Efficient carbon capture is an essential step in many energy-related processes. Here, we use molecular dynamics simulations and free energy analysis to investigate the inherent implication of the ZIF-8/glycol slurry based adsorption and absorption hybrid technique for carbon capture. Our results reveal that the formation of two-layer ordered hydrogen bond (HB) networks of glycol molecules on the ZIF-8 surface is the physical origin of the high efficiency of using ZIF-8/glycol slurry for carbon capture. It is found that the film composed of two-layer HB networks acts as a selective gatekeeper, allowing the penetration of CO2 molecules but efficiently blocking CH4. The interaction between the HB-forming solvent and ZIF-8 is the key to the formation of the semipermeable film, while the solute–solvent interaction is essential for film crossing. Finally, we discuss the basis for the design of highly efficient nanopore/slurry system for filtering and separation technologies. The uncovered mechanism for the hybrid technique opens up an exciting strategy for highly efficient CO2 separation.

•Hydrate formation conditions in pure water and methanol-water mixtures were measured.•The inhibiting effect of methanol on hydrate formation conditions was investigated.•The performance of four existed hydrate models for three sour gases was inspected.•A method for predicting hydrate formation in methanol-water solutions was proposed.Preventing hydrate plugging is of great concern in the natural gas production and transportation industries. In this paper, hydrate formation conditions of three CH4/CO2/H2S/N2 sour gases with different H2S and CO2 contents in both distilled water and methanol-water solutions were measured. The temperature range was varied from 270.15 to 292.15 K, and three mass fractions (15 wt%, 20 wt% and 30 wt%) of methanol in methanol-water solutions were considered. The experimental results showed that the hydrate formation conditions of tested quaternary sour gases are related to the concentrations of gas components to a large extent, in which H2S played the most important role. The addition of methanol in water significantly increased the gas mixtures' hydrate formation pressures, in which the inhibiting effect of methanol on the hydrate formation pressures showed a nonlinear relation to its concentration in the aqueous solution. For simulating the hydrate formation of tested sour gases in distilled water, both the PVTSim and Chen-Guo model showed acceptable prediction precisions in water, but they did not perform well for the methanol-water mixtures. At the same time, a calculation method based on the original Chen-Guo model for predicting the hydrate formation conditions of CH4/CO2/H2S/N2 quaternary sour gases in methanol-water solutions was proposed in this work. The modeling results are in excellent agreement with the experimental data.

•2-methylimidazole-glycol solution is proposed to capture CO2.•CO2 solubility in 2-methylimidazole-glycol solution was measured.•CO2 separation efficiency in 2-methylimidazole-glycol was investigated.•CO2 capture mechanism was determined through Fourier-transform infrared technology.Here, we report the fabrication of a stable CO2 capture medium by dissolving 2-methylimidazole in glycol. It was found that 2-methylimidazole dissolved in glycol well, and their mixture showed good flowability. For single CO2 absorption in the 10 wt% 2-methylimidazole-glycol solution, the molar ratio between CO2 and 2-methylimidazole molecules reached 0.79 at 293.15 K and 18.73 bar. It was supposed that an alkylcarbonate salt formed upon contact between 2-methylimidazole, glycol and CO2. This CO2 absorption mechanism was further characterized by using the Fourier-transform infrared technology. For CO2/N2 (20.65/79.36 mol%) gas mixture separation in the 30 wt% 2-methylimidazole-glycol solution, the equilibrium partial pressure of CO2 in the gas phase could be decreased to as low as 0.10 bar with the selectivity of CO2 over N2 reaching 239 at 293.15 K and 4.55 bar. More importantly, the CO2 uptake in 2-methylimidazole-glycol solution was reversible, CO2 was readily desorbed from the solution through vacuuming at room temperature and with no apparent liquid loss. This work opens up the possibility of applying imidazole-alcohols mixtures to CO2 capture.

By using Zeolitic imidazolate framework-67(ZIF-67)/water-ethylene glycol slurry, we separated methane/ethylene gas mixtures by the absorption-adsorption method. The influences of temperature, mass fraction of ZIF-67 in ZIF-67/water-ethylene glycol slurry, pressure, and gas-slurry ratio on the separation performance were studied systematically. We found lower temperature, mass fraction of ZIF-67 of 0.15, higher gas–liquid ratio, and higher operation pressure are suitable for separation of methane/ethylene mixture. The experimental results show by using the slurry we synthesized, the mole fraction of ethylene in gas phase decreases from 74.8 to 53.0%, the selectivity coefficient reaches 10 (much higher than that of solid ZIF-67), and more than 68% of C2H4 can be recovered after one stage of the absorption-adsorption process. Sorption enthalpy of methane and ethylene at low loading is only 11 and 17 kJ/mol, respectively. In addition, the crystal structure of ZIF-67 remains intact after 20 adsorption/desorption cycles.

The performance of zeolitic imidazolate framework-8 (ZIF-8) for CO2 capture under three different conditions (wetted ZIF-8, ZIF-8/water slurry, and ZIF-8/water–glycol slurry) was systemically investigated. This investigation included the study of the pore structure stability of ZIF-8 by using X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and Raman detection technologies. Our results show that the CO2 adsorption ability of ZIF-8 could be substantially increased under the existence of liquid water. However, the structure characterization of the recovered ZIF-8 showed an irreversible change of its framework, which occurs during the CO2 capture process. It was found that there is an irreversible chemical reaction among ZIF-8, water, and CO2, which creates both zinc carbonate (or zinc carbonate hydroxides) and single 2-methylimidazole crystals, and therefore the pore structure of ZIF-8 collapses. It is suggested therefore that care must be taken when using ZIF-8 or products containing ZIF-8 for gas capture, gas separation, or other applications where both water and acid gases coexist.

The self-preservation effect experiments in water + diesel oil dispersion systems for methane hydrate were carried out with the particle size ranging from tens to more than 100 μm. The influence of water cuts (low water cuts of 10, 20, and 30 vol % or high water cuts of 95, 99, and 100 vol %) and types of inhibitors (tetra-n-butylammonium bromide or Lubrizol) on the dissociation kinetics in oil and water suspensions were examined. The addition of surfactants, especially those able to lower the size of droplets or hydrate particles in low water cut suspension systems, could remarkably hinder the self-preservation effect by surface adsorption and alterations in structures and morphologies of ice film. For higher water cut systems with or without surfactants, the enhanced self-preservation effect was observed in comparison to lower water cuts. Systems with oil exhibited a declined effect in contrast to pure water systems. The ice-shielding mechanism for hydrate dissociation is supported by the effects of surfactants and water cuts on dispersion and agglomeration properties as well as the size of hydrate particles.

Two new kinetic hydrate inhibitors (KHIs) named PVP-B and PVP-BP were synthesized successfully and compared with commercial KHIs such as PVP and Inhibex 55W. PVP-B and PVP-BP are both the ramifications of PVP. They were obtained by introducing a t-butyl group and both t-butyl and phenyl groups into PVP molecules, respectively. The hydrate inhibition performances of KHIs were assessed in a sapphire cell through two kinds of onset times of hydrate formation: TVO, the time when hydrate crystals are initially observed by naked eyes, and TPD, the time when rapid and continuous dropping of system pressure begins. TVO was used to evaluate an KHI’s inhibition performance to hydrate nucleation. The length of time period from TVO to TPD was used to evaluate a KHI’s inhibition performance to hydrate growth. The results demonstrate that both PVP-B and PVP-BP are superior to PVP and Inhibex 55W; PVP-BP has better inhibition performance than PVP-B because of the stronger steric hindrance effects of the phenyl group in PVP-BP molecules than those of the t-butyl group in PVP-B molecules. Additionally, we found TVO does not always increase with the increasing dosage of KHI while TPD does. The most suitable dosage of PVP-BP was determined to 0.5 wt % or so, at which the nucleation of hydrate is inhibited most perfectly. Finally, we demonstrated glycol could be used as synergist to improve the performance of PVP-BP remarkably. The most suitable dosage of glycol was determined to be 0.9 wt % or so. Our work presents not only two new KHIs but also important insights into the inhibition mechanism of KHIs.

CO2-in-water emulsions formed from polyoxyethylene sorbitan monooleate (Tween 80) and sodium dodecyl sulfate (SDS) were evaluated in terms of stabilization time and emulsion droplet distribution. A CO2 emulsion with 0.5 wt % SDS + 5 wt % Tween 80 was found to be more stable than the other emulsions. With an increase in the stirring rate or pressure or a decrease in the temperature, the size of CO2 emulsion droplets tended to be smaller, and the stabilization time of the CO2 emulsion increased. With the optimized CO2 emulsion, the CH4–CO2 replacement reaction in a hydrate-bearing quartz sand sample was performed in a three-dimensional reactor. In the emulsions, the function of Tween 80 is to make the emulsion much more hydrophilic and reduce the flow resistance of the emulsion, whereas that of SDS is to make the newly formed hydrate particles/layers more granular and looser. The results indicate that, for hydrate reservoirs located in the stability fields of both CO2 hydrate and methane hydrate, the replacement efficiency of the CO2 emulsion can reach 47.8%, higher than those of liquid CO2 and gaseous CO2. The addition of 3.35 wt % salt to the emulsion can prevent the pore spaces of sediments from being blocked.

•Water-in-oil (W/O) emulsion was used for the separation of CO2/H2 mixtures.•The synergistic effect of CP and TBAB was used to improve separation ability.•The maximum separation factor of CO2 over H2 reached 103.•H2 could be purified from 53.2 to 97.8 mol% through a two-stage separation.CO2/H2 mixtures, such as integrated gasification combined cycle (IGCC) syngas, were separated via hydrate formation in water-in-oil (W/O) emulsions. The oil phase was composed of diesel and cyclopentane (CP). Span 20 was used to disperse the aqueous phase or hydrate in the oil phase, and tetra-n-butyl ammonium bromide (TBAB) was added to produce a synergistic effect with CP. The experimental results show that the presence of TBAB can remarkably increase the separation ability and improve the flow behavior of the hydrate slurry. The most suitable contents of TBAB in the aqueous phase and water in the emulsion were determined to be 0.29 mol% and 35 vol%, respectively. The maximum separation factor of CO2 over H2 was 103, which is much higher than the literature values for separating CO2/H2 gas mixture via hydrate formation. After a two-stage separation, hydrogen was enriched from 53.2 to 97.8 mol%. The influence of temperature, pressure, and the initial gas–liquid volume ratio on the separation ability and hydrate formation rate were investigated in detail. In addition, a criterion for choosing the suitable operation conditions was suggested based on both phase equilibrium and kinetic factors. Based on this criterion, the suitable operation temperature, pressure, and gas–liquid volume ratio for the separation of CO2/H2 are approximately 270.15 K, 3–5 MPa, and 80–100, respectively.

The anti-agglomeration performance of single or compounded commercial chemical additives with/without the addition of alcohol as a co-surfactant was evaluated using a sapphire cell and an autoclave reactor with a focused beam reflectance measurement (FBRM) probe. Five kinds of gas hydrate morphologies, clumpy-like, slush-like, flocculent-like, slurry-like, and powder-like, were found during evaluating the effect of the commercial additives in (water + oil) systems. The experimental results showed that AEO-3 combined with some commercial chemical additives, especially Span 20, exhibits good anti-agglomeration performance. The hydrate slurry thus formed has a high stability and will not result in agglomeration for a long period of time. A compounded inhibition mechanism, in which one of the components disperses water droplet in the oil phase and the other component prevents formed hydrate from agglomeration, was proposed. A new structure of hydrate anti-agglomerant was designed according to the evaluating results of the single or compounded commercial additives with/without the addition of alcohol material.

Constant composition expansion measurements were performed using a high pressure fluid PVT system for two gas condensate samples collected from two reservoir fields in China to determine the dew point pressures and compressibility factors at different temperatures, where the experimental pressures were up to 95 MPa. For the samples studied, the experimental results showed that the dew point pressures decrease with the increase of temperature and the gas compressibility factors increase with the increase of pressure in single phase zone at four temperatures. Thermodynamic models that combining equations of state with different kinds of plus-fraction splitting and critical properties of pseudo-components characterization methods were compared and optimized to describe the phase behavior and volumetric properties of gas condensate samples under high-pressure and high-temperature conditions. The preferred model has a better description for dew point pressures and gas compressibility factors with the total average absolute deviation for two samples of 0.64% and 1.56%, respectively.Highlights► Constant composition expansion tests are performed under pressure up to 95 MPa. ► Dew point pressure and compressibility factor for two gas condensates are obtained. ► Thermodynamic models that combined EOS with characterization methods are compared. ► Models to describe gas condensate samples under HPHT conditions are optimized.

•Fractal theory is introduced to describe hydrate phase equilibria in porous media.•The shape of pore is supposed to hold a fractal feature as von Koch curve.•Laplace equation is adopted by considering the surface effect of the pore edge shape.•The hydrate phase equilibria calculations for silica gel are superior to those of VDW-Platteeuw type models.A thermodynamic model based on the reaction-adsorption two-step formation mechanism was improved by introducing fractal theory to predict hydrate phase equilibria in porous media. The surface effect on phase equilibrium of porous media system was modified by considering the shape of the pore edge, which was supposed to hold a fractal feature as von Koch curve. A fractional dimension Laplace equation was established in describing the phase equilibrium conditions of methane, ethane, propane, and carbon dioxide hydrates in silica gel pores. The calculated results showed that when the shape of the pore edge is assumed as spherical, the calculations by the thermodynamics model developed are close or a little superior to those of traditional van der Waals-Platteeuw type models. When the surface effect of the pore edge shape is introduced, the absolute average deviations for silica gel systems can be further decreased.

•Volumetric properties of two reservoir fluid samples were measured with pressure up to 116 MPa.•Dew point pressures at four temperatures for condensate gas sample are obtained.•Correlations and thermodynamic model for describing gas compressibility factor under high pressure were compared.•The thermodynamic model recommended is most suitable for fluids produced from reservoirs with a wide pressure range.The volumetric properties of two reservoir fluid samples collected from one condensate gas well and one natural gas well were measured under four groups of temperatures, respectively, with pressure up to 116 MPa. For the two samples examined, the experimental results show that the gas compressibility factor increases with the increase of pressure. But the influence of the temperature is related to the range of the experimental pressure. It approximately decreases with the increase of temperature when the pressure is larger than (45 to 50) MPa, while there is the opposite trend when the pressure is lower than (45 to 50) MPa. The dew point pressure was also determined for the condensate gas sample, which decreases with the increase of temperature. The capabilities of four empirical correlations and a thermodynamic model based on equation of state for describing gas compressibility factor of reservoir fluids under high pressure were investigated. The comparison results show that the thermodynamic model recommended is the most suitable for fluids whatever produced from high-pressure reservoirs or conventional mild-pressure reservoirs.

The exploitation of methane hydrate formed under the same conditions is simulated experimentally to investigate the gas production behavior at different injected solutions (hot water, saline solution, and ethylene glycol) with continuous injection mode using a three-dimensional quiescent reactor. The influence of hot water temperature, injection rate, and injected solution type on the gas production and the energy efficiency are examined. The results show that the gas production increases with the increased hot water temperature, but the influence of temperature is weakened when it is beyond 313.0 K. An optimal injection rate also exists for the gas production. Hydrate dissociation with NaCl solution has the highest gas production compared with Na2SO4 and ethylene glycol when under the same conditions. The comparison of energy efficiency between the continuous injection mode and the interval injection mode shows that the former mode with double wells is more in favor of hydrate dissociation and gas production.Highlights► Hydrate exploitation is simulated with continuous injection mode using a 3-D reactor. ► An optimal water temperature and injection rate exists for gas production of hydrate. ► Compared with Na2SO4 and EG, injecting NaCl solution has the highest gas production. ► The continuous injection mode is more in favor of gas production of hydrate.

We report for the first time an experimental investigation of gas storage in porous graphene with nanomeshes. High capacity methane storage (236 v(STP)/v) and a high selectivity to carbon dioxide adsorption were obtained in the nanomesh graphene with a high specific surface area (SSA) and a SSA-lossless tightly stacking manner.

Four reservoir samples under ultra-high-pressure and high-temperature conditions were collected from condensate gas fields in China. Constant-composition expansion tests were performed to determine the phase behavior and volumetric properties of reservoir fluid using an ultra-high-pressure fluid PVT test system. The compressibility factor and dew-point pressure were obtained at four temperatures for four samples. The range of pressure was from 22.03 to 118.89 MPa. For the samples studied, the experimental results showed that the dew-point pressure decreased with increasing temperature and the compressibility factors increased with increasing pressure but decreased with increasing temperature at a given high reduced pressure. A thermodynamic model based on an equation of state was developed to describe the volumetric properties and phase behavior of the condensate gas under ultra-high-pressure conditions. The calculated results are in good accordance with the experimental data, which is important for the development of condensate gas reservoirs in ultra-high-pressure environments.

An experimental setup was developed to in situ measure the evolvement of the electrical resistivity during hydrate formation process to aid the interpretation of the influence of hydrate saturation on the electrical properties of the sediment. Five hydrate samples under different initial brine saturations of 12, 20, 30, 40, and 50% were formed from free methane gas and brine with 3.35 wt % NaCl in the 60–80 mesh sandy sediment, during which the variations of electrical resistivity were in situ measured. It was observed that the resistivity of the hydrate-bearing sediment increases with the increase of hydrate saturation during the hydrate formation process and finally achieves a constant value for each group of hydrate sample. For hydrate samples at different initial brine saturations, the values of electrical resistivity are larger, even though hydrate saturation is lower if free methane gas exists in sediment pores. Based on the measured electrical resistivity of hydrate-bearing sediment before and after brine injection at different hydrate saturations, parameters in Archie equation were determined to describe the relationship between resistivity and hydrate saturation, which would be helpful for mapping the hydrate concentration in the sediment through resistivity logging data from hydrate deposits.

Using molecular dynamics simulations on the microsecond time scale, we investigate the nucleation and growth mechanisms of CO2 hydrates in a water/CO2/silica three-phase system. Our simulation results indicate that the CO2 hydrate nucleates near the three-phase contact line rather than at the two-phase interfaces and then grows along the contact line to form an amorphous crystal. In the nucleation stage, the hydroxylated silica surface can be understand as a stabilizer to prolong the lifetime of adsorbed hydrate cages that interact with the silica surface by hydrogen bonding, and the adsorbed cages behave as the nucleation sites for the formation of an amorphous CO2 hydrate. After nucleation, the nucleus grows along the three-phase contact line and prefers to develop toward the CO2 phase as a result of the hydrophilic nature of the modified solid surface and the easy availability of CO2 molecules. During the growth process, the population of sI cages in the formed amorphous crystal is found to increase much faster than that of sII cages, being in agreement with the fact that only the sI hydrate can be formed in nature for CO2 molecules.

The gas production from methane-hydrate-bearing sediment by injecting ethylene glycol (EG) solution was investigated using a three-dimensional experimental apparatus. Eight experimental runs were performed to examine the influence of operation conditions on hydrate dissociation by EG injection. The variations of pressure and temperature distribution in the reactor stimulated by the injected EG were obtained for the gas production process of the hydrate. The variation trend of temperature in the injection stage shows a shape of a “well” because of heat transfer and hydrate dissociation. The appearance sequence of temperature “well” and “well” depth is different for every port at different depths and radii. The effects of the concentration and quantity of EG and soaking time on the gas production ratio are examined. It shows that there exists an optimal value of the mass ratio of injected EG solution to initial water, where a maximum gas production ratio appears. When other conditions are similar, the gas amount produced by hydrate dissociation increases with the increase of the inhibitor concentration. The gas production efficiency increases with the decrease of the EG quantity and the increase of the EG concentration.

An experimental apparatus was developed to measure P-wave velocity (VP) of gas-hydrate-bearing sediment. Tetrahydrofuran (THF) was added to quicken the hydrate formation in the porous media and to synthesize hydrate-bearing sediments with uniform distribution. Methane acted as a free gas to participate in the hydrate formation. Five experimental runs were performed to examine the influence of sediment grain size and THF concentration on VP. The P-wave velocity and the amplitude for the first arrival wave signal were collected in real time during hydrate formation process. The experimental data showed that VP increases monotonically with the increase of hydrate saturation in the sediment pore space and finally tends to be a constant value. This final VP value increases with the increase of initial THF content, but the effect of sand grain size on VP is inconclusive. The variations of amplitude for the first arrival wave signal with elapsed time during hydrate formation illustrates that the amplitude increases with the increase of hydrate saturation until it attains a maximum value and then decreases gradually due to the effect of free methane gas penetrating into the hydrate-bearing sediment. The acoustic velocity of THF-hydrate filled sediment was also predicted based on the extended contact cement theory. The predicted results were close to the experimental data obtained in this work.

The equilibrium and dynamic interfacial tension of water/n-octane plus sorbitan monolaurate (the commercial name of Span 20) were measured using the pendant drop technique at four temperatures, (274.2, 278.2, 282.2, and 293.2) K. The concentration range of Span 20 is from (0.014 to 1.41) g·kg−1. The experimental results showed that Span 20 has excellent interface activity and a pronounced dynamic effect on the water/n-octane interface. The critical micelle concentration of Span 20 at different temperatures was determined, and it shifts toward lower value with the increase of temperature. The dynamic interfacial tension data show that, at the beginning of the adsorption process, it was only diffusion-controlled. At the near-equilibrium stage, there exists an adsorption barrier due to the strong molecular interaction of Span 20.

The solubility of five groups of natural gas in reservoir formation water was measured under (333.2 to 393.2) K and (15.0 to 43.6) MPa. The formation water and corresponding natural gas were sampled in situ from a China oilfield. The range of total dissolved solids (TDS) of the reservoir formation water samples is from (8744 to 80634) mg·kg−1. The mole fraction of methane in natural gas ranges from 0.7575 to 0.9449. The experimental results show that, with the increase of temperature and pressure, the solubility of natural gas in formation water goes through a minimum value under the experimental conditions. The solubility of natural gas in formation water was also influenced by a composition of gas phase and TDS of liquid phase. The salt effect becomes more significant in the high-pressure region.

In this paper, we report microsecond molecular dynamics simulations of the kinetic pathway of CO2 hydrate formation triggered by hydroxylated silica surfaces. Our simulation results show that the nucleation of the CO2 hydrate is a three-stage process. First, an icelike layer is formed closest to the substrates on the nanosecond scale. Then, on the submicrosecond timescale, a thin layer with intermediate structure is induced to compensate for the structure mismatch between the icelike layer and the final stable CO2 hydrate. Finally, on the microsecond timescale, the nucleation of the first CO2 hydrate motif layer is generated from the intermediate structure that acts as nucleation seeds. We also address the effects of the distance between two surfaces.

The pressure dependence of interfacial tension between methane and octane at five temperatures has been determined using the pendant-drop method. The experimental results show that the interfacial tension value decreases with the increased temperature and pressure. The higher temperature and pressure has a positive contribution on weakening the intermolecular interaction between methane and octane. The surface excess concentration for methane on octane at different temperatures and pressures and the surface free energies of adsorption for (methane + octane) were calculated and compared with the (methane + water) system. The calculated results show that methane is more preferred for the adsorption on octane than on water.

A three-dimensional middle-size reactor was used to simulate gas production from methane hydrate-bearing sand by hot-water cyclic injection. The gas production process and energy efficiency in the whole process, which was divided into injecting hot water, closing well, and producing gas (three steps), were investigated using 16 thermocouples distributed in hydrate-bearing sand samples. The experimental results indicates that the overall temperature trend increases with hot-water injection and decreases with gas production. The temperature distribution and fluctuation in the reactor depend upon the location of the injecting/producing well as well as the porosity and permeability of hydrate samples. Heat transfer is controlled by hot-water seepage flow during the injection of hot water. The affecting factors on the energy efficiency, such as hydrate saturation, hydrate sample temperature, hot-water temperature, mass of hot water injected, and well pressure, were examined. It was found that, when other conditions are similar, the energy efficiency ratio increases with the increase of the hydrate-bearing sand saturation and hydrate sample temperature but decreases with the increase of the hot-water temperature and well pressure.

A thermodynamics model is improved to predict the hydrate−water−gas equilibria in micropores based on the reaction−adsorption two-step formation mechanism by considering the effect of capillarity. The interfacial tension values between hydrate and water for different gas species are determined by the Gibbs−Thomson relationship or generalized as a linear function of temperatures. The hydrate phase equilibrium conditions for carbon dioxide, methane, ethane, propane, and (6.7% methane + 2.1% ethane + 91.2% propane) gas mixtures in porous media with different pore diameters predicted by the thermodynamics model developed in this work are equivalent to or superior to those of the traditional van der Waals−Platteeuw type models if the interfacial tension values are fixed. The absolute average deviations for all porous media systems can be decreased to 5.08% after a linear relation of interfacial tension and temperatures are introduced. It was found that the interfacial tension values between hydrate and water are in the magnitude order of propane, ethane, carbon dioxide, and methane, which is related to hydrate structure type and the occupancy of linked cavities for different gas species.

The hydrate inhibition effect of three kinetic inhibitors (inhibex 301, 501, and 713) was assessed from (CH4 + C2H6 + C3H8) gas mixture + brine systems using a high pressure sapphire cell. The onset time of hydrate formation was determined by visual observation method and pressure drop profile method, respectively. The experimental results demonstrated that the onset time was able to be determined by the visual observation method all the time while the pressure drop profile method failed to detect the onset time clearly and correctly at lower temperatures. In some cases, the initial appearance of hydrate crystals cannot induce a clear break in the pressure–time relationship curve. The onset time measured by the visual observation method is usually shorter than or at least the same as that determined by the pressure drop profile method. The inhibiting effect on the growth of hydrate crystals can be shown by the difference of the onset time obtained by the two methods. The maximum tolerated subcooling of each inhibitor was also investigated based on the onset time. It was found that inhibex 301 behaves as the best inhibitor that can tolerate the maximum subcooling of 8.3 K at 0.5 wt% and 10.6 K at 1.0 wt%, respectively. The maximum subcooling for inhibex 501 is 6.8 K at 0.5 wt% and 6.6 K at 1.0 wt%, respectively. Inhibex 713 has relatively poor inhibiting effect among the three inhibitors with the maximum subcooling of less than 3.5 K at 0.5 wt% and 5.1 K at 1.0 wt%, respectively.

The interfacial tensions between methane and aqueous solutions of different contents of VC-713 (a terpolymer of N-vinylpyrrolidone, N-vinylcaprolactam, and dimethylamino-ethyl-methacrylate) were measured at different temperatures and pressures in the hydrate formation region. The surface adsorption free energies of methane were calculated accordingly in order to investigate the effect of this kinetic inhibitor on the nucleation of hydrate. The results show that the presence of VC-713 lowers the interfacial tension, increasing the concentration of methane on the surface of the aqueous phase, and thus promotes nucleation of hydrate at the gas/liquid interface. Additionally, the measured interfacial tension data suggest that VC-713 tends not to form micelles in water. Subsequently, the lateral growth rate of hydrate film on the surface of a methane bubble suspended in the aqueous phase was measured at different pressures to investigate the effect of VC-713 on the growth of hydrate. The results show that the lateral growth rate of hydrate film from aqueous VC-713 solution is much lower than that from pure water, demonstrating that VC-713 significantly inhibits the hydrate growth. The mechanism of the inhibition is also discussed.The kinetic inhibitor VC-713 acts as a surfactant, although it can decrease gas/liquid interface tension, it tends not to form micelles in water.

Vapor−hydrate equilibria data for the methane + hydrogen + tetrahydrofuran + water system were obtained using the sapphire cell device. The influence of temperature, pressure, initial gas−liquid volume ratio, and the mole fraction of tetrahydrofuran was investigated to evaluate the separation efficiency by forming hydrate at different operating conditions. The experimental results show that decreasing operating temperature or the initial gas−liquid volume ratio and increasing operating pressure or initial tetrahydrofuran mole fraction will be effective to increase the content of hydrogen in the vapor phase. But their influence on the recovery degree of hydrogen in the vapor phase and the operating cost should also be taken into account synthetically when designing a separation process.

Interfacial tension of CH4 + kinetic inhibitors, inhibex 301, and inhibex 501 systems was measured at different concentrations of hydrate inhibitors using the pendant-bubble method. The temperature and pressure ranges were (274.2 to 282.2) K and (0.1 to 20.1) MPa, respectively. The experimental data show that the interfacial tension between methane and aqueous solution of hydrate inhibitor decreases with the increase of pressure and inhibitor concentration. It implied that the interface behavior of inhibex 301 and inhibex 501 is similar to a surfactant. The presence of inhibex 301 in water makes the interfacial tension decrease more remarkably compared with inhibex 501.

Dissociation kinetic behavior of methane hydrate was studied at 268.15 K using thermal method in a closed quiescent middle-sized reactor of 10 L, which with a multi-deck cell-type vessel as the internals and coiled copper tubes placed inside assuring hydrate form or dissociate in all cells of the vessel simultaneously to reduce or eliminate the scale-up effect. A dramatically reduced dissociation rate phenomenon – “buffered dissociation” due to the ice melting was observed. The influences of the water temperature, the heating rate, the quantity of hydrate, and the dissociation pressure upon the dissociation rate and the extent of the buffering effect were investigated experimentally to reveal the gas production mechanism from hydrate below the ice point. The experimental results indicate that the rate of heat transfer and the thermodynamic driving force were the key rate-limiting factors for hydrate dissociation in the closed reactor. The buffering effect of gas production can be eliminated and the dissociation rate can be increased by increasing the temperature of the heating water and lowering the dissociation pressure. However, the temperature buffering behavior cannot be eliminated.

To separate gas mixture through forming hydrate is a new technology, which might be applicable in recovering economically valuable gas components, such as hydrogen and ethylene from refinery gases. A set of equipment was designed and constructed for the experimental study of gas mixtures separation efficiency via forming hydrate. Batch operation method with/without gas–liquid circulating can be adopted. The separation efficiency for hydrogen + methane gas mixtures in pure water, 1, and 6 mol% tetrahydrofuran (THF) in initial aqueous solution were examined at different temperature, pressure, and feed gas composition conditions. The results showed that high operation pressure is needed for separation from pure water, which is only used to the initial hydrogen enriched. With the existence of THF in aqueous solution, hydrogen was remarkably enriched in vapor phase by a single equilibrium stage, especially when 6 mol% THF added. Based on the extended Chen-Guo hydrate model, an algorithm for vapor-hydrate flash calculation was developed to simulate the single equilibrium stage separation of gas mixtures. Good predict precision can be obtained by the simulating algorithm, which was then extended to simulate the experimental conditions that cannot be achieved by the limit of device.

In this work, the absorption-hydration hybrid method was used to recover (hydrogen + nitrogen) from (hydrogen + nitrogen + methane + argon) tail gas mixtures of synthetic ammonia plant through hydrate formation/dissociation. A high-pressure reactor with magnetic stirrer was used to study the separation efficiency. The influences of the concentration of anti-agglomerant, temperature, pressure, initial gas-liquid volume ratio, and oil-water volume ratio on the separation efficiency were systematically investigated in the presence of tetrahydrofuran (THF). Anti-agglomerant was used to disperse hydrate particles into the condensate phase for water-in-oil emulsion system. Since nitrogen is the material for ammonia production, the objective production in our separation process is (hydrogen + nitrogen). Our experimental results show that by adopting appropriate operating conditions, high concentration of (hydrogen + nitrogen) can be obtained using the proposed technology based on forming hydrate.

It is of great significance to study gas hydrate because of following reasons. (1) Most organic carbon in the earth reserves in the form of natural gas hydrate, which is considered as a potential energy resource for the survival of human being in the future. (2) A series of novel technologies are based on gas hydrate. (3) Gas hydrate may lead to many hazards including plugging of oil/gas pipelines, accelerating global warming up, etc. In this paper, the latest progresses in exploration and exploitation of natural gas hydrate, the development of hydrate-based technologies including gas separation, gas storage, CO2 sequestration via forming hydrate, as well as the prevention of hydrate hazards are reviewed. Additionally, the progresses in the fundamental study of gas hydrate, including the thermodynamics and kinetics are also reviewed. A prospect to the future of gas hydrate research and application is given.

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.

•Hydrate slurry presents obvious shear-thinning behaviour with increase of hydrate ratio.•Hydrate slurry is transported as a solid dispersion system with addition of AAs.•A Herschel-Bulkley-type equation was built by considering the hydrate volume fraction.•Shutting-down/restarting tests show that the hydrate slurry is easily and safely restarted.•Hydrate slurry exhibits obvious thixotropic behaviour with increasing shutting-down time.The flow characteristics and rheological properties of natural gas hydrate slurry, with initial water cuts ranging from 5 to 30 vol%, were investigated in a flow loop. The experimental results indicate that the hydrate slurry can be considered a pseudoplastic fluid and presents more obvious shear-thinning behaviour with the increase in the hydrate volume fraction. The study on the fluid morphology demonstrated that the original structure of the water-in-oil emulsion is destroyed by the formation of gas hydrate, and the hydrate slurry is ultimately transported as a solid dispersion system. An empirical Herschel–Bulkley-type equation that considers the hydrate volume fraction was developed to improve the description of the rheological properties of the hydrate slurry. The apparent viscosities of the hydrate slurry calculated by the new equation were in accordance with the experimental data. Shutting-down/restarting tests using three shutting-down times (2 h, 4 h, and 8 h) were performed. The experimental results indicate that the hydrate slurry can be easily and safely restarted from the static state after a long shutting-down period and exhibits obvious thixotropic behaviour with increasing shutting-down time.

We report for the first time an experimental investigation of gas storage in porous graphene with nanomeshes. High capacity methane storage (236 v(STP)/v) and a high selectivity to carbon dioxide adsorption were obtained in the nanomesh graphene with a high specific surface area (SSA) and a SSA-lossless tightly stacking manner.