Emeritus Professor Marcel Maeder

The Henry's Coefficient of CO2 is a fundamental property, it quantifies the equilibrium between the partial pressure of CO2 in the gas phase and the concentration of dissolved free CO2 in the liquid phase. This value is critical for simulation and may lead to improvements in the process of post combustion capture of CO2. However, because of its reactive nature, CO2 reacts in amine solutions and forms multiple carbon-containing species, so that the free CO2 concentration is very small and difficult to determine. The ¿N2O analogy¿ has been used to estimate the physical solubility of CO2 based on the assumption that the two gases behave similarly in water and amine solutions. In the current study, a direct way is proposed for the determination of the Henry's Coefficient of CO2 in MEA solutions. The method only requires vapor-liquid equilibrium (VLE) measurements of the MEA-CO2-H2O system; and all equilibrium/protonation constants for all solution reactions are sourced from the open literature while activity coefficients for all ionic species were estimated using the Davis equation. The free CO2 concentration in solution can be computed using the total MEA concentration, total CO2 concentration 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, total CO2 concentration and temperature. VLE calculations were performed using the liquid-phase equilibrium calculations and the polynomial function calculating the Henry's Coefficient, and the results show the VLE calculations are adequate to describe the VLE data.

A promising option to improve post combustion capture (PCC) is to use inorganic catalysts to accelerate the absorption process of CO2, in particular the reaction between CO2 and water to form carbonic acid. In this study, the efficiency of sulfate and vanadate on enhancing the hydration reaction of CO2 to form H2CO3 for PCC has been studied at 25°C by stopped-flow spectrophotometry. The catalytic rate constants were determined to be 14.2(2) and 277(6)M-1s-1 for sulfate and vanadate, respectively. Their application to PCC was further investigated by simulating the effect of these catalysts on the absorption process under PCC operating conditions. Vanadate was confirmed to have a greater efficiency toward CO2 hydration than sulfate and also a series of other inorganic catalysts reported earlier.

Piperazine (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 0.5M) on the mass transfer of CO2 into 3.0M NH3 solutions over a range of pre-loaded CO2 concentrations of 0.9M 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.

The Post Combustion Capture (PCC) process is currently the most applicable and effective approach to reduce emissions of CO2 from fossil fuel electricity generation. If the rate of CO2 hydration can be accelerated, the size of the absorber column can be reduced accordingly. Carbonic anhydrase type II (CAII), being one of the most active enzymes known for the reaction of CO2 with H2O, is a promising catalyst for CO2 absorption in aqueous solution for PCC application. In this study, the catalytic efficiencies of human (hCAII), wild type bovine (wtbCAII), and a series of wtbCAIIs (bCAM1-bCAM9) structurally modified in an attempt to improve thermal and salt stability were determined by stopped-flow spectrophotometry at 25°C. wtbCAII and hCAII were found to have the highest activity for the hydration of CO2 while bCAM9 was the least efficient catalyst (catalytic rate constants of 1.198(6)×108, 1.14(1)×108 and 0.77(2)×108M-1s-1, respectively). The remaining mutants have similar catalytic efficiencies for CO2 hydration and fall in the range of 8.63(6)×107-1.072(2)×108M-1s-1. In addition to these results, this study also verifies the analytical method developed for the study of enzymatic systems.

Carbon dioxide (CO2) post combustion capture and storage is the most mature technology option for the mitigation of CO2 emissions from fossil fuel electricity generation. Typically CO2 separation at low pressure is achieved by reactive chemical absorption using aqueous amines. In this work benzylamine (BZA) has been assessed in terms of the chemical and physical properties relevant for its application as an aqueous amine CO2 absorbent. BZA was found to have similar reaction kinetics with CO2 to monoethanolamine (MEA) (kcarb=7600M-1s-1 at 35°C and Ea=38kJmol-1) and similar carbamate stability but with a ~40% larger enthalpy of protonation. It was also found to be less corrosive and have lower viscosity and heat capacity. Significant performance gains relative to MEA 30wt% were predicted by using BZA in a formulation with either MEA or 2-amino-2-methyl-1-proponal (AMP) with predicted reductions in reboiler duty up to 13%, improvements in mass transfer up to 20% and low corrosion potential.

Global efforts to reduce carbon dioxide emissions stemming from the combustion of fossil fuels have acknowledged and focused on the implementation of post combustion capture (PCC) technologies utilizing aqueous amine solvents to fulfill this role. The cyclic diamine solvent piperazine has received significant attention for application as a CO2 capture solvent, predominantly for its rapid reactivity with CO2. A thorough investigation of alternative but simpler cyclic amines incorporating a single amine group into the cyclic structure may reveal further insight into the superior kinetic performance of piperazine and the wider applicability of such cyclic solvents for PCC processes. One such example is the cyclic monoamine 3-piperidinemethanol (3-PM). To facilitate the evaluation of 3-PM as a capture solvent requires knowledge of the fundamental chemical parameters describing the kinetic and equilibrium of the reactions occurring in solutions containing CO2 and 3-PM. Additionally, in parallel with the preceding, experimental measurements of CO2 absorption into 3-PM solutions, including mass transfer and vapor-liquid equilibrium measurements, can be used to validate the CO2 absorption performance in 3-PM solutions and compared to that of monoethanolamine (MEA) under similar conditions. The present study is focused in two parts on (a) determination of fundamental kinetic and equilibrium constants via the analysis of stopped-flow kinetic and quantitative equilibrium measurements via 1H/13C nuclear magnetic resonance (NMR) spectroscopy and (b) experimental measurements of CO2 absorption into 3-PM solutions via wetted wall column kinetic measurements, vapor-liquid equilibrium measurements, and corresponding physical property data including densities and viscosities of the amine solutions over a range of concentrations and CO2 loadings. Fundamental kinetic rate constants describing the reaction of CO2 with 3-PM are significantly faster than MEA at similar temperatures (3-PM = 32 × 103 M-1 s-1, extrapolated to 40 °C from kinetic data between 15.0 and 35.0 °C; MEA = 13 × 103 M-1 s-1, 40 °C). Conversely, the equilibrium constants describing the reaction between bicarbonate and amine, often termed carbamate stability constants, are significantly lower for 3-PM than MEA at similar temperatures. Overall CO2 absorption rates in 3.0 M solutions of 3-PM and MEA, assessed in overall CO2 mass transfer coefficients, are lower in the former case over the entire range of CO2 loadings from 0.0 to 0.4 mol of CO2 per mol of amine. The reduced absorption rates in the 3-PM solutions can be attributed to higher solution viscosities and thus corresponding reductions in CO2 diffusion. CO2 absorption and cyclic capacities in 3.0 M solutions of 3-PM and MEA were found to be significantly higher in the case of 3-PM. The larger CO2 capacities are attributed to the lower stability 3-PM carbamate and the formation of larger amounts of bicarbonate compared to MEA. Overall, the larger CO2 absorption capacity, cyclic capacity, and rapid kinetics with CO2 position 3-PM as an attractive CO2 capture solvent.

Amino 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 (¿Hm0) and entropies (¿Sm0) 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. © 2014 American Chemical Society.

The presence of SO2 in flue gas has a negative impact on typical CO2 capture processes utilising aqueous amines. For countries like Australia, that do not have flue gas desulphurisation, retrofitting such equipment to existing coal fired power stations is of the order of 100s of millions of dollars. In this work a new process configuration is described in which SO2 is absorbed into a fraction of the absorbent used for CO2 capture. SO2 absorption is carried out in the bottom of the absorber column into a bleed stream from the bulk solvent, and a recycle flow allows the absorbent to be near saturation in SO2. This high concentration allows a number of low cost options for sulphur removal from the absorbent such as chemical treatment and precipitation, with predicted capital and operating costs in the range of 100s of thousands for a full scale plant. The principles underlying the concept, specifically kinetic selectivity of SO2 absorption over CO2 and reactivity with amines, have been studied in the laboratory. Also a rate based model of an absorber column has been used to model the behaviour of SO2 from the application of the process concept to CSIRO's CO2 capture pilot plant at Loy Yang power station. The laboratory and modelling results support both its feasibility and utility.

Formulated aqueous amine solvents incorporating two or more individual amines into a single blended solvent are often suggested as alternative options to mono-amine solvents employed for CO2 capture processes. Using software developed in Matlab and existing knowledge of chemical equilibria, investigations into the fundamental properties of formulated amine solvents and their interaction with CO2 have been carried out. A simple solvent formulation containing equimolar amounts of monoethanolamine (MEA), Amine 1, and a sterically hindered / tertiary amine, Amine 2, has been investigated here. Specifically, the role of Amine 2 in the overall equilibrium behaviour of amine blends is discussed. Systematic variations of the protonation constants and reaction enthalpies of Amine 2 over a range of typical values has revealed that significant improvement can be made in terms of absorption capacity, cyclic capacity, and overall absorption enthalpy in the blends when compared to the standard 5.0M MEA solvent.

The presence of SO2 in flue gas has a negative impact on typical CO2 capture processes utilising aqueous amines. For countries like Australia, that do not have flue gas desulfurisation, retrofitting such equipment to existing coal fired power stations is of the order of 100's of millions of dollars. In this work a new process configuration is described in which SO2 is absorbed into a fraction of the absorbent used for CO2 capture. SO2 absorption is carried out in the bottom of the absorber column into a bleed stream from the bulk solvent, and a recycle flow allows the absorbent to be near saturation in SO2. This high concentration allows a number of low cost options for sulfur removal from the absorbent such as chemical treatment and precipitation. The principles underlying the concept, have been studied in the laboratory, and a rate based model of an absorber column has been used to model the SO2 removal column section. The laboratory and modelling results support both the feasibility and utility of the process concept.