Non-ideal hydrate solid solution model for a multi-phase equilibria program, A

Abstract

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 thirty years.The standard approach in using these models to predict hydrate equilibrium does not explicitly include them in the flash calculation.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.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, via an activity coefficient.This approach, which allows for distortion of the hydrate from its standard state, gives a more accurate composition of the hydrate and significantly improves hydrate formation predictions at high pressure.As a direct result of accounting for a changing hydrate volume, the cage radii are functions of the hydrate volume.Direct incorporation of spectroscopic data is crucial for parameter optimization in the model.We have included the hydrate phase in a Gibbs energy minimization, multi-phase flash routine.A total of ten phases (3 fluid, 3 hydrate, and 4 pure solid) are accounted for in the flash routine.This allows for hydrate phase properties to be calculated at any temperature and pressure (not just at the formation boundary) and with any other coexisting phases.Development of the hydrate prediction program, CSMGem, was also part of this work.The program was written in Visual Fortran and linked with Visual Basic for ease of use.A comparison of CSMGem and four commercially available hydrate prediction programs was made with over 1650 hydrate formation data points and several other types of hydrate data.CSMGem compares favorably with the other programs.An analysis of all hydrates of methane, ethane, and propane was performed at seafloor temperatures (277.6 K).Interesting phenomena occur (i.e.structural transitions and pseudo-retrograde hydrates).The idea of using hydrates to separate close-boiling compounds is also proposed.