(490a) Global Phase Diagrams of Binary and Ternary Mixtures of CH4, CO2, and H2S: Experimental Measurements and Modeling

Natural gas is the fastest growing fossil fuel (1.6% p.a.) and the sole with an increasing share in primary energy (24% up to 26%) between 2016 and 2040. A valid option for natural gas storage and transportation is the Liquefied Natural Gas (LNG) technology. To illustrate, in March 2018 the global liquefaction nameplate capacity reached 369.4 MTPA and the global liquefaction capacity of proposed projects starting in the early-2020s reached 875 MTPA.

Recent studies have shown that 40% of the remaining natural gas reserves are sour and/or with high CO₂ content (15-80%). In these low-quality gas fields, also the hydrogen sulfide content may be high, even up to about 15%. When the CO₂ content in raw gas streams is high, classical sweetening technologies become too much energy demanding, impacting on production costs. Hence, over the last decades, several new gas purification processes, based on low-temperature separations, have been studied and proposed as potential solutions to overcome this issue. These purification processes, as well as natural gas liquefaction technologies, operate in regions of temperatures and pressures where the possible formation of solid phases (mostly rich in CO₂) may occur.

In this scenario, a detailed description of the phase behavior of systems containing methane, carbon dioxide, and hydrogen sulfide is necessary to perform the correct process design of new gas purification and liquefaction technologies that are required to allow the profitable production of commercial-grade gas from low quality natural gas reserves.

In this work, original experimental measurements of phase equilibria involving solid phases for the CH₄+CO₂+H₂S system have been obtained by means of a static-analytic apparatus for temperatures between 183.63 K and 207.59 K, and pressures from 1.46 MPa up to 4.32 MPa.

In addition to that, the global phase equilibrium behavior of the corresponding binary mixtures has been modeled by means of the Solid-Liquid-Vapor Equation of State (SLV EoS), proposed by Yokozeki A. [Analytical equation of state for solid-liquid-vapor phases, International Journal of Thermophysics, 2003], and of the GERG2008 EoS coupled with a G^E model for the solid phase.

For the application of the SLV EoS, binary interaction parameters have been regressed using literature fluid-fluid and solid-fluid equilibrium data of the CH₄+CO₂, CH₄+H₂S, and CO₂+H₂S binary systems. The parameters of the G^E model for the solid phase have been regressed using literature solid-fluid equilibrium data of the aforementioned binary systems. Then, the simulation results from both the models have been compared with the literature fluid-fluid equilibrium values and the unpublished solid-fluid equilibrium data of the CH₄+CO₂+H₂S system proposed in this work.

This work aims at proposing a map of the phase equilibrium behavior of the three binary systems, thus providing a rapid access to the evaluation of the change of the equilibria as function of pressure and temperature. Furthermore, the modeling of the ternary system points out the complexity of the phase equilibrium behavior in a wide range of temperature and pressure and the multitude of fluid-fluid, solid-fluid, solid-solid, solid-solid-fluid and solid-fluid-fluid equilibria occurring according to temperature, pressure, and composition.