Control of Methane Hydrate Formation at the Molecular Level

47110-G10
Control of Methane Hydrate Formation at the Molecular Level

Tadanori Koga, State University of New York at Stony Brook

Text Box: Fig.1 High pressure cell for in-situ neutron reflectivity set up at the NG7 beamline at NIST. Natural gas hydrates owe their existence to the ability of water molecules to assemble via hydrogen bonding and form polyhedral cavities in which trapped methane molecules reside[1,2]. Though an initial interest in understanding gas hydrate formation focused on flow assurance to avoid gas pipeline plugging, it is now being considered for applications such as an alternative to desalination technology[3], potentially a huge natural gas reserve[2], and natural gas transport alternative to liquefied natural gas due to high energy density (160 cubic meters of natural gas per cubic meter of gas hydrate at STP)[4]. A recognizable problem in utilizing the versatility of gas hydrate routes is the uncertainty in the hydrate formation process that can take from few minutes to several days. In this proposed research, we aim to understand the mechanism of the formation at the molecular scale, which have not been elucidated yet.

As a model system, we are focusing on a stationary interface separating methane (hydrophobic) and liquid water where the hydrate formation develops. The experimental technique used is neutron reflectivity (NR), which is sensitive to interfacial structures under compressed gases. At the same time, laser light scattering (LS) is also utilized as a complementary technique to study the growth of the hydrate crystals at the micron-scale. In order to mimic environmental conditions (0<T<10�C and 5<P<15MPa) for laboratory-based experiments, we have built a high-pressure cell made of stainless steel (Fig.1).

Text Box: Fig.2 Scattering profiles for the D2O/CD4 system at T=5.5�C (a) t=300 min at P=0.1 MPa (without CD4); (b) t=10min at P=5.5MPa. The specular and diffuse components correspond to the strike lines located at the middle and the rest of the scattering area, respectively. The color of red indicates higher intensity than yellow. In-situ NR experiments using the high-pressure cell were carried out at the NG7 beamline of the National Institute of Standards and Technology (Gaithersburg, MD). Deuterated methane (CD₄) and deuterated oxide (D₂O) were used for NR due to low absorption for neutrons and a scattering contrast. A two-dimensional detector was utilized to characterize the layer structures normal to the surface (specular components) as well as lateral surface structures (diffuse components) simultaneously. Fig. 2 shows the representative scattering profiles from a D₂O/CD₄ system at T=5.5�C. We found the drastic increase in the diffuse scattering occurred at P>P_c (the critical pressure of 3.8 MPa) within 10 min exposure (Fig. 2b) compared to those at P<P_c where no time evolution of the scattering profile was observed for 10 h (Fig. 2a). The change indicates the existence of the rough surface at the nanometer scale due to the formation of methane hydrate at the interface. In addition, the nucleation times for the surface hydrate formation were estimated to be about 10 min and remained unchanged regardless of the pressures used forthe experiments (Fig.3).

Text Box: Fig.3 Pressure dependence of the induction time for the formation of methane hydrate obtained by in-situ NR and LS experiments at T=5.5�C. Further, we studied the formation of the hydrate structures at the micron-scale by using LS. For LS experiments, the reflected beam from the D₂O/CD₄ interface was monitored by using a photosensitive detector as a function of time. It was found that the formation of the micron-scale surface structures caused a sharp discontinuity in the intensity, leading to the induction time at the given environments. As summarized in Fig.3, the induction time determined by the LS experiments increased up to several days as approaching the critical pressure, which can be explained by the classical nucleation theory and has been previously observed in bulk methane hydrate experiments. Thus, this is the first time, to the best of our knowledge, to demonstrate experimentally that the nanometer-scale methane hydrate crystals can be formed rapidly even near the phase boundary. We are currently studying the effect of surfactants, which are known to accelerate the formation of the macroscopic methane hydrate crystals and could control the growth of the nanometer-scale hydrate crystals as well.

References

1. Sloan, E.D., 1998. Clathrate Hydrates of Natural gases, 2^nd Ed., Marcel Dekker, New York.

2. "Gas Hydrates and Clathrates", 2007. J. Pet. Sci.& Eng...56(1-3), D. Mahajan and C. Taylor, Eds.

3. Osegovic, J.P., Tatro, S.R., Holman, S.A., Ames, A.L., Max, M.D. J. Pet. Sci.& Eng. 56(1-3), 42-6.

4. Gudmundsson, J.S., Borrehaug, A., 1996. Frozen hydrate for transport of natural gas. Proc. of 2^nd Int. Conf. Natural Gas Hydrates, 415-22.

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Home > Reports Back to Table of Contents 47110-G10 Control of Methane Hydrate Formation at the Molecular Level Tadanori Koga, State University of New York at Stony Brook Natural gas hydrates owe their existence to the ability of water molecules to assemble via hydrogen bonding and form polyhedral cavities in which trapped methane molecules reside[1,2]. Though an initial interest in understanding gas hydrate formation focused on flow assurance to avoid gas pipeline plugging, it is now being considered for applications such as an alternative to desalination technology[3], potentially a huge natural gas reserve[2], and natural gas transport alternative to liquefied natural gas due to high energy density (160 cubic meters of natural gas per cubic meter of gas hydrate at STP)[4]. A recognizable problem in utilizing the versatility of gas hydrate routes is the uncertainty in the hydrate formation process that can take from few minutes to several days. In this proposed research, we aim to understand the mechanism of the formation at the molecular scale, which have not been elucidated yet. As a model system, we are focusing on a stationary interface separating methane (hydrophobic) and liquid water where the hydrate formation develops. The experimental technique used is neutron reflectivity (NR), which is sensitive to interfacial structures under compressed gases. At the same time, laser light scattering (LS) is also utilized as a complementary technique to study the growth of the hydrate crystals at the micron-scale. In order to mimic environmental conditions (0<T<10�C and 5<P<15MPa) for laboratory-based experiments, we have built a high-pressure cell made of stainless steel (Fig.1). In-situ NR experiments using the high-pressure cell were carried out at the NG7 beamline of the National Institute of Standards and Technology (Gaithersburg, MD). Deuterated methane (CD₄) and deuterated oxide (D₂O) were used for NR due to low absorption for neutrons and a scattering contrast. A two-dimensional detector was utilized to characterize the layer structures normal to the surface (specular components) as well as lateral surface structures (diffuse components) simultaneously. Fig. 2 shows the representative scattering profiles from a D₂O/CD₄ system at T=5.5�C. We found the drastic increase in the diffuse scattering occurred at P>P_c (the critical pressure of 3.8 MPa) within 10 min exposure (Fig. 2b) compared to those at P<P_c where no time evolution of the scattering profile was observed for 10 h (Fig. 2a). The change indicates the existence of the rough surface at the nanometer scale due to the formation of methane hydrate at the interface. In addition, the nucleation times for the surface hydrate formation were estimated to be about 10 min and remained unchanged regardless of the pressures used forthe experiments (Fig.3). Further, we studied the formation of the hydrate structures at the micron-scale by using LS. For LS experiments, the reflected beam from the D₂O/CD₄ interface was monitored by using a photosensitive detector as a function of time. It was found that the formation of the micron-scale surface structures caused a sharp discontinuity in the intensity, leading to the induction time at the given environments. As summarized in Fig.3, the induction time determined by the LS experiments increased up to several days as approaching the critical pressure, which can be explained by the classical nucleation theory and has been previously observed in bulk methane hydrate experiments. Thus, this is the first time, to the best of our knowledge, to demonstrate experimentally that the nanometer-scale methane hydrate crystals can be formed rapidly even near the phase boundary. We are currently studying the effect of surfactants, which are known to accelerate the formation of the macroscopic methane hydrate crystals and could control the growth of the nanometer-scale hydrate crystals as well. References 1. Sloan, E.D., 1998. Clathrate Hydrates of Natural gases, 2^nd Ed., Marcel Dekker, New York. 2. "Gas Hydrates and Clathrates", 2007. J. Pet. Sci.& Eng...56(1-3), D. Mahajan and C. Taylor, Eds. 3. Osegovic, J.P., Tatro, S.R., Holman, S.A., Ames, A.L., Max, M.D. J. Pet. Sci.& Eng. 56(1-3), 42-6. 4. Gudmundsson, J.S., Borrehaug, A., 1996. Frozen hydrate for transport of natural gas. Proc. of 2^nd Int. Conf. Natural Gas Hydrates, 415-22. Back to top Terms of Use Security Privacy Contact Copyright © 2009 American Chemical Society

47110-G10 Control of Methane Hydrate Formation at the Molecular Level

47110-G10
Control of Methane Hydrate Formation at the Molecular Level