Hydrate Formation Rate Research Articles

Methane Hydrate Formation in Hollow ZIF-8 Nanoparticles for Improved Methane Storage Capacity

Methane hydrate has been extensively studied as a potential medium for natural gas storage and transportation. Due to their high specific surface area, tunable porous structure, and surface chemistry, metal–organic frameworks are ideal materials to exhibit the catalytic effect for the formation process of gas hydrate. In this paper, hollow ZIF-8 nanoparticles are synthesized using the hard template method. The synthesized hollow ZIF-8 nanoparticles are used in the adsorption and methane hydrate formation process. The effect of pre-adsorbed water mass in hollow ZIF-8 nanoparticles on methane storage capacity and the hydrate formation rate is investigated. The storage capacity of methane on wet, hollow ZIF-8 is augmented with an increase in the mass ratio of pre-adsorbed water and dry, hollow ZIF-8 (RW), and the maximum adsorption capacity of methane on hollow ZIF-8 with a RW of 1.2 can reach 20.72 mmol/g at 275 K and 8.57 MPa. With the decrease in RW, the wet, hollow ZIF-8 exhibits a shortened induction time and an accelerated growth rate. The formation of methane hydrate on hollow ZIF-8 is further demonstrated with the enthalpy of the generation reaction. This work provides a promising alternative material for methane storage.

Enhanced CO2 sequestration based on hydrate technology with pressure oscillation in porous medium using NMR

Carbon dioxide (CO2) is a dominant greenhouse gas in the atmosphere that contributes to global warming. A promising approach to mitigate CO2 emissions is CO2 capture and storage (CCS) through clathrate hydrate crystallization under the seafloor; however, numerous issues regarding the mechanisms of concentration, efficiency, and stability of CO2 hydrate sequestration in seafloor sediments remain under dispute. This study employed low-field nuclear magnetic resonance (NMR) measurements to observe the in-situ formation of CO2 hydrate using the pressure oscillation method in porous media and evaluate the carbon sequestration efficiency. Our results indicate that CO2 hydrates are preferentially formed in large pore spaces, further hindering the subsequent gas contact with water in isolated pores. Additionally, a high initial water saturation is more conducive to high-quantity CO2 hydrate capture and sequestration in a pressure variation environment with a higher driving force. The proposed pressure oscillation method could effectively break the mass transfer barriers in the later stage of hydrate formation with the help of CO2 solubility fluctuations, significantly increase the rate of later hydrate formation, and shorten the period of hydrate sequestration.

Evaluation of Conditions to Prevent the Formation of Gaseous Hydrates Using Kinetic Inhibitors

Hydrate formation in the oil and gas industry poses serious operational problems. These problems include clogging of the gas transmission path and its pressure drop. There are several ways to prevent the formation of hydrates, one of which is the use of kinetic inhibitors. These compounds inhibit the growth of hydrate crystals and prevent them from enlarging. The most important of these compounds are polyvinyl caprolactum (PVCAP) and a ternary polymer of vinyl pyrrolidene, vinyl caprolactum and dimethylaminoethyl methacrylate (VC-713). In this study, using the energy balance around a hydrate cluster and simplification, a mathematical equation for the rate of hydrate formation is obtained. The resulting equation has two parameters, model nt0 and s, which are determined using the experimental data of Ledhos et al. . In the following, using the values ​​obtained for the above parameters, the conditions for hydrate formation in raw maroon without the presence of inhibitors and also with their presence are predicted. The results show that if PVCAP inhibitors are used in raw maroon, the rate of hydrate formation is about 0.005 gmol / lit, which is 70% lower than in the case without inhibitors. For inhibitor VC-713, the amount of hydrate formed will be zero to 0.004 gmol / lit, which results in a 70 to 100% reduction in hydrate formation compared to conditions without inhibitor. Therefore, the results indicate the proper performance of the VC-713 inhibitor. Also, the effect of temperature and pressure operating parameters on the rate of hydrate formation was investigated and, as expected, increasing temperature and decreasing pressure will reduce the rate of hydrate formation.

Superabsorbent polymer for improved CO2 hydrate formation under a quiescent system

The hydrate formation rate and conversion yield are essential factors for hydrate-based gas storage and accordingly various thermodynamic and kinetic promoters have been extensively investigated. However, for hydrate-based CO2 capture, carbonate ions induced from dissolved CO2 in water hinder the performance of kinetic promoters (e.g., sodium dodecyl sulfate (SDS)). In this work, we introduce a superabsorbent polymer (SAP) matrix that provides an extremely high interfacial area between a sorbed tetrahydrofuran (THF) solution and gaseous CO2. The SAP enhanced the formation rate of binary THF-CO2 hydrate, despite the absence of mechanical agitation. We varied the concentration of THF, temperature, reactor volume, and the ratio of solution to the reactor for a systematic study and explored their effects on the CO2 storage capacity and the induction time. We observed that highly concentrated THF and a larger reactor with a high gas-liquid contact area enhanced the CO2 storage capacity and formation rate. Raman and PXRD spectroscopic analyses were employed to investigate the guest distribution and crystalline structure of the THF-CO2 hydrates synthesized under various conditions. All the binary hydrates were structure-II (sII) hydrates, and there was no pure structure-I (sI) CO2 hydrate. Based on the spectroscopic results, we developed a theoretical model that could calculate the amount of enclathrated CO2 and dissolved CO2 in liquid phases separately. This work reveals that SAP has excellent potential for rapid CO2 capture in hydrate media with a high water-to-hydrate conversion ratio.

Gas hydrate nucleation in acoustically levitated water droplets

We report subcooling-dependent measurements of hydrate formation on water droplets suspended via acoustic levitation in high-pressure natural gas. Ninety independent visual induction time measurements enabled the construction of induction time distributions with nucleation rates extracted at subcoolings of (12, 13.4 and 14.5) K. Using models from classical nucleation theory, kinetic and thermodynamic nucleation parameters were determined for freely suspended droplets and subsequently compared to those measured in more traditional stirred reactors. Scaling of the observed nucleation rates by each system’s gas–water interfacial area led to improved consistency between the datasets. However, we show that while hydrate formation rates in systems with solid containing surfaces are determined by a small number of nucleation sites with low nucleation work, formation rates in levitated droplets are set by sites that have higher nucleation work but are more numerous. This leads to distinct subcooling-dependencies for nucleation rates in the two systems. These results provide insight both into hydrate promotion and avoidance strategies that are relevant to multiple applications.

Molecular dynamics simulations of the effects of metal nanoparticles on methane hydrate formation

Understanding the properties of methane hydrate formation is important for the storage and transport of methane in the form of gas hydrates. In this study, nanosecond simulations were performed to investigate the formation of methane hydrates in the presence of Fe, Cu, Ag and their corresponding oxide nanoparticles. The effects of different nanoparticle and nanoparticle concentration on the rate and amount of CH4 hydrate formation were explored. The simulation results show that metal nanoparticles have a mixed effect on the kinetic improvement of methane hydrate generation process, which varies with the type and concentration of metal nanoparticles. Compared to the pure water system, the formation of methane hydrate was promoted to different degrees in the metal nanoparticle system, in the order of nano-Ag > nano-Cu > nano-Fe, which is mainly attributed to the excellent thermal conductivity of the metal itself. For the single metal oxide nanoparticle system, the promotion effect of nano-Ag2O was not obvious, while the promotion effect of nano-CuO was the best, and the promotion degree is in the order of nano-CuO > nano-FeO > nano-Ag2O. For the multiple metal oxide nanoparticle systems, the two nano-FeO system showed faster growth rate and more hydrate cages formed, which was better than the single nano-FeO and nano-Fe systems. The nano-CuO system showed the same results. However, the promotion effect of nano-Ag2O was slightly smaller than that of nano-Ag at the particle size and concentration studied in this work.

Gas-liquid asynchronous cooling promoting gas hydrate formation with high energy efficiency and its promoting mechanism

Hydrate technology has a bright future in the development of high quality production and life for human being. However, the slow hydrate formation rate and low gas consumption are the biggest two obstacles to the commercial application of the technology. In this study, the macro experimental investigation, simulation calculation and microscopic analysis are combined to study the process of the hydrate formation and the effects of gas molecules at different cooling ways on the hydrate nucleation process from the perspective of the transformation of free-water to bound-water. Macroscopically, the difference of gas–liquid temperature makes the heat transfer process between gas and liquid at the interface more active, which is more conducive to the diffusion of gas to liquid; and microscopically, the transformation of free-water to bound-water and the distance between oxygen atoms dOO of adjacent water molecules in the hydrate nucleation are the two key factors affecting the rate of hydrate formation and gas consumption. The results of this study not only propose a more efficient hydrate formation way, but also provide direction for solving the key bottleneck of hydrate technology so that it possible for the hydrate technology from laboratory research to industrial application.

Key Areas of Gas Hydrates Study: Review

Gas hydrates are widespread all over the world. They feature high energy density and are a clean energy source of great potential. The paper considers experimental and theoretical studies on gas hydrates in the following key areas: formation and dissociation, extraction and transportation technologies of natural methane hydrates, and ignition, and combustion. We identified a lack of research in more areas and defined prospects of further development of gas hydrates as a promising strategic resource. One of the immediate problems is that there are no research findings for the effect of sediments and their matrices on hydrate saturation, as well as on gas hydrate formation and dissociation rates. No mathematical models describe the dissociation of gas hydrates under various conditions. There is a lack of research into the renewal and improvement of existing technologies for the easier and cheaper production of gas hydrates and the extraction of natural gas from them. There are no models of gas hydrate ignition taking into account dissociation processes and the self-preservation effect.

Geological characterization of natural gas hydrate bearing sediments and their influence on hydrate formation and dissociation

Lithology of host sediments associated with gas hydrate reservoirs have a significant effect on the hydrate formation. The Krishna-Godavari (K-G), Mahanadi and Andaman basin in India have been proven to be rich in natural gas hydrate. In this study, a detailed investigation was carried out for the mineralogical analysis of the sediments collected from K-G, Mahanadi and Andaman basin along with hydrate formation in sediments. The study reveals that, the majority of the minerals in K-G basin samples are clay minerals and quartz. The presence of pyrite crystal (FeS2) in the K-G basin sediment is the indication of anoxic and sulfide environment. Spares fragments of microfossil like diatoms and spicules of fresh-water sponges have been identified from K-G basin sediments. Mahanadi basin is also consisting of clay minerals & quartz, and proportion of clay minerals are higher than K-G basin sediment. Andaman basin is mainly composed of clay and calcite minerals with little amount of quartz and feldspar. The rheological behavior of suspensions of sediments in water was investigated as it has significant effect on the drilling operation through the formation. Considering the similar mineralogical composition, gas hydrate formation and dissociation behavior were investigated in colloidal suspension containing sediments collected from K-G basin. Hydrate formation studies indicated that the presence of sediment solution significantly decreased pressure needed for hydrate formation compared to formation in pure water alone. It was observed that the salinity has delayed the hydrate formation as salt is considered as hydrate inhibitor. The rate of hydrate formation was observed to be high in case of sediment aqueous solution than the pure water. Whereas, in case of sediment solution with salinity shows lower formation rate than both pure water and sediment aqueous solution. The studies are relevant to the origin and stability of natural gas hydrate deposits known to exist in deep permafrost and marine sediments, where the effect of sediment surfaces is largely unknown.

Recyclable and high-efficiency methane hydrate formation promoter based on SDS-coated superparamagnetic nano-Fe3O4

Gas hydrate has been proposed as an effective medium for gas separation, desalination, gas storage and so on, yet the low reaction rate has been hindering the industrialization of hydrate-based technology. Hence various kinds of promoters, especially nano particles, have been introduced and investigated to increase the rate of hydrate formation. However, the recyclability of these promoters has not been given sufficient attention from an environmental friendly and economic point of view. In this study, a recyclable magnetic Fe3O4 nanoparticle coated with SDS (SDS-coated Fe3O4) was synthesized and successfully used for hydrate formation promotion as well as cycle experiments. The results showed that SDS-coated Fe3O4 exhibited more efficient promotion for hydrate formation than bare Fe3O4 in the studied concentration range. Especially, the induction time and reaction time can be significantly reduced to 77.6 ± 24.1 and 36 ± 3 min, respectively, and the methane storage capacity can be maintained up to 130 ± 5.9 v/v with the concentration of 20 g/L SDS-coated Fe3O4. Further comparison of bare Fe3O4 and SDS-coated Fe3O4 during cycle experiments showed that after 5 cycles, the methane storage capacity and reaction time in presence of SDS-coated Fe3O4 can still be maintained as 119.5 ± 8 v/v and 110 ± 10 min, which were even better than the performance of bare Fe3O4 just after two cycles. To sum up, SDS-coated Fe3O4 can be recycled and reused many times for the hydrate formation, indicating that the SDS-coated Fe3O4 has a potential to increase the recyclability and its economy for promoters in the hydrate-based technology.

Experimental study on the formation characteristics of CO2 hydrate in porous media below the freezing point: Influence of particle size and temperature on the formation process and storage capacity

AbstractCO2 storage in form of hydrate in stratigraphic sediment has been considered to be one of the effective strategies against global warming and mitigating CO2 emission, which has attracted extensive research interest in the field of greenhouse gas (GHG) reduction and natural gas hydrate exploitation in permafrost regions. How the formation characteristics of CO2 hydrate influences the storage process is a fundamental issue related to the hydrate‐based technology of CO2 sequestration and storage in the permafrost regions. In this study, the formation experiments of CO2 hydrate were carried out in porous media below freezing point under the condition of different particle sizes and temperatures. The influence of different factors on the formation rate, conversion rate, and gas storage capacity of CO2 hydrate were studied through experiments. It was indicated that temperature and ice particle size had a significant effect on the formation characteristics of CO2 hydrate in porous media below the freezing point. However, it did not mean that the greater the degree of supercooling was the better the hydrate formation would be. The formation rate and conversion rate of hydrate were relatively higher when the temperature approached to the freezing point. In contrast, the gas storage capacity of CO2 hydrate was higher than that above the freezing point. The average formation rate, conversion rate, and gas storage capacity of CO2 hydrate were obtained under the temperature of 270.15 K, which was 3.68 × 10−4 mol h−1, 40.47%, and 75.09 L/L, respectively. When the ice particle size was 700 µm under the same particle size of quartz sand, the conversion rate and gas storage capacity of CO2 hydrate was the maximum, reached to 49.69% and 92.19 L/L, respectively. These results provide greater insights into the hydrate‐based technology of CO2 sequestration and storage in sediments.

The research on the natural gas hydrate dissociation kinetic from hydrate-sediments/seawater slurries

Due to the weak cementation of natural gas hydrate (NGH) reservoir in the seabed, during both the drilling and development processes, a multiphase (gas hydrate-sediments-seawater-natural gas mixture) flow is more likely to exist in the NGH reservoir and development wells, understand the natural gas hydrate particle dissociation kinetic in the hydrate-sediments/seawater slurries is important. In this work, first, a new NGH phase behavior research experimental apparatus contains a strong magnetic stirrer was built. By using this, we for the first time proposed to form NGH reservoir in the water saturated sands under stirring in the laboratory, which showed a much higher hydrate formation rate and water to hydrate conversation ratio than the usual static formation method. After that, the CH4 hydrate-sands/seawater slurries were produced, the influence of pressures, temperatures, the size of sands, and stirring rates on CH4 hydrate dissociation kinetic in the slurry were investigated. The dissociation rate of CH4 hydrate-sands mixed particles markedly increased with decreasing operation pressure from 3 MPa to 0.1 MPa. The average dissociate rate reached 1762.57 mL/min at 0.1 MPa. It is interesting that unlike to the single NGH reservoir, no hydrate secondary formation and ice formation phenomenon arose in the hydrate-sands/seawater slurries during the CH4 hydrate dissociation process. The increase of temperature and stirring rate substantially stimulated the dissociation of CH4 hydrate particles in the slurry, while the sands’ size showed little effect. In addition, a high precision kinetic model for describing the dissociation of CH4 hydrate in hydrate-sands/seawater slurries was built, in which the influence of pressure, temperature and stirring rate on the hydrate apparent dissociation rate constant was considered at the same time.

Investigation of the formation characteristics of methane hydrate in frozen porous media

The replacement technique of CO2–CH4 hydrate in naturally occurring gas hydrates is a promising method for achieving CO2 storage and CH4 recovery in permafrost regions. Revealing the formation characteristics of CH4 hydrate becomes an urgent need in porous medium below the freezing point. For this purpose, the formation experiment was carried out under different conditions and the formation characteristics were systematically investigated. The result indicated that the initial pressure was the key factor affecting the formation process of CH4 hydrate. And the final pressure drop was greater at higher initial pressure conditions. The final pressure drop reached to 0.92 MPa when the initial pressure was 9 MPa under the same temperature conditions. The results also illustrated that, the formation rate of hydrate was the fastest when the temperatures were closely to the freezing point. The average formation rate of CH4 hydrate reached to 0.646 mmol/h when the temperature was 273.15 K. Moreover, the smaller the particle size was, the higher the final conversion rate of ice. The final conversion rate reached to 56.5% when the initial pressure was 9 MPa. The results are of significance for exploiting natural gas hydrate in the permafrost regions. Highlights The formation characteristics of methane were studied in porous media below the freezing point. The higher initial pressure could enhance the final conversion rates of ice. The formation rate of methane was the fastest when the temperature was closely to the freezing point. The pressure disturbance could improve the formed amount of methane hydrate in porous media.

Formation and occurrence characteristics of methane hydrate in the complex system of sodium dodecyl sulfate and porous media

ABSTRACT In recent years, porous media have attracted a lot of attention due to the enhanced kinetics of hydrate formation. However, how porous media promote hydrate formation is still an open topic. In this study, two different porous media, activated alumina and glass beads (average particle size 1, 3, 5 mm), were mixed with sodium dodecyl sulfate (SDS) to promote methane hydrate formation (275.15 K, 6 MPa). The smaller the particle size is, the better the promoting effect of porous media is, corresponding to faster formation kinetics and higher gas consumption. The average hydrate formation rate (2.3 mmol/min) and T90 (136 min) in the 1 mm activated alumina system was 2.6 and 0.4 times higher than the average hydrate formation rate (0.9 mmol/min) and T90 (349 min) in the 3 mm activated alumina system, respectively. Better performance of activated alumina particles than that of glass beads because of its rich micropores and unique electric double layer distribution in solution. The gas storage density of hydrate in the 1 mm alumina system can reach 129.13 Vg/Vh, while the highest gas storage density of hydrate in the 1 mm glass bead system was 122.35 Vg/Vh, which was similar to that of hydrate in the 3 mm activated alumina system (123.63 Vg/Vh). The gas consumption of the system was not affected by mass transfer but was controlled by particle size and solution volume with the presence of SDS. Under supersaturated and saturated conditions, it is found that the porous media bed and hydrate layer moved upward under the action of capillary force and methane gas driving force. This work provides a reference for the formation and occurrence of hydrates with the presence of porous media and surfactants.

Influences of different co-promoters on the mixed methane hydrate formation with salt water at moderate conditions

Effects of co-promoters on the mixed methane-THF hydrate formation in the presence of 3.5 wt% NaCl and 5.56 mol% THF solution were investigated at 8 MPa and 288.2 K in an unstirred reactor. The addition of NaCl resulted in the two-step hydrate formation, and it is the first stage that adversely affected the hydrate formation rate. However, the final gas uptake in this system was similar to that without NaCl. The synergistic effects on the kinetic promotion by adding a co-promoter including sodium dodecyl sulfate (SDS) and amino acids (valine, leucine, and methionine) were then studied. The results showed that the addition of the co-promoters reduced the induction time, minimized the length of the first stage formation, and increased the hydrate formation rate. Moreover, the induction time and the length of the first stage formation decreased along with the increase in the co-promoter concentration. However, the addition of SDS had the adverse effects by decreasing about half of the final gas uptake and the water to hydrate conversion for the system without SDS. Moreover, the addition of methionine resulted in the best performance as a kinetic promoter on the hydrate formation in the system with THF and NaCl. Up to 95 % of methane gas can be recovered from the hydrates formed with and without the co-promoters. The hydrate formation morphology illustrated the same pattern in all cases.

Combined effect of NaCl and sodium dodecyl sulfate on the mechanism and kinetics of methane hydrate formation in an unstirred system

Interest in studying an effect of the mixed NaCl + SDS additive on gas hydrate formation is associated with the direct use of saline waters and seawater in hydrate-based technologies for the storage and transportation of natural gases. We performed studies of methane hydrate formation without stirring using pressures of 8, 12 and 16 MPa for water, aqueous solutions of NaCl and SDS, and the mixed additive of NaCl + SDS. A 3 wt% NaCl solution, as a model for seawater, and a 0.1 wt% SDS solution were used. The mixed additive was 3 wt% NaCl+0.1 wt% SDS. Data on the methane solubility, induction time of methane hydrate formation, water-to-hydrate conversion and methane hydrate growth rate, as well as data on morphology of methane hydrate formed with and without additives have been presented. It was found that the mechanism of methane hydrate growth in NaCl solution under static conditions was determined by the morphology of hydrate film at the gas-liquid interface and was pressure dependent. A liquid penetrable hydrate film at the gas-liquid interface formed in the presence of NaCl at moderate pressures (8 and 12 MPa) that allowed capillary-driven hydrate growth with a high rate and water-to-hydrate conversion. At high pressures (16 MPa), methane hydrate growth was limited by formation of impenetrable hydrate film at the gas-liquid interface, that restricted hydrate growth. In contrast to the existing concepts that the addition of NaCl to SDS solutions enhances surfactant micellization and accelerate hydrate formation, we found that SDS micelles did not form under the studied methane hydrate-forming conditions with NaCl. Evidence for SDS hydrate formation in the NaCl + SDS solution have been presented. Mechanisms of gas hydrate nucleation and growth in NaCl + SDS solution taking into account SDS hydrate formation and permeability of gas hydrate film at the gas-liquid interface are discussed.

Hydrate-based gas separation for working fluid mixtures: Application to composition-adjustable organic Rankine cycle

Composition-adjustable organic Rankine cycle (ORC) is deemed to improve the overall energy efficiency compared to the conventional ORC, bringing more energy saving and emission reduction. Inspired by hydrate-based gas separation, the hydrate-based composition regulation of working fluid was proposed and investigated experimentally using R22/R125 mixtures as feed gas. To improve the hydrate formation kinetics, stirring was employed and boosted the initial hydrate formation rate by about 200% compared with the unstirred cases. Cases with more R22 fraction in the feed gas showed faster initial hydrate formation rate. Partition of components between gas and hydrate phases was observed in a kinetic separation manner, i.e., R22 led the hydrate formation at the early stage while R125 was enclathrated more at the later stage. With proper formation time (45 – 135 min), the R125 fraction in the gas phase could increase by 9.5 – 18.7 mol% from various feed gas compositions (30 – 70 mol%). In the hydrate phase R22 was typically enriched with the best separation factor varied between 2.71 and 10.27. Compared to hydrate formation at a constant temperature (275.5 K), the temperature ramping down mode (from 278.5 K to 275.5 K) slowed down the hydrate formation kinetics but amplified the separation performance, with the best R22 separation factor 2.84 times higher than that of the constant-temperature case achieved at the same time. Visual images revealed a “wall-climbing” hydrate growth pattern for R125/R22 mixed hydrate formation particularly in unstirred conditions. These results support the proposal of a hydrate-based composition-adjustable ORC and could lead to further development of this technology with practical insights.

Sequestration and Storage of Carbon Dioxide Using Hydrate Formation Method in the Presence of Copper Oxide Nanoparticles

The nanofluid-based gas hydrate formation process employing copper oxide (CuO) nanoparticles have been experimentally investigated in this work. Different concentrations of nanofluids are injected into the reactor at the operating condition of 29 bar, 274.15 K, and impeller speed of 100 rpm. It was observed that the kinetics of the carbon dioxide hydrate formation process was greatly affected by the nanoparticles. The remarkable point was that at a very low concentration of 20 ppm, a considerable improvement on the carbon dioxide hydrate formation kinetic without using any surfactant was obtained. At the concentration of 20 ppm, the values of the initial rate of hydrate formation, growth time, and induction time were 0.0495, 194.5, and 4.4 min, respectively, which these results can be of great importance for the use of carbon dioxide hydrate in various industries. The results indicated that the kinetics of gas hydrate formation was also severely influenced by the impeller speed and initial gas pressure. The rate of CO2 captured in the hydrate crystalline lattice is also modeled by the first-order kinetic model. It was seen that this model can be used to predict the rate of hydrate formation with considerable accuracy.

Study on the growth kinetics of methane hydrate in pure water system containing ZIF-8.

The hydrate formation rate is the key to the implementation of solid gas storage and transportation technology by the hydrate method. As a MOF material with strong hydrothermal stability, ZIF-8 has been proved to play a significant role in promoting the nucleation and growth of hydrate. However, the growth kinetics promotion mechanism and growth law of methane hydrate in the ZIF-8 promoter system have not been clarified at present. Therefore, the growth kinetics experiment of methane hydrate in the ZIF-8 promoter system was carried out in a high-pressure visualization reactor to systematically study the effects of the ZIF-8 concentration, undercooling degree, and pressure on the growth law of methane hydrate. The experimental results showed that: (1) the concentration of ZIF-8 had a significant shortening effect on the induction period of methane hydrate. With the increase in the ZIF-8 concentration, the induction time of methane hydrate was shortened from 5.85 h to 0.85 h. The methane gas consumption showed a gradually increasing trend at first, and then with the reaction going on, a dense hydrate film was formed at the gas–liquid interface, which increased the mass transfer resistance, resulting in the increase in the methane gas consumption gradually becoming stable. There were four stages in the growth process of hydrate, namely rapid formation, slow formation, secondary formation, and end formation and the optimal dosage of ZIF-8 promoter exists, and the optimal critical specific content was 0.02 mg mL−1. (2) With the increase of undercooling, the induction time of methane hydrate decreased significantly, and the increase in the methane consumption also showed an increasing trend at first and then decreased. (3) With the increase of the system pressure to 7 MPa, the induction time of methane hydrate decreased from 1.02 h at 6 MPa to 0.2 h at 7 MPa, and the decrease rate was 80.8%, which was mainly due to the presence of “OPEN GATE” in the ZIF-8 accelerator. With the increase of the system pressure, the pore opening of the ZIF-8 material increased, the adsorption of methane increased, and the nucleation and growth of methane hydrate were promoted.

Effect of sodium tripolyphosphate (STPP) and tetrasodium pyrophosphate (TSPP) on the formation kinetics of CO2 hydrate in bulk and porous media in the presence of pure water and seawater relevant for CO2 sequestration

Enhanced kinetics of carbon dioxide (CO2) hydrate formation will assist in developing technologies for CO2 gas storage, carbon capture and sequestration (CCS), and potentially for methane production from natural gas hydrates. The present work is focused on understanding the kinetics of CO2 hydrate formation in both bulk and porous media with pure water and seawater under the influence of non-toxic and low-cost inorganic emulsifiers or builders. Sodium tripolyphosphate (STPP) and tetrasodium pyrophosphate (TSPP) was used in a very low concentration of 0.2 wt.% to promote its economic viability. The experiments were carried out in a 300 mL stirred tank reactor and 100 mL porous bed reactor with an initial pressure of 3.5 MPa and 274.65 K with pure water and seawater, respectively. The porous bed was made with 0.15 mm silica sand with 70% water saturation. Both STPP and TSPP showed significant improvement in induction time for CO2 hydrate formation. Moreover, STPP showed reasonable enhancement in CO2 gas uptake compared to TSPP in both bulk and porous bed with both pure water and seawater. The average rate of hydrate formation was also better for STPP with multiple growth peaks. The results obtained will enhance hydrate-based technologies for efficient CO2 sequestration in shallow subsea sediments.