Stability of the hydrate layer formed on the surface of a CO2 droplet in high-pressure, low-temperature water

✍ Scribed by H. Teng; A. Yamasaki; Y. Shindo

• Publisher: Elsevier Science
• Year: 1996
• Tongue: English
• Weight: 768 KB
• Volume: 51
• Category: Article
• ISSN: 0009-2509
• DOI: 10.1016/0009-2509(96)00358-2

✦ Synopsis

Stability of the hydrate layer formed on the surface of a CO2 droplet in high-pressure and low-temperature water is studied. The criterion for stability of the hydrate layer is found to be Xcno2/> 0.098, where X~o: is the mole fraction of CO2 in the hydrate layer. Because COz molecules are not bonded in the hydrate lattice and are smaller than the free diameters of most cavities in the hydrate lattice, CO2 molecules in the hydrate tend to diffuse from the hydrate into water; thus, hydrate formation does not stop mass transfer of CO2 from the droplet into water, albeit the rate of mass transfer is reduced dramatically. Xcno~ is regulated by mass transfer of CO2 from the droplet into the hydrate and from the hydrate into water. Accordingly, density and stability of the hydrate layer are influenced significantly by changes in Xcno2. CO2 hydrate is found to be a nonstoichiometric compound. In a freshly formed hydrate layer, Xcno2 = 0.115 and X~o ~ drops as the hydrate crystal grows. When the thickness of the hydrate layer reaches a critical value at which x n = 0.098, the hydrate layer becomes unstable and collapses into hydrate clusters. Re-establish-CO2 ment of hydrates on the droplet is almost instantaneous. The hydrate layer undergoes a continuous cycle of collapse and re-establishment during the droplet dissolution; thus, it is thermodynamically unstable. However, rapid hydrate formation can maintain hydrates on the droplet, making the hydrate layer chemical-kinetically stable. Since the hydrate layer is thin and the critical thickness is close to that of a freshly formed hydrate layer, the thickness of the hydrate layer appears unchanged over the entire dissolution process.