Self-preservation Effect Research Articles

Promoted Disappearance of CO2 Hydrate Self-Preservation Effect by Surfactant SDS

The capture, storage and utilization of CO2 through hydrate-related technology is a promising approach to addressing the global warming issue. Dissociation is required after the transportation of CO2 gas in the form of a self-preserving hydrate. In order to investigate the dissociation behaviors as the self-preservation effect is removed, CO2 hydrates were frozen, and then the self-preservation effect was removed through uniform heating. An evident dependence of hydrate dissociation duration on the initial dissociation rates after losing the preservation effect was observed. The results in the silica gel powder and sodium dodecyl sulphate solution showed significant reductions in the initial dissociation temperatures and a slight decrease in the initial dissociation rates when compared with those of pure water. The reductions in the former were 2.88, 2.89, and 5.73 °C in silica gel, sodium dodecyl sulphate, and a combination of the two, respectively, while the reductions in the latter were 0.12, 0.12, and 0.16 mmol/min, respectively. As the results are inconsistent with the conventional mechanism elucidating a self-preservation effect, the ice shell theory was hence further supplemented by introducing innovative contribution factors—nonenclathrated liquid water and gas molecules dissolved inside. These findings are expected to provide references for CO2 gas transportation and usage of the self-preservation effect.

The depressurization of natural gas hydrate in the multi-physics coupling simulation based on a new developed constitutive model

Natural gas hydrate has become a hotspot for researchers because of its considerable economic benefits and potential strategic significance. In the hydrate exploitation process, the hydrate-bearing sediments get damaged, and the coupling effect of multi-physical fields is involved. In this study, a damage constitutive model of hydrate-bearing sediments is established by considering decomposition damage, loading damage, and the influence of residual strength. Based on this damage constitutive model, a Thermo-Hydro-Chemo-Mechanical (THCM) multi-field coupling model considering the sediment damage is constructed on the COMSOL simulation platform. Compared with the experimental data, the THCM coupling model is demonstrated and employed to study the depressurization process of horizontal wells in permafrost regions. During the hydrate exploitation model, changes in displacement, saturation, average pore pressure, and temperature at different mining positions are recorded and analyzed. The results indicate that the damaging of sediments has a significant effect on soil deformation and temperature. After the sediment damaging effect is considered, the minimum temperature of the extraction points is reduced by 22.5% at most. The maximum deformation of the extraction points is about 11.8% greater than that without considering the sediment damage effect. Moreover, the model reveals the self-preservation effect of gas hydrate. The temperature of the surrounding soil at the exploitation site decreases due to hydrate decomposition, which would inversely promote hydrate formation. These numerical models in this study can be applied to estimate the strength of sediments and predict the reservoir settlement in the process of hydrate depressurization.

Investigating regularities of gas hydrate ignition on a heated surface: Experiments and modelling

Interrelated heat transfer processes are numerically studied under conditions of chemical reactions and exothermic and endothermic phase transformations at methane gas hydrate ignition in the course of conductive heating. The dependences of the main characteristic of the process – the ignition delay time on the heating surface temperature, the sample heating rate, the activation energy of the fuel vapour oxidation reaction and the pre-exponential multiplier of the fuel vapour oxidation reaction are established. The limiting values of the main parameters of the energy source, under which the ignition conditions are implemented steadily, are revealed. It is shown that as the heater surface temperature increases, the ignition delay times decrease non-linearly. Self-preservation of the gas hydrate leads to an increase in the ignition delay time. The effect of self-preservation depends on the initial temperature of the heater.

Self-preservation Effect of Gas Hydrates

This work was performed to improve the storage and transportation technology of gas hydrates in nonequilibrium conditions. At atmospheric pressure and positive ambient temperature, they gradually dissociate into gas and water. Simulation of the gas hydrate dissociation will determine optimal conditions for their transportation and storage, as well as minimize gas loss. Thermodynamic parameters of adiabatic processes of forced preservation of pre-cooled gas hydrate blocks with ice layer were determined theoretically and experimentally. Physical and mathematical models of these processes were proposed. The scientific novelty is in establishing quantitative characteristics that describe the gas hydrates thermophysical parameters thermophysical characteristics influence on the heat transfer processes intensity on the interphase surface under conditions of gas hydrates dissociation. Based on the results of experimental studies, approximation dependences for determining the temperature in the depths of a dissociating gas hydrate array have been obtained. Gas hydrates dissociation mathematical model is presented.

Insights into the self-preservation effect of methane hydrate at atmospheric pressure using high pressure DSC

The self-preservation effect of CH4 hydrate can facilitate the solidified storage and transport of natural gas at mild temperature and pressure conditions. However, the factors affecting the self-preservation of CH4 hydrate are complex and the impacts are not well known. In this work, a high pressure micro-differential scanning calorimeter (HP μ-DSC) was employed to elucidate the impact of preservation time, preservation temperature, and heating rate on the self-preservation of CH4 hydrate at atmospheric pressure (0.1 MPa) from the perspective of thermal analysis. It was found that CH4 hydrate can be stably preserved at 0.1 MPa in the temperature range from 253.15 to 268.15 K, and the self-preservation effect of CH4 hydrate was independent of the preservation time while no heat was injected into the preservation system. This will avail long-distance storage and transport of natural gas under low energy consumption conditions. In addition, two dissociation regimes of CH4 hydrate were identified when varying the heating rate from 0.1 K/min to 1 K/min. At the small heating rate of 0.1 K/min, the ice layer melted prior to the dissociation of CH4 hydrate. When the heating rate was increased to 0.5 and 1 K/min, the ice layer and CH4 hydrate melted simultaneously. Therefore, a large heating rate can be used to recover CH4 quickly from the dissociation of CH4 hydrate and thus the operating time will be saved.

Theoretical study of the self-preservation effect in CF4 gas hydrates

This work studies thermodynamic properties of CF4 gas hydrates using the statistical thermodynamic model of inclusion compounds, developed in our laboratory and closely associated with Quasiharmonic Lattice Dynamic (QLD) and Molecular Dynamic Simulation (MD). Thermal expansion coefficient of CF4 gas hydrate is calculated at different hydrate composition. The thermal expansion coefficient is shown to be lesser than the thermal expansion coefficient of CH4 hydrates, but larger than ice Ih, that still indicates the presence of the self-preservation effect. The thermal expansion coefficient is very sensitive to the composition of the hydrate and can vary in a wide range. The same is true for the hydrate lattice parameter as well; it is shown that at certain conditions it may be even less than for hypothetical empty hydrate structure. The self-preservation effect may be applied for many aspects related to CF4 compound, especially for storage and transportation purposes in producing semiconductor electronics.

Thermal expansion of ice based gas hydrates

This work aims at studying thermal expansion of ice Ih based gas hydrates that play important role in self-preservation effect of classical gas hydrates. Linear thermal expansion of ice Ih based hydrogen hydrates are calculated using a very effective statistical thermodynamic model of gas hydrates developed by the authors of this work. Molecular structures of all studied systems are obtained by MD simulations, and all parameters required by the model are obtained by using Quasiharmonic Lattice Dynamic approach. In accordance with our calculations, introduction of hydrogen into the system not only significantly affects the unit cell parameter of ice, but also increases linear thermal expansion and makes it comparable to the classical gas hydrates such as CH4, CO2, etc.

In situ PXRD analysis on the kinetic effect of PVP-K90 and PVCap on methane hydrate dissociation below ice point

Kinetic hydrate inhibitors (KHIs) have been developed as an alternative for the prevention of hydrate plug in natural gas transportations due to economic and environmental factors. To understand the kinetic performance of KHIs comprehensively, dissociations of CH4 hydrates in the presence of PVP-K90 and PVCap were investigated below ice point. Cryo-SEM and in situ PXRD were used to provide a microscopic insight on the dissociation kinetics. Results showed that a typical CH4 hydrate dissociation at 268 K could be divided into 4 stages. The self-preservation effect took place in the second stage accompanied with an rise in the ratio of Ih(002) to Ih(100) peaks from 0.5 to 1.1. About 30% of CH4 hydrates were suggested to dissociate into plate-like ice to form ice coatings on hydrate surface. In the presence of PVP-K90 or PVCap, the self-preservation stage reduced greatly and the initial dissociation rates of CH4 hydrates were found enhanced as the concentrations of the KHIs increased from 0.5 to 2.0 wt%. SEM images revealed that PVP-K90 was suggested to be included in the small ice crystals embedded in hydrate phase and hinder the connections of the plate-like ice crystals on hydrate surface, while PVCap was found to induce a dendritic growth of CH4 hydrate, leading to a surge in the specific surface area of CH4 hydrates which was not beneficial to the formation of ice coatings on hydrate phase. Therefore, both PVP-K90 and PVCap promoted hydrate dissociation below ice point by inhibiting the formation of self-preservation effect.

Experimental research on self-preservation effect of methane hydrate in porous sediments

The self-preservation is considered as an advantageous property for natural gas hydrate transportation and storage. However, it may also bring serious troubles for well drilling and hydrate exploitation. In this work, various factors affecting the self-preservation effect of CH4 hydrate were investigated by using HP μ-DSC. The results indicate the presence of porous sediments does not influence the anomalous self-preservation region of CH4 hydrate. The CH4 hydrate dissociation rate increases with the decreased initial water content and quartz sand particle size as a whole. However, the self-preservation could be still found in a very low initial water content (10 vol%) and small-particle sediments (25–38 μm) condition. On the other hand, an enhanced self-preservation effect and excessive pressure phenomenon were unexpectedly found in bentonite and kaolin. The hydrate can still maintain high metastability with hardly any decomposition after the pressure was released to atmospheric pressure. In situ Raman and CCD camera were used for the further study of the mechanism. We speculate the interaction of hydrogen bonds between bentonite and hydrate might be the main reason for this abnormal phenomenon. The knowledge gained in this work is significant for the comprehension of CH4 hydrate dissociation in porous sediments below ice point, and provides a better understanding of the self-preservation mechanism.

Adsorption-Hydration Sequence Method for Methane Storage in Porous Material Slurry.

Porous materials are deemed to be capable for promoting hydrate formation, while for the purpose of hydrate-based gas storage, those systems containing porous materials often cannot meet the requirement of high storage density. To increase the storage density, an adsorption-hydration sequence method was designed and systematically examined in this study. Methane storage and release in ZIF-8 slurries and fixed beds were investigated. The ZIF-8 retained 98.62%, while the activated carbon lost 62.17% of their adsorption capacities in slurry. In ZIF-8 fixed beds, methane storage density of 127.41 V/Vbed was acquired, while the gas loss during depressurization accounted for 21.50% of the gas uptake. In the ZIF-8 slurry, the storage density was effectively increased with the adsorption-hydration sequence method, and the gas loss during depressurization was much smaller than that in fixed beds. In the slurry, the gas uptake and gas loss decreased with the decrease of the chilling temperature. The largest gas uptake and storage density of 78.84 mmol and 133.59 V/Vbed were acquired in the slurry with ZIF-8 content of 40 wt.% at 268.15 K, meanwhile, the gas loss just accounted for 14.04% of the gas uptake. Self-preservation effect was observed in the slurry, and the temperature for the slowest gas release was found to be 263.15 K, while the release ratio at 10 h reached to 43.42%. By increasing the back pressure, the gas release rate could be effectively controlled. The gas release ratio at 1.1 MPa at 10 h was just 11.08%. The results showed that the application of adsorption-hydration sequence method in ZIF-8 slurry is a prospective manner for gas transportation.

Metastable state of gas hydrate during decomposition: A novel phenomenon

Abstract Natural gas hydrates are solid compounds with cage-like structures formed by gas and water. An intriguing phenomenon that gas hydrates can dissociate at a low rate below the ice freezing point has been viewed as the metastability of hydrate. The mechanisms of hydrate metastability have been widely studied, and many mechanisms were proposed involving the self-preservation effect, supercooled water-gas-hydrate metastable equilibrium, and supersaturated liquid–gas-hydrate system etc. The metastable state of hydrate could be of crucial significance in the kinetics of hydrate formation and decomposition, heat and mass transfer during gas production processes, and the application of hydrate-based technique involving desalination, energy storage and transportation, and gas separation and sequestration. Few researches have systematically considered this phenomenon, and its mechanism remains unclear. In this work, various mechanisms and hypothesis explaining the metastable state of gas hydrates were introduced and discussed. Further studies are still required to reveal the intrinsic nature of this metastable state of gas hydrate, and this work could give some implications on the existing theory and current status of relevant efforts.

Diffusion model of gas hydrate dissociation into ice and gas that takes into account the ice microstructure

A diffusion model of gas hydrate dissociation into ice and gas is presented, which allows us to simulate the self-preservation effect of gas hydrates. This model takes into account that the forming ice layer has a porous structure that changes during the process of gas hydrate dissociation. Three problems of gas hydrate dissociation into ice and gas were considered separately: the dissociation of a spherical gas hydrate particle, the dissociation of a cylindrical gas hydrate, and the dissociation of a flat gas hydrate layer. For each problem, a solution in the framework of a quasi-stationary approximation is obtained. From the comparison of the calculated data with the available experimental data on the kinetics of dissociation of a powdery methane hydrate into ice and methane, the molecular diffusion coefficient of methane in ice and the diffusion coefficient of methane through the system of pores in the forming ice layer were estimated. Based on this estimation, an explanation of the anomalous preservation thermal regime of methane hydrate is provided.

Theoretical investigation of nitrogen gas hydrates outside of stability zone

Using a combination of Quasiharmonic Lattice Dynamic and Molecular Dynamic Simulation methods in conjunction with well developed statistical thermodynamics theory the thermodynamic properties of nitrogen gas hydrates has been calculated, with the main focus on the thermal expansion coefficient. It is shown that nitrogen gas hydrate displays larger value of the thermal expansion coefficient in comparison with hexagonal ice Ih and empty gas hydrate lattice, moreover this value is very close to one of the carbon dioxide and to other hydrates that have self-preservation effect. Exactly because of this difference in the thermal expansion coefficient it is possible to confirm existence of the self-preservation effect. The self-preservation effect can be used for creation of low-cost technology of hydrates storage and transportation technology applicable for many gas types.

Mathematical Model of the Effect of Self-Preservation of Gas Hydrates

A diffusion model of dissociation of a plane layer of gas hydrate into ice and gas has been presented, which permits modeling the effect of self-preservation of gas hydrates. In this model, the gas-hydrate dissociation into ice and gas is described with account taken of the internal kinetics of the process and of the pore structure of the formed ice layer. Calculated data obtained within the framework of a quasi-stationary approximation for the cases of dissociation of plane layers of methane hydrate and carbon-dioxide hydrate into ice and gas have been given as an example.

Synthesis of Methane Hydrate from Ice Powder Accelerated by Doping Ethanol into Methane Gas

Clathrate hydrate is considered to be a potential medium for gas storage and transportation. Slow kinetics of hydrate formation is a hindrance to the commercialized process development of such applications. The kinetics of methane hydrate formation from the reaction of ice powder and methane gas doped with/without saturated ethanol vapor at constant pressure of 16.55 ± 0.20 MPa and constant temperature ranging from −15 to −1.0 °C were investigated. The methane hydrate formation can be dramatically accelerated by simply doping ethanol into methane gas with ultralow ethanol concentration (<94 ppm by mole fraction) in the gas phase. For ethanol-doped system 80.1% of ice powder were converted into methane hydrate after a reaction time of 4 h, while only 26.6% of ice powder was converted into methane hydrate after a reaction time of 24 h when pure methane gas was used. Furthermore, this trace amount of ethanol could also substantially suppress the self-preservation effect to enhance the dissociation rate of methane hydrate (operated at 1 atm and temperatures below the ice melting point). In other words, a trace amount of ethanol doped in methane gas can act as a kinetic promoter for both the methane hydrate formation and dissociation.

Use of Transformer Oil and “Dry Water” to Store and Transport Methane Hydrate

In this paper a new system is proposed for the storage and transportation of gas in the hydrate form (for the case of methane hydrate). The use of solid particles (Aerosil ® R202) is most effective as promoters of hydrate formation for the intended purposes. The ratio of water to transformer oil in the initial systems was constant (1:1 by weight); the content of Aerosil® R202 in “dry water” was 5 wt. %. From the obtained results it follows that the presence of “dry water” in the system accelerates the formation of the hydrate, while the transformer oil provides a self-preservation effect and thereby slow decomposition of the hydrate phase. The proportion of methane hydrate subjected to self-preservation in the presence of transformer oil is 80 times higher than for the hydrate obtained from “dry water” without any additives. The obtained data can be used in the development of hydrate methods of gas storage and transportation.

Design of Small Nanoparticles Decorated with Amphiphilic Ligands: Self-Preservation Effect and Translocation into a Plasma Membrane.

Design of nanoparticles (NPs) for biomedical applications requires a thorough understanding of cascades of nano-bio interactions at different interfaces. Here, we take into account the cascading effect of NP functionalization on interactions with target cell membranes by determining coatings of biomolecules in biological media. Cell culture experiments show that NPs with more hydrophobic surfaces are heavily ingested by cells in both the A549 and HEK293 cell lines. However, before reaching the target cell, both the identity and amount of recruited biomolecules can be influenced by the pristine NPs' hydrophobicity. Dissipative particle dynamics (DPD) simulations show that hydrophobic NPs acquire coatings of more biomolecules, which may conceal the properties of the as-engineered NPs and impact the targeting specificity. Based on these results, we propose an amphiphilic ligand coating on NPs. DPD simulations reveal the design principle, following which the amphiphilic ligands first curl in solvent to reduce the surface hydrophobicity, thus suppressing the assemblage of biomolecules. Upon attaching to the membrane, the curled ligands extend and rearrange to gain contacts with lipid tails, thus dragging NPs into the membrane for translocation. Three NP-membrane interaction states are identified that are found to depend on the NP size and membrane surface tension. These results can provide useful guidelines to fabricate ligand-coated NPs for practical use in targeted drug delivery, and motivate further studies of nano-bio-interactions with more consideration of cascading effects.

Thermal Conductivity of Frozen Sediments Containing Self-Preserved Pore Gas Hydrates at Atmospheric Pressure: An Experimental Study

The paper presents the results of an experimental thermal conductivity study of frozen artificial and natural gas hydrate-bearing sediments at atmospheric pressure (0.1 MPa). Samples of hydrate-saturated sediments are highly stable and suitable for the determination of their physical properties, including thermal conductivity, due to the self-preservation of pore methane hydrate at negative temperatures. It is suggested to measure the thermal conductivity of frozen sediments containing self-preserved pore hydrates by a KD-2 needle probe which causes very little thermal impact on the samples. As shown by the special measurements of reference materials with known thermal conductivities, the values measured with the KD-2 probe are up to 20% underestimated and require the respective correction. Frozen hydrate-bearing sediments differ markedly in thermal conductivity from reference frozen samples of the same composition but free from pore hydrate. The difference depends on the physical properties of the sediments and on changes in their texture and structure associated with the self-preservation effect. Namely, it increases proportionally to the volumetric hydrate content, hydrate saturation, and the percentage of water converted to hydrate. Thermal conductivity is anisotropic in core samples of naturally frozen sediments that enclose visible ice-hydrate lenses and varies with the direction of measurements with respect to the lenses. Thermal conductivity measurements with the suggested method provide a reliable tool for detection of stable and relict gas hydrates in permafrost.

Microscopic measurements on the decomposition behaviour of methane hydrates formed in natural sands.

In this work, the decomposition behaviour of methane hydrate in porous media was investigated microscopically using powder X-ray diffraction, cryogenic scanning electron microscopy and in situ Raman spectroscopy. The effect of grain sizes on the decomposition of methane hydrate was measured. The results showed that bulk hydrates could exist stably at 223 K and atmospheric pressure because of the self-preservation effect. However, hydrate formed in sands was relatively easier to decompose because it had a higher equilibrium pressure compared with bulk hydrate at the same temperature. In this case, there would be a higher decomposition driving force. Interestingly, the complete decomposition time for hydrate formed in sands did not decrease with the decrease in particle size. The shortest decomposition time was observed for the sands with the particle size range of 38–55 μm, which was less than 30 minutes. Moreover methane hydrate was found to decompose faster in the porous medium containing 3.5 wt% NaCl, which suggested that there was almost no self-preservation effect. In situ Raman measurements showed that the integrated intensity ratio of methane in large and small cages (AL/AS) did not change during the decomposition process, suggesting that the methane hydrate crystal units decomposed as an entity in sands. This study provided important data as a basis for drilling fluid technology in hydrate mining.

Macroscopic defects upon decomposition of CO2 clathrate hydrate crystals.

Micrometer- and submicrometer-sized pores and macroscopic defects like cracks and tubular channels can be found in a variety of clathrate hydrates (hydrates for short) during formation and decomposition. Their origin, their evolution in time, and their effect on hydrate decomposition kinetics are unclear. We used time-lapse micro computed tomography (μCT) in combination with temperature control and pressure monitoring to study the formation and evolution of pores and macroscopic defects in decomposing CO2 hydrates at subzero (Celsius) temperature. Our results suggest that the decomposition of hydrates is always accompanied by the formation of pores and an increase of the apparent hydrate volume by more than 3%. Hydrate decomposition often starts with the formation of cracks inside the hydrate and not necessarily at the free surface of the hydrate, which frequently remains intact for a long period and seems to be self-preserved in some regions. Decomposition spreads out from these cracks in a uniform fashion yielding a variety of macroscopic features. In some cases, the propagating decomposition front seems to get blocked by planar barriers inside the hydrate yielding regions with high resistance against decomposition. This, together with a generally heterogeneous distribution of decomposition resistant regions, challenges the shrinking core model of hydrate decomposition as well as the popular ice-rind theory used to explain the effect of self-preservation.