Volumetric Investigations of High-Pressure Propane and Acid Gas Fluids

Abstract

Changes in chemical equilibria are important in handling high-pressure acid gas fluids and natural gas liquids, particularly, when the fluids contain other trace components, such as H2O, COS and CS2. Beyond ideal gas application, high-pressure reaction equilibria require fugacity coefficients which can be calculated with reference equations of state, provided that reliable mixing parameters are available. Densimetric/Volumetric measurements are one way to validate or reoptimize the mixing parameters to be subsequently used for investigating reaction equilibria in a dense fluid. In this thesis, the volumetric properties of the binary systems of CS2, COS and H2S with dense C3H8 alongside impurities of CO2, CS2, and COS dissolved in dense H2S are reported. Additionally, the thesis explores the equilibrium limits for formation of COS and CS2 in high-pressure H2S fluids containing CO2. For the densimetric measurements for a CS2 + C3H8 system, the data were used to calculate the apparent molar volumes, which were assumed to approximate the partial molar volumes at infinite dilution. These partial molar volumes were used to optimize the adjustable parameters in an infinite correlation equation based on the generalized Krichevskii parameter. The measurements of the COS and H2S in C3H8 fluid were used to compare calculations performed with the mixing coefficients from Kunz & Wagner (2012). Calculations for apparent molar volumes showed an agreement within ±0.4% at high-pressures without the need for re-optimization for these systems. The measured densities of the various compositions of (x(CO2) = 0.0982, 0.270, and 0.496) for the CO2 + H2S binary mixtures were used to compare the data from Stouffer et al. and Nazeri et al. The comparison showed an agreement within ±2% at high pressures and up to 4% in the vicinity of the critical point or high compressibility region. Also, the densities were used to calculate excess molar volumes and further used to ascertain the accuracy of the mixing coefficients from Kunz and Wagner (2012). The comparison of the measured data and the calculated values showed good agreement to within ±2 cm3∙mol-1. Furthermore, for this binary mixture, the COS formation from the equilibrium reaction of H2S and CO2 was measured. The measured COS equilibrium concentrations were low over the entire experimental conditions and suggests that the reaction of H2S and CO2 to form COS was not experimentally significant. For the volumetric measurements of CS2, COS or H2O dissolved in dense H2S fluid, the volumetric measurements were used to validate/optimize (i) a molar volume equation of CS2 at infinite dilution, and (ii) the mixing coefficients from Kunz & Wagner (2012). Using the determined/validated coefficients to calculate fugacity coefficients, the equilibrium limits of COS and CS2 formation in dense H2S were explored at conditions of T = 298.15 and 773.15 K and p = 0.1 and 10 MPa. These results highlight that equilibrium concentrations of COS and CS2 increases with increasing temperature and pressure. Although the equilibrium amounts of CS2 generated were very low (in the ppb range), the amounts of COS generated can be quite significant at high temperatures and/or when the acid gas mixture is compressed to dense phase. Lastly, with the optimized/validated mixing coefficients, an empirical method for estimating non-hydrocarbon acid gas mixing coefficients was developed for circumstances where there are no or poor experimental data. The newly developed empirical method showed good comparison to the calculated values performed with the optimized/validated coefficients.