Insight into the thermo-physics of gas hydrates: Three phase equilibrium in presence of electrolyte

Abstract

Although salt ions severely distort the gas hydrate lattice, this phenomenon is poorly realized in the existing statistical thermodynamics models. We propose a mechanistic model by addressing a novel concept of instability in the hydrate-water film arising due to salt exclusion. The consequent water reorientation in the unstable lattice due to excluded ions is described in terms of the stress buildup in the hydrate lattice using Poirier’s theory of crystal creep. Tolman’s theory is reworked to account for the combined influence of guest and salt concentration on the surface energy of the phases in contact. Together with an empirical conformational term, the three phase hydrate equilibrium is modelled. To describe the liquid phase non-ideality, the UNIQUAC and Pitzer activity coefficient models are combined. For vapour phase, Patel and Teja equation of state is used, whereas for hydrate phase, Klauda and Sandler model is utilized. The particle swarm optimization is hybridized with simplex search algorithm to identify the model parameters. The proposed thermo-physical hydrate model exhibits an improved performance over the existing models, which is quantified in terms of absolute average relative deviation (AARD) with respect to the experimental data.