Experimental Study on the Formation Kinetics of Methane Hydrates in

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Experimental study on formation kinetics of methane hydrates in the presence of tetrabutyl ammonium bromide Lingli Shi, Xiaodong Shen, Jiaxiang Ding, and De-Qing Liang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01213 • Publication Date (Web): 20 Jul 2017 Downloaded from http://pubs.acs.org on July 23, 2017

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Experimental study on formation kinetics of methane hydrates in the presence of tetrabutyl ammonium bromide Lingli Shia,b,c, Xiaodong Shena,b,c,d, Jiaxiang Dinga,b,c,d, Deqing Lianga,b,c,* a

CAS Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences,

Guangzhou 510640, China b

Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou

510640, China c

Guangzhou Center for Gas Hydrate Research，Chinese Academy of Sciences, Guangzhou 510640, China

d

University of Chinese Academy of Sciences, Beijing 100049, China

* Corresponding Author. E-mail address: [email protected]. (D. Liang)

ABSTRACT:

Tetrabutyl ammonium bromide (TBAB), could form semiclathrate hydrate under milder conditions

as compared to gas hydrate and thus effectively improve the thermodynamic stability of gas hydrate. To evaluate the effect of TBAB on the formation kinetics of methane hydrates, the kinetic properties of TBAB + CH4 semiclathrate hydrate were investigated with an isobaric method at 7.0 MPa with salt mass fraction and subcooling degree varying from (0.05 to 0.60) and from (4 K to 10 K), respectively. The result showed that with higher salt concentration and subcooling degree, the induction time presented its stochastic property, while the normalized gas consumption and gas consumption rate were relatively constant and higher, respectively. In addition, the Raman spectroscopy, PXRD device, and a cryo-SEM were employed to identify the molecular behavior and observe the hydrate’s morphology. The spectra of Raman and PXRD revealed that the CH4 molecules were encaged in dodecahedron cages and its

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addition induced the orthorhombic pattern, which had the larger potential gas capacity, rather than the tetragonal one. The microstructure pictures presented that the addition of TBAB could make the hydrate’s surface more ordered and tight, as a contrary to that of inhibitors which led to a more chaotic and porous surface. These results could provide important information for hydrate based industrial applications, such as gas separation and storage.

Keywords: tetrabutyl ammonium bromide, methane hydrate, hydrate formation kinetics 1. Introduction Gas hydrate, formally known as clathrate hydrate, is a nonstoichiometric crystalline compound that consists of water (host) and gas (guest, such as CH4, CO2 and N2) molecules.1 So far, three main crystalline structures are identified as cubic structure І (sІ), cubic structure П (sП) and hexagonal structure H (sH) to house the gas molecules according to the size, shape and natures of gas molecules. Recently, methane hydrate’s key feature of significantly high volumetric gas storage capacity (~160 v/v STP methane) has received rather massive attention from both science and engineering fields for numerous applications, such as gas capture and storage,2-7 seawater desalination,8 refrigeration,9 and energy storage.10, 11 However, the condition of high pressure and low temperature is generally the main concern about the described applications. Presently, many studies focusing on hydrate-based CH4 storage have been reported, representing that adding thermodynamic promoters was an effective method to stabilize hydrates. Among the thermodynamic promoters, quaternary ammonium salts (QAS), especially tetrabutyl ammonium bromide (TBAB),12-28 were perhaps the most-widely proposed hydrate promoters due to their good promotion effect and the properties of environmental friendly and non-volatile as compared to THF,29, 30 another known hydrate thermodynamic promoter. QAS were reported to form semiclathrate hydrate (SCH) with water molecules at atmospheric pressure. SCH was not exactly similar to gas hydrate but shared many of the physical and structural properties as ordinary clathrate hydrate.31 The main difference between them was caused by the performance of QAS. In SCH, the lattice was a polyhedral water-anion framework built by water molecules and anions through hydrogen bonding, and the cations were located in four compartment cavities composed by tetrakaidecahedron cages (T, 51262) and pentakaidecahedron cages (P, 51263), leaving the dodecahedron cages (D, 512) vacant. Thus the QAS performed a dual function, forming both a part of the lattice and acting as guest molecules hosted in structural cavities. It was reported that the most common structures of SCH were hexagonal structure-І (HS-І), tetragonal structure-І (TS-І) and cubic


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superstructure-І (CSS-І) with unit cell formula of 2P·2T·3D·40H2O, 4P·16T·10D·172H2O, and 48T·16D·368H2O, respectively. TBAB was reported to be formed on the basis of HS-І and CSS-І water lattice in the low salt content region and high salt content region, respectively.32 In the unit cell of TBAB SCH (TBAB·38H2O), the host framework, built by Br¯ and water molecules together, consists of 6 D cages, 4 T cages and 4 P cages. The TBA+ cation was trapped in four-compartment cavities of 2T·2P and the small D cages were vacant to enclathrate gas molecules.33 Within the past decade, particular attention has been paid to TBAB as a prospective thermodynamic hydrate promoter. Several researchers have reported that TBAB could enhance the stability of gas hydrate by greatly lowering the equilibrium pressure.13, 34-42 While compared with the hydrate’s thermodynamic properties which were time-independent, the time-dependent formation kinetics were still not completely understood. Trueba et al.43 studied the formation kinetics (hydrate nucleation, hydrate growth and gas storage capacity) of TBAB + H2 SCH under the influence of two salt concentrations (2.6 mol% and 3.7 mol%) and reported that kinetics were favored at higher concentrations. In 2013, Park et al.44 investigated and compared the kinetics of TBAB + H2 + CO2 SCH and TBAF + H2 + CO2 SCH. The results indicated that TBAB was a better additive in terms of the gas storage, whereas TBAF was a better additive for thermodynamic stability. Besides, it should be noted that the additive could perform as both kinetic promoter and thermodynamic inhibitor. In 2014, Farhang et al.45 reported that the sodium halide anions which were known as thermodynamic inhibitors performed as kinetic promoters by enhancing the kinetics of CO2 hydrate formation at low concentration. This reference review indicated an imperative need for studying the kinetics of SCH, due to the fact that the influence of salt concentrations on formation kinetics of SCH over a wide concentration range has not been fully studied. The focus of this study was to investigate the formation kinetics of methane hydrates in the presence of TBAB. Prior to the kinetic study, the three-phase equilibrium conditions of TBAB + CH4 hydrates at various concentrations were measured to determine the thermodynamic stability. Then, the formation kinetics of TBAB + CH4 were investigated to evaluate the effect of TBAB. In addition, the Raman spectroscopy, PXRD device, and cryo-SEM were employed to identify the molecular behavior and hydrate’s morphology. 2 Experimental section 2.1 Experimental materials


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Table 1 reported the suppliers and purity of the chemicals used in this work. All of these materials were used as received and without any further purification. Deionized water made in the laboratory, after careful degassing, and TBAB were weighed on an electronic balance with the uncertainty of ± 0.01g to prepare desired TBAB aqueous solution with w (TBAB mass fraction) = (0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, and 0.60). Table 1. List of the Materials Used for the Experiments. chemical name TBAB

chemical structure

CH4 water

purity

source

> 0.98 (mass fraction)

Tokyo Chemical Industry Co., Ltd.

> 0.999 (mole fraction) deionized

Guangzhou Shiyuan Gases Co., Ltd

2.2 Experimental apparatus The experimental apparatus used for the measurements of hydrate phase equilibrium condition and kinetics was shown in Figure 1, and it has been described in detail in our previous work.46 The core of the apparatus was the hydrate formation reactor, a cylindrical vessel made of stainless steel with inner volume of 307 cm3. The reactor was designed to operate at pressures up to 20 MPa. There were two see-through quartz windows (16 mm × 80 mm each) which allowed lighting inside the reactor and making video documentation during hydrate formation reinforced on front and back of the reactor. A stirrer driven by a DC motor was equipped at the top of the reactor to mix the fluid and hydrate crystals in the reactor. The reactor was immersed in a glycol-water bath to maintain the temperature of the cell at a prescribed level. A buffer tank equipped with a pump and a glycol-water bath and a pressure reducing valve (Tescom, ER5000SІ-1) allowing maintaining a constant pressure with a precision of ± 0.5% relative were employed to regulate the conditions of test gas induced into the cell during the experiment. A gas flow meter (Bronkhorst, EL-FLOW Digital) was used to record the volumetric gas consumption. A platinum resistance with an uncertainty of ± 0.1 K and a pressure transducer with an uncertainty of ± 0.024 MPa were applied to measure the temperature and pressure of the system, respectively. All the data of temperature, pressure, gas consumption and process video were collected through a data acquisition system (DAQ) and saved at preset sampling intervals on a computer.


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Figure 1. Schematic diagram of the experimental setup. 2.3 Experimental Procedures 2.3.1 Measurement of hydrate phase equilibrium condition The measurement of hydrate phase equilibrium condition was conducted by employing the isochoric pressure search method. The procedure was the same as that described in references 47-50. The test aqueous solution was introduced into the vacuum reactor after the reactor was rinsed with distilled water and the aqueous solutions twice, respectively. Then the reactor was pressurized up to the desired pressure with CH4 and the stirring was started. The system was cooled to form hydrates. Once the hydrate formation was finished, the system was heated gradually at about 0.1 K and each temperature was kept constant for (3 to 4) h to achieve a steady equilibrium state in the reactor. Eventually, the point at which the slope of p-T curve sharply changed was reported as the equilibrium condition. 2.3.2 Measurements of hydrate formation kinetics The measurement of hydrate formation kinetics was carried out with an isobaric method which was reported in reference 46. Prior to the test, the reactor was thoroughly washed with deionized water and the aqueous solution twice, respectively. The vacuum reactor was then charged with 100 mL of aqueous solution and cooled to the experimental temperature. After the temperature was kept constant for about 0.5 h, the reactor was pressurized to the experimental pressure (7.0 MPa) through the pressure reducing valve. During the experiment, the pressure was kept constant with the help of the pressure reducing valve (Tescom, ER5000SІ-1). Once the temperature change caused by gas phase was removed, the stirrer was switched on and set to 360 rpm. The gas consumption collected by the gas flow meter increased due to the formation of hydrate. The end of the hydrate formation could be checked through the stabilized temperature and gas consumption. All the experiments were repeated three times for each system with different w and T to ensure that the experiment results were reliable and reproducible.


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2.3.3 Raman, PXRD and SEM measurement The hydrate samples were prepared in a 100 mL high-pressure reactor with 20 mL test solutions at p = 7.0 MPa and subcooling degree (∆T) = 10 K. To prevent the dissociation of hydrate shell, the samples were stored in liquid nitrogen. A confocal Raman spectrometer (Horbia, LabRAM HR) equipped with a multichannel air-cooled CCD detector was used to measure the Raman spectra at 203 K. 532 nm from an Ar+ laser performed as the excitation source and the laser had 50 mW intensity. An X’pert Highscore Plus (PANalaytical) diffractometer was employed to measure the structural characterization of the hydrate samples at 183 K. The measurement was carried out in the θ/2θ step scan mode with a step width of 0.017° over a 2θ range of 5° – 80° using Cu Kα radiation and parallel beam optics. A cryo-SEM (Hitachi, S-4800) was used to observe the hydrate’s morphology. Before observation, the sample was roughly grinded in a ceramic mortar which was full of liquid nitrogen to ensure the original hydrate morphology without dissociation. The observation was carried out at 100 K in a vacuum environment. 3. Results and Discussion 3.1 Phase equilibrium condition of (TBAB + CH4) hydrate The stability conditions of (TBAB + CH4) SCH were systematically measured with w ranging from (0.05 to 0.60). The results were tabulated in Table 2 together with the data reported in references.15, 51 It was found that the equilibrium temperatures of (TBAB + CH4) SCH were all higher than that of pure CH4 hydrate (283.25K at 7.25 MPa), indicating that TBAB performed as a thermodynamic promoter and remarkably enhanced the thermal stability. In addition, a lower stabilization effect was shown as w exceeded 0.35. It revealed that the highest stabilization effect appeared at the stoichiometric concentration of TBAB SCH, which was TBAB·38H2O. This phenomenon has been observed and reported by many researchers for systems including QAS.46, 47 Table 2. Phase equilibrium data of (TBAB + CH4) SCH. w 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.60 0.10 (reference 51) 0.35 (reference 15) 0.407(reference 51 ) 0.60 (reference 51 )

T/K 290.1 290.6 292.3 292.9 293.0 293.2 293.3 293.0 291.3 290.75 293.8 292.95 291.35

P/MPa 7.01 6.97 6.97 6.90 7.08 7.10 7.10 7.07 7.04 7.09 6.892 6.80 7.20


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3.2 Formation kinetics of (TBAB + CH4) hydrate The formation kinetics of (TBAB + CH4) hydrate were measured at 7.0 MPa with w and ∆T varying from (0.05 to 0.60) and from (4 K to 10 K), respectively. The investigated kinetics included the parameter of induction process (induction time, IT) and those of growth process (normalized gas consumption and normalized gas consumption rate). As shown in Figure 2, the higher ∆T was, the lower the experimental temperature was. Thus a higher ∆T meant a higher driving force, and was expected to present a better kinetic performance, including shorter induction time and faster formation rate.

Figure 2. Kinetic experimental conditions represented on the T-w curve at 7.0 MPa pressure.

Figure 3. Induction time (IT) of TBAB + CH4 hydrate formation process with various ∆T and w.


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Figure 4. Normalized gas consumption (NG) of TBAB + CH4 hydrate formation process with various ∆T and w. The IT was caused by the fact that the hydrate formation process required a supersaturation environment, which was dependent on the pressure and temperature differences between the equilibrium conditions and the operation conditions. In this work, IT was calculated as the time elapsed from the beginning of the stirring until a significant temperature increase was detected in the system, which could also be confirmed with the assistant of visual observation through the see-through quartz windows. As shown in Figure 3, in each system, the IT generally decreased with the increase of ∆T, showing that a higher ∆T resulted in a shorter IT. On the other hand, it should be noted that w also influenced the IT but the influence was complicated. At the same ∆T, it did not show any uniform rules considering the effect of w on IT. In systems with ∆T = 4 K and w = (0.15, 0.20, and 0.40), the hydrate did not nucleate even in 12 h after stirring started. In systems with ∆T = (6, 8 and 10) K, according to the change of IT the experimental solutions could be divided into two groups: one was solutions with w < 0.20 and the other was those with w > 0.20. The length of IT generally increased in the first group and decreased in the second group with the increase of w. We supposed that this phenomenon might be caused by both the well-known stochastic property of IT and the structural properties of SCH.


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Figure 5. Gas consumption rate (r) and gas uptake of TBAB + CH4 hydrate formation process with w = 0.35 and ∆T = 4 K in three repeated studies. Normalized gas consumption (NG) was the parameter of hydrate growth process and obtained through the following equation: NG=TGC /nw

⑴.

nw /18.0153

⑵.

TGC was the total gas consumption collected through the gas flow meter. nw was the mole number of water in the studied system, and it could be calculated with equation (2) with the help of solutions density given by Belandria.52 As shown in Figure 4, the NG was almost same in different systems except that in systems with T below 282 K, which was the condition under which pure CH4 hydrate could be formed. This phenomenon revealed that although the addition of TBAB could enhance the stability of CH4 hydrate, it could not improve the formation kinetics, especially for the NG.


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Figure 6. Gas consumption rate (r) of TBAB + CH4 hydrate formation process with various ∆T and w. To further study the properties of hydrate formation process, the gas consumption rate (r) was calculated with the equation: r=dTGC/dt

⑶.

t was the time elapsed from the end of hydrate nucleation stage. With the assistance of r, firstly, we examined the repeatability of hydrate growth stage. Then we studied the influences of ∆T and w on r in different systems. Figure 5 displayed the change of r and gas uptake during hydrate growth stage under the same conditions (w = 0.35, ∆T = 4 K) in three repeated experiments. During the three experiments, both the change and the value of r were almost the same. Thus the repeatability of hydrate growth stage was well verified. Figure 6 presented the change of r in hydrate growth process under various ∆T and w. With the same w, in system with higher ∆T, the value of r was higher and the growth time was shorter, which meant that the hydrate was formed faster. Under the same ∆T, system with


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higher w showed higher r and shorter growth time. In addition, it should be noted that in each systems, two relatively high peaks appeared in the r-t figures. The former showed up in the early period while the latter appeared near the end of growth stage. This phenomenon was caused by the driving force and the experimental apparatus. In the early period, the hydrates were formed rapidly, which corresponded to a high gas consumption rate, increased system temperature and decreased driving force. When the heat was taken off by the glycol-water bath, the latter peak appeared due to the increased driving force and gas which was charged to keep constant pressure. The comparison of the former peaks suggested that a slightly stronger hydrate nucleation occurred in the memory solutions. 3.3 Structural analysis of (TBAB + CH4) hydrate

Figure 7. Raman spectroscopic observation on the hydrates formed from TBAB solutions and CH4 with various w. Figure 7 showed the Raman spectra of TBAB + CH4 hydrate samples. It could be seen that small 512 cages (corresponding to the Raman shift at 2915-1) appeared in all systems, while the large 51262 cages (corresponding to the Raman shift at 2905-1) only showed up in systems with w = (0.00 and 0.05), in which pure CH4 hydrate was formed. This implied that the addition of TBAB could leave small cages vacant to trap gas molecules and the large 51262 cages was distorted and combined to encage TBA+. This result was consistent with previous report of the properties of SCH. To further study the structure of SCH, PXRD measurement was carried out. Figure 8 presented the PXRD patterns for the hydrate samples formed with various w. Systems with w = (0.05 to 0.60) formed TBAB + CH4 SCH, and the difference between these patterns could be seen at 2θ angles below 20°. Samples with w = (0.30, 0.35 and


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0.60) presented an orthorhombic Pmma pattern (a = 20.472 Å, b = 11.703 Å, c = 11.706 Å), similar to the orthorhombic TBAB hydrate

Figure 8. Powder X-ray diffraction (PXRD) patterns of TBAB + CH4 hydrates formed with various w. . The peaks at 2θ = (7.6°, 8.9°, 11.5°, 16.8°, 17.4°, and 18.7°) were assigned to the Miller index (hkl) of (001, 101, 201, 311, 121 and 401), respectively. Muromachi et al.53 have reported the PXRD pattern of TBAB + CH4 SCH with w = 0.32, and it was also an orthorhombic Pmma pattern. In 2016, Oshima et al.54 presented the PXRD patterns for pure TBAB SCH with w varying from 0.10 to 0.35. It showed that TBAB SCH was formed with an orthorhombic pattern at w lower than 0.18, and the pattern significantly changed into the tetragonal one as w exceeded 0.22. Comparing these results, it was possible that the difference was caused by the addition of guest gas, since in our study and Oshima’s experiment, CH4 was added. It was supposed that the guest gas could induce the formation of the orthorhombic phase, as the orthorhombic structure (corresponding to TBAB·38H2O with 3 D cages) had the larger potential gas capacity among the TBAB hydrate structures. The tetragonal TBAB + CH4 hydrates (corresponding to TBAB·26H2O with 2 D cages)32 was obtained only at w = 0.40, and for systems with w = (0.05 to 0.25), the patterns mainly consisted of hexagonal ice and the typical SCH structures were not observed. Considering that the hydrate samples were prepared at different temperatures, the forming conditions might influence hydrate structures. Therefore, other analytical techniques, such as in situ Raman spectroscopy and in situ XRD measurement, might be useful for understanding the structural changes in TBAB + CH4 SCH.


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Figure 9. Microstructure pictures of TBAB + CH4 hydrates formed with various w. Given that the addition of CH4 induced the orthorhombic pattern with w exceeding 0.25 showed by the PXRD spectra, to further compare the properties of TBAB + CH4 SCH, a cryo-SEM was employed to observe the hydrate’s morphology of systems with w = (0.25 to 0.60). As shown in Figure 9, with the increase of w, the hydrate surface became rougher and more uneven. Compared with the pictures reported by Zhang,55 TBAB + CH4 hydrate was relatively ordered and tight. Thus the additive of TBAB could make the hydrate’s surface more ordered, while the additive of inhibitor led to a more chaotic and porous surface. In addition, samples with w exceeding 0.35 presented surfaces with veins, showing that the SCH was formed and grew in order. These results might provide important information for hydrate based industrial applications, such as gas separation and storage. 4 Conclusions In this study, the formation kinetics of TBAB + CH4 SCH have been investigated with an isobaric method. The IT, parameter of induction process, generally decreased with the increase of ∆T. The NG and r, parameters of growth process, were relatively constant and higher, respectively, with the increase of w and ∆T. In addition, the micro structural information of TBAB + CH4 SCH was detected by employing a Raman spectrometer, a PXRD spectrometer, and a cryo-SEM. The Raman spectra showed that the SCH owned the same small 512 cages (corresponding to the Raman shift at 2915-1) and left it vacant to capture gas molecules. The PXRD spectra revealed


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that the orthorhombic pattern was induced by the addition of guest gas as it had the larger potential gas capacity than the tetragonal one. Cryo-SEM pictures indicated that the addition of TBAB could make the hydrate’s surface more ordered and tight. These results could provide more important information for hydrate based industrial applications.

Author information Corresponding author E-mail: [email protected]. Tel.: +86 20 8705 7669. Fax: +86 20 8705 7669. Notes The authors declare no competing financial interest.

Acknowledgments The authors acknowledge the supports by the National Natural Science Foundation of China (51606198), Guangdong Natural Sciences of Foundation (2016A030310126), and Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (Y607jb1001). References (1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases. 3rd ed. CRC Press, Taylor & Francis Group: Boca Raton 2008. (2) Xu, C.-G.; Li, X.-S.; Lv, Q.-N.; Chen, Z.-Y.; Cai, J. Hydrate-based CO2 (carbon dioxide) capture from IGCC (integrated gasification combined cycle) synthesis gas using bubble method with a set of visual equipment. Energy 2012, 44 (1), 358-366. (3) Dabrowski, N.; Windmeier, C.; Oellrich, L. R. Purification of Natural Gases with High CO2 content using gas hydrates. Energy Fuels 2009, 23 5603-5610. (4) Nakayama, T.; Tomura, S.; Ozaki, M.; Ohmura, R.; Mori, Y. H. Engineering investigation of hydrogen storage in the form of clathrate hydrates: Conceptual design of hydrate production plants. Energy Fuels 2010, 24 (4), 2576-2588. (5) Zhang, B.; Wu, Q. Thermodynamic promotion of tetrahydrofuran on methane separation from low-concentration coal mine methane based on hydrate. Energy Fuels 2010, 24 (4), 2530-2535.


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Zhan,

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