Aqueous Alkanolamine Research Articles

Application of the perturbed chain-SAFT equation of state for modeling CO2 solubility in aqueous monoethanolamine solutions

CO2 removal by treatment of acid gases by aqueous alkanolamines is a very significant operation from industrial and environmental point of view. To attain a comprehensive thermodynamic model of the CO2–MEA–H2O in a wide range of temperature and CO2 partial pressures, Perturbed Chain-Statistical Associating Fluid Theory (PC-SAFT) EOS is applied to predict the absorption of carbon dioxide by MEA (MonoEthanolAmine). In order to find the best association scheme for MEA in PC-SAFT EOS, three pure parameter sets for MEA in the 2B, 3B and 4C association schemes are determined in temperature range 303.15–443.15K. Temperature independent binary interaction parameters have been adjusted in the VLE calculation for three schemes of MEA and two schemes of water. Binary VLE calculations show the 3B scheme for MEA and the 4C scheme for water indicate the best prediction in the MEA–H2O system. Excess enthalpy data for aqueous MEA are predicted by kij, which has been adjusted in VLE calculations. The 3B scheme for MEA and the 4C scheme for water also are used to find CO2 solubility in the ternary system of CO2–MEA–H2O system. Ideal Smith–Missen algorithm has been applied to find the concentration of all species in chemical equilibrium. Results show the 3B association scheme for MEA and the 4C association scheme for water in PC-SAFT EOS have better agreement with binary and ternary experimental data. PC-SAFT EOS is able to anticipate the CO2 solubility in the CO2–MEA–H2O system without any regression in the ternary system. The CO2 solubility in ternary system is compared to e-NRTL as a common thermodynamics model. The average absolute partial pressure deviations for PC-SAFT and e-NRTL are calculated around 36% and 42%, respectively.

Incorporating Pitzer equations in a new thermodynamic model for the prediction of acid gases solubility in aqueous alkanolamine solutions

In gas sweetening, acid gases such as CO2 and/or H2S are usually removed by “chemical” absorption through aqueous amine solutions such as N-Methyldiethanolamine (MDEA) solution. Reliable prediction of equilibrium properties (vapor–liquid equilibrium and species distribution) is needed for a rigorous design of such absorption processes. Information on energy requirements can also be obtained from a reliable vapor–liquid equilibrium thermodynamic model. The currently used methods for correlating/predicting the simultaneous solubility of H2S and CO2 in aqueous MDEA solutions require accurate experimental solubility data of single and mixed gases which, in general, confine their applicability in the experimental region. The purpose of this paper is to develop a new theoretical thermodynamic model based on incorporating thermodynamic relationships that correlates the equilibrium and solubility constants to the Gibbs free energy of reaction, leading to an enhanced predictive capability of the model. In this work the Pitzer model is used to account for activity and specific ion interactive forces. This will allow taking into account the effect of the presence of all cations and anions such as thermally stable salts, dissolved organic species and amine degradation products that are usually encountered in absorption units. The suggested model has been verified through comparison with literature data for CO2 and H2S absorption. The presented model can be a very powerful tool that could be of significant importance in the design of amine absorption processes as well as in simulations of the operating variables for optimization of gas sweetening systems.

Mass transfer performance of CO2 absorption by alkanolamine aqueous solution for biogas purification

The CO2 absorption of an aqueous alkanolamine solution is an important research topic in the field of biogas purification. In this paper, an experimental packed tower was set up to investigate the high-concentration CO2 absorption of an ethanolamine (MEA) solution. A mathematical model of the enhancement factor based on a second-order chemical reaction was proposed. This model was used to calculate and analyse the enhancement factor and the mass transfer performance of high-concentration CO2 absorption by an MEA aqueous solution, respectively. The results show that the assumption of a pseudo-first-order reaction is not suitable for the high-concentration CO2 absorption process and the enhancement factor obtained via mathematical prediction by this model is in agreement with the experimental value. Moreover, the high-concentration CO2 absorption of the MEA aqueous solution is a liquid-controlled process. Furthermore, parameters such as gas phase overall mass transfer coefficient KG, liquid phase mass transfer coefficient kL and enhancement factor E increase with increasing MEA concentration but decrease with increasing CO2 concentration. When the gas flow rate is increased, the above parameters initially increase, then decrease, and finally level-off gradually. The results of this study may provide meaningful insights for the research and application of biogas purification technology.

Comparative study: Absorption enthalpy of carbon dioxide into aqueous diisopropanolamine and monoethanolamine solutions and densities of the carbonated amine solutions

Absorption processes using aqueous alkanolamines are under intense research for new gas purification and carbon capture applications. In this work continuous flow calorimetry (SETARAM C80) and vibrating tube densimetry were used to measure absorption enthalpies of carbon dioxide (CO2) into aqueous amine solutions and the final densities of carbonated amine solutions. We present new data for monoethanolamine (MEA) and diisopropanolamine (DIPA) solutions of 0.1–0.2 mass fraction MEA and 0.1–0.3 mass fraction DIPA. CO2 loadings used in the study were 0–1.0molCO2/mol amine. New experimental data presented herein will facilitate the validation and/or further development of thermodynamic models describing the absorption phenomena.

Simultaneous measurement solubility of carbon dioxide + hydrogen sulfide into aqueous blends of alkanolamines at high pressure

Treatment of the sour natural gas is a major step in natural gas processing so that the acid gases such as H2S and CO2 are removed from natural gas stream. The acid gases are harmful to environment and destroy the production equipment so that their presence in gas stream leads to corrosion and lowering heating value. On the other hand, for reliable and optimum design of separation equipment, primarily sufficient and accurate equilibrium data of the acid gases solubility in the aqueous alkanolamines is required. In this work, the simultaneous solubility of the H2S+CO2 in the alkanolamine mixtures is measured at 343K and total pressure range of 0.1–2.1MPa. The blends are studied as the aqueous mixtures of N-methyldiethanolamine (MDEA)+2-amino-2-methyl-1-propanol (AMP)+Piperazine (Pz) and the aqueous mixtures of diisopropanolamine (DIPA), AMP and Pz. For the acid gas solubility measurements, a high pressure static apparatus is used through a volumetric method. The mass fraction of the total alkanolamine is fixed at 0.45 and the results are presented as the partial pressure of each acid gas against its loading (mole acid gas/total mole amine) and mole fraction. The influence of the AMP and Pz on the aqueous DIPA-based and MDEA-based systems are studied so that it is observed that the absorption of the CO2 in the aqueous alkanolamine enhances through separate blending of the AMP and Pz with the aqueous system of MDEA or DIPA and the absorption of the H2S reduces in both of the aqueous DIPA-based and MDEA-based systems.

Representation of CO2 Absorption in Sterically Hindered Amines

Post-combustion capture technology implemented at carbon-rich power plants offers an alternative for mitigating CO2 emissions. Aqueous alkanolamines such as monoethanolamine and N-methyldiethanolamine are utilized to chemically absorb CO2. However, current laboratory practice for evaluating new absorbents is laborious and time consuming. In this paper, we presented a possible relationship between acid dissociation constant, Ka and the CO2 absorption affinity of sterically hindered amines. We demonstrated that addition of hydroxyl and methyl groups to AMP decreases the absorption affinity of sterically hindered amines towards CO2. This finding adds to our understanding in trying to find a new and better CO2 absorbent.

Possible Pathways for Oxidative Degradation of β-hydroxyl Alkanolamine for Post-combustion CO2 Capture

Solvent degradation due to the presence of oxygen is an important issue for amine-based absorption/stripping process for post-combustion carbon dioxide capture from flue gas streams. There are still some knowledge gaps in the understanding of alkanolamine oxidation so far. To explore possible degradation mechanism of alkanolamines, oxidative degradation of aqueous β-hydroxyl alkanolamine solutions without CO2 loading was investigated in the presence of 250kPa O2 at 120°C. The alkanolamines include monoethanolamine (MEA), 2-amino-2-methyl-1-propanol (AMP), 2-amino-1-propanol (APN), and 2- amino-1-butanol (ABN). The anionic oxidation products were identified by anion chromatography. A possible degradation pathway for the oxidative degradation of the investigated amines is proposed to account for the formation of the carboxylic acid products.

High pressure measurement and thermodynamic modeling the solubility of H2S in the aqueous N-methyldiethanolamine + 2-amino-2-methyl-1-propanol + piperazine systems

Removal of hydrogen sulfide and carbon dioxide from sour gas is important in the natural gas treatment process and capturing of acid gases from flue gases. Moreover, for the design and construction of gas contactor equipment, experimental data of the solubility of H2S and CO2 in aqueous alkanolamines are needed. In this work using a volumetric static method, solubility of H2S in aqueous blends of N-methyldiethanolamine (MDEA), 2-amino-2methyl-1-propanol (AMP) and piperazine (Pz) with different compositions are obtained at 313.15, 328.15 and 343.15K over the pressure range of 1–21bar. The composition of the MDEA is fixed at 25mass % and the composition of the AMP and Pz are changed as (20, 15, 10, 5)mass % and (0, 5, 10, 15)mass %, respectively, so that the sum of mass fractions of all amines (MDEA+AMP+PZ) is kept constant. The experimental data are presented as the partial pressure of H2S against acid gas loading (moles H2S/total moles of amine). Considering the present results at the given conditions, it is observed that in the gas loading region less than one, blending Pz with an aqueous solution of the MDEA+AMP leads to reduction of absorption of the H2S.

Analysis of Laplace–Young equation parameters and their influence on efficient CO2 capture in membrane contactors

Based on wetting-related properties like liquid surface tension, contact angle, membrane breakthrough pressure and chemical stability, this work aims to perform a thorough analysis of these properties on different potential membrane/liquid combinations in order to develop an appropriate way to select the best conditions to elude the unwanted wetting phenomenon in membrane contactors (MC). From new experimental data and a systematic review from literature, several significant results were obtained. First, a new and very simple classification method for the estimation of surface tension of aqueous amine, alcohol or alkanolamine solutions was developed. Second, in addition to polytetrafluoroethylene (PTFE) and polypropylene (PP) (to a lesser extend), laminated PTFE/PP membranes were found to be an interesting alternative to be considered in plate MC because the lamination process leads to higher contact angle values. Finally, a new criterion for long-term performance of gas absorption in MC was proposed. It was estimated that a ratio between the liquid overpressure and the nominal breakthrough pressure less than 1.5% could be a useful criterion to prevent membrane wetting and several actions were suggested to respect it.

Reversible Ionic Liquid Stabilized Carbamic Acids: A Pathway Toward Enhanced CO2 Capture

Capturing CO2 from flue gas streams under near ambient conditions—e.g. coal-fired power plants—has traditionally involved the use of aqueous alkanol amine solutions. Aqueous solvent-based processes are energy intensive, as solvent regeneration can be costly. With growing concern over climate change, alternative CO2 capture technologies that are energy and cost efficient are required. We have established that nonaqueous silylamines can be used to efficiently and reversibly capture and release CO2 via the formation of reversible ionic liquids. We now report their unique, enhanced CO2 uptake at room temperature under 1 atm of CO2, as silylamines exhibit CO2 capture capacities greater than that expected from the conventional stoichiometry of a 2:1 amine to CO2 mole ratio. Experimental evidence is presented supporting the formation of a carbamic acid species in equilibrium with an ionic liquid network of ammonium–carbamate ion pairs to give a 3:2 amine to CO2 mole ratio. This is the first report of the stabilization of carbamic acid by reversible ionic liquids. Stabilization of carbamic acid leads to a significant increase in CO2 capacity (30% on average) over conventional amine solutions for CO2 capture.

Solubility and density of carbon dioxide in different aqueous alkanolamine solutions blended with 1-butyl-3-methylimidazolium acetate ionic liquid at high pressure

Using a static high pressure equilibrium cell, the new set of the experimental data is obtained for solubility of carbon dioxide in the aqueous mixtures of different type of alkanolamines such as N-methyldiethanolamine (MDEA), Diethanolamine (DEA), Diisopropanolamine (DIPA), 2-amino-2-methyl-1-propanol (AMP), and 1-butyl-3-methylimidazolium acetate ionic liquid [bmim] [acetate]. The solubility of CO2 in the aqueous MDEA+[bmim][acetate], DEA+[bmim][acetate], DIPA+[bmim][acetate] and AMP+[bmim][acetate] solutions with 0–10wt% of ionic liquid and 30–40wt% alkanolamine are carried out at 1 to 40bar and 323.15K. Moreover, the temperature dependency of density for these solutions is measured from 293.15 to 343.15K with 10K intervals.The results of the CO2 solubility are represented by partial pressure of CO2 against the loading and mole fraction of CO2. The solubility of the CO2 in all of the aqueous alkanolamine+ionic liquid solutions decreases with increasing weight percent of ionic liquid. Also, the results show that the maximum decrease of CO2 loading belongs to addition of [bmim] [acetate] to the aqueous MDEA solution. Finally, the results of density of the alkanolamine+ionic liquid solutions present that the density enhances with increasing concentration of [bmim][acetate] and reduces linearly with enhancing temperature.

Equilibrium Approach for CO2 and H2S Absorption with Aqueous Solutions of Alkanolamines: Theory and Parameter Estimation

Acid gas removal from gas streams is mainly executed industrially via aqueous alkanolamines absorption, whose modeling often involves ionic nonequilibrium chemical reactions and mass/heat interfacial transfers. This demands high computational efforts and often nonavailable physical parameters. Also, the role of ions, created by weak dissociations, is not well understood, and despite the high pressures, concentrations, and loadings, idealities are often recruited, such as ideal laws for gas and mass action. In this work, these issues are circumvented by prescribing only molecular species incorporated into a chemical theory equilibrium framework using full thermodynamics via cubic equations of state. Nonvolatile molecular Complex species are formed via equilibrium reactions involving acid gas, water, and alkanolamines (AGWA) systems. The approach is calibrated with large sets of AGWA data, partial pressures, and loadings, by searching equilibrium constants via maximum likelihood nonlinear implicit parameter estimation. These constants are used to address ideal gas properties of Complex species, which are necessary to model energy effects in AGWA separations.

Thermal regeneration of alkanolamine solutions in a rotating packed bed

In the present study, a 30wt% monoethanolamine (MEA) aqueous solution that was loaded with CO2 was thermally regenerated in a rotating packed bed (RPB). The effects of rotational speed, liquid flow rate, reboiler temperature, and pressure on the regeneration efficiency and regeneration energy were investigated. The experimental results indicated that the regeneration efficiency of a conventional packed bed could be achieved by an RPB apparatus that was only one-tenth of the volume of the conventional packed bed. Moreover, compared with a conventional packed bed, the RPB apparatus consumed less energy per unit ton of CO2 that was desorbed from an aqueous alkanolamine solution. Virtually the same regeneration efficiencies were found regardless of whether an RPB was present or absent from the regeneration apparatus; however, relative to the process without the RPB, the process with the RPB demonstrated a large advantage of 64% in terms of regeneration energy, indicating the dramatic potential of an RPB for the reduction of energy consumption. A CO2-loaded aqueous solution consisting of a blend of 20wt% diethylenetriamine+10wt% piperazine was also examined in the study. Compared with a 30wt% MEA aqueous solution, this blended solution demonstrated a more effective CO2 capturing ability, higher regeneration efficiency, and lower consumption of regeneration energy, suggesting that the proper choice of absorbent was an extremely important consideration for the capture of CO2.

Comparison of Carbon Dioxide Absorption in Aqueous MEA, DEA, TEA, and AMP Solutions

The separation and capture process of carbon dioxide from power plants is garnering interest as a method to reduce greenhouse gas emissions. In this study, aqueous alkanolamine solutions were studied as absorbents for CO2 capture. The solubility of CO2 in aqueous alkanolamine solutions was investigated with a continuous stirred reactor at 313, 333 and 353 K. Also, the heat of absorption (−ΔHabs) between the absorbent and CO2 molecules was measured with a differential reaction calorimeter (DRC) at 298 K. The solubility and heat of absorption were determined at slightly higher than atmospheric pressure. The enthalpies of CO2 absorption in monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), and 2-amino-2-methyl-1-propanol (AMP) were 88.91, 70.44, 44.72, and 63.95, respectively. This investigation showed that the heat of absorption is directly related to the quantity of heat for absorbent regeneration, and is dependent on amine type and CO2 loading.

Modeling CO2 and H2S solubilities in aqueous alkanolamine solutions via an extension of the Cubic-Two-State equation of state

An extension of the Cubic-Two-State (CTS) equation of state was employed to evaluate the fugacities of ionic species in solution. The equation contains three terms relating to the various intermolecular interactions occurring in electrolyte solutions: a short range non-specific term represented by the Soave–Redlich–Kwong equation of state, a specific association term described by the two-state association model and a Debye–Hückel primitive model term for long range ion–ion interactions. The resulting equation, named eCTS, has six adjustable parameters: three for the non-specific part, two for the association term and one for the ionic term. Then, the eCTS EoS was used to describe both chemical and phase equilibria for systems including acid gases (H2S and CO2), alkanolamines (monoethanolamine, diethanolamine and methyldiethanolamine) and water. Measured saturation pressures, liquid densities and binary and ternary vapor–liquid equilibria data were used to estimate the model's pure and binary parameters. Low deviations from experimental data were observed for pure, binary and ternary systems. Finally, the eCTS was employed to predict the behavior of quaternary VLE with mixtures containing CO2–H2S and alkanolamine blends. Except for the prediction of CO2 partial pressures over aqueous alkanolamine blends, the eCTS gave good predictions of experimental quaternary data over a broad range of temperatures and alkanolamine concentrations.

Computational investigation of carbon dioxide absorption in alkanolamine solutions

We investigated CO2 absorption in aqueous alkanolamine solutions using density functional theory with dielectric continuum solvation models (SMD/IEF-PCM and COSMO-RS). We varied the alkyl chain length (m = 2, 3, 4) and the alcohol chain length (n = 2, 3, 4) in the alkanolamine structures, H(CH2) m NH(CH2) n OH. Using the SMD/IEF-PCM/B3LYP/6-311++G(d,p) and COSMO-RS/BP/TZVP levels of theory, our calculations predict that the product of CO2 absorption (carbamate or bicarbonate) is strongly affected by the alcohol length but does not differ significantly by varying the alkyl chain length. This prediction was confirmed experimentally by (13)C-NMR. The observed sensitivity to the alcohol chain length can be attributed to hydrogen bonding effects. The intramolecular hydrogen bonds of HN · · · HO, NH2 (+) · · · OH, and NCOO(-) · · · HO induce ring structure formation in neutral alkanolamines, protonated alkanolamines, and carbamate anions, respectively. The results from our studies demonstrate that intramolecular hydrogen bonds play a key role in CO2 absorption reactions in aqueous alkanolamine solutions.

Modeling CO2 solubility in Aqueous Methyldiethanolamine Solutions by Perturbed Chain-SAFT Equation of State

CO2 capture by aqueous alkanolamines treating is one of the prevalent methods to reduce carbon dioxide emissions and to help environmental problems. For realizing more the thermodynamics of the CO2–MDEA–H2O, the PC-SAFT equation of state was used to simulate the absorption of carbon dioxide by MDEA (methyldiethanolamine). A correlation for temperature-dependent binary interaction parameter were calculated by excess enthalpy data for aqueous MDEA at low temperatures (lower than 350K), and then this binary interaction parameter used to predict phase equilibria of ternary aqueous mixtures of MDEA with carbon dioxide. Smith–Missen algorithm and PC-SAFT EOS have been used to determine concentration of species in chemical equilibrium and physical equilibrium, respectively. In addition, for determining parameter sets of MDEA, vapor pressure and saturated liquid density data were used and different and probable association schemes were considered in parameter estimations. Results show 4(2:2,0:0) association scheme for MDEA and 4(2:2) association scheme for water have better agreement with binary and ternary VLE experimental data.

Equilibria of MEA, DEA and AMP with Bicarbonate and Carbamate: A Raman Study

Species distribution analysis in carbonated alkanolamine solution is import in order to estimate accurate thermodynamic properties. The applicability of Raman spectroscopy as an analytical tool for speciation of carbonated aqueous alkanolamine systems was studied. Alkanolamine solutions loaded with HCO3– were measured using ClO3− as internal standard. The same solutions were measured with 13C-NMR spectroscopy. The molar scattering intensity factors for HCO3, CO3 and MEA- and DEA-carbamate were found to be 0.1973, 0.2901, 0.0632 and 0.0400 respectively. Characteristic bands of these species were identified at 1017, 1067, and 1162 cm−1. A lower detection limit for HCO3– anions in MEA and DEA solution was observed. Bands for potential quantification of free- and protonated alkanolamine are identified in the region of 2800 - 3800 cm−1.

Densities, Viscosities, and Refractive Indices of Aqueous Alkanolamine Solutions as Potential Carbon Dioxide Removal Reagents

To determine the possibility of improvement of the process of CO2 removal by absorption with alkanolamines, densities, viscosities, and refractive indices of four aqueous alkanolamine solutions (water + monoisopropanolamine (MIPA), diisopropanolamine (DIPA), triisopropanolamine (TIPA), or diethanolamine (DEA)) were measured in the temperature range (298.15 to 343.15) K and at atmospheric pressure. Due to the corrosive effect of increased concentration of alkanolamine solutions, the mass fractions of alkanolamines were selected as w = 0.05, 0.1, 0.15, 0.20, and 0.25. The measured data were regressed with polynomial equations. Obtained results provide necessary data for industrial equipment design and process optimization regarding greenhouse gases (GHG), but also it is a foundation for further investigation of the potential replacement of usually used monoethanolamine with other mixtures of alkanolamine solutions presented within this work.

CO2 Capture in Alkanolamine-RTIL Blends via Carbamate Crystallization: Route to Efficient Regeneration

One of the major drawbacks of aqueous alkanolamine based CO(2) capture processes is the requirement of significantly higher energy of regeneration. This weakness can be overcome by separating the CO(2)-captured product to regenerate the corresponding amine, thus avoiding the consumption of redundant energy. Replacing aqueous phase with more stable and practically nonvolatile imidazolium based room-temperature ionic liquid (RTIL) provided a viable approach for carbamate to crystallize out as supernatant solid. In the present study, regeneration capabilities of solid carbamates have been investigated. Diethanolamine (DEA) carbamate as well as 2-amino-2-methyl-1-propanol (AMP) carbamate were obtained in crystalline form by bubbling CO(2) in alkanolamine-RTIL mixtures. Hydrophobic RTIL, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf(2)N]), was used as aqueous phase substituent. Thermal behavior of the carbamates was observed by differential scanning calorimetry and thermogravimetric analysis, while the possible regeneration mechanism has been proposed through (13)C NMR and FTIR analyses. The results showed that decomposition of DEA-carbamate commenced at lower temperature (∼55 °C), compared to that of AMP-carbamate (∼75 °C); thus promising easy regeneration. The separation of carbamate as solid phase can offer two-way advantage by letting less volume to regenerate as well as by narrowing the gap between CO(2) capture and amine regeneration temperatures.