In this work, the dissociation behavior of methane hydrate in quartz sand sediment by injecting a thermodynamic inhibitor, methanol (MeOH), was investigated using a one-dimensional experimental apparatus. The experimental results indicated that the hydrate dissociation process included four stages: free gas production, methanol dilution, major hydrate dissociation, and residual gas production. The overall liquid production rate was smaller than the injection rate during the whole production process. The cumulative gas produced from hydrate under methanol solution injection was adjusted with the reference experiment. A new strategy of the adjustment of the experimental runs was introduced, which was based on the ratio of the water and methanol solution injection rates. In general, with the increase of the methanol injection rate and the methanol concentration, the cumulative hydrate-originating gas produced increased. During the major hydrate dissociation stage, the production efficiency was enhanced continuously with the increase of the injection rate and concentration of the methanol solution, while the methanol efficiency increased and reached a maximum value when the concentration was 60 wt % and then gradually decreased.

The kinetic characteristics of methane hydrate formation in porous media are investigated in three experimental apparatus with different scales. A total of 18 experimental formation runs are implemented in these apparatus (6 runs in each device). The kinetic model proposed by Li et al. [Chem. Eng. Sci., 2014, 105, 220–230] is employed to simulate these hydrate formation processes, and the applicability of this model in different hydrate deposits is confirmed by comparing the experimental data with the numerical prediction. It is found that the reaction rate constant k decreases sharply with the increase of the size of the apparatus, which indicates that the hydrate formation rate is inversely proportional to the scale of the hydrate deposit. In addition, both the initial phase saturations and the properties of the porous media show extraordinary effects on the formation kinetics of methane hydrate.

On the basis of the hot brine in situ seafloor prepared for marine NGHs exploitation, formation kinetic behavior of cyclopentane (CP)–methane hydrate was studied at various depths of ocean water. The effects of the temperature, pressure, gas–liquid ratio, and ratio of CP/liquid phase on gas uptake were investigated to reveal the affecting factors of the hydrate rapid formation. The experimental results indicated that the driving force played an important role in the hydrate formation. In addition, the CP/liquid phase ratio of 5% was beneficial to gas uptake. When the conditions of driving force and the CP/liquid phase ratio were very favorable for hydrate formation, the gas uptakes slightly change with the gas–liquid ratio. In contrast, a smaller gas–liquid ratio was conducive to gas uptake.

Gas hydrate technology is considered as a promising technology in the fields of gas storage and transportation, gas separation and purification, seawater desalination, and phase-change thermal energy storage. However, to date, the technology is still not commercially used mainly due to the low gas hydrate formation rate and the low gas uptake. In this study, the effect of hydrate promoters on gas uptake was systematically studied and analyzed based on hydrate-based CH4 storage and CO2 capture from CO2/H2 gas mixture experiments. Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and gas chromatography (GC) were employed to analyze the microstructures and gas compositions. The results indicate that the effect of the hydrate promoter on the gas uptake depends on the physical and chemical properties of the promoter and gas. A strong polar ionic promoter is not helpful towards obtaining the ideal gas uptake because a dense hydrate layer is easily formed at the gas–liquid interface, which hinders gas diffusion from the gas phase to the bulk solution. For a weak polar or non-polar promoter, the gas uptake depends on the dissolution characteristics among the different substances in the system. The lower the mutual solubility among the substances co-existing in the system, the higher the independence among the substances in the system; this is so that each phase has an equal chance to occupy the hydrate cages without or with small interactions, finally leading to a relatively high gas uptake.

•CO2/H2 is proven to be feasible to recovery CH4 from NGHs by replacement.•Replacement includes CH4 hydrate dissociation followed by CO2 hydrate formation.•CH4 recovery efficiency of more than 71% is obtained in CH4-CO2/H2 replacement.•H2 plays promotion role in the replacement instead of occupying hydrate cages.Methane (CH4) recovery from natural gas hydrates (NGHs) by CO2-CH4 replacement is considered as a win-win technology for producing CH4 and sequestrating CO2 synchronously. In this investigation, simulated Integrated Gasification Combined Cycle (IGCC) syngas of CO2/H2 gas mixture is used to replace CH4 from simulated methane hydrate which is formed in pure water at 274.15 K and 4.5 MPa. The changes of concentrations of CH4, CO2 and H2 in gas phase during the replacement process are supervised by Gas Chromatograph (GC), and the gas hydrates are determined through in situ Raman. Meanwhile, the CH4 recovery and the replacement mechanism are qualitatively analyzed. The results indicate that, on one hand, the replacement consists of two steps, CH4 hydrate dissociation at the first and followed by CO2 hydrate formation, on the other hand, the CH4 recovery from CH4-CO2/H2 replacement is more than 71% which is significantly higher than that from CH4-CO2 replacement. Notably, no H2 is found in the hydrate phase in the replacement process, which implying that H2 does not compete with CH4 molecules occupying hydrate cages but plays promotion role in CO2-CH4 replacement.

•Hydrate dissociation by heat stimulation with different well-spacing are studied.•Optimized well-spacing should equal to the maximum range of hydrate dissociation.•An integrated factor is proposed for evaluating the gas recovery method.•Maximum range for hydrate dissociation in reservoir by heat stimulation is verified.Due to the vast amount of recoverable natural gas predicated (∼3000 TCM) in natural gas hydrate on earth, natural gas hydrate has the potential to become the next generation of unconventional source of fuel. Recently years, laboratory researches are still underway to advance our understanding of the theory and technology for natural gas hydrate exploitation. In this work, experiments of methane hydrate dissociation by heat stimulation with different well-spacing were firstly performed in the Cubic Hydrate Simulator (CHS). The five-spot vertical wells (5-wells) and the dual vertical wells (D-wells) were applied as the multi-well strategies. The well spacing of D-wells is twice as larger as that of the 5-wells. The influences of well spacing on the production behaviors and the heat transfer characteristics during hydrate dissociation are analyzed. The experimental results indicate that a maximum range for hydrate dissociation exists during hydrate dissociation by heat stimulation method, which is in direct proportion to the heat injection rate (Rinj). The optimized well-spacing should equal to the maximum range of hydrate dissociation. For maximizing average gas production rate per well, the larger well spacing and the higher Rinj benefit for gas recovery. On the other hand, for minimizing the heat consumption per unit of gas production (HGP), the moderate Rinj and the shorter well spacing benefit for gas recovery. In order to evaluate the gas recovery by heat stimulation with different well spacing and Rinj, an evaluation factor is firstly proposed, which considers the combined effect of the gas production rate per well, the HGP, and the hydrate dissociation ratio. By calculation, the optimal experimental conditions for hydrate recovery in experiments are the injection rate of 40 mL/min with the longer well spacing (D-wells). The study of temperature distribution verifies the maximum range for hydrate dissociation in reservoir by heat stimulation.

•We develop an apparatus for hydrate formation and permeability measurement with water.•We measure water permeability with methane hydrate in three different quartz sands.•Permeability features are different with hydrate saturation higher or lower than 10%.•Simple cubic sphere pack with uniform grain diameter was used in the model.•We develop a new relationship between water permeability and hydrate saturation.Natural gas hydrates widely distributed in marine sediments and permafrost areas have attracted global attentions as potential energy resources. The permeability of sediments with or without hydrate is an essential and critical parameter that could determine the technical and economical feasibility of gas recovery from hydrate reservoirs. The saturation of hydrate in the solid phase significantly affects the pore size, the pore volume, the distribution of reservoir pore throat size, etc., which are key factors determining the permeability of the hydrate-bearing deposit. In this study, the absolute permeability and the water effective permeability were experimentally measured with fluid water under a serials of hydrate saturations (0–31% in volume). Hydrate saturations were controlled and calculated precisely based on the amount of injected and produced gas/water, and the system pressure and temperature. Unconsolidated quartz sands with different particle size (200–300, 300–450, 450–600 µm) were used as the porous media. The absolute permeabilities of the above quartz sands were 21.11, 35.53 and 52.32 Darcies, respectively. The experimental results indicated that the characteristics of the permeability were different with the hydrate saturation lower and higher than 10%. When the hydrate saturation increased from 0 to 10%, there was a sudden drop for the permeability, which indicated that the appearance and the existence of the solid hydrate phase in the porous media affected the permeability significantly. On the other hand, this effect lightened when the hydrate saturation higher than 10%. With different hydrate crystal growth habit, a new relationship between the ratio of the permeability in the presence and the absence of hydrate and the hydrate saturation was developed. Two patterns of the pore filling models with the hydrate saturation lower and higher than 10% were used to fit the measured experimental data. The overall relationship and the values of the saturation exponent were continuous and consistent with hydrate saturation lower than 31%.

•Hydrate-based acidic gases capture with synergic additives was investigated.•Synergic additives was comprised by TMS and TBAB.•CO2 and H2S could be synchronously captured.•Synergic additives enhance the dissolution and diffusion of acidic gases.•Synergic additives promoted hydrate-based capture process.The widespread need for carbon dioxide (CO2) and hydrogen sulfide (H2S) removal from potential gaseous fuel processes associated with upgrading of natural gas, biogas and landfill gas has led to a continuing interest in developing acid gas capture technologies. This work experimentally investigated the hydrate-based acidic gases (CO2 and H2S) capture for clean methane (CH4) fuel from biogas or natural gas with new synergic additives, which comprised physical gas solvent (TMS) and traditional hydrate promoter (TBAB). The results show that, with the synergic additives, the equilibrium hydrate formation pressures were moderated by about 90% relative to pure water, the selectivity of CO2 over CH4 and the selectivity of H2S over CH4 could achieve 18.56 and 11.38, respectively. Compared with TBAB, the synergic additives could improve the hydrate formation rate and the gas storage capacity by 149% and 84%, respectively. Furthermore, the promotion effect could be enhanced when with the help of H2S. It has been shown that CO2 and H2S could be synchronously captured through the hydrate formation process. It will be of importance to the fundamental study of enhancing gas hydrate formation process, and of practical significance for the hydrate-based application industry.Download high-res image (75KB)Download full-size image

In this work, the formation kinetics of cyclopentane (CP) + methane hydrate is studied. CP is used as a promoter to accelerate the hydrate formation. The total methane consumption, the induction time, and the formation rate were investigated under different hydrate formation conditions in NaCl solution. The results indicated that the pressure driving force could increase the gas consumption and shorten the induction time. Meanwhile, the induction time could be greatly influenced by the pressure driving force at a lower temperature. Especially, it could be shortened to a minimum value of 110 s with the increase of the pressure driving force at a fixed operating condition (CP concentration, 7.45%; NaCl solution concentration, 3.50%; and temperature, 298.15 K). Moreover, the hydrate formation rate would be accelerated with the increase of the stirring rate by its promotion in the dissolution and dispersion of methane. Finally, a higher CP concentration was favorable for the rapid hydrate formation of CP + CH4 binary hydrates. The amount of CP used could determine the amount of methane incorporated into the hydrate phase.

The Pilot-Scale Hydrate Simulator (PHS), a three-dimensional 117.8 L pressure vessel, was applied to study the methane hydrate dissociation with different reservoir temperatures and different production pressures in the sandy sediment. The volume of the vessel is big enough to simulate the field-scale gas production from hydrate reservoir. The depressurization method and the depressurization assisted with heat stimulation method were performed as the hydrate dissociation methods. Three different temperatures, which are 4.7 °C, 8.8 °C, and 13.0 °C, were selected as the reservoir temperatures. The range of temperature in this work is the most common temperatures of hydrate reservoir in the ocean sediment. The experimental results indicate that, for the depressurization method, the temperature drop in the reservoir during hydrate dissociation is the key factor for the amount of hydrate dissociation in the depressurization (DP) stage and the rates of hydrate dissociation in the constant-pressure (CP) stage, which can be enhanced by the increase of the temperature drop. With the same production pressure, rate of hydrate dissociation in the experiment with higher reservoir temperature is quicker. With a same pressure drop below hydrate dissociation pressure, the rate of hydrate dissociation in the experiment with the lower reservoir temperature is quicker. For the depressurization assisted with heat stimulation method, the hydrate dissociation rate in the heat stimulation (HS) stage mainly depends on the temperature difference between the injection temperature and the hydrate dissociation temperature corresponding to the production pressure. The larger temperature difference causes the larger hydrate dissociation rate in the HS stage. In addition, the effect of reservoir temperature on the rate of hydrate dissociation is smaller than that of production pressure in the HS stage.

•Purification hydrogen from syngas by hydrate process with synergic additives.•Thermodynamic, kinetic, separation experiments and microcosmic analysis were studied.•The formation rate, storage capability and gas selectivity are remarkably improved.•The hydrate-based separation process was enhanced due to the addition of DMSO.Hydrated-based separation process has recently raised as a novel technology for hydrogen (H2) purification from mixture gases containing carbon dioxide (CO2). However, the practical application requires suitable hydrate formation condition, hydrate formation rate, gas selectivity, gas storage capacity and so on. This work presents the effect of synergic additives on the hydrate-based syngas purification process based on the thermodynamic, kinetic, purification characteristic studies as well as microcosmic structure analysis. The synergic additives comprise traditional hydrate promoter (tetra-n-butyl ammonium bromide, TBAB) and gas solvent (dimethyl sulfoxide, DMSO). 40.2 mol% CO2/H2 binary mixtures were selected as the simulated syngas. The results show that the synergic additives (TBAB-DMSO) can reduce the equilibrium hydrate formation pressure of the simulated syngas by 80%. Compared to single TBAB for the hydrate-based purification process, the synergic additives could improve the gas consumption rate of the unit system, gas storage capability and gas selectivity by about 32%, 105% and 51% respectively, and decrease the loss ratio of H2 by 1.45%. The Raman analysis reveals that the simulated syngas forms semiclathrate framework with TBAB-DMSO. DMSO only performs as a gas solvent during the gas dissolution and diffusion process, and not participates in the hydrate framework formation.Graphical abstract for the manuscript entitled “Hydrate-based hydrogen purification from simulated syngas with synergic additives” by Zhiming Xia, Xiaosen Li, Zhaoyang Chen, Kefeng Yan, Chungang Xu and Jing Cai.

The experimental study on phase equilibrium conditions for semi-clathrate hydrates formed in the systems of CH4/N2 gas mixture and tetra-butyl-ammonium fluoride (TBAF) with different ratios (0.210, 0.293 and 0.500 mol%) to water was conducted in this paper. The experiments were carried out under the conditions of 281.15–291.15 K and 0.30–3.70 MPa. The structures of the hydrates were characterized using in-situ Raman spectroscopy at 276.15 K and 2.50 MPa. At a certain given pressure, the equilibrium temperature of the semi-clathrate hydrates formed in the systems with 0.500 mol% TBAF is significantly higher than that of semi-clathrate hydrates formed the systems with 0.210 or 0.293 mol% TBAF, and even higher than that of structure II hydrates formed in the systems with THF-SDS. Moreover, TBAF has more positive influence on enhancing dissociation enthalpies of the hydrates than THF. Due to the quite weak signal of N–N triple bond vibration or/and seldom N2 molecules encaged into the cavities, only CH4 molecules could be determined clearly in the mixed hydrates. In addition, the crystal morphology of the semi-clathrate hydrates is affected by the ratio of TBAF to water.

Decomposition conditions of methane hydrate in marine sediments from South China Sea were measured using multi-step decomposition method. Four different samples of the marine sediments were used in the experiments. The pore distribution, the surface area, particle size and the surface texture were measured and observed. The experimental results indicated that the final decomposition temperatures are shifted lower than those for bulk hydrates at the same pressure for different marine sediments and different water saturations. Temperature shifts are more negative for smaller initial water saturation. The surface textures and pore of the sediments both affect the equilibrium condition of methane hydrate. Using the Clausius-Clapeyron equation, the enthalpy of hydrate dissociation in marine sediments was calculated. It was found that the enthalpy of hydrate dissociation in marine sediments with lower water saturation is higher than that at the bulk state.

•Hydrate formation conditions in nanofluids was discussed in the article.•An improved method was used to determine the hydrate formation conditions.•A model was proposed to predict the conditions of hydrate formation.Stepwise heating method was used to determine the hydrate phase equilibrium in the water containing graphite nanoparticles. The results show that, compared with deionized water at a given temperature, a significant upward shift of the formation pressure of the CO2 hydrate formed in graphite nanoparticles suspensions was observed, but the phase equilibrium curves of CO2 hydrate formation in the different graphite nanoparticles concentrations were basically consistent. In this regard, graphite nanoparticles have a negative effect on the gas hydrate formation. In addition, an improved hydrate thermodynamic model was proposed to predict the hydrate equilibrium conditions under the system of graphite nanoparticles suspensions. The results show that the maximum average deviation of pressure predicted by model is 3.39, but the minimum average deviation is 2.21. It indicates that the data of the model prediction was good agreements with the data obtained from experiments.

In present work, the phase equilibrium of methane + cyclopentane (CP) + timethylene sulfide (TMS) hydrate were measured in NaCl solution at the temperature range from 286.42 to 303.77 K and the pressure varying from 1.16 to 12.49 MPa. The experimental data were measured with an isochoric T-cycle method. The phase equilibrium pressure of organic compounds (VCP:V TMS = 4:1) + CH4 hydrate increases with the temperature and the NaCl concentration. When the temperature was higher, the effect of temperature and NaCl concentration on the phase equilibrium pressure was more remarkable. The dissociation enthalpies of organic compounds (VCP:VTMS = 4:1) + CH4 hydrate in 3.5, 5.0, 7.0, 10% (mass fraction) NaCl solution were calculated through the Clausius−Clapeyron equation based on the phase equilibrium data. The dissociation enthalpy decreases with the increase of either the temperature or the NaCl concentration. The crystal structure of organic compounds (VCP:VTMS = 4:1) + CH4 hydrate was determined by using Raman spectroscopy. Methane was present only in the small sII cavity, cyclopentane and timethylene sulfide were all in the large sII cavities.

The behavior of hydrate formation in porous sediment has been widely studied because of its importance in the investigation of reservoirs and in the drilling of natural gas hydrate. However, it is difficult to understand the hydrate nucleation and growth mechanism on the surface and in the nanopores of porous media by experimental and numerical simulation methods. In this work, molecular dynamics simulations of the nucleation and growth of CH4 hydrate in the presence of the surface and nanopores of clay are carried out. The molecular configurations and microstructure properties are analyzed for systems containing one H2O hydrate layer (System A), three H2O hydrate layers (System B), and six H2O hydrate layers (System C) in both clay and the bulk solution. It is found that hydrate formation is more complex in porous media than in the pure bulk solution and that there is cooperativity between hydrate growth and molecular diffusion in clay nanopores. The hydroxylated edge sites of the clay surface could serve as a source of CH4 molecules to facilitate hydrate nucleation. The diffusion velocity of molecules is influenced by the growth of the hydrate that forms a block in the throats of the clay nanopore. Comparing hydrate growth in different clay pore sizes reveals that the pore size plays an important role in hydrate growth and molecular diffusion in clay. This simulation study provides the microscopic mechanism of hydrate nucleation and growth in porous media, which can be favorable for the investigation of the formation of natural gas hydrate in sediments.

•A novel 3-D high-pressure reactor with a quick-opening component is adopted.•CH4 hydrate decomposition in sandy and natural silty clay sediment is studied.•Pressure, temperature and gas production behaviors in both sediments are studied.•Hydrate in silty clay sediment shows a dynamic decomposition condition.•A radial shrinkage effect of hydrate decomposition is observed in both sediments.Extraction of methane from hydrate-bearing sediment (HBS) has aroused increasing interest around the world. However, the decomposition of gas hydrate may cause significant sediment deformation, which is one of the main obstructions for the hydrate exploitation. To investigate the decomposition behaviors of methane hydrate in different sediments and study the sediment deformation during hydrate decomposition, two contrast experiments were carried out in a novel three-dimensional (3-D) high-pressure reactor with a quick-opening component. In this study, synthetic sandy sediment and natural silty clay sediment sampled from the Shenhu Area, South China Sea were used as the HBS. For hydrate in sandy sediment, the pressure-temperature (P-T) relationship is consistent with that of bulk hydrate. However, in silty clay sediment, the hydrate decomposition conditions shift into a higher pressure region than that in sandy sediment, which is mainly due to the small pore particle size and the presence of salinity. In addition, hydrate decomposition conditions in silty clay sediment also vary at different positions in the reactor because of the effects of salinity, organics, minerals and the uneven distribution of the pores and pore particles. Hence, it is considered that there is a variable equilibrium for the methane hydrate deposits in silty clay sediment. Further, Radial Shrinkage Effect of Hydrate Decomposition (RASHEHD) was observed in both experiments. It is possibly a combined consequence of gas seepage and the cementation effect of hydrate.

This study presents the three-dimensional (3D) cubic hydrate simulator (CHS) to analyze the methane dissociation phenomena from hydrate-bearing sediment with different hydrate saturation. The experiments by depressurization have been carried out at the environmental temperature of 281.15 K, the dissociation pressure of 5.0 MPa, and in the hydrate saturation range of 17.0–43.2%. The hydrate dissociation process consists of two periods: the depressurizing period and the steady-pressure period. In the depressurizing period, the hydrate dissociation occurs in the whole reactor. The cumulative gas production is similar, and the gas production rate is affected by the depressurization rate and the water production in this period. The cumulative water production increases with the decrease of the hydrate saturation in the whole depressurization process. In the steady-pressure period, the cumulative gas production increases with the increase of the hydrate saturation. The average gas production rate first increases with the increase of the hydrate saturation and then decreases at hydrate saturation of 43.2%. The water production during the steady-pressure period only occurs in the experiments with the high hydrate saturation. The temperatures in different regions in the reactor change with similar degrees in the depressurizing period and have the similar lowest values for different experiments. In the steady-pressure period, the temperatures increase gradually from the inner-wall region to the center region. On the basis of the calculation of the energy balance, it was found that the ratio of the sensible heat of the reservoir to the latent heat of the hydrate dissociation decreases with the increases of the hydrate saturation and the dissociation pressure.

The kinetics of the formation of cyclopentane (CP) + methane (CH4) hydrates are studied to find the optimum condition for the rapid hydrate formation under the submarine condition (NaCl mass fraction of 0.035, around 277.15 K and 12 MPa). The influences of the CP/liquid phase volume ratio and the volume of the liquid phase on the amount of gas uptake and CH4 consumption rate are determined. The results show that the high volume ratio of CP/liquid phase is favorable for the rapid formation of CP + CH4 hydrates. There is an optimal ratio of the liquid phase volume for obtaining the highest gas consumption. Raman spectrum analyses for CP + CH4 hydrates are carried out to examine the hydrate structure and guest molecule occupation. The results show that CP mainly occupies the large cavities (51264) and CH4 is encapsulated in the small cavities (512) of the structure II hydrates. When the volume ratio of the CP/liquid phase is 0.05, CP with CH4 also occupies the large cavities (51264).

In this study, the different saturations of hydrate samples were formed in a cubic hydrate simulator (CHS) filled with silica sand. Subsequently, the hydrate was dissociated by depressurization in conjunction with warm water stimulation using dual horizontal wells. The hydrate dissociation process includes the depressurizing period and the injection period (the constant-pressure period). Hydrate was dissociated simultaneously in the whole reservoir during the depressurizing period. Meanwhile, gas production in the depressurizing period is mainly determined by the depressurizing rate, and it has little relation to the hydrate saturation (when the hydrate saturation ranges from 15.5% to 39.1%). During the injection period, more gas can be produced for the reservoir with the higher hydrate saturation, whereas the highest average gas production rate can be obtained for the reservoir with the middle-higher hydrate saturation. With respect to the gas production in the depressurizing period, gas production in the injection period is the dominant factor affecting the whole production efficiency in the experiment. In addition, the energy ratio only increases with the increase of the hydrate saturation in the prior stage of the constant-pressure period, and the final energy ratio with the middle-higher hydrate saturation is the maximum. Moreover, energy analysis indicates that heat injection plays the leading role for hydrate dissociation in the constant-pressure production period when the initial hydrate saturation is higher than 32.4%.

Due to the consumption of fossil fuels, an alternative energy source is necessary for the world’s continuous development. Methane hydrates, a vast energy resource that exists in deep-ocean or permafrost sediments containing approximately 10000 Gt of carbon, are a potential energy source for the future. However, economically and safely producing methane from gas hydrate deposits is still not on the drawing board. The main reasons include (1) low methane production efficiency, (2) low methane production, (3) poor production sustainability. Thus, it is pressing to develop methane production technology and/or approaches to improve methane production efficiency. In this paper, we comprehensively review the research on methane production from gas hydrates, including the research on the characteristics of gas hydrate reservoirs, production methods, numerical simulations and field production tests. The different investigations are analyzed and relevant comments and suggestions are proposed accordingly.

Hydrate-based CO2 separation and capture from gas mixtures containing CO2 has gained growing attention as a new technology for gas separation, and it is of significance for reducing anthropogenic CO2 emissions. Previous studies of the technology include the thermodynamics and kinetics of hydrate formation/dissociation, hydrate formation additives, analytical methods, separation and capture progress, equipment and applications. Presently, the technology is still in the experimental research stages, and there are few reports of industrial application. This review examines research progress in the hydrate formation process and analytical methods with a special focus on laboratory studies, including the knowledge developed in analog computation, laboratory experiments, and industrial simulation. By comparing the various studies, we propose original comments and suggestions on further developing hydrate-based CO2 separation and capture technology.

Technology of hydrate-based CO2 separation and capture is considered as a green and economical gas separation technology and is extensively studied. Most of the previous studies involving the aspects of thermodynamics and kinetics of the CO2 gas hydrate formation were carried out with small reactors, whereas few studies were carried out with pilot-scale equipments. In this paper, a pilot-scale CO2 separation from flue gas by the hydrate method is reported. By the equipment, we successfully realize CO2 separation from flue gas. By a two-stage hydrate separation process, the CO2 concentration can be enhanced to approximately 90.0% from 17.0%. The effects of the operating pressure and gas flow rate on CO2 recovery are also investigated by comparing and analyzing the data of the CO2 concentration, gas consumption, and CO2 recovery. The higher pressure results in the higher CO2 recovery, and there is an optimal ratio of the gas flow rate to the fluid flow rate for obtaining the highest gas consumption. The results achieved in this paper will be an important foundation to further develop the continuous CO2 hydrate separation process.

Gas hydrate formation behaviors in sediments were investigated in a three-dimensional reactor using the cooling method. The characteristics of the temperature change, the heat transfer, the gas consumption, and the electrical resistance change in the hydrate formation process were studied. The results show that the temperature in the reactor gradually decreases from the near-wall to the center in the cooling process. The gas hydrate preferentially forms in the inner-wall regions of the reactor, which has a low temperature in the cooling process, and then the formation spreads to the surrounding area. On the basis of the balance of energy, it was found that the temperature change was roughly equal to the values calculated in the early stage of the hydrate formation. At the beginning of the hydrate formation, the hydrate formation rates at different places are very different from each other. The gas consumption rate first rises and then falls as the supercooling degree increases during the whole process. The changes of the temperature and resistance illustrate that the methane hydrate inhomogeneously distributes in the reactor.

An energy-efficient hydrate-based warm brine preparation method in situ seafloor was first proposed for marine natural gas hydrates (NGH) exploitation. The detailed preparation process and key technologies are discussed. The optimal hydrate-former, cyclopentane + CH4, is viable for preparing warm brine under various seawater depths. The heating coefficient of the warm brine preparation reaches 3.0. The NGH production performance by depressurization in conjunction with the prepared warm brine stimulation was studied by numerical simulation. The warm brine stimulation accelerates gas production. The gas production behavior performs better with the higher salinity and temperature. However, these positive effects are limited by the direct seepage of the brine from the injection well to the production well. The massive water production from the overburden and underburden layers causes low RGW and energy efficiency. Compared to the conventional hot brine injection, the good performance of the warm brine injection confirms the feasibility of the new method.

•The production behaviors of methane hydrate are investigated in the 3-D simulator.•The different methods are used for hydrate production.•The gas/water production, efficiency, recovery, and production rate are analyzed.•The heat stimulation combining depressurization in 5-spot well is the optimal method.A novel method for hydrate production named “Five-spot thermal huff and puff (HP-5S)” is designed and employed to investigate the behaviors of hydrate dissociation in the Cubic Hydrate Simulator (CHS). This method uses the thermal huff and puff method in a five-spot well system. In addition, the experiments with the methods of the five-spot thermal huff and puff in conjunction with depressurization (HP-5S-D), the heat stimulation with a five-spot well (HS-5S), the heat stimulation in conjunction with depressurization with a five-spot well (HS-5S-D), the thermal huff and puff (HP), and the huff and puff in conjunction with depressurization (HP-D), are also carried out in this work. The energy efficiencies, thermal efficiencies, gas recoveries, and average gas production rates are used to evaluate these production methods. The analysis of hydrate decomposition shows that the thermal huff and puff method in a five-spot well system is superior to that in a single vertical well on the aspects of the energy efficiency, thermal efficiency, gas recovery, and average gas production rate. The HP-5S-D method, which can obtain the highest gas recovery, thermal efficiency, and energy efficiency, is the optimal method for hydrate production in this work.

In this work, we measured the phase equilibrium data of trimethylene sulfide + methane hydrates in brine water systems with a NaCl mass fraction of 0.035, 0.070, 0.100, and 0.120 by using T-cycle methods. The dissociation enthalpies of trimethylene sulfide–methane hydrates were calculated by the Clausius–Clapeyron equation based on the measured phase equilibrium data. The equilibrium pressure of trimethylene sulfide + methane hydrate is much lower than that of the cyclopentane + methane hydrate at the same NaCl concentration in the aqueous solution. Hence, trimethylene sulfide may be a promising water-insoluble chemical promoter. The phase equilibrium pressure of trimethylene sulfide + methane hydrate increases as the temperature rises at the same NaCl concentration. In addition, the concentration of NaCl has more remarkable influence on the equilibrium pressure at a higher temperature. The formation/dissociation enthalpies of trimethylene sulfide + methane hydrate decrease with the increases of the temperature and NaCl concentration, respectively.

The behaviors of methane hydrate formation in porous media are investigated in three-dimensional vessels. The effects of the fugacity difference, the water–gas ratio, and the volume of the vessel on the formation rate of methane hydrate are studied. The results show that the formation rates are disproportionate to the fugacity differences but in proportion to the volume of the vessel and the change of the initial gas–water ratio has little effect on the rate of hydrate formation. Meanwhile, according to the discussion about the temperatures and resistances in hydrate reservoir, it is confirmed that methane hydrate forms from the boundaries to the center of the vessel. Furthermore, a new method is designed to form methane hydrate samples in porous sediments with high hydrate/water saturation and low gas saturation.

Phase equilibrium data were measured for the mixed organic hydrates with methane. The organic compounds studied are cyclopentane, cyclopentyl chloride, cyclopentanone, methylcyclopentane, cyclohexene, tetrahydrothiophene, and trimethylene sulfide, which are water-insoluble. It is found that the organic compounds + methane hydrates are more easily formed than the methane hydrate. Therefore, these organic compounds act as the hydrate formation promoters. The hydrate promotion effectivenesses with methane as a help gas can be summarized in the following order: trimethylene sulfide > cyclopentane > tetrahydrothiophene > cyclopentanone > cyclopentyl chloride and cyclohexene > methylcyclopentane. Trimethylene sulfide has the biggest effect of hydrate promotion among the promoters studied in this work. It may be a promising water-insoluble chemical promoter. It is noted that the promotion effect is heightened with the decrease of the molecular size for the seven compounds. The hydrate dissociation enthalpies were determined via the Clausius–Clapeyron equation based on the different experimental equilibrium data. The absolute value of slope k plays an important role on the ΔdissHm.

In November 2008, gas hydrate samples were recovered during the scientific expedition conducted in the Qilian Mountain Permafrost. It is expected that this area will become a strategic area of gas hydrate production in China. However, the evaluation of the hydrate deposits in the area as a potential energy resource has not been completed. Using numerical simulation and currently available data from site measurements, we preliminarily estimate the gas production potential of these hydrate reservoirs. The cumulative volume of the produced CH4 at the well in the early production period (0–10 years) is more than 50% of the total gas production from the well. The gas hydrate dissociation mainly occurs in the vicinity of the horizontal well and along a cylindrical interface. There is a ringlike structure of ice around the horizontal well during the gas production process. With the decrease of the gas production rate, the ice starts to melt from the bottom to the top under the influence of the geothermal gradient. The sensitivity analysis demonstrates the dependence of production on the intrinsic permeability and the initial deposit temperature. Economic assessment based on the cumulative volumetric gas production and the gas-to-water ratio indicates that it is not economically viable to produce gas from this hydrate deposit using single horizontal well by depressurization.

A novel three-dimensional 117.8-L pressure vessel, which is called a Pilot-Scale Hydrate Simulator (PHS), is developed to investigate the gas production performance from hydrate-bearing porous media using the huff and puff method through both experimental and numerical simulations. The methane gas and deionized water are injected into the pressure vessel to synthesize methane hydrate. The grain sizes of the quartz sand in the vessel are between 300 and 450 μm. The huff and puff stages, including the injection, the soaking, and the production, are employed for hydrate dissociation. A single vertical well at the axis of the PHS is used as the injection and production well. The whole experiment consists of 15 huff and puff cycles. The numerical simulation results agree well with the experiment. Both the experimental and numerical simulation results indicate that the injected water is mainly restricted around the well during the injection stage. The system pressure fluctuates regularly in each cycle, and the secondary hydrate is formed under the pressurization effect caused by the hot water injection in the injection stage. The gas production rate maintains approximately stable in a relatively long period. The sensitivity analysis indicates that the gas production can be enhanced with high intrinsic permeability of the deposit or by raising the temperature of the injected hot water. The mass of the water produced in each cycle has little difference and is manageable when using the huff and puff method.

Dissociation processes of methane hydrate in porous media using the depressurization method are investigated by a combination of experimental observations and numerical simulations. In situ methane hydrate is synthesized in the Pilot-Scale Hydrate Simulator (PHS), a three-dimensional (3D) 117.8-L pressure vessel. During the experiment, constant-pressure depressurization method is used during the hydrate dissociation. A vertical well at the axis of the PHS is used as the production well. The initial hydrate and aqueous saturations before dissociation are SH0 = 27% and SA0 = 37% in volume, respectively. The hydrate dissociates continuously under depressurization and there is little hydrate remaining in the PHS. The hydrate dissociation is an analog of a moving boundary ablation process, and the hydrate dissociation interface separates the hydrate dissociated zone containing only gas and water from the undissociated zone containing the hydrate. The temperature increases in the hydrate dissociated zone near the boundaries, while that in the hydrate undissociated zone around the PHS center basically remains constant. The numerical results of the cumulative gas produced, the remaining hydrate in the deposit, and the temperature spatial distribution all agree well with the experiments, which completes the validation of the mathematical model and numerical codes employed in this study. The heat transfer from the surroundings is predominant in our experimental and numerical cases. The analysis of sensitivity to the intrinsic permeability and the initial hydrate saturation of the numerical simulation are investigated.

In this work, the effects of stirring and bubbling methods on hydrate-based carbon dioxide (CO2) separation from integrated gasification combined cycle (IGCC) synthesis gas are compared. Then, an integrated process of bubbling in conjunction with temperature fluctuation is proposed and adopted in the experiments, which are conducted in bench and scaled-up equipment. The experimental results show that the bubbling method has a similar positive effect on the CO2 separation as the stirring method. The optimal volume ratio of tetra-n-butylammonium bromide (TBAB) solution to the reactor shifts to 0.75 after the volume of the reactor is enlarged 100-fold, and at that ratio, the total 15.3 mol of gas is consumed and the mole concentration of CO2 in the gas phase reduces from 40.0 to 13.2%. The results indicate that the integrated process and scaled-up equipment are feasible for hydrate-based CO2 separation.

In this work, the gas production behaviors of methane hydrate in porous media were investigated in the three-dimensional cubic hydrate simulator (CHS) with a single well using the different methods, depressurization (DP), regular huff and puff (HP), huff and puff in conjunction with depressurization (HP-D), and huff and puff with no-soaking (HP-NS), at 281.15 K and hydrate saturation of 33.5%. The temperature spatial distributions and the hydrate spatial distributions in the hydrate reservoirs, the amount of gas and water produced, and the gas and water production rates in the gas production process were measured. In addition, the gas production efficiencies with the four methods were evaluated by calculating the gas recoveries, average production rates, thermal efficiencies, and energy efficiencies. It was found that HP-D is the optimal method for gas hydrate recovery.

To determine the appropriate operating conditions for separating methane (CH4) from drainage coal-bed methane (CBM) mixed with air, a hydrate-based methane separation method is proposed. The amount of gas uptake, CH4 concentration in decomposed gas phase, CH4 split fraction, and CH4 separation factor are investigated at the initial operating pressure range of 1.50–4.50 MPa and 279.15 K in the presence of sodium dodecyl sulfate (SDS) with the concentration range of 0–1000 ppm in 1.0 mol % tetrahydrofuran (THF) aqueous solution. The results indicate that the 1.0 mol % THF aqueous solution with the addition of 300 ppm SDS at 2.50 MPa and 279.15 K is the optimal condition for recovering CH4 from the drainage CBM via hydrate formation. Under the condition, the final amount of the gas uptake, the CH4 concentration in decomposed gas phase, the CH4 split fraction, and the CH4 separation factor after a one-stage hydrate-based separation are up to 0.1364 mol, 69.93 mol %, 86.44%, and 10.77, respectively. The result illustrates that the hydrate-based CH4 separation is a promising method to recover CH4 from the drainage CBM at mild conditions. Moreover, CH4 recovered from the drainage CBM can be directly utilized in industry.

The effects of the flow rates of cyclopentane (CP) and methane gas, operating pressure, and experimental temperature on the formation of cyclopentane-methane hydrate were investigated using a novel bubble column reactor. The results indicated that, with either an increase of the operating pressure or a decrease of the experimental temperature, the induction time decreased whereas the cumulative gas consumption and gas conversion ratio increased. In addition, with an increase of the flow rate of methane gas, the induction time decreased, whereas the cumulative gas consumption increased. The gas conversion ratio increased first from 61.5% to 93.1% when the flow rate of methane gas increased from 159 to 234 mL/min and then decreased from 93.1% to 83.2% when the flow rate of methane gas continuously increased from 234 mL/min to 308.9 mL/min. This decrease was attributed to an increase of the discharge rate of methane from the reactor. It was noted the average methane consumption rate could be effectively enhanced with an increase of the flow rate of gas at relatively low pressure and high temperature. The flow rate of CP had a minimal effect on the cumulative gas consumption and gas conversion ratio. However, an increase of the flow rate of CP could reduce the induction time.

The synergic effect of Cyclopentane (CP) and Tetra-n-butyl Ammonium Bromide (TBAB) on the hydrate-based carbon dioxide (CO2) separation from IGCC (Integrated Gasification Combined Cycle) syngas is investigated by measuring the gas uptake and the power X-ray diffraction (PXRD) patterns in this work. The CP with CP/TBAB solution ratio of 5 vol% added into the 0.29 mol% TBAB solution can remarkably increase the gas uptake at 4.0 MPa and 274.65 K. The PXRD patterns of the semi-clathrate (sc) hydrate and structure II (sII) hydrate are obtained for the CP/TBAB/gas/H2O system. The synergic effect of the CP and the TBAB includes two aspects: On one hand, the CP molecules housed in the hollow centers of the large cavities together with TBAB cations (TBA+) make the sc hydrate more stable. On the other hand, the TBA+ displaced out of the large cavities by the CP molecules make the ionization reaction of TBAB in the solution going toward the reverse direction. Thus, the more TBAB molecules exist in the solution and form the more sc hydrate, resulting in the considerable increase of the gas uptake.Highlights► The PXRD patterns of CP-TBAB hydrate are obtained. ► The synergic effect of CP-TBAB on the hydrate formation is verified. ► CP competes TBA+ with large cavities resulting in the hydrate more stable.

Quaternary salts can form semi-clathrate hydrates, caging gas molecules in the empty small cages, which have the potential for the separation of mixtures, such as the simulated flue gas [CO2 (17 mol %)/N2 mixtures]. To enhance the CO2 separation from CO2/N2 binary mixtures, three quaternary salts, tetra-n-butylammonium bromide (TBAB), tetra-n-butylphosphonium bromide (TBPB), and tetra-n-butylammonium nitrate (TBANO3), are investigated at different operating conditions by a one-stage hydrate separation process. The results indicate that the induction time for each quaternary salt system can be shortened to less than 5 min under the optimal operating condition. Meanwhile, each quaternary salt can significantly promote the CO2 separation under its optimal condition. TBANO3 displays the strongest capability in terms of gas consumption and CO2 separation with the pressure drop of 0.72 MPa and the highest split fraction of 67% and separation factor of 15.54 compared to the other two salts. Besides, CO2 can be further removed from 17 to 7 mol % in the presence of TBANO3. TBPB also has a potential effect on CO2 separation with the pressure drop of 0.57 MPa and the separation factor of 14.06. The result demonstrates that TBANO3 and TBPB are two better additives for efficient hydrate capture of CO2.

This work presents the thermodynamic study of separating CH4 and CO2 from the simulated landfill gas (LFG) [CO2 (0.45) + CH4 (0.55)] based on hydrate crystallization in the presence of tetra-n-butyl ammonium bromide (TBAB), tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), and their mixtures. The mole fractions of TBAB, THF, and DMSO aqueous solutions were fixed at 0.0234, 0.0556, and 0.0165, respectively. The equilibrium hydrate formation conditions were measured by T-cycle method in the temperature range of (274.15 to 294.95) K and the pressure ranges up to 6.72 MPa. The gas phase in the crystallizer at the equilibrium points was also sampled and analyzed. For the additives with the fixed concentrations studied in this work, it was found that both TBAB and THF can remarkably reduce the equilibrium hydrate formation pressure of LFG mixture gas, but the effect of THF is better than that of TBAB in the high temperature region, while DMSO have no obvious pressure drop effect on the equilibrium hydrate formation conditions but can promote the solubility of CO2 in the solution. However, the mixture additives of TBAB + DMSO and THF + DMSO can not only remarkably promote the solubility of CO2 but also remarkably reduce the equilibrium hydrate formation pressure of CO2 + CH4 + H2O hydrate. Moreover, the pressure drop effect of THF + DMSO is better than that of TBAB + DMSO on the CO2 + CH4 + H2O equilibrium hydrate formation in the high temperature region.

The gas production behavior from methane hydrate in a porous sediment by depressurization was investigated in a three-dimensional (3D) cubic hydrate simulator (CHS) at 281.15 K, hydrate saturation of 33.1%, and a production pressure range of 4.5–5.6 MPa. The results show that the gas production process consists of three periods: free gas production, mixed gas (free gas and gas dissociated from the hydrate) production, and gas production from the hydrate dissociation. The temperature in the near-well region in the 3D hydrate reservoir changes during gas production in five stages. In the first period, the free gas in the system is released and the temperature change is not significant. In the second period, the temperature increases because of the reformation of the hydrate. In the third period, the temperature at each measuring point decreases quickly to the lowest value because of the considerable dissociation of the hydrate. The fourth period is the thermostatic hydrate dissociation period. During this period, the temperature at each point remains constant. In the fifth period, the hydrate has almost dissociated completely and the temperatures gradually increase to the environmental temperature of 281.15 K. There is no thermostatic dissociation period in the far-from-well region, in which the temperature at each measuring point gradually increases after it reaches the lowest value. In the third period of gas production, the temperatures in the near-well region are lower than those in the far-from-well region. In the gas production process, the resistances in the hydrate reservoir change with the hydrate dissociation and the flow of the gas and water. It can also be found that the gas production rate and the cumulative gas production increase with the decrease of the pressure. The gas hydrate dissociation in the gas production process is mainly controlled by the rate of the pressure reduction in the system and the heat supplied from the ambient. There is significant water production in the free gas production process. However, there is little water production in the hydrate dissociation process.

In this work, the decomposition behavior of methane hydrate in porous media was investigated in the three-dimensional cubic hydrate simulator (CHS) using the huff and puff method with a single well, which is divided into three stories, distributed in the center line position of the reactor. The thermal huff and puff process is analyzed, and it is concluded that each huff and puff cycle includes three stages: heat injection, soaking, and gas production. The injected heat spreads out from the injection point. The region impacted by heat diffusion enlarges as the number of huff and puff cycles increases and eventually reaches the largest impact region. After that, the region impacted by heat diffusion is no longer increasing with continuous heat injection. The change characteristics of the injection temperature, pressure, resistance ratio, and other related parameters during the injection are obtained. The result also verifies that the hydrate decomposition process is a moving boundary ablation process in the three-dimensional level.

The dissociation kinetics of methane hydrate in the silica gels as the porous media are studied when the temperatures are above the quadruple-phase (hydrate(H)–water(LW)–ice(I)–vapor(V)) point temperatures. The dissociation experiments were carried out by depressurization in the temperature range of 269.15–278.15 K and the initial formation pressure range of 4.1–11.0 MPa. Four silica gels, with pore sizes of 9.03, 12.95, 17.96, and 33.2 nm (the particle size distribution is 0.105–0.15 mm) were used. The experimental results shows that the rate of methane released from the hydrate dissociation increases with the increase of the initial formation pressure, the decrease of the environmental temperature, and the increase of the pore size. The temperature in the hydrate crystallizer first immediately has an obvious drop in the process of the hydrate dissociation and then rises gradually to the environmental temperature after it reaches the lowest temperature point. Based on the fractal theory and the shrinking-core model, a fractional dimension shrinking-core model is established for the correlation and prediction of the dissociation kinetic behaviors of methane hydrate in the porous media above the corresponding quadruple-phase point temperatures. The results are in good agreement with the experimental data.

Trapping CO2 in hydrates is a promising approach to reduce the greenhouse gas emissions. This work presents the efficient separation process of CO2 from the simulated fuel gas (39.2 mol % CO2/H2 binary mixture) based on the hydrate crystallization in the presence of tetra-n-butylammonium bromide (TBAB). The experiments were carried out in the TBAB concentration range of 0.14−1.00 mol %, the temperature range of 275.15−285.15 K, the driving force range of 1.00−4.50 MPa, the gas/liquid phase ratio range of 0.86−6.47, and the hydrate growth time from 15 to 120 min. The results indicate that the increase of the TBAB concentration or the driving force can enhance the separation efficiency, except when the TBAB concentration is above 0.29 mol % or the driving force is above 2.50 MPa. The lower gas/liquid phase volume ratio and the hydrate growth time can also promote gas consumption. However, H2 more competitively encages into the hydrate phase with time. In addition, the temperature change has little effect on the CO2 separation efficiency with the fixed driving force. It is worth noting that the one-stage hydrate formation/decomposition process for the fuel gas in the presence of 0.29 mol % TBAB at 278.15 K and 2.50 MPa driving force could obtain a 96.85 CO2-rich gas and a 81.57 mol % H2-rich gas. The split fraction (SFr) and separation factor (SF) of CO2 are 67.16% and 136.08, respectively. On the basis of the data of the separation efficiency, a hybrid conceptual process for precombustion capture based on only one hydrate formation/decomposition stage coupled with membrane separation is presented.

The equilibrium hydrate formation conditions for methane in the presence of the aqueous solutions of the five ionic liquids with the mass fraction of 0.1 have been investigated. The data were measured using an isochoric method in the pressure range of (3 to 17) MPa and the temperature range of (276.15 to 289.15) K. It is found that the additions of the ionic liquids shift the methane hydrate equilibrium phase boundary to the temperature and pressure conditions that are unfavorable for the hydrate formation. The dialkylimidazolium-based ionic liquids with the hydroxylated cations exhibit an enhanced effectiveness in inhibiting hydrate formation. For the tetraalkylammonium-based ionic liquids, ones with the shorter alkyl substituents of the cations perform better thermodynamic inhibition effects than ones with the hydroxylated longer alkyl substituents of the cations. Among all of the ionic liquids studied, tetramethyl-ammonium chloride is the most effective one, which is comparable with ethylene glycol.

The Shenhu Area is located in the Pearl River Mouth Basin, the northern continental slope of the South China Sea. In 2007, gas hydrate samples were recovered during the scientific expedition conducted by the China Geological Survey in the area. Using numerical simulation and currently available data from site measurements, including the water depth, thickness of the hydrate-bearing layer (HBL), sediment porosity, salinity, and pressures and temperatures at key locations, we developed preliminarily estimates of the production potential of these hydrates as gas-producing resource. We used measurements of ambient temperature in the sediments to determine the local geothermal gradient. Evidence from this and other field studies showed that the initial pressure distribution followed the hydrostatic gradient. Direct measurements from core samples provided estimates of the initial hydrate saturation and of the intrinsic permeabilities in the various strata of the system. The hydrate accumulations in the Shenhu Area appear to be hydrate deposits involving a single HBL within fine-textured sediments and boundaries (overburden and underburden layers) which have the same intrinsic permeabilities with the HBL. We investigated gas production from the Shenhu hydrates by means of depressurization and thermal stimulation using a single horizontal well placed in the middle of the HBL. The simulation results indicated that the hydrates dissociate along cylindrical interfaces around the well and along horizontal dissociation interfaces at the top and bottom of the HBL. Production is invariably lower than that attainable in a confined system, and thermal stimulation is shown to affect only a limited region around the well. The sensitivity analysis demonstrates the dependence of production on the level of depressurization, the initial hydrate saturation, the intrinsic permeability of the HBL, the temperature of the well, and the initial temperature of the HBL. A general observation is that gas production is low and is burdened with significant water production, making the hydrate accumulations at the Shenhu Area unattractive production targets with current technology.

To determine the suitable operating conditions for the hydrate-based CO2 separation process from a fuel gas mixture, the hydrate nucleation and growth kinetics of the simulated fuel gas (39.2 mol % CO2/H2 gas mixture) in the presence of tetra-n-butyl ammonium bromide (TBAB) are investigated. The experiments were conducted at the TBAB concentration range of 0.14−1.00 mol %, the temperature range of 275.15−282.45 K, the driving force range of 1.00−4.50 MPa, the gas/liquid phase ratio range of 0.86−6.47, and the hydrate growth time of 15−120 min. It is found that the addition of TBAB not only shortens the induction time and accelerates the hydrate growth rate, but also enhances CO2 encaged into the hydrate. However, the number of total moles of gas consumed and the number of moles of CO2 transferred into the hydrate slurry phase decrease with the increase of the TBAB concentration when the TBAB concentration is above 0.29 mol %. The induction time reduces, and the number of moles of gas consumed, the hydrate formation rate, and the number of moles of CO2 encaged into hydrate phase increase with the increase of the driving force. However, when the driving force is more than 2.5 MPa, H2 prefers to go into the hydrate phase with the increase of the driving force, as compared to CO2. In addition, the temperature has little effect on the hydrate formation process.

The equilibrium hydrate formation conditions for the gas mixture of CO2 and H2 with tetrabutyl ammonium bromide (TBAB) are measured. The data show that TBAB can reduce the gas hydrate formation pressure as an additive with the mole fraction of (0.14, 0.21, 0.29, 0.50, 1.00, and 2.67) %. The experiments were carried out in the temperature range of (274.05 to 288.55) K and the pressure range of (0.25 to 7.26) MPa. The equilibrium hydrate formation pressure of the CO2 + H2 + TBAB mixture is remarkably lower than that of the CO2 + H2 mixture at the same temperature and decreases with the increase in the concentration of TBAB. In addition, to avoid the formation of pure TBAB hydrate, in which there are no CO2 or H2, for the above gas mixture, the formation conditions of the pure TBAB hydrate with TBAB mole fraction from (0.14 to 2.67) % were also measured.

The phase equilibrium data of cyclopentane (CP) + methane hydrates in brine water with NaCl mass fractions of w = 0, 0.035, 0.070, and 0.100 were measured in the temperature range (284.4 to 301.3) K using a visual high-pressure phase equilibrium apparatus. The dissociation enthalpies of CP + methane hydrates in hot brine water were determined via the Clausius−Clapeyron equation based on these phase equilibrium data. The effect of help gas and salinity on the phase equilibrium of CP hydrate and CP + methane hydrates was studied, respectively. Liquid CP forms hydrates with small-molecule help gas at a temperature region much higher than the quadruple point of pure CP hydrate. Methane partially occupied the small cages of structure II hydrate, which accelerates the nucleation and growth of CP + methane hydrates and increases the stability of the hydrates. The phase equilibrium pressure of CP + methane hydrates increases with the increase in temperature, and it increases linearly with the increase in NaCl concentration in solutions. The higher the temperature, the more remarkable effect the temperature and salinity has on the phase equilibrium pressure. The dissociation enthalpy decreases with the increase in temperature and the NaCl concentration.

The acid–base stabilities of Al13 and Al30 in polyaluminum coagulants during aging and after dosing into water were studied systematically using batch and flow-through acid–base titration experiments. The acid decomposition rates of both Al13 and Al30 increase rapidly with the decrease in solution pH. The acid decompositions of Al13 and Al30 with respect to H+ concentration are composed of two parallel first-order and second-order reactions, and the reaction orders are 1.169 and 1.005, respectively. The acid decomposition rates of Al13 and Al30 increase slightly when the temperature increases from 20 to ca. 35 °C, but decrease when the temperature increases further. Al30 is more stable than Al13 in acidic solution, and the stability difference increases as the pH decreases. Al30 is more possible to become the dominant species in polyaluminum coagulants than Al13. The acid catalyzed decomposition and followed by recrystallization to form bayerite is one of the main processes that are responsible for the decrease of Al13 and Al30 in polyaluminum coagulants during storage. The deprotonation and polymerization of Al13 and Al30 depend on solution pH. The hydrolysis products are positively charged, and consist mainly of repeated Al13 and Al30 units rather than amorphous Al(OH)3 precipitates. Al30 is less stable than Al13 upon alkaline hydrolysis. Al13 is stable at pH < 5.9, while Al30 lose one proton at the pH 4.6–5.75. Al13 and Al30 lose respective 5 and 10 protons and form [Al13]n and [Al30]n clusters within the pH region of 5.9–6.25 and 5.75–6.65, respectively. This indicates that Al30 is easier to aggregate than Al13 at the acidic side, but [Al13]n is much easier to convert to Alsol–gel than [Al30]n. Al30 possesses better characteristics than Al13 when used as coagulant because the hydrolysis products of Al30 possess higher charges than that of Al13, and [Al30]n clusters exist within a wider pH range.

The gas production behavior from methane hydrate in porous sediment by injecting the brine with the salinity of 0−24 wt % and the temperature of −1 to 130 °C was investigated in a one-dimensional experimental apparatus. The results show that the gas production process consists of three periods: the free gas production, the hydrate dissociation, and the general gas reservoir production. The hydrate dissociation accompanies the temperature decrease with the injection of the brine (NaCl solution), and the dissociation duration is shortened with the increase of the salinity. With the injection of hot brine, instantaneous hydrate dissociation rate also increases with the increase of the salinity. However, while the NaCl concentration is beyond a certain value, the rate has no longer continued increasing. Thermal efficiency and energy ratio for the hydrate production can be enhanced by injecting hot brine, and the enhanced effectiveness is quite good with the injection of high salinity at lower temperature.

The perturbed-chain statistical associating fluid theory and density-gradient theory are used to construct an equation of state (EOS) applicable for the phase behaviors of carbon dioxide aqueous solutions. With the molecular parameters and influence parameters respectively regressed from bulk properties and surface tensions of pure fluids as input, both the bulk and interfacial properties of carbon dioxide aqueous solutions are satisfactorily correlated by adjusting the binary interaction parameter (kij). Our results show that the constructed EOS is able to describe the interfacial properties of carbon dioxide aqueous solutions in a wide temperature range, and illustrate the influences of temperature, pressure, and densities in each phase on the interfacial properties.

We employ two thermodynamic approaches, based on the equal fugacities and the equal activities, to predict the gas hydrate equilibrium dissociation conditions in the porous media. The predictions are made for the hydrate systems, CH4/H2O, C2H6/H2O, C3H8/H2O, CO2/H2O, CH4/CO2/H2O, C3H8/CH4/C2H6/H2O, and CH4/CH3OH/H2O. For the non-hydrate phase, we used the Trebble–Bishnoi equation in the fugacity approach and the Soave–Redlich–Kwong equation in the activity approach. For the hydrate phase, the van der Waals–Platteeuw model incorporated with the capillary model of Llamedo et al. [M. Llamedo, R. Anderson, B. Tohidi, Am. Mineral. 89 (2004) 1264–1270] was used in the two approaches. The predictions are found to be in satisfactory to good agreement with the experimental data. The predictive ability of the fugacity approach is better than that of the activity approach.

This article investigates the gas production behavior from methane hydrate (MH) in porous sediment by injecting ethylene glycol (EG) solution with the different concentrations and the different injection rates in an one-dimensional experimental apparatus. The results suggest that the gas production process can be divided into the four stages: (1) the initial injection, (2) the EG diluteness, (3) the hydrate dissociation, and (4) the remained gas output. Nevertheless, the water production rate keeps nearly constant during the whole production process. The production efficiency is affected by both the EG concentration and the EG injection rate, and it reaches a maximum with the EG concentration of 60 wt %.

A new method, a molecular thermodynamic model based on statistical mechanics, is employed to predict the hydrate dissociation conditions for binary gas mixtures with carbon dioxide, hydrogen, hydrogen sulfide, nitrogen, and hydrocarbons in the presence of aqueous solutions. The statistical associating fluid theory (SAFT) equation of state is employed to characterize the vapor and liquid phases and the statistical model of van der Waals and Platteeuw for the hydrate phase. The predictions of the proposed model were found to be in satisfactory to excellent agreement with the experimental data.

•Effects of particle and pore sizes on hydrate formation in silica gel were studied.•Hydrate formation rate and final gas uptake increase as the pore size increases.•The hydrate formation rate deceases dramatically as the particle size increases.•Three discrete formation periods were found in silica gel with large particle size.•The durations are basically same at different conditions with small particle size.The formation behaviors of methane hydrate in porous media with different particle and pore sizes were studied in a closed system in the temperature range of 267.15–278.15 K. The silica gels were applied as the porous media for the experiments, in which the diameter ranges of the silica gel particles are 0.105–0.150 mm, 0.150–0.200 mm and 0.300–0.450 mm, respectively, and the mean pore diameters are 9.03 nm, 12.95 nm, 17.96 nm and 33.20 nm, respectively. The formation processes of methane hydrate show different behaviors in porous silica gels with different particle sizes. For the particle diameter range of 0.300–0.450 mm, three formation periods were observed for the experiments with the high driving force. For the particle diameter ranges of 0.105–0.150 mm and 0.150–0.200 mm, there is no remarkable discrete formation period, the hydrate durations of different experiments are basically same at the bath temperature above the freezing point and decrease with the increase of the temperature at the bath temperature below the freezing point. The formation rate of methane hydrate increases with the decrease of the particle size and the increase of the pore size. The final gas consumption increases with the increase of the mean pore diameter but is slightly affected by the particle size.

The equilibrium hydrate formation conditions for CO2/H2 gas mixtures with different CO2 concentrations in 0.29 mol% TBAB aqueous solution are firstly measured. The results illustrate that the equilibrium hydrate formation pressure increases remarkably with the decrease of CO2 concentration in the gas mixture. Based on the phase equilibrium data, a three stages hydrate CO2 separation from integrated gasification combined cycle (IGCC) synthesis gas is investigated. Because the separation efficiency is quite low for the third hydrate separation, a hybrid CO2 separation process of two hydrate stages in conjunction with one chemical absorption process (absorption with MEA) is proposed and studied. The experimental results show H2 concentration in the final residual gas released from the three stages hydrate CO2 separation process was approximately 95.0 mol% while that released from the hybrid CO2 separation process was approximately 99.4 mol%. Thus, the hybrid process is possible to be a promising technology for the industrial application in the future.

A new method of temperature fluctuation is proposed to promote the process of hydrate-based CO2 separation from fuel gas in this work according to the dual nature of CO2 solubility in hydrate forming and non-hydrate forming regions The temperature fluctuation operated in the process of hydrate formation improves the formation of gas hydrate observably. The amount of the gas consumed with temperature fluctuation is approximately 35% more than that without temperature fluctuation. It is found that only the temperature fluctuation operated in the period of forming hydrate leads to a good effect on CO2 separation. Meanwhile, with the proceeding of hydrate formation, the effect of temperature fluctuation on the gas hydrate gradually reduces, and little effect is left in the completion term. The CO2 separation efficiencies in the separation processes with the effective temperature fluctuations are improved remarkably.

•The production behaviors of methane hydrate are obtained in the 3-D simulator.•The different methods with a five-spot well are used for hydrate production.•The water and gas production, efficiency, recovery, production rate are analyzed.•The heat stimulation in conjunction with depressurization is the optimal method.The cubic hydrate simulator (CHS) is used to study the methane hydrate production behaviors in porous media with three different methods, which are the depressurization (DP) using a single well, the heat stimulation using five-spot well (HS-5S), and the heat stimulation in conjunction with depressurization using five-spot well (H&D-5S), respectively. During these experiments, the initial hydrate saturations are all 35%; environmental temperatures are all 281.15 K; and the production pressures are ranged from 4.7 MPa to 7.4 MPa. The injection temperature (Tinj) and the rate of hot water injection (Rinj) in the experiments with the HS-5S and H&D-5S methods are 10 ml/min and 160 °C, respectively. The analysis of hydrate decomposition shows that almost all of the hydrate can be decomposed in the hydrate reservoirs with the above methods. The H&D-5S method, which can obtain the largest volume of gas production, the highest gas production rate, and the shortest production time, is the optimal method for hydrate production in this work.

•A new kinetic model is established for hydrate formation in porous media.•The intrinsic reaction rate constant k0 is obtained.•The validity of the proposed model is verified with the experimental data.•The new model performs better than the other kinetic models.A new kinetic model is established to investigate into the characteristics of methane hydrate formation in the porous media. In this model, the hydrate formation is mainly controlled by the mass transfer at the gas–water contact area, which is formed between the liquid and the “gas bubbles” in the pores. Six experimental formation runs are carried out in a three-dimensional pressure vessel, the Pilot-Scale Hydrate Simulator (PHS). The experimental data for four of the six runs are correlated with the kinetic model, and the intrinsic reaction rate constant k0 and the activation energy ΔEa are determined to be approximately 8.06 kg/(m2 Pa s) and 8.09×104 J/mol, respectively. The system pressures, the hydrate formation rates, and the amount of the remaining free methane gas at other conditions are predicted from the obtained k0 and ΔEa. Compared with the experimental data from other two runs, the predicted results are in good agreement, which verifies the reliability of the proposed kinetic model for the prediction of methane hydrate formation in porous media. Sensitivity analysis indicates that the hydrate formation is strongly dependent on the kinetic parameters of k0 and the reduction exponent β.

•The scaling criteria for methane hydrate reservoir are built.•The scaling criteria are verified by the experiments in two 3-D simulators.•The scaling criteria are used for predicting gas production of real hydrate reservoir.•Methane of 1.168 × 106 m3 is produced from the hydrate reservoir after 13.9 days.The Cubic Hydrate Simulator (CHS), a three-dimensional 5.8 L cubic pressure vessel, and the Pilot-Scale Hydrate Simulator (PHS), a three-dimensional 117.8 L pressure vessel, are used for investigating the production processes of hydrate. The gas production behaviors of methane hydrate in the porous media using the thermal stimulation method with a five-spot well system are studied. The experimental conditions are designed by a set of scaling criteria for the gas hydrate reservoir. The experimental results verify that the scaling criteria for gas hydrate production are reliable. The scaling criteria are used for predicting the production behavior of the real-scale hydrate reservoir. In the model of the real-scale hydrate reservoir with the size of 36 m × 36 m × 36 m, methane of 1.168 × 106 m3 (STP) is produced from the hydrate reservoir during 13.9 days of gas production. It is obtained that the gas recovery is 0.73, and the final energy efficiency is 9.5.

•Hydrate dissociation by depressurization in different-scale reservoirs was executed.•Single horizontal well was performed for production well.•Entropy generation model during depressurization process was established.Based on the hydrate conditions of the South China Sea, hydrate samples were synthesized in the Cubic Hydrate Simulator (CHS) and the Pilot-Scale Hydrate Simulator (PHS), and hydrate dissociation experiments by depressurization with single horizontal well were carried out. In order to illuminate the characteristic of the irreversible energy loss during the gas production process in a large-scale hydrate simulator and a smaller hydrate simulator, the entropy generation model was established. Results show that the evolutions of the pressure, temperature, gas production, and water production during hydrate dissociation process with different scales of hydrate reservoir are similar. Moreover, entropy generation in the mixed gas release stage is the largest. In addition, in the dissociated gas release stage, the ratio of entropy generation decreases remarkably with the increase of the hydrate reservoir scale, and constant-pressure depressurization method is favorable for hydrate dissociation in a larger scale hydrate reservoir.