The next generation of hydrate prediction I. Hydrate standard states and incorporation of spectroscopy

The van der Waals and Platteeuw hydrate equation of state, coupled with the classical thermodynamic equation for hydrates, has been used in the prediction of hydrate formation for over 40 years. These equations describe a hydrate to liquid water phase change, where the hydrate is always treated as an ideal solid solution. Several limitations of this method have been removed in a new derivation of the model. In this work, a direct derivation of the standard empty hydrate lattice fugacity has been given. This allows for description of the hydrate phase itself, instead of a specific phase change. The ideal solid solution assumption is removed by defining a specific volume of the standard hydrate lattice. The activity of water in the hydrate is a function of the energy difference between the real and standard lattice. This approach, which allows for distortion of the hydrate from its standard state, is believed to give a more accurate composition of the hydrate. We propose to make the cage radii a linear function of the hydrate lattice parameter. Direct incorporation of spectroscopic data is crucial for parameter optimization in the model. Preliminary predictions with the new model are presented that show the wider applicability of the approach. H-V and I-H equilibrium can be calculated using this method as well as a correct description of high pressure behavior. This paper is the first in a series of four that the authors believe to be a more complete prescription for hydrate modeling. Therefore, this part only pertains to the theory of the hydrate model. Subsequent papers will discuss: (1) the aqueous phase model; (2) incorporation of the models into a multi-phase flash routine and (3) regressed parameter values and a comparison of our models with four commercial hydrate prediction programs. (C) 2002 Elsevier Science B.V. All rights reserved.