CO2 Capture by Water-Lean Amino Acid Salts: Absorption

Environmental and Carbon Dioxide Issues

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CO2 capture by water-lean amino acid salts: absorption performance and mechanism Hui Guo, Hui Li, and Shufeng Shen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01012 • Publication Date (Web): 24 May 2018 Downloaded from http://pubs.acs.org on May 27, 2018

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CO2 capture by water-lean amino acid salts: absorption performance and mechanism Hui Guo, Hui Li and Shufeng Shen* School of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, P.R. China

ABSTRACT Water as a solvent remains the weakest point of CO2 capture systems using aqueous chemical absorbents due to large heat capacity and latent heat of water. Water-lean absorbents including low volatile solvent have been considered as potential use for CO2 capture in terms of reducing energy consumption. In this work, we have examined the capture performance of several water-lean amino acid salts. Ethylene glycol (EG) with high boiling point and low specific heat was used to replace water. The physical properties and the absorption and desorption rate of CO2 were determined and evaluated for potassium lysinate (LysK)/EG/water and potassium prolinate (ProK)/EG/water. Aqueous monoethanolamine (MEA), ProK and LysK systems were used for comparison. The samples from the absorption test and continuous absorption-desorption cycles were analyzed by NMR spectra to provide insight into the product species and the stability of absorbent. Absorption mechanisms and degradation species were also discussed as well as the main issues such as high viscosity of CO2-free/loaded absorbents and high desorption temperature related to these EG-based water-lean systems. Keywords: CO2 capture; Water-lean solvent; Ethylene glycol; Potassium prolinate; Potassium lysinate

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1. Introduction Global warming resulted from the main greenhouse gas, CO2, has attracted a lot of attention in the past decade. The main source of CO2 is from fossil fuel-fired power stations. The resulting environmental problems underlie the urgent global demand for reduction of CO2 emission. Carbon capture, utilization and storage (CCUS) is currently the only available technology that can reduce the emission of CO2 [1,2]. The most common and proven absorbents are the aqueous amine-based solvents, typically monoethanolamine (MEA) [3]. The typical carbon capture process consists of two columns connected in series, in which CO2 in the flue gas is selectively absorbed by aqueous amines in an absorber and then the CO2-loaded solutions are heated to high temperature (e.g. 393K or higher) by steam supply from the power plant in a stripper where CO2 is released and the resulting CO2-lean liquids are continuously pumped back to the absorber. The main drawback associated with this process is high energy penalty paid for CO2 capture, accounting for about 70% of the total cost, which is also the main reason that CCUS has not been performed yet on a large scale [1,4]. It is believed that the major contribution to energy penalty is the heat duty of reboiler for solvent regeneration. This energy is used to break the chemical bond between CO2 and MEA at high stripper temperature, including necessary sensible heat and latent heat of vaporization of solvents [2]. Additionally, this regeneration causes severe degradation of the MEA absorbent, resulting in its rapidly deteriorating capture performance [5]. From the viewpoint of energy consumption, the use of water as a solvent is the weakest point of these aqueous systems because of large heat capacity and latent heat of water [6-8]. Thus, using water-lean absorbent systems by replacing water with organic solvents with low specific heat and volatility seem to be an attractive alternative to prevent the high energy consumption. Several research groups have recently investigated some non-aqueous amine solutions for improvement of energy-intensive CO2 capture process. Sada et al. [9] studied the kinetics of the CO2 absorption with ethanolamines in alcohol solvents such as methanol, ethanol and 2-propanol. Hwang et al. [10] and Park et al. [11] measured the absorption rate of CO2 into primary, secondary and tertiary alkamolamine in polar organic solvents including ethanol, butanol, ethylene glycol, propylene glycol


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and propylene carbonate. Tan et al. [12] developed a mixture system of MEA and triethylene glycol for CO2 capture. Yu et al. [13] studied a mixture system of piperazine (PZ) with diethylene glycol in a rotating packed bed. It has been proved that glycols as solvent results in low energy consumption with nearly no solvent evaporation and avoidance of thermal degradation. Zheng et al. [14,15] and Barzagli et al. [16] have also developed nonaqueous systems of sterically hinderd amine (i.e. 2-amino-2-methyl-1-propanol) with glycols as CO2 absorbent to reduce energy consumption. Chen et al. [17] has proposed N,N-diethylethanolamine (DEEA) and N-ethylmonoethanolamine (EMEA) as a nonaqueous mixture for CO2 absorption, and found that DEEA shows more superior than other nonaqueous solvents such as alcohols and glycols. However, poor CO2 absorption performance is generally observed for organic solvent–based absorbents due to their high solvent viscosity. Moreover, the use of high volatile organics to dissolve the CO2 carrier seems to be impractical for industrial use. It is also reported that this energy consumption for the latent heat of vaporization of water generally depends on the amount of water present in the aqueous absorbent mixture. The regeneration energies vary from 3.29 GJ/ton CO2 for 30 wt% MEA to 3.01 GJ/ton CO2 for 40 wt% MEA aqueous solutions [18]. More recent advancements have led to the development of water-lean amine solutions [6]. Blended absorbents consisting of amines (MEA, PZ, etc), water and organic solvents such as alcohols, ethers and glycols have been considered [19-22]. The mixed solvent made of water and organics would avoid the sharp increase of viscosity for the CO2-loaded absorbent and increase CO2 mass transfer during absorption. Thus, the regeneration energy penalty may be reduced without losing absorption performance. Lin et al. [19] have investigated several water-lean blend systems and found that a PZ/diethylenetriamine/methanol/water blend achieved a high absorption-desoption efficiency and a low regeneration energy penalty. The regeneration energy penalty was estimated to be 1.84 GJ/ton CO2. Kang et al. [20,21] have investigated quasi-aqueous MEA solutions based on ethylene glycol with low vapor pressure to reduce the amount of water to 50% or less. Quantitative estimation suggests that 30 wt% MEA/water/ethylene glycol absorbents can reduce regeneration energy by 20–30% than aqueous 30 wt% MEA solution. Phase separation solvent of aqueous (amine and ether) solution was also developed for energy-efficient CO2 capture [22]. Recently, aqueous amino acid salts


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are of great interest as alternative absorbent for CO2 capture due to their intrinsic basicity, high capacity and kinetics, good resistance to thermal and oxidative degradation, biodegradability, and commercial availability [6,23-26]. Water-lean AAS system might be one of possible methods to reduce energy consumption in close to viable capture costs for post-combustion CO2 capture. In our previous work, we have proposed potassium prolinate (ProK)/ethanol/water solution as a water-lean absorbent [27,28]. This solvent can undergo a liquid-to-solid phase change during CO2 absorption. Low-temperature regeneration for solid phase enriched in about 50 % of the total CO2 captured may significantly lower the energy consumption. However, associated issues, such as ethanol vapor loss and complicated solid-phase transfer process, need to be considered in future study. In this work, we have examined the capture performance using several AAS based water-lean systems. EG with high boiling and low specific heat was considered to replace water. A comparison of physicochemical properties of solvents is shown in Table 1 [29,30].The physical properties and the absorption and desorption rate of CO2 were determined for LysK/EG/water and ProK/EG/water and compared with their aqueous systems and aqueous MEA. The samples from the absorption test and continuous absorption – desorption cycles were analyzed by NMR spectra to provide insight into the product species and the stability of absorbent at a high regeneration temperature.

2. Experimental

2.1. Materials and absorbent preparation

L-Proline (Pro, 99.28% HPLC purity, CAS No.147-85-3), L-Lysine (Lys, ≥ 98% HPLC purity,

CAS No.56-87-1), MEA (99.1% GC purity, CAS No.141-43-5), potassium hydroxide (KOH, ≥ 95% GR, CAS No.1310-58-3), ethylene glycol (EG, 99.27% GC, CAS No.107-21-1) and ethanol (EthOH, 99.71%, CAS No.64-17-5) were purchased from Aladdin reagent, China. N2 (99.99%, v/v) and CO2 (99.995%, v/v) were obtained commercially. Standard N2/CO2 mixed gas (19.98 % CO2) was purchased from Nanjing Special Gas Factory Co., Ltd. Pure N2, S1 and/or pure CO2 were used for calibration of non-dispersive infra-red (NDIR) CO2 gas analyzers (GXH-3011N, 0 – 20%, uncertainty


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1.0% FS, Institute of Beijing Huayun Analytical Instrument; SKS-BA-CO2, 0 – 100%, uncertainty 1.0% FS, Guangdong Skesen Gas Detection Equipment Co., Ltd.). All the reagents were used without further purification. Water was produced from Merck-Millipore Aquelix 5. Electronic analytical balances (OHAUS, CP214, Scout SE 1501F) were used for weight measurements. 0.495 M sulfuric acid standard solution was used to measure CO2 loading. The amino acid salt solutions (water/ethanol/EG as solvent) were prepared by dissolving amino acid in solvent with an equimolar amount of KOH in a volumetric flask at 298 K. Aqueous MEA (2.0M) solution was also prepared and used for comparison in this study.

2.2. Screening experiments for CO2 absorption Absorption screening tests can offer initial estimation for selection of potential absorbent systems before further characterization. The experiments apparatus was operated at 298, 303 and 313 K and near atmospheric pressure, as shown in Fig.1a. Ethanol and ethylene glycol were selected for the preparation of water-lean absorbents, and their solvent/phase behaviors during CO2 absorption and relative absorption capacity were investigated compared with their aqueous systems. A gas mixture of N2/CO2 (14.5 ± 0.5 vol% CO2) was firstly sent into a 250 mL glass reactor that heated to 298K at a flow rate of about 800 mL/min. The CO2 concentration exiting the reactor was analyzed and recorded using an online CO2 analyzer after drying the gas using a cold trap and a drying tube. Then, the absorbent (200 mL) was fed into the reactor to start the absorption. The operation of CO2 absorption would stop until the outlet CO2 concentration was above 95% of the inlet CO2 concentration. After the absorption test, the samples were taken to quantify the absorption capacity. The amount of CO2 captured in the absorbent solution or slurry solution, defined as mol CO2 / mole absorbent and mol CO2 / kg unloaded solution, was measured by a modified Chittick CO2 apparatus [27]. 2.3. Solubility of CO2 and N2O Equilibrium CO2 solubility in the potential absorbent was performed in a thermostatted stirred cell reactor. A schematic diagram of the experimental set-up was presented in Fig. 2. The method and


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procedure was reported in our previous work [26-28,31]. The set-up consists of a vessel (Vv, 1.33 L) for storing the CO2 gas and a reactor (VR, 0.81 L) with a magnetic-drive stirrer with two GS4200-USB pressure transducers (0 – 3.5 bar absolute, ESI) with a stated uncertainty of 0.2 kPa. The temperatures (TV and TR) were recorded by PT-100 thermocouples (WZP-293) and a recorder (MIK200D) from MEACON China. In each run, a known mass and volume (Vs, about 0.50 L) of solution was fed into the reactor and degassed by vacuum at 298 K. Then, the system was allowed to come to the vapor-liquid equilibrium at controlled temperature and this pressure was referred as the initial system pressure. Then, the CO2 gas was filled in the reactor and the added amount of CO2 can be calculated from CO2 stored vessel by pressure changes. After vapor-liquid equilibrium reached again, the CO2 partial pressure at the calculated CO2 loading (α) can be obtained. By adding more CO2 in the reactor to change the CO2 loading, a set of equilibrium data can be achieved. PCO2 = Ptotal − Pinitial

α=

(1)

VV  P1 P2  (VR − VS ) PCO2  − − RTV  z1 z2  zeq RTR

(2)

nAAS

where PCO2 is the partial pressure of CO2 present in the gas phase at vapor-liquid equilibrium. α is defined as the mole of CO2 per mole of amino acid salt. z1 , z2 and zeq are the compressibility factors of gas at different conditions, respectively, which were calculated using the Peng-Robinson equation of state. Solubility of N2O in the absorbent was measured in the same experimental set-up. The detailed method was reported in our previous work [23].

2.4. CO2 absorption and desorption experiments The absorption (313K and 333K)-desorption (353K) experiments for potential absorption systems were further investigated on a screening apparatus at near atmospheric pressure, as shown in Figure 1. This method can provide the detailed information of the absorbents on the absorption capacity, the relative absorption and desorption rate, and cyclic capacity. Aqueous MEA were used as a reference system for comparison. For CO2 absorption experiment, N2 and CO2 gases controlled by mass flow controllers (D07-7C, Beijing Sevenstar, China, with ± 1.5% F.S. accuracy), are mixed in


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the mixing tank with a total flow rate of 540 mL/min. Then the gas mixture (13 vol% CO2) was sent to a 250 mL reactor controlled by thermostat bath (DF-101S, Yuhua, China, with ± 1.0% F.S. accuracy). The outlet gas stream was cooled by a Graham condenser (C544300, Synthware, Beijing). The CO2 concentration was determined by the GXH-3011N CO2 analyzer. Aqueous unloaded water-lean absorbent (100 mL) was preheated to the absorption temperature and quickly introduced into the reactor to start the absorption process, which was monitored by measuring the CO2 concentration of the outlet gas along with time. Absorption was finished at 90 min or stopped when solid precipitate appeared. Meanwhile, liquid samples were taken for CO2 loading analysis. Then, the reactor was removed overhead and back to the water bath when the temperature 353 K was reached. CO2 desorption was started by bubbling pure N2 in the loaded solution with a flow rate of 0.2 L /min. The CO2 concentration was monitored by the CO2 gas analyzer. The duration time was 60 min and then liquid samples were taken. The next absorption-desorption process started using CO2 lean loading of solution instead of zero CO2 loading. Desorption performance was also investigated by the thermo-desorption technique. Additionally, in order to evaluate the thermal stability of the absorbents, eight cycles of absorption (313K or 333K for 120 min) – desorption (433 K for 90 min) was also investigated in an apparatus (Fig.1b). Nitrogen as overhead carrying gas at 0.2 L/min was used.

2.5. Analytical methods

Density measurements were carried out using a digital oscillating tube density meter (Anton Paar, DMA-4100M). Viscosity measurements were performed using a digital rolling ball microviscometer (Anton Paar, Lovis 2000M/ME). Certified standard S3 and N26, purchased from CANNON Instrument Company USA and Sigma-Aldrich China respectively, was used to calibrate the density and viscosity meters. The water content in the water-lean solvents was analyzed by Karl Fischer method using an automatic titrator (Shanghai Anting Electronic Instrument, ZSD-2) [17,22]. CO2 loading (α) of samples is expressed as mol CO2 / mole absorbent and mol CO2 / kg CO2-free absorbent solution. By using the Chittick CO2 apparatus [27], the amount of captured CO2 in the sample of known volume and mass was measured by acid titration in a flask which is connected to a


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graduated gas measuring tube and an adjustable leveling bulb reservoir. The pressure in the flask, measured by a digital pressure manometer (GM520, Shenzhen Jumaoyuan Science and Technology Co., Ltd.), was kept to the normal atmosphere pressure by adjusting the liquid level. Meanwhile, the concentrations of AAS and MEA were measured using an automatic potentiometric titrator (ZDJ-5, INESA Scientific Instrument Co., Ltd). Based on the logged data, the absorption and desorption rate at a given time, rCO2, with units of mole /(kg solution• s), is calculated as: In QCO − 2

rCO2 =

Out QN 2 yCO 2 Out 1 − yCO 2

(3)

ms × 22.40

where ms is the amount of CO2-free absorbent solution in the flask, kg. QCO2In and QN2 are the volumetric flow rates in standard liter per second (SLS) of CO2 and N2 fed into the reaction flask from mass flow controller, respectively and yCO2Out is the CO2 molar fraction in the outlet gas stream as a function of time from the CO2 analyzer. Then, the CO2 loadings (αrich and αlean) in mol CO2 / kg absorbent solution can also be calculated by integration over period of time, expressed as: t

α rich = ∫ rCO 0

2 ,absorption

t

α lean = α rich − ∫ rCO 0

(4)

dt

2 , desorption

(5)

dt

The CO2 cyclic capacity, αcyc, can be defined by the difference of CO2 rich loading after absorption and CO2 lean loading after desorption, with units in mole CO2 per mole absorbent or mole CO2 per kg CO2-free absorbent solution. The errors between the measured and the calculated were within 5%. 1

H NMR and

13

C NMR experiments were carried out on a Bruker Avance 500 spectrometer. It

equipped with a 5 mm high-sensitivity triple-resonance TCI probe that maintained at 298.0 K. D2O was used for deuterium lock and deuterated (3-(trimethylsilyl)-2,2,3,3-tetradeuteropropionic acid sodium salt, TMSP-d4, was used as external reference at 0.00 ppm.


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3. Results and discussion

3.1. Physicochemical properties of amino acid salt-based solutions

The physical properties such as density and viscosity of water-lean ProK-EG and LysK-EG solutions at 303 and 313 K are presented in Table 2. Aqueous and ethanol-based water-lean systems for MEA, LysK and ProK were also investigated for comparison. It can be seen that the EG-based water-lean systems have much higher viscosity than other systems studied, which probably results in a lower mass transfer rate for CO2 absorption. For pure EG, the viscosity is 13.66 mPa s at 303K and 9.47 mPa s at 313K. The addition of amino acid salts greatly increased its viscosity, especially for LysK. As expected, the viscosity of these absorbents decreases rapidly with the increasing temperature. For 2.0M ProK-EG and 2.0M LysK-EG at 333K, their viscosities decrease to 14.01 and 22.23 mPa respectively (not shown in Table 2). Physical solubility of CO2 can not be measured directly because the chemical absorbents can react with CO2. The solubility of CO2 was estimated based on a N2O analogy experiment because of the non-reacting N2O with similarities in molecular volume, configuration and electronic structure with CO2 [32-34]. It was found that N2O solubility in AAS-based absorbents decreases with increasing the solvent polarity (i.e. EthOH< EG< water). Although ethanol as a volatile solvent can substantially increase the CO2 physical solubility, it may result in solvent loss and environmental issues during CO2 absorption. A series of screening tests using 1.0M absorbent in different solvents (i.e. water, ethanol and EG) have been carried out under constant conditions (298 K and near atmospheric pressure). Absorption behaviors and relative absorption capacity were shown in Fig.3. It should be pointed out that, in these experiments, the setup was firstly purged with pure N2 before simulated flue gas was switch into the absorbent. Therefore, the profiles of outlet CO2 concentration can also roughly reflect the absorption rate and efficiency along with the CO2 loading. As can be seen from Fig.3a, the use of ethanol instead of water showed more efficient in CO2 removal than aqueous ProK system, resulting in the lower CO2 concentration at outlet gas stream. When the solid precipitation occurred, the enhanced absorption rate


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of CO2 was observed. High physical solubility of CO2 in the solvents can enhance the CO2 chemical absorption in ProK-based solutions under low CO2 partial pressure (about 15kPa) conditions. For LysK/EthOH system, a white-sticky precipitation formed once CO2 was absorbed and the gas disperser can be easily blocked. For the low volatile EG systems at 298K, their viscosities are about ten times higher than those for the aqueous and ethanolic systems. Both water-lean LysK/EG and ProK/EG solutions showed lower absorption rate of CO2. Increasing absorption temperature may improve the absorption performance because of highly temperature-dependant viscosity for these systems. During CO2 absorption, no phase change was observed for ProK/EG solutions. Solid phase was observed for LysK/EG solutions although its formation was a slow process. As seen from Fig.3b, the absorption loading of aqueous MEA (1.0M) was about 0.65 mol mol-1, which was consistent with the literature [35]. The measured CO2 loadings of LysK/EG and ProK/EG were about 1.05 and 0.66 mol mol-1 absorbent, respectively.

3.2. Solubility of CO2 in water-lean absorbents

The experiments on CO2 solubility in 2.5 M MEA solutions at 313 K and 373 K were used to validate the experimental method in this work. The experimental data are presented in Fig. 4a, along with the reported in open literature [35,36]. As seen that the experimental data from present work are in line with literature data, especially for the CO2 partial pressure above 1.0 kPa, which suggests that the experimental setup and method in this work are reliable. The vapor-liquid equilibrium data for the CO2–AAS–EG–H2O systems were determined at temperatures of 298 –353 K over CO2 partial pressures of 1–150 kPa. The graphical representation of VLE data is shown in Fig. 4 in terms of CO2 loading (α) against CO2 partial pressure. It should be pointed out that the initial pressure (Pinitial) equals to the sum of solvent vapor pressure (Ps) and the residual gas pressure. The total pressures in the equilibrium cell were not higher than 160 kPa. Therefore, the ideal gas for gas phase was considered. It can be seen that CO2 solubility in LysK/EG system is about twice more than that in ProK/EG system, which was probably due to the structural feature of LysK with two amino groups. Similar observation about CO2 solubility in aqueous ProK


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and LysK has been reported in previous work [25]. As expected, CO2 loading increases with the increasing CO2 partial pressures over the solutions and decreases with the increase in temperature. Only for LysK/EG system, solid was found after the equilibrium cell was opened to discharge the CO2 loaded solution. However, it was unknown at which point the phase change happened. With VLE data for 2.0M ProK+EG solutions at the temperature range of 313-353K, a semi-empirical model was correlated with R2 = 0.996. Then, the heat of absorption of CO2 can also be roughly estimated by a simplified Gibbs-Helmholtz relationship for abosrbent selection.

ln PCO2 = 13.7245 −   ∂ ln PCO2  ∂ 1 T 

( )

α 8402.9 + 15.9131α − 20.3303α 2 + 7447.12 T T

  = −∆H diff = − 8402.9 + 7447.12α  R α

(6)

(7)

where PCO2 represents the partial pressure of CO2 in the gas phase over the solution at CO2 loading α, R is the universal gas constant and T is the equilibrium temperature. The -∆Hdiff values for 2.0M ProK/EG at the loading range investigated are about 32-70 kJ/mol CO2 absorbed, which are lower than 80-85 kJ/mol for conventional 30 mass % MEA [37].

3.3 CO2 capture performance with water-lean ProK+EG and LysK+EG absorbents

The absorption behavior, capacity, absorption rate as a function of time were investigated in the apparatus (Fig.1a) at 313 and 333K for 2.0M absorbent (i.e. ProK and LysK) in EG solvent. Aqueous 2.0M MEA, ProK and LysK was used for comparison. The inlet flowrate of simulated flue gas with 13.5 ± 0.5 v/v% CO2 was about 32.5 L h-1. Generally, absorption process was operated for 90 min. However, once the absorbent solution turned cloudy for LysK/EG system, the absorption was stopped. These profiles are shown in Fig. 5. For comparison in a batch absorption process, the outlet CO2 concentration in the gas stream is related to the removal efficiency, which reveals the absorption performance of any absorbent system. The elapsed time for removal efficiency higher than 80% (t80) in a specific reactor is generally used for comparison of absorption performance [31]. It can be seen from Fig. 5 that the aqueous 2.0M


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MEA and aqueous 2.0M LysK systems maintained the highest removal efficiency and absorption rate in mole /(kg • s) during the first 30 min, and the t80 is about 32min and 55min respectively. However, for EG-based absorbents, t80 is about 15min and 25min for LysK and ProK at 313K, respectively. Surprisingly, for absorption at 333K, t80 is increased to 36min and 26min for LysK and ProK, respectively. Removal efficiency and absorption rate were greatly enhanced for LysK/EG system at 333K. It is probably due to the fact that, the decreased viscosity of its solution could be favorable for the mass transfer between the simulated gas and the liquid solution and then enhance the absorption rate. When the elapsed time is about 80min, phase change was observed for the LysK/EG system, where the CO2 loading is about 1.4 mol CO2/kg fresh solution. Unlike the LysK/EG system, the similar absorption behaviors for ProK/EG and aqueous MEA were observed. After t80, the outlet CO2 concentration in the gas stream increased rapidly and reached the 95% of the inlet value at about 50min. After then, the CO2 loadings turned stable, 0.9 and 1.1 mol CO2/kg solution for ProK/EG and aqueous MEA respectively. Probably due to relatively low viscosity, aqueous ProK and LysK showed fast absorption rate and high absorption capacity. Absorption capacity for aqueous 2.0M ProK and aqueous 2.0M LysK was measured to be about 1.1 and 1.8 mol CO2/kg solution, respectively. Desorption profiles under the same operating conditions are presented in Fig. 6. In these desorption experiments, CO2 desorption at 353K for the CO2 loaded MEA solution was used as a reference for comparison. It can be seen from Fig.6a that CO2 concentration in the stripping gas stream increased rapidly to the peak and then decreased gradually with the elapsed time. The areas under the desorption curves can also reflect the amount of CO2 released from the solutions (Fig.6b). The desorption efficiency was about 34%, 16% and 45% for aqueous ProK-EG, LysK-EG and aqueous MEA, respectively. For aqueous ProK and aqueous LysK, the desorption efficiency was about 30%. It should be pointed out that the properties of CO2-loaded solutions such as viscosity and solvent vapor pressure are quite different, which can affect the desorption rate and performance. The viscosities of CO2-loaded ProK-EG and LysK-EG were measured to be 10.5 and 20.7 mPa•s at 353 K, which are far higher than that of aqueous MEA (about 1.1 mPa•s). EG has lower volatility than water. The estimated vapor pressure of EG is about 0.68 kPa at 353K, but 47.4 kPa for water. It is the fact that the generated


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solvent vapor and carry-over condenser can promote CO2 desorption by N2 stripping through gas effect. Effect of desorption temperature on solvent regeneration was investigated in an apparatus (Fig.1b) for CO2-loaded ProK-EG and LysK-EG solutions. In these runs, the CO2 loaded solutions were heated up to the desired temperatures (± 2.0K) within 10 min. As can be seen from Fig. 7, most of CO2 was released within 20min, and the higher desorption temperature, the more CO2 released from the solutions. The calculated CO2 loadings from equation 5 matched well with the measured data for the resulting solution (Fig. 7c). For ProK-EG system, CO2 loading changed from 0.96 to 0.28 mol CO2/kg solution at 433K. For LysK-EG system, the resulting CO2 loading was about 0.37 from 1.25 mol CO2/kg solution. Thus desorption efficiency could enhance up to about 70%, which is comparable with that of the aqueous MEA system (about 68%) at 373K. Although the CO2 desorption was operated at 433K, the solvent vapor pressure of EG was only about 30kPa. The enhancing CO2 release is mainly due to the increasing driving force, since the equilibrium CO2 partial pressure increase with the increasing temperature at a certain CO2 loading. Small amount of solvent evaporation during the desorption will reduce the regeneration energy consumption. More experiments are required to confirm this potential in future studies.

3.4 Absorption-desorption cycles

The regenerability of these absorbents was comprehensively investigated by means of continuous cycles of absorption-desorption to evaluate the long-term absorption behavior and CO2 cyclic capacity by a temperature-swing process. For N2-assisted desorption on ProK-EG system, the absorption-desorption cycle was repeated four times. For thermal-desorption at 433K, eight continuous cycles of absorption (120 min)–desorption (90 min) were performed. The results were shown in Fig.8. As can be seen from Fig.8a, the absorption capacity and cyclic capacity reveals that the absorption (313K)-desorption (353K) performance of this absorbent is relatively stable considering the measuring uncertainty of CO2 loading. For the continuous cycles with desorption at 433K, the absorption capacity was maintained at about 0.88 mol CO2/kg for ProK-EG system and


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Page 14 of 34

1.35 mol CO2/kg for LysK-EG system, except for the first cycle. It was also observed that the cyclic capacity was about 0.50 mol CO2/kg for ProK-EG system and 0.85 mol CO2/kg for LysK-EG system. However, the cyclic capacity decreased slightly for two systems due to the increasing CO2-lean loading after each cycle, as shown in Fig.8b,c. These suggest that desorption efficiency decreased a little after each cycle for 90min. However, increasing the desorption time or temperature, the CO2-lean loading can decrease to the same level. It should be pointed out that no additional water and EG were added into the regenerated solutions to compensate the solvent loss due to evaporation, which may result in the change of physicochemical properties of CO2-loaded solution. The viscosity and the concentration of AAS would have a little increase. For comparison, the cyclic capacity of aqueous 2.0M MEA was calculated to be about 0.70 mol CO2/kg solution for absorption (313K)-desorption (373K) cycle (see Fig.7c). For the most used 5.0M MEA, the estimated cyclic capacity was 1.25-1.50 mol CO2/kg solution for absorption (313K)-desorption (393K) conditions [35]. Thus, considering a limited solubility of AAS in EG-based water-lean solvent, these developed absorbents are expected to have lower CO2 cyclic capacity than aqueous 5.0M MEA although they have the potential for reducing energy consumption in the CO2 capture process. 3.5 Reaction mechanism of CO2 capture into water-lean absorbents based on NMR spectroscopy

3.5.1 NMR spectra analysis of the carbonated species in CO2-loaded absorbents

Reaction mechanisms of CO2 absorption into the aqueous and non-aqueous absorbents are quite different in the reported literature [16,23,24,27,38,39]. To understand the reaction mechanism of CO2 absorption into the water-lean absorbents, 13C NMR spectra analysis was performed before and after absorption. The effect of a little amount of water on the resulting products was discussed. The results are shown in Figs.9-10. The nonequivalent C atoms of ProK, LysK and EG are labeled in these Figures. For CO2-free ProK-EG system, the free prolinate ion (Pro-) was indicated by the chemical shift (C1-C5) in 13C NMR spectra at 27.9, 33.5, 48.8, 64.3 and 184.9 ppm, and the carbon signal for EG were observed at 65.3 ppm (Fig.9 a,b). When CO2 was absorbed by the solution, “doubled”


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signals (i.e. 26.7, 31.9, 49.8, 64.0 and 177.9 ppm) were found and two new signals were observed in the range of 162–166 ppm with low intensity resonances. The chemical shift of 165.7 ppm was assigned to carbonyl carbon of Pro-carbamic acid or carbamate product. A small carbon signal at 164.7 ppm, which was attributed to the newly formed bicarbonate-carbonate species, suggested water still took part in CO2 absorption even in such a low content (Table 1). Similar to LysK-EG system, six nonequivalent C atoms (C1-C6) in lysinate ion (Lys-) were easily assigned at 186.3, 58.8, 42.9, 37.0, 34.2 and 25.0 ppm, as already reported for aqueous solutions [26]. The pH of fresh solutions was about 12.9 and decreased as the CO2 loading increased. LysK possesses an active α-amino group (pKa 9.2) and a ε-amino group(pKa 10.7) in the side chain. The molecules with deprotonated amine groups are expected to react with CO2. “Tripled” signals were observed as well as three new signals at 161.9, 166.4 and 167.4 ppm. The latter two chemical shifts are attributed to the cabamic acids/carbamates and the week signal at 161.9 ppm is ascribed to bicarbonate species. These results suggested that CO2 can react with ProK and LysK in EG solvent forming main products, i.e. carbamic acids or carbamates. Moreover, bicarbonate can be found due to the presence of a little amount of water. Unlike the non-aqueous amines using alcohols as solvent [16,38,39] and ProK-ethanol absorbent [27], monoalkyl carbonate (EG-CO3-) with carbon signals in the range of 159–161 ppm was not observed for both ProK-EG and LysK-EG systems. CO2 uptake by the water-lean AAS absorbent using EG as a solvent was proposed as follows:

 → AAS-N(R)COOH CO 2 + AAS-N(R)H ← 

(8)

 → AAS-N(R)COO − + AAS-N(R)H 2 + AAS-N(R)COOH + AAS-N(R)H ← 

(9)

 → AAS-N(R)H 2 + + HCO 3− CO 2 + H 2 O + AAS-N(R)H ← 

(10)

3.5.2 NMR spectra of the CO2 absorption-desorption

To provide more information about the increasing loading of the regenerated solutions for each cycle, we reported here the

13

C NMR and 1H NMR spectra of the samples from the 8th cycle of

absorption-desorption. Samples treated by concentrated acid and base to release CO2 completely were


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Page 16 of 34

also tested compare the fresh absorbent samples for 1H NMR spectra analysis. The results are shown in Figs.9-11. As can be seen that bicarbonate species disappears in the two CO2-lean absorbent after desorption at 433K and week signals for carbonyl carbon of carbamates were observed due to the low CO2 loadings (about 0.5-0.7 mol CO2/kg). Notably, some additional carbon signals were observed at 171.1, 102.8 and 55-60 ppm for ProK-EG system. In the 1H NMR spectra of ProK-EG system, additional peaks were also found compared with the fresh sample, indicated that new compounds might exist in the regenerated solution. For LysK-EG system, negligible change was observed in the spectra. Acidic degradation products of EG such as glycolic acid and formic acid are possible based on the NMR spectra analysis. Since the decomposition temperature of ProK and LysK is in the range of 473-500K, the desorption at 433K for both ProK-EG and LysK-EG systems is not recommended for a long-term run.

4. Conclusions In the present study, CO2 absorption and desorption characteristics using LysK/EG and ProK/EG water-lean absorbents were investigated under post-combustion capture conditions. EG-based water-lean systems have much higher viscosity than other systems studied, although they have shown some advantages of high physical solubility of CO2 and low solvent vapor pressure. Similar to the aqueous systems, CO2 solubility in LysK/EG system is greater than that in ProK/EG system. A semi-empirical VLE model for 2.0M ProK/EG at 313-353K was proposed and the estimated heat of absorption of CO2 was about 32-70 kJ/mol CO2 absorbed. Absorption behaviours and CO2 removal efficiency for ProK/EG and aqueous MEA were similar. However, phase change was observed for the LysK/EG system at about 1.4 mol CO2/kg fresh solution. Increasing absorption temperature could improve the absorption performance for both EG-based systems. Desorption profiles from N2-assisted and thermal desorption tests showed that desorption efficiency was worse than that for aqueous MEA system at the same operational conditions. Thermal- desorption at 433K for EG-based systems, desorption performance can be comparable with


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that of the aqueous MEA system at 373K. But, solvent vapor pressure of EG was still lower than that of water in this case, resulting in low energy consumption due to small amount of solvent evaporation. The data from several absorption-desorption cycles suggest the absorbents can be reused for CO2 capture. It was also observed that desorption efficiency decreased a little after each cycle of absorption (313K)-desorption (433K), probably due to no water addition to compensate the solvent evaporation resulting in the change of properties of CO2-loaded solutions. Carbamic acids or carbamates, assigned at 165–167 ppm, are the main species in the CO2-loaded absorbents by 13C NMR analysis. The formation of bicarbonate-carbonate species suggest water takes part in CO2 absorption even in such a low content. Monoalkyl carbonate (EG-CO3-) was not observed for both ProK-EG and LysK-EG systems. Acidic degradation products of EG such as glycolic acid and formic acid are possible at 433K for a long-term run based on the NMR spectra analysis. Although the EG-based water-lean absorbents have potential for energy-efficient CO2 capture, high viscosity of these systems and high desorption temperature are still the challenges for the practical use. Derivative solvents such as glycol ethers with relatively low viscosity and advanced phase change systems could be considered for improvement in future.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]; Tel.: +86 311 88632183. Fax: +86 311 88632183.

Funding The authors would like to acknowledge Key Program of Hebei Provincial Natural Science Foundation (Grant No. B2018208154) and Training Program for Talent Engineering of Hebei Province (Grant No. A2017002022) for financial support.

Notes The authors declare no competing financial interest.


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References [1]

Vitillo, J. G.; Smit, B.; Gagliardi, L. Introduction: Carbon capture and separation. Chem. Rev. 2017, 117, 9521–9523.

[2]

Idem, R.; Supap, T.; Shi, H.; Gelowitz, D.; Ball, M.; Campbell, C.; Tontiwachwuthikul, P. Practical experience in post-combustion CO2 capture using reactive solvents in large pilot and demonstration plants. Int. J. Greenh. Gas Control. 2015, 40, 6–25.

[3]

Mumford, K.A.; Wu, Y.; Smith, K.H.; Stevens, G.W. Review of solvent based carbon-dioxide capture technologies. Front. Chem. Sci. Eng. 2015, 9, 125–141.

[4]

Rochelle, G. T. Amine scrubbing for CO2 capture. Science 2009, 325, 1652–1654.

[5]

Vega, F.; Sanna, A.; Navarrete, B.; Maroto-Valer, M. M.; Cortés, V. J. Degradation of amine-based solvents in CO2 capture process by chemical absorption. Greenh. Gas Sci. Technol. 2014, 4, 707–733.

[6]

Heldebrant, D. J.; Koech, P. K.; Glezakou, V. -A.; Rousseau, R.; Malhotra, D.; Cantu, D. C. Water-lean solvents for post-combustion CO2 capture: fundamentals, uncertainties, opportunities, and outlook. Chem. Rev. 2017, 117, 9594–9624.

[7]

Kang, M.-K; Cho, J.-H; Lee, J.-H; Lee, S.-S; Oh. K.-J. Kinetic reaction characteristics of quasi-aqueous and nonaqueous sorbents for CO2 absorption using MEA/H2O/ethylene glycol. Energy Fuel 2017, 31, 8383–8391.

[8]

Barzagli, F.; Mani, F.; Peruzzini, M. A comparative study of the CO2 absorption in some solvent-free alkanolamines and in aqueous monoethanolamine (MEA). Environ. Sci. Technol. 2016, 50, 7239−7246.

[9]

Sada, E.; Kumazawa, H.; Han, Z. Q.; Matsuyama, H. Chemical kinetics of the reaction of carbon dioxide with ethanolamines in non-aqueous solvents. AIChE J. 1985, 31, 1297–1303.

[10] Hwang, K. S.; Park, S. W.; Park, D. W.; Oh, K. J.; Kim, S. S. Absorption of carbon dioxide into diisopropanolamine solutions of polar organic solvents. J. Taiwan Inst. Chem. Eng. 2010, 41, 16–21. [11] Park, S. W.; Lee, J. W.; Choi, B. S.; Lee, J. W. Absorption of carbon dioxide into non-aqueous solutions of N-methyldiethanolamine. Korean J. Chem. Eng. 2006, 23, 806–811. [12] Tan, J.; Shao, H.; Xu, J.; Du, L.; Luo, G. Mixture absorption system of monoethanolamine-triethylene glycol for CO2 capture. Ind. Eng. Chem. Res. 2011, 50, 3966–3976. [13] Yu, C. H.; Wu, T. W.; Tan, C. S. CO2 capture by piperazine mixed with non-aqueous solvent diethylene glycol in a rotating packed bed. Int. J. Greenh. Gas Control. 2013, 19, 503–509. [14] Zheng, C.; Tan, J.; Wang, Y. J.; Luo, G. S. CO2 Solubility in a mixture absorption system of 2-amino-2-methyl-1-propanol with glycol . Ind. Eng. Chem. Res. 2012, 51, 11236–11244.


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[15] Zheng, C.; Tan, J.; Wang, Y. J.; Luo, G. S. CO2 solubility in a mixture absorption system of 2-amino-2-methyl-1-propanol with ethylene glycol. Ind. Eng. Chem. Res. 2013, 52, 12247–12252. [16] Barzagli, F.; Mani, F.; Peruzzini, M. Efficient CO2 absorption and low temperature desorption with non-aqueous solvents based on 2-amino-2-methyl-1-propanol (AMP). Int. J. Greenh. Gas Control 2013, 16, 217–223. [17] Chen, S.; Chen, S.; Fei, X.; Zhang, Y.; Qin, L. Solubility and characterization of CO2 in 40 mass % N-ethylmonoethanolamine solutions: Explorations for an efficient nonaqueous solution. Ind. Eng. Chem. Res. 2015, 54, 7212−7218. [18] Abu-Zahra, M. R. M.; Schneiders, L. H. J.; Niederer, J. P. M.; Feron, P. H. M.; Versteeg, G. F. CO2 capture from power plants part I. A parametric study of the technical performance based on monoethanolamine. Int. J. Greenh. Gas Control. 2007, 1, 37–46. [19] Lin, P. H.; Wong, D. S. H. Carbon dioxide capture and regeneration with amine/alcohol/water blends. Int. J. Greenh. Gas Control 2014, 26, 69–75. [20] Kang, M.; Jeon, S.; Cho, J.; Kim, J.; Oh. K. Characterization and comparison of the CO2 absorption performance into aqueous, quasi-aqueous and non-aqueous MEA solutions. Int. J. Greenh. Gas Control 2017, 63, 281–288. [21] Kang, M.; Cho, J.; Lee, J.; Lee, S.; Oh. K. Kinetic reaction characteristics of quasi-aqueous and nonaqueous sorbents for CO2 absorption using MEA/H2O/ethylene glycol. Energy Fuel 2017, 31, 8383–8391. [22] Machida, H.; Oba, K., Tomikawa, T,; Esaki, T,; Yamaguchi, T,; Horizoe, H. Development of phase separation solvent for CO2 capture by aqueous (amine + ether) solution. J. Chem. Thermody. 2017, 113, 64–70. [23] Shen, S.; Yang, Y.; Bian, Y.; Zhao, Y. Kinetics of CO2 absorption into aqueous basic amino acid salt: potassium salt of lysine solution. Environ. Sci. Technol. 2016, 50, 2054 –2063. [24] Majchrowicz, M. E.; Kersten, S.; Brilman, W. Reactive absorption of carbon dioxide in L-prolinate salt solutions. Ind. Eng. Chem. Res. 2014, 53, 11460–11467. [25] Shen, S.; Yang, Y.; Wang, Y.; Ren, S.; Han, J.; Chen, A. CO2 absorption into aqueous potassium salts of lysine and proline: density, viscosity and solubility of CO2. Fluid Phase Equilib. 2015, 399, 40−49. [26] Shen, S.; Zhao, Y.; Bian, Y.; Wang, Y.; Guo, H.; Li, H. CO2 absorption using aqueous potassium lysinate solutions: Vapor - liquid equilibrium data and modelling. J. Chem. Thermody. 2017, 115, 209–220. [27] Shen, S.; Bian, Y.; Zhao, Y. Energy-efficient CO2 capture using potassium prolinate/ethanol solution as a phase-changing absorbent. Int. J. Greenh. Gas Control 2017, 56, 1–11. [28] Bian, Y.; Zhao, Y.; Shen, S. Characteristics of potassium prolinate + water + ethanol solution as a phase changing absorbent for CO2 Capture. J. Chem. Eng. Data 2017, 62 (10): 3169–3177. [29] Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 90th ed., (CD-ROM, version 2010); CRC Press: Boca


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Energy & Fuels 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

Page 20 of 34

Raton, FL, 2010. [30] Poling, B. E.; Thomson, G. H.; Friend, D.G..; Rowley, R. L.;Wilding, W. V. Perry’s Chemical Engineers Handbook, 8th ed.; The McGraw-Hill-Companies Inc.: Columbus, USA, 2011; pp 2–55. [31] Zhao, Y.; Bian, Y.; Li, H.; Guo, H.; Shen, S.; Han, J.; Guo, D. A comparative study of aqueous potassium lysinate and aqueous monoethanolamine for postcombustion CO2 capture. Energy Fuel 2017, 31, 14033–14044. [32] Laddha, S. S., Diaz, J. M., Danckwerts, P. V. The N2O analogy: the solubilities of carbon dioxide and nitrous oxide in aqueous solutions of organic compounds. Chem. Eng. Sci. 1981, 36, 228–229. [33] Bian, Y., Shen, S. CO2 absorption into a phase change absorbent: Water-lean potassium prolinate/ethanol solution. Chin. J. Chem. Eng. 2018, https://doi.org/10.1016/j.cjche.2018.02.022. [34] Monteiro, J.G.M-S.; Svendsen, H.F. The N2O analogy in the CO2 capture context: literature review and thermodynamic modelling considerations. Chem. Eng. Sci. 2015, 126, 455–470. [35] Lee, J. I.; Otto, F. D.; Mather. A. E. Equilibrium between carbon dioxide and aqueous monoethanolamine solutions. J. Appl. Chem. Biotechnol 1976, 26, 541–549. [36] Song, H. J.; Lee, M. G.; Kim, H. Density, viscosity, heat capacity, surface tension, and solubility of CO2 in aqueous solutions of potassium serinate. J. Chem. Eng. Data 2011, 56, 1371–1377. [37] Luo, W.; Yang, Q.; Conway, W.; Puxty, G.; Feron, P.; Chen, J. Evaluation and modeling of vapor−liquid equilibrium and CO2 absorption enthalpies of aqueous designer diamines for post combustion capture processes. Environ. Sci. Technol. 2017, 51, 7169−7177. [38] Barbarossa, V.;

Barzagli, F.; Mani, F.; Lai, S.; Stoppioni, P.; Vanga, G.

Efficient CO2 capture by non-aqueous

2-amino-2-methyl-1-propanol (AMP) and low temperature solvent regeneration, RSC Adv. 2013, 3, 12349–12355. [39] Barzagli, F.; Mani, F.; Peruzzini, M. Improved solvent formulations for efficient CO2 absorption and low-temperature desorption, ChemSusChem 2012, 5, 1724 – 1731.


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Captions Table 1 Physicochemical property data of solvents Table 2 Physical properties and CO2 absorption capacity of aqueous and water-lean absorbent solutions Fig.1 Schematic diagram of the absorption-desorption apparatus. (a) CO2 absorption and desorption using mimic flue gas or N2 via gas disperser, (b) desorption setup using a heating mantle in the cycling runs. Fig.2 Schematic diagram of experimental apparatus for CO2 solubility. Fig.3 Comparison of CO2 absorption capacity in aqueous and water-lean absorbents (1.0M). The CO2 concentration in the simulated flue gas was 14.5 ± 0.5 mol %. (a) the outlet CO2 concentration curves versus time; (b) measured CO2 loadings. Fig.4 Comparison of CO2 solubility in different absorbent solutions: (a) aqueous 2.5M MEA solutions at 313 and 373 K compared with literature data [35,36], (b) 2.0M absorbent in EG solvent at 313, 333, 353 K. Fig.5 CO2 absorption performance using different absorbent solutions. The CO2 concentration in the simulated flue gas was 13.0 ± 0.2 mol %. (a) the outlet CO2 concentration curves versus time; (b) integration curves of CO2 loading; (c) absorption rate with loading in mol CO2 kg-1 absorbent solution. Fig.6 CO2 desorption curves for different absorbent solutions at 353 K with N2 flowrate of 0.2L min-1. (a) the outlet CO2 concentration curves versus elapsed time; (b) integration curves of CO2 loading with measured data. Fig.7 Comparison of thermal desorption performance for different absorbent solutions at various temperatures. (a) desorption profile for 2.0M ProK-EG, (b) desorption profile for 2.0M LysK-EG, (c) integration curves of CO2 loading with measured data. Fig.8 Absorption-desorption performance for ProK-EG and LysK-EG water-lean absorbents. CO2 desorption conditions: (a) 353K and 0.2 L min-1 N2; (b) thermal desorption for ProK-EG system at 433K; (c) thermal desorption for LysK-EG system at 433K.Cyclic loading (∆α) is defined as the CO2 loading difference between the CO2-rich and the CO2-lean. Fig.9

13

C NMR spectra of ProK+EG solutions in D2O. (a) ethylene glycol; (b) CO2-free ProK+EG solution; (c)

CO2-loaded ProK+EG solution; (d) sample after eight absorption-desorption cycles. Fig.10

13

C NMR spectra of LysK+EG solutions in D2O. (a) ethylene glycol; (b) CO2-free LysK +EG solution; (c)

CO2-loaded LysK +EG solution; (d) sample after eight absorption-desorption cycles. 1

Fig.11 H NMR spectra of ProK-EG and LysK-EG solutions in D2O. ProK-EG: (a) fresh absorbent, (b) CO2-loaded solution after eight absorption-desorption cycles, (c) CO2-free cycling absorbent by pH adjustment; LysK-EG: (d) fresh absorbent, (e) CO2-loaded solution after eight absorption-desorption cycles, (f) CO2-free cycling absorbent by pH adjustment.


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Table 1 Physicochemical property data of solvents a Molecular

Molar mass

Boiling point at

∆vapH (bp),

Specific heat (Cp),

Vapor pressure

structure

(g·mol-1)

101.3 kPa, K

kJ kg-1

kJ kg-1 K-1

at 293 K, Pa b

Water

H2O

18.02

373.2

2256.4

4.18

2339

EthOH

CH3CH2OH

46.07

351.4

837.0

2.44

5899

EG

HOCH2CH2OH

62.07

470.5

813.6

2.39

7.4

Solvent

a

Data were obtained from the literature [29,30].

b

Vapor pressure at 293 K was calculated from the proposed correlation in the reference [30].


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Table 2 Physical properties and CO2 absorption capacity of aqueous and water-lean absorbent solutions Absorption performance a

Absorbent CA, M

T, K

-3

ρ, g cm

η, mPa s

3

HN2O, kPa m kmol

-1

H2O, wt%

(A+S) MEA+H2O

α, mol mol-1

α, mol kg-1

Phase change

1.0

298

1.0002

1.2260

—

93.8

0.66-0.73

—

N

2.0

313

0.9964

0.9012

—

87.8

—

1.13

N

5.0

313

1.0036

1.5780

5613

70.0

0.52

2.24

N

ProK+ H2O

1.0

298

1.0547

1.3800

5843 b

85.5

0.79

—

N

LysK+ H2O

1.0

298

1.0560

1.5747

6056

82.6

1.43

—

N

ProK+ EthOH

1.0

298

0.8712

2.7309

1066

2.56

0.67

0.78

Y

2.0

303

0.9431

5.5773

1394

4.58

—

1.32

Y

1.0

303

1.1458

24.919

4039

2.40

0.66

—

N

2.0

313

1.1764

29.019

5202

3.98

—

0.93

N

1.0

303

1.1427

34.727

4584

2.47

—

1.20

Y

2.0

313

1.1689

51.674

6567

4.71

—

1.39

Y

ProK+ EG

LysK + EG

a

CO2 capacity α is defined as the moles of CO2 per mol of absorbent or per kg of sample solution.

b

data measured at 303 K.

c

Standard uncertainties u are u (T) = 0.5 K, u (C) = 0.05 mol L-1, u (ρ) = 0.0002 g·cm-3, ur (η) = 3.0 %, ur(H) = 3.0 %, u(wH2O) = 0.2 mass%, u (α) = 0.05 mol kg-1.


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Fig. 1

(a)

(b)

Fig.1 Schematic diagram of the absorption-desorption apparatus. (a) CO2 absorption and desorption using mimic flue gas or N2 via gas disperser, (b) desorption setup using a heating mantle in the cycling runs.


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Fig. 2

Fig.2 Schematic diagram of experimental apparatus for CO2 solubility.


25

Energy & Fuels

Fig. 3 (a)

Outlet CO2 concentration (%)

15

10

ProK/water ProK/ethanol ProK/EG LysK/water LysK/EG MEA/water

5

0 0

50

100 t (min)

150

200

1.6 (b)

CO2 loading α (molCO2/mol absorbent)

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

Page 26 of 34

1.2

0.8

0.4

0.0 ProK/water

ProK/ethanol

ProK/EG

LysK/water

LysK/EG

MEA/water

Fig. 3 Comparison of CO2 absorption capacity in aqueous and water-lean absorbents (1.0M). The CO2 concentration in the simulated flue gas was 14.5 ± 0.5 mol %. (a) the outlet CO2 concentration curves versus time; (b) measured CO2 loadings.


26

Page 27 of 34

Fig. 4

1000

(a)

CO2 partial pressure (kPa)

100

10

2.5M MEA 313 K, this work 313 K, Lee et al. (1976) 313 K, Song et al .(2011) 373 K, this work 373 K, Lee et al. (1976) 373 K, Song et al .(2011)

1

0.1 0.0

0.3

0.6 0.9 CO2 loading (mol of CO2/mol of MEA)

1.2

1000

CO2 partial pressure(kPa)

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

Energy & Fuels

100

2.0 M absorbent ProK-EG, 313 K ProK-EG, 333 K ProK-EG, 353 K LysK-EG, 313 K LysK-EG, 333 K

(b)

10

1 0.0

0.3

0.6

0.9

1.2

CO2 loading (mol of CO2/mol of absorbent)

Fig.4 Comparison of CO2 solubility in different absorbent solutions: (a) aqueous 2.5M MEA solutions at 313 and 373 K compared with literature data [35,36], (b) 2.0M absorbent in EG solvent at 313, 333, 353 K.


27

Energy & Fuels

Fig. 5 (a)

15

CO2 concentration (%)

12

9

6

2.0M MEA, 313K 2.0M LysK+EG, 313K 2.0M LysK+EG, 333K 2.0M ProK+EG, 313K 2.0M ProK+EG, 333K 2.0M LysK+H2O, 313K 2.0M ProK+H2O, 313K

3 80% removal

0 0

20

40

60

80

100

t (min) 1.5

-1

CO2 loading (mol.kg )

(b)

1.0

2.0M MEA, 313K 2.0M LysK+EG, 313K 2.0M LysK+EG, 333K 2.0M ProK+EG, 313K 2.0M ProK+EG, 333K

0.5

0.0 0

20

40

60

80

100

t (min)

50

(c)

40 -1

-1

10 .RCO2 (mol .kg .s )

30

2.0M MEA, 313K 2.0M LysK+EG, 313K 2.0M LysK+EG, 333K 2.0M ProK+EG, 313K 2.0M ProK+EG, 333K

20

5

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

Page 28 of 34

10

0 0.0

0.5

1.0

1.5

-1

CO2 loading (mol .kg )

Fig. 5 CO2 absorption performance using different absorbent solutions. The CO2 concentration in the simulated flue gas was 13.0 ± 0.2 mol %. (a) the outlet CO2 concentration curves versus time; (b) integration curves of CO2 loading; (c) absorption rate with loading in mol CO2 kg-1 absorbent solution.


28

Page 29 of 34

Fig. 6

30

Outlet concentration of CO2 (%)

(a)

25

2.0M ProK-EG 2.0M LysK-EG 2.0M MEA 2.0M ProK-H2O

20

2.0M LysK-H2O

15

10

5

0 0

20

40

60

t (min)

2.0 (b)

2.0M ProK-EG 2.0M LysK-EG 2.0M MEA 2.0M ProK-H2O

1.6

2.0M LysK-H2O -1


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

Energy & Fuels

1.2

0.8

0.4 0

20

40

60

t (min)

Fig. 6 CO2 desorption curves for different absorbent solutions at 353 K with N2 flowrate of 0.2L min-1. (a) the outlet CO2 concentration curves versus elapsed time; (b) integration curves of CO2 loading with measured data.


29

Energy & Fuels

Fig. 7 60


(a)

2.0M ProK-EG 393K 413K 433K

40

20

0 0

10

20

30

40

50

60

t (min)

60

2.0M LysK-EG 393 K 413 K 433 K


(b)

40

2.0 M MEA 373K

20

0 0

10

20

30

40

50

60

t (min)

1.5

2.0M LysK-EG (c)

393 K 413 K 433 K

1.2

2.0 M MEA 373K

-1


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

Page 30 of 34

0.9

0.6

0.3

0.0 0

10

20

30

40

50

60

t (min)

Fig.7 Comparison of thermal desorption performance for different absorbent solutions at various temperatures. (a) desorption profile for 2.0M ProK-EG, (b) desorption profile for 2.0M LysK-EG, (c) integration curves of CO2 loading with measured data.


30

Page 31 of 34

Fig. 8 1.5 2.0M ProK-EG

(a) Absorption 313K Desorption 353K, N2 Cyclic capacity

-1

CO2 loading (mol kg )

1.2

0.9

0.6

0.3

0.0 1

2

3

4

Cycle number 2.0M ProK+EG Absorption 313K Desorption 433K Cyclic capacity

1.2

0.9

-1


(b)

0.6

0.3

0.0 1

2

3

4

5

6

7

8

Cycle number 2.0

2.0M LysK+EG Absorption 333K Desorption 433K Cyclic capacity

(c) 1.5 -1


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

Energy & Fuels

1.0

0.5

0.0 1

2

3

4

5

6

7

8

Cycle number

Fig.8. Absorption-desorption performance for ProK-EG and LysK-EG water-lean absorbents. CO2 desorption -1

conditions: (a) 353K and 0.2 L min N2; (b) thermal desorption for ProK-EG system at 433K; (c) thermal desorption for LysK-EG system at 433K.Cyclic loading (∆α) is defined as the CO2 loading difference between the CO2-rich and the CO2-lean.


31

Energy & Fuels 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

Page 32 of 34

Fig. 9

Fig.9 13C NMR spectra of ProK+EG solutions in D2O. (a) ethylene glycol; (b) CO2-free ProK+EG solution; (c) CO2-loaded ProK+EG solution; (d) sample after eight absorption-desorption cycles.


32

Page 33 of 34 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

Energy & Fuels

Fig. 10

Fig.10 13C NMR spectra of LysK+EG solutions in D2O. (a) ethylene glycol; (b) CO2-free LysK +EG solution; (c) CO2-loaded LysK +EG solution; (d) sample after eight absorption-desorption cycles.


33

Energy & Fuels 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

Page 34 of 34

Fig. 11

Fig.11 1H NMR spectra of ProK-EG and LysK-EG solutions in D2O. ProK-EG: (a) fresh absorbent, (b) CO2-loaded solution after eight absorption-desorption cycles, (c) CO2-free cycling absorbent by pH adjustment; LysK-EG: (d) fresh absorbent, (e) CO2-loaded solution after eight absorption-desorption cycles, (f) CO2-free cycling absorbent by pH adjustment.


34

CO2 Capture by Water-Lean Amino Acid Salts: Absorption

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