Clathrate Hydrates: A Hope for the Fuel Industry and Great Ecological Hazard

Introduction

Water is a mysterious substance; being hydrophilic (by definition) it can capture hydrophobic or hydrophobic-hydrphilic species forming different kinds of hydrate inclusion compounds. Hydrate inclusion compounds can be formed by a variety of molecules and molecular associates, which are commonly referred to as guests. Common to all these compounds is the presence of a more or less complex framework built of hydrogen-bonded water molecules. This framework is commonly referred to as the host framework. An example of one of the hydrate inclusion compounds with a relatively simple water framework is shown in Figure 1. One of the most studied and practically important classes of hydrate inclusion compounds are clathrate hydrates, in which the interaction of the guest components and the host's water framework is predominantly of a van der Waals nature. This class of compounds will be discussed below.

Figure 1.

An example of water structure containing hydrophobic-hydrphilic compound (complex of 18-crown-6 with ammonium fluoride). Water molecules, shown as small light-green balls, form hydrogen-bonded aggregates containing in their outer space hydrophobic parts. Hydrogen bonds are shown as light-green sticks. Nitrogen atoms marked in blue, orange – fluoride, red – macrocycle oxygen and gray – carbon atoms. Blue lines display hydrogen bonds between water oxygen atoms (light green), fluoride and nitrogen.

Structures, Phase Diagrams And Properties

Structures of gas hydrates represent itself open crystal frameworks built by the hydrogen-bonded water molecules. These frameworks have molecular-size cavities in which molecules of hydrate former are located (Figure 2). The structures of hydrates are usually depicted in the form of polyhedra stuck together along the edges, the vertices of which correspond to the oxygen atoms of water molecules and the edges represent hydrogen bonds. Hydrates of three structural types are found in nature, usually designated sI, sII and sH (Sloan & Koh, 2008; Carroll, 2014). The vast majority of natural hydrates have sI or sII.

Figure 2.

Crystalline frameworks of the main structural types of gas hydrates. The vertices of the polyhedra are oxygen atoms of water molecules, the edges are hydrogen bonds. Shown below are the types of polyhedra that make up a given crystal structure, their designations, and the characteristic dimensions of the internal space available for the inclusion of guest molecules. The bottom shows the number of polyhedra of each type in one unit cell and the number of water molecules that make up the unit cell.

sI hydrates are formed by natural gas with a predominant content of methane as well as other gases such as ethane, carbon dioxide, xenon, etc. Natural gas with a high content of ethane, propane, and butanes forms sII hydrates, which have larger cavities. Freons, chlorine derivatives of methane, tetrahydrofuran, and other large molecules also form hydrates of this structure. Finally, the rare examples of natural sH hydrates are associated with the presence of heavier hydrocarbons in the feed gas. Under the laboratory conditions, hydrates of this structure can be obtained, for example, from adamantane + xenon or isoamyl alcohol + methane. The conditions under which the gas hydrates can exist are described by phase diagrams. In the simplest case of a two-component system hydrate former - water, the temperature and pressure range corresponding to the stability of the hydrate is limited by two or three equilibrium lines (Figure 3). These lines describing the three-phase equilibria are related to ice - hydrate - gaseous hydrate former (line 1, Figure 3), liquid water - hydrate - gaseous hydrate former (line 2, Figure 3), and liquid water - hydrate - liquid hydrate former (line 3, Figure 3).