Kinetic Measurements on CO2 Hydrate Formation in the Presence of

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Environmental and Carbon Dioxide Issues

Kinetic measurements on CO2 hydrate formation in the presence of Tetra-n-butyl Ammonium Bromide Xuebing Zhou, Zhen Long, cuiping tang, and De-Qing Liang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01914 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 1, 2018

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Kinetic measurements on CO2 hydrate formation in the presence of Tetra-n-butyl Ammonium Bromide Xuebing Zhoua,b,c,d, Zhen Longa, Cuiping Tanga, Deqing Liang*a,b,c,d a

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China b

c

CAS Key Laboratory of Gas Hydrate, Guangzhou 510640, PR China

Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, PR China

d

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

*Corresponding author. Tel.: +86 20 87057669; fax: +86 20 87057669. E-mail: [email protected].

Abstract: Ionic clathrate hydrates are promising materials for hydrate–based CO2 capture due to their mild forming conditions. In this work, hydrate crystallizations from different TBAB solutions were measured. The effect of TBAB concentration on hydrate nucleation time, hydrate growth and gas storage capacity were evaluated. The TBAB concentration ranging from 0.3 to 7.5 mass% was found to be ideal for an efficient hydrate formation. The TBAB hydrates were assumed to form first in the liquid bulk which induced the growth of simple CO2 hydrate. In dilute TBAB solutions, guest ions in TBAB tended to inhibit the hydrate nucleation and increase the induction time, while concentrated TBAB solutions were found to reduce gas storage capacities. PXRD measurements revealed that the orthorhombic TBAB hydrate preferred to form where TBAB concentration was below 35 mass%. When the concentration was above 40 mass%, tetragonal and trigonal TBAB hydrate dominated the solid hydrate phase. In Raman measurements, C-H stretching modes of TBA+ cation in different 1

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hydrate structures were characterized. CO2 was proved to be captured by tetragonal and orthorhombic TBAB hydrates which was suggested to be beneficial for hydrate stabilization. Keywords: Gas hydrate; Hydrate-based technology; Carbon dioxide

1. INTRODUCTION The anthropogenic emissions of carbon dioxide (CO2) and other hydrocarbons, such as methane (CH4) released from refineries and wells, are key contributors to global climate change. In 2014, the global CO2 concentration rose up to 400 ppm and it continues to increase.1 Unfortunately, traditional fossil fuels still dominates power and industrial systems. With growing public awareness of climate change, carbon capture and storage (CCS) technology has been developed and become a one of the most attractive technologies for decarburization.2-4 A gas hydrate or clathrate hydrate is a kind of crystalline solid that can capture CO2 and stably preserved at high pressures and low temperatures in deep-sea sediments. It consist of the host framework of hydrogen-bonded water molecules and the gases including CH4 and CO2 as guest molecules. According to the size of the guest molecules, different types of hydrate structures (the canonical sI, sII and sH) can be formed, which are the combinations of several differently shaped cages such as dodecahedron (512) cages and tetrakaidecahedron (51262) cages.5 It means that a certain type of hydrate structure will capture the gas molecules with selectivity. Meanwhile, gas hydrates can be reusable by shifting the pressure above or below the equilibrium pressure to absorb or release the gases, which is suggested be suitable for carbon capture. The introduction of ionic salts makes hydrate-based carbon capture more appealing. As guest 2


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molecules, the cations of ionic salts are mainly composed of alky groups which can be encaged by the host lattice. The anions, usually halide anions, are involved in the water framework through hydrogen bonding by replacing one or two water molecules.6-8 This combination mode, named ionic clathrate hydrates, allow the crystal structures to remain stable at around 285 K without guest gas molecules. The pressure needed for CO2 capture is also greatly reduced. Among a vast variety of ionic salts, tetra-n-butylammonium bromide (TBAB) get widely studied due to its low toxicity and good stability.9,

10

There are 2 typical TBAB hydrates structures with hydrate number of 26 and 38.

TBAB·38H2O belongs to an orthorhombic structure which contains 6 512-cages, 4 51262-cages and 4 51263-cages per unit cell, while TBAB·26H2O belongs to a tetragonal structure which has 10 512-cages, 16 51262-cages and 4 51263-cages in a crystal unit.11 The structural variety should result from the difference in the way of including bromide anions and arranging tetra-n-butyl ammonium cations.12, 13 Since the TBAB hydrates only leave 512-cages vacant for gas capture, the selectivity is more evident than that of sI hydrates.14 Specifically, the vacant 512-cages, which are squeezed by the cations in the adjacent cages, show well selectivity for CO2.15-17 TBAB hydrates are thus proposed to be applied in ecological CO2 enrichment system for greenhouse production.18 More widely, they get intensively studied in the separation of CO2 from the synthetized CO2/H2 gas mixtures or flue gas produced in integrated gasification combined cycle (IGCC).19-23 Both of the testing temperature and separation efficiencies are found to be greatly enhanced comparing to those of the sI hydrates.24-26 These studies offers a promising technology for CO2 capture, but understanding about the TBAB hydrates formation is still far from enough due to the complex kinetic behaviors. Nguyen et al.27 noticed an unexpected inhibition behavior during hydrate crystallization formed with CO2 in dilute TBAB solutions. 3


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Structural transitions of the mixed TBAB hydrates are found to occur when noble gases are introduced at high pressures.28-30 Due to the polymorphic phases of TBAB hydrates, tetragonal and orthorhombic structures are found to easily coexist during crystallization.31-34 The CO2 storage capacity and the selectivity of TBAB hydrates are suggested to irregularly depend on hydrate forming pressures and TBAB concentrations in aqueous solutions. Unpredictable kinetics of TBAB hydrate formation increase the difficulties in scalability studies and prevent large scale development of the proposed technologies. In this work, we focused on the formation kinetics of carbon dioxide gas hydrates with TBAB. Effects of TBAB on the hydrate nucleation and growth were measured by varying TBAB concentration from 0 to 60% in mass. The crystallographic properties of the formed hydrates were also characterized by powder X ray diffraction (PXRD) and Raman spectroscopy. Based on the experimental results, an optimal TBAB concentration for hydrate formation was found, which has short nucleation time, high growth rate and relatively large gas storage capacity. The structural change caused by TBAB concentration will be discussed which is instructive to hydrate-based CO2 capture.

2. EXPERIMENTAL SETCTION 2.1 Kinetic measurements The reagents of research-grade TBAB (0.98 mass fraction purity) was supplied by Tokyo Chemical Industry Co., Ltd. The doubly distilled, deionized water was made in the laboratory. 18 kinds of aqueous solutions with the TBAB concentrations ranging from 0 to 60 mass% were prepared. The uncertainties of the TBAB concentrations were controlled within ±5×10-4 mass%. The CO2 (0.999 mole fraction purity) was purchased from the Puyuan Gas company (Guangzhou, China). All 4


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materials were used without further purification. A schematic diagram of the experimental apparatus for the hydrate crystallization was shown in Fig. 1. The apparatus was specially designed to accurately measure the pressure and temperature during hydrate formation. The hydrate forming reactor was made of 316 stainless steel and had an internal volume of 179 mL. A magnetic spin bar was placed at the bottom of the reactor and driven by an external magnet. The gas in the reactor was introduced from a 2.3 L gas reservoir which was used to precool the gas. A platinum resistance thermometer (PT 100) with an accuracy of ±0.1 K and a Setra

smart pressure transducer (model SS2, Boxborough, MA) with an uncertainty of ±0.01 MPa were inserted from the top of the reactor and the gas reservoir respectively. The temperature of the apparatus were controlled by an air bath with an accuracy of ±0.1 K. The reactor was first rinsed 3 times with distilled water, and 30 ±0.02 grams of prepared aqueous solution was charged to the reactor. After properly sealed, the reactor was put into the air bath, connected to gas reservoir, evacuated and chilled down to 275.2 K. When the temperature was stable, CO2 in the gas reservoir was slowly injected in the reactor till the desired pressure was reached. In this work, the initial pressure of all the experiments were set at 3.00 ±0.05 MPa. The experiment was considered to commence once the stirrer was started. The initial conditions for different experimental runs were listed in Table 1. Through a whole experimental run, the temperature and pressure were recorded at a regular time interval of 5 seconds. A typical hydrate formation usually went through a nucleation stage, followed by a crystal growth stage as seen in Fig.2. In the first stage, a part of gas molecules dissolved in the bulk aqueous phase, but no visible hydrate crystals could be found. Within the framework of the classical nucleation theory, unstable water clusters without reaching the limited size grew or perished repeatedly during this 5


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stage.35, 36 The time lapse between the start of stirrer and the onset of crystal growth was taken as the induction time. The crystal growth stage started with the appearance of the first stable hydrate crystal. Driven by the difference between the experimental pressure and the equilibrium pressure, free gas and water molecules kept consuming for hydrate growth which was accompanied by a temperature spike and a continuous decrease in pressure. In this work, the hydrate formation was considered to be finished when the pressure decrease was less than 0.1 MPa within 24 hours. Since the pressure after crystallization was still above the equilibrium pressure of CO2 hydrate at 275.2 K, the cease of the hydrate growth was considered to be caused by lack of free water molecules. To better evaluate the averaged kinetic performance, the hydrate formation from the solution of different TBAB concentrations was repeated 5 times. The parameters obtained were presented in averaged values.

2.2 PXRD and Raman measurements. The hydrate samples used for PXRD and Raman analysis were taken from the hydrate formation in kinetic measurements. As hydrate formation finished, the reactor was chilled down to about 253 K and depressurized. Once exposed to atmospheric pressure, the hydrate samples were quickly transferred into liquid nitrogen and finely ground. The structure of the hydrate samples were characterized by an PXRD apparatus (PANalytical V.B., X’pert PRO MPD) equipped with an Anton Paar refrigerating device using Cu Kα radiation (λ=1.5406 Å). The measurements were performed at 120 K in the θ/2θ step scan mode with a step width of 0.017° over a 2 range of 5-70°. The Raman spectra were obtained from a confocal Raman spectrometer (Horiba, LabRAM HR) equipped with a 1800 grooves/mm grating and a multichannel air-cooled charge-coupled device (CCD) detector. The Raman shifts were collected at a spectral resolution of 1.45 6


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cm-1. The Ar+ laser beam (wavelength: 512 nm, power: 50 mW) condensed to about 2 µm spot from the object lens (Olympus 50×) was irradiated to the sample surface. The hydrate sample was first dense packed in a sapphire sample holder in liquid nitrogen and loaded into a stage (Linkam BCS) precooled at 233 K, which was enough to prevent hydrate sample from fast dissociation.37

3. RESULTS AND DISCUSSION 3.1 Microscopic analysis Crystallizations from TBAB solutions usually yielded a variety of crystal structures. At least 4 possible hydrate structures could be found in the formed hydrate samples in this work. Based on literature data, a brief list of the formed crystal properties was given in Table 2.12, 15, 38-40 The cubic structure referred to the sI hydrate which required highest pressure to remain stable among the formed hydrates. However, having the smallest cell volume and 8 empty cages per unit cell as seen in Fig. 3(a), the sI hydrate were still a competitive structure for gas storage. Orthorhombic structure is one of the main structure for gas storage in TBAB hydrates. Each cell contained 2 TBAB ions, leaving 6 512-cages vacant for light gases as seen in Fig. 3(c). The TBA+ cation was located at the center of two 51262 – cages and two 51263-cages.15, 39 TBAB·36H2O and TBAB·38H2O were two orthorhombic hydrates and reported to be isostructural.9, 12, 39 Due to the formation of metastable phases and the sensitivity of ionic clathrate hydrates to small changes of environmental parameters, tetragonal structure had not been

determined so far. According to the refinement from Rodionova et al.12, 6 TBAB ions were included and 6 512-cages were vacant per unit cell. The TBA+ cations were either encaged by two 51262-cages and two 51263-cages or two 512-cages and two 51262-cages. The hydration number of the

tetragonal structure was suggested to vary from 24.5 to 32.4. As seen in Fig. 3(d), empty 512-cages 7


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were found in the tetragonal structure, but not continuous, which would inhibit the gas diffusion in hydrate phase. The TBAB hydrates with small hydration numbers usually came from the crystallization in highly concentrated TBAB solutions.38 Only one type of such TBAB hydrates was solved which was trigonal with a hydration number of 21/3. No hydrate cages was found in the unit cell. By comparing the PXRD patterns obtained in each group of experiments, the PXRD patterns could be generally categorized into 4 groups as seen in Table 1. In dilute solutions where the TBAB concentration was less than 1 mass%, the cubic Pm3n dominated the solid phase which was assigned to be sI hydrate as seen in Fig. 4. Only a small fraction of orthorhombic Pmma which was nCO2·TBAB·38H2O could be noticed. As the TBAB concentration increased from 1 mass% to 35 mass%, the intensities of the diffraction pattern of orthorhombic Pmma increased, which indicated a rise in the volume of nCO2·TBAB·38H2O in the formed sample as seen in Fig. 5.41 At the same time, no other diffraction patterns of TBAB hydrates could be clearly observed which agreed with literature data.15 At atmospheric pressure, the orthorhombic Pmma and the tetragonal P4/m were reported to coexist42, 43, however, the orthorhombic Pmma was suggested to be the preferred structure under pressurized CO2. The diffraction pattern of tetragonal P4/m became obvious when the TBAB concentration was above 40 mass% as seen in Fig. 6. Meanwhile, a small fraction of trigonal R3c could also be observed

from the diffraction pattern. Neglecting the gas molecules in hydrate phase, the mass percent of TBAB should be 32 in orthorhombic Pmma and ranged from 35.9 to 42.2 in tetragonal P4/m. In this case, the concentrated TBAB was suggested to avoid a large crystallization of orthorhombic Pmma, but chose to form tetragonal P4/m or trigonal R3c to stabilize the guest molecules. 8


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The diffraction pattern of trigonal R3c could be clearly seen in Fig. 7. The mass percent of

TBAB was 88 in trigonal R3c so that the combination of trigonal R3c and tetragonal P4/m in the solid phase would be suitable. Results from PXRD measurements showed that orthorhombic Pmma preferred to form in dilute TBAB solutions, while diffraction patterns of tetragonal P4/m and trigonal R3c could only be clearly

observed in the samples formed from highly concentrated solutions. However, the spectra obtained from Raman measurements indicated that tetragonal P4/m could coexist with orthorhombic Pmma in dilute solutions. Since the thermodynamic conditions were set to allow either ionic clathrate hydrate or canonical sI hydrates to form, the coexistence of the different hydrate structures showed their good compatibility in the formed solid phase. In addition, the CO2 distributions and the features of the hydrate structures in Raman spectra had been characterized. Fig. 8 showed the Raman spectrum of simple CO2 hydrate where the structure was cubic Pm3n. The presence of CO2 in the simple CO2 hydrate could recognized by two peaks at 1277.5 and 1382.7 cm-1, which agreed with our previous work.37,

44

The O-H stretching modes of water molecules

covering the spectral range from 2800 to 3600 cm-1 formed two broad peaks at about 3120.8 and 3350.9 cm-1. The simple CO2 hydrate crystals could easily found in the samples formed from TBAB solution in which the concentration was below 15 mass%. As the concentration increased, the amount of the simple CO2 hydrate crystals became less, although the cubic Pm3n could still be found. Fig. 9 and Fig. 10 compared the Raman spectra of two types of ionic clathrate hydrates with or without CO2. The presence of CO2 did not bring a significant change to the spectra, but induced a shift toward lower frequencies for the peaks of both CO2 and water molecules. Meanwhile the intensity of lower frequency peak of water molecules grew relatively, suggesting an increase in the stability of 9


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hydrogen bond. Therefore, CO2 molecules were beneficial to stabilize the crystal structure of ionic clathrate hydrates. In addition, the ionic clathrate hydrates without CO2 were usually observed in the samples formed from concentrated solutions. These hydrates were assumed to exist in some large TBAB hydrate crystals which were formed without participation of CO2 molecules so that the hydrates inside the crystals could not get CO2 molecules before they were recovered due to the slow diffusion rate of CO2 in hydrate phase. Fig. 11 compared the Raman spectra of the 4 main crystal structures obtained in this work. In the spectra range from 700 to 1600 cm-1, the splitting of the peaks at 1326.8 cm-1 and 1448.1 cm-1 was observed in orthorhombic and tetragonal TBAB hydrate relative to trigonal TBAB hydrate as seen in Fig. 11(a) which was accord with previous work.45 The trigonal R3c was the only structure where the

TBA+ cation was not encaged so that the alkyl groups in TBA+ cation closely gathered and formed a broad band of C-H stretching vibration modes from 2850 to 3050 cm-1. Only 4 peaks could be clearly distinguished. In tetragonal TBAB hydrate, the each alkyl group in TBA+ cation was fixed by hydrate cages so that the peaks of at 2872.7 cm-1 and 2927.5 cm-1 were split into 3 and 2 peaks respectively. Meanwhile, the new peak was emerged at 3019.9 cm-1. In orthorhombic TBAB hydrates, more water molecules were added to the hydrate cages and TBA+ cations were fixed in only one form of combined hydrate cages, which simplified the environment of each alkyl group in hydrate cages. Therefore, the peaks at 2960.8 cm-1 was also divided into 3 peaks. In this case, the band of C-H stretching vibration modes showed 4 peaks in trigonal, 8 peaks in tetragonal and 10 peaks in orthorhombic TBAB hydrates, which was suggested to be an efficient way to distinguish the TBAB hydrates. In addition, the Raman peaks of water molecules in TBAB hydrates was found to shift toward 10


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higher frequency relative to simple CO2 hydrate as seen in Table 3 and Fig. 11(b). Chazallon et al.

45

noted that high ratio of hydrogen bond hexagonal rings resulted in the shift of water peaks toward higher frequency while the high ratio of pentagonal rings drove the peaks toward the other side. Based on this theory, tetragonal TBAB hydrate did not have higher ratio of large cages than that of orthorhombic TBAB hydrate. However, some hydrogen bond rings in tetragonal TBAB hydrate were found to be even larger than hexagonal rings. The angle between hydrogen bond was larger. In trigonal TBAB hydrates, no rings was found, but the water peaks continued to shifting toward higher frequency. Therefore, the shift in the peaks of water molecules was assumed to largely depend on the angles between the hydrogen bonds. 3.2 Kinetic measurements Although TBAB was considered as an ideal thermodynamic promoter, the ion-induced changes of water dynamics could not be neglected, especially for the kinetics of hydrate formation.27 Fig. 12 showed the averaged induction time of hydrate crystallizations in different TBAB solutions. As an ionic salts, TBAB inhibited the hydrate nucleation when the concentration was below 0.1 mass%. The induction time rose evidently with the increase of TBAB concentration. In such dilute TBAB solutions, the clusters of TBAB-CO2 hydrates were not able to capture enough amount of ionic guests which was beneficial to the formation of stable hydrate nuclei.9 These free ionic guests then tend to weaken the stability of other hydrate clusters and inhibit the formation of simple CO2 hydrate. As seen in Fig. 12, the inhibition effect reached the highest at 0.1 mass% TBAB. As the TBAB concentration continued to rise from 0.3 to 15 mass%, the induction time decreased dramatically which was shorter than that of simple CO2 hydrate formation. In this range of concentration, the ionic guests started to show their promoting behavior to hydrate formation. 11


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TBAB-CO2 hydrates was assumed to form before or simultaneously with simple CO2 hydrate and function as the seeding of gas hydrate formation.22, 46, 47 Associating with the inhibition effect of TBAB in dilute solutions, it was reasonable to believe that there should be a critical concentration above which the TBAB-CO2 hydrates could form. And this critical TBAB concentration should be around 0.1 mass%. When TBAB concentration was above 20 mass%, the induction time can hardly be noticed. In this concentration range, the driving force for TBAB-CO2 hydrates formation got significantly increased.48 TBAB hydrates, no matter what kind of structure they were, could crystallization without partition of CO2 at 275.2 K, especially when the stirrer was started. Therefore, the efficiency of hydrate formation generally were determined by hydrate growth. The gas consumption rate, a common indicator of hydrate growth rate, were calculated from the pressure and temperature data over time using P-R equation of state.49, 50 As a CO2 consumption profile was obtained, the initial 10 minutes was selected to calculate the gas consumption rate. It is the period where gas consumption rate reaches the highest. The gas consumption rate remain almost constant in this period. Therefore, a linear regression was performed to fit a straight line of the gas consumption profile. Fig. 13 showed the CO2 consumption rates at the initial period of hydrate growth stage. Similarly, the obtained CO2 consumption rates could be divided into 3 groups. In the dilute solutions where TBAB concentration was below 0.1 mass%, the CO2 consumption rates were generally around 0.01 mol/hour. When TBAB concentration increased up to 0.3-7.5 mass%, the rates doubled. Then the rate decreased with the increasing concentration as the concentration was above 10 mass%. When the concentration surpassed 15 mass%, the CO2 consumption rate generally fixed at 0.0018 mol/hour. 12


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Comparing with the CO2 consumption rates obtained in pure water which was 0.006 ±0.0006

mol/hour, the ones obtained in the TBAB solutions below 15 mass% got improved to varying degrees, suggesting that TBAB was beneficial to hydrate growth. Although the ions in TBAB would inhibit the hydrate nucleation in dilute solutions, the water clusters formed around hydrophobic cations of TBAB might be useful to the growth of hydrate nuclei.27, 51 As for the TBAB solution above 15 mass%, the low CO2 consumption rates were probably caused by the reduced amount of simple CO2 hydrate formed at the initial stage and low storage capacity of TBAB hydrates. To comprehensively understand the kinetic performance of hydrate formation in each system, the CO2 consumption rates were correlated with induction times as seen in Fig. 14. The kinetic behaviors generally fell into 3 categories. In dilute solutions (0~0.1 mass% TBAB), the hydrate formation took long induction time due to the inhibition effect of the ionic guests. As the gas dissolution rates in the induction period were not included in the rates of hydrate growth, the consumption rates were found to be low. In the concentrated solution (15~60 mass% TBAB), the induction time got greatly reduced, but the CO2 consumption rates were quite limited by the low storage capacity of TBAB hydrates and the reduced amount of simple CO2 hydrates. When the TBAB concentration was among 0.3 and 10 mass% TBAB, the CO2 consumption rates generally tripled the ones in dilute solutions and the induction times were not significant. The total CO2 consumptions and hydration numbers obtained in each hydrate forming system were shown in Fig. 15. As discussed in microscopic analysis, the hydration numbers of both types of TBAB hydrates were high above the canonical sI hydrates. Therefore, more water would be consumed for TBAB hydrates with the increasing TBAB concentration. At the same time, the mass of the injected solution is fixed in each experiment. The total CO2 consumption would be inevitably reduced. As seen 13


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in Fig. 15, the solutions kept high storage capacities when TBAB concentration was below 5 mass%. Then the storage capacities got a dramatic decrease as the concentration increased above 7.5 mass%. In this case, keeping the TBAB concentration below 5 mass% would be suitable for the applications in gas separation and storage. Based on the above analysis, it could be concluded that TBAB concentration had a great influence on the kinetics of hydrate formation. When TBAB concentration was below 0.1 mass%, a long induction time would prolong the crystallization and the hydrate growth rate did not reach a desired value. While increasing the TBAB concentration above 7.5 mass% could significantly reduce the induction time, but the solution would lose their competitiveness at the performance in gas storage and hydrate growth rates. Therefore, the best TBAB concentration for hydrate formation was found to range from 0.3-7.5 mass% which took a moderate induction time, a fast hydrate growth rate and a high storage capacity into account.

4. CONCLUSIONS Kinetics of hydrate crystallizations from different kinds of TBAB solutions were measured at 275.2 K and 3.0 MPa. The effect of TBAB concentration on hydrate nucleation time, hydrate growth and gas storage capacity was evaluated. The crystallographic properties of the formed hydrate samples were characterized using PXRD and Raman spectroscopy. The conclusions were drawn as follows: Based on microscopic analysis, the simple CO2 hydrate and 3 different kinds of TBAB hydrates were formed by changing the TBAB concentration from 0 to 60 mass%. The simple CO2 hydrate and orthorhombic TBAB hydrate which had relatively large gas storage capacity preferred to form when TBAB concentration was below 35 mass%. Trigonal TBAB hydrate which could not capture the gas 14


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molecules was found to coexist with tetragonal TBAB hydrate in concentrated TBAB solutions. Capturing CO2 was suggested to be beneficial to stabilize the structures of ionic clathrate hydrates. In macroscopic measurements, the TBAB concentration ranging from 0.3 to 7.5 mass% was found to be the ideal for an efficient hydrate formation. The TBAB hydrates were assumed to form first in the liquid bulk which induced the growth of simple CO2 hydrate. However, in a dilute solution where TBAB concentration was below 0.3 mass%, the ionic guests could inhibit hydrate nucleation which prolonged the nucleation stsage, while a concentrated TBAB solution would induce the formation of tetragonal and tetragonal TBAB hydrates which could significantly reduce gas consumption rates and gas storage capacity. AUTHOR INFORMATION Corresponding Author *Tel.: +86 20 8705 7669. Fax: +86 20 8705 7669. E-mail: [email protected]. Funding Sources This work was supported by the National Natural Science Foundation of China (51706230). Notes The authors declare no competing financial interest. REFERENCES (1) Zhang, Z. H.; Huisingh, D. Carbon dioxide storage schemes: Technology, assessment and deployment. J Clean Prod 2017, 142, 1055-1064. (2) Sholl, D. S.; Lively, R. P., Seven chemical separations to change the world. Nature 2016, 532 (7600), 435-437. (3) Yang, J. H.; Okwananke, A.; Tohidi, B.; Chuvilin, E.; Maerle, K.; Istomin, V.; Bukhanov, B.; 15


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Cheremisin, A. Flue gas injection into gas hydrate reservoirs for methane recovery and carbon dioxide sequestration. Energ Convers Manage 2017, 136, 431-438. (4) Li, Q.; Wu, Z. S.; Li, X. C. Prediction of CO2 leakage during sequestration into marine sedimentary strata. Energ Convers Manage 2009, 50 (3), 503-509. (5) Sloan, E. D.; Carolyn, A. K. Clathrate Hydrates of Natural Gases; CRC Press: Boca Raton, FL, 2008. (6) Deschamps, J.; Dalmazzone, D. Dissociation enthalpies and phase equilibrium for TBAB semi-clathrate hydrates of N2, CO2, N2 + CO2 and CH4 + CO2. J Therm Anal Calorim 2009, 98 (1), 113-118. (7) Lee, S.; Lee, Y.; Park, S.; Seo, Y. Structural Transformation of Isopropylamine Semiclathrate Hydrates in the Presence of Methane as a Coguest. J Phys Chem B 2012, 116 (45), 13476-13480. (8) Xu, C.-G.; Zhang, S.-H.; Cai, J.; Chen, Z.-Y.; Li, X.-S. CO2 (carbon dioxide) separation from CO2–H2 (hydrogen) gas mixtures by gas hydrates in TBAB (tetra-n-butyl ammonium bromide) solution and Raman spectroscopic analysis. Energy 2013, 59, 719-725. (9) Hashimoto, H.; Yamaguchi, T.; Ozeki, H.; Muromachi, S., Structure-driven CO2 selectivity and gas capacity of ionic clathrate hydrates. Sci Rep-Uk 2017, 7, 1-10. (10) Shi, X. J.; Zhang, P. Crystallization of tetra-n-butyl ammonium bromide clathrate hydrate slurry and the related heat transfer characteristics. Energy Conversion and Management 2014, 77, 89-97. (11) Takeshi, S.; Hironobu, M. Dissociation and Nucleation of Tetra-n-butyl Ammonium Bromide Semi-clathrate Hydrates at High Pressures. Chemical Engineering Data 2017, 62 (9), 2721-2725. (12) Rodionova, T. V.; Komarov, V. Y.; Villevald, G. V.; Karpova, T. D.; Kuratieva, N. V.; Manakov, A. Y. Calorimetric and Structural Studies of Tetrabutylammonium Bromide Ionic Clathrate Hydrates. J 16


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Phys Chem B 2013, 117 (36), 10677-10685. (13) Rodionova, T.; Komarov, V.; Villevald, G.; Aladko, L.; Karpova, T.; Manakov, A. Calorimetric and Structural Studies of Tetrabutylammonium Chloride Ionic Clathrate Hydrates. J Phys Chem B 2010, 114 (36), 11838-11846. (14) Rodionova, T. V.; Sizikov, A. A.; Komarov, V. Y.; Villevald, G. V.; Karpova, T. D.; Manakov, A. Y. Semiclathrate Hydrates in Tri-n-butylphosphine Oxide (TBPO)-Water and TBPO-Water-Methane Systems. J Phys Chem B 2017, 121 (18), 4900-4908. (15) Muromachi, S.; Udachin, K. A.; Shin, K.; Alavi, S.; Moudrakovski, I. L.; Ohmura, R.; Ripmeester, J. A. Guest-induced symmetry lowering of an ionic clathrate material for carbon capture. Chem Commun 2014, 50 (78), 11476-11479. (16) Muromachi, S.; Udachin, K. A.; Alavi, S.; Ohmura, R.; Ripmeester, J. A. Selective occupancy of methane by cage symmetry in TBAB ionic clathrate hydrate. Chem Commun 2016, 52 (32), 5621-5624. (17) Park, S.; Lee, S.; Lee, Y.; Seo, Y. CO2 capture from simulated fuel gas mixtures using semiclathrate hydrates formed by quaternary ammonium salts. Environ Sci Technol 2013, 47 (13), 7571-7577. (18) Takeya, S.; Muromachi, S.; Maekawa, T.; Yamamoto, Y.; Mimachi, H.; Kinoshita, T.; Murayama, T.; Umeda, H.; Ahn, D. H.; Iwasaki, Y.; Hashimoto, H.; Yamaguchi, T.; Okaya, K.; Matsuo, S. Design of Ecological CO2 Enrichment System for Greenhouse Production using TBAB + CO2 Semi-Clathrate Hydrate. Energies 2017, 10 (927), 1-12. (19) Li, Z. Y.; Xia, Z. M.; Li, X. S.; Chen, Z. Y.; Cai, J.; Li, G.; Lv, T., Hydrate-Based CO2 Capture from Integrated Gasification Combined Cycle Syngas with Tetra-n-butylammonium Bromide and Nano-Al2O3. Energ Fuel 2018, 32 (2), 2064-2072. 17


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(20) Kim, S. M.; Lee, J. D.; Lee, H. J.; Lee, E. K.; Kim, Y., Gas hydrate formation method to capture the carbon dioxide for pre-combustion process in IGCC plant. Int J Hydrogen Energ 2011, 36 (1), 1115-1121. (21) Gholinezhad, J.; Chapoy, A.; Tohidi, B. Separation and capture of carbon dioxide from CO2/H2 syngas mixture using semi-clathrate hydrates. Chem Eng Res Des 2011, 89 (9), 1747-1751. (22) Duc, N. H.; Chauvy, F.; Herri, J. M. CO2 capture by hydrate crystallization - A potential solution for gas emission of steelmaking industry. Energy Conversion and Management 2007, 48 (4), 1313-1322. (23) Belandria, V.; Mohammadi, A. H.; Richon, D. Compositional analysis of the gas phase for the CO2+N2+tetra-n-butylammonium bromide aqueous solution systems under hydrate stability conditions. Chem Eng Sci 2012, 84, 40-47. (24) Linga, P.; Kumar, R. N.; Englezos, P. Gas hydrate formation from hydrogen/carbon dioxide and nitrogen/carbon dioxide gas mixtures. Chem Eng Sci 2007, 62 (16), 4268-4276. (25) Babu, P.; Kumar, R.; Linga, P. Pre-combustion capture of carbon dioxide in a fixed bed reactor using the clathrate hydrate process. Energy 2013, 50, 364-373. (26) Li, Q.; Fan, S. S.; Wang, Y. H.; Lang, X. M.; Chen, J. CO2 Removal from Biogas Based on Hydrate

Formation

with

Tetra-n-Butylammonium

Bromide

Solution

in

the

Presence

of

1-Butyl-3-Methylimidazolium Tetrafluoroborate. Energ Fuel 2015, 29 (5), 3143-3148. (27) Nguyen, N. N.; Nguyen, A. V.; Nguyen, K. T.; Rintoul, L.; Dang, L. X. Unexpected inhibition of CO2 gas hydrate formation in dilute TBAB solutions and the critical role of interfacial water structure. Fuel 2016, 185, 517-523. (28) Babaee, S.; Hashemi, H.; Mohammadi, A. H.; Naidoo, P.; Ramjugernath, D. Experimental 18


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Measurement and Thermodynamic Modeling of Hydrate Dissociation Conditions for the Argon plus TBAB plus Water System. J Chem Eng Data 2014, 59 (11), 3900-3906. (29) Jin, Y.; Nagao, J. Change in the Stable Crystal Phase of Tetra-n-butylammonium Bromide (TBAB) Hydrates Enclosing Xenon. J Phys Chem C 2013, 117, (14), 6924-6928. (30) Jin, Y.; Kida, M.; Nagao, J. Phase Transition of Tetra-n-butylammonium Bromide Hydrates Enclosing Krypton. J Chem Eng Data 2016, 61 (1), 679-685. (31) Oyama, H.; Shimada, W.; Ebinuma, T.; Kamata, Y.; Takeya, S.; Uchida, T.; Nagao, J.; Narita, H. Phase diagram, latent heat, and specific heat of TBAB semiclathrate hydrate crystals. Fluid Phase Equilibr 2005, 234 (1-2), 131-135. (32) Lee, S.; Park, S.; Lee, Y.; Lee, J.; Lee, H.; Seo, Y. Guest Gas Enclathration in Semiclathrates of Tetra-n-butylammonium Bromide: Stability Condition and Spectroscopic Analysis. Langmuir 2011, 27 (17), 10597-10603. (33) Jin, Y.; Kida, M.; Nagao, J. Phase Equilibrium Conditions for Clathrate Hydrates of Tetra-n-butylammonium Bromide (TBAB) and Xenon. J Chem Eng Data 2012, 57 (6), 1829-1833. (34) Ma, Z. W.; Zhang, P.; Wang, R. Z.; Furui, S.; Xi, G. N. Forced flow and convective melting heat transfer of clathrate hydrate slurry in tubes. Int J Heat Mass Tran 2010, 53 (19-20), 3745-3757. (35) Samanta, A.; Tuckerman, M. E.; Yu, T. Q.; E, W. N. Microscopic mechanisms of equilibrium melting of a solid. Science 2014, 346 (6210), 729-732. (36) Walsh, M. R.; Koh, C. A.; Sloan, E. D.; Sum, A. K.; Wu, D. T. Microsecond Simulations of Spontaneous Methane Hydrate Nucleation and Growth. Science 2009, 326 (5956), 1095-1098. (37) Zhou, X. B.; Long, Z.; Liang, S.; He, Y.; Yi, L. Z.; Li, D. L.; Liang, D. Q. In Situ Raman Analysis on the Dissociation Behavior of Mixed CH4-CO2 Hydrates. Energ Fuel 2016, 30 (2), 1279-1286. 19


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(38) Janusz, L.; Yu, K. V.; V., R. T.; A., D. Y.; S., A. L. The Structure of Tetrabutylammonium Bromide Hydrate (C4H9)4NBr21/3H2O. Journal of Supramolecular Chemistry 2002, 2, 435-439. (39) Shimada, W.; Shiro, M.; Kondo, H.; Takeya, S.; Oyama, H.; Ebinuma, T.; Narita, H. Tetra-n-butylammonium bromide-water (1/38). Acta Crystallogr C 2005, 61, O65-O66. (40) Udachin, K. A.; Ratcliffe, C. I.; Ripmeester, J. A. Structure, dynamics and ordering in structure I ether clathrate hydrates from single-crystal X-ray diffraction and H-2 NMR spectroscopy. J Phys Chem B 2007, 111 (39), 11366-11372. (41) Takeya, S.; Fujihisa, H.; Yamawaki, H.; Gotoh, Y.; Ohmura, R.; Alavi, S.; Ripmeester, J. A. Phase Transition of a Structure II Cubic Clathrate Hydrate to a Tetragonal Form. Angew Chem Int Edit 2016, 55, (32) 9287-9291. (42) Oshima,

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Tetra-n-butylammonium Bromide + Tetra-n-butylammonium Chloride Mixed Semiclathrate Hydrates. Journal of Chemical & Engineering Data 2016, 61 (9), 3334-3340. (43) Shi, L. L.; Yi, L. Z.; Shen, X. D.; Wu, W. Z.; Liang, D. Q. Dissociation Temperatures of Mixed Semiclathrate Hydrates Formed with Tetrabutylammonium Bromide Plus Tetrabutylammonium Chloride. J Chem Eng Data 2016, 61 (6), 2155-2159. (44) Zhou, X. B.; Lin, F. H.; Liang, D. Q. Multiscale Analysis on CH4-CO2 Swapping Phenomenon Occurred in Hydrates. J Phys Chem C 2016, 120 (45), 25668-25677. (45) Chazallon, B.; Ziskind, M.; Carpentier, Y.; Focsa, C. CO2 Capture Using Semi-Clathrates of Quaternary Ammonium Salt: Structure Change Induced by CO2 and N2 Enclathration. J Phys Chem B 2014, 118 (47), 13440-13452. (46) Jacobson, L. C.; Hujo, W.; Molinero, V. Amorphous Precursors in the Nucleation of Clathrate 20


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Hydrates. J Am Chem Soc 2010, 132 (33), 11806-11811. (47) Babu, P.; Chin, W. I.; Kumar, R.; Linga, P. Systematic Evaluation of Tetra-n-butyl Ammonium Bromide (TBAB) for Carbon Dioxide Capture Employing the Clathrate Process. Ind Eng Chem Res 2014, 53 (12), 4878-4887. (48) Ye, N.; Zhang, P. Equilibrium Data and Morphology of Tetra-n-butyl Ammonium Bromide Semiclathrate Hydrate with Carbon Dioxide. J Chem Eng Data 2012, 57, (5), 1557-1562. (49) ZareNezhad, B.; Varaminian, F. A unified approach for description of gas hydrate formation kinetics in the presence of kinetic promoters in gas hydrate converters. Energ Convers Manage 2013, 73, 144-149. (50) ZareNezhad, B.; Varaminian, F. A generalized macroscopic kinetic model for description of gas hydrate formation processes in isothermal-isochoric systems. Energ Convers Manage 2012, 57, 125-130. (51) Guo, G. J.; Zhang, Y. G.; Liu, C. J.; Li, K. H. Using the face-saturated incomplete cage analysis to quantify the cage compositions and cage linking structures of amorphous phase hydrates. Phys Chem Chem Phys 2011, 13 (25), 12048-12057.

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Table 1 Initial conditions for crystallization and main crystal structures found in hydrate sample from PXRD TBAB concentration

Initial pressure

Temperature

(mass%)

(MPa)

(K)

0

3.00

275.2

Cubic

0.01

3.00

275.2

Cubic

0.05

3.00

275.2

Cubic

0.1

3.00

275.2

Cubic

0.3

3.00

275.2

Cubic

0.5

3.00

275.2

Cubic

1

3.00

275.2

Cubic, Orthorhombic

2.5

3.00

275.2

Cubic, Orthorhombic

5

3.00

275.2

Cubic, Orthorhombic

7.5

3.00

275.2

Cubic, Orthorhombic

10

3.00

275.2

Cubic, Orthorhombic

15

3.00

275.2

Cubic, Orthorhombic

20

3.00

275.2

Cubic, Orthorhombic

25

3.00

275.2

Cubic, Orthorhombic

30

3.00

275.2

Cubic, Orthorhombic

35

3.00

275.2

Cubic, Orthorhombic,

40

3.00

275.2

Tetragonal, Orthorhombic

60

3.00

275.2

Tetragonal, Trigonal

Crystal systems

22


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Energy & Fuels

Table 2 The possible crystal structure in the hydrate sample Lattice

Ideal unit cell formula

Space Crystal system

parameters group

512-cage

51262-cage

51263-cage

H2O

2

6

0

46

6

4

4

76/72

10

16

4

147-194.4

0

0

0

7

(Å) Cubic

Pm3n

a≈ 12 a≈ 21

Orthorhombic

Pmma

b≈ 13 c≈ 12 a≈ 23

Tetragonal*

P4/m c≈ 38 b≈ 13

Trigonal

R3c c≈ 13

*The crystal structure was not solved and the data were summarized from Rodionova et al.12.

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Table 3 Wavenumbers and assignments of the Raman peaks of CO2 and O-H stretching modes of water molecules in different types of hydrates. CO2

H2O

ν (cm-1)

ν (cm-1)

Crystal system

Cubic

1277.5

1382.7

3120.8

3350.9

Orthorhombic

1274.6

1379.8

3142.5

3354.3

Tetragonal

1274.9

1378.8

3162.8

3373.6

Trigonal

-

-

3183.0

3395.2

24


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Energy & Fuels

Fig. 1 Schematic diagram of the hydrate forming system: (1) reactor, (2) gas reservoir, (3)

thermostatic air bath, (4, 6) thermometer, (5, 7) pressure transducer, (8) magnetic stirrer, (9) data acquisition, (10) vacuum pump, (11) gas cylinder, (12) pressure relief valve, (13) safety valve, and (14-17) needle valve.

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Fig. 2 A typical Hydrate formation from TBAB solution (0.5 mass% TBAB).

26


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Fig. 3 Cation of TBA+ and hydrate structures with gas capacities. (a) cubic Pm3n; (b) cation of TBA+; (c) orthorhombic Pmma; (d) tetragonal P4/m. Cages with gas capacity are painted with color.

27


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Fig. 4 PXRD patterns of the sample formed from 1 mass% TBAB solution.

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30


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Fig. 8 The Raman spectrum of simple CO2 hydrate.

32


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Fig. 9 The Raman spectra of orthorhombic structure with and without CO2.

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Fig. 10 The Raman spectra of tetragonal structure with and without CO2.

34


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Fig. 11 Comparison of the Raman spectra of different hydrate structures. (a) spectra from 700 to 1600 cm-1; (b) spectra from 2600 to 3800 cm-1.

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Fig. 12 The averaged induction time of hydrate crystallization formed from different TBAB solutions.

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Fig. 13 CO2 consumption rates versus TBAB concentration.

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Fig. 14 Correlation between the CO2 consumption rate and induction time.

38


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Fig. 15 Total CO2 consumptions and hydration numbers obtained from different TBAB solutions.

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TOC

40


Kinetic Measurements on CO2 Hydrate Formation in the Presence of

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