoS predicts the compressibility factors and speeds of sound data of natural gas mixtures with best accuracy among the other EoSs. The average absolute error was found to be 0.47% for predicting the compressibility factors and 0.70% for the speeds of sound data. The proposed EoS was also used to predict thermal and equilibrium properties. For predicting isobaric heat capacity, Joule-Thomson coefficient, dew points and flash yields of natural gas mixtures, the predictive accuracy of the EoS is comparable to the predictive accuracy of the Redlich-Kwong-Soave (RKS) EoS or one of its variants. For predicting saturated liquid density of LNG mixtures, however, the accuracy of predictions is between the RKS and Peng-Robinson (PR) EoSs.
Numerical procedures for natural gas accurate thermodynamic properties calculation
Journal of Engineering Thermophysics, 2012
Natural gas (NG) is a mixture of 21 elements and widely used in the industries and domestics. Knowledge of its thermodynamic properties is essential for designing appropriate process and equipments. In this study, the detailed numerical procedures for computing most thermodynamic properties of natural gas are discussed based on the AGA8 equation of state (EOS) and thermodynamics relationships. To validate the procedures, the numerical values are compared with available measured values. The validations show that the average absolute percent deviation (AAPD) for density calculations is 0.0831%, for heat capacity at the constant pressure is 0.87%, for heat capacity at the constant volume is 1.13%, for Joule-Thomson coefficient is 1.93%, for speed of sound is 0.133%, and for enthalpy is 1.06%. Furthermore, in this work, the new procedures are presented for computing the entropy and internal energy. Due to lack of experimental data for these properties, the validation is done for pure methane. The validation shows that AAPD is 0.078% and 0.0133% for internal energy and entropy, respectively.
GERG Project: Wide-range reference equation of state for natural gases
GAS UND …, 2003
A group of European gas companies, GERG, supported the development of a new equation of state for the thermodynamic properties of natural gases covering the gas and liquid region including the vapour-liquid phase equilibrium. The new equation, GERG02, was developed on the basis of a multi-fluid approximation using pure substance equations for each component and experimental data for binary mixtures only. Therefore, the representation of multicomponent mixture data is predictive. The results calculated with the new equation for thermal and caloric properties of natural gas mixtures show substantial improvements in comparison to the AGA8-DC92 and the GERG88 equation, which are known to be the current internationally accepted standard for density or compression factor calculations at pipeline conditions (see ISO 12213-part 2 and 3, 1997). The new reference equation allows high accuracy calculations for thermal and caloric properties in the homogeneous region (gas, liquid and supercritical) and also enables calculations for the vapour-liquid equilibrium. The reference equation can be used as a database or reference for technical applications and processes with natural gases, LNG, LPG, natural gas vehicles and hythane mixtures.
Comparison of the GERG-2008 and Peng-Robinson Equations of State for Natural Gas Mixtures
This work compares two equations of state applicable to natural gas mixtures, namely the GERG-2008 equation of state (EoS), which was proposed as a high-accuracy reference model, and the traditional Peng-Robinson (PR) EoS. This comparison is done in terms of the accuracy of calculated properties such as pressure and density with respect to experimental data from the literature, as well as in vapor-liquid equilibria (VLE) calculations. It was found that the GERG-2008 EoS gives better results in comparison with PR for the calculation of density and pressure, generating deviations in the range from 0.1 to 1%. For the VLE calculations, the accuracy of GERG-2008 was slightly better than PR. However, this accuracy is accompanied with increased mathematical complexity, resulting in increased computational time: 2 to 6 times higher. This is due to the fact that the calculation of molar density of GERG-2008 requires an iterative calculation step for the liquid and vapor phases, which makes the resolution of the VLE calculation slower.
Journal of Physical and Chemical Reference Data, 2019
A new mixture model (EOS-LNG) for the accurate representation of thermodynamic property data of multicomponent natural gas mixtures in the liquid state is presented. The mathematical approach of the GERG-2008 equation of state of Kunz and Wagner is adopted and new binary-specific functions for methane + n-butane, methane + isobutane, methane + n-pentane, and methane + isopentane are developed. The representation of all experimental data available in the literature for the corresponding binary systems is carefully analyzed so that these functions can also be applied at fluid states beyond the liquefied natural gas (LNG) region. The EOS-LNG represents all available binary and multicomponent data in the LNG region within their specified experimental uncertainty, which is significantly more accurate than the GERG-2008 model. The main focus was given to the representation of new density data measured between 100 K and 180 K with a maximum pressure of 10 MPa. Deviations from the EOS-LNG presented here do not exceed 0.02% for binary data and 0.05% for multicomponent systems. Deviations of calculated values of these data from experimental data in other fluid regions are similar to or better than those calculated with the GERG-2008 model.
Industrial & Engineering Chemistry Research, 2022
This work demonstrates the need for accurate thermodynamic models to reliably quantify changes in the thermophysical properties of natural gas when blended with hydrogen. For this purpose, a systematic evaluation was carried out on the predictive accuracy of three well-known models, the Peng−Robinson equation of state (EoS), the multiparameter empirical GERG-2008 model, and the molecular-based polar soft-SAFT EoS, in describing the thermodynamic behavior of mixtures of hydrogen with commonly found components in natural gas. Deviations between the calculated properties and experimental data for phase equilibria, critical loci, second-order derivative properties and viscosities are used to determine the accuracy of the models, with polar soft-SAFT performing either equally or better than the other two examined models. The evaluation for the effect of H 2 content on the properties of methane, simulated as natural gas at conditions for transportation, reveals higher changes in blend density and speed of sound with increasing H 2 content within 5% change per 5 mol % H 2 added, while viscosity is the least affected property, changing by 0.4% for every 5 mol % H 2 .
A Study on Gas Compressibility Factor for Gas-Condensate Systems
Gas compressibility factor is the most important gas property. Its value is required in many petroleum engineering calculations. There are many different sources of gas compressibility factor value such as experimental measurements, equations of state, charts, tables, intelligent approaches and empirical correlations methods. In absence of experimental measurements of gas compressibility factor values, it is necessary for the petroleum engineer to find an accurate, quick and reliable method for predicting these values. This study presents a new gas compressibility factor explicit empirical correlation for gas-condensate reservoir systems above dew point pressure. This new correlation is more robust, reliable and efficient than the previously published explicit empirical correlations. It is also in a simple mathematical form. The predicted value using this new correlation can be used as an initial value of implicit correlations to avoid huge number of iterations. This study also presents evaluation of the new and previously published explicit correlations.