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Clathrate hydrate is a kind of environment-friendly material for storing hydrogen under a certain condition of temperature and pressure. In this work, tetrahydrofuran aqueous solution with concentration of 3.0 mol% was adopted to investigate hydrogen storage process. Moreover, thermal property of hydrate was measured by high pressure differential scanning calorimeter, and mechanism of hydrate-based hydrogen storage was studied by in-situ Raman. Especially, gas uptake, morphology and structures change of compounds from gas/liquid interface towards hydrate layer were monitored in the process of hydrate formation. Thermal experiments illustrate that thermal data for tetrahydrofuran-hydrogen binary hydrate under extra high pressures could be effectively obtained by high pressure differential scanning calorimeter, moreover, memory effect shows no influence on thermal state of hydrate but weakly affects water aggregation. Kinetics and microscopic experiments illustrate that a special pressure drop and some tetrahydrofuran hydrates with unstable structure can be found under conditions of 273.15 K and 14.53 MPa. The pressure drop involves into hydrogen molecules tunneling movement among hydrate cavities. Moreover, hydrogen molecules show a positive effect on binary hydrate stability. Further, the density of 1.875 g/Lwater shows that hydrogen storage process via clathrate hydrate is an excellent method to store hydrogen.

Energy shortage and carbon emission reduction are the two big problems in the development of human society. The technologies involving CH4-CO2 binary hydrate is considered to be promising for CH4 recovery and carbon emission reduction. The DSC, Raman, FTIR, Cryo-SEM and PXRD are employed to investigate the thermal process, the micro structure and compositions of the CH4-CO2 hydrate formation and decomposition. The investigations reveal that there are not one kind of hydrate but rather multi-kinds of hydrates coexistence during the hydrate formation. The mechanism of gas hydrate formation could be considered as, under a certain condition, the component with lower enthalpy prior to entrap the cavities to stabilize the hydrate cages in the process of constructing hydrate cages by water molecules, and once the relevant cages are stabilized, the hydrates thereby exist. To fully disperse the oil additive (e.g. CP) into water can effectively improve the gas consumption and enhance CO2 separation efficiency in the process of CH4-CO2 binary hydrate formation. The methods presented here can also be employed for other fields such as hydrate-based sea-water desalination, CO2 separation and H2 purification from IGCC syngas, gas transportation, and other fields.

Hydrate-based carbon dioxide (CO2) capture and hydrogen (H2) purification is a promising technology in clean energy fields. In this work, in order to reveal the effect and mechanism of tetrahydrofuran (THF) on the hydrate-based CO2 separation from Integrated Gasification Combined Cycle (IGCC) syngas, the CO2/H2/THF hydrates formation processes were studied with and without memory effect. According to the pressure drop curves, there appear two pressure plateaus in the CO2/H2/THF hydrate formation processes. Furthermore, with the usage frequency of the THF solution increasing, the plateau effects are more ambiguous and difficult to be observed. It is interesting that the Raman spectra for CO2 and H2 molecules also reveal slim Raman shifts between the two different hydrate plateaus. According to the powder X-ray diffraction (PXRD) patterns, indeed, the detail Miller indices indicates that CO2/H2/THF hydrate mainly forms THF•16.8 H2O structure in the first plateau, while mainly forms THF•17 H2O structure in the second plateau. The reason for this phenomenon is mainly the influence of CO2, its large molecular size and the localized tension it causes in the water network of the small cages which can enhance the storage capability for the large cages of THF hydrate. The experimental results illustrate that the highest Split fraction (S.Fr) is 69.02% obtained at 6 MPa/284.85 K (memory effect), and this work highlights that the memory solution are more suitable for industrial application.

Memory effect of gas hydrate is a double-edged sword in hydrate-based application and natural gas hydrates exploitation. In this work, in order to acquire a comprehensive understanding of memory effect, we conduct a series of experiments on hydrate reformation and dissociation under different grain filling patterns and thermal history conditions. Experimental results reveal that the memory effect can not only shorten the induction time of hydrate nucleation but also significantly reduce the hydrate formation rate by enhancing the homogeneous distribution of gas hydrate in pores. The homogeneous hydrate distribution under memory effect has been further investigated and evaluated by the hydrate heterogeneity degree and dead-end porosity for the first time. More importantly, the decrease in heterogeneity degree and dead-end porosity driven by memory effect shows significant effects on hydrate dissociation behaviors. On the one hand, the improved homogeneous distribution of gas hydrate under memory effect impairs the heat transfer from the environment to hydrate-bearing sediments, thereby reducing the hydrate dissociation rate. On the other hand, the decreased dead-end porosity can lead to the expansion of fluid flow channels in hydrate-bearing sediments, thus increasing the hydrate dissociation rate. These findings are significant for efficient and secure gas production in field tests since the violent gas/water flow in reservoirs would lead to the rapid hydrate reformation during gas production from hydrate-bearing reservoirs.

In order to study the mechanism of methane hydrate formation in porous media, the formation experiments of methane hydrate in porous media at the constant pressure were performed in the temperature range of 274.15–276.15 K and the pressure range of 5–8 MPa. The silica gels with the average pore diameters of 129.5, 179.6, and 332 A were used as the porous media for the experiments. The experimental results indicate that the final gas consumption increases with the increase of the formation pressure and the decrease of the formation temperature. Based on the shrinking core model, the reaction-controlled kinetic model and the diffusion-controlled kinetic model for hydrate formation in silica gels were built, respectively. The reaction-controlled kinetic model well fits the kinetic data in 129.5 A and 179.6 A silica gels, and the diffusion-controlled model well fits the kinetic data in 332 A silica gels with a relatively high regression coefficient (R2 > 0.99). The formation rate of the methane hydrate is controlled by the gas diffusion process in 129.5 A and 179.6 A silica gels, and is controlled by the reaction process in 332 A silica gels.

Hydrate carbon dioxide (CO2) separation is a promising method for reducing carbon emission. In this work, water-solubility of tetrahydrofuran (THF) was added into water to generate the single gas/liquid interface. In order to understand hydrate nucleation and crystallization well, CO2 concentration in the residual gaseous phase was measured, morphology of the hydrate formation was filmed, and structure changes of compounds around the gas/liquid interface was monitored by in situ Raman spectrometer. Two groups of experiments were carried out at 274.15 K and 4.0 MPa in the systems with and without gas supply. The experimental results illustrate that hydrate formation is completed in 5 h according to CO2 concentration, gas consumption and morphology, however, the compound transition and hydrate crystallization are still in process from the microstructure point of view. For the system with gas supply, the hydrates initially occur in the gas/liquid interface due to stable gas flux in the boundray layer, where Raman spectra change regularly at the beginning. Such stable gas flux has a positive impact on changing water aggregation. This change of water aggregation benefits for the original structures in the process of hydrate nucleation. With the hydrate formation, the hydrate nucleation interface is moving from the gas/liquid interface towards the THF solution. Otherwise, for the system without gas supply, no obvious hydrate was observed in the gas/liquid interface, and Raman spectra around the interface change with the saltation from gaseous phase towards the THF solution. For the two systems, THF hydrates form prior to the multi-hydrates and keep forming, and both intensity of Raman peaks around the interfaces is the weakest.

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.

In this work, the hydrate-based methane (CH4) separation from coal mine methane (CMM) gas mixture was carried out by bubbling with a scale-up equipment (SHW-II). The influences of gas/liquid volume ratios (0.25 and 0.60), gas bubble sizes (diameter: 20, 50 and 100 μm) and gas flow rates (7.50, 16.13 and 21.50 mL/min/L) on gas consumption and CH4 recovery were systematically investigated at 277.15 K and 1.50 MPa. The hydrate formation morphology was filmed by a camera and the hydrate structure was determined by powder X-ray diffraction (PXRD). Gas bubbles generated when gas mixture flowed into bulk solution through a bubble plate from the bottom of SHW-II. Initially, the gas hydrates formed at the bubble boundary and grew up as the shell around the bubble with the continuously rising of the gas bubble, and finally accumulated in the interface between the gaseous phase and solution. The experimental results showed that the THF/CH4/N2 hydrate in SHW-II presented structure II (sII). The gas/liquid volume ratio, gas bubble size and gas flow rate had influences on gas consumption and CH4 recovery. The increase of gas/liquid volume ratio resulted in the decrease of gas consumption and CH4 recovery, while the increase of gas flow rate caused the decrease of gas consumption. Both the maximum gas consumption and CH4 recovery were achieved at the gas bubble with diameter of 50 μm. The optimal operating condition for large-scale CH4 separation via clatharate hydrate was comprehensively defined as the gas/liquid volume ratio of 0.25, the gas bubble diameter of 50 μm and the gas flow rate of 16.13 mL/min/L at 277.15 K and 1.50 MPa.

BaSed on the hot brine in situ seafloor prepared for marine NGHs exploitation, the formation of hydrates and the characteristics of dissolved ions were investigated for the cyclopentane (CP)-methane-NaCl solution (3.5%) system. Both the gas consumption and the solution salinity influenced by two factors - the flow rate of gas (Q(g)) and the mass fraction of CP (M-CP)-were discussed. On one hand, the gas consumption went up at a lower Mcp (3.950 wt%) while dropped down at a higher M-CP (8.340 or 18.775 wt%) with the increase of Q(g). Nevertheless, higher mass fraction of CP behaved more favorable for the gas consumption. On the other hand, there would be a similar trend that the salinity of remaining liquid increased firstly and then decreased with the reaction time at any fixed Q(g) and M-CP, which might be attributed to the adsorption of Na+ and Cl- on the surface of hydrate. Furthermore, PXRD analysis of the hydrate was conducted to confirm this explanation. And it was confirmed that the ion of Na+ or Cl- did not play any role in the construction of hydrate cages. Meanwhile, CP was enclosed in large cavities (51264s) while CH4 was mainly enclosed in the small cavities (512). (C) 2016 Elsevier Ltd. All rights reserved.

The combined styles of depressurization and electrical heating have an important influence on hydrate recovery and energy use in hydrate exploitation. However, the efficient combined styles of depressurization and electrical heating have not been achieved at present. In this work, six combined styles of depressurization and electrical heating were designed. In order to determine efficient combined styles, a depressurized vertical wellbore and a heated horizontal wellbore were used to model these combined styles and further to dissociate hydrate-bearing samples prepared by the excess-water method. The results showed that electrical heating should be started before depressurization. Specifically, considering hydrate saturation increase of 0.327–2.47% in the hydrate stability region, electrical heating was proposed to start at the onset of fresh hydrate formation. Subsequently, the soaking through electrical heating was performed at a pressure below the equilibrium pressure at the ambient temperature, which increased the averaged hydrate dissociation rate by 7.72%. A lower shut-in pressure for the soaking could enlarge the effective heating radius in samples to improve hydrate dissociation. During depressurization, no electrical heating reduced the averaged water production rate by 80.99% and increased energy efficiency by 18.06%. So electrical heating was proposed to stop in the temperature recovering stage, but whether it was used or not in the temperature reducing stage should depend on exploiting conditions, due to secondary hydrate formation and ice formation at a lower back pressure. This work may offer some reference on the arrangement of depressurization and electrical heating in future field tests for hydrate exploitation.

Treatment and subsequent use of biogas are garnering huge interest for both energy recovery and mitigation of environmental impact. However, separation process is pivotal for increasing its calorific value and removing CO2. This work presents the kinetic and separation efficiency study as well as microcosmic structure analysis for purifying simulated biogas (45.0 mol% CO2/CH4 binary mixture) through hydrate crystallization approach. Particularly, synergic additives comprise gas solvent (dimethyl sulfoxide (DMSO)) and traditional hydrate promoter (tetrahydrofuran (THF) or tetra-n-butyl ammonium bromide (TBAB)) were proposed to enhance the hydrate-based separation process. The promotion mechanism was explored through in-situ Raman spectroscopy. The residual gas phase and the decomposition gas phase from the hydrate slurry were sampled and analyzed. Based on the experimental data, the gas storage capacity, unit system gas consumed rate, gas selectivity and separation efficiency were calculated for evaluating the separation process. It was found that, the synergic additives could promote the mixture hydrate formation process due to DMSO (acid gas solvent) could improve both rate and selectivity of CO2 during the dissolution and diffusion processes. In addition, the Raman analysis reveals that the simulated biogas forms structure II hydrate and semiclathrate framework with THF–DMSO and TBAB–DMSO respectively, and CH4 molecules are only found in the smaller (512) cages of the mixture hydrates. It is inferred that DMSO just performs as an acid gas solvent during the gas dissolution and diffusion processes but not participate in the hydrate framework formation. It will be of practical interest in relation to resolving the bottleneck of hydrate-based biogas purification technology and of potential importance for the industry application of gas hydrate.

In this work, temperature characteristics during hydrate formation were investigated in the cyclopentane-methane system using large-scale equipment with bubbling. The volume in each experiment was increased by a factor of 80 compared with previous work. A group of scaled-up experiments, under the optimal condition based on experiments in the smaller crystallizer, was carried out to verify the scale-up potential of hydrate-based thermal fluid production process. Another group of scaled-up experiments were performed with different variables (volume ratio of cyclopentane to water, gas/liquid ratio and pressure) to figure out the initial hydrate formation interface and optimize the temperature of the thermal fluid. Experimental results illustrate that hydrate-based thermal fluid production process can be scaled up by a factor of 80, however, the temperature of the thermal fluid needs to be optimized by adjusting variables accordingly. Moreover, the liquid cyclopentane/water interface more than the gas/liquid interface affects hydrate formation and hydrate accumulation. In particular, the hydrate formation interface initially occurs on the water side near the liquid cyclopentane/water interface. A large amount of hydrate accumulates in the bulk liquid cyclopentane phase, forming new hydrate formation interfaces. Thermal fluid with a specific temperature range can be produced based on the heat released and conducted from the hydrate formation interfaces.

Hydrate-based carbon dioxide (CO2) separation from gas mixture has been extensively investigated for it being process simple and environmentally friendly. However, as the concentration of CO2 in the gas mixture decreases, the condition of the hydrate formation becomes very harsh. Therefore, it is significantly difficult for a single hydrate-based method to separation CO2 completely. In this work, the first set of device for continuously separating CO2 from gas mixture was developed on the base of the method of hydrate combined with chemical absorption. The process feasibility of the combined method and the device were proved through experimental study on CO2 separation from integrated gasification combined cycle (IGCC) syngas. The experimental results also indicated CO2 could be completely separated from the balance component, and the estimated energy cost for CO2 separation with the combined method is about ¥209 per ton CO2, which is lower than that with cryogenic separation process by about 30.0%. The study provides scientific data and theoretical guidance for the industrial application of hydrate-based CO2 separation and capture in future.

Investigation of the optimal injection temperature for the hydrate dissociation plays a significant role in the gas hydrate exploitation in the practical field. In this work, the experiments of hydrate dissociation by depressurization in conjunction with thermal stimulation (DT) with the different injection temperatures are carried out in a Cubic Hydrate Simulator (CHS). Evaluation of the entropy production minimization (EPM), the energy ratio and the thermal efficiency are employed to investigate into the optimized injection temperature for hydrate dissociation. The thermal efficiency decreases with the increase of the injection temperature. The optimal injection temperatures for the hydrate dissociation from the points of the maximization of the energy ratio and the minimization of the entropy production, which are equivalent to maximizing the energy production and minimizing the energy consumption, respectively, are 38.8 °C and 37.9 °C. The results of evaluations from the two aspects are in a quite good agreement. Thus, the warm water injection of approximately 38–39 °C is suitable for hydrate dissociation with the DT method, and the hot water injection beyond 39 °C is uneconomical for hydrate dissociation.

Natural gas hydrate, as an unconventional energy resource, has generated considerable research interest. It is generally accepted that depressurization method is the most practical and economically promising way to produce gas from gas hydrate sediments. Rates of hydrate dissociation by depressurization depend on heat transfer rate, and the heat transfer during hydrate dissociation mainly includes heat conduction and heat convection. In this paper the Pilot-Scale Hydrate Simulator (PHS), with an inner volume of 117.8 L, was applied to investigate the influence of heat conduction and heat convection on hydrate dissociation. Different thermal boundary conditions and different flow directions during gas recovery from hydrate reservoir by depressurization were performed in the PHS. In addition, the method of studying the effect of different directions of heat convection by changing well locations was firstly proposed in this paper. It was obtained from experimental results that the hydrate dissociation rate with an isothermal boundary is faster than that with a semi-adiabatic boundary, and heat conduction is the dominant factor in hydrate dissociation by depressurization in the constant pressure stage. The influence of heat convection on hydrate dissociation in the constant pressure stage may not be obvious, but during the depressurizing stage, the opposite direction of fluid flow and heat transfer can promote hydrate reformation, and has effect on fluid flow characteristics inside the reservoir. These findings can provide theoretical references for field tests of exploiting natural gas hydrate.

The cubic hydrate simulator (CHS) is used to study the methane hydrate production behaviors in porous media by the thermal stimulation with a five-spot well system. The hot water injection rates range from 10.0 to 40.0 ml/min. The thermal stimulation process is analyzed, and the conclusions are that the hydrate decomposition boundary moves from the central point to the surroundings gradually and finally covers almost the entire hydrate field in the CHS during the thermal stimulation process. The heat conduction plays a more significant role than the convection for the heat diffusion in the thermal stimulation process. The increasing injection rate of the hot water enhances the rate of hydrate decomposition, shortens the production time, and decreases the water production volumes, while it has little influence on the final gas production volumes. Furthermore, the change of the hot water injection rate (R-inj)has little influence on the final gas recovery, however, the higher R-inj leads to the higher average production rate and the lower energy efficiency. (C) 2013 Elsevier Ltd. All rights reserved.

The gas production behavior from methane hydrate in the sediment by depressurization was investigated in a novel pilot-scale hydrate simulator (PHS), a three-dimensional pressure vessel of 117.8 L. Experimental results are compared with those in a cubic hydrate simulator (CHS) with the effective volume of 5.8 L to reveal the dependence of the production behavior on the size of the hydrate reservoir. Results show that the gas production processes in the two simulators consist of three periods: the free gas production, mixed gas (free gas and gas dissociated from the hydrate) production and gas production from hydrate dissociation. The first and second periods are mainly controlled by the pressure reduction rate. The heat conduction from the ambient is main driving force to dissociate the hydrate in the third period. The cumulative gas production in the third period with the PHS and CHS is much higher than those in the first and second periods. However, the gas production rate in the period is low. The duration for gas production with the PHS is approximately 20 times as many as that with the CHS. Water production behavior with the PHS is different with that with the CHS during the gas production. The system temperature change tendency with the PHS is the same with that with the CHS during the gas production. The unique difference is that there is also a temperature rise period with the CHS.