Experimental Investigation To Elucidate Why Tetrahydrofuran Rapidly

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Experimental Investigation to Elucidate Why Tetrahydrofuran Rapidly Promotes Methane Hydrate Formation Kinetics: Applicable to Energy Storage Asheesh Kumar, Nagu Daraboina, Rajnish Kumar, and Praveen Linga J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11995 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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The Journal of Physical Chemistry

Experimental Investigation to Elucidate Why Tetrahydrofuran Rapidly Promotes Methane Hydrate Formation Kinetics: Applicable to Energy Storage Asheesh Kumara, Nagu Daraboinab, Rajnish Kumarc and Praveen Lingaa* a

Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117 585, Singapore

b

McDougall School of Petroleum Engineering, The University of Tulsa, Tulsa, OK, U.S.A. c

Chemical Engineering and Process Development Division, CSIR – National Chemical Laboratory, Pune, India

*Corresponding author: email: [email protected] (P. Linga) ABSTRACT Methane storage as SNG (solidified natural gas) in the form of clathrate hydrates is an emerging, economically feasible and environmentally benign technology. Mixed tetrahydrofuran (THF)methane (CH4) hydrates offer a paradigm shift to milder storage conditions and faster hydrate formation kinetics, providing a promising scenario to scale up the SNG technology. In this work, we synthesize mixed THF-CH4 hydrates in a high pressure micro-differential scanning calorimeter (HP µ-DSC) to validate the two-step mechanism for production of mixed THF-CH4 hydrate identifying the synergistic effect of THF and CH4. Heat flow change during hydrate formation and dissociation of mixed THF-CH4 hydrates formed in presence of 5.56 mol% THF (stoichiometric composition) was monitored. The two step-mechanism of mixed THF-CH4 hydrate formation was further confirmed by pressure-temperature profile and visual observations with a sample volume scale up of about 350 times that of µ-DSC experiments.

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1. INTRODUCTION Clathrate hydrates are crystalline non-stoichiometric inclusion compounds formed by guest molecules (like CH4, THF etc.) and hydrogen bonded water molecules (host). Depending on the size and composition of guest molecules, clathrates may form cubic structures (sI or sII), or hexagonal structure (sH)1-3. Solidified natural gas (SNG) via clathrate hydrates offers an excellent opportunity to store NG/methane on a large scale4-5. Methane gas hydrates have a volumetric capacity of approximately 173 v/v at STP in the structure I (sI) hydrates 6. Stability of the synthesized hydrates at temperatures close to ambient conditions plays an important role as far as economics are concerned. In this direction, Stern et al. reported the anomalous selfpreservation of methane hydrates which greatly suppress the hydrate dissociation at sub-zero temperature (242-271 K at 1 atm)7. Based on this self-preservation phenomenon, the production of natural gas hydrate for energy storage has also been demonstrated by various research groups 5, 8-10

. Recently Mimachi et al. reported natural gas hydrate pellets storage for 3 months at 85-253

K under atmospheric pressure11. However, storage of natural gas hydrates at milder temperatures is an ongoing effort. A paradigm shift to develop a large scale stationary energy storage system via SNG technology will be to move away from the sI structure domain in search of milder formation and storage conditions. However, it is noted that this will result in a reduction in storage capacity as the structural change requires an additional guest that will occupy some fraction of the hydrate cages. One such promoter is tetrahydrofuran (THF)12. THF is one of the most commonly researched promoter in gas hydrate formation systems12-14. THF readily forms sII hydrates by occupying the large cage of sII hydrates. There are a number of literature works that present the thermodynamic data for mixed THF- CH4 hydrates.15-17 Susilo et al.18 reported based on


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thermodynamic modeling that it is possible to tune methane content in mixed THF- CH4 hydrates along with the ability to shift to higher temperature and lower pressure conditions. Recently, we have reported methane storage in structure II hydrates demonstrating their formation at near ambient temperatures mainly attributed to the dual role of THF that provides both thermodynamic stability and kinetic promotion19-20. The superior stability of mixed THF-CH4 hydrates at temperatures much higher than the anomalous self-preservation temperatures of pure methane hydrates was demonstrated in a recent study21. For pure THF hydrates (THF.17 H2O) the decomposition temperature is 277.5 K at 1.0 bar where THF molecules occupy the large cages and all the small cages are presumably empty. In the mixed hydrate of THF-CH4 the small cages would be occupied by methane molecules, thus imparting better stability to the resulting hydrate. The maximum decomposition temperature observed by Larionov et al. was 377.2K at 15.0 kbar for mixed THF- CH4 hydrates22. These mixed hydrates predominantly form structure-II (sII) hydrate. Recently, Moryama et al. reported the formation of structure-II hydrates for CH4 + deuterated tetrahydrofuran mixed hydrate with the small-cage occupancy of CH4 in the structureII mixed hydrate reported to increase with increasing pressure

23

. However, the coexistence of

pure and mixed hydrates (sI and sII) have also been reported in the literature which depends on the concentration of THF and phase boundary conditions 24-26. In the present work, high pressure differential scanning calorimetry (HP-DSC) was employed to characterize the mixed THF- CH4 hydrates to elucidate the kinetic promotion of THF/methane system. HP-DSC has been considered an innovative rapid technique to measure the thermodynamic stability of gas hydrates, moreover the endothermic dissociation can also predict the presence of mixed hydrate structure in such systems

27-31

. Recently, CH4 − CO2 swapping

mechanism has also been explored by measuring the heat flow change during the CH4 −CO2



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replacement process using a high pressure micro-differential scanning calorimeter (HP µ-DSC) 32

.

In this study, heat flow change during hydrate formation and dissociation process of mixed THFCH4 hydrates formed in presence 5.56 mol% THF (stoichiometric composition) was monitored employing a HP µ-DSC. To further examine the mechanism of mixed THF-CH4 hydrates formation, we performed the experiments in a large scale unstirred reactor (UTR) configuration.

2. EXPERIMENTAL SECTION 2.1.Materials Methane gas with the purity of 99.9% was procured from Singapore Oxygen Air Liquide Private Ltd (SOXAL). Tetrahydrofuran (THF) of 99.99% purity was purchased from Fisher Chemicals. De-ionized water was used in all the experiments. 2.2. Apparatus:

Figure 1 presents the schematic diagram of the experimental setup. A high-pressure microdifferential scanning calorimeter (HP µ-DSC7 Evo, Setaram Inc.) was used in this study. The details of the calorimetry is described elsewhere in the literature33. Briefly, the highly sensitive HP µ-DSC provides a temperature range from -45 up to 120°C. The high-pressure cell is designed for a pressure of up to 400 bars and is made of Hastelloy C276. A specific high pressure gas panel is used to pressurize the cell. The HP µ-DSC has a resolution of 0.02 µW and a temperature deviation of ±0.2 K. Further, the hydrate formation-dissociation experiments were also performed in an unstirred reactor (UTR) configuration. The details of the UTR experimental setup is available in our previous paper19, brief description is presented in the supporting information Section A1 (schematic given in figure S1).


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Figure 1. Schematic diagram of experimental setup 2.3.Procedure: 150 µl of water/solution was charged to the sample cell. Then, the DSC sample cell was purged with the methane gas at least three times to remove any residual air. The sample cell was then pressurized with methane to the desired pressure. Isothermal temperature program was used in all the experiments as established in the literature

27, 34-35

. In this method the temperature was

dropped from 303.2K to 263.2 K at the rate of 1 K/min and kept constant at 263.2 K for 4 h. The hydrate nucleation events were observed as onset of exothermic peaks. After completion of



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hydrate formation (~4 h), the temperature was gradually raised to 308.2 K at the rate of 0.10 K/min to observe hydrate decomposition as onset of endothermic peaks. Similarly some experiments were performed by dropping temperature from 308.2K to 258.2 K at the rate of 1 K/min for hydrate formation and decomposition by rising temperature from 258.2 K to 308.2 K at 0.10 K/min. All the experiments were carried out at three different pressures (6.8, 5.0 and 3.0 MPa). 3. RESULTS AND DISCUSSION Using HP µ-DSC, we obtained typical DSC thermograms along with heat flow and temperature profile, presenting hydrate formation and dissociation phase for 5.56 mol% THF system at a constant pressure of 6.8 MPa (Figure 2a). 5.56 mol% THF is the stoichiometric amount of THF (THF.17H2O) in which large cages are occupied by THF molecules and small cages of sII hydrates are vacant (Pure THF hydrates). However in presence of methane, the small cages are occupied by CH4 molecules and can significantly improve the thermodynamic stability of sII hydrates (Mixed THF-CH4 hydrates) compared to pure sI methane hydrates.


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Figure 2. (a) Typical DSC thermograms of Water+5.56 mol% THF system along with heat flow and temperature profile for hydrate formation and decomposition using a temperature ramping program from 303.2 K to 263.2 K at 1.0 K/min (b) & 263.2 K to 308.2 K at 0.10 K/min (c) and 6.8 MPa pressure of methane. As can be seen in the Figure 2b, while reducing the temperature from 303.2K to 263.2 K, a sharp rise in the heat flow (an exothermic peak) represents the nucleation event and when temperature was increased from 263.2 K to 308.2 K (Figure 2c), we observe two endothermic peaks (along with unexpected exothermic peak) for hydrate dissociation. First endothermic peak at ~ 278K corresponds to pure THF hydrate with empty small cages (sII). To further confirm the pure THF hydrate dissociation peak, we performed the experiments with 5.56 mol% THF solution at atmospheric pressure (presented as Figure S2) and at 6.8 MPa pressure of helium (presented as



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Figure S3). On the other hand, we attribute the second endothermic peak at ~ 299 K (in Figure 2c) to mixed THF-methane hydrates in which methane molecules occupy the small cages of sII hydrates. Further to distinguish the mixed THF-methane hydrate (sII) endothermic peak from pure methane hydrate peak (sI), we performed the experiments with pure water and methane; the dissociation thermograms of pure methane hydrates has been presented in the supplementary information Figure S4). As seen in Figure S4, we observed a peak at ~282.6 K which corresponds to the dissociation temperature of pure methane hydrates at the experimental pressure of 6.8 MPa. The appearance of pure THF hydrtae in the DSC experiment can be attributed to considerably less mass transfer limitation for pure THF hydrate formation compared to THF-methane mixed hydrate formation and the high driving force in terms of sub-cooling allowed rapid formation of THF hydrate . It is noted that we did not observe any other change in the heat flow further in the formation experiment (Figure 2b). During the decomposition, we observe the melting of pure THF hydrates (at ~278 K and 6.6 h). An interesting observation is that right after the melting of pure THF hydrates, we observe hydrate formation (or re-crystallization) as illustrated by an exothermic peak around 7.5 h. This exothermic peak can only be attributed to the formation of mixed THF-CH4 hydrates (Figure 2c). This is an interesting observation and to the best of our knowledge has never been reported before. In a recent simulation study done by Wu et al.36, it has been shown that due to high concentration of THF and hydrogen bonding interactions between THF and water, pure THF hydrates are not very stable initially. Our current work provides an experimental proof that a hydrate forming gas (Methane) can stabilize THF hydrate (mixed hydrate) by occupying the small (512) cages while the large (512 64) cages are stabilized by THF 36. However, when a nonhydrate former is used as a help gas (Helium in place of methane, refer to Figure S3) THF


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hydrate stability could not be improved. This is a classic case of synergetic effect of methane and THF, which not only improves thermodynamic stability; it also improves hydrate growth kinetics significantly. Recently, Veluswamy et al. reported a rapid formation of mixed THF-CH4 hydrate formation while performing the kinetic experiments coupled with morphological observations when experiments were performed at 283.2 K and 7.3 MPa20. The experimental condition employed by Veluswamy et al. eliminated the possibility of formation of pure sI methane hydrates20. This work, thus sheds new insights into why rapid formaiton of mixed methane/THF hydrates were observed at experimental conditions above the formation of pure THF hydrate formation conditions with aqueous solution and provides evidence of the two-step hydrate growth mechanism observed and postulated in literature20. The absolute cage occupancies for mixed THF-CH4 (sII hydrates) confirmed by Seo et al.26, was found to be ᶿs,ch4 =0.3684, ᶿ =0 and ᶿ

L,THF

=0.9948 (no methane in large cage). Prasad et al.

24

L,ch4

have also confirmed that

methane molecules did not occupy the large cage for 5.88 mol% THF in Raman spectrum and that there was no co-existance of sI hydrates (pure methane hydrates) and sII hydrates at stoichiometric amount of THF experimented at 8.0 MPa and 260 K.



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Figure 3. DSC thermograms for Water+5.56 mol% THF system along with heat flow and temperature profile for hydrate formation and decomposition obtained using a temperature ramping program from 303.2K to 263.2 K at 1.0 K/min (section i of figure 3a) & 263.2 K to 308.2 K at 0.10 K/min (section ii of figure a) and 308.2K to 258.2 K (section iii of figure 3a) then heating to 308.2 K at 0.10 K/min (section iv of figure a). Figure 3b presents the pressure profile. Figure 3c and 3d shows the comparison of hydrate formation (section i and iii) and dissociation (section ii and iv) thermograms respectively. We have also performed the experiments at 5.0 and 3.0 MPa pressure. Figure 3 presents the DSC thermograms for Water+5.56 mol% THF system at 3.0 MPa pressure of methane along with heat flow and temperature profile for hydrate formation and decomposition obtained using a temperature ramping program from 303.2K to 263.2 K at 1.0 K/min (section i of Figure 3a) & 263.2 K to 308.2 K at 0.10 K/min (section ii of Figure 3a) and 308.2K to 258.2 K (section iii of Figure 3a) then heating to 308.2 K at 0.10 K/min (section iv of Figure 3a). Figure 3b presents the 10 ACS Paragon Plus Environment

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pressure profile which is almost constant throughout the experiment. Figure 3c and 3d shows the comparison of hydrate formation (section i and iii) and dissociation (section ii and iv) thermograms respectively, which clearly differentiate the ice, pure THF hydrates and mixed THF-CH4 hydrates. Further, Figure 4 presents the comparison of hydrate dissociation thermograms of 5.56 mol% THF+ water system at various pressures (6.8, 5.0 and 3.0 MPa). As can be seen in the figure, the amount of hydrate formed was different and recrystallization (peak area) during melting of pure THF hydrates reduces because of low driving force.

Figure 4. Comparison of hydrate dissociation thermograms of 5.56 mol% THF+ water system at various pressures (6.8, 5.0 and 3.0 MPa) To further examine the mechanism of mixed THF-CH4 hydrates at a large scale, we performed experiments in a lab scale unstirred reactor (UTR) configuration with a four step experimental procedure. Experiment was designed in four steps, first step is formation of pure THF hydrates where 5.56 mol% THF-water solution was cooled at 275.2K temperature and atmospheric pressure and stirred vigorously at 600 rpm for few minutes then stirring was stopped (Figure 11 ACS Paragon Plus Environment


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5a). Step two, reactor was pressurized with methane at 3.2 MPa and 275.2 K (at this condition, pure methane hydrate is not thermodynamically stable) as shown in Figure 5b. We did not observe any pressure drop up to ~5 hour of experimental run which reveals that pure THF hydrates are stable at the increased pressure of 3.2 MPa and it was not possible for methane molecules to occupy the small cages of pure THF hydrates (sII) (mass transfer limitation). This corroborates to what we observed in the HP µ-DSC measurements. Now in the third step, temperature was increased to 283.2K to dissociate the pure THF hydrates (Figure 5c). It can be seen in Figure 5c, as the temperature crosses the boundary conditions of pure THF hydrates (~278K), growth of mixed THF-CH4 hydrates takes places which results in rapid pressure drop due to enclathration of methane molecule in the small cages and subsequent formation of mixed THF-CH4 hydrates (sII). These results are in excellent agreement with HP µ-DSC observations presented earlier. Further, hydrates were dissociated after completion of step III, pressure of the reactor was reduced and simultaneously temperature was increased up to 298.2K (Figure 5d).


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Figure 5. (a) Pressure-Temperature profile of an experiment conducted in a UTR configuration using 5.56 mol% THF solution in which section (I) presents formation of pure THF hydrates while cooling the solution at atmospheric pressure. Section (II) shows insertion of methane at 3.2 MPa and 275.2K temperature (Figure 5b). Section (III) presents the melting of pure THF hydrates by rising the temperature up to 283.2K and simultaneously formation of mixed THFCH4 hydrates (Rapid pressure drop) (Figure 5c). Section (IV) shows the hydrate dissociation profile (Figure 5d). In order to critically examine the visual changes, we captured the images of reactor contents during these four steps, namely, THF hydrate formation, methane insertion, THF hydrate melting and hydrate dissociation. Figure 6 presents the recorded observations of these four steps at selected time intervals. In the step I of Figure 6, we can observe the formation of pure THF hydrates as transparent crystals (figure 6c-f). (Refer supplementary figure S5 for step II)Step III presents the visual observations of pure THF hydrate dissociation and the formation of mixed 13 ACS Paragon Plus Environment


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THF-CH4 hydrates taking place simultaneously; during this process, the translucent pure THF hydrate transforms into whitish mixed THF-CH4 hydrates (Figure 6n-r) and results in a significant drop in pressure (refer to Figure 5c for the pressure drop). For detailed visual observations of step III refer to supplementary information (Figure S6 and four Video files: Step III_V1-V4). The final fourth step presents the morphological observations of melting of mixed THF-CH4 hydrates. We have presented six videos in supporting information for visualization of the changes during the dissociation (Video files: Step IV_V1-V4).

Figure 6. Visual observations of the rector contents for experiment conducted in a UTR configuration using 5.56 mol% THF solution in which step I shows the images while the solution was cooled at atmospheric pressure to ~275.2K. Step II shows the gas insertion step (pressurization with methane to 3.2 MPa at 275.2K, Figure S5) and visual images of preserved THF hydrates. Step III presents the melting of pure THF hydrates when the temperature was increased to 283.2K and simultaneously formation of mixed THF-CH4 hydrates. Step IV shows the visual presentation of hydrate dissociation. 14 ACS Paragon Plus Environment

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4. CONCLUSION In summary, we employed a high-pressure micro-differential scanning calorimetry (HP µ-DSC) to investigate the formation and dissociation behavior of mixed THF-CH4 hydrates formed by using the stoichiometric concentration of THF. For stoichiometric amount of THF (5.56 mol%) in water, the formation of two type of structure II were observed (pure THF and mixed THF-CH4 hydrates) which follow the two-step mechanism of mixed THF-CH4 hydrates formation through synergistic effect of these two guest molecules. Further, this mechanism of mixed THF-CH4 hydrate formation was also confirmed by employing an UTR configuration along with visual observations with a volumetric sample scale-up of about 350 times. The current study can be considered to be the first attempt to utilize HP µ-DSC as an indirect method for mechanism elucidation of mixed hydrates. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Supplementary information includes the following –Sections A1 detail the experimental setup and methods (Unstirred reactor configuration). Figure S1 present the schematic diagram of unstirred reactor configuration. Figure S2 presents the Pure THF hydrate formation for 5.56 mol% THF system through multiple runs and enlarged figure presents the ice melting temperature and THF hydrate dissociation temperature for this system. Figure S3 shows the comparison of pure THF hydrate dissociation peak for 5.56 mol% THF at atmospheric pressure and 6.8 MPa pressure of helium and methane. Figure S4 presents the pure methane hydrate dissociation thermograms at 6.8 MPa pressure. Figure S5 and S6 shows the details of step II and



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III of Figure 5. Videos presents the step III (real time videos V1-V4) and step IV (real time videos V1-V4) of Figure 5. ACKNOWLEDGMENTS The work was funded in part under the Energy Innovation Research Programme (EIRP, Award No. NRF2015EWTEIRP002-002), administrated by the Energy Market Authority (EMA) and funded by the National Research Foundation (NRF), Singapore. The authors also acknowledge the support from the National University of Singapore (R-261-508-001-646/733) for the DSC equipment. REFERENCES 1. Sloan, E. D.; Koh, C. A., Clathrate Hydrates of Natural Gases. CRC Press: 2007. 2. Englezos, P., Clathrate Hydrates. Ind. Eng. Chem. Res. 1993, 32, 1251-1274. 3. Ripmeester, J. A.; Tse, J. S.; Ratcliffe, C. I.; Powell, B. M., A New Clathrate Hydrate Structure. Nature 1987, 325, 135-136. 4. Wang, W.; Bray, C. L.; Adams, D. J.; Cooper, A. I., Methane Storage in Dry Water Gas Hydrates. J. Am. Chem. Soc. 2008, 130, 11608-11609. 5. Gudmundsson, J. S., Method for Production of Gas Hydrates for Transportation and Storage. Google Patents: 1996. 6. Susilo, R.; Ripmeester, J. A.; Englezos, P., Methane Conversion Rate into Structure H Hydrate Crystals from Ice. AIChE Journal 2007, 53, 2451-2460. 7. Stern, L. A.; Circone, S.; Kirby, S. H.; Durham, W. B., Anomalous Preservation of Pure Methane Hydrate at 1 Atm. The Journal of Physical Chemistry B 2001, 105, 1756-1762. 8. Kanda, H. Economic Study on Natural Gas Transportation with Natural Gas Hydrate (Ngh) Pellets; 2006. 9. Katoh, Y.; Horiguchi, K.; Iwasaki, T.; Nagamori, S., Process for Producing Gas Hydrate Pellet. Google Patents: 2011. 10. Lee, J. D.; Kim, H. J.; Kim, S. R.; Hong, S. Y.; Park, H. O.; Ha, M. K.; Jeon, S. K.; Ahn, H.; Woo, T. K., Apparatus and Method for Continuously Producing and Pelletizing Gas Hydrates Using Dual Cylinder. Google Patents: 2013. 11. Mimachi, H.; Takahashi, M.; Takeya, S.; Gotoh, Y.; Yoneyama, A.; Hyodo, K.; Takeda, T.; Murayama, T., Effect of Long-Term Storage and Thermal History on the Gas Content of Natural Gas Hydrate Pellets under Ambient Pressure. Energy Fuels 2015, 29, 4827-4834. 12. Gough, S. R.; Davidson, D. W., Composition of Tetrahydrofuran Hydrate and the Effect of Pressure on the Decomposition. Can. J. Chem. 1971, 49, 2691-2699. 13. Lee, H.; Lee, J.-w.; Kim, D. Y.; Park, J.; Seo, Y.-T.; Zeng, H.; Moudrakovski, I. L.; Ratcliffe, C. I.; Ripmeester, J. A., Tuning Clathrate Hydrates for Hydrogen Storage. Nature 2005, 434, 743-746. 16 ACS Paragon Plus Environment

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Experimental Investigation To Elucidate Why Tetrahydrofuran Rapidly

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