equilibrium studies of natural gas components and water by W.M. Deaton and E.M. Frost (US Bureau of Mines) [97], Donald Katz (University of Michigan), Riki Kobayashi (Rice University), and coworkers. These important advances in the engineering and thermodynamics of gas hydrates, particularly for species in natural gas, are covered admirably in Dendy Sloan and Carolyn Koh's book on Clathrate Hydrates of Natural Gases [98]. This work resulted in general procedures for the prediction of solid hydrate formation under engineering conditions, usually based on gas gravity, and recipes for their prevention. The state of the art of this approach to hydrate prediction and prevention can be found, e.g. in the Handbook of Natural Gas Engineering (1959) [99]. This work would only develop along more knowledge‐based lines once the hydrate crystal structures and a theoretical description in the form of the “solid solution” theory became available in the late 1950s. Further development took place in describing guest–host interactions e.g. the work of McKoy and Sinanoglu on the use of the Kihara potential in the solid solution theory as an alternative to the Lennard‐Jones 12–6 potential [100]. The Kihara potential is similar to the Lennard‐Jones potential but considers molecules to have a hard, impenetrable core at the center. The modeling of phase equilibria was advanced considerably by Parrish and Prausnitz [101] who introduced the Kihara potential into the van der Waals and Platteeuw model and developed a particularly useful form for the computation of hydrate phase equilibria and cage occupancies. The first molecular simulations on clathrate hydrates were performed in 1972 by Tester, Bivens, and Herrick who used canonical Monte Carlo simulations to explicitly calculate guest–water interaction terms in the cages for use in the van der Waals–Platteeuw theory, without using the approximate Kihara potential form [102]. These simulations were followed in 1982 by the work of John S. Tse and Donald Davidson who improved the intermolecular potential representations of the water–guest interactions in the Monte Carlo simulations [103]. The first molecular dynamics simulations on hydrate phases were performed shortly afterward on structural characterizations of methane hydrate by J.S. Tse et al. [104].
With a good understanding of the nature of clathrate hydrates in hand, impressive strides were made in both experimental and predictive work on the phase equilibria of hydrates. In the 1960s and 1970s, a new generation of hydrate researchers arose in the United States and Canada: Gerald D. Holder in Pittsburgh, E. Dendy Sloan Jr. at the Colorado School of Mines (CSM), Donald B. Robinson at the University of Alberta, Raj Bishnoi at the University of Calgary. Much of the emphasis of these researchers was to deal with flow assurance problems involving hydrates: hydrate inhibition, hydrate plug prevention, and decomposition. In many of the laboratories mentioned above, hydrates became a continuing research theme which in many cases still continues – sometimes again with a new generation of hydrate researchers.
Another stream of hydrate research emerged when the US Office of Saline Water started a number of projects on the desalination of sea water. Hydrates were seen as one route to producing fresh water, with some advantages over the straightforward freezing of salt water to exclude salt. Allan Barduhn (University of Syracuse) investigated a large number of potential hydrate formers as potential desalting agents, including a number of chlorofluorocarbons (CFCs). Although hydrate desalination went as far as the pilot plant stage in the 1960s, so far a viable technology has not been developed.
2.9 Clathrate Hydrates in Nature
Hydrates of natural gas in nature were the subject of some speculation [105] before Yuri F. Makogon first reported the existence of natural gas hydrates associated with the Messoyakha gas field in Siberia in 1965 [106]. Onshore permafrost hydrates in Canada [107] and marine gas hydrates associated with sediment on the continental margins [108] were reported soon after. Their ubiquitous nature soon kindled worldwide interest in hydrates as to their role in the geosphere and especially as a huge new global source of natural gas. This broadened hydrate research to include the earth and ocean sciences, geotechnical engineering, reservoir modeling, and other related fields. Some of the laboratories mentioned above quite naturally also became involved in this new stream of hydrate research.
Other naturally occurring hydrates, those of air, were predicted by Stanley L. Miller in 1969 to occur deep inside glaciers, at a depth where air bubbles trapped in the ice disappeared [109]. He also suggested that CO2 hydrate may well occur on Mars, and methane hydrate on the outer planets and their satellites [110, 111]. In 1952, Armond H. Delsemme and Pol F. Swings, then at the University of Liège further speculated that gas hydrates may be responsible for the steady outgassing of the heads of comets on approaching the sun [112].
2.10 Summary and Observations
In the early years, hydrate research was carried out without sophisticated apparatus and depended largely on the acuity of the observer and the attention to experimental detail. The human factor is also displayed clearly in the various wrong‐headed ideas that arose, and the stubborn adherence to these that outlasted their usefulness sometimes by over a century. The major achievements of early hydrate scientific research were:
1 The initial observation of gas hydrates as new materials, followed by building the hydrate guest inventory to some 40 species. This culminated in the development of thermodynamic models to describe heterogeneous equilibria in hydrates. The peak years of activity were 1870–1903 when some 370 scientific papers were published, see Figure 2.11. This left the state of knowledge on hydrates quite capable of understanding practical problems in the oil and gas industry such as hydrate plug formation and prevention as initiated by Hammerschmidt [96].
2 After a very active period of research ending in about 1903, scientific work on hydrates dropped off somewhat for close to 30 years, likely because of an impasse reached in the determination of hydrate compositions and the lack of structural knowledge. During this time, the work of Hammerschmidt attracted attention and industry initiated related work on the clathrate hydrates. What was of critical importance for further progress was structural information on the hydrate phases, and this finally became available in the early 1950s.Figure 2.11 The number of publications related to gas (clathrate) hydrates between 1810 and 1970 from Schröder's review [1] and later literature sources. More than 500 publications appeared during this time span.
3 With the knowledge of heterogeneous equilibria and hydrate structures, it did not take long for the development of the “solid solution” theory capable of describing and predicting hydrate properties (1953–1959).
As well, in the early years of hydrate research, many interesting observations were made, but often not understood. Some of these early “mysteries” have been clarified, but others have persisted until today. Also, there are a number of recent “discoveries” that in fact were reported by the early hydrate researchers. These include:
1 Memory effects in the (re)formation of hydrate phases in reactors;
2 Self‐preservation where hydrates are seen to remain metastable under conditions where they are not thermodynamically stable;
3 The presence of air as an unintended helper gas during hydrate preparation stabilizes the hydrate phase;
4 The storage of unstable species in hydrate form (ClO2);
5 Hydrogen as helper gas (hydrogen storage);
6 Preparation of and stability conditions of hydrates with large guest molecules (structure H hydrates);
7 The possibility of separation of gases in mixtures by selective incorporation into the hydrate phase.
Recent progress in the study of clathrate hydrates has been characterized by the use of sophisticated methods and instrumentation such as single‐crystal X‐ray and neutron diffraction, powder X‐ray diffraction, solution and solid‐state NMR,
