Selection and Characterization of New Absorbents for Carbon Dioxide Capture

Abstract

Removal of acidic gases, in particular CO2, is an important industrial operation. Carbon dioxide is produced in large quantities by fossil–fuel–fired power plants, steel production, the production of petrochemicals, cement production, and natural gas purification. The global climate change, where CO2 is found to be a major contributor, is one of the most important and challenging environmental issues facing the world community. This has motivated intensive research on CO2 capture and storage.

Carbon dioxide capture by an absorption process is one of the most common industrial technologies today. Recent economic studies (Desideri and Corbelli, 1998) indicate that the process will also remain competitive in the future. One of the key improvements under development is new, faster and more energy–efficient absorbents. A chemical to be used as a commercial absorbent must have high net cyclic capacity, high absorption rate for CO2, and good chemical stability. Alkanolamines are the most commonly used chemical absorbents for the removal of acidic gases today.

In the first part of this thesis, an experimental screening of new absorbents for CO2 capture was performed by absorption of CO2 into both single absorbents and absorbent mixtures for amine–based and non–amine–based systems at 40 °C. From testing of ∼30 systems, it was found that an aqueous 30 mass % AEEA {2-(2-aminoethyl-amino ethanol} solution seems to be a potentially good absorbent for capturing CO2 from atmospheric flue gases. It offers high absorption rate combined with high absorption capacity. In addition to AEEA, MMEA (2-(methylamino)ethanol) also needs to be considered. It could have a good potential when used in contactors where the two phases are separated, like in membrane contactors, whereas indications from the study showed foaming tendencies that will make it difficult to use in ordinary towers. AEEA as the selected absorbent obtained from the screening tests was further investigated to determine its vapor–liquid equilibrium characteristics. The experimental and modeling study of the solubility of CO2 in aqueous AEEA is described in the second part of the thesis. From the VLE data, it is shown that AEEA does not only offer high absorption rate combined with high absorption capacity in terms of CO2 loading but also offers higher cyclic capacity and lower regeneration energy requirement for some cases studied compared to MEA. In addition, a VLE thermodynamic modeling of the aqueous

AEEA solution was performed by use of a modified Deshmukh–Mather model (Deshmukh and Mather, 1981) as well as NMR analyses to determine the species distribution in the liquid phase as function of CO2 loading. A two–stage calculation was performed to model the VLE of the CO2–AEEA–H2O system. The first stage of the calculation was the regression of the parameters involved in the temperature dependency of the chemical equilibrium constants without binary interaction parameters taken into account. As seen from the results, the model provides a very good representation of the experimental data over a range of temperatures from 40 to 120 °C. The second regression of the VLE data was then performed to evaluate the binary interaction parameters i.e. the short–range terms in the Deshmukh–Mather model. However, only minor improvements in the overall fit were achieved.

In the last part of the thesis, an experimental kinetic study of the CO2–AEEA–H2O system was performed using a string of disc contactor over a range of temperatures from 32 to 49 °C for various concentrations of AEEA. The reaction mechanism used for interpretation of the kinetics was the single step and termolecular mechanism approach proposed by Crooks and Donnellan (1989) and reviewed by da Silva and Svendsen (2004). The results showed that the observed pseudo–first order rate constant is in good agreement with the equation proposed for this mechanism. In addition, the physical properties, density and viscosity, have been measured to determine the physico–chemical parameters. The solubility of N2O in AEEA was also measured to estimate the solubility of CO2 in AEEA solution.