A Model for the Equilibria among Fluid Phases for Water + n-Alkane Systems

D.J. ESCOBAR GARCÍA; S.B. RODRIGUEZ-REARTES; M.S. ZABALOY

Virtual

Conferencia; PetroPhase2021 (Virtual): The 21st International Conference on Petroleum Phase Behavior and Fouling; 2021

ExxonMobil

The fluid phase equilibrium for systems containing hydrocarbons and water has a fundamental importance, e.g., in the transportation and processing of natural gas and liquified petroleum gas1. A potential issue is the undesired formation of ice or hydrates1, and even of liquid aqueous phases. Models for the multicomponent systems of interest rely on binary parameters which are typically obtained from experimental binary phase equilibrium information. In particular, the description through models, of the fluid phase equilibria of binary water + n-alkane systems over wide enough ranges of conditions, is difficult due to the significant asymmetry of the system. Such asymmetry is evident from, e.g., the experimentally known very low mutual solubilities in liquid state (liquid-liquid equilibrium). To describe water + n-alkane equilibria, equations of state (EOS) cubic with respect to the molar volume (v) (EOSs of the van der Waals family) coupled to flexible but inconsistent2 mixing rules have been used1. The inconsistency comes from the lack of compliance with the restriction of mixture molar volume invariance2. The alternative of resorting to EOSs which are not cubic with respect to the molar volume is not desirable, due to other inconsistent situations such as the multiplicity of Maxwell loops at a given temperature3. The purpose of this work is to capture the experimental behavior of water+n-alkane systems without giving up the relative simplicity of EOSs cubic in v. To that end, it is proposed to use mixing rules cubic with respect to mole fraction (CMRs)4,5. CMRs are consistent2 and have a degree of flexibility that enables them to capture the behavior of highly asymmetric systems, such as the CO2 + n-alkane systems5. The chosen PVT relationship is the generalized Redlich−Kwong−Peng−Robinson Equation of State (RK-PR-EOS5) coupled to CMRs used both, for the attractive and for the repulsive mixture parameters. The interaction parameters of the attractive term were allowed to be temperature dependent. Results are illustrated for the systems ethane + water and propane + water. These systems present heterogeneous azeotropy. We conclude that an EOS cubic with respect to v, such as the RK-PR-EOS, is able to represent very well the fluid phase behavior of water + n-alkane systems if it is combined with CMRs. References: [1] A. Chapoy; S. Mokraoui; A. Valtz; D. Richon; A. H. Mohammadi; B. Tohidi, Fluid Phase Equilibria, 226 (2004) 213 .[2] M.S. Zabaloy; J.H. Vera, Fluid Phase Equilibria 119 (1996), 27.[3] R. Span, Multiparameter Equations of State. Springer. Berlín (2000)[4] M.S. Zabaloy. Ind. Eng. Chem. Res. 47 (2008) 5063 .[5] M. Cismondi; S. B. Rodríguez-Reartes; J. M. Milanesio; M. S. Zabaloy. Ind. Eng. Chem. Res. 51 (2012), 6232 .