Seong-Pil Kang

Formation characteristics of synthesized natural gas hydrates in meso- and macroporous silica gels.

Phase equilibria and formation kinetics of the natural gas hydrate in porous silica gels were investigated using the natural gas composition in the Korean domestic natural gas grid. The hydrate-phase equilibria in the porous media are found to shift to the inhibition area than that in the bulk phase. The measured phase equilibrium data, combined with the Gibbs-Thomson equation, were used to calculate the hydrate-water interfacial tension. The value was estimated to be 59.74 +/- 2 mJ/m(2) for the natural gas hydrate. In addition, the inhibition effect is observed to be more significant in the meso-sized pore than the macro-sized one. In the formation kinetics, it was found that the hydrate formation reached the steady-state in a short period of time without mechanical stirring. Furthermore, the formation rate was found to be faster at 275.2 K than 273.2 K even though the driving force at 273.2 K is larger than that of 275.2 K. Even though the porous silica gels have smaller volume than other methods for gas storage, the gas consumption was found to be significantly enhanced in this study (for example, 120 vol/vol for the silica gels and 97 vol/vol for wet activated carbon). In this regard, using porous silica gels can be a potential alternative for gas storage and transportation as a nonmechanical stirring method. Although this investigation was performed with the natural gas composition in the Korean domestic grid, the results can also be expanded for designing or operating any hydrate-based process using various gas compositions.

Spectroscopic identification on cage occupancies of binary gas hydrates in the presence of ethanol.

Ethanol has been widely used for inhibiting gas hydrate formation due to its cost and efficiency. However, recent research showed that ethanol can act as a hydrate former when coguested with CH(4) molecules at various ethanol concentrations. Herein, we report tuning phenomenon of the gas hydrate in the presence of ethanol by means of spectroscopic measurements. On the basis of the experimental results, it is verified that ethanol molecules cannot inhibit hydrate formation effectively, but enhance the gas storage in the hydrate phase when a much less amount of the inhibitor than the stoichiometric concentration is used. The cage occupancies of binary hydrate systems in the presence of a thermodynamic inhibitor, showing similar guest behaviors in the presence of a promoter such as tetrahydrofuran (THF), can provide useful information on the molecular behaviors of guest species.

Inhibition of natural gas hydrates in the presence of liquid hydrocarbons forming structure H.

The effects of LMGS (large molecule guest substance) amount on the thermodynamics of natural gas hydrates, as well as structural characteristics of mixed hydrates of LMGS and natural gas, have been studied. The addition of 1.7 wt % neohexane (NH) to water induced inhibition of natural gas hydrates, and this inhibition effect increased with increased addition of NH up to 7.8 wt %. However, the hydrate equilibrium condition changed slightly when the concentration of NH further increased from 7.8 to 14.5 wt %. Investigations on structural characteristics were carried out by analyzing (13)C NMR spectra of mixed hydrates formed from the mixture of natural gas and NH. They indicate that two hydrate structures of II and H coexist simultaneously, and the ratio of structure H to II decreased from 0.97 to 0.43 when the NH concentration decreased from 14.5 to 7.8 wt %. In addition, it was confirmed that ethane, propane, and iso-butane gas molecules do not participate in the formation of structure H and only enclathrated in large cages of structure II. These results indicate the existence of multiple hydrate structures, which must be considered in many industrial applications when mixed hydrates are formed from multicomponent gas mixtures and liquid hydrocarbons.

13C NMR analysis of C2H6+C2H4+THF mixed hydrate for an application to separation of C2H4 and C2H6

Mixed hydrates (C2H4+5.56mol% THF, and C2H4+C2H6+5.56mol% THF) were analyzed using 13C MAS NMR spectroscopy. The hydrates were formed using a variety of feed gas compositions (100% C2H4; 20% C2H4+80% C2H6; 40% C2H4+60% C2H6; 60% C2H4+40% C2H6; and 80% C2H4+20% C2H6). According to the peak identification results, C2H4 molecules can occupy both the small and large cavities in the sI and sII hydrate structures, while C2H6 molecules can occupy only the large cavities of sI. Moreover, the mole fraction of C2H4 in the hydrate matrix was found to increase with increasing feed ratio of C2H4. On the basis of the NMR analysis, a hydrate-based process for separating C2H4 and C2H6 by repeated hydrate formation and dissociation was proposed. For cases with a feed-gas mixture with 20% C2H4 and 80% C2H6, a recovery of more than 88% C2H4 in the gas mixture could be achieved after five cycles of hydrate-based separation.

Influence of polymeric additives on paraffin wax crystallization in model oils

Wax deposition, precipitation, and gelation make the transport of crude oil in pipelines challenging. The effect of several ethylene copolymers, and small molecules with a long alkyl chain, on wax formation was investigated for n-C32H66 in decane and de-aromatized white oil. Addition of a small amount of EVA (ethylene-co-vinyl acetate) copolymers delayed nucleation by reducing the onset temperature and the wax appearance temperature. They modified the wax crystal-structure and morphology from large plates to tiny particles by adsorbing to the wax surfaces and inhibiting growth. Viscosity and the pour-point were improved by inhibiting the formation of large aggregates. It was demonstrated that the content of vinyl acetate groups in EVA copolymers affected wax crystallization. The small molecules, propylene copolymers, and ethylene copolymers with ethyl acrylate, maleic anhydride, and ethylene glycol showed a weak inhibiting effect. The effect of wax inhibitors was determined by the content and by the type of structure-disturbing groups in the copolymers.

Performance Evaluation of Kinetic Hydrate Inhibitors for Well Fluids Experiencing Hydrate Formation

Offshore flowlines transporting hydrocarbons have to be operated very carefully to avoid the formation of gas hydrates as they are considered one of the largest concerns for flow assurance engineers. The oil and gas industry is generally relying on chemical injection for hydrate inhibition; however hydrate blockages can occur in many different places of offshore production system due to unexpected circumstances. Once hydrate blockage formed considerable efforts are required to dissociate the hydrate via depressurization. Because residual hydrate structures known as gas hydrate precursors will be present in the aqueous phase after dissociation, the risk of hydrate re-formation becomes extremely high. Although the KHIs are becoming popular in many fields as hydrate inhibitors are considered not effective to inhibit the hydrate formation in the presence of residual hydrate structures, so that the use of KHIs for shut-in and restart operations is not recommended. In this study, new experimental procedures composed of three stages are designed to simulate the dissociation of hydrate blockages and transportation of well fluids experiencing hydrate formation. The obtained experimental results have shown that gas hydrates are rapidly re-formed when the temperature of dissociated water falls into the hydrate formation region. With an injection of KHIs before transporting the well fluids, the subcooling increased significantly indicating the possible use of KHIs for transporting the well fluids after dissociation of hydrate blockage. Moreover, the inhibition performance of KHIs is also investigated with two different gases to study the effect of gas composition. This study is confirmed that KHIs are possible candidate to prevent the hydrate re-formation in well fluids experiencing hydrate formation if the KHI is carefully evaluated.

Investigation of xenon and natural gas hydrate as a storage medium to maintain the enzymatic activity of the model proteins

To medically use and store proteins like enzymes, long-term maintenance of their activity must be considered. We examined the effectiveness of several methods for preserving the activity of three model-protein solutions. Solutions of catalase, L-lactate dehydrogenase, and carbonic anhydrase were used to form gas hydrates with xenon and natural gas. These enzyme aqueous solutions showed inhibitory effects on hydrate formation, and exhibited significant differences in induction time as well. The hydrates formed of enzyme solutions with xenon or natural gas are expected to have a better preservation effect than storage at room temperature and in liquid nitrogen. Changes in the activity of enzymes stored under different conditions were measured in relation to storage time. Storage in hydrate was good for maintaining the activity of catalase and L-lactate dehydrogenase. For carbonic anhydrase, the activity at room temperature was generally similar to that after storage in gas hydrate, but storing it in liquid nitrogen produced better results. For certain enzymes, storage in gas hydrates is expected to be a more effective method of maintaining activity than protein storage methods like freeze-drying, which causes mechanical damage to the protein.

PHASE EQUILIBRIA AND FORMATION KINETIS OF CARBON DIOXIDE, METHANE, AND NATURAL GAS IN SILICA GEL PORES

Hydrate phase equilibria for the CO2, CH4 and natural gas in silica gel pores of nominal pore diameters 6, 30 and 100 nm were measured, and compared with the calculated results based on van der Waals and Platteeuw model. At a specific temperature, three-phase hydrate–water-rich liquid–vapor (HLV) equilibrium curves for pore hydrates were shifted to the higher pressure condition depending on pore sizes when compared with those of bulk hydrates. The activities of water in porous silica gels were modified to account for capillary effect, and the calculated results were in good agreement with the experimental data. To investigate the formation kinetics of each system, the isobaric method was applied. It was found that there were no difference in structure between hydrate in silica gel pore and that in bulk free state. Results showed that hydrate formation in the silica gel pores indicated significantly faster rates, intensively reduced induction times, increased gas consumption and conversion of water to hydrate as compared to hydrate formation in bulk free water or fine ice powder. Utilizing these superior characteristics, formation of hydrate in porous material is expected to present the process on gas separation or storage.

COMPLEX COEXISTENCE BEHAVIOR OF STRUCTURE I AND H HYDRATES

13 C NMR spectroscopic analysis was carried out to clarify the formed hydrate structure in specific conditions on hydrate phase diagram of ternary methane, neohexane, and water system. The obtained NMR spectra at three different conditions suggested that both structure I and H were formed simultaneously and coexisted at 273.6 K and 50 bar. But, for both conditions of 273.6 K, 25 bar and 283.1 K, 50 bar the formed hydrate was identified as structure H only. These results showed that the pure CH4 hydrate of structure I was formed and coexisted with mixed CH4+neohexane hydrate of structure H in low temperature and high pressure region after passing through the phase boundary of pure CH4 hydrate. We have examined the structure coexistence at 273.6 K and 50 bar with other structure H formers of isopentane, methylcyclopentane, and methylcyclohexane. In case of isopentane, the obtained NMR spectrum showed that structure I and H coexisted and the amount of methane molecules in structure I was two times as many as in cages of structure H. However, there were no resonance lines of structure I when methylcyclohexane formed structure H with methane molecules.