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An accurate description of the phase behavior of the CH 4 1 H 2 S system is given for temperatures from 70 K to the critical temperature of H 2 S and pressures up to 250 MPa. The study includes the solid phases of CH 4 and H 2 S. A global pressure-temperature diagram is presented. The types of temperature-composition and pressure-composition phase diagrams that can be encountered in the studied temperature and pressure ranges have been described. The temperature and pressure ranges where the phase behavior of the system changes have been identified and a representative phase diagram is presented for each range. Phase diagrams have been obtained through the solid-liquid-vapor equation of state proposed by Yokozeki.

International audienceDesign and optimization of cryogenic technologies for biogas upgrading require accurate determination of freeze-out boundaries. In cryogenic upgrading processes involving dry ice formation, accurate predictions of solid–liquid, solid–vapor, and solid–liquid–vapor equilibria are fundamental for a correct design of the heat exchanger surface in order to achieve the desired biomethane purity. Moreover, the liquefied biogas production process, particularly interesting for cryogenic upgrading processes due to the low temperature of the obtained biomethane, requires an accurate knowledge of carbon dioxide solubility in liquid methane to avoid solid deposition. The present work compares two different approaches for representing solid–liquid, solid–vapor, and solid–liquid–vapor equilibria for the CH4−CO2 mixture. Model parameters have been regressed in order to optimize the representation of phase equilibrium at low temperatures, with particular emphasis to the equilibria involving a solid phase. Furthermore, the extended bibliographic research allows determining the regions where more accurate data are needed

In a previous paper, authors used molecular simulation data for Lennard-Jones fluids for the regression of the binary interaction parameters of the LJ-SLV-EoS. The binary interaction parameters of the EoS have been expressed as simple functions of the ratios σ 11 /σ 22 and ε 11 /ε 22. This procedure allows obtaining a qualitative prediction of the solid-liquid phase behavior of mixtures composed of simple fluids. This work presents the predicted phase diagrams including solid phases for binary mixtures composed of argon, oxygen, nitrogen, krypton, xenon, and methane. Predictions are in qualitative agreement with the phase behavior documented by the experimental data available from the literature. The adopted procedure allows producing a qualitative reasonable phase diagram for mixtures knowing the Lennard-Jones parameters of the mixture components. The comparison with literature data shows that the adopted procedure is suitable for predicting the solid-liquid behavior of the mixture, distinguishing among eutectic, solid solution, solid-liquid azeotrope.

In 2003, A. Yokozeki proposed an Equation of State capable of representing solid, liquid, and vapor phases. This equation represents an innovative approach of including solid in phase diagrams. The capability of this Equation of State, named SLV-EoS, of giving a qualitative correct representation of phase diagrams for binary mixtures of Lennard-Jones spheres has been tested in this work. The SLV-EoS has been used for producing the phase diagrams of binary Lennard-Jones mixtures with diameter ratio σ 11 /σ 22 ranging from 0.85 to 1, and well-depth ratio ε 11 /ε 22 ranging from 0.625 to 1.6 at reduced pressure P * = Pσ 11 3 / ε 11 = 0.002. The obtained phase diagrams have been compared with literature data obtained by Monte-Carlo simulation. The comparison shows the incapability of the SLV-EoS with null binary interaction parameters of predicting solid-liquid azeotrope and eutectic in the investigated range of σ 11 /σ 22 and ε 11 /ε 22. Binary interaction parameters have been regressed to allow the SLV-EoS giving a qualitative representation of the three types of phase diagrams: solid solution, solid azeotrope and simple eutectic. Binary interaction parameters are shown being smoothed functions of σ 11 /σ 22 and ε 11 /ε 22 , allowing the prediction of the phase behavior for other Lennard-Jones mixtures or real mixtures of simple fluids.

A Gibbs free energy equation of state (EoS) for phase I of solid methane has been developed with an original functional form. The EoS has a validity range from 21 to 300 K and up to 5000 MPa. The EoS parameters have been regressed on the literature data of molar volume, isothermal compressibility, thermal expansion, and isobaric and isochoric heat capacity. To the knowledge of the authors, this EoS is the most complete published model for phase I of solid methane in terms of the number of data of different thermodynamic properties used for the regression of the parameters and extension of the temperature and pressure validity range. The EoS represents the thermodynamic data within their estimated experimental uncertainty, and it maintains a physically correct behavior when extrapolated to high temperatures and pressures. The developed EoS for solid methane has been coupled with the reference equation of state for the fluid phases developed by Setzmann and Wagner in 1991 for calculating the melting and sublimation curves, and the predictions are in good agreement with the available data.

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The functional form developed in Stringari Stringari J. Chem. Eng. Data20216611571171 has been applied for obtaining a Gibbs free energy equation of state (EoS) for phase I of solid benzene. The EoS has a validity range from 15 K up to the temperature and pressure of the solid II–solid I–liquid triple point, that is, 488.3 K and 1164.6 MPa. The EoS parameters have been regressed on literature data of molar volume, isothermal compressibility, thermal expansion, and isobaric and isochoric heat capacities. The average absolute deviation between the EoS and the primary data is 6.8% for isothermal compressibility, 5.5% for isobaric thermal expansion, 0.9% for molar volume, 2.7% for isobaric heat capacity, and 4.9% for isochoric heat capacity. The EoS maintains a physically correct behavior in the whole range of temperature and pressure of existence of phase I of solid benzene. The developed EoS for solid benzene has been coupled with the reference equation of state for the fluid phases developed by Thol Thol High Temp.-High Press2012418197 for calculating the melting and sublimation curves. Predictions of sublimation pressures are in excellent agreement with the recommended values of Ružička Ružička J. Chem. Thermodynamics2014684047 and those of melting temperatures matches very well the values obtained from the auxiliary melting equation in the validity limits of the EoS of Thol Thol High Temp.-High Press2012418197.