(148af) A Comparison of Variable Reference Potential Models for Gas Hydrates

Gas hydrates are crystalline compounds formed from water and (generally) light non-polar gases such as methane, nitrogen, and propane. In these compounds, the gas occupies the interstices of a solid water lattice. Since the original work of van der Waals (1959) thermodynamic models for gas hydrates generally treat the reference properties of the empty hydrate lattice as fixed or independent of the gas which occupies the hydrate. Since 1989, Holder and co-workers have proposed that the correct empty lattice to use in the model is dependent upon the guest that occupies that lattice and thus the reference properties should vary from guest to guest. A number of models developed by many investigators have overtly or more subtly used the variable reference model to predict and/or correlate experimental vapor pressures. A comparison of these models shows that limitations in the models still exist and these limitations still require additional theoretical work. Reference properties from various models show considerable agreement but also a great deal of divergence in unexpected areas. The reference entropy, a term not generally presented in the development of models can serve as a means of identifying weaknesses in existing programs.