Hydrate

1- Introduction

In its most general sense, a hydrate is a compound containing water. For example, there is a class of inorganic compounds called “solid hydrates,” ionic solids where the ions are surrounded by water molecules and form crystalline solids. hydrates are composed of a small molecule and water.

Hydrates are crystalline solid compounds formed from water and smaller molecules. They are a subset of compounds known as clathrates or inclusion compounds. A clathrate compound is one in which a molecule of one substance is enclosed in a structure built from molecules of another substance.

Even though the clathrates of water, the so-called hydrates, are the focus of this work, they are not the only clathrate compounds. For example, urea forms interesting inclusion compounds as well.

Although hydrates were probably encountered by others earlier, credit for their discovery is usually given to the famous English chemist, Sir Humphrey Davy. He reported of the hydrate of chlorine in the early 19th century. In particular, he noted that the ice-like solid formed at temperatures greater than the freezing point of water, and that the solid was composed of more than just water.

Davy’s equally famous assistant, Michael Faraday, also studied the hydrate of chlorine. In 1823, Faraday reported the composition of the chlorine hydrate.

Although his result was inaccurate, it was the first time the composition of a hydrate was measured.

Throughout the 19th century, hydrates remained an intellectual curiosity. Early efforts focused on finding which compounds formed hydrates and under what temperatures and pressures they would form. Many of the important hydrate former were discovered during this era.

Among the 19th-century hydrate researchers who deserve mention are the French chemists Villard and de Forcrand, who measured the hydrate conditions for a wide range of substances.

However, it would not be until the 20th century that the industrial importance of gas hydrates would be established.

Hydrates

In combination with water, many of the components commonly found in natural gas form hydrates. One of the problems in the production, processing, and transportation of natural gas and liquids derived from natural gas is the formation of hydrates. However, the importance of natural gas hydrates was not apparent in the early era of the gas business.

During that time in the natural gas business, gas was produced and delivered at relatively low pressure. Thus, hydrates were never encountered. In the twentieth century, with the expansion of the natural gas industry, the production, processing, and distribution of gas became high-pressure operations.

Under pressure, it was discovered that pipelines and processing equipment were becoming plugged with what appeared to be ice, except the conditions were too warm for ice to form. It was not until the 1930s that Hammerschmidt clearly demonstrated that the “ice” was actually gas hydrates. Moreover,

the hydrates were a mixture of water and the components of natural gas.

In the petroleum industry, the term hydrate is reserved for substances that are usually gaseous at room temperature. These include methane, ethane, carbon dioxide, and hydrogen sulfide. This leads to the term gas hydrates and to one of the popular misconceptions regarding these compounds. It is commonly believed that non-aqueous liquids do not form hydrates. However, liquids may

also form hydrates. An example of a compound that is liquid at room conditions, yet forms a hydrate, is dichlorodifluoromethane (Freon 12).

The water molecule

Many of the usual properties of water can be explained by the structure of the water molecule and the

consequences of this structure.

Of particular interest to us is that the structure of the water molecule leads to the possibility of hydrate formation. we will demonstrate that water does indeed have some unusual properties.

The Shape of the Water Molecule and the Hydrogen Bond

Virtually all of the unusual properties of water noted earlier can be explained by the shape of the water molecule and the interactions that result from its shape.
The water molecule consists of a single atom of oxygen bonded to two hydrogen atoms, as depicted in the above Figure. In the water molecule, the bond between the oxygen and hydrogen atoms is a covalent bond, which is essentially a shared pair of electrons. The angle between the two hydrogen atoms in the water molecule is about 105°.
These electrons induce negative charges on the oxygen molecule and a small positive charge on the hydrogen atoms. The induced electrostatic charges on the molecule. Thus, the water molecules will tend to align with a hydrogen molecule lining up with an oxygen.

This aligning of the hydrogen and oxygen atoms is called a “hydrogen bond.” The hydrogen bond is essentially an electrostatic attraction between the molecules.
It should be noted that each water molecule has two pair of unbond electrons and thus has two hydrogen bonds—two water molecules “stick” to each water molecule. The hydrogen bond is only 1/10 or 1/20 as strong as a covalent bond, which is what holds the oxygen and hydrogen atoms together in the water molecule, but this is still strong enough to explain the properties discussed earlier.

The hydrogen bonds are particularly strong in water although they are present in other substances, such as the alcohols. It is for this reason that the normal boiling points of the alcohols are significantly larger than their paraffin analogues.

When the water molecules line up, they form a hexagonal pattern. This is the hexagonal crystal structure discussed earlier. From elementary geometry, it is well known that the angle between the sides of a regular hexagon is 120°, which is greater than the 105° angle in the water molecule. This seeming paradox is overcome because the hexagonal pattern of the water molecules is not planar. The hexagonal pattern of the water molecules in the ice crystal is shown in the next Figure. In this figure, the circles represent the water molecules and the lines the hydrogen bonds.