Volume and non-equilibrium crystallization of clathrate hydrates
Résumé
Phase equilibria involving clathrate hydrates are usually modeled by using equations and tools from classical thermodynamics. For given temperature and pressure conditions (T and P), the equilibria between the different phases (hydrate, liquids, vapor, etc.) are obtained by minimizing the Gibbs free energy or, if there is no ambiguity, by equalizing the chemical potentials (or fugacities) of each component in each phase, as described in numerous studies .
The usual approach for modeling liquid hydrate equilibrium (LHE) consists of determining the equilibrium curve (PT) for a given gas composition. In addition, for this given gas composition, the hydrate cage filling is commonly determined from a Langmuir-like approach. Furthermore, the compositions of the fluid phases in equilibrium with the hydrate are also of interest. Gibbs phase law states that the degree of freedom (or number of independent intensive variables) is 2 + n – Φ, where n is the number of components and Φ is the number of phases at equilibrium. As a result, considering the vapor–liquid hydrate equilibrium, the hydrate composition xH (fraction of guest molecules in occupied cavities) can be seen as a function of the temperature T (or pressure P) and the liquid (or gas) composition.