Article pubs.acs.org/jced
Characteristics of Potassium Prolinate + Water + Ethanol Solution as a Phase Changing Absorbent for CO2 Capture Yangyang Bian, Yue Zhao, and Shufeng Shen* School of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, P. R. China S Supporting Information *
ABSTRACT: Potassium prolinate + water + ethanol (ProK/W/Eth) solution has potential use as a liquid-to-solid phase-changing absorbent for energyefficient CO2 capture process. In this study, the equilibrium solubility of CO2 in ProK/W/Eth solutions was measured in a stirred equilibrium cell at a temperature range from 293 to 343 K and CO2 partial pressures up to 200 kPa. Density and viscosity were also measured at temperatures from 293 to 333 K and ProK molality up to 6.09 mol·kg−1 ethanol. Moreover, the CO2 loading and cyclic capacity were compared with aqueous monoethanolamine (MEA) and aqueous ProK solution. The solubility data were well-represented using a Soft model with AAD within 8.0%. The data of density and viscosity were also correlated with empirical models. The predicted results matched well the experimental data with AAD within 0.37% for density and 1.38% for viscosity.
1. INTRODUCTION For decades, global environmental problems especially global warming have been the focus of social controversy.1 Carbon dioxide emissions from industrial activities such as fossil fuelfired power plants are mainly contributors to global climate change. Chemical absorption method is one of the most feasible methods to reduce carbon emissions from power plants. Due to having chemically reactive amino group toward to CO2, amine-based solvents particularly monoethanolamine (MEA) have been widely used for the removal of CO2 from gas mixtures. Absorption and stripper columns connected in series constitute the typical CO2 capture process, where high energy consumption is required for water vaporization during the regeneration for solvent reuse.2−4 Hence, energy-efficient methods need to be developed for commercial deployment for CO2 capture process. In recent years, phase-changing absorbents have been recognized as potential solvents for energy-efficient capture process. When they react with CO2, the homogeneous liquid solution can produce a distinct liquid−liquid or liquid−solid phase separation. Under the ideal conditions, the absorbed CO2 can be enriched into one solid phase. Only regenerating CO2-rich phase can reduce the evaporation of solvents and energy consumption.5,6 Researchers have invested great interest for aqueous amino acid salts (AAS) and their precipitating systems (such as potassium of sarcosine,7−11 lysine,12,13 and proline7,10,14,15) as alternative solvents, since AAS have fast kinetics with CO2, along with good properties such as good resistance to oxidation and low toxicity. However, the promising process using precipitating amino acid solvents for CO2 capture has a limiting window of operation, where solid precipitation only occurred at higher concentrations and CO2 partial © 2017 American Chemical Society
pressures, for example, 4.0 M for potassium sarcosinate and 3.5 M for potassium prolinate at 293 K and 11.0 kPa CO2 partial pressure.10,11 Our latest study found that potassium prolinate + water + ethanol (ProK/W/Eth) solution as a novel phase-changing absorbent could easily enrich CO2 into a solid phase with high absorption capacity and fast kinetics.16,17 Physicochemical properties are essential for kinetics of CO2 absorption, simulation, and design of capture process. The solubility of CO2 in aqueous amino acid salts such as lysinate,12,13,18 prolinate,12,14,15,19 and sarcosinate9,10,20 has been widely reported recently. However, the data of density, viscosity, and CO2 solubility for ProK/W/ Eth solutions have not been reported yet in literature. In this work, we characterized the ProK/W/Eth solution at various conditions. The CO2 solubility in ProK/W/Eth solutions was measured in a temperature range from 298 to 343 K and CO2 partial pressure range from 1.0 to 200 kPa. We also investigated the density and viscosity of CO2-free and CO2loaded (before solid precipitation) ProK/W/Eth solutions in a molality range of 0.66−6.09 mol·kg−1 ethanol and a temperature range of 293−333 K. The molality is defined as mols of ProK or CO2 per kg of ethanol, referred to m1 and m2, respectively. Experimental data of density and viscosity were correlated with empirical correlations. The soft model was used to represent the CO2 solubility data as a function of temperature and CO2 loading.
2. EXPERIMENTAL SECTION 2.1. Materials and Solution Preparation. L-Proline (Pro, 99.28% HPLC purity, CAS No.147−85−3), potassium hydroxide Received: March 16, 2017 Accepted: July 20, 2017 Published: August 4, 2017 3169
DOI: 10.1021/acs.jced.7b00267 J. Chem. Eng. Data 2017, 62, 3169−3177
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Table 1. Chemical Samples Used in This Study chemical name
molar mass (g·mol−1)
CAS no.
source
puritya (mass fraction)
L-proline potassium prolinate potassium hydroxide monoethanolamine ethanol sulfuric acid carbon dioxide
115.13 153.24 56.11 61.08 46.07 98.08 44.01
147-85-3
Aladdin Industrial Inc. synthesis in this work Aladdin Industrial Inc. Aladdin Industrial Inc. Aladdin Industrial Inc. Tianjing Yongda reagent Shijiazhuang Xisanjiao oxygen generation station
0.9928 >0.97c 0.9999 0.9912 0.9971 0.98 0.9999
1310-58-3 141-43-5 64-17-5 7664-93-9 124-38-9
analysis methodb HPLC titrationc trace metals analysis GC GC
a
The purity in mass fraction was provided by the supplier. bThe analysis method for mass fraction was provided by the suppliers. HPLC: highperformance liquid chromatography, GC: gas chromatography. cThe purity in mass fraction was analyzed by acid titration.
(KOH, Semiconductor grade, 99.99% metals basis), ethanol (EthOH, 99.71% GC purity), and monoethanolamine (MEA, 99.12% GC purity) were purchased from Aladdin reagent, China. S3 viscosity standard (Lot No. 15101) was purchased from CANNON Instrument Company, USA. Carbon dioxide (CO2, 99.99 mass %) was obtained from Shijiazhuang Xisanjiao oxygen generation station, China. Water was produced from Merck-Millipore Aquelix 5. All of the reagents were used without further purification. A chemical sample description is given in Table 1. The ProK/W/Eth solutions were prepared by dissolving proline in ethanol with an equimolar amount of KOH in a volumetric flask at 298 K. An electronic analytical balance (OHAUS, CP214) were used for weight measurements. The CO2-loaded solutions for density and viscosity measurement were prepared by bubbling CO2 from a gas cylinder in a flask, and samples were taken at different absorption time before precipitation occurred. The CO2 loading (α) of the solutions is defined as the moles of CO2 per mole ProK, mol CO2/mol ProK. In a modified Chittick CO2 apparatus,21 the amount of captured CO2 in the sample of known volume and mass was measured through the acid titration in the reaction flask which is connected to a graduated gas measuring tube and adjustable leveling bulb reservoir. The colored nonreactive liquid level is controlled by a digital pressure manometer (GM520, Shenzhen Jumaoyuan Science and Technology Co., Ltd.). Meanwhile, the concentrations of ProK were titrated using an automatic potentiometric titrator (ZDJ-5, INESA Scientific Instrument Co., Ltd.). 2.2. CO2 Solubility. The solubility of CO2 in ProK/W/Eth solutions was measured in a stainless steel equilibrium cell with stirrers for the gas phase and liquid phase. The apparatus mainly consists of a stainless steel vessel (Vv, 0.813 L) for storing the CO2 gas, a equilibrium cell (Ve, 1.335 L) for vapor− liquid−solid equilibrium of CO2 loaded ProK/W/Eth system, and two temperature-controlled water baths (±0.1 K), as shown in Figure 1. The pressures and temperatures of vessel and equilibrium cell were recorded by two pressure transducers (PX409-050AUSBH, Omega Engineering Inc.; GS4200-USB, 0−3.5 bar absolute, ESI) and PT-100 thermocouples (±0.1 K), respectively. In each experiment, the equilibrium cell was degassed by a vacuum pump (2XZ-1, Shanghai Shuang’e, China) at 278 K. Then, a known mass of solution (about 0.40−0.45 kg) was fed into equilibrium cell and quickly degassed again at 278 K until equilibrium cell pressure dropped to about 2.0 kPa. Then, the equilibrium cell was heated to the desired experimental temperature (298, 313, 333, 343 K). When the system came to vapor−liquid equilibrium at the experimental temperature, the stable pressure Pinitial was recorded as the
Figure 1. Experimental apparatus of CO2 solubility.
Figure 2. Comparison of CO2 solubility in 15.3 mass % MEA solution between this work and literature at 313.2 K: ■, this work, run 1 (15.0 mass % MEA used); ●, this work, run 2; red ○, Lee et al. (1976); green △, Shen and Li (1992); blue ◇, Portugal et al. (2009); pink ☆, Song et al. (2011); brown □, Aronu et al. (2011) (15.0 mass % MEA used).
solvent vapor pressure of CO2-free absorbent solution. The initial temperature and pressure in CO2 gas vessel were recorded as Tv and P1, respectively. Then, the CO2 was injected into equilibrium cell, and the pressure in the vessel decreased down to P2. The total amount of CO2 gas entered into the equilibrium cell can be calculated from eq 1. After vapor−liquid equilibrium reached, the total pressure Ptotal in equilibrium cell was obtained. The equilibrium partial pressure of CO2, P*CO2, in the gas phase can be calculated by the difference between the Ptotal and the Pinitial at the controlled temperature. At this point, 3170
DOI: 10.1021/acs.jced.7b00267 J. Chem. Eng. Data 2017, 62, 3169−3177
0.500 0.526 0.552 0.585 0.615 0.660 0.714 0.755 0.802 0.839 0.901 0.946 0.973 0.997 1.023 1.064
0.430 0.452 0.474 0.503 0.529b 0.568b 0.614b 0.649b 0.689b 0.721b 0.775b 0.813b 0.836b 0.857b 0.880b 0.914b
3171
11.26 12.20 12.78 13.73 13.69 14.20 15.92 18.40 20.04 24.73 34.27 43.41 53.52 62.52 74.85 97.62
Ptotal (kPa) 1.69 2.64 3.21 4.16 4.13 4.64 6.35 8.83 10.47 15.16 24.70 33.84 43.96 52.95 65.28 88.05
P*CO2 (kPa) 0.308 0.372 0.408 0.450 0.472 0.501 0.527b 0.565b 0.610b 0.644b 0.681b 0.712b 0.765b 0.803b 0.826b 0.846b 0.867b 0.905b
α (mol·mol−1) 0.359 0.433 0.475 0.524 0.549 0.583 0.613 0.657 0.710 0.749 0.792 0.828 0.890 0.934 0.960 0.984 1.009 1.053
m2 (mol·kg−1)
313.5 K
20.96 22.36 23.20 23.69 24.71 25.62 25.80 27.55 30.72 35.21 40.83 47.74 58.35 68.43 80.33 90.60 104.60 124.50
Ptotal (kPa) 2.07 3.48 4.32 4.81 5.82 6.74 6.91 8.67 11.83 16.32 21.94 28.85 39.47 49.54 61.45 71.72 85.71 105.62
P*CO2 (kPa) 0.234 0.307 0.370 0.405 0.424 0.467 0.495 0.521 0.558 0.600 0.632 0.667 0.695 0.741
α (mol·mol−1) 0.272 0.357 0.431 0.471 0.493 0.543 0.576 0.606 0.649 0.698 0.736 0.776 0.808 0.862
m2 (mol·kg−1)
333.5 K
48.69 50.56 53.04 55.80 57.28 60.66 62.87 63.64 66.77 73.99 81.47 90.52 102.92 124.10
Ptotal (kPa) 2.13 3.99 6.48 9.24 10.71 14.10 16.31 17.08 20.21 27.42 34.90 43.95 56.35 77.54
P*CO2 (kPa) 0.150 0.232 0.305 0.367 0.401 0.420 0.463 0.490 0.514 0.550 0.591 0.621 0.656 0.684 0.727
α (mol·mol−1) 0.174 0.270 0.355 0.427 0.467 0.488 0.538 0.570 0.598 0.640 0.688 0.723 0.763 0.796 0.846
m2 (mol·kg−1)
343.5 K
71.79 74.66 77.05 81.55 85.96 88.50 91.39 95.65 98.15 103.12 112.35 123.30 134.01 145.18 172.08
Ptotal (kPa)
1.61 4.49 6.88 11.38 15.78 18.33 21.22 25.48 27.98 32.95 42.18 53.13 63.83 75.00 101.90
P*CO2 (kPa)
α is the total CO2 loading in the system CO2 + ProK + water + ethanol solution; m1 is the molality of ProK and water in ethanol, and m2 is the molality of CO2 in the saturated potassium prolinate + water + ethanol solution, mol·kg−1 ethanol. Standard uncertainties u are u(a) = 0.010 mol·mol−1, u(Ptotal) = 0.25 kPa, u(P*CO2) = 0.25 kPa, u(T) = 0.2 K, u(m1) = 0.002 mol·kg−1, and u(m2) = 0.015 mol·kg−1. The initial pressures of equilibrium cell Pinitial are respectively 9.57, 18.89, 46.57, and 70.18 kPa at temperatures of 298.5, 313.5, 333.5, and 343.5 K. bIn these points, solid phase precipitates were observed. The reported α and m2 characterize total amount of CO2 in the liquid and solid phases.
a
m2 (mol·kg−1)
α (mol·mol−1)
298.5 K
Table 2. Experimental Values of Molality m2 and Loading α (Moles of CO2 per Mole of Potassium Prolinate) of CO2 in the Saturated Potassium Prolinate + Water + Ethanol Solution at the Molality of Potassium Prolinate and Water m1 = 1.440 mol·kg−1 Ethanol, Total Pressure Ptotal, Partial Pressure of CO2 P*CO2, and Temperature Ta
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DOI: 10.1021/acs.jced.7b00267 J. Chem. Eng. Data 2017, 62, 3169−3177
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Table 3. Experimental Values of Molality m2 and Loading α (Moles of CO2 per Mole of Potassium Prolinate) of CO2 in the Saturated Potassium Prolinate + Water + Ethanol Solution at the Molality of Potassium Prolinate and Water m1 = 3.359 mol·kg−1 Ethanol, Total Pressure Ptotal, Partial Pressure of CO2 P*CO2, and Temperature Ta 298.3 K
323.3 K
α (mol·mol−1)
m2 (mol·kg−1)
Ptotal (kPa)
P*CO2 (kPa)
0.321 0.370 0.425b 0.482b 0.534b 0.586b 0.736b 0.780b 0.826b 0.844b 0.863b 0.877b 0.888b
0.685 0.790 0.907 1.027 1.138 1.250 1.570 1.664 1.762 1.799 1.840 1.869 1.894
10.92 11.25 11.55 11.81 12.02 12.33 13.81 15.53 26.86 27.45 46.62 72.92 98.48
1.35 1.68 1.99 2.24 2.46 2.76 4.24 5.97 17.30 17.89 37.06 63.36 88.92
343.3 K
α (mol·mol−1)
m2 (mol·kg−1)
Ptotal (kPa)
P*CO2 (kPa)
0.152 0.229 0.307 0.372 0.435 0.508 0.583 0.651 0.693 0.732 0.757 0.782 0.810 0.858
0.323 0.489 0.653 0.793 0.927 1.083 1.242 1.385 1.475 1.559 1.612 1.666 1.725 1.827
27.16 27.80 28.58 30.68 33.45 34.68 38.84 50.96 62.43 82.77 93.00 105.90 122.40 178.50
1.98 2.62 3.40 5.50 8.27 9.50 13.66 25.78 37.25 57.59 67.82 80.72 97.22 153.32
α (mol·mol−1)
m2 (mol·kg−1)
Ptotal (kPa)
P*CO2 (kPa)
0.068 0.135 0.206 0.272 0.335 0.394 0.451 0.507 0.558 0.606 0.651 0.702 0.741
0.143 0.286 0.436 0.577 0.712 0.837 0.957 1.077 1.185 1.287 1.383 1.491 1.572
66.45 68.10 69.49 71.57 74.92 83.16 84.12 92.45 104.02 120.00 151.21 201.62 258.02
2.34 3.99 5.38 7.46 10.81 19.05 20.01 28.34 39.91 55.89 87.10 137.51 193.91
α is the total CO2 loading in the system CO2 + ProK + water + ethanol solution; m1 is the molality of ProK and water in ethanol, and m2 is the molality of CO2 in the saturated potassium prolinate + water + ethanol solution, mol·kg−1 ethanol. Standard uncertainties u are u(a) = 0.010 mol·mol−1, u(Ptotal) = 0.25 kPa, u(P*CO2) = 0.25 kPa, u(T) = 0.2 K, u(m1) = 0.002 mol·kg−1, and u(m2) = 0.034 mol·kg−1. The initial pressures of equilibrium cell Pinitial are respectively 9.57, 25.18, and 64.11 kPa at temperatures of 298.3, 323.3 and 343.3 K. bIn these points, solid phase precipitates were observed. The reported α and m2 characterize the total amount of CO2 in the liquid and solid phases. a
meter before daily measurement. The calibration was validated by measuring ethanol, the pure water, and S3 viscosity standard. 2.4. Viscosity Measurement. The viscosity of potassium prolinate/ethanol solution was measured by using a digital rolling ball microciscometer (Lovis 2000ME, Anton Paar) with the stated precision 1.0%. The temperature was controlled at 293.15−333.15 K with a precision of ±0.01 K. The viscosity measurement of pure ethanol, the pure water, and S3 viscosity standard was to validate the viscometer.
the total CO2 loading (α) can be obtained from the ratio of the mole of CO2 in the absorbent solution to the mole of ProK. Then, by adding more CO2 into the equilibrium cell, the new vapor−liquid equilibrium and a serial of data points for P*CO2 vs α can be obtained. The solubility of CO2 can be calculated as follows: nadded =
ng =
VV ⎡ P1 P⎤ ⎢ − 2⎥ RTV ⎣ z1 z2 ⎦
(1)
3. MODELS 3.1. Solubility Model Representation. The equilibrium solubility of CO2 for vapor−liquid−solid systems (CO2-loaded ProK/W/Eth solution) could be represented using an empirical Soft model as given in eq 5:
* (Ve − Vs) PCO 2 z 3RTR
(2)
* = Ptotal − Pinitial PCO 2
* = A ln α + k1 + ln PCO 2
nadded − ng α= nProK added
(3)
(4)
B 1 + k 2 exp( −k 3 ln α)
(5)
where P*CO2 is the equilibrium partial pressure of CO2, kPa; α is the CO2 loading, mol·mol−1; A and B are adjustable constants; k1, k2, and k3 are the functions of temperature. This empirical model has been successfully used to predict the CO2 partial pressure for the PZ−AMP−H2O and CO2−KSar−H2O systems.20,23 3.2. Density and Viscosity Models. To better understand the effect of ProK or CO2 concentration, the ratio ρsolution·ρethanol−1 and ηsolutionηethanol−1 were used to correlate the experimental data.24 ρsolutionρethanol−1 and ηsolutionηethanol−1 were respectively referred as the ratios of density and viscosity of CO2-free (or CO2-loaded) ProK/W/Eth solution to those of solvent ethanol. The effect of temperature can be minimized using the ratio. Then, the ratios can be correlated as a function of the ProK or CO2 concentration from experimental data. The expressions can be obtained as follows:
g
where n and n are the added amount of CO2 and the amount of CO2 in the gas phase in the equilibrium cell, respectively. P*CO2 is the CO2 partial pressure of in the gas phase in equilibrium cell. z1, z2, and z3 are the compressibility factors of CO2 gas, which are calculated by the Peng−Robinson equation of state using the critical temperature of 304.21 K, critical pressure of 7383 kPa, and acentric factor of 0.2236.22 It should be pointed out that, in the case of solid formation, the CO2 loading (α) is referred to the ratio of the total moles of CO2 in the system (both liquid and solid phase) to the mole of ProK. 2.3. Density Measurement. A digital oscillating tube dendimeter (DMA-4100 M, Anton Paar) having a stated precision of ±1.0 × 10−4 g·cm−3 was used to measure the densities of potassium prolinate/ethanol solutions. The temperature was controlled at 293.15−333.15 K with a precision of ±0.01 K. Dry air and deionized water were used to check the density
ρsolution ρethanol 3172
= b1 + b2m + b3m2
(6) DOI: 10.1021/acs.jced.7b00267 J. Chem. Eng. Data 2017, 62, 3169−3177
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Table 5. Comparison of the CO2 Loading and Cyclic Capacity for Different Absorption Systemsa absorbent system
loading capacity (mol CO2/mol ProK or MEA)
cyclic capacity (mol CO2/mol ProK or MEA)
absorption− desorption temperature (K)
0.80
0.50
298−343
1.32b 0.52
0.30
313−393
0.81
0.37
298−383
3.359 m1 ProK, this work 3.359 m1 ProK17 aqueous 30 mass % MEAc aqueous 27.4 mass % ProKd a
Cyclic capacity is defined as the difference of CO2 loadings at a given CO2 partial pressure 10 kPa at absorption and desorption temperature, respectively. bData from Shen et al. (2017).17 The loading capacity is defined as moles of CO2 per kilograms of sample solution. cData from Aronu et al. (2011).29 dData from Majchrowicz et al. (2012).34
The similar models have been applied in aqueous (piperazine + CO2) solutions.24 The average absolute deviation deviations (AAD) between experimental data and the model predicted can be calculated by AAD(%) =
1 n
n
∑ i=1
yi exp − yi mod yi exp
(8)
yiexp
where n is the numbers of experimental points and and yimod are the experimental data and model predictions, respectively.
4. RESULTS AND DISCUSSION 4.1. Reliability Test of the Experimental Method. To validate the experimental method in this work, the solubility of CO2 in 15 mass% (2.5M) MEA at 313.2 K was measured in the equilibrium apparatus and compared with the data in open literature.25−29 The results are shown in Figure 2 and Supporting Information (SI) Table S1. It can be observed that the experimental results are in good agreement with the published data in the investigated pressure range. Therefore, the solubility method and apparatus are feasible and reliable. Experimental data of density and viscosity of ethanol, deionized water, and S3 viscosity standard at temperatures of 293−353 K were also measured and compared with the literature data.30−33 The experimental data and the comparison with literature data are listed in SI Table S2. It can be seen that the measured data match well with the values from literature and the AADs are within 0.05% for density and within 1.50% for viscosity. 4.2. Equilibrium Solubility of CO2 in ProK/W/Eth Solutions. Equilibrium solubility data are essential to a novel
Figure 3. Solubility of CO2 into the ProK + water + ethanol solution. 1.440 mol·kg−1 ProK(m1) (a): ■, 293.5 K; red ●, 313.5 K; brown ▲, 333.5 K; blue ▼, 343.5 K; , calculated from soft model. 3.359 mol·kg−1 ProK(m1) (b): blue ▲, 293.3 K; red ▼, 323.3 K; brown ◆, 343.3 K; − − −, calculated from soft model 1; , predicted from soft model 2 correlated only at 293.3 K in Table 4.
⎛η ⎞ ln⎜⎜ solution ⎟⎟ = c1m + (c 2m + c3m ln(m))/T ⎝ ηethanol ⎠
(7)
where ρsolution and ηsolution are the density and viscosity of CO2-free (or CO2-loaded) ProK/W/Eth solution, respectively; ρethanol and ηethanol are the density and the viscosity of ethanol as solvent, respectively; m is the molality of ProK or CO2 in ethanol, mol·kg−1; b1, b2, b3 and c1, c2, c3 are adjustable constants.
Table 4. Fitted Parameters in Equation 5 for the Soft Model for the CO2 Saturated Potassium Prolinate + Water + Ethanol Solution at Molality of Potassium Prolinate and Water m1 = 1.440 mol·kg−1 and 3.359 mol·kg−1 parameter
1.440m1, this work
A B k1 k2 k3
1.61 10.01 −3734/T + 14.15 exp(−1186/T + 4.56) 3963/T − 10.13
3.359 m1, this work 1.01 7.00 −4497/T + 16.46a exp(909/T − 3.01)a 1261/T + 0.21a
1.53b 0.17b 17.0b
aqueous 63.5 mass % KSarc
aqueous 30 mass % MEAd
3.036 8.720 −15237/T + 43.67 exp(−2794/T + 5.476) −167.47/T + 4.515
1.8 10 −9156/T + 28.03 exp(−6146/T + 15.00) 7527/T − 16.94
a The parameters was fitted at 323.3 and 343.3 K as soft model 1. bThe parameters was only fitted at 298.3 K as soft model 2. cSoft model parameters for the aqueous 63.5 mass% KSar system from Ma’mun (2014).20 dSoft model parameters for the aqueous 30 mass% MEA system from Bruder et al. (2011).23
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Table 6. Experimental Values of Density ρ for the System (CO2 + ProK + Water + Ethanol) at Molality of ProK and Water m1, Molality m2 of CO2, Temperature T, and Pressure 101.35 ± 0.5 kPaa T (K) m1 (mol·kg−1) 0 0.661 1.533 2.367 3.349 4.564 6.092 3.349 3.349 3.349 3.349
α (mol·mol−1) 0 0 0 0 0 0 0 0.049 0.109 0.249 0.301
m2 (mol·kg−1) 0 0 0 0 0 0 0 0.164 0.365 0.834 1.008
293.15 0.7895 0.8339 0.8754 0.9150 0.9470 0.9835 1.0206 0.9476 0.9497 0.9543 0.9561
298.15 0.7852 0.8296 0.8712 0.9109 0.9429 0.9794 1.0166 0.9435 0.9455 0.9502 0.9520
303.15 0.7808 0.8254 0.8670 0.9067 0.9388 0.9753 1.0126 0.9394 0.9414 0.9461 0.9478
308.15
313.15
318.15
323.15
328.15
333.15
0.7765 0.8211 0.8627 0.9025 0.9347 0.9712 1.0085 0.9352 0.9373 0.9420 0.9437
ρ/(g·cm−3) 0.7721 0.8167 0.8585 0.8983 0.9305 0.9671 1.0045 0.9310 0.9331 0.9378 0.9395
0.7677 0.8124 0.8542 0.8940 0.9263 0.9630 1.0004 0.9268 0.9289 0.9336 0.9353
0.7632 0.8079 0.8498 0.8897 0.9221 0.9588 0.9963 0.9226 0.9247 0.9294 0.9311
0.7587 0.8035 0.8454 0.8854 0.9178 0.9546 0.9922 0.9184 0.9204 0.9251 0.9268
0.7541 0.7989 0.8410 0.8810 0.9135 0.9504 0.9880 0.9141 0.9161 0.9208 0.9225
a m1 is the molality of ProK and water in ethanol, α is the total CO2 loading in the system, m2 is the molality of CO2 in the saturated potassium prolinate + water + ethanol solution, mol·kg−1 ethanol. Standard uncertainties u are u(ρ) = 0.0002 g·cm−3, u(T) = 0.02 K, u(m1) = 0.002 mol·kg−1, u(α) = 0.010 mol·mol−1, u(m2) = 0.033 mol·mol−1.
Table 7. Experimental Values of Viscosity η for the System (CO2 + ProK + Water + Ethanol) At Molality of ProK and Water m1, Molality m2 of CO2, Temperature T, and Pressure 101.35 ± 0.5 kPaa T (K) −1
m1 (mol·kg ) 0 0.661 1.533 2.367 3.349 4.564 6.092 3.349 3.349 3.349 3.349
−1
α (mol·mol ) 0 0 0 0 0 0 0 0.049 0.109 0.249 0.301
−1
m2 (mol·kg ) 0 0 0 0 0 0 0 0.164 0.365 0.834 1.008
293.15 1.1936 1.9737 3.0826 4.8490 7.0064 11.173 18.392 7.0354 7.2675 7.8449 8.0891
298.15 1.0854 1.7727 2.7309 4.2314 6.0131 9.3990 15.118 6.0403 6.2233 6.7096 6.8995
303.15 0.9897 1.5952 2.4407 3.7099 5.1955 7.9805 12.570 5.2239 5.3766 5.7793 5.9382
308.15
313.15
318.15
323.15
328.15
333.15
0.9036 1.4411 2.1814 3.2729 4.5011 6.8513 10.568 4.5399 4.6710 5.0099 5.1396
η/(mPa·s) 0.8277 1.3083 1.9552 2.8975 3.9600 5.9083 8.9733 3.9793 4.0895 4.3711 4.4808
0.7590 1.1923 1.7599 2.5853 3.4893 5.1532 7.6884 3.5073 3.5992 3.8410 3.9329
0.6978 1.0877 1.5893 2.3209 3.0927 4.5249 6.6351 3.1064 3.1841 3.3912 3.4695
0.6434 0.9953 1.4418 2.0895 2.7520 3.9839 5.7706 2.7635 2.8303 3.0068 3.0755
0.5942 0.9120 1.3091 1.8835 2.4586 3.5296 5.0542 2.4668 2.5213 2.6755 2.7351
m1 is the molality of ProK and water in ethanol, α is the total CO2 loading in the system, m2 is the molality of CO2 in the saturated potassium prolinate + water + ethanol solution, mol·kg−1 ethanol. Standard uncertainties ur(η) = 0.015, u(T) = 0.02 K, u(m1) = 0.002 mol·kg−1, u(α) = 0.010 mol·mol−1, u(m2) = 0.033 mol·mol−1. a
low CO2 loading range where precipitation occurs. However, when the CO2 loadings greater than 0.7 mol/mol, the CO2 partial pressure increases greatly with the increasing absorption amount of CO2. Moreover, more solid precipitation was obtained at 298.3 K. In our previous work,17 the main compositions in solid were qualitatively identified as bicarbonate and carbamate by NMR experiments. Separation of the resulting products from the liquid solution will benefit the CO2 absorption from the view of thermodynamic equilibrium. CO2 partial pressures are expected to lower than those that no solid precipitation happens. Therefore, an increasing absorption rate of CO2 was also observed when the precipitation occurred.17 The solubility data were correlated using the Soft model. The fitted model parameters were presented in Table 4. The predictions from the Soft model were also shown in Figure 3a,b. Figure 3a shows a good agreement between experimental data and predicted values for the concentration of 1.440 m1 with 8.0% AAD. Large deviations were observed at low temperature and low partial pressure range due to the effect of precipitation, even worse (dashed line in Figure 3b) for 3.359 m1
absorbent, which are the fundamental for kinetic study and the process design for operating conditions. The vapor−liquid− solid equilibrium data of CO2 in ProK/W/Eth solutions were obtained with a temperature range from 298.4 to 343.4 K and CO2 partial pressure up to 200 kPa. The molality concentrations of ProK investigated were 1.440 mol·kg−1 ethanol and 3.359 mol·kg−1 ethanol. In this work, m1 is referred to the molality of ProK and water, mol·kg−1 ethanol. The solubility data are shown in Tables 2 and 3. The graphical representations are also presented in Figure 3a,b. It can be observed that the CO2 partial pressures increase with increasing the CO2 loading. For a given CO2 partial pressure, the CO2 loading decreases as the temperature increases. Similar trends were found at different temperatures for 1.440 m1 ProK. The effect of solid precipitation on CO2 partial pressure can be seen at low temperatures but not obvious. However, the solubility of CO2 in the higher concentration (i.e., 3.359 m1) of ProK/W/Eth solution shows different trends at low temperature (i.e., 298.3 K) and high temperature (i.e., 343.3 K). As observed in Figure 3b, the CO2 solubility curve at 298.3 K appears a relax slope at the relatively 3174
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ProK/W/Eth solution at 298 K. Different curve patterns were applied to represent the experimental points. So model parameters were also obtained to fit the experimental data at low and high temperature, respectively. As presented in Figure 3b, the predicted values match well the experimental data over the investigated ProK concentration and temperature range. The average absolute deviation (AAD) is within 7.5%. The cyclic capacity (Δα) is defined as the difference of CO2 loadings under 10 kPa CO2 partial pressure at absorption and desorption temperature, respectively. It can be estimated from the solubility curves.34 The CO2 loading capacity and cyclic capacity for 3.359 m1 ProK/W/Eth, aqueous 27.4 mass % (2M) ProK,35 and aqueous 30 mass % (5M) MEA solution29 were estimated and given in Table 5. The equilibrium loading at 10 kPa can reach 0.80 for ProK/W/Eth, 0.52 for MEA, and 0.81 for ProK/water, respectively. It is noted that the obtained CO2 loading at 10 kPa CO2 partial pressure in this study is larger than the previous results (i.e., 1.32 mol·kg−1 solvent) which is about 0.66 mol·mol−1 on mole ProK basis.17 The reason may be due to the different measuring methods. In this work, the data were obtained in equilibrium cell to ensure vapor−liquid−solid phase equilibrium. It is also found that the cyclic capacity of ProK/W/Eth solution is the highest and at a narrower operating temperature range from 298 to 343 K. The effect of temperature on CO2 loading of ProK/W/Eth solution is much greater than those of aqueous ProK and MEA solution. Hence, ProK/W/Eth solution as a phase changing absorbent has a favorable characteristic for CO2 capture. 4.3. Density and Viscosity. Density and viscosity data are generally required for the calculation of mass transfer and process simulation. The densities and viscosities of CO2-free and CO2-loaded ProK/W/Eth solutions were measured at temperatures from 293 to 333 K and the molalities of ProK and water, m1, up to 6.09 mol·kg−1 ethanol. The results are presented in Tables 6−7 and Figures 4−7. As expected, densities decrease with the increasing temperature and increase with the increasing ProK concentrations. The viscosities also show similar trends in the investigated temperature and concentration range. It was noted that the absorption of CO2 into the
Figure 5. Viscosity of CO2-free ProK + water + ethanol solution at temperature T and molality of ProK and water m1, mol·kg−1: ■, pure ethanol; red ●, 0.661 m1; green ▲, 1.533 m1; blue ▼, 2.367 m1; pink ◆, 3.349 m1; dark green ◀, 4.564 m1; dark red ▶, 6.092 m1; , calculated from eq 10.
Figure 6. Density of CO2-loaded ProK + water + ethanol solution (m1 = 3.349 mol·kg−1) at temperature T and molality of CO2 (m2): ■, 293.15 K; ●, 298.15 K; ▲, 303.15 K; ▼, 308.15 K; ⧫, 313.15 K; ◀, 318.15 K; ▶, 323.15 K; ★, 328.15 K; *, 333.15 K; , calculated from eq 11.
absorbents can also affect the properties of the solutions. In this work, 3.349 m1 ProK/W/Eth solutions were used to investigate the effect of CO2 concentration (m2) on its properties before phase change. As shown in Figures 6−7, the densities and viscosities increase greatly with the addition of CO2 at a given temperature before the occurrence of solid precipitation. The experimental data were then correlated as a function of the ProK concentration (m1, mol·kg−1 ethanol) and CO2 concentration (m2, mol·kg−1 ethanol) as follows: ρsolution = 1.005 + 0.0762m1 − 0.0046m12 ρethanol (9) ⎛η ⎞ ln⎜⎜ solution ⎟⎟ = −0.3417m1 + (305.8m1 − 41.73m1 ⎝ ηethanol ⎠
Figure 4. Density of CO2-free ProK + water + ethanol solution at temperature T and molality of ProK and water m1, mol·kg−1: ■, pure ethanol; red ●, 0.661 m1; green ▲, 1.533 m1; blue ▼, 2.367 m1; pink ◆, 3.349 m1; dark green ◀, 4.564 m1; dark red ▶, 6.092 m1; , calculated from eq 9.
× ln(m1))/T 3175
(10) DOI: 10.1021/acs.jced.7b00267 J. Chem. Eng. Data 2017, 62, 3169−3177
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00267. Solubility of CO2 in aqueous 15.3 mass % MEA at 313.2 K; density and viscosity of ethanol, deionized water and S3 viscosity standard; estimation for parameter values in eqs 9−12 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]; tel.: +86 311 88632183. Fax: +86 311 88632183. ORCID
Shufeng Shen: 0000-0003-0625-133X Figure 7. Viscosity of CO2-loaded ProK + water + ethanol solution (m1 = 3.349 mol·kg−1) at different temperature T and molality of CO2 (m2): ■, 293.15 K; ●, 298.15 K; ▲, 303.15 K; ▼, 308.15 K; ⧫, 313.15 K; ◀, 318.15 K; ▶, 323.15 K; ★, 328.15 K; *, 333.15 K; , calculated from eq 12.
ρCO ‐ loaded 2
ρCO ‐ free
Funding
The authors would like to acknowledge Hebei Provincial Natural Science Foundation for Distinguished Young Scholars of China (B2015208067), Hebei Provincial Science and Technology Research Project of College and University (QN2015070), and Postgraduate Innovation Fund Project of Hebei Province for financial support (CXZZSS2017085).
= 0.9998 + 0.0109m2 + 0.0076m22
2
⎛η ⎞ CO2 ‐ loaded ⎟ = 0.0934m2 + 0.1645m22 ln⎜⎜ ⎟ η ⎝ CO2 ‐ free ⎠
(11)
Notes
The authors declare no competing financial interest.
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REFERENCES
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where ρsolution and ηsolution are the density and viscosity of ProK/ W/Eth solution, respectively; ρethanol and ηethanol are the density and the viscosity of ethanol as solvent, respectively; m1 is the molality of ProK and water, mol·kg−1 ethanol. For eqs 11−12, m2 is the molality of CO2 in ethanol for 3.349 m1 ProK/W/Eth solution, mol·kg−1 ethanol; ρCO2‑loaded and ηCO2‑loaded are the density and viscosity of 3.349 m1 ProK/W/Eth solution with different CO2 molality, respectively; ρCO2‑free and ηCO2‑free are the density and the viscosity of 3.349 m1 ProK/W/Eth solution, respectively. Estimations for the fitted parameter values in these equations are listed in SI, Table S3. The predictions from the correlations are also shown in Figures 4−7. It can be observed that the predicted results are in good agreement with experiment data and the average absolute deviation (AAD) are within 0.37% for density and 1.38% for viscosity, respectively.
5. CONCLUSIONS In the present study, the density and viscosity of ProK/W/Eth solution were measured at temperatures from 293 to 333 K and concentrations up to 6.092 m1. The proposed models can represent well the experimental data with 0.37% AAD for density and 1.38% AAD for viscosity. The solubility of CO2 in ProK/W/Eth solution was obtained over temperatures ranging from 298 to 343 K and CO2 partial pressures from 1.0 to 200 kPa. Solubility data were also well-represented by the Soft model within 8.0% AAD. The behavior of phasechanging ProK/W/Eth absorbent has shown a higher cyclic capacity and better performance for CO2 capture process compared with MEA. Experiments and evaluation of absorption heat and energy consumption for solvent regeneration are still in progress. 3176
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