Full Title: Kinetics of Gas Hydrate Nucleation and Growth
Author(s): Hailu Kebede Abay
Publisher(s): University of Stavanger
Publication Date: January 1, 2011
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Description (excerpt):
Gas hydrates are ice-like crystalline compounds that are composed of water molecules (host) with encaged gas molecules (guests). Despite gas hydrates look like ice, they are quite different from ice as they can exist at temperatures well above the ice point and also with their structures. The most common gas hydrate crystal structures are structure I (sI, cubic), structure II (sII, cubic), and structure H (sH, hexagonal) of which sII is the commonest. Physical and chemical properties of these structures are different [1]. The present work fo- cuses only on sI and sII hydrates. Generally, these structures are composed of cavities formed from hydrogen-bonded water molecules where the guest molecule is trapped in the host. A specific number of water molecules is associated with each gas molecule due to the framework of the crystal type. For example, the ideal cubic unit cell of sI hydrate consists of two small pentagonal dodecahedron (512) cavities and six large tetrakaidecahedron (51262) cavities with a total of 46 water molecules. SII hydrate is composed of 16 small (512) cavities and 8 large hexakaidecahedron (51264) cavities with a total of 136 water molecules. Each cavity is formed from hydrogen-bonded water molecules. Fig. 1.1 shows 512, 51262 and 51264 cavities of sI and sII hydrates. The smallest cavity can ac- commodate one guest molecule of suitable size and shape but the largest cavity can accommodate two guests of suitable combination in size. At high pressures and at lower temperatures, the process of filling these cages by a guest molecule starts when hydrates begins to nucleate and grow which is called the process of hydrate formation.