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Solidified natural gas (SNG) via clathrate hydrates has been proposed as an alternative approach for natural gas storage and transportation. In this work, the roles of 5.56 mol% pyrrolidine were investigated for the methane hydrate formation in terms of thermodynamics and kinetics along with morphology. The results showed that pyrrolidine generally improved the thermodynamic stability of mixed methane hydrates, enhancing the formation at milder conditions than those of pure methane hydrates. To demonstrate the kinetic performance of pyrrolidine, the experiments were performed at 8 MPa and 285.2 K in a quiescent configuration. The results showed that a very short induction time and a rapid rate of hydrate formation with desirable methane uptake were achieved. In addition, a distinct methane bubble with breathing effect, assisting methane gas to interact with the bulk solution, was observed during the hydrate formation morphology. The comparative kinetic study with 5.56 mol% tetrahydrofuran (THF) was also carried out under the same driving force and the same experimental condition. Surprisingly, although THF gave a high methane uptake, pyrrolidine could give more than 10 times higher rate of hydrate formation at the same driving force; moreover, it provided a competitive rate with THF at the similar formation condition. Figure G1 Thermodynamics, kinetics, and morphology of the mixed methane hydrate formation in the presence of 5.56 mol% pyrrolidine. [Display omitted] • Up to ten-time increase in the rate of methane hydrate formation with pyrrolidine compared to tetrahydrofuran (THF). • A new insight on the roles of pyrrolidine in methane hydrate formation through morphology investigation. • A shift in the methane hydrate phase equilibrium in the presence of pyrrolidine. • A clear comparison on the roles of pyrrolidine and THF in the methane hydrate formation. [ABSTRACT FROM AUTHOR]

Foam generation from the presence of sodium dodecyl sulfate (SDS) as a kinetic promoter in methane hydrate formation can be potentially reduced by using mixed surfactants. Mixtures of SDS with nonionic surfactants were proposed to solve this problem. Polyoxyethylene (n) lauryl ether (EO 3 and EO 5) and alkyl poly glycol ether (APG) were chosen to mix with SDS in different concentrations at 8 MPa and 4 °C in the quiescent system for both kinetics and morphology studies. The result showed that the addition of EO 3 resulted in the slight increase for the induction time but no change in the formation rate. Adding EO 5 showed the stochastic phenomenon on hydrate formation kinetics. In the presence of APG, the induction time increased up to ten times and lowered the hydrate formation rate compared to only 0.25 wt% SDS. The methane uptakes were about the same with all conditions. The morphology showed similar hydrate formation and dissociation patterns with all investigated solutions. Interestingly, the mixture of nonionic surfactants with SDS decreased the foam formation compared with the system with only SDS. The addition EO 3 showed the gradual decrease in the foam height with the higher concentration of EO 3. Adding the highest EO 5 concentration showed the optimum foam reduction compared with all conditions. Moreover, the presence of APG showed the similar effects of foam reduction with EO 5. • Kinetics and morphology during the methane hydrate formation and dissociation in the presence of mixed surfactants. • A large amount of foam generated during gas recovery from the presence of sodium dodecyl sulfate. • The foam formation decreased with the addition of nonionic surfactant. • The formation kinetics hardly affected in the presence of mixed surfactants. [ABSTRACT FROM AUTHOR]

Two widely investigated methane hydrate promoters, tetra-n-butyl ammonium bromide (TBAB) and cyclopentane (CP), for methane hydrate formation and dissociation were comparatively investigated in the quiescent reactor at 2.5 °C and 8 MPa. The results indicated that the increase in the mass fraction TBAB decreased the induction time. However, it did not significantly affect the methane uptake. In the presence of CP, the increase in the CP concentration resulted in an increase in the induction time due to the increasing thicknesses of the CP layer in the unstirred reactor. Moreover, the methane uptake was varied proportionally with the CP concentration. The addition of TBAB resulted in a higher methane uptake than that of CP, since the presence of TBAB provided the cavities in the hydrate structure to accommodate the methane gas during the hydrate formation better than that of CP. On the contrary, the presence of CP significantly increased the induction time. Although the methane recovery remained relatively the same regardless of TBAB and CP concentrations, the recovery was higher in the presence of TBAB. [ABSTRACT FROM AUTHOR]

The kinetics of methane hydrate formation in the presence of methyl ester sulfonate (MES), a bio-based anionic surfactant, was evaluated in an unstirred reactor. MES concentration was varied from 1 mM to 8 mM (0.029 wt% to 0.232 wt%) and its effect on the kinetics of the methane hydrate formation was observed at 277.2 K and 8 MPa. MES drastically improved the kinetics of methane hydrate formation and methane consumption. The increase in the MES concentration dramatically decreased the induction time as well. In addition, the morphology of methane hydrate formation observed with the different MES concentrations was presented. Hydrate nucleation was observed at the gas/liquid interface followed by the hydrate growth in the upward direction into the gas phase and then downward direction into the liquid phase, respectively. Dissociation characteristics and methane recovery were not significantly different for different MES concentrations. Image 1 • Biodegradable MES surfactant significantly enhanced the kinetics of methane hydrate formation in an unstirred tank reactor • Increase in MES concentration reduced the induction time and increased the gas uptake. • Morphology observations during the methane hydrate formation and dissociation in presence of MES are presented [ABSTRACT FROM AUTHOR]

[Display omitted] • Two-stage hydrate formation in the presence of salt. • The slow formation evidenced by the length of the first-stage growth. • The length of the first-stage growth decreased by incorporating an amino acid or SDS. • Synergism of an amino acid and THF in the presence of salt. Effects of co-promoters on the mixed methane-THF hydrate formation in the presence of 3.5 wt% NaCl and 5.56 mol% THF solution were investigated at 8 MPa and 288.2 K in an unstirred reactor. The addition of NaCl resulted in the two-step hydrate formation, and it is the first stage that adversely affected the hydrate formation rate. However, the final gas uptake in this system was similar to that without NaCl. The synergistic effects on the kinetic promotion by adding a co-promoter including sodium dodecyl sulfate (SDS) and amino acids (valine, leucine, and methionine) were then studied. The results showed that the addition of the co-promoters reduced the induction time, minimized the length of the first stage formation, and increased the hydrate formation rate. Moreover, the induction time and the length of the first stage formation decreased along with the increase in the co-promoter concentration. However, the addition of SDS had the adverse effects by decreasing about half of the final gas uptake and the water to hydrate conversion for the system without SDS. Moreover, the addition of methionine resulted in the best performance as a kinetic promoter on the hydrate formation in the system with THF and NaCl. Up to 95 % of methane gas can be recovered from the hydrates formed with and without the co-promoters. The hydrate formation morphology illustrated the same pattern in all cases. [ABSTRACT FROM AUTHOR]

Methane/THF hydrate formation at different temperature and pressure in an unstirred reactor. • The kinetic data of methane/THF hydrates in different hydrate structure domains were reported. • Substantial methane uptake observed in methane/THF hydrates even at lower pressures. • Synergistic effect of tetrahydrofuran (THF) and methyl sulfonate (MES) was presented. • 4 mM MES surfactant facilitates rapid methane/THF hydrate formation at 293.2 K. In this work, the effects of temperature and pressure on the methane hydrate formation and dissociation with the presence of 5.56 mol% THF solution in the quiescent condition is investigated. The experiments were conducted at 8 MPa and at 277.2, 283.2, 288.2 and 293.2 K. The results show that the methane uptake increased with the increase in the experimental temperature due to at the fact that higher experimental temperature showed the multiple hydrate nucleation and effective heat distribution to the surrounding. The experiment at 277.2 K showed the fastest induction time and shortest time to reach 90% of the methane uptake (t 90). The highest methane uptake was observed at 293.2 K. Moreover, the effects of experimental over pressure on the methane hydrate formation was investigated by varying the pressure from 8 to 4 MPa at 288.2 K. The result showed that the pressure did not affect the methane uptake but the rate of formation at the higher pressure is higher than at lower pressure. Furthermore, this work reports the synergistic effects of kinetic promotion of methane hydrate formation by coupling THF with methyl ester sulfonate (MES) at 293.2 K. The results showed that the presence of MES increases both induction time and rate of hydrate formation. The dissociation experiment was conducted after the formation by increasing the temperature to 308.2 K. The results showed that up to 98% of methane could be recovered. [ABSTRACT FROM AUTHOR]

[Display omitted] • A well-defined amino acid hydrophobicity ranking for CH4-THF hydrate formation. • A proposed mechanism for roles of amino acid hydrophobicity on the hydrate formation. • A morphology insight on effects of different amino acids on the hydrate formation. • Extremely stable hydrates for a long-term storage and transportation. This report reveals the success of the formation of mixed methane-THF hydrates at room temperature (298.2 K) and a pressure of 9.2 MPa equipped with a hybrid stirring in the presence of amino acids as kinetic promoters. Five different amino acids in increasing order of hydrophobicity: glutamic acid, valine, leucine, methionine, and tryptophan, were used to investigate their effects on methane uptake, formation, and dissociation kinetics. The addition of hydrophobic amino acids (tryptophan, methionine, and leucine) promotes the hydrate formation to completion, which doubly increases the methane uptake compared to that without amino acids and with hydrophilic amino acids (glutamic acid and valine) at the same condition. The results indicate that the hydrophobicity has a positive contribution to the methane uptake by creating a hydrophobic area, enhancing hydrate growth to completion. Moreover, the time taken to complete hydrate by 90 percent has a negative correlation to the molecular weight of the amino acids. The morphological studies demonstrate that the hydrates with hydrophobic amino acids were more consolidated than the addition of hydrophilic amino acids. Lastly, the hydrates with hydrophobic amino acids show excellent stability for 30 days even if they are stored at a temperature higher than their equilibrium temperature at atmospheric pressure. The small amount of gas lost was due to intrinsic loss only. This opens the possibility for cost-reduction for methane hydrate in commercialization. [ABSTRACT FROM AUTHOR]

Natural gas (NG) is the cleanest burning fossil fuel and its usage can significantly reduce CO 2 emissions from power plants. With its widespread use, there is an ever increasing need to develop technologies to store NG on a large scale. NG storage via clathrate hydrates is the best option for a large scale storage system because of its non-explosive nature, mild storage conditions, high volumetric capacity and being an environmentally benign process. In this work, we demonstrate a new method to achieve rapid methane hydrate formation in an unstirred tank reactor configuration (UTR) at moderate temperature and pressure conditions employing tetrahydrofuran (THF) as a promoter. For the first time, THF is reported to act both as a thermodynamic and an excellent kinetic promoter for methane hydrate formation. We demonstrate a multi-scale experimental validation of our method to a volumetric sample scale-up factor of 120 and internal reactor diameter scale-up factor of 10. Further, new insights on the dissociation behavior of the hydrates are reported. There is a competitive edge for storing NG via clathrate hydrates compared to compressed natural gas storage both in terms of cost and safety. [ABSTRACT FROM AUTHOR]

The presence of porous media such as activated carbon has been known to enhance the hydrate formation rate and the amount of gas consumption for methane storage applications. However, information on the effects of porous media particle size on the formation and dissociation kinetics is very limited or not available. Activated carbons with different particle sizes, 250–420 µ m, 420–841 µ m, and 841–1680 µ m, were used in order to investigate their effects on the methane hydrate formation and dissociation. The methane hydrate formation experiments were conducted at 8 MPa and 4 °C in a quiescent fixed bed crystallizer. The result showed that the experiment conducted with the 841–1680 µ m activated carbon showed the highest average water conversion to hydrate, 96.5%, due to its large interstitial pore space between the activated carbon particles, which were observed by the two-step methane consumption. In contrast, the highest average percentage of methane recovery, 98.1%, was achieved in the methane hydrate dissociation experiment conducted with the 250–420 µ m activated carbon. [ABSTRACT FROM AUTHOR]

Methane hydrate formation in the presence of 5.56 mol% THF and amino acids at 298 K and 8 MPa [Display omitted] • Mixed CH 4 -THF hydrate formation kinetic enhancement with amino acids. • Five-time increase in the mixed CH 4 -THF hydrate formation rate with amino acids. • Morphology insight on the mixed CH 4 -THF hydrate formation with amino acids. • No foam formation after the mixed CH 4 -THF hydrate dissociation with amino acids. Application of amino acids to copromote methane hydrate formation with tetrahydrofuran (THF) and their effects on the dissociation were investigated at room temperature. Hydrophobic side chain amino acids (valine, leucine, and methionine) were experimented together with 5.56 mol% THF on the hydrate formation kinetics and morphology at 293.2 K and 8 MPa in an unstirred reactor. Results show that all hydrophobic side chain amino acids affected the hydrate formation kinetics by increasing the formation rate at the same extent as surfactants. In addition, the presence of amino acids resulted in almost five times higher formation rate than with only 5.56 mol% THF. However, there were no significant difference hydrate formation rate with different amino acid concentrations. Surprisingly, the formation with certain hydrophobic side chain amino acids, leucine and methionine, showed multiple-step hydrate growth profiles, which resulted in the high methane uptake as compared to 5.56 mol% THF. The hydrate formation morphology in the presence of hydrophobic side chain amino acids illustrated different hydrate growth patterns from the THF solution and was totally different for each of the amino acid. Moreover, the presence of hydrophobic side chain amino acid did not affect the methane hydrate dissociation. [ABSTRACT FROM AUTHOR]