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Llovell F.,Research Center | Marcos R.M.,Rovira i Virgili University | Vega L.F.,Research Center | Vega L.F.,Carburos Metalicos Air Products Group
Journal of Physical Chemistry B | Year: 2013

The evaluation of phase equilibria and solubility properties through theoretical approaches is a well-known field, where a significant amount of models are able to describe them with a good degree of accuracy. However, the simultaneous calculation of transport properties together with thermodynamic phase properties still remains a challenge, due to the difficulties in describing the behavior of properties like the viscosity of fluids with the same approach. In this work, the free-volume theory (FVT) has been coupled with the soft-SAFT equation for the first time to extend the capabilities of the equation to the calculation of transport properties. The theory has been first tested using simulation data of the viscosity of the Lennard-Jones (LJ) fluid and LJ chains over a wide range of temperature and pressure. Good agreement has been found at all chain lengths, except for some deviations at near-zero density values. Several trends of the viscosity parameters with the length of the chain are identified, allowing the prediction of other chain fluids. Finally, the new equation has been applied to the n-alkanes family, where viscosity is a key property, and results are compared with experimental data. The three viscosity parameters were fitted to viscosity data of the pure fluid at several isotherms or isobars, whereas the density and pressure (or temperature) were taken from the soft-SAFT output. Again, the effect of these parameters on the viscosity has been investigated and compared with results obtained for the LJ chains and with previous work of other authors. The new equation performs very well in all cases, with a global average absolute deviation of 2.12% and shows predictive capabilities for heavier compounds. This empowers soft-SAFT with new capabilities, allowing the equation to calculate phase, interfacial, and transport properties with the same model and degree of accuracy. © 2013 American Chemical Society. Source

Llovell F.,Research Center | Marcos R.M.,Rovira i Virgili University | Vega L.F.,Research Center | Vega L.F.,Carburos Metalicos Air Products Group
Journal of Physical Chemistry B | Year: 2013

In a previous paper (Llovell et al.J. Phys. Chem. B, submitted for publication), the free-volume theory (FVT) was coupled with the soft-SAFT equation of state for the first time to extend the capabilities of the equation to the calculation of transport properties. The equation was tested with molecular simulations and applied to the family of n-alkanes. The capability of the soft-SAFT + FVT treatment is extended here to other chemical families and mixtures. The compositional rules of Wilke (Wilke, C. R.J. Chem. Phys. 1950, 18, 517-519) are used for the diluted term of the viscosity, while the dense term is evaluated using very simple mixing rules to calculate the viscosity parameters. The theory is then used to predict the vapor-liquid equilibrium and the viscosity of mixtures of nonassociating and associating compounds. The approach is applied to determine the viscosity of a selected group of hydrofluorocarbons, in a similar manner as previously done for n-alkanes. The soft-SAFT molecular parameters are taken from a previous work, fitted to vapor-liquid equilibria experimental data. The application of FVT requires three additional parameters related to the viscosity of the pure fluid. Using a transferable approach, the α parameter is taken from the equivalent n-alkane, while the remaining two parameters B and Lv are fitted to viscosity data of the pure fluid at several isobars. The effect of these parameters is then investigated and compared to those obtained for n-alkanes, in order to better understand their effect on the calculations. Once the pure fluids are well characterized, the vapor-liquid equilibrium and the viscosity of nonassociating and associating mixtures, including n-alkane + n-alkane, hydrofluorocarbon + hydrofluorocarbon, and n-alkane + hydrofluorocarbon mixtures, are calculated. One or two binary parameters are used to account for deviations in the vapor-liquid equilibrium diagram for nonideal mixtures; these parameters are used in a transferable manner to predict the viscosity of the mixtures. Very good agreement with available experimental data is found in all cases, with an average absolute deviation ranging between 1.0% and 5.5%, even when the system presents azeotropy, reinforcing the robustness of the approach. © 2013 American Chemical Society. Source

Llovell F.,CSIC - Institute of Materials Science | Llovell F.,Autonomous University of Barcelona | Valente E.,Autonomous University of Barcelona | Vilaseca O.,CSIC - Institute of Materials Science | And 4 more authors.
Journal of Physical Chemistry B | Year: 2011

In a previous work (Andreu and Vega, J. Phys. Chem.B2008, 111, 16028), we presented a simple model for the imidazolium-based ionic liquids (ILs) with the bis(trifluorosulfonyl)imide anion [Tf2N]- in the context of the soft-SAFT equation of state. The model was successfully used to predict the solubility of several gases in these ILs. However, the small amount of experimental data made the predictions less accurate when going into more complex mixtures and one or two fitted binary parameters were needed in some cases. In this work, we have reparameterized our previous model and evaluated its reliability to predict the behavior of these ionic liquids in binary mixtures with other associating compounds. Model parameters for the ionic liquids were estimated using new experimental density data at atmospheric pressure in an extended range of temperatures, from 273 until 473 K, consistent within the range of temperatures previously measured by other authors. The new set of molecular parameters has been tested to predict the density of several members of the family at higher pressures up to 60 MPa with the same degree of accuracy than at atmospheric values. In addition to density-temperature data, interfacial tensions and the isothermal compressibility of some compounds were predicted in reasonable good agreement with experimental data. The molecular parameters of the pure compounds were used then, in a predictive manner, to describe the behavior of binary mixtures with other imidazolium ionic liquids, changing either the cation or the anion. Predictions for some mixtures with methanol, ethanol, and water were compared with experimental data, providing an excellent description of the systems, with no fitting to mixture data in almost all the cases. The excellent results obtained in this work reinforce the need to have accurate data, showing that molecular based models can be used to assess the validity of these data. In addition, this work also shows that a simple model in which the physics of the system is kept is good enough to describe the complex behavior of associating mixtures of ionic liquids, without the need of additional parameters that may obscure the real physics of the system. © 2011 American Chemical Society. Source

Oliveira M.B.,University of Aveiro | Llovell F.,Research Center | Coutinho J.A.P.,University of Aveiro | Vega L.F.,Research Center | Vega L.F.,Carburos Metalicos Air Products Group
Journal of Physical Chemistry B | Year: 2012

In this work, the soft statistical associating fluid theory (soft-SAFT) equation of state (EoS) has been used to provide an accurate thermodynamic characterization of the pyridinium-based family of ionic liquids (ILs) with the bis(trifluoromethylsulfonyl)imide anion [NTf2]-. On the basis of recent molecular simulation studies for this family, a simple molecular model was proposed within the soft-SAFT EoS framework. The chain length value was transferred from the equivalent imidazolium-based ILs family, while the dispersive energy and the molecular parameters describing the cation-anion interactions were set to constant values for all of the compounds. With these assumptions, an appropriate set of molecular parameters was found for each compound fitting to experimental temperature-density data at atmospheric pressure. Correlations for the nonconstant parameters (describing the volume of the IL) with the molecular weight were established, allowing the prediction of the parameters for other pyridiniums not included in the fitting. Then, the suitability of the proposed model and its optimized parameters were tested by predicting high-pressure densities and second-order thermodynamic derivative properties such as isothermal compressibilities of selected [NTf2] pyridinium ILs, in a large range of thermodynamic conditions. The surface tension was also provided using the density gradient theory coupled to the soft-SAFT equation.Finally, the soft-SAFT EoS was applied to describe the phase behavior of several binary mixtures of [NTf2] pyridinium ILs with carbon dioxide, sulfur dioxide, and water. In all cases, a temperature- independent binary parameter was enough to reach quantitative agreement with the experimental data. The description of the solubility of CO2 in these ILs also allowed identification of a relation between the binary parameter and the molecular weight of the ionic liquid, allowing the prediction of the CO 2 + C12py[NTf2] mixture. The good agreement with the experimental data shows the excellent ability of the soft-SAFT EoS to describe the thermophysical properties of ILs as well as their phase behavior. Results prove that this equation of state can be a valuable tool to assist the design of ILs (in what concerns cation and anion selection) in order to obtain ILs with the desired properties and, consequently, enhancing their potential industrial applications. © 2012 American Chemical Society. Source

Pereira L.M.C.,University of Aveiro | Oliveira M.B.,University of Aveiro | Llovell F.,Research Center | Vega L.F.,Research Center | And 2 more authors.
Journal of Supercritical Fluids | Year: 2014

The capabilities of the soft-SAFT EoS to accurately describe the thermophysical properties of ionic liquids (ILs) and the phase equilibria of their mixtures with greenhouse gases is extended in this work to address the CO2 and the N2O solubilities in [C4mim] + ILs from different anion families. In addition to the commonly studied [BF4]- and [NTf2]- anions, the solubility of these gases in ILs with the anions [N(CN)2] -, [SCN]- and [Ac]- is also studied and compared among them, searching for the best system for separation purposes. A coarse-grained molecular model is proposed within the soft-SAFT framework for each newly studied IL based on structural information, guidance obtained from quantum calculations and previous work. The most adequate set of molecular parameters are selected from the ILs density description and from the ability to reproduce the N2O/CO2 solubilities in these ILs at the lowest and highest temperatures for which experimental data are available. A discussion about the association molecular parameters values and their relation with the anion nature is also presented. With these molecular models, the description of the high pressure phase equilibria of the binary systems composed of the two gases and the ILs referred above are described with soft-SAFT for the remaining isotherms. For most systems, the equilibria behavior of the mixtures is predicted without using any binary parameter. When good agreement with the experimental data is not achieved, a single temperature independent binary parameter is enough to allow a good description. Finally, Henry's law constants are calculated from soft-SAFT to evaluate the selectivity of those ILs for the CO2/N2O separation. © 2014 Elsevier B.V. Source

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