Browsing by Author "Burns, Robert"
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Item Investigation of Metal Ion Additives on the Suppression of Ammonia Loss and CO2 Absorption Kinetics of Aqueous Ammonia-based CO2 Capture(2017-01-01) Yu, Hai; Clifford, Sarah; Puxty, Graeme; Maeder, Marcel; Burns, Robert; Conway, William; Li, LichunAqueous ammonia is an attractive absorbent for the capture of CO2 from industrial flue gases. However evaporative ammonia loss limits the application of the ammonia-based CO2 capture process. The use of metal ion additives (where M(II) = Cu(II), Zn(II) and Ni(II)), has been reported previously for the effective suppression of ammonia loss in the post combustion capture of CO2. In this study, we have investigated the absorption of CO2 into various M(II)/NH3 solutions, the resulting loss of ammonia to evaporation, and have attempted to rationalise the observed effects using an extended chemical model that includes all chemical reactions in solution. The theoretical and experimental investigations suggest that complexation of ammonia by metal ions effectively reduces the concentration of free ammonia, resulting in reduced evaporative loss but also reduced reactivity towards CO2. 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 the ammonia loss as well as the CO2 absorption rate. The effects on CO2 absorption capacity at 25 and 80 °C, cyclic capacity and regeneration energy are also simulated and discussed.
Item Kinetic and Equilibrium Reactions of a New Heterocyclic Aqueous 4-Aminomethyltetrahydropyran (4-AMTHP) Absorbent for Post Combustion Carbon Dioxide (CO2) Capture Processes(2017-10-02) Conway, William; Yu, Hai; Burns, Robert; Maeder, Marcel; Puxty, Graeme; Clifford, Sarah; Li, LichunAqueous amine absorbent processes remain at the forefront of existing technologies for the removal of CO2 from industrial and large-scale power generation flue gas streams. It is essential that improvements in amine-based absorbent technologies are made in order to reduce both capital and operational costs. Intimate understanding of the fundamental chemical behavior of new amine absorbent systems is an intelligent pathway toward higher efficiency amine-based CO2 capture processes. Herein, we investigate and report for the first time the complete temperature-dependent kinetic and equilibrium behavior of a new heterocyclic amine 4-aminomethyltetrahydropyran (4-AMTHP), with CO2, in aqueous solutions. Stopped-flow spectrophotometry, 1H NMR spectroscopy, and potentiometric titration measurements performed over the temperature range 25.0–45.0 °C and the corresponding rate constants for the reversible formation of the carbamic acid, together with equilibrium constants describing the stability of the carbamate, and the protonation of the amine are reported here. Thermodynamic analysis of the resulting constants using the Eyring, Arrhenius, and van’t Hoff relationships has revealed the activation energies, enthalpies, and entropies for the reactions, allowing a comparison to the industrial standard monoethanolamine (MEA). From the kinetic data, the performance of 4-AMTHP was found to be superior to MEA and in line with the established Brønsted relationship between the second-order rate constant and the protonation constant or basicity of the amine. The largely negative protonation enthalpy (−47 kJ/mol), among the key chemical drivers for CO2 regeneration, is again superior to MEA (−41 kJ/mol). Together, a combination of kinetic and equilibrium properties of 4-AMTHP strongly position 4-AMTHP as a promising candidate for more intensive evaluations as a CO2 capture absorbent.
Item The Effect of Piperazine (PZ) on CO2 Absorption Kinetics into Aqueous Ammonia Solutions at 25.0 °C(2015-05-01) Yu, Hai; Maeder, Marcel; Clifford, Sarah; Burns, Robert; Puxty, Graeme; Conway, William; Li, LichunPiperazine (PZ) has been reported as an effective rate promoter in the aqueous ammonia-based solvent process for the post combustion capture (PCC) of CO2. However, the detailed promotion effect of PZ on CO2 absorption into partially loaded ammonia solutions and the mechanism of this process are still unclear. In an effort to determine the detailed promotion effect of PZ in aqueous ammonia-based solvents, overall mass transfer coefficients (KG) describing the absorption of CO2 into aqueous PZ/NH3 solutions were determined using a wetted-wall column apparatus at 25 °C. The effect of added PZ (from 0 to 0.5 M) on the mass transfer of CO2 into 3.0 M NH3 solutions over a range of pre-loaded CO2 concentrations of 0.9 M at 25 °C are reported in this work. The fast kinetic reactions of CO2(aq) with blended solutions containing PZ/NH3 were investigated using stopped-flow spectrophotometry at 25.0 °C. Analysis of the kinetic measurements using a chemical model which incorporates the complete reaction sets of the individual amines with CO2 (i.e., NH3-CO2-H2O and PZ-CO2-H2O) resulted in good agreement with the experimental data. The contribution distribution from each reactive species was calculated based on the proposed reaction scheme of the PZ-NH3-CO2-H2O system. Results show that both the PZ/PZH+ and PZCO2− /PZCO2H pathways make contributions to the promotion of CO2 absorption into PZ promoted aqueous NH3 solutions. Importantly, the reactive piperazine mono-carbamate species, PZCO2− /PZCO2H, which is present in the CO2-loaded mixtures of PZ/NH3, plays an important role in the promotion of CO2 absorption into CO2-loaded aqueous NH3 solutions. The mass transfer simulation results reveal that there are additional reactions occurring in the gas–liquid interface and gas phase due to the volatility of NH3, which requires further improvement on the simulation model.
Item The Henry Coefficient of CO2 in the MEA-CO2-H2O System(2017-07-01) Yu, Hai; Clifford, Sarah; Puxty, Graeme; Burns, Robert; Maeder, Marcel; Li, LichunThe Henry Coefficient of CO2 is a fundamental property and crucial for the accurate simulation of the absorption and desorption of CO2 in MEA solutions related to post combustion capture (PCC). The free CO2 concentration in amine solutions is very small and is difficult to be measured since it reacts with the solution. The “N2O analogy” is traditionally applied to estimate the physical solubility of CO2 based on the assumption that the two gases behave similarly in amine solutions. We propose a direct way to determine Henry Coefficient of CO2 in MEA solutions as an alternative to N2O analogy. The method only requires vapor liquid equilibrium measurements of the MEA-CO2-H2O system. Based on the total MEA concentration, loading and known equilibrium constants the free CO2 can be computed which allows the determination of the Henry Coefficient. A 6-parameter polynomial is used to approximate the Henry Coefficient as a function of the total MEA concentration and total CO2 concentration at 40 oC.