Phase equilibria with hydrate formation in H2O + CO2 mixtures modeled with reference equations of state

Abstract

Formation of gas hydrates is an important feature of the water–carbon dioxide system. An accurate description of thermodynamic properties of this system requires a consistent description of both fluid (liquid, vapor, and supercritical fluid) and solid states (ice, dry ice, and hydrates) and of their respective phase equilibria. In this study, we slightly modified and refitted the gas hydrate model by A.L. Ballard, E.D. Sloan [Fluid Phase Equilib. 194 (2002) 371–383] to combine it with highly accurate equations of state (EoS) in form of the Helmholtz energy and Gibbs energy for other phases formed in the water–carbon dioxide system. The mixture model describing the fluid phases is based on the IAPWS-95 formulation for thermodynamic properties of water by W. Wagner, A. Pruß [J. Phys. Chem. Ref. Data 31 (2002) 387–535] and on the reference EoS for CO2 by R. Span, W. Wagner [J. Phys. Chem. Ref. Data 25 (1996) 1509–1596]. Both pure-fluid equations are combined using newly developed mixing rules and an excess function explicit in the Helmholtz energy. Pure-component solid phases were modeled with the IAPWS formulation for water ice Ih by R. Feistel, W. Wagner [J. Phys. Chem. Ref. Data 35 (2006) 1021–1047] and with the dry ice EoS by A. Jäger, R. Span [J. Chem. Eng. Data 57 (2012) 590–597]. Alternatively, the hydrate model was combined with the GERG-2004 EoS [O. Kunz, R. Klimeck, W. Wagner, M. Jaeschke, GERG technical monograph 15, VDI Verlag GmbH, Düsseldorf, 2007]. Since the gas hydrate model uses the fugacity of the gas component in the coexisting phase as an input variable, the accuracy of the predicted phase equilibria was significantly improved by using highly accurate EoSs for coexisting phases. The new hydrate model can be used in a temperature range of 150–295K and at pressures up to 500MPa. Together with the models describing the fluid and pure solid phases it allows for the desired accurate and consistent description of all phases and phase equilibria including, e.g., flash calculations into two and three phase regions.