Gas Hydrate Type

Chlorine is a greenish gas. It is moderately soluble in water with which it reacts (see page 476). When chlorine is passed into dilute solutions of CaCl2 at 0°, feathery crystals of chlorine hydrate, Cl2 TJHjO, are formed. This substance is a clathrate of the gas-hydrate type (see page 160), having all medium holes and 20% of the small holes in the structure filled with chlorine molecules. 

The Formation of Ciathrates Having a Water Host Lattice 16.2.2.1. Ciathrates of the Gas Hydrate Type. 

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). 

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.

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. 

The main problem of hydrate formation will arise in pipelines transporting natural gas, because gas hydrates are solids and will leave deposits. The solid deposits reduce the effective diameter of the pipeline and can therefore restrict or even clog the flow properties. Furthermore, the formation of condensates, hydrates, or ice may occur in the course of decompression of natural gas stored in natural reservoirs (e.g., in salt caverns). The operation of oil and gas pipelines in the deep sea is significantly complicated by the formation of gas hydrates [1204]. Experience indicates that large gas hydrate plugs in gas and oil pipelines form most actively during the period of an unforeseen long shut-down. In static conditions, three types of hydrate crystals can be formed [1153] ... 

Of special interest in recent years has been the analysis of natural gas hydrates that form in marine sediments and polar rocks when saline pore waters are saturated with gas at high pressure and low temperature. Large and 5D-variations of hydrate bound methane, summarized by Kvenvolden (1995) and Milkov (2005), suggest that gas hydrates represent complex mixtures of gases of both microbial and thermogenic origin. The proportions of both gas types can vary significantly even between proximal sites. 

Uchida, T. Moriwaki, M. Takeya, S. Ikeda, I.Y. Ohmura, R. Nagao, J. Minagawa, H. Ebinuma, T. Narita, H. Gohara, K. Mae, S. (2004). Two-Step Formation of Methane-Propane Mixed Gas Hydrates in a Batch-Type Reactor. AIChE J. 50(2), 518-523. 

Alkali-Metal-Free Solutions. Films of CD PbS are usually p-type as deposited. One early suggestion to explain this was that the alkali metal ions used in the deposition solution (as NaOH or KOH) act as a p-type dopant [33]. Based on this supposition, Bloem deposited PbS from a solution of PbAci, hydrazine hydrate, and thiourea (free of Na or K). The as-deposited films were initially n-type but changed to p-type on exposure to air. Attempts to dope the films permanently n-type by adding trivalent ions to the deposition solution were unsuccessful. However, by depositing the films on a substrate coated with trivalent ions (such as Al, In, Ga), n-type behavior could be maintained for a considerable time. PbS p-n junctions were fabricated using this approach (see Chap. 9). 

Although beryllium and magnesium salts do not form stable mctal-ammines yet they unite with ammonia, forming additive compounds of the hydrate type which are sometimes referred to as ammoniates or ammonio-compounds. These appear to be of the same type as the metal-anunines, and the difference seems to be merely one of stability. The ammonio-compounds are formed by the addition of ammonia gas to dry or fused salt, and most of them decompose with liberation of ammonia when dissolved in water. 

Section of the framework in gas hydrates of type I. Each vertex represents an O atom, and along each edge there is an H atom... 

All common natural gas hydrates belong to the three crystal structures, cubic structure I (si), cubic structure II (sll), or hexagonal structure H (sH) shown in Figure 1.5. This chapter details the structures of these three types of hydrate and compares hydrates with the most common water solid, hexagonal ice Ih. The major contrast is that ice forms as a pure component, while hydrates will not form without guests of the proper size. 

In a recent ocean hydrate formation state-of-the-art summary, Trehu et al. (2006) listed the effects of fluid flow and sediment lithology. Ocean hydrate deposits are distributed on a spectrum between two types in ocean sediments (1) focused high flux (FHF) gas hydrates, and (2) distributed low flux (DLF) gas hydrates. In FHF hydrates the gas comes from a large sediment volume channeled through a high-permeability sand to the point of hydrate formation, and these hydrates are typically in the upper tens of meters of the sediment. In contrast, the DLF hydrates are generated near where the hydrates are formed, and fluid flow is responsible for movement of the gas within the gas hydrate occurrence zone (GHOZ). 

There are several types of environments on Earth where significant water exists at prevalent low temperatures such that ice and liquid aqueous solutions commonly coexist permafrost, snow, glaciers, lake and river ice, sea ice, and parts of the atmosphere (polar troposphere, global upper troposphere, and stratosphere). In addition, the deep sea floor occurs at temperatures very close to the freezing point of water. For example, temperatures in the oceanic abysses hover around 2°C at a maximum hydrostatic pressure of 1100 bars (10,660 m) in the Mariana Trench (Yayanos, 1995). Table 4.1 summarizes some of these environments. Furthermore, in some permafrost and sea-floor environments, the presence of nonpolar gases under pressure can stabilize a modified form of ice known as gas hydrates even where temperatures are not quite low enough for ordinary ice to form. 

We emphasize that it is not anhydrous salts but salt hydrates (and gas hydrates) that have this important thermal insulating property. Cool climates, which tend to support high hydration states, favor this type of process. Cold evaporitic basins are ideal, and so we find that Mars is where this phenomenology is most likely to have widespread relevance. 

A basic building block in many clathrate hydrates is a pentagonal dodecahedron of water molecules (Fig. 7). This structure is found in gas hydrates of structure I (12 A) and structure II (17 A) types... 

A hydrate is a type of chemical compound called a clathrate, defined as a solid molecular compound in which one component is trapped in the cavities of cagelike crystals of another component. In the natural gas hydrate, water molecules form the cage, and hydrocarbon molecules are the trapped components). 

The lattice of the host in the form it takes in the clathrate is usually thermodynamically unstable by itself—that is, with the holes empty. It is stabilized by inclusion of the guest molecules, and it is of obvious interest in connection with the nonstoichiometry of clathrates to consider the extent to which the cavities in the host lattice must be filled before the system achieves thermodynamic stability. The cavities in the host lattice may all be identical in size and environment, as in the hydroquinone clathrates, or they may be of more than one kind. The gas hydrates, for example, have two possible structures, in each of which there are two sorts of cavity, van der Waals and Platteeuw (15) have developed a general statistical theory of clathrates containing more than one type of cavity. 

The situation with the gas hydrates is rather more complicated, since two types of cavity exist, and Equation 5 becomes... 

Table 21.1. Molecules which form types I and II gas hydrates [762]...

The gas hydrates occur in three types of structures, named type I, type n and type H. The type I has 512 and 51262 polyhedra associated by face-sharing in the ratio 1 3, whereas in type II, 512 and 5,264 polyhedra are combined by face-sharing in the ratio 2 1. These structures have cubic symmetry. The recently discovered type H [779] has hexagonal symmetry and is isostructural with the hexagonal clathrasil-dodecasil 7-H [780]. The host lattice contains 512, 435663 and a larger 51268 void which can accommodate larger organic molecules than the type II cubic structure. 

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