Gaurav
Bhattacharjee‡
,
Marcus N.
Goh‡
,
Sonia E. K.
Arumuganainar
,
Ye
Zhang
and
Praveen
Linga
*
Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117 585, Singapore. E-mail: praveen.linga@nus.edu.sg
First published on 27th October 2020
The continuously increasing trend of natural gas (NG) consumption due to its clean nature and abundant availability indicates an inevitable transition to an NG-dominated economy. Solidified natural gas (SNG) storage via combustible ice or clathrate hydrates presents an economically sound prospect, promising high volume density and long-term storage. Herein, we establish 1,3-dioxolane (DIOX) as a highly efficient dual-action (thermodynamic and kinetic promoter) additive for the formation of clathrate (methane sII) hydrate. By synergistically combining a small concentration (300 ppm) of the kinetic promoter L-tryptophan with DIOX, we further demonstrated the ultra-rapid formation of hydrates with a methane uptake of 83.81 (±0.77) volume of gas/volume of hydrate (v/v) within 15 min. To the best of our knowledge, this is the fastest reaction time reported to date for sII hydrates related to SNG technology and represents a 147% increase in the hydrate formation rate compared to the standard water–DIOX system. Mixed methane–DIOX hydrates in pelletized form also exhibited incredible stability when stored at atmospheric pressure and moderate temperature of 268.15 K, thereby showcasing the potential to be industrially applicable for the development of a large-scale NG storage system.
Broader contextThe emergence of natural gas as a key player in the current energy landscape presents a rare opportunity for the development of new and robust gas storage technologies. Gas hydrates or combustible ice-based solidified natural gas (SNG) technology can realize the compact and safe long-term storage of natural gas using eco-friendly water as the major raw material (>94%). However, its practical application has been limited by problems in forming natural gas hydrates at a rapid rate, and subsequently ensuring their prolonged stability. Herein, we report 1,3-dioxolane (DIOX) as a dual-action chemical promoter for the formation of methane sII hydrate, offering excellent thermodynamic and kinetic enhancement ability. A small amount (300 ppm) of kinetic promoter, L-tryptophan, was added to the scheme to further help in achieving ultra-rapid hydrate formation rate. A mixed methane–DIOX hydrate pellet stored at atmospheric pressure in a conventional freezer for 8 days remained highly stable, thereby demonstrating the industrial applicability, scalability and ease of operation of this approach. |
Nature has stored methane gas in the form of natural gas hydrates for millions of years, although in a slow manner, resulting in the accumulation of a huge energy resource.8–10 Gas hydrates are crystalline inclusion compounds, where under suitable conditions, cages made of water molecules may host guest gas molecules.11 Gas hydrates made of methane or natural gas are also known as combustible ice. With the appropriate tuning of the formation conditions and the identification of suitable promoters, the formation of gas hydrates can be accelerated. Thus, NG stored in the form of hydrate as solidified natural gas (SNG) has emerged as an option for its large volume and long-term storage.12–14 Tetrahydrofuran (THF) has proven to be a stable thermodynamic promoter for H2 storage via the formation of clathrate hydrates.15,16 Recently, THF has also been demonstrated as a dual-action (thermodynamic and kinetic promotion) promoter for methane storage.17 sII methane hydrate is formed rapidly in presence of THF over a wide temperature range17 and has been shown to be stable near the ambient pressure of 0.15 MPa and at 271.5 K.18
However, despite the use of THF in many industrial chemical processes,19,20 its role in the formation of hydrates has often been questioned considering its carcinogenicity,21 high volatility,22,23 and corrosive nature,24 which impede its utilization in the development of large-scale technology. Thus, it is necessary to identify cleaner alternatives to THF without compromising the vital dual functionality. Accordingly, 1,3-dioxolane (DIOX) is a heterocyclic compound closely related to THF, where the carbon atom at the 3-position of THF is replaced with an oxygen atom.25 It has similar water solubility as THF, but is less volatile and toxic (Table S1 in the ESI†). DIOX on its own can stabilize the sII hydrate.26 However, in the literature, there is only one study on the phase equilibrium of the methane–DIOX system, where 5.0 mol% of DIOX was found to be optimal as a thermodynamic promoter among the investigated concentrations in the range of 0.99 mol% to 20.02 mol%.27
Herein, we report the rapid methane uptake for the formation of mixed methane–DIOX hydrates via a detailed kinetic study, and elucidate the mechanism for this rapid enhancement by combining crystal morphology and in situ Raman Sepctroscopy observations. Further, we report the ultra-rapid formation of mixed methane–DIOX hydrates under mild operating conditions with exceptionally high gas uptake, which was achieved together with the use of L-tryptophan. The hydrate formed was also analyzed using the powder X-ray diffraction (p-XRD) technique for structure identification. Finally, this study demonstrated the production of an sII methane–DIOX hydrate pellet together with the monitoring of its stability over an eight-day period for the first time.
Fig. 1 Three-phase (H–Lw–V) equilibrium points for methane–water29 and methane–DIOX/water systems27 together with the experimental data obtained in the present study for the methane–DIOX/water system containing a stoichiometric concentration of DIOX (5.56 mol%). |
The visual images captured during a typical hydrate formation experiment for the methane–DIOX/water system is presented in Fig. 2b, which provide insight into the physical hydrate growth patterns. Additionally, a video of the visual observation of hydrate formation is presented in the ESI† (Video SV1). For the methane–DIOX/water system under quiescent growth conditions, hydrate masses propagated from the three-phase (solid–liquid–gas) interface points on both sides of the reactor (as observed through the viewing window, see visual image at t = 7 min) inwards towards the center, where they eventually meet, and thereon grew as bulk hydrates. Interestingly, at about 7 min in Fig. 2b, a significant amount of hydrates can be observed in the reactor, but the methane uptake is only about 9.12 v/v, [11.1% (±0.76%)] of the total methane uptake. Subsequently, 88.9% (±0.76%) of the methane uptake occurs after 7 min of the formation experiment. These calculations are based on the assumption that the methane uptake at the 35 minute mark is the total methane uptake since beyond this point (Fig. 2b), the hydrate growth observed is minimal. It should be noted that this particular pattern of methane uptake and hydrate growth was consistent for all the experiments, as seen by the very small standard error in Fig. 2a.
For comparison, we also conducted experiments using the traditional stirred-tank operation mode for the growth phase, and the results are presented as Fig. S2 in the ESI.† As observed in Fig. S2 (ESI†), the inclusion of stirring does not provide significant advantages to the hydrate growth characteristics, with the hydrate growth rate being slightly better for the stirred system compared to quiescent growth. Specifically, t90 for the stirred growth condition is 22.89 (±0.69) min, whereas this is achieved in 32.45 (±3.70) min for the quiescent (unstirred) growth condition. The kinetic performance data for individual experiment runs conducted in the stirred-tank operation mode is presented in Table S3 in the ESI.† A video of the visual observation of hydrate formation in the stirred-tank operation is also presented in the ESI† (Video SV2). As can be seen in Video SV2 (ESI†), within 2 min the reactor was full of hydrates, but the average methane uptake for this system at this point was only 12.56 v/v, [15.2% (±0.94%)] of the total methane uptake.
For the experimental conditions employed for hydrate formation (283.15 K and 7.2 MPa), from a thermodynamic viewpoint, it would be practically impossible to form pure methane (sI) hydrates (refer Fig. 1). Thus, only mixed methane–DIOX (sII) hydrate formation could occur. To confirm this independently, the produced hydrate was analyzed using both the powder X-ray diffraction (p-XRD) and in situ Raman spectroscopy techniques, and the results are presented in the following section.
We independently analysed the hydrates formed from the methane–DIOX/water system under the current experimental conditions (283.15 K and 7.2 MPa) via in situ Raman spectroscopy. The obtained spectrum is presented in Fig. 2d, which confirms the formation of only sII hydrates using the investigated methane–DIOX/water system under the aforementioned experimental conditions. The presence of sII hydrates was determined by the signature C–H stretching at 2914.3 cm−1 of methane gas in the small (512) cages of sII hydrates. The absence of any other specific methane signatures such as the C–H stretching at 2904.85 (±0.33) cm−1, which is a characteristic of methane occupancy in the large (51262) cages of sI hydrate,32 confirms the presence of only sII hydrates. This in situ Raman spectroscopy study provided the first complete Raman spectrum and associated analysis for mixed methane–DIOX hydrates. The detailed analysis of the observed Raman spectra is presented in the ESI† (refer to pages S5–S9).
Fig. 3 (a) Hydrate growth under quiescent conditions for methane–THF hydrates (in pink) and methane–DIOX hydrates (in blue) at 283.15 K and an initial driving force of ∼6.2 MPa, where the continuous lines represent the average data of three experiments and the vertical shaded regions represent the standard deviation of three experimental data sets. (b) Three-phase (H–Lw–V) equilibrium points for methane–DIOX/water (5.56/94.44 mol%) and methane–THF/water (5.56/94.44 mol%) systems.33 (c) Comparison of the relevant physical and safety parameters of DIOX34–36 and THF21,23 for their use as dual-action promoters for hydrate formation. (d) Molecular structure and formula of DIOX and (e) molecular structure and formula of THF. |
The kinetic gas uptake data in Fig. 3a evidently indicates that the presence of DIOX results in rapid methane hydrate growth similar to that observed with THF. However, this observation can only feasibly be put into practice if DIOX exhibits other clear advantages compared to THF, for example, with regards to the operational and safety hazards of THF outlined in the Introduction. Accordingly, Fig. 3c compares various relevant physical and safety parameters of DIOX and THF for their application in this technology. The data presents similar water solubility for both compounds, which is expected owing to their similar molecular structures (Fig. 3d and e for DIOX and THF, respectively), but more importantly, the significantly lower volatility and toxicity of DIOX compared to THF. Moreover, DIOX is classified as non-carcinogenic36 compared to THF, which is a confirmed animal carcinogen with unknown relevance to humans21 (refer to Table S1, ESI† for details of the properties presented in Fig. 3c). Lower toxicity and non-carcinogenicity of DIOX imply obvious safety benefits, and furthermore, its lower volatility indicates both safety and possible recyclability advantages due to lower promoter loss between continuous cycles. Therefore, considering all the available information, the DIOX/water system appears to be an attractive alternative to the significantly more toxic THF for utilization in SNG technology for gas storage application.
Fig. S3 (ESI†) presents images of the methane–DIOX hydrate system undergoing growth, in both the quiescent and stirred regimes, together with quantified methane uptake volumes and percentages for the related time periods. As seen in Fig. S3a (ESI†) representing quiescent growth, at about 7 min, although there was visibly lots of hydrates in the reactor, the average methane uptake at this stage is only about 9.12 v/v, [11.1% (±0.76%)] of the total methane uptake. Step 1 of growth (as described in the preceding paragraph) was also observed for the stirred system (see Fig. S3b, ESI†), where at 2 min of hydrate growth (the time around when the stirring of the contents ceased, refer to Video SV2, ESI†), the reactor already appeared to contain a considerable amount of hydrate mass, while the average methane uptake was only 12.56 v/v, [15.2% (±0.94%)] of the total methane uptake. If we assume that the enclathration (or cage loading) of DIOX and methane occurs at the same rate in the large (for DIOX) and small (for methane) cages, i.e. 1:2 (DIOX:methane), the methane uptake presented in Step 1 in Fig. S3 (ESI†) (refer to the bar charts) corresponds to a conversion of ∼2.62 mL (for quiescent growth) and 3.53 mL (for stirred growth) of the 32.4 mL solution present in the reactor. However, contradictorily, we observed considerable amount of hydrate mass in the reactor for both operating modes (quiescent and stirred).
In Step 2 of the hydrate growth process, accelerated enclathration of methane molecules in the hydrate structure (small cages of sII hydrate) occurs. The visual images together with the methane uptake presented in Fig. S3 (ESI†) indicate the occupation of methane molecules in the small cages of the formed sII hydrates during Step 2 of hydrate growth. Quantifiably, on average, 88.9% (±0.76%) and 84.8% (±0.94%) of the total methane uptake in the hydrate phase for quiescent growth and stirred growth, respectively, occurred during Step 2. The stirred system ensures better gas–liquid contact due to rigorous mixing right after nucleation, and thus it is plausible to expect that the transition from Step 1 to Step 2 would be much quicker for mixed methane–DIOX hydrate growth under the fully stirred condition compared to that under the quiescent condition.
In situ Raman spectroscopy experiments were independently carried out on the mixed methane–DIOX hydrate system for an independent perspective on the proposed two-step hydrate growth mechanism. A detailed discussion on the same is presented in the ESI† (pages S5–S9). The time-dependent Raman spectra obtained at various intervals for the first 30 min of mixed methane–DIOX hydrate growth at 7.2 MPa pressure and 283.2 K are presented in Fig. S5 (ESI†). As mentioned previously, when the hydrates nucleate, we expect both the DIOX and CH4 molecules to start moving into the hydrate structure and stabilize the hydrate cages, i.e. both DIOX and CH4 trigger hydrate nucleation and initial growth. Actually, this is something we also observed in our prior work on a similar mixed methane–THF (sII) hydrate system.18,37 As seen in Fig. S5b (ESI†), at the 1 min mark post-nucleation, a small signal appeared at 2914.3 cm−1, indicating methane occupancy in the small cages of the sII hydrates, thus confirming the formation of hydrates in the system. The signatures for DIOX enclathration in the large cages were also observed in the Raman spectra, as shown in Fig. S5a (ESI†), and are discussed in detail in the ESI.† It can be seen in Fig. S5b (ESI†) that the peak intensity for methane in the 512 cages increases significantly throughout hydrate growth from 1 min after nucleation, as reflected by the Raman spectra obtained at selected periods during the hydrate growth process. This suggests significant enclathration of methane molecules into the solid hydrate phase as hydrate growth proceeds right up to the 30 min upper limit mark, as shown in Fig. S5 (ESI†), and this molecular level observation is consistent with the results of the methane uptake experiments performed at the macroscopic scale for similar periods. On the other hand, the various peak intensities for DIOX in the 51264 cages did not exhibit a significant increase during this period, indicating that a significant percentage of the DIOX enclathration in the hydrates may have already occurred during the short initial hydrate growth period right after nucleation. The independent conclusions drawn from the in situ Raman spectroscopy study are consistent with the observed two-step growth mechanism for mixed methane–DIOX hydrate formation.
Thus, based on the methane uptake, visual observations, and in situ Raman spectra analysis for methane loading in the hydrate structure, a schematic to describe the two-step hydrate growth for the mixed methane–DIOX hydrate system is presented in Fig. 4. The visual observation of the reactor contents, methane uptake and Raman spectra for the DIOX and methane signals in sII hydrate structure for 2 min and 30 min post-hydrate nucleation (stirred growth condition) are also presented in Fig. 4 to support the proposed mechanism of hydrate growth. The two-step hydrate growth mechanism comprises a preferential DIOX over methane enclathration step (Step 1) and a subsequent rapid and sustained methane enclathration step (Step 2). The methane uptake across both periods represented independently is consistent with the corresponding methane peak intensity changes. Similarly, the lack of significant change in the DIOX peaks in the respective Raman spectra across the represented periods also corroborates the large volumes of the hydrates visually observed immediately after nucleation.
Fig. 5a represents the gas uptake (quiescent hydrate growth) obtained for a methane–DIOX/water system containing 300 ppm L-tryptophan and its comparison with the gas uptake (quiescent hydrate growth) for the standard system in the present study, i.e. “the methane–DIOX/water system without any additional kinetic promoter”. As observed, the presence of L-tryptophan greatly boosted the hydrate formation kinetics by inducing ultra-rapid hydrate growth even under quiescent operation. Fig. 5b presents a comparison of the t90 and methane uptake rate between the standard case and the solution with 300 ppm L-tryptophan. In the presence of L-tryptophan, mixed methane–DIOX hydrate formation reached 90% completion (t90) in 12.11 (±0.79) min after nucleation (also see Table S5 in the ESI†), which is faster by a factor of 2.7 times compared to the standard system without any L-tryptophan. The normalized gas uptake rate comparison for the t90 periods between the two systems, also presented in Fig. 5b, indicates a 147% increase in the hydrate formation rate for the DIOX/water/L-tryptophan mixture compared to the DIOX/water standard system.
Upon the introduction of L-tryptophan, the morphology observed during quiescent growth (Fig. 5c) indicates a porous and flexible hydrate structure; this is the distinctive crystal morphology characteristic to which we attribute the excellent kinetic promotion observed for mixed methane–DIOX hydrate formation when L-tryptophan was added to the system. The images of the system at 7 and 10 min post-nucleation show a hydrate structure that appears strongly consolidated near the three-phase interface points (reactor walls) and loosely stacked towards the centre of the reactor. This is indicative of a mechanism known as “capillary suction”, which is unique to porous hydrates, wherein underlying water or solution from the aqueous phase is drawn through channels present in the hydrate microstructure towards the top of the hydrate layer, thus facilitating further hydrate growth. The propagation of this growth in the present case clearly occurred from the three-phase (solid–liquid–gas) interface points, i.e. reactor walls, towards the centre of the reactor. Two videos are presented in the ESI† (Videos SV3 and SV4) to illustrate the ultra-rapid hydrate growth in the presence of L-tryptophan under the quiescent condition. As can be seen in the videos, the hydrates were formed extremely fast and exhibited a porous and flexible nature. The aforementioned capillary suction mechanism manifested as the underlying water or solution migrating to the top of the hydrate layer, facilitating further gas–water contact and ensuring fast, sustained hydrate formation, can actually be observed in Videos SV3 and SV4 (ESI†). Although the underlying unreacted water or solution is transported through the channels present in the hydrate microstructure towards the top of the hydrate layer, the overlying gas may use the same channels to move into the already formed hydrate structure. This would further enhance the rate of gas uptake, and consequently, enhance the overall kinetics of hydrate formation. Thus, the kinetic promotion activity of L-tryptophan is mainly based on its ability to enable the easy migration of both gas and water/solution through the system via the formation of a distinctive porous and flexible hydrate crystal structure. It should be noted that the porous hydrate crystal structure observed for the system containing the amino acid L-tryptophan in the present study is a classical signature of amino acid kinetic promotion.38
Although the presence of 300 ppm L-tryptophan resulted in ultra-rapid mixed methane–DIOX hydrate formation, the final gas uptake achieved exhibited a minute drop of about 3.7% (see Fig. 5a). The slight dip observed can be attributed to the mass transfer resistance to methane induced by the ultra-rapid rate of hydrate formation in the presence of L-tryptophan, which is also corroborated by the huge mass of hydrates observed in the morphology videos (Videos SV3 and SV4, ESI†). Subsequently, p-XRD characterization was performed for the hydrates formed using the methane–DIOX/water/L-tryptophan system, with the typical sII hydrate pattern observed again. This confirms the fact that even in the presence of the kinetic promoter L-tryptophan, only sII mixed methane–DIOX hydrates were formed under the experimental conditions (283.15 K temperature and initial pressure of 7.2 MPa). The representative XRD pattern is shown as Fig. 5d. Further augmenting the findings from the p-XRD characterization are the results from the in situ Raman spectroscopy experiment carried out for hydrate formation using the methane–DIOX/water/L-tryptophan system, keeping the experimental temperature and pressure conditions constant. The details of this experiment together with the time-dependent in situ Raman spectra obtained at regular intervals during the hydrate growth process are included in the ESI,† Fig. S7, and its associated discussion. According to Fig. S7 (ESI†), it can be further confirmed that the presence of L-tryptophan in the system did not change the hydrate structure formed (mixed methane–DIOX (sII) hydrates in the present case), but rather only provides pure kinetic enhancement to the process.
The recyclability of the DIOX/water/L-tryptophan solution for mixed methane hydrate formation is presented in Fig. 5e. Under quiescent operation, ultra-rapid hydrate growth and a predictable pattern were observed for both the fresh (cycle 1) and repeat (cycle 2) solution states. The t90 for the repeat runs of the DIOX/water/L-tryptophan system is 13.44 (±0.42) min post-nucleation, with a gas uptake at t90 of 75.11 (±0.48) v/v. In comparison, for the fresh runs, the t90 and gas uptake at t90 are 12.11 (±0.79) min post-nucleation and 75.43 (±0.69) v/v, respectively. The final gas uptake achieved for cycles 1 and 2 using the DIOX/water/L-tryptophan solution is also similar, i.e. 83.81 (±0.77) v/v for the fresh runs and 83.46 (±0.53) v/v for the repeat runs. The closeness in the kinetic data obtained for the fresh and repeat cycles using the DIOX/water/L-tryptophan system firmly establishes its exceptional recyclability potential. This should a vital contributor towards the successful scale-up of the technology under consideration currently since recycling the solution implies considerable economic conservation. The individual kinetic performance parameters for the repeat runs of the DIOX/water/L-tryptophan system are provided in Table S6 in the ESI.† With regards to the hydrate growth pattern of the guest molecules, the methane–DIOX system with L-tryptophan also appears to follow the two-step mechanism presented for the methane–DIOX system. This can be clearly seen from the initial slow methane uptake rates in Fig. 5e (<5 min), while Videos SV3 and SV4 (ESI†) reveal the presence of a considerable amount of hydrates at 5 min post-nucleation, following which an ultra-rapid and sustained methane uptake can be observed. The majority of the methane uptake (>90%) into the hydrate phase evidently occurs after 5 min of hydrate growth. Moreover, the evolution trends in the time-dependent in situ Raman spectra for the methane–DIOX/water/L-tryptophan system presented in Fig. S7 (ESI†) show strong similarity to that observed for the in situ Raman spectroscopy runs conducted for the methane–DIOX/water systems (Fig. S5 and S6, ESI†). It can be observed in Fig. S7 (ESI†) that while the peak intensity for methane trapped in the small cages of the sII hydrates increases significantly right from the start of hydrate growth up to the 15 min upper limit hydrate growth mark shown in this figure, the various peak intensities for the DIOX trapped in the large cages of the sII hydrates do not exhibit a significant increase over the same period. This indicates that the majority of the DIOX enclathration into the hydrate phase occurred during an initial short hydrate growth period immediately following hydrate nucleation, whereas methane enclathration into the hydrate phase occurred throughout the hydrate growth process. Combining the gas uptake trends obtained with the evolution of the time-dependent in situ Raman spectra observed, it can be concluded for hydrate formation from the methane–DIOX/water/L-tryptophan system that, similar to the methane–DIOX/water system, hydrate formation follows the distinct two-step hydrate growth mechanism. This includes, (a) a preferential DIOX over methane enclathration step (Step 1) immediately after hydrate nucleation, where the majority of the large cages of the formed sII hydrates get filled with DIOX, and only a small amount of methane molecules get enclathrated into the hydrate phase during this period, occupying a few of the small cages of the hydrates, and (b) a rapid and sustained methane enclathration step (Step 2), in which the majority of methane uptake in the small cages of the formed sII hydrates occurs.
We also tested the recyclability of the DIOX/water/L-tryptophan system for multiple hydrate formation cycles, where two sets of experiments were conducted, demonstrating 10 (1C1 to 1C10) and 7 (2C1 to 2C7) hydrate formation cycles. The results revealed that the gas uptake at the end of 45 min of hydrate growth remained practically the same for these systems, even after undergoing multiple cycles of hydrate formation. For cycles 1C1 to 1C10, the average gas uptake achieved at the end of 45 min of hydrate growth was 84.06 (±0.95) v/v, whereas that for cycles 2C1 to 2C7 was 84.02 (±0.65) v/v. These findings demonstrate the strong recyclability characteristics of the DIOX/water/L-tryptophan system. The individual kinetic performance parameters of the two sets of multiple cycle experiments (1C1 to 1C10 and 2C1 to 2C7) performed for the DIOX/water/L-tryptophan system are presented in the ESI,† Table S7.
Two supplemental studies were conducted to culminate the investigation on mixed methane–DIOX hydrate formation in the presence of L-tryptophan. In the first study, the effect of L-tryptophan concentration on the mixed methane–DIOX hydrate formation kinetics was considered, and it was found that 300 ppm is the optimum L-tryptophan concentration for the current experimental investigation. The results and relevant discussion are presented in the ESI† (Fig. S8 and Table S8). The second supplemental study involved the comparison of the kinetic performance parameters of mixed methane–DIOX hydrate formation and mixed methane–THF hydrate formation, each in the presence of 300 ppm L-tryptophan. The overall conclusion from this study is that considering that the initial driving force for hydrate formation is kept constant, the methane–DIOX/water/L-tryptophan system kinetically outperforms the methane–THF/water/L-tryptophan system, with a significant advantage obtained in the t90 period for the system containing DIOX and L-tryptophan. The results and relevant discussion are presented in the ESI† (Fig. S9 and Table S9).
We investigated the stability of mixed methane–DIOX hydrate pellets stored at atmospheric pressure and a moderate storage temperature of 268.15 K. Cylindrical mixed methane–DIOX hydrate pellets in the presence of L-tryptophan were synthesized using a unique, custom-designed bench-scale SNG technology prototype available in our lab. The details of the apparatus used, and the procedure followed for the synthesis of these pellets are provided in the ESI.† Specifically, a pellet having a length of 5.4 cm (Fig. 6a), diameter of 5.0 cm (Fig. 6b), and weight of 91.57 g (Fig. 6c) was produced and stored in a separate storage vessel, maintained at atmospheric pressure and storage temperature of 268.15 K, and monitored to demonstrate the stability of the mixed methane–DIOX hydrates. We observed a volumetric gas uptake of 82.47 v/v for the pellet based on the gas uptake analysis at the end of hydrate formation.
If a hydrate pellet was unstable under the aforementioned storage pressure and temperature conditions, it would dissociate, thus releasing the trapped gas into the storage vessel, which would consequently lead to an increase in the pressure inside the vessel. The amount of gas in the stored hydrate pellet was 82.47 v/v, which would translate to a pressure increase in the storage vessel of 973.29 kPa on complete dissociation. Conversely, no or a gradual pressure increase in the storage vessel would indicate good stability of the hydrate under the experimental storage conditions. Fig. 6d shows the stability of the mixed methane–DIOX hydrate pellet monitored over a period of eight days. At the first data point (beginning of the storage period), the temperature of the storage vessel was recorded as 255 K, while the pressure inside the storage vessel was 3.25 kPa (gauge pressure). The initial low temperature in the vessel is due to the fact that the storage vessel was pre-cooled at 253.15 K before transferring the produced hydrate pellet into it and subjecting the vessel containing the pellet to the pre-determined storage conditions (vessel closed at atmospheric pressure and kept inside a freezer maintained at the storage temperature). Quickly the temperature inside the storage vessel reached the desired experimental storage temperature of 268.15 K, and we also observed a slight increase in the pressure inside the storage vessel at this point (19.22 kPa; second data point in Fig. 6d depicting the hydrate storage period), which can be predominantly attributed to residual gas expansion due to the temperature increase rather than hydrate dissociation. From here on, both the temperature and pressure inside the storage vessel remained largely constant until the end of the 8 day hydrate storage period, thus signifying that the hydrate pellet was highly stable for the tested period. For the last five days of storage, the pressure in the storage vessel was extremely stable at 44.87 (±5.09) kPa. This demonstrates the exceptional stability of the mixed methane–DIOX hydrate pellet. It should be noted that the dotted pressure line at the top of Fig. 6d represents the expected pressure if all the gas stored in the pellet dissociates and evolves into the storage vessel.
Thus, for the first time, we demonstrate the highly stable storage of mixed methane–DIOX hydrate pellets at atmospheric pressure. Through the identification of DIOX as a dual-action promoter and synergistically combining it with a small amount of L-tryptophan, we addressed both the bottlenecks pertaining to SNG technology by (a) ensuring ultra-rapid hydrate formation under moderate pressure and temperature conditions and (b) ensuring highly stable storage of formed hydrates under moderate pressure and temperature conditions.
Clathrate hydrates can be synthesized as three structures, namely sI, sII and sH. Methane by itself forms sI hydrates, while in order to form sII and sH hydrates, methane requires a thermodynamic promoter (or co-guest). In our work, we illustrated DIOX as a co-guest for mixed methane (sII) hydrate formation. According to the knowledge that the only hydrate structure to exclusively house methane is sI, it becomes obvious that the storage capacity for sI hydrates is the highest, followed by sII and sH hydrates, respectively. A detailed comparison of the storage capacity (volumetric) and storage temperature at 1 atmosphere (atm) pressure for the three hydrate structures with methane is presented in Table S11 in the ESI.† As seen in this table, the key advantage of methane storage as sII hydrates is the requirement of much milder conditions for these hydrates to remain stable and non-reliance on the self-preservation effect, as demonstrated in our mixed methane-DIOX hydrate stability test. Moreover, it is also possible to tune the methane uptake in the large cages of sII hydrates through innovation in experiment process design and optimization of the promoter concentration,16 which can lead to an increase in the gas storage capacity offered by this particular hydrate structure.
Similar to CNG, adsorption-based large-scale gas storage ANG systems will need to be kept at high pressures. This is the great advantage of SNG, where SNG formed with sII hydrates is extremely stable at atmospheric pressure and moderate temperature. For the first time, we demonstrated the exceptionally stable storage of mixed methane–DIOX SNG pellets at atmospheric pressure in the present work. From an economic viewpoint, the cost of a storage vessel made of stainless steel for methane storage as ANG or CNG (pressure rating of 7.5 MPa) works out to be in excess of 5.5 times the cost of a vessel for SNG storage as sII mixed methane hydrate (pressure rating of 1.0 MPa) having identical volume, dimensions, orientation and materials.39 These details are included in the ESI† (Table S12). The practical use of large-scale high-pressure storage tanks is not advisable due to the explosive nature of these systems. The scale-out (such as CNG for laboratory use) approach will result in an increase in the cost for both CNG and ANG for large-scale operation. On the other hand, there are industry standards for large-scale storage tanks that can be readily adopted for SNG, for example, liquefied natural gas (stored at 0.2–0.5 MPa and 111.2 K) storage tanks are designed for a pressure of 1.0 MPa. Another significant advantage of SNG technology compared to ANG is that the former mainly uses water as the solvent (>94 mol%) with the addition of two small quantities of thermodynamic and kinetic promoters. Even if industrial grade water is produced from seawater by desalination, its cost is less than USD$1.13 per tonne (or per m3),40 making the economics of SNG technology highly feasible considering the raw materials.
Thus, in addition to the ultra-rapid formation of SNG via sII methane–DIOX hydrates, the low cost of its storage tank, its high degree of stability under mild storage conditions and safety firmly cement SNG technology as an efficient option for large-scale methane storage.
Pressure and temperature data were recorded over the course of hydrate formation using a data acquisition system. The pressure drop inside the system was used to calculate the amount of gas consumed due to hydrate formation. As hydrate formation proceeded, the pressure inside the system dropped as more and more gas was incorporated into the solid hydrate phase. Since the experiments were carried out in batch mode, i.e. the system pressure was not replenished at any point during the experiments, hydrate formation ceased when there was not sufficient driving force in the system to sustain the process. Once hydrate formation was completed, the excess gas in the system was vented over a period of 3–5 min and an external chiller was used to regulate the reactor temperature back up to 298.15 K (ambient temperature) to facilitate hydrate dissociation. The reactor pressure and temperature were likewise recorded during hydrate dissociation, which occurred over a period of 1.5–2 h. This concluded the first cycle of formation and dissociation, and for any solution used, was referred to as the fresh run. When a solution previously used for a hydrate formation-dissociation cycle was used in a subsequent cycle, the second run for the same solution was referred to as the repeat run.
Herein, the mixed methane–DIOX hydrate formation experiments were conducted in the SNG technology prototype to obtain a compact hydrate pellet, which was then stored to demonstrate the stability of the mixed methane–DIOX (sII) hydrates. Accordingly, 100 mL of a 5.56 mol% DIOX aqueous solution together with 1000 ppm L-tryptophan was prepared and introduced into the hydrate formation zone of the SNG prototype using the solution entry port. This corresponded to target masses of 81.38 g and 19.74 g for water and DIOX, respectively, while L-tryptophan was weighed relative to the total weight of liquid used. We observed at the 32.4 mL solution scale that the kinetic promotion provided by the presence of L-tryptophan when used in a concentration of up to 1000 ppm did not make a significant difference to the overall gas uptake. The hydrate formation experiments were conducted at a temperature of 283.15 K and initial pressure of 7.2 MPa, i.e. the hydrate formation zone was operated under these temperature and pressure conditions. The experimental temperature and pressure were so chosen as it would be virtually impossible to form pure methane (sI) hydrates under these conditions (refer to Fig. 1). Prior to the pelletization process, the hydrate pelletization zone was cooled to the storage temperature of 268.15 K. Once hydrate formation was completed and the hydrate pelletization zone had cooled to the desired hydrate storage temperature, the excess gas inside the hydrate formation zone was vented and the ball valve separating the two zones was opened. The piston present then extruded the formed hydrate particles into the hydrate pelletization zone followed by compacting them into a solid hydrate pellet, as described in the previous paragraph. The hydrate pelletization zone was then opened and the compacted hydrate pellet was pushed and collected into a pre-cooled storage vessel. The storage vessel having a total volume of 1029.58 mL was then transferred to a conventional freezer maintained at a storage temperature of 268.15 K, where the storage vessel containing the compacted hydrate pellet was left untouched so as to study the stability of the mixed methane–DIOX hydrate pellet formed. The storage vessel was equipped with a pressure transducer, WIKA A-10, and thermocouple, Omega T type, connected to a data logger, which were used to record the pressure and temperature inside the storage vessel at 20 s intervals throughout the hydrate pellet storage period, respectively. The pressure increase inside the storage vessel was representative of hydrate pellet dissociation, which involves the release of free gas into the storage vessel. For the current work, we reported the stability data for the mixed methane–DIOX hydrate pellet for a storage period of 8 days.
(1) |
(2) |
(3) |
(4) |
(5) |
Mr.hydrate = (136 × 18) + (8 × 74.08) + (16 × 16), |
∴ρhydrate = 1.057 (g cm−3) |
Normalized gas uptake rate, NRt = Rt × K (v v−1 h−1) | (6) |
The in situ Raman spectroscopy experiments were conducted in fully stirred operation (600 rpm) at an experimental temperature of 283.15 K and initial pressure of 7.2 MPa (similar to the experimental conditions used for the gas uptake experiments). Also, the same as the gas uptake experiments, a solution volume of 32.4 mL comprising target masses of 26.37 g and 6.39 g for water and DIOX, respectively, was loaded into the crystallizer for in situ Raman spectroscopy measurements. For mixed methane–DIOX hydrate formation in the presence of L-tryptophan, the solution additionally contained 300 ppm L-tryptophan as a kinetic promoter, which was calculated relative to the total weight of the liquid used to make up the solution. Once the solution loading was complete, the crystallizer was tightly closed and allowed to reach the desired experimental temperature (making use of the external refrigerator) prior to the injection of methane gas. The location of the Raman probe was pre-fixed before the solution was introduced into the crystallizer and was chosen to ensure that the probe was as close to the solution interface as possible, while also being fully submerged in the solution. The eventual selected location of the Raman probe in the present study was consistent with that used in a previous study published by our group, and thus a representative schematic illustration of it is available in the literature.46 A 3 cm stirrer bar controlled using a magnetic stirring plate positioned underneath the crystallizer was used to provide agitation to the system. Care was taken to ensure that the stirrer bar did not interfere with the Raman probe present inside the system. Once the desired experimental temperature was reached, the crystallizer was flushed with methane gas through rapid pressurization and depressurization cycles to remove any air present inside the system, following which methane gas injection was carried out slowly until the desired experimental pressure was reached, making sure that the temperature of the system remained close to the desired experimental temperature. When methane gas pressurization was completed, the system was isolated, i.e. the gas inlet valve was closed (refer schematic available in the literature)46 and data acquisition was started. Additionally, at this point, both stirring of the system (600 rpm) and Raman signal acquisition were also simultaneously initiated. The real-time Raman spectrometer was set to record Raman spectra at 20 s intervals throughout the hydrate formation process. Hydrate nucleation was determined using three simultaneous markers, i.e. the familiar characteristic pressure drop and temperature spike of hydrate nucleation, and the appearance of characteristic Raman spectral signatures for one or more hydrate guests (methane or DIOX) incorporated into the hydrate structure.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ee02315a |
‡ Equal contribution from authors. |
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