Comparative Study of CO2 Capture by Aqueous and Nonaqueous 2

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Applied Chemistry 2

A comparative study of CO capture by aqueous and non-aqueous AMP-based absorbents carried out by C NMR and enthalpy analysis 13

Francesco Barzagli, Claudia Giorgi, Fabrizio Mani, and Maurizio Peruzzini Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00552 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 3, 2019

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Industrial & Engineering Chemistry Research

A comparative study of CO2 capture by aqueous and non-aqueous AMP-based absorbents carried out by 13C

NMR and enthalpy analysis

Francesco Barzagli a,*, Claudia Giorgi b, Fabrizio Mani a, Maurizio Peruzzini a,c a

National Research Council, ICCOM Institute, via Madonna del Piano 10, 50019 Sesto F.no,

Florence, Italy b

University of Florence, Department of Chemistry, via della Lastruccia 3, 50019 Sesto F.no,

Florence, Italy c

National Research Council, DSCTM, piazzale Aldo Moro 7, 00185 Rome, Italy

*To whom correspondence should be addressed. E-mail: [email protected]

KEYWORDS carbon dioxide capture • heat of CO2 reaction • 2-amino-2-methyl-1-propanol •

13C

NMR

speciation • amine carbamates

ACS Paragon Plus Environment

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

The CO2 uptake by single 2-amino-2-methyl-1-propanol (AMP) and its blends with 2(ethylamino)ethanol (EMEA) or N-methyl-2,2’-iminodiethanol (MDEA) has been investigated both in aqueous and non-aqueous solutions, and compared with aqueous 2-aminoethanol (MEA), the most used sorbent in CCS processes. The loading capacity, the rate of absorption and the heat of CO2 absorption have been experimentally determined for all the amine solutions.

13C

NMR

analysis allowed the identification of the carbonated species formed in solution and to evaluate their relative amount. The most promising sorbents have been further tested in a continuous cycle of absorption and desorption carried out in packed columns, in order to verify their CO2 (15% in N2) capture efficiency. Thanks to their good CO2 loading, high rate of reaction with CO2 and low heat of absorption, the AMP-EMEA blend solutions, both in water and in organic diluents, are good candidates for CO2 capture as an alternative to the conventional aqueous MEA solution.

1. INTRODUCTION CO2 separation from gas mixtures is an important and widespread industrial process, unavoidable in ammonia synthesis, natural gas purification, hydrogen manufacture from watergas shift and coal gasification, methane production from landfill wastes.1-9 Moreover, in order to curb the greenhouse gas emissions aimed at mitigating the global warming, the development of technologies for the capture of CO2 produced by anthropogenic activities (CCS) should not be further delayed, in accordance with the Paris Agreement that recommended keeping the global mean temperature well below 2°C above pre-industrial levels, in order to limit the risks and impacts of climate change.10,11


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The chemical capture by aqueous alkanolamines is currently considered the most feasible option for CO2 separation from gas mixtures , but its widespread use is limited by the high operating costs, mostly due to the absorbent regeneration and water evaporation.12-18 As a part of our experimental studies towards CO2 capture from gas mixtures, focused on combining a high capture efficiency with a potential cut of the energy for absorbent regeneration, we recently investigated the performances of different alkanolamines, dissolved in water and in 2-(2methoxyethoxy)ethanol (DEGMME).19 By continuing these studies, it was interesting to us to compare the performances of single 2-amino-2-methyl-1-propanol (AMP) and its blends (1:1, on molar scale) with 2-(ethylamino)ethanol (EMEA) or N-methyl-2,2’-iminodiethanol (MDEA), both in aqueous and non-aqueous solutions. AMP is a primary amine and its aqueous solutions have been studied for a long time as a CO2 capture absorbent because of its favorable properties, namely the high loading capacity, the low regeneration energy needed 20,21 and the high thermal stability.22,23 We selected EMEA, due to its high reaction rate and its low heat of reaction with CO2, and the tertiary amine MDEA, because of its high loading capacity and its low heat of CO2 absorption, even if its reactivity towards CO2 is lower than primary or secondary.24 The CO2 capture features of the aforementioned alkanolamines have been evaluated both in aqueous and non-aqueous solution. Replacing water with organic diluents has the potential to redirect the reaction with CO2 towards AMP carbamate or alcohol carbonate, that require less energy to be regenerated than the species formed in aqueous solution. In addition, the lower heat capacity and vapor pressure of organic diluents compared to water, as well as the lower equipment corrosion, contribute to reduce the energy penalty of the CO2 capture process.23 For this purpose, a suitable organic diluent should be chosen taking into account the solubility of the carbonated products, the high boiling temperature, the low cost and the avoidance of foaming


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problems. Ethylene glycol (EG) has been proven to be necessary in order to solubilize the carbonated species formed during CO2 absorption; however, with the objective of reducing its high viscosity (μ = 16.1 mPa s, at 25°C) we employed 1:1 (v/v) mixtures of EG with either 1propanol (PrOH) or diethylene glycol monomethyl ether (DEGMME), maintaining the boiling temperature up to 150 °C. A lower viscosity of the absorbent enhances the absorption efficiency because of a better liquid-gas mass transfer. The present study reports a series of batch experiments aimed at measuring some of the most important parameters for the sequestration of CO2 from a gas mixture: the CO2 loading capacity, the CO2 capture rate and the heat of CO2 absorption. Subsequently, the sorbents with the best features were further evaluated in closed cycles of continuous CO2 capture and sorbent regeneration, in order to determine the efficiency of the CO2 capture process. 13C NMR spectroscopy, a powerful non-invasive analytical technique, has been used to evaluate the different carbonated species occurring in solution as a function of the different absorbents, obtaining information about the reaction mechanism.25-27 The absorption performances of aqueous and non-aqueous solutions were compared to those of aqueous 30 wt% 2-aminoethanol (MEA), the most used sorbent of any CCS process. In our opinion, this experimental study may provide useful results to select suitable absorbents for industrial applications. Moreover, the present work reports the one of the first systematic study for the determination of the heat of CO2 capture for amine solutions in organic diluents.


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2. MATERIALS AND METHODS 2.1.

General information

All the alkanolamines and the diluents (Sigma-Aldrich) were reagent grade and were utilized without any further purification. Pure CO2 and N2 (Sapio Srl) were used to simulate flue gas. The amount of CO2 in the gas stream was determined by using a Varian CP-4900 gas-chromatograph, calibrated with 15% and 40% CO2/air (v/v) reference mixture (Rivoira Spa) and 100% CO2 reference gas (Sapio Srl). The total amine concentration in all solutions was 3.0 mol dm–3, except for aqueous 30 wt% ethanolamine (MEA, 4.9 mol dm–3). In particular, amine blends of AMP with EMEA were 1:1 molar ratio (AMP 1.5 mol dm–3 and EMEA 1.5 mol dm–3) and similarly AMP with MDEA (AMP 1.5 mol dm–3 and MDEA 1.5 mol dm–3). Mixed non-aqueous diluents, EG-PrOH and EG-DEGMME, were 1:1 on volume scale. 2.2.

Determination of Loading

The batch experiments aimed at determining the CO2 loading value, defined as the ratio between the maximum amount of CO2 captured and the amount of amine in the absorbent (mol/mol), have been carried out with an apparatus and a procedure carefully described in a previous work.19 A volume of 0.025 dm3 of the tested sorbent was placed into an home-made glass absorber, maintained at 40 °C by means of a thermostatted Julabo F33-MC bath, and was reacted with a continuous stream of pure CO2, injected at a flow rate as low as 15.0 dm3 h-1, to limit amine loss or foaming. In order to ensure that the equilibrium was reached, the experiment was stopped after 8 hours. The total volume of CO2 captured was determined with a gastight apparatus as described in a previous work.19 From the measured volume of CO2, it is possible to compute the amount (mol) of CO2, and therefore the loading value at 40°C.


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2.3.

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Evaluation of the CO2 capture profile as a function of time

The reaction rate between sorbents and CO2 has been evaluated with a procedure and an apparatus comprehensively described in a previous work.19 A simplified sketch of the apparatus used is reported in Figure 1A.

Figure 1. (A) Simplified sketch of the apparatus for the determination of the rate of CO2 capture and (B) schematic diagram of the process for the CO2 capture efficiency determination.

A volume of 0.015 dm3 of the appropriate solution (0.045 mol of the amine) was introduced from a pressure-equalizing dropping funnel into a 2.300 dm3 flask filled with pure CO2 and maintained at 25 °C with a water bath. After the introduction of the sorbent and the activation of a magnetic stirrer, the reaction with CO2 began. A digital pressure gauge continuously measured the decrease in pressure inside the flask, due to the consumption of CO2, and by the consequence enabled us to evaluate the CO2 capture as a function of time. The pressure was measured until a steady state was reached (after 50 minutes in our experiments).


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2.4.

Determination of the heat of CO2 absorption

As already reported in literature,28-30 the Gibbs-Helmholtz equation (1) allows to estimate the heat of CO2 absorption (ΔHabs) into a solution without calorimetric measures, by using experimental data of CO2 partial pressure (PCO2) at different temperature (T) and at the same CO2 equilibrium loading values.31,32 d(ln(PCO2) 1

d(T)

=

∆Habs R

(1)

The experiments to evaluate the ΔHabs of the different sorbents were carried out with the same procedure of the loading experiments (section 2.2): the desired CO2 partial pressures, ranging from 10.13 to101.33 kPa, were obtained from pure CO2 and N2 by using two gas mass flow meters (Aalborg) equipped with gas controllers (Cole Parmer); the exploited temperatures , comprised between 20 and 40°C, were maintained by means of a Julabo F33-MC thermostatted bath. The values of CO2 equilibrium loading were obtained at different CO2 partial pressures and temperatures until at least three measures gave the same loading value under different conditions. The slope of the plot between ln(PCO2) and 1/T at the same CO2 equilibrium loading allow us to obtain the ΔHabs value. This procedure has been previously validated19 by comparing the heat of absorption of aqueous 30 wt% MEA that we obtained (-83.24 kJ mol-1) with experimental data of other authors.24,33 2.5.

Determination of the CO2 absorption efficiency

The CO2 capture efficiency of the different sorbents was evaluated in a continuous cycles of CO2 absorption and desorption experiment, carried out as described in previous works19,23 into two home-built glass cylinders (absorber and desorber) equipped with a jacket and packed with


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glass rings (diameter 5 mm). A simplified sketch of the system is depicted in Figure 1B. A volume of 0.400 dm3 of the tested solution was circulated continuously between the two columns at the flow rate of 0.30 dm3 h–1 by means of a double head peristaltic pump (Masterflex). A thermostatted liquid, circulating through the jackets of the columns, ensured that the desired temperatures (absorber at 40 °C, desorber at 110 °C) were kept constant for the whole experiment. The amine solution reacts into the absorber with a gas mixture containing 15% (v/v) CO2 in N2, continuously injected from the bottom of the column at a flow rate of 29.0 dm3 h–1 (0.180 molCO2 h–1 at 22 °C). After the reaction, the loaded sorbent was sent to the desorber for its regeneration, and subsequently was recycled back to the absorber. The gas exiting from the top of the absorber was analyzed by means of a gas chromatograph at intervals of 10 minutes to measure the CO2 concentration. The ratio between captured and injected CO2 provides the percentage of absorption efficiency. The experiment was stopped after 24-36 h, when the reactions of CO2 absorption and sorbent regeneration reached a steady state and the capture efficiency (%) remained unchanged over time. The viscosity of the starting and of the carbonated amine solutions at the end of each cyclic experiments was measured at 40 °C with a Gilmont “Falling Ball Type” Viscometer by using a procedure already described and validated in a previous work.34

2.6.

13C

NMR Spectroscopy

The 13C NMR analysis of the absorbent solutions were performed with a Bruker Avance III 400 spectrometer operating at 100.613 MHz with a procedure previously described35,36 that


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allowed us to identify the carbonated species formed in solution and to evaluate their relative amount.37-39 Details of the experimental settings and procedures are reported in the Supporting Information.

3. RESULTS AND DISCUSSION 3.1.

Chemical equilibria in solutions and CO2 loading

The CO2 capture by aqueous amine solutions can be described by the following equations: 2AmH + CO2 ⇄ AmCO2− + AmH2+

(2)

AmH + CO2 + H2O ⇄ HCO3− + AmH2+

(3)

AmCO2− + CO2 + 2H2O ⇄ 2HCO3− + AmH2+

(4)

HCO3− + AmH ⇄ CO32− + AmH2+

(5)

CO2 + CO32− + H2O ⇄ 2HCO3−

(6)

where AmH, AmCO2− and AmH2+ denote the free amine, the amine carbamate and the protonated amine, respectively. Equations (2) and (4) do not apply to the tertiary amine MDEA which is unable to form carbamate.40 In anhydrous conditions, only primary and secondary amines can react with CO2 to produce amine carbamates [equation (2)]. Tertiary amines are not able to react directly with CO2 in the absence of water, but they can still behave as bases for the amine deprotonation when mixed


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with primary or secondary amines [equation (7); AmH denotes primary or secondary amine, Am* denotes tertiary amine]: AmH + Am* + CO2 ⇄ AmCO2− + Am*H+

(7)

The same acid-base reaction also occurs in aqueous solutions of blended tertiary and primary (or secondary) amines.40 Additionally, alcohol could react with CO2 uptake in the presence of a base (i.e. the amine) to produce alkyl carbonates: AmH + CO2 + ROH ⇄ AmH2+ + ROCO2−

(8)

where R denotes an alkyl group. The loading value is the maximum CO2 absorption capacity of an amine solution and it is expressed as the ratio between the mol of CO2 captured and the mol of amine in the absorbent. The results are reported in Table 1. The 13C-NMR analysis has been used to identify the species formed at the end of the experiment and to evaluate their relative amount. Table 1. CO2 loading at 40°C of the different 3.0 mol dm–3 AMP-based solutions, compared with 30 wt% aqueous MEA. diluent amine

H2O

EG-PrOH

EG-DEGMME

AMP

0.875

0.723

0.607

AMP-EMEA

0.861

0.665

0.585

AMP-MDEA 0.813

0.456

0.398

MEA

--------

---------

0.609


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On the basis of the results reported in Table 1, it is possible to identify two general trends as a function of the different amine solutions: the loading values decreases in the order H2O > EGPrOH > EG-DEGMME and in the order AMP > AMP-EMEA > AMP-MDEA > MEA. These results can be explained on the basis of the 13C NMR data. As an exemplification, in Figure 2 are reported the spectra of 3.0 mol dm-3 AMP-EMEA solutions at the end of the loading experiments carried out in the three different diluents: water, EG-PrOH and EG-DEGMME. Similar spectra for AMP and AMP-MDEA are provided in Supporting Information.

Figure 2. 13C NMR spectra of AMP-EMEA solutions in H2O, EG-PrOH and EG-DEGMME at the end of the loading experiment. Asterisks refer to the carbon atoms of amine carbamate. C


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denotes the carbonyl atoms of amine carbamate or alkyl carbonate. b/c refers to the bicarbonate/carbonate ion. The height of the peaks at 158-165 ppm is not in scale. As already reported in a previous study,19 the loading in water is higher than in organic diluents. In water, as also stated by other authors,41 bicarbonate was the only product found at the end of the experiment (Figure 2), without a detectable amount of the carbamate of the two amines. The steric hindrance near the amine functionality and the strong excess of CO2 in solution make the carbamates of the two amines less stable than bicarbonate [equation (3) and equations (2) + (4)]; the consequence is a substantial enhancement of CO2 loading (theoretical loading value of 1). On the contrary, in organic diluents the formation of bicarbonate cannot occur and the reaction between the amines and CO2 leads to the amine carbamates, with a theoretical loading value of 0.5 [equation (2)]. Loading values higher than 0.5 (Table 1) are due to the reaction between CO2 and the hydroxyl groups of alcohols and glycols that form alkyl carbonates [equation (8)]. In Figure 2 are shown the chemical shifts of EMEA carbamate (δ = 164.4 ppm) and those of the carbonates of EG, PrOH and DEGMME. There are no appreciable amounts of AMP carbamate: AMP rather favours the deprotonation of the alcohols and allows the formation of the alkyl carbonates. The higher loading value in EG-PrOH compared with EGDEGMME is a clear evidence of the greater reactivity of CO2 and PrOH compared to DEGMME, as shown by the greater intensity of the peak of PrOCO2− than that of DEGMME carbonate in their 13C NMR spectra (Figure 2). In order to better understand the effect of the different diluents on the reaction mechanism with CO2, some samples of solution were taken at different steps, from the beginning to the end, of the loading experiment. Figure 3 shows the spectra of the carbonyl atoms of AMP-EMEA after 1, 2, 3, 4, 5, 10, 30 and 210 minutes both in water (Figure 3A) and in EG-DEGMME (Figure 3B).


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During the first few minutes of the experiment, when the amines are in great excess with respect to CO2, aqueous solution of AMP-EMEA (Figure 3A) quickly reacts to form mainly EMEA carbamate [equation (2), signal at 164.4 ppm] and a smaller amount of carbonate/bicarbonate ions [equations (3) + (5)]. The signal at 165-166 ppm of fast exchanging bicarbonate/carbonate ion corresponds approximately to 20-30% of bicarbonate.39

Figure 3. 13C NMR spectra of the carbonyl zone of AMP-EMEA solutions in H2O (A) and in EGDEGMME (B) at various steps of the loading experiment. C denotes the carbonyl atoms of amine carbamate or alkyl carbonate. b/c refers to the bicarbonate/carbonate ion.

By continuing the experiment, when the amount of free amine in solution decreases, the reactions of CO2 with carbamate and carbonate ions in solutions are right hand shifted [equations


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(4) and (6)], and consequently the amount of bicarbonate increases. Figure 3A clearly shows that the carbamate peak (C

EMEA)

decreases over time while the peak of the fast exchanging

bicarbonate/carbonate (b/c) increases in intensity and moves to high field, to chemical shift values that correspond to higher percentages of bicarbonate.39 At the end of the experiment the carbamate peak (C

EMEA)

is no longer detectable (Figure 3A,

210’), which means that all the carbamate has been converted into bicarbonate. The signal of the fast exchanging bicarbonate/carbonate after 210 minutes at 161.4 ppm indicates nearly 85% of bicarbonate and 15% of carbonate, which explains the high loading value in water. As expected, in organic diluent (Figure 3B) the reaction mechanism is quite different: the reaction starts with the formation of only one product, the EMEA carbamate (δ = 164.4 ppm). The formation of AMP carbamate is not observed and AMP acts as the base for the formation of EMEA carbamate [equation (2)]. After a few minutes, when a large part of the initial EMEA has been converted to carbamate, the amount of EG carbonate increases [equation (8)], as indicated by the carboxyl signal (C

EG,

δ = 159.7 ppm) which becomes increasingly more intense over time. The

formation of DEGMME carbonate (C DEGMME, δ = 159.3 ppm) is definitely lower than that of EG carbonate and it is observed when CO2 becomes in strong excess. From the comparison of the CO2 loading of the different amines in the same diluent, AMPMDEA results the worse absorbent in each diluent (Table 1), because of the lower basicity of the tertiary amine MDEA with respect to AMP and EMEA, and the inability of MDEA to produce the carbamate derivative, as clearly shown in Figure S1 in Supporting Information. It’s noteworthy that in water, where comparison is possible, the loading values of AMP and its blends (in the range 0.875-0.813) are always higher than the one of MEA (0.609), which forms more carbamate than bicarbonate, even with an excess of CO2 (Figure 4). As previously reported,19


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from the careful integration of the NMR peaks of each -CH2- resonance of aqueous MEA, the relative amount of MEA carbamate with respect to total MEA is 37% on molar scale.

Figure 4. 13C NMR spectra of AMP, AMP-EMEA, AMP-MDEA and MEA aqueous solutions at the end of the loading experiment. Asterisks refer to the carbon atoms of amine carbamate. b/c refers to the bicarbonate/carbonate ion. C denotes the carbonyl atom of MEA carbamate. The height of the peaks at 158-166 ppm is not in scale.

Similarly, from the peak integration of the carbonyl group, MEA carbamate (C

MEA,

δ = 164.3

ppm) is 64% on molar scale with respect to the sum of carbamate and bicarbonate/carbonate ion


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(b/c, δ = 160.6 ppm). In summary, aqueous MEA reacts with CO2 to form more carbamate [equation (2), theoretical loading value of 0.5] than bicarbonate [equation (3), theoretical loading value of 1], and consequently its loading value is always lower than AMP and its blends. The loading of single AMP is appreciably higher than AMP–MDEA both in aqueous and non-aqueous solutions (Table 1): the tertiary amine MDEA is unable to produce carbamate and because of its lower basicity, it does not give as much bicarbonate as AMP in water. The loading of single AMP is comparable to that of its blends with EMEA in aqueous solution because AMP and EMEA have a quite similar basicity, and consequently give analogous amount of bicarbonate. The small reduction of the loading of the AMP-EMEA blend in non-aqueous solutions compared to single AMP can be tentatively explained by the amount of EMEA carbamate which does not compensate the reduced amount of alkyl carbonates formed by AMP, due to the half concentration of AMP in the blends (1.5 mol dm-3 instead of 3.0 mol dm-3). On this subject, we can point out that alkyl carbonates are by far the prevailing carbonated species in EG-PrOH and EG-DEGMME solutions of AMP (Figure S2, Supporting Information). 3.2.

CO2 capture profile as a function of time

The uptake of CO2 as a function of time by the different amine solutions has been measured at 25 °C as described in section 2.3; the results are reported in Figure 5. Figure 5B shows the starting absorption rates, relative to the first 30 seconds of the experiment for each sorbent, when the amine is in strong excess and the rate values are quite high for all the solutions. As a general finding, for the same amine or amine blends, the starting rate of CO2 uptake in organic diluents is always higher than in water, with no appreciable differences between EG-PrOH


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and EG-DEGMME diluents. Moreover, the addition of EMEA to AMP increases the reaction speed, while MDEA has the opposite effect. The solutions that cannot form carbamates, such as aqueous AMP and AMP-MDEA, show a low reaction rate. The highest rates are observed for AMP-EMEA in organic diluents, where the respective carbamate is formed during the first few seconds of absorption, as previously observed in Figure 3.


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Figure 5. (A) variation of the ratio between mol of CO2 captured and mol of amine in solution over time. “aq” denotes aqueous solutions, “e-p” denotes EG-PrOH solutions, “e-d” indicates EGDEGMME solutions. (B) Detail of the first 30 seconds of the CO2 uptake. All these findings are congruent with other previous data19 and suggest that the carbamate is more kinetically favoured than both bicarbonate and alkyl carbonate, and the starting reaction rate is therefore higher in systems where carbamates are easily formed.


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Since the free amine decreases with the absorption of CO2, in the presence of water CO2 can react with the amine carbamate, thus increasing the amount of bicarbonate formed [equation (4)]. By the consequence, the amount of absorbed CO2 increases in aqueous solutions (Figure 5A) and significantly overcomes that in organic diluents. At the end of the experiment, according also to the data reported in Table 1, the highest absorption capacities are obtained by the three aqueous systems, where bicarbonate is the prevailing species.

3.3.

Heat of CO2 absorption

One of the most important parameters for the assessment of the energy needed in a CO2 capture system is the heat of CO2 absorption (ΔHabs). The total heat (Qtot) needed for the regeneration of the sorbent can be approximated as: Qtot = Qsens + Qvap + ΔHdes

(9)

where Qsens is the sensible heat needed to increase the temperature of the solution from the absorption temperature (Tabs) to the desorption temperature (Tdes), Qvap is the heat required to form the stripping vapor in the desorber and ΔHdes is the CO2 desorption heat, which could be considered equal to ΔHabs, in absolute value.24,29,30,42 In this work the heat of absorption for the different amine solutions has been computed as described in section 2.4 by using the Gibbs-Helmholtz equation; the obtained values are reported in Table 2. All the determined values of CO2 equilibrium loading at different PCO2 and temperatures are provided in Table S1 in Supporting Information. Moreover, Figure S3 in Supporting Information provides an exemplification of the application of the Gibbs-Helmholtz equation for several solutions.


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The ΔHabs value of aqueous AMP obtained in this work (-73.01 kJ mol-1) is in good agreement with data reported by other authors.24,30,33

Table 2. Heat of CO2 absorption ΔHabs (kJ mol-1) calculated for the different amine solutions. diluent

H2O

amine

EG-PrOH

EG-DEGMME

AMP

-73.01

-66.39

-59.14

AMP-EMEA

-70.10

-63.00

-54.08

AMP-MDEA -57.64

-55.69

-52.94

MEA

-83.24

Sorbents made by amine blends feature heat of absorption (absolute value) lower than that of AMP solutions; in particular, the tertiary amine MDEA lowers the -ΔHabs much more than the secondary amine EMEA. Moreover, the experimental results reported in Table 2 indicate that, for the same amine or amine blend, the different diluents significantly influence the heat of absorption, with absolute values that decrease in order H2O > EG-PrOH > EG-DEGMME, which indicates a lower energy for sorbent regeneration in organic diluents. It’s noteworthy that all the formulations show absolute values of ΔHabs lower than aqueous MEA (Table 2).

3.4.

Comparison of the CO2 absorption efficiency


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An ideal sorbent for CO2 capture should combine high loading capacity, high absorption rate and low energy demand for regeneration. In Figure 6 are summarized the results of the batch experiments here reported.

Figure 6. Summary of the data obtained in the batch experiments for each amine solution. “aq” indicates aqueous solutions, “e-p” indicates EG-PrOH solutions, “e-d” indicates EG-DEGMME solutions.

Although the blends of AMP with the tertiary amine MDEA can be easier regenerated because of their lower heat of reaction with CO2, these absorbents are unsuitable for an amine scrubbing process that works in continuous because of their low rate of CO2 capture and their low loading in organic diluents. Considering the loading capacity, the absorption rate and the heat of CO2 absorption, the AMP-EMEA-based sorbents are good candidates for CO2 capture processes as


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alternatives to the conventional aqueous MEA solution. In particular, AMP-EMEA in EGDEGMME shows a very low heat of CO2 absorption (absolute value), yet attaining a quite similar reaction rate than in EG-PrOH and in water. In order to have a more realistic interpretation of the performances of the AMP-EMEA solutions in H2O and in EG-DEGMME, their CO2 absorption efficiency from a gas mixture (15% CO2 v/v) has been evaluated in continuous cycle of CO2 capture and sorbent regeneration, and compared with single AMP and aqueous MEA. The solutions are continuously circulated in the apparatus described in section 2.5 between the absorber (at 40°C) and the desorber (at 110°C), until the reactions of CO2 capture and amine regeneration reached a steady state and the measured capture efficiency remained unchanged over time. Moreover, for each formulation we have measured the viscosity of: (i) the starting solution, (ii) the loaded solution coming from the absorber at the end of the experiment and (iii) the regenerated solution coming from the desorber at the end of the experiment. In Table 3 are summarized the results obtained.

Table 3. CO2 absorption efficiency (Tabs=40°C; Tdes=110°C) and viscosity values (at 40°C). Viscosity at 40°C (mPa s) Amine MEA AMP

AMP-EMEA

Diluent

Abs Eff % starting

loaded

regenerated

H2O

99.4%

1.66

2.28

2.08

H2O

93.0%

1.90

2.37

1.99

EG-DEGMME

72.4%

9.22

14.06

10.56

H2O

99.6%

1.81

2.26

1.92

EG-DEGMME

85.9%

7.95

14.07

10.74


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The measured value of viscosity for aqueous MEA and AMP well agree with data from literature.43,44 As a general remark, the increased viscosity of the sorbents after CO2 capture ranges between 25% and 37% with respect to the unloaded sorbents. Furthermore, the viscosity of the regenerated sorbents greater than the starting solutions clearly indicates that the regeneration of the carbonated amines is not complete because of the predominant kinetic constraints. As already found in our previous work,19 the CO2 capture efficiency of the same amine is higher in water than in EG-DEGMME (Table 3), likely due to the greater viscosity of the organic diluents, which greatly limits the mass transfer between gas and liquid.45 Nonetheless, it is important to underline that the CO2 capture in organic diluents is still a feasible process.45,46 The AMP-EMEA blend is always more efficient than single AMP in the same diluent (Table 3), even if the loading capacity of AMP-EMEA is slightly lower than AMP (Table 1). That result is a clear evidence that kinetic constraints of AMP may overcome its more favourable thermodynamic properties (Figure 5). At the same operating conditions, in a continuous cycle of absorption and desorption, 3.0 mol dm-3 aqueous solution of AMP-EMEA achieves the same CO2 capture efficiency of aqueous 30 wt% MEA (4.9 mol dm-3), but with a lower energy demand for the regeneration due to the low absolute value of heat of CO2 absorption (Table 2). The CO2 capture efficiency of AMP-EMEA in EG-DEGMME is slightly lower (85.9%), but with the advantage of a possible considerable energy saving, due to the very low (absolute value) heat of CO2 absorption (-54.08 kJ mol-1). Moreover, an approximately 25% less sensible heat could be easily calculated (see Supporting Information), due to the lower heat capacity of organic diluents compared to water. In addition, the energy saving should be further improved thanks to the neglectable vaporization of EG and DEGMME at the regeneration temperature of 110 °C, compared to the evaporation of water.34


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It should be pointed out that these non-aqueous alkanolamine systems can tolerate moisture up to 5% (v/v) of water, as already determined in previous works.34,47,48 4. CONCLUSIONS With the aim of developing innovative formulation with low energy consumption for efficient CO2 capture, the features of single 2-amino-2-methyl-1-propanol (AMP) and its blends with 2(ethylamino)ethanol (EMEA) or N-methyl-2,2’-iminodiethanol (MDEA) has been exploited both in aqueous and non-aqueous solutions and compared with those of the most used sorbent in conventional CCS processes, the aqueous 2-aminoethanol (MEA). An accurate 13C NMR analysis has allowed us to follow the reaction process step by step and to establish a correlation between the CO2 capture performances and the distribution of the different species in the amine/CO2/diluent system. As a general finding, for the same amine or amine blend, the CO2 loading is higher in water than in organic diluents, due to the bicarbonate formation, while the higher reaction rate in organic diluents is due to the faster formation of the amine carbamate compared to bicarbonate. In order to evaluate the impact of these factors on the CO2 capture efficiency from a gaseous mixture, we performed experiments in continuous cycles of absorption and sorbent regeneration, to simulate a real process. Moreover, the determination of the heat of CO2 absorption (ΔHabs) allowed us to compare the regeneration energy of the different solutions. Considering their good CO2 capture efficiency and low heat of absorption, the AMP-EMEA blends, either in water and in EG-DEGMME, gave reasonable indications of potential advantages over conventional aqueous 30 wt% MEA. However, a careful assessment of costs and benefits will be required before such sorbent formulations can be implemented in a commercial system.


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ASSOCIATED CONTENT Supporting Information. 13C NMR analysis settings and procedures; comparison of the sensible heat of aqueous 30 wt% MEA and AMP-EMEA in EG-DEGMME; spectra of 3.0 mol dm-3 AMP and AMP-MDEA solutions at the end of the loading experiments carried out in the three different diluents; exemplification of the application of the Gibbs-Helmholtz equation; determined values of CO2 equilibrium loading at different PCO2 and temperatures. ACKNOWLEDGMENT For the financial support, the authors thank ICCOM Institute of National Research Council, Regione Toscana – POR FSE 2014-2020 as well as the industry partner STM Technologies srl. For the facilities, the Department of Chemistry, University of Florence, is gratefully acknowledged.

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TABLE OF CONTENTS (TOC)


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Comparative Study of CO2 Capture by Aqueous and Nonaqueous 2

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