Modeling the dissociation conditions of carbon dioxide + TBAB, TBAC, TBAF, and TBPB Semiclathrate Hydrates

The thermodynamic approach developed by Paricaud [J. Phys. Chem. B 2011, 115, 288-299] is applied to predict the dissociation conditions of semiclathrate hydrates made with tetra-n-butyl ammonium bromide (TBAB), tetra-n-butyl ammonium chloride (TBAC), tetra-n-butyl ammonium fluoride (TBAF), and tetra-n-butyl phosphonium bromide (TBPB). The SAFT-VRE equation of state is used to describe the properties of fluid phases, and a good description of osmotic and mean activity coefficients of electrolyte solution is obtained. The temperature-composition diagrams of water + tetra-n-alkylammonium/alkylphosphonium salt binary systems are well described by the model. Group contribution methods are proposed to predict the fusion enthalpies and the congruent melting points of semiclathrate hydrates. The van der Waals and Platteeuw theory is combined with the model to calculate the dissociation conditions of carbon dioxide semiclathrate hydrates. The liquid-vapor-hydrate three phase lines can be accurately described over wide ranges of pressure and salt concentrations, by optimizing only one parameter per hydrate phase.