TABLE OF CONTENTS Hydrates are classified by the arrangement of the water molecules in the crystal, and hence the crystal structure. There are two types of hydrates commonly encountered in the petroleum business. These are called Type I and Type II, sometimes referred to as Structure I and II. A third type of hydrate that may also be encountered is Type H (Structure H), but it is much less common.
Table 2-1 provides a quick comparison between Type I and Type II hydrates. These hydrates are reviewed in more detail throughout this chapter. Figure 2-1 shows the types of polyhedral cages involved in Type I and II hydrates. The information in Table 2-1 and Figure 2-1 will become clearer as the reader covers the entire chapter.
| Type I | Type II | |
|---|---|---|
| Water Molecules per Unit Cell | 46 | 136 |
| Cages per Unit Cell | ||
| 2 | 16 |
| 6 | 8 |
| Theoretical Formula [ ] | ||
| X 5 H 2O | X 5 ? H 2O |
| 0.1481 | 0.1500 |
| X 7 ? H 2O | X 17 H 2O |
| 0.1154 | 0.0556 |
| Cavity Diameter ( ) | ||
| 7.9 | 7.8 |
| 8.6 | 9.5 |
| Volume of Unit Cell (m 3) | 1.728 10 -27 | 5.178 10 -27 |
| Typical Formers | CH 4, C 2H 6, H 2S, CO |
