Thermodynamic Modeling of CO2-Rich Natural Gas Fluid Systems

Abstract

This chapter presents a survey on equation of state (EOS) for high-pressure natural gas (NG), pure CO2, and CO2-rich NG systems. The cubic EOS’s—PR-EOS, RK-EOS, SRK-EOS—are discussed in detail, with formulas for residual properties used in NG engineering. The text also discusses EOS’s of higher complexity for pure CO2 and CO2-rich NG: Span–Wagner EOS (SW-EOS), cubic-plus-association EOS (CPA-EOS), and GERG-2004/2008-EOS. EOS performances and applicability are qualitatively discussed. Some EOS’s are quantitatively compared for high-pressure pure CO2—including the critical neighborhood—in terms of density, enthalpy, isobaric heat capacity, and sound speed. The performance of the most popular EOS for NG processing—PR-EOS—is evaluated for CO2-rich NG, by comparing its predictions with available experimental vapor–liquid equilibrium (VLE) data—bubble-point, dew-point, and critical pressures—for the binary CH4–CO2 in wide ranges of compositions and temperatures. The binary CH4–CO2 is interesting because the literature has a reasonable amount of VLE CH4–CO2 data, and it is a prototype system for representation of high-pressure dry CO2-rich NG. The PR-EOS is also demonstrated with CH4–CO2 systems for predicting VLE envelopes and three-dimensional color maps on plane P × T for density, enthalpy, isobaric heat capacity, sound speed, isothermal compressibility, and isobaric expansivity of single-phase fluid.