Gas Hydrate Formation Research Articles

Robust and comprehensive predictive models for methane hydrate formation condition in the presence of brines using black-box and white-box intelligent techniques

Accurate estimation of gas hydrate formation condition is crucial for many reasons. In one hand, the gas hydrate formation is a promising approach for gas separation, cold energy storage, seawater desalination, etc. On the other hand, in some industries, this phenomenon may cause some challenges, such as pipelines blockage. The current study was undertaken to develop trustworthy predictive tools for methane hydrate formation temperature (MHFT) in the presence of brines. To achieve this target, 1051 experimental data pertinent to the methane hydrate equilibrium in 26 single and multiple brines over an extensive range of operating conditions were amassed from published studies. Four simple factors, including pressure, brine concentration, salt molecular weight and salt melting temperature were defined as the input variables, and the black-box intelligent techniques, including gaussian process regression (GPR), multilayer perceptron (MLP) and radial basis function (RBF) were employed to link MHFT to the mentioned factors. A plethora of graphical and statistically-based assessments were performed to determine the accuracy level of the proposed models. While all intelligent models had excellent performances, the one established by the GPR method showed the best agreements with experimental data, and gave the AARE and R2 values of 0.14% and 99.17%, respectively, in the testing step. The foregoing model was capable of predicting MHFT over a broad range of conditions with relative deviations mostly below 0.10%. Additionally, it showed an excellent performance in describing the physical variations of MHFT versus operating factors. The authority of the analyzed databank and the presented model was asserted via the William's plot investigation. Moreover, a sensitivity analysis was undertaken in order to determine the most fundamental factors in controlling MHFT. Eventually, a simple mathematical correlation was also derived based on the white-box intelligent approach of gene expression programming (GEP), which allowed the accurate estimation of MHFT with an AARE of 0.56%.

Formation and phase equilibria of gas hydrates confined in hydrophobic nanoparticles

The promotion of gas hydrate formation is of fundamental importance to facilitate its great potential applications in, for example, gas separation, transportation and energy storage. The employment of Dry Water (DW), which is a powdery Pickering system composed of macro water droplets surrounded by stabilising hydrophobic nanoparticles, is considered an effective method for enhancing hydrate formation. The promotion mechanism underlying this observation, however, has yet to be well understood. In this work, the effect of DW on both formation kinetics and thermodynamic stability of gas hydrates was investigated using a micro differential scanning calorimetry (µDSC) in CH4 and CO2 hydrate systems, using two different hydrophobic nanoparticles. Distinctly shorter induction times and higher conversion ratios were observed for DW compared to bulk water. The DW system stabilised by more hydrophobic nanoparticles showed benefits in accelerating nucleation, while the other system, which consisted of smaller DW droplets, led to a superior enhancement of hydrate growth. Besides the effect on formation kinetics, it was also noted that DW influenced the µDSC hydrate dissociation peak characteristics, including melting points and melting peak shapes. This phenomenon suggested that the centre- and surface-droplet hydrates in DW may have different structures, due to the “hydrophobic effect” of the nanoparticles structuring the near-surface water, so exhibiting melting points corresponding to bulk hydrate or to a more structured surface hydrate (with raised melting point), respectively. To support this “hydrophobic effect” hypothesis, a series of hydrate and ice formation experiments were conducted in porous media having different hydrophobicity, the results of which demonstrated that hydrates adjoining confined hydrophobized surfaces, in other systems similar to DW, also displayed higher melting temperatures.

Natural Structural Transition of Gas Hydrates From sI to sII in the Deep Seafloor

AbstractThe evolution of gas hydrates influenced by the seawater environment is unknown. We present a model of structural transformation from sI hydrate to sII hydrate due to the influence of seawater environment and vent fluid in nature through in situ experiments of gas hydrate formation in the Haima cold seep area. The in situ experimental results indicate that gas hydrates preferentially form as sI hydrates even in cold seep environments where C2+ hydrocarbons are present. During subsequent evolution, the sI hydrates could restructured at the effect of seawater environment and vent fluid, causing transformation to sII hydrates under the influence of hydrate stability. The supply of gas and direct contact with seawater environment are critical factors for structural transformation. Such structural transformation is the result of gas hydrates seeking thermodynamic stability and may be common in active cold seep areas.

Analytical Prediction of Gas Hydrate Formation Conditions for Oil and Gas Pipeline

Analytical Prediction of Gas Hydrate Formation Conditions for Oil and Gas Pipeline

Kinetics and morphology of gas hydrate formation from MEG solution in under-inhibited systems

Natural gas hydrate blockages pose a major risk to deepwater oil and gas development. Thermodynamic under-inhibition is a cost-effective hydrate management strategy, but the impact of mono ethylene glycol (MEG) on hydrate formation kinetics and mass transfer at the gas–water interface requires further understanding for hydrate-related flow assurance. This study investigates the kinetics and morphology of hydrate growth from MEG solutions using in situ micro X-ray CT and a macro transparent reactor. MEG concentration emerged as the primary factor influencing hydrate growth morphology. Findings revealed that MEG alters the growth morphology of gas hydrates by inhibiting the formation of a hydrate film at the gas–liquid interface and inducing flocculent hydrate growth within the solution. MEG can also induce the formation of low-saturation hydrates, with no significant self-inhibition observed at concentrations below 20 %. Over time, a dense hydrate shell tended to form on the surface of the hydrate lump, increasing the risk of blockage. Additionally, it was observed that subcooling also affects the hydrate growth morphology; higher subcooling accelerates the hydrate growth rate and increases the density of the formed hydrate. This research explored the mechanisms underlying hydrate formation in under-inhibited systems, aiming to extend the safe operational boundary for oil and gas transportation and provide theoretical support for deepwater hydrate management.

An investigation on hydrate prediction and inhibition: An industrial case study

AbstractThis investigation reports the first study to predict natural gas hydrate formation using both Aspen HYSYS® and HydraFlash software for various gas compositions and thermodynamic inhibitors (monoethylene glycol [MEG] concentrations at 10, 20, 30, and 40 wt.% and methanol concentrations at 10 and 20 wt.%). The simulated predictions are compared with the results of the experimental data in the literature. It has been shown that HydraFlash software can accurately predict hydrate formation conditions for a given industrial case, without having to carry out costly experimental work. This work also evaluated the effect of inhibitors and it appears that inhibitor type and concentration are determined according to condition of gas composition. MEG is consequently selected as the most ideal hydrate inhibitor for the industrial case. This also was confirmed through COSMO‐RS studies in which the sigma profile and sigma potential of the considered inhibitors were obtained and presented using density functional (DFT) calculations to verify the hydrogen bonding affinities of the inhibitors to water molecules. HydraFlash was utilized to predict the dissociation conditions of hydrates under the influence of a high concentration of MEG inhibition, reaching up to 40 wt.% at 313 K and a pressure of 311.1 bar. Finally, it is shown that both software packages are quite accurate and useful tools for the prediction of hydrate for simple systems. However, HydraFlash can simulate more complex systems, including different types of salts at higher pressures. Investigation results indicate insightful guidance for accurately predicting hydrate dissociation under simulated conditions.

Controlling Effect of Particle Size on Gas Hydrate Enrichment in Fine‐Grained Sediments

AbstractThe particle size of sediments below the seabed is a crucial factor affecting the formation and enrichment of gas hydrates. Apart from the formation and enrichment law of gas hydrate in coarse‐grained sediments (dominated by a sandy‐sized fraction), in the fine‐grained sediments (<62.5 μm) which accounts for more than 90% of offshore gas hydrate resources globally, the control effect of sediment particle size on gas hydrate is still unclear. Therefore, understanding the relationship between the fine‐grained sediment particle size and gas hydrate enrichment is essential for revealing the global distribution and dynamic evolution of gas hydrates. Here, we analyzed the vertical gas hydrate saturation, particle size parameters of sediments, whole‐rock minerals, and clay mineral components based on drilling data and sediment samples from fine‐grained gas hydrate reservoirs (GHRs) in the Shenhu area of the northern South China Sea. The results show that in fine‐grained sediments, the coarse particles cannot improve the reservoir quality or enrich the gas hydrate because many fine particles fill the intergranular pores formed by the coarse particles. Meanwhile, the fine particles were dominated by clay minerals, especially in the illite/smectite mixed layer, which significantly reduced the permeability of the sediment layer and was not conducive to the enrichment of gas hydrates. Moreover, sedimentary processes directly control the sediment particle size and mineral composition, which play an essential role in controlling GHRs at the macroscale. In the fine‐grained sediments, very fine sediments (<8 μm) have a more significant negative impact on gas hydrate enrichment.

Influence of natural gas component changes on hydrate generation temperature and pressure

Abstract During the transportation of natural gas through pipelines, there is a potential for the formation of gas hydrates. These hydrates can result in pipeline blockages, causing economic losses and significant safety hazards. Therefore, it holds crucial reference value to understand the conditions under which hydrates are formed and to grasp the laws governing their generation for effective prevention and control. This study employs the P-P hydrate thermodynamic model to compute the temperature and pressure at which natural gas generates hydrates. We also explore the impact of changes in the composition of natural gas on the temperature and pressure required for hydrate formation. This helps us discern the sensitivity of hydrate formation conditions to various components. Our findings indicate that propane, isobutane, and H2S substantially influence hydrate formation under the same percentage of alteration. For instance, a 3% increase in these three components results in a temperature increase of 2.36°C, 2.83°C, and 2.14°C, respectively, for hydrate formation at 100 bar.

Artificial neural network modelling of natural gas dehydration process

A multi-input–multi-output artificial neuron network (MIMO-ANN) model has been developed for process monitoring and improvement on a natural gas glycol dehydration process. The MIMO-ANN model was based on a steady-state process simulation model constructed in commercial software Aspen HYSYS. A set of training data was generated with the converged simulation model for the training of the MIMO-ANN model in Python. The process input of the model includes lean glycol recirculation rate and purity along with wet gas inlet pressure. On the other hand, the process output considered includes dehydrated gas water and aromatics content, dehydrated gas hydrate formation temperature and water dew point, stripping gas flow rate as well as reboiler duty. The overall mean squared error (MSE) of the MIMO-ANN model was calculated as 1.79. The best-fit line that highly overlaps with a 45° diagonal line is constructed with a correlation coefficient (R2) score of more than 0.999 for all studied process output testing datasets. The mean absolute percentage error (MAPE) of the predicted outputs is generally less than 1%, except for dehydrated gas water dew point (16.63%) and stripping gas flow rate (11.55%), due to error predictions attempted on pseudo-zero values. This successfully displays an exceptional predictive performance of the MIMO-ANN model developed in this work which can be further deployed as an online dashboard for real-time monitoring tool.

Application of four intelligent models for the estimation of formation pressure of natural gas hydrate

Natural gas hydrate is a clean and efficient resource that contains lots of heat energy. The production, processing and transportation of natural gas can be influenced dramatically by natural gas hydrate. Natural gas hydrate will accumulate at the bottom of the pipeline, even block the pipeline and reduce the flow rate of the natural gas. This article presents four intelligent models named GWO-SVM (Grey Wolf Optimizer connected with Support Vector Machine), GWO-LSSVM (Grey Wolf Optimizer connected with Least Squares Support Vector Machine), GA-LSSVM (Genetic Algorithm connected with Least Squares Support Vector Machine), and GWO-RBF (Grey Wolf Optimizer connected with Radial Basis Function) to predict the formation pressure of natural gas hydrate in different systems. In order to determine the degree of influence of each input variable on the formation pressure of natural gas hydrate, this paper uses Spearman correlation analysis method to calculate the correlation coefficients between the formation pressure of natural gas hydrate and various influencing factors. The contradistinction among different intelligent models shows that the GWO-LSSVM model has the exceptional performance of the smallest average absolute relative deviation (AARD=2.89%), the minimum standard deviation (SD=0.05) with all 244 data of natural gas hydrate formation pressure in different systems. When compared to six commonly used empirical models for predicting formation pressure in common natural gas hydrate, the GWO-LSSVM model has the highest prediction accuracy, with an AARD of only 1.82%. Furthermore, when compared to three thermodynamic models employed for predicting formation pressure in sour natural gas hydrate, the GWO-LSSVM model has the highest prediction accuracy, with an AARD of 2.38%. The leverage method demonstrates that four intelligent models have excellent robustness in reflecting the inner relationships among natural gas hydrate formation pressure and influential factors. The four intelligent models for predicting hydrate formation pressure will be helpful to find hydrate formation pressure in different systems.

Calculation of the required methanol consumption during the flow of wet hydrocarbon gas in a horizontal pipeline

One of the main problems that have to be solved during the development of hydrocarbon deposits is the formation of gas hydrates in pipelines. In this regard, the article presents a preventive method to struggle against the formation of gas hydrate deposits on the pipes inner walls associated with the supply of a hydrate formation inhibitor to the gas stream. The research was conducted on the basis of a mathematical model of the wet hydrocarbon gas flow in a horizontal pipeline. The research object is to determine the minimum required consumption of methanol, in which there is no formation of gas hydrate deposits on the channel inner walls. The practical significance of this study is that it is aimed at reducing the risks associated with the formation of gas hydrates in pipelines. The numerical implementation of a mathematical model of natural gas flow in a horizontal channel is based on a sequential solution of a system of four differential equations by the Runge-Kutta method of 4 orders of accuracy, followed by a search for sequential approximations of the minimum inhibitor flow rate, in which the "gas + water ↔ gas hydrate phase" transition process does not occur on the inner surface of the channel. The article presents a calculation of the proportion of a hydrate formation inhibitor in the liquid phase by solving a cubic equation using the Cardano method. Based on the computational experiments results, graphs were constructed and interpreted of the dependencies of the minimum inhibitor consumption on the soil temperature, inlet gas pressure, total water concentration in the gas flow, initial gas temperature and total gas flow rate.

Development and application of wellbore flow assurance monitoring system for combustible ice production in deepwater

The commercial development of combustible ice, as the most promising strategy energy, requires not only efficient gas production technology but also a safe flow assurance system. For the depressurization development of combustible ice in the sea area, the issues of wellbore fluid carrying sand, hydrate re-formation in the wellbore, and instability of wellbore flow, are the key problems leading to flow obstacles in the wellbore and affecting the normal production. Therefore, some measures must be taken to effectively monitor and analyze the flow situation in the wellbore during the development of combustible ice in the sea. In this paper, a Wellbore Flow Assurance Monitoring System (WFAMS) is developed to display the fluid flow conditions in the wellbore in real-time during the gas hydrate production, including temperature, pressure, flow rate, the carrying sand amount, etc. Furthermore, the system is capable of online monitoring for potential plug risks, including sand plugs and hydrate blockages caused by gas hydrate formation, as well as assessing flow stability. Additionally, it can provide valuable information, such as production adjustment data and the required injection dosage of thermodynamic inhibitors to prevent hydrate formation. In the 2020 South China Sea production test, this system was used for depressurization production and successfully predicted wellbore flow conditions. This enabled field engineers to monitor flow risks and adjust production plans in a timely manner, ensuring smooth test progress.

An Extensive Review on Gas Hydrates: Recent Patents, Properties, Formation, Detection, Production, Importance, and Challenges

Introduction:: Gas Hydrates, or Clathrate Hydrates, have been the subject of increasing scientific and industrial attention due to their potential as an alternative energy source, their role in climate change, and their association with geohazards. The growth of new indigenous gas supply sources could impart a significant positive ripple effect on a country's economy, ecological balance, and energy landscape. This burgeoning interest has led to a surge in research and development, resulting in numerous patents related to the extraction, processing, and utilization of gas Hydrates. Objectives:: This review paper aims to provide an up-to-date, comprehensive overview of the properties, formation, detection, production, importance, challenges, and patent landscape of Gas hydrates. The integration of patented technologies into the field underscores the importance of intellectual property in shaping the future of energy, environment, and economic development. Methods:: Patented technologies in this field are contributing to making this resource more accessible and commercially viable. Moreover, the development of gas hydrates as an energy source could act as a safeguard for manufacturing jobs that are sensitive to gas prices, with proprietary technologies enhancing the efficiency and sustainability of the production process. Results:: On the environmental front, an uptick in the consumption of natural gas, known for its cleaner combustion, could herald positive change. Patented innovations in clean and efficient extraction and utilization methods for Gas Hydrates are instrumental in reducing the environmental impact. From the standpoint of energy security, a larger domestic slice of the energy pie, complemented by an extensive array of gas supply alternatives, could equip the nation to better navigate the unpredictable terrain of future energy scenarios. Conclusion:: The strategic patenting of key technologies in the exploration, production, and application of Gas Hydrates ensures competitive advantage and fosters innovation, driving forward the energy industry's evolution.

Gas solubility enhancement and hydrogen bond recombination regulated by terahertz electromagnetic field for rapid formation of gas hydrates

The low solubility of methane in water and the formation of hydrate film at methane-water interface restrict interfacial mass transfer between methane and water, thereby impeding the rapid formation of hydrate. Herein, THz electromagnetic field (TEMF) is introduced to destroy hydrogen bonds through resonance with water molecules for decreasing methane-water interfacial tension (IFT) and improving methane solubility in water. Thereafter, TEMF is removed to restore hydrogen bonding interaction for hydrate nucleation within bulk-phase water rather than at methane-water interface, thereby accelerating hydrate formation. The results show that a TEMF at 15.8 THz can increase methane solubility by 50-fold and provide more nucleation sites for hydrate formation by lowering the IFT by 15-fold. Furthermore, the hydrate nucleation follows “blob” nucleation at low solubility and Local Structuring Mechanism nucleation at high solubility. The higher solubility of methane shortens the induction period of hydrate nucleation form 58.5 ns to 29 ns. Particularly, small 512 cages form preferentially regardless of nucleation pathway. The underlying mechanism for TEMF-induced the acceleration of hydrate formation is attributed to the transition of water phase from a disordered to an ordered structure. This transition arises from the resonance of the TEMF with water molecules, disrupting the hydrogen bond between water molecules and providing sufficient driving force for their rearrangement after removing the TEMF. In rearrangement process, weakly hydrogen-bond interactions correspondingly are converted into strongly hydrogen-bond ones. These findings are expected to improve the understanding of hydrate formation mechanism and promote the applications of terahertz technology in gas hydrates.

Comparison of methanol and ethylene glycol effectiveness as chemical inhibitors in the prevention of gas hydrates in well testing barge DT-05 well Z Mahakam field

Gas hydrate is one of the production problems that are often encountered in the management of natural gas wells. Well Z is a natural gas well in the Mahakam field which routinely be tested periodically to determine the ability of flow rate and the production parameters using well testing barge. However, there are often for the formation of gas hydrate formed in several test components in well testing barge. This study aims to determine the effectiveness use of 2 inhibitor chemicals, which are methanol and glycol in handling gas hydrate in the testing barge. Natural gas flow originating from wellhead / upstream 5 MMScfd and temperature 122 °F and the operating pressure 2187 psia. From the simulation results, gas hydrate will form at 46.33 °F and it is obtained that the gas hydrate formation was predicted at the choke manifold. Fluid stream passing through the choke manifold had decreased the pressure into 254 psi and there is also a decrease in temperature from 122 °F to 41.88 °F. Methanol injection develops free of potential hydrates to form in flowlines. The temperature at which hydrates are formed changes to 33.88 °F. In the other hand, after Ethylene glycol injection in flowline, hydrates formation temperature dropped to 0.48 °F. From these results it can be resumed that the application of Ethylene Glycol injection on the process is more effective lowering the temperature of the formation of hydrates in well testing barge.

Thermodynamic phase equilibria study of Hythane (methane + hydrogen) gas hydrates for enhanced energy storage applications

Hydrogen blending with natural gas has become an effective technique for incorporating the characteristics of natural gas and facilitating the storage and transportation of hydrogen gas. The transportation of hydrogen and natural gas (methane) blend, also referred to as Hythane, presents numerous challenges, advantages, and disadvantages that differ based on the technology employed. The current research showcased a thorough examination of the thermodynamics involved in studying the three phase equilibrium analysis of gas hydrate technology for storing Hythane gas mixtures. Three different Hythane compositions varying in hydrogen percentage were selected, and the isochoric gas hydrate phase equilibria study was performed. The experimental results of phase equilibria and thermodynamic modelling using CPA EOS in software were compared. A similar study was also conducted in the presence of a 5.56 mol % THF solution to understand the reduction in the pressure requirement for the phase equilibria. Results from the mixed Hythane+THF gas hydrate findings suggested stable gas hydrate formation at much lower pressure requirements, increasing the credibility of storing Hythane mixtures in gas hydrates at moderate conditions. The gas compositions for each experiment were analyzed using gas chromatography to evaluate the presence of the desired gas in the hydrate phase. Heat exchange during the gas hydrate dissociation was analyzed using a high pressure differential scanning calorimeter (DSC). The change in the heat release temperature also indicates the stability of the gas hydrate, and with increasing hydrogen gas in the blend, the gas hydrate stability decreased. The Clausius-Clapeyron equation was employed to compute the heat of gas hydrate dissociation, and a comparison was made with the heat of dissociation obtained from the DSC study. The experimental investigations on Hythane gas hydrates were performed to find the hydrogen storage inside the gas hydrate phase and compare it with the previous studies of pure hydrogen hydrates. To demonstrate the Hythane gas transportation, Hythane gas hydrate pellets were formed, followed by their storage in a controlled environment to observeany changes in pressure and temperature. The Hythane gas hydrate pellets demonstrated exceptional stability of Hythane gas presenting an intriguing prospect for utilizing gas hydrate technology in Hythane gas transportation.

Effects of differential tectonic activities on overpressure evolution in the deep-water area of the Qiongdongnan Basin: implications for gas hydrate accumulation

The lack of data on the complex tectonics of the Qiongdongnan Basin has thus far restricted our understanding of the overpressure, deep hydrocarbon and shallow gas hydrate distribution. We therefore combined integrated seismic, well and borehole test data with basin models to clarify the relationship between tectonic activity and overpressure evolution, as well as the potential effects of the subsurface pressure on gas hydrate accumulation. The results show differences in tectonic activity and pressure characteristics between the eastern and western basins. Large numbers of faults and magmatic intrusions have developed in the post-rift layer of the eastern basin (the Baodao to Changchang sub-basins) since the Late Miocene ( c. 10.5 Ma), although they seldom formed in the western basin (the Lengdong to Lingshui sub-basins). Correspondingly, the eastern basin was characterized by lower overpressures (up to 40 MPa) and the western basin displayed higher overpressures (up to 110 MPa). The results indicate that tectonic activity since c. 10.5 Ma caused the relief of overpressure in the eastern basin. In stark contrast, the overpressure in the western basin increased significantly due to the lack of structural conduits and rapid sedimentation of the thick Pliocene and Quaternary deposits. Numerical models reveal that the faults and gas chimneys associated with the intrusions constituted shallow plumbing systems through which deep natural gas was able to migrate upwards. The formation of gas hydrates relied on the migration of deep gas to the hydrate stability zone below the seafloor. The relatively lower overpressured zones due to pressure relief in the eastern basin are therefore considered the next areas potentially favourable for the formation of gas hydrates. Thematic collection: This article is part of the Emerging knowledge on the tectonics of the South China Sea collection available at: https://www.lyellcollection.org/topic/collections/south-china-sea

CO2 capture through gas hydrate formation in the presence of polyethyleneimine-surface-grafted clay

CO2 capture through gas hydrate formation was performed using a stirring-type vessel in the presence of nanoclay and polyethyleneimine-grafted nanoclay. First, the grafting process was implemented via the wet impregnation method. The nanoparticles were characterized using FT-IR, FESEM, EDS, BET, XRD, TGA, Zeta potential, and DLS analyses. The characterization analyses confirmed surface-grafting of the nanoclay. Then, the hydrate formation experiments were implemented at three various nanoparticle loadings (200, 400, and 500 ppm). Water-to-hydrate conversion, CO2 gas consumption, apparent rate constant, and hydrate storage capacity were obtained and compared. Prior to conducting the CO2 hydrate formation experiments in the presence of nanoparticles, the rocking-type and stirring-type vessels were compared to evaluate the hydrate formation rate (using pure water). Stirring-type vessel exhibited higher hydrate formation rates than rocking-type vessel along with improved gas storage performance which resulted in a 22.4% increase in CO2 consumption. Thus, the effect of additives was examined using the stirring-type vessel. Surface-grafted nanoclay led to higher CO2 consumption compared to nanoclay at all concentrations. Using 500 ppm of modified nanoclay, grafted by 50 mass% polyethyleneimine (PEI) solution as additive, the maximum enhancement of CO2 consumption in the hydrate phase in comparison with pure water was obtained, along with the greatest water to hydrate conversion value (39.67). This study introduces a new technique by modifying nanoclay with PEI to improve CO2 storage in gas hydrate phase, presenting potential advancements in gas storage technology.

Effects of multi-walled carbon nanotubes on microstructure transformation of water before carbon dioxide hydrate formation

There is no consensus on the micro mechanism of gas hydrate formation. The essence of water conversion into hydrates is the transformation of disorderly arranged water into the water with specific structure under a certain condition. This study used different multi-walled carbon nanotubes (MWCNTs) as solid promoters for gas hydrate formation. By comparing the effects of nanotubes modified with hydroxyl, carboxyl and amidogen on the microstructure transformation of water before CO2 hydrate formation, the micro mechanism and influence rules of gas hydrate formation were systematically study. The results indicated that the conversion of water to hydrates mainly manifested as the mutual conversion of strong/weak hydrogen bonding water. Due to the Brownian motion and dispersion of MWCNTs, the hydrogen bonds of the weak hydrogen-bonded water weakened (the corresponding peak blue shifts), and the distance between water molecules (dO−O) expanded, forming relatively loose hydrate, which was conducive to gas diffusion in the solid hydrate phase and increased the final gas consumption. Meanwhile, the strengthening of strong hydrogen bonded water increased the rate of hydrate formation.

Selective CO2 Capture from CO2/N2 Gas Mixtures Utilizing Tetrabutylammonium Fluoride Hydrates.

Gas hydrates, a type of inclusion compound capable of trapping gas molecules within a lattice structure composed of water molecules, are gaining attention as an environmentally benign gas storage or separation platform. In general, the formation of gas hydrates from water requires high-pressure and low-temperature conditions, resulting in significant energy consumption. In this study, tetrabutylammonium fluoride (TBAF) was utilized as a thermodynamic promoter forming a semi-clathrate-type hydrate, enabling gas capture or separation at room temperature. Those TBAF hydrate systems were explored to check their capability of CO2 separation from flue gas, the mixture of CO2 and N2 gases. The formation rates and gas storage capacities of TBAF hydrates were systematically investigated under various concentrations of CO2, and they presented selective CO2 capture behavior during the hydrate formation process. The maximum gas storage capacities were achieved at 2.36 and 2.38 mmol/mol for TBAF·29.7 H2O and TBAF·32.8 H2O hydrate, respectively, after the complete enclathration of the feed gas of CO2 (80%) + N2 (20%). This study provides sufficient data to support the feasibility of TBAF hydrate systems to be applied to CO2 separation from CO2/N2 gas mixtures based on their CO2 selectivity.