soft-SAFT Equation Research Articles

A consistent framework to characterize the impact of co-solvents in the key process thermophysical properties of choline chloride-based DESs

In the pursuit of environmentally greener solvents, Deep Eutectic Solvents (DESs) have emerged as suitable solutions for diverse industrial applications. The reliable prediction of the physicochemical properties of DESs, affecting the design and operation of processes, is a challenging task that can be accelerated through computational modelling, reducing the amount of experimental work. The purpose of this contribution is to introduce a straightforward yet robust coarse-grained molecular model, able to characterize the thermodynamic properties of several choline chloride-based DESs, including the effect of water and alcohols as co-solvents. This is carried out using the soft-SAFT equation of state by developing an accurate and transferable parametrization. A description of the key properties of pure DESs and their mixtures, including density, speed of sound, isentropic compressibility, viscosity and activity coefficients, is provided and discussed thoroughly, revealing good agreement with the available experimental data. This methodology allows the screening of the thermophysical properties of DESs, whose knowledge is essential pre-requisite for process design.

Systematic study of the effect of the co-solvent on the performance of amine-based solvents for CO2 capture

• Polar soft-SAFT predictive model are presented for CO 2 solubility in water-lean solvents. • Model parameters for absorption process accurately correlate with co-solvent dielectric constant. • Assessment of co-solvent effects on the CO 2 absorption process. • CO 2 solubility, degree of speciation, and enthalpy of absorption are obtained for all solvents. • Compared to aqueous amine, ethylene glycol is a promising co-solvent with acceptable trade-offs. Absorption and separation by aqueous amines is seen today as a mature technology for CO 2 capture at large scale. However, it still suffers from some drawbacks, especially the high energy required for the regeneration of the solvent. In this sense, water-free and water-lean solvents are attractive alternatives to replace them due to their potential in reducing this regeneration energy; however, systematic studies on the effect of the co-solvent on the required thermodynamic properties to optimize the separation process are still missing. In this work, we use the polar soft-SAFT equation of state for modeling CO 2 chemisorption in a range of aqueous, water-free, and water-lean solvents, formulated using glycol co-solvents. In addition to accurately capturing the CO 2 solubility in these solvents, the model allows to isolate the effect of a given organic diluent, and its ratio with water, on CO 2 solubility, degree of speciation, and enthalpy of absorption. We find that ethylene glycol is a promising co-solvent, as the reduction in CO 2 solubility is minimized, while exhibiting the largest decrease in enthalpy of absorption than other water-lean and water-free solvents, irrespective of the employed amine. This is the first study in which the effect of the co-solvent has been systematically evaluated at post-combustion capture conditions, helping in the rational design of novel solvents for CO 2 capture.

Assessment of Low Global Warming Potential Refrigerants for Drop-In Replacement by Connecting their Molecular Features to Their Performance.

The use of hydrofluorocarbons (HFCs) as an alternative for refrigeration units has grown over the past decades as a replacement to chlorofluorocarbons (CFCs), banned by the Montreal’s Protocol because of their effect on the depletion of the ozone layer. However, HFCs are known to be greenhouse gases with considerable global warming potential (GWP), thousands of times higher than carbon dioxide. The Kigali Amendment to the Montreal Protocol has promoted an active area of research toward the development of low GWP refrigerants to replace the ones in current use, and it is expected to significantly contribute to the Paris Agreement by avoiding nearly half a degree Celsius of temperature increase by the end of this century. We present here a molecular-based evaluation tool aiming at finding optimal refrigerants with the requirements imposed by current environmental legislations in order to mitigate their impact on climate change. The proposed approach relies on the robust polar soft-SAFT equation of state to predict thermodynamic properties required for their technical evaluation at conditions relevant for cooling applications. Additionally, the thermodynamic model integrated with technical criteria enable the search for compatibility of currently used third generation compounds with more eco-friendly refrigerants as drop-in replacements. The criteria include volumetric cooling capacity, coefficient of performance, and other physicochemical properties with direct impact on the technical performance of the cooling cycle. As such, R1123, R1224yd(Z), R1234ze(E), and R1225ye(Z) demonstrate high aptitude toward replacing R134a, R32, R152a, and R245fa with minimal retrofitting to the existing system. The current modeling platform for the rapid screening of emerging refrigerants offers a guide for future efforts on the design of alternative working fluids.

Searching for suitable lubricants for low global warming potential refrigerant R513A using molecular-based models: Solubility and performance in refrigeration cycles

• Accurate molecular thermodynamic model for R513A/lubricants in refrigeration cycles. • Rigorous energetic investigation of refrigeration cycles with thermodynamic postulates. • PEC5 optimal lubricant for R513A, in view of solubility, miscibility and viscosity. • Procedure allowed quantifying impact of the presence of lubricant on the COP. • Presence of PEC5 in the mixture up to 5% weight led to COP decrease of 3%. Following environmental regulations, refrigerant R134a (Global Warming Potential GWP = 1430), commonly used in air conditioning applications, is required to be replaced by refrigerants with lower GWP. Among them, R513A is investigated as an excellent alternative, with a GWP reduction of 56% compared to R134a. However, further studies are needed in order to assess its use for this application. In this work, the molecular-based polar soft-SAFT equation has been employed to predict the vapour-liquid equilibria (VLE) of R513A and selected lubricants, searching for the optimal lubricant and their performance on a vapour compression refrigeration cycle. First, the binary VLE of the commercial lubricants from the Pentaerythritol Esters (PECs) and Polyethylene Glycol Dimethyl Ethers (PEGDMEs) families with R1234yf and R134a were assessed versus available experimental data, by developing the appropriate molecular models and parameters. Then, the models were used to predict the properties of the ternary mixtures containing the two refrigerants and the lubricants. PEC5 was found to be the optimal lubricant for R513A, considering a balance between solubility, miscibility and viscosity. Molecular modelling combined with thermodynamic and energetic analysis allowed to calculate the performance of a vapour compression refrigeration cooling cycle, quantifying the impact of the presence of the lubricant on the COP. The presence of PEC5 in the mixture was not significant, staying below a COP decrease of 3% in all cases considered, while for TrEGDME, its presence reduced the COP up to 6.1%. The procedure used here allows tuning the performance of the refrigerant/lubricant pair, in a step forward on the search for low GWP refrigerants in air conditioning systems.

Screening Tool for Water-Free and Water-Lean Amine-Based Solvents for CO2 Capture

Owing to the high regeneration energy of conventional aqueous amines, water-free and water-lean solvents have emerged as potential alternatives for absorption CO2 capture processes. Given the wide array of possible solvents and required properties, rapid and reliable screening tools are required. Hence, this contribution is focused on demonstrating the capabilities of the soft-SAFT equation of state as a reliable thermodynamic model for the accurate representation of CO2 chemisorption in these novel solvents. Calculations of CO2 cyclic capacity and total energy of regeneration allow evaluating the performance of these solvents relative to their aqueous counterpart as a first screening tool. Results indicate that at the same conditions, transition the MEA-based solvent from aqueous, to water-lean, and finally to non-aqueous, lead to decreased cyclic capacity but also lower energy of regeneration.

Effect of size disparity on the gas-liquid interfacial properties of Lennard-Jones monomer plus dimer mixtures

The interfacial properties of Lennard-Jones monomer plus dimer mixtures at the gas-liquid (gl) interfaces are determined by the direct molecular dynamics (MD) and the density gradient theory (DGT), coupled with the soft-SAFT (soft-statistical associating fluid theory) equation of state. It is demonstrated that both approaches exhibit excellent mutual agreement in the entire pressure and concentration ranges. According to the presented results, the leading effect of increasing the geometrical disparity between the constituent dimer and monomer (σ223/σ113) at the gl state is sharpening of the interface increase and expansion of the biphasic coexistence region. Besides that, decreasing of the size disparity and increasing the mixture asymmetry result in a wider phase envelope in terms of densities and molar fractions. In addition, the interfacial thickness and the surface tension grow with reduction of σ223/σ113, which is a direct consequence of increasing the system asymmetry.

Thermodynamic and process modeling of the recovery of R410A compounds with ionic liquids

• A soft-SAFT model is used to describe the solubility of R410A compounds in ILs. • Henry constants, the enthalpy and entropy of dissolution are predicted. • [C 2 mim][Tf 2 N] is chosen based on selectivity and solubility predictions. • The ternary diagram IL-R32-R125 is built to evaluate the competitive selectivity. • A process recovery simulation unit using [C 2 mim][Tf 2 N] is optimized. European regulations are limiting the use of hydrofluorocarbons (HFCs) as refrigerants because of their elevated global warming potentials (GWPs). Apart from their substitution by other compounds with lower environmental impact, one of the plausible approaches to meet the legal requirements is the formulation of new refrigerant blends containing a low GWP compound (e.g., hydrofluoroolefins) mixed with one HFC that provides the necessary thermodynamic characteristics to act as an effective refrigerant. Thus, the recovery and reuse of HFCs seems a promising approach to increase their lifespan and reduce their production. However, current regeneration technologies that are based on distillation are highly energy-intensive. Therefore, the development of novel separation processes to selectively separate HFCs is needed for the practical implementation of circular economy principles in the use of refrigerants. This work is a step forward on using ionic liquids (ILs) to selectively separate F-gases. The advanced molecular-based soft-SAFT equation of state (EoS) is used as a tool to assess the potential of ILs as a solvent platform for the selective separation of the compounds forming the R410A refrigerant blend: R32 and R125. Soft-SAFT is employed to model the absorption of these HFCs into different ILs with different fluorinated anions. Ternary diagrams are then predicted for the absorption of R32/R125 mixtures into selected ILs to evaluate the competitive selectivity between both compounds. Based on this study, a potential ionic liquid candidate is chosen and a process simulation is performed to estimate the approximate energy cost of the separation and recovery process.

Isobaric vapor-liquid equilibrium of water + glymes binary mixtures: Experimental measurements and molecular thermodynamic modelling

In this work, new experimental data on the isobaric vapor-liquid equilibria (VLE) of binary aqueous systems, with six different glycol ethers (glymes), some of which are currently used in the Selexol process, were measured at three different pressures, namely 0.05, 0.07, and 0.1 MPa.From the experimental data, the water activity coefficients were estimated using the modified Raoult's law and used to infer about the effect of the glymes structure on their interactions with water. Moreover, using a coarse-grain molecular model previously proposed in the framework of the soft-SAFT equation of state (EoS) for both glycols and glymes, the experimental data were successfully correlated with a single state-independent binary interaction parameter and average absolute deviations from the experimental data of 1.30 K. Furthermore, the model was used in a predictive manner to obtain the water activity coefficients in the whole composition range, providing useful insights into the systems non-ideality.

Functionalization of fluorinated ionic liquids: A combined experimental-theoretical study

We present new experimental and modelling data concerning imidazolium based-FILs synthesized with a hydroxyl group in the end of the cationic hydrogenated side chain and compared them with the analogous non-functionalized FILs in order to verify their suitability in the biomedical field. The thermophysical and thermodynamic properties of the neat compounds and the self-aggregation behaviour of FILs in aqueous solutions were measured and compared with theoretical results from the soft-SAFT equation of state, in good agreement with each other. Results showed that the presence of the hydroxyl group increases the density and viscosity of pure compounds and aqueous mixtures, whereas the thermal stability, melting, free volume, ionicity and self-aggregation behaviour decrease. These properties are improved with respect to the conventional perfluorosurfactants for the desired application, due to the full miscibility in water and the promising improved biocompatibility.

A consistent thermodynamic molecular model of n-hydrofluoroolefins and blends for refrigeration applications

This work presents a thermodynamic model that characterizes 4th-generation hydrofluoroolefins (HFOs)-based refrigerants with the molecular-based soft-SAFT equation of state (Blas and Vega, 1998) as well as its application in process simulations for a selected refrigeration application. The evaluation of the HFOs has been done building on a molecular model transferred from the equivalent hydrofluorocarbons (HFCs), taking advantage of the similarities between the two chemical families. The model has been used to calculate all thermophysical properties of the selected HFOs relevant for their application as refrigerants, including the saturated density, vapor pressure, heat capacity, speed of sound, surface tension and viscosity, providing good agreement with experimental available data. In addition, phase equilibria, interfacial behavior and viscosity calculations have been performed for blends between HFCs and the two most common HFOs, R1234yf and R1234ze(E). The obtained thermodynamic properties have been used for a process simulation of a vapor compression refrigeration system, comparing the 3rd generation refrigerant R410A with these 4th generation blends, including the Coefficient of Performance for different cases, in order to establish the best alternative to R410A. Overall, this work shows how molecular modeling tools can be used now a day, as a complementary tool to generate reliable data for process simulation, in this case related to the search for alternative refrigerants.

Polar soft-SAFT: theory and comparison with molecular simulations and experimental data of pure polar fluids.

The consideration of polar interactions is of vital importance for the development of predictive and accurate thermodynamic models for polar fluids, as they govern most of their thermodynamic properties, making them highly non-ideal fluids. We present here for the first time the extension of the soft-SAFT equation of state (EoS), named polar soft-SAFT, to explicitly model intermolecular polar interactions (dipolar and quadrupolar), using the approach of Jog and Chapman (P. K. Jog and W. G. Chapman, Mol. Phys., 1999, 97(3), 307-319). The theory is first validated using molecular simulation data for a wide range of polar model systems including Stockmayer fluids, LJ dimers with dipole, and quadrupolar LJ fluids, for a wide range of thermophysical properties such as liquid density, vapour pressure, surface tension and heat capacities. Excellent agreement between polar soft-SAFT and simulation data has been obtained for all examined fluids and properties for systems exhibiting low to intermediate polar strength, while the agreement deteriorates at very high polar strengths. Once validated with simulations, the equation has been applied to calculate vapour-liquid equilibria (VLE), surface tension and second-order derivative properties of systems such as 2-ketone and methane chloride families as showcases for dipolar fluids and the benzene family for quadrupolar fluids, finding very good agreement with experimental data. In order to preserve the robustness of the model, the experimental value of the dipole or quadrupole was used in these calculations, while the additional parameter for the polar fluids was set a priori rather than included in the fitting procedure. The excellent agreement found with simulations and experiments empowers the soft-SAFT equation with new capabilities for the development of robust and accurate molecular models of polar fluids of industrial relevance.

Combining soft-SAFT and COSMO-RS modeling tools to assess the CO2–SO2 separation using phosphonium-based ionic liquids

The development of efficient CO2 separation techniques from post-combustion flue gases is a key area of research to green-house gas control. However, CO2 capture is typically affected by the presence of other acid impurities, such as traces of SO2. In that sense, this work assesses CO2 separation from a CO2/SO2 mixture with a set of phosphonium-based ILs. Two different modeling tools, soft-SAFT and COSMO-RS, have been used cooperatively to study the CO2 gas separation on ILs. From one side, the soft-SAFT equation of state, which has been employed for the first time in this family of ILs, has been used to effectively reproduce the absorption properties of these promising CO2 absorbents in a wide range of pressures/temperatures. Additionally, COSMO-RS, employed to evaluate the charge distribution so as to develop representative models for soft-SAFT, has been capable of reproducing the low-pressure absorption region in a purely predictive way. In both cases, the enthalpy and entropy of dissolution and the selectivity of the mixtures are predicted. Also, several ternary diagrams have been built to analyze different acid gas compositions.

Vapor Liquid Equilibria of Binary Mixtures of 1-Butyl-3-methylimidazolium Triflate (C4mimTfO) and Molecular Solvents: n-Alkyl Alcohols and Water.

Isobaric vapor liquid equilibria (VLE) of binary mixtures of the ionic liquid (IL) 1-butyl-3-methylimidazolium trifluoromethanesulfonate (C4mimTfO) with either water or short chained n-alkyl alcohols (methanol, ethanol, propan-1-ol, and butan-1-ol) are described in this study. Two different microebulliometers and a classical VLE apparatus were compared and the VLEs were determined in the composition range 0.4 ≤ x(solvent) ≤ 1 at three different pressure levels ( p = 500 mbar, 700 mbar, and 1000 mbar). The experimental data were modeled using the soft-SAFT equation of state, which was able to accurately describe the nonideal behavior of these mixtures. The combined experimental-modeling results obtained contribute to establish the structure-property relationship between the C4mimTfO and n-alkyl alcohol molecules and to infer about its influence on the phase behavior of these solvents.

Soft-SAFT Transferable Molecular Models for the Description of Gas Solubility in Eutectic Ammonium Salt-Based Solvents

Research on new sustainable technologies focused on carbon capture is a priority in most industrial processes, particularly in the combustion of fossil fuels. One challenging option is the replacement of corrosive, volatile, and nonbiodegradable amine-based solvents by the more tunable, greener, and less expensive deep eutectic solvents (DESs). However, the large amount of eutectic mixture combinations requires an intense experimental characterization. The implementation of semipredictive theoretical models to describe the physicochemical properties is a useful tool to reduce the amount of experimental work. In this contribution, the well-established soft-SAFT equation of state is applied to develop consistent molecular models of different eutectic ammonium chloride and bromide salt-based DESs with ethylene glycol and levulinic acid to describe their density and the solubility of CO2 and SO2 in them. In all cases, DESs are modeled as a mixture of two independent entities, overcoming the limitations of the...

A consistent and transferable thermodynamic model to accurately describe CO2 capture with monoethanolamine

In this work, a consistent and transferable thermodynamic model to describe the carbon dioxide (CO2) capture process with monoethanolamine (MEA) has been successfully developed within the framework of the soft-SAFT equation of state. The effect of the association interactions of the amine and alcohol groups contained in MEA have been isolated and characterized by studying the 1-alkylamines and 1-alkanols family. In a similar manner, the interaction between these functional groups and water has also been determined and later transferred to aqueous MEA. At the end, a reliable and consistent set of molecular parameters has been obtained for MEA. The CO2 absorption process is described in terms of the aggregation of the reactants driven by the presence of strong intermolecular interactions. Descriptions of the density, vapor pressure and surface tension in these systems provide excellent agreement with the available experimental information. Also, a very good estimation of the CO2 loading in MEA is found in the range of operating conditions. Predictions on the mixing enthalpies and the enthalpy of absorption are also given. This transferable model will allow the prediction of the CO2 absorption process in other alkanolamines in the absence of experimental data.

Accurate viscosity predictions of linear polymers from n-alkanes data

The design of processes involving heavy alkanes and polymers relies on the accurate knowledge of their viscosity, as it strongly affects the flowing behavior, and hence, the final performance. Most classical viscosity modeling methods are unable to predict the behavior at high-pressure, high-temperature conditions, neither to capture the change in the slope of the zero-shear viscosity with the molecular weight. We present here results of coupling two molecular theories, the soft-SAFT Equation of State and the Free-Volume Theory (FVT), for viscosity calculations of n-alkanes and linear polyethylene up to 32,100g/mol, in a transferable manner. A new procedure, named Spider-Web, is implemented to obtain reliable parameters for the FVT. The procedure provides accurate predictions for the viscosity of n-alkane mixtures in agreement with experimental data, including high pressure and high temperature conditions. The molecular model is also able to predict the dependency of the entanglement molecular weight with temperature and pressure.

Accurate description of thermophysical properties of Tetraalkylammonium Chloride Deep Eutectic Solvents with the soft-SAFT equation of state

The potential to reduce the environmental impact in industrial processes has led to investigations on greener solvents for a variety of applications that conventionally use organic compounds. In recent years, Deep Eutectic Solvents (DESs) have emerged as a promising new class of solvents and have opened a new field of research. DESs are systems formed from an eutectic mixture of Lewis or Brønsted acids and bases, which can contain a variety of anionic and/or cationic species. Due to a strong hydrogen-bonding effect, their melting point is significantly lower than the melting point of the individual components. DESs are non-volatile, biodegradable and do not react with water, becoming potential candidates for a wide variety of applications. Their inexpensive synthesis is an additional advantage versus other compounds. Unfortunately, the mechanisms behind their physicochemical properties are still not well understood. Consequently, there are almost no available theoretical models able to accurately capture the main physicochemical properties of these compounds. In this work, a novel methodology based on the soft-SAFT statistical mechanics-based equation of state is used to propose different molecular models of variable complexity to describe the density, surface tension, viscosity, and CO2 solubility of several Tetraalkylammonium Chloride based DESs. Two approaches are explored: treating the DESs as a pseudo-pure compound or describing them as a mixture of two independent constituents. The transferability of the molecular parameters between DESs to describe their thermodynamic behavior is checked in order to predict the properties of new possible DESs and their mixtures if experimental data are not available. A very good description of the density, surface tension, viscosity and gas solubility is found in all cases. This methodology provides a new platform to qualitatively screen multiple physicochemical properties of new DESs in a rapid, efficient and reliable manner.

Evaluation of the solvent structural effect upon the vapor –liquid equilibrium of [C4C1im][Cl] + alcohols

A systematic isobaric vapor-liquid equilibrium (VLE) study of seven binary mixtures of 1-butyl-3-methylimidazolium chloride, [C4C1im][Cl], and methanol, propan-1-ol, propan-2-ol, butan-1-ol, butan-2-ol, tert-butanol (2-methylpropan-2-ol), iso-butanol (2-methylpropan-1-ol) and pentan-1-ol, was carried out at three different system pressures (0.1, 0.07 and 0.05 MPa). Activity coefficients were estimated from the boiling temperatures of the binary mixtures. soft-SAFT equation of state was used to describe the experimental VLE data and all together, allowed to infer and understand the effect of the alcohol alkyl chain length and structural isomerism on the molecular interactions between the IL and the alcohols.

New measurements and modeling of high pressure thermodynamic properties of glycols

New experimental density data of six glycols, namely ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TriEG), tetraethylene glycol (TeEG), pentaethylene glycol (PeEG) and hexaethylene glycol (HeEG), and a polyethyleneglycol (PEG 400) were determined in a wide range of temperatures (283–363 K) and pressures (0.1–95) MPa. The experimental density data was first correlated, as function of temperature and pressure, using the modified Tait-Tammann equation, and the derivative properties, such as isobaric thermal expansion coefficients and the isothermal compressibility, were estimated. It is shown that the isobaric thermal expansivity converges to a cross-over point that, although commonly observed for non-associating compounds, is here reported for the first time for the studied glycols. This denotes the presence of hydrogen bonds, mainly dominated by dispersive interactions, breaking and decreasing intermolecular interactions as the temperature and the number of glycols ethoxy groups increase.The study is completed with the modeling of the experimental data using the soft-SAFT equation of state. A molecular model, considering the glycol molecules as LJ chains with one associating site at each of the compounds' end groups (hydroxyl groups) is proposed for all the glycols, allowing the EoS to provide an excellent description of the glycols pVT surface. Additionally, the optimized parameters were correlated with the compound's molecular weight, providing a good prediction of the PEG400 density and the compounds' derivative properties.

Global phase behaviour in methane plus n-alkanes binary mixtures

The methane plus linear alkanes series is one of the simpler and most common type of mixtures presented in the industry and its accurate representation is paramount because of the diversity of its applications. From a theoretical perspective, its structural simplicity due to the low degree of synergy and the absence of short-range dispersive forces suggests that a good model should represent these systems with acceptable accuracy. Based on these ideas and the relatively regular behaviour of n-alkanes, these systems have become particularly suitable for trial calculation in convoluted models, in order to verify their capabilities in both quantitative and qualitative predictions. In this contribution, the soft-SAFT equation of state is used for the construction of the serial prediction domain global phase diagram (spd-GPD) of the methane plus linear alkanes series. The main goal is to address the capability of the equation of state to predict the global behaviour of the binary mixtures until methane plus n-decane, with special emphasis in accurately reproducing the mechanism and topology of mixtures. All systems shown here are predicted with quantitative accuracy, including a complete description of their taxonomy. The information obtained from the equation of state allows the validation of some predictions of the spd-GPD in the absence of experimental data. Following this analysis, the double retrograde behaviour lies between the methane plus propane and methane plus n-butane systems, while the tricritical transition of the series is found between the methane plus n-pentane and methane plus n-hexane mixtures. Also, the presence of the molar density inversion phenomenon is predicted to begin between the methane plus n-hexane and methane plus n-heptane systems, while no signals of mass density inversions are found on these mixtures.