Modeling the vapor-liquid equilibrium and association of nitrogen dioxide/dinitrogen tetroxide and its mixtures with carbon dioxide

We have used in this work the crossover soft-SAFT equation of state to model nitrogen dioxide/dinitrogen tetraoxide (NO2/N2O4), carbon dioxide (CO2) and their mixtures. The prediction of the vapor – liquid equilibrium of this mixture is of utmost importance to correctly assess the NO2 monomer amount that is the oxidizing agent of vegetal macromolecules in the CO2 + NO2 / N2O4 reacting medium under supercritical conditions. The quadrupolar effect was explicitly considered when modeling carbon dioxide, enabling to obtain an excellent description of the vapor-liquid equilibria diagrams. NO2 was modeled as a self associating molecule with a single association site to account for the strong associating character of the NO2 molecule. Again, the vapor-liquid equilibrium of NO2 was correctly modeled. The molecular parameters were tested by accurately predicting the very few available experimental data outside the phase equilibrium. Soft-SAFT was also able to predict the degree of dimerization of NO2 (mimicking the real NO2/N2O4 situation), in good agreement with experimental data. Finally, CO2 and NO2 pure compound parameters were used to predict the vapor – liquid coexistence of the CO2 + NO2 / N2O4 mixture at different temperatures. Experimental pressure – CO2 mass fraction isotherms recently measured were well described using a unique binary parameter, independent of the temperature, proving that the soft-SAFT model is able to capture the non-ideal behavior of the mixture

Soft-SAFT modeling of vapour liquid equilibria of nitriles and their mixtures

Nitriles are strong polar compounds showing a highly non-ideal behavior, which makes them challenging systems from a modeling point of view; in spite of this, accurate predictions for the vapor-liquid equilibria of these systems are needed, as some of them, like acetonitrile (CH3CN) and propionitrile (C2H5CN), play an important role as organic solvents in several industrial processes. This work deals with the calculation of the vapor - liquid equilibria (VLE) of nitriles and their mixtures by using the crossover soft-SAFT Equation of State (EoS). Both polar and associating interactions are taken into account in a single association term in the crossover soft-SAFT equation, while the crossover term allows for accurate calculations both far from and close to the critical point. Molecular parameters for acetonitrile, propionitrile and n-butyronitrile (C3H7CN) are regressed from experimental data. Their transferability is tested by the calculation of the VLE of heavier linear nitriles, namely, valeronitrile (C4H9CN) and hexanonitrile (C5H11CN), not included in the fitting procedure. Crossover soft-SAFT results are in excellent agreement with experimental data for the whole range of thermodynamic conditions investigated, proving the robustness of the approach. Parameters transferability has also been used to describe the isomers n-butyronitrile and i-butyronitrile. Finally, the nitriles soft-SAFT model is further tested in VLE calculation of mixtures with benzene, carbon tetrachloride and carbon dioxide, which proved to be satisfactory as well

Phase Behavior of strongly associating systems

The modeling of associating fluids has been an active area of research for several decades. Attention has gradually shifted from the so called chemical theories, where molecular association is treated as a chemical reaction, to molecular models where association naturally arises from strong attractive intermolecular forces; among the last ones the Statistical Associating Fluid Theory (SAFT) and related approaches are becoming very popular. We will present calculations performed with the soft-SAFT EoS [F.J. Blas and L.F. Vega, Ind. Eng. Chem. Res. 37 (1998) 660-674.] to simulate the equilibrium thermodynamic properties of the acetic acid and the nitriles family (two classes of strongly associating compounds) as well as their mixtures[K. Jackowski and E. Wielogorska, Journal of Molecular Structure355 (1995) 287-290.]. Carboxylic acids form stable double hydrogen bridged dimers which in the gas phase exist in equilibrium with the monomers. Molecular association in liquid phase of the nitriles family is interesting as they are important organic solvents which are soluble in water without any limits. Pure-component molecular parameters are obtained by fitting the equation to available experimental data. The equation enables to search for physical trends, allowing the transferability of the parameters. The complex behavior of these mixtures is also investigated with the same approach

Modelling of a dynamic multiphase flash: the positive flash. Application to the calculation of ternary diagrams

A general and polyvalent model for the dynamic simulation of a vapor, liquid, liquid-liquid, vapor-liquid or vapor-liquid-liquid stage is proposed. This model is based on the -method introduced as a minimization problem by Han & Rangaiah (1998) for steady-state simulation. They suggested modifying the mole fraction summation such that the same set of governing equations becomes valid for all phase regions. Thanks to judicious additional switch equations, the -formulation is extended to dynamic simulation and the minimization problem is transformed into a set of differential algebraic equations (DAE). Validation of the model consists in testing its capacity to overcome phase number changes and to be able to solve several problems with the same set of equations: calculation of heterogeneous residue curves, azeotropic points and distillation boundaries in ternary diagrams

Application of the Statistical Associating Fluid Theory (SAFT) for the calculation of the vapour-liquid equilibrium: Modeling of the N2O4-NO2 reacting system

Accurate thermophysical properties are mandatory for all industrial applications. However, experimental data are often scarce and models are needed for the estimation of the properties. Such is the case in supercritical processes like the selective oxidation of vegetal macromolecules in mixture NO2/N2O4 – supercritical CO2 [1]. Currently, the process optimization is incomplete due to the unability to assess experimental the fraction of free NO2 monomer in the reactor. Statistical Associating Fluid Theory (SAFT) is a powerful equation of state model for thermodynamic property and phase equilibria calculations for fluid mixtures. It has been used to describe with an equation of state approach non polar and polar compounds, with particular attention to associating compounds. In this work we use a modified version of the SAFT EOS, the so called soft-SAFT equation of state extended by a crossover treatment to take into account the long density fluctuation encountered when the critical region is approached [2]. The equation is applied to predict vapour liquid equilibria for NO2 that can self associate into N2O4 [3]. Both molecules NO2 and N2O4 are rigid molecules and the self association equilibrium is quickly achieved [4]. The procedure of modeling of such system consists in considering the monomer NO2 as an associating fluid with a strong association strength that represents the covalent bond between the N-N atoms. The system is successfully modeled by crossover soft-SAFT EOS by a set of parameters of only NO2 with a self association to form the dimmer (N2O4). The equation is also applied to a mixture NO2-N2O4/CO2 where good agreement is obtained with experimental data of the CO2 mass fraction

Modeling the phase equilibria of nitriles by the soft-SAFT equation of state

Nitriles are strong polar compounds, and some of them, like acetonitrile (CH3CN) and propionitrile (C2H5CN), play an important role as organic solvents in several industrial processes. There are challenging systems to investigate from the modeling point of view, given the highly non-ideal intermolecular interactions they present. This work deals with results concerning calculations of the vapor - liquid equilibrium (VLE) for nitriles using a modified version of the SAFT Equation of State (EoS): the soft-SAFT EoS, chosen because of its accuracy in modeling associating fluids. In this work, both polar and associating interactions are taken into account in a single association term in the equation. Molecular parameters for acetonitrile, propionitrile and n-butyronitrile (C3H7CN) were regressed from experimental data. Their transferability is tested by the calculation of the VLE of heavier linear nitriles, namely, valeronitrile (C4H9CN) and hexanonitrile (C5H11CN), not included in the fitting procedure. soft-SAFT results are in excellent agreement with experimental data, proving the robustness of the approach

Solubility of greenhouse and acid gases on the [C(4)mim][MeSO4] ionic liquid for gas separation and CO2 conversion

Ionic liquids (ILs) are an exciting class of compounds of high interest from a technological point of view. One of the applications that is raising more interest is their possible use as solvents to carry out the conversion of CO2 into more valuable compounds. Theoretical approaches provide an attractive option to screen ILs properties and give quick answers to guide the experiments, becoming a crucial tool for process design. This work illustrates a practical example based on the solubility of greenhouse and acid gases on the butylmethylimidazolium methylsulfate [C(4)mim][MeSO4] IL, in order to study its feasibility for gas separation and conversion. A simple but reliable molecular model is presented for the ionic liquid based on structural information and molecular simulations, and coarse-grained models are used to model the different gases. The absorption of relevant gases for the separation/conversion process (CO2, CH4, CO, H-2, SO2, H2S) in [C(4)mim][MeSO4] is modeled and compared with experimental data using a minimum amount of binary data. From this information, the ternary diagrams of [C(4)mim][MeSO4] with CO2 and the acid gases SO2 and H2S are predicted, and the selectivity of CO2 by respect all the gases is evaluated, with particular attention to the contaminants above mentioned. (C) 2015 Elsevier B.V. All rights reserved

Phase equilibria description of biodiesels with water and alcohols for the optimal design of the production and purification process

The water solubility in fatty acid esters and biodiesels and the vapor-liquid equilibria (VLE) of fatty acid ester + methanol/ethanol systems, formed at biodiesel production and purification industrial units, were modeled in this work to complete the development and application of the soft-SAFT equation of state (EoS) to the design and optimization of biodiesel production plants. The soft-SAFT EoS is able to accurately describe the water solubility and the VLE of methanol and ethanol with a large number of binary systems, composed of a variety of esters, in wide ranges of temperature, including near/supercritical conditions, with the use of just one binary interaction parameter. This parameter is chain length dependent for the water systems, while it is constant for the methanol and the ethanol binary mixtures. An entirely predictive scheme is proposed for the soft-SAFT EoS water solubility in biodiesels description, being only necessary to know the biodiesel fatty acid esters composition to determine the required molecular and binary interaction parameters. A new association scheme to explicitly consider the solvation phenomenon between esters and water/alcohols is also proposed. The results obtained in this work with soft-SAFT are clearly superior, regarding qualitative and quantitative agreement with the experimental data and predictive ability of the model, to those obtained with other similar modeling approaches, namely with the Cubic-plus-Association equation of state (CPA EoS) and other SAFT-type EoSs, as discussed throughout the work. The soft-SAFT EoS is shown here as a valuable tool to assist the design of biodiesel purification units, specifically for applications in the biodiesel washing, drying and alcohol removal, in order to obtain the biodiesel with the quality specifications required by the international standards. (C) 2014 Elsevier Ltd. All rights reserved

Assessing the N2O/CO2 high pressure separation using ionic liquids with the soft-SAFT EoS

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 [C(4)mim](+) 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. (C) 2014 Elsevier B.V. All rights reserved