Gas Hydrates Clathrates

FIGURE 11.17 Phase behavior of the water-methane system [14, p 164] in the hydrate-forming region. Here V stands for vapor, H for hydrate, I for ice, and L for liquid. See the text for a discussion of this figure. 

The areas are marked on the figure. The curves, (clockwise from the upper right corner) represent the (V, L, H) equilibrium, the (V, L, 1) equilibrium, the (V, 1, H) equilibrium and the (1, H, L) equilibrium. The quadruple point represents the (V, L, 1, H) equilibrium. 

On this figure two of the phase boundaries have been carefully measured, as the data points show. The two nearly-vertical boundaries at 32°F represent the ordinary freezing curve for water, drawn on the assumption that the small amount of methane that dissolves in water does not influence the shape of this curve much. At high pressures this curve bends to the left (Problem 11.34) but over the range shown it is practically straight and vertical. 

This is one of the simplest phase diagrams for hydrate formation, many others are more complex [14], mostly because the hydrates have at least three different crystalline 

LIQUID-LIQUID, LIQUID-SOLID, AND GAS-SOLID EQUILIBRIUM 

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Solvation, folding of biological molecules Gas hydrates (clathrates)... 

A consideration of thermodynamic properties of the aqueous solution of rare gases and hydrocarbons led to the iceberg model for water structure around nonpolar molecules [139], which later had to be abandoned (see Part IV, Chap. 23.4). The gas hydrate clathrate structures described in Part IV, Chap. 21 provided... 

The model of icebergs around nonpolar solute molecules in aqueous solution is clearly not a very realistic one. However, if solutions of hydrocarbons (or noble gases) are cooled, then the solid phase that sometimes separates out consists of a so-called gas hydrate (clathrate), in which water provides a particular kind of hydrogen-bonded framework containing cages that are occupied by the nonpolar solute molecules. Obviously, such gas hydrates (clathrates) represent more realistic models for the phenomenon of hydrophobic hydration [176]. 

Quinol (hydroquinone) crystal was the first composite to be called a clathrate. Nowadays, this term has been adopted for many complexes which consist of a host molecule (forming the basic frame) and a guest molecule (set in the host molecule by interaction). The clathrate that is of interest to this study is the clathrate hydrate, also referred to as gas hydrate. Clathrate hydrates were discovered in 1810 by Sir Humphrey Davy. In his lecture to the Royal Society in 1810, he said that he had found, by several experiments, that the solution of chlorine gas in water freezes more readily than pure water [9]. 

Gas hydrate clathrates are formed by cooling water in presence of the guest species. The conditions of existence of the hydrates are conveniently represented by means of a pressure-temperature diagram sueh as Fig. 2. The domain of stability of the hydrate is located on the left part of curves II and III. Three data must be known in order to construct this diagram ... 

Ice-like gas hydrates (clathrates) also have been found at numerous sites along the convergent plate margins (e.g., Suess et al. 1997). Methane hydrates are stable in solid form only in a narrow temperature-pressure window (Dickens and Quinby-Hunt 1994 Fig. 14.3 in chapter 14). In theory, Im of methane hydrate can contain up to 164 m of methane gas at standard conditions (Kvenvolden 1993). It has been estimated that the amount of carbon in gas hydrates considerably exceeds the total of carbon occurring in all known oil, gas and coal deposits worldwide (Kvenvolden and McMenamin 1980 Kvenvolden 1988). This raises the possibility that gas hydrates may be a future energy source of global importance. 

Hesse, R., and Harrison, W.E., 1981. Gas hydrates (clathrates) causing pore-water freshening and oxygen istope fractionation in deep-water sedimentary... 

Gas hydrates (clathrates) may technically be considered as an alternative form of ice that has the ability to entrap relatively large volumes of gas within cavities in the hydrate crystal matrix. The entrapped guest molecules (gas) stabilize the structure by means of van der Waals interactions, and combinations of the different unit cells give rise to structures I, II (175-177), and H (178). The most common gas to form gas hydrates is methane, but ethane, propane, butane, carbon dioxide, nitrogen, and many other types of gases may also give rise to gas hydrates. 

Glathrate Formation. Ethylene oxide forms a stable clathrate with water (20). It is non stoichiometric, with 6.38 to 6.80 molecules of ethylene oxide to 46 molecules of water iu the unit cell (37). The maximum observed melting poiat is 11.1°C. An x-ray stmcture of the clathrate revealed that it is a type I gas hydrate, with six equivalent tetrakaidecahedral (14-sided) cavities fully occupied by ethylene oxide, and two dodecahedral cavities 20—34% occupied (38). 

According to these authors all gas hydrates crystallize in either of two cubic structures (I and II) in which the hydrated molecules are situated in cavities formed by a framework of water molecules linked together by hydrogen bonds. The numbers and sizes of the cavities differ for the two structures, but in both the water molecules are tetrahedrally coordinated as in ordinary ice. Apparently gas hydrates are clathrate compounds. 

HC1 2H20 and HC1 3H20 it readily forms a hydroquinone clathrate. Ammonia, on the other hand, does not form clathrates with either water or hydroquinone. Molecules with a very low polarizability (He, Ne, H2) are not known to form clathrate solutions by themselves, but they do help to stabilize the clathrate of a more polarizable solute simultaneously present.47 It is almost needless to say that in the following we shall only consider those hydrates which are in fact clathrates and which are frequently referred to as gas hydrates/ although the molecules of certain volatile liquids may also be included. 

In the next section we shall give a brief account of the crystal structure of the hydroquinone clathrates and of the gas hydrates, as far as is needed for a proper understanding of the subsequent parts. The reader who is interested in the phenomenology of other clathrate compounds should consult one of the many review articles7,8 39 on inclusion compounds. 

Let us consider a clathrate crystal consisting of a cage-forming substance Q and a number of encaged compounds ( solutes ) A, B,. . ., M. The substance Q has two forms a stable modification, which under given conditions may be either crystalline (a) or liquid (L), and a metastable modification (ft) enclosing cavities of different types 1,. . ., n which acts as host lattice ( solvent ) in the clathrate. The number of cavities of type i per molecule of Q is denoted by vt. For hydroquinone v — for gas hydrates of Structure I 1/23 and v2 = 3/23, for those of Structure II vx = 2/17 and v2 = 1/17. 

From the condition 21a it immediately follows that if the clathrate is formed in the presence of a number of compounds which are potential solutes, i.e., sufficiently small to have 0 for some i, all these compounds contribute to its stability. As has already been pointed out by Barrer and Stuart4 this at once explains the stabilizing influence of "Hilfsgase" such as air, C02, or H2S on the formation of gas hydrates discussed by Villard49 and von Stackelberg and Meinhold.47 If there is only one solute, Eq. 21a with the = sign determines the minimum vapor pressure fiA necessary to make the clathrate stable relative to Qa. Since all cavities contribute to the stabilization, one cannot say that this minimum pressure is controlled by a specific type of cavity. 

Let us explicitly consider the two important cases of hydro-quinone clathrates and gas hydrates. 

Gas hydrates are a special form of clathrates. Here water is the host molecule. The first gas hydrate (with chlorine) was described in 1818 by Sir Humphrey Davy. Naturally-occurring gas hydrates in Siberia are methane hydrates. 

Clathrates and, in particular, gas hydrates can be decomposed very easily by dissolving or melting the crystal lattice of the host molecule. 

The same applies to the historic gas-hydrates (hydrate clathrates, Fig. 5)17,18). However, on principle, only such molecules are suited for inclusion into the complicated H-bridge networks of gas-hydrates which do not interfere with the H-bridges of water, but have a hydrophobic nature. More recent hosts related to this inclusion principle are given in Chapter 3 of this book. 

Properties and extraction processes At high pressures and low temperatures, water and gas form an ice-like mixture, called gas hydrate, also known as clathrate or simply hydrate. Hydrates are a crystalline, solid substance composed largely of water... 

Natural gas (methane) can be obtained from gas hydrates. Gas hydrates are also called clathrates or methane hydrates. Gas hydrates are potentially one of the most important energy resources for the future. Methane gas hydrates are increasingly considered a potential energy resource. Methane gas hydrates are crystalline solids formed by combination of methane and water at low temperatures and high pressures. Gas hydrates have an iee-hke crystalline lattiee of water molecules with methane molecules trapped inside. Enormous reserves of hydrates can be foimd imder eontinental shelves and on land under permafrost. The amount of organic... 

An important advantage of the inclusion complexes of the cyclodextrins over those of other host compounds, particularly in regard to their use as models of enzyme-substrate complexes, is their ability to be formed in aqueous solution. In the case of clathrates, gas hydrates, and the inclusion complexes of such hosts as urea and deoxycholic acid, the cavity in which the guest molecule is situated is formed by the crystal lattice of the host. Thus, these inclusion complexes disintegrate when the crystal is dissolved. The cavity of the cyclodextrins, however, is a property of the size and shape of the molecule and hence it persists in solution. In fact, there is evidence that suggests that the ability of the cyclodextrins to form inclusion complexes is dependent on the presence of water. Once an inclusion complex has formed in solution, it can be crystallized however, in the solid state, additional cavities appear in the lattice, as in the case of the hosts previously mentioned, which enable the inclusion of further guest molecules. ... 

Several hundred to several thousand feet beneath the ocean floor in permafrost and continental edge regions lies a potentially vast source of natural gas in excess of 10 cubic meters of gas hydrates, consisting largely of methane clathrate (53-55). Gas... 

Tanasawa, I. Takao, S. (2002). Clathrate hydrate slurry of tetra-n-butylammonium bromide as a cold storage material. Proc. 4th Int. Conf. Gas Hydrates, 963-967, Yokohama, Japan. 

As follows from his laboratory notes, the first discovered clathrate hydrate (of chlorine) was observed, but not recognized, by Davy in 1810. Then Cl2, Br2, so2) co2, ch3ci, ch4, c2h, and numerous other gases were shown to form clathrate hydrates [22, 23]. Contrary to inorganic stoichiometric hydrates, those involving hydrocarbons are both non-stoichiometric and crystalline. In addition, gas hydrate composition was found to depend on temperature, pressure, and some... 

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