The addition of a strong acid to an aqueous alkanolamine solvent is common in different acid gas removal applications to improve the selectivity toward H2S compared to CO2 and thus reach more stringent H2S specifications. The purpose of solvent acidification is to shift the equilibrium of the reactions. In this work, the vapor-liquid equilibria (VLE) of CO2 and H2S in aqueous solutions of N-methyldiethanolamine (MDEA) with 0.5 and 1.0 mol % sulfuric acid (H2SO4) were measured at temperatures of 323 and 353 K. A reduced solubility for both acid gases was observed for experiments with H2SO4 compared with experiments without acidification. The experimental VLE results were used to develop a thermodynamic model based on e-NRTL. The model accurately predicts the experimental data in this work and from other authors. Additionally, qualitative kinetic experiments were performed, and they have shown limited impact on the absorption rates of both acid gases. The study improves our understanding of the impact of strong acids on thermodynamics and kinetics and on acid gas removal processes at the industrial scale.

Acid gas chemical absorption with aqueous amine solvents is an important industrial technology for gas processing and the capture of CO2. Vapor-liquid equilibrium (VLE) reflects the efficiency of solvents and is essential to model the thermodynamics of the absorption and solvent regeneration process. In this study, several machine learning (ML) approaches were used to develop VLE models for acid gas absorption in aqueous methyldiethanolamine (MDEA) and piperazine (Pz), namely, CO2-MDEA-H2O, H2S-MDEA-H2O, CO2-H2S-MDEA-H2O, and CO2-Pz-H2O systems. New experimental data are presented for the CO2-MDEA-H2O and H2S-MDEA-H2O ternary systems, and they are used to compare the accuracy of the ML models to that of an earlier reported activity coefficient (e-NRTL)-based thermodynamic model. For the quaternary system CO2-H2S-MDEA-H2O, the ML models and the physical model are compared using experimental data from the literature because the physical model of the quaternary system is only trained on the ternary experimental systems. For this system, e-NRTL predicted CO2 & H2S VLE with RMSEs of 0.32 and 0.43 log10(mPa), whereas the ML model trained on CO2-H2S-MDEA-H2O had a RMSE of 0.39 and 0.21, respectively. The results indicate that the optimal ML approach is not systematically more accurate than the physics-based model.

Removing hydrogen sulfide (H2S) from gas streams is a critical task in many industrial processes, e.g. in natural gas cleaning where it is generally carried out together with the removal of carbon dioxide (CO2) by reactive absorption. The most common solvents for the reactive CO2 absorption are aqueous amine solutions, sometimes with alcohols as cosolvents. The sour gases H2S and CO2 dissociate in these basic solutions, leading to high solubilities, which are further enhanced by chemical reactions of CO2 with many amines. It is usually assumed that H2S only dissociates in these solvents but does not react chemically as CO2 does; however, no experimental proof of this hypothesis has been given in the literature so far. Therefore, we have carried out an NMR study of the speciation of aqueous amine solutions containing N-methyldiethanolamine (MDEA), monoethanolamine (MEA), piperazine (PZ), and ethanol (EOH), which were loaded with H2S at pressures between 2.7 and 4.8 bar at 323.15 K. To identify possible chemical reactions of H2S with the solvents, the resulting NMR spectra were compared with the NMR spectra of the unloaded aqueous amine solutions. The dissociation of H2S in the basic amine solutions leads to protonation of the amines and strong NMR peak shifts, but no reaction products of H2S with MDEA, MEA, PZ, and EOH were found. We expect that this is also true for other amines and alcohols and confirms the hypothesis that the solubility of H2S in the studied solvent class is not affected by chemical reactions with the solvents.

The removal of CO2 and H2S from a gas by absorption with an aqueous alkanolamine solution is an energy-intensive process. The addition of a physical cosolvent might lower the regeneration energy but also affects the thermodynamics and kinetics of acid gas absorption. In this work, we study the impact on the thermodynamics and kinetics of CO2 and H2S absorption of the addition of 1,3-dimethyl-2-imidazolidinone (DMI) to the aqueous 2-(2-diethylaminoethoxy)ethanol (DEAE-EO) solvent. A comparison is made with the impact of DMI on an aqueous methyldiethanolamine (MDEA) solvent. In all cases, DMI reduced the solubility and the absorption rate of the acid gases. DMI also reduced the initial kinetic H2S/CO2 selectivity, although this effect seems much more pronounced in aqueous MDEA than in aqueous DEAE-EO. The equilibrium thermodynamic H2S/CO2 selectivity is higher in the presence of DMI.

We explore the impact of force field parameters and reaction equilibrium on the scaling behavior towards the critical point in reactive binary systems, focusing on NO2/N2O4. This system can be considered as a special single-component system since NO2 and N2O4 are in chemical equilibrium via the chemical reaction 2NO(2) (sic) N2O4. We simplify the system by representing both components as single LJ particles, achieving excellent agreement with densities computed using molecular simulations in which all-atom force fields were used. We investigate the effect of force field parameters (epsilon and sigma-) on phase behavior and show that the critical exponent remains constant, which means that intermolecular interactions do not affect the scaling to the critical point when the chemical reaction takes place. We also investigate the sensitivity of the reaction equilibrium constant and show that even small changes in isolated molecule partition functions lead to large differences in chemical equilibria. We show that the critical exponent beta is different for systems with different reaction equilibrium constants, so a careful parameterization of beta needed for an accurate computation of critical temperatures of reactive mixtures. We perform a screening of reactive binary mixtures for a wide range of ideal gas reaction equilibrium constants, revealing key insights into the thermodynamic behavior and critical properties. Thereby we facilitate the efficient screening of reactive binary mixtures for various applications. Our results emphasize the importance of accurately parameterizing and provide valuable insights into the critical scaling behavior of complex reactive systems.

A complete study of thermophysical properties concerning the 2-(2-Diethylaminoethoxy)ethanol (DEAE-EO) + water binary system is realized. Density, speed of sound, dynamic and kinematic viscosities and refractive index measurements have been performed at atmospheric pressure, using a vibrating tube densitometer, a falling ball viscosimeter, and a refractometer, for pure DEAE-EO, pure water, and for aqueous solutions of DEAE-EO, from 278.15 to 323.15 K. The thermal expansion was calculated from density data. Excess Gibbs energy of flow and the corresponding excess entropy of flow were also calculated considering dynamic viscosity and density data. Excess molar properties (volume, isobaric expansion coefficient, Gibbs energy of flow and square of refractive index) were calculated and the Redlich-Kister equations were applied to correlate the data. Thermophysical properties of aqueous DEAE-EO (50 mol%) and Methyldiethanolamine (MDEA) (50 mol%) solutions were compared for their application in absorption of acid gasses.

We explore the impact of force field parameters and reaction equilibrium on the scaling behavior towards the critical point in reactive binary systems, focusing on NO2/N2O4. This system can be considered as a special single-component system since NO2 and N2O4 are in chemical equilibrium via the chemical reaction 2NO2⇌N2O4. We simplify the system by representing both components as single LJ particles, achieving excellent agreement with densities computed using molecular simulations in which all-atom force fields were used. We investigate the effect of force field parameters (ɛ and σ) on phase behavior and show that the critical exponent β remains constant, which means that intermolecular interactions do not affect the scaling to the critical point when the chemical reaction takes place. We also investigate the sensitivity of the reaction equilibrium constant and show that even small changes in isolated molecule partition functions lead to large differences in chemical equilibria. We show that the critical exponent β is different for systems with different reaction equilibrium constants, so a careful parameterization of β is needed for an accurate computation of critical temperatures of reactive mixtures. We perform a screening of reactive binary mixtures for a wide range of ideal gas reaction equilibrium constants, revealing key insights into the thermodynamic behavior and critical properties. Thereby we facilitate the efficient screening of reactive binary mixtures for various applications. Our results emphasize the importance of accurately parameterizing β and provide valuable insights into the critical scaling behavior of complex reactive systems.

A complete study of thermophysical properties concerning the 1,3-dimethyl-2-imidazolidinone (DMI) + water binary system is realized. Density, speed of sound, dynamic viscosities, and refractive index measurements have been performed at atmospheric pressure, using a vibrating tube densitometer, a falling ball viscosimeter, and a refractometer for pure DMI, pure water, and for aqueous solutions of DMI, from 278.15 to 323.15 K. The thermal expansion was calculated from density data. Excess Gibbs energy of flow and the corresponding excess entropy of flow were also calculated considering dynamic viscosity and density data. Excess molar properties (volume, thermal expansion, Gibbs energy of flow, and square of refractive index) were calculated, and the Redlich-Kister equations were applied to correlate the data. The analysis of the behavior of the excess properties allows us to identify the most relevant molecular interaction in mixtures rich in DMI and in mixtures rich in water. Our results show that DMI can be classified as a liquid structure maker and that molecules of water are certainly inserted in the DMI network at finite dilution.

Acid gas absorption with aqueous amines is an important industrial technology for natural or flue gas treatment. To lower the high energy required for the regeneration process it has previously been proposed to replace a significant part of the water by a physical co-solvent with a lower specific heat. To investigate this idea, as well as the overall impact of adding a physical co-solvent, a detailed process model for CO2 and H2S absorption using an aqueous mixture of N-methyl diethanolamine (MDEA) - ethylene glycol (EG) is developed based on lab-scale experimental measurements of physical, thermodynamic, and kinetic properties. Those experiments indicate that the addition of EG reduces the solubility of the acid gases and that, despite the higher viscosity of the solvent, the impact on the rate of acid gas absorption is minor. Pilot plant tests further confirmed this. The vapor liquid equilibrium (VLE) experiments were used to regress the thermodynamic electrolyte e-NRTL (Non-Random Two-Liquid) model parameters. In parallel, the kinetic measurements allowed to develop a rate-based model. All models were implemented in a commercial process simulator that was validated with high-pressure absorp-tion-regeneration pilot tests with the solvent composition of 34 wt% MDEA, 44.5 wt% EG and 21.5 wt% H2O and with the raw gas composition of 2.5 mol% H2S and 2.5 mol% CO2. Comparison with aqueous MDEA (50 wt% MDEA, and 50 wt% H2O) shows that a reduction of 25 % in reboiler duty can be obtained when adding EG. Nevertheless, an 80 % increase in solvent flow rate is needed to get a similar acid gas absorption performance, and a higher co-absorption of hydrocarbons is observed.

We developed an open-source chemical reaction equilibrium solver in Python (CASpy, https://github.com/omoultosEthTuDelft/CASpy) to compute the concentration of species in any reactive liquid-phase absorption system. We derived an expression for a mole fraction-based equilibrium constant as a function of excess chemical potential, standard ideal gas chemical potential, temperature, and volume. As a case study, we computed the CO2 absorption isotherm and speciation in a 23 wt % N-methyldiethanolamine (MDEA)/water solution at 313.15 K, and compared the results with available data from the literature. The results show that the computed CO2 isotherms and speciations are in excellent agreement with experimental data, demonstrating the accuracy and the precision of our solver. The binary absorptions of CO2 and H2S in 50 wt % MDEA/water solutions at 323.15 K were computed and compared with available data from the literature. The computed CO2 isotherms showed good agreement with other modeling studies from the literature while the computed H2S isotherms did not agree well with experimental data. The experimental equilibrium constants used as an input were not adjusted for H2S/CO2/MDEA/water systems and need to be adjusted for this system. Using free energy calculations with two different force fields (GAFF and OPLS-AA) and quantum chemistry calculations, we computed the equilibrium constant (K) of the protonated MDEA dissociation reaction. Despite the good agreement of the OPLS-AA force field (ln[K] = −24.91) with the experiments (ln[K] = −23.04), the computed CO2 pressures were significantly underestimated. We systematically investigated the limitations of computing CO2 absorption isotherms using free energy and quantum chemistry calculations and showed that the computed values of μiex are very sensitive to the point charges used in the simulations, which limits the predictive power of this method.

Experimentally measuring the diffusivities of CO2 and H2S in aqueous alkanolamine solutions presents an extremely challenging task. To overcome this challenge, we performed Molecular Dynamics (MD) simulations to study the effects of temperature and N-methyldiethanolamine (MDEA) concentration on self-diffusivities of CO2 (DCO2) and H2S (DH2S) in aqueous MDEA solutions. We compute the densities and viscosities of aqueous MDEA solutions for an MDEA concentration range of 10-50 wt% and a temperature range of 288-333 K showing an excellent agreement with experimental data from literature. We compute the self-diffusivity of MDEA (DMDEA) in aqueous MDEA solutions and our findings show that the computed values of DMDEA are in excellent agreement with experimental and simulation results from literature. The self-diffusivities DCO2 and DH2S in aqueous MDEA solutions are computed for a wide range of temperatures and MDEA concentrations and our results show that both DCO2 and DH2S depend significantly on temperature and MDEA concentration. We also show that both CO2 and H2S diffuse slower in aqueous MDEA solutions than in aqueous MEA solutions. By comparing the radial distribution functions of CO2, H2S, water, and MDEA, we show that H2S has stronger interactions with the surrounding molecules than CO2, which makes H2S diffuse slower in aqueous MDEA solutions. We also investigate the densities and viscosities of acid gas loaded aqueous MDEA solutions and self-diffusivities of the reaction products of CO2 and H2S with aqueous MDEA solutions. We show that the self-diffusivities of CO2-loaded solutions significantly decrease with increasing CO2 loading while the self-diffusivities of H2S-loaded solutions do not change with changing H2S loading. Our results will be helpful in the design and optimization of acid gas removal units.

In the treatment of many fuel gases such as biogas, natural gas, syngas, and so on, tertiary alkanolamines play an important role in the selective removal of H2S with respect to CO2. The selectivity might be required for various reasons: to respect more stringent H2S specifications, to optimize the performance of the Claus unit, to lower the cost of CO2 capture, and so on. The H2S/CO2 selectivity is mainly kinetic and, to a lesser extent, thermodynamic. A novel experimental setup has been put in place to measure the time evolution of the simultaneous absorption of H2S and CO2. The results of an extensive experimental campaign with 18 different aqueous tertiary alkanolamine solvents (13 mol % amine, 87 mol % H2O) are presented. Although the absorption of H2S is expected to be a very fast proton transfer, a significant variation in H2S absorption rates and thus in selectivity is observed. This could not only be explained by the pKa or the viscosity of the amines. Therefore, an accurate quantitative molecular simulations-based kinetic model is developed and validated. The study allowed us to better understand the molecular origin of selectivity, as well as to identify amines with a higher selectivity than aqueous MDEA (MethylDiEthanolAmine), the standard industrial selective solvent.

We investigated the effect of temperature and monoethanolamine (MEA) concentration on the self-diffusivity of acid gases, CO2 , and H2S in aqueous MEA solutions. For this purpose, we computed densities of pure MEA and 30 wt% MEA/water solutions while scaling the LJ energy (epsilon) parameter and point charges of MEA. Results show that with a scaling factor of 0.80 applied to the point charges of MEA, computed densities agree well with the experimental ones from literature. This was tested by computing viscosities and the self-diffusivity of pure MEA and 30 wt% MEA/water solutions and comparing these with experiments. We showed that the scaling factor of 0.80 also works well for predicting transport properties of MEA/water solutions. Finally, we computed self-diffusivities of infinitely diluted CO2 and H2S for temperatures ranging from 293-353 K and MEA concentrations of 10-50 wt%. Our results show that the self-diffusivity of both acid gases depends significantly on the temperature and MEA concentration in the solution. The results of this study will contribute to the development of more efficient acid gas treatment processes.

The chemical absorption of CO2 and H2S in aqueous tertiary amines is a well-known acid-base reaction. Kinetic and vapor-liquid equilibrium experiments show that the addition of an amide such as HMPA, which is known to be a strong liquid structure maker, significantly inhibits the acid-base reactions. The impact is more pronounced for CO2 than for H2S absorption. Despite the presence of water in the solvent, the absorption becomes almost physical. Due to hydrogen bonding and the hydrophobic effect, each amide molecule is involved in a cluster containing several water molecules, thus rendering the water molecules less available to participate in the reaction and to solvate HS- and HCO3- ions. This effect is absent when ethylene glycol, a weak structure maker, is added, even in large quantities. This study demonstrates the importance of solvent structure in the study of chemical reactions. State-of-the-art molecular dynamics simulations of the water-HMPA system could not reproduce the strongly negative excess volume of the mixture. This illustrates the need for more accurate force fields to simulate the structuring effect and their impact on chemical reactions.

Abstract The contribution of CO2 present in natural gas to the total GHG emissions of the gas plant can be significant (up to 20% for Middle East Gas). When both H2S and CO2 are present, the CO2 capture cost is currently very high. The method developed to tackle this challenge is an innovative architecture of the acid gas treatment chain based on a new ultra-selective absorption process. The redesigned acid gas treatment architecture is detailed and compared with available technologies on the market. The technical cost of avoided CO2 can be reduced by up to 70% by using the new ultra-selective process compared to the available CO2 capture technology. This new acid gas treatment architecture can reduce by up to 20% the energy consumption to capture CO2. A new ultra-selective solvent has been developed using computational screening tools combined with experiments measuring physical, operational, thermodynamic, and kinetic properties. Those serve as input of a process simulation tool validated with medium-pressure absorber and regenerator pilot plant data. The new solvent is currently at TRL4 validation, a selectivity of more than 75% (CO2 slippage) has been observed on the pilot plant, with a H2S specification in the treated gas kept around 5 ppm. This result is the consequence of both a new solvent formulation (selective amine) and optimal operating conditions. There is, moreover, no increase in regeneration duty for the new solvent.