Chapter two - Hydrate types and formers

Abstract

This chapter discusses the types of hydrates and formers. Hydrates are classified by the arrangement of the water molecules in the crystal, and hence, the crystal structure. Two types of hydrates are commonly encountered in the petroleum business: Type I and Type II, sometimes referred to as Structure I and II. A third type of hydrate that also may be encountered is Type H (also known as Structure H), but it is much less common. Type I are made from two types of cages: dodecahedron, a 12-sided polyhedron where each face is a regular pentagon, and tetrakaidecahedron, a 14-sided polyhedron with 12 pentagonal faces and two hexagonal faces. The dodecahedral cages are smaller than the tetrakaidecahedral cages; thus, the dodecahedra are often referred to as small cages whereas the tetrakaidecahedra cages are referred to as large cages. The structure of the Type II hydrates is significantly more complicated than that of the Type I. Some of the common Type I hydrate formers include methane, ethane, carbon dioxide, and hydrogen sulfide. The Type II hydrates are also constructed from two types of cages. The unit structures of a Type II hydrate are: dodecahedron, a 12-sided polyhedron where each face is a regular pentagon, and hexakaidecahedron, a 16-sided polyhedron with 12 pentagonal faces and four hexagonal faces. The dodecahedral cages are smaller than the hexakaidecahedron cages. Among the common Type II formers in natural gas are nitrogen, propane, and isobutane. Type H hydrates are much less common than Type I or II. To form this type of hydrate requires a small molecule, such as methane, and a Type H former.