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Accurate results for fluids at thermal equilibrium, valid both outside and inside the critical region, are difficult to obtain due to the singular nature of this region. However, for simple models like interacting hard spheres and the Ising model high accuracy has been obtained by use of SCOZA (self-consistent Ornstein-Zernike approximation). Here this approach is extended to one component fluids more generally. Outside the critical region good analytic approximations may be obtained by use of statistical mechanical results that are adjusted to experimental data and/or computer simulations. However, such approximations typically will be of mean field type in the sense that the usual singular behavior in the critical region and in the transition to it, is not present. Here the SCOZA is generalized such that approximations accurate outside the critical region are utilized as basic input in the SCOZA problem. The singular behavior is connected to correlations of increasing range that develop as the critical point is approached. These correlations contribute both to the internal energy and compressibility of the fluid, and they give another contribution to the SCOZA partial differential equation. Accurate results in the critical region are expected to be obtained by adjusting one or a few parameters. • The SCOZA is extended to fluids more generally. Earlier it was developed to improve upon the MSA. • By this extension also equations of state known well outside the critical region, can be input to the SCOZA problem. • The SCOZA gives a rather accurate description of the critical region with its singular behavior. • Solution of the SCOZA problem is expected to give accurate results also in the critical region for the fluid of interest. [ABSTRACT FROM AUTHOR]

A diverse substance library containing extensive PVT data for 77 pure components was used to critically evaluate the performance of a quartic equation of state and other four famous cubic equations of state in critical region. The quartic EOS studied in this work was found to significantly superior to the others in both vapor pressure prediction and saturated volume prediction in vicinity of critical point.

An equation of state with near-critical corrections is developed for water (H 2 O) and carbon dioxide (CO 2 ). The equation is constrained by the critical conditions and optimized with highly accurate pressure–volume–temperature ( PVT ) and phase equilibrium data at temperatures from the triple point to 647.096 K and pressures from zero to 80 MPa. The resulting equation is systematically checked in the above P – T range. For H 2 O, the vapor pressures are reproduced within 0.032%, with an average deviation of only 0.005%; the average deviations of the saturated vapor and liquid volumes, and the volumes in the single-phase regions are 0.047%, 0.019% and 0.058%, respectively. The maximum deviations of the above-mentioned volumes are 1.26%, 0.91% and 1.06%, respectively, which are all in the immediate vicinity of the critical point. Below 520 K, the deviations of PVT and saturation properties are all within 0.06%. For subcritical CO 2 , the average and maximum deviations of vapor pressures are 0.011%, 0.077%, respectively; the average deviations of the saturated vapor and liquid volumes, and the volumes in the single-phase regions are 0.099%, 0.058% and 0.126%, respectively. The largest deviations of the above volumes are 0.230%, 0.173% and 0.358%, respectively. For supercritical CO 2 below 650 K, the average volume deviation is 0.080%. The equation also agrees very well with many high-quality density and phase equilibrium data not used to fit the equation. The equation gives excellent prediction of fugacity coefficients, residual enthalpies and entropies and heats of vaporization, and can be extrapolated to 200 MPa for H 2 O, or to 100 MPa for CO 2 . The equation can serve as a good starting point for the thermodynamic modeling of many industrial and geological processes, including carbon capture and sequestration. [ABSTRACT FROM AUTHOR]

In recent years, the use of nitrogen has increased as gas injection to recover oil fluids near the critical point. The behavior of hydrocarbon mixture phases in the critical region shows very interesting complex phenomena when facing a recovery project with nitrogen. Therefore, it is important to have experimental information of the PVTx thermodynamic variable, often scarce, for this type of critical phenomena. This paper reports the experimental measures of the volumetric behavior and phases of synthetic fluid in a nitrogen injection process. The experiment was performed at laboratory scale, and it obtained variations on the saturation pressure, gas oil ratio, density and composition of the hydrocarbon phase when nitrogen was injected at molars of 10, 20, 30 and 40% on different volumetric portions of the mother sample. In addition, the data obtained experimentally was used to demonstrate the capacity of tune to compositional models. The data provided represents a valuable contribution to the understanding of phenomena associated with retrograde and near critical regions, as well as their use in tuning and developing more elaborate models such as Cubic Equations of State (EOS). It is worth highlighting the importance of this data in the potential processes of nitrogen, CO2, and lean gas injection, which require knowledge of the gas-oil ratio, saturation pressures, density and composition of the fluid in current production. The identification of the phenomena shown, represent a potential application to the modeling of displacements and maintaining the pressure in the improved recovery when scaling up the laboratory data to the field / reservoir conditions.En años recientes se ha incrementado el uso de nitrógeno como gas de inyección para recuperar fluidos de petróleo cercanos al punto crítico. El comportamiento de fases de mezclas de hidrocarburos en la región crítica presenta fenómenos complejos de gran interés cuando se trata de encarar un proyecto de recuperación con nitrógeno. Es conveniente por tanto, tener información experimental de las variables termodinámicas PVTx, muchas veces escasas para este tipo de fenómenos críticos. El presente artículo reporta las medidas experimentales del comportamiento volumétrico y de fases realizadas sobre un fluido sintético en un proceso de inyección de nitrógeno. El experimento realizado a escala de laboratorio obtuvo las variaciones de la presión de saturación, relación gas aceite, densidad y composición de la fase de hidrocarburos al inyectar nitrógeno al 10, 20, 30 y 40% molar sobre diferentes porciones volumétricas de la muestra madre. Adicionalmente se utilizaron los datos obtenidos en forma experimental para demostrar la capacidad de ajuste a modelos composicionales. Los datos suministrados representan un aporte valioso en el entendimiento de los fenómenos asociados a las regiones retrogradas y cercanamente crítica, además de su uso en el ajuste y desarrollo de modelos más elaborados como las Ecuaciones Cúbicas de Estado (EOS). Es de resaltar la importancia que representan estos datos en los procesos potenciales de inyección de nitrógeno, CO2, y gas pobre, donde se requiere el conocimiento de las relación gas-aceite, presiones de saturación, densidades y composiciones del fluido en producción actual. La identificación de los fenómenos presentados representa una aplicación potencial al modelamiento de desplazamientos y mantenimiento de presión en el recobro mejorado al escalar los datos de laboratorio a condiciones de campo-yacimiento.

Abstract: In our previous work [S. Azzouz, A. Rizi, A. Acidi, A. Abbaci, St. Cerc. St. CICBIA 11 (2) (2010) 235–241], we developed an interim thermodynamic property formulation for the supercritical n-hexane which incorporates non-analytic scaling laws in the critical region and reproduces the thermodynamic properties of n-hexane far away from the critical region. However, it appears that this equation of state gives unphysical values for critical amplitudes such as that of the compressibility above the critical point. In this work, we present a modification of this equation of state based on the pressure data of Grigoriev et al. group and those of Abdulagatov, the isochoric specific heat data of Amirkhanov et al. as well as the isobaric specific heat of Gerasimov et al. Comparison with different sets of thermodynamic-property data available is given. [Copyright &y& Elsevier]

Abstract: In the present work, we have extended a crossover parametric equation of state proposed by Agayan et al. [V.A. Agayan, M.A. Anisimov, J.V. Sengers, Phys. Rev. E 64 (2001) 026125] in order to take account of the asymmetric nature of fluids. To this end, we have used the concept of complete scaling which implies that the scaling fields are a combination of all physical fields. To test this extended equation of state, it has been applied to carbon dioxide and nitrogen fluids. In addition, by using this equation of state we have studied the temperature behaviour of diameter of the coexistence curve in close to and far from the critical point and compared it with one of the recent approach to nitrogen fluid. [Copyright &y& Elsevier]

The compressibility factor was calculated in a wide range of parameters including the critical region using the equations of state obtained previously for real gas and conditions and limitations formulated by the authors for the form (structure) of the equation of state. It was determined that maximal deviations of compressibility factor values calculated by these analytical equations from the experimental (tabulated) values occur mainly within the critical region. A corresponding correction was introduced into the initial equation, and the analytical equation accurately describing experimental data in a wide range of real gas state parameters, including the critical region, was derived. It was shown by the example of analysis of high-accuracy experimental data on thermal properties of helium in the critical region that data description by the proposed analytical equation is at least as good as that obtained by the equation of fluctuation theory (scaling). It is shown that the proposed equation meets the classical conditions at the critical point. [ABSTRACT FROM AUTHOR]

Abstract: In our previous work [S.B. Kiselev, J.F. Ely, Fluid Phase. Equilib. 222–223 (2004) 149], we developed a generalized cubic (GC) EoS for pure fluids, which incorporates non-analytic scaling laws in the critical region and reproduces the thermodynamic properties of pure fluids with high accuracy, including the asymptotic scaling behavior of the isochoric heat capacity in the one- and two-phase regions. However, it appears that in some cases the GC EoS can give unphysical behavior when extrapolated to high temperatures and densities. In this work, we present a modification of the generalized cubic EoS, which in the critical region is physically equivalent to the GC EoS developed earlier, but is more reliable in its extrapolation to high temperatures. [Copyright &y& Elsevier]

Abstract: The thermodynamic properties of fluids are predicted using global equations of state. Among these thermodynamic properties, we consider the densities of the liquid and vapor phases, pressure, the caloric properties such as specific heat at constant volume and specific heat at constant pressure and the speed of sound. In the present work, we apply the crossover theory to these thermodynamic properties and give a comparison of the crossover model equation of state with the experimental thermodynamic property data of ethane. [Copyright &y& Elsevier]

We have developed a generalized cubic (GC) EOS for pure fluids, which incorporates non-analytic scaling laws in the critical region and in the limit ρ→0 is transformed into the ideal gas equation EOS. The GC EOS contains 10 adjustable parameters and reproduces the thermodynamic properties of pure fluids with high accuracy, including the asymptotic scaling behavior of the isochoric heat capacity in the one- and two-phase regions. Unlike the crossover cubic EOS developed earlier [Fluid Phase. Equilibr. 147 (1998) 7], the GC EOS is based on the crossover sine model and can be extended into the metastable region for representing analytically connected van der Waals loops. In addition, using the GC EOS and the decoupled-mode theory (DMT) we have developed a generalized GC + DMT model, which reproduces the singular behavior of the thermal conductivity of pure fluids in and beyond the critical region. Unlike the DMT model based on the asymptotic crossover equation of state CREOS-97, the GC + DMT model is valid in the entire fluid state region at T≥Tb (where Tb is the binodal temperature), and at ρ→0 reproduces the dilute gas contributions for the transport coefficients. [Copyright &y& Elsevier]

A parametric crossover model is adapted to represent the thermodynamic properties of pure D2O in the extended critical region. The crossover equation of state for D2O incorporates scaling laws asymptotically close to the critical point and is transformed into a regular classical expansion far from the critical point. An isomorphic generalization of the law of corresponding states is applied to the prediction of thermodynamic properties and the phase behavior of D2O + H2O mixtures over a wide region around the locus of vapor-liquid critical points. A comparison is made with experimental data for pure D2O and for the D2O + H2O mixture. The equation of state yields a good representation of thermodynamic property data in the range of temperatures 0.8 Tc( x)≤ T≤1.5 Tc( x) and densities 0.35ρc( x)≤ρ≤1.65ρc( x). [ABSTRACT FROM AUTHOR]