Extension of a hydrate model for structure H applied to multiparameter equations of state

Abstract

This work introduces an extension of an existing thermodynamic model for gas hydrates based on the van der Waals and Platteeuw (vdWP) approach for structure H (sH) hydrates. A model for the hydrate volume is adapted to the hexagonal unit cell with a universal correlation for the thermal expansion, guest-specific lattice parameters at reference conditions, a compressibility relation given in terms of the Murnaghan equation of state (EOS), and a multi-layered description of the hydrate cavities. Available experimental data for the lattice parameters are represented within ±0.05 Å, while molecular simulation predictions show larger deviations. The results allow for the determination of lattice parameters for 10 help gases and 15 large-guest molecule substances (LGMSs), which enables the application to a wide range of hydrate-forming mixtures. In combination with multiparameter EOS for fluid phases, i.e., the IAPWS-95 for water, the proposed vdWP-type model is validated by comparing predicted hydrate formation conditions with experimental data for methylcyclohexane or 2-methylbutane containing mixtures, demonstrating good agreement. The fluid EOS for methylcyclohexane systems with water, carbon dioxide, and methane and 2-methylbutane with water is revised.