Measurement and prediction of gas hydrate and hydrated salt equilibria in aqueous ethylene glycol and electrolyte solutions

Abstract

A recently developed method in modelling electrolyte solutions is extended to include phase behaviour of aqueous solutions containing hydrated salts (e.g., calcium chloride) and organic hydrate inhibitors (e.g., ethylene glycol). A novel salt precipitation model applicable to various hydrated salts is presented. The precipitation model takes into account various precipitates of hydrated salts over a wide range of temperature (i.e., - 20 –120 ∘ C). Due to lack of the required experimental data in the literature, new experimental data have been generated. These data, which have been used in determining the binary interaction parameters between salts and organic inhibitors, include; freezing point depression, boiling point elevation, and salt solubility in the aqueous solutions containing salts and organic inhibitors. The extended thermodynamic model is capable of predicting complex vapour–liquid–solid equilibria (VLSE) for aqueous electrolytes and/or organic inhibitor solutions over a wide range of pressure, temperature and inhibitor concentration. In addition, in order to establish the effect of a combination of salts and organic inhibitors on the locus of incipient hydrate-liquid water-vapour (H–L W –V) curve, reliable equilibrium data have been generated for one quaternary system, methane/water/calcium chloride/ ethylene glycol at pressures up to 50 MPa. These data along with various independent literature data are used to validate the predictive capabilities of the model for phase behaviour and hydrate equilibria. Good agreement between experimental data and predictions is observed, demonstrating the reliability of the developed model.