Ceria (CeO(2)) is a promising catalyst for the reduction of carbon dioxide (CO(2)) to liquid fuels and commodity chemicals, in part because of its high oxygen storage capacity, yet the fundamentals of CO(2) adsorption, activation, and reduction on ceria surfaces remain largely unknown. We use density functional theory, corrected for onsite Coulombic interactions (GGA+U), to explore various adsorption sites and configurations for CO(2) on stoichiometric and reduced ceria (110), the latter with either an in-plane oxygen vacancy or a split oxygen vacancy. We find that CO(2) adsorption on both reduced ceria (110) surfaces is thermodynamically favored over the corresponding adsorption on stoichiometric ceria (110), but the most stable adsorption configuration consists of CO(2) adsorbed parallel to the reduced ceria (110) surface at a split oxygen vacancy. Structural changes in the CO(2) molecule are also observed upon adsorption. At the split vacancy, the molecule bends out of plane to form a unidentate carbonate with the remaining oxygen anion at the surface; this is in stark contrast to the bridged carbonate observed for CO(2) adsorption at the in-plane vacancy. Also, we analyze the pathways for CO(2) conversion to CO on reduced ceria (110). The subtle difference in the energies of activation for the elementary steps suggest that CO(2) dissociation is favored on the split vacancy, while the reverse process of CO oxidation may favor the formation of the in-plane vacancy. We thus show how the structure and properties of the ceria catalyst govern the mechanism of CO(2) activation and reduction.

This study determines the energy benefit of advanced stripper configurations for 8 m PZ, 7 m MDEA/2 m PZ, and 5 m MDEA/5 m PZ. Three novel configurations were tested: (1) interheated stripper, (2) two-stage flash with cold rich bypass, and (3) two-stage flash with cold rich bypass and a low temperature adiabatic flash. Generally, increasing the complexity improved process performance, but in some cases the improvements were too marginal to justify the additional capital cost. Configuration 3 has the added benefit of removing entrained oxygen before feeding the solvent to the high temperature flash vessels, but its energy performance is very sensitive to operating conditions. This paper also describes the creation of a thermodynamic, hydraulic, and kinetic model in Aspen Plus ® that predicts experimental data for 8 m PZ, 7 m MDEA/2 m PZ, and 5 m MDEA/5 m PZ over operationally significant temperature and loading ranges. The next step in this work will include conducting techno-economic studies to quantify the capital and operating cost tradeoffs associated with these novel configurations.

Explanations for the mass-transfer behavior of 2-amino-2-methyl-1-propanol (AMP) are conflicting, despite extensive study of the amine for CO2 capture. At equilibrium, aqueous AMP reacts with CO2 to give bicarbonate in a 1:1 ratio. Although this is the same stoichiometry as a tertiary amine, the reaction rate of AMP is 100 times faster. This work aims to explain the mass-transfer behavior of AMP, specifically the stoichiometry and kinetics. An eNRTL thermodynamic model was used to regress wetted-wall column mass-transfer data with two activity-based reactions: formation of carbamate and formation of bicarbonate. Data spanned 40–100 °C and 0.15–0.60 mol CO2/mol alk. The fitted carbamate rate constant is 3 orders of magnitude greater than the bicarbonate rate constant. Rapid carbamate formation explains the kinetics, while the stoichiometry is explained by the carbamate reverting in the bulk liquid to allow CO2 to form bicarbonate. Understanding the role of carbamate formation and diffusion in hindered amines enables optimization of the solvent amine concentration by balancing viscosity and free amine concentration. This improves absorber design for CO2 capture.

Amine scrubbing is a necessary technology to offset CO 2 emissions from fossil-fuel power plants. Of the many solvents studied, hindered amines are of particular interest for their marriage of the capacity of tertiary amines with rates a hundredfold greater than tertiary amines. The relatively rapid rates of hindered amines have not been adequately explained, despite their extensive use in commercial solvents. This work seeks to explain the rapid rate of mass transfer of 2-piperidineethanol (2PE) and uses this rationale to draw general conclusions on hindered amines.Quantitative 13 C NMR data were collected to determine the equilibrium of carbamate in 30 wt.% 2PE. Using these data along with VLE and pK a data, a rigorous thermodynamic model of 8 molal 2PE was built with electrolyte-NRTL and activity-based kinetics. Wetted-wall column flux data were fit to create the activity-based mass transfer model. Using this comprehensive model, the mass transfer rate was examined through sensitivity studies and Brønsted correlations.This work shows that 2PE forms a more stable carbamate than 2-amino-2-methyl-1-propanol. The carbamate reaction is the most significant component of mass transfer at 40 °C. The Brønsted correlation for carbamate reactions of unhindered amines predicts the rate of carbamate reaction of 2PE, but the Brønsted correlation for bicarbonate underpredicts the regressed rate. The CO 2 solubility is fit with five parameters with an ARD of 0.84%. The kinetics are fit with a carbamate-and a bicarbonateforming reaction with an ARD of 7.03%.The chief conclusions are: 1) that the rapid mass transfer of hindered amines is due to the formation of carbamate and the high pK a of the amine, 2) the carbamate formation rate appears unimpeded by steric hindrance and is predicted by a Brønsted correlation, suggesting that hindered amines react in the same manner as unhindered amines. Keywords: separations; CO 2 capture; eNRTL ; wetted-wall column; hindered amine; carbamate stability Sample preparationAmine solutions were prepared from 2-piperidineethanol (with purity ≥96%) supplied by TCI Europe and were used as received without further purification. Amine solutions were prepared with distilled water, and the resulting solution was 30 wt.% amine. The solutions were loaded with CO 2 (grade 5.0) supplied by AGA Gas (AGA Gas GmbH, Hamburg, Germany). The amine concentration was determined by acid-

No abstract