Browsing by Author "Maeder, Marcel"
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Item A New Approach to the Equilibrium Study of Iron(III) Thiocyanates Which Accounts for the Kinetic Instability of the Complexes Particularly Observable Under High Thiocyanate Concentrations(2016-04-24) Clifford, Sarah; Maeder, Marcel; de Berg, Kevin C.The kinetic instability of iron(III) thiocyanate complexes in aqueous solution, evidenced by the loss of solution colour with time, has made it difficult to determine reliable equilibrium data. A technique, which measures the absorbance of these solutions as a function of time within milliseconds of mixing Fe3+ with SCN- and which extrapolates to an initial spectrum at time zero, has been designed to overcome these difficulties and is described in this paper. Two complexes, Fe(SCN)2+ and Fe(SCN)2+, have been identified upon analysing the absorbance data for thiocyanate concentrations up to 0.25 M at 25oC and an ionic strength of 0.5 M. Equilibrium constants, K1 = and K2 = have been determined and comparisons made with the literature.
Item Development and Evaluation of a Novel Method for Determining Absorbent Composition in Aqueous Ammonia-Based CO2 and SO32– and SO42– Loaded Capture Process Solutions via FT-IR Spectroscopy(2018-08-18) Puxty, Graeme; Maeder, Marcel; Clifford, Sarah; Yu, Hai; Conway, William; Bennett, Robert; Li, LichunCO2 capture using aqueous ammonia is a potentially attractive option for emissions reductions from energy production and industrial processes. From an operational perspective, the capture absorbent must be monitored continuously to maintain the maximum efficiency of the capture process. In practice the composition of the absorbent is typically evaluated offline and retrospectively via wet chemistry methods, delaying any necessary variations to the process conditions to maintain maximum efficiency. Online absorbent monitoring methods incorporating spectroscopy via Raman or Fourier transform infrared (FT-IR) are attractive options due to their rapid response times and flexibility of the resulting output to be incorporated directly into process control packages. The present study outlines an evaluation of the FT-IR spectroscopic technique with analysis via partial least squares regression (PLSR) for a range of dilute to concentrated aqueous ammonia absorbents from ∼0.3–6.0 M and over a range of CO2 loadings from ∼0.0–0.6 mol CO2/mol NH3. The water concentration in the samples ranges from ∼35.2–55.2 M. The effect of interfering SOx species on the FT-IR method has been evaluated by incorporating dissolved SO32– and SO42– components into the solutions from 0.0–1.5 M. The analysis results in accurate concentrations for all analytes. The robustness of the analysis results has been evaluated and discussed. Additionally, FT-IR spectroscopy with PLSR was compared with conventional titration methods for a selected series of mixed NH3/CO2 standard solutions and a series of liquid samples from a bench-scale CO2 absorption process. At low concentrations where the total NH3 concentration is less than 4.0 M and the total CO2 concentration is less than 1.5 M, both the combined PLSR with FT-IR method and the conventional potentiometric titration methods were suitable for the evaluation of the liquid compositions. However, at concentrations out of the low concentration range, the combined PLSR and FT-IR method was proven to have a robustness and accuracy greater than those of the conventional potentiometric titration methods. Therefore, given the simplicity and rapid turnaround of FT-IR spectroscopy in combination with PLSR, we consider this to be a superior and flexible technique for monitoring of CO2 loaded aqueous ammonia solutions.
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 Protonation Constants and Thermodynamic Properties of Amino Acid Salts for CO2 Capture at High Temperatures(2014-07-11) Feron, Paul; Norman, Sarah; Maeder, Marcel; Yu, Hai; Puxty, Graeme; Wei, Chiao-Chien; Xu, Dong-Yao; Yang, NanAmino acid salts have greater potential for CO2 capture at high temperatures than typical amine-based absorbents because of their low volatility, high absorption rate, and high oxidative stability. The protonation constant (pKa) of an amino acid salt is crucial for CO2 capture, as it decreases with increasing absorption temperature. However, published pKa values of amino acid salts have usually been determined at ambient temperatures. In this study, the pKa values of 11 amino acid salts were determined in the temperature range of 298–353 K using a potentiometric titration method. The standard-state molar enthalpies (ΔHm0) and entropies (ΔSm0) of the protonation reactions were also determined by the van’t Hoff equation. It was found that sarcosine can maintain a higher pKa than the other amino acids studied at high temperatures. We also found that the CO2 solubilities and overall mass-transfer coefficients of 5 m′ sarcosinate (moles of sarcosine per kilogram of solution) at 333–353 K are higher than those of 30% MEA at 313–353 K. These results show that some possible benefits can be produced from the use of sarcosine as a fast solvent for CO2 absorption at high temperatures. However, the pronotation reaction of sarcosine is the least exothermic among those of all amino acids studied. This could lead to a high regeneration energy consumption in the sarcosinate-based CO2 capture process
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.
Item The Thermodynamic Formation Constants for Iron(III) Thiocyanate Complexes at Zero Ionic Strength(2017-09-01) Clifford, Sarah; Maeder, Marcel; de Berg, Kevin C.The successive thermodynamically relevant standard formation constants for Fe(SCN)2+ and Fe(SCN)2+ have been determined at 250C using absorbance spectra at time zero to reduce the impact of kinetic instability. Average ionic strengths of 0.0581M, 0.0955 M, 0.1756 M, 0.2506 M, 0.5004 M, and 1.0011 M were used in the study to enable extrapolation to zero ionic strength. Extrapolation to zero ionic strength was supported by the application of the Specific Ion Interaction theory (SIT) developed by Bronsted, Guggenheim and Scatchard. Application to the iron(III) thiocyanate system led to a log value of (2.85 0.08) and a log value of (1.51 0.13). The change in the empirical specific ion interaction coefficients associated with , , is (-0.29), and that associated with , , is (-0.18). The log value is lower than those published in the literature due to the fact that, for this study, the equilibrium data were based on initial spectra captured before kinetic instability significantly interfered.