Thermodynamic Studies of CO2 + TBAB + Water ... - ACS Publications

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Thermodynamic Studies of CO2 + TBAB + Water System: Experimental Measurements and Correlations Wei Lin,† Didier Dalmazzone,*,† Walter Fürst,† Anthony Delahaye,‡ Laurence Fournaison,‡ and Pascal Clain‡ †

ENSTA ParisTech-UCP, 828 Boulevard des Maréchaux, 91762 Palaiseau Cedex, France LGP2ES (EA 21), IRSTEA-GPAN, Parc de Tourvoie BP 44, 92136 Antony Cedex, France

‡

ABSTRACT: Thermodynamic properties of TBAB + CO2 + water system are important for using TBAB + CO2 semiclathrate hydrates in some promising hydrate-based techniques. This work is an experimental and modeling study of two fundamental properties in vapor−liquid phase equilibrium of TBAB + CO2 + water, that is, CO2 solubility in TBAB solution and density of CO2-saturated TBAB solution. With the use of a PVT system, CO2 solubility was measured; thus, Henry’s constant was obtained at temperatures from 288.1 K to 303.1 K and TBAB mass fraction from 0.10 to 0.40. Our results showed that CO2 is either salted-in or salted-out by the presence of TBAB, depending upon the temperature and TBAB concentration. For the density, we present experimental data for TBAB + water system at T = 283.15 K to 303.15 K with pressures up to 5 MPa. The density of CO2-saturated TBAB aqueous solution was calculated with the help of Duan’s model, which takes into account the effect of pressure and temperature to calculate the apparent molar volume of dissolved CO2 in pure water.

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INTRODUCTION Clathrate hydrates are crystalline compounds, in which water molecules construct host lattices with inclusion of small guest molecules, such as CH4 and CO2. The host lattice is a hydrogen-bonded network, which is stabilized by van der Waals forces between water molecules and the enclathrated molecule.1 Clathrate hydrate slurries (CHS) are two-phase fluids containing liquid and clathrate hydrate particles, in which the cooling energy is stored as latent heat. Clathrate hydrate slurries can be used as secondary refrigerants in the airconditioning system to reduce the use of primary refrigerants, for example, hydrofluorocarbons (HFCs).2,3 Compared with the current secondary refrigerants, the major advantages of CHS lie in (1) high phase change temperature (>273.1 K) resulting in a higher coefficient of performance than that of ice slurries and (2) higher thermal storage density than that of single-phase fluids, such as chilled water.2−4 Previous works investigated CO2 hydrate slurries because CO2 hydrates have a large melting heat of 374 kJ per kg of hydrates.4 Semiclathrate hydrates that are formed from quaternary ammonium salts are also studied due to their appropriate melting temperature for air-conditioning application (278 to 285 K at atmospheric pressure).2,5,6 The rheological investigation of CO2 hydrate slurries and tetra-nbutyl ammonium bromide (TBAB) hydrate slurries showed that these CHS are fluid enough to flow easily in the secondary refrigeration loop.7−10 Recently, the double hydrates of CO2 + tetrahydrofuran11 and CO2 + tetrabutylammonium halide12,13 have received much attention as they are thermodynamically more stable than single semiclathrate hydrates and CO2 © 2013 American Chemical Society

hydrates. In this work, we address the CHS formed from the CO2 + TBAB + water system. The behavior of clathrate hydrate slurries is largely dependent on the solid mass fraction that is the quantity of solid particles in suspension. This parameter is essential to quantify the latent energy stored in the fluids, and also to control the flowing conditions of the hydrate slurries.8,10 Therefore, Marinhas et al. developed a model to estimate the CO2-contained hydrate mass fraction in the hydrate slurries.14,15 This model is based on the CO2 mass balance at different thermodynamic equilibrium conditions in a closed or open system. For modeling the hydrate mass fraction, it is necessary to know the gas solubility and density of the liquid phase; so this work presents recent results regarding gas solubility and density of the CO2 + TBAB + water system. On the other hand, these solubility data are also useful to validate the models that were proposed to calculate the equilibrium properties of the CO2 + TBAB + water system.16 In the current work, CO2 solubility were measured systematically at the TBAB mass fraction from 0.10 to 0.40, T = (288.1 to 303.1) K and P = (0.5 to 3.0) MPa (total pressure). The density of TBAB aqueous solution was measured at T = (283.1 to 303.1) K and P = (0.1 to 5.0) MPa. By using the solubility and density data, the density of CO2-saturated TBAB solutions was calculated with the help of Duan’s model.17 Received: March 21, 2013 Accepted: June 25, 2013 Published: July 10, 2013 2233

dx.doi.org/10.1021/je400272k | J. Chem. Eng. Data 2013, 58, 2233−2239

Journal of Chemical & Engineering Data

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Article L xCO2 = nCO /ntotal 2

EXPERIMENTAL SECTION Experimental Apparatus and Procedure for CO2 Solubility Measurement. A schematic of the experimental apparatus is shown in Figure 1. The apparatus mainly consists

where ntotal is the total number of moles in the liquid sample. Experimental Apparatus and Procedure for Density Measurement. The density measurements were performed by using a vibrating-tube densimeter (Anton Paar, model DMA HPM). The oscillation period was determined with an uncertainty of 10−6 ms, corresponding to a density precision of 10−5 g·cm−3. The densimeter was calibrated by the method developed by L.A. Galicia-Luna et al.;18 water and dichloromethane (CH2Cl2) were used as reference fluids in the calibration. Water was distilled first and then degassed twice by using freeze−pump−thaw process while dichloromethane was used without further purification. The density of water was available from the Web site of NIST (http://webbook.nist.gov/ chemistry/fluid). The density of dichloromethane, presented by Anton Paar, was obtained from Dermiriz’s work.19 After the tube of the densimeter was carefully cleaned with water and acetone, the sample was introduced into the densimeter by the syringe pump, which regulated the pressure of the sample. When the thermal equilibrium was reached, the oscillation period was recorded and then temperature was adjusted to a new condition. Each data point was measured three times, and the average value was given. To verify the calibration procedure, we measured the density of 1-propanol and THF at the temperature range of 293.1 K to 308.1 K at atmospheric pressure. The results showed that the average absolute deviations (AAD) between literature data and our results were 0.2 % and 0.4 % for the THF and 1-propanol, respectively. Uncertainty Analysis. For the solubility measurement, the standard uncertainty was estimated as follows: for pressure, the u(P) was 0.003 MPa; for temperature, the u(T) was 0.1 K. For the CO2 solubility, the source of random error came mainly from pressure, as the calculated sensitivity coefficient of pressure was far greater than that of temperature. The relative standard uncertainty of solubility was estimated to be 2 %. If the standard uncertainty is assumed to be normally distributed, the coverage factor is taken as 2; thus, each expanded uncertainty was estimated to be U(T) = 0.2 K, U(P) = 0.006 MPa and U(xCO2) = 4 %. These expanded uncertainty correspond to a level of confidence of 95 %. The standard uncertainty of density was estimated by the standard deviation of the mean from three measurements, that is, u(ρ) is ± 0.0001 g·mL−1. The coverage factor is 4.30 given that the degrees of freedom is 2 and level of confidence is 95 %; thus, the expanded uncertainty U(ρ) was estimated to be ± 0.0004 g·mL−1. Materials. Tetra-n-butylammonium bromide (TBAB, 98 %) was purchased from Fluka Company. 1-Propanol (98.5 %), tetrahydrofuran (98.5 %), and dichloromethane (99.5 %) were obtained from Prolabo Company. N45 grade CO2 (purity > 0.99995) was purchased from Air Liquide. All these chemicals were used without further purification. Distilled water was used to prepare the liquid sample. The sample mass was weighed by an electronic balance with a precision of 0.01 mg. All the information such as source and purity for each chemical sample studied in this paper is summarized in Table 1.

Figure 1. Schematic diagram of experimental devices: (1) thermostat; (2) syringe pump; (3) thermostat; (4) reactor; (5) data acquisition and PC; (6) overhead stirrer; (7) CO2 gas cylinder.

of an equilibrium reactor equipped with an overhead stirrer, which has two blade impellers located in the liquid and vapor phase, respectively. The reactor, made of stainless steel, has a volume of 0.620 L and was immersed into a thermostat consisting of an ethylene glycol/water (20 wt %) mixture. The syringe pump was temperature-regulated using a circulating thermostat (Julabo, F10) and allowed the precise measurement of the volume of sample solution (with a precision of 0.01 mL). A resistance temperature detector probe with an accuracy of 0.1 K was used to measure the temperature of the liquid phase. The pressure was measured by a pressure transducer (Druck, PTX 611) with a precision of 0.001 MPa and an accuracy of ± 0.01 % of the given span (0 to 3 MPa). A data acquisition unit (AOiP PC-10) was connected to a PC to record system temperature and pressure every 5 s. The reactor and its loading lines were evacuated first to remove the air, then CO2 was charged into the reactor until a desired pressure was reached. Once the temperature and pressure of the reactor was stabilized, we denoted it as state zero (P0, T0). While the temperature of syringe pump was kept constant at T0, the liquid with known volume and TBAB concentration was injected into the reactor using the syringe pump. Then the stirrer was started to accelerate the dissolution process. When the pressure reached a constant value (unchanged for 6 h), the dissolution equilibrium was attained and the corresponding pressure and temperature were recorded as P1, T1. Subsequently, the temperature was changed to reach another equilibrium condition. The number of moles of CO2 dissolved in the aqueous phase is given by eq 1. L nCO = 2

P0V0 P (V − Vl ) − 1 0 Z0RT0 Z1RT1

(2)

(1)

where V0 is the total volume of the reactor; Vl is the volume of sample solution at T1 and can be obtained from the syringe pump. The effect of CO2 dissolution on the volume change of the liquid phase was neglected. The compressibility factor Z was computed by the Peng−Robinson equation of state. The EoS data for CO2 was obtained from the NIST chemistry book. (http://webbook.nist.gov/cgi/cbook.cgi?ID=C124389&Units= SI&Mask=1#Thermo-Gas). The mole fraction of CO2 in the liquid phase was represented by eq 2.

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EXPERIMENTAL RESULTS CO 2 Solubility in TBAB Solution and Henry’s Constant. First, the experimental equipment and procedure were checked by measuring CO2 solubility in pure water at 2234



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Table 1. Source, Abbreviation, Purity for Each Chemical Sample Reported in This Work chemical name

abbreviation

tetra-n-butylammonium bromide 1-propanol

TBAB

tetrahydrofuran

THF

initial mass fraction

source Fluka Company Prolabo Company Prolabo Company Prolabo Company Air Liquide

dichloromethane carbon dioxide

0.98 0.985 0.985 0.995 > 0.99995

Figure 2. CO2 mole fraction in the TBAB solutions and pure water at various temperatures. For wTBAB = 0.40: ×, 288.1 K; ▲, 293.1 K; □, 298.1 K; ◆, 303.1 K. For pure water: solid line, 288.1 K; dotted line, 303.1 K, calculated by the model of ref 21.

293.1 K with total pressures up to 2 MPa. The results showed that our Henry’s constant, 141.8 MPa, is close to that from ref 20, that is, 143.7 MPa. This indicates that the apparatus and procedure used are reliable. CO2 solubility measurements were conducted at wTBAB = 0.1 to 0.40 and a temperature range from 288.1 K to 303.1 K. All the isothermal p−x data sets are tabulated in Table 2. Figure 2 shows the experimental data for wTBAB = 0.40 at different temperatures, together with the data for pure water calculated by Diamond and Akinfiev’s model.21 From Figure 2, a noticeable feature is that TBAB exhibits both a salting-in and salting-out effect on CO2 while the presence of most salts tends to decrease the solubility of CO2. This complex salinity effect is temperature-dependent, changing from saltingin at 303.1 K to salting-out at 288.1 K. Indeed, this result is not surprising as TBAB was reported to salt-in CH4 in water at 298.1 K but salt-out CH4 at 283.1 K.22 As a consequence of salinity effect transition, the influence of temperature on CO2 solubility becomes weaker with the presence of 40 wt % TBAB compared with that in pure water.

With the experimental solubility data, Henry’s constant of CO2, KH, can be calculated by eq 3, where f CO2 is CO2 fugacity in the vapor phase in MPa and computed using the Peng− Robinson EoS; P is the total pressure of vapor phase in MPa; PHsat2O is the vapor pressure of pure water at temperature T in MPa and was obtained from NIST database; v∞ CO2 is the partial molar volume of CO2. KH =

fCO

2

∞ xCO2 exp[vCO (P − PHsat2O)/RT ] 2

(3)

It should be noted that eq 3 was used on the basis of two assumptions: (1) The concentration of the CO2 is sufficiently small so that the activity coefficient of CO2 is unity. (2) The

Table 2. Experimental Vapor−Liquid Equilibrium Data at Temperature T, Pressure P, and CO2 Mole fraction xCO2 for the TBAB + CO2 + Water System wTBAB = 0.10 b

a

c

d

wTBAB = 0.20 c

wTBAB = 0.30

d

P /MPa

xCO2 (·10 )

P /MPa

xCO2 (·10 )

P /MPa

xCO2 (·10 )

P /MPa

xCO2d (·103)

288.1 288.1 288.1 288.1 288.1 293.1 293.1 293.1 293.1 293.1 293.1 298.1 298.1 298.1 298.1 298.1 298.1 303.1 303.1 303.1 303.1 303.1

0.504 0.700 0.946 1.484 1.871 0.524 0.728 0.984 1.130 1.543 1.948 0.544 0.754 1.021 1.17 1.594 2.018 0.563 0.779 1.056 1.644 2.085

3.85 5.10 6.93 10.33 12.44 3.54 4.64 6.39 7.20 9.43 11.55 3.24 4.27 5.88 6.61 8.84 10.78 2.98 3.94 5.46 8.35 10.14

0.562 0.827 1.063 1.601 1.789 0.582 1.034 1.449 2.173 2.400 2.835 0.603 1.135 1.490 1.708 1.901

3.87 5.69 7.14 10.2a 11.41a 3.59 6.46 8.70 11.70 13.19 14.80 3.29 6.20 8.33 8.96 9.92

0.746 1.012 1.434 1.941 2.523 0.768 1.054 1.476 2.009 2.044 2.620 0.789 1.091 1.518 2.066 2.694

5.21 7.07 9.77a 12.46a 16.22a 4.87 6.23 9.40 11.48 11.76 14.46 4.66 6.43 8.96 11.02 13.99

0.639 0.851 1.160 1.491 1.976 0.656 1.000 1.383 1.757 2.014 2.552 0.673 1.024 1.571 1.831 2.619

4.41 5.78 7.70a 9.88a 12.74a 4.23 6.55 8.92 11.0 12.61 14.80 4.08 6.25 9.24 10.7 14.6

0.621 0.908 1.169 1.759 1.954

3.09 4.63 5.83 8.47 9.38

0.810 1.133 1.558 2.114 2.155

4.46 6.54 8.69 10.95 11.06

0.689 0.900 1.252 1.610 2.136

3.96 5.15 6.95 8.99 11.78

3

c

d

wTBAB = 0.40

T /K

3

3

c

CO2 solubility for the metastable vapor−liquid equilibrium. bU(T) = 0.2 K. cU(P) = 0.006 MPa. dU(xCO2) = 4 %. 2235



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partial molar volume of CO2, v∞ CO2, is independent of pressure and salinity. In our case, CO2 solubility is less than 2 mol %, which is small enough to be consistent with the assumptions. Regarding salinity, it is quite low as the TBAB mass fraction of 0.40 corresponds to a low mole fraction of 0.036; besides, according to Malinin,23 the salinity has a weak influence on the partial molar volume of CO2. Therefore, v∞ CO2 is calculated using an empirical equation only dependent on temperature, as suggested by Dhima et al.24 The dependence of KH on temperature was correlated by a two-term polynomial, as shown in eq 4 and the correlated parameters are given in Table 3. The parameters for pure water

Table 4. Experimental Values of Density ρ at Temperature T, Pressure P and Mass Fraction of TBAB for TBAB + Water System ρa/g·cm−3 P/MPa

T/K

wTBAB = 0.10

wTBAB = 0.14

wTBAB = 0.20

wTBAB = 0.30

wTBAB = 0.40

0.1 0.1 0.1 0.1 0.1 0.1 0.1 1.0 1.0 1.0 1.0 1.0 2.0 2.0 2.0 2.0 2.0 3.0 3.0 3.0 3.0 3.0 4.0 4.0 4.0 4.0 4.0 5.0 5.0 5.0 5.0 5.0

303.15 298.15 293.15 291.15 288.15 285.15 283.15 303.15 298.15 293.15 291.15 288.15 303.15 298.15 293.15 291.15 288.15 303.15 298.15 293.15 291.15 288.15 303.15 298.15 293.15 291.15 288.15 303.15 298.15 293.15 291.15 288.15

1.0032 1.0049 1.0062 1.0067 1.0076 1.0083 1.0087 1.0037 1.0053 1.0067 1.0071 1.0080 1.0041 1.0057 1.0070 1.0076 1.0084 1.0045 1.0061 1.0075 1.0080 1.0089 1.0050 1.0065 1.0079 1.0084 1.0092 1.0054 1.0069 1.0083 1.0087 1.0097

1.0063 1.0084 1.0100 1.0109 1.0116 1.0126 1.0129 1.0066 1.0087 1.0104 1.0112 1.0121 1.0070 1.0091 1.0108 1.0116 1.0125 1.0074 1.0095 1.0112 1.0120 1.0129 1.0077 1.0099 1.0116 1.0124 1.0133 1.0081 1.0103 1.0121 1.0127 1.0137

1.0119 1.0142 1.0163 1.0170 1.0181 1.0193 1.0199 1.0124 1.0145 1.0167 1.0174 1.0186 1.0128 1.0149 1.0170 1.0178 1.0190 1.0132 1.0153 1.0175 1.0182 1.0194 1.0136 1.0157 1.0178 1.0186 1.0197 1.0140 1.0161 1.0182 1.0188 1.0201

1.0207 1.0237 1.0263 1.0274 1.0290

1.0286 1.0319 1.0353 1.0367 1.0385

1.0212 1.0240 1.0268 1.0277 1.0294 1.0216 1.0244 1.0271 1.0281 1.0297 1.0220 1.0248 1.0275 1.0285 1.0301 1.0224 1.0251 1.0279 1.0288 1.0304 1.0227 1.0255 1.0282 1.0291 1.0307

1.0292 1.0323 1.0357 1.0370 1.0390 1.0295 1.0327 1.0360 1.0374 1.0394 1.0299 1.0330 1.0364 1.0378 1.0397 1.0303 1.0333 1.0368 1.0381 1.0401 1.0307 1.0337 1.0371 1.0384 1.0404

Table 3. Adjusted Coefficients for eq 4, Temperature Dependence of Henry’s Constant for CO2 in Aqueous TBAB Solutions wTBAB

A

B

0a 0.05b 0.10b 0.10 0.20 0.30 0.40

13.460 11.981 11.981 11.434 10.742 9.743 8.238

−2491.9 −2028.0 −2009.9 −1886.1 −1669.7 −1389.6 −995.5

temperature range (283.1 (281.1 (281.1 (288.1 (288.1 (288.1 (288.1

to to to to to to to

372.1) 291.1) 291.1) 303.1) 303.1) 303.1) 303.1)

K K K K K K K

relative coefficient

0.997 0.995 0.962 0.995

a Correlated by the data from ref 20. bCorrelated by the data from ref 25.

were correlated by the Henry’s constant from Carroll,20 and the parameters for wTBAB = 0.05 and 0.10, measured by Thiam,25 are also presented in Table 3. From Table 3, the values of A and B in our work showed contrary tendency with a TBAB concentration increase, that is, decrease and increase, respectively. Besides, the data for pure water and wTBAB = 0.05 and 0.10 from Thiam follow a similar trend. B ln(KH/MPa) = A + (4) T /K a

Density of TBAB + Water System. Under the pressure range of 0.1 MPa to 5 MPa, the density measurement of aqueous TBAB solution was performed as a function of temperature (283.1 K to 303.1 K) and TBAB mass fraction (wTBAB = 0.10 to 0.40). The density data are given in Table 4, where the data for wTBAB = 0.30 and 0.40 at low temperature and atmospheric pressure is not given because the temperature is very close to the equilibrium temperature of TBAB semiclathrate hydrates. Belandria et al.26 determined the density of TBAB aqueous solution at atmospheric pressure at nine temperatures (293.15 K to 333.15 K) and 10 TBAB concentrations (wTBAB = 0.0697 to 0.50). The agreement between their data and ours is good as the absolute deviations (AD) are less than 0.1 % at wTBAB = 0.10, 0.20, and 0.40 and less than 0.4 % at wTBAB = 0.30, respectively. Density of CO2 + TBAB + Water System. On the basis of thermodynamic theory, the density of binary solution of CO2 + water can be expressed as 1 ρCO − H O 2

2

=

φ xCO2V CO 2

MT

+

molecular weight of CO2 aqueous solution, expressed as MT = xCO2MCO2 + xH2OMH2O. By assuming CO2 + TBAB + water system as pseudobinary solutions of TBAB solution and CO2, eq 5 can be rewritten as eq 6 to calculate the density of CO2saturated TBAB solution. 1 ρCO − TBAB − H O 2

2

2

=

φ xCO2V CO 2

MT

+

M TBAB − H2O(1 − xCO2) ρTBAB − H O M T 2

(6)

where MTBAB−H2O and MT are the molecular weight of TBAB solution without and with CO2, respectively. ρTBAB−H2O is the density of CO2-free TBAB solution and was obtained through a linear correlation derived from experimental data in Table 4. M TBAB − H2O =

M H2O(1 − xCO2) ρH O M T

U(ρ) = 0.0004 g·mL−1.

x H2O x TBAB M TBAB + MH O x TBAB + x H2O x TBAB + x H2O 2

M T = xCO2MCO2 + x TBABM TBAB + x H2OM H2O

(5)

VφCO2 −1

where is the apparent molar volume of dissolved CO2 in 3 cm ·mol . MCO2 is the molecular weight of CO2; MT is

(7) (8)

In eq 6, all the parameters are known except for the apparent molar volume of CO2. To simplify the calculation, VφCO2 in eq 6 2236



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DISCUSSION Salinity Effect of TBAB on CO2 Solubility. In general, the gas solubility in the presence of a salt can be represented by the Sechenov equation. For salting-in, the Sechenov coefficient, ks, is negative and for salting-out it is positive.

is approximate with the apparent molar volume of CO2 in pure water. According to Duan et al.,17 this approximation is valid at CO2 concentration < 3 mol % and low salinity. In the open literature, there are some empirical or semitheoretical models that were proposed to calculate the apparent molar volume of CO2 in water at infinite dilution.17,27−29 The model of Duan et al. was chosen to calculate the value of VφCO2 from the following considerations: (1) The applied conditions of the model of Duan et al. cover our experimental range. (2) The model of Duan et al. considered the effect of pressure and temperature on VφCO2, whereas the pressure effect is neglected in most of other models. (3) The accuracy of the model of Duan et al. is good compared with other empirical models, as the predicted VφCO2 is found to be consistent with the experimental data of Song et al.30 and Hnedkovsky et al.31 The model of Duan et al. is shown as eqs 9 and 10, where VH2O is water volume, P is pressure in MPa, A1 and A2 are temperature-dependent, and the detailed parameters are referred to the work of Duan et al. φ V CO = VH2O(1 + A1 + A 2 P) 2

Article

ln(KH/KH0) = ksC TBAB

(11)

K0H

where denotes Henry’s constant of CO2 in pure water at the same temperature as KH and was obtained from ref 20. CTBAB is the TBAB concentration. The logarithm of the Henry’s constant ratio ln(KH/K0H) is plotted in Figure 4 against TBAB mass fraction. As seen from

(9)

Ai = Ai1T 2 + Ai2 T + Ai3 + Ai 4 T −1 + Ai5T −2 (i = 1, 2)

(10)

Following eqs 6 to 10, the density of the CO2-saturated TBAB solution was calculated with the measured solubility and density data. The calculated results for wTBAB = 0.40 are illustrated in Figure 3, together with the measured density of

Figure 4. Plot of ln(KH/K0H) versus TBAB mass fraction for CO2 in aqueous TBAB solutions at different temperatures: ◊, 288.1 K; ■, 293.1 K; △, 298.1 K; ●, 303.1 K.

Figure 4, at lower temperature of 288.1 and 293.1 K, only salting-out effect was observed regardless of TBAB concentration; while at higher temperature of 298.1 and 303.1 K, ln(KH/KH0) decreases with TBAB concentration increase, resulting in a transition from salting-out to salting-in. Our results suggested that increasing temperature and TBAB concentration facilitates the salting-in process. In analogy, Wen and Hung22 reported that with the presence of 24.4 wt % TBAB, the increment of CH4 solubility rose from 4 % to 26.7 % when the temperature increased from 288.1 to 308.1 K. Compared with inorganic salts, TBAB shows a weaker saltingout effect on CO2 solubility; for instance, the sechenov coefficients at 298.1 K are 0.1 L·mol−1 and 0.02 L·mol−1 for NaCl32 and TBAB (wTBAB = 0.20), respectively. The mechanism of TBAB salinity effect on CO2 solubility was analyzed as follows: TBA+ ions that have alkyl chains and low charges exhibit hydrophobe-like character. It is believed that the change of water structure brought about by the presence of quaternary ammonium ions is an important factor responsible for the salting-in process.33−36 On the other hand, Wen and Hung22 proposed a “cage effect” to explain the salting-out effect of TBAB on CH4, which was observed at 278.1 K and wTBAB = 0.11 to 0.20. This cage is not for the small solutes but for hiding the hydrocarbon chain of TBA+ ions and by doing so the hydrocarbon solubility will be reduced. From the above, we deduced that there is a competition between two opposite trends with temperature and TBAB concentration change, that is, TBAB reorganizing the water molecule to accommodate more CO2 or enhancing the water structure thereby repelling CO2. At a certain temperature and TBAB concentration, these two competing factors might have similar contributions, which cancel out the salinity effect on CO2 solubility. As a result, the value of ln (KH/K0H) is close to zero,

Figure 3. Density of CO2-saturated and CO2-free TBAB aqueous solutions as a function of pressure (wTBAB = 0.40).

CO2-free TBAB solution. As seen in Figure 3, the dissolution of CO2 raises the density of aqueous TBAB solution, and the density difference between CO2-saturated and CO2-free TBAB solution increases with a pressure increase. Generally speaking, the density change of the aqueous solution due to gas dissolution relies on the apparent mass density of dissolved gas and aqueous solution. By dividing the CO2 molecular weight with the apparent molar volume of CO2, the apparent mass density of CO2 was calculated to be approximately 1.25 g·cm−3. This value is higher than the density of TBAB aqueous solution, that is, approximately 1.04 g·cm−3 (from Table 4); therefore, the dissolution of CO2 in TBAB aqueous solution produced a density increases. 2237



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as examples for wTBAB = 0.40 at 293.1 K and wTBAB = 0.20 at 303.1 K. Salinity Effect of TBAB on Density Change. In the literature, there are some empirical models to describe the density change between CO 2 -free and CO 2 -bearing brine.27,30,37 Following Teng and Yamasaki’s method,37 the calculated density data was re-evaluated to define a density change, Δρ, as the density difference between CO2-saturated TBAB solution, ρCO2−TBAB−H2O and CO2-free TBAB solution, ρTBAB−H2O under the same p, T conditions. In the case of pure water, CO2 solubility was calculated by Diamond and Akinfiev’s model,21 ρCO2−H2O was calculated by eqs 5, 9, and 10, and ρH2O was obtained from the NIST database. Figure 5 illustrates the influence of TBAB concentration on the density change that was caused by CO2 dissolution. At a

density data, the density of CO2-saturated TBAB solution was calculated with the help of the model of Duan et al. It was found that at the given TBAB concentration, the density change of TBAB solution caused by CO2 dissolution increased linearly with CO2 mole fraction, but was independent of temperature and pressure. The derived density change analyses should be tested with further experiments.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 5. Density change of CO2-saturated TBAB solution against CO2 mole fraction at different TBAB mass fractions: +, pure water; ○, wTBAB = 0.1; ◆, wTBAB = 0.2; △, wTBAB = 0.3; ■, wTBAB = 0.4.

given TBAB concentration, it was found that the density change of TBAB solution and pure water (Δρ) can be fitted using a linear function with CO2 mole fraction, independent of temperature and pressure. Our results were similar to those obtained by Song et al.30 as they found the density change of a 3.5 wt % seawater solution is linearly dependent on the CO2 mass fraction (wCO2) at 276.1 K and 283.1 K within the pressure range from 4 to 13 MPa. Besides, their results showed that the slope of density change versus wCO2 is independent of salinity when comparing the slopes in 3.5 wt % seawater and pure water. However, in our work, the salinity influences the density change of CO2-saturated TBAB solution, that is, the slope of density change versus xCO2 decreases with TBAB concentration increase. Our results indicated that CO 2 dissolution causes less density increase of TBAB solution at higher TBAB concentration.

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CONCLUSIONS The solubility of CO2 in aqueous TBAB solutions was studied at temperature ranges from 288.1 K to 303.1 K with pressures up to 3 MPa. The complex salinity effect of TBAB on CO2 solubility was observed, that is, changing from salting-in to salting-out by decreasing TBAB concentration and temperature. This result was attributed to the special structure of TBAB that exhibits both hydrophobic and hydrophilic character. The density of TBAB aqueous solution was determined over T = (283.15 to 303.15) K at pressures up to 5 MPa. With a combination of the experimental solubility and 2238



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