The current method of choice for large-scale carbon dioxide (CO2) capture is amine-based chemisorption, typically in packed columns, with the benchmark solvent being aqueous solutions of a primary alkanolamine: monoethanolamine (MEA). In this contribution, we use the statistical associating fluid theory for potentials of variable range (SAFT-VR) to describe the fluid phase behavior of MEA + H2O + CO2 mixtures. The physical chemistry of CO2 in aqueous solutions of amines is highly complex owing to the chemical equilibria between the various species that are formed in solution at ambient conditions. We explicitly consider the multifunctional nature of MEA and, in so doing, are able to represent accurately the thermodynamic properties and phase equilibria of this highly nonideal mixture over a wide range of temperatures, pressures, and compositions. MEA is modeled as an associating chain molecule formed from homonuclear spherical segments with six distinct association sites incorporated to mediate the asymme...

Modeling the Fluid Phase Behavior of Carbon Dioxide in Aqueous Solutions of Monoethanolamine Using Transferable Parameters with the SAFT-VR Approach

The goal of this work was to check the ability of the soft-SAFT equation to capture the solubility of carbon dioxide (CO2) in ionic liquids (ILs) with a simple model for the IL. Imidazolium-based ILs were modeled as Lennard−Jones chains with an associating site mimicking the specific interactions between the cation and the anion as a pair. The CO2 was modeled as a Lennard−Jones chain with explicit quadrupolar interactions, with parameters taken from a previous work (Dias, A. M. A.; et al. Ind. Eng. Chem. Res. 2006, 45, 2341); no specific interaction between the ILs and the CO2 molecules was necessary to reproduce the solubility of CO2 in these ILs, even at high pressures. The parameters for the pure components were estimated with data from literature for two different families of ILs:  the 1-alkyl-3-methylimidazolium tetrafluoroborate [Cn-mim][BF4] and the 1-alkyl-3-methylimidazolium hexafluorophosphate [Cn-mim][PF6]. The study was performed in a systematic manner:  First, the equation was used to model t...

Capturing the Solubility Behavior of CO2 in Ionic Liquids by a Simple Model

A modified version of the statistical associating fluid theory (SAFT), the so-called soft-SAFT equation of state (EOS), has been extended by a crossover treatment to take into account the long density fluctuations encountered when the critical region is approached. The procedure, based on White's work from the renormalization group theory [Fluid Phase Equilibria 75, 53 (1992); L. W. Salvino and J. A. White, J. Chem. Phys. 96, 4559 (1992)], is implemented in terms of recursion relations where the density fluctuations are successively incorporated. The crossover soft-SAFT equation provides the correct nonclassical critical exponents when approaching the critical point, and reduces to the original soft-SAFT equation far from the critical region. The accuracy of the global equation is tested by direct comparison with molecular simulation results of Lennard-Jones chains, obtaining very good agreement and clear improvements compared to the original soft-SAFT EOS. Excellent agreement with vapor-liquid equilibrium experimental data inside and outside the critical region for the n-alkane series is also obtained. We provide a set of transferable molecular parameters for this family, unique for the whole range of thermodynamic properties.

Thermodynamic properties of Lennard-Jones chain molecules: renormalization-group corrections to a modified statistical associating fluid theory.

We present here the extension of the crossover soft-statistical associating fluid theory (soft-SAFT) equation of state to mixtures, as well as some illustrative applications of the methodology to mixtures of particular scientific and technological interest. The procedure is based on White's work (White, J. A. Fluid Phase Equilib. 1992, 75, 53) from the renormalization group theory, as for the pure fluids, with the isomorphism assumption applied to the mixtures. The equation is applied to three groups of mixtures:  selected mixtures of n-alkanes, the CO2/n-alkane homologous series, and the CO2/1-alkanol homologous series. The crossover equation is first applied to the pure components of the mixtures, CO2 and the 1-alkanol family, while an available correlation is used for the molecular parameters of the n-alkane series (Llovell et al. J. Chem. Phys 2004, 121, 10715). A set of transferable molecular parameters is provided for the 1-alkanols series; these are accurate for the whole range of thermodynamic con...

Global Fluid Phase Equilibria and Critical Phenomena of Selected Mixtures Using the Crossover Soft-SAFT Equation

The purpose of this work is twofold: (1) to provide an accurate molecular model for water within the soft-SAFT equation of state [Blas, F. J.; Vega, L. F. Mol. Phys. 1997, 92, 135; Llovell, F., et al. J. Chem. Phys. 2004, 121, 10715] and (2) to check the capability of this molecular-based equation of state for capturing the solubility minima of n-alkanes in water experimentally found at room temperature for these mixtures. Water was modeled as a Lennard-Jones sphere with four associating sites, with parameters obtained by fitting to experimental vapor−liquid equilibrium data. Special care was taken to the value of these parameters depending on the range of applicability of the equation, which turned out to be essential for accurate predictions for mixtures. A correlation available in the literature was used for the molecular parameters of the n-alkane series. The crossover soft-SAFT equation was able to accurately describe the phase behavior of water near to and far from the critical point, up to 350 K. I...

Capturing the solubility minima of n-alkanes in water by soft-SAFT.

An equation of state, expressed in the form of a recursion relation for the Helmholtz free energy density, is presented. This equation of state represents the free energy density as an initial mean field free energy density, plus a sum of density fluctuation correction terms which serve both to dress the initial mean field equation of state into a more accurate analytic equation of state away from the critical region, and to provide the characteristic asymptotic singular behavior in the critical region. Numerical solutions of the recursion relations are described and presented as pressure isotherms and the calculated density coexistence curve. A comparison is made with simple fluid data for temperatures in the range 0.7≤T/Tc≤1.5 and densities in the range 0≤ρ/ρc≤2.5. In particular, good agreement is found near the critical point. Three critical exponents, δ, γ, and β, are calculated and found to be in good agreement with simple fluid data as well as with renormalization group results for the 3D Ising model.

Liming W. Salvino

Papers

Calculation of density fluctuation contributions to thermodynamic properties of simple fluids