13C NMR analysis and gas uptake measurements of pure and mixed gas hydrates: Development of natural gas transport and storage method using gas hydrate

¹³C NMR spectra were obtained for pure CH₄, mixed CH₄+THF, and mixed CH₄+Neohexane hydrates in order to identify hydrate structure and cage occupancy of guest molecules. In contrast to the pure CH₄ hydrates, the NMR spectra of the mixed CH₄+THF hydrate verified that methane molecules could occupy only the small portion of 5¹² cages because the addition of THF, water-soluble guest component, to aqueous solution prevents the complete filling of methane molecules into small cages. Furthermore, from these NMR results one important conclusion can be made that methane molecules can’t be enclathrated at all in the large 5¹²6⁴ cages of structure II. In addition, gas uptake measurements were carried out to determine methane amount consumed during pure and mixed hydrate formation process. The moles of methane captured into pure CH₄ hydrate per mole of water were found to be similar to the full occupancy value, while the moles of methane captured into the mixed CH₄+THF hydrate per moles of water were much lower than the ideal value. The overall results drawn from this study can be usefully applied to storage and transportation of natural gas.     

Phase behavior and structure transition of the mixed methane and nitrogen hydrates

Pure methane and nitrogen form structure I and II hydrate, respectively, and therefore the structure type of mixed gas hydrate was found to largely depend on their relative gas composition. In addition to the structural difference of size and shape, each hydrate structure shows different capacity to store the guest molecules. In this study, we investigated phase and structural behaviors according to the composition of methane+nitrogen gas mixture. Three-phase (H-Lw-V) equilibria of solid hydrate, water-rich liquid and vapor phase containing 25.24 mol%, 28.51 mol%, 31.23 mol% and 40.39 mol% of methane were determined at various temperatures (in the range from 273.30 K to 285.05 K) and pressures (from 8.325 MPa to 20.700 MPa). 13C solid-state NMR spectroscopy and powder XRD method were performed to identify the formed structure of hydrate samples. The experimental results showed that gas hydrate of the methane+nitrogen mixture changes its structure from sI to sII between 25.24 mol% and 28.51 mol% of methane concentration. These results of phase behavior and structure identification for the mixed gas hydrates are expected to be very helpful in evaluating the feasibility of exploitation of methane gas from natural gas hydrate and the separation process using gas hydrate as a storage-media     

Structure identification of binary 1-propanol+methane hydrate using neutron powder diffraction

Alcohols are frequently used in hydrate communities as thermodynamic hydrate inhibitors, but some alcohol molecules are also known to be hydrate formers with a help gas. In this study, the crystal structures of binary 1-propanol+methane hydrates at various temperatures were identified using neutron powder diffraction analysis with Rietveld refinement. Characteristic behaviors of the guest molecules in the hydrate structure were also analyzed to verify possible host-guest interactions from the refinement results. The results showed that the thermal factors of host water and guest methane increased continuously as the temperature increased. However, the isotropic thermal factors (B values) of 1-propanol were abnormally high compared to those of methane in the small cages of structure II (sII) hydrates, which could be because the 1-propanol molecules were off-centered in the large cages of sII hydrates. This implies that hydrogen bonding interactions between host and guest molecules can occur in the large cages of sII hydrates. The present findings may lead to a better understanding of the nature of guest-host interactions that occur in alcohol hydrates.     

Effect of multi-walled carbon nanotubes on methane hydrate formation

1 m³ of methane hydrate can be decomposed into a maximum of 216 m³ of methane gas under standard conditions. If these characteristics of hydrates are utilized in the opposite sense, natural gas can be fixed into water in the form of a hydrate solid. Therefore, the use of hydrates is considered to be a great way to transport and store natural gas in large quantities. However, when methane hydrate is formed artificially, the amount of gas that is consumed is relatively low, due to the slow reaction rate between water and methane gas. Therefore, for practical purposes in the application, the present investigation focuses on increasing the rate of formation of the hydrate and the amount of gas consumed by adding multi-walled carbon nanotubes (MWCNTs) to pure water. The results show that when 0.004 wt% of multi-walled carbon nanotubes was added to pure water, the amount of gas consumed was about 300% higher than that in pure water and the hydrate formation time decreased at a low subcooling temperature.     

Spectroscopic analysis of carbon dioxide and nitrogen mixed gas hydrates in silica gel for CO2 separation

In this study solid-state NMR spectroscopy was used to identify structure and guest distribution of the mixed N₂ + CO₂ hydrates. These results show that it is possible to recover CO₂ from flue gas by forming a mixed hydrate that removes CO₂ preferentially from CO₂/N₂ gas mixture. Hydrate phase equilibria for the ternary CO₂–N₂–water system in silica gel pores were measured, which show that the three-phase H–L_w–V equilibrium curves were shifted to higher pressures at a specific temperature when the concentration of CO₂ in the vapor phase decreased. ¹³C cross-polarization (CP) NMR spectra of the mixed hydrates at gas compositions of more than 10 mol% CO₂ with the balance N₂ identified that the crystal structure of mixed hydrates as structure I, and that the CO₂ molecules occupy mainly the abundant 5¹²6² cages. This makes it possible to achieve concentrations of more than 96 mol% CO₂ gas in the product after three cycles of hydrate formation and dissociation.     

X射线衍射分析聚乙烯吡咯烷酮对水合物分解过程的影响

深水油气资源的勘探开发以及开采过程中的环保要求,使得天然气水合物动力学抑制剂使用不可避免,含动力学抑制剂的分解研究对水合物生成后的解堵具有重要的指导意义。本文在高压反应釜内采用甲烷和丙烷混合气,合成天然气水合物,并用X射线粉晶衍射仪分析了含动力学抑制剂聚乙烯吡咯烷酮的水合物分解过程。结果显示甲烷和丙烷气体会形成SⅡ型水合物,但伴随有SⅠ型甲烷水合物的生成;添加动力学抑制剂后,水合物的分解速率变慢,在-60℃,添加0.5%的聚乙烯吡咯烷酮后,分解起始的20 min内,无抑制剂体系水合物分解可达69%,而在含抑制剂体系分解约18%;SⅡ型甲烷丙烷混合气水合物分解过程中晶胞各晶面分解速率相同,没有偏好性,水合物笼作为一个整体分解,添加抑制剂不改变这种分解方式,仍以整体分解。     

Characterization of gas hydrates with PXRD, DSC, NMR, and Raman spectroscopy

Structure I (sI) and H (sH) hydrates containing methane were synthesized and characterized with PXRD, DSC, NMR, and Raman spectroscopy. Three well-known large molecule guest substances (LMGSs) were selected as sH hydrate formers: 2,2-dimetylbutane (NH), methylcyclohexane (MCH), and tert-butyl methyl ether (TBME). The solid phase analysis confirmed the presence of sH hydrate whenever a LMGS was present. The presence of a non-hydrate former (n-heptane) did not affect the methane hydrate structure or cage occupancies. Ice to hydrate conversion was limited when the LMGS amount was less than stoichiometric and synthesized at low methane pressure, but nearly complete conversion was achieved with temperature ramping and excess LMGS. The methane occupancies were found to depend on the type of LMGS and increased with pressure. The hydrate with TBME was found to have the smallest methane content followed by the hydrates with NH and MCH. Both NMR and Raman identified methane and LMGS signals from the hydrate phase, however, the cage occupancy values of sH hydrate can only be obtained from NMR spectroscopy. The hydrate structures, ice to hydrate conversion, gas content in hydrate and cage occupancy from the various measurements are consistent with each other.     

Gauche conformation of acyclic guest molecules appearing in the large cages of structure-H clathrate hydrates

In the present study, measurements and analyses were made of the High-Power Decoupling (HPDEC) solid-state ¹³C NMR spectra of structure-H (sH) methane hydrates with isopentane, one of the simplest and smallest acyclic large guest molecules, and methylcyclohexane (MCH), a commonly used cyclic guest molecule that is larger than isopentane. From the spectroscopic information, clear and definite evidence for the molecular conformation of acyclic guest molecules that are sufficiently small so as to be entrapped into the structure-H large cage (sH-L) was expected. The ¹³C NMR chemical shift change was additionally checked through the use of a hydrogen-hydrogen steric perturbation model. From the overall results, we concluded that one of the smallest acyclic guest molecules, isopentane, participating in the formation of a structure-H clathrate hydrate is encaged, confirming the gauche conformation in large cavities. The present results strongly suggest that the guest position and structure in hydrate cages are greatly influenced by both short-range interactions between guest molecules and cage frameworks and long-range interactions between small and large guests. Accordingly, cage dynamics must be carefully considered when a specific sH hydrate is designed and synthesized for the purpose of tuning material properties.     

Structure and thermal expansion of natural gas clathrate hydrates

We report on the structural properties of natural gas hydrate crystals from the Sea of Okhotsk. Using powder X-ray diffraction (PXRD), it was determined that sediments from four locations contained type I gas hydrate, which encage mostly methane (96-98%) and a small amount of carbon dioxide. For all hydrates, the lattice constant was estimated to be at 113 K, which approximately equals that of pure methane hydrate. The result is in good agreement with the structure of artificially synthesized methane + carbon dioxide mixed-gas hydrates. These results suggest that the lattice constant of the natural gas hydrate does not change due to a change of CO₂ gas content. In addition, the thermal expansion of the sampled hydrate was measured for the temperature range of 83-173 K, and the resulting density of the hydrate crystal at 273 K was estimated to be . These results are essential for applying natural gas hydrates as an alternative natural fuel resources.     

Gas hydrates: A cleaner source of energy and opportunity for innovative technologies

The global energy system is characterized by a gradual de-carbonization and move to cleaner burning technologies: from wood to coal to oil and to natural gas. A final destination characterized by the term“hydrogen economy” is desired. Gas hydrate found in the earth’s crust is considered a source of natural gas that is essentially 100% methane (CH₄) gas. Natural gas hydrate estimates worldwide range from 10,000 to 40,000 trillion cubic meters (TCM). Efforts are underway to exploit this resource. These methane hydrates in the earth’s crust also have the potential to be a significant factor in global climate change. Moreover, gas hydrates offer opportunities for the development of innovative technologies (separation of CO₂ from CO₂/N₂ and CO₂/H₂ mixtures, CO₂ sequestration, natural gas transportation and storage and H₂ storage). In this work we assess the progress towards exploitation of gas hydrates as a resource for methane (cleaner energy) and summarize the state of the art with respect to the role of gas hydrates in the development of innovative technologies.     

Ultra-stability of gas hydrates at 1 atm and 268.2 K

This paper details creation of methane sI hydrates that are much more stable at 1 atm and 268.2 K than any previously reported. Extraordinarily stable natural gas sII hydrates at 1 atm and 268.2-270.2 K are reported for the first time. Test innovations that achieved ultra-stabilities give insight into hydrate self-preservation mechanisms. Water-surfactant liquid solutions were used to nucleate hydrate crystals that adsorbed as extremely small particles on surfaces of high thermal conductivity. The small hydrate particles packed and consolidated symmetrically upon Al or Cu cylindrical surfaces, minimizing internal void spaces and fractures in the accumulated 250-400 g hydrate mass. Resulting hydrate stability window is 268.2-270.2 K at 1 atm. Methane sI, as well as natural gas sII, hydrates exhibit only minimal decomposition upon reducing confining system pressure to 1 atm in the 268.2-270.2 K stability window. Total gas that evolved after 24 h at 1 atm in the stability window typically amounted to less than 0.5% of originally stored gas, and this ultra-stability was shown to persist when the test was allowed to run 256 h before terminating. The entire methane sI or natural gas sII hydrate mass remains stable during pressure reduction to 1 atm, whereas previous reports defined hydrate anomalous stability for only about 50% of fractional hydrate remnants.     

Study on methane hydrate formation using ultrasonic waves

Methane hydrate is a kind of gas hydrate and has the crystal structure I. 1 m³ of methane hydrate can be decomposed to a maximum of 172 m³ of methane gas in standard conditions. If this characteristic of methane hydrate is reversely utilized, natural gas, which mainly consists of methane gas, is fixed into water in the form of hydrate solid. However, when methane hydrate is formed artificially by simply reversing its process of natural generation, the amount of methane gas consumed owing to hydrate formation is fairly low, which would be problematic for its massive synthesis and application. In this study, experiments are carried out with the goal of increasing the amount of gas consumed by using ultrasonic waves. The power for maximum gas consumption was observed at 150 W, and the amount of gas consumed was four times higher than that at 0 W at the subcooling temperature of 0.5 K. The ultrasonic waves are more effective at the subcooling temperature of 5.7 K than at the subcooling temperature of 0.5 K, and are another effective method for enhancing methane hydrate formation and reducing the hydrate formation time.     

初始压力对冰冻石英砂中CO2水合物生成特性的影响

利用冰冻石英砂模拟冻土水合物的赋存条件，研究了压力对二氧化碳水合物生成特性的影响，在300 mL高压水合物反应釜中于271 K下进行了多组CO2液化压力以上及以下的霰状冰粉包裹的石英砂中水合物生成实验。结果表明，充入的CO2未液化时，初始压力越大，水合反应速率越快，压力越早达稳定状态；充入压力达液化压力后，注入的CO2越多，水合反应速率越快。压力作为水合反应的驱动力，压力越高水合物生成越多，冰的最终转化率越高。采用CO2置换冻土区中甲烷水合物时，控制压力低于液化压力或注入过量的CO2，置换效果更好。     

Methane hydrates: A future clean energy resource

Natural gas has been considered as the best transition fuel into the future carbon constraint world. The ever-increasing demand for natural gas has prompted expanding research and development activities worldwide for exploring methane hydrates as a future energy resource. With its vast global resource volume (~ 3000 trillion cubic meter CH₄) and high energy storage capacity (170 CH₄ v/v methane hydrate), recovering energy from naturally-occurring methane hydrate has attracted both academic and industry interests to demonstrate the technical feasibility and economic viability. In this review paper, we highlight the recent advances in fundamental researches, seminal discoveries and implications from on-going drilling programs and field production tests, the impending knowledge gaps and the future perspectives of recovering energy from methane hydrates. We further emphasize the current scientific, technological and economic challenges in realizing long-term commercial gas production from methane hydrate reservoir. The continuous growth of the corresponding experimental studies in China should target these specific challenges to narrow the knowledge gaps between laboratory-scale investigations and reservoir-scale applications. Furthermore, we briefly discuss both the environmental and geomechanical issues related to exploiting methane hydrate as the future energy resource and believe that they should be of paramount importance in the future development of novel gas production technologies.     

Investigation on the effect of oxalic acid,succinic acid and aspartic acid on the gas hydrate formation kinetics

Gas hydrate reserves are potential source of clean energy having low molecular weight hydrocarbons trapped in water cages. In this work, we report how organic compounds of different chain lengths and hydrophilicities when used in small concentration may modify hydrate growth and either act as hydrate inhibitors or promoters. Hydrate promoters foster the hydrate growth kinetics and are used in novel applications such as methane storage as solidified natural gas, desalination of sea water and gas separation. On the other hand, gas hydrate inhibitors are used in oil and gas pipelines to alter the rate at which gas hydrate nucleates and grows. Inhibitors such as methanol and ethanol which form strong hydrogen bond with water have been traditionally used as hydrate inhibitors. However, due to relatively high volatility a significant portion of these inhibitors ends up in gas stream and brings further complexity to the safe transportation of natural gas. In this study, organic additives such as oxalic acid, succinic acid and L-aspartic acid (all three) having—COOH group(s) with aspartic acid having an additional—NH₂ group, are investigated for gas hydrate promotion/inhibition behavior. These compounds are polar in nature and thus have significant solubility in liquid water; the presence of weak acidic and water loving (carboxylic/amine groups) moieties makes these organic acids an excellent candidate for further study. This study would pave ways to identify a novel(read better) promoter/inhibitor for gas hydrate formation. Suitable thermodynamic conditions were generated in a stirred tank reactor coupled with cooling system; comparison of gas hydrate formation kinetics with and without additives were carried out to identify the effect of these acids on the formation and growth of hydrates. The possible mechanisms by which these additives inhibit or promote the hydrate growth are also discussed.     

烃类水合物导热特性的分子动力学模拟

采用分子动力学模拟方法Green-Kubo理论计算了263.15 K、3 MPa,sⅠ乙烷水合物、乙烯水合物的导热,给出密度和热导率值。从主客体分子和晶体结构（致密性、规整程度）对导热的影响等角度研究了烃类水合物（甲烷水合物、乙烷水合物、乙烯水合物）导热的特性。结果显示化学性质相似、分子量相差不大的烃类形成的水合物,其导热具有相似的温度压力依赖关系和晶体结构相关关系。对于sⅠ型水合物,水分子对水合物导热的影响远远超过客体分子对导热的影响。水合物的分子量越大,水合物密度越大,热导率越大。水合物晶体越致密、晶格越规整,热导率越大。     

Effect of synthesized silver nanoparticles in promoting methane hydrate formation at 4.7 MPa and 5.7 MPa

Using gas hydrates as materials for storage and transportation of natural gas have attracted much attention in recent years. However, there are two barriers in industrializing this new method. Firstly, methane hydrate induction time is relatively high. On the other hand the amount of gas trapped in methane hydrate crystals is too low. In this survey, silver nanoparticles were synthesized using a chemical reduction method and introduced to the hydrate reactor. Experiments were conducted at initial reactor pressures of 4.7 MPa and 5.7 MPa. At each pressure three independent experiments were performed. According to the results, in the presence of silver nanoparticles, methane hydrate induction time decreased by 85% and 73.9%, and the amount of methane trapped in hydrate crystals increased by 33.7% and 7.4% at the pressures of 4.7 MPa and 5.7 MPa respectively.     

Fundamental challenges to methane recovery from gas hydrates

The growing use of natural gas, cleanest of all available fossils fuels, is already raising concern regarding the long-term supply of this precious resource. The amount of methane in gas hydrates is much greater than all other presently known sources of methane. This paper describes some fundamental challenges, the location, magnitude, and feasibility of recovery, which must be addressed to recover methane from dispersed hydrate sources. For methane recovery, we briefly describe kinetic models of methane hydrate decomposition for temperature and pressure conditions that mimic in situ methane hydrate stability. We also propose the catalytic role of sediment impurities, if any, in inducing nucleation sites for hydrate formation. The availability of plentiful methane is important to avoid future energy crises, such as that which crippled the world economy three decades ago.     

Mechanism of methane hydrate formation in the presence of hollow silica

Methane hydrates are studied extensively as a prospective medium for storing and transporting natural gas due to their inherent advantages, including high volumetric energy storage density, being environmentally benign and extremely safe method compared to conventional compression and liquefaction methods. Enhanced formation kinetics of methane hydrates has been reported in hollow silica due to the increased gas/liquid contact surface area available for efficient conversion of water to hydrates. This work elucidates the mechanism of methane hydrate formation in light weight hollow silica. Hollow silica-to-water ratio was varied and its effect on the methane hydrate formation/dissociation morphology was observed. There exists a critical hollow silica-to-water ratio (1 : 6) beyond which the hydrates preferentially crystallize on the top of the bed by drawing water from the interstitial pores, whereas below this ratio the hydrate formation occurs within the bed between inter-particular spaces of hollow silica. Due to the very low bulk density, a small fraction of hollow silica was observed to be displaced from the bed during the hydrate formation above the critical hollow silica to water ratio.     

Energy efficiency analysis of natural gas hydrates production method

The decomposition of natural gas hydrates is a phase change process, which involves the consumption and conversion of various forms of energy, such as electrical energy, chemical energy, and thermal energy. In order to evaluate the economy capacity of natural gas hydrates exploitation, an exergy model was established to calculate the energy efficiency ratio (EER) of hydrate production method. The CO₂ replacement method is taken as a case study to introduce the calculation equation and flow chart of energy efficiency ratio in any production period. The amount of CO₂ injection, gas production and mole fraction of methane in produced gas are three key parameters in the process of CO₂ replacement. The ratio between the amount of gas production and CO₂ injection is defined as production injection ratio to eliminate the influence of deposit size. This work studied the influence of production injection ratio and the mole fraction of methane in produced gas on EER. The results show that the EER of gas hydrates production by CO₂ replacement is between 0.31 and 6.4 under the set conditions, and it increases with the increase of production injection ratio. In addition, increasing the mole fraction of methane in produced gas can reduce the energy consumption for gas separation and increase EER. Therefore, there are two effective ways to increase EER of CO₂ replacement through controlling the amount of gas production and the mole fraction of methane in produced gas. The EER model is established to provide guidance for the optimization of gas hydrate mining process.