Minjun Cha

Kinetics of methane hydrate replacement with carbon dioxide and nitrogen gas mixture using in situ NMR spectroscopy.

In this study, the kinetics of methane replacement with carbon dioxide and nitrogen gas in methane gas hydrate prepared in porous silica gel matrices has been studied by in situ (1)H and (13)C NMR spectroscopy. The replacement process was monitored by in situ (1)H NMR spectra, where about 42 mol % of the methane in the hydrate cages was replaced in 65 h. Large amounts of free water were not observed during the replacement process, indicating a spontaneous replacement reaction upon exposing methane hydrate to carbon dioxide and nitrogen gas mixture. From in situ (13)C NMR spectra, we confirmed that the replacement ratio was slightly higher in small cages, but due to the composition of structure I hydrate, the amount of methane evolved from the large cages was larger than that of the small cages. Compositional analysis of vapor and hydrate phases was also carried out after the replacement reaction ceased. Notably, the composition changes in hydrate phases after the replacement reaction would be affected by the difference in the chemical potential between the vapor phase and hydrate surface rather than a pore size effect. These results suggest that the replacement technique provides methane recovery as well as stabilization of the resulting carbon dioxide hydrate phase without melting.

Catastrophic Growth of Gas Hydrates in the Presence of Kinetic Hydrate Inhibitors

The effect of the concentration of kinetic hydrate inhibitors, polyvinylpyrrolidone (PVP), and polyvinylcaprolactam (PVCap) on the onset and growth of synthetic natural gas hydrates is investigated by measuring the hydrate onset time and gas consumption rate. Although the hydrate onset time is extended by increasing the concentration from 0.5 to 3.0 wt % for both PVP and PVCap, the growth rate of hydrates shows that the different tendency depends on the type of kinetic hydrate inhibitor and its concentration. For PVCap solution, the hydrate growth was slow for more than 1000 min after the onset at the concentration of 0.5 and 1.5 wt %. However, the growth rate becames almost 8 times faster at the concentration of 3.0 wt %, representing the catastrophic growth of hydrate just after the hydrate onset. (13)C NMR spectra of hydrates formed at 3.0 wt % of PVP and PVCap indicate the existence of both structures I and II. Cage occupancy of methane in large cages of structure II decreases significantly when compared to that for pure water. These results suggest that increasing the concentration of KHI up to 3.0 wt % may induce the earlier appearance of catastrophic hydrate growth and the existence of metastable structure I; thus, there needs to be an upper limit for using KHI to manage the formation of gas hydrates.

Superoxide Ions Entrapped in Water Cages of Ionic Clathrate Hydrates

In the present work, we first described the stable entrapment of the superoxide ions in gamma-irradiated (Me(4)NOH + O(2)) clathrate hydrate. Owing to peculiar direct guest-guest ionic interaction, the lattice structure of gamma-irradiated (Me(4)NOH + O(2)) clathrate hydrate shows significant change of lattice contraction behavior even at relatively high temperature (120 K). Such findings are expected to provide useful information for a better understanding of unrevealed nature (such as icy nanoreactor concept, ice-based functional material synthesis and lattice tuning by specific ionic guests) of clathrate hydrate fields.

Macroscopic investigation of water volume effects on interfacial dynamic behaviors between clathrate hydrate and water.

This study investigated the effects of the water volume on the interfacial dynamics between cyclopentane (CP) hydrate and water droplet in a CP/n-decane oil mixture. The adhesion force between CP hydrate and various water droplets was determined using the z-directional microbalance. Through repetition of precise measurements over several cycles from contact to detachment, we observed abnormal wetting behaviors in the capillary bridge during the retraction process when the water drop volume is larger than 100 μL. With the increase in water droplet volumes, the contact force between CP hydrate and water also increases up to 300 μL. However, there is a dramatic reduction of increasing rate in the contact forces over 300 μL of water droplet. With the addition of the surfactants of sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) to the water droplet, the contact force between CP hydrate and solution droplet exhibits a lower value and a transition volume of the contact force comes with a smaller solution volume of 200 μL. The water volume effects on the liquid wetting of the probe and the size of capillary bridges provide important insight into hydrate growth and aggregation/agglomeration in the presence of free water phase inside gas/oil pipelines.

Thermal expansivity of ionic clathrate hydrates including gaseous guest molecules.

Although thermal expansion is a key factor in relation to the host-guest interaction of clathrate hydrates, few studies have investigated the thermal behavior of ionic clathrate hydrates. The existence of ionic species in these hydrates creates a unique host-guest interaction compared to that of nonionic clathrate hydrates. It was revealed that X-ray diffraction cannot be used for research of tetramethylammonium hydroxide clathrate hydrates due to damage of the cations by the X-ray, which results in abnormal thermal expansion of the ionic clathrate hydrates. Hence, in the present work, the thermal expansivities of binary sII Me(4)NOD·16D(2)O and sI DClO(4)·5.5D(2)O were measured by neutron powder diffraction (NPD) in order to shed light on their thermal behavior. General correlations for the thermal behaviors of given structures were established and lattice expansions depending on the guests were compared between ionic and nonionic clathrate hydrates. The peculiar change in the thermal expansivity of binary DClO(4)·5.5D(2)O was also considered in relation to the host-guest configuration.

Magnetic transition and long-time relaxation behavior induced by selective injection of guest molecules into clathrate hydrates.

Magnetic molecules physisorbed into low-dimensional nanostructures of microporous materials such as graphite and metal-organic frameworks have been verified to exhibit an unusual magnetic behavior. We demonstrate that the selective injection of both magnetic and nonmagnetic guest molecules into the water-ice cages of clathrate hydrates to form a 3D superstructure with tetrahedral and diamond-like sublattices can modify the inherent magnetism.

Discrete Magnetic Patterns of Nonionic and Ionic Clathrate Hydrates

In this communication, the charge transfer phenomenon from ionic host lattice to nonionic guest molecule was observed by magnetization and Raman spectroscopy measurements for nonionic and ionic clathrate hydrates. The present findings on the magnetic property of nonionic guest molecules in ionic hydrate might provide important information on the unrevealed nature of host-guest interaction in ionic hydrate systems. The charge transfer occurring between ionic host and nonionic guest molecules will open up interesting application fields for ionized hydrate complexes and activated secondary guest molecules.

Hydrophobic Particle Effects on Hydrate Crystal Growth at the Water–Oil Interface

This study introduced hydrophobic silica nanoparticles (SiNPs) into an interface of aqueous and hydrate-forming oil phases and analyzed the inhibition of hydrate crystal growth after seeding the hydrate slurry. The hydrate inhibition performance was quantitatively identified by micro-differential scanning calorimetry (micro-DSC) experiments. Through the addition of 1.0 wt% of SiNPs into the water-oil interface, the hydrate crystal growth only occurred around the seeding position of cyclopentane (CP) hydrate slurry, and the growth of hydrate crystals was retarded. Upon a further increase in the SiNP concentration up to 2.0 wt%, the SiNP-laden interface completely prevented hydrate growth. We observed a hollow conical shape of hydrate crystals with 0.0 and 1.0 wt% of SiNPs, respectively, but the size and shape of the conical crystals was shrunken at 1.0 wt% of silica nanoparticles. However, the conical shape did not appear with an increased nanoparticle concentration of 2 wt%. These findings can provide insight into hydrate inhibition in oil and gas delivery lines, possibly with nanoparticles.

Inclusion of thiophene as a co-guest in a structure II hydrate with methane gas

Thiophene and its derivatives are found in crude oils and their molecular sizes indicate they could be clathrate hydrate formers. In this study, the formation of clathrate hydrates in the presence of thiophene and methane gas is identified through spectroscopic and thermodynamic investigations. Powder X-ray diffraction (PXRD) is used in order to identify the crystal structure of the binary (thiophene + CH4) clathrate hydrate, and PXRD patterns of the binary (thiophene + CH4) clathrate hydrate indicate the formation of structure II hydrate with methane gas. The unique inclusion and distribution of guest molecules such as thiophene and methane are verified through Raman spectroscopy. The thermodynamic stability of the binary (thiophene + CH4) clathrate hydrate is also investigated by using high pressure differential scanning calorimetry. The experimental results are valuable for a better understanding of the crystal structure, host–guest interaction and stability conditions in the binary (thiophene + CH4) clathrate hydrate.

SWAPPING CARBON DIOXIDE FOR COMPLEX GAS HYDRATE STRUCTURES

Large amounts of CH4 in the form of solid hydrates are stored on continental margins and in permafrost regions. If these CH4 hydrates could be converted into CO2 hydrates, they would serve double duty as CH4 sources and CO2 storage sites. Herein, we report the swapping phenomena between global warming gas and various structures of natural gas hydrate including sI, sII, and sH through C solid-state nuclear magnetic resonance, and FT-Raman spectrometer. The present outcome of 85% CH4 recovery rate in sI CH4 hydrate achieved by the direct use of binary N2 + CO2 guests is quite surprising when compared with the rate of 64 % for a pure CO2 guest attained in the previous approach. The direct use of a mixture of N2 + CO2 eliminates the requirement of a CO2 separation/purification process. In addition, the simultaneously-occurring dual mechanism of CO2 sequestration and CH4 recovery is expected to provide the physicochemical background required for developing a promising large-scale approach with economic feasibility. In the case of sII and sH CH4 hydrates, we observe a spontaneous structure transition to sI during the replacement and a cage-specific distribution of guest molecules. A significant change of the lattice dimension due to structure transformation induces a relative number of small cage sites to reduce, resulting in the considerable increase of CH4 recovery rate. The mutually interactive pattern of targeted guest-cage conjugates possesses important implications on the diverse hydratebased inclusion phenomena as clearly illustrated in the swapping process between CO2 stream and complex CH4 hydrate structure.