Modelling of gas clathrate hydrate equilibria using the electrolyte non-random two-liquid (eNRTL) model
Résumé
A thermodynamic framework for modelling clathrate hydrate equilibria involving electrolytes is presented. In this framework, the gas phase is described by using the Soave-Redlich-Kwong equation of state, while the gas solubility in the liquid phase is estimated by means of a Henry's law approach. The liquid phase non-idealities are accounted for by using the semi-empirical electrolyte non-random two-liquid (eNRTL) excess Gibbs energy model. The van der Waals and Platteeuw model is used for the hydrate phase. This three-phase equilibrium model has been implemented in a new Java-based in-house programme. The main focus of the present work is the influence of the electrolytes on the incipient hydrate forming conditions. Therefore, the most recent version of the eNRTL model is thoroughly discussed. The model equations are presented in detail to facilitate future implementation and further development of this model, since the eNRTL modelling approach is quite new in the context of gas hydrate calculations. The correctness of the programme implementation is rigorously studied and verified by comparing the results with results of selected examples in the literature. At last, calculations are performed on solid-aqueous liquid-gas phase equilibria of selected systems of the type {water + salt + gas}, {water + salt1 + salt2 + gas}, {water + salt + CH4 + CO2} and {water + salt1 + salt2 + CH4 + CO2} with salt = NaCl, KCl, CaCl2 and gas = CH4, CO2) comprising a gas clathrate hydrate phase. The results are in good agreement with experimental p-T-hydrate-liquid-gas phase equilibrium data found in the literature, with average absolute relative deviations between experimental and calculated pressures ranging from 1% to 15%.
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