Toward Intelligent CO2 Capture Solvent Design ... - ACS Publications

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Toward Intelligent CO2 Capture Solvent Design through Experimental Solvent Development and Amine Synthesis Qi Yang,† Graeme Puxty,‡ Susan James,† Mark Bown,† Paul Feron,‡ and William Conway*,‡ †

CSIRO Manufacturing, Ian Wark Laboratory, Bayview Avenue, Clayton, Victoria 3168, Australia CSIRO Energy, P.O. Box 330, Newcastle, New South Wales 2300, Australia

‡

S Supporting Information *

ABSTRACT: 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.

1. INTRODUCTION Ongoing emissions of CO2 into the atmosphere from the combustion of fossil fuels remains at the forefront of environmental concerns in the context of global climate change. As an intermediate strategy to reduce the impact of the CO2 emissions from energy generation, postcombustion capture of CO2 (PCC) from coal fired power station flue gas streams has been advocated. Typical PCC technologies incorporate aqueous amine based solvent systems into an integrated absorber−stripper configuration and currently represent the most advanced means of implementing carbon capture and storage (CCS) on a commercial scale in the near term.1−3 Aqueous amine solvents, including monoethanolamine (MEA) and methyldiethanolamine (MDEA), have been used historically for the separation of CO2 and H2S from gas streams in the oil and natural gas industries. The cyclic diamine piperazine (PZ) has also been applied widely both as a rate promoting additive4,5 and as a “state-of-the-art” absorbent in an advanced absorption process in its own right.6 These amine solvents represent the foundation for the transition to the removal of CO2 from power station flue gases.7 The anticipated application of aqueous amine solvents for large scale CO2 © 2016 American Chemical Society

capture processes faces several technical challenges. Low CO2 partial pressures (CO2 content) present in flue gas demand highly reactive amines able to uptake large amounts of CO2 with low physical driving force, while fulfilling the requirement to release CO2 effectively during the desorption process with the minimum of energy consumption.8 Additionally, and unlike traditional industrial applications, power station flue gas streams contain additional acidic and oxidative gases, among other flue gas impurities, including NOx, SOx, and O2.9,10 The exposure of amine solvents to these flue gas impurities can accelerate degradation of the solvent, resulting in operational issues, inefficient capture, and concerning environmental issues. These issues impose additional restriction onto the stability of amine solvents. It is well-known that the chemical structure of amines plays an influential role in the fundamental chemistry and physical properties of their behavior in CO2 capture applications.11−13 In terms of their reactivity with CO2, primary and secondary amines react directly (with CO2) to form carbamic acid, which Received: April 12, 2016 Revised: June 16, 2016 Published: August 24, 2016 7503

DOI: 10.1021/acs.energyfuels.6b00875 Energy Fuels 2016, 30, 7503−7510

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amines, requires at most one amine molecule for each molecule of CO2 captured. This scenario is demonstrated in eq 8, where NX1 and NX2 represent the amine groups which can react with CO2, and the H+ generated from the reaction with CO2, respectively. Additionally, given the presence of two or more amine groups in the molecule, these amines are able to react with more than one molecule of CO2, providing an additional amine molecule is available in the solution to absorb the corresponding protons generated from the reaction. The preference for such scenarios is strongly dependent on steric hindrance and the pH of the solution, so not all diamines will exhibit this behavior. We have previously described the complete chemical mechanism for the absorption of CO2 by the typical diamine PZ.16

subsequently deprotonates to the carbamate by exchanging a proton with a second amine molecule. This pathway is described in eqs 1 and 2, with the overall reaction given by eq 3. The major benefit of the carbamate pathway is the rapid reaction rate of the amino group(s) with CO2. However, the benefits of the rapid reaction rates must be balanced by the stability of the carbamate and the energy required to release CO2 during desorption. R1R 2NH + CO2 (aq) ⇄ R1R 2NCO2 H

(1)

R1R 2NCO−2 + H+ ↔ R1R 2NCO2 H

(2)

2R1R 2NH + CO2 (aq) ⇄ R1R 2NCO−2 + R1R 2NH+

(3)

Some sterically hindered, and all tertiary amines, do not react directly with CO2 in aqueous solution; rather, these amines act only to accept the protons released from the reaction of CO2 with water to form bicarbonate. This second pathway is described in eqs 4−6, with the overall reaction described given by eq 7. In these equations R1, R2, and R3 represent substituents attached via an N−C bond. This pathway significantly increases the amount of CO2 which can be absorbed by the amine solution and also needs a smaller energy input to desorb CO2. The trade-off that exists for this pathway is the slow reaction rate (compared to primary and secondary amines) and the requirement for rate promoters. CO2 (aq) + H 2O ⇄ H 2CO3

(4)

HCO−3 + H+ ↔ H 2CO3

(5)

R1R 2R3N + H+ ↔ R1R 2R3NH+

(6)

R1R 2R3N + CO2 + H 2O ⇄ R1R 2R3NH+ + HCO−3

(7)

X1N‐‐‐NX2

+ CO2 ⇄ (CO−2 )X1N‐‐‐NX2(H+)

(8)

The benefit of the diamine scenario is in the smaller quantity (mass) of material in the solutions required to achieve similar total concentrations of reactive nitrogen (amine) and, thus, similar CO2 capacities to more concentrated solvents involving a single monoamine. This also has benefits in terms of energy requirement, as less material needs to be circulated and heated in the stripper to achieve the same CO2 removal. While PZ has been widely recognized for its rapid reactivity and low energy requirement,16,17 significant scope still exists for the development of alternative diamine solvents. A purposedesigned diamine or triamine molecule which utilizes specific structural characteristics could potentially be synthesized to offer improved energy performance. In parallel, the structural modification of amine molecules can also address characteristics that enhance amine stability and reduce the potential environmental impacts of CO2 capture.18 This work aims to investigate the potential improvement in the absorption− desorption cyclic performance of a suite of novel amine solvents through strategic structural design, compound synthesis, and solvent screening methods. Following synthesis, the cyclic capacities of 30 new amines were determined here using a simple absorption−desorption apparatus. CO2 cyclic capacity is the single most important solvent factor affecting the capital and operating costs of a CO2 capture process.19 An increased cyclic capacity reduces operating costs by reducing the required solvent flow rate and therefore the stripping and pumping energy requirements, and reduces capital costs by allowing smaller diameter columns and smaller heat exchangers and pumps. The information presented here is valuable to guide the assessment and selection of promising amine solvents for further investigation of CO2 absorption rates and vapor liquid equilibrium behavior.

In the case of moderately hindered amines, reaction with CO2 to form a thermodynamically unstable carbamate that hydrolyses to bicarbonate and free amine is possible. Steric hindrance has two general modes of action: restricting access of CO2 to the amine group; and/or reducing the stability of the carbamate once formed. Importantly, increased steric hindrance around the amine group is typically associated with decreased absorption rates. However, this is not always the case, particularly for heterocycles,14 and these amines promote the conversion of carbamates to bicarbonate, which can result in larger CO2 absorption capacities.15 A combination of the above amine behaviors is sought; however, this is impossible to achieve in a single molecule containing only a single amine group. In efforts to combine the fast reaction kinetics of carbamate forming amines with the superior CO2 capacity and energy requirement of sterically hindered and tertiary amines, the use of blended amine solvents and diamines is often considered. The former involves the combination of two or more individual amine molecules into a new single solvent mixture while the latter involves the incorporation of two or more amine groups into a single molecular structure. Blended solvents are often preferred due to the ease with which the composition can be varied and the commercial availability of suitable molecules. The latter scenario involving single molecules containing multiple amine groups is attractive due to the reduced quantity of molecules required to absorb the same amount of CO2 as a monoamine blend. The main difference can be observed in the reaction mechanism for diamines which, unlike the case for mono-

2. EXPERIMENTAL SECTION 2.1. Chemicals. The chemicals used in synthesis were purchased from Sigma-Aldrich, Strem Chemicals, or Oakwood (varying purities). Where required, the chemicals were purified before use. CO2 and N2 gases were purchased from BOC gases (>99.0%). 2.2. Amine Synthesis. Briefly, the following section describes the pathways employed in the synthesis of the amine molecules here. Specific details of the reaction conditions and synthetic methods can be found in the Supporting Information. The new amine compounds have been developed from two core structures: 4-aminopiperidine (4AP) (1) and 4-aminomethylpiperidine (4-AMP) (2). Both of these amines have been reported previously as excellent CO2 absorbents in aqueous systems.20−22 These two structures contain two different amino groups, one embedded in the ring and another connected to the ring directly (4-AP) or via a methylene carbon (4-AMP). These two 7504


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Energy & Fuels heterocyclic skeletons are structurally similar to piperidine derivatives that have been reported previously14,23 to have favorable equilibrium properties and are capable of high CO2 loadings. Additionally, they also have partial structural similarity to PZ. Steric hindrance has an additional and strong impact on the stability of carbamates. Increasing hindrance results in lower amounts of carbamate at equilibrium; thus, CO2 preferentially exists (depending on the pH of the solution) as bicarbonate ions. The inherent advantage of this scenario is the improvement in the CO2 absorption capacity of the solvent.15 The two skeletons proposed here incorporate the favorable features of heterocycles while providing a workable amine site for molecular modification. The introduction of substituents to the amino groups of the skeleton amines, denoted here as amines (1) and (2), generates five subgroups of amines listed as amines (3) to (7) in Figure 1. Further

modification of the amino functional groups with selected substituents introduces changes to the amine characteristics including the protonation constants (pKa values), molecular polarity, water solubility, viscosity, and boiling points. The substituents proposed here include alkyl, hydroxylalkyl, alkyl carboxylic acid, and alkylheterocyclic moieties. The resulting amines within subgroups (3) to (7) are represented by the letters a to i to specify their substituents. Overall, 30 amines have been synthesized in this work in total (see Supporting Information). 2.3. NMR Acquisition and Analysis. Hydrogen (1H) and quantitative carbon (13C) NMR spectra were acquired on a Bruker Avance Ascend 400 NMR spectrometer operating at a frequency of 400.13 MHz (1H) and 100.62 MHz (13C). Proton 1H spectra were obtained as the sum of 32 scans with an interscan delay of 3.73 s. To accurately quantify carbon signals using quantitative 13C NMR spectroscopy, it is critical that the recovery time of each scan (which is the sum of the acquisition time (Bruker parameter AQ) and the relaxation delay (Bruker parameter D1)) has to be sufficiently large to allow complete relaxation of the signal. Relaxation time is affected by factors such as carbon environment and NMR spectrometer frequency; therefore, the relaxation time (T1) should be measured and the longest relaxation time of each sample (T1max) used to determine the recovery time (AQ + D1) that satisfies (AQ + D1) ≥ 5 × T1 for all carbons in the molecule. Inverse-gated 1H-decoupled 13C spectra were obtained at a pulse angle of 30° (zgig30 pulse program, Bruker) as the average of 32 scans with a minimum pulse delay time (D1) of 70 s, which corresponds to a value (AQ + D1) ≥ 5 × T1max of the T1 relaxation time of the slowest relaxing carbon (typically carbamate/HCO3−) for most samples, including all of our amines at the measured concentration. The desired temperature of the sample was maintained internally within the instrument (±1.0 °C) during the spectral acquisitions. 1 H and 13C NMR spectra were processed individually using Bruker Topspin software. A baseline correction (polynomial) was applied to all spectra prior to integration. The molar ratio of species was calculated by dividing the integration of the specific carbonyl signal(s) by the sum of the relative integrals of the carbon signals for all amine

Figure 1. Five subgroups of amines prepared from skeletons (1) and (2), where R equals one of the following: a, Et; b, Pr; c, (CH2)2OH; d, (CH2)3OH; e, (CH2)6OH; f, CH2CO2H; g, [(CH2)2]N[(CH2)2]2O; h, (CH2)2N(CH3)2; i, 4-(1H-imidazyl)ethylene.

Figure 2. General schematic of the CO2 absorption and desorption apparatus. 7505


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Figure 3. Improvement in cyclic capacity of amines as a percentage relative to PZ according to 9eq 9. to the flask jacket: 40.0 °C for absorption and 90.0 °C for desorption. Once the internal liquid temperature was stable, a mixed gas containing 10% CO2 v/v in N2 was first saturated with water by bubbling through a Dreschel bottle and then introduced to the amine solution in the flask, which also contained an egg-shaped magnetic stirring bar. The gas flow rate was controlled to 50.0 mL/min using Bronkhorst mass flow controllers for CO2 and N2, and the stirring speed of the solution was maintained at 900 rpm. Absorption was performed over 18 hours at 40.0 °C, which was sufficient for the solutions to have reached saturation with the CO2 gas stream. At this point the gas supply was removed and the reaction liquid sampled for NMR analysis. The remaining amine solution was subjected to desorption at 90.0 °C by elevating the temperature of the water passing through the jacket of the flask. Samples were withdrawn during the desorption measurements (2.0, 5.0, 10.0, 30.0, and 60.0 min, with the latter representing equilibrium or no further changes in CO2 loading) and transferred immediately to NMR tubes which were capped and sealed to avoid loss of amine or CO2 from the solutions. The cyclic capacity was determined from the difference in the amount of CO2 dissolved into the amine solution during absorption, and that remaining in the amine solution after desorption. Quantitative 13C NMR spectroscopy was employed for the analysis of dissolved CO2 concentrations in the lean and rich samples.24 The NMR measurements were additionally able to provide the chemical speciation of the dissolved CO2 as carbonate/bicarbonate or carbamate/carbamic acid, and the amine as carbamate/carbamic acid/unreacted amine.

species. Using this method, the CO2 loading of the solutions was normalized in each case based on a mol/mol (CO2/amine) or mass (CO2 gram/amine g) basis. 1,4-Dioxane was added to each NMR tube to act as the external standard. 2.4. CO2 Absorption and Cyclic Capacity Evaluation. Aqueous PZ solutions (2.0 M) were prepared by dissolving PZ solid in deionized water using sonication at 40 °C followed by adjusting the liquid volume accurately in volumetric glassware when the solution was cooled to 25 °C. This solution was then dispensed to the reaction flask prior to any precipitation. Aqueous solutions (4.0 M) of MEA and 2.0 M aqueous solution of the 30 amines and skeleton amines (1) and (2) were prepared in volumetric glassware using deionized water. Potassium salt solutions of four amino acids (compounds 3f, 5f, 6f, and 7f) were prepared by dissolving the appropriate quantity of the amino acid together with a stoichiometric amount of potassium hydroxide in deionized water in volumetric glassware. The CO2 absorption−desorption experiments were conducted using the laboratory scale reactor system illustrated in Figure 2. The system was designed to allow simple evaluation of CO2 absorption and desorption over a large temperature range (50 °C swing between absorption and desorption) in the laboratory. While the current system does not necessarily represent optimized absorption and desorption conditions observed during continuous absorption− desorption cycles in a large experimental laboratory reactor or pilot plant scale, the experimental setup allows for the controlled evaluation and comparison of the amine cyclic capacity at fixed temperature conditions with limited quantities of amine. Evaluation and comparison with the standard cyclic diamine PZ, and the linear alkanolamine MEA, over a similar temperature range reveals the relative potential for improvement with the new amines. Any amines which can successfully demonstrate improvements in CO2 absorption and cyclic capacity despite the conservative desorption temperature represent promising candidates for further investigation at larger scale (high cyclic capacity at low desorption temperatures is desirable) if and when larger amine quantities can be synthesized or become available commercially. A 10.0 mL aliquot of the 2.0 M amine solutions was placed into a 50.0 mL jacketed pear-shaped flask equipped with a condenser. The temperature of the solution inside the reactor was controlled by circulating water at the required temperatures from a thermal regulator

3. RESULTS AND DISCUSSION Thirty synthesized amines, MEA, PZ, and two skeleton amines, (1) and (2), were investigated for their CO2 absorption and cyclic capacities using quantitative 13C NMR spectroscopy. Potassium salts of four amino acids (3f-K, 5f-K, 6f-K, and 7f-K) were also tested with the same experimental methods after the free amino acids were converted to potassium salts. It should be noted that several of the amines are available from commercial suppliers, however not in large enough quantities for these measurements, making synthesis necessary. Intimate details of 7506


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Energy & Fuels the synthetic methods and reactions for the amines synthesized in this work are available in the Supporting Information. In the CO2 absorption and desorption experiments, PZ and all designer amines were tested at 2.0 M concentration, which corresponded to 4.0 N (normal) concentration of reactive nitrogen for diamines and 6.0 N for the two amines, 3h and 3i, which contained three reactive nitrogens in their molecules. Amine 3i has four nitrogen atoms in its structure; however, due to the acidic nature of the NH group of the 1-H-imidazyl substituent, it was not considered to be active in terms of CO2 absorption reactions. MEA was evaluated at 4.0 M concentration, representing a 4.0 N active nitrogen concentration to largely match the concentrations of the majority of the amines in this study. Cyclic capacity results are presented in (a) normality units (mole of CO2 per mole of active nitrogen atom), reflecting the relative activity of the amino functionality in each amine compound, and (b) mass ratio (grams of CO2 per gram of amine), to reflect the influence of amine molecular weight on cyclic capacity. Three amine compounds (3b, 3f, and 3i) and one amino acid salt (potassium salt of 3f-K) formed precipitates during CO2 absorption, and their cyclic capacities were unable to be measured accurately using the current method. The cyclic capacity results of the remaining 30 amines were obtained and are given in Table S1 of the Supporting Information. The percentage improvement in cyclic capacity of designer amines is defined by eq 9 as the CO2 cyclic capacity of an amine relative to PZ. The relative cyclic capacities of MEA and compounds (1) and (2), in comparison with PZ were also calculated following the same method. These results are presented graphically in Figure 3. %Improvement in Cyclic Capacity Cycdesigner amine − Cyc PZ = 100 × Cyc PZ

(9)

All amines demonstrated an increase in cyclic capacity compared to PZ on a molar basis. Eight of the amines achieved more than a 200% improvement in their cyclic capacity on a molar basis, with the largest improvement representing a 273% increase by amine 5e. When the amine molecular weight changes are considered, eight amines showed negative changes in their mass based cyclic capacities. However, these mainly resulted from the amino acids with large and unreactive spectator counterions (potassium salts) contributing to their molecular masses. From Figure 3, 21 of the amines displayed an increase in mass based cyclic capacity compared to PZ. Of these amines, six achieved more than 80% improvement, with the largest improvement being 106% for amine 5a. The overall success in relative improvements in molar cyclic capacity of these amines demonstrates the designer amine process and the capability to achieve higher efficiency cyclic absorption− desorption of CO2 at the specified conditions here. 3.1. Assessment of Designer Amine Structure and Impact on Cyclic Capacity. Figure 4 shows the structures of the 9 amines that yielded a cyclic capacity increase >75% relative to PZ. Six of these 9 are based on skeleton (1), suggesting the NHx group be preferentially located directly adjacent to the ring. In terms of substitution, the NHx group being a primary or secondary amine is clearly preferred. For the ring bound N, there is no clear preference for it being a secondary or tertiary amine. In terms of the substituents themselves, Et, (CH2)2OH, or (CH2)2N(CH3)2 appear in 7 of the 9 amines. Figure 5 shows the % improvement in mass based

Figure 4. These 9 amines had a >75% improvement in both mole and mass based CO2 cyclic capacity relative to PZ. Molecules based on skeleton (1) have the substructure indicated in black/bold, and those based on skeleton (2) have that in gray/bold.

cyclic capacity relative to PZ of these 9 amines. In order of mass based cyclic capacity: 5a > 3a > 3c > 5b > 7a > 6a > 6h > 5c > 3h. 5a is something of a standout, with 3a to 7a being similar, followed by a ∼10% drop to 6a−3h, which are similar. Despite the larger structure and molecular mass, amine 3h was able to achieve a reasonable improvement in cyclic capacity over PZ. 3.2. Intimate Assessment of Chemical Speciation in Designer Amine Solutions. Perhaps of greater interest than the overall cyclic capacity are the relative contributions of specific CO2 species to the cyclic capacity. The individual concentrations of carbamate (total concentration resulting from both mono- and dicarbamates in the case of structures containing more than one active primary and/or secondary nitrogen group) and the total concentration of HCO3−/CO32−, at both absorption and desorption conditions from 13C NMR data have been evaluated here. The concentration of free CO2(aq) is negligible under the equilibrium conditions, and its contribution to the overall CO2 loading and cyclic capacity can 7507


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Figure 5. % improvement in mass based cyclic capacity for the 9 amines with >75% improvement.

Figure 6. Species cycled (mol/mol) expressed as a percentage relative to PZ.

in carbamate concentration after desorption (∼10% and ∼5% improvement over PZ) while amine 6a experienced an increase in carbamate concentration after desorption (∼25% increase in carbamate concentration at 90 °C over PZ) despite maintaining a large overall cyclic capacity (negative contribution of carbamate to the overall cyclic capacity). Conceivably, the observed increase in carbamate could relate to the relative stabilities of the individual carbamates as a function of pH (induced by both temperature changes and changes in CO2 loading). In this example it is feasible to consider that CO2 is transferring between the two amine groups depending on the above stated conditions, resulting in a negligible change or increase in carbamate concentration following desorption. Further knowledge surrounding the chemical behavior of the amine solvent,s including carbamate stability constants and protonation constants, and their corresponding enthalpies, will reveal further insight into the possible pathways for CO2 in designer amines and the observed cyclic capacities here. In

be ignored. The amounts of each component, expressed as a percentage relative to the amount of each species cycled by PZ on a mol/mol basis as calculated by 9eq 9, are shown in Figure 6. In the case of diamines capable of multiple reactions with CO2 (more than one possible carbamate per molecule), the amount of carbamate expressed here represents the sum of all carbamates. All 9 amines demonstrated improvements in the amount of bicarbonate cycled between absorption and desorption conditions relative to PZ, with the largest result being some ∼962% improvement from amine 6a. This large contribution to the cyclic capacity stemming from the energetically favorable HCO3−/CO32− regeneration pathway here should translate to an overall energy benefit. The trend in terms of bicarbonate/ carbonate follows 6a > 7a > 3h > 3c > 6h > 5a > 5c > 3a > 5b. Six of the 9 amines experienced improvements in the amount of carbamate cycled, with the largest improvement being amine 6h (∼160%). Amines 3h and 7a experienced only minor changes 7508


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Coal Association Low Emissions Technology Limited and the Australian Government through the Clean Energy Initiative.

particular, the protonation enthalpies of amines are strongly coupled to its propensity to regenerate CO2, where larger enthalpies typically result in larger pH swings and larger cyclic capacities. Based on overall cyclic capacity data and bicarbonate/ carbonate cycling amines 6a, 7a, and 3h represent promising solvents to pursue. From this cohort, the choice of preferred structure will also likely depend upon other factors. For example, if low vapor pressure (volatility of amine) and maximum aqueous solubility are identified as priorities, the alkanolamines 3c and 5c will likely be favored. If the formation of nitrosamines in a high NOx flue gas is priority, then the secondary amines would best be excluded, leaving 5a, 5b, and 5c. The impact of kinetic properties and overall bulk absorption behavior (CO2 mass transfer and vapor liquid equilibrium) should also be considered when making an assessment of the overall suitability of the designer amines for use in CO2 capture processes. This is not currently possible with the small amount of each amine available. However, we are pursuing options for synthesis at larger scale that will afford sufficient material for a more detailed characterization. A detailed assessment of the synthetic pathways and the impact of this on the final cost of amine production is underway. 3.3. Conclusions. Through synthesis of amines and studies of their cyclic CO2 absorption−desorption ability, we have successfully demonstrated significant improvement in the cyclic capacity of amine solvents for CO2 capture processes compared to the most commonly used diamine, PZ, and the most commonly used amine, MEA. This would lead to reductions in operating costs via a reduced solvent flow rate and associated energy demand, and capital costs via smaller column diameters, heat exchangers, and pumps. This indicates that molecular synthesis based on knowledge of CO2−amine chemistry is an effective way to design amine molecules suited to CO2 capture processes that give better performance than existing commercially available compounds. Eight amines achieved greater than 200% improvement in cyclic capacity over PZ (expressed as moles of CO2/mol of nitrogen), with the largest improvement achieving a 273% increase. More experimental data of the CO2−amine−water vapor−liquid equilibria and kinetics data of these promising amine systems is required to more precisely understand their performance at optimized process conditions.

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(1) Bhown, A. S.; Freeman, B. C. Analysis and Status of PostCombustion Carbon Dioxide Capture Technologies. Environ. Sci. Technol. 2011, 45, 8624−8632. (2) Liang, Z.; Rongwong, W.; Liu, H.; Fu, K.; Gao, H.; Cao, F.; Zhang, R.; Sema, T.; Henni, A.; Sumon, K.; Nath, D.; Gelowitz, D.; Srisang, W.; Saiwan, C.; Benamor, A.; Al-Marri, M.; Shi, H.; Supap, T.; Chan, C.; Zhou, Q.; Abu-Zahra, M. R.; Wilson, M.; Olson, W.; Idem, R.; Tontiwachwuthikul, P. Recent Progress and New Developments in Post-Combustion Carbon-Capture Technology with Amine Based Solvents. Int. J. Greenhouse Gas Control 2015, 40, 26−54. (3) Shakerian, F.; Kim, K.-H.; Szulejko, J. E.; Park, J.-Y. A Comparative Review Between Amines and Ammonia as Sorptive Media for Post-Combustion CO2 Capture. Appl. Energy 2015, 148, 10−22. (4) Dash, S. K.; Samanta, A.; Samanta, A. N.; Bandyopadhyay, S. S. Absorption of carbon dioxide in piperazine activated concentrated aqueous 2-amino-2-methyl-1-propanol solvent. Chem. Eng. Sci. 2011, 66, 3223−3233. (5) Bougie, F.; Lauzon-Gathier, J.; Iliuta, M. C. Acceleration of the reaction of carbon dioxide into aqueous 2-amino-2-hydroxymethyl-1,3propanediol solutions by piperazine addition. Chem. Eng. Sci. 2009, 64, 2011−2019. (6) Freeman, S. A.; Dugas, R.; Van Wagener, D. H.; Nguyen, T.; Rochelle, G. T. Carbon Dioxide Capture with Concentrated, Aqueous Piperazine. Int. J. Greenhouse Gas Control 2010, 4, 119−124. (7) IEA. Prospects for CO2 Capture and Storage; OECD/IEA: 2004. (8) Dutcher, B.; Fan, M.; Russell, A. G. Amine-Based CO2 Capture Technology Development from the Beginning of 2013A Review. ACS Appl. Mater. Interfaces 2015, 7, 2137−2148. (9) Fine, N. A.; Goldman, M. J.; Rochelle, G. T. Nitrosamine Formation in Amine Scrubbing at Desorber Temperatures. Environ. Sci. Technol. 2014, 48, 8777−8783. (10) Resnik, K. P.; Yeh, J. T.; Pennline, H. W. Aqua ammonia process for simultaneous removal of CO2, SO2 and NOx. Int. J. Environ. Technol. Manage. 2004, 4, 89−104. (11) Conway, W.; Wang, X.; Fernandes, D.; Burns, R.; Lawrance, G.; Puxty, G.; Maeder, M. Toward Rational Design of Amine Solutions for PCC Applications:The Kinetics of the Reaction of CO2 with Cyclic and Secondary Amines in Aqueous Solution. Environ. Sci. Technol. 2012, 46, 7422−7429. (12) Conway, W.; Wang, X.; Fernandes, D.; Burns, R.; Lawrance, G.; Puxty, G.; Maeder, M. Toward the Understanding of Cheimical Absorption Processes for Post-Combustion Capture of Carbon Dioxide: Electronic and Steric Considerations from the Kinetics of Reactions of CO2(aq) with Sterically Hindered Amines. Environ. Sci. Technol. 2013, 47, 1163−1169. (13) Couchaux, G.; Barth, D.; Jacquin, M.; Faraj, A.; Grandjean, J. Kinetics of Carbon Dioxide with Amines. I. Stopped-Flow Studies in Aqueous Solutions. A Review. Oil Gas Sci. Technol. 2014, 69, 865−884. (14) Conway, W.; Beyad, Y.; Maeder, M.; Burns, R.; Feron, P.; Puxty, G. CO 2 Absorption into Aqueous Solutions Containing 3Piperidinemethanol: CO2Mass Transfer, Stopped-Flow Kinetics, 1H/13C NMR, and Vapor−Liquid Equilibrium Investigations. Ind. Eng. Chem. Res. 2014, 53, 16715. (15) Hook, R. J. An Investigation of Some Sterically Hindered Amines as Potential Carbon Dioxide Scrubbing Compounds. Ind. Eng. Chem. Res. 1997, 36, 1779−1790. (16) Conway, W.; Fernandes, D.; Burns, R.; Lawrance, G.; Puxty, G.; Maeder, M. Reactions of CO2 with Aqueous Piperazine Solutions: Formation and Decomposition of Mono- and Dicarbamic Acids/ Carbamates of Piperazine at 25.0 °C. J. Phys. Chem. A 2013, 117, 806− 813. (17) Rochelle, G. T.; Chen, E.; Freeman, S. A.; Van Wagener, D. H.; Xu, Q.; Voice, A. Aqueous Piperazine as the New Standard for CO2 Capture Technology. Chem. Eng. J. 2011, 171, 725−733.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b00875. Syntheses of designer amines, and cyclic capacity and energy performance of amines (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +61 2 49 606098. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors also wish to acknowledge financial assistance provided through both CSIRO Energy Flagship and Australian National Low Emissions Coal Research & Development (ANLEC R&D). ANLEC R&D is supported by Australian 7509


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Energy & Fuels (18) Lepaumier, H.; Picq, D.; Carrette, P.-L. New Amines for CO2 Capture. I. Mechanisms of Amine Degradation in the Presence of CO2. Ind. Eng. Chem. Res. 2009, 48, 9061−9067. (19) Raynal, L.; Bouillon, P.-A.; Gomez, A.; Brouti, P. From MEA to demixing solvents and future steps, a roadmap for lowering the cost of post-combustion carbon capture. Chem. Eng. J. 2011, 171, 742−752. (20) Singh, P.; Versteeg, G. F. Structure and activity relationships for CO2 regeneration from aqueous amine-based absorbents. Process Saf. Environ. Prot. 2008, 86, 347−359. (21) Singh, P.; Niederer, J. P. M.; Versteeg, G. F. Structure and activity relationships for amine-based CO2 absorbents-II. Chem. Eng. Res. Des. 2009, 87, 135−144. (22) Yoshiyama, R.; Tanoura, M.; Watari, N.; Fujii, S.; Inoue, Y.; Sakano, M.; Ichihara, T.; IIjima, M.; Mimura, T.; Yagi, Y.; Ogura, K. Absorbent Liquid, and Apparatus and Methods for Removing CO2 or H2S from Gas with Use of Absorbent Liquid. US patent, US 8,506,683 B2 (Aug. 13, 2013). (23) Puxty, G.; Rowland, R.; Allport, A.; Yang, Q.; Bown, M.; Burns, R.; Maeder, M.; Attalla, M. Carbon Dioxide Postcombustion Capture: A Novel Screening Study of the Carbon Dioxide Absorption Performance of 76 Amines. Environ. Sci. Technol. 2009, 43, 6427− 6433. (24) Yang, Q.; Bown, M.; Abdelselam, A.; Winkler, D.; Puxty, G.; Attalla, M. A Carbon-13 NMR Study of Carbon Dioxide Absorption and Desorption with Aqueous Amine Solutions. Energy Procedia 2009, 1, 955−962.

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