Dissociation Rate Of Methane Hydrate Research Articles

Experimental study on the intrinsic dissociation rate of methane hydrate

This work developed a new method that can efficiently separate gas-liquid-hydrate mixture and obtain solid gas hydrate, making it possible to measure the intrinsic dissociation rate of gas hydrate directly. The results demonstrate that the classical Kim’s model can reflect the relationship between driving force and hydrate dissociation rate when at same temperature, but can’t accurately reveal the effect of temperature on hydrate decomposition rate when under the same driving force. In Kim’s model, the driving force of hydrate dissociation was expressed as the difference between equilibrium fugacity (feq) and the fugacity of guest molecules in gas phase (fg). A new hydrate dissociation model was established in this work, and the driving force of hydrate dissociation was modified into (feq-fg)/feq, which has a higher universality than feq-fg. The intrinsic rate constant and activation energy of methane hydrate decomposition were determined to be 18905.35 mol/(m2‧s) and 22.149 kJ/mol, respectively.

Investigation on the coupling effects of mass transfer limitation and the presence of ice on methane hydrate dissociation inside pore

The exploitation of natural gas hydrates has bottlenecks such as low gas production efficiency and poor sustainability. The scientific issues behind this bottleneck are the continuous mass transfer of methane in the water and the presence of ice phase. A model is proposed to investigate the characteristics of mass transfer limitation and ice formation/melting during methane hydrate dissociation and their effects on hydrate dissociation. A numerical simulation has been carried out on the experimental research, and the reliability of proposed model has been validated by comparing with the experiment data, and consistent results have been obtained. Cumulative methane generation, methane generation rate and volume fraction of phases at local location, distribution of pressure, temperature and phases inside pore were clarified. The findings showed that the dissociation of methane hydrate is very complicated in the presence of the ice phase, and the effect of mass transfer limitation on the hydrate dissociation has become more complicated. These two factors have a great impact on the dissociation rate of methane hydrate. The original pressure and temperature distribution changes due to the ice formation/melting, resulting in the difference in the pressure and temperature at the inner and outer layer inside the pore. Ice formation in the inner layer weakens the delaying and limiting effect of mass transfer limitation on the hydrate dissociation, and ice melting in the outer layer promotes the delaying and limiting effect of mass transfer limitation on the dissociation of the hydrate. The presence of the ice phase weakens the mass transfer limitation effect.

Effect of the surfactants on hydrate formation during methane storage process

Enhanced hydrate formation rate by adding surfactant for achieving hydrate-based methane storage and transportation has drawn much attention in recent years. In this study, the effect of cocamidopropyl dimethylamine (CPDA), Sodium dodecyl sulfate (SDS), dodecyl trimethyl ammonium bromide (DTAB), and hexadecyl trimethyl ammonium bromide (HTAB) on induction time, the formation rate, water conversion ratio, and dissociation rate of methane hydrate have been investigated. And all the results have been compared with the water system without any additives. After adding surfactant into the aqueous solution, the hydrate formation is promoted, which lead to higher the hydrate formation rate and more water converted to be hydrate. 69.96wt% water was converted to be hydrate and its formation rate is 31.58mmol/min in 1.0wt%CPDA aqueous solution, in water without any additives system, it is 29.44wt%, 11.45mmol/min, respectively. Besides, the highest dissociation rate is also found in CPDA aqueous solution system, it is 42.2mmol/min and is 2.16 times than that of the pure water system.

Experimental study of methane hydrate dissociation in porous media with different thermal conductivities

Methane hydrate dissociation is an endothermic reaction. Therefore, one of important factors for methane hydrate dissociation is the heat transfer rate. In order to study the heat transfer characteristics of porous media on methane hydrate dissociation, experiments of methane hydrate dissociation using depressurization are conducted in porous media with different thermal conductivities, including quartz sand (0.926 W/(m·K)), white corundum (28.82 W/(m·K)) and silicon carbide (41.9 W/(m·K)). Experimental results show that, during the depressurization stage (DS), the temperature difference among different positions in quartz sand is larger than that in white corundum and silicon carbide. Since the heat transfer rate of quartz sand is smallest among three kinds of sand. A low temperature zone at the center of the reactor is observed in quartz sand compared to white corundum and silicon carbide. During the constant pressure stage (CPS), as the thermal conductivity of porous media increases, the temperature rising rate increases, and the duration of the CPS decreases. The dissociation rate of methane hydrate is controlled by the heat transfer rate of sediments during the CPS. The minimum gas production rate is obtained from the experimental in quartz sand, and the maximum gas production rate is obtained from the experiment in silicon carbide. This result indicates that the dissociation rate of hydrate increased with the increase of the thermal conductivity of porous media. Meanwhile, the overall rate constants (koverall) for different runs are calculated to quantify the dissociate rate of methane hydrate during the CPS. As the thermal conductivities of porous media increase, the overall rate constant of methane hydrate dissociation increases. The results of this study are important for understanding the effects of thermal conductivity of porous media on hydrate dissociation in actual field. Furthermore, it can also be used for the validation of numerical simulation in future.

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.

Dissociation behaviors of methane hydrate in marine sediments from South China Sea under constant pressure

The dissociation behaviors of methane hydrate under constant pressure in the marine sediments from the South China Sea were experimental studied. The experiments were carried out in a high-pressure reactor with the effective volume of 416 cm3. The marine sediments with mean pore diameter of 12.178 nm and total pore volume of 4.997×102 mL/g, surface area of 16.412 m2/g were used for the experiments. The mass fraction of water in the marine sediments is 40%. The methane hydrate was formed at the initial pressure of 14.4 MPa and dissociated by depressurization through the gas release from the reactor to the supply vessel with the volume of 1091 cm3. The moles of the gas released were calculated by the pressure change in the supply vessel. The dissociation experiments were carried out at the temperature range of 274.15–281.15 K and the dissociation pressure range of 3.0–5.0 MPa. The experimental results show that the hydrate begins to dissociate during the depressurization period, resulting in the significant decrease of the temperature in the reactor to be below the melting-point due to that the hydrate dissociation is an endothermal process. The lowest temperature during the hydrate dissociation in marine sediments is significantly lower than that in silica sands. The methane hydrate can dissociate when the pressure is higher than the equilibrium dissociation pressure for bulk methane hydrate. The dissociation rate of methane hydrate, the final gas dissociated from the hydrate and the final hydrate dissociation ratio increase with the decrease of dissociation pressure. For the experiments with different experimental temperature, the hydrate dissociation rate increases with the increase of the experimental temperature, and the final gas dissociated from hydrate is affected by the initial hydrate saturation in the marine sediments. The hydrate can dissociate completely when the dissociation pressure is lower than the equilibrium hydrate dissociation pressure corresponding to the experimental temperature for the bulk hydrate.

Methane Hydrate Intrinsic Dissociation Kinetics Measured in a Microfluidic System by Means of in Situ Raman Spectroscopy

The present work investigates the dissociation kinetics of methane (sI) hydrate in a thermoelectrically cooled microfluidic system with in situ Raman spectroscopy. The dissociation profile of methane (sI) hydrate was measured under different Reynolds numbers (0.42–4.16), pressures (60.2 to 80.1 bar), and temperature driving force ([Teq – 0.1 K] to [Teq + 0.3 K]). A theoretical model was derived from first-principles to describe the contributions of heat transfer and intrinsic kinetics on the dissociation rate of methane hydrate. It was observed that the dimensionless ratio of heat transfer to the intrinsic dissociation rate depended on the initial thickness of methane hydrate, temperature driving force, pressure, and the time. Intrinsic kinetics dominated where the initial thickness of methane hydrate was in the range of 10 μm and the temperature driving force was low. Increasing initial thickness of methane hydrate resulted in a switch to a heat-transfer-limited dissociation. Our results support that the “memory effect” previously reported is the result of dissociation limited by intrinsic kinetics. Furthermore, this work introduces a new laboratory method of unusually sensitive microscale control of the dissociation conditions, while generating molecular-level insight. Our findings support that microfluidics with in situ Raman spectroscopy are excellent laboratory tools to understand methane hydrate dissociation with potentially much broader utility to the field. Methane hydrates play an important role in energy production and storage and the atmospheric and oceanic sciences.

Direct numerical simulation of methane hydrate dissociation in pore-scale flow by using CFD method

The objective of this work is to establish a new pore-scale (m–μm) model for estimating the dissociation rate of methane hydrate (MH) synthesized in laboratory-scale sediment samples. Finite volume method (FVM) with unstructured mesh were constructed in a representative face-centered cubic unit. The surface model of MH reported by Sean et al. (2007) has been employed. In the bulk flow, concentration of methane in water flow was analyzed by computational fluid dynamics (CFD) method. However, only water flow and solid MH are simulated without considering ice or gas phase. In this study, tentative cases with porosity 0.74, 0.66, and 0.49 is individually considered as representative cubic unit of MH array. The initial temperature, 253.15 (K) of MH pellets inside the cubic unit dissociated due to the driving force of fugacity variation, ex. 0.56 and 0.54 (MPa) while warm water of 282.15 and 276.15 (K) flow in. In the calculation, periodic conditions are imposed at surfaces of inlet/right/front sides updated every time step. The flux of methane at the surface are all regarded as being dissolved into the water in this high pressure state, and compared to Kim et al. (1987)’s correlation at Reynolds no. of about 50. Results of dissociation flux in cases 5 and 6 with porosity 0.49 show good agreements with Kim’s correlation. However, as the porosity increases, flux increases due to the fast transport in bulk flow such as cases 1–4 in this study. If the transport process in bulk flow is faster than dissociation rate, then the surface flux becomes saturated as Reynolds no. greater than 100 in this work.

2-6 反応場の違いに対するメタンハイドレート分解速度の比較(Session 2 天然ガス)

2-6 反応場の違いに対するメタンハイドレート分解速度の比較(Session 2 天然ガス)

Dissociation behaviors of methane hydrate formed from NaCl solutions

The properties of gas hydrates with additives such as electrolytes and surfactants have been studied because their properties are greatly affected by additives. Although sodium chloride is a well known thermodynamic inhibitor of gas hydrate formation and a main component of seawater, little is known about its influence on gas hydrate dissociation. In this study, we formed methane hydrate containing sodium chloride up to 2.7 wt% and conducted a storage test at 253 K under ambient pressure. We used phase contrast X-ray computed tomography, cryo-SEM, and powder X-ray diffraction to investigate the stability of methane hydrate with up to a few percent of sodium chloride. It was revealed that the dissociation rate of methane hydrate containing sodium chloride was faster than that of pure methane hydrate. In addition, smaller particles of methane hydrate exhibited more influence of sodium chloride on dissociation. These experimental results obtained in this study may be caused by occurrences of NaCl solutions even at 253 K.

Effect of heat transfer on the kinetics of methane hydrate dissociation

The dissociation of methane hydrate under external pressure of 1bar is studied experimentally. Non-isothermal dissociation is fundamentally different from the quasi-isothermal case. The increase in the density of heat flux from 255 to 13700W m−2 results in 9-fold increase in the dissociation rate of methane hydrate. Different variations of clathrates dissociation may be observed depending on the heat flux magnitude: (1) without self-preservation (high heat fluxes), (2) a partial self-preservation with one minimum of dissociation rate, and (3) a partial self-preservation with two minimums (low heat fluxes). When describing dissociation kinetics of the spherical granules, it is important to know the time dependence of the ice layer thickness growth. It is shown that not the curvature, but the heat flux value regulates the dissociation rate and the change in diffusion. A drastic change in dissociation rate is caused by a pressure decrease in pores.

The Dependence of the Dissociation Rate of Methane-SDS Hydrate below Ice Point on Its Manners of Forming and Processing

The dissociation rates of methane hydrates formed with and without the presence of sodium dodecylsulfate (methane-SDS hydrates), were measured under atmospheric pressure and temperatures below ice point toinvestigate the influence of the hydrate production conditions and manners upon its dissociation kinetic behavior. The experimental results demonstrated that the dissociation rate of methane hydrate below ice point is strongly dependent on the manners of hydrate formation and processing. The dissociation rate of hydrate formed quiescentlywas lower than that of hydrate formed with stirring; the dissociation rate of hydrate formed at lower pressure was higher than that of hydrate formed at higher pressure; the compaction of hydrate after its formation lowered its stability, i.e., increased its dissociation rate. The stability of hydrate could be increased by prolonging the time periodfor which hydrate was held at formation temperature and pressure before it was cooled down, or by prolonging thetime period for which hydrate was held at dissociation temperature and formation pressure before it was depressurized to atmospheric pressure. It was found that the dissociation rate of methane hydrate varied with the temperature(ranging from 245.2 to 272.2 K) anomalously as reported on the dissociation of methane hydrate without the presence of surfactant as kinetic promoter. The dissociation rate at 268 K was found to be the lowest when the manners and conditions at which hydrates were formed and processed were fixed.

Stabilization of Methane Hydrate by Pressurization with He or N2 Gas

The behavior of methane hydrate was investigated after it was pressurized with helium or nitrogen gas in a test system by monitoring the gas compositions. The results obtained indicate that even when the partial pressure of methane gas in such a system is lower than the equilibrium pressure at a certain temperature, the dissociation rate of methane hydrate is greatly depressed by pressurization with helium or nitrogen gas. This phenomenon is only observed when the total pressure of methane and helium (or nitrogen) gas in the system is greater than the equilibrium pressure required to stabilize methane hydrate with just methane gas. The following model has been proposed to explain the observed phenomenon: (1) Gas bubbles develop at the hydrate surface during hydrate dissociation, and there is a pressure balance between the methane gas inside the gas bubbles and the external pressurizing gas (methane and helium or nitrogen), as transmitted through the water film; as a result the methane gas in the gas bubbles stabilizes the hydrate surface covered with bubbles when the total gas pressure is greater than the equilibrium pressure of the methane hydrate at that temperature; this situation persists until the gas in the bubbles becomes sufficiently dilute in methane or until the surface becomes bubble-free. (2) In case of direct contact of methane hydrate with water, the water surrounding the hydrate is supersaturated with methane released upon hydrate dissociation; consequently, methane hydrate is stabilized when the hydrostatic pressure is above the equilibrium pressure of methane hydrate at a certain temperature, again until the dissolved gas at the surface becomes sufficiently dilute in methane. In essence, the phenomenon is due to the presence of a nonequilibrium state where there is a chemical potential gradient from the solid hydrate particles to the bulk solution that exists as long as solid hydrate remains.

Effect of mixed compounds on methane hydrate formation and dissociation rates and storage capacity

To use gas hydrates for storage and transport of gas, the hydrate should be reasonably stable. Surfactants were found to increase the hydrate formation rate and storage capacity. On the other hand, they accelerate its decomposition rate. This study is concerned with methods of decreasing the dissociation rate of methane hydrate formed in the presence of sodium dodecyl sulfate (SDS). Using SDS solution with concentration of 500 ppm, the hydrate formation rate increased by the factor grater than 35 and the storage capacity was doubled. Below the ice point however, the stability of hydrate decreased effectively. While after 10 h 2.3 mol% of methane hydrate in pure water was dissociated at 268.2 K, the dissociation percentage was 9.7 wt.% for the hydrate formed in the SDS solution. By addition of a minor amount of xanthan or starch (so as to reach a concentration of 100–300 ppm in the solution) the dissociation rate of the methane hydrate was decreased effectively.

CFD and experimental study on methane hydrate dissociation Part I. Dissociation under water flow

AbstractDissociation processes of methane hydrate under water flow conditions were investigated by a combination of experimental observations and numerical simulations using computational fluid dynamics (CFD). In Part I of this study, the dissociation process induced by water flow at pressures above the three‐phase [hydrate (H)–liquid water (Lw)–vapor (V)] boundary in an isothermal x–P phase diagram is discussed. Dissociation experiments were carried out with a methane hydrate ball (diameter ≅ 10 mm) suspended in a flow cell, and the overall dissociation rate of methane hydrate without bubble formation was measured under various conditions of pressure, temperature, and volumetric flow rate of water. A linear phenomenological rate equation in the form of the product of the dissociation rate constant kbl and the molar Gibbs free energy difference ΔG, between the hydrate phase and the ambient aqueous phase, was derived by considering the Gibbs free energy difference as the driving force for the dissociation. The molar Gibbs free energy difference was expressed by the logarithm of the ratio of the concentration of methane dissolved in water at the hydrate surface to the solubility of methane in the aqueous solution in equilibrium with the hydrate. The dissociation rate constant kbl was determined from the experimental results of the overall dissociation rate combined with the numerical simulation results of the concentration profile of methane by the CFD method. The obtained dissociation rate constant was found to be independent of the ambient water flow rate, indicating that the rate constant is intrinsic for the hydrate dissociation within the conditions examined in this study. The rate constant was independent of the pressure, whereas the temperature dependency was described by an Arrhenius‐type equation with the apparent activation energy of 98.3 kJ/mol. © 2006 American Institute of Chemical Engineers AIChE J, 2007

Effect of different surfactants on methane hydrate formation rate, stability and storage capacity

The effects of anionic surfactants sodium dodecyl sulfate (SDS) and linear alkyl benzene sulfonate (LABS), cationic surfactant cetyl trimethyl ammonium bromide (CTAB) and non-ionic surfactant ethoxylated nonylphenol (ENP) on the formation, dissociation and storage capacity of methane hydrate have been investigated. Each surfactant was tested with 3 concentrations 300, 500 and 1000 ppm and it has been found that SDS, when prepared with these three concentrations speeds up the hydrate formation rate effectively. LABS increases the hydrate formation rate at 500 and 1000 ppm but decreases it at 300 ppm. CTAB and ENP have promotion effect on hydrate formation rate at 1000 ppm but decrease it at 300 and 500 ppm. Hydrate stability tests have been performed at three temperatures 268.2, 270.2 and 272.2 K with and without surfactant promoters. The results show that all tested additives increase the dissociation rate of methane hydrate below the ice point. CTAB has the minimum and LABS the maximum effect on the methane hydrate dissociation rate. Experimental results on hydrate gas content revealed that maximum storage capacity of 165 V/V is obtained with 1000 ppm of CTAB in water.

Effect of surfactant on the formation and dissociation kinetic behavior of methane hydrate

The effects of anionic surfactant sodium dodecyl sulfate (SDS) on the formation/dissociation kinetic behaviors of methane hydrate have been studied experimentally, with an emphasis put on dissociation kinetic behavior below ice point. The experimental results on hydrate formation show that the formation rates of methane hydrate could be speeded up by adding SDS to water and a critical SDS concentration of 650 ppm corresponding to a maximum storage capacity of 170V/V is determined. The SDS concentrations are fixed at this value in preparing hydrate samples for all dissociation tests. The dissociation experiments have been performed in two ways, at atmospheric pressure where the dissociation rates are determined by measuring the accumulative evolved gas volume, and in a closed system where the dissociation rates are determined by measuring the increasing system pressure profiles. For comparison, the dissociation tests with respect to two different cases, with and without the presence of SDS, are done in parallel. The results from tests in the first way show that the presence of SDS increases the dissociation rate of methane hydrate in whole temperature region below ice point. The results for the second way are somewhat different. The presence of SDS increases the dissociation rate and meta-stable system pressure in temperature region lower than 269.4 K . But when temperature is equal to or higher than 269.4 K , SDS speeds up the dissociation process only in beginning period, it turns to suppress the dissociation of methane hydrate several hours later and leads to a lower meta-stable system pressure compared with the case of without SDS. The experiments in closed system also demonstrate that the dissociating system approaches a meta-stable state with a pressure much lower than equilibrium dissociation pressure.

In Situ Raman Spectroscopy Investigation of the Dissociation of Methane Hydrate at Temperatures Just below the Ice Point

Dissociation kinetics of methane hydrates was investigated by using in situ Raman spectroscopy at temperatures just below the melting point of ice. Measurements of decomposition rates were performed using finely powdered hydrate samples with a diameter range of 100−250 μm. It was found that the dissociation rate of methane hydrate at 0.1 MPa is considerably faster than that at 0.25 and 0.5 MPa. A kinetic model using non steady-state approximation and a diffusion-controlled regime is presented for describing the decomposition behavior of methane hydrates. This approach would be effective for modeling the dissociation of methane hydrate in the early stages of dissociation. The diffusion coefficients of methane molecules through the hydrate surface coated with ice are estimated to be 1.7 × 10-13, 6.7 × 10-14, and 2.9 × 10-14 m2/s at 272.65, 271.15, and 268.15 K, respectively.

メタンハイドレートの分解過程の数値モデル(オーガナイズドセッション12 CO_2排出抑制技術に関する熱工学)

We developed a new model for the dissociation rate of methane hydrate (MH) at its surface. It was incorporated to our numerical flow simulation code, which features a finite volume method, unstructured mesh and very thin layer (VTL) cells at the surface. The VTL is aimed to predict the mass transfer of methane for high Schmidt number. For solving the unknown intrinsic dissociation rates in our model, we conducted the laboratory experiments of MH decomposition and obtained the bulk flux of methane into water. By applying the numerical code, the intrinsic dissociation rates were determined under various flow conditions of ambient water, such as pressure, temperature, and flow rate.