Thermodynamic model of phase equilibria of tetrabutyl ammonium halide (fluoride, chloride, or bromide) plus methane or carbon dioxide semiclathrate hydrates

• The model is for TBAF/TBAC/TBAB hydrate with methane or carbon dioxide.
• The vdW–P model and e-NRTL activity model are applied in the semiclathrate hydrate systems.
• The equations of Langmuir constants for three kinds of large cavities are proposed.
• The overall predicted data match well with the corresponding experimental data.

In this work, based on the van der Waals–Platteeuw (vdW–P) theory, a thermodynamic approach is proposed to determine the phase equilibria of semiclathrate hydrates formed with tetrabutyl ammonium halide (fluoride, chloride, or bromide) and guest gas (CH4 or CO2). The Peng–Robinson equation of state (PR-EoS) and electrolyte-Non-Random Two-Liquid (e-NRTL) activity model are employed to obtain the fugacity of gaseous hydrate formers and activity coefficients of species in the aqueous phase, respectively. In addition, two modifications for evaluations of vapor pressure of water in the empty hydrate lattice and Langmuir constants relating to the salt concentrations in aqueous solution and temperature are applied. It is shown that the model results are in acceptable agreement with the experimental data on phase equilibria for studied systems over wide temperature, pressure, and salt concentration ranging from 280.1 to 304.8 K, from 0.172 to 9.932 MPa, and from 0.02 to 0.35, respectively.