Propiedades interfaciales y equilibrio de fase de promotores/inhibidores de hidratos mediante dinámica molecular

Abstract

There is an increasing interest in the gas hydrates study due to their energetic and environmental applications. The thermodynamic stability conditions of these hydrates can be widely modified using additives which can promote or inhibit their formation. Tetrahydrofuran (THF) is one of the most known and used hydrate promoters. However, a very limited number of studies have been devoted to the determination of its thermodynamic properties, and to the study of the phases equilibria of their mixtures with the rest of the compounds which form the gas hydrate (water, H2O, methane, CH4, carbon dioxide, CO2,...). In this aspect, the molecular simulation and the theoretical formalisms are able to proportionate not only macroscopic information, but also microscopic information, about the phases equilibria, and the interfacial properties of the binary mixtures of THF with H2O, CH4, and CO2. As a first approximation to understand the thermodynamic properties of the THF+CO2(2), CH4(2), and + H2O(2) binary mixtures, the high pressures phases diagrams of these systems were obtained using the equation of state SAFT-VR (Statistical Associating Fluid Theory-Variable Range). In this work, the thermodynamic behaviour of these mixture systems was studied from a theoretical point of view. The theoretical predictions obtained were used as a start point in the following works. We have also studied the ability of different THF models, taken from the literature, to determine their interfacial properties through the direct simulation of the vapor-liquid interface. The THF was modeled using six different molecular models, three of them based on the united-atoms approach and the other three based on a coarse-grained approach. One of the united-atoms models was proposed in this study and it is an approximate rigid and planar version of the original TraPPE-UA THF model (Transferable Potentials for Phase Equilibria-United Atoms) proposed by Keasler et al. [J. Phys. Chem. B 115, 11234 (2012)]. For the six studied THF model, we examined the density profiles, the coexistence densities, interfacial thickness and the surface tension in terms of temperature. This rigid version was able to provide similar results as the original flexible model at the same time that it provides faster simulations. In order to validate the theoretical predictions obtained to the THF+CO2 binary mixture, we have measured experimentally the interfacial tension, the coexistence densities and the relative Gibbs adsorption at two temperatures (298.15 and 353.15 K) and at several pressures. In addition, density profiles were calculated applying the Square Gradient Theory. The experimental results were used, together with the theoretical predictions obtained using SAFT-VR, as a start point in the study of the THF+CO2 binary mixture using molecular dynamic simulation. These simulations were carried out at the same thermodynamic conditions at which the experiments were performed. THF was modeled using the original and the rigid version of the TraPPE-UA THF model. The agreement between molecular dynamic simulation results, using both models, with the experimental results and the theoretical predictions were excellent in the majority of the cases. Following the steps of the previous works, we have studied experimentally, and using molecular dynamic simulation, the interfacial properties and the phase equilibria of the binary mixture of THF+CH4 at 300 and 370 K at several pressures. In this study, the THF was only modeled using the rigid TraPPE-UA THF model due to this one provides equally acceptable results than the original flexible Keasler’s model, but it needs lesser simulation times. Again, the agreement between simulation, theoretical and experiment results was excellent in the majority of the thermodynamic studied conditions. At this point, it is important to mention that before this work there were not experimental or simulation results for the THF+CH4 binary mixture. On the other hand, the family of the 1−alkanols has been widely used as hydrate inhibitors. For the H2O+1−alkanol binary mixture (from 1−butanol to 1−heptanol), we have studied the interfacial properties (density profiles, coexistence densities, and interfacial tensions) and the phases equilibria. The results obtained from molecular dynamics simulation were compared with experimental results taken from the literature.