Modeling gas hydrate equilibria in electrolyte solutions

Abstract

Virtually all gas hydrate models use a statistical thermodynamic approach to describe solid phase gas hydrates. An alternative to this model is a “solid solution” model based on classical thermodynamics. Various models are used to describe associated gas phase and aqueous electrolyte properties. None of the electrolyte models, however, are state-of-the-art with respect to gas/electrolyte interactions. The objectives of this work were to (1) develop a classical thermodynamic approach for gas hydrate equilibria, (2) incorporate these hydrate chemistries into a state-of-the-art electrolyte model, and (3) validate the gas hydrate/electrolyte model. The model used the Pitzer approach to estimate as functions of temperature and pressure: (1) activity coefficients for cations, anions, and soluble gases, and (2) the activity of water. This model explicitly considers interactions between soluble gases and electrolytes. The Duan model was used to characterize the gas phase. Solubility products were calculated for CH 4⋅6H 2O ( T = 180 – 298 K ) and CO 2⋅6H 2O ( T = 180 – 283 K ) . A “solid solution” model was used to define mixed CH 4–CO 2 gas hydrates. Model fits to electrolyte/gas hydrate experimental data were an improvement over previously published models. For example, average errors for four salt–CO 2⋅6H 2O systems were reduced from 7.2 ( ± 4.1 ) % to 3.0 ( ± 1.8 ) % . Similarly, average errors for 13 salt–CH 4⋅6H 2O systems were reduced from 4.3 ( ± 1.1 ) % to 3.0 ( ± 0.6 ) % . The results of this work demonstrate the critical importance of the electrolyte component in gas hydrate models.