Thermodynamic Properties of Double Semiclathrate Hydrates Formed

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

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Thermodynamic Properties of Double Semiclathrate Hydrates Formed with Tetrabutylphosphonium Chloride + CH4 Lingli Shi,†,‡,§,∥ Jiaxiang Ding,†,‡,§,∥,⊥ and Deqing Liang*,†,‡,§,∥ †

Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China ‡ Guangzhou Institute of Energy Conversion, §Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, and ∥Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China ⊥ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ABSTRACT: Phase equilibrium conditions for tetrabutylphosphonium chloride (TBPC) + CH4 double semiclathrate hydrates (SCHs) were determined to evaluate the stabilization effect of TBPC for potential applications of hydrate-based gas separation and storage. The equilibrium data revealed that TBPC strongly enhances the stability of CH4 hydrates by increasing the equilibrium temperature at a given pressure, and the system showed the strongest stabilization effect at a TBPC mass fraction of 0.30. The dissociation enthalpies of (TBPC + CH4) SCHs were calculated from the experimental data obtained in this work using the Clausius−Clapeyron equation and the Peng−Robinson equation of state. It was found that the mean dissociation enthalpies for (TBPC + CH4) SCHs with TBPC mass fractions of 0.05 to 0.50 were (135.49 to 158.32) kJ·(mol of CH4)−1, which were much higher than that of the pure CH4 hydrate. In addition, to identify the molecular behavior and hydrate structure, Raman spectroscopy measurements were conducted at 203 K. The spectra demonstrated that (TBPC+ CH4) SCHs were formed in tetragonal structure I with small 512 cages trapping CH4 molecules.

1. INTRODUCTION

SCH formers consist of a large number of ammonium salts, phosphonium salts, sulfonium salts, and amines. They are known to form SCHs of varying structures depending on salt ratios. SCHs are not exactly similar to gas hydrates, but they share many of the physical and structural properties as ordinary clathrate hydrates. The main difference between them is caused by the dual function of SCH formers. In SCHs, the host lattice is built by water molecules and anions together, trapping the cations in four-compartment large cavities and leaving the small cages vacant. Thus, the SCH formers perform a dual function of forming part of the host lattice and acting as guest molecules.This is in contrast to ordinary hydrates, whose host lattice is composed solely of water. In addition, SCHs have hydrogen-bonding interactions between the host and guest molecules, which are much stronger than the van der Waals forces in ordinary hydrates. SCHs were first found by Fowler et al.13 in 1940 and structurally determined by Jeffery14 in 1969. Among SCH formers, tetrabutylammonium salts (TBAX, X = F, Cl, Br) have been targeted by many researchers for applications and intensively investigated with respect to not only thermody-

Clathrate hydrates, or simply hydrates, are crystalline inclusion compounds stabilized by guest species in the cavities of cages composed of hydrogen-bonded water molecules.1 More than 130 guest components are known to be able to form clathrate hydrates with water, and they can be mainly categorized into three typical structures, namely, structure I with space group Pm3n, structure II with space group Fd3m, and structure H with space group P6/mmm, which are formed depending on the shape, size, and nature of the guest molecules. Because of their physical properties, such as guest-substance selectivity, large gas storage capacity, and large values of the enthalpy of formation/ dissociation, clathrate hydrates have been studied as promising media for many engineering practices to solve energy and environmental problems, for example, CO2 sequestration,2,3 gas separation/storage/transportation,4,5 seawater desalination,6−9 and cool energy storage.10−12 However, there are still a few challenges, and the major one is to form hydrate crystals under moderate conditions of pressure and temperature, since gas hydrates are typically stable under conditions of high pressure and low temperature. A technique using semiclathrate hydrate (SCH) formers, a group of nonvolatile additives, to remarkably reduce the equilibrium pressure and increase the equilibrium temperature has been proposed. © XXXX American Chemical Society

Received: August 17, 2017 Accepted: October 3, 2017

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DOI: 10.1021/acs.jced.7b00738 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Experimental Data in the Open Literature for SCHs Formed with TBAX (X = F, Cl, Br) and Water at Different Concentrations SCH former

guest gas

w

T range/K

P range/MPa

TBAB

CH4 CO2 N2 CH4 CO2 N2 CH4 CO2 N2

0.05−0.60 0.04−0.65 0.05−0.57 0.05−0.45 0.04−0.50 0.10−0.50 0.02−0.45 0.02−0.45 0.05−0.45

279.7−298.2 279.1−291.8 281.0−294.5 281.3−295.2 279.4−293.3 278.8−292.4 285.1−304.7 285.1−302.3 290.5−303.4

0.24−41.4 0.10−4.56 0.47−25.0 0.61−10.56 0.36−4.61 0.61−13.57 1.05−9.62 0.53−4.98 2.05−10.24

TBAC

TBAF

refs 15−22 15, 19, 15, 19, 30−34 25, 30, 30, 33, 39, 40 25, 32, 29, 40

20, 23−28 22, 23, 26, 29 33−38 36 39, 40

Table 2. List of the Materials Used for the Experiments chemical name

purity

source

tetrabutylphosphonium chloride (TBPC: C16H36ClP) CH4 deionized water

82.2% in water (mass fraction) 99.9% (mole fraction)

Tokyo Chemical Industry Co., Ltd. Guangzhou Shiyuan Gases Co., Ltd. laboratory-made

2.2. Experimental Apparatus. A schematic diagram of the experimental apparatus is shown in Figure 1. It allowed

namic properties but also kinetic properties. Table 1 presents a concise review of some extensively studied SCH systems at variable salt concentrations in the open literature.15−40 It shows that these SCH formers present excellent stabilization effects on gas hydrates. In addition to the TBAX salts, tetrabutylphosphonium salts (TBPX)41−44 also form SCHs with water molecules, and it was reported that TBPX salts were favorable in terms of gas storage capacity. 35,45 Deschamps and Dalmazzone45 reported that the amount of H2 stored in (TBPB + H2) SCHs could be 2 times higher than that in the (TBAB + H2) SCHs. Mayoufi et al.35 observed that (TBPB + CO2) SCHs could store 2 to 4 times more CO2 per water molecule than other SCHs. Compared with TBPB, TBPC was reported to have a stronger stabilization effect. However, the experimental information was quite limited. In 2011, Sakamoto et al.46 studied the equilibrium temperatures of pure TBPC SCHs. In 2014, Ye and Zhang38 focused on the stabilization effect of TBPC on CO2 hydrates, and Iino et al.47 reported the equilibrium conditions of (TBPC + CH4/CO2/N2) SCHs with w (salt mass fraction) = 0.36 and pressures ranging from (0.19 to 5.02) MPa. This literature review indicates an imperative need to generate more phase equilibrium data on TBPC SCHs with varying mole fractions and higher pressures to reveal the properties of TBPC, design effective processes, and optimize the thermodynamic models. In this study, the dissociation temperatures of (TBPC + CH4) double SCHs were investigated systematically at mass fractions and pressures ranging from (0.05 to 0.50) and (2.06 to 10.54) MPa, respectively. The dissociation enthalpies of (TBPC + CH4) SCHs were calculated from the experimental data obtained in this work using the Clausius−Clapeyron equation and the Peng−Robinson equation of state. In addition, Raman spectroscopy was employed to identify the molecular behavior and hydrate structure.

Figure 1. Schematic diagram of the experimental apparatus: DA, data acquisition system; V1 to V3, valves; LN, liquid nitrogen.

measurement of phase equilibrium curves within the temperature range from (253 to 399) K and pressure range from (0 to 25) MPa. The core part of the reactor was made of 316 stainless steel with an internal volume of 100 cm3. A magnetic stirrer was used to mix the contents of the reactor. The cell temperature and pressure were measured by a platinum resistance thermometer with an uncertainty of ±0.1 K and a TF01 400A absolute pressure transducer with an uncertainty of ±0.024 MPa, respectively. The uncertainties were obtained by calculating the deviations between the collected data and standard data from standard devices. The ranges of the calibration for temperature and pressure were from (273.0 to 293.0) K and (0.00 to 13.00) MPa, respectively. The measured data were collected from the acquisition system and saved at preset sampling intervals (10 s) on a computer. 2.3. Phase Equilibrium Measurement. The experimental method used to measure the equilibrium conditions was the isochoric pressure-search method, and the procedures were the same as those described in previous studies.36,42,48−50 Prior to each test, the reactor was thoroughly cleaned twice with deionized water and the aqueous solution. Then about 30 mL of the test solution was added to the vacuum reactor, and the reactor was pressurized with CH4 to the desired pressure. Then the system temperature was decreased to form hydrates. Once

2. EXPERIMENTAL SECTION 2.1. Materials Preparation. The suppliers and purities of the materials used in this work are described in Table 2. The desired TBPC aqueous solutions were prepared with freshly distilled and degassed water made in the laboratory and a purchased TBPC solution using an electronic balance with a reading uncertainty of ±0.1 mg. B



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Table 3. Phase Equilibrium (Liquid−Hydrate−Vapor) Data for (TBPC + CH4) Double SCHs Formed at Different TBPC Compositionsa w

T/K

P/MPa

w

T/K

P/MPa

w

T/K

P/MPa

0.05

282.9 285.1 287.1 288.1 289.4 285.5 288.1 289.9 291.1 292.1

2.23 4.06 5.93 7.80 9.93 2.17 4.13 6.08 8.04 10.35

0.20

288.1 290.7 292.4 293.6 294.4 288.6 291.1 292.8 294.0 294.9

2.08 4.08 6.11 8.34 10.30 2.08 4.14 6.19 8.29 10.54

0.36

288.1 290.7 292.4 293.8 294.4 286.0 288.4 290.1 291.3 292.1

2.06 4.12 6.13 8.43 10.23 2.11 4.10 6.10 8.21 9.79

0.10

a

0.30

0.50

The uncertainties u are u(T) = ±0.1 K and u(P) = ±0.024 MPa.

the hydrate formation was finished, the system was heated gradually at 0.1 K and each step was kept constant for about 4 h to achieve a steady equilibrium state. Eventually, the equilibrium point of the hydrate was determined by the sharp change in the slope of the plotted pressure−temperature curve. 2.4. Raman Measurements. The double SCH samples were prepared at a pressure of 6.0 MPa. To prevent dissociation of the hydrate shell, the samples were preserved in liquid nitrogen when transported or prepared for microstructural measurement in our laboratory. Raman spectroscopy measurements were carried out at 203 K using a confocal Raman spectrometer (LabRAM HR, Horiba) equipped with a multichannel air-cooled CCD detector. An Ar+ laser source emitting a 532 nm line was used at a power of 50 mW. The silicon crystal standard of 520.7 cm−1 was employed to calibrate the subtractive spectrograph. The sample was placed in the sample disk, which could keep the sample at the temperature of 203 K.

strongly enhances the stability of CH4 hydrates by increasing the equilibrium temperature. For example, at a pressure (P) of 4 MPa, the equilibrium temperature was 277.24 K for pure CH4 hydrate, while with the addition of TBPC (w = 0.10) it increased to 288.1 K. Besides, the stabilization effect was different in systems with different w. It was strengthened as w increased from 0.10 to 0.30 and then lessened as w increased from 0.30 to 0.50. In other words, The TBPC aqueous solution with w = 0.30 showed the strongest effect of improving the hydrates’ thermodynamic stability. For instance, when P was 6.0 MPa, the equilibrium temperature increased from 281.24 K for CH4 hydrate to (287.1, 289.9, 292.4, 292.8, 292.4, and 290.1) K for (TBPC + CH4) double SCHs with w = 0.05, 0.10, 0.20, 0.30, 0.36, and 0.50, respectively. Meanwhile, a number of experimental data reported by Iino et al.47 are also plotted in Figure 2. Some disagreements between the our equilibrium conditions data and those of Iino were observed. These disagreements were small (about 0.5 K), and they might be caused by differences in the experimental apparatus. Thus, it could be considered that the data measured in this work are consistent with those reported in ref 47. On the other hand, Figure 3 illustrates the temperature phase boundaries versus TBPC mass fraction in the ternary system TBPC + CH4 + H2O at variable CH4 pressures. It can be seen that as a result of the

3. RESULTS AND DISCUSSION 3.1. Experimental Phase Equilibrium Data for (TBPC + CH4) SCHs. The results for the hydrate equilibrium (liquid− hydrate−vapor) conditions for the TBPC + CH4 + H2O system are reported in Table 3 and plotted in Figure 2 together with those for pure CH4 hydrates.51,52 It can be seen that TBPC

Figure 3. Temperature (T) phase boundaries vs TBPC mass fraction (w) in the ternary system TBPC + CH4 + H2O at variable CH4 pressures to show the effect of TBPC on the hydrate equilibrium conditions.

Figure 2. Hydrate equilibrium data for (TBPC + CH4) hydrates along with those for pure CH4 hydrates reported in refs 51 and 52. C



Article

presence of a CH4 atmosphere, the dissociation temperatures of TBPC SCHs were also increased by (3.4 to 13.8) K depending on the pressure. Such stabilization could be explained by the inclusion of gas into available empty cavities of the SCH and the interactions between the gas and the lattice via van der Waals forces. This phenomenon, called gas enclathration, has been observed and studied for many gaseous species with a number of similar hydrates formed from TBAX (X = F, Cl, Br) and others. For further understanding, Figure 4 compares our data with those for TBAC/TBPB/TBAB + CH4 double SCHs at the salt

Figure 5. Semilogarithmic plots of pressure vs reciprocal temperature for TBPC + CH4 + H2O systems.

Table 4. Enthalpies of Dissociation for CH4 in TBPC Solutions and Pure Water system TBPC

Figure 4. Experimental equilibrium data for double SCHs formed with CH4 and various SCH formers.

pure water51,52

concentrations corresponding to their highest stabilization effects. As shown, all of the SCH formers stabilized the CH4 hydrates to different degrees. Among them, the effect of TBPB was weakest, and the strongest-to-weakest order of their stabilization effects is TBAC > TBAB > TBPC > TBPB. Thus, the cation is assumed to have a greater effect on the trend of stabilization effects and the anion to have a minor impact. This phenomenon was the same as that reported in ref 48. 3.2. Dissociation Enthalpy of (TBPC + CH4) SCHs. The dissociation enthalpy (ΔH) of the (TBPC + CH4) double SCHs was determined using the Clausius−Clapeyron equation: d ln p ΔH =− d(1/T ) zR

w

ΔH/kJ·(mol of CH4)−1

T range/K

0.05 0.10 0.20 0.30 0.36 0.50

135.49 141.83 156.16 158.32 154.58 152.61 57.48

282.9−289.4 285.5−292.1 288.1−294.4 288.6−294.9 288.1−294.4 286.0−292.1 273.4−286.4

participates in the cage structure mainly through hydrogen bonding and the fact that the hydrogen-bonding interactions between the host and guest are much stronger than the van der Waals forces in ordinary hydrates. Besides, different enthalpies were observed at various TBPC concentrations. It should be noted that eq 1 is a simplification of the Clapeyron equation. Only the methane gas phase was considered, and the dissociation enthalpy was supposed to be constant over the temperature range of interest. Thus, the difference in these enthalpies might largely result from the volume changes and the ignorance of the amount of dissolved methane. Therefore, the calculated enthalpies should be considered as a first approximation. Considering the fact that direct measurement was difficult, more useful and reliable dissociation enthalpy calculations from experimental data are a goal of future work. 3.3. Raman Spectra of (TBPC + CH4) SCHs. Raman spectra were measured to identify the molecular behavior and hydrate structure. The Raman spectra are shown in Figure 6. It is well-known that pure CH4 hydrate is formed in structure I with two peaks at (2905 and 2915) cm−1, corresponding to CH4 in the large and small cages, respectively. As shown in Figure 6, the peak at 2913 cm−1, corresponding to the C−H vibration of CH4 molecules in small cages, demonstrated that the CH4 molecules were encaged in the dodecahedron (512, D) cavities of TBPC SCHs. The slight shift in wavenumber (2915 cm−1 to 2913 cm−1) might be caused by distortion of the 512 cages. It should be noted that this phenomenon was observed previously. In 2015, Kim et al.34,54 studied the spectra of (TBAC + CO2) SCHs and (TBAC + CO2 + N2) SCHs, and the results showed that the addition of TBAC caused a slight shift in wavenumber from (1275 to 1272) cm−1. Besides, in

(1)

where R is the gas constant and z is the compressibility factor of CH4 gas, which was calculated using the Peng−Robinson equation of state. The values of d ln P/d(1/T) were set equal to the slopes of the straight lines shown in Figure 5. The straight lines represent the best linear fits of the semilogarithmic plots of hydrate dissociation pressure versus reciprocal absolute temperature (ln P versus 1/T) with the correlation coefficient R2 close to 1. The mean enthalpies of dissociation for (TBPC + CH4) double SCHs are shown in Table 4 along with that for the pure CH4 hydrates. It should be noted that the calculated ΔH value for CH4 hydrates agrees well with the reported values, measured by Handa53 via calorimetry (54.19 kJ·(mol of CH4)−1) and calculated by Du and Wang22 (59.39 kJ·(mol of CH4)−1). As shown, the ΔH values of the SCHs were systematically larger than that of the CH4 hydrates. This was caused by the special structure of SCHs in which TBPC D



Article

Notes

The authors declare no competing financial interest.

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Figure 6. Raman spectra of (TBPC + CH4) hydrates formed with various w.

2008, Hashimoto et al.55 illustrated and analyzed the differences between Raman spectra of hexagonal structure I (HS-I) and tetragonal structure I (TS-I) formed with different TBAB solutions. The peaks around (1100 and 1400) cm−1 could be used to distinguish the crystal types. As depicted in Figure 6, the peaks appeared around (1100 and 1414) cm−1, which revealed that the (TBPC + CH4) SCHs were formed in TS-I.

4. CONCLUSIONS In this work, phase equilibrium conditions of (TBPC + CH4) SCHs were determined at w = 0.05, 0.10, 0.20, 0.30, 0.36, and 0.50. The experiments were conducted using the isochoric pressure-search method over the temperature range of (282.9 to 294.9) K and the pressure range of (2.06 to 10.54) MPa. The results showed that TBPC has a stabilization effect on CH4 hydrates. As w increased, the effect first became stronger and then lessened. The dissociation enthalpies were calculated from the experimental data using the Clausius−Clapeyron equation and the Peng−Robinson equation of state. The mean dissociation enthalpies for (TBPC + CH4) SCHs were (135.49 to 158.32) kJ·(mol of CH4)−1 for w = 0.05 to 0.50. In addition, Raman spectroscopy measurements were carried out at 203 K, and the spectra revealed that (TBPC + CH4) SCHs were formed in TS-I with small 512 cages trapping CH4 molecules. These results could provide important information for hydrate-based industrial applications such as gas separation and storage.

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AUTHOR INFORMATION

Corresponding Author

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

Lingli Shi: 0000-0003-3374-6485 Deqing Liang: 0000-0001-7534-4578 Funding

This work was supported by the National Natural Science Foundation of China (51606198 and 51376182), the Natural Science Foundation of Guangdong Province (2016A030310126), and the Fund of the Key Laboratory of Gas Hydrate, Chinese Academy of Sciences (Y607jb1001). E



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Thermodynamic Properties of Double Semiclathrate Hydrates Formed

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