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, vibrational (IR and Raman) and electronic spectroscopy, molecular dynamics simulation, and quantum mechanical computations. These techniques and what they teach us about clathrate hydrates are the subjects of the remainder of this book. While new methodologies produce an unprecedented wealth of detail on the structure and dynamics of clathrate hydrate phases, the lessons and constraints of the past centuries of clathrate hydrate research should not be forgotten.
References
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21 21