Gas Hydrates 1

Fundamentals, Characterization and Modeling

Daniel Broseta, Livio Ruffine, Arnaud Desmedt, Daniel Broseta, Livio Ruffine, Arnaud Desmedt

• Publisher: Wiley-ISTE
• Year: 2017
• Print ISBN: 9781848219694
• eBook ISBN: 9781119427438
• Edition: 1
• Topic: Sciences physiques
• Subtopic: Énergie

Gas hydrates, or clathrate hydrates, are crystalline solids resembling ice, in which small (guest) molecules, typically gases, are trapped inside cavities formed by hydrogen-bonded water (host) molecules. They form and remain stable under low temperatures – often well below ambient conditions – and high pressures ranging from a few bar to hundreds of bar, depending on the guest molecule. Their presence is ubiquitous on Earth, in deep-marine sediments and in permafrost regions, as well as in outer space, on planets or comets. In addition to water, they can be synthesized with organic species as host molecules, resulting in milder stability conditions: these are referred to as semi-clathrate hydrates. Clathrate and semi-clathrate hydrates are being considered for applications as diverse as gas storage and separation, cold storage and transport and water treatment.

This book is the first of two edited volumes, with chapters on the experimental and modeling tools used for characterizing and predicting the unique molecular, thermodynamic and kinetic properties of gas hydrates (Volume 1) and on gas hydrates in their natural environment and for potential industrial applications (Volume 2).

1
Neutron Scattering of Clathrate and Semiclathrate Hydrates

1.1. Introduction

Neutron scattering is a standard tool when dealing with the microscopic properties of the condensed matter at the atomic level. This comes from the fact that the neutron matches with the distances and energy scales, and thus with the microscopic properties of most solids and liquids. Neutrons, with wavelengths in the order of angstroms, are capable of probing molecular structures and motions and increasingly find applications in a wide array of scientific fields, including biochemistry, biology, biotechnology, cultural heritage materials, earth and environmental sciences, engineering, material sciences, mineralogy, molecular chemistry, solid state and soft matter physics.

The striking features of neutrons can be summarized as follows. Neutrons are neutral particles. They interact with other nuclei rather than with electronic clouds. They have (de Broglie) wavelengths in the range of interatomic distances. They have an intrinsic magnetic moment (a spin) that interacts with the unpaired electrons of magnetic atoms. Their mass is in the atomic mass range. They carry, thus, similar energies and momentum than those of condensed matter, and more specifically of gas hydrates.

As gas hydrates are mainly constituted of light elements (H, O, C, etc.), in situ neutron scattering appears as a technique particularly suited to their study. In the case of diffraction (i.e. structural properties), while the identification of these light atoms by X-ray diffraction requires the presence of heavy atoms and is therefore extremely complicated, neutron diffraction (NP) is highly sensitive to them due to the interaction of the neutrons with nuclei rather than with electron clouds. Moreover, most of the matter is “transparent” to neutron beams. Such a feature provides advantages for studying gas hydrates when a heavy sample environment is required (e.g. high pressure, low temperature). For instance, X-ray powder diffraction studies are usually restricted to small sample volumes, as large sample volumes would be associated with a strong absorption and unwanted scattering from the pressure cell. Neutron techniques allow studies of bulk processes in situ in representative volumes, hence with high statistical precision and accuracy [STA 03, HEN 00, GEN 04, FAL 11]. Furthermore, although alteration of some types of ionic clathrate hydrates (or semiclathrates), such as the splitting of the tetra-alkylammonium cations into alkyl radicals [BED 91, BED 96], by X-ray irradiation has been reported, neutrons do not damage sample.

Finally, future developments in gas hydrate science will be based on the understanding, at a fundamental level, of the factors governing the specific properties of gas hydrates. In this respect, the investigation of gas hydrate dynamics is a prerequisite. At a fundamental level, host–guest interactions and coupling effects, as well as anharmonicity, play an important role. These phenomena take place over a broad timescale, typically ranging from femtoseconds to microseconds. Investigating the dynamics (intramolecular vibrations, Brownian dynamics, etc.) of gas hydrates thus requires various complementary techniques, such as NMR or Raman spectroscopy, and indeed inelastic and quasi-elastic neutron scattering (QENS), especially when it comes to encapsulating light elements such as hydrogen or methane in water-rich structures.

In this chapter, the recent contributions of neutron scattering techniques in gas hydrate research are reviewed. After an introduction to neutron scattering techniques and theory, an overview of the accessible information (structural and dynamical properties) by means of neutron scattering is provided. Then, selected examples are presented, which illustrate the invaluable information provided by neutron scattering. Some of these examples are directly related to existing or possible applications of gas hydrates.

1.2. Neutron scattering

Both nuclear and magnetic neutron interactions are weak: strong but at very short length scale for the nuclear interaction and at larger scale for the magnetic interaction. In that respect, the probed sample can be considered as transparent to the neutron beam. This highly non-destructive character combined with the large penetration depth, both allowed because of the weak scattering, is one of the main advantages of this probe.

Nuclear scattering deals with nuclear scale interaction and hence presents no wave vector dependent form factor attenuation allowing to offer high momentum transfers for diffraction or specific techniques such as deep inelastic neutron scattering (also known as neutron Compton scattering).

Neutron spectroscopic techniques range from the diffraction of large objects using small-angle scattering, usually made with long incident wavelengths (cold neutrons), to direct imaging through contrast variation (neutron tomography), usually made with short wavelengths (hot neutrons) and going through ordinary diffraction and inelastic scattering in the intermediate wavelength range.

In that respect, neutron scattering complements without necessarily overlapping the other available spectroscopic techniques such as nuclear magnetic resonance (NMR). If one naturally thinks about X-ray for structure determination, neutrons are very competitive for inelastic scattering and even essential for magnetic scattering both in the diffraction and inelastic modes.

The main drawback that contrasts with the numerous advantages comes from the intrinsic relative flux limitation of neutron sources, and thus, this type of spectroscopy can only be performed at dedicated large-scale facilities.

1.2.1. A basic ideal scattering experiment

In a generic experiment (Figure 1.1), a beam of monochromated neutrons with single energy (E_i) is directed on a sample. The scattered neutrons are collected along direction (angles θ and ϕ) and analyzed by energy difference with the incident energy by using a detector, covering a solid angle ΔΩ of the sphere, which measures the analyzed neutron intensity. The measured intensity in the solid angle spanned by the detector and in a final energy interval ΔE_f in this simple gedanken experiment ...

Table of contents

• Cover
• Table of Contents
• Title
• Copyright
• Preface
• 1 Neutron Scattering of Clathrate and Semiclathrate Hydrates
• 2 Spectroscopy of Gas Hydrates: From Fundamental Aspects to Chemical Engineering, Geophysical and Astrophysical Applications
• 3 High-Resolution Optical Microscopy of Gas Hydrates
• 4 Calorimetric Characterization of Clathrate and Semiclathrate Hydrates
• 5 Thermodynamic Modeling of Solid-Fluid Equilibria: From Pure Solid Phases to Gas Semiclathrate Hydrates
• 6 Volume and Non-Equilibrium Crystallization of Clathrate Hydrates
• List of Authors
• Index
• End User License Agreement