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Please use this identifier to cite or link to this item: http://hdl.handle.net/1959.13/935995

Title
An equilibrium study of carbamate formation relevant to post combustion capture of CO₂
Author/Creator
Fernandes, Debra
Institution
University of Newcastle. Faculty of Science & Information Technology, School of Environmental and Life Sciences
Description
Research Doctorate - Doctor of Philosophy (PhD)
Description
The global community is currently facing a significant challenge in the form of climate change. The increasing emissions of greenhouse gases, especially carbon dioxide CO₂ is threatening the constitution of the Earth’s climate. This fosters the need for the removal of CO₂ from coal-fired power plants as it is the largest contributor to global CO₂ emissions. One possible option for mitigating climate change is by CO₂ capture and sequestration (CCS), employing post-combustion capture of CO₂ (PCC). PCC is a mature technology for the capture of CO₂, as it is currently used in industry for gas-sweetening processes. The typical flue gas in power plants consists of about 80% N₂ and about 15% CO₂, with the remainder mainly unused O2. For PCC purposes, separation of the two gases N₂ and CO₂ is important for compression, transportation and storage of CO₂. This can be achieved by reversible chemical absorption using amine-based solvents. Application of chemical absorption technology to power plants is not straight forward and poses several new challenges for chemists and chemical engineers, especially with the high cost associated with the process. From a chemist’s point of view for PCC to be efficient the three main requirements are: 1) a fast reaction rate - this is the rate at which CO₂ interacts with the amine in aqueous solutions. For an ideal process the absorption of CO₂ has to be fast in order to minimise the size of the absorber column, 2) the stoichiometry of the amine-CO₂ interaction has to be 1:1 leading to a high loading capacity, and 3) the regeneration of the amine in the stripper column, the energy requirement of which is related to the protonation/deprotonation of the amine should be as low as possible, leading to a low cost and more efficient capture process. A substantial number of studies on the interactions of amines and CO₂ have been published in the literature. However, most of the studies focus on the empirical functions but lack a mechanistic approach. As a consequence, the mechanism of the amine-CO₂ interaction is not clear. This thesis focuses on the molecular kinetics, the equilibria of carbamate formation in the reaction between amine and CO₂ in aqueous solution, and the protonation constant of the amine. The amines investigated can be classified as primary, sterically-hindered primary, secondary, substituted-cyclic secondary and tertiary amines, with the aim of elucidating the possible effects of their chemical structures, electronic and steric effects, hydrogen bonding and substitution on the reaction rate of CO₂ absorption, carbamate stability and protonation/deprotonation of the amine. As a result of this thesis we developed a complete reaction scheme in homogenous solution for the absorption of CO₂(aq) with H₂O/OH and amine. The reaction scheme is complicated and involves a number of kinetically observable reactions, defined by rate and equilibrium constants and protonation equilibria that are all coupled together. A detailed explanation of the scheme is given in all of the papers and also in the Introduction to this thesis. The rate and equilibrium constants for a number of amines was investigated using stopped-flow spectrophotometry as this technique is capable of monitoring fast reactions occurring at the milliseconds time scale, while ¹H NMR spectroscopy was used to monitor slower reactions. Monoethanolamine (MEA) and ammonia (NH₃) were investigated from 15°C to 45°C, analysis of the rate and equilibrium constants in terms of the Arrhenius, Eyring, and van’t Hoff relationships gave the relevant thermodynamic parameters. For sterically-hindered amines, substituted cyclic amines and piperazine a Brønsted correlation relating the protonation constant of the amines to the carbamic acid formation rate and equilibrium constants at 25°C were established. The resulting values are reported in this thesis (Papers 3, 4, 5, 7, 8 and 9). A separate temperature dependence study of the equilibrium constant for the formation of carbamate and the protonation/deprotonation of the carbamate was undertaken using ¹H NMR spectroscopy. The outcome of the study was the determination of the equilibrium constants and thermodynamic parameters such as enthalpy, entropy and Gibbs free energy of reaction. A ∆Hm°-∆Sm° plot generates a linear correlation for carbamate formation and this relationship helps provide a guide to the selection of an amine(s) solvent for CO₂ capture, in terms of enthalpy considerations. A linear ∆Hm°-∆Sm° plot also occurs for carbamate protonation. All the relevant values are detailed in Papers 1 and 6. The basicity of the amine is a very important characteristic in the absorption/desorption process; hence potentiometric titrations were used in the determination of the protonation constants of amines from 15 °C to 45 °C. The resulting protonation constants, enthalpies, entropies and Gibbs free energies are given in Paper 2. Also trends in ∆Hmo are correlated with systematic changes in composition and structure of the selected series of amines/alkanolamines, while ∆Hm°-∆Sm° plots generated linear correlations for the mono-, di-, and trialkanolamines, the –CH₂OH and –CH₂CH₂OH substituted piperidines, and the alkylamines. These relationships provide a guide to the selection of an amine(s) solvent for CO₂ capture. Wherever possible, a comparison with the literature values for the kinetic, carbamate stability and the amine protonation are given in the papers.
Relation
University of Newcastle Research Higher Degree Thesis
Date
2012
Keyword(s)
post combustion capture;  alkanolamines;  protonation constants;  equlibrium;  thermodynamic;  kinetic;  thesis by publication
Resource Type
thesis
Rights
Copyright 2012 Debra Fernandes
Identifier
http://hdl.handle.net/1959.13/935995
Language
eng
 
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"An equilibrium study of carbamate formation relevant to post combustion capture of CO₂"
 
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