Article pubs.acs.org/jced
Solubility of Carbon Dioxide in LiF−Li2CO3 Molten Salt System Zhongning Shi,* Wentao Deng, Xiaowen Song, Xianwei Hu, Bingliang Gao, and Zhaowen Wang School of Metallurgy, Northeastern University, Shenyang 110819, China ABSTRACT: The solubility of carbon dioxide in a LiF−Li2CO3 molten salt system was determined using a pressure differential method. With increasing holding temperature, the solubility of CO2 in LiF−Li2CO3 melts decreases rapidly. The influence of the alkali metal fluoride of XF(X = K, Na) content from 5% to 25% on the CO2 solubility was considered. The CO2 solubility in LiF−Li2CO3− XF melts decreased with increasing temperature and concentration of XF. The calculated thermodynamic data for CO2 dissolved in these melts revealed that the reaction between CO2 and a carbonate ion was exothermic. At 630 °C, KF decreased the CO2 solubility more effectively than NaF. When the mole fraction of LiF was 50% in LiF−Li2CO3, the CO2 solubility reached a maximum of 6.8 × 10−4 (molCO2/molmelt) at 640 °C.
■
INTRODUCTION With the rapid development of modern industry, a huge amount of fossil fuels such as coal, oil, and natural gas are consumed, and at the same time, increasing amounts of CO2 are released into the atmosphere. Carbon dioxide is the main gas that causes the greenhouse effect and may also destroy the ozone layer. The Earth’s ecological environment has been significantly affected by global warming caused by the largescale emissions of CO2. Therefore, in some studies, the reduction of CO2 emissions and the rational utilization of CO2 have been investigated. Molten salt electrolysis is one of the tools used to decompose CO2. Ingram1 electrolyzed carbon in a Li2CO3−Na2CO3−K2CO3 melt via molten salt electrolysis. Kawamura2 obtained carbon from a cathode via electrodeposition in a LiCl−KCl−K2CO3 melt. With respect to carbonate molten salts, Li2CO3 is a superior electrolyte because it exhibits a higher theoretical decomposition voltage among the investigated carbonate molten salts.3 Furthermore, the addition of fluoride into carbonate molten salts as a supporting electrolyte reduces the liquidus temperature of an electrolyte system. Molten salt electrolysis is an important method for the comprehensive utilization of CO2, therefore, the study of molten salts for the electrolysis of carbon dioxide to produce carbon and oxygen is crucial for the transformation of the carbon cycle and the reduction of carbon dioxide emissions. Moreover, we can use this process to convert CO2 into C and O2 on Mars, where CO2 is dominant in the atmosphere. Li4 et al. obtained O2 and C by the electrolysis of CO2 in a LiF− Li2CO3 melt. From the viewpoint of utilization, the CO2 solubility has a significant influence on the electrolytic process, therefore, it is necessary to study the CO2 solubility in the LiF− Li2CO3 molten salt system. According to Henry’s law, the concentration of CO2 physically dissolved in a molten salt is related to its partial pressure: © XXXX American Chemical Society
CO2(g) ↔ CO2(M)
(1)
m PCO = KHSH(CO2)U 2
(2)
Here, subscripts (g) and (M) represent gas and molten carbonate phases, respectively. KH is Henry’s constant (MPa·L· mol −1), SH(CO2) is the physical solubility of CO2 in the molten salt (molCO2/molmelt), U is a parameter for unit conversion of physical quantity (mol·L−1) and Pm CO2 is the equilibrium partial pressure of CO2 (MPa). Some solubility data of CO2 in melts are shown in Table 1. The CO2 dissolution process includes both physical solubility and chemical solubility. The product obtained by the reaction between CO2 and carbonate molten salts is generally regarded as a dicarbonate (C2O52−) anion when CO2 dissolves in molten carbonates. Raman spectroscopy of the molecular structure of dicarbonate revealed six individual overlapping theoretical bands.10 Peeters11 studied the formation reaction by starting from a carbonate anion and CO 2 and found that it thermodynamically favors the formation of a dicarbonate (C2O52−) anion. Moreover, the formation of dicarbonate ions was proposed: CO2(g) + CO32 −(M) ↔ C2O52 −(M)
(3)
12
Claes studied the solubility and solvation of CO2 in a molten 43.5Li2CO3−31.5Na2CO3−25.0K2CO3 (mol %) eutectic mixture at 700 °C. The physical solubility was evaluated to be 2 × 10−3 mol·L−1 from the peak current density for the electrochemical reduction of CO2 in a saturated melt, and the measured chemical solubility of CO2 was (9.3 ± 1.0) × 10−2 Received: January 16, 2016 Accepted: July 29, 2016
A
DOI: 10.1021/acs.jced.6b00043 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
Table 1. Solubility Data of CO2 in Melts T/°C 880 800 700 1000 350
KH−1 (mol·L−1·MPa
melts
−1
)
−2
9.5 × 10 1.81 3.56 × 10−2 9.9 × 10−3 1.07 × 10−5
Na2CO3 (NaK)2CO3 (LiNaK)2CO3 Na3AlF6 NaNO3
determination technique
ref
manometric manometric quenching radiotracer simplified elution
Andresen5 Claes6 Appleby7 Numata8 Sada9
Table 2. Information of All Samples chemical name
source
grade
reported purity/wt %a
Li2CO3 LiF KF NaF CO2
Sinopharm Aladdin Aladdin Aladdin Jiahe Gas Products
≥99% ≥99% ≥99% ≥99% ≥99.9%
99 99.2 99.1 99.1 99.9
purification method vacuum vacuum vacuum vacuum none
drying drying drying drying
analysis method
analysis purity/wt %b
titration (HCl) assayc assayd assaye gas chromatograph
99.9 99.5 99.2 99.7 99.95
a
Reported by the manufacturer on the certificate of analysis. bAnalyzed by the authors after purification treatment. cAnalyzed by measuring the content of impurities (sulfate, silicon dioxide, iron and heavy meatal Pb). dAnalyzed by measuring the content of impurities (chloride, HF, K2CO3 and heavy meatal Pb). eAnalyzed by measuring the content of impurities (potassium, sulfate, chloride, iron and heavy meatal Pb).
mol·L−1. Owing to this reaction, the chemical solubility was approximately 50 times larger than the physical solubility. Therefore, an analysis of the CO2 dissolution process in molten carbonates is needed. The correct value of the CO2 solubility is essential in understanding what is occurring in molten carbonates. Highly sensitive data can be obtained by the pressure differential method at a stable temperature condition. This study aims to determine the CO2 solubility in LiF− Li2CO3 melts and the influence of additives on the solubility. We believe that our study will provide a theoretical basis for the selection of process parameters for carbon electrodeposition and oxygen preparation by CO2 electrolysis in LiF−Li2CO3 under a CO2 atmosphere.
(volume = 668 mL). To verify that the apparatus and chemicals were fit for the experiment, the equilibrium dissociation pressure of pure molten lithium carbonate at 735 °C was measured and determined to be (1.552 ± 0.038) × 10−3 MPa, compared to a value of 1.541 × 10−3 MPa that was measured by Janz13 at the same temperature, illustrating that the relative error rate in our experiments is acceptable. The volume of CO2 in the stainless steel container and duct was calibrated with CO2. The calibration process is described as follows. The apparatus was carefully evacuated and the airtightness of the apparatus was controlled by GUQ-pneumatic high vacuum ball valves (here after referred to as GUQ). When GUQ(2) and GUQ(4) are closed, the CO2 gas cylinder inflates the container to pressure P1. By opening GUQ(4), CO2 is allowed to fill the bellows, and the pressure changes from P1 to P2. The corresponding free volume of CO2 in the stainless steel container and duct (Vg) is easily calculated from the volume of bellows (V1) and pressures P1 and P2 measured before and after the opening of GUQ(4), respectively:
■
EXPERIMENTAL SECTION All chemicals, including LiF, Li2CO3, NaF, and KF were dried at 673 K for 4 h. All salts were handled in a glovebox under a dry argon atmosphere. The information on all samples is shown in Table 2. The solubility of carbon dioxide in binary LiF−Li2CO3 and LiF−Li2CO3−XF (X = K, Na) molten salts was measured using a pressure differential method, which is derived by the change of gaseous pressure at different temperatures in the apparatus. The apparatus is shown in Figure 1. The volume of the bellows was calibrated with water before being fixed to the apparatus
PV 1 g = P2V1 + P2Vg
(4)
The container contains m (g) of the mixture melt that has been previously fused under an argon atmosphere in the stainless steel container. The reactor is connected to a vacuum device that lowers the internal pressure to 0.04 Pa. The measurements were performed within a temperature interval of approximately 80 °C, starting at 20 to 30 °C above the liquidus temperature of the melts. The volume (Vm) of molten mixture is assumed to be thermostated at the experimental temperature (T) in the apparatus from which the gas has been carefully evacuated. Vm = m/ρ1
(5)
The density of mixed molten salts is ρ1, obtained from Song.14 If CO2 is introduced in the apparatus at an exact known initial pressure P1 and temperature T1, CO2 will start to dissolve in the melt and the pressure will decrease until an equilibrium pressure P2 is reached at temperature T2. The number of moles of CO2 in the apparatus can be calculated from n1 = P1(Vg − Vm)/RT1 ,
Figure 1. Device for measuring the solubility of CO2. B
n2 = P2(Vg − Vm)/RT2
(6)
DOI: 10.1021/acs.jced.6b00043 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
The number of moles of CO2 in the apparatus is n1 before CO2 dissolves in the melt and n2 after CO2 dissolves in the melt, the solubility of CO2 in carbonate molten salts is readily calculated from S(CO2) = (n 2 − n1)/n(melt)
Table 4. Equilibrium Pressure and Solubility of Carbon Dioxide in LiF−Li2CO3 Meltsa xLiF
(7)
mol %
°C
30 30 30 30 40 40 40 40 50 50 50 50 60 60 60
680 690 700 710 660 680 700 720 640 660 680 700 700 710 720
Prior to the solubility measurement, the liquidus temperature of a molten carbonate system was estimated using the thermal analysis method. The detailed description of the methods can be found in Solheim et al.,15 and the results are given in Table 3. Table 3. Liquidus Temperature of LiF−Li2CO3−XF Molten Salt Systema xXF
xLiF
xLi2CO3
liquidus temperature
XF
mol %
mol %
mol %
°C
NaF NaF NaF NaF NaF KF KF KF KF KF
5 10 15 20 25 5 10 15 20 25
47.5 45 42.5 40 37.5 47.5 45 42.5 40 37.5
47.5 45 42.5 40 37.5 47.5 45 42.5 40 37.5
601.5 588.3 569.8 552.8 527.6 601.3 585.5 557.7 539.5 519.2
T
equilibrium pressure
S(CO2)
MPa
molCO2/molmelt
5.74 6.08 6.37 6.72 5.52 5.86 6.44 6.75 5.02 5.54 6.01 6.54 6.82 7.05 7.39
× × × × × × × × × × × × × × ×
10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2
5.7 5.3 5.2 5.0 5.7 5.3 4.9 4.6 6.8 6.4 6.1 5.7 4.5 3.9 3.5
× × × × × × × × × × × × × × ×
10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4
Standard uncertainties u are u(T) = 1 °C, u(xLiF) = 0.02, u(equilibrium pressure) = 4 × 10−4 MPa, u(S(CO2)) = 0.10S(CO2)/ (molCO2/molmelt). a
Table 5. Equilibrium Pressure and Solubility of Carbon Dioxide in LiF−Li2CO3−KF Meltsa
Standard uncertainties u are u(liquidus temperature) = 0.1 °C, u(xXF) = 0.02, u(xLiF) = 0.02, u(xLi2CO3) = 0.02.
xKF
xLiF
xLi2CO3
mol %
mol %
mol %
°C
5 5 5 5 10 10 10 10 15 15 15 15 20 20 20 20 25 25 25 25 25
47.5 47.5 47.5 47.5 45 45 45 45 42.5 42.5 42.5 42.5 40 40 40 40 37.5 37.5 37.5 37.5 37.5
47.5 47.5 47.5 47.5 45 45 45 45 42.5 42.5 42.5 42.5 40 40 40 40 37.5 37.5 37.5 37.5 37.5
630 650 670 690 610 630 650 670 580 600 620 640 570 590 610 630 540 560 580 600 630
a
■
RESULTS AND DISCUSSION CO2 Solubility in LiF−Li2CO3 Binary Molten Salt System. The CO2 solubility in a LiF−Li2CO3 molten salt system, for which the mole fraction of LiF (xLiF) is in the range between 30% and 60%, was determined. As summarized in Table 4, the CO2 solubility decreases with increasing temperature. The solubility reaches the maximum value of 6.8 × 10−4 (molCO2/molmelt) at 640 °C when the mole fraction of LiF is 50%. CO2 Solubility in LiF−Li2CO3−XF (X = K, Na) Molten Salt System. The CO2 solubility in a molten LiF−Li2CO3 binary mixture was investigated under various conditions when the LiF:Li2CO3 mole ratio was equal to 1:1 to reduce the liquidus temperature of a molten mixture and maximize the CO2 solubility. To reduce the liquidus temperature of the molten carbonate system, alkali metal fluoride was added. Then, the CO2 solubility in a LiF−Li2CO3−XF (X = K, Na) molten salt system was determined. The mole fraction of XF ranged from 5% to 25%, and the results are shown in Tables 5 and 6. As summarized in Figure 2, the effect of temperature on the CO2 solubility in the LiF−Li2CO3−XF (X = K, Na) molten salt system is negative, the CO2 solubility decreases when the content of XF increases. Compared to a binary molten LiF− Li2CO3 eutectic mixture at 640 °C, the addition of 15 mol % KF reduces the CO2 solubility to 9.5 × 10−5 (molCO2/molmelt). The effect of XF content on the CO2 solubility in LiF− Li2CO3−XF at 630 °C is shown in Figure 3. The CO2 solubility decreases with increasing alkali metal fluoride concentration, and KF decreases the CO2 solubility more effectively than NaF.
T
Equilibrium pressure
S(CO2)
MPa
molCO2/molmelt
6.48 6.74 7.02 7.41 6.80 7.06 7.31 7.52 6.41 6.74 6.96 7.35 6.35 6.72 6.87 7.02 6.65 6.90 7.12 7.35 7.60
× × × × × × × × × × × × × × × × × × × × ×
10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2
2.98 2.45 2.09 1.50 2.59 1.97 1.58 1.22 2.03 1.63 1.31 9.48 1.50 1.18 9.01 6.21 1.13 1.04 8.08 5.54 4.15
× × × × × × × × × × × × × × × × × × × × ×
10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−5 10−4 10−4 10−5 10−5 10−4 10−4 10−5 10−5 10−5
Standard uncertainties u are u(T) = 1 °C, u(xKF) = 0.02, u(xLiF) = 0.02, u(xLi2CO3) = 0.02, u(equilibrium pressure) = 4 × 10−4 MPa, u(S(CO2)) = 0.10S(CO2)/(molCO2/molmelt). a
Vapor−Liquid Equilibrium Time. Equilibrium time refers to the elapsed time that a system needs to achieve a constant pressure at a constant temperature. When the reading from a pressure gauge does not change, the system is known to reach an equilibrium state. The pressure changes over time in different composition melts recorded in Table 7. The time from the introduction of CO2 to the system until the system reaches C
DOI: 10.1021/acs.jced.6b00043 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
Table 6. Equilibrium Pressure and Solubility of Carbon Dioxide in LiF−Li2CO3−NaF Meltsa xNaF
xLiF
xLi2CO3
T
mol %
mol %
mol %
°C
5 5 5 5 10 10 10 10 15 15 15 15 20 20 20 20 25 25 25 25 25
47.5 47.5 47.5 47.5 45 45 45 45 42.5 42.5 42.5 42.5 40 40 40 40 37.5 37.5 37.5 37.5 37.5
47.5 47.5 47.5 47.5 45 45 45 45 42.5 42.5 42.5 42.5 40 40 40 40 37.5 37.5 37.5 37.5 37.5
630 650 670 690 610 630 650 670 590 610 630 650 580 600 620 640 550 570 590 610 630
Equilibrium pressure
S(CO2)
MPa
molCO2/molmelt
6.17 6.74 7.13 7.66 6.66 6.82 7.03 7.21 6.74 7.05 7.42 7.76 6.72 6.80 6.94 7.12 6.92 7.18 7.42 7.59 7.82
× × × × × × × × × × × × × × × × × × × × ×
10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2
3.84 3.05 2.54 1.88 3.24 2.73 2.18 1.64 2.36 2.12 1.61 1.17 1.87 1.47 1.20 9.13 1.26 1.07 1.04 7.01 5.96
× × × × × × × × × × × × × × × × × × × × ×
10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−5 10−4 10−4 10−4 10−5 10−5
Figure 3. Effect of XF concentration on the CO2 solubility in LiF− Li2CO3−XF(X = K, Na) melts (xLiF = xLi2CO3 = 0.5−0.5xXF) at 630 °C.
Carbonate ions capture and transport CO2. The formation of dicarbonate ions (eq 3) can also occur by the above-mentioned dissociation equilibrium. The activity of carbonate ions is assumed to be equal to the mole fraction of carbonate ions in the mixed melt (xCO2− ) when 3 it was mixed with chloride, the activity coefficient is assumed to 1, and the carbonate melts were considered to be completely disintegrated. K is the quilibrium constant of eq 3 after the equilibrium solubility is reached in carbonate melts.
Standard uncertainties u are u(T) = 1 °C, u(xNaF) = 0.02, u(xLiF) = 0.02, u(xLi2CO3) = 0.02, u(equilibrium pressure) = 4 × 10−4 MPa, u(S(CO2)) = 0.10S(CO2)/(molCO2/molmelt).
a
m K = aC2O52−/(PCO x 2−) 2 CO3
(10)
Pm CO2
Here, is the partial pressure of CO2 in the gaseous phase at equilibrium. The chemical solubility and physical solubility of carbon dioxide are considered, giving eq 11:
equilibrium can be observed in Figure 4; the equilibrium time ranged between 2 and 3 h. Thermodynamic Properties of Solutions. The simultaneous occurrence of CO2 and C2O52− in carbonate melts was considered. This means that the dissociation of carbonate ions occurs in two ways, two conjugate acids of a solvent were produced.12 The dissociation equilibrium is as follows: CO32 −(M) ↔ CO2(g) + O2 −(M) 2CO32 −(M) ↔ C2O52 −(M) + O2 −(M)
S(CO2) = Sr(CO2) + SH(CO2)
(11)
where the physical solubility, SH(CO2), and chemical solubility, Sr(CO2), can be obtained by eqs 2 and 12, respectively. We can 2− take the concentration of C2O2− 5 (C(C2O5 )) as the activity of C2O2− , which obeys Henry’s law in dilute ideal solution. 5
(8)
m Sr(CO2) = C(C2O52−) = KPCO x 2− 2 CO3
(9)
(12)
The equilibrium constant K can then be calculated by
Figure 2. Effect of temperature on carbon dioxide solubility in melts: (a) LiF−Li2CO3−KF (xLiF = xLi2CO3 = 0.5−0.5xKF) and (b) LiF−Li2CO3−NaF (xLiF = xLi2CO3 = 0.5−0.5xNaF). D
DOI: 10.1021/acs.jced.6b00043 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
Table 7. Pressure Changes (MPa) over Time (h) in Different Composition Meltsa pressure/MPa sample IDb
0.5 h
1.0 h
1.5 h
2.0 h
2.5 h
3.0 h
3.5 h
a b c d
0.0591 0.0456 0.0551 0.0438
0.0627 0.0486 0.0602 0.0497
0.0637 0.0495 0.0627 0.0571
0.0638 0.0500 0.0639 0.0660
0.0637 0.0502 0.0664 0.0682
0.0638 0.0501 0.0671 0.0702
0.0637 0.0501 0.0670 0.0701
Standard uncertainties u are u(T) = 1 °C, u(xKF) = 0.02, u(xNaF) = 0.02, u(xLiF) = 0.02, u(xLi2CO3) = 0.02, u(time) = 0.1 h, u(pressure) = 0.0002 MPa. bIdentification: (a) 40%LiF−60%Li2CO3 system, (b) 50%LiF−50%Li2CO3 system, (c) 40%LiF−40%Li2CO3−20%NaF system, and (d) 40% LiF−40%Li2CO3−20%KF system. a
Figure 4. Equilibrium time in different composition melts: (a) 40%LiF−60%Li2CO3 system; (b) 50%LiF−50%Li2CO3 system; (c) 40%LiF−40% Li2CO3−20%NaF system; and (d) 40%LiF−40%Li2CO3−20%KF system.
K=
S(CO2) − SH(CO2) m PCO x 2− 2 CO3
=
S(CO2) m PCO2xCO32−
−
1 UKHxCO32−
where the total pressure P can be obtained from the equilibrium partial pressure Pm CO2 and the fugacity coefficient ΦCO2(T,P) can be obtained from SRK EOS.18
(13)
The vapor pressure of the melts is considered to be negligible, thus, the gaseous phase is assumed to be pure CO2. This does not allow the direct determination of Henry’s constant; however, the method for determining the constant can be obtained from previous research.16,17 The application of vapor−liquid equilibrium conditions results in extended Henry’s law for CO2: KH(T , P)aCO2(T ) = fCO (T , P) 2
ln Φ = ln
(16)
where A=
2
P /Pc aP = 0.42747α(T ) 2 2 RT (T /Tc)2
(14)
B = bP /RT = 0.08664
The fugacity of pure CO2 in the melts at equilibrium temperature and pressure (f CO2(T,P)) is calculated from the following equation: fCO (T , P) = P ΦCO2(T , P)
f A ⎛⎜ B⎞ = Z − 1 − ln(Z − B) − ln 1 + ⎟ ⎝ P B Z⎠
(15) E
P /Pc T /Tc
(17)
(18)
a = acα(T )
(19)
ac = 0.42747R2Tc 2/Pc
(20) DOI: 10.1021/acs.jced.6b00043 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
b = 0.08664RTc/Pc
Table 9. Thermodynamic Properties of CO2 Absorbed in LiF−Li2CO3−XF (X = K, Na) Melts at 630 °Ca
(21)
Moreover, P=
XF
RT a − V−b V (V + b)
XF
(22)
PV Z= RT
(23)
α(T )1/2 = 1 + (0.48508 + 1.5517ω − 0.15613ω 2) × (1 − Tr1/2)
(24)
Tr = T /Tc
(25)
where R is the gas constant, T is the equilibrium temperature, a and b are the EOS constants, V is the molar volume, Tc is the critical temperature of CO2, Pc is the critical pressure, Tr is the reduced temperature, α(T) expresses the temperature dependence of the parameter a, ω is the acentric factor, and Z is the compressibility factor. Once the vapor pressure of mixed melt in the vapor phase can be negligible, the Henry’s law constant for CO2 can then be obtained by KH(T , P) =
lim
P → PM = 0
⎡ f (T , P ) ⎤ ⎢ CO2 ⎥ ⎢⎣ C(CO2)(T ) ⎥⎦
(27)
Δsol G = ΔHG + Δr G = RT ln(KH(T , P)/P 0) + RT ln(K (T , P)/P 0) (28)
Δsol H = ΔHH + Δr H ⎛ ∂ ln(K (T , P)/P 0) ∂ ln(K (T , P)/P 0) ⎞ H ⎟ = R⎜ + ∂(1/T ) ∂(1/T ) ⎠ ⎝
P
(29)
Δsol S = (Δsol H − Δsol G)/T
xLiF
ΔG
ΔH
ΔS
mol %
kJ·mol−1
kJ·mol−1
kJ·K−1·mol−1
a
0.5 0.6 0.7 0.5
−19.9 ± 0.2
KF
10
45
58.1 ± 1.4
−16.7 ± 0.6
KF
15
42.5
61.8 ± 1.3
−14.0 ± 0.3
KF
20
40
65.9 ± 1.3
−9.4 ± 0.3
KF
25
37.5
68.4 ± 1.5
−6.1 ± 0.1
NaF
5
47.5
53.5 ± 0.5
−26.3 ± 0.3
NaF
10
45
55.6 ± 0.8
−21.2 ± 0.3
NaF
15
42.5
59.2 ± 1.4
−16.8 ± 0.5
NaF
20
40
61.9 ± 1.0
−10.5 ± 0.2
NaF
25
37.5
65.7 ± 0.8
−7.2 ± 0.1
(−6.3 ± 10−1 (−6.6 ± 10−1 (−7.0 ± 10−1 (−7.4 ± 10−1 (−7.6 ± 10−1 (−6.2 ± 10−1 (−6.4 ± 10−1 (−6.7 ± 10−1 (−7.0 ± 10−1 (−7.4 ± 10−1
0.1) × 0.1) × 0.2) × 0.1) × 0.2) × 0.2) × 0.1) × 0.2) × 0.1) × 0.2) ×
CONCLUSIONS The solubility of carbon dioxide in binary LiF−Li2CO3 and LiF−Li2CO3−XF (X = K, Na) molten salts was measured using a pressure differential method. The solubility value of carbon dioxide in those compositions was found to be smaller than in other pure carbonate systems. Physical and chemical solubility is responsible for CO2 absorption in the molten carbonate; however, the chemical solubility is higher than physical solubility due to the formation of dicarbonate ions. In this paper, the influence of XF (X = K, Na) content and temperature on CO2 solubility was considered. The solubility of CO2 in the LiF−Li2CO3 melts decreased with increasing temperature over 25 °C−100 °C under different component compositions. The CO2 solubility in LiF−Li2CO3−XF (X = K,
Table 8. Thermodynamic Properties of CO2 Absorbed in LiF−Li2CO3 Melts at 700°Ca
± ± ± ±
55.4 ± 0.9
5
■
(30)
Here, subscripts H and r represent the thermodynamic properties of physical solubility and chemical solubility,
−41.6 −37.7 −33.5 −31.5
47.5
KF
respectively. Table 8 shows that the Gibbs free energy change (ΔG) for absorbing CO2 in LiF−Li2CO3 melts at 700 °C is positive, indicating that the reaction between CO2 and CO32− is an antidromic spontaneous reaction; in other words, the smaller is the value of ΔG is for CO2 in 50%LiF + 50%Li2CO3, the more effective is the formation reaction of dicarbonate (C2O52−). Correspondingly, the solubility of carbon dioxide in LiF−Li2CO3 reaches its maximum under the same conditions. The enthalpy values indicate that the reaction between CO2 and CO32− is exothermic, thus, the formation of C2O52− ions will be restrained. As the temperature rises, the carbon dioxide solubility in LiF−Li2CO3 melts will significantly decrease. Table 9 shows that the values of the Gibbs free energy change (ΔG) for CO2 absorbed in LiF−Li2CO3−XF (X = K, Na) melts at 630 °C gradually increase with increasing alkali metal fluoride concentration. It is observed that the data of NaF are smaller than that of KF at the same XF concentration. The result agrees with measurement of CO2 solubility; that is to say, KF diminishes the CO2 solubility more effectively than NaF in the LiF−Li2CO3−XF(X = K, Na) melts. For reducing the system’s liquidus temperature, the addition of much XF to the carbonate molten salt is unreasonable.
The thermodynamic properties of CO2 absorbed in the molten LiF−Li2CO3 binary mixture and LiF−Li2CO3−XF (X = K, Na) melts under various conditions are determined using eqs 28−30, and the results are presented in Tables 8 and 9.
1.3 1.0 1.7 0.8
ΔS kJ·K−1·mol−1
(26)
m KH(T , P) = ΦCO2(T , P)PCO /C(CO2) 2
± ± ± ±
ΔH kJ·mol−1
Standard uncertainties u are u(T) = 1 °C, u(xXF) = 0.02, u(xLiF) = 0.02.
According to the results of Zhang, we can take the concentration of CO2 (C(CO2)) as the activity of CO2 in the melts, which expressed obeys the Henry’s law in dilute ideal solution. From eq 2, eq 26 can be changed into
58.3 57.5 54.9 55.0
ΔG kJ·mol−1
xLiF
a
17
30 40 50 60
xXF
mol % mol %
(−6.4 (−6.2 (−6.0 (−5.9
± ± ± ±
0.2) 0.2) 0.1) 0.2)
× × × ×
10−1 10−1 10−1 10−1
Standard uncertainties u are u(T) = 1 °C, u(xLiF) = 0.02. F
DOI: 10.1021/acs.jced.6b00043 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
(11) Peeters, D.; Moyaux, D.; Claes, P. Solubility and solvation of carbon dioxide in the molten Li2CO3/Na2CO3/K2CO3 (43.5:31.5:25.0 mol-%) eutectic mixture at 973 K II. Theoretical Part. Eur. J. Inorg. Chem. 1999, 1999, 589−592. (12) Claes, P.; Moyaux, D.; Peeters, D. Solubility and solvation of carbon dioxide in the molten Li2CO3/Na2CO3/K2CO3 (43.5:31.5:25.0 mol-%) eutectic mixture at 973 K I. Experimental Part. Eur. J. Inorg. Chem. 1999, 1999, 583−588. (13) Janz, G. J.; Lorenz, M. R. Equilibrium dissociation pressures of molten lithium carbonate. J. Chem. Eng. Data 1964, 9, 94−95. (14) Song, X. W.; Deng, W. D.; Liu, Z. H.; Shi, Z. N.; Gao, B. L.; Hu, X. W.; Wang, Z. W. Density of lithium fluoride−lithium carbonatebased molten salts. Chem. Pap. 2015, 69, 1101−1107. (15) Solheim, A.; Rolseth, S.; Skybakmoen, E.; StØen, L.; Sterten, Å.; StØre, T. Liquidus Temperature and alumina solubility in the system Na3AlF6-AlF3-LiF-CaF2-MgF2. Metall. Metall. Mater. Trans. B 1996, 27, 739−744. (16) Kumelan, J.; Kamps, Á . P.-S.; Tuma, D.; Maurer, G. Solubility of CO2 in the ionic liquids [bmim][CH3SO4] and [bmim][PF6]. J. Chem. Eng. Data 2006, 51, 1802−1807. (17) Zhang, S. J.; Chen, Y. H.; Ren, R. X.-F.; Zhang, Y. Q.; Zhang, X. p.; Zhang, X. Solubility of CO2 in sulfonate ionic liquids at high pressure. J. Chem. Eng. Data 2005, 50, 230−233. (18) Soave, G. Equilibrium constants from a modified Redkh-Kwong equation of state. Chem. Eng. Sci. 1972, 27, 1197−1203.
Na) melts decreased with increasing mole fraction of XF concentration from 5% to 25%, as well as with increasing temperature beyond the liquidus temperature of melts from 20 to 90 °C. The CO2 solubility is more effectively decreased by KF than by NaF at 630 °C. The calculation of thermodynamic data for CO2 dissolving in the LiF−Li2CO3 melts shows that the reaction of CO2 and carbonate ion is exothermic. The higher is the percentage of Li2CO3, the more negative is the heat and entropy for dissolving carbon dioxide in LiF− Li2CO3−XF (X = K, Na) melts. This shows that the interaction between carbon dioxide with the molten salt is strong. The CO2 solubility reaches a maximum value of 6.8 × 10−4 (molCO2/molmelt) at 640 °C when the mole fraction of LiF is 50% of the LiF−Li2CO3 melt.
■
AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 24 83686464. Fax: +86 24 83686464. E-mail:
[email protected]. Funding
The authors would like to acknowledge the financial support from the National Natural Science Foundation of China (Nos. 51322406, 51434005, 51574070 and 51474060), the Program for New Century Excellent Talents (NCET-13-0107), Ministry of Education of China, and the Fundamental Research Funds for the Central Universities (No. N140205001 and No. L1502014). Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Ingram, M. D.; Baron, B.; Janz, G. J. The electrolytic deposition of carbon from fused carbonates. Electrochim. Acta 1966, 11, 1629− 1639. (2) Kawamura, H.; Ito, Y. Electrodeposition of cohesive carbon films on aluminum in a LiCl-KCl-K2CO3 melt. J. Appl. Electrochem. 2000, 30, 571−574. (3) Ijije, H. V.; Sun, C. G.; Chen, G. Z. Indirect electrochemical reduction of carbon dioxide to carbon nanopowders in molten alkali carbonates: Process Variables and Product Properties. Carbon 2014, 73, 163−174. (4) Li, L. X.; Shi, Z. N.; Gao, B. l.; Xu, J. L.; Hu, X. W.; Wang, Z. W. Preparation of carbon and oxygen by carbon dioxide electrolysis in LiF-Li2CO3 eutectic molten salt. J. Electrochem. Soc. 2016, 190, 655− 658. (5) Andresen, R. E. Solubility of oxygen and sulfur dioxide in molten sodium sulfate and oxygen and carbon dioxide in molten sodium carbonate. J. Electrochem. Soc. 1979, 126, 328−334. (6) Claes, P.; Thirion, B.; Glibert, J. Solubility of CO2 in the molten Na2CO3-K2CO3 (42 mol %) eutectic mixture at 800°C. Electrochim. Acta 1996, 41, 141−146. (7) Appleby, A. J.; Drune, C. Solubilities of oxygen and carbon monoxide in carbonate melts. J. Electrochem. Soc. 1980, 127, 1655− 1659. (8) Numata, H.; Bockris, J. O. Interactions of gases in molten salts: carbon dioxide and oxygen in cryolite alumina melts. Metall. Trans. B 1984, 15, 39−46. (9) Sada, E.; Katoh, S.; Yoshii, H.; Takemoto, I.; Shiomi, N. Solubility of carbon dioxide in molten alkali halides and nitrates and their binary mixtures. J. Chem. Eng. Data 1981, 26, 281−283. (10) Zhang, L. L.; Huang, X. Y.; Qin, C. Y.; Brinkman, K.; Gong, Y. H.; Wang, S. W.; Huang, K. V. First spectroscopic identification of pyrocarbonate for high CO2 flux membranes containing highly interconnected three dimensional ionic channels. Phys. Chem. Chem. Phys. 2013, 15, 13147−13152. G
DOI: 10.1021/acs.jced.6b00043 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
