Optimization and Characterization of an Amino Acid Ionic Liquid and

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Optimization and characterization of an amino acid ionic liquid and polyethylene glycol blend solvent for precombustion CO2 capture: experiments and model fitting Muhammad Usman, Huang Huang, Jun Li, Magne Hillestad, and Liyuan Deng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02457 • Publication Date (Web): 26 Oct 2016 Downloaded from http://pubs.acs.org on October 27, 2016

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Industrial & Engineering Chemistry Research

Optimization and characterization of an amino acid ionic liquid and polyethylene glycol blend solvent for pre-combustion CO2 capture: experiments and model fitting

Muhammad Usmana, Huang Huanga, Jun Lib, Magne Hillestada, Liyuan Denga* a

Department of Chemical Engineering, Norwegian University of Science and Technology, Sem Sælandsvei 4, 7491 Trondheim, Norway b

Shanghai Research Institute of Petrochemical Technology, 1658 Pudong Beilu, Shanghai 201208 China

Abstract Amino acid ionic liquids are green solvents with low toxicity and properties suitable for precombustion CO2 capture, such as high CO2 absorption, good thermal stability, and negligible vapor pressure. However, the high viscosity and the relatively high cost have hampered their industrial applications. This work systematically studied an amino acid ionic liquid tetrabutylphosphonium glycinate ([P4444][Gly]) and its blends with a low-cost and less viscous co-solvent polyethylene glycol (PEG400). The concentration of the blend solvent was optimized with respect to CO2 solubility, regeneration efficiency and cyclic capacity. The solubility of CO2 in [P4444][Gly], PEG400 and their blends of four different concentrations was measured experimentally for a temperature range of 60-140oC and up to a pressure of 17bar. The results show that CO2 solubility increases with increasing the ionic liquid concentration in the blend and decreases with increasing the temperature. The optimum CO2 absorption was determined to be 30 wt.% of [P4444][Gly] in the blend. The regeneration study of 30wt%[P4444][Gly]-70wt%PEG400 for three cycles verified their reusability in the process and confirmed that the reaction between [P4444][Gly], and CO2 can be reversed at 140oC. The CO2 absorption capacity of the blend absorbent is up to a loading of 1.23 mol of CO2/mol of Abs. The parameter fitting of the experimental data using empirical correlations was evaluated and these correlations are developed in particular to predict the blend solvent of an amino acid ionic liquid-PEG400 system based on the ionic liquid concentration and temperature.

Key words: Pre-combustion CO2 capture; Amino acid ionic liquids; PEG; CO2 Solubility; Cyclic sorption capacity. 1 ACS Paragon Plus Environment


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Highlights

1. The absorption-desorption of CO2 in a [P4444][Gly]-PEG400 blend of various concentrations was studied. 2. The physical and thermal properties of [P4444][Gly], PEG400 , and blends were systematically characterized. 3. The cyclic capacity and regeneration of the optimized solvent were evaluated. 4. The models fit well with the experimental data for density, viscosity, and solubility.

Graphical Abstract:

2 ACS Paragon Plus Environment

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1. Introduction Prolific carbon dioxide emissions have led to global climate change due to strong dependence on fossil fuels for energy requirements. These CO2 emissions of

originate from the

combustion of fossil fuels in the petroleum industry, power plants, and the petrochemical industry1. These emissions can be substantially reduced by employing carbon dioxide capture, which is an energy intensive process and demands higher operational costs 2. Researchers are striving for an energy efficient and cost effective CO2 capture process. Three different routes for CO2 capturing techniques are under investigation, namely pre-combustion, postcombustion, and oxy-fuel combustion processes. These processes vary in term of the source and conditions of the CO2 capture. Post-combustion CO2 capture is a widespread and well-studied capturing method. However, the pre-combustion CO2 capture process offers the advantages of higher CO2 concentration and hence higher CO2 partial pressure, resulting in a greater driving force for separation and smaller footprints. Several technologies are being investigated for CO2 capture techniques on lab and industrial scales, including absorption, adsorption, cryogenics and membrane separations

3-11

. Among them, the chemical absorption of CO2 is the most investigated and

mature technology

12, 13

. Aqueous alkanol-amine solutions are considered established

chemical absorbents in the industry for CO2 capture due to their high reactivity and low cost. However, aqueous amine solutions face obstacles in their application due to their low cyclic capacity, equipment corrosion, solvent degradation, high solvent regeneration cost and loss of solvent by entrainment. To overcome these challenges, a new class of solvents known as ionic liquids are being applied for CO2 capture. Ionic liquids have gained much attention in recent years in various industrial sectors

14-25

.

Ionic liquids are molten salts at a temperature less than 100oC. They possess appealing and diverse characteristics such as undetectable vapor pressure, high thermal and chemical stability, tunable characteristics and strong absorption capacity. Ionic liquids can be tailored for a particular process by adjusting their cations and anions. A diverse range of ionic liquid cations and anions have been reported for CO2 separation

26-29

. Room temperature ionic

liquids, task-specific ionic liquids and supported ionic liquid membranes have been investigated for CO2 capture in the literature 30-34. Amino acid ionic liquids are green solvents with low toxicity and properties suitable for precombustion CO2 capture, such as high CO2 absorption, good thermal stability, and negligible 3 ACS Paragon Plus Environment


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vapor pressure 35, 36. Amino acid based ionic liquids have an N-H group like amines and react with CO2 to enhance its absorption capacity. The reaction between amino acid ionic liquids and CO2 was reported by Zhang et al.

37

as a two-step process. In the first step, a proton

transfers from the N-atom in glycine to the oxygen atom of the carboxyl group in glycine, while the second step involves the proton sharing between the oxygen atoms of CO2 and the carboxyl group in the anion. However, the high viscosity and relatively high cost of amino acid ionic liquids have become

major obstacles in the development of their industrial

applications. Phosphonium-based amino acid ionic liquids have proven to possess a lower viscosity compared to other types of ionic liquids, but still higher than amines 38. One way to deal with the high viscosity is to blend the amino acid anion ionic liquids with low molecular weight organic solvents to reduce the viscosity. Li et al. 39 investigated the amino acid(sodium glycinate)-ionic liquid([Bmim][BF4]) for CO2 absorption with different blending ratios and came to the conclusion that a high absorption capacity was obtained for a molar ratio of 4:1 (amino acid-IL). Sistla et al.

35

reported CO2

absorption in aminated anion ionic liquids, which resulted in a CO2 absorption capacity (mass of the CO2/mass of an ionic liquid) in amino acid ionic liquids of 70% more than that of primary amine-cation ionic liquids and 600% more than tertiary amine-cation ionic liquids. Zhang et al.

40

studied four dicationic amino acid ionic liquids for CO2 absorption and found

a nearly equimolar CO2 absorption in the ionic liquids accompanied by the reversibility of the absorbent. They also addressed the enhancement in CO2 absorption capacity if water is added to the ionic liquids due to the decrease in the viscosity of the liquid. Polyethylene glycols were investigated for CO2 solubility 40. Considering the above mentioned characteristics and our previous study 41, amino acid based ionic liquid tetradecylphosphonium glycinate [P4444][Gly] is proposed for pre-combustion CO2 capture applications at high temperature and pressure. However, as ionic liquids possess high viscosity, which makes them challenging for this application, the amino acid- based ionic liquid [P4444][Gly] is blended with the low molecular weight polymer PEG400 in this work, to reduce the viscosity of the absorbent and enhance the CO2 sorption. The PEG400 has a comparable CO2 absorption capacity, good thermal stability and negligible vapor pressure as ionic liquids 42, and [P4444][Gly] has been reported to react chemically with CO2 37; thus the blend of [P4444][Gly] and PEG400 will combine both chemical absorption and physical sorption; a CO2 absorption capacity higher than both solvents is expected.


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The present work investigated and optimized the CO2 absorption capacity for the blends of [P4444][Gly] and PEG400 at pre-combustion CO2 capture conditions. The thermal stability of the amino acid ionic liquid-PEG400 blends were measured up to a temperature of 200oC. The density of these blends was measured for a temperature range of 20-80oC at ambient pressure, while their viscosity was tested for temperature range of 20-90oC. The density, viscosity, and solubility data are fitted by concentration and temperature-based empirical correlations. The models fitted very well to the experimental data. The CO2 absorption capacity in these blends was tested at 60, 100, and 140oC, and pressures up to 18bar. The CO2-absorption experiments were also run in descending pressure range to validate the experimental setup and procedure. The regeneration of the absorbent is a key consideration in the selection of the absorption process. The regeneration of the 30wt%[P4444][Gly]-70wt%PEG400 blend was investigated in three cycles of absorption-desorption. Absorption was maintained at 60oC and desorption at 140oC for the measurements. The cyclic capacity and CO2 removal efficiency of the [P4444][Gly], PEG400 and blends were evaluated.

2. Experimental 2.1 Materials The carbon dioxide was purchased from PARAXair with a nominal purity of 99.99%. The polyethylene glycol was obtained from Sigma Aldrich with a purity of 99%. The ionic liquid [P4444][Gly] was synthesized in the laboratory. The procedure of ionic liquid synthesis is the same as described in

38,

43

. The other raw materials used for synthesis are

Tetrabutylphosphonium-hydroxide solution (40wt% in water), acetonitrile (99.8%), methanol (99.8%), and deuterium oxide (99.9atom%), and were purchased from Sigma Aldrich and used as received.

2.2 Methods 2.2.1

Synthesis of tetrabutylephosphonium glycinate ([P4444][Gly])

The ionic liquid [P4444][Gly] was synthesized in the lab using the same method as reported in 43, 44

. The tetrabutylphosphonium hydroxide solution [P4444][OH] (with 40% water by weight)

was mixed drop-wise to a marginally excess equimolar aqueous glycine solution. This mixture was allowed to stir for 12 hours at room temperature to become well mixed. Water evaporation of this mixture was carried out at 45oC in a rotary evaporator for 5-6 hours. Acetonitrile and methanol with a volume ratio of 9:1 was poured into this water evaporated 5 ACS Paragon Plus Environment


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mixture and stirred vigorously for 9 hours. The undissolved excess amino acid salt was removed from this mixture by filtration using filter paper. The filtrate product was further evaporated to remove solvents and dried in a vacuum oven at 70oC for 48 hours. The purity and chemical identity of the ionic liquid was analyzed by NMR using a Bruker Avance DPX 400MHz NMR spectrometer. The sample for NMR analysis was prepared by mixing D2O with the synthesized ionic liquid. First, D2O was dissolved in the ionic liquid sample at room temperature in a NMR tube and subjected to ultrasonic mixing to ensure a constant concentration of the sample. The 1H proton NMR spectrum was obtained at 25oC with 16 number of scans, a pulse duration of 10.5µsec, and an acquisition time of 3.95secc. The 1H NMR spectrum was analyzed using TopSpin software. The 1H proton NMR spectrum was compared with the reported values of the spectrum peaks for accuracy. The ionic liquid was stored in a desiccator to avoid moisture uptake before use. The 1H NMR spectrum of the synthesized ionic liquid obtained from the NMR analysis is shown in Figure 1.

Figure 1. The 1H NMR spectrum of the ionic liquid [P4444][Gly] from the NMR analysis

The first chemical shift at 0.92ppm showed the four CH3 groups with 12 protons attached to the P and the second shift at a range of 1.34-1.52ppm representing four CH2 groups with 16 protons. The third chemical shift at 2.13-2.23ppm stands for CH2 attached to P in with 8 protons. The last chemical shift at 3.0ppm is the functional group N-CH2-CO2 with 2 protons. Through a comparison of adjusting the result and the literature, the figure clearly shows 4 peaks with similar δ values on the x-axis as reported by Zhang et al.45. The 1HNMR results agree with the literature and indicate a successful synthesis of [P4444][Gly]. There are also


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two small hetero peaks in the scan but they are insignificant. The synthesis and refining of ionic the liquid [P4444][Gly] was thus authenticated by the 1HNMR data. 2.2.2 Density The densities of all the samples were measured by an Anton Paar DMA 4500M density meter within a temperature range of 20- 80oC at atmospheric pressure. This density meter consists of a U-tube with a Pt-100 thermocouple (accuracy of ±0.01oC). The measuring volume is approximately 9.5ml and the sample needs to be filled in the test vial and covered by a cap (Usman, 2012). The available temperature range is set in the apparatus at an operating temperature of 20-80°C.

2.2.3 Viscosity The viscosity measurements were carried out with a Rheometer (ARG2 by TA instruments). All the samples were tested within a temperature range of 20-90oC at atmospheric pressure with an accuracy of ±2%. 2.2.4 Thermal decomposition temperature measurement The thermal decomposition temperatures of the ionic liquids and their blends with PEG400 were measured by a thermo gravimetric analyzer (Q500, TA instruments) with an accuracy of ±0.01%. A known sample amount (10-15mg) was placed in a platinum pan and TGA measurements were performed under a nitrogen atmosphere (60mL/min) over a temperature range of 21-400oC at a heating rate of 10oC/min. 2.2.5 CO2 solubility The CO2 solubility experiments were performed with an in-house built apparatus as shown in Figure 2. Both the CO2 absorption and desorption tests were carried out using the same apparatus, which mainly consists of two steel vessels; one for gas storage (80.7±0.1cm3) and the other for gas absorption into liquid (33.9±0.1cm3). The temperature of both vessels was controlled by silicon oil baths with an accuracy of ± 0.1K. The pressure of the system was observed by a mounted pressure transducer (Series 33X, Keller Co.) with an accuracy limit of ±0.3kPa. A magnetic stirrer was used in the gas absorption vessel to stir the mixture at a speed of 450rpm.



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Figure 2.The experimental setup for CO2 solubility. 1: Gas cylinder, 2/3: Oil bath, 4: Gas storage vessel, 5: Absorption vessel, 6: Magnetic stirrer, 7: Pressure transmitter, 8: Computer program. Reprinted from 46

The whole apparatus was first evacuated by using a vacuum pump for about one hour and was checked for leakages. The gas was then stored in the vessel at a certain pressure and pressure (Ps) was allowed to become stabilized in the vessel. The middle valve between the two vessels was opened; the magnetic stirrer was turned on and equilibrium was allowed to be achieved. The equilibrium pressure is termed Pe. The difference in pressure values (Ps-Pe) gives the CO2 being absorbed in the liquid by converting the pressure into moles using the PVT data with the Soave Redlich Kwong (SRK) equation of state. The amount of CO2 absorbed in the absorbent can be estimated by the difference between number of moles of CO2 in the gas vessel and number of moles of the CO2 in the gas phase at equilibrium. The same procedure was adapted for all the data points. Desorption experiments were performed when the system was at the maximum concerned pressure. The gas was released into a vacuum line by closing the middle valve first. Then the gas storage vessel was allowed to achieve stable pressure following the opening of the middle valve and achieving equilibrium in the absorption vessel. The same steps were followed while the gas was desorbed up to a minimum achievable pressure. All the experimental data were recorded using the LabView computer program. 2.2.6 Error analysis The error analysis for density, viscosity and CO2-solubility measurements were evaluated by calculating the ratio of standard deviation and average value of two or more observations. The inherent errors were mentioned for each instrument used for measurements in section 2.2. Density and viscosity measurements were run at least twice to reproduce the data with an accuracy of 0.05% and 3% respectively. As it is stated in the experimental procedure, CO2 gas at various pressures was filled in the gas vessel manually in this study. It was challenging to replicate the exact same pressure point for the CO2-solubility measurements for each 8 ACS Paragon Plus Environment

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temperature and concentrations of ionic liquid due to difficulty in controlling the pressure. For this reason, at least two measurements were commonly performed to produce the data for CO2-solubility and ensure the accuracy, but not able to calculate the standard deviation for variables. The evaluated maximum uncertainties for the reported CO2-loading value and equilibrium partial pressure of CO2 are 3% and 5%, respectively, according to the deviation found from the reproduction of at least two measurements.

3. Results and discussion 3.1 Density of [P4444][Gly] and blends The density of a solvent is an important physical property for the evaluation of reaction kinetics and mass transfer as well as the estimation of the CO2 solubility in the solvent. In this work the pressure drop method is being employed to estimate the CO2 solubility in the ionic liquid and blends, so the density of the solvent is required to calculate the gas phase volume. The densities of [P4444][Gly], PEG400 and their blends were measured for a temperature range of 20-80oC with an accuracy of ±1% at ambient pressure conditions (please see Table S1 in the Supplementary information). The density of deionized water was also measured along with the [P4444][Gly] -PEG400 blends to ensure the reliability of the apparatus and validation of the experiments. The density data of deionized water agreed well with the literature with a standard deviation of ±0.2%. The measured density data is fitted as a function of the mole fraction of the ionic liquid at different temperatures using the empirical correlation given in equation 2 47 and presented in Figure 3:

= +

1 − ∗ +

+ (2)

Where ω1 is the weight fraction of the ionic liquid [P4444][Gly] in the blend and b1,b2,b3,b4 , and b5 are the fitted parameters of equation 2. The objective function is defined as the absolute average relative deviation, and is given as:

=

1

*

! +,

"#$% − &'( " ∗ 100 #

%$The fitted parameters of Equation 2 are provided in Table 1. Table 1. Model parameters for the density correlation in Equation 2 as a function of the mole fraction of the ionic liquid in the blend



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Parameters

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b1

b2

b3

b4

b5

AARD%

0.8142

78.1176

12.8995

36.2356

-2.5529

0.166

The density of PEG400 is greater than [P4444][Gly] at a constant temperature. The results from Figure 3 indicate that the density reduces as the concentration of PEG400 in the blend decreases, or the temperature increases. The density correlation given in Equation 2 defined the experimental data very well with an AARD of 0.166% for all the concentrations and temperatures. A comparison of the experimental and modeled density data is presented in Figure 4, which shows that the model predicts the densities very well.

Figure 3. Density of [P4444][Gly], PEG400, and blends as a function of increasing ionic liquid concentration in the blend at 20, 30, 40, 60, and 80oC, ▲:20oC, +:30oC, ■:40oC, ●:60oC, ♦: 80oC. The uncertainty of the density is 0.05%.


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Figure 4. Parity plot between the experimental and the predicted density of [P4444][Gly] and blends with PEG400

The density of PEG400 in this work is compared with the literature accuracy of the test rig, as shown in Figure 5

48-50

to verify the

48-50

. For example, Wu et. al.48 reported the

density of PEG400 1.1122, 1.0973, 1.0815 g/cm3 at 40, 60 and 80oC, respectively, which are in strong agreement with the observed values in this work. The density data for [P4444][Gly] are not available in the literature.

Figure 5. Density of PEG400 at a temperature range of 20-80oC and a comparison with the literature data, ■: This work, ►: Wu et al.48, ●: Zhang et al 50,▼:Han et al. 49



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3.2 Viscosity Viscosity is an important parameter in the selection of the solvent for CO2 capture. Viscosity of the ionic liquid [P4444][Gly], PEG400, and their blends were measured for a temperature range of 20-90oC (Table S2). The results for these measurements are presented in Figure 6. The experimental viscosity data are correlated by Equation 347 and the fitted parameters are presented in Table 2. The criteria for fitting are to minimize the sum of square error deviation, where the deviation is given as (-exp--modeled /-exp. - = 81 +

9.$

+

9 $ $

: . 1 +

9

+

9

+

9; $ $

(3)

Table 2. Fitted model parameters for viscosity as a function of the mole fraction of the ionic liquid in the blend

Parameters

k1

k2

k3

k4

k5

721.184

171519.275

-2290.084

996741.678

52581.902

It can be seen from Figure 6 that the viscosity of the solvent increases as the blend becomes more concentrated with the ionic liquid, and reduces as the temperature rises. The viscosity of PEG400 is the lowest of all, and that of [P4444][Gly] with the least PEG400 shows the maximum. The viscosity of the [P4444][Gly]-PEG400 mixtures has a maximum value of 358.4mPa.s at a temperature of 20oC, and this reduces to 22.5 mPa.s at a temperature of 90oC. Under this condition, the viscosity of the absorbents becomes comparable to that of aqueous alkanol amine solutions.


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Figure 6. Viscosity of the systems as a function of temperature, ■; PEG400, ●; 70wt%PEG40030%[P4444][Gly], ▲; 50wt%PEG400-50wt%[P4444][Gly], ▼; 30wt%PEG400-70wt%[P4444][Gly], lines; fitted model. The uncertainty of the viscosity is 3%.

From Figure 6 it can be observed that the viscosity reduces drastically at a temperature lower than 70oC, and then the reduction becomes less significant at higher temperatures.

Figure 7. Parity plot between the experimental and modeled viscosity

The model defined for viscosity as a function of concentration and temperature adequately correlates the experimental data with square error deviations of 3.77%. A parity plot of viscosity is shown in Figure 7, which clearly demonstrates the accuracy of the model. 3.3 Thermal decomposition temperature As the pre-combustion process takes place at high temperature conditions (180-200oC), the thermal stability of an absorbent at a temperatureof 200oC is a critical consideration in the selection of absorbent.



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Figure 8. Thermal gravimetric analysis of [P4444][Gly], PEG400 and blends (N2 flow =60mL/min), ; −: [P4444][Gly], −: PEG400, −: 30wt%[P4444][Gly]-70wt%PEG400, −: 50wt%[P4444][Gly]-50wt%PEG400, −: 70wt%[P4444][Gly]-30wt%PEG400

Conventional alkanol amine absorbents are reported to either have degradation or cause corrosion of the equipment at elevated temperatures generally claimed to have high thermal stability

51-53

. However, ionic liquids are

54, 55

. The thermal stability of the ionic liquid

[P4444][Gly], PEG400, and their blends was performed by thermos-gravimetric analysis. The results obtained are presented in Figure 8. The point of thermal degradation is termed Tonset , calculated from the Universal Thermal Analysis software. Table 3. Tonset of [P4444][Gly], PEG400, and blends ω(weight fraction

Tonset (oC)

of [P4444][Gly] in blend) 0

280.97

0.3

262.70

0.5

270.70

0.7

281.14

1.0

296.30

Tonset is defined by the intercept of the baseline of the weight loss and the tangent line derived from the weight loss vs temperature curve. The actual thermal degradation is said to started before Tonset 56. The Tonset of all the samples are listed in Table 3. All the samples showed good thermal stability up to 200oC. The thermal stability of the pure PEG400 is the best out of all the samples. The thermal stability of the pure ionic liquid [P4444][Gly] showed the 14 ACS Paragon Plus Environment

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lowest value of Tonset; however, even the highest weight loss was only 0.197%, which is very low.

3.4 Effect of pressure on CO2 solubility The effect of pressure on CO2 solubility in the mixtures of [P4444][Gly] and PEG400 is investigated and the experimental results of CO2 solubility in terms of loading up to a pressure of 18bar are presented in Figure 9 (Table S3). The loading of CO2 (α) is represented by mole of CO2 per mole of absorbent. The CO2 solubility in the [P4444][Gly] ionic liquid and PEG400 are correlated by the following empirical correlation given by Equation 4, and this correlation is based on the concentration and temperature dependent terms. < =>' = < ? + @ +

A

(4)

B9 .CD

In this correlation, A and B are the concentration dependent parameters, defined by Equations 5 and 6, while k1, k2 and k3 are the functions of temperature and are presented in Equations 7, 8, and 9. = ' + . + .

(5)

E = E' + E + E + EF F

(6)

@ = @G + @

(7)

@ = exp @G . + @

(8)

@F =

9H

+ @F

(9)

The fitted parameters derived from the experimental data for this correlation are shown in Table 4. Figure 9 indicates the effect of pressure on CO2 solubility in the increasing trend. This incremental trend in CO2 loading with respect to pressure is true both for the physical (PEG400) and chemical absorption ([P4444][Gly] blends with the PEG400) cases. The loading of CO2 was observed to be within the range of 0.013- 1.051 (mol-CO2/mol-Abs) for a pressure range of 0.87 to 17.36 bar at 60oC. Table 4. Fitted parameters for CO2 solubility in the system as a function of weight fraction and temperature Parameters

Ao

A1

A2

Bo

B1

B2

B3



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Parameters

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0.9074

1.1680

7.5970

-0.4795

8.7843

-69.530

k1a

k1b

k2a

k2b

k3a

k3b

-1028.481

11.908

-49.318

2.2616

1370.94

-1.6161

36.547

The high solubility of ionic liquid blends with PEG400 resulted from the chemical reaction between the ionic liquid and CO2. As the reaction proceeded, the CO2 solubility increased and it showed a maximum when the concentration of the [P4444][Gly] in the blend was the highest (70%). The chemical reaction between the ionic liquid and CO2 takes place at lower pressures, and a further increase in pressure could only enhance the CO2 solubility due to the physical absorption of CO2.

Figure 9. Effect of pressure on CO2 loading (molCO2/molabs) at a temperature of 60oC; ■:PEG400, ●: 70wt%PEG400-30wt%[P4444][Gly], ▼: 50wt%PEG400-50wt%[P4444][Gly], ►:30wt%PEG40070wt%[P4444][Gly]. The uncertainties of the CO2-loading value and equilibrium partial pressure of CO2 are 3% and 5%, respectively.

It was observed to take a longer time for equilibrium to be attained at low pressures. This effect could be the result of approaching high viscosity with the increased concentration of ionic liquid in the blend and the formation of carbamate species due to the chemical reaction taking place between the ionic liquid and CO2. The higher viscosity of the absorbent will 16 ACS Paragon Plus Environment

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limit the mass transfer and reaction kinetics, and thus the time required to attain equilibrium would be significantly larger.

3.5 Effect of temperature and composition of ionic liquid in the blend The ionic liquid was blended with PEG400 in different weight percentages to examine the optimum CO2 absorption in the blends. The temperature and composition of the absorbents are key factors insensitively studying CO2 solubility. The experimental results for the equilibrium partial pressure of CO2 in 70 wt.% PEG400-30 wt.% [P4444][Gly] as a function of CO2-loading are presented in Figure 10 for temperatures 60, 100, and 140oC. The experimental data are presented as data points, and the lines are the fitted experimental data resulted from the correlation defined by Equation 4. The CO2 absorption capacity dropped as the temperature was increased from 60 to 140oC (Table S4). The CO2 loading was shown to have a value of 0.6 at 60oC, which is the maximum value at all the temperatures studied in this work.

Figure 10. Effect of temperature on CO2 solubility in 70wt%PEG400-30wt%[P4444][Gly], ►:60oC, ●: 100oC, ▼: 140oC. The uncertainties of the CO2-loading value and equilibrium partial pressure of CO2 are 3% and 5%, respectively.



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(a)

(b) Figure 11. Effect of blend composition on CO2 solubility at 100oC (a) and 140oC (b); ■:PEG400, ●: 70wt%PEG400-30wt%[P4444][Gly], ▼: 50wt%PEG400-50wt%[P4444][Gly], ►:30wt%PEG40070wt%[P4444][Gly]. The uncertainties of the CO2-loading value and equilibrium partial pressure of CO2 are 3% and 5%, respectively.

The CO2 solubility for each concentration of ionic liquid in the blend is presented by Figures 11 a and b, at temperatures 100oC and 140oC, respectively (Tables S5 and S6). The CO2 18 ACS Paragon Plus Environment

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solubility is enhanced as the weight percentage of the ionic liquid increases in the blend at all the pressures and temperatures studied. The CO2 -loading of PEG400 is the minimum (0.28) in Figure 11 b. The addition of the ionic liquid increases the CO2 absorption due to the chemical reaction taking place between the ionic liquid and CO2. The glycinate anion contains one primary amine group, which reacts with CO2 and eventually boosts the absorption capacity. As [P4444][Gly] comprises a glycine anion, the reaction between the ionic liquid and CO2 should have the same stoichiometry as primary amine (i-e CO2 loading of 0.5). It can be observed from Figure 11(a) that a CO2-laoding of 0.5 could only be achieved for concentrations of ionic liquid in the blend higher than 50wt% at all pressures and 60oC. In the case of 30wt% of ionic liquid in the blend, this ratio could be attained at a CO2 pressure greater than 16bar.The maximum CO2-loading achieved was 1.08 in the case of 70wt% of the ionic liquid in the blend, which is about 50% more absorption capacity than that of conventional amines. The plots of 50 and 70wt% of ionic liquid in the blend show a bigger plateau at higher pressures, which demonstrates the dominance of the chemical absorption in the ionic liquid.

The model used for CO2 solubility in terms of equilibrium pressure

describes the experimental data quite well at all temperatures. Temperature also plays a key role in the CO2 solubility in the absorbent. The CO2 capacity of the absorbent decreases as the temperature increases. The CO2-loading is reduced to 0.7 from 1.08 when the temperature approaches to 140oC.

3.6 Cyclic capacity of the solvent As the absorbent will be used in a continuous absorption-desorption process, the cyclic capacity of the absorbent is an important parameter to be addressed. The cyclic capacity must be sufficiently high so that the solvent can be reused in a continuous absorption-desorption cycle. The CO2 removal efficiency should also be taken into account along with the cyclic capacity for the optimization of the blend composition. The cyclic capacity and CO2 removal efficiency for all four concentration of [P4444][Gly]-PEG400 were studied, as presented in Figure 12. The cyclic capacity of the absorbent can be determined by the difference between the CO2 loading at absorption conditions and that at the stripping conditions. The CO2 removal efficiency is calculated from the following correlation: IJ KLMNO< PPQRQ RS =

IJ OTMKU − IJ TVKQU ∗ 100 IJ OTMKU

The cyclic capacity experiments were carried out by testing CO2 absorption at 60oC and stripping at 140oC with relative propagation error of 3.2% and 5.2% for physical and 19 ACS Paragon Plus Environment


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chemical absorption cases respectively. It can be observed from Figure 12 that the CO2 removal efficiency goes down from 96% to 12% as the concentration of the ionic liquid is enhanced in the blend, but the cyclic capacity follows the opposite trend up to an ionic liquid concentration of 50wt%.

Figure12. The performance analysis of solvents according to the CO2 removal efficiency and cyclic capacities ● cyclic absorption capacity, ■ CO2 removal efficiency

The cyclic capacity is reduced as the concentration of ionic liquid is increased beyond 50 wt.%. The reason for this could be that equilibrium was not achieved completely in the tests of the solvents with higher ionic liquid concentrations. The performance of 30 wt.% and 50 wt.% ionic liquid in the blend are comparable, which indicates that the best performance of the absorbent in terms of cyclic capacity and CO2 removal efficiency can be achieved in 3050 wt.% of ionic liquid in the blend. As the ionic liquid is quite viscous compared to PEG400, and a higher amount of ionic liquid in blend will lead to a higher viscosity of the blend and hence low mass transfer rates, a blend of 30 wt.% of ionic liquid is considered the optimal absorbent, and is further studied in this work. Pinto et al.

57

conducted experiments for the

absorption and desorption of physical and chemical absorbent ionic liquids. According to their studies, the cyclic capacity of an ionic liquid follows the same trend as shown here. The cyclic capacity goes to a minimum for the highest concentration of chemical absorbent. The maximum attainable cyclic capacity in their system is 0.22 which is smaller than the cyclic capacity of the absorbent in this work.


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3.7 Regeneration of the solvent Solvent regeneration consumes most of the energy in an absorption process using aqueous solvents. Aqueous alkanolamine solvents have the issue of high energy consumption due to solvent evaporation and degradation at high temperatures in desorbers 51. As the ionic liquid [P4444][Gly] and PEG400 have negligible vapor pressures, solvent evaporation is no longer an issue, which makes the absorption process significantly more energy efficient. The 30wt% [P4444][Gly]-70wt%PEG400 blend was investigated for a regeneration study.

Figure13. Regeneration efficiency of the solvent 30wt%[P4444][Gly]-70wt%PEG400 in three absorptiondesorption cycles; CO2 absorption at 60oC and desorption at 140oC

CO2 was first absorbed into the blend absorbent at 60oC and a pressure about 16bar and desorbed CO2 at 140oC and a pressure of 1bar. The CO2 rich solvent from the first cycle was then fed into the second cycle and so on. Three cycles were run following the same procedure. The results for CO2 absorption in three cycles are shown in Figure 13, where the CO2 loading is plotted against the number of regeneration cycles. It can be clearly seen from Figure 13 that there is no appreciable differences in the CO2 loading and absorbent for all three cycles. The differences in the CO2 loading after the first and the second cycles as well as the second and the third cycles, are fairly small and within the range of the error bar (5%), most likely resulting from the uncertainty in measuring the pressures. Moreover, the tested blend solvent has physical and chemical absorption; the absorption capacity can also be expected to vary to a small extent. These results also conclude that chemical reaction between an ionic liquid and CO2 can be reversed at a high temperature (140oC), and the solvent can be reused during the absorption process. The regeneration efficiency of the combined physical and chemical 21 ACS Paragon Plus Environment


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absorbent solvent (amino acid based ionic liquid) is reported by

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39

, and according to their

findings the absorbent lost 19% of CO2 after the first regeneration cycle. However, the regeneration efficiency of the absorbent in this work is approximately 95% after the first cycle and 87.9% after three cycles.

3.8 Comparison with the literature The AAIL and PEG400 blends showed proficient CO2 solubility, which are suitable for CO2 separation at elevated temperature and pressures. However, data for blended solvents are scarcely reported at similar conditions. A comparison has been made in Figure 14 with few reported data of blended amine-IL solvents, where the CO2 solubility is represented by the CO2 loading (mole of CO2/mole of absorbent).

Fig14. Comparison of CO2 solubility in the ionic liquid-PEG400 blend and the reported literature at 60oC, ▪;70wt%[P4444][Gly]-30wt%PEG400, red line; 30wt%MEA-10wt%[C2OHmim][DCA]58, blue line; 4M MDEA1M[gua][OTf]59.

Monoethanolamine and methyldiethanolamine are the most industrially used solvents for CO2 absorption. This work is compared with the blend of MEA-IL and MDEA-IL absorbents. The ionic liquid-PEG400 blend tested in this work exhibit high CO2 solubility in comparison with the 30wt%MEA-10wt%[C2OHmim][DCA] and 4M MDEA-1M[gua][OTf. Both the reported ionic liquid-amine absorbents are seen to chemically react with CO2. The 4M MDEA1M[gua][OTf] has a lower CO2 solubility than 30wt%MEA-10wt%[C2OHmim][DCA].


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4. Conclusions The four different blending ratios of [P4444][Gly] and PEG400 in the blend solvent and their CO2 absorption at temperatures of 60, 100 and 140oC were reported. The density of [P4444][Gly] is less than water, while that of PEG400 is more than DI water and [P4444][Gly]. The viscosity of the blend solvent reduces with the addition of PEG400. These blends of [P4444][Gly] and PEG400 exhibited high thermal stability in tests up to 200oC, which makes them feasible for pre-combustion applications at elevated operating temperatures. CO2 absorption can be achieved to a loading of 1.23 mol CO2/mol Abs. at 60oC at a pressure of 17bar if pure [P4444][Gly] is used, but the cyclic capacity of the solvent and CO2 removal efficiency are low. Optimum CO2 absorption could be observed between the concentration range of 30 wt%[P4444][Gly] and 50 wt%[P4444][Gly] in the blend based on the cyclic capacity and CO2 removal efficiency, but 30 wt.% [P4444][Gly] was chosen as the optimal solvent considering the high viscosity and cost of the ionic liquid. The correlation derived on the basis of concentration and temperature terms fitted well with the experimental data in the observed temperature and pressure ranges. This validates the accuracy of the experimental data. The regeneration of the absorbent was verified by a temperature swing from 60 to 140oC and a pressure swing from 16 to 1bar, which concludes in the reversibility of the reaction and reuse of the absorbent.59

Supplementary information The supplementary information for this article can be found online at ACS publications website, including: Table S1. Density of [P4444][Gly], PEG400, and blends as a function of increasing ionic liquid concentration in the blend at 20, 30, 40, 60, and 80oC Table S2. Viscosity of the systems (PEG400, 70wt%PEG400-30%[P4444][Gly], 50wt%PEG400-50wt%[P4444][Gly], 30wt%PEG400-70wt%[P4444][Gly]) as a function of temperature. Table S3. Effect of pressure on CO2 loading (molCO2/molabs) at a temperature of 60oC for PEG400, 70wt%PEG400-30wt%[P4444][Gly], 50wt%PEG400-50wt%[P4444][Gly], 30wt%PEG400-70wt%[P4444][Gly]. Table S4. Effect of temperature on CO2 solubility in 70wt%PEG400-30wt%[P4444][Gly].



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Table S5. Effect of blend composition on CO2 solubility at 100oC. Table S6. Effect of blend composition on CO2 solubility at 140oC.

Acknowledgement This work is supported by the Research Council of Norway through the CLIMIT program (MCIL-CO2 project, 215732).

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Optimization and Characterization of an Amino Acid Ionic Liquid and

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