A Simple Cubic Equation of State for Hydrocarbons and Other Compounds (original) (raw)
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The Canadian Journal of Chemical Engineering, 2013
Precise descriptions of the thermodynamic properties of pure fluids require accurate definition of vapour-pressure and phase volumes as well as residual volumes, enthalpies and entropies. While carefully fitted multi-parameter equations of state (EOS), such as Benedict-Webb-Rubin-Starling fulfil these requirements, cubic EOSs usually do not. On the other hand, cubic EOSs are widely used in the oil industry, due to their simplicity and reliability in most vapour-liquid equilibrium calculations. For thermal oil recovery processes and the natural gas industry, the choice of EOS becomes important for predicting thermodynamic properties, such as isobaric and isochoric heat capacities, sound velocity and the Joule-Thomson coefficient. In this study, eight cubic EOSs which most of them are used in commercial reservoir simulators are selected for evaluation of their capability in the prediction of second-order derivative thermodynamic properties at different temperatures and pressures, using pure components frequently found in petroleum and natural gas mixtures. It is shown that none of the cubic EOSs could accurately predict all of the stated parameters, especially below the critical point. All EOSs failed to show the extrema in the derivative properties. However, among these equations the Yu-Lu and Schmidt-Wenzel EOSs were found to have more reliable predictions in most of the cases.
A Universal Equation of State for Hydrocarbons
Journal of Computer Chemistry Japan
a simple, universal equation of state (EoS) for hydrocarbons is proposed. this EoS allows estimation of the effective potential parameters ε and σ in the Lennard-Jones function as analytic functions of n, the number of electrons in the hydrocarbon molecule. Using these parameters, the thermodynamic properties of hydrocarbons may be calculated based on the EoS for a perfect liquid, as already proposed in the Lennard-Jones system. the thermodynamic properties estimated in this manner for various hydrocarbons have been found to be reasonably similar to experimentally obtained values.
RK-, SRK-, & SRK-PR-TYPE EQUATION OF STATE FOR HYDROCARBONS, BASED ON SIMPLE MOLECULAR PROPERTIES
A database made of 429 different hydrocarbons was utilized to fit their critical pressure and temperature as a function of molecular weight and carbon atomic fraction. The critical parameters (T_c^2)/P_c and T_c/P_c , appearing in Redlich-Kwong, Soave-Redlich-Kwong, and Redlich-Kwong-Peng-Robinson equation of state (EoS), were replaced by two curve-fitted equations. The afore-mentioned original equations were tested and contrasted versus their counterparts as far as predicting the molar volume of both liquid and vapor at a given pair of pressure and temperature for a given hydrocarbon. Using either an empirical or a reference datum for the estimated molar volume, it was found that the substituted equation did predict the volumetric properties with accuracy as good as the original equation. The replacement will ease the calculation of volumetric properties of liquid and vapor with a reasonable accuracy. It was found that, in general, all models, the original and the substituted/modif...
An equation of state for property prediction of alcohol–hydrocarbon and water–hydrocarbon systems
Journal of Petroleum Science and Engineering , 2001
Equations of state have been widely used in the petroleum and chemical industries for thermodynamic property calculation. In the presence of polar substances that self-associate through hydrogen bonding (such as water or alcohol), equations of state are of very limited use. One way to account for the association is to consider the equation of state to be formed of two contributions: physical and chemical. In this work, we develop an equation of state consisting of two terms as proposed by Andreko [Fluid Phase Equilib. 65 (1991) 89], a chemical and a physical term, for correlation of thermodynamic properties of mixtures containing an associating species. This equation of state is used to correlate vapor pressure data for a number of associating molecules, such as alcohol and water, as well as bubble point pressure data for binary water – hydrocarbon and alcohol – hydrocarbon systems. The results obtained are in good agreement with the experimental data and requiring the use of only one adjustable parameter for each binary system. D
Thermophysical Properties of Ethane from Cubic Equations of State
Revista de Chimie (Bucharest), 2013
Vapour-liquid equilibrium and thermophysical properties of ethane were predicted, along the saturation curve and in the single-phase region. Five cubic equations of state were used: Soave-Redlich-Kwong (SRK), Peng-Robinson (PR), Schmidt-Wenzel (SW), Treble-Bishnoi-Salim) and GEOS3C. A wide comparison with NIST recommended data was made. This study shows that the cubic EOSs lead to reasonable predictions of thermophysical properties of ethane, resting simple enough for applications. Because ethane is both an industrially important fluid and the second member of the interesting alkane series, we have considered that it is necessary to examine the possibility to reproduce its PVT (pressure-volume-temperature) and thermophysical properties using simple thermodynamic models as cubic equations of state. In the classical thermodynamic framework it is possible to develop relations to calculate the Helmholz and Gibbs energies, enthalpies, entropies, fugacity coefficients and other thermophysical properties of fluids. Such relationships together with equations of states (EOS) can be applied to obtain departure functions of thermodynamic properties [1]. Then the "true" thermodynamic properties are calculated for pure components and mixtures. In our previous works [2-5], thermophysical properties of many pure fluids and mixtures were predicted by cubic equations of state. In the last two decades, a progress has been made in implementing molecular theories for the development of new generations of thermodynamic models, such as Statistical Association Fluid Theory (SAFT) in equations of state. Most of the SAFT models are not entirely theoretical, because their molecular parameters are obtained by fitting the experimental vapor pressures and other thermodynamic data [6]. On the other hand, cubic equations of state exhibit an overall robustness in predicting various thermophysical properties over a wide range of temperatures and pressures, with the exception of certain phenomena related to heat capacity, sound velocity and Joule-Thomson coefficient [2]. In this work, vapour-liquid equilibrium and thermophysical properties were predicted, along the saturation curve and in the single-phase region for ethane. Five cubic equations of state were used: Soave-Redlich-Kwong (SRK) [7], Peng-Robinson (PR) [8], Schmidt-Wenzel (SW) [9], Treble-Bishnoi-Salim (TBS) [10] and GEOS3C [11]. A wide comparison with recommended NIST (National Institute of Standard and Technology, USA) data [12], considered as pseudo experimental data, was made. The NIST data were obtained by using a multiparameter equation of state developed by Friend, Ingham, and Ely [13]. The authors claim that this equation of state is accurate from about 90 K to 625 K for pressures less than 70 MPa and was developed by considering PVT, second virial coefficient, heat capacity, and sound speed data. * email: v_feroiu@chim.upb.ro This study shows that the cubic EOSs lead to reasonable predictions of thermophysical properties of ethane, resting simple enough for applications. The GEOS3C equation of state The GEOS3C equation of state is a general form [1, 14] for the cubic equations of state with two, three and four parameters: (1) The four parameters a, b, c, d for a pure component are expressed by: (2) (3) The GEOS3C equation is based on the GEOS equation [14] and uses a new temperature function [11]: β(T r)= 1 + C 1 y + 2 3 2 3 C y + C y for T r ≤ 1 (4) (T r)= 1 + C 1 y for T r > 1 (5) (6) The expressions of the parameters Ω a , Ω b , Ω c , Ω d are: (9) Riedel's criterion, α c , is calculated using the acentric factor from the equation: (7) (8)
New equation of state is accurate for VLE, PVT calculations of oil mixtures
Oil & Gas Journal, 2002
A new cubic molecular-based equation of srare (MMM) accru:ate.ly calculates pressure, volume, temperature (PVT) and vapor-liquid equilibrium (VLE) data for pure and mixed fluids. Comparisons to the Redlich-Kwong (RK) equation of state, which is considered the best two-constant-parameter equation of state, indicated that the MMM equation of state was more accurate. This article reports application of the MMM equation of state to pure and mixed fluid calculations of interest to the oil and gas industries. Comparison of the results to those obtained using the RK and Peng-Robinson (PR) equations of srate showed that MMM is more accurate.
Liquid-state theory of hydrocarbon-water systems: application to methane, ethane, and propane
The Journal of Physical Chemistry, 1992
We have studied the structural and bulk thermodynamic properties of hydrocarbon (methane, ethane, and propane)-water systems as well as pure water using the site-site Omstein-Zemike (SSOZ) equation under a variety of different closure relations in order to compare the quantitative predictive Capabilities of the various closures. For the hydrocarbon-water systems, the simple point-charge (SPC) potential was used to model water, and the optimized potentials for liquid simulation (OPLS) were used to model the hydrocarbons. For pure water, predictions were also made for other water potential models. We solved the SSOZ equation with the hypemetted-chain (HNC) closure to determine the pair correlation functions of water. We then analyzed the structural and bulk thermodynamic properties of methane, ethane, and propane at infinite dilution in water using various closure relations for the hydrocarbonwater pair correlation functions. We find that the HNC closure, which is the closure that has been utilized almost exclusively to predict bulk thermodynamic properties of interaction-site fluids, performs rather poorly. Specifically, we find that the HNC closure consistently underpredicts the magnitudes of both the solute partial molar volume and the solute-solvent interaction energy, grossly overpredicts the magnitude of the residual chemical potential, and gives the incorrect sign of the enthalpy of solution. On the other hand, we find that two recently developed closures, the Martynov-Sarkisov (Mas) and Ballone-Pastore-Galli-Gazzillo ( B E G ) closures, which have not been utilized so far in conjunction with the SSOZ equation, yield reasonable predictions of the structural and bulk thermodynamic properties of the hydrocarbon-water systems studied. In particular, utilizing the SSOZ-BEG equation, the predicted temperature variation of the residual chemical potential over the relatively broad range 5-80 OC was found to be in very good agreement with the experimental data. Note that the residual chemical potential is directly related to the Henry's law constant, which, in tum, can be utilized to predict solubilities. In addition, we have developed an analytical expression for the residual chemical potential, appropriate for interaction-site fluids, in terms of pair and direct correlation functions at full coupling for the various closures examined in this paper. To date, an expression of this type was available only for the HNC closure. This new expression facilitates the calculation of the residual chemical potential by eliminating the previous need to perform a numerical integration over the coupling constant, thus making the computation of the chemical potential simpler and more efficient. Finally, we have also tested the accuracy of the equivalent-site approximation (ESA), a perturbation method which was developed by Curro and Schweizer to study long polymeric chains by treating all the sites in a given molecule as equivalent, on the predictions of the structural and bulk thermodynamic properties of propane at infinite dilution in water. Note that of all the n-alkanes, propane poses the most severe challenge to the ESA. Interestingly, we find that, already for propane, the ESA yields predictions of bulk thermodynamic properties which are within 5% of those obtained using the rigorous calculations.
Fluid Phase Equilibria, 2010
A new three-parameter cubic equation of state is presented by combination of a modified attractive term and van der Waals repulsive expression. Also a new alpha function for the attractive parameter of the new EOS is proposed. The new coefficients of alpha function and the other parameters of the attractive term are adjusted using the data of the saturated vapor pressure and liquid density of almost 60 pure compounds including heavy hydrocarbons. The new EOS is adopted for prediction of the various thermophysical properties of pure compounds such as saturated and supercritical volume, enthalpy of vaporization, compressibility factor, heat capacity and sound velocity. Following successful application of the new EOS for the pure components, using vdW one-fluid mixing rules, the new EOSs are applied to prediction of the bubble pressure and vapor mole fraction of the several binary and ternary mixtures. The accuracy of the new EOS for phase equilibrium calculation is demonstrated by comparison of the results of the present EOSs with the PT, PR, GPR and SRK cubic EOSs.
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.