water absorbent

Applied Energy 257 (2020) 113962

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CO2 separation using a hybrid choline-2-pyrrolidine-carboxylic acid/ polyethylene glycol/water absorbent Yifeng Chena,b, Yunhao Suna,b, Zhuhong Yangb, Xiaohua Lub, , Xiaoyan Jia, ⁎

a b

T

⁎

Energy Engineering, Division of Energy Science, Lulea University of Technology, 97187 Lulea, Sweden Key Laboratory of Material and Chemical Engineering, Nanjing Tech University, Nanjing 210009, China

HIGHLIGHTS

GRAPHICAL ABSTRACT

O interacts strongly with [Cho] • H[Pro]/PEG200 in the hybrid absor2

bent.

formation of carbamate is domi• The nated especially at low CO pressure. presence of H O results in the • The formation of bicarbonate. hybrid absorbent has advantages • The on the CO solubility and absorption 2

2

enthalpy.

2

ARTICLE INFO

ABSTRACT

Keywords: Ionic liquid Carbon dioxide Property Solubility Modeling

Developing novel hybrid absorbents is essential for CO2 separation. In this study, the density and viscosity of a hybrid absorbent (choline-2-pyrrolidine-carboxylic acid/polyethylene glycol/water ([Cho][Pro]/PEG200/H2O)) were measured experimentally, and its CO2 solubility was also determined. The excess mole volume and excess Gibbs energy of activation of the hybrid absorbent were further estimated to understand the molecular structure and interactions between [Cho][Pro]/PEG200 and H2O. The CO2 solubilities in [Cho][Pro]/PEG200 and [Cho] [Pro]/H2O were analyzed and described using the Redlich–Kwong non-random-two-liquid (RK-NRTL) model. Furthermore, the CO2 solubility in the hybrid absorbent was predicted using the RK-NRTL model and was compared with the new experimental results for verification. The effect of H2O on the CO2 absorption performance was further analyzed. The performance and cost of the hybrid absorbent were compared with those of other commercialized CO2 absorbents. In addition, the recyclability of the hybrid absorbent for CO2 separation was studied. The results of this study indicated that the hybrid absorbent could be promising for CO2 separation.

1. Introduction Fossil fuels play an essential role in the global energy production, as over 85% energy is generated using fossil fuels [1]. The utilization of fossil fuel has led to the rapid increase in CO2 emissions, and CO2 separation is often used to mitigate CO2 emissions [2,3]. Moreover, renewable energy sources and fuels have attracted more attention ⁎

worldwide owing to their environmental friendliness, while CO2 separation is often required to purify gas before further converting it or using it as a vehicle fuel [4,5]. Therefore, CO2 separation is important for CO2 emissions mitigation as well as renewable energy and fuel production. For the past several decades, technologies, such as adsorption, absorption, membrane separation, and cryogenics, have been widely

Corresponding authors. E-mail addresses: [email protected] (X. Lu), [email protected] (X. Ji).

https://doi.org/10.1016/j.apenergy.2019.113962 Received 14 April 2019; Received in revised form 6 August 2019; Accepted 2 October 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.

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Pv n a

Nomenclature Abbreviations P T H xi Z R yi Ps Tc Pc V

pressure, MPa temperature, K Henry’s constant, MPa mole fraction of component i in the liquid phase compression factor gas constant, 8.314 J·mol−1·K−1 mole fraction of component i in the gas phase saturated vapor pressure, MPa critical temperature, K critical pressure, MPa mole volume, cm3·mol−1

b K M

vapor pressure, MPa molar amount, mol a constant that corrects for attractive potential of molecules of component a constant that corrects for volume of component chemical equilibrium constant molecular weight

Greek Letters Ρ η φ γ

developed and commercialized for CO2 separation. The preferred method of removing CO2 from gas mixtures is using absorption/desorption towers containing effective absorbents owing to the high efficiency and continuity of the absorption process [6]. Generally, absorbents can be divided into two categories. Amines [7,8], potassium carbonate [9], and ammonia [10] are typical chemical absorbents, while physical absorbents [11,12] mainly include water, methanol, dimethyl ether of polyethylene glycol (DEPG), N-methyl-2-pyrrolidone, and propylene carbonate. Amine scrubbing is the most commonly used technology for CO2 separation from gas mixtures featuring low CO2 partial pressure values, such as flue gas [8,13]. However, the aminebased separation technology is prone to volatility and corrosion, and results in secondary pollution [14,15]. On the contrary, physical-absorbent scrubbing is preferred for gaseous streams featuring high CO2 partial pressure owing to the low energy demand for solvent regeneration, while processes of physical-absorbent scrubbing usually involve high investments and huge amounts of circulated absorbent [16,17]. Therefore, developing environmental friendly and cost-effective absorbents for CO2 separation is still a research hot spot. Ionic liquids (ILs) are low melting point salts, which have exhibited incredible potential for industrial applications owing to their extremely low vapor pressure, high thermal stability, and tunable property [18–20]. Since the CO2 solubility in 1-butyl-3-methylimidazolium hexafluorophosphate was first reported by Blanchard et al. [19], intensive efforts have been made toward designing and synthesizing suitable ILs for CO2 separation, such as pyrrolidinium-[21], imidazolium-[22], quaternary ammonium-[23] and quaternary phosphoniumbased [24]. The current research is mainly focused on the following areas [25–27]: (1) the molecular structure design and synthesis of functional ILs for improving CO2 separation performance; (2) experimental measurements and modeling predictions of physicochemical properties, gas solubility, and kinetics of multicomponent systems involving gases, ILs, and other co-solvents; and (3) developing IL-based technologies and evaluating their feasibility based on energy utilization and economic performances. Considering the high cost and potential toxicity of traditional ILs [28], the utilization of affordable and green ILs has attracted more attention. Choline-based ILs are a class of representative ILs that are derived from nontoxic and biodegradable materials, and one type of choline-based ILs is deep eutectic solvents (DESs). Choline chloride/ urea (ChCl/Urea) [29,30], choline chloride/lactic acid [31], choline chloride/guaiacol [32], and choline chloride/furfuryl alcohol [33] have been prepared by mixing two compounds at temperatures ranging from 333.15 to 353.15 K. Usually, DESs can achieve high CO2 absorption capacities only at relatively high pressures (above 3 MPa), and their absorption rate is quite low. The other type of choline-based ILs includes [Cho][Pro], choline-serine, and choline-glycine, which have been synthesized using choline and various amino acids. The viscosity

density, g·cm−3 viscosity, Pa·s fugacity coefficient activity coefficient

of such choline-based ILs is usually higher than several hundred centipoise [34], and therefore, it could be difficult to achieve high CO2 separation efficiency in practice using conventional absorption/desorption towers [35–37]. Recently, combining task-specific ILs with other molecular solvents has been proposed as a new method of overcoming the challenge caused by the high viscosity of ILs [38], and therefore, DEPG [39], PC [38] and sulfolane [40] could be used as co-solvents alongside ILs. Polyethylene glycol (PEG) is a widely used green solvent in the pharmaceutical and food industries, and its properties (e.g. density and viscosity) can be tuned by changing its molecular weight. Among them, PEG200 has some unique properties, such as extremely low vapor pressure [41], moderate molecular weight and viscosity [42], and high CO2 capacity [43]. In fact, PEG200 has been used alongside amine as co-solvent for CO2 separation to improve the cyclic capacity and regeneration efficiency of the process [44]. Previous studies [45,46] have indicated that PEG200 could be a promising co-solvent for decreasing the viscosity of [Cho][Pro], while [Cho][Pro]/PEG200 could still exhibit high CO2 absorption capacity, high CO2/CH4 selectivity, and moderate absorption enthalpy. However, the viscosity of [Cho][Pro]/ PEG200 (200 mPa·s at 308.15 K) is still too high for practical applications using an absorption/desorption towers. Furthermore, adding H2O to ILs is considered to be a promising and favorable option for overcoming the drawbacks caused by the high viscosity of ILs. The viscosities of some ILs (e.g., 1-butyl pyridinium tetrafluoroborate and 1-butyl 3-methyl pyridinium tetrafluoroborate) can decrease by 50% by adding only 2 vol% water to them [47]. The presence of H2O can also lead to some changes in the CO2 absorption capacities. For example, H2O can only cause a slight decrease in the CO2 absorption capacity [48] of trihexyl(tetradecyl)phosphonium asparaginate prolinate. On the contrary, an increase in CO2 solubility of bis(2-dimethylaminoethyl)ether bis(trifluoromethylsulfonyl)imide was observed in the presence of H2O [49]. In addition, H2O is an impurity of ILs owing to their strong hygroscopicity, and CO2 gas streams (e.g., flue gas, biogas, and biosyngas) are generally saturated with H2O [50]. Therefore, studying the properties and CO2 absorption performance of [Cho][Pro]/PEG200/ H2O should be an interesting endeavor. To develop appropriate absorbents for CO2 separation, some factors should be considered. The properties, gas solubility and absorption rate of an absorbent are three key factors that could significantly influence the cost of CO2 separation by determining the size of absorption equipment and recirculated amount of absorbent [51]. Therefore, determining absorbent properties such as density, viscosity, and CO2 solubility are the prerequisites [52]. To the best of our knowledge, the properties of PEG200 and PEG200/H2O have been reported in the literature, and the CO2 solubilities in PEG200, PEG200/H2O, [Cho][Pro]/ H2O and [Cho][Pro]/PEG200 have also been determined experimentally. However, the properties of and CO2 solubility in the hybrid 2

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absorbent have not been studied in detail. Excess properties, such as the excess mole volume (VE) and excess Gibbs energy of activation (ΔG*E), allow a comprehensive understanding of the molecular structure interactions that determine the thermo-physical properties of substances [53]. The VE values of ChCl/ Urea and H2O are negative, and the minimum value corresponds to the mole fraction of water of 0.65, which indicates strong intermolecular interactions between H2O and ChCl/Urea [54]. Similar phenomena were observed for other ILs, such as 1-propyl-2,3-dimethylimidazolium tetrafluoroborate [55], and 2,3-N-epoxypropyl-N-methyl-2-oxopyrro lidinium acetate ([EPMpyr][OAC]) [56]. Similarly, large positive values of ΔG*E were observed for the mixture of 1,1,3,3-tetramethylguanidium lactate and H2O, which usually indicate the formation of intermolecular hydrogen bonds between the IL ions and H2O [57]. Representing CO2 solubility using a thermodynamic model is required for energy utilization evaluation and separation process design. Equations of state (e.g., Redlich–Kwong (RK) [58] and Peng–Robinson [59]) are often used to describe the non-ideal behavior of the components of the gas phase, while non-random-two-liquid (NRTL) [60], electrolyte NRTL [61], Pitzer [62] and universal quasi-chemical [63] models are used to describe the non-ideal behavior of the species in the liquid phase. Additionally, some advanced equations of state such as Statistical Associating Fluid Theory [64] (SAFT), Perturbed-Chain SAFT [65] (PC-SAFT), electrolyte PC-SAFT [66–68] (ePC-SAFT) and Cubic Plus Association [69] are also used for describing the phase equilibrium, which usually less adjustable parameters are needed. The CO2 solubilities in H2O, PEG200 and H2O/PEG200 have been correlated with the RK-NRTL model in one of our previous studies [70], while the description of the CO2 solubility in the hybrid absorbent is still unavailable. The objective of this study was to develop a novel hybrid absorbent for CO2 separation. To achieve this, [Cho][Pro]/PEG200 (mass ratio = 1:2) was selected owing to its large CO2 absorption capacity and moderate absorption enthalpy, the density (ρ) and viscosity (η) of the hybrid absorbent were measured, and the VE and ΔG*E values were calculated. Moreover, the CO2 solubility in the hybrid absorbent was measured and also predicted using the RK-NRTL model and was compared with the experimental results for verification. Furthermore, the effect of H2O on the CO2 absorption capacity was analyzed, and the hybrid absorbent was recycled using a multi-cycle experiment. The performance and cost of the hybrid absorbent were compared with those of other commercialized CO2 absorbents.

2. Experiments and theory 2.1. Materials preparation Aqueous choline hydroxide solution (46 wt%) was obtained from Sigma Aldrich, L-proline (99 wt%) was supplied by the Chinese National Medicine Corporation, analytical grade PEG200 (average molecular weight of 200 g·mol−1) was supplied by the Guandong Guanghua Sci-Tech Corporation, and analytical grade ethyl acetate and phosphorus pentoxide (P2O5) were purchased from Shanghai Lingfeng Chemical Reagent Co. Ltd·H2O was purified in our laboratory using a reverse osmosis membrane, and CO2 (> 99.99 wt%) was purchased from the Nanjing Tianhong gas factory. The synthesis procedure of [Cho][Pro] was similar to that described in one of our previous papers [46], and the identified structure was in good agreement with the results reported in the literature [34,71]. Before experiments, [Cho][Pro] and PEG200 were dried in a vacuum dryer containing P2O5 at 343.15 K for 72 h, and their H2O contents were subsequently determined using the Karl Fischer titration method employing a Shanghai Peiou V100 moisture meter. The H2O contents of the dried [Cho][Pro] and PEG200 were smaller than 0.2 and 0.06 wt%, respectively. The absorbents were prepared by mixing [Cho][Pro] with PEG200 at the weight ratio of 1:2, and then different amounts of H2O were added to the mixture at the temperature of 298.15 K. 2.2. Density and viscosity measurements Both density and viscosity were measured at temperatures ranging from 298.15 to 333.15 K and at atmospheric pressure. The temperature accuracy was ± 0.01 K. The density was measured using a density meter (Anton Paar DMA 5000, Anton Paar Co., Austria) and the expanded uncertainty (Uc) was estimated to be Uc (ρ) = 0.0001 g·cm−3 (level of confidence = 0.95). The viscosity was determined using an Anton Paar AMVn Automated Mircoviscometer, and the relative expanded uncertainty (Ur) was estimated to be Ur (η) = 0.005 mPa·s (level of confidence = 0.95). 2.3. CO2 solubility and recycling measurements The schematic diagram of the set-up for CO2 solubility measurements is illustrated in Fig. 1. An equilibrium cell, a gas reservoir, and a magnetic stirrer were placed in a water bath featuring a temperature control system. During the absorption experiments, the hybrid

Fig. 1. Schematic diagram of CO2 solubility and recycling measurement set-up.

3

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absorbent was injected into the equilibrium cell, then the cell was degassed using a vacuum pump. Afterward, CO2 was introduced into the cell, and the stirrer was opened. When the CO2 pressure remained constant for 1 h, it was assumed that the absorption equilibrium was achieved. For the recycling experiment, the hybrid absorbent, after absorption, was added to a three-necked flask which was placed in an oil bath that was heated to 383.15 K using a magnetic stirrer. The temperature was maintained for 60 min. A condenser was used to minimize the loss of water by evaporation. To maintain the constant H2O content of the absorbent for the next absorption-desorption experiment, a small amount of H2O was added to the flask. The entire recycling experiment was performed at atmospheric pressure, and the temperature deviation was 0.01 K. The CO2 solubility in the hybrid absorbent was calculated with Eqs. (1)–(2):

nCO2 = x CO2 =

(P 0

P v )(VA Z1 RT

VL)

(P e

P v )(VA Z2 RT

VL)

nCO2 nCO2 + nPEG200 + n H2 O + n[Cho][Pr o] 0

CO2 (l) + R1 R2 NH (l) CO2 (l) + 2R1 R2 NH (l)

CO2 (l) + R1 R2 NH (l) + H2 O (l)

K1 =

K2 =

(1)

CO2

= ln

CO2

ln

CO2

(2)

ln

H2 O

= PHs 2 O x H2 O

H2 O

(9)

R1 R2 NH2 HCO3 x R1 R2 NH2 HCO3 CO2 x CO2 H2 O x H2 O R1 R2 NH x R1 R2 NH

(10)

=Z

1

ln(Z

bP / RT )

(

a /R2T 2.5 ) ln(1 + bP / ZRT ) b/ RT

(11)

where RT V b

a T 0.5V (V + b)

Z = PV / RT a= i

(3)

j

yi yj (ai aj )0.5, ai = 0.42748

b= i

(4)

yi bi , bi = 0.08644

R2Tci2.5 Pci

RTci Pci

(12)

In Eq. (11), V is the mole volume, a is the constant used for the correction of the attractive potential of gas molecules, b is the constant used for correcting the volume, and Tci and Pci are the critical temperature and pressure. The Tci and Pci values for CO2 and H2O are 304.2 and 647.45 K and 7.38 and 22.05 MPa, respectively. In this study, all species were assumed to be molecules, for simplification, and their activity coefficients in the liquid phase were represented using the NRTL model, as follows:

where P is the system pressure, HCO2, mix (T , P ) is Henry’s constant of CO2 in the mixed solvents at temperature T and pressure P, CO2 is the fugacity coefficient of CO2 in the vapor phase, CO2 is the asymmetric activity coefficient of CO2 in the liquid phase, CO2 is the activity coefficient of CO2 in the liquid phase, CO2 is the infinite dilute activity coefficient of CO2 in the liquid phase, and yCO2 and x CO2 are the mole fractions of CO2 in the vapor and liquid phases, respectively. It was reported that the saturated vapor pressure of PEG200 at 368.0 K is just 9.9 Pa [41]. Therefore, a negligible mole fraction of PEG200 in the vapor phase was assumed, i.e. only CO2 and H2O are comprised in the vapor phase. The phase equilibrium of H2O can be expressed as follows:

PyH2 O

(8)

In this study, the non-ideal behavior of the components in the vapor and liquid phases were described with RK equation of state and NRTL model, respectively. The reliability of RK-NRTL models for representing the CO2 solubilities in H2O, ILs and IL/H2O has been confirmed in our previous work [75,76]. The expression of RK equation [58] used for calculating the fugacities of CO2 and H2O in the vapor phase are shown in Eqs. (11)–(12).

2.4.1. Theory The CO2 solubility in the solvent can be expressed as

ln

R1 R2 NCOONH2 R1 R2 x R1 R2 NCOONH2 R1 R2 2 CO2 x CO2 ( R1 R2 NH x R1 R2 NH )

P=

CO2

R1 R2 NH2 HCO3 (l) K2

and

e

= HCO2, mix (T , P ) xCO2

(7)

The chemical equilibrium constants (K1 and K2) can be expressed using the following equations:

2.4. Modelling CO2 solubility

CO2

R1 R2 NCOONH2 R1 R2 (l) K1

Furthermore, depending on the H2O content, the zwitterion can be deprotonated using H2O instead of R1R2NH to form bicarbonate, and the overall reaction for the process can be expressed as follows:

where P and P are the CO2 pressures in the cell at the states of initial and equilibrium, respectively. Pv is the vapor pressure of the hybrid absorbent before the CO2 injection. VL and VA are the volumes of the hybrid absorbent and cell (53.01 ml), respectively. R is the universal gas constant. Z1 and Z2 are the CO2 compressibility factors corresponding to the states of initial and equilibrium, respectively, which were calculated using the second virial coefficients [72].

PyCO2

(6)

R1 R2 NCOOH (l)

m

ln

i

=

j=1 m

m

+ Gli xl

l=1

(5)

m

ji Gji x j

Gij x j m

j=1 l =1

Glj xl

ij

xr

r=1 m l=1

rj Grj

Glj xl

(13)

where Gij = exp( ij ) and Gji = exp( ji ) α was assumed to be 0.2 in this study, Gij , Gji , ij , and ji are binary interaction parameters, and ij and ji are temperature-dependent parameters, i.e.,

where is the saturated vapor pressure of H2O, and its value was retrieved from the literature [73]. As reported [20], the absorption mechanism of CO2 in a pure amino acid IL consist of one CO2 molecule combining with two IL molecules (i.e., the reaction mole ratio = 1:2), while the CO2 absorption in an amino acid salt solution could be described as a two-step zwitterion mechanism [74]. [Cho][Pro] could be abbreviated as R1R2NH owing to the presence of L-proline, and can be considered to be a secondary amine, while PEG200/H2O is considered to be the co-solvent. As indicated in Eq. (6), the zwitterion (i.e., carbamate) is generated first through the reaction of CO2 with R1R2NH, and is further deprotonated by the base in the aqueous solution. If the zwitterion is obtained using R1R2NH, the overall reaction between CO2 and [Cho][Pro]/PEG200 can be expressed using Eq. (7).

PHs 2 O

ij

= mij + nij / T

ji

= mji + nji / T

(14)

where m and n are the coefficients to describe the temperature-dependent binary interaction parameters. To further decrease the number of binary parameters, in this study, it was assumed that R1R2NH, R1R2NCOONH2R1R2, and R1R2NH2HCO3 were the same substance in the NRTL model owing to their similar molecular structures, and the binary NRTL interactions were decreased to those between four species: CO2, H2O, PEG200, and [Cho][Pro]. 4

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2.4.2. Model parameterizing In this study, to calculate gas solubility, the HCO2, mix value, the chemical equilibrium constants (K1 and K2), and the binary interaction parameters of CO2/[Cho][Pro], H2O/[Cho][Pro] and PEG200/[Cho] [Pro] in NRTL (τij and τji) need to be obtained. The procedure to obtain these parameters was described as the following steps: (1) The HCO2, mix values of CO2 in [Cho][Pro]/PEG200, [Cho][Pro]/H2O and [Cho][Pro]/PEG200/H2O were assumed to be the same as those of HCO2, PEG200 , HCO2, H2 O , and HCO2, PEG200 H2 O , respectively, for simplification. This is reasonable as the physical CO2 solubility in [Cho][Pro] only occupies a small proportion and the molecular numbers of PEG200 and H2O are dominated in the different systems (i.e. [Cho][Pro]/PEG200, [Cho][Pro]/H2O and [Cho][Pro]/ PEG200/H2O). (2) The K1 value was obtained from the fitting of the CO2 solubility in [Cho][Pro]/PEG200 up to 0.1 MPa by assuming CO2 = 1. This was done at each temperature, and then the temperature-dependent K1 values were obtained, i.e. A and B in Table 2. The binary parameters of CO2/[Cho][Pro] and PEG200/[Cho][Pro] in NRTL were obtained from the fitting of the CO2 solubility in [Cho][Pro]/ PEG200 over the entire pressure range based on the obtained K1 at each temperature. The temperature-dependent binary interaction parameters were further obtained, i.e. mCO2,[Cho][Pro], nCO2,[Cho][Pro], nPEG200,[Cho][Pro], mPEG200,[Cho][Pro], m[Cho][Pro], CO2 , n[Cho][Pro], CO2 , m[Cho][Pro], PEG200 and n[Cho][Pro], PEG200 in Eq. (14); (3) The K2 value was obtained from the fitting of the CO2 solubility in [Cho][Pro]/H2O up to 0.1 MPa based on the obtained temperaturedependent K1, where it was also assumed that CO2 = 1. This was done at each temperature, and then the temperature-dependent K2 values were obtained, i.e. A and B in Table 2. The binary NRTL parameters of H2O/[Cho][Pro] were obtained from the fitting of the CO2 solubility in [Cho][Pro]/H2O over the entire pressure range based on the obtained temperature-dependent K1 and K2. The temperature-dependent binary NRTL interaction parameters were further obtained, i.e. m H2 O,[Cho][Pro], n H2 O,[Cho][Pro], m[Cho][Pro], H2 O and n[Cho][Pro], H2 O in Eq. (14).

Fig. 2. Temperature dependence of density (ρ) of [Cho][Pro]/PEG200 (1) and H2O (2) mixtures.

VE =

(x1 M1 + x2 M2)

x1 M1

m

1

x2 M2 2

(15)

where the subscripts 1, 2, and m represent [Cho][Pro]/PEG200, H2O, and the hybrid absorbent, respectively, and M is the molecular weight. The values of M for [Cho][Pro]/PEG200 and H2O are 201.3 and 18.0 g·mol−1, respectively. The calculated VE values are listed in Table S1 and illustrated in Fig. 3. The Redlich-Kister type polynomial expression [77] was used to correlate the values of VE, and VE at constant temperature can be expressed as 4

V E = x1 x2

An (x1 n=0

x2) n

(16)

where n is the number of estimated parameters, and A is the equation coefficient. The fitted parameters are listed in Table S2. The curve of VE of the studied system was determined to be similar to those of other solvents [78,79]. All VE values were negative, which implied that the strong inter-interactions between [Cho][Pro]/PEG200 and H2O resulted in volume reductions, and the interactions may be Hbonding. As the temperature increased, VE increased. This may attribute to the decrease in inter-interactions being more prominent compared to the decrease in intra-interactions as the temperature increased. This observation was consistent with that for the system of ChCl/Urea-H2O which has been reported by Yadav et al. [78]. Additionally, as the amount of H2O increased, VE decreased, reached a minimum value, and then increased. The minimum VE value was observed in the H2O-rich region (x2 ≈ 0.65–0.85) within the studied temperature range. Arumugam et al. [56] also determined that the minimum VE values of the [EPMpyr][OAC]/H2O and [EPMpyr][OAC]/methanol mixtures were reached when the mole fractions of H2O and methanol were in the

3. Results and discussion 3.1. Properties of the hybrid absorbent ([Cho][Pro]/PEG200/H2O) The absorbents used in this study included [Cho][Pro], PEG200, H2O, [Cho][Pro]/PEG200, [Cho][Pro]/H2O, PEG200/H2O, and the hybrid absorbent. The densities and viscosities of PEG200, H2O and PEG200/H2O have been studied and reported, and no further studies were required. As mentioned in the previous section, [Cho][Pro]/ PEG200 (mass ratio = 1:2) was selected as absorbent, then the properties of the hybrid absorbent featuring different H2O mole fractions were measured. 3.1.1. Density At atmospheric pressure, the density of the hybrid absorbent was measured in the 298.15 to 333.15 K temperature range over the entire H2O compositional range, and the results are listed in Table S1 and depicted in Fig. 2. As illustrated in Fig. 2, the density decreased monotonically as the temperature increased for a given H2O mole fraction (x2). This was expected owing to thermal expansion. On the contrary, the density decreased nonlinearly as x2 increased, and it was determined that the density decreased significantly when x2 increased from 0.8481 to 1. Therefore, the effect of H2O on the density of the hybrid absorbent was significant at high H2O contents. To reveal the depth of the interactions between [Cho][Pro]/PEG200 and H2O, particularly at high H2O contents, VE was calculated using the density, as follows:

Fig. 3. Excess mole volume (VE) of [Cho][Pro]/PEG200 (1) and H2O (2) mixtures as function of the mole fraction of H2O (x2) (☆, 298.15 K; ▾, 303.15 K; △, 308.15 K; ▲, 313.15 K; ○, 318.15 K; ●, 323.15 K; □, 328.15 K; ■, 333.15 K). 5

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G

E

= RT [ln(

m Vm

x1 ln( 1 V1)

x2 ln( 2 V2)]

(17)

E

The calculated ΔG* values are listed in Table S3 and illustrated in Fig. 5. All ΔG*E values were positive within the studied temperature range, which may attribute to the formation of complexes owing to the specific intermolecular H-bonding between [Cho][Pro]/PEG200 and H2O [80,81]. The confirmation of such complex formation was important for the theoretical model developed, and will be the focus of our future research. Based on our literature survey of the advanced experimental and theoretical studies for other ILs [82,83], the addition of H2O to [Cho][Pro]/PEG200 could be described as following these three steps: (1) the H-bonding network in [Cho][Pro]/PEG200 is destroyed and ionic clusters form; (2) ionic clusters dissociate into ion-pairs, which are surrounded by H2O; and (3) the ionic pairs become the dominant form of ILs in aqueous solutions. The maximum ΔG*E value was achieved for water-rich mixtures (x2 ≈ 0.65–0.85) within the studied temperature range. This was consistent with the VE data. Moreover, ΔG*E exhibited different changing trends before and after it reached the maximum value. Based on the calculated values of VE and ΔG*E, the interaction between [Cho][Pro]/PEG200 and H2O can be speculated to occur as follows: (i) inter-interactions are stronger than intra-interaction, and (ii) most probably, a complex structure forms for water-rich mixtures (x2 ≈ 0.65–0.85). This speculated mechanism was confirmed in one of our recent studies [84], where we concluded that the addition of H2O could significantly modify the microstructure of ILs.

Fig. 4. Temperature dependence of viscosity (η) of [Cho][Pro]/PEG200 (1) and H2O (2) mixtures.

0.6–0.8 and 0.6–0.8 ranges, respectively. 3.1.2. Viscosity In addition to the CO2 absorption capacity, viscosity is another important factor as it could affect the rate of gas-liquid mass-transfer in absorption/desorption towers. In this study, the viscosity of the hybrid absorbent was also measured from 298.15 to 333.15 K over the entire H2O compositional range at atmospheric pressure. The measured viscosity data points are listed in Table S3 and depicted in Fig. 4. As expected, the monotonic decrease in viscosity of the hybrid absorbent as the temperature increased was observed particularly at low H2O contents. The viscosity of the hybrid absorbent also significantly decreased as the amount of added H2O increased. At 298.15 K, the viscosity of the hybrid absorbent decreased by approximately 60% as H2O mole fraction increased from 0 to 0.5633 (i.e., by 10.0 wt%). For practical CO2 separation using conventional absorption/desorption towers, the viscosity of the solvent should typically be smaller than 50 cP. Based on the results listed in Table S3 and illustrated in Fig. 4, when the mole fraction of H2O increased up to 0.8431 (i.e., by 32.0 wt%), the viscosities of the hybrid absorbent were all lower than 20 mPa·s. This low viscosity allowed the hybrid absorbents to be used in conventional absorption/desorption towers. This also implied that the addition of H2O was a promising way to overcome the high viscosity of [Cho][Pro]based absorbents. The ΔG*E values were calculated using Eq. (17) from the measured viscosities. The ΔG*E values were also fitted using a Redlich − Kistertype polynomial expression [77], and the results are listed in Table S4.

3.2. Phase equilibria In this study, seven absorbents ([Cho][Pro], PEG200, H2O, [Cho] [Pro]/PEG200, [Cho][Pro]/H2O, PEG200/H2O, and the hybrid absorbent) were used for CO2 absorption. The CO2 solubility in [Cho][Pro] was excluded in this study owing to the high viscosity of [Cho][Pro]. The CO2 solubilities in H2O, PEG200, and PEG200/H2O have been analyzed extensively, and no further studies were conducted for them. The CO2 solubilities in [Cho][Pro]/PEG200 and [Cho][Pro]/H2O were described using the RK-NRTL model, based on the available experimental data. Furthermore, the CO2 solubility in the hybrid absorbent was measured experimentally, predicted using the RK-NRTL model, and the experimental results were compared with the theoretical ones for verification. 3.2.1. Experimental measurement validation Before determining the CO2 solubility in the hybrid absorbent, the CO2 solubility in H2O at 308.15 K was measured to determine the accuracy of the apparatus. The CO2 solubility values measured in this study were compared with the data reported in the literature [54,85] (see Fig. 6), and the results indicated the reliability of the measurements in this study. 3.2.2. CO2 solubility in [Cho][Pro]/PEG200 The CO2 solubility in [Cho][Pro]/PEG200 has been measured experimentally and compared with the data reported in the literature (Table 1). The CO2 solubility in [Cho][Pro]/PEG200 was measured by Li et al. [45] (T = 308.15–353.15 K, P = 0.005–0.011 MPa, mPEG200/ m[Cho][Pro] = 1–3) and we also reported such results in one of our previous studies [46] (T = 308.15–338.15 K, P = 0.044–2.40 MPa, mPEG200/m[Cho][Pro] = 2). The experimental results reported in the literature have been compared in one of our previous studies [46], and all the reliable data points were selected for further correlation. To obtain the value of K, in this study, Henry’s constant of CO2 in the hybrid absorbent (HCO2, mix ) was determined using other methods instead of fitting together with K. Generally, there are two methods to estimate HCO2, mix . One is the N2O analogy, where Henry’s constant of N2O in the hybrid absorbent is determined based on the experimentally measured solubility values [88]. The other is to determine the value of

Fig. 5. Excess Gibbs activation energy (ΔG*E) of [Cho][Pro]/PEG200 (1) and H2O (2) as function of the mole fraction of H2O (x2) (★, 298.15 K; ▾, 303.15 K; △, 308.15 K; ▲, 313.15 K; ○, 318.15 K; ●, 323.15 K; □, 328.15 K; ■, 333.15 K). 6

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Table 3 Binary NRTL parameters for CO2-[Cho][Pro], H2O-[Cho][Pro], PEG200-[Cho] [Pro]. NRTL CO2 H2O PEG200

[Cho][Pro] [Cho][Pro] [Cho][Pro]

mij

nij

mji

nji

13.77 6.11 −5.12

−4999.44 1294.08 1895.36

−18.80 −1.58 1.01

5387.57 −2381.42 965.36

Fig. 6. Solubility of CO2 in H2O at 308.15 K: ●, measured in this study; ○, results from Valtz et al. [85]; ■, results from Xie et al. [54]. Table 1 CO2 solubility in [Cho][Pro]/PEG200, [Cho][Pro]/H2O, and the hybrid absorbent. Gas

CO2

Solvent

[Cho][Pro]/PEG200

[Cho][Pro]/H2O

hybrid absorbent

References

[45,46]

[86,87]

This study

Fig. 7. CO2 solubility in [Cho][Pro]/PEG200 (experimental data vs model estimation). Results from Li et al. [45]: ●, mPEG200/m[Cho][Pro] = 3, ○, mPEG200/ m[Cho][Pro] = 2, ▲, mPEG200/m[Cho][Pro] = 1; results from Chen et al. [46]: □, mPEG200/m[Cho][Pro] = 2.

Table 2 Parameters of chemical equilibrium constants (K1 and K2). lnK = A + B/T

A

B

K1 K2

−6.08 −15.06

4760.49 7299.74

for evaluating the energy needs for the separation process. It was reported that the energy used for the amine regeneration accounts for 70–80% of the total operating costs owing to the high Habs values of primary and secondary amines [51]. Based on the temperature-dependent K1 and HCO2, PEG200 (T ) values, the enthalpy of CO2 in [Cho][Pro]/ PEG200 ( Habs ) (mass ratio = 1:2) could be calculated using the following equation

HCO2, mix from the Henry’s constants of CO2 in the pure solvents using the mixing rule [89]. In this model, even though [Cho][Pro] can physically absorb small amounts of CO2, Henry’s constant of CO2 in [Cho][Pro]/ PEG200 was assumed to be the same as that of CO2 in PEG200 for simplification. This was attributed to the following two considerations: (1) the physical CO2 solubility in [Cho][Pro] only occupies a small proportion; and (2) the molecular numbers of PEG200 is double that of [Cho][Pro]. The Henry’s constant value of CO2 in PEG200 (HCO2, PEG200 (T )) was reported in one of our previous studies [70], and can be described using Eq. (18). ln HCO2, PEG200 (T ) = 7.1554

1524.49/ T

Habs = HPhy + HChem =

R

ln HCO2, PEG200 (T ) (1/ T )

-

ln K (1/ T ) (19)

−1

For the hybrid absorbent Habs = −53.92 kJ·mol , and the physical and chemical contributions to it were −12.67 and −41.25 kJ·mol−1, respectively. In one of our previous studies [46], Habs was estimated to be −51.89 kJ·mol−1 based on the reaction equilibrium thermodynamic model, and the corresponding ARD value was 3.8%, which was comparable with the value obtained in this study. This indicated that the parameters obtained in this work were reliable.

(18)

Eq. (3) was used to describe the CO2 solubility in [Cho][Pro]/ PEG200, and the chemical equilibrium constant (K1) and binary NRTL parameters (CO2/[Cho][Pro] and PEG200/[Cho][Pro]) were set to be adjustable parameters. During parameter fitting, first, the K1 values were obtained from the fitting of CO2 solubility at low pressure (< 0.1 MPa), where it was assumed that CO2 = 1, i.e., NRTL did not contribute; second, using the fitted K1 value, the binary NRTL parameters were obtained from the fitting of the CO2 solubility over the entire pressure range. The obtained temperature-dependent K1 values and binary NRTL parameters are listed in Tables 2 and 3. Preliminary studies have been conducted, and the exclusion of the NRTL model resulted in large deviations of the theoretically estimated results from the experimental measurements. The CO2 solubility results, including both experimental measurements and theoretical estimations were in agreement, are illustrated in Fig. 7. The average relative deviations (ARDs) were 3.98%, 5.02% and 6.83% for the mass ratios of [Cho][Pro] to PEG200 of 1:1, 1:2, and 1:3 (measured by Li et al. [45]), respectively. The ARD of the experimental results in one of our previous studies was 2.92%[46]. The enthalpy of CO2 absorption ( Habs ) is another critical property

3.2.3. CO2 solubility in [Cho][Pro]/H2O The CO2 solubility in [Cho][Pro]/H2O has been measured experimentally and the literature-reported results are listed in Table 1. The CO2 solubilities in [Cho][Pro]/H2O were measured by Yuan et al. [86] (T = 308.15 K, P = 0.005–1.5 MPa, and w[Cho][Pro] = 0.05–0.3) and Li et al. [46] (T = 303.15–333.15 K, P = 0.015–0.70 MPa, and w[Cho][Pro] = 0.3). Our preliminary studies indicated that including the CO2 solubility data at T = 333.15 K, P = 0.0157 MPa, and m[Cho][Pro] = 0.3 did not generate reasonable fitting results. Additionally, the CO2 solubility data at CO2 partial pressures higher than 0.7 MPa were also excluded from the model to keep the pressure range consistent with that used for the solubility of CO2 in [Cho][Pro]/ PEG200/H2O (which will be described in the following section). Considering the obvious difference in molecular weight between [Cho][Pro] and H2O, most molecules of the [Cho][Pro]/H2O system were H2O molecules in the above studied composition range. Therefore, when modelling, Henry’s constant of CO2 in [Cho][Pro]/H2O was 7

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The calculated enthalpy of bicarbonate formation (Eq. (8)) was −60.69 kJ·mol−1, which was much higher than the enthalpy of carbonate formation (−41.25 kJ·mol−1) (Eq. (7)). Moreover, more energy would be consumed during desorption when bicarbonate was the majority component of CO2-based products. Therefore, the distribution of CO2-based species and the effect of H2O will be further clarified in the next section. 3.2.4. CO2 solubility in [Cho][Pro]/PEG200/H2O The CO2 solubility in the hybrid absorbent was measured in this study (T = 308.15–333.15 K, P = 0–0.58 MPa, and x2 = 0.3892–0.8481). All solubility data points are listed in Table S5 and illustrated in Fig. 9, where x CO2 is the mole fraction of CO2 in the hybrid absorbent. As expected, high CO2 solubility was achieved at high pressure and low temperature. Owing to the chemical reaction between CO2 and the absorbent, relatively high CO2 solubility could be obtained at low pressure (usually lower than 0.1 MPa), while the solubility of CO2 increased inconspicuously as the pressure increased at relatively high pressure. When x2 = 0.3892, the mole fraction of CO2 at 308.15 K and 0.1 MPa was 0.097, which was approximately 58.4 wt% of the total CO2 absorption capacity at 308.15 K and 0.548 MPa. When the temperature increased to 338.15 K, the CO2 solubility at 0.1 MPa (mainly chemical absorption) was approximately 52.8 wt% of the total CO2 capacity at 0.548 MPa. This indicated the co-occurrence of chemical and physical absorption processes, where chemical absorption predominated within the studied temperature and pressure ranges owing to the relatively high alkalinity of the proline-derived IL. The CO2 solubility in the hybrid absorbent could be predicted using Henry’s constant, K1 and K2, and the binary NRTL parameters (CO2/ H2O, CO2/PEG200, CO2/[Cho][Pro], H2O/PEG200, H2O/[Cho][Pro], and PEG200/[Cho][Pro]). During modelling, it was assumed, again, that the presence of [Cho][Pro] in the solvent would not affect Henry’s constant of CO2 in PEG200/H2O. In one of our previous studies [70], Henry’s constant of CO2 in PEG200/H2O was determined according to the experimental solubility data. Therefore, HCO2, mix could be estimated using Eq. (21) by correlating the Henry’s constant values at different

Fig. 8. CO2 solubility in [Cho][Pro]/H2O (experimental data vs model estimation). ●, the results from Li et al. [87]; □, the results from Yuan et al. [86].

assumed to be the same as that of CO2 in H2O (HCO2, H2 O (T )), which has been reported in the literature [90], and could be described using Eq. (20).

ln HCO2, H2 O (T ) = 156.9

8477.7/ T

21.9574 ln(T ) + 0.00578T

(20)

The temperature-dependent K1 and binary NRTL parameters for CO2/[Cho][Pro] were retrieved from the previous section, the chemical equilibrium constant (K2) and binary NRTL parameters (H2O/[Cho] [Pro]) were set to be adjustable parameters and were obtained from the CO2 solubility in [Cho][Pro]/H2O using a strategy similar to that described in Section 3.2.2. The obtained temperature-dependent K2 and binary NRTL parameters for CO2/[Cho][Pro] are listed in Tables 2 and 3. Again, the exclusion of the NRTL model could lead to large deviations between the experimental and theoretical results. The results illustrated in Fig. 8 indicate that the modeling results were in good agreement with the experimental ones. The ARDs of the experimental results measured by Yuan et al. [86] and Li et al. [46] were 15.06% and 2.59%, respectively.

Fig. 9. Experimental and predicted solubilities of CO2 in [Cho][Pro]/PEG200 (1) and H2O (2) mixtures. Symbols: measured in this study, ■, 308.15 K, ○, 318.15 K, ●, 328.15 K, □, 338.15 K; Curves: prediction values using NRTL model. 8

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H2O contents.

(

ln HCO2, mix = x H2 O ln HCO2, H2 O (T ) + xPEG200 ln HCO2,PEG200 (T ) + x H2 O 1

distributions is illustrated in Fig. 11(b). When the temperature increased from 308.15 to 338.15 K, the contribution of R1R2NH2HCO3 remained almost constant. Moreover, the formation of R1R2NH2HCO3 was not sensitive to temperature. It was also interesting to note that the contribution of R1R2NCOONH2R1R2 increased instead of decreasing. Generally, high temperature is unfavorable for the CO2 absorption. This phenomenon could be explained by the contribution of the physical CO2 absorption decreasing more significantly than the formation of R1R2NCOONH2R1R2.

)

x H2 O ·( 3)

(21) As depicted in Fig. 9, the measured solubility values of CO2 in [Cho] [Pro]/PEG200/H2O agree well with the predicted CO2 solubility, and the corresponding ARDs are 5.11%, 3.34%, 6.04%, and 7.09% for the H2O mole fractions of 0.3892, 0.5259, 0.7695 and 0.8481, respectively. Therefore, the theoretical prediction in this study was reliable. For gas separation, the selectivity of CO2 to other gases in the mixture (CO2/N2 and CO2/CH4) is also a key factor. The solubilities of CH4 and N2 in [Cho][Pro]/PEG200[46] and PEG200[70] have been analyzed in our previous studies. Combining the previous research results with those in this study, the solubilities of CH4 and N2 could be

(22)

(23)

predicted and then the selectivity could be obtained.

Two reactions could occur for the hybrid absorbent, and the binding of CO2 to [Cho][Pro] is depicted in Eqs. (22) and (23). Based on the experimental measurements and theoretical modelling, the mechanism and H2O effect on the CO2 absorption of the hybrid absorbent could be attributed to the following: (i) the formation of R1R2NCOONH2R1R2 (Eq. (22)) dominated throughout the entire absorption process and (ii) the presence of H2O shifted the CO2 absorption toward strong chemical absorption (i.e., the formation of bicarbonate, which caused the CO2 absorption capacity and absorption enthalpy to increase simultaneously). In fact, there are some ions species (i.e. [Cho]+, [Pro]−, HCO3−, H+, OH− and so on) existed in the systems of CO2/[Cho][Pro]/H2O and CO2/[Cho][Pro]/PEG200/H2O. In our future work, the models such as e-NRTL [91] and ePC-SAFT [66,67] will be used for the description of the CO2 solubility in the hybrid absorbent with the consideration of ions.

3.2.5. Effect of H2O content on CO2 solubility To further analyze the physical and chemical contributions to the CO2 solubility, the species in the CO2/hybrid absorbent system at the temperature of 308.15 K and for x2 = 0.3892 are illustrated in Fig. 10. The mole fractions of PEG200 and H2O were excluded because their concentrations remained almost constant throughout the entire absorption process. At 308.15 K, the CO2 mole fraction increased linearly, following Henry’s law, and the mole fraction of [Cho][Pro] decreased sharply as the CO2 pressure increased owing to the chemical reaction between [Cho][Pro] and CO2. When the CO2 pressure was lower than 0.1 MPa, the concentration of R1R2NCOONH2R1R2 first increased as the pressure increased, and then decreased slightly. On the contrary, the concentration of R1R2NH2HCO3 increases slowly as the pressure increased. Therefore, R1R2NCOONH2R1R2 was the dominant species in the CO2/hybrid absorbent system. This was probably due to the formation of R1R2NH2HCO3 being more difficult than that of R1R2NCOONH2R1R2 at this temperature and H2O mole fraction. Moreover, the CO2 mole fraction surpassed that of R1R2NCOONH2R1R2 in the liquid phase when the pressure was higher than 0.35 MPa, which indicated that the hybrid absorbent also presented excellent physical absorption performance. To further examine the contributions of the physical dissolution and chemical reaction on the CO2 absorption capacity, the distributions of CO2-based species in the hybrid absorbent are illustrated in Fig. 11. As depicted in Fig. 11(a), for x2 of 0.3892 and the temperature of 308.15 K, the contribution of R1R2NCOONH2R1R2 to the absorption capacity was large at low CO2 partial pressure values. The contribution of the physical CO2 dissolution increased while the contribution of R1R2NCOONH2R1R2 decreased sharply as the pressure increased. The contribution of R1R2NH2HCO3 was only a small part of the total absorption capacity for the studied case (x2 = 0.3892). Therefore, the formation of R1R2NCOONH2R1R2 prevailed at low x2 values, and the physical absorption predominated at high pressure. When x2 increased from 0.3892 to 0.8481, the percentage of R1R2NH2HCO3 increased significantly while the percentage of R1R2NCOONH2R1R2 remained almost the same. The high H2O content could promote the generation of bicarbonate, and therefore, the physical absorption performance of the hybrid absorbent was weakened. Therefore, the addition of H2O to the hybrid absorbent could enhance the CO2 absorption owing to the stronger chemical absorption. At the same time, the effect of temperature on the species

3.3. Absorbents comparison Preliminary screening of different absorbents for CO2 separation is required, and should mainly rely on properties such as, density, viscosity, CO2 capacity, and absorption enthalpy. Considering that the [Cho][Pro]-based absorbents analyzed in this study possess both physical and chemical absorptions features, commercial solvents, such as, 30 wt% monoethanolamine (MEA), 30 wt% methyldiethanolamine

Fig. 10. Species distribution in [Cho][Pro]/PEG200/H2O at 308.15 K and for x2 = 0.3892. 9

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Fig. 11. CO2-based species contribution in the hybrid absorbent. (a) 308.15 K and (b) x2 = 0.3892.

(MDEA), DEPG, H2O, and PEG200 were selected for comparison. Since it would be unreasonable to select the same operational conditions as references for the physical and chemical absorbents, in this study, the temperature and CO2 partial pressure of 313.15 K and 0.01 MPa, respectively, were selected for 30 wt% MEA and 30 wt% MDEA because these are typical operational conditions for CO2 separation from the flue gas. These conditions were also used for [Cho] [Pro]-based absorbents. In addition, the temperature and CO2 partial pressure of 293.15 K and 0.28 MPa were selected as reference for H2O, PEG200, and DEPG because these are typical operational conditions for biogas upgrading using high pressure water scrubbing. All the properties of the absorbents are summarized in Table 4 for comparison. The viscosity of 30 wt% [Cho][Pro]/PEG200 was the highest (97.15 mPa·s), which was not appropriate for CO2 separation using absorption/desorption towers, and thus adding water was required to decrease its viscosity. The 30 wt% MEA solution presented the largest CO2 absorption capacity and absorption enthalpy, which were attributed to the high alkalinity of MEA. Furthermore, physical solvents such as H2O, PEG200, and DEPG exhibited low CO2 absorption capacity and absorption enthalpy. Even though its CO2 absorption capacity was high and its absorption enthalpy was low, 30 wt% MDEA could not be considered to be a promising solvent owing to its low absorption kinetics [74]. On the contrary, [Cho][Pro]-based absorbents presented high absorption kinetics because [Cho][Pro] could be considered to be a secondary amine owing to the molecular structure of proline. Meanwhile, all the [Cho][Pro]-based absorbents have a relatively low absorption enthalpy (less than − 57 kJ·mol−1) compared with 30 wt% MEA solution (−84 kJ·mol−1), which indicates that less energy is demanded for the regeneration process. The CO2 absorption capacity increased by 60% when PEG200 in 30 wt% [Cho][Pro]/PEG200 was completely replaced with H2O. In addition, the viscosity of 30 wt% [Cho][Pro]/PEG200 decreased from 97.15 to 2.42 mPa·s, while the CO2 absorption enthalpy increased only by 5.6%. This indicated that 30 wt%

[Cho][Pro]/H2O presented advantages for CO2 separation from the CO2 absorption capacity and enthalpy point of view. However, the evaporation of H2O at high temperature should be considered, while PEG200 was preferred owing to its extremely low vapor pressure compared to that of H2O, MEA, and MDEA [41]. Therefore, this implied that the hybrid absorbent might be promising for CO2 separation at relatively high temperature. At the same time, it was also reported that ILs containing the [Pro]− anion might present faster absorption rate than the aqueous amine [102]. In addition, the cost of the hybrid absorbent was also compared with those of commercial solvents. The price of [Cho][Pro] was estimated to be approximately 10,000 $·t−1 with an industrial scale production in the future. Based on the results listed in Table 4, the cost of the hybrid absorbent was much higher compared to those of 30 wt% MEA and 30 wt% MDEA, but was competitive with those of other traditional solvents, such as DEPG. Overall, the advantages of using the hybrid absorbent for CO2 separation could attribute to: (1) its environmental compatibility and low vapor pressure; (2) lower energy utilization compared to that of the aqueous amine owing to its lower CO2 absorption enthalpy; and (3) faster absorption rate compared to those of the aqueous amine and other physical solvents owing to the [Pro]− anion. 3.4. Recyclability of the hybrid absorbent In addition to the properties and CO2 absorption performance of the absorbents, their recyclability is another important factor for industrial applications. The regeneration experiment for [Cho][Pro]/PEG200/ H2O (x2 = 0.8481) was performed at 383.15 K and atmospheric pressure, and the results are depicted in Fig. 12. After five cycles, the hybrid absorbent was able to maintain a relative steady absorption capacity, which demonstrated its usability. The properties, CO2 solubility, and stability of the hybrid absorbent

Table 4 Properties and costs of different absorbents. Solvent

ρ (kg·m−3)

η·103 (Pa·s)

c (g CO2·g−1 solvent)

−ΔHabs (kJ·mol−1)

Cost ($·ton−1)

30 wt% MEAa 30 wt% MDEAa 30 wt% [Cho][Pro]/H2Oa 30 wt% [Cho][Pro]/PEG200a 30 wt% [Cho][Pro]/PEG200/H2Oa (w H2 O = 20 wt%) 30 wt% [Cho][Pro]/PEG200/H2Oa (w H2 O = 50 wt%) H2Ob PEG200b DEPGb

1010 [92] 1017 [92] 1026 1115 1110 1030 997.8 1124 1055.4

1.62 [92] 1.95 [92] 2.42 97.15 15.01 5.12 1.0 66 7.58

0.108 [93] 0.055 [98] 0.030 0.0187 0.0213 0.0292 0.00453 0.00872 0.015

84 [94,95] 55 [94,95] 56.92 53.90 56.06 56.76 17.34 12.67 11.49

375 [96,97] 1095 [99] 3000 4260 3900 3540 0.5 [100] 1800 3500 [101]

a b

At 313.15 K and PCO2 = 0.01 MPa. At 298.15 KPCO2 = 0.28 MPa.

10

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Fig. 12. Recyclability of the hybrid absorbent (x2 = 0.8481).

were systematically analyzed in this study. Combined with the results obtained in our previous work [70], the model can be used to describe the properties, gas solubility, and selectivity required for process simulation. In our future work, the kinetics analysis, process simulation, and pilot testing will be conducted to develop a process that will facilitate the separation of CO2 from gas streams, such as flue gas, biogas, or synthetic gas. 4. Conclusion A systematic study of the hybrid absorbent, choline-2-pyrrolidinecarboxylic acid/polyethylene glycol/water ([Cho][Pro]/PEG200/H2O) was performed to develop a hybrid absorbent for CO2 separation. The density, viscosity, and CO2 solubility of the hybrid absorbent were measured experimentally. In addition, the excess mole volume and excess Gibbs energy of activation of the hybrid absorbent were calculated. The solubility of CO2 in [Cho][Pro]/PEG200 and [Cho][Pro]/ H2O was analyzed and described using the RK-NRTL model. The solubility of CO2 in the hybrid absorbent was predicted using the RK-NRTL model and verified using the newly obtained experimental results. It was determined that H2O interacted strongly with [Cho][Pro]/PEG200, and a complex structure may be generated for water-rich (x2 ≈ 0.65–0.85) mixtures. The formation of R1R2NCOONH2R1R2 dominated the entire CO2 absorption process, and the presence of H2O resulted in the formation of bicarbonate (i.e., the CO2 absorption capacity and absorption enthalpy increasing simultaneously). In addition, the performances and costs of the hybrid absorbent and other commercial solvents used for CO2 separation were compared using their properties, and the recycling of the hybrid absorbent was also performed. The CO2 solubility and absorption enthalpy of the hybrid absorbent were advantageous, and therefore, [Cho][Pro]/PEG200/ H2O could be considered to be a promising absorbent for CO2 separation. The systematic analysis in this study will facilitate the simulation, evaluation, and comparison of the performance of the hybrid absorbent using comprehensive process simulation for its practical use for real gas streams. Acknowledgements We would like to thank the National Natural Science Foundation of China (21729601, 21776123, 21136004, 21476106, 21428601, and 21490584). Y. Chen thanks Kempe foundation in Sweden for financial support, and X. Ji thanks Swedish Energy Agency. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apenergy.2019.113962. 11

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