The effect of hydrate promoters on gas uptake

Rapid Gas Hydrate Formation-Evaluation of Three Reactor Concepts and Feasibility Study

Gas hydrates show great potential with regard to various technical applications, such as gas conditioning, separation and storage. Hence, there has been an increased interest in applied gas hydrate research worldwide in recent years. This paper describes the development of an energetically promising, highly attractive rapid gas hydrate production process that enables the instantaneous conditioning and storage of gases in the form of solid hydrates, as an alternative to costly established processes, such as, for example, cryogenic demethanization. In the first step of the investigations, three different reactor concepts for rapid hydrate formation were evaluated. It could be shown that coupled spraying with stirring provided the fastest hydrate formation and highest gas uptakes in the hydrate phase. In the second step, extensive experimental series were executed, using various different gas compositions on the example of synthetic natural gas mixtures containing methane, ethane and propane. Methane is eliminated from the gas phase and stored in gas hydrates. The experiments were conducted under moderate conditions (8 bar(g), 9–14 °C), using tetrahydrofuran as a thermodynamic promoter in a stoichiometric concentration of 5.56 mole%. High storage capacities, formation rates and separation efficiencies were achieved at moderate operation conditions supported by rough economic considerations, successfully showing the feasibility of this innovative concept. An adapted McCabe-Thiele diagram was created to approximately determine the necessary theoretical separation stage numbers for high purity gas separation requirements.

Enhanced CH4-CO2 Hydrate Swapping in the Presence of Low Dosage Methanol

CO2-rich gas injection into natural gas hydrate reservoirs is proposed as a carbon-neutral, novel technique to store CO2 while simultaneously producing CH4 gas from methane hydrate deposits without disturbing geological settings. This method is limited by the mass transport barrier created by hydrate film formation at the liquid–gas interface. The very low gas diffusivity through hydrate film formed at this interface causes low CO2 availability at the gas–hydrate interface, thus lowering the recovery and replacement efficiency during CH4-CO2 exchange. In a first-of-its-kind study, we have demonstrate the successful application of low dosage methanol to enhance gas storage and recovery and compare it with water and other surface-active kinetic promoters including SDS and L-methionine. Our study shows 40–80% CH4 recovery, 83–93% CO2 storage and 3–10% CH4-CO2 replacement efficiency in the presence of 5 wt% methanol, and further improvement in the swapping process due to a change in temperature from 1–4 °C is observed. We also discuss the influence of initial water saturation (30–66%), hydrate morphology (grain-coating and pore-filling) and hydrate surface area on the CH4-CO2 hydrate swapping. Very distinctive behavior in methane recovery caused by initial water saturation (above and below Swi = 0.35) and hydrate morphology is also discussed. Improved CO2 storage and methane recovery in the presence of methanol is attributed to its dual role as anti-agglomerate and thermodynamic driving force enhancer between CH4-CO2 hydrate phase boundaries when methanol is used at a low concentration (5 wt%). The findings of this study can be useful in exploring the usage of low dosage, bio-friendly, anti-agglomerate and hydrate inhibition compounds in improving CH4 recovery and storing CO2 in hydrate reservoirs without disturbing geological formation. To the best of the authors’ knowledge, this is the first experimental study to explore the novel application of an anti-agglomerate and hydrate inhibitor in low dosage to address the CO2 hydrate mass transfer barrier created at the gas–liquid interface to enhance CH4-CO2 hydrate exchange. Our study also highlights the importance of prior information about methane hydrate reservoirs, such as residual water saturation, degree of hydrate saturation and hydrate morphology, before applying the CH4-CO2 hydrate swapping technique.

Kinetics Investigation of Carbon Dioxide Hydrate Formation Process in a New Impinging Stream Reactor

The absorption of carbon dioxide by hydrates is considered as one of the potential methods for carbon capture and storage. In this work, a new impinging stream reactor was designed to investigate the characteristics of carbon dioxide hydrate formation process. The experiments were carried out at different pressure, temperature and impinging strength. It was shown that the carbon dioxide hydrate formation process could be enhanced by the impinging stream technique. With the increased of impinging strength, both gas consumption and hydration rate were increased. In addition, initial pressure and temperature also had an effect on the carbon dioxide hydrate formation process. Moreover, the kinetics of carbon dioxide hydrate formation was discussed. When the initial pressure was 3.5 MPa and impinging strength was 0.21, the activation energy was 24.74 kJ/mol, which was similar to the experimental data available in the literature.

Effects of micro-bubbles on the nucleation and morphology of gas hydrate crystals

Gas hydrate is usually regarded as a huge potential energy resource that has promising industrial applications in gas separation, storage, and transportation. Previous research studies have shown that a gas hydrate phase transition is mainly controlled by heat and mass transfer, while there are limited works on the mass transfer effects of gas micro-bubbles on hydrate crystallization. In this study, variations in the microscopic morphology of the hydrate crystal growth in a liquid-gas interface were observed using a microscope imaging system. The results indicated that the nucleation of the hydrate first tends to occur at the bubble surface. The cooling rates increased exponentially with the crystal growth rates and played an important role in the morphology of the hydrate crystal growth. In addition, the hydrate crystals tended to grow in the direction of the bubbles affected by the Ostwald ripening effects, which suggested that bubbling was an efficient measure to promote the application of hydrate-based technologies. In turn, reducing the concentration of the bubbles on the surface of the hydrate, inhibiting their generation, and enhancing the process of gas mass transfer in water around the hydrate surface were also conducive to further accelerate the decomposition of the hydrate, which may provide some guidance for the resource exploitation of gas hydrate.

Molecular Dynamics Simulation Study on the Growth of Structure II Nitrogen Hydrate

Crystal growth of N₂ hydrate in a three-phase system consisting of N₂ hydrate, liquid water, and gaseous N₂ was performed by molecular dynamics simulation at 260 K. Pressure influence on hydrate growth was evaluated. The kinetic properties including the growth rates and cage occupancies of the newly formed hydrate and the diffusion coefficient and concentration of N₂ molecules in liquid phase were measured. The results showed that the growth of N₂ hydrate could be divided into two stages where N₂ molecules in gas phase had to dissolve in liquid phase and then form hydrate cages at the liquid-hydrate interface. The diffusion coefficient and concentration of N₂ in liquid phase increased linearly with increasing pressure. As the pressure rose from 50 to 100 MPa, the hydrate growth rate kept increasing from 0.11 to 0.62 cages·ns^-1·Å^-2 and then dropped down to around 0.40 cages·ns^-1·Å^-2 once the pressure surpassed 100 MPa. During the hydrate formation, the initial sII N₂ hydrate phase set in the system served as a template for the subsequent growth of N₂ hydrate so that no new crystal structure was found. Analysis on the cage occupancies revealed that the amount of cages occupied by two N₂ molecules increased evidently when the pressure was above 100 MPa, which slowed down the growth rate of hydrate cages. Additionally, a small fraction of defective cages including two N₂ molecules trapped in 5¹²6⁵ cages and three N₂ molecules trapped 5¹²6⁸ cages was observed during the hydrate growth.