Abstract
CO2 recovery, applied in processes such as the CO2 separation from natural gas or the CO2/N2 separation from power plant flue gas streams, is of great importance for technical, economic, and environmental reasons. Among the available CO2 capture technologies, chemical absorption using aqueous amine solutions is considered the most viable and mature technology, due to its extensive use in the gas processing industry. However, this method presents limitations, such as significant solvent losses, due to amine volatility and degradation, and high energy demands for solvent regeneration. Various solvent systems were investigated to overcome these drawbacks, such as novel amines, amine mixtures and amine blends with ionic liquids or deep eutectic solvents. Another promising alternative method for CO2 recovery is using polymeric membranes due to inherent merits over the traditional chemical absorption, such as low energy demands, easy maintenance, and portability. However, their widespread us ...
CO2 recovery, applied in processes such as the CO2 separation from natural gas or the CO2/N2 separation from power plant flue gas streams, is of great importance for technical, economic, and environmental reasons. Among the available CO2 capture technologies, chemical absorption using aqueous amine solutions is considered the most viable and mature technology, due to its extensive use in the gas processing industry. However, this method presents limitations, such as significant solvent losses, due to amine volatility and degradation, and high energy demands for solvent regeneration. Various solvent systems were investigated to overcome these drawbacks, such as novel amines, amine mixtures and amine blends with ionic liquids or deep eutectic solvents. Another promising alternative method for CO2 recovery is using polymeric membranes due to inherent merits over the traditional chemical absorption, such as low energy demands, easy maintenance, and portability. However, their widespread use is limited, mainly due to low separation efficiency and poor stability. Another major drawback is the plasticization effect of such membranes in the presence of CO2, as well as other highly plasticizing components, such as hexane and toluene, included in some gas streams. The introduction of a liquid in a supporting polymeric membrane is a promising approach to improve the efficiency of such membranes. The aim of the present thesis was the experimental and theoretical investigation of liquid solvents and polymeric membranes for efficient CO2 recovery from industrial gas streams. In this context, four approaches were used, three relevant to CO2 chemical absorption in liquid solvent systems and one relevant to membrane separation. Twelve systems were investigated in total. This PhD thesis is divided into two major parts, one for the introduction to the topic and to the used experimental methods (Chapters 1-3) and the experimental one (Chapters 4-7). In Chapter 1 a short introduction in used solvents, presenting physical and chemical CO2 absorption, and polymeric membranes used for the CO2 separation is presented. Next, the experimental methods used for the materials’ characterization and a review of the methods utilized for the measurement of CO2 solubility in liquid solvents and in polymer membranes are presented (Chapter 2). Chapter 3 describes the experimental apparatus, which is based on the pressure decay method, used in this study for the experimental measurement of the CO2 solubility in liquid solvents is briefly described and the theoretical background for thermodynamic modelling is presented. Regarding the experimental part, the CO2 solubility in pure amine aqueous solutions was experimentally measured and a rigorous thermodynamic model was applied for the theoretical description (Chapters 4). These results were used to predict the CO2 absorption in blended amine solutions (Chapters 4). The model developed for the amine mixtures was used to describe the solubility of more complex systems, such as aqueous ionic liquid (IL) + amine mixtures or deep eutectic solvents (DES) (Chapters 5). Choline Glycine (ChGly), a known non-toxic and biodegradable ionic liquid, was the connection item between two experimental parts of the study, i.e., chemical absorption in liquid solvents and membrane separation. ChGly was synthesized via a novel procedure, using choline chloride as precursor, for the first time. It was used for the preparation of ionic liquid (IL) + amine blends (Chapter 5) and as an additive in cellulose acetate membranes (Chapter 6). Finally, in Chapter 7 the results of this study are presented and simultaneously the perspectives for future work are formulated. In the next paragraphs, the major aspects of the investigation are presented. Initially, in this study, the solubility of CO2 in aqueous solutions of 2-amino-2-methyl-1-propanol (AMP) and 3-methylaminopropylamine (MAPA), which were recently suggested as constituents of novel phase change solvent mixtures, was experimentally measured at 298, 313, 323 and 333 K and in a wide range of pressures, up to approximately 700 kPa (Chapter 4). Since the available literature experimental data for MAPA aqueous solutions are very limited, the experimental results of this study were compared to respective literature data only for AMP aqueous systems and a rather satisfactory agreement was observed. The new experimental data were correlated with the Cubic plus Association (CPA) equation of state and the modified Kent-Eisenberg models. It was observed that both models rather satisfactorily correlate with the experimental data, with the Kent-Eisenberg model presenting more accurate correlations. Subsequently, new experimental data were obtained for the CO2 solubility in aqueous solutions of amine blends containing N-methyldiethanolamine (MDEA), AMP and 3-amino-1-propanol (MPA) at 298, 313, 323 and 333 K and pressures up to approximately 800 kPa (Chapter 4). The modified Kent-Eisenberg model, parameterized using experimental data solely for single amine solutions, was subsequently applied to predict the CO2 absorption in blended amine solutions. Satisfactory model predictions were observed. The average absolute deviations from the experimental data of this study range between 2 and 5%. Furthermore, the effect of amine content on the CO2 absorption ability of ionic liquid-Amine blends was investigated (Chapter 5). Two ionic liquids (IL) were used, i.e. the 1-butyl-3-methylimidazolium hydrogen sulfate, [Bmim][HSO4], and ChGly. New experimental data were presented for two [Bmim][HSO4] + AMP aqueous systems, one [Bmim][HSO4] + MAPA and two ChGly + AMP aqueous systems, which were modelled with the modified Kent-Eisenberg model. In the case of [Bmim][HSO4] based blends, it was shown that, starting from an aqueous amine solution, the replacement of a small part of the amine by [Bmim][HSO4] significantly reduces the CO2 solubility, expressed in moles of the absorbed CO2 per mole of amine, and the CO2 solubility in the bulk, expressed as moles of CO2 per kg of solvent. On the contrary, in the case of ChGly based blends, the replacement of a small part of the amine by ChGly increases the CO2 solubility, expressed in moles of the absorbed CO2 per mole of amine. Furthermore, it was shown that, if a small part of water is replaced by ChGly, the CO2 solubility in the bulk, expressed as moles of CO2 per kg of solvent, is not significantly altered. Such replacement is expected to reduce the vapor pressure of the solvent and, since ChGly is non-toxic, the new solvent is not expected to be more environmentally hazardous. Two main factors were taken into consideration for explaining such observations: the effect of the IL salt on the basicity of the solution and the new sites for interactions, physical or/and chemical, with CO2 that are introduced upon the addition of the salt. The modified Kent-Eisenberg model satisfactorily correlates the experimental data showing deviations that range between 2.0 – 11.6 % in all cases. The model predictions for the speciation in the loaded solutions reveal that the unreacted amine content is very low at CO2 partial pressures of 1 kPa and that the increase of CO2 solubility at higher partial pressures is attributed to the hydrolysis of the carbamate and the molecular CO2 dissolution. Moreover, the effect of water and choline chloride (ChCl) on the CO2 absorption ability of amine based deep eutectic solvents was investigated (Chapter 5). New experimental data are presented for four ChCl-MPA aqueous solutions of various water contents and for one aqueous ChCl-MAPA solution, which, subsequently, were modelled with the modified Kent-Eisenberg model. It was shown that, starting from an aqueous amine solution, the replacement of a small part of MPA by ChCl slightly increases the CO2 solubility, while the replacement of a small part of MAPA does not significantly influence the CO2 solubility, expressed in moles of the absorbed CO2 per mole of amine. However, in both cases, the overall absorption ability of the resulting ChCl-amine blends, in terms of moles of CO2 per kg of solvent, is decreased. Furthermore, it was shown that if a small part of water is replaced by choline chloride, the CO2 solubility in the bulk, expressed in moles of CO2 per kg of solvent, is not significantly altered. Such replacement is expected to reduce the vapor pressure of the solvent and, since choline chloride is non-toxic, the new solvent is not expected to be more environmentally hazardous. Two main factors were taken into consideration when explaining the results: the effect of the choline chloride salt on the basicity of the solution and the new sites for physical interactions with CO2 that are introduced upon the addition of the salt. Furthermore, the addition of water favours the chemical absorption, as the experimental results show increased CO2 solubility, in terms of moles of the absorbed CO2 per mole of amine. However, the increased chemical absorption is not enough to compensate for the rather low CO2 solubility in water and, consequently, the overall absorption ability of the aqueous DES solution, expressed as moles of CO2 per kg of solvent, decreases. The modified Kent-Eisenberg model satisfactorily correlates the experimental data showing deviations that range between 0.4 – 6.6% in all cases. The model predictions for the speciation of the loaded solutions reveal that the unreacted amine content is very low at CO2 partial pressures of 1 kPa and that the increase of CO2 solubility at higher partial pressures is attributed to the hydrolysis of the carbamate and the molecular CO2 dissolution. Finally, two ionic liquids (IL) were used for the preparation of cellulose acetate (CA)-IL composite membranes for potential CO2 capture applications, ChGly and [Bmim][HSO4], since they present adequate CO2 dissolution ability (Chapter 6). The first IL is commercially available, whereas the latter, ChGly, was synthesized via an innovative process (Chapter 5), as mentioned above. Several composite membranes were prepared through the solvent casting technique and characterized by a variety of methods including, thermogravimetry, calorimetry, FTIR spectroscopy and X-ray diffraction. The CO2 sorption in the composite membranes was experimentally measured using the Mass-Loss-Analysis (MLA) technique. The characterization results showed that ILs strongly interact with the C=O groups of CA, which exhibit high affinity with CO2. Upon the addition of an IL in the polymer, there is an interplay of favorable CA-IL interactions acting competitively to the favorable CO2-CA and CO2-IL intermolecular interactions. Such behavior results in the appearance of extrema, if the CO2 sorption is plotted against the IL content, i.e., a maximum appears for the ChGly containing membranes and a minimum for the [Bmim][HSO4], containing membranes. In all cases, ChGly membranes present higher CO2 solubility than neat CA and [Bmim][HSO4] containing membranes. Besides these aspects, ChGly exhibits additional advantages over the other IL, such as non-toxicity, biodegradability and low cost of the precursor chemicals. Thus, it seems that the combination of ChGly with the eco-friendly and low-cost CA is a promising approach for effective and sustainable CO2 capture applications.
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