(180l) Modeling the Phase Behavior, Excess Enthalpies and Henry's Constants of the H2O + H2S Binary Mixture Using the SAFT-VR+D Approach

Due to quality regulations and environmental restrictions, sulphur compounds must be monitored and removed from the final products in a range of industrial operations, including waste treatment plants, in pulp and paper operations, and in natural gas and petroleum reservoirs. Therefore, an accurate knowledge of the phase behavior of mixtures of water and hydrogen sulphide is crucial to plant design and operation within the chemical industry [1]. Additionally, both hydrogen sulphide and water are very interesting molecules to study due to the presence of their permanent dipole moment and hydrogen bonding interactions. The SAFT-VR+D approach [2] is based on a version of the statistical associating fluid theory that was developed to model associating dipolar fluids by explicitly accounting for dipolar interactions and their effect on the thermodynamics and structure of a fluid. In the SAFT-VR+D approach this is achieved through the use of the generalized mean spherical approximation (GSMA) to describe a reference fluid of dipolar associating square well segments. In the present work, we modify the SAFT-VR+D approach to study mixtures of dipolar associating fluids of arbitrary size, based on the work of Cummings and Blum [3], and to explicitly account for the dipolar interactions of each polar compound and examine the high-pressure phase diagram and other thermodynamic properties of the binary systems of water + hydrogen sulphide. Both water and hydrogen sulphide are modeled as associating spherical molecules with four off-centre sites to mimic hydrogen bonding and an embedded dipole moment (μ) to describe their polarity. Using simple Lorentz-Berthelot combining rules, the theory is able to predict the phase behavior of the H₂O + H₂S system from pure component parameters: type III phase behavior according to the classification of Scott and van Konynenburg [4] with a three-phase line at low temperatures and its corresponding upper critical end point, and two separate critical lines, one at high temperatures that runs continuously from a gas-liquid critical line to a liquid-liquid critical line at high pressures and a second gas-liquid critical line located at low temperatures. The phase behavior is predicted by the SAFT-VR+D approach in excellent agreement with the experimental data, without requiring binary interaction parameters. In addition, the theory is able to predict in a qualitative manner the excess molar enthalpies and Henry's constants as a function of temperature.

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[2] H. Zhao and C. McCabe, Journal of Chemical Physics 125, 104504 (2006); H. Zhao, Y. Ding, and C. McCabe, Journal of Chemical Physics 127, 084514 (2007).

[3] P. T. Cummings and L. Blum, Journal of Chemical Physics 84, 1833 (1986).

[4] P. H. van Konynenburg and R. L. Scott, Philosophical Transactions of the Royal Society of London, Series A A298, 495 (1980); R. L. Scott and P. H. van Konynenburg, Discussions of the Faraday Society 49, 87 (1970).