Toward Intelligent CO₂ Capture Solvent Design through Experimental Solvent Development and Amine Synthesis

In order to improve toward efficient large scale CO₂ capture applications, the largest uncertainties with postcombustion carbon dioxide capture (PCC) still surround the chemical reactivity and reaction rate of the solvent, the large parasitic energy penalty introduced during the regeneration of CO₂ from the solvents, and the stability of the amine solvent to resist degradation in the presence of trace impurities present in the flue gas. Heterocyclic amines are a class of molecules that have inherently superior kinetic reactivity with CO₂ but, importantly, have demonstrated desirable energy performance and degradation resistance. The current work is focused on further understanding of the chemical behavior of diamine and triamine solvents during CO₂ absorption and desorption from laboratory scale measurements. In this study we have proposed and prepared a series of cyclic diamine and triamine derivatives which can potentially offer reductions in solvent related costs associated with the PCC process. Thirty amines were synthesized and their CO₂ absorption and cyclic capacities determined between 40 and 90 °C using a small reactor with analysis of the solutions performed using quantitative ¹³C and ¹H NMR spectroscopy. Cyclic capacity results indicate the majority of the amines are capable of increases in CO₂ uptake and cycle (when expressed as molar or mass ratios) compared to piperazine (PZ, the most commonly used diamine) and monoethanolamine (MEA, the standard amine to which all other amines are compared) over a similar temperature swing. Eight of the amines demonstrated significant improvements with 200% or greater improvement in cyclic capacity over PZ (expressed as moles of CO₂/mol of nitrogen), with the largest improvement achieving a 273% increase. The intimate chemical behavior of the amines was examined by considering the relative contributions of specific CO₂ species to the cyclic capacity. Nine of the amines investigated showed significant improvements in the amount of the targeted bicarbonate product cycled between 40 and 90 °C compared to PZ. Despite the unoptimized and conservative desorption conditions utilized here, the results demonstrate that CO₂ can be regenerated from cyclic amines without the requirement for excessive regeneration temperatures as is the case for PZ (∼150 °C to achieve optimum cyclic capacity). The results here demonstrate the potential for improved amine solvents via amine synthesis and future development pathways through intelligent molecular design

Evaluation and Modeling of Vapor–Liquid Equilibrium and CO₂ Absorption Enthalpies of Aqueous Designer Diamines for Post Combustion Capture Processes

Novel absorbents with improved characteristics are required to reduce the existing cost and environmental barriers to deployment of large scale CO₂ capture. Recently, bespoke absorbent molecules have been specifically designed for CO₂ capture applications, and their fundamental properties and suitability for CO₂ capture processes evaluated. From the study, two unique diamine molecules, 4-(2-hydroxyethylamino)­piperidine (A4) and 1-(2-hydroxyethyl)-4-aminopiperidine (C4), were selected for further evaluation including thermodynamic characterization. The solubilities of CO₂ in two diamine solutions with a mass fraction of 15% and 30% were measured at different temperatures (313.15–393.15 K) and CO₂ partial pressures (up to 400 kPa) by thermostatic vapor−liquid equilibrium (VLE) stirred cell. The absorption enthalpies of reactions between diamines and CO₂ were evaluated at different temperatures (313.15 and 333.15 K) using a CPA201 reaction calorimeter. The amine protonation constants and associated protonation enthalpies were determined by potentiometric titration. The interaction of CO₂ with the diamine solutions was summarized and a simple mathematical model established that could make a preliminary but good prediction of the VLE and thermodynamic properties. Based on the analyses in this work, the two designer diamines A4 and C4 showed superior performance compared to amines typically used for CO₂ capture and further research will be completed at larger scale

CO₂ Absorption into Aqueous Solutions Containing 3‑Piperidinemethanol: CO₂ Mass Transfer, Stopped-Flow Kinetics, ¹H/¹³C NMR, and Vapor–Liquid Equilibrium Investigations

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

Process Modeling of an Advanced NH₃ Abatement and Recycling Technology in the Ammonia-Based CO₂ Capture Process

An advanced NH₃ abatement and recycling process that makes great use of the waste heat in flue gas was proposed to solve the problems of ammonia slip, NH₃ makeup, and flue gas cooling in the ammonia-based CO₂ capture process. The rigorous rate-based model, RateFrac in Aspen Plus, was thermodynamically and kinetically validated by experimental data from open literature and CSIRO pilot trials at Munmorah Power Station, Australia, respectively. After a thorough sensitivity analysis and process improvement, the NH₃ recycling efficiency reached as high as 99.87%, and the NH₃ exhaust concentration was only 15.4 ppmv. Most importantly, the energy consumption of the NH₃ abatement and recycling system was only 59.34 kJ/kg CO₂ of electricity. The evaluation of mass balance and temperature steady shows that this NH₃ recovery process was technically effective and feasible. This process therefore is a promising prospect toward industrial application

Protonation Constants and Thermodynamic Properties of Amino Acid Salts for CO₂ Capture at High Temperatures

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 (p<i>K</i>_a) of an amino acid salt is crucial for CO₂ capture, as it decreases with increasing absorption temperature. However, published p<i>K</i>_a values of amino acid salts have usually been determined at ambient temperatures. In this study, the p<i>K</i>_a 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 (Δ<i>H</i>_m⁰) and entropies (Δ<i>S</i>_m⁰) of the protonation reactions were also determined by the van’t Hoff equation. It was found that sarcosine can maintain a higher p<i>K</i>_a than the other amino acids studied at high temperatures. We also found that the CO₂ solubilities and overall mass-transfer coefficients of 5 <i>m</i>′ 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

Insights into the Chemical Mechanism for CO₂(aq) and H⁺ in Aqueous Diamine Solutions - An Experimental Stopped-Flow Kinetic and ¹H/¹³C NMR Study of Aqueous Solutions of <i>N</i>,<i>N</i>‑Dimethylethylenediamine for Postcombustion CO₂ Capture

In an effort to advance the understanding of multiamine based CO₂ capture process absorbents, we report here the determination of the kinetic and equilibrium constants for a simple linear diamine <i>N,N</i>-dimethylethylenediamine (DMEDA) via stopped-flow spectrophotometric kinetic measurements and ¹H/¹³C NMR titrations at 25.0 °C. From the kinetic data, the formation of monocarbamic acid (DMEDACOOH) from the reaction of DMEDA with CO₂(aq) is the dominant reaction at high pH > 9.0 (<i>k</i>₇ = 6.99 × 10³ M^–1·s^–1). Below this pH, the formation of protonated monocarbamic acid (DMEDACOOH₂) via the pathway involving DMEDAH⁺ and CO₂(aq) becomes active and contributes to the kinetics despite the 107-fold decrease in the rate constant between the two pathways. ¹H and ¹³C NMR spectra as a function of decreasing pH (increasing HCl concentration) at 25.0 °C have been evaluated here to confirm the protonation events in DMEDA. Calculations of the respective DMEDA nitrogen partial charges have also been undertaken to support the NMR protonation study. A comparison of the DMEDA kinetic constants with the corresponding data for piperazine (PZ) reveals that despite the larger basicity of DMEDA, the enhanced and superior kinetic performance of PZ with CO₂(aq) above its predicted Bronsted reactivity is not observed in DMEDA