Vapour-liquid and liquid-liquid phase equilibria and interfacial properties of the tetrahydrofuran + methane mixture from molecular dynamics simulation

Thermodynamic hydrate promoters (THP) are an important class of substances that allow to stabilize gas hydrates (nonstoichiometric crystalline inclusion compounds). Tetrahydrofuran, or simply THF, is one of the most known and... [ view full abstract ]

Thermodynamic hydrate promoters (THP) are an important class of substances that allow to stabilize gas hydrates (nonstoichiometric crystalline inclusion compounds). Tetrahydrofuran, or simply THF, is one of the most known and used THP since many years. In particular, THF is able to reduce the equilibrium pressure dramatically and it makes it possible to treat gas hydrates under mild pressure conditions.

From a theoretical modelling point of view, computer simulation offers insight into atomic level phenomena that are not observable from an experimental point of view. This provides unique and invaluable information about the underlying physicochemical principles governing hydrate behaviour, including thermodynamic properties, phase equilibria, and interfacial properties, among many others. This work is a preliminary study to predict the phase equilibrium of THF hydrates and hydrates of mixtures of THF with methane using molecular dynamics simulation. Since fluid-fluid interfacial properties, and particularly surface tension, are particularly sensitive to subtle differences in molecular details, we studied the capability of two different molecular models of THF to predict the interfacial behaviour of the system.  

In this work, we determine the vapour-liquid and liquid-liquid phase equilibria and interfacial properties of the tetrahydrofuran + methane binary mixture at different extreme thermodynamic conditions in which the methane hydrate would be stable. THF is modelled following the united-atom approach. We use two different models, the original flexible TraPPE model and an approximated model in which bending and torsional degrees of freedom of molecules are frozen. We perform molecular dynamics simulations in the canonical ensemble to determine the most representative vapour-liquid and liquid-liquid interfacial properties, including the interfacial tension, density profiles, coexistence phase diagrams, and interfacial thickness as functions of pressure. Theoretical
predictions obtained from molecular dynamics simulation are compared
with predictions from the Statistical Associating Theory for potentials
of Variable Range (SAFT-VR) and experimental data.