Thermodynamics is important for the design of amine scrubbing CO₂ capture processes. CO₂ solubility and amine volatility in aqueous amines were measured at high temperature and pressure. A rigorous thermodynamic model was developed for MEA-CO₂-H₂O in Aspen Plus®. CO₂ solubility at 80-190°C was obtained from total pressure measurements. Empirical models as a function of temperature and loading were developed for CO₂ solubility from 40 to 160°C in aqueous monoethanolamine (MEA), piperazine (PZ), 1-methylpiperazine (1MPZ), 2-methylpiperazine (2MPZ), PZ/2MPZ, diglycolamine® (DGA®), PZ/1MPZ/1,4-dimethylpiperazine (1,4-DMPZ), and PZ/methyldiethanolamine (MDEA). The high temperature CO₂ solubility data for MEA is comparable to literature and compatible with previous low temperature data. For MEA and PZ, amine concentration does not have obvious effects on the CO₂ solubility. The heat of CO₂ absorption derived from these models varies from 66 kJ/mol for 4 m (molal) PZ/4 m 2MPZ and to 72, 72, and 73 kJ/mol for MEA, 7 m MDEA/2 m PZ, and DGA. The heat of absorption estimated from the total pressure data does not vary significantly with temperature. At 0-0.5 loading ([alpha]), 313-413 K, 3.5-11 m MEA (mol fraction x is 0.059-0.165), the empirical model of MEA volatility is ln(PMEA/xMEA) = 30.0-8153/T-2594[alpha]²/T. In 7 m MEA with 0.2 and 0.5 loading, PMEA is 920 and 230 Pa at 120°C. At 0.3-0.5 loading, the enthalpy of MEA vaporization, -[Delta]Hvap,MEA, is about 70-73 kJ/mol MEA. At 0.25-0.4 loading, 313-423 K, 4.7-11.3 m PZ (x is 0.078-0.169), the empirical model of PZ volatility is ln(PPZ/xPZ) = -123+21.6lnT+20.2[alpha]-18174[alpha]²/T. In 8 m PZ with 0.3 and 0.4 loading, PPZ is 400 and 120 Pa at 120°C, and 2620 and 980 Pa at 150°C. At 0.25-0.4 loading, -[Delta]Hvap,PZ is about 85-100 kJ/mol PZ at 150°C and 66-80 kJ/mol PZ at 40°C. [Delta]Hvap,PZ has a larger dependence on CO₂ loading than [Delta]Hvap,MEA in rich solution because of the more complex speciation/reactions in PZ at rich loading. Specific heat capacity of 8 m PZ is 3.43-3.81 J/(g•K) at 70-150°C. Two new thermodynamic models of MEA-CO₂-H₂O were developed in Aspen Plus® starting with the Hilliard (2008) MEA model. One (Model B) includes a new species MEACOOH and it gets a better prediction than the other (Model A) for CO₂ solubility, MEA volatility, heat of absorption, and other thermodynamic results. The Model B prediction matches the experimental pKa of MEACOOH, and the measured concentration of MEACOO-/MEACOOH by NMR. In the prediction the concentration of MEACOOH is 0.1-3% in 7 m MEA at high temperature or high loading, where the heat of formation of MEACOOH has effects on PCO₂ and CO₂ heat of absorption. Model B solved the problems of Model A by adding MEACOOH and matched the experimental data of pKa and speciation, therefore MEACOOH may be considered an important species at high temperature or high loading. Although mostly developed from 7 m MEA data, Model B also gives a good profile for 11 m (40 wt%) MEA. / text

To screen amine solvents for application in CO2 capture from coal-fired power plants, the equilibrium CO2 partial pressure and liquid film mass transfer coefficient were characterized for CO2-loaded and highly concentrated aqueous amines at 40 – 100 °C over a range of CO2 loading with a Wetted Wall Column (WWC). The acyclic amines tested were ethylenediamine, 1,2-diaminopropane, diglycolamine®, methyldiethanolamine (MDEA)/Piperazine (PZ), 3-(methylamino)propylamine, 2-amino-2-methyl-1-propanol and 2-amino-2-methyl-1-propanol/PZ. The cyclic amines tested were piperazine derivatives including proline, 2-piperidineethanol, N-(2-hydroxyethyl)piperazine, 1-(2-aminoethyl)piperazine, N-methylpiperazine (NMPZ), 2-methylpiperazine (2MPZ), 2,5-trans-dimethylpiperazine, 2MPZ/PZ, and PZ/NMPZ/1,4-dimethylpiperazine (1,4-DMPZ). The cyclic CO2 capacity and heat of CO2 absorption were estimated with a semi-empirical vapor-liquid-equilibrium model. 5 m MDEA/5 m PZ, 8 m 2MPZ, 4 m 2MPZ/4 m PZ and 3.75 m PZ/3.75 m NMPZ/0.5 m 1,4-DMPZ were identified as promising solvent candidates for their large CO2 capacity, fast mass transfer rate and moderately high heat of absorption. The speciation in 8 m 2MPZ and 4 m 2MPZ / 4 m PZ at 40 °C at varied CO2 loading was investigated using quantitative 1H and 13C nuclear magnetic resonance (NMR) spectroscopy. In 8 m 2MPZ at 40 °C over the CO2 loading range of 0 – 0.37 mol CO2/mol alkalinity, more than 75% of the dissolved CO2 exists in the form of unhindered 2MPZ monocarbamate, and the rest is in the form of bicarbonate and dicarbamate; 19% - 56% of 2MPZ is converted to 2MPZ carbamate at 0.1 - 0.37 mol CO2/mol alkalinity. A rigorous thermodynamic model was developed for 8 m 2MPZ in the framework of the Electrolyte Nonrandom Two-Liquid (ENRTL) model. At 40 °C, the reaction stoichiometry for 2MPZ and CO2 is around 2 at lean loading but diminishes to 0 at rich loading. Bicarbonate becomes the major product at CO2 loading greater than 0.35 mol/mol alkalinity. The predicted heat of CO2 absorption is 75 kJ/mol at 140 °C and decreases with temperature when CO2 loading is above 0.25. The mass transfer rate data for 8 m 2MPZ was represented with a rate-based WWC model created in Aspen Plus®. The reaction rate was described with termolecular mechanism on an activity basis. With minor CO2 loading adjustment and regression of pre-exponential kinetic constants and diffusion activation energy, a majority of the measured CO2 fluxes in the WWC experiments were fitted by the model within ±20% over 40 – 100 °C and 0.1 – 0.37 mol CO2/mol alkalinity. The diffusion activation energy for 8 m 2MPZ at the rich loading is about 28 kJ/mol. The activity-based reaction rate constant at 40 °C for 2MPZ carbamate formation catalyzed by 2MPZ is 1.94×1010 kmol/m3•s. The calculated liquid film mass transfer coefficients are in close agreement with the experimental values. The liquid film mass transfer rate is dependent on the diffusion coefficients of amine and CO2 to the same extent at lean loading and 40 °C. The sum of the powers for the two diffusivities is approximately equal to 0.5 over the loading range of 0 – 0.4 mol CO2/mol alkalinity. The sum of the powers for the dependence of the liquid film mass transfer coefficient on the carbamate formation rate constants (k2MPZ-2MPZ and k2MPZCOO--2MPZ) approaches 0.5 at very lean loading at low temperature, but it decreases as CO2 loading and temperature is increased. At 100 °C, the physical liquid film mass transfer coefficient is the most important factor that determines the liquid mass transfer rate. The pseudo-first order region shifts to higher range of physical liquid film transfer coefficient as temperature increases. / text

Ce travail s'inscrit dans le cadre du développement de nouveaux procédés de désacidification de gaz naturel à travers l'étude thermodynamique des équilibres entre les gaz acides (CO2 et H2S) et un solvant mixte. Ce solvant résulte de la combinaison d'un solvant chimique (solution aqueuse de diéthanolamine) et d'un solvant physique (méthanol). Couplant équilibres chimique et physique, la modélisation de ces systèmes nécessite l'élaboration de modèles complexes et une large gamme de données d'équilibre liquide-vapeur. La première partie de ce travail est consacrée à l'acquisition de données expérimentales de solubilité des gaz acides dans le solvant mixte. En étendant le champ d'application d'un appareillage fonctionnant en méthode synthétique, nous avons acquis des données originales de solubilité des mélanges CO2 + H2S dans des solvants eau-diéthanolamine-méthanol. Dans la seconde partie, la modélisation simultanée des équilibres chimiques et physiques est réalisée. L'approche hétérogène développée combine le modèle NRTL-Electrolyte pour représenter la phase liquide avec l'équation d'état de Peng-Robinson pour décrire le comportement de la phase vapeur. La procédure d'ajustement des paramètres est graduelle si bien que le système complet eau-diéthanolamine-méthanol-CO2-H2S est décrit de manière prédictive, les paramètres du modèle NRTL-Electrolyte ayant été déterminés sur des systèmes inférieurs. L'effet de solvatation du méthanol est aussi mis en valeur. Nous nous sommes également intéressés au calcul des chaleurs d'absorption et à l'extension de notre modèle à d'autres alcanolamines / This work is related to the development of new processes about gas sweetening with hybrid solvents coupling a chemical one (aqueous solution of diethanolamine) with a physical one (methanol). In the liquid phase, CO2 and H2S react with diethanolamine so that the VLE description of these systems is quite complex and requires experimental data. The first part of this work is dedicated to the experimental determination of acid gases + hybrid solvent solubility data. The experimental apparatus was improved to study H2S solubility and CO2 + H2S mixtures solubility in water-diethanolamine-methanol solvents. In the second part, the simultaneous representation of chemical and phase equilibria was realised. The heterogeneous approach developed here combines the Peng-Robinson equation of state for the vapour phase with the Electrolyte-NRTL model for the liquid phase. Parameters are fitted gradually so that the entire system water-diethanolamine-methanol-CO2-H2S system is extrapolated using Electrolyte-NRTL parameters determined by fitting experimental acid gas partial pressures of lower systems. Methanol effect is also described. An original calculation of heat of absorption was also developed. We extended the use of our approach to study others alkanolamines