Equilibrio de fases y propiedades interfaciales de sistemas moleculares de interés industrial

Resumen

Since the last years in the 20th century, the interest in biodiesel has increased because of its use as a renewable fuel and as a form of decreasing greenhouse gas emissions. Biodiesel is renewable, biodegradable, non-toxic and applicable to actual diesel engines. Besides, biodiesel can be transported, kept and treated in an easier way than diesel. Biodiesel is mainly composed of methyl esters (FAMEs), on the contrary, diesel derives from petroleum which is mainly composed of n-alkanes and aromatic hydrocarbons. In this work, the molecular simulation technique is used to study systems based on n-alkanes and systems based on methyl esters. The chosen technique for the study is called Molecular Dynamics, this technique has been used in the program GROMACS (4.6.1 version) with a canonical ensemble (NVT). On one hand, binary mixtures of methane and n-alkane (n-decane, n-dodecane, n-tetradecane and n-hexadecane) were studied at 344.15K of temperature and pressures between 0.1 and 30 MPa. On the other hand, the phase equilibria and interfacial properties from a family of methyl esters (methyl acetate, methyl propionate, methyl butyrate, methyl valerate, methyl hexanoate and methyl heptanoate) were studied at different temperatures, for pure substances and binary mixtures of these methyl esters with water. The molecular models used for describing n-alkanes were Coarse-Grained models with Mie potential. Whereas the methyl esters were described as TraPPE-UA models (Transferable Potentials for Phase Equilibria-United Atoms), where the parametrizations for different groups have been developed in relation to similar substances. In the case of methyl esters, these parameters used before in similar systems, like ketones or ethers, have never been combined to predict the presented methyl esters behavior. The study of these models by molecular simulation allows to obtain the phase equilibria and interfacial properties of the systems and to compare them with experimental data. For n-alkanes, a decrease of the density in the liquid phase is observed while the pressure increases and an increment of the density in the liquid phase and interfacial tension are observed while the length of the chains increases. Another observation for n-alkanes is the existence of methane adsorption in the interfacial region, which increases with the pressure until the saturation limit is reached. For pure systems of methyl esters, an accurate prediction of the phase coexistence and properties of interest are observed, in comparison with experimental data. Between these properties of interest can be found coexistence densities, vapour pressure, interfacial tension, interfacial entropy and interfacial enthalpy. In the case of binary mixtures of water with methyl esters, data obtained for coexistence densities and molar fractions are excellent, these are better while the length of the alkyl chain is bigger. Density profiles show the expected adsorption effect. However, interfacial tension results overestimate the experimental data.