Hydrate Phase Equilibria of the Ternary CH4 + NaCl + Water, CO2 + NaCl + Water and CH4 + CO2 + Water Mixtures in Silica Gel Pores

Anti-Agglomeration Effects of Biodegradable Surfactants from Natural Sources on Natural Gas Hydrate Formation

Kinetic hydrate inhibitors (KHI) and anti-agglomerants (AA) rather than thermodynamic hydrate inhibitors (THI) are often used for flow assurance in pipelines. This is because they require much lower dosages than thermodynamic inhibitors. Although the hydrate-phase equilibria are not affected, KHI and AA prevent the formed hydrate crystals from growing to a bulky state causing pipeline blockage. However, these KHIs might have huge environmental impact due to leakages from the pipelines. In this study, two biodegradable AA candidates from natural sources (that is, lecithin and lanolin) are proposed and their performances are evaluated by comparing them with and without a conventional AA (Span 80, sorbitan monooleate). At 30% and 50% water cut, the addition of AA materials was found to enhance the flow characteristics substantially in pipelines and hardly affected the maximum value of the rotational torque, respectively. Considering the cost-effective and environmental advantages of the suggested AA candidates over a conventional AA such as Span 80, the materials are thought to have potential viability for practical operation of oil and gas pipelines. However, additional investigations will be done to clarify the optimum amounts and the action mechanisms of the suggested AAs.

Investigation on methane hydrate formation in silica gel particles below the freezing point

Herein, methane hydrate formation in silica gel was studied in the temperature and pressure range of 253.1–268.1 K and 4.0–6.0 MPa, respectively. The stability of the hydrate and the morphology of methane hydrate formed in silica gel were analyzed by P-XRD and cryo-SEM technology. An NG_t of 0.150 mol mol⁻¹ and the conversion of water to hydrate completely were realized at 253.1 K and 6 MPa. But the fastest NR₁₂₀ of 52.96 mol min⁻¹ m⁻³ and shortest T₉₀ of 160 min were achieved at 263.1 K and 6 MPa. The NG_t of 0.136 and 90.93% water conversion to hydrate were realized at 263.1 K and 6 MPa. The temperature range of 263.1–268.1 K was the optimal temperature for methane hydrate formation and dissolution. From P-XRD patterns and cryo-SEM images, it was confirmed that most of the cubic ice was formed on the silica gel surface and it was metastable. All the silica gel spherical surfaces were covered with intermittent ice particles. Most of the methane hydrate was formed on the interconnection surface between silica gel particles rather than on the single silica gel spherical surface. The methane hydrate formed on the silica gel surface decomposed faster than pure water methane hydrate.

Herein, methane hydrate formation in silica gel was studied in the temperature and pressure range of 253.1–268.1 K and 4.0–6.0 MPa, respectively.

Decomposition Characterizations of Methane Hydrate Confined inside Nanoscale Pores of Silica Gel below 273.15 K

The formation and decomposition of gas hydrates in nanoscale sediments can simulate the accumulation and mining process of hydrates. This paper investigates the Raman spectra of water confined inside the nanoscale pores of silica gel, the decomposition characterizations of methane hydrate that formed from the pore water, and the intrinsic relationship between them. The results show that pore water has stronger hydrogen bonds between the pore water molecules at both 293 K and 223 K. The structure of pore water is conducive to the nucleation of gas hydrate. Below 273.15 K, the decomposition of methane hydrate formed from pore water was investigated at atmospheric pressure and at a constant volume vessel. We show that the decomposition of methane hydrate is accompanied by a reformation of the hydrate phase: The lower the decomposition temperature, the more times the reformation behavior occurs. The higher pre-decomposition pressure that the silica gel is under before decomposition is more favorable to reformation. Thus, reformation is the main factor in methane hydrate decomposition in nanoscale pores below 273.15 K and is attributed to the structure of pore water. Our results provide experimental data for exploring the control mechanism of hydrate accumulation and mining.

Inhibited phase behavior of gas hydrates in graphene oxide: influences of surface and geometric constraints

Structural changes of water confined in graphene oxide to gas hydrates were investigated using low temperature XRD. It was revealed that the phase equilibrium points of gas hydrates strongly inhibited due to the surrounding nano-sized voids and the hydrophilic surface of graphene oxide.

Experimental measurement and thermodynamic modeling of the mixed CH4 + C3H8 clathrate hydrate equilibria in silica gel pores: effects of pore size and salinity

We measured hydrate phase equilibria for the ternary CH(4) (90%) + C(3)H(8) (10%) + water mixtures in silica gel pores with nominal diameters of 6.0, 15.0, 30.0, and 100.0 nm and for the quaternary CH(4) (90%) + C(3)H(8) (10%) + NaCl + water mixtures of two different NaCl concentrations (3 and 10 wt %) in silica gel pores with nominal diameters of 6.0, 15.0, and 30.0 nm. The CH(4) (90%) + C(3)H(8) (10%) hydrate-water interfacial tension (sigma(HW)) of 42 +/- 3 mJ/m(2) was obtained through the Gibbs-Thomson equation for dissociation within cylindrical pores. With this value, the experimental results were in good agreement with the calculated ones based on the van der Waals and Platteeuw model. A correction term for the capillary effect and a Pitzer model for electrolyte solutions were adopted to calculate the activity of water in the aqueous electrolyte solutions within silica gel pores. At a specified temperature, three-phase H-L(W)-V equilibrium curves of pore hydrates were shifted to higher-pressure regions depending on pore sizes and NaCl concentrations. From the cage-dependent (13)C NMR chemical shifts of enclathrated guest molecules, the mixed CH(4) (90%) + C(3)H(8) (10%) gas hydrate was confirmed to be structure II.

Gas hydrate growth and dissociation in narrow pore networks: capillary inhibition and hysteresis phenomena

Marine sediments hosting gas hydrates are commonly fine-grained (silts, muds, clays) with very narrow mean pore diameters (∼0.1 µm). This has led to speculation that capillary phenomena could play an important role in controlling hydrate distribution in the seafloor, and may be in part responsible for discrepancies between observed and predicted (from bulk phase equilibria) hydrate stability zone (HSZ) thicknesses. Numerous recent laboratory studies have confirmed a close relationship between hydrate inhibition and pore size, stability being reduced in narrow pores; however, to date the focus has been hydrate dissociation conditions in porous media, with capillary controls on the equally important process of hydrate growth being largely neglected. Here, we present experimental methane hydrate growth and dissociation conditions for synthetic mesoporous silicas over a range of pressure–temperature (PT) conditions (273–293 K, to 20 MPa) and pore size distributions. Results demonstrate that hydrate formation and decomposition in narrow pore networks is characterized by a distinct hysteresis: solid growth occurs at significantly lower temperatures (or higher pressures) than dissociation. Hysteresis takes the form of repeatable, irreversible closed primary growth and dissociation PT loops, within which various characteristic secondary ‘scanning’ curve pathways may be followed. Similar behaviour has recently been observed for ice–water systems in porous media, and is characteristic of liquid–vapour transitions in mesoporous materials. The causes of such hysteresis are still not fully understood; our results suggest pore blocking during hydrate growth as a primary cause.

Efficient recovery of CO2 from flue gas by clathrate hydrate formation in porous silica gels

Thermodynamic measurements and NMR spectroscopic analysis were used to show that it is possible to recover CO2 from flue gas by forming a mixed hydrate that removes CO2 preferentially from CO2/N2 gas mixtures using water dispersed in the pores of silica gel. Kinetic studies with 1H NMR microimaging showed that the dispersed water in the silica gel pore system reacts readily with the gas, thus obviating the need for a stirred reactor and excess water. Hydrate phase equilibria for the ternary CO2-N2-water system in silica gel pores were measured, which show that the three-phase hydrate-water-rich liquid-vapor equilibrium curves were shifted to higher pressures at a specific temperature when the concentration of CO2 in the vapor phase decreased. 13C cross-polarization NMR spectral analysis and direct measurement of the CO2 content in the hydrate phase suggested that the mixed hydrate is structure I at gas compositions of more than 10 mol % CO2, and that the CO2 molecules occupy mainly the more abundant 5(12)6(2) 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. 1H NMR microimaging showed that hydrate yields of better than 85%, based on the amount of water, could be obtained in 1 h when a steady state was reached, although approximately 90% of this yield was achieved after approximately 20 min of reaction time.