Mixed Hydrate Research Articles

Experimental study of the formation and decomposition of mixed gas hydrates in water-hydrocarbon system

The phase transition of gas hydrate with two or more guests is associated with gas recovery, storage and separation. In this work, the formation and decomposition of binary and ternary gas hydrates containing CH4, C2H6 and C3H8 were investigated experimentally in high-pressure reactors equipped with visual windows. The gas compositions during mixed hydrates formation and decomposition were analyzed to track the occupation of guests in hydrate structures and the release of guests from hydrate cages. It was found that of CH4 was adsorbed in the hydrate cages constructed by C2H6 and C3H8 in the initial stage. In the depressurized decomposition of CH4-C3H8 hydrates and CH4-C2H6-C3H8 hydrates slurry, CH4 and C2H6 molecules were preferentially released from hydrate structures while more C3H8 molecules were able to stay in hydrate phase, which was in accordance with the order of the diameter ratio of guest molecule to hydrate cage. The preservation of CH4-C3H8 hydrates and CH4-C2H6-C3H8 hydrates above 0 ℃ was observed under a low pressure. It was proposed that the reconstitution or reformation of hydrate layer rich of C3H8 may encrust the remaining hydrate particles, preventing hydrate from further quick decomposition. In-situ Raman spectra confirmed the enrichment of C3H8 molecules at the surface of CH4-C3H8 hydrates, which revealed that the metastable hydrate layer rich of C3H8 molecules may act as mass transfer resistance of gas release from hydrate decomposition. Differently, the decompositions of CH4-C2H6 hydrates and C2H6-C3H8 hydrates were uniform as different guests were produced concurrently without preservation. The new insights into mixed hydrates decomposition in this work may provide a new basis for mixed gas production from gas hydrate, and gas storage and separation with hydrate.

Timing and Abundance of Clathrate Formation Control Ocean Evolution in Outer Solar System Bodies: Challenges of Maintaining a Thick Ocean within Pluto

Clathrate hydrates may represent a sizable fraction of material within the icy shells of Kuiper Belt objects and icy moons. They influence the chemical and thermal evolution of subsurface oceans by locking volatiles into the ice shell and by providing more thermal insulation than pure water ice. We model the formation of these crystalline compounds in conditions relevant to outer solar system objects, using Pluto as an example. Although Pluto may have hosted a thick ocean in its early history, Pluto’s overall heat budget is probably insufficient to preserve liquid today if its outer shell is pure water ice. One previously proposed reconciliation is that Pluto’s ocean has a winter jacket: an insulating layer of methane clathrate hydrates. Unfortunately, assessments of the timing, quantity, and type of clathrate hydrates forming within planetary bodies are lacking. Our work quantifies the abundance of clathrate-forming gases present in Pluto’s ocean from accreted ices and volatiles released during thermal metamorphism throughout Pluto’s history. We find that if Pluto formed with the same relative abundances of ices found in comets, then a buoyant layer of mixed methane and carbon dioxide clathrate hydrates may form above Pluto’s ocean, though we find it insufficient to preserve a thick ocean today. In general, our study provides methodology for predicting clathrate formation in ocean worlds, which is necessary to predict the evolution of the ocean’s composition and whether a liquid layer remains at present.

Kinetics studies of methane hydrate formation and dissociation in the presence of cyclopentane

The work systematically investigates the kinetic of methane(CH4) hydrate formation in the presence of cyclopentane（CP）by measuring induction time, hydrate formation rate, and gas uptake. The results showed that there are two steps for CH4 hydrate formation in the presence of CP: Step 1, pure CP hydrate formation. Step 2, gas enters the empty cages of pure CP hydrates to form CP/CH4 mixed hydrates. Besides, the rapid hydrate formation rate and high CH4 uptake are observed from hydrate formation in this work. At the optimum conditions of 288.15 K and CP mole fractions of 5.6 mol%, the gas uptake and the time required for completion of hydrate formation are 131.80 mmol/mol H2O and 138 min, respectively. Relative to the highest reported gas uptake obtained in the presence of thermodynamic promoter (72.0 mmol/mol H2O), the value increased by 83%. More strikingly, the work shows that high gas uptake hydrates formed at relatively low temperatures can be stored and transported at relatively high temperatures, as high as 298.15 K, without significantly reducing the gas uptake, thereby providing one of the cost-effective methods for hydrate storage and transportation, that is forming hydrates at relatively low temperatures and storing and transporting hydrates at relatively high temperatures. Further, we proposed a schematic of the process chain of SNG(storing natural gas via solid hydrate) technology in vehicular and detailed discuss the current status of SNG technology development. It can be concluded that using the strategy that storage and transportation hydrates at a low-pressure condition instead of at ambient pressure (1.0 atm) may be a more realistic choice to industrialize SNG technology.

Unraveling the Role of Natural Sediments in sII Mixed Gas Hydrate Formation: An Experimental Study.

Considering the ever-increasing interests in natural gas hydrates, a better and more precise knowledge of how host sediments interact with hydrates and affect the formation process is crucial. Yet less is reported for the effects of sediments on structure II hydrate formation with complex guest compositions. In this study, experimental simulations were performed based on the natural reservoir in Qilian Mountain permafrost in China (QMP) due to its unique properties. Mixed gas hydrates containing CH4, C2H6, C3H8, and CO2 were synthesized with the presence of natural sediments from QMP, with quartz sands, and without sediments under identical p-T conditions. The promoting effects of sediments regardless of the grain size and species were confirmed on hydrate formation kinetics. The ice-to-hydrate conversion rate with quartz sand and natural QMP sediments increased by 23.5% and 32.7%, respectively. The compositions of the initial hydrate phase varied, but the difference became smaller in the resulting hydrate phases, having reached a steady state. Beside the structure II hydrate phase, another coexisting solid phase, neither ice nor structure I hydrate, was observed in the system with QMP sediments, which was inferred as an amorphous hydrate phase. These findings are essential to understand the mixed gas hydrates in QMP and may shed light on other natural hydrate reservoirs with complex gas compositions.

Influence of the memory effect during CO2/CH4 mixed gas hydrate reformation process

The study of the memory effect can help to clarify the mechanisms of synthesis and decomposition of the CO2/CH4 mixed gas hydrate for mixed gas separation, CO2 sequestration, and CH4 recovery from unconventional natural gas reservoirs. To examine the impact of the memory effect on the CO2/CH4 mixed gas hydrate synthesis in terms of induction time, gas consumption rate, change of gas components, and separation factor, we conducted initial and second CO2/CH4 mixed gas hydrate synthesis experiments in a porous medium and deionized water system. The influence characteristics of the memory effect on CO2/CH4 hydrate synthesis under the predesigned decomposition temperatures (5 ∼ 25 °C) and pressures (0.5, 1.0, and 1.5 MPa) were investigated under decomposition times of 0.5 and 1.0 h. Compared to the induction time of the initial synthesis, the induction time of the second synthesis was greatly shortened, reaching a maximum of 87.42%. The induction time decreased with decreasing decomposition temperature and pressure in the same decomposition period. The decomposition time had a large effect on the peak value of the induction time during the second synthesis process. Besides, the memory effect was greatly affected by the decomposition time in depressurization cases, and the 0.5 h decomposition time was significantly better than that of 1.0 h. The memory effect had the strongest promotion effect on the second synthesis when the decomposition pressure and time were 1.0 MPa and 0.5 h, respectively. Furthermore, the memory effect improved the nucleation rate of the CO2/CH4 mixed gas hydrate and increased the amount of CO2 hydrate synthesis to 1.3 times that of the initial synthesis. Also, the memory effect influenced the hydrate to exhibit stronger CO2 selectivity, which provides a novel approach for the separation of mixed gases via the hydrate method.

Comparative studies on the formation and dissociation of mixed and methane gas hydrates using clay and zeolite

Comparative studies on the formation and dissociation of mixed and methane gas hydrates using clay and zeolite

Microwave-assisted rapid synthesize of cadmium–thallium mixed fluoride hydrate complex and its physiochemical properties

Herein we report a facile and rapid, cost-effective microwave-assisted synthesis of mixed cadmium-thallium fluoride hydrate compound using a homemade vessel with a special protective outer container. The physicochemical properties of the prepared compound were investigated by several characterization techniques. Single crystal X-ray diffraction measurement was performed to investigate the crystal structure of the prepared compound. The XEOL intensity shows a good linear response as a function of the delivered X-ray dose, which indicates that this material can be highly desirable for radiation detection applications. The fitting of scintillation decay kinetic from a241Am α− radiation source, shows the fastest decay time constant of 40 ns (16%) with medium and slow components of 380 ns (72%) and 1.5 μs (12%) at room temperature. The pulse height spectrum shows the photopeak of α− particles produced by the 241Am radiation source at 5.48 MeV.

Highly-selective xenon–krypton separation using hydrate-based technology

Xenon (Xe) and krypton (Kr) are used in many important industries such as lighting, lasers, double glazing for insulation, and medicine. As Xe and Kr are found in extremely low concentrations in the atmosphere, only small quantities of the mixture of these gases are obtained as byproducts from air separation processes. As the further separation of Kr from Xe is achieved via a costly energy-intensive cryogenic distillation method, the development of a new cost-effective approach for Xe–Kr separation is essential. In this study, we demonstrated that clathrate hydrates can be used as a promising Xe trap to separate a Xe–Kr mixture and produce Kr-rich vapors. The Xe and Kr guest distributions in mixed clathrate hydrates were experimentally identified using powder X-ray diffraction, solid-state 129Xe NMR spectroscopy, and gas chromatography. The mixed Xe + Kr hydrates have a Pm3¯n structure, termed structure I (sI), regardless of the concentration of Xe in the feed vapor in the range of 3% to 100%, and the sI hydrate selectively captures Xe from the feed mixture. The phase equilibria of the mixed Xe + Kr hydrates were also investigated, and it was confirmed that the hydrate-based Xe–Kr separation process requires milder temperature conditions than the cryogenic distillation process. Overall, the findings in this work suggest that the hydrate-based Xe–Kr separation process is an excellent alternative to the energy-intensive cryogenic distillation method.

Experimental investigation on the synergistic influence of tetra-n-butyl ammonium bromide(TBAB) and cyclopentane(CP) in hydrate-based gas separation

To enhance the CO2 separation efficiency of hydrate-based gas separation (HBGS) technology, the kinetic process of hydrate formation from synthesized CH4/CO2/N2 gas mixtures in the presence of cyclopentane (CP) and tetra-n-butyl ammonium bromide (TBAB) and gas separation efficiency were studied at an initial pressure of 7.0 MPa and temperatures ranging from 275.1 to 290.1 K. The crystallographic properties of the mixed hydrates were also evaluated by Raman, powder X-ray diffraction(XRD) and cryo-scanning electron microscop(cryo-SEM). Results showed that the combination of CP and TBAB tended to inhibit the separation efficiency compared with pure CP. The maximum CH4 recovery was 1.37 and the CO2 split ratio was 0.791 when using CP and TBAB solution. The order of CO2 split ratio in the different systems followed the order SDS > CP > TBAB/CP > CP/SDS > TBAB. Meanwhile, the adsorption capacity and gas consumption also decreased with an increase in TBAB concentration. Raman and XRD tests revealed that sI, sII, and semi-clathrate hydrates were found to coexist after hydrate formation and no other types of hydrates was formed. CH4 could only enter the small cages (512) of the hydrate. SEM images showed that the hydrate crystals were ellipsoid-shaped with different sizes and stacked together to form a globular structure, suggesting that the specific surface area of the hydrate crystals enhanced with the increasing of TBAB concentration. These findings can be used to evaluate the feasibility of applying TBAB and CP in HBGS technology.

Mechanisms of the memory effect of clathrate hydrates

The memory effect in the nucleation of gas hydrates refers to the phenomenon that gas hydrates nucleate easier or faster in the water that has a history of gas hydrate formation than in fresh water that has no such history. Although several hypotheses have been proposed to explain the cause of the memory effect over the last several decades, it remains the longest-standing mystery in the nucleation of gas hydrates. Supersaturation of the guest gas has been considered to occur in a memory system, however, the extent of its impact on the memory effect has been unclear. In the current research, we used linear cooling ramp experiments under isobaric conditions to determine the nucleation curves of CO2 hydrate in quiescent water in a stainless-steel sample cell and in quiescent quasi-free water droplets supported by a bulk liquid of perfluoromethyldecalin contained in a Teflon sample cell. From systematically comparing the nucleation curves of both freshly prepared samples and samples with a history of CO2 hydrate formation in both the stainless-steel sample cell and the quasi-free water droplets, as well as the nucleation curves of methane – propane mixed gas hydrates previously reported, a coherent picture emerged. The impurity imprinting component, with the interfacial gaseous states as the entity of the “impurity”, and the guest supersaturation component, with or without nanobubbles in the bulk water, are comparable in size, albeit the former is the larger one.

Desalination of produced water via CO2 + C3H8 hydrate formation

Recent years have seen increased interest in gas hydrate production as a viable saline water desalination technique. Produced water is the liquid that is extracted from oil and gas fields and is typically salty. The management of produced water is a significant aspect of the petroleum industry because of its potential to serve as a source of fresh water for oil-producing nations with limited access to clean water, growing environmental concerns, and the strict laws governing the discharge of produced water into the environment. In this study, the effect of mixed gas (CO2 + C3H8 (70:30)) hydrate formation in treating produced water and kinetic studies on induction time, moles of gas consumed, rate, water recovery, and water-to-hydrate conversion were experimentally investigated. An aqueous solution with the same ion concentrations was prepared and experiments were performed at 275.15 K/277.15 K, 2.0 MPa, and 450 rpm. The findings unveiled that with an increase in salinity, the water recovery decreased showing the hindering effect of salts on mixed hydrate formation by reducing the solubility of mixed gas due to the strong electrostatic force of attraction between salt ions and water. A maximum of 67.3% water recovery was observed in 2.8 wt% produced water solution at 275.15 K, 2.0 MPa, and 450 rpm. Furthermore, the induction time increased by 52.64% in treating 2.8 wt% and a decrease in moles of gas consumed, water recovery, and water-to-hydrate conversion by 6.99%, 2.08%, and 9.69% compared to the deionized water system at 275.15 K. A removal efficiency of 47%-63% is achieved by using CO2 + C3H8 and the sequence of metal cation removal is in the order K+ > Na+ > Mg2+ > Ca2+ and anions SO42− > Cl− at 275.15 K. The results also showed that with a rise in the experimental temperature to 277.15 K the removal efficiency increased though the amount of hydrate formed is less. This study illustrates that the formation of mixed gas (CO2 + C3H8) hydrate can be used for treating produced water based on the water recovery, removal efficiency, electrical conductivity, and total dissolved solids results with lower energy consumption.

Effects of X (X = H2S, SO2 and N2O) mole fractions on adsorption behavior and phase equilibrium properties of CO2 + X mixed gas hydrate

As a remedy for air pollution caused by flue gas, Hydrate Based Gas Separation (HBGS) technology is under development. Among many issues in HBGS, the prediction for the adsorption and dissociation conditions of hydrate and the impact from mixed gas on the formation of carbon dioxide hydrate are crucial to understand the sequestration and separation of CO2. In this work, the occupancy isotherms of the mixed systems of CO2 + H2S, CO2 + SO2 and CO2 + N2O with different mole fractions of H2S, SO2 and N2O in sI and sII hydrates are examined. The results show that X (H2S, SO2 and N2O) in flue gases can increase the absorption of mixed gas; CO2 + H2S in the sI and sII hydrates can be categorized as the one-site Langmuir type. The calculated abundance ratio of CO2 to X vary with the mole fraction of impurity gas and pressure, which provide the prerequisite information for prediction of gas recovery yield at different conditions during the CO2 + X gas separate process. The phase equilibria of clathrate hydrates of mixed gases are predicted and the hydration numbers are determined. The results show that the dissociation pressure of mixed hydrates decreases as the impurity gas mole fraction increases, indicating that SO2, H2S and N2O gases can all promote the formation of CO2 hydrate. Additionally, inspection indicates that the mixed flue gas can be efficiently separate by clathrate hydrate.

Visualization of CH4/CO2 hydrate dissociation and reformation during multistep depressurization assisted by pore-scale X-ray computed tomography

CH4–CO2 swapping is a promising and safe technique to produce CH4 gas and store CO2 without destabilizing hydrate-bearing sediments. However, inefficient CH4 recovery due to low CO2 diffusion prevents this technique from large-scale application. Multistep depressurization (MD) is therefore performed on CH4/CO2 hydrates after hydrate swapping to improve CO2 diffusion. This work presents pore-scale visualization of CH4/CO2 hydrates during MD by non-destructive X-ray computed tomography (CT) and combines the visual results with mixed hydrate kinetics to study hydrate morphology evolution and fluid migration. Influences of particle sizes (2–5 mm) and L-methionine (3000 ppm) on dissociation parameters were examined at reducing depletion pressures (8.5–59.4 bar) and constant temperatures (0.5–1.3 °C). The results of CT images showed enlargements of hydrate or gas phase with depletion pressures below CH4 hydrate stability pressures, complying with the pressure variations during MD. These variations of hydrate and fluid phases as well as pressure responses were the most obvious for particle size of 3 mm, resulted from good pore connectivity and relatively efficient mass transfer. L-methionine had no direct effect on CH4 production but enhanced CO2 storage, with the highest both CH4 gaseous mole fraction (86.1 mol%) and CO2 storage ratio (88.8%) at depletion pressure of 22.6 bar. This work clarified five-type pressure variations coupled with phase changes, providing firm evidence for hydrate dissociation/reformation and fluid migration that greatly affected hydrate exploitation. These understandings of varying morphologies and compositions of CH4/CO2 hydrates during MD would be applicable to help hydrate energy harvest and carbon emission mitigation.

Blended hydrate seed and liquid promoter for the acceleration of hydrogen hydrate formation

The slow formation rate of hydrogen clathrate hydrate is one of the most challenging factors impeding the utilization of clathrate hydrate as a hydrogen storage media. Blending of a hydrate seed solution and a liquid phase thermodynamic promoter is introduced in this study to promote the formation of hydrogen hydrates for safe hydrogen storage. Cyclopentane hydrate seeds induced instantaneous growth of cyclopentane + hydrogen mixed hydrates by providing nucleation sites for hydrate formation. Increasing the amount of hydrate seeds accelerated the formation rate of hydrates at the early stage of the reaction because the surface area of hydrate crystals increased. However, the total storage capacity was not so high because the diffusion of hydrogen molecules into empty cages of pre-constructed hydrate crystals was difficult. This diffusion limitation was overcome by the addition of the cyclopentane liquid phase promoter for the simultaneous formation of cyclopentane and hydrogen hydrates with their porous vertical growth. Tetrahydrofuran, one of the most commonly used organic promoters for hydrogen hydrates, was not suitable for the blended hydrate seed and liquid promoter system because its miscibility with water hindered the formation of hydrogen-bond cages and did not induce vertical growth of porous hydrates. This study provides insights into the role of liquid promoter blended with hydrate seeds in enhancing formation of hydrogen hydrates as sustainable hydrogen storage.

Experimental and Simulation Study for the Dissociation Behavior of Gas Hydrates─Part II: sII Mixed Gas Hydrates

This study is an extension of our research on the dissociation behavior of sI CH4 hydrates (Part I). In this study, investigations of the dissociation of binary and multicomponent structure II hydrates were carried out. A novel approach involving in situ and ex situ Raman spectroscopic measurements together with molecular dynamics simulations were applied for a systematic assessment of the hydrate dissociation and gas release behavior due to the destabilization of hydrate cavities. The hydrate decomposition was induced by thermal stimulation to mimic the warming of oceans and atmosphere brought by climate changes. The interactions between the released gas, aqueous phase, and hydrates were described. The results demonstrated that in the vicinity of the hydrate surface, the liquid phase was oversaturated with the gas molecules and some unstable partial hydrate cavities were also formed. Consequently, the release of gas was temporarily stopped or very slow. Throughout the process, the hydrate underwent decomposition–reformation–continuing decomposition until the crystal disappeared. A faster breakdown of small cavities (512) was recorded, leading to an increase in the large-to-small cavity ratio during dissociation. Moreover, the results indicated a faster release of CH4 molecules compared to C3H8 over the dissociation process of the sII CH4–C3H8 hydrate. This could be due to the higher diffusivity of CH4 molecules from the hydrate surface to the gas phase as well as its lower potential to stabilize the cavities compared to C3H8. The release of CH4 molecules was also faster compared to CO2 and C2H6 molecules in the sII mixed hydrate, leading to changes in the hydrate composition throughout the process.

Molecular hydrogen storage in binary H2-CH4 clathrate hydrates

Clathrate hydrate has emerged as a promising candidate for hydrogen storage, but the major challenge is the high pressure required for its formation. The addition of natural gas can significantly reduce the formation pressure, but also can improve the energy density. In this work, we estimated the hydrogen storage capacity of the H2-CH4 binary hydrate by performing the first-principles calculations and simulations. The thermodynamical and mechanical stability of the hydrate was studied as a function of the cage occupancy of both 512 and 51264 cages. For the most stable structure, the molar ratio of H2 and H2O is 0.353:1. The H2-CH4 binary hydrate can maintain its structure during dynamics under the moderate temperature and pressure. The self-preservation effect was observed at ∼270 K, which can be used for the hydrogen storage and transport. These findings provide a better understanding of the mixed hydrate as a viable hydrogen storage technology, which could enable us to achieve a sustainable hydrogen economy.

Nucleation Curves of Carbon Dioxide Hydrate in the Absence of a Solid Wall

Nucleation kinetics of clathrate hydrate is poorly understood because of the difficulties in determining the essential parameter, nucleation rate. Nucleation rate enables quantitative comparisons of the impacts of various additives and solid walls on nucleation. We made a setup that determines the nucleation curves of structure I─forming CO2 hydrate in quiescent quasi-free water droplets supported by a bulk of liquid perfluoromethyldecalin under isobaric conditions. The results were compared to the nucleation rates of CO2 hydrate in quiescent water that was in direct contact with stainless-steel walls. We assessed the convergence of the nucleation curves with the increasing numbers of nucleation data and compared our results to the nucleation rates of methane/propane mixed gas hydrate in quiescent quasi-free water droplets and the nucleation rates of gas hydrates of various guest types in the presence of solid walls reported in the literature. We found that (1) 400 nucleation events were sufficient to construct a reliable nucleation curve; (2) the addition of stainless-steel walls promoted the nucleation kinetics of CO2 hydrate, as it did to methane/propane mixed gas hydrate; (3) the kinetic parameter was significantly lower than the theoretically expected value, whereas the thermodynamic parameter was comparable to both the theoretically expected value and the experimentally determined value reported in the literature; and (4) CO2 hydrate nucleated over a substantially shallower supercooling range and had higher nucleation rates than those of methane/propane mixed gas hydrate, both in the presence and in the absence of a solid wall.

Continuous hydrate-based CO2 separation from H2 + CO2 gas mixture using cyclopentane as co-guest

CO2 separation from H2 + CO2 gas mixture is a necessary technology for a stable supply of blue hydrogen. Among CO2 separation technology, hydrate-based CO2 separation is an environmentally and economically superior technology. Cyclopentane (CP) was selected as guest compound to enable hydrate-based CO2 separation under thermodynamic conditions closer to normal pressure and temperature. Hydrate-based continuous CO2 separation experiments are conducted in the H2 + CO2 + CP + H2O system at 284 K. The results of these experiments showed that mole fraction of H2 in the gas phase reached 0.9 at the steady state in the H2 + CO2 + CP + H2O system after the CO2 was extracted via hydrate formation. At the same time, more H2 was incorporated into the mixed hydrate during this process as the concentration of H2 in the gas phase increases post hydrate formation. Therefore, it was demonstrated that hydrate-based CO2 separation was successful in H2 + CO2 + CP + H2O system at 284 K but there is also increase of energy costs caused by H2 loss. The PXRD results identified H2 + CO2 + CP hydrate as structure II hydrate. Although H2 + CO2 + Tetrahydropyran (THP) hydrate also forms structure II hydrate, more H2 is encapsulated in the H2 + CO2 + CP mixed hydrate than in the H2 + CO2 + THP mixed hydrate. This difference may be caused by the difference in the lattice constants of CP and THP hydrates, which arises from the difference in molecular size of CP and THP, from the differences between the large guest–host interactions, and from differences arising from hydrogen bonding of THP with the host water framework which can decrease the H2 holding capacity. In this study, it was revealed that the molecular diameter of the guest molecules need also to be considered in addition to the phase equilibrium conditions of the hydrates when selecting guest compound of hydrates.

Synthesis of mixed methane-ethylene hydrate with ice powder

The study of gas hydrate researched by the ice powder method is helpful for us to understand the storage, separation, transportation and transformation of gas on the solid surface. In this work, the change regulation has been studied that temperature, pressure, hydrate saturation, and conversion rate in the formation process of methane-ethylene mixed hydrate, which exploring the internal action mode of the methane-ethylene mixed hydrate. The experimental results show that under the condition of −4.0 °C and 4 MPa, the conversion rates of methane, methane-ethylene (with mole ratios 2, 1, and 0.5), and ethylene hydrate are 5.30%, 11.95%, 16.57%, 19.93%, and 26.84%, respectively. And more, the conversion efficiencies of hydrate are 0.36, 0.81, 0.96, 1.58, and 6.58, respectively. The above data show that with the increase of ethylene content, the formation of hydrate is easier, which proves that ethylene can promote the formation efficiency of methane hydrate. The above results have reference value for understanding the formation of natural gas hydrates, so as to promote the development and application of gas hydrate. • There is no induction period in the process of ice powder generating methane, ethylene, and methane-ethylene mixed hydrate. • The ethylene molecules in the methane hydrate formation process can increase the reaction rate of the system. • It provides theoretical support for the development and utilization of natural gas hydrates.

An equation of state and a phase diagram for gas-hydrate-forming slurries in gas-shortage

Slurries of gas hydrates, especially mixed hydrates of CO2 and TBPB (Tetra-n-Butyl Phosphonium Bromide), are currently under consideration for secondary refrigeration. Secondary refrigeration circuits contain wide sections where the two-phase slurry (solid + liquid), separated from the gas phase, undergoes transformations that would if gas were present, lead to both hydrate formation and gas dissolution. The absence of gas hinders these processes: the slurry is in gas shortage. The analysis demonstrates that, although thermodynamically distant from three-phase equilibria, slurries in gas-shortage are in equilibrium. For describing their thermodynamic state, the notion of CO2 partial pressure in a gas mixture is introduced, which leads to an equation of state and a new phase diagram with temperature or enthalpy versus CO2 mass-fraction. This diagram complements those commonly used for three-phase systems. The thermal behavior of slurries in gas shortage is then investigated, leading to the possibility of future experimental validation of the present analysis.