Large pressure depression of methane hydrate by adding 1,1-dimethylcyclohexane

The structure-H hydrate of 1,1-dimethylcyclohexane (DMCH) helped by methane has been investigated in a temperature range of 274.6-289.3 K and pressure range up to 6.7 MPa. The present results suggest that 1,1-DMCH is a suitable additive which makes a mild-pressure handling of natural-gas hydrate possible.     

Gas hydration of trans-1,2-dimethylcyclohexane with cooperative assistance of methane and cis-1,2-dimethylcyclohexane

It has been regarded that the limit of the largest cage occupancy for the structure-H hydrate is between the 1,2-dimethylcyclohexane stereo-isomers, because the cis-isomer is able to generate the structure-H hydrate in the presence of methane while the trans-isomer is not. In the present study, gas hydration of trans-1,2-dimethylcyclohexane in the presence of methane and cis-1,2-dimethylcyclohexane is found from stability boundaries for the structure-H hydrate system.     

Dual function inhibitors for methane hydrate

The performance of five imidazolium-based ionic liquids as a new class of gas hydrate inhibitors has been investigated. Their effects on the equilibrium hydrate dissociation curve in a pressure range of 30-110 bar and the induction time of hydrate formation at 114 bar and a high degree of supercooling, i.e., about 25 °C, are measured in a high-pressure micro differential scanning calorimeter. It is found that these ionic liquids, due to their strong electrostatic charges and hydrogen bond with water, could shift the equilibrium hydrate dissociation/stability curve to a lower temperature and, at the same time, retard the hydrate formation by slowing down the hydrate nucleation rate, thus are able to act as both thermodynamic and kinetic inhibitors. This dual function is expected to make this type of inhibitors perform more effectively than the existing inhibitors.     

Icosahedron cage occupancy of structure-H hydrate helped by Xe -1,1-, cis -1,2-, trans-1,2-, and cis-1,4-dimethylcyclohexanes

Four mixtures of 1,1-, cis-1,2-, trans-1,2-, and cis-1,4-dimethylcyclohexanes (hereafter abbreviated DMCH) including H₂O and Xe have been investigated in a temperature range over 274.5 K and a pressure range up to 2.7 MPa. The 1,1-DMCH and cis-1,2-DMCH generate the structure-H hydrate in the temperature range up to 295.2 and 280.2 K, respectively. Especially, very large depression of equilibrium pressure has been observed in the structure-H 1,1-DMCH hydrate system. On the other hand, neither trans-1,2-DMCH nor cis-1,4-DMCH generates the structure-H hydrate in the present temperature range. It is an important finding that the cis-1,4-DMCH does not generate the structure-H hydrate in the presence of Xe, while the mixture of cis-1,4-DMCH and methane generates the structure-H hydrate.     

Surfactant effects on hydrate formation in an unstirred gas/liquid system: An experimental study using methane and sodium alkyl sulfates

This paper reports an experimental study on the effects of surfactant additives on the formation of a clathrate hydrate in a quiescent methane/liquid-water system, which was initially composed of a 300-cm³ aqueous phase and an ∼640-cm³ methane-gas phase, then successively provided with methane such that the system pressure was held constant. The surfactants used in the present study were three sodium alkyl sulfates appreciably different in the alkyl chain length—they were sodium dodecyl sulfate (abbreviated as SDS), sodium tetradecyl sulfate (abbreviated as STS) and sodium hexadecyl sulfate (abbreviated as SHS). For each surfactant added to water up to, at most, 1.82-3.75 times the solubility, we performed visual observations of hydrate formation simultaneously with the measurements of methane uptake due to the hydrate formation. The qualitative hydrate-formation behavior thus observed was almost the same irrespective of the species as well as the initial concentration of the surfactant used; i.e., thick, highly porous hydrate layers were formed and grew on the horizontal gas/liquid interface and also on the test-chamber wall above the level of the gas/liquid interface. In each experimental operation, hydrate formation continued for a limited time (from ∼6 to ) and then practically ceased, leaving only a small proportion (typically 15% or less) of the aqueous solution unconverted into hydrate crystals. The variations in the time-averaged rate of hydrate formation (as measured by the rate of methane uptake) and the final water-to-hydrate conversion ratio with the initial concentration of each surfactant were investigated. Moreover, we examined the promotion of hydrate formation with the aid of a water-cooled cold plate, a steel-made flat-plate-type heat sink, vertically dipped into the aqueous phase across the gas/liquid interface.     

相变浆液中甲烷水合物的生成过程强化

利用相变材料（PCM）正十四烷的固液相变过程,吸收甲烷水合释放的热量,实现了直接换热强化水合过程的目的。正十四烷与水混合制成相变乳液（PCE）,经冷却后形成浆液。在半间歇水合器中,测定并计算了甲烷水合物在此浆液中的收率和生成速率。为了提高计算的准确性,设计了一套PVT装置,通过减压法实验测定了低温条件下甲烷在正十四烷中的溶解度。实验结果表明：低温条件下,甲烷在正十四烷中的溶解度与压力基本呈线性关系;相比于间接传热方式下的水合过程,相变浆液中甲烷水合物收率及生成速率得到了有效提升。     

Stability boundaries of gas hydrates helped by methane—structure-H hydrates of methylcyclohexane and cis-1,2-dimethylcyclohexane

The four-phase coexistence curves for the structure-H hydrates of methylcyclohexane and cis-1,2-dimethylcyclohexane in the presence of methane are measured in the temperature range 274.09- and pressure range 1.42-. Very large pressure reductions from the pure methane hydrate are observed by forming structure-H hydrates. The present investigation on the trans-1,2-dimethylcyclohexane system reveals that the limit of the largest-cage occupancy for the structure-H hydrate is laid between the 1,2-dimethylcyclohexane stereo-isomers.     

Enclathration of hydrogen by organic-compound clathrate hydrates

The properties of hydrogen enclathration by cyclic ethers and acetone clathrate hydrates were investigated by powder X-ray diffraction, Raman spectroscopic analysis and volumetric analysis. Powder X-ray diffraction profiles indicate that the hydrates are structure-II hydrates. The variation in lattice constant by hydrogen occupation was investigated. This result indicates that inclusion of H₂ atom within empty small cage changes size of host cages depending on type of guest molecule. Raman results show that the samples formed binary clathrate hydrate of hydrogen and each organic compound. The amount of encaged H₂ was found to be comparable to that of H₂–THF binary hydrate. The trend of the changes for lattice constants is not related to the amount of encaged H₂. These results suggest that the organic compounds investigated in this study can be used as alternatives to THF for H₂ enclathration.     

Thermodynamic Properties of Ionic Semiclathrate Hydrate Formed with Tetrabutylammonium Propionate

The phase equilibrium temperature and dissociation heat of tetrabutylammonium propionate (TBAPr) hydrate are reported. TBAPr hydrate is a type of ionic semiclathrate hydrates and also could potentially be used as thermal energy storage material. The temperature‐composition phase diagram of the TBAPr hydrate was determined in a defined range of mass fractions. Considering the dissociation heat of differential scanning calorimetry (DSC) measurements, multiple peaks of heat flow were observed in the TBAPr‐water system at the TBAPr mass fraction lower than 0.35, and there was a single peak at the mass fraction higher than 0.37.     

Evidence of liquid water formation during methane hydrates dissociation below the ice point

Dissociation of small methane hydrate samples formed from water droplets of size 0.25-2.5 mm has been investigated below the ice melting point in the temperature range of 240-273 K, where the self-preservation effect is observed for bulk hydrates. The experiments included optical microscopy observations combined with P-T measurements of the dissociation conditions for the methane hydrates. For the first time, the formation of supercooled liquid water during the hydrate dissociation was reliably detected in the temperature range of 253-273 K. The formation of the liquid phase was visually observed. The induction time of the ice nucleation for the metastable liquid water depended from the dissociation temperature and a size of water droplets formed during the hydrate dissociation. It was found that in the temperature range of 253-273 K values of the dissociation pressure for the small hydrate samples fall on the extension of the water-hydrate-gas equilibrium curve into the metastable region where supercooled water exist. The average molar enthalpy of 51.7 kJ/mol for the dissociation of the small methane hydrate samples in the temperature range of 253-273 K was calculated using Clausius-Clapeyron equation. This value agrees with the enthalpy of dissociation of bulk methane hydrates into water and gas at temperatures above 273 K.     

离子液体与PVP K90复合抑制剂对甲烷水合物的生成影响

注入抑制剂是油气行业解决管道输送过程因水合物生成而引发的堵塞问题最常用的方法。但现有大多数动力学抑制剂(KHIs)存在抑制性能不足、高过冷度条件下会失效等问题,可应用场合大大受限。离子液体作为绿色溶剂对甲烷水合物具有良好的热力学抑制作用。为改进KHIs的性能,提出将离子液体与KHIs复合。本文实验考察8.0 K过冷度、两种浓度下[1.0%(质量)、2.0%(质量)]离子液体N-丁基-N-甲基吡咯烷四氟硼酸盐([BMP][BF4])、聚乙烯基吡咯烷酮(PVP K90)以及二者复配构成的复合型抑制剂对甲烷水合物抑制规律,得到了最佳组分配比。利用粉末X射线衍射(PXRD)和低温激光拉曼光谱测量了不同抑制剂体系中形成的甲烷水合物晶体微观结构和晶穴占有率,发现添加抑制剂不会改变sI型甲烷水合物晶体结构,但会影响水合物晶体的大、小笼占有率和水合数。结合宏观动力学实验和微观结构测试结果,揭示离子液体与PVP K90复合抑制剂的抑制机理。     

Effect of complex inhibitors containing ionic liquids and PVP K90 on formation of methane hydrate

Injecting inhibitors is the most commonly used method in the oil and gas industry to solve the problem of blockage caused by hydrate formation during pipeline transportation. However, most of the kinetic hydrate inhibitors (KHIs) are strictly limited by weak inhibition performance and low subcooling. Ionic liquids, a kind of green solvent, have been recognized to act as excellent thermodynamic inhibitors on methane hydrate formation. So, it is proposed to add the ionic liquids into KHIs to improve their overall performance. In this paper, the kinetic effects of an ionic liquid N-butyl-N-methylpyrrolidine tetrafluoroborate ([BMP][BF₄]), a commercial kinetic inhibitor polyvinyl pyrrolidone (PVP K90) and their mixtures with different mass ratios on the methane hydrate formation were experimentally studied at 8.0 K subcooling and two concentrations [1.0%(mass) and 2.0%(mass)]. The best mass ratio of the compound inhibitor was determined. Moreover, the crystal structures and cage occupancy characteristics of methane hydrates formed without and with inhibitors at different mass concentrations and composition ratios were measured by using powder X-ray diffraction (PXRD) and low-temperature Laser Raman spectrometers. It was found that the addition of inhibitors did not change the crystal structure of methane hydrate, but affected the cage occupancies and hydration numbers. Based on the results from macroscopic kinetics and microscopic structure tests, the inhibition mechanism of compound inhibitors was proposed.     

Structural changes in soot particles induced by diode laser irradiation

The effects of laser radiation on soot in a vacuum were studied by Raman spectroscopy and transmission electron microscopy. It was found that defective carbon onions could be formed simply by irradiation of focused common diode lasers with intensity much lower (10³-10⁶ W/cm²) than those used in the previous work. A modified Melton model was introduced to analyze the formation of onions. The results show a threshold value of laser intensity (5.49 × 10³ W/cm²), above which sublimation and rearrangement play a dominant role in the formation of onions. Such onions can also be formed with laser intensities below the threshold since neighboring graphene layers may merge and grow by capturing interstitial carbon atoms. The deposited soot is damage resistant to the diode laser radiation in air due to the rapid formation of carbon onions.     

Induction time,storage capacity,and rate of methane hydrate formation in the presence of SDS and silver nanoparticles

In this communication, the kinetic parameters of methane hydrate formation (induction time, quantity and rate of gas uptake, storage capacity (SC), and apparent rate constant) in the presence of sodium dodecyl sulfate (SDS), synthetized silver nanoparticles (SNPs), and mixture of SDS?+?SNPs have been studied. Experimental measurements were performed at temperature of 273.65?K and initial pressure of 7?MPa in a 460?cm³ stirred batch reactor. Our results show that adding SDS, SNPs and their mixture increases the quantity of gas uptake, water to hydrate conversion, and SC of methane hydrate formation, noticeably. Using 300?ppm SDS increases the SC and the quantity of methane uptake 615, and 770%, respectively, compared with pure water. Investigating the hydrate growth rate at the start of hydrate formation process shows that, using SNPs, SDS, and their mixture increases the initial apparent rate constant of hydrate rate, considerably. Our results show that the system of methane?+?water?+?SDS 500?ppm?+?SNPs 45?µM represents the maximum value of initial apparent rate constant, compared with other tested systems.     

Intensification and kinetics of methane hydrate formation under heat removal by phase change of n‐tetradecane

A method of direct heat removal resulting from the phase change of n‐tetradecane was used to intensify the heat transfer during hydrate formation. The growth rates of methane hydrate in aqueous slurries containing 25–45 wt % of solid n‐tetradecane were investigated at pressures between 4.70 and 6.46 MPa (gauge) and near the fusion point of solid n‐tetradecane. Methane hydrate growth started at a practically constant rate, which became variable after a sudden increase. Two rate laws were established to correlate with the experimental data for the constant and variable rate stages. The methane hydrate growth rates achieved with solid n‐tetradecane were significantly enhanced compared with those obtained under indirect heat removal. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3441–3450, 2015     

Extraction of methane hydrate energy by carbon dioxide injection-key challenges and a paradigm shift

Significant effort including field work has been devoted to develop a natural gas extraction technology from natural gas hydrate reservoirs through the injection of carbon dioxide. Natural gas hydrate is practically methane hydrate. The hypothesis is that carbon dioxide will be stored as hydrate owing to its favorable stability conditions compared to methane hydrate. Although the dynamics of the CO₂/CH₄ exchange process are not entirely understood it is established that the exchange process is feasible. The extent is limited but even if the CH₄ recovery is optimized there is a need for a CH₄/CO₂ separation plant to enable a complete cyclic sequence of CO₂ capture, injection and CH₄ recovery. In this paper we propose an alternative paradigm to the Inject (CO₂)/Exchange with (CH₄)/Recover (CH₄) one namely Recover (CH₄) first and then Inject (CO₂) for Storage.     

Storage capacity of hydrogen in tetrahydrothiophene and furan clathrate hydrates

The storage capacity of hydrogen in the tetrahydrothiophene and furan hydrates was investigated by means of pressure-volume-temperature measurement. The hydrogen-absorption rate of tetrahydrothiophene and furan hydrates is much larger than that of tetrahydrofuran hydrate in spite of same crystal structure (structure-II). The storage amount of hydrogen at 275.1 K is about 1.2 mol (hydrogen)/mol (tetrahydrothiophene or furan hydrate) (∼0.6 mass%) at 41.5 MPa, which is coincident with that of tetrahydrofuran hydrate.     

Storage capacity of hydrogen in tetrahydrofuran hydrate

The storage capacity of hydrogen in tetrahydrofuran hydrate was investigated by means of pressure-volume-temperature (p-V-T) measurement and Raman spectroscopic analysis. We carried out two measurement strategies using Raman spectroscopic analysis. One was isothermal pressure-swing absorption using tetrahydrofuran hydrate at 277.15 K, and the other was the preparation of a single crystal of hydrogen+tetrahydrofuran mixed gas hydrate from compressed hydrogen and tetrahydrofuran aqueous solutions along the stability boundary. The storage amount of hydrogen at 277.15 K obtained from the p-V-T measurement is about 1.6 mol (hydrogen)/mol (tetrahydrofuran) (about 0.8 mass%) at 70 MPa, and isothermal Raman spectroscopic measurement reveals that it reaches the maximum value of 2.0 mol (hydrogen)/mol (tetrahydrofuran) at about 85 MPa. These results agree well with those for a single crystal of hydrogen+tetrahydrofuran hydrate.     

Structural changes on the surfaces of borosilicate glasses induced by gamma-ray irradiation

Vitrification is a kind of glass that can solidify high-level radioactive waste (HLW). As the basic material of vitrification, borosilicate glass was studied extensively. To keep HLW away from the biosphere, the tolerance of borosilicate glass to irradiation is important. In this work, various samples of borosilicate glass with different compositions were irradiated with gamma rays at ambient temperature to study their stability. The hardness, moduli, and microscopic changes on surfaces of the borosilicate glasses were measured at specific absorbed doses. Upon the gamma irradiation, the structural changes on surfaces of borosilicate glasses were identified, which were strongly influenced by the composition of borosilicate glasses. The results demonstrate that gamma irradiation, as well as beta irradiation, might strongly influence the properties of vitrification. The irradiation effects on vitrification induced by gamma irradiation should be paid more attention to than before.