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Characterization and comparison of the CO2 absorption performance into aqueous, quasi-aqueous and non-aqueous MEA solutions
International Journal of Greenhouse Gas Control 63 (2017) 281–288
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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
Characterization and comparison of the CO2 absorption performance into aqueous, quasi-aqueous and non-aqueous MEA solutions Min-Kyoung Kanga, Soo-Bin Jeona, Joon-Hyung Chob, Jin-Seop Kimc, Kwang-Joong Ohd,
MARK
⁎
a
Institute of Environmental Studies, Pusan National University, San 30, Jangjeon-dong, Busan 609-735, Republic of Korea Division of Creative Low Impact Development and Management for Ocean Port City Infrastructures, Pusan National University, San 30, Jangjeon-dong, Busan 609-735, Republic of Korea c Department of Geological Sciences, Pusan National University, San 30, Jangjeon-Dong, Geumjeong-Gu, Busan 609-735, Republic of Korea d Department of Environmental Engineering, Pusan National University, San 30, Jangjeon-Dong, Geumjeong-Gu, Busan 609-735, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Keywords: Carbon dioxide Quasi-aqueous Non-aqueous Organic alcohol Absorption Renewable energy
Herein, we investigated quasi-aqueous and non-aqueous absorbents based on organic solvents to reduce the amount of water used in wet CO2 capture by amine- and water-based sorbents to < 50%. Solvent optimization was performed through screening and vapor pressure tests, and the performances of quasi-aqueous and nonaqueous sorbents prepared using selected organic solvents were evaluated in terms of CO2 absorption rates and equilibrium absorption capacities. All absorbents were subjected to thermogravimetric analyses before and after CO2 absorption to determine their thermal stabilities. Moreover, the prepared absorbents were characterized in terms of reaction heat, sensible heat, and latent heat of evaporation, which were used to determine their regeneration energies. The obtained results revealed that quasi-aqueous/non-aqueous absorbents are superior to conventional water-based ones, exhibiting maximized CO2 absorption and lower regeneration energies.
1. Introduction The amount of CO2 released into the atmosphere by the continuous use of fossil fuels is increasing every year. To mitigate this problem, carbon capture and storage (CCS) technologies have been developed to capture and store CO2, preventing it from entering the atmosphere (Lepaumier et al., 2009; Reynolds et al., 2012). At present, absorption is the most practical and quick method of capturing CO2 from combustion flue gas, mostly performed using alkanolamines (Hendriks, 1994) such as monoethanolamine (MEA), Nmethyldiethanolamine (MDEA), diethanolamine (DEA), and 2-amino-2methyl-1-propanol (AMP). Absorbents based on aqueous solutions of such amines exhibit highly efficient and fast CO2 absorption, are sufficiently technologically advanced for commercialization, and can be processed on a large scale. Among these absorbents, MEA-based ones are the most popular due to their low cost and fast carbon dioxide capture. However, the amine functional group of MEA is strongly bound to the absorbent used in the current process, requiring energy-consuming amine regeneration (separation) at temperatures above 120 °C(Zhang J et al., 2011). Moreover, this regeneration causes decomposition of the MEA absorbent, resulting in its rapidly deteriorating performance. Consequently, the capture process is very expensive, requiring ⁎
Corresponding author. E-mail address: [email protected] (K.-J. Oh).
http://dx.doi.org/10.1016/j.ijggc.2017.05.020 Received 28 December 2016; Received in revised form 12 April 2017; Accepted 26 May 2017 1750-5836/ © 2017 Elsevier Ltd. All rights reserved.
continuous absorbent supply to compensate for the consumed amounts (Hartono et al., 2007; Munoz et al., 2009; Strazisar et al., 2003). The use of water as a solvent also poses a problem due to its high specific and latent heat, which is reflected in the high energy required to dissipate the chemical bond between CO2 and the absorbent (Lin and Wong, 2014). As a result, heating to high temperatures accounts for 40–60% of the total energy consumption associated with the CO2 absorption process (Peeters et al., 2007; Singh et al., 2009), with only a small amount of latent heat consumed by water heating. Thus, to make the absorption process more energy-efficient, the absorbent needs to be regenerated at temperatures below 80–100 °C. The above problems can be mitigated by using organic solvents to replace water in conventional amine-based absorbents, resulting in much lower CO2 recovery and solvent regeneration energies(Barbarossa et al., 2013; Barzagli et al., 2013; Lail et al., 2012, 2011). However, significantly lower CO2 absorption is generally observed for organic solvent–based absorbents due to their high net absorbent exchange rate, necessitating the development of new absorbents with high thermal/ chemical stability and low vapor pressure to overcome the abovementioned disadvantages(Heldebrant et al., 2011, 2008, 2009; Jessop et al., 2005). This study aims to address the problems associated with the use of amine-based aqueous sorbents for wet CO2 capture, evaluating the
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Nomenclature
α
Symbols
Acronyms
B′ NA
PCO2 PN2 Peq
VLE MEA MDEA DEA AMP TGA DSC
Bronsted base Absorption rate of species A at the gas–liquid interface for a chemical reaction in the continuous phase, [kmol m2 s−1] Partial pressure of carbon dioxide Initial pressure Equilibrium pressure
2.2. CO2 absorption rate Eq. (5) shows that the absorption rate of CO2 may be calculated from its molar flux (NA) from the gas phase into the liquid phase, partial pressure(PCO2, g ) , and the overall mass transfer coefficient. For irreversible absorption, the equilibrium CO2 pressure in bulk liquid (P* CO2 ) equals zero (McCabe et al., 2005), and Eq. (5) can be simplified to Eq. (6):
2. Theoretical background 2.1. CO2 absorption mechanism
NA = K G P (yA − yA* )
The reactions of CO2 in aqueous solutions of primary alkanolamines (Danckwerts, 1979), follow the generally accepted zwitterion mechanism (Eq. (1)) originally proposed by Caplow (1968) and reintroduced by Danckwerts (1979).
RNH2+COO−
+ B′ ⇔
RNHCOO−
PCO2, lm =
(1)
+
PCO2, in − PCO2, out ln (PCO2, in/ PCO2, out )
(7)
Here, subscripts “in” and “out” correspond to gas flows to and from the wetted wall column (WWC), respectively. The bulk CO2 pressure in the gas phase can be calculated from the logarithmic mean (PCO2, lm , Eq. (7)). In the corresponding graphical representation, the abscissa denotes the average partial pressure of CO2, and the ordinate represents its molar flux into the liquid phase, with KG obtained from the slope of the linear fit.
(2)
B′H+
(5)
* 2 ) (6) NA = K G (PCO2, g − PCO
The above reaction comprises two steps, i.e., the formation of CO2amine zwitterions (Eq. (2)) and their base-catalyzed deprotonation (Eq. (3)).
CO2 + RNH2 ⇔ RNH2+COO−
Vapor–liquid equilibrium Monoethanolamine N-Methyldiethanolamine Diethanolamine 2-Amino-2-methyl-1-propanol Thermogravimetric analysis Differential scanning calorimetry
greatly influenced by pH, being increased by increasing solution basicity. Despite the slow CO2 absorption, absorbent regeneration is relatively easy, and the extent of its degradation in repeated absorption/ regeneration experiments is low. The reaction is influenced by the functional groups of the organic alcohol, since they can change its basicity, and by the nature of the amine absorbing agent, which affects the CO2 absorbing ability, absorption rate, and absorbent regenerability. The acid dissociation constant of the sorbent (pKa) is a measure of its basicity, which indicates how well the sorbent can react with CO2, an acidic gas. Low sorbent basicity complicates the deprotonation of the amphoteric salt produced from the amine and CO2, and one mole of CO2 per mole of amine can remain bonded as a result. For organic alcohols, the presence of a hydroxyl group makes the NeH bond of the above amphoteric compound unstable. In addition, amine basicity can be increased by increasing the electron density around the aminic nitrogen.
properties (e.g., specific heat) of a quasi-aqueous absorbent, which utilizes ≤50% water as a solvent compared to conventional aqueous absorbents, and a non-aqueous absorbent, which utilizes an organic solvent in place of water. In the first step, the absorption and regeneration efficiencies of MEA solutions in ethanol, methanol, ethylene glycol, and glycerol were investigated. The amount of absorbent lost per unit reaction time was determined to minimize loss to the gas phase, which is a major issue of the absorption/regeneration process, and an optimum organic alcohol was selected to replace water in the preparation of quasi-aqueous and non-aqueous sorbents. The performance of the thus obtained absorbents was evaluated in terms of their absorption rates and absorption capacities, and thermogravimetric analyses (TGA) were performed before and after CO2 absorption to gain insight into their thermal stabilities. In addition, the heat of reaction, sensible heat, and latent heat of evaporation of the investigated absorbents were calculated to derive their regeneration energy. Thus, quasi-aqueous and non-aqueous sorbents were optimized for maximum regeneration performance and low regeneration energy to investigate the feasibility of their use in actual CO2 absorption processes.
CO2 + 2RNH2 ⇔ RNHCOO− + RNH3+
CO2 loading ratio, [mol CO2/mol amine]
(3)
−
where B′ = amine, OH , or H2O (Blauwhoff et al., 1983). The equilibrium loading capacities of primary and secondary alkanolamines are stoichiometrically limited to 0.5 mol CO2/mol amine (Eq. (1)). For normal primary amines, such as MEA, the produced carbamates are quite stable. Organic alcohols lack NeH bonds and thus cannot directly react with CO2 to form carbamates. Instead, these alcohols are converted to trialkylammonium hydroxides ([R1R2R3NH]OH) by a reaction with water, and the produced OHe reacts with CO2 to bind it as bicarbonate (Eq. (4)).
R1 R2 R3 N + CO2 + H2 O ⇔ R1 R2 R3 N+H + HCO3−
2.3. CO2 equilibrium absorption capacities The absorption capacities of different absorbents were experimentally determined by measuring the pressure changes between the initial and equilibrium states of the system. Subsequently, the partial pressure of CO2 (PCO2 , atm) was calculated from the difference between initial (PN2 ) and equilibrium absorption (Peq) pressures.
PCO2 = (Peq − PN2)
(13)
The obtained value was used in combination with the ideal gas equation of state to determine the extent of CO2 absorption (Eq. (14)). The amount of absorbed CO2 (nCO2 , mol) was calculated by multiplying its volume (V) by the difference between CO2 injection pressure
(4)
The above reaction requires a lower temperature and is slower than that of carbamate formation. Furthermore, the rate of CO2 absorption is 282
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(PN2 + CO2 ) and the equilibrium pressure (Peq), and dividing the obtained value by RT.
nCO2 =
(PN2 + CO2 − Peq) V RT
(14)
The absorption capacity (α) could then be obtained by dividing nCO2 by the number of moles of the amine:
α = nCO2 /(moleofamine )
(15)
The rate of absorption (NA) and the amount of absorbed gas were calculated from the difference between the absorption and the flow of gas from the reactor into the reactor's initial flow. Conversion of the above amount into moles allowed the absorption per reactor unit area at the gas–liquid interface area to be obtained. The absorption rate was calculated using the following equation, with the initial volumetric absorption rate, V(t1)/t1, obtained from the cumulative volume of gas that had flown through the CO2 analyzer:
NA =
PT − PW V (t1) SRT
Fig. 1. Apparatus used for screening experiments. (1) Water bath of absorber, (2) water bath of desorber, (3) saturator, (4) CO2 cylinder, (5) N2 cylinder, (6) CO2 analyzer, and (7) condenser.
Ethyl alcohol (99.5%), methyl alcohol (99.5%), ethylene glycol (99.0%), and glycerol (99.0%), were supplied by Samchun Pure Chemicals (Korea). Aqueous solutions were prepared using distilled water. Commercial grade CO2 and N2 gases with purities of 99.99% were used.
(16)
where PT is the atmospheric pressure, PW is the pressure of water vapor, S is the surface area of the liquid phase, and V(t1) is the cumulative volume of gas absorbed during absorption time t1.
3.2. Screening test Ethyl alcohol, methyl alcohol, ethylene glycol, and glycerol were selected for the preparation of quasi-aqueous/non-aqueous sorbents, and their relative CO2 absorption rates and absorption/regeneration capacities were investigated using screening tests. A schematic diagram of the experimental setup is presented in Fig. 1. The absorbent (200 mL) was fed into a 500-mL glass reactor and heated to 313 K using a water bath. A mixture of N2/CO2 (15 vol% CO2) was fed into the reactor at a flow rate of 500 mL/min. After 30 min of CO2 absorption, the reactor was moved to another water bath for absorbent regeneration. A regeneration temperature of 353 K was used, since the lipophilic amine could be regenerated at a lower temperature than the alkanolamine. During regeneration, the absorbent was stripped with N2 gas for ∼30 min, and the gas exiting the absorption/regeneration reactor was analyzed using a CO2 analyzer (ZRF model, Fuji Electric, Japan). After the absorption/regeneration test, the CO2 loading ratio was analyzed using an automatic titrator (702 SM Titrino, Metrohm, Riverview, FL, USA) to quantify the regeneration efficiency.
2.4. Regeneration energy The enthalpy of CO2 absorption by the amine absorbent, i.e., the heat of reaction, was calculated using the Gibbs-Helmholtz equation (Eq. (17)), as reported by Mathonat et al. (1998):
⎡ ∂lnPCO ⎤ ΔH 2 ⎥a =⎢ R ⎢ ∂ 1 ⎥ T ⎣ ⎦
()
(17)
where PCO2 is the equilibrium partial pressure of CO2, T is the absolute temperature of the system, a is the CO2 loading, and R is the universal gas constant. In addition to differential calculations, the integral of the temperature change is determined by the heat of the dissolution reaction and can be calculated as:
Q = m ·C p·ΔT
(18)
where Q is the heat of absorption, m is mass of absorbed CO2, Cp is the absorbent heat capacity after absorption of CO2, and ΔT is the temperature change after injection of CO2 into the equilibrium cell. The biggest disadvantage of chemical absorption is the large energy required for absorbent regeneration, which is the main factor determining process design and economic feasibility. The phase change heats of pristine and CO2-loaded absorbents were measured using differential scanning calorimetry (DSC), and the regeneration heat of aqueous amine solution was calculated using the difference between the abovementioned values (Eqs. (19) and (20)). ' ΔHamine − s = ΔHamine − s × Xamine − s
(19)
ΔHreg = (ΔHamine + H2 O + CO2 − ΔHamine − s ') ÷ yCO2
(20)
3.3. Wetted wall column A WWC similar to that used by Choi et al. (2012) was employed to investigate the absorption rate, with the experimental setup shown in Fig. 2. The central gas–liquid contactor was constructed from a stainless steel tube (91 mm long, outer diameter 12.6 mm). The column was surrounded by a cylindrical thick glass wall, and the entire chamber was surrounded by a second glass wall with a liquid (such as paraffin oil) flowing between the two walls as a heat transfer medium. The absorbent was pumped upwards into the column and flowed out at the top, supplying liquid to the bottom of the chamber. The gases came in cross-current contact with the liquid and subsequently exited at the top. During the experiment, a water bath was used to maintain the absorbent and paraffin oil at a constant temperature. The pressure inside the reactor was measured with pressure transducers (MGI/MGAMP series, accuracy = ± 0.1 kPa) installed in the reactor and feeder. The concentration of CO2 in the feed gas stream equaled 1–20 vol%, and gas flow rates were controlled using mass flow controllers (5850E, Brooks Instrument, Hatfield, PA, USA). CO2 concentrations were analyzed using a gas chromatograph (7890A, Agilent Technologies), with a 30 m × 0.32 m packed column (GS-GasPro, Agilent Technologies) equipped with a thermal conductivity detector (TCD) utilized as the GC column.
where ΔHamine−s is the difference of absorbent phase change heat (kJ/ kg), ΔHamine−s' is the corrected difference of absorbent phase change heat (kJ/kg), ΔHamine+H2O+CO2 is the difference of phase change heat for absorbent + CO2 (kJ/kg), ΔHreg is the heat regeneration difference (kJ/(mol CO2)), Xamine−s is the mole fraction of absorbent, and yCO2 is the mole fraction of CO2. 3. Materials and methods 3.1. Materials Analytical grade MEA (99%) was supplied by Sigma-Aldrich (USA). 283
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an SDT-Q600 analyzer and a thermal analysis controller (TA Instruments). TGA and DSC measurements were performed at 25–250 °C, using nitrogen as a purge gas at a flow rate of 100 mL/min and 40 mg absorbent samples. The heating rate was kept constant at 2 °C/min. All data point measurements were performed in triplicate, and the average value was reported.
4. Results and discussion 4.1. Screening of organic alcohols Herein, we tried to select an organic solvent to replace water and obtain a quasi-aqueous/non-aqueous CO2 sorbent. The absorption/regeneration efficiency was qualitatively screened, with the results obtained for MEA as a sorbent (concentration = 30 wt%) and ethanol, methanol, ethylene glycol, and glycerol as organic alcohols shown in Fig. 4. In these systems, the absorption efficiency of CO2 (15 vol%) was measured at 313 K over 30 min. Fig. 4a shows that the lowest CO2 outlet gas concentrations equaled 1.51, 1.95, 4.38, 5.03, and 5.74% for ethylene glycol, glycerol, methyl alcohol, ethyl alcohol, and H2O, respectively, revealing that higher absorption efficiencies were obtained for organic solvents than for the conventionally used water. Moreover, faster CO2 saturation was also observed for organic solvents. This behavior probably originated not only from the effect of the amine absorbent but also from that of organic alcohol functional groups, which affected basicity. Fig. 4b shows sorbent regeneration efficiencies obtained for 30 min N2 injection at 353 K. The use of organic alcohols instead of water resulted in significantly higher CO2 removal efficiencies compared to that of the conventional water absorbent. The waterborne sorbent had to be regenerated at high temperature (393 K) due to the high specific heat of water attributed to hydrogen bonding. Therefore, its regeneration efficiency at low temperature (353 K) was considered to be low. On the other hand, organic alcohols do not exhibit overly high specific heats,
Fig. 2. Apparatus used for the wetted wall column experiment. (1) N2 cylinder, (2) CO2 cylinder, (3) mass flow controller, (4) mixing chamber, (5) saturator, (6) wetted wall column, (7) water bath, (8) absorbent inflow, (9) absorbent outflow, (10) paraffin oil inflow, (11) paraffin oil outflow, (12) gas inflow, (13) gas outflow, (14) condenser, (15) GC/TCD, (16) thermocouple, and (17) pressure transducer.
3.4. Vapor–liquid equilibrium The vapor–liquid equilibrium (VLE) system was used to test the CO2 absorption equilibrium for all absorbents (Fig. 3), comprising a saturator for feeding CO2 into the reactor and the reactor itself. The reactor (height = 160 mm) was located in a temperature-controlled vessel. Four 5 mm-wide baffles and a two-blade impeller (70 mm × 20 mm) were installed inside the reactor. Temperature was measured with an accuracy of ± 0.1 K using a K-type thermocouple. Initially, CO2 gas was filled into the supplier and preheated by the electric heater. After removing the remaining reactor gas using a vacuum pump, 300-mL aliquots of each solution were added into the reactor using a syringe. CO2 loadings of each solution were calculated at different reactor pressures, and CO2 pressures in the reactor and supplier were determined using a pressure data logger (PR2000, MadgeTech, USA). 3.5. Vaporization The employed vapor pressure apparatus and the followed procedure are described elsewhere (Seo et al., 2012). The absorbent vapor pressure was measured using the abovementioned pressure data logger with an accuracy of ± 0.25% at 293–393 K. Experimental data supplied by the pressure transducer were automatically stored in a computer. Absorbent losses were determined at constant temperature (293–393 K) and relative humidity (65%). After 300 mL of the absorbent was fed into the reactor, it was heated to the desired temperature. The reactor was maintained at constant humidity using a thermal humidity chamber, and the absorbent concentration was analyzed using an automatic titrator (702 SM Titrino, Metrohm, Riverview, FL, USA). 3.6. TGA and DSC characterization TGA/DSC were used to measure the heat flow and weight change of samples (Kim et al., 2011). Moreover, DSC was employed to determine the difference in the heat required to increase the temperature of the sample and the reference as a function of temperature, which was used to calculate the heat of phase change. The TGA apparatus consisted of
Fig. 3. Apparatus used for characterizing the vapor–liquid equilibrium. (1) N2 cylinder, (2) CO2 cylinder, (3) mass flow controller, (4) supplier, (5) magnetic drive, (6) controller, (7) reactor, (8) pressure transducer, (9) computer, and (10) vacuum pump.
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CO2 outlet concentration [ % ]
to the former. Thus, the vapor pressure of this absorbent was the highest due to unfavorable hydrogen bond formation conditions. On the other hand, in the case of organic alcohols, one hydroxyl group (–OH) is present in the molecular structures of ethyl and methyl alcohols, two are present in ethylene glycol, and three in glycerol, with the number of hydroxyl groups being negatively correlated with absorbent vapor pressure. These results are considered to be affected by the chemical structure in the case of absorbent loss. Glycerol showed the lowest vapor pressure, which, however, was not much different from that of ethylene glycol, indicating that the absorption efficiency of the latter alcohol was higher than in screening experiments. Therefore, in this study, we selected ethylene glycol as an optimal organic solvent to replace water and prepare quasi-aqueous/ non-aqueous sorbents.
MEA/H2O MEA/Ethyl alcohol MEA/Methyl alcohol MEA/Ethylene glycol MEA/Glycerol
14 12 10 8 6 4 2
(a) 0
0
5
10
15
20
25
4.3. CO2 absorption rate
30
Reaction time [ min ]
CO2 outlet concentration [ % ]
14
As stated above, quasi-aqueous/non-aqueous sorbents were prepared using ethylene glycol as an optimum organic solvent instead of water. Absorption rates were determined for 30 wt% MEA/H2O, 30 wt % MEA/H2O/ethylene glycol (quasi-aqueous), and 30 wt% MEA/ethylene glycol (non-aqueous) sorbents at 293, 303, 313, and 323 K. Fig. 6 shows absorption rates of the above absorbents at various reaction temperatures and constant CO2 partial pressure (15 kPa), revealing that MEA/H2O, MEA/H2O/ethylene glycol, and MEA/ethylene glycol sorbents exhibited values of 6.88–9.63 × 10−6, 9.15 × 10−6, and 10.86–14.21 × 10−6 kmol m−2 s−1, respectively, which increased with increasing temperature. This behavior can be explained by the fact that the CO2 uptake entails a chemical reaction, the rate of which increases with temperature. The above absorption rate increase is considered to be linear due to the increased amounts of solvent and solute capable of absorbing CO2 and diffusion/collision at the gas–liquid interface. In addition, the absorption rates of water-based and quasi-aqueous/ non-aqueous absorbents were compared, with the corresponding differences between water-based and low-water absorbents, quasi-aqueous and non-aqueous absorbents, and between water-based and non-aqueous absorbents determined as 2.27–3.24 × 10−6, 1.34–1.71 × 10−6, and 3.98–4.58 × 10−6 kmol m−2 s−1, respectively. The absorption rate of the MEA/ethylene glycol non-aqueous sorbent was the highest. The variation of these results is explained by the corresponding variations of basicity, which depends on the functional groups of ethylene glycol. Thus, the presence of a hydroxyl group destabilizes the
(b)
12 10 8 6 4
MEA/H2O MEA/Ethyl alcohol MEA/Methyl alcohol MEA/Ethylene glycol MEA/Glycerol
2 0
0
5
10
15
20
25
30
Reaction time [ min ] Fig. 4. Variation of outlet CO2 concentration as a function of reaction time. (a) Absorption test at 313 K and (b) regeneration test at 353 K.
resulting in larger regeneration efficiencies at the same temperature.
300
4.2. Absorbent vapor pressure as a function of organic alcohol type CO2 uptake by the CO2 absorption process gets reduced due to absorbent vapor pressure. The absorbent vapor pressure also lowers absorbent performance due to volatilization (Hagaman et al., 2003). Therefore, the effect of vapor pressure was investigated at reaction temperatures of 293–393 K for four representative organic alcohols, namely ethyl alcohol, methyl alcohol, ethylene glycol, and glycerol. The effect of vapor pressure on the initial pressure and the difference in pressure due to the increase of reaction temperature was examined for direct absorbent injection. As shown in Fig. 5, vapor pressures of MEA/H2O, MEA/ethyl alcohol, MEA/methyl alcohol, MEA/ ethylene glycol, and MEA/glycerol mixtures equaled 8.97–250.90, 3.77–166.38, 2.03–138.20, 1.41–90.10, and 1.38–80.14 kPa, respectively, with increasing. The change in vapor pressure was largest for the conventional MEA/H2O absorbent, decreasing in the order of ethyl alcohol > methyl alcohol > ethylene glycol > glycerol. These results can be explained by the dependence of absorbent hydrophilicity on their chemical structure. For the aqueous MEA sorbent, the formation of hydrogen bonds between the amine (–NH2) and hydroxyl groups (–OH) is influenced by the alkyl groups present close
MEA/H2O MEA/Ethyl alcohol MEA/Methyl alcohol MEA/Ethylene glycol MEA/Glycerol
Vapor Pressure [ kPa ]
250
200
150
100
50
0
20
30
40
50
60
70
80
90
100
110
Temperature [¡É ] Fig. 5. Vapor pressure of absorbents as a function of temperature.
285
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16
increased difference in CO2 loadings (effective CO2 loading) between these two regions favors CO2 removal at elevated temperatures. MEA/H2O, MEA/H2O/ethylene glycol, and MEA/ethylene glycol absorbents exhibited ratios of ∼0.14, 0.43, and 0.43 mol at an atmospheric pressure of 101.3 kPa, respectively, with a value of 0.50 mol determined for the aqueous sorbent. Therefore, the difference in the absorption balance load of the non-aqueous sorbent was the largest, meaning that its absorption/regeneration ability was superior to that of the water-based absorbent.
MEA/H2O MEA/H2O/Ethylene glycol MEA/Ethylene glycol
NCO2 ¡¿106 [ kmol m-2 s-1 ]
14
12
10
4.5. TGA During TGA, a temperature program is applied to a sample, and its mass change caused by chemical reactions that produce volatile or gaseous products is measured as a function of time or temperature. Herein, the thermal stabilities of 30 wt% MEA/H2O, 30 wt% MEA/ H2O/ethylene glycol, and 30 wt% MEA/ethylene glycol absorbents were evaluated (Muhammad et al., 2008, 2009). Fig. 8a shows TGA results for different absorbents, revealing that aqueous, quasi-aqueous, and non-aqueous absorbents were stable up to 100, 130, and 140 °C, respectively, and indicating that thermal stability was negatively correlated with water content. In view of the above, it was concluded that the thermal properties of the absorbent were more dominant than those of water. When the absorbent is stripped at high temperatures, its concentration can be changed due to deterioration or evaporation, directly impacting CO2 absorption. Fig. 8b shows the TGA results obtained for CO2-saturated sorbents, revealing that the onset of their mass loss is observed at higher temperatures than that of unsaturated sorbents. This finding is attributed to absorbent destabilization due to the formation of carbamate.
8
6
293
303
313
323
Temperature [ K ] Fig. 6. Rate of CO2 absorption by various absorbents as a function of reaction temperature.
binding of the amphoteric compound produced by the reaction of the amine with CO2. Increasing the electron density of the aminic nitrogen causes the basicity of the corresponding amine to increase. Amine basicity is considered to be positively correlated with reactivity toward CO2, which is an acidic gas, with high basicities resulting in increased CO2 absorption rates. 4.4. CO2 absorption equilibrium The performance of quasi-aqueous/non-aqueous sorbents was evaluated using 30 wt% MEA, MEA/H2O/ethylene glycol, and MEA/ ethylene glycol sorbents, with absorption equilibrium experiments performed at 313 (absorption region) and 353 K (regeneration region). Since the results of the screening test in Section 4.1 suggested that the absorbent was effectively regenerated at 353 K, this temperature was further used for regeneration throughout this study. Fig. 7 shows CO2 absorption capacities (moles of CO2 absorbed per mole of absorbent) at 313 and 353 K determined for aqueous and quasiaqueous/non-aqueous absorbents, revealing values of ∼0.68 mol/mol for MEA/H2O and 0.87 mol/mol for MEA/H2O/ethylene glycol at ∼101.3 kPa, with the highest value (∼0.90 mol/mol) observed for MEA/ethylene glycol. MEA stoichiometrically absorbs up to 0.5 mol/ mol CO2 in aqueous solution as a result of carbamate formation. However, the produced carbamate is strongly destabilized by the alkyl group of the amine. Therefore, when MEA absorbs more than 0.5 mol/ mol of CO2, hydrolysis occurs, releasing the free amine and bicarbonate and resulting in the measured solubility being higher than the theoretical one. For organic alcohols in quasi-aqueous/non-aqueous absorbents, if the hydroxyl group in the molecule affects the direct reaction of amine and CO2, then the functional group influences the absorption of CO2 due to hydrogen bonding. On the other hand, the CO2 absorption capacities of MEA/H2O, MEA/H2O/ethylene glycol, and MEA/ethylene glycol sorbents at 353 K equaled ∼0.54, 0.44, and 0.40 mol/mol, respectively, indicating that the carbamate produced during CO2 absorption is thermally unstable. This instability was reflected in the fact that the amount of CO2 absorbed at 353 K was reduced compared to that absorbed at 313 K due to the weak binding and low stability of bicarbonate and the exothermicity of CO2 absorption. As evidenced by the above numbers, the nonaqueous absorbent exhibited the best regeneration performance. The absorbing capacity and the absorbing capacity of each of the absorbing regions were compared, with good absorbent performance characterized by a high loading of absorbed CO2 in the absorbing region and a small loading in the regeneration region. Furthermore, the
4.6. Regeneration energy The regeneration energy of CO2 absorbents is the energy required to remove CO2 in the regeneration tower, featuring contributions from the absorption reaction heat, the sensible heat consumed to heat the absorbent, and the latent heat of vaporization to generate steam in the regenerator. Herein, we quantitatively assessed the regeneration energy of the common water-based absorbent and quasi-aqueous/non-aqueous absorbents (Sakwattanapong et al., 2005; Song et al., 2008). Fig. 9 shows the total regeneration energy for each sorbent, with the Reaction at 313 K
Reaction at 353 K
MEA/H2O
MEA/H2O
MEA/H2O/Ethylene glycol
MEA/H2O/Ethylene glycol
MEA/Ethylene glycol
MEA/Ethylene glycol
CO2 Partial pressure [kPa]
150
100
50
0 0.0
0.2
0.4
0.6
0.8
1.0
CO2 loading ratio [ mol CO2/mol amine ] Fig. 7. Equilibrium CO2 absorption capacities of different absorbents at various CO2 partial pressures at 313 and 353 K.
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heat is attributed to the high binding energy between the absorbent and CO2. Consequently, a large amount of energy is consumed in sorbent regeneration as it is separated from the bound CO2. Sensible heat is the largest contributor to regeneration energy, equaling 2573 kJ/kg CO2 for the aqueous sorbent, 1823 kJ/kg CO2 for the quasi-aqueous sorbent, and 1693 kJ/kg CO2 for the non-aqueous sorbent. Compared to the water-based absorbent, quasi-aqueous and non-aqueous absorbents showed values lower by 750 and 880 kJ/kg CO2, respectively. The latent heats of vaporization for aqueous, quasi-aqueous, and non-aqueous absorbents equaled 706, 423, and 354 kJ/kg CO2, respectively, with the corresponding regeneration energies estimated as 5226, 3981, and 3590 kJ/kg CO2. The water-based absorbent possessed the highest specific and latent heat. Quasi-aqueous and non-aqueous sorbents, utilizing organic solvents instead of water, required much less energy to regenerate CO2 than water-based sorbents, being advantageous for reducing the energy consumption associated with CO2 capture.
MEA/H2O MEA/H2O/Ethylene glycol MEA/Ethylene glycol
Weight Loss [ - ]
15
10
5
(a ) 0 20
40
60
80
100
120
140
160
180
200
Temperature [ ¡É ] 20
5. Conclusions
MEA/H2O_CO2 MEA/H2O/Ethylene glycol_CO2
Herein, we have addressed the problems posed by using conventional amine-based aqueous absorbents, reducing their water content to 50% or less, and developed highly efficient quasi-aqueous and nonaqueous absorbents with low regeneration energies using organic solvents to replace water. Screening experiments were conducted to select organic solvents suitable to replace water in the above sorbents. The absorption efficiency of ethylene glycol was higher than that of other organic alcohols by ∼90%, with the corresponding regeneration efficiency equaling ∼88.9%. Therefore, taking both absorption and regeneration efficiencies into account, ethylene glycol was selected as the best organic solvent for quasi-aqueous and non-aqueous sorbents. The absorption/regeneration efficiency was related to the CO2 absorption rate of quasi-aqueous/non-aqueous absorbents, which increased in the order of aqueous > quasi-aqueous ≫ non-aqueous. The absorption/regeneration performance was evaluated by absorption equilibrium experiments, with larger absorption load differences corresponding to better absorbent balance. As a result, the largest absorption balance difference was observed for the non-aqueous absorbent (∼0.50 mol). The reaction heat, sensible heat, and latent heat of evaporation were analyzed to quantitatively estimate the regeneration energy of quasiaqueous and non-aqueous absorbents and understand the ways of its reduction. The regeneration energies of aqueous, quasi-aqueous, and non-aqueous absorbents were estimated as ∼5226, 3981, and 3590 kJ/ kg CO2, respectively, being lower for quasi-aqueous and non-aqueous sorbents than for the water-based one and suggesting that the regeneration energy of the actual CO2 capture process can be reduced by 23.8–31.3%. Thus, we have demonstrated that quasi-aqueous and nonaqueous sorbents maximize CO2 absorption efficiency and exhibit low regeneration energies.
MEA/Ethylene glycol_CO2
Weight Loss [ - ]
15
10
5
(b) 0 20
40
60
80
100
120
140
160
180
200
Temperature [ ¡É ] Fig. 8. TGA curves of absorbents (a) without CO2, (b) saturated with CO2.
Regeneration energy [ kJ/kg CO 2 ]
6000 Heat of reaction Sensible heat Heat of vaporization
5000
4000
3000
2000
Acknowledgments 1000
This work was supported by the Brain Korea 21 Plus Project in the Division of Creative Low Impact Development and Management for Ocean Port City Infrastructures. Moreover, we acknowledge a two-year research grant from Pusan National University.
0 MEA/H2O
MEA/H2O/Ethylene glycol
MEA/Ethylene glycol
References
Fig. 9. Total regeneration energies of absorbents.
contribution of reaction heat equaling ∼84.8–86.9 kJ mol−1 for the aqueous sorbent, 70.3–73.5 kJ mol−1 for the quasi-aqueous sorbent, and 60.4–64.3 kJ mol−1 for the non-aqueous sorbent. The high reaction
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alcohol/water blends. Greenhouse Gas Control 26, 69–75. Mathonat, C., Majer, V., Mather, A.E., Grolier, J.P.E., 1998. Use of flow calorimetry for determining enthalpies of absorption and the solubility of CO2 in aqueous monoethanolamine solutions. Ind. Eng. Chem. Res. 37, 4136–4141. McCabe, W., Smith, J., Harriott, P., 2005. Unit Operations of Chemical Engineering, seventh ed. McGraw-Hill Book Co, Boston. Muhammad, A., Abdul Mutalib, M.I., Wifred, C.D., Murugesan, T., Shafeeq, A., 2008. Viscosity Refractive Index, Surface Tension, and Thermal Decomposition of Aqueous N-Methyldiethanolamine Solutions from (298. 15–338.15) K. J. Chem. Eng. Data 53, 2226–2229. Muhammad, A., Abdul Mutalib, M.I., Murugesan, T., Shafeeq, A., 2009. Thermophysical Properties of Aqueous Piperazine and Aqueous (N-Methyldiethanolamine + Piperazine) Solutions at Temperatures (298.15–338.15) K. J. Chem. Eng. Data. 54, 2317–2321. Munoz, D.M., Portugal, A.F., Lozano, A.E., Campa, J.G.D.L., Abajo, J.D., 2009. New liquid absorbents for the removal of CO2 from gas mixtures. Energy Environ. Sci. 2, 883–891. Peeters, A.N., Faaij, A.P.C., Trukenburg, W.C., 2007. Techno-economic analysis of natural gas combined cycles with post-combustion CO2 absorption, including a detailed evaluation of the development potential. Int. J. Greenhouse Gas Control 1, 396–417. Reynolds, A.J., Verheyen, T.V., Adeloju, S.B., Meuleman, E., Feron, P., 2012. Towards commercial scale postcombustion capture of CO2 with monoethanolamine solvent: key considerations for solvent management and environmental impacts. Environ. Sci. Technol. 46, 3643–3654. Sakwattanapong, R., Aroonwilas, A., Veawab, A., 2005. Behavior of reboiler heat duty for CO2 capture plants using regenerable single and blended alkanolamines. Ind. Eng. Chem. Res. 44, 4465–4473. Seo, J.-B., Jeon, S.-B., Kim, J.-Y., Lee, G.-W., Jung, J.-H., Oh, K.-J., 2012. Vaporization reduction characteristics of aqueous ammonia solutions by the addition of ethylene glycol, glycerol and glycine to the CO2 absorption process. J. Environ. Sci. 24, 494–498. Singh, P., Niederer, J., Versteeg, G.F., 2009. Structure and activity relationships for amine-based CO2 absorbents-II. Chem. Eng. Res. Des. 87, 135–144. Song, H.-J., Lee, S., Park, K., Lee, J., Spah, D.C., Park, J.-W., Filburn, T.P., 2008. Simplified estimation of regeneration energy of 30 wt.% sodium glycinate solution for carbon dioxide absorption. Ind. Eng. Chem. Res. 47, 9925–9930. Strazisar, B.R., Anderson, R.R., White, C.M., 2003. Degradation pathways for monoethanolamine in a CO2 capture facility. Energy Fuel. 17, 1034–1039. Zhang, J., Agar, D.W., Zhang, X., Geuzebroek, F., 2011. CO2 absorption in biphasic solvents with enhanced low temperature solvent regeneration. Energy Procedia 4, 67–74.
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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
Characterization and comparison of the CO2 absorption performance into aqueous, quasi-aqueous and non-aqueous MEA solutions Min-Kyoung Kanga, Soo-Bin Jeona, Joon-Hyung Chob, Jin-Seop Kimc, Kwang-Joong Ohd,
MARK
⁎
a
Institute of Environmental Studies, Pusan National University, San 30, Jangjeon-dong, Busan 609-735, Republic of Korea Division of Creative Low Impact Development and Management for Ocean Port City Infrastructures, Pusan National University, San 30, Jangjeon-dong, Busan 609-735, Republic of Korea c Department of Geological Sciences, Pusan National University, San 30, Jangjeon-Dong, Geumjeong-Gu, Busan 609-735, Republic of Korea d Department of Environmental Engineering, Pusan National University, San 30, Jangjeon-Dong, Geumjeong-Gu, Busan 609-735, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Keywords: Carbon dioxide Quasi-aqueous Non-aqueous Organic alcohol Absorption Renewable energy
Herein, we investigated quasi-aqueous and non-aqueous absorbents based on organic solvents to reduce the amount of water used in wet CO2 capture by amine- and water-based sorbents to < 50%. Solvent optimization was performed through screening and vapor pressure tests, and the performances of quasi-aqueous and nonaqueous sorbents prepared using selected organic solvents were evaluated in terms of CO2 absorption rates and equilibrium absorption capacities. All absorbents were subjected to thermogravimetric analyses before and after CO2 absorption to determine their thermal stabilities. Moreover, the prepared absorbents were characterized in terms of reaction heat, sensible heat, and latent heat of evaporation, which were used to determine their regeneration energies. The obtained results revealed that quasi-aqueous/non-aqueous absorbents are superior to conventional water-based ones, exhibiting maximized CO2 absorption and lower regeneration energies.
1. Introduction The amount of CO2 released into the atmosphere by the continuous use of fossil fuels is increasing every year. To mitigate this problem, carbon capture and storage (CCS) technologies have been developed to capture and store CO2, preventing it from entering the atmosphere (Lepaumier et al., 2009; Reynolds et al., 2012). At present, absorption is the most practical and quick method of capturing CO2 from combustion flue gas, mostly performed using alkanolamines (Hendriks, 1994) such as monoethanolamine (MEA), Nmethyldiethanolamine (MDEA), diethanolamine (DEA), and 2-amino-2methyl-1-propanol (AMP). Absorbents based on aqueous solutions of such amines exhibit highly efficient and fast CO2 absorption, are sufficiently technologically advanced for commercialization, and can be processed on a large scale. Among these absorbents, MEA-based ones are the most popular due to their low cost and fast carbon dioxide capture. However, the amine functional group of MEA is strongly bound to the absorbent used in the current process, requiring energy-consuming amine regeneration (separation) at temperatures above 120 °C(Zhang J et al., 2011). Moreover, this regeneration causes decomposition of the MEA absorbent, resulting in its rapidly deteriorating performance. Consequently, the capture process is very expensive, requiring ⁎
Corresponding author. E-mail address: [email protected] (K.-J. Oh).
http://dx.doi.org/10.1016/j.ijggc.2017.05.020 Received 28 December 2016; Received in revised form 12 April 2017; Accepted 26 May 2017 1750-5836/ © 2017 Elsevier Ltd. All rights reserved.
continuous absorbent supply to compensate for the consumed amounts (Hartono et al., 2007; Munoz et al., 2009; Strazisar et al., 2003). The use of water as a solvent also poses a problem due to its high specific and latent heat, which is reflected in the high energy required to dissipate the chemical bond between CO2 and the absorbent (Lin and Wong, 2014). As a result, heating to high temperatures accounts for 40–60% of the total energy consumption associated with the CO2 absorption process (Peeters et al., 2007; Singh et al., 2009), with only a small amount of latent heat consumed by water heating. Thus, to make the absorption process more energy-efficient, the absorbent needs to be regenerated at temperatures below 80–100 °C. The above problems can be mitigated by using organic solvents to replace water in conventional amine-based absorbents, resulting in much lower CO2 recovery and solvent regeneration energies(Barbarossa et al., 2013; Barzagli et al., 2013; Lail et al., 2012, 2011). However, significantly lower CO2 absorption is generally observed for organic solvent–based absorbents due to their high net absorbent exchange rate, necessitating the development of new absorbents with high thermal/ chemical stability and low vapor pressure to overcome the abovementioned disadvantages(Heldebrant et al., 2011, 2008, 2009; Jessop et al., 2005). This study aims to address the problems associated with the use of amine-based aqueous sorbents for wet CO2 capture, evaluating the
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Nomenclature
α
Symbols
Acronyms
B′ NA
PCO2 PN2 Peq
VLE MEA MDEA DEA AMP TGA DSC
Bronsted base Absorption rate of species A at the gas–liquid interface for a chemical reaction in the continuous phase, [kmol m2 s−1] Partial pressure of carbon dioxide Initial pressure Equilibrium pressure
2.2. CO2 absorption rate Eq. (5) shows that the absorption rate of CO2 may be calculated from its molar flux (NA) from the gas phase into the liquid phase, partial pressure(PCO2, g ) , and the overall mass transfer coefficient. For irreversible absorption, the equilibrium CO2 pressure in bulk liquid (P* CO2 ) equals zero (McCabe et al., 2005), and Eq. (5) can be simplified to Eq. (6):
2. Theoretical background 2.1. CO2 absorption mechanism
NA = K G P (yA − yA* )
The reactions of CO2 in aqueous solutions of primary alkanolamines (Danckwerts, 1979), follow the generally accepted zwitterion mechanism (Eq. (1)) originally proposed by Caplow (1968) and reintroduced by Danckwerts (1979).
RNH2+COO−
+ B′ ⇔
RNHCOO−
PCO2, lm =
(1)
+
PCO2, in − PCO2, out ln (PCO2, in/ PCO2, out )
(7)
Here, subscripts “in” and “out” correspond to gas flows to and from the wetted wall column (WWC), respectively. The bulk CO2 pressure in the gas phase can be calculated from the logarithmic mean (PCO2, lm , Eq. (7)). In the corresponding graphical representation, the abscissa denotes the average partial pressure of CO2, and the ordinate represents its molar flux into the liquid phase, with KG obtained from the slope of the linear fit.
(2)
B′H+
(5)
* 2 ) (6) NA = K G (PCO2, g − PCO
The above reaction comprises two steps, i.e., the formation of CO2amine zwitterions (Eq. (2)) and their base-catalyzed deprotonation (Eq. (3)).
CO2 + RNH2 ⇔ RNH2+COO−
Vapor–liquid equilibrium Monoethanolamine N-Methyldiethanolamine Diethanolamine 2-Amino-2-methyl-1-propanol Thermogravimetric analysis Differential scanning calorimetry
greatly influenced by pH, being increased by increasing solution basicity. Despite the slow CO2 absorption, absorbent regeneration is relatively easy, and the extent of its degradation in repeated absorption/ regeneration experiments is low. The reaction is influenced by the functional groups of the organic alcohol, since they can change its basicity, and by the nature of the amine absorbing agent, which affects the CO2 absorbing ability, absorption rate, and absorbent regenerability. The acid dissociation constant of the sorbent (pKa) is a measure of its basicity, which indicates how well the sorbent can react with CO2, an acidic gas. Low sorbent basicity complicates the deprotonation of the amphoteric salt produced from the amine and CO2, and one mole of CO2 per mole of amine can remain bonded as a result. For organic alcohols, the presence of a hydroxyl group makes the NeH bond of the above amphoteric compound unstable. In addition, amine basicity can be increased by increasing the electron density around the aminic nitrogen.
properties (e.g., specific heat) of a quasi-aqueous absorbent, which utilizes ≤50% water as a solvent compared to conventional aqueous absorbents, and a non-aqueous absorbent, which utilizes an organic solvent in place of water. In the first step, the absorption and regeneration efficiencies of MEA solutions in ethanol, methanol, ethylene glycol, and glycerol were investigated. The amount of absorbent lost per unit reaction time was determined to minimize loss to the gas phase, which is a major issue of the absorption/regeneration process, and an optimum organic alcohol was selected to replace water in the preparation of quasi-aqueous and non-aqueous sorbents. The performance of the thus obtained absorbents was evaluated in terms of their absorption rates and absorption capacities, and thermogravimetric analyses (TGA) were performed before and after CO2 absorption to gain insight into their thermal stabilities. In addition, the heat of reaction, sensible heat, and latent heat of evaporation of the investigated absorbents were calculated to derive their regeneration energy. Thus, quasi-aqueous and non-aqueous sorbents were optimized for maximum regeneration performance and low regeneration energy to investigate the feasibility of their use in actual CO2 absorption processes.
CO2 + 2RNH2 ⇔ RNHCOO− + RNH3+
CO2 loading ratio, [mol CO2/mol amine]
(3)
−
where B′ = amine, OH , or H2O (Blauwhoff et al., 1983). The equilibrium loading capacities of primary and secondary alkanolamines are stoichiometrically limited to 0.5 mol CO2/mol amine (Eq. (1)). For normal primary amines, such as MEA, the produced carbamates are quite stable. Organic alcohols lack NeH bonds and thus cannot directly react with CO2 to form carbamates. Instead, these alcohols are converted to trialkylammonium hydroxides ([R1R2R3NH]OH) by a reaction with water, and the produced OHe reacts with CO2 to bind it as bicarbonate (Eq. (4)).
R1 R2 R3 N + CO2 + H2 O ⇔ R1 R2 R3 N+H + HCO3−
2.3. CO2 equilibrium absorption capacities The absorption capacities of different absorbents were experimentally determined by measuring the pressure changes between the initial and equilibrium states of the system. Subsequently, the partial pressure of CO2 (PCO2 , atm) was calculated from the difference between initial (PN2 ) and equilibrium absorption (Peq) pressures.
PCO2 = (Peq − PN2)
(13)
The obtained value was used in combination with the ideal gas equation of state to determine the extent of CO2 absorption (Eq. (14)). The amount of absorbed CO2 (nCO2 , mol) was calculated by multiplying its volume (V) by the difference between CO2 injection pressure
(4)
The above reaction requires a lower temperature and is slower than that of carbamate formation. Furthermore, the rate of CO2 absorption is 282
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(PN2 + CO2 ) and the equilibrium pressure (Peq), and dividing the obtained value by RT.
nCO2 =
(PN2 + CO2 − Peq) V RT
(14)
The absorption capacity (α) could then be obtained by dividing nCO2 by the number of moles of the amine:
α = nCO2 /(moleofamine )
(15)
The rate of absorption (NA) and the amount of absorbed gas were calculated from the difference between the absorption and the flow of gas from the reactor into the reactor's initial flow. Conversion of the above amount into moles allowed the absorption per reactor unit area at the gas–liquid interface area to be obtained. The absorption rate was calculated using the following equation, with the initial volumetric absorption rate, V(t1)/t1, obtained from the cumulative volume of gas that had flown through the CO2 analyzer:
NA =
PT − PW V (t1) SRT
Fig. 1. Apparatus used for screening experiments. (1) Water bath of absorber, (2) water bath of desorber, (3) saturator, (4) CO2 cylinder, (5) N2 cylinder, (6) CO2 analyzer, and (7) condenser.
Ethyl alcohol (99.5%), methyl alcohol (99.5%), ethylene glycol (99.0%), and glycerol (99.0%), were supplied by Samchun Pure Chemicals (Korea). Aqueous solutions were prepared using distilled water. Commercial grade CO2 and N2 gases with purities of 99.99% were used.
(16)
where PT is the atmospheric pressure, PW is the pressure of water vapor, S is the surface area of the liquid phase, and V(t1) is the cumulative volume of gas absorbed during absorption time t1.
3.2. Screening test Ethyl alcohol, methyl alcohol, ethylene glycol, and glycerol were selected for the preparation of quasi-aqueous/non-aqueous sorbents, and their relative CO2 absorption rates and absorption/regeneration capacities were investigated using screening tests. A schematic diagram of the experimental setup is presented in Fig. 1. The absorbent (200 mL) was fed into a 500-mL glass reactor and heated to 313 K using a water bath. A mixture of N2/CO2 (15 vol% CO2) was fed into the reactor at a flow rate of 500 mL/min. After 30 min of CO2 absorption, the reactor was moved to another water bath for absorbent regeneration. A regeneration temperature of 353 K was used, since the lipophilic amine could be regenerated at a lower temperature than the alkanolamine. During regeneration, the absorbent was stripped with N2 gas for ∼30 min, and the gas exiting the absorption/regeneration reactor was analyzed using a CO2 analyzer (ZRF model, Fuji Electric, Japan). After the absorption/regeneration test, the CO2 loading ratio was analyzed using an automatic titrator (702 SM Titrino, Metrohm, Riverview, FL, USA) to quantify the regeneration efficiency.
2.4. Regeneration energy The enthalpy of CO2 absorption by the amine absorbent, i.e., the heat of reaction, was calculated using the Gibbs-Helmholtz equation (Eq. (17)), as reported by Mathonat et al. (1998):
⎡ ∂lnPCO ⎤ ΔH 2 ⎥a =⎢ R ⎢ ∂ 1 ⎥ T ⎣ ⎦
()
(17)
where PCO2 is the equilibrium partial pressure of CO2, T is the absolute temperature of the system, a is the CO2 loading, and R is the universal gas constant. In addition to differential calculations, the integral of the temperature change is determined by the heat of the dissolution reaction and can be calculated as:
Q = m ·C p·ΔT
(18)
where Q is the heat of absorption, m is mass of absorbed CO2, Cp is the absorbent heat capacity after absorption of CO2, and ΔT is the temperature change after injection of CO2 into the equilibrium cell. The biggest disadvantage of chemical absorption is the large energy required for absorbent regeneration, which is the main factor determining process design and economic feasibility. The phase change heats of pristine and CO2-loaded absorbents were measured using differential scanning calorimetry (DSC), and the regeneration heat of aqueous amine solution was calculated using the difference between the abovementioned values (Eqs. (19) and (20)). ' ΔHamine − s = ΔHamine − s × Xamine − s
(19)
ΔHreg = (ΔHamine + H2 O + CO2 − ΔHamine − s ') ÷ yCO2
(20)
3.3. Wetted wall column A WWC similar to that used by Choi et al. (2012) was employed to investigate the absorption rate, with the experimental setup shown in Fig. 2. The central gas–liquid contactor was constructed from a stainless steel tube (91 mm long, outer diameter 12.6 mm). The column was surrounded by a cylindrical thick glass wall, and the entire chamber was surrounded by a second glass wall with a liquid (such as paraffin oil) flowing between the two walls as a heat transfer medium. The absorbent was pumped upwards into the column and flowed out at the top, supplying liquid to the bottom of the chamber. The gases came in cross-current contact with the liquid and subsequently exited at the top. During the experiment, a water bath was used to maintain the absorbent and paraffin oil at a constant temperature. The pressure inside the reactor was measured with pressure transducers (MGI/MGAMP series, accuracy = ± 0.1 kPa) installed in the reactor and feeder. The concentration of CO2 in the feed gas stream equaled 1–20 vol%, and gas flow rates were controlled using mass flow controllers (5850E, Brooks Instrument, Hatfield, PA, USA). CO2 concentrations were analyzed using a gas chromatograph (7890A, Agilent Technologies), with a 30 m × 0.32 m packed column (GS-GasPro, Agilent Technologies) equipped with a thermal conductivity detector (TCD) utilized as the GC column.
where ΔHamine−s is the difference of absorbent phase change heat (kJ/ kg), ΔHamine−s' is the corrected difference of absorbent phase change heat (kJ/kg), ΔHamine+H2O+CO2 is the difference of phase change heat for absorbent + CO2 (kJ/kg), ΔHreg is the heat regeneration difference (kJ/(mol CO2)), Xamine−s is the mole fraction of absorbent, and yCO2 is the mole fraction of CO2. 3. Materials and methods 3.1. Materials Analytical grade MEA (99%) was supplied by Sigma-Aldrich (USA). 283
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an SDT-Q600 analyzer and a thermal analysis controller (TA Instruments). TGA and DSC measurements were performed at 25–250 °C, using nitrogen as a purge gas at a flow rate of 100 mL/min and 40 mg absorbent samples. The heating rate was kept constant at 2 °C/min. All data point measurements were performed in triplicate, and the average value was reported.
4. Results and discussion 4.1. Screening of organic alcohols Herein, we tried to select an organic solvent to replace water and obtain a quasi-aqueous/non-aqueous CO2 sorbent. The absorption/regeneration efficiency was qualitatively screened, with the results obtained for MEA as a sorbent (concentration = 30 wt%) and ethanol, methanol, ethylene glycol, and glycerol as organic alcohols shown in Fig. 4. In these systems, the absorption efficiency of CO2 (15 vol%) was measured at 313 K over 30 min. Fig. 4a shows that the lowest CO2 outlet gas concentrations equaled 1.51, 1.95, 4.38, 5.03, and 5.74% for ethylene glycol, glycerol, methyl alcohol, ethyl alcohol, and H2O, respectively, revealing that higher absorption efficiencies were obtained for organic solvents than for the conventionally used water. Moreover, faster CO2 saturation was also observed for organic solvents. This behavior probably originated not only from the effect of the amine absorbent but also from that of organic alcohol functional groups, which affected basicity. Fig. 4b shows sorbent regeneration efficiencies obtained for 30 min N2 injection at 353 K. The use of organic alcohols instead of water resulted in significantly higher CO2 removal efficiencies compared to that of the conventional water absorbent. The waterborne sorbent had to be regenerated at high temperature (393 K) due to the high specific heat of water attributed to hydrogen bonding. Therefore, its regeneration efficiency at low temperature (353 K) was considered to be low. On the other hand, organic alcohols do not exhibit overly high specific heats,
Fig. 2. Apparatus used for the wetted wall column experiment. (1) N2 cylinder, (2) CO2 cylinder, (3) mass flow controller, (4) mixing chamber, (5) saturator, (6) wetted wall column, (7) water bath, (8) absorbent inflow, (9) absorbent outflow, (10) paraffin oil inflow, (11) paraffin oil outflow, (12) gas inflow, (13) gas outflow, (14) condenser, (15) GC/TCD, (16) thermocouple, and (17) pressure transducer.
3.4. Vapor–liquid equilibrium The vapor–liquid equilibrium (VLE) system was used to test the CO2 absorption equilibrium for all absorbents (Fig. 3), comprising a saturator for feeding CO2 into the reactor and the reactor itself. The reactor (height = 160 mm) was located in a temperature-controlled vessel. Four 5 mm-wide baffles and a two-blade impeller (70 mm × 20 mm) were installed inside the reactor. Temperature was measured with an accuracy of ± 0.1 K using a K-type thermocouple. Initially, CO2 gas was filled into the supplier and preheated by the electric heater. After removing the remaining reactor gas using a vacuum pump, 300-mL aliquots of each solution were added into the reactor using a syringe. CO2 loadings of each solution were calculated at different reactor pressures, and CO2 pressures in the reactor and supplier were determined using a pressure data logger (PR2000, MadgeTech, USA). 3.5. Vaporization The employed vapor pressure apparatus and the followed procedure are described elsewhere (Seo et al., 2012). The absorbent vapor pressure was measured using the abovementioned pressure data logger with an accuracy of ± 0.25% at 293–393 K. Experimental data supplied by the pressure transducer were automatically stored in a computer. Absorbent losses were determined at constant temperature (293–393 K) and relative humidity (65%). After 300 mL of the absorbent was fed into the reactor, it was heated to the desired temperature. The reactor was maintained at constant humidity using a thermal humidity chamber, and the absorbent concentration was analyzed using an automatic titrator (702 SM Titrino, Metrohm, Riverview, FL, USA). 3.6. TGA and DSC characterization TGA/DSC were used to measure the heat flow and weight change of samples (Kim et al., 2011). Moreover, DSC was employed to determine the difference in the heat required to increase the temperature of the sample and the reference as a function of temperature, which was used to calculate the heat of phase change. The TGA apparatus consisted of
Fig. 3. Apparatus used for characterizing the vapor–liquid equilibrium. (1) N2 cylinder, (2) CO2 cylinder, (3) mass flow controller, (4) supplier, (5) magnetic drive, (6) controller, (7) reactor, (8) pressure transducer, (9) computer, and (10) vacuum pump.
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CO2 outlet concentration [ % ]
to the former. Thus, the vapor pressure of this absorbent was the highest due to unfavorable hydrogen bond formation conditions. On the other hand, in the case of organic alcohols, one hydroxyl group (–OH) is present in the molecular structures of ethyl and methyl alcohols, two are present in ethylene glycol, and three in glycerol, with the number of hydroxyl groups being negatively correlated with absorbent vapor pressure. These results are considered to be affected by the chemical structure in the case of absorbent loss. Glycerol showed the lowest vapor pressure, which, however, was not much different from that of ethylene glycol, indicating that the absorption efficiency of the latter alcohol was higher than in screening experiments. Therefore, in this study, we selected ethylene glycol as an optimal organic solvent to replace water and prepare quasi-aqueous/ non-aqueous sorbents.
MEA/H2O MEA/Ethyl alcohol MEA/Methyl alcohol MEA/Ethylene glycol MEA/Glycerol
14 12 10 8 6 4 2
(a) 0
0
5
10
15
20
25
4.3. CO2 absorption rate
30
Reaction time [ min ]
CO2 outlet concentration [ % ]
14
As stated above, quasi-aqueous/non-aqueous sorbents were prepared using ethylene glycol as an optimum organic solvent instead of water. Absorption rates were determined for 30 wt% MEA/H2O, 30 wt % MEA/H2O/ethylene glycol (quasi-aqueous), and 30 wt% MEA/ethylene glycol (non-aqueous) sorbents at 293, 303, 313, and 323 K. Fig. 6 shows absorption rates of the above absorbents at various reaction temperatures and constant CO2 partial pressure (15 kPa), revealing that MEA/H2O, MEA/H2O/ethylene glycol, and MEA/ethylene glycol sorbents exhibited values of 6.88–9.63 × 10−6, 9.15 × 10−6, and 10.86–14.21 × 10−6 kmol m−2 s−1, respectively, which increased with increasing temperature. This behavior can be explained by the fact that the CO2 uptake entails a chemical reaction, the rate of which increases with temperature. The above absorption rate increase is considered to be linear due to the increased amounts of solvent and solute capable of absorbing CO2 and diffusion/collision at the gas–liquid interface. In addition, the absorption rates of water-based and quasi-aqueous/ non-aqueous absorbents were compared, with the corresponding differences between water-based and low-water absorbents, quasi-aqueous and non-aqueous absorbents, and between water-based and non-aqueous absorbents determined as 2.27–3.24 × 10−6, 1.34–1.71 × 10−6, and 3.98–4.58 × 10−6 kmol m−2 s−1, respectively. The absorption rate of the MEA/ethylene glycol non-aqueous sorbent was the highest. The variation of these results is explained by the corresponding variations of basicity, which depends on the functional groups of ethylene glycol. Thus, the presence of a hydroxyl group destabilizes the
(b)
12 10 8 6 4
MEA/H2O MEA/Ethyl alcohol MEA/Methyl alcohol MEA/Ethylene glycol MEA/Glycerol
2 0
0
5
10
15
20
25
30
Reaction time [ min ] Fig. 4. Variation of outlet CO2 concentration as a function of reaction time. (a) Absorption test at 313 K and (b) regeneration test at 353 K.
resulting in larger regeneration efficiencies at the same temperature.
300
4.2. Absorbent vapor pressure as a function of organic alcohol type CO2 uptake by the CO2 absorption process gets reduced due to absorbent vapor pressure. The absorbent vapor pressure also lowers absorbent performance due to volatilization (Hagaman et al., 2003). Therefore, the effect of vapor pressure was investigated at reaction temperatures of 293–393 K for four representative organic alcohols, namely ethyl alcohol, methyl alcohol, ethylene glycol, and glycerol. The effect of vapor pressure on the initial pressure and the difference in pressure due to the increase of reaction temperature was examined for direct absorbent injection. As shown in Fig. 5, vapor pressures of MEA/H2O, MEA/ethyl alcohol, MEA/methyl alcohol, MEA/ ethylene glycol, and MEA/glycerol mixtures equaled 8.97–250.90, 3.77–166.38, 2.03–138.20, 1.41–90.10, and 1.38–80.14 kPa, respectively, with increasing. The change in vapor pressure was largest for the conventional MEA/H2O absorbent, decreasing in the order of ethyl alcohol > methyl alcohol > ethylene glycol > glycerol. These results can be explained by the dependence of absorbent hydrophilicity on their chemical structure. For the aqueous MEA sorbent, the formation of hydrogen bonds between the amine (–NH2) and hydroxyl groups (–OH) is influenced by the alkyl groups present close
MEA/H2O MEA/Ethyl alcohol MEA/Methyl alcohol MEA/Ethylene glycol MEA/Glycerol
Vapor Pressure [ kPa ]
250
200
150
100
50
0
20
30
40
50
60
70
80
90
100
110
Temperature [¡É ] Fig. 5. Vapor pressure of absorbents as a function of temperature.
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increased difference in CO2 loadings (effective CO2 loading) between these two regions favors CO2 removal at elevated temperatures. MEA/H2O, MEA/H2O/ethylene glycol, and MEA/ethylene glycol absorbents exhibited ratios of ∼0.14, 0.43, and 0.43 mol at an atmospheric pressure of 101.3 kPa, respectively, with a value of 0.50 mol determined for the aqueous sorbent. Therefore, the difference in the absorption balance load of the non-aqueous sorbent was the largest, meaning that its absorption/regeneration ability was superior to that of the water-based absorbent.
MEA/H2O MEA/H2O/Ethylene glycol MEA/Ethylene glycol
NCO2 ¡¿106 [ kmol m-2 s-1 ]
14
12
10
4.5. TGA During TGA, a temperature program is applied to a sample, and its mass change caused by chemical reactions that produce volatile or gaseous products is measured as a function of time or temperature. Herein, the thermal stabilities of 30 wt% MEA/H2O, 30 wt% MEA/ H2O/ethylene glycol, and 30 wt% MEA/ethylene glycol absorbents were evaluated (Muhammad et al., 2008, 2009). Fig. 8a shows TGA results for different absorbents, revealing that aqueous, quasi-aqueous, and non-aqueous absorbents were stable up to 100, 130, and 140 °C, respectively, and indicating that thermal stability was negatively correlated with water content. In view of the above, it was concluded that the thermal properties of the absorbent were more dominant than those of water. When the absorbent is stripped at high temperatures, its concentration can be changed due to deterioration or evaporation, directly impacting CO2 absorption. Fig. 8b shows the TGA results obtained for CO2-saturated sorbents, revealing that the onset of their mass loss is observed at higher temperatures than that of unsaturated sorbents. This finding is attributed to absorbent destabilization due to the formation of carbamate.
8
6
293
303
313
323
Temperature [ K ] Fig. 6. Rate of CO2 absorption by various absorbents as a function of reaction temperature.
binding of the amphoteric compound produced by the reaction of the amine with CO2. Increasing the electron density of the aminic nitrogen causes the basicity of the corresponding amine to increase. Amine basicity is considered to be positively correlated with reactivity toward CO2, which is an acidic gas, with high basicities resulting in increased CO2 absorption rates. 4.4. CO2 absorption equilibrium The performance of quasi-aqueous/non-aqueous sorbents was evaluated using 30 wt% MEA, MEA/H2O/ethylene glycol, and MEA/ ethylene glycol sorbents, with absorption equilibrium experiments performed at 313 (absorption region) and 353 K (regeneration region). Since the results of the screening test in Section 4.1 suggested that the absorbent was effectively regenerated at 353 K, this temperature was further used for regeneration throughout this study. Fig. 7 shows CO2 absorption capacities (moles of CO2 absorbed per mole of absorbent) at 313 and 353 K determined for aqueous and quasiaqueous/non-aqueous absorbents, revealing values of ∼0.68 mol/mol for MEA/H2O and 0.87 mol/mol for MEA/H2O/ethylene glycol at ∼101.3 kPa, with the highest value (∼0.90 mol/mol) observed for MEA/ethylene glycol. MEA stoichiometrically absorbs up to 0.5 mol/ mol CO2 in aqueous solution as a result of carbamate formation. However, the produced carbamate is strongly destabilized by the alkyl group of the amine. Therefore, when MEA absorbs more than 0.5 mol/ mol of CO2, hydrolysis occurs, releasing the free amine and bicarbonate and resulting in the measured solubility being higher than the theoretical one. For organic alcohols in quasi-aqueous/non-aqueous absorbents, if the hydroxyl group in the molecule affects the direct reaction of amine and CO2, then the functional group influences the absorption of CO2 due to hydrogen bonding. On the other hand, the CO2 absorption capacities of MEA/H2O, MEA/H2O/ethylene glycol, and MEA/ethylene glycol sorbents at 353 K equaled ∼0.54, 0.44, and 0.40 mol/mol, respectively, indicating that the carbamate produced during CO2 absorption is thermally unstable. This instability was reflected in the fact that the amount of CO2 absorbed at 353 K was reduced compared to that absorbed at 313 K due to the weak binding and low stability of bicarbonate and the exothermicity of CO2 absorption. As evidenced by the above numbers, the nonaqueous absorbent exhibited the best regeneration performance. The absorbing capacity and the absorbing capacity of each of the absorbing regions were compared, with good absorbent performance characterized by a high loading of absorbed CO2 in the absorbing region and a small loading in the regeneration region. Furthermore, the
4.6. Regeneration energy The regeneration energy of CO2 absorbents is the energy required to remove CO2 in the regeneration tower, featuring contributions from the absorption reaction heat, the sensible heat consumed to heat the absorbent, and the latent heat of vaporization to generate steam in the regenerator. Herein, we quantitatively assessed the regeneration energy of the common water-based absorbent and quasi-aqueous/non-aqueous absorbents (Sakwattanapong et al., 2005; Song et al., 2008). Fig. 9 shows the total regeneration energy for each sorbent, with the Reaction at 313 K
Reaction at 353 K
MEA/H2O
MEA/H2O
MEA/H2O/Ethylene glycol
MEA/H2O/Ethylene glycol
MEA/Ethylene glycol
MEA/Ethylene glycol
CO2 Partial pressure [kPa]
150
100
50
0 0.0
0.2
0.4
0.6
0.8
1.0
CO2 loading ratio [ mol CO2/mol amine ] Fig. 7. Equilibrium CO2 absorption capacities of different absorbents at various CO2 partial pressures at 313 and 353 K.
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heat is attributed to the high binding energy between the absorbent and CO2. Consequently, a large amount of energy is consumed in sorbent regeneration as it is separated from the bound CO2. Sensible heat is the largest contributor to regeneration energy, equaling 2573 kJ/kg CO2 for the aqueous sorbent, 1823 kJ/kg CO2 for the quasi-aqueous sorbent, and 1693 kJ/kg CO2 for the non-aqueous sorbent. Compared to the water-based absorbent, quasi-aqueous and non-aqueous absorbents showed values lower by 750 and 880 kJ/kg CO2, respectively. The latent heats of vaporization for aqueous, quasi-aqueous, and non-aqueous absorbents equaled 706, 423, and 354 kJ/kg CO2, respectively, with the corresponding regeneration energies estimated as 5226, 3981, and 3590 kJ/kg CO2. The water-based absorbent possessed the highest specific and latent heat. Quasi-aqueous and non-aqueous sorbents, utilizing organic solvents instead of water, required much less energy to regenerate CO2 than water-based sorbents, being advantageous for reducing the energy consumption associated with CO2 capture.
MEA/H2O MEA/H2O/Ethylene glycol MEA/Ethylene glycol
Weight Loss [ - ]
15
10
5
(a ) 0 20
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100
120
140
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200
Temperature [ ¡É ] 20
5. Conclusions
MEA/H2O_CO2 MEA/H2O/Ethylene glycol_CO2
Herein, we have addressed the problems posed by using conventional amine-based aqueous absorbents, reducing their water content to 50% or less, and developed highly efficient quasi-aqueous and nonaqueous absorbents with low regeneration energies using organic solvents to replace water. Screening experiments were conducted to select organic solvents suitable to replace water in the above sorbents. The absorption efficiency of ethylene glycol was higher than that of other organic alcohols by ∼90%, with the corresponding regeneration efficiency equaling ∼88.9%. Therefore, taking both absorption and regeneration efficiencies into account, ethylene glycol was selected as the best organic solvent for quasi-aqueous and non-aqueous sorbents. The absorption/regeneration efficiency was related to the CO2 absorption rate of quasi-aqueous/non-aqueous absorbents, which increased in the order of aqueous > quasi-aqueous ≫ non-aqueous. The absorption/regeneration performance was evaluated by absorption equilibrium experiments, with larger absorption load differences corresponding to better absorbent balance. As a result, the largest absorption balance difference was observed for the non-aqueous absorbent (∼0.50 mol). The reaction heat, sensible heat, and latent heat of evaporation were analyzed to quantitatively estimate the regeneration energy of quasiaqueous and non-aqueous absorbents and understand the ways of its reduction. The regeneration energies of aqueous, quasi-aqueous, and non-aqueous absorbents were estimated as ∼5226, 3981, and 3590 kJ/ kg CO2, respectively, being lower for quasi-aqueous and non-aqueous sorbents than for the water-based one and suggesting that the regeneration energy of the actual CO2 capture process can be reduced by 23.8–31.3%. Thus, we have demonstrated that quasi-aqueous and nonaqueous sorbents maximize CO2 absorption efficiency and exhibit low regeneration energies.
MEA/Ethylene glycol_CO2
Weight Loss [ - ]
15
10
5
(b) 0 20
40
60
80
100
120
140
160
180
200
Temperature [ ¡É ] Fig. 8. TGA curves of absorbents (a) without CO2, (b) saturated with CO2.
Regeneration energy [ kJ/kg CO 2 ]
6000 Heat of reaction Sensible heat Heat of vaporization
5000
4000
3000
2000
Acknowledgments 1000
This work was supported by the Brain Korea 21 Plus Project in the Division of Creative Low Impact Development and Management for Ocean Port City Infrastructures. Moreover, we acknowledge a two-year research grant from Pusan National University.
0 MEA/H2O
MEA/H2O/Ethylene glycol
MEA/Ethylene glycol
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Fig. 9. Total regeneration energies of absorbents.
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