in the temperature dependence of the thermal conductivity of hydrate phases, but absent in ice, are discussed in detail in this chapter.
Chapter 16 presents with selected potential applications of clathrate hydrate compounds, including flow assurance, natural gas recovery, desalination, concentration of aqueous solutions, and the storage of natural gas, hydrogen, and gas separation. This chapter is meant to be an overview of some applications, which will be well studied in the near future.
The authors' work described in this book has contributions from many colleagues, staff, and students at the National Research Council of Canada who are named in Chapter 1. The contributions of these individuals are gratefully acknowledged. The work described would not have been possible without the material support of the National Research Council of Canada from ∼1960 to 2011. As in any field, the progress in clathrate hydrate research is the result of collaboration and contributions from researchers in many countries. We hope to have given proper representation of contributions from researchers from all parts of the world. We apologize for any omissions, which are the result of the limited scope of some of the discussions in this book.
We would like to acknowledge the contributions of Donald Davidson (1925–1986) of the National Research Council of Canada as one of the pioneers in modern hydrate research in Canada. Don (i) initiated a multi‐technique approach to studying gas hydrates; (ii) provided mentorship to generations of hydrate researchers at the NRC; and (iii) wrote a number of monographs and book chapters, which are exemplary for their clarity and are still useful today. At the time, the National Research Council was a fertile multidisciplinary environment where cutting‐edge dielectric, NMR, single‐crystal X‐ray crystallographic, and simulation techniques were being developed, and almost immediately being used in understanding these substances. After Don's passing, this tradition was maintained at the NRC.
The editors would like to thank the authors of the chapters in this book for their contributions and their patience with our numerous requests for editorial changes. We would particularly like to thank Christopher I. Ratcliffe, Amadeu K. Sum, Peter Englezos, and Dennis D. Klug who also commented extensively on chapters other than their own.
We would like to thank our wives Beth and Dorothy for support and dealing with the seemingly unending demands on our time while we worked on this book. We thankfully acknowledge their indirect, but important contributions in getting this book project completed.
1 An Introduction to Clathrate Hydrate Science
John A. Ripmeester1, Saman Alavi1, 2, and Christopher I. Ratcliffe1
1 National Research Council of Canada, 100 Sussex Drive, Ottawa, ON, K1A 0R6, Canada
2 University of Ottawa, Department of Chemistry and Biomolecular Science, STEM Complex, 150 Louis‐Pasteur Pvt., Ottawa, ON, K1N 6N5, Canada
1.1 Introduction
The first intersection of clathrate hydrates and human endeavor took place in the late 1700s. A number of researchers (natural philosophers) working on the solubility of newly discovered airs (gases) observed unexpected ice‐like solids formed above the freezing point of ice when certain gases were passed into cold water or when such a solution was frozen. Davy identified these solids as two‐component water–gas compounds and named them “gas hydrates.” After some 140 years and much research, these solids were shown to be clathrates, materials where small molecules (guests) are trapped in an ice‐like lattice (host) consisting of hydrogen‐bonded water cages. During the time between initial discovery and final identification, gas hydrates confounded researchers by having a number of properties that countered concepts derived from mainstream chemistry. For instance, the hydrates were non‐stoichiometric, the water‐to‐gas ratios were not small whole numbers, and they decomposed upon heating or depressurization to give back the unchanged starting materials. The lack of chemical bonds between the water and the gas in the hydrates suggested that these were not real chemical compounds and in fact were the first examples of “chemistry beyond the molecule” – supramolecular compounds.
From phase equilibrium studies, we now know that when many gases and water are in contact under appropriate pressure (P) and temperature (T) conditions, a solid hydrate will form. The gas hydrates store gases, including natural gas, very efficiently with one volume of solid hydrate storing some 160 volumes of gas at standard temperature and pressure (STP). Since a number of gas hydrates are found naturally, this class of materials can be taken to be an unusual type of mineral. There are many sites in the geosphere where natural gas and water are in contact under the conditions required to form gas hydrate. Locations where this occurs are in sediments offshore of continental margins, under permafrost, and in some deep freshwater lakes.
Well before the discovery of natural gas hydrate in the geosphere, oil and gas engineers encountered blocked natural gas pipelines during cold weather operation which was initially attributed to ice formation from moisture in wet gas. Knowledge of earlier work on solid methane hydrate led Hammerschmidt in the 1930s to the correct explanation for the pipeline blocks – they were made of solid methane hydrate rather than ice. Since then, during the exploration, production and transport phases of hydrocarbon resources, blockage by natural gas hydrate formation has become a well‐known hazard, resulting in possible serious damage and loss of life, for example in the Deepwater Horizon oil spill of 2010. Much research has been carried out to prevent or manage hydrate formation in pipelines. Other problems related to gas hydrates have been identified, including marine geohazards, such as submarine landslides, and sudden gas releases from hydrate formations.
Because of the vast amounts of trapped natural gas in hydrate form globally, gas hydrates have been evaluated to be a significant unconventional natural gas resource. Hydrate deposits have been mapped in many locations around the world, usually using geotechnical methods that depend on the location of unexpected solid hydrate–liquid water interfaces which act as seismic reflectors. Test wells for gas production have been drilled in the Mackenzie Delta, Canada (Mallik 2L38), Alaska, the Nankai trough offshore Japan, and offshore China. Many problems have been encountered in producing gas from hydrate reservoirs, including the development of the best techniques for destabilizing the solid hydrate and capturing the resulting gas. Some hydrate deposits, often associated with hydrocarbon seeps or vents, exist as outcrops on the seafloor. Whereas most of the methane in hydrate reservoirs is of biogenic origin, hydrates associated with seeps or hot vents are formed from thermally altered hydrocarbons originally residing in deeper reservoirs. Some hydrate outcrops are home to specialized biological ecosystems where microbes feed on hydrocarbons and these in turn become a food source for “ice‐worms.”
Besides the marine and terrestrial natural gas hydrates, there are gas hydrates of air (mainly N2 and O2) deep inside glaciers. The hydrate zone starts at pressures where gas bubbles in the ice disappear. There has been much speculation regarding the existence of hydrates in extra‐terrestrial space, that is, on Mars, Titan, Enceladus, and the heads of comets. One of the best candidates for finding such a hydrate would appear to be that of CO2. Although lots of spectroscopic data exist for free solid CO2 and ice in extra‐terrestrial space, no sign of CO2 hydrate has been found, see Chapters 3 and 13 for possible reasons.
The large gas capacity and water as cheap working fluid make gas hydrates interesting materials for industrial applications. Gas hydrates are generally selective for guest molecule adsorption which allows the separation of gas mixtures. Much as salt is excluded when brine is frozen in desalination processes, gas hydrates have the same ability, but now the properties of the solid phase can be adjusted. For example, depending on the choice of guest, the hydrate freezing point can be well above the ice point, so saving on refrigeration costs. The same principle applies to cool energy storage where