Water Content of CO2 in Equilibrium With Liquid Water and/or Hydrates

Abstract

Summary. Experimentally measured water content in CO2-rich fluid in the gaseous or liquid state in equilibrium with liquid water or hydrate is presented for pressures ranging from 100 to 2,000 psia [0.69 to 13.79 MPa] and temperatures from -19 to 77 degrees F [-28.33 to 25.0 degrees C]. The water content of the CO2-rich phase along the three-phase equilibria, i.e., liquid water/liquid CO2/gas to the three-phase critical endpoint, is also reported. The experimental results from this study on the water content in the CO2-rich phases have been combined with earlier research results of the CO2/water binary system in the hydrate-free region from 77 to 200 degrees F [25.0 to 93.33 degrees C] and pressures to 3,000 psia [20.69 MPa] to produce a comprehensive pressures to 3,000 psia [20.69 MPa] to produce a comprehensive plot. plot. The high degree of complexity in the phase behavior of a CO2/water binary system, which exhibits several pairs of equilibrium phases for the conditions, is shown. To make the data more intelligible than they are in their raw form, the data are presented in terms of the pressure enhancement of the water content presented in terms of the pressure enhancement of the water content along isotherms. Finally, the activity coefficients of water in the CO2-rich phases are summarized. Introduction Information on the equilibrium water content of CO2-rich fluids is necessary over a broad range of pressures and temperatures to complete specifications for the processing and transportation of the fluid associated with CO2 EOR projects. Fig. 1 presents the pressure/temperature projection for CO2/ water binary system along its univariant three phase loci and in two-phase regions, obviously showing the extent and variety of phase equilibria most relevant to EOR technologists. As Fig. 1 phase equilibria most relevant to EOR technologists. As Fig. 1 shows, the phase behavior of the CO2/water system is much more complicated over the natural gas/water system because of the occurrence of multiple condensed phases-e.g., the existence of two liquid phases, one composed of liquid water, liquid CO2, and a gaseous CO2 phase at high temperatures, and one composed of solid hydrate/liquid CO2, and solid hydrate/gas CO2 at low temperatures. Consequently, establishing the water-content specifications was difficult and challenging. The water content of CO2 was first measured by Wiebe and Gaddy, who flowed expanded CO2-rich samples through a train of absorbents and determined the weight gained by the train. In this study, a special experimental apparatus used by Galloway et al. and a water analysis scheme devised by Bloch and Lifland were used to measure the water content in the CO2-rich phases. The data reported by Wiebe and Gaddy have recently been supplemented in the fluid CO2/water region by Kobavashi et al. and by Gillespie and Wilson. Calculation methods for the prediction of hydrate formation conditions have been developed by van der Waals and Platteeuw, Saito et al., and many others. Dewan's correlation yields a good estimate of the water content from the phase density and other thermodynamic parameters in the fluid CO2/water region. parameters in the fluid CO2/water region. Equations of state (EOS's), such as those formulated by Peng and Robinson and Soave, are frequently used: however, we used available tabulations of thermodynamic properties of CO2 provided by Angus et al. to reduce the errors resulting from the provided by Angus et al. to reduce the errors resulting from the use of an EOS. Experimental Apparatus and Procedure The experimental apparatus initially used by Galloway et al. to measure hydrate numbers has been modified by Sloan et al. Kobayashi et al., and Song and Kobayashi to measure the water content of gases in equilibrium with hydrate. A line diagram of the apparatus is shown in Fig 2. Equilibrium in the fluid CO2/water system was established by rotating the cylindrical autoclave containing carefully selected and aligned ball bearings. The equilibration process in the fluid CO2/hydrate region is considerably more complicated than that of the fluid CO2/water region. It was necessary first to establish gas/hydrate equilibrium by converting all the metastable water into hydrate crystals by an extended rotation of the autoclave. A tandem pump (shown in Fig. 2) was used to circulate the fluid phase throughout the system and to store CO2-rich fluid of the same composition. but not at the same temperature as the equilibrium fluid phase in the cell. A second hand pump was used to charge or to phase in the cell. A second hand pump was used to charge or to add pure CO2 to the cell. The elimination of metastable liquid water was confirmed by many successive analyses of samples (up to 30) taken from the equilibrium gas phase. The sampling and analyses are difficult to perform because of the low water concentration, the long sample line, and the tedious modified chromatographic analytic technique developed by Bloch and Lifland. The determination of water content in the CO2-rich phase in the liquid CO2/hydrate region was undertaken after all the studies of the gas CO2/hydrate region were completed. To re-establish equilibrium in the liquid CO2/hydrate region, pure CO2 was slowly added to the autoclave to the desired pressure. Owing to the greater water-carrying capacity of liquid CO2 than gaseous CO2 at the same temperature, a small amount of the hydrate phase decomposes in the re-equilibration process at a new pressure. Because the density difference between CO2 in the hydrate phase and CO2 in the liquid phase was estimated to be small (Fig. phase and CO2 in the liquid phase was estimated to be small (Fig. 3), it is necessary to stir the contents of the autoclave gently. In this study, all the water-content measurements were made at states where the density of CO2 in hydrate, was greater than the density of fluid CO2, . However, could be greater than at high pressures, as shown in Fig. 3. The subsequent experimental condition was always chosen in the direction of increasing water concentrations in the CO2-rich phase because the elimination of metastable water from the system containing dense CO2 was almost impossible. Experimental Results Experimentally measured water contents in the CO2-rich phases are presented in Table 1, and the phases in equilibrium are identified. In general, a series of phase equilibrium data was taken along isobars (lines of constant pressure), although a few experimental points were obtained along the three-phase, H2O(l)/CO2(l)/CO2(g), locus. Experimental data from this and earlier studies are plotted along isobars in Fig. 4 and plotted isothermally in Fig. 5. SPEFE P. 500