Three-Phase Lw-H-V Equilibrium Calculations

The conditions of three-phase (Lw-H-V) equilibrium are the most useful. In particular, methods to calculate the temperature and pressure at which hydrates form from a given gas composition and free water have both industrial and academic applications. 

There are comparatively few measurements of the hydrate phase composition, due to experimental difficulty. Hydrate phase difficulties arise because water is often occluded in the hydrate mass, separation of hydrate and water is difficult, and the hydrate phase of mixtures is often inhomogeneous in experiments. Consequently, the ratio of water to hydrocarbon is often inaccurate. As discussed in Chapter 6, only over the last two decades have experimental techniques (e.g., diffraction and NMR and Raman spectroscopy) become accurate enough to determine the degree of filling of hydrate cavities with different types of molecules. 

In this section two prediction techniques are discussed, namely, the gas gravity method and the Kvsi method. While both techniques enable the user to determine the pressure and temperature of hydrate formation from a gas, only the KVSI method allows the hydrate composition calculation. Calculations via the statistical thermodynamics method combined with Gibbs energy minimization (Chapter 5) provide access to the hydrate composition and other hydrate properties, such as the fraction of each cavity filled by various molecule types and the phase amounts. 

Because the two prediction techniques of this chapter were determined over half a century ago, they apply only to si and sH hydrates, without consideration of the more recent sH, which always contains a heavier component. For structure H equilibrium, only the statistical thermodynamics method of Chapter 5 is available for prediction of hydrate pressure, temperature, and composition. 

The two calculation techniques in this chapter may be regarded as successive approximations to hydrate phase equilibrium, increasing both in accuracy and in 

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