Optimization and Characterization of an Amino Acid Ionic Liquid and

Article pubs.acs.org/IECR

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*,† †

Department of Chemical Engineering, Norwegian University of Science and Technology, Sem Sælandsvei 4, 7491 Trondheim, Norway ‡ Shanghai Research Institute of Petrochemical Technology, 1658 Pudong Beilu, Shanghai 201208, China S Supporting Information *

ABSTRACT: Amino acid ionic liquids are green solvents with low toxicities and properties suitable for precombustion CO2 capture, such as high CO2 absorption, good thermal stability, and negligible vapor pressure. However, their high viscosities and relatively high costs have hampered their industrial applications. In this work, we systematically studied the amino acid ionic liquid tetrabutylphosphonium glycinate ([P4444][Gly]) and its blends with the low-cost and less viscous cosolvent poly(ethylene glycol) (PEG400). The concentration of the blend solvent was optimized with respect to CO2 solubility, regeneration efficiency, and cyclic capacity. The solubilities of CO2 in [P4444][Gly], PEG400, and their blends of four different concentrations were measured experimentally in the temperature range of 60−140 °C and up to a pressure of 17 bar. The results showed that the CO2 solubility increased with increasing ionic liquid concentration in the blend and decreased with increasing temperature. The optimum CO2 absorption was determined to occur at 30 wt % of [P4444][Gly] in the blend. The regeneration study of the 30 wt % [P4444][Gly]−70 wt % PEG400 blend for three cycles verified its reusability in the process and confirmed that the reaction between [P4444][Gly] and CO2 can be reversed at 140 °C. The CO2 absorption capacity of the blend absorbent was found to be up to a loading of 1.23 mol of CO2/mol of absorbent. The parameter fitting of the experimental data using empirical correlations was evaluated, and these correlations were developed in particular to predict the blend solvent of an amino acid ionic liquid−PEG400 system based on the ionic liquid concentration and temperature.

1. INTRODUCTION Prolific carbon dioxide emissions due to a strong dependence on fossil fuels for energy requirements have led to global climate change. These CO2 emissions originate from the combustion of fossil fuels in the petroleum industry, power plants, and the petrochemical industry.1 Such emissions can be substantially reduced by employing carbon dioxide capture, which is an energy-intensive process and demands higher operating 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, precombustion, postcombustion, and oxy-fuel combustion processes. These processes vary in terms of the source and conditions of CO2 capture. Postcombustion CO2 capture is a widespread and wellstudied capture method. However, the precombustion CO2 capture process offers the advantages of higher CO 2 concentrations and hence higher CO2 partial pressures, resulting in a greater driving force for separation and smaller footprints. Several technologies are being investigated for CO2 capture on the laboratory and industrial scales, including © 2016 American Chemical Society

absorption, adsorption, cryogenics, and membrane separations.3−11 Of these, the chemical absorption of CO2 is the most investigated and mature technology.12,13 Aqueous alkanolamine solutions are considered to be established chemical absorbents in the industry for CO2 capture because of their high reactivity and low costs. However, aqueous amine solutions face obstacles in their application because of their low cyclic capacities, equipment corrosion, solvent degradation, high solvent regeneration costs, and loss of solvent by entrainment. To overcome these challenges, a new class of solvents known as ionic liquids is 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 temperatures of less than 100 °C. They have appealing and diverse characteristics such as undetectable vapor pressures, high thermal and chemical stabilities, tunable characteristics, Received: Revised: Accepted: Published: 12080

June 27, 2016 October 17, 2016 October 26, 2016 October 26, 2016 DOI: 10.1021/acs.iecr.6b02457 Ind. Eng. Chem. Res. 2016, 55, 12080−12090

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Industrial & Engineering Chemistry Research

measured up to a temperature of 200 °C. The densities of these blends were measured for the temperature range of 20−80 °C at ambient pressure, and their viscosities were tested for the temperature range of 20−90 °C. The density, viscosity, and solubility data were fitted by concentration- and temperaturebased empirical correlations. The models were found to fit the experimental data very well. The CO2 absorption capacities in these blends were tested at 60, 100, and 140 °C and at pressures up to 18 bar. CO2 absorption experiments were also run in a decreasing pressure range to validate the experimental setup and procedure. The regeneration of the absorbent is a key consideration in the selection of an absorption process. The regeneration of the 30 wt % [P4444][Gly]−70 wt % PEG400 blend was investigated for three cycles of absorption− desorption. Absorption was maintained at 60 °C, and desorption was performed at 140 °C for the measurements. The cyclic capacities and CO2 removal efficiencies of [P4444][Gly], PEG400, and their blends were evaluated.

and strong absorption capacities. 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 toxicities and properties suitable for precombustion CO2 capture, such as high CO2 absorption, good thermal stability, and negligible vapor pressure.35,36 Amino-acid-based ionic liquids have a N− H group similarly to 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 to be 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, whereas the second step involves proton sharing between the oxygen atoms of CO2 and the carboxyl group of the anion. However, the high viscosities and relatively high costs of amino acid ionic liquids have become major obstacles in the development of their industrial applications. Phosphoniumbased amino acid ionic liquids have viscosities that are lower than those of other types of ionic liquids but still higher than those of amines.38 One way to deal with the high viscosity is to blend the amino acid anion ionic liquids with low-molecularweight organic solvents to reduce the viscosity. Li et al.39 investigated the amino acid (sodium glycinate)− ionic liquid ([Bmim][BF4]) system 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 and Khanna35 reported the absorption of in aminated anion ionic liquids, which resulted in a CO2 absorption capacity (mass of CO2/mass of ionic liquid) in amino acid ionic liquids of 70% more than that in primary amine−cation ionic liquids and 600% more than that in tertiary amine−cation ionic liquids. Zhang et al.40 studied four dicationic amino acid ionic liquids for CO2 absorption and found the nearly equimolar absorption of CO2 in the ionic liquids accompanied by the reversibility of the absorbent. They also addressed the enhancement in CO2 absorption capacity when water was added to the ionic liquids because of the decrease in the viscosity of the liquid. Poly(ethylene glycol)s have been investigated for CO2 solubility.40 Considering the above-mentioned characteristics and our previous study,41 the amino-acid-based ionic liquid tetradecylphosphonium glycinate [P4444][Gly] is proposed for precombustion CO2 capture applications at high temperature and pressure. However, because ionic liquids have high viscosities, which make them challenging for this application, the aminoacid-based ionic liquid [P4444][Gly] was blended with the lowmolecular-weight polymer poly(ethylene glycol) (PEG400) in this work to reduce the viscosity of the absorbent and enhance the CO2 sorption. The PEG400 has a CO2 absorption capacity comparable to those of ionic liquids, good thermal stability, and negligible vapor pressure,42 and [P4444][Gly] has been reported to react chemically with CO2;37 thus, blends of [P4444][Gly] and PEG400 will combine both chemical absorption and physical sorption, so a CO2 absorption capacity higher than that obtained in either solvent is expected. The present work investigated and optimized the CO2 absorption capacity for blends of [P4444][Gly] and PEG400 under precombustion CO2 capture conditions. The thermal stabilities of the amino acid ionic liquid−PEG400 blends were

2. EXPERIMENTAL SECTION 2.1. Materials. Carbon dioxide was purchased from Praxair with a nominal purity of 99.99%. Poly(ethylene 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 was the same as described in refs 38 and 43. The other raw materials used for synthesis were tetrabutylphosphonium hydroxide solution (40 wt % in water), acetonitrile (99.8%), methanol (99.8%), and deuterium oxide (99.9 atom %), which 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 laboratory using the same method as reported in refs 43 and 44. A solution of tetrabutylphosphonium hydroxide [P4444][OH] (with 40% water by weight) was mixed dropwise into a marginally excess equimolar aqueous glycine solution. This mixture was allowed to stir for 12 h at room temperature to become well mixed. Water evaporation of this mixture was carried out at 45 °C in a rotary evaporator for 5−6 h. A mixture of acetonitrile and methanol in a volume ratio of 9:1 was poured into this water-evaporated mixture, and the overall mixture was stirred vigorously for 9 h. 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 70 °C for 48 h. The purity and chemical identity of the ionic liquid was analyzed by NMR spectrometry using a Bruker Avance DPX 400 MHz 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 an NMR tube and subjected to ultrasonic mixing to ensure a constant concentration of the sample. The 1 H NMR spectrum was obtained at 25 °C with 16 scans, a pulse duration of 10.5 μs, and an acquisition time of 3.95 s. The 1H NMR spectrum was analyzed using TopSpin software. The 1H 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. The first chemical shift at 0.92 ppm represents four CH3 groups with 12 protons attached to the P atom, and the second 12081

DOI: 10.1021/acs.iecr.6b02457 Ind. Eng. Chem. Res. 2016, 55, 12080−12090

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Figure 2. Experimental setup for CO2 solubility measurements: (1) gas cylinder, (2,3) oil bath, (4) gas storage vessel, (5) absorption vessel, (6) magnetic stirrer, (7) pressure transmitter, (8) computer program. Reproduced with permission from ref 46.Copyright 2012 American Chemical Society.

was observed with a mounted pressure transducer (series 33X, Keller Co.) with an accuracy limit of ±0.3 kPa. A magnetic stirrer was used in the gas absorption vessel to stir the mixture at a speed of 450 rpm. The whole apparatus was first evacuated using a vacuum pump for about 1 h and was checked for leaks. The gas was then stored in the vessel at a certain pressure, and the 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 denoted as Pe. The difference in pressure values (Ps − Pe) gives the amount og CO2 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 was estimated from the difference between the number of moles of CO2 in the gas vessel and the number of moles of CO2 in the gas phase at equilibrium. The same procedure was employed for all of the data points. Desorption experiments were performed when the system was at the maximum observed pressure. The gas was released into the vacuum line by closing the middle valve first. Then, the gas storage vessel was allowed to achieve a 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 experimental data were recorded using the LabView computer program. 2.2.6. Error Analysis. The error analysis for density, viscosity, and CO2 solubility measurements was performed by calculating the ratio of the 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 stated in the experimental procedure, CO2 gas at various pressures was filled into the gas vessel manually in this study. It was challenging to replicate the exact same pressure point for the CO2 solubility measurements for each temperature and concentration of ionic liquid because of the difficulty in controlling the pressure. For this reason, at least two measurements were commonly performed to produce the data for CO2 solubility and ensure accuracy, but it was not possible to calculate the standard deviations for variables. The evaluated maximum uncertainties for the reported CO2 loading value and equilibrium partial pressure of CO2 are 3% and 5%,

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

shift in the range of 1.34−1.52 ppm represents four CH2 groups with 16 protons. The third chemical shift at 2.13−2.23 ppm indicates CH2 attached to P with eight protons. The last chemical shift at 3.0 ppm corresponds to the functional group N−CH2−CO2 with two protons. Through a comparison of the adjusted results and the literature, the figure clearly shows four peaks with δ values on the x axis similar to those reported by Zhang et al.45 The 1H NMR results agree with the literature and indicate the successful synthesis of [P4444][Gly]. There were also two small hetero peaks in the scan, but they are insignificant. The synthesis and refining of the ionic liquid [P4444][Gly] was thus authenticated by the 1H NMR data. 2.2.2. Density. The densities of all of the samples were measured on an Anton Paar DMA 4500 M density meter within the temperature range of 20−80 °C at atmospheric pressure. This density meter consists of a U-tube with a Pt-100 thermocouple (accuracy of ±0.01 °C). The measuring volume was approximately 9.5 mL, and the sample needs to be placed in a test vial and covered with 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 of the samples were tested within the temperature range of 20−90 °C at atmospheric pressure with an accuracy of ±2%. 2.2.4. Thermal Decomposition Temperature Measurements. The thermal decomposition temperatures of the ionic liquids and their blends with PEG400 were measured with a thermogravimetric analyzer (Q500, TA Instruments) with an accuracy of ±0.01%. A known amount of sample (10−15 mg) was placed in a platinum pan, and thermogravimetric analysis (TGA) measurements were performed under a nitrogen atmosphere (60 mL/min) over the temperature range of 21− 400 °C at a heating rate of 10 °C/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 consisted of two steel vessels, one for gas storage (80.7 ± 0.1 cm3) and the other for gas absorption into liquid (33.9 ± 0.1 cm3). The temperatures of both vessels were controlled by silicon oil baths with an accuracy of ±0.1K. The pressure of the system 12082


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Industrial & Engineering Chemistry Research respectively, according to the deviation found from the reproduction of at least two measurements.

Table 1. Model Parameters for the Density Correlation in Eq 1 as a Function of the Mole Fraction of the Ionic Liquid in the Blend

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 was employed to estimate the CO2 solubility in the ionic liquid and blends, so the density of the solvent was required to calculate the gas-phase volume. The densities of [P4444][Gly], PEG400, and their blends were measured for the temperature range of 20−80 °C with an accuracy of ±1% under ambient-pressure conditions (see Table S1 in the Supporting Information). The densities of deionized water and the [P4444][Gly]−PEG400 blends were also measured to ensure the reliability of the apparatus and validate the experiments. The density data for deionized water agreed well with the literature with a standard deviation of ±0.2%. The measured density data were fitted as a function of the mole fraction of the ionic liquid at different temperatures using the empirical correlation47 ⎡b b b ⎛ ω ⎞2 ⎤ ρ = b1 + 2 (1 − ω1) exp⎢ 3 + 4 ω1 + b5⎜ 1 ⎟ ⎥ ⎝T ⎠ ⎦ T T ⎣T (1)

parameter

value

b1 b2 b3 b4 b5 AARD (%)

0.8142 78.1176 12.8995 36.2356 −2.5529 0.166

the density decreases as the concentration of PEG400 in the blend decreases and as the temperature increases. The density correlation given in eq 1 fitted the experimental data very well with an AARD of 0.166% for all of 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.

where ω1 is the weight fraction of the ionic liquid [P4444][Gly] in the blend and b1, b2, b3, b4, and b5 are fitted parameters of eq 1. The density data and fitting are presented in Figure 3. The

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

The density of PEG400 measured in this work was compared with the literature48−50 to verify the accuracy of the test apparatus, as shown in Figure 5.48−50 For example, Wu et al.48 reported the density of PEG400 as 1.1122, 1.0973, and 1.0815 g/cm3 at 40, 60, and 80 °C, respectively, which are in good agreement with the values observed in this work. Density data for [P4444][Gly] are not available in the literature. 3.2. Viscosity. Viscosity is an important parameter in the selection of the solvent for CO2 capture. The viscosities of the ionic liquid [P4444][Gly], PEG400, and their blends were measured for the temperature range of 20−90 °C (Table S2). The results for these measurements are presented in Figure 6. The experimental viscosity data were correlated by the equation47

Figure 3. Densities of [P4444][Gly], PEG400, and blends as a function of increasing ionic liquid concentration in the blend at (olive triangles) 20 °C, (blue plusses) 30 °C, (red squares) 40 °C, (gray circles) 60 °C, and (green diamonds) 80 °C. The uncertainty in the density is 0.05%.

objective function is defined as the absolute average relative deviation, given by AARD (%) =

1 n

n

∑ i=1

|ρexp, i − ρmod, i | ρexp, i

× 100

⎛ ⎛ k k kxx ⎞ k1x1 k x x 2⎞ η = ⎜1 + + 2 12 2 ⎟ exp⎜1 + 3 + 42 + 5 13 2 ⎟ ⎝ T T T ⎠ T T ⎠ ⎝

(2)

where the subscripts exp and mod indicate the experimental and modeled results, respectively. The fitted parameters of eq 1 are provided in Table 1. At a constant temperature, the density of PEG400 is greater than that of [P4444][Gly]. The results in Figure 3 indicate that

(3)

and the fitted parameters are presented in Table 2. The criterion for the fitting was to minimize the sum of square error deviation, where the deviation is given as by (ηexp − ηmod)/ηexp. 12083


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had a maximum value of 358.4 mPa·s at a temperature of 20 °C, and this decreased to 22.5 mPa·s at a temperature of 90 °C. Under these conditions, the viscosities of the absorbents became comparable to those of aqueous alkanol amine solutions. From Figure 6, it can be observed that the viscosity decreased dramatically at temperatures lower than 70 °C, and then the reduction became less significant at higher temperatures. 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.

Figure 5. Densities of PEG400 in the temperature range of 20−80 °C and comparison with literature data: (black squares) this work, (blue triangles) Wu et al.,48 (red circles) Zhang et al.,50 (green triangles) Han et al.49

Figure 7. Parity plot between the experimental and modeled viscosities.

3.3. Thermal Decomposition Temperature. As the precombustion process takes place under high-temperature conditions (180−200 °C), the thermal stability of an absorbent at a temperature of 200 °C is a critical consideration in the selection of the absorbent. Conventional alkanol amine absorbents are reported to either experience degradation or cause equipment corrosion at elevated temperatures.51−53 However, ionic liquids are generally claimed to have high thermal stability.54,55 The thermal stabilities of the ionic liquid [P4444][Gly], PEG400, and their blends were determined by thermogravimetric analysis. The results obtained are presented in Figure 8. The point of thermal degradation is termed Tonset and was calculated using TA Instruments’ Universal Analysis software. Tonset is defined by the intercept of the baseline of the weight loss and the tangent line derived from the weight loss versus temperature curve. The actual thermal degradation is said to start before Tonset.56 The Tonset values of all of the samples tested are listed in Table 3. All of these samples showed good thermal stability up to 200 °C. The thermal stability of pure PEG400 was the best among all of the samples. The pure ionic liquid [P4444][Gly] exhibited the 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 effects of pressure on the CO2 solubility in mixtures of [P4444][Gly] and

Figure 6. Viscosities of the systems as a function of temperature: (black squares) PEG400, (red circles) 70 wt % PEG400−30 wt % [P4444][Gly], (blue triangles) 50 wt % PEG400−50 wt % [P4444][Gly], (green triangles) 30 wt % PEG400−70 wt % [P4444][Gly], (lines) fitted model. The uncertainty in the viscosities is 3%.

Table 2. Fitted Model Parameters for Viscosity as a Function of the Mole Fraction of the Ionic Liquid in the Blend parameter

value

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 increased as the blend became more concentrated in the ionic liquid and decreased as the temperature rose. The viscosity of PEG400 was the lowest among all of the solvents tested, and that of [P4444][Gly] with the lowest amount of PEG400 was the highest. The viscosity of the [P4444][Gly]−PEG400 mixtures 12084


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mole of absorbent. The CO2 solubilities in the [P4444][Gly] ionic liquid and PEG400 were correlated by the following empirical correlation based on concentration- and temperaturedependent terms B ln PCO2 = A ln α + k1 + 1 + k 2α k3 (4) In this correlation, A and B are concentration-dependent parameters, defined by the equations A = A 0 + A1ω + A 2 ω 2

(5)

B = B0 + B1ω + B2 ω 2 + B3ω3

(6)

whereas k1, k2, and k3 the functions of temperature, given by the equations Figure 8. Thermal gravimetric analysis of [P4444][Gly], PEG400, and blends (N2 flow = 60 mL/min): (pink line) [P4444][Gly], (black line) PEG400, (red line) 30 wt % [P4444][Gly]−70 wt % PEG400, (blue line) 50 wt % [P4444][Gly]−50 wt % PEG400, (green line) 70 wt % [P4444][Gly]−30 wt % PEG400.

a

Tonset (°C)

0 0.3 0.5 0.7 1.0

280.97 262.70 270.70 281.14 296.30

(7)

⎤ ⎡ ⎛1⎞ k 2 = exp⎢k 2a⎜ ⎟ + k 2b⎥ ⎝ ⎠ ⎦ ⎣ T

(8)

k 3a + k 3b (9) T The fitted parameters derived from the experimental data for this correlation are reported in Table 4. Figure 9 indicates that

Table 3. Tonset Values of [P4444][Gly], PEG400, and Their Blends ωa

⎛1⎞ k1 = k1a⎜ ⎟ + k1b ⎝T ⎠

k3 =

Table 4. Fitted Parameters for CO2 Solubility in the System as a Function of Weight Fraction and Temperature

Weight fraction of [P4444][Gly] in the blend.

PEG400 were investigated, and the experimental results of CO2 solubility in terms of loading up to a pressure of 18 bar are presented in Figure 9 (also see Table S3). The loading of CO2 (α) is represented in terms of the number of moles of CO2 per

parameter

value

parameter

value

A0 A1 A2 B0 B1 B2 B3

0.9074 1.1680 7.5970 −0.4795 8.7843 −69.530 36.547

k1a k1b k2a k2b k3a k3b

−1028.481 11.908 −49.318 2.2616 1370.94 −1.6161

increasing pressure results in an increasing trend in CO2 solubility. This incremental trend in CO2 loading with respect to pressure is true for both the cases of physical absorption (PEG400) and chemical absorption ([P4444][Gly] blends with PEG400). The loadings of CO2 were observed to be within the range of 0.013−1.051 molCO2/molabs for the pressure range of 0.87−17.36 bar at 60 °C. The high solubility of the ionic liquid in 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 [P4444][Gly] in the blend was the highest (70%). The chemical reaction between the ionic liquid and CO2 takes place at lower pressures, so that a further increase in pressure could enhance the CO2 solubility only through the physical absorption of CO2. It was observed that the time required for the system to attain equilibrium was longer at low pressures. This effect could be a result of the increase in viscosity with increasing concentration of ionic liquid in the blend and the formation of carbamate species through the chemical reaction taking place between the ionic liquid and CO2. A higher viscosity of the absorbent limits the mass-transfer and reaction kinetics, thus

Figure 9. Effects of pressure on the CO2 loading (molCO2/molabs) at a temperature of 60 °C: (gray squares) PEG400, (red circles) 70 wt % PEG400−30 wt % [P4444][Gly], (blue triangles) 50 wt % PEG400−50 wt % [P4444][Gly], (pink triangles) 30 wt % PEG400−70 wt % [P4444][Gly]. The uncertainties in the CO2 loading values and equilibrium partial pressures of CO2 are 3% and 5%, respectively. 12085


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Industrial & Engineering Chemistry Research resulting in a significantly longer time required to attain equilibrium. 3.5. Effects 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 the absorbent composition are key factors in carefully 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 of 60, 100, and 140 °C. The

Figure 10. Effects of temperature on the CO2 solubility in 70 wt % PEG400−30 wt % [P4444][Gly]: (pink triangles) 60 °C, (black circles) 100 °C, (blue triangles) 140 °C. The uncertainties in the CO2 loading values and equilibrium partial pressures of CO2 are 3% and 5%, respectively.

experimental data are presented as data points, and the lines are the fitted experimental data resulting from the correlation defined by eq 4. The CO2 absorption capacity dropped as the temperature was increased from 60 to 140 °C (Table S4). The CO2 loading was found to have a value of 0.6 at 60 °C, which was the maximum value for all of the temperatures studied in this work. The CO2 solubilities for each concentration of ionic liquid in the blend are presented in panels a and b of Figure 11 at temperatures of 100 and 140 °C, respectively (Tables S5 and S6). The CO2 solubility was enhanced as the weight percentage of the ionic liquid in the blend increased at all of the pressures and temperatures studied. In Figure 11b, one can see that the CO2 loading in PEG400 was the minimum (0.28). The addition of the ionic liquid increased the CO2 absorption as a result of 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] contains a glycine anion, the reaction between the ionic liquid and CO2 should have the same stoichiometry as that between a primary amine and CO2 (i.e., a CO2 loading of 0.5). It can be observed from Figure 11a that a CO2 loading of 0.5 could be achieved only for concentrations of ionic liquid in the blend higher than 50 wt % at all pressures and 60 °C. In the case of 30 wt % of the ionic liquid in the blend, this ratio could be attained at CO2 pressures greater than 16 bar. The maximum CO2 loading achieved was

Figure 11. Effects of blend composition on the CO2 solubility at (a) 100 and (b) 140 °C: (gray squares) PEG400, (red circles) 70 wt % PEG400−30 wt % [P4444][Gly], (blue triangles) 50 wt % PEG400−50 wt % [P4444][Gly], (pink triangles) 30 wt % PEG400−70 wt % [P4444][Gly]. The uncertainties in the CO2 loading values and equilibrium partial pressures of CO2 are 3% and 5%, respectively.

1.08 in the case of 70 wt % of the ionic liquid in the blend, which represents about 50% higher absorption capacity than for conventional amines. The plots for the blends with 50 and 70 wt % of the ionic liquid show larger plateaus 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 decreased as the temperature was increased. The CO2 loading was reduced to 0.7 from 1.08 when the temperature approached 140 °C. 3.6. Cyclic Capacity of the Solvent. As CO2 absorbents are intended for use 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 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 12086


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system was 0.22, which is smaller than the cyclic capacity of the absorbent in this work. 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 not an issue for this system, which makes the absorption process significantly more energyefficient. The 30 wt % [P4444][Gly]−70 wt % PEG400 blend was investigated in a regeneration study. CO2 was first absorbed into the blend absorbent at a temperature of 60 °C and a pressure of about 16 bar and desorbed CO2 at a temperature of 140 °C and a pressure of 1 bar. 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 the three cycles are shown in Figure 13, where the CO2 loading is plotted

capacity for the optimization of the blend composition. The cyclic capacity and CO2 removal efficiency for all four concentrations of [P4444][Gly]−PEG400 were studied, as presented in Figure 12. The cyclic capacity of an absorbent can be determined by the difference between the CO2 loading under absorption conditions and that under stripping conditions.

Figure 12. Performance analysis of solvents according to the (blue squares) CO2 removal efficiency and (black circles) cyclic absorption capacity.

The CO2 removal efficiency was calculated as CO2 removal efficiency = × 100

CO2 absorbed − CO2 stripped CO2 absorbed (10)

The cyclic capacity experiments were carried out by testing CO2 absorption at 60 °C and stripping at 140 °C, with relative propagation errors of 3.2% and 5.2% for the cases of physical and chemical absorption, respectively. It can be observed from Figure 12 that the CO2 removal efficiency decreased from 96% to 12% as the concentration of the ionic liquid in the blend was enhanced, but the cyclic capacity followed the opposite trend up to an ionic liquid concentration of 50 wt %. The cyclic capacity was reduced as the concentration of ionic liquid was increased beyond 50 wt %. The reason for this finding could be that equilibrium was not achieved completely in the tests of the solvents with higher concentrations of ionic liquid. The performances of the blends with 30 and 50 wt % of the ionic liquid were found to be comparable, which indicates that the best performance of the absorbent in terms of cyclic capacity and CO2 removal efficiency can be achieved for 30−50 wt % of the ionic liquid in the blend. Given that the ionic liquid is quite viscous compared to PEG400 and that a higher amount of ionic liquid in the blend will lead to a higher viscosity of the blend and, hence, low mass-transfer rates, the blend with 30 wt % of the ionic liquid was considered to be the optimal absorbent and was further studied in this work. Pinto et al.57 conducted experiments for the absorption and desorption of ionic liquids that function as physical and chemical absorbents. According to their studies, the cyclic capacity of an ionic liquid follows the same trend as shown here. The cyclic capacity reaches a minimum for the highest concentration of chemical absorbent. The maximum attainable cyclic capacity in their

Figure 13. Regeneration efficiency of the solvent 30 wt % [P4444][Gly]−70 wt % PEG400 in three absorption−desorption cycles involving CO2 absorption at 60 °C and desorption at 140 °C.

against the number of regeneration cycles. It can be clearly seen from Figure 13 that there were no appreciable differences in the CO2 loading and absorbent among the three cycles. The differences in the CO2 loading between the first and second cycles as well as between the second and third cycles were fairly small and within the range of the error bars (5%), most likely resulting from the uncertainty in measuring the pressures. Moreover, the tested blend solvent exhibits both physical and chemical absorption, so the absorption capacity can also be expected to vary to a small extent. These results also indicate that chemical reaction between the ionic liquid and CO2 can be reversed at high temperature (140 °C) and that the solvent can be reused during the absorption process. The regeneration efficiency of a combined physical- and chemical-absorbent solvent (an amino-acid-based ionic liquid) is reported by Li et al.,39 and according to their findings, the absorbent lost 19% of its CO2 absorption capacity after the first regeneration cycle. In contrast, the regeneration efficiency of the absorbent in this work was approximately 95% after the first cycle and 87.9% after three cycles. 3.8. Comparison with the Literature. The blends of amino acid ionic liquid and PEG400 in this work showed 12087


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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. A correlation derived on the basis of concentration- and temperature-dependent terms fitted the experimental data well 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 140 °C and a pressure swing from 16 to 1 bar, which demonstrated the reversibility of the reaction and possibility for reuse of the absorbent.59

proficient CO2 solubility, indicating that they are suitable for CO2 separations at elevated temperatures and pressures. However, data for blended solvents have scarcely been reported at similar conditions. A comparison is made in Figure 14 with the few reported data on blended amine−IL solvents, where the CO2 solubility is represented by the CO2 loading (moles of CO2 per mole of absorbent).

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02457. Densities of [P4444][Gly], PEG400, and blends as a function of increasing ionic liquid concentration in the blend at 20, 30, 40, 60, and 80 °C (Table S1), viscosities of the systems PEG400, 70 wt % PEG400−30 wt % [P4444][Gly], 50 wt % PEG400−50 wt % [P4444][Gly], and 30 wt % PEG400−70 wt % [P4444][Gly] as a function of temperature (Table S2), effects of pressure on the CO2 loading (molCO2/molabs) at a temperature of 60 °C for PEG400, 70 wt % PEG400−30 wt % [P4444][Gly], 50 wt % PEG400−50 wt % [P4444][Gly], and 30 wt % PEG400−70 wt % [P4444][Gly] (Table S3), effects of temperature on the CO2 solubility in 70 wt % PEG400−30 wt % [P4444][Gly] (Table S4), effects of blend composition on CO2 solubility at 100 °C (Table S5), and effects of blend composition on CO2 solubility at 140 °C (Table S6) (PDF)

Figure 14. Comparison of the CO2 solubility in the ionic liquid− PEG400 blend and the reported literature at 60 °C: (black squares) 70 wt % [P4444][Gly]−30 wt % PEG400, (red line) 30 wt % MEA−10 wt % [C2OHmim][DCA],58 (blue line) 4 M MDEA−1 M [gua][OTf].59

Monoethanolamine (MEA) and methyldiethanolamine (MDEA) are the most industrially used solvents for CO2 absorption. The results obtained in this work were compared with those reported for MEA−IL and MDEA−IL blend absorbents. The ionic liquid−PEG400 blend tested in this work exhibits a higher CO2 solubility than the reported 30 wt % MEA−10 wt % [C2OHmim][DCA]58 and 4 M MDEA−1 M [gua][OTf]59 blends {where [C2OHmim][DCA] is 1-(2hydroxyethyl)-3-methyl-imidazolium dicyanamide and [gua][OTf] is guanidinium trifluoromethanesulfonate}. Both of the reported ionic liquid−amine absorbents also chemically react with CO2. The 4 M MDEA−1 M [gua][OTf] blend was found to have a lower CO2 solubility than the 30 wt % MEA−10 wt % [C2OHmim][DCA] blend.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

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

4. CONCLUSIONS Four different blending ratios of [P4444][Gly] and PEG400 in blend solvents and the CO2 absorption behaviors of these solvents at temperatures of 60, 100, and 140 °C were reported in this work. The density of [P4444][Gly] was found to be less than that of water, whereas the density of PEG400 is more than those of both deionized water and [P4444][Gly]. The viscosity of the blend solvent decreased with increasing addition of PEG400. The blends of [P4444][Gly] and PEG400 exhibited high thermal stability in tests up to 200 °C, making them feasible for precombustion applications at elevated operating temperatures. CO2 absorption could be achieved to a loading of 1.23 molCO2/molabs at a temperature of 60 °C and a pressure of 17 bar when pure [P4444][Gly] was used, but the cyclic capacity of the solvent and its CO2 removal efficiency were low. The optimum CO2 absorption was observed within the concentration range of 30−50 wt % of [P4444][Gly] in the blend based

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