- Title
- Equilibria and kinetics of the interactions of CO₂ with aqueous ammonia and other amine solutions
- Creator
- Li, Lichun
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2018
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The development of cost-effective carbon dioxide (CO2) capture strategies to achieve significant reduction in atmospheric CO2 levels is one of the main global challenges for the near future. Post combustion capture (PCC) of CO2 offers a short-term option to help the reduction of CO2 emissions from fossil fuel power plants. Employing aqueous ammonia (NH3) based solutions has been recognised as a promising option because of its key advantages for PCC, compared to the current benchmark solvent - monoethanolamine (MEA). Advantages include higher CO2 absorption removal capacity, lower regeneration energy, as well as the chemical stability of NH3 in the presence of SO2, NO2 and oxygen in the flue gas. There are, however, two major challenges limiting the large scale deployment of aqueous NH3-based PCC technology; the low CO2 absorption rate and the high solvent loss via evaporation. Therefore, this research is focused on these areas to further advance and accelerate the commercialization of aqueous NH3-based PCC processes. This thesis focuses on improving the physio-chemical properties of aqueous ammonia based solvents through the addition of additives to reduce ammonia loss via evaporation and/or to increase the CO2 absorption rate. In addition, this work lead to the development of a direct way to determine the Henry Coefficient of CO2 in aqueous amine solutions and this was validated in the MEA-CO2-H2O system. 1. Attempts to suppress ammonia loss by additional of metal ions (M(II) = Zn(II), Ni(II) Cu(II)) was experimentally measured and theoretically simulated with a comprehensive model of the M(II)-NH3-CO2-H2O system. The theoretical and experimental investigations both suggest that complexation of ammonia by metal ions effectively reduces the ammonia loss via evaporation. However, because of the reduction of concentration of free ammonia in solution, the reactivity towards CO2 also decreased. Simple reduction of the total NH3 concentration to the same concentration of free NH3 concentration as that obtained by the addition of M(II) showed a similar reduction in ammonia loss and CO2 absorption rate. 2. Piperazine (PZ) has been reported as an effective rate promoter in the aqueous ammoniabased solvent process for the post combustion capture (PCC) of CO2. The wetted-wall column technique was employed to examine the mass transfer coefficient of PZ-promoted aqueous ammonia solutions. Results showed that PZ is an effective promoter for both pure and CO2-loaded aqueous ammonia solutions. Stopped-flow spectrophotometry was used to further investigate the promotion mechanism of PZ on CO2 absorption into aqueous ammonia solutions. A comprehensive reaction mechanism for the PZ-NH3-CO2-H2O system revealed that the simple combination of all reactions involved in the individual PZ-CO2-H2O and NH3-CO2-H2O systems can quantitatively explain the reaction mechanism between CO2 and blended PZ/NH3 solutions. As suggested from the modelled results, the reactive piperazine mono-carbamate species (PZCO2/PZCO2H), in high CO2-loaded blended PZ/NH3 solutions, acts as the dominant rate promoter, enhancing CO2 absorption into aqueous ammonia solutions. 3. It was noted from previous publications, that the family of cyclic amine absorbents i.e. piperazine (PZ), 3- and 4-piperidinealkanols (3-PM, 4-PM), had the properties of fast kinetics and high absorption capacity. As such they were recognised to be an attractive family of absorbents for CO2 absorption on their own and also blended solvents with aqueous ammonia solutions. Herein we investigate and report for the first time the complete temperature dependent kinetic and equilibrium behaviour of a new heterocyclic amine 4- aminomethyltetrahydropyran (4-AMTHP), with CO2, in aqueous solutions at 25, 35 and 45 oC. Furthermore, a comprehensive model describing all reactions in the 4-AMTHP-NH3-CO2-H2O system was also developed to rationalise the experimental kinetics results. The combination of kinetics and enthalpy properties of 4-AMTHP showed this amine to be a promising candidate for CO2 absorbance on its own and as a solvent blend with aqueous ammonia. 4. The Henry's Coefficient of CO2 is a fundamental property and crucial for the reliable simulation of the absorption and desorption of CO2 in amine/ammonia solutions related to post combustion capture (PCC). CO2 is a reactive gas. When mixing with aqueous amine solutions, several species are formed including the carbamic acid/carbamate species, carbonate, bicarbonate, carbonic acid, and CO2(aq). In this chapter, a direct way to determine Henry's Coefficient of CO2 in amine solutions was proposed as a superior alternative to the well-known "N2O analogy" method. The method requires published vapour liquid equilibrium (VLE) measurements of the amine-CO2-H2O system and MEA was chosen as it is the most extensively studied amine and there are abundantly published results on the VLE data of the MEA-CO2-H2O system. The free CO2 concentration in solution can be computed using the total MEA concentration, loading and temperature, which allows the determination of the Henry’s Coefficient from the CO2 partial pressure. A 10-parameter polynomial is used to approximate the Henry's Coefficient as a function of the total MEA concentration, loading and temperature.
- Subject
- carbon dioxide; capture strategies; atmospheric CO₂; post combustion capture
- Identifier
- http://hdl.handle.net/1959.13/1384925
- Identifier
- uon:32142
- Rights
- Copyright 2018 Lichun Li
- Language
- eng
- Full Text
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