Aspen Plus Modeling Predicts Carbon Dioxide Solubility in Novel Deep Eutectic Solvent

Thermodynamic modeling conducted with Aspen Plus has demonstrated high accuracy in predicting carbon dioxide solubility across varying pressures and temperatures in an aqueous deep eutectic solvent. The research team examined a formulation comprising ethanolamine hydrochloride and diethylenetriamine in a one-to-nine molar ratio diluted with thirty percent water by volume. By pairing the Peng-Robinson equation of state for the vapor phase with the UNIFAC group contribution model for the liquid phase, the investigators successfully correlated experimental phase equilibria, achieving a coefficient of determination of 0.9937. Key thermodynamic parameters, including Henry’s law constants, absorption enthalpy, absorption entropy, and excess Gibbs free energy, were comprehensively determined. Furthermore, comparative evaluations against conventional thirty percent monoethanolamine indicated substantially lower solvent regeneration energy requirements, confirming the solvent’s potential as an energy-efficient alternative for industrial carbon capture.

Atmospheric emissions of carbon dioxide from anthropogenic activities represent a primary driver of global climate change, with fossil fuel combustion in power generation facilities and cement manufacturing plants accounting for eighty-six percent of total releases. Recent atmospheric monitoring indicates that carbon dioxide concentrations have surpassed the acceptable ceiling of 350 parts per million, contributing directly to a global mean temperature rise of 0.8 degrees Celsius. This warming trajectory has precipitated severe environmental consequences across the globe, including accelerating sea level rise, persistent regional droughts, and increased frequencies of destructive hurricanes. In response to these climatic threats, establishing effective and scalable technological approaches for capturing carbon dioxide from industrial point sources, particularly post-combustion exhaust streams, has become an urgent focus within chemical engineering research.

Among the diverse physical and chemical methodologies proposed for point-source emissions control, including solid adsorption, polymeric membrane separation, and cryogenic fractionation, chemical absorption using liquid solvents remains the most reliable and widely applied technology. Most critically, the substantial thermal energy required to strip absorbed gas and regenerate spent alkanolamine solutions imposes heavy parasitic power loads on host facilities, compelling researchers to pursue alternative solvent formulations with reduced energy footprints.

To overcome the inherent liabilities of conventional amines and expensive ionic liquids, deep eutectic solvents have emerged as an adaptable class of sorbents characterized by tunable physicochemical properties. While deep eutectic solvents share favorable attributes with ionic liquids, such as low melting points and high thermal stability, they offer distinct advantages including low preparation costs, minimal toxicity, and higher environmental biodegradability. The exploration of eutectic mixtures for gas capture began in 2008, when researchers documented carbon dioxide solubility in a choline chloride and urea system in a one-to-two ratio, inaugurating extensive international investigations into tailored eutectic formulations for sustainable carbon sequestration.

The vapor-liquid equilibrium data analyzed in this investigation originated from experimental measurements performed on an aqueous deep eutectic solvent prepared from ethanolamine hydrochloride and diethylenetriamine in a one-to-nine molar ratio with thirty percent water by volume. The molecular architecture of diethylenetriamine contains two primary amine groups and one secondary amine group, providing a higher functional amine density than simpler alkanolamines such as monoethanolamine. This elevated concentration of active nitrogen centers readily facilitates chemical reaction with carbon dioxide through the formation of diethylenetriamine carbamate species, enhancing total equilibrium absorption capacity. Isobaric phase equilibrium diagrams at twelve bars illustrate that at forty degrees Celsius, carbon dioxide solubility reaches a mole fraction of approximately 0.25 in the aqueous eutectic solvent, compared to only 0.15 in a conventional thirty percent monoethanolamine solution.

Accurate thermodynamic modeling of vapor-liquid equilibria is essential for evaluating, designing, and optimizing commercial gas absorption towers and stripping columns. However, simulating deep eutectic solvents within Aspen Plus remains challenging because these complex fluid mixtures are governed by intricate intermolecular networks featuring strong dipole-dipole interactions and extensive hydrogen bonding between solvent constituents and water. Because critical parameters and interaction coefficients for this specialized eutectic system were absent from standard Aspen databases, the researchers adopted the UNIFAC group contribution framework. This structural approach defines the solvent mixture as an assembly of distinct functional groups rather than a single molecule, providing the mathematical flexibility needed to model liquid-phase non-ideality while relying on the Peng-Robinson equation of state to characterize non-ideal behavior in the coexisting gas phase.

Before initiating regression calculations within the Aspen Plus computational environment, the researchers estimated several indispensable critical and physical parameters for the aqueous eutectic solvent that were missing from commercial chemical libraries. Key thermophysical constants, including molecular weight, normal boiling temperature, critical pressure, critical temperature, and the acentric factor, were systematically calculated by implementing the modified Lydersen-Joback-Reid estimation method coupled with the Lee-Kesler mixing rules. In addition, essential temperature-dependent transport and thermodynamic properties, encompassing liquid molar volume, dynamic viscosity, ideal gas heat capacity, and liquid surface tension, were determined through established empirical correlations. These rigorous property estimations provided the foundational thermophysical framework required by Aspen Plus to perform stable and physically consistent phase equilibrium calculations across the broad operating envelopes characteristic of industrial gas-liquid contactors.

Parameter regression of the experimental vapor-liquid equilibrium dataset was executed within the Aspen Plus Data Regression System to determine the temperature-dependent energy interaction parameters—designated as aij, bij, cij, dij, and eij—governing the UNIFAC model formulation. The regression procedure optimized these energy parameters across wide pressure and temperature ranges by minimizing differences between measured and estimated liquid-phase carbon dioxide mole fractions. Model precision was evaluated using the percentage of absolute relative error alongside the coefficient of determination. At a system temperature of 303.15 Kelvin, the fitted model demonstrated exceptional correlation with measured values, achieving an overall R² of 0.9937. Furthermore, the calculated average absolute relative error was 1.61 percent at 303.15 Kelvin and rose only to 2.64 percent at 333.15 Kelvin, proving that model variance remained satisfactorily controlled across the evaluated thermal range.

A detailed examination of the parity distribution between experimentally observed and computationally predicted carbon dioxide mole fractions revealed minor, localized anomalies within the high-concentration region spanning mole fractions from 0.70 to 0.80. Specifically, the PR-UNIFAC framework slightly underestimated experimental solubility values up to a liquid mole fraction of 0.65, followed by modest overestimation between mole fractions of 0.65 and 0.80, before returning to near-exact agreement for mole fractions exceeding 0.80. The authors observed that because thermodynamic models inherently generate smooth, continuous mathematical curves, localized deviations within the 0.70 to 0.80 interval most likely stemmed from experimental measurement uncertainties at elevated gas loadings rather than systematic model failure. Despite these minor local variances, the high overall coefficient of determination confirmed that the regressed UNIFAC parameter set reliably tracked phase behavior across the full solubility spectrum.

The study also evaluated Henry’s law constants across different operating temperatures by determining the limiting slope of carbon dioxide partial pressure plotted against liquid-phase mole fraction as gas concentration approached zero. Because Henry’s constant exhibits an inverse mathematical relationship with gas solubility, smaller constant values signify enhanced gas dissolution in the liquid phase. The computational analysis established that the lowest Henry’s law constant occurred at the lowest evaluated temperature of 303.15 Kelvin, confirming superior carbon dioxide absorption capacity under cooler conditions. Conversely, Henry’s constant reached its highest value at elevated temperatures, verifying that carbon dioxide solubility in the eutectic solvent decreases progressively as thermal energy rises. This temperature sensitivity confirms the thermodynamic feasibility of absorbing carbon dioxide efficiently at mild absorber temperatures while facilitating thermal desorption at higher temperatures during cyclic stripping.

Thermodynamic state changes associated with the absorption process were quantified through Van ’t Hoff linear regression analyses relating log-transformed equilibrium pressures to reciprocal temperature and logarithmic temperature. For carbon dioxide loadings of α ≥ 1.2, the data conformed closely to the linear assumption of constant absorption enthalpy over the investigated temperature range. At a loading of α = 3.5 and a temperature of 298.15 Kelvin, the calculated enthalpy of absorption was -8.52 kilojoules per mole, with the negative sign confirming an exothermic dissolution process driven by strong intermolecular affinity. The calculated entropy of absorption was likewise negative at -0.02682 kilojoules per mole-Kelvin, reflecting increased structural ordering as gaseous carbon dioxide enters the liquid solvent matrix. Furthermore, standard Gibbs free energy calculations yielded negative values across all evaluated conditions, demonstrating that carbon dioxide dissolution proceeds spontaneously.

The thermodynamic properties determined for this aqueous eutectic solvent carry profound practical implications when contrasted directly with conventional amine benchmarks. Published literature values indicate that conventional monoethanolamine exhibits a heat of absorption reaching 88.91 kilojoules per mole of carbon dioxide at 298.15 Kelvin, an energy barrier roughly ten times higher than the -8.52 kilojoules per mole exhibited by the aqueous eutectic solvent. Because the heat of absorption represents the dominant thermal contribution to the overall regeneration duty of an absorption facility, the lower enthalpy requirement of the deep eutectic solvent substantially decreases the thermal energy needed to strip absorbed gas during solvent recovery.

Subject of Research: Thermodynamic modeling of carbon dioxide solubility in an aqueous deep eutectic solvent

Article Title: Prediction of CO2 solubility in aqueous deep eutectic solvent using Aspen modelling approach

Article References: Nasrifar, K., Murshid, G., Al-Salmani, S., & Tan, L. S. (2025). Prediction of CO2 solubility in aqueous deep eutectic solvent using Aspen modelling approach. Discover Chemical Engineering, 6(1), Article 3. https://doi.org/10.1007/s43938-025-00106-8

Image Credits: AI Generated

DOI: 10.1007/s43938-025-00106-8

Keywords: carbon dioxide capture, deep eutectic solvents, Aspen Plus, Peng-Robinson equation of state, UNIFAC model, diethylenetriamine, ethanolamine hydrochloride, vapor-liquid equilibrium, Henry's constant, enthalpy of absorption, entropy of absorption, Gibbs free energy, monoethanolamine, solvent regeneration, greenhouse gas mitigation, carbamate formation, phase equilibria