Gases hydrates

Gas bubble Capacity 10 200 Gas bubbles in liquid gas, for example, butane, propane 

The organic carbon sources at seeps (methane, petroleum, other hydrocarbon gases, solid gas hydrates) are derived primarily from accumulated sedimentary organic carbon and thus are photosynthetic in origin (Tunnicliffe etal., 2003). In addition, microbial oxidation of reduced compounds at/or above the seafloor requires photosynthetically produced oxygen. Thus, while these ecosystems are primarily based on chemosynthesis (Aharon, 2000), they are not completely independent from photosynthesis-dominated pelagic ecosystems. 

A common feature associated with cold seeps is the presence of gas hydrates, which are naturally occurring solids comprised essentially of natural gas, mainly methane, trapped in frozen, crystalline water (see reviews by Kvenvolden, 1993 and Buffet, 2000). The occurrence of gas hydrates is controlled by an interrelation among temperature, pressure and composition, and they are stable in solid form only in a narrow range of these conditions (Kvenvolden, 1993). Because of these restrictions, gas hydrates are common mainly in polar and deep 

Dissolution of these metastable gas hydrates is a natural consequence of tectonic uplift of accretionary prisms at plate margins and is likely to be partly responsible for the extensive methane plumes above these sites (Herzig Hannington, 2000). The amount of methane that is slowly released from these hydrates can be enough to support a dependent biological community (Suess etal., 1999). 

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Sulphur dioxide, SO2, m.p. — 72-7°C, b.p. — I0"C. Colourless gas with characteristic smell. Formed by burning S, metal sulphides, H2S in air or acid on a sulphite or hydrogen sulphite. Powerful reducing agent, particularly in water. Dissolves in water to give a gas hydrate the solution behaves as an acid - see sulphurous acid. Used in the production of SO3 for sulphuric acid. 

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). 

Englezos, P., Kalogerakis, N., Dholababhai, P.D. and Bishnoi, P.R., 1987a. Kinetics of fonuation of methane and ethane gas hydrates. Chemical Engineering Science, 42(11), 2647-2658. 

Noble gas hydrates are formed similarly when water is frozen under a high pressure of gas (p. 626). They have the ideal composition, [Gg(H20)46], and again are formed by Ar, Kr and Xe but not by He or Ne. A comparable phenomenon occurs when synthetic zeolites (molecular sieves) are cooled under a high pressure of gas, and Ar and Kr have been encapsulated in this way (p. 358). Samples containing up to 20% by weight of Ar have been obtained. 

Gas hydrates are an ice-like material which is constituted of methane molecules encaged in a cluster of water molecules and held together by hydrogen bonds. This material occurs in large underground deposits found beneath the ocean floor on continental margins and in places north of the arctic circle such as Siberia. It is estimated that gas hydrate deposits contain twice as much carbon as all other fossil fuels on earth. This source, if proven feasible for recovery, could be a future energy as well as chemical source for petrochemicals. 

Dagani, R. Gas hydrates eyed as future energy source, Chemical and Engineering News, March 6, 1995, p. 39. 

Non-conventional gas is natural gas found in unusual underground conditions, such as very impermeable reservoirs which require massive stimulation in order to be recovered, or in underground occurrences of gas hydrates, or dissolved in formation water, or coal-bed methane, or gas from in-situ gasification of coal. 

The accurate composition of the gas hydrates for a long time remained a controversial subject, since direct analysis leads to ambiguous results owing to decomposition of the hydrate and/or inclusion of mother liquor in the crystals. Thus it was firmly believed that the nonstoichiometric compositions of gas hydrates found experimentally were all due to errors in the analysis. But more recent determinations of the composition by the indirect... 

For a detailed account of the work on gas hydrates and references prior to 1925 the reader is referred to Schrdder s historical account of this subject,40 while later references are given in von Stackelberg s papers.4 49... 

The mechanism responsible for the formation of gas hydrates became clear when von Stackelberg and his school 42 49 in Bonn succeeded in determining the x-ray diffraction patterns of a number of gas hydrates and Claussen6 helped to formulate structural arrays fitting these patterns. Almost simultaneously Pauling and Marsh26 determined the crystal structure of chlorine hydrate. 

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. 

Fig. 2. Unit cell of a gas hydrate of Structure I according to von Stackelberg and Muller.48 For the sake of clarity only the elements lying in the nearer half of the unit cell have been drawn. The smaller dots indicate the tetrahedrally surrounded water molecules, the larger dots represent the centers of the two types of cavities.

Although the agreement between the observed x-ray intensities and those calculated from the aforementioned model is not perfect, the latter is thought to be essentially correct. The composition of a gas hydrate of Structure I is then characterized by... 

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. 

The value of Ay for gas hydrates of Structure I reported in Table II could thus be derived30 with the aid of Eq. 25 with v — 3/23 from the composition Br2 8.47 H20 of the bromine hydrate following from Miss Mulders accurate study19 of the system Br2-f-H20 cf. Section III.C.(l). It should be possible to derive the value of Ay for hydrates of Structure II in the same way from the equilibrium composition of the SFe hydrate unfortunately the equilibrium composition of this hydrate is not known. The value of Ay for hydrates of Structure II reported in the table has been derived from the vapor pressure of the SF6 hydrate using some further assumptions (cf. Section III.C.(2)(b)). 

In this equation, AH is not known, but Eucken (as quoted by von Stackelberg48) suggested, AH = 0, and comparison of experimental and calculated heats of hydrate formation30 certainly supports a low value of AH. The variation of the composition of a gas hydrate along the three-phase line ice-hydrate-gas will therefore be small. (The variation along the three-phase line hydrate-aqueous liquid-gas is larger, cf. Section III.C.(l).)... 

Recently30 results were given of an application of the present theory to some gas hydrates of Structure I. In this case one has to consider two kinds of cavities for which we assumed, again using the approximations 40 and 41,... 

TABLE V. Dissociation Pressures and Degrees of Occupation of Gas Hydrates at 273°K30... 

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