Phase Equilibrium of Cyclopentane + Carbon Dioxide Binary Hydrates

Jul 17, 2017 - Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China. ∥. Guangzhou Cente...
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Phase Equilibrium of Cyclopentane + Carbon Dioxide Binary Hydrates in Aqueous Sodium Chloride Solutions Ye Zhang,†,‡,§,∥,⊥ Shu-Mei Sheng,†,‡,§,∥,⊥ Xiao-Dong Shen,†,‡,§,∥ Xue-Bing Zhou,†,‡,§,∥ Wen-Zhi Wu,†,‡,§,∥ Xiao-Ping Wu,# and De-Qing Liang*,†,‡,§,∥ †

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China CAS Key Laboratory of Gas Hydrate, Guangzhou 510640, China § Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China ∥ Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, China ⊥ Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, China # School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China ‡

ABSTRACT: This work aims to report the cyclopentane(CP)-carbon dioxide binary sII hydrate phase equilibrium data in the presence of aqueous sodium chloride solutions at five different mass fractions (0.00, 0.035, 0.07, 0.10, 0.15 and 0.25) in the range of temperature from (269.8 to 292.4) K and pressure from (1.18 to 3.33) MPa. An isochoric pressure-search method is applied for the experimental measurements. The obtained equilibrium data of CO2 hydrate in pure water are compared with those from the literature. The good agreement validates that our experimental apparatus and approach are well-established and reliable. It is indicated that the inhibiting effect of NaCl yields to the promotion effect of cyclopentane except for the case of 0.25 mass fraction of aqueous NaCl solution.

1. INTRODUCTION Gas hydrates are nonstoichiometric inclusion compounds formed by water and guest molecules (such as CO2, CH4, cyclopentane, and cyclohexane, et al.) under suitable temperature and pressure environments. These compounds are separated into three common types: sI (structure I), sII (structure II), and sH (structure H), which consist of different sized and shaped cages.1,2 Because of huge reserves and excellent characteristic, gas hydrates have been proposed for many applications such as energy recovery, storage and transportation, CO2 capture and separation, refrigerant and cold thermal energy, frozen food, and so forth.3−6 An attractive potential application of gas hydrates is desalination of seawater or industry salty water,7,8 as well as the pretreatment process of traditional desalination technologies (such as reverse osmosis9). Seawater desalination of gas hydrate is based on salts rejection effects. The salts are retained in the concentrated solution when gas hydrates constantly form in seawater under lower temperature and higher pressure, coupled with a physical process to separate the gas hydrates from the remaining concentrated salty water. Although the energy consumption and economic cost of the hydrate desalination process could be competitive with other desalination technologies such as distillation and reverse osmosis,10−15 there are two main challenges to the application of hydrate-based desalination technology: to increase the efficiency of salts removal15,16 and to decrease the cost.17 The © 2017 American Chemical Society

main contributory factor to the total cost of hydrate desalination process comes from the refrigeration and pressurization required to form a hydrate. Therefore, it is necessary to investigate hydrates under mild phase equilibrium conditions. On one hand, the dissociation points of single component hydrates, such as CO2, CH4, propane, cyclopentane and so forth, usually occur in higher pressure or/and low temperature. Additionally, salts/electrolytes such as NaCl/ MgCl2/KCl18−20 existent in real circumstances suppress gas hydrate phase equilibrium conditions. However, some thermodynamic promoters such as cyclopentane(CP),21 tetrahydrofuran(THF),22 and tetra-butyl ammonium bromide (TBAB)23 have been found to substantially improve hydrate phase equilibrium conditions. Therefore, we conducted experiments to investigate the dissociation point of gas hydrates containing gas and thermodynamics promoters in aqueous salt solution. Some binary hydrate-containing promoters have been investigated by researchers. Cha et al.8 suggested that cyclopentane−CO2 and cyclohexane−CO2 binary hydrate have mild hydrate formation temperature and fast formation kinetics than CO2 simple hydrate. Chen et al.24 measured the phase equilibrium data and dissociation enthalpies for cycloReceived: May 2, 2017 Accepted: July 6, 2017 Published: July 17, 2017 2461

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pentane + methane + water + NaCl systems and Lv et al.25 studied the formation kinetics of the same systems. Also, Cai et al.26,27 investigated the thermodynamics and kinetics of a CP− methane binary hydrate in fresh water and brine solutions separately. Sabil et al.28 investigated the contending effects of NaCl and THF on the phase equilibrium of hydrates of a quaternary system containing CO2 + THF + NaCl + H2O. It was observed that the promoting effect of tetrahydrofuran suppressed the inhibiting effect of sodium chloride at low concentrations of sodium chloride. Godishala et al.29 measured the equilibrium data of semiclathrate hydrate of carbon dioxide (CO2) in varying concentrations of TBAB and aqueous sodium chloride solutions. It was observed that the inhibiting effect of the NaCl was abated with the addition of TBAB. Compared to TBAB and THF, CP has low toxicity and is insoluble in water, which makes it easily separated from the desalinated water. However, the equilibrium data of CP-CO2 binary hydrate in salty water is limited. Negma et al.30 established the thermodynamic model based on measurements of the dissociation conditions of the ternary systems comprising CO2 + sodium chloride (NaCl), or calcium chloride (CaCl2), or magnesium chloride (MgCl2) + water + cyclopentane at salt concentrations of (10, 15, and 20) wt %, which gave some new insights and data for the hydrate-based desalination process. Recently, Zheng et al.31 investigated the characteristics of CO2 hydrate formation with CP in porous media in the presence of 0.03 mass fraction of aqueous NaCl solution. They concluded that the recommended optimal molar ratio of CP was 0.01, in which the increase in equilibrium temperature was more than 10 K, and the decrease in hydrate saturation was less than 2%. The objective of this work is to obtain CO2−CP sII binary hydrate phase equilibrium data in the presence of aqueous sodium chloride solutions to aid in developing a hydrate-based desalination process. Carbon dioxide hydrate phase equilibrium date in pure water was measured and compared with literature data. A preliminary literature review showed a paucity of the data concerning this system.

Figure 1. Schematic diagram of experimental apparatus. DA, data acquisition; PC, personal computer; PT, pressure transducer; TS, temperature sensor; R, reactor; MR, magnetic rotor; MS, magnetic stirrer; CB, cool bath; VP, vacuum pump; GC, gas cylinder.

The cell mixtures were fully agitated by a magnetic stirrer driven by an external magnet. A platinum resistance thermometer (PT100) with an uncertainty of ±0.1 K was inserted into the cells to measure the system temperature. The cell pressure was measured by a pressure transmitters (CYB20S) ranging from (0 to 15) MPa with an uncertainty of ±0.02 MPa. The pressure and temperature of the reactor were displayed and stored in a personal computer through an Agilent data acquisition. First, the experimental apparatus was thoroughly washed with deionized water, and adequately dried with by air oven. The cell was loaded with 30 mL of NaCl aqueous solution (or deionized water) and 9 mL of CP. Then the reactor was purged with carbon dioxide three times and evacuated with a vacuum pump to ensure the absence of air in the reactor and to remove dissolved gases in the solution. Finally the cell was pressurized to an expected initial value with carbon dioxide and the system temperature was slowly lowered to form hydrates. Abrupt pressure drop and temperature increase were the sign of hydrate formation. After sufficient time had elapsed (no decrease in pressure), the system temperature was gradually increased with a step of 0.2 K to dissociate hydrate. When the temperature increased to approximately 2 K below the expected equilibrium temperature, the temperature was increased with an increment of 0.1 K and the pressure was kept constant for about 3 h to achieve a steady equilibrium state at each temperature. After the hydrate dissociated completely, the increase in pressure was only due to gas expansion. Consequently, a pressure−temperature diagram was obtained for each experimental run, in which the dissociation points could be determined with the isochoric pressure-search method.19,32,33 The point that the slope of temperature− pressure changed sharply was considered as the hydrate dissociation point. The initial pressure value was changed and the procedure was repeatedly performed to achieve other dissociation points.

2. EXPERIMENTAL SECTION 2.1. Materials. The supplier and the purity of chemicals used in this work were listed in Table 1. Deionized water was Table 1. Chemicals Used in This Work chemical name

purity

carbon dioxide 0.9999 (volume fraction) cyclopentane 0.99 (mass fraction) soldium 0.995 (mass chloride fraction) deionized water

supplier Guangzhou Yigas Gases Co. Ltd. Aldrich- Sigma Guangzhou Second Chemical Reagent Factory, China laboratory-made

made in the laboratory with a resistivity of 18.25 mΩ·cm−1. The required amounts of each component were weighed by an electronic analytical balance with a reading uncertainty of ±0.1 mg. All materials were used without further purification. 2.2. Experimental Apparatus and Procedure. The schematic diagram of the experimental apparatus was shown in Figure 1. The main part of the apparatus was a cylindrical reactor with an interior volume of 100 mL which was immersed in a temperature controlled water bath. It was made of 316 stainless steel and can withstand pressure as high as 25 MPa.

3. RESULTS AND DISCUSSION To check the accuracy of experimental apparatus and approach used in this work, the carbon dioxide hydrate phase equilibrium data in pure water along with H−LW−V equilibrium boundary was measured. The experimental data were listed in Table 2 and plotted in Figure 2 together with those data reported from Zha et al.,34 Dholabhai et al.35, Heuvel et al.,36 and Shen et al.37 The data obtained in the present study agreed well with those 2462

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Table 2. Phase Equilibrium Data of CO2 Hydrate in Pure Watera

a

T/K

P/MPa

279.9 282.7 280.5 282.1 281.2 278.8

2.64 4.00 2.94 3.70 3.26 2.33

Uncertainties u are u(T) = ± 0.1 K, u(p) = ± 0.02 MPa.

Figure 3. Four-phase (H−LW−LHC−V) equilibrium conditions of CPCO2 binary hydrate in pure water and in aqueous NaCl solution; ◀, CP + CO2 in pure water, this work; ■, CP + CO2 + 0.035 mass fraction NaCl, this work; ●, CP + CO2 + 0.07 mass fraction NaCl, this work; ▲, CP + CO2 + 0.10 mass fraction NaCl, this work; ▼, CP + CO2 + 0.15 mass fraction NaCl, this work; ◆, CP + CO2 + 0.25 mass fraction NaCl, this work; +, CO2 in pure water, ref 35.

pentane and NaCl. As shown in Figure 3, for each equilibrium line, both the CP-CO2 binary hydrate dissociation pressure and the rate of rise in pressure increased with the increase of temperature. It was apparent that CP acted as a thermodynamic promoter to improve the CO2 hydrate phase equilibrium condition, and the CP-CO2 binary hydrate phase equilibrium condition was inhibited by the addition of NaCl. Moreover, the inhibition effect increased with the increase of the mass concentrations of NaCl, and the difference in temperature of CP-CO2 binary hydrate in the presence and in the absence of NaCl, at about 1.85 MP was observed to be around 1.5 K for 0.035 mass fraction NaCl solution, 3.3 K for 0.07 mass fraction NaCl solution, 5.2 K for 0.10 mass fraction NaCl solution, 8.5 K for 0.15 mass fraction NaCl solution, and 19.1 K for 0.25 mass fraction NaCl solution. Similar to the ice freezing point depression equation in the dilute solution, CP-CO2 binary hydrate dissociation points at five different pressures can be calculated by the proposed equation:

Figure 2. Phase equilibrium conditions for the CO2 hydrate in pure water and a comparison with literature data: ●, this work; □, ref 34; △, ref 35; ×, ref 36; +, ref 37.

given in the literature. This validated that our experimental apparatus and approach were well-established and reliable. The CP-CO2 binary hydrate phase equilibrium data measured in this study along the hydrate (H)−liquid water (LW)−liquid hydrocarbon (LHC)−vapor rich in carbon dioxide (V) boundary were tabulated in Table 3 and plotted in Figure 3. The CO2 hydrate equilibrium data in pure water were also presented to illustrate the competitive effect between cycloTable 3. Experimental Values of Four-Phase (H−LW−LHC− V) Equilibrium Pressure P, and Temperature, T, in Pure Water and in Aqueous NaCl Solutiona wNaCl

T/K

P/MPa

wNaCl

T/K

P/MPa

0.00

289.3 292.4 292.0 291.3 290.4 287.9 289.0 289.7 290.3 290.7 286.2 287.2 288.0 288.5 288.9

1.36 3.33 2.82 2.33 1.85 1.41 1.87 2.27 2.65 3.06 1.42 1.89 2.31 2.68 3.02

0.10

284.4 285.3 285.9 286.4 286.8 281.1 282.0 282.6 283.1 283.3 269.8 270.8 271.4 271.8 272.0

1.40 1.79 2.15 2.58 2.98 1.48 1.88 2.11 2.43 2.81 1.18 1.50 1.82 2.13 2.45

0.035

0.07

0.15

0.25

ΔT = kmNaCl

(1)

where ΔT is the difference in temperature of CP-CO2 binary hydrate at constant pressure in the presence and in the absence of NaCl; k is a constant (k = 2.74 K·kg·mol−1) which could be acquired by fitting measured data; mNaCl is molality of solute NaCl (in mol·kg−1). First additional data are used to verify eq 1 so that the limited data measured in this work can be regressed by the following equation: ln(P /MPa) = A /(T /K) + B

(2)

where P is pressure and T is the temperature. As shown in Figure 4, it is believed that k is almost independent of the pressure under the tested pressure range and related to the guest molecules. Although the equation works well for mNaCl < 3.02 mol·kg−1 (15 wt %) with maximum deviation of 0.3 K, the maximum deviation of 3.7 K is noticeable for mNaCl = 5.7 mol· kg−1 (25 wt %). This is due to the molality of NaCl being too high to regard the aqueous NaCl solution as dilute solution, so the equation is not applicable for higher concentrations of aqueous NaCl solution.

Uncertainties u are u(w) = ±0.001, u(T) = ±0.1 K, u(P) = ±0.02 MPa. a

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Figure 5. Phase equilibrium conditions of CP-CO2 and THF-CO2 binary hydrate in similar concentration of aqueous NaCl solution systems: ■, CP + CO2 + 0.035 wt % NaCl, this work; □, 0.05 mol % THF + CO2 + 0.035 wt % NaCl, ref 28; ●, CP + CO2 + 0.07 wt % NaCl, this work; ○, 0.05 mol % THF + CO2 + 0.068 wt % NaCl, ref 28; ▲, CP + CO2 + 0.10 wt % NaCl, this work; △, 0.05 mol % THF + CO2 + 0.100 wt % NaCl, ref 28; ▼, CP + CO2 + 0.15 wt % NaCl, this work; ▽, 0.05 mol % THF + CO2 + 0.159 wt % NaCl, ref 28; ◆, CP + CO2 + 0.25 wt % NaCl, this work; ◇, 0.06 mol % THF + CO2 + 0.265 wt % NaCl, ref 28.

Figure 4. Effect of molality of NaCl on decrease in temperature of CP + CO2 binary hydrate at five different pressures. The solid straight line was obtained by using linear eq 1.

Basically, the inhibiting effect of NaCl yielded to the promotion effect of CP at low concentrations, but for the case of 0.25 mass fraction of NaCl, the equilibrium line shifted to low temperature compared with the line of CO2 hydrate in pure water which indicated a predominant inhibiting effect of NaCl over the promotion effect of CP. The effect of NaCl on CP-CO2 binary hydrate could be explained by the following aspects. For one thing, due to ionization of NaCl in solution, the chloride and sodium ions interacted with the dipoles of the water molecules with a Coulombic bond which was stronger than either the hydrogen bond or the van der Waals forces that caused clustering around the apolar solute molecule. The stronger Coulombic bond caused ion clustering and inhibited the formation of hydrate since water was attracted to ions more than water was attracted to hydrate structure. For another thing, the clustering also caused a decrease in solubility of carbon dioxide in water, a phenomenon known as “salting-out”. Both ion clustering and salting out combine to require substantially more subcooling for hydrates to form.2 Therefore, the inhibiting effect of NaCl increased with the increase of concentration of NaCl and finally exceeded the promoting effect of CP. As we know, CP, THF, and TBAB were three common thermodynamic promoters, the phase equilibrium of THF-CO2 binary hydrate, and semiclathrate hydrate of TBAB-CO2 in the presence of aqueous NaCl solution had been investigated by Sabil 28 and Godishala29 separately. In this work, the comparison with TBAB was omitted because the intersection among these equilibrium lines of TBAB, CP, and THF was complicated. However, a clear comparison of equilibrium lines between CP-CO2 and THF-CO2 binary hydrate with similar mass fraction of aqueous NaCl solution was presented in Figure 5, in which the mole concentration of THF (0.05 mol % and 0.06 mol %) were near the optimum and carbon dioxide was vapor for all systems. The shaded symbols represented the equilibrium date of CP-CO2 binary hydrate and the unshaded symbols corresponded to that of THF-CO2 binary hydrate. As shown, the CP-CO2 binary hydrates were expected to be more stable than THF-CO2 binary hydrates at relatively low mass fraction of aqueous NaCl solution except the case of 0.25 mass fraction of NaCl. In addition, 0.035 mass fraction of NaCl solution was used to simulate seawater and 0.10 mass fraction was enough for a simulation of high salty produced water8,27 so

the decrease of cooling to form CP-CO2 binary hydrates in seawater and in high salty produced water was approximately 1.5 K or 8.5 K, respectively. From the thermodynamic aspect, it was suggested that CP-CO2 binary hydrate could be a promising candidate for hydrate-based seawater desalination as well as high salty wastewater desalination. However, the interaction between CP and NaCl was not clear and more research was still needed.

4. CONCLUSIONS In this work, we reported CP-CO2 binary hydrate phase equilibrium data in the presence of aqueous NaCl solution at six mass fraction (0.00, 0.035, 0.07, 0.10, 0.15, and 0.25) along the H−LW−LHC−V boundary in the temperature range of (269.8 to 292.4) K and in the pressure range of (1.18 to 3.33) MPa. An isochoric pressure-search method was applied in all of the measurements. It was indicated that the inhibiting effect of NaCl yielded to the promotion effect of cyclopentane except for the case of 0.25 mass fraction of aqueous NaCl solution. It was hoped that the equilibrium data acquired in this work could contribute to developing the application of CP-CO2 binary hydrate-based desalination technology. A kinetic study of CPCO2 binary hydrate in brine will be performed in the future.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 20 8705 7669. Fax: +86 20 8705 7669. E-mail: [email protected]. ORCID

De-Qing Liang: 0000-0001-7534-4578 Funding

This work was supported by the National Natural Science Foundation of China (51376182), Scientific cooperative project by CNPC and CAS (2015A-4813) Notes

The authors declare no competing financial interest. 2464

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DOI: 10.1021/acs.jced.7b00404 J. Chem. Eng. Data 2017, 62, 2461−2465