Determination of CO2 Solubility in Water and NaCl Solutions under

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Determination of CO2 Solubility in Water and NaCl Solutions under Geological Sequestration Conditions Using a Fused Silica Capillary Cell with in Situ Raman Spectroscopy Junliang Wang,† Benben He,† Lifeng Xie,† Ke Bei,†,‡ Guixuan Li,† Zuhua Chen,§ I.-Ming Chou,∥ Chunmian Lin,† and Zhiyan Pan*,† †

College of Environment, Zhejiang University of Technology, Hangzhou 310032, China College of Life and Environmental Science, Wenzhou University, Wenzhou 325035, China § Petroleum Exploration and Development Research Institute, Huadong Branch Company of China Petroleum & Chemical Co., Ltd., Nanjing 210011, Jiangsu, China ∥ Laboratory for Experimental Study under Deep-sea Extreme Conditions, Institute of Deep-sea Science and Engineering, Chinese Academy of Sciences, Sanya 572000, Hainan, China

J. Chem. Eng. Data Downloaded from pubs.acs.org by ALBRIGHT COLG on 04/25/19. For personal use only.

‡

ABSTRACT: To investigate the solubility of carbon dioxide (CO2) in geological fluids at different temperatures and pressures, we developed a method for measuring solubility using an optically transparent fused silica capillary cell as a high-pressure optical cell combined with Raman spectroscopy under geological sequestration conditions (temperatures of up to 353.15 K and pressure of up to 30.0 MPa). On the basis of the fact that the band intensity of an active Raman species is proportional to its concentration, we determined the linear correlation between the known CO2 concentration and the ratio of the Raman peak heights of CO2 to H2O (νCO2/νH2O) in homogeneous CO2−H2O and CO2−H2O−NaCl systems. Our results indicate that the Raman peak height ratio is a function of the CO2 concentration and that the pressure and temperature do not significantly affect the relationship between the CO2 concentration and νCO2/νH2O within the experimental P−T−x conditions. The νCO2/νH2O values of the CO2-saturated solutions were determined by simulating the CO2 geological sequestration T−P−x conditions. Then, the CO2 solubilities were calculated using the linear relationship. Our results indicate that the method is feasible and that the solubility of CO2 in H2O or NaCl solutions decreases with increasing temperature, increases with increasing pressure, and decreases with increasing salinity. On the basis of a comparison between our experimental data and the results of the previous model, our method provides satisfactory results.

1. INTRODUCTION

are thought to have the largest storage capacity potential for CO2 sequestration.13 In order to estimate the total amount of CO2 that can be stored in a reservoir, it is necessary to accurately access the solubility of CO2 in H2O and in brine under reservoir conditions. In addition, it is also necessary to understand the details of the numerous CO2-related geological processes occurring in the reservoir.14,15 The solubility of CO2 in aqueous solutions has been studied for decades.16 The experimental methods developed in these studies can be divided into two categories: analytical methods

The worldwide environmental problems caused by global warming have attracted a great deal of attention. Because of the combustion of fossil fuels, the concentration of CO2, which is a greenhouse gas, in the atmosphere has increased significantly, leading to dramatic climate changes. Thus, finding ways to reduce the CO2 content of the atmosphere has become a topic of great interest in the research community.1,2 CO2 capture and storage has been proposed as a promising technique for reducing CO2 emissions.3,4 Geological storage is generally regarded as the most mature option for isolating the CO2 captured from the atmosphere. The geological reservoirs being studied include depleted oil and gas reservoirs,5,6 coal beds,7 deep saline aquifers,8 ocean storage,9 and mineral carbonation.10−12 Deep saline aquifers © XXXX American Chemical Society

Received: January 5, 2019 Accepted: April 4, 2019

A

DOI: 10.1021/acs.jced.9b00013 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

and synthetic methods.14 The analytical methods can be performed by collecting samples from an equilibrium cell and analyzing them at atmospheric pressure or by using a physicochemical analysis method in a pressure equilibrium cell. These analyses commonly include volumetric analysis, gravimetric analysis, chemical titration, and chromatographic analysis.17−22 Volumetric analysis includes measuring the volume of CO2 precipitated from a volume of sample solution at room temperature and atmospheric pressure and using the Bunsen absorption coefficient to correct for the CO2 remaining in solution.17,18 Prutton and Savage used volumetric analysis to measure the solubility of CO2 in calcium chloride−water solutions at 348.15, 373.15, and 393.15 K and at pressures of up to 70 MPa.19 Gravimetric analysis involves weighing the samples collected from a high-pressure cell, releasing the CO2 from the solution, and weighing the solution sample again, which allows for the determination of the amount of CO2 in the solution. Truche et al. used gravimetric analysis to measure the solubility of CO2 in NaCl solutions at temperatures of less than 453.15 K and pressures of less than 15 MPa.20 Chemical titration involves the use of an alkaline reagent with a known concentration, for example, NaOH or Ba(OH)2, to neutralize the CO2 in a solution, and then, the amount of CO2 is determined based on the amount of OH− consumed. Mohammadian et al. used potentiometric titration to determine the solubility of CO2 in brines (0−15 000 ppm NaCl) at temperatures of 333.15−373.15 K and pressures of up to 25 MPa.21 The spectroscopic method involves the use of spectroscopy to analyze the CO2 content released from the solution. Servio and Englezos employed gas chromatography to investigate the solubility of CO2 in water in the presence of a gas hydrate.22 Synthetic methods involve the preparation of a mixture with a known CO2 concentration and the observation of its phase behavior in an equilibrium cell. One common synthetic method is the PVT method, which determines the solubility of CO2 using the P−V−T parameters of CO2 before and after phase equilibrium. Nighswander et al. used this method to determine the solubility of CO2 in water and in a 1.0 wt % NaCl solution at pressures of up to 10 MPa and temperatures of 353.15− 473.15 K.23 Conventional CO2 solubility measurement methods have advantages, but they also have some shortcomings. For instance, in the volumetric analysis and gravimetric analysis methods, the sampling needs to be conducted during the experiment, which causes depressurization and breaks the original equilibrium of the system, leading to uncertainty in the obtained solubility data. Traditional cells have relatively large volumes, which result in large temperature gradients, slow mass-transfer rates, and heterogeneous concentrations. In addition, these methods cannot be operated continuously and are often time-consuming. Raman spectroscopy provides a nondestructive in situ analysis technique. This method is used in many fields because of its practicality, high efficiency, and sensitivity.24 Detailed information on compounds, for example, composition and molecular orientation, can be obtained by analyzing the Raman spectrum.25 In addition, confocal Raman spectroscopy can be used to quantitatively monitor the concentration of gases such as CO2 and CH4 in aqueous solutions over a wide range of temperatures and pressures because the band intensity of an active Raman species is proportional to its concentration in a fluid.20,26 The Raman peak intensity ratio reflects the varying concentration of the solute and solvent,27 which is useful in determining the phase behavior of the aqueous solution.

Several previous studies have used Raman spectroscopy to quantitatively or qualitatively analyze the dissolution property of CO2 and CH4 in water, brine, and model oil.28−35 However, the application of Raman spectroscopy also has its limitations, which is often limited while analyzing samples containing highly fluorescent substance because strong noise from fluorescence obscures the Raman signal.36 In addition, the fused silica capillary cell has superior mass and heat transfer, pressure, and temperature resistance, which can reduce the temperature gradient and reagent consumption, and it is easily adapted for use in target applications as a high-pressure optical cell (HPOC).37,38 In this study, based on the advantages of using the fused silica capillary cell as an HPOC, we present a simple and fast methodology for determining the solubility of CO2 in H2O and brines using in situ Raman spectroscopy. We synthesized unsaturated homogeneous CO2−H2O and CO2−H2O−NaCl systems with known compositions. Then, we employed Raman spectroscopy to verify the phase equilibrium and obtained a series of Raman spectra at fixed temperatures and pressures. The relationship between the Raman peak intensity ratio (based on the CO2 Fermi dyad, νCO2, and the O−H stretching vibration band of H2O, νO−H) and the known CO2 concentration was determined. By acquiring the Raman peak intensity ratio of H2O to CO2 in the CO2-saturated solution at different temperatures and pressures using the obtained relationship between the peak intensity ratio and CO2 concentration, the solubilities of CO2 in H2O and NaCl brine solutions under various geological conditions were calculated.

2. MATERIALS AND METHODS 2.1. Materials. Table 1 presents the chemicals used in this work. All chemicals were purchased commercially and used Table 1. Chemicals Used in This Work chemical name CO2 NaCl a

source

purity/wt %

Pujiang Special gas Co., Ltd Xilong Scientific Co., Ltd

99.995 99.5a

a

M/g·mol−1 44.01 58.44

Purities given by supplier; no further purification was performed.

immediately after receiving. The ultrapure water (resistivity 18.2 MΩ cm) used as the solvent was prepared in the laboratory using a secondary reverse osmosis system (UPT-II-20, Ulupure, China). NaCl (purity 99.5%) was supplied by Xilong Scientific Co., Ltd. (Guangdong, China) and dried in an oven at T = 373.15 K and at ambient pressure before using it. CO2 (purity 99.995%) was purchased from Pujiang Special Gas Co., Ltd. (Shanghai, China). The type TSP300665 silica capillary tubing (665 μm in outer diameter and 300 μm in inner diameter with polyimide coating) was purchased from Polymicro Technologies LLC (Phoenix, AZ, USA). All the valves and the high-pressure stainless steel tubing were purchased from High Pressure Equipment Co. (Catalog nos. 15-11AF1, 15-15AF1, and 15-9A1-30). NaCl solutions were prepared gravimetrically with the relative uncertainties in mass being below 0.01%. Thus, the uncertainty of molality was most probably limited by the purity of the salt and was taken to be approximately 0.5%. 2.2. Apparatus. An experimental setup was designed to investigate the solubility of CO2 in solutions (Figure 1). The apparatus is composed of a phase equilibrium vessel (volume is adjustable) with a 70 MPa pressure generator, a quantitative pump (30 MPa full scale), a circulation pump, a fused silica B



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Figure 1. Schematic of the in situ CO2 solubility determination system.

Figure 2. Changes in the phase state during heating and depressurization (a → d), cooling and pressurization (e → h), in 1 mol·kg−1 NaCl solution in the HPOC.

Figure 3. Raman spectra of the CO2−H2O system at 313.15 K and 9.0 MPa from 0 to 40 min.

capillary cell as the HPOC combined with a heating−cooling stage (Linkam, CAP500, UK), and a confocal Raman spectrometer (HORIBA Jobin Yvon, HR800, France). The temperature of the aqueous solution in the HPOC was controlled via the heating−cooling stage (Linkam, T95, UK, accurate to ±0.1 K) in conjunction with a digital temperature controller. The pressure was adjusted using a 70 MPa pressure generator and measured using a Setra 206 pressure transducer (accurate to ±0.13%). The pressure in the 30 MPa quantitative pump was also measured by a Setra 206 pressure transducer (accurate to ±0.13%). 2.3. Experimental Procedures. The solubility of CO2 in an aqueous solution under simulating geological conditions (temperature of up to 353.15 K, pressure of up to 30.0 MPa, and NaCl concentration of up to 3.0 M) was measured in a circulating equilibrium system with an HPOC (665 μm O.D., 300 μm I.D., and ∼2 cm-long window). To construct an HPOC, a ∼27 cm-long section was cut from the silica capillary tubing. The polyimide layer on the surface of the ∼2 cm-long tube in the middle of the HPOC was removed using an oxyhydrogen flame for later observation and Raman analysis. The HPOC, with both ends connected to the high-pressure line, was inserted into the heating−cooling stage, which was fixed on a Raman optical platform. The relationship between the known CO2 concentration of the homogeneous aqueous solution and the Raman peak height ratio of CO2 to H2O (νCO2/νH2O) was initially determined at temperatures ranging from 303.15 to 353.15 K and at pressures ranging from 3.0 to 30.0 MPa. The following steps were used: (1) the system, including the pressure lines, was evacuated using a vacuum pump and (2) a certain amount of solution (60 mL of ultrapure water, or 1−3 M NaCl solution)

Figure 4. (a) Image of the CO2-saturated solution in the HPOC. (b) Raman spectra of the CO2−H2O system at the three positions shown in (a) at 313.15 K and 9.0 MPa.

was added to the phase equilibrium vessel from its top under vacuum at room temperature (∼20 °C). Then, the volume of the phase equilibrium vessel was adjusted and the vessel was filled up with the solution. (3) Quantitative CO2 gas was injected into the system using a 30 MPa quantitative pump (the pressure of CO2 was generally lower than 30 MPa). The amount of CO2 C



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Table 2. Raman Peak Height Ratios of the Different CO2 Concentrations at Different Pressures and at Temperatures from 303.15 to 353.15 K in a Homogeneous CO2−H2O System Raman peak height ratio −1

CCO2/mol·kg 0.210 0.380 0.607 0.885 1.077

P/MPa

303.15 K

313.15 K

323.15 K

333.15 K

343.15 K

353.15 K

SD

15.0 20.0 15.0 20.0 16.0 20.0 15.0 20.0 19.0 24.0

0.0302 0.0307 0.0534 0.0533 0.0906 0.0907 0.1258 0.1255 0.1530 0.1536

0.0302 0.0304 0.0521 0.0534 0.0890 0.0895 0.1251 0.1261 0.1524 0.1524

0.0299 0.0302 0.0531 0.0521 0.0884 0.0905 0.1249 0.1249 0.1520 0.1516

0.0296 0.0296 0.0521 0.0521 0.0905 0.0892 0.1249 0.1244 0.1516 0.1514

0.0292 0.0289 0.0519 0.0527 0.0891 0.0879 0.1243 0.1250 0.1511 0.1514

0.0285 0.0281 0.0516 0.0519 0.0883 0.0882 0.1241 0.1236 0.1488 0.1504

0.0007 0.0010 0.0007 0.0007 0.0010 0.0012 0.0006 0.0009 0.0015 0.0011

Table 3. Raman Peak Height Ratio of the Different CO2 Concentrations at Different Pressures and at Temperatures from 303.15 to 353.15 K in the NaCl Solution (b = 1 mol·kg−1) Raman peak height ratio −1

CCO2/mol·kg 0.278 0.420 0.567 0.700 0.965

P/MPa

303.15 K

313.15 K

323.15 K

333.15 K

343.15 K

353.15 K

SD

14.0 20.0 15.0 25.0 18.0 25.0 19.0 26.0 25.0 30.0

0.0417 0.0420 0.0627 0.0629 0.0845 0.0842 0.1020 0.1020 0.1396 0.1398

0.0397 0.0411 0.0621 0.0629 0.0835 0.0837 0.1010 0.1014 0.1386 0.1392

0.0405 0.0409 0.0616 0.0626 0.0833 0.0839 0.1011 0.1007 0.1388 0.1382

0.0409 0.0407 0.0612 0.0619 0.0827 0.0824 0.1001 0.0999 0.1385 0.1371

0.0403 0.0400 0.0608 0.0614 0.0833 0.0823 0.0983 0.0996 0.1367 0.1377

0.0403 0.0398 0.0610 0.0608 0.0814 0.0821 0.0987 0.0991 0.1366 0.1370

0.0007 0.0008 0.0007 0.0009 0.0010 0.0009 0.0014 0.0011 0.0012 0.0011

Figure 5. Raman peak intensity ratio at different CO2 concentrations and temperatures from 303.15 to 353.15 K in (a) H2O (■, 0.210 mol·kg−1; ●, 0.380 mol·kg−1; ▲, 0.610 mol·kg−1; ▼, 0.885 mol·kg−1; and ◆, 1.077 mol·kg−1), (b) 1 mol·kg−1 NaCl (■, 0.280 mol·kg−1; ●, 0.420 mol·kg−1; ▲, 0.570 mol·kg−1; ▼, 0.700 mol·kg−1; and ◆, 0.965 mol·kg−1), (c) 2 mol·kg−1 NaCl (■, 0.310 mol·kg−1; ●, 0.450 mol·kg−1; ▲, 0.570 mol·kg−1; ▼, 0.700 mol·kg−1; and ◆, 0.792 mol·kg−1), and (d) 3 mol·kg−1 NaCl (■, 0.240 mol·kg−1; ●, 0.360 mol·kg−1; ▲, 0.475 mol·kg−1; ▼, 0.572 mol·kg−1; and ◆, 0.675 mol·kg−1).

was controlled by the initial pressure, which was measured using a Setra 206 pressure transducer. During the experiment, the void volume of the quantitative pump and pipe gap were

known. First, valves V1 and V2 were opened and valve V3 was turned off (see Figure 1); then, CO2 was injected into the quantitative pump and the pipe gap at room temperature, and D



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from the edge of the CO2 bubbles) at temperatures ranging from 303.15 to 353.15 K and pressures ranging from 3.0 to 30.0 MPa. A HR800 Raman spectrometer equipped with a 531.95 nm laser excitation (frequency-doubled Nd:YAG, 20 mW) and a charge-coupled device (CCD) detector (multichannel, air cooled) was used for the in situ analysis of the CO2−H2O and CO2−H2O−NaCl systems. Raman spectra in the range of 1100−4000 cm−1 were obtained under various T−P conditions to obtain the peak intensity ratio of νCO2 to νO−H after the system reached phase equilibrium. Raman spectra were obtained for 20−30 s with two accumulations per spectrum to maintain a higher signal-to-noise ratio. Five independent measurements were performed for each set of T−P−x conditions, and the final presented Raman spectra intensities are the averages of these measurements.

the amount of injected CO2 was calculated from the measured initial pressure using the gas equation of state (accurate to ±0.13%). Then, valve V2 was turned off and valve V3 was opened until CO2 in the quantitative pump was completely injected into the phase equilibrium vessel, and the pressure was measured again in order to calculate the amount of remaining CO2 in the pipe gap using the gas equation of state. The amount of CO2 injected into the system is the difference between the initial and remaining amounts of CO2. (4) The mixture was compressed to ∼35.0 MPa by a 70 MPa pressure generator connected to the phase equilibrium vessel, and the magnetic stirrer in the equilibrium system was turned on for several hours to ensure the homogeneity of the solution; (5) the circulating pump was used to circulate the mixture for about 15 min; (6) the circulating pump was turned off, and the Raman spectra of the CO2 Fermi dyad and the O−H stretching band of the H2O were obtained, until the value of νCO2/νH2O remained almost unchanged; and (7) the temperature of the HPOC was set to 303.15 K using a digital temperature controller, and then, νCO2 and νO−H were measured. The solution was pressurized to approximately 5 MPa, and the Raman spectra were recorded again; and (8) the experiment was repeated to obtain the Raman spectra of the CO2-bearing homogeneous solution under different pressures and at temperatures of 303.15−353.15 K. The CO2 concentration was plotted versus the Raman peak intensity ratio of the H2O to NaCl solutions, and the equation of the calibrated line was obtained for each solution. The relationship between the CO2 concentration and the Raman peak intensity ratio was plotted and fitted using the equation η = I /CCO2

3. RESULTS AND DISCUSSION 3.1. Phase Equilibrium Verification for the CO2Bearing Solution. Phase equilibrium plays a key role in measuring the solubility of CO2 in an aqueous solution. In this study, we used the peak height ratio of CO2 to H2O to determine whether or not the CO2-bearing solution reached phase equilibrium. The verification of the phase equilibrium of the saturated CO2-bearing solution includes (1) observations of the changes in the volume of CO2 bubbles caused by heating or depressurization and (2) investigations of the variations in the Raman peak height ratio. It is apparent that the system will form CO2 bubbles during heating or depressurization (Figure 2). The Raman spectra in the HPOC were continuously obtained under specific temperature and pressure conditions once the bubble volume is stabilized. As Figure 3 shows, the Raman spectra were acquired at position 2 (see Figure 4a) over a 40 min period at 313.15 K and 9.0 MPa in a CO2−H2O system. The νCO2/νH2O values were 0.1720, 0.1735, 0.1741, 0.1742, and 0.1744 and the variation over time was negligible, especially from 20 to 40 min, indicating that the system had almost reached equilibrium after 20 min. To confirm the feasibility of this method, additional tests were performed to check the Raman intensity ratios (νCO2/ νH2O) at different positions within a single HPOC. As shown in Figure 4, the νCO2/νH2O values were 0.1747, 0.1742, and 0.1758 at three positions, and the standard deviation (SD) was

(1)

where I is the Raman peak intensity ratio (I = νCO2/νH2O, νCO2 is the Raman peak height of the upper band of the CO2 Fermi dyad and νH2O is the Raman peak height of the O−H stretching band of H2O) and CCO2 is the concentration of CO2 in the aqueous solutions in mol·kg−1. To obtain the CO2 solubility at a certain temperature and pressure, the νCO2/νH2O values of the saturated CO2−H2O or CO2−H2O−NaCl system were investigated using the following steps: (1) after evacuating the vessel, a certain amount of H2O or NaCl solution was loaded into the phase equilibrium vessel, and excessive amounts of CO2 were added to the system using a 30 MPa quantitative pump; (2) the temperature the system was maintained at room temperature (∼293.15 K), and the equilibrium kettle was compressed to maintain the system at pressures of greater than 30 MPa; (3) the magnetic stirrer in the equilibrium system was turned on for several hours to ensure that the system reached phase equilibrium; (4) the circulating pump was used to circulate the mixture for about 15 min; and (5) the temperature was increased or the pressure was decreased to form CO2 bubbles in the HPOC. Figure 2 shows the formation of CO2 bubbles during the heating and depressurization process (a → d) and the dissolution of the CO2 bubbles during the cooling and pressurization process (e → h); (6) the desired temperature and pressure were maintained until the CO2-saturated solution in the HPOC reached phase equilibrium; and (7) the Raman spectra of the CO2 Fermi dyad and the O−H stretching band of the H2O were obtained near the CO2 bubbles (about 100 μm away

Figure 6. Raman height intensity ratio of different CO2 concentrations in H2O and 1−3 mol·kg−1 NaCl solutions. ▼, H2O; ■, 1 mol·kg−1 NaCl; ●, 2 mol·kg−1 NaCl; and ▲, 3 mol·kg−1 NaCl. E



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solution along the outer circulation line for about 15 min after allowing the system to equilibrate for several hours. Then, the Raman spectra of νCO2 and νO−H were obtained. The Raman spectra were obtained until the resulting peak height ratio remained almost unchanged, indicating that the system had reached equilibrium. 3.2. Relationship between CO2 Concentration and Peak Intensity Ratio. The νCO2/νH2O ratio is proportional to the concentration of CO2 in the aqueous solution. The Raman spectra were obtained for CO2-bearing solutions with known compositions from 303.15 to 353.15 K at defined pressures to establish the relationship between CO2 concentration and νCO2/νH2O. The main factors affecting this relationship are pressure, temperature, and salinity. As listed in Table 2, the deviations of the νCO2/νH2O ratios of the CO2−H2O system were generally less than 2% at certain temperatures and CO2 concentrations. As for the CO2−H2O− NaCl system, the deviations of the νCO2/νH2O ratios were also generally within 2% for a constant salinity (1 mol·kg−1 NaCl solution in Table 3, data for the 2 and 3 mol·kg−1 NaCl solutions is not shown), temperature, and CO2 concentration. Therefore, it can be deduced that the pressure has little effect on the relationship between the νCO2/νH2O ratios and the CO2 concentration. In addition, the SD of the νCO2/νH2O ratios was less than 1.5 × 10−3 at constant CO2 concentrations in water and 1−3 mol·kg−1 NaCl solutions at temperatures of 303.15− 353.15 K (Figure 5), which are within the limits of experimental error. In other words, it can be concluded that the temperature also has little effect on the νCO2/νH2O ratios of the homogeneous CO2-bearing solutions in this experiment. As a result, the νCO2/νH2O ratios of the homogeneous CO2bearing solutions were independent of temperature and pressure. Thus, the relationship between the νCO2/νH2O ratio

Table 4. Equations Used To Fit the CO2 Concentration vs the Peak Intensity Ratiosa fitting equation

system CO2−H2O

R2

y = 0.1416x

(2)

0.9993

CO2−H2O−NaCl (b = 1 mol·kg )

y = 0.1417x

(3)

0.9994

CO2−H2O−NaCl (b = 2 mol·kg−1)

y = 0.1405x

(4)

0.9998

y = 0.1368x

(5)

0.9997

−1

−1

CO2−H2O−NaCl (b = 3 mol·kg ) a

The uncertainties of CO2 concentrations in the prepared sample solutions are estimated based on the deviation of calculation using the gas equation of state, and standard uncertainties u are u(T) = 0.01 K, u(P) = 0.039 MPa, and u(b) = 0.0005 b.

Figure 7. Raman spectra of the CO2-saturated H2O at temperatures from 303.15 to 353.15 K and at 7.0 MPa. From top to bottom are 303.15, 313.15, 323.15, 333.15, 343.15, and 353.15 K, respectively.

8.2 × 10−4. This result further confirms that the system reached phase equilibrium. In addition, the phase equilibrium verification of the homogeneous solution is similar to that of the CO2-saturated solution. The circulating pump was used to circulate the

Figure 8. Peak height ratio of the CO2-saturated solutions at temperatures from 303.15 to 353.15 K and pressures from 3.0 to 30.0 MPa. CO2 is saturated in (a) H2O, (b) 1 mol·kg−1 NaCl, (c) 2 mol·kg−1 NaCl, and (d) 3 mol·kg−1 NaCl. ■, 303.15 K; ●, 313.15 K; ▲, 323.15 K; ▼, 333.15 K; ◆, 343.15 K; and ◀, 353.15 K. F



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Table 5. Determined CO2 Solubility in H2O at Temperatures from 303.15 to 353.15 K and Pressures from 3.0 to 30.0 MPaa CO2 solubility/mol·kg−1 P/MPa 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 15.0 15.5 17.0 17.5 20.0 21.0 23.0 24.0 26.0 27.0 30.0

303.15 K 0.745 0.855 0.940 1.006 1.085 1.155 1.205 1.258 1.303 1.312 1.326 1.334 1.341 1.346 1.354 1.361 1.370 1.376 1.382 1.388 1.394 1.399 1.415 1.420 1.430 1.437 1.456 1.462 1.476 1.482 1.496 1.502 1.519

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.013 0.011 0.017 0.009 0.008 0.008 0.007 0.007 0.012 0.011 0.008 0.011 0.005 0.013 0.005 0.015 0.007 0.020 0.010 0.014 0.007 0.017 0.008 0.028 0.005 0.019 0.019 0.007 0.015 0.006 0.016 0.009 0.006

313.15 K 0.616 0.697 0.767 0.827 0.889 0.946 1.003 1.053 1.099 1.152 1.195 1.217 1.229 1.244 1.257 1.265 1.271 1.281 1.289 1.295 1.300 1.309 1.321 1.326 1.337 1.344 1.365 1.373 1.386 1.393 1.409 1.416 1.446

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.012 0.010 0.011 0.017 0.008 0.007 0.008 0.003 0.011 0.011 0.008 0.017 0.007 0.005 0.006 0.014 0.006 0.006 0.004 0.014 0.003 0.009 0.005 0.008 0.002 0.003 0.008 0.006 0.014 0.003 0.003 0.005 0.003

323.15 K 0.525 0.597 0.657 0.721 0.773 0.819 0.868 0.909 0.954 0.999 1.039 1.075 1.111 1.139 1.172 1.187 1.199 1.205 1.213 1.222 1.230 1.237 1.248 1.257 1.273 1.277 1.301 1.308 1.324 1.333 1.348 1.356 1.378

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.011 0.008 0.005 0.009 0.013 0.005 0.018 0.009 0.004 0.007 0.003 0.006 0.013 0.010 0.006 0.012 0.006 0.006 0.006 0.013 0.006 0.004 0.005 0.007 0.005 0.014 0.018 0.007 0.010 0.007 0.014 0.005 0.002

333.15 K 0.448 0.516 0.571 0.628 0.682 0.727 0.775 0.812 0.855 0.893 0.928 0.962 0.996 1.025 1.062 1.087 1.112 1.130 1.142 1.150 1.158 1.165 1.185 1.190 1.207 1.214 1.243 1.252 1.273 1.278 1.295 1.300 1.326

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.006 0.006 0.007 0.013 0.012 0.016 0.017 0.013 0.015 0.006 0.004 0.004 0.009 0.004 0.009 0.006 0.011 0.004 0.009 0.017 0.006 0.018 0.009 0.009 0.004 0.012 0.014 0.008 0.010 0.003 0.024 0.005 0.013

343.15 K 0.400 0.452 0.508 0.554 0.608 0.650 0.695 0.737 0.781 0.821 0.855 0.886 0.916 0.944 0.975 0.997 1.021 1.042 1.059 1.071 1.084 1.095 1.119 1.130 1.149 1.154 1.188 1.202 1.225 1.237 1.254 1.261 1.286

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.005 0.014 0.007 0.013 0.007 0.011 0.004 0.010 0.004 0.011 0.012 0.010 0.005 0.012 0.011 0.010 0.005 0.009 0.010 0.011 0.006 0.013 0.003 0.017 0.007 0.015 0.011 0.009 0.016 0.005 0.012 0.004 0.003

353.15 K 0.352 0.406 0.453 0.499 0.548 0.595 0.630 0.669 0.706 0.746 0.781 0.820 0.845 0.873 0.899 0.927 0.946 0.969 0.987 1.005 1.025 1.034 1.056 1.076 1.100 1.111 1.150 1.172 1.193 1.207 1.230 1.240 1.268

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.009 0.005 0.010 0.008 0.011 0.006 0.006 0.012 0.014 0.013 0.002 0.007 0.010 0.004 0.003 0.007 0.013 0.007 0.007 0.013 0.008 0.006 0.010 0.015 0.004 0.015 0.017 0.008 0.013 0.006 0.007 0.009 0.014

a

Standard uncertainties u are u(T) = 0.01 K and u(P) = 0.039 MPa, and the uncertainties of CO2 solubility are estimated based on the average deviation of five measurements of Raman peak height ratio for each measurement at the T and P condition.

Figure 8. The CO2 solubilities were calculated based on the equations listed in Tables 4−8. It can be seen from Figure 9 that the CO2 solubility of the aqueous solutions increased with increasing pressure, increased with decreasing temperature, and decreased with increasing NaCl concentration. The measured CO2 solubilities ranged from 0.352 to 1.519 mol·kg−1 in H2O, from 0.306 to 1.277 mol·kg−1 in the 1 mol·kg−1 NaCl solution, from 0.255 to 1.032 mol·kg−1 in the 2 mol·kg−1 NaCl solution, and from 0.214 to 0.891 mol·kg−1 in the 3 mol·kg−1 solution. Within the investigated temperature and pressure ranges, the observed decrease in CO2 solubility with increasing salinity can be ascribed to the salting out effect.39 The CO2 solubility decreases with increasing temperature, which may due to a similar arrangement of adjacent (H2O + CO2) molecules.21 Such an arrangement could result from the limited number of neighboring CO2 molecules because of the anisotropic distribution of the water molecules highly linked by hydrogen bonding.30 The effect of pressure on CO2 solubility is more complex. The CO2 solubility increases almost linearly with increasing pressure at a constant temperature within the pressure range of ∼3.0 to 7.4 MPa. This result can be explained by Henry’s law.40 However, when the pressure exceeds ∼7.4 MPa, the tendency of CO2 solubility to increase with increasing pressure significantly

and the CO2 concentration was determined without considering the effects of pressure and temperature. As shown in Figure 6, there is a positive linear correlation between the CO2 concentration and the νCO2/νH2O ratio (R2 > 0.999). However, the influence of salinity cannot be ruled out because at a constant CO2 concentration, the νCO2/νH2O ratio decreases with increasing salinity (Figure 6). The equation of the fitted curve of CO2 concentration versus the Raman νCO2/νH2O ratio was obtained (Table 4). 3.3. Determination of CO2 Solubility in H2O and NaCl Solutions. Although the relationship between the concentration of CO2 and νCO2/νH2O ratio is known, confocal Raman spectroscopy approach is quite flexible and allows for the determination of the CO2 solubility in H2O and NaCl solutions based on the Raman peak height ratio of the CO2-saturated solutions. The Raman spectra of H2O and CO2 were obtained for CO2-saturated solutions at temperatures of 303.15−353.15 K and pressures of 3.0−30.0 MPa. For example, Figure 7 shows the Raman spectra of CO2-saturated H2O at a constant pressure of 7.0 MPa and temperatures from 303.15 to 353.15 K. Then, the νCO2/νH2O ratios were calculated and plotted in G



Article

Table 6. Determined CO2 Solubility in the NaCl Solution (b = 1 mol·kg−1) at Temperatures from 303.15 to 353.15 K and Pressures from 3.0 to 30.0 MPaa CO2 solubility/mol·kg−1 P/MPa 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 15.0 17.0 20.0 23.0 26.0 30.0

303.15 K 0.604 0.743 0.865 0.966 1.038 1.054 1.068 1.083 1.093 1.106 1.119 1.138 1.159 1.188 1.220 1.243 1.277

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.006 0.015 0.008 0.009 0.008 0.012 0.015 0.019 0.014 0.015 0.009 0.016 0.017 0.015 0.013 0.018 0.020

313.15 K 0.498 0.623 0.722 0.812 0.893 0.951 0.978 0.995 1.005 1.014 1.031 1.050 1.070 1.096 1.122 1.151 1.194

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.009 0.011 0.010 0.012 0.014 0.011 0.009 0.014 0.016 0.012 0.009 0.015 0.007 0.012 0.011 0.007 0.022

323.15 K 0.435 0.537 0.618 0.702 0.767 0.832 0.885 0.930 0.940 0.953 0.965 0.987 1.013 1.038 1.066 1.092 1.138

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.006 0.005 0.013 0.013 0.012 0.012 0.009 0.018 0.017 0.005 0.006 0.006 0.008 0.011 0.016 0.007 0.011

333.15 K 0.376 0.471 0.549 0.614 0.683 0.738 0.790 0.835 0.876 0.904 0.916 0.944 0.970 0.998 1.027 1.050 1.096

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.007 0.012 0.013 0.007 0.012 0.010 0.010 0.012 0.010 0.006 0.016 0.013 0.016 0.014 0.013 0.012 0.009

343.15 K 0.339 0.427 0.485 0.559 0.620 0.679 0.723 0.761 0.804 0.830 0.851 0.894 0.923 0.965 1.001 1.029 1.069

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.009 0.013 0.007 0.010 0.008 0.009 0.013 0.007 0.008 0.014 0.007 0.007 0.006 0.006 0.007 0.020 0.018

353.15 K 0.306 0.373 0.440 0.511 0.566 0.621 0.677 0.710 0.752 0.786 0.810 0.857 0.891 0.933 0.968 1.003 1.044

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.007 0.006 0.004 0.003 0.007 0.007 0.014 0.012 0.009 0.011 0.012 0.011 0.006 0.007 0.017 0.014 0.013

a

Standard uncertainties u are u(T) = 0.01 K, u(P) = 0.039 MPa, and u(b) = 0.0005 b, and the uncertainties of CO2 solubility are estimated based on the average deviation of five measurements of Raman peak height ratio for each measurement at the T and P condition.


303.15 K 0.506 0.622 0.722 0.794 0.849 0.865 0.879 0.890 0.900 0.909 0.917 0.933 0.950 0.971 0.991 1.009 1.032

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.002 0.005 0.005 0.005 0.008 0.007 0.004 0.008 0.004 0.004 0.003 0.004 0.003 0.006 0.004 0.004 0.006

313.15 K 0.422 0.515 0.592 0.670 0.736 0.786 0.808 0.820 0.830 0.840 0.850 0.868 0.886 0.909 0.930 0.949 0.972

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.004 0.0077 0.005 0.004 0.004 0.006 0.003 0.003 0.004 0.008 0.004 0.007 0.006 0.006 0.003 0.005 0.007

323.15 K 0.363 0.447 0.514 0.585 0.641 0.694 0.743 0.772 0.786 0.797 0.808 0.826 0.843 0.867 0.888 0.908 0.931

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.005 0.009 0.008 0.004 0.006 0.002 0.004 0.006 0.009 0.010 0.003 0.003 0.009 0.006 0.004 0.004 0.006

333.15 K 0.317 0.393 0.455 0.518 0.570 0.617 0.658 0.696 0.730 0.757 0.769 0.789 0.807 0.836 0.858 0.883 0.905

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.005 0.005 0.009 0.008 0.002 0.003 0.007 0.005 0.005 0.005 0.005 0.008 0.004 0.008 0.003 0.005 0.004

343.15 K 0.283 0.347 0.407 0.466 0.513 0.558 0.603 0.641 0.670 0.697 0.719 0.751 0.774 0.807 0.837 0.862 0.885

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.008 0.006 0.006 0.009 0.008 0.004 0.006 0.007 0.007 0.007 0.005 0.005 0.005 0.006 0.008 0.003 0.004

353.15 K 0.255 0.315 0.372 0.425 0.472 0.518 0.562 0.596 0.627 0.657 0.678 0.715 0.747 0.787 0.819 0.845 0.868

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.008 0.010 0.008 0.003 0.009 0.010 0.004 0.006 0.004 0.004 0.005 0.004 0.011 0.006 0.009 0.009 0.006

a


by pressure at temperatures of 303.15−313.15 K (near the critical temperature). Conversely, the density increases slowly with increasing pressure at temperatures of 323.15−333.15 K (above the critical temperature). The greater the density of the solute, the closer it is to the liquid−liquid system, and the less its solubility is affected by external pressure.19 Wiebe and Gaddy also confirmed that when the characteristics of the CO2 phase approach that of a liquid, the CO2 solubility in the liquid is slightly affected by pressure.17 3.4. Verification of the Method with a Thermodynamic Model. To verify the feasibility and accuracy of the method, we compared the obtained solubility of CO2 in H2O with those calculated using the models developed by Duan and

decreases within the temperature range of 303.15−313.15 K, whereas at temperatures from 323.15 to 353.15 K, this tendency gradually decreases. Within the lower pressure range of 3.0− 7.4 MPa, the CO2 solubility is proportional to the pressure of the system under isothermal conditions. Within the higher pressure range of ∼7.4 to 30.0 MPa, the variation in the increasing tendency of CO2 solubility with increasing pressure at different temperatures can be illustrated by the physical properties of supercritical CO2. When the temperature and pressure exceed the critical point of CO2 (Tc = 304.41 K, Pc = 7.38 MPa), the density of CO2 decreases with increasing temperature and increases with increasing pressure.41 Therefore, the density of supercritical CO2 is significantly influenced H



Article


303.15 K 0.426 0.530 0.610 0.678 0.720 0.735 0.747 0.757 0.764 0.773 0.784 0.798 0.807 0.831 0.848 0.867 0.891

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

313.15 K

0.007 0.006 0.006 0.005 0.003 0.006 0.003 0.006 0.002 0.005 0.003 0.003 0.002 0.012 0.008 0.005 0.010

0.366 0.444 0.520 0.585 0.632 0.673 0.690 0.703 0.713 0.723 0.731 0.745 0.760 0.779 0.800 0.820 0.842

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

323.15 K

0.006 0.007 0.009 0.010 0.003 0.012 0.006 0.008 0.009 0.005 0.010 0.011 0.009 0.006 0.003 0.004 0.002

0.320 0.391 0.454 0.507 0.556 0.596 0.631 0.660 0.671 0.683 0.692 0.710 0.723 0.744 0.766 0.782 0.806

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.005 0.008 0.012 0.011 0.007 0.005 0.009 0.010 0.005 0.004 0.003 0.007 0.006 0.006 0.004 0.007 0.002

333.15 K 0.275 0.335 0.396 0.448 0.493 0.534 0.572 0.601 0.627 0.652 0.663 0.685 0.698 0.717 0.739 0.756 0.782

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.005 0.008 0.004 0.010 0.006 0.004 0.007 0.005 0.007 0.005 0.008 0.006 0.010 0.008 0.004 0.002 0.008

343.15 K 0.244 0.305 0.356 0.406 0.449 0.487 0.524 0.556 0.581 0.606 0.620 0.652 0.671 0.701 0.724 0.744 0.767

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.003 0.007 0.005 0.008 0.006 0.010 0.011 0.009 0.005 0.008 0.005 0.010 0.013 0.005 0.003 0.005 0.007

353.15 K 0.214 0.280 0.325 0.375 0.419 0.456 0.492 0.522 0.548 0.572 0.593 0.620 0.648 0.680 0.706 0.727 0.755

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.004 0.004 0.009 0.005 0.009 0.007 0.004 0.008 0.005 0.007 0.010 0.003 0.008 0.006 0.005 0.002 0.007

a


Figure 9. Determined CO2 solubility at temperatures from 303.15 to 353.15 K and pressures from 3.0 to 30.0 MPa in solutions of (a) H2O, (b) 1 mol·kg−1 NaCl, (c) 2 mol·kg−1 NaCl, and (d) 3 mol·kg−1 NaCl. ■, 303.15 K; ●, 313.15 K; ▲, 323.15 K; ▼, 333.15 K; ◆, 343.15 K; and ◀, 353.15 K.

Table 9. Deviations between the Experimental CO2 Solubility in Water and the Data from the Duan Model P/MPa 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5

303.15 K (%)

313.15 K (%)

323.15 K (%)

0.16 1.75 1.30 −0.29 0.24 0.54 −0.28 −0.18 −0.45 0.19

1.03 0.78 0.07 −1.12 −1.27 −1.55 −1.16 −1.07 −0.90 0.28

2.20 2.12 1.19 1.32 0.64 −0.41 −0.28 −0.87 −0.53 0.12 I

333.15 K (%) 1.27 1.81 1.67 1.42 2.02 1.01 1.54 0.70 1.04 1.11

343.15 K (%) 2.96 1.00 2.20 0.65 2.29 1.14 1.89 2.13 2.85 3.15

353.15 K (%) 0.95 0.48 1.28 0.06 1.86 1.83 1.68 1.05 1.69 1.89



Article

Table 9. continued P/MPa

303.15 K (%)

313.15 K (%)

323.15 K (%)

333.15 K (%)

343.15 K (%)

353.15 K (%)

8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 15.0 15.5 17.0 17.5 20.0 21.0 23.0 24.0 26.0 27.0 30.0 mean deviation

0.56 0.64 0.52 0.36 0.34 0.33 0.39 0.32 0.17 0.15 0.05 −0.10 −0.46 −0.56 −1.24 −1.22 −1.76 −2.46 −3.03 −3.37 −3.89 −4.25 −5.13 1.56

0.85 0.07 0.92 1.46 1.88 1.86 1.75 1.93 1.93 1.91 1.64 1.77 1.09 0.94 0.21 0.27 −0.56 −0.83 −1.60 −1.86 −2.30 −2.55 −2.74

0.54 0.84 1.56 1.70 2.04 2.68 2.98 2.80 2.83 2.92 2.93 2.87 2.04 2.14 1.76 1.53 0.86 0.46 −0.20 −0.37 −0.95 −1.12 −1.90

1.16 1.18 1.80 1.89 3.11 3.40 3.76 1.59 1.90 1.96 1.97 1.95 1.74 1.50 1.12 1.14 0.84 0.48 0.22 −0.36 −0.80 −1.33 −1.78

3.19 2.78 2.97 2.72 3.57 3.34 3.49 3.39 3.37 3.79 3.46 3.12 1.54 1.45 0.28 −0.20 −0.90 −0.98 −1.33 −1.28 −1.70 −1.96 −2.29

2.61 3.11 3.07 2.58 2.92 2.90 2.88 2.64 2.76 2.38 2.61 1.98 0.82 1.53 0.33 0.29 −0.68 −0.30 −1.19 −1.17 −1.49 −1.68 −2.19

Table 10. Deviations between the Experimental CO2 Solubility in Water and the Data from the Akinfiev and Diamond Model P/MPa

303.15 K (%)

313.15 K (%)

323.15 K (%)

333.15 K (%)

343.15 K (%)

353.15 K (%)

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 15.0 15.5 17.0 17.5 20.0 21.0 23.0 24.0 26.0 27.0 30.0 mean deviation

−2.72 −1.19 −1.69 −3.18 −2.60 −2.16 −2.73 −2.42 −2.01 −2.65 −2.28 −2.19 −2.22 −2.34 −2.24 −2.20 −2.01 −2.02 −2.02 −1.99 −1.98 −2.02 −2.09 −2.11 −2.49 −2.38 −2.80 −3.04 −3.32 −3.52 −3.79 −4.03 −4.54 1.65

−0.98 −1.22 −2.16 −3.31 −3.58 −3.80 −3.44 −3.28 −2.97 −1.59 −0.70 −0.94 −1.36 −1.13 −0.85 −0.93 −1.06 −0.89 −0.85 −0.85 −1.07 −0.89 −1.38 −1.46 −1.97 −1.85 −2.31 −2.50 −3.03 −3.18 −3.42 −3.59 −3.49

0.73 0.78 −0.54 −0.34 −1.31 −2.26 −2.3 −2.93 −2.61 −1.93 −1.46 −0.98 −0.17 0.24 1.37 1.28 1.12 0.68 0.46 0.45 0.37 0.24 −0.63 −0.53 −0.85 −1.05 −1.52 −1.85 −2.35 −2.45 −2.89 −3.01 −3.60

0.09 1.00 0.22 0.22 0.33 −0.48 −0.34 −1.24 −0.97 −0.94 −0.91 −0.74 −0.28 −0.02 1.09 1.45 1.89 1.99 1.64 1.19 0.81 0.49 −0.26 −0.60 −1.18 −1.20 −1.59 −1.92 −2.14 −2.69 −3.07 −3.57 −3.94

1.94 0.60 0.86 −0.16 0.74 −0.01 0.13 0.28 0.90 1.14 1.11 0.95 0.80 0.78 1.33 1.09 1.22 1.31 1.08 0.65 0.37 0.12 −1.05 −1.01 −1.79 −2.16 −2.55 −2.59 −2.95 −2.97 −3.56 −3.93 −4.63

0.18 0.77 0.05 −0.38 0.32 1.01 −0.02 −0.07 −0.18 0.51 0.59 1.49 0.92 0.81 0.66 0.98 0.54 0.60 0.33 0.22 0.37 −0.33 −2.40 −1.65 −2.56 −2.51 −3.08 −2.61 −3.36 −3.32 −3.65 −3.88 −4.56

J



Article

solubility of CO2 by determining the relationship between the known CO2 concentration and the νCO2/νH2O ratio of the CO2-bearing solution and calculating the νCO2/νH2O ratio of the CO2-saturated solutions. The solubilities of CO2 in H2O and NaCl solutions at temperatures from 303.15 to 313.15 K and pressures from 3.0 to 30.0 MPa were obtained. By comparing our experimental data with data calculated from the Duan model and the Akinfiev and Diamond model, we conclude that our results are in good agreement with these accepted models, which further confirms the feasibility of our experimental method. In situ Raman spectroscopy is clearly a viable technique for assessing the solubility of CO2 under geological conditions, which makes it a promising prospect for measuring the solubility of CO2 in more complicated brine systems (including single- and multi-salts or in other aqueous solutions) at wider pressure and temperature ranges. This technique can also provide more basic data on the geological storage of CO2. CO2 is easily dissolved in deep saline aquifers. The dissolved CO2 is regarded as the major form of sequestration, but a number of issues have been raised regarding the potential hazards to groundwater from CO2 or formation of fluid leakage and from brine displacement during the geological storage of CO2.52 In addition, the CO2 solubility depends on the temperature, pressure, and salinity of the reservoir. It is well-known that the mineralogical compositions of deep saline aquifers are complex53 and that the salinity, temperature, and pressure of a reservoir usually increase with depth. Therefore, problems involving the environmental safety of injected CO2 and CO2 solubility in actual brine systems in a wider range of temperatures and pressures should be investigated further.

Figure 10. Data obtained in this experiment plotted with previous measurements for comparison. , this work; △, ref 1 T = 323.15 K; ▽, refs17,18 T = 313.15 K; ◆, refs17,18 T = 323.15 K; ●, ref 28, T = 313.15 K; ○, ref 28, T = 333.15 K; ▲, ref 28, T = 353.15 K; ▼, ref 48, T = 313.15 K; ★, ref 49, T = 303.15 K; ■, ref 49, T = 323.15 K; □, ref 49, T = 333.15 K; ◇, ref 50, T = 323.15 K; ◀, ref 51, T = 323.15 K; ☆, ref 51, T = 333.15 K; and ▶, ref 51, T = 353.15 K.

Sun, and Akinfiev and Diamond.42−44 These models are suitable for use in sequestration environments, and they are considered to estimate the CO2 solubility more accurately.45 The comparisons are shown in Tables 9 and 10. As shown in Tables 9 and 10, the deviations between the experimental CO2 solubility in H2O and those calculated from the above two models were primarily within 3%, with a few exceptions of more than 5%. The mean deviations of the two models were 1.56 and 1.65%. In addition, an interesting deviation was found. The CO2 solubility data from our experiment was in good agreement with the model data at lower pressures; however, with increasing pressure, the deviation of the data increased, especially for pressures greater than 20.0 MPa. For example, the deviation at 303.15 K and 5.0 MPa is 0.24% relative to the Duan model and −2.60% compared to the Akinfiev and Diamond model. However, these values become −5.13 and −4.54%, respectively, at 303.15 K and 30.0 MPa. Darwish and Hilal, and Bastami et al. also found that prediction models such as the Duan model have larger deviations in predicted CO2 solubility at higher pressures.46,47 New parameters should be used in the future to improve the model for aqueous solutions, so that they can be used to accurately calculate the solubility of CO2 in geological fluids over a wider range of temperatures and pressures. In addition, we also compared our results with other reported data. As shown in Figure 10, the experimental data are in good agreement with the results of Liu et al.,1 Wiebe and Gaddy,17,18 Guo et al.,28 Ferrentino et al.,48 Bando et al.,49 Koschel et al.,50 and Bamberger et al.51 Therefore, the feasibility of the experimental method is confirmed.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-571-88320061. ORCID

Zhiyan Pan: 0000-0002-1623-5133 Funding

Financial support for this research was provided by the Natural Science Foundation of the Zhejiang Province of China (Y17D030003), the Natural Science Foundation of China (no. 21377116), the Knowledge Innovation Program (SIDSSE-201302), and the Hadal-trench Research Program of the Chinese Academy of Sciences (XDB06060100). Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Liu, Y.; Hou, M.; Yang, G.; Han, B. Solubility of CO2 in aqueous solutions of NaCl, KCl, CaCl2 and their mixed salts at different temperatures and pressures. J. Supercrit. Fluid. 2011, 56, 125−129. (2) Zhang, S.; Shen, Y.; Shao, P.; Chen, J.; Wang, L. Kinetics, thermodynamics, and mechanism of a novel biphasic solvent for CO2 capture from flue gas. Environ. Sci. Technol. 2018, 52, 3660−3668. (3) Kopp, A.; Class, H.; Helmig, R. Investigations on CO2 storage capacity in saline aquifers : Part 1. Dimensional analysis of flow processes and reservoir characteristics. Int. J. Greenh. Gas Con. 2009, 3, 263−276. (4) Nghiem, L.; Yang, C.; Shrivastava, V.; Kohse, B.; Hassam, M.; Card, C. Risk mitigation through the optimization of residual gas and solubility trapping for CO2 storage in saline aquifers. Energy Procedia 2009, 1, 3015−3022.

4. CONCLUSIONS In this study, we developed a method for measuring CO2 solubility in aqueous solutions using an HPOC, a heating− cooling stage, a quantitative pump, a circulating equilibrium system, and a confocal Raman spectrometer. The experimental results demonstrate the feasibility of obtaining Raman spectra at different positions and times within the HPOC to examine the phase equilibrium at each T−P point. In addition, this method is shown to be an effective means of obtaining the K



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Determination of CO2 Solubility in Water and NaCl Solutions under

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