Nucleation Curve of Carbon Dioxide Hydrate from a Linear Cooling Ramp Method.

Formation of gas hydrate is a first-order phase transition that starts with nucleation. Understanding of nucleation is of interest to many in chemical and petroleum industries, as nucleation, while beneficial in many chemical processes, is detrimental in flow assurance of oil and natural gas pipelines. A primary difficulty in the investigation of gas hydrate nucleation has been the inability of researchers to compare nucleation rates of gas hydrates across various systems of different scales and complexities, which in turn has been limiting the ability of researchers to study the nucleation process itself. In this study, a first-generation high-pressure automated lag time apparatus (HP-ALTA MkI) was used to determine the nucleation curve of structure I (sI) - forming carbon dioxide hydrate. The instrument subjected a quiescent water sample of well-defined dimensions to a large number of linear cooling ramps under isobaric conditions, and detected and recorded carbon dioxide hydrate formation temperature distributions. A survival curve was constructed from the measured ensemble, and a nucleation curve was derived from the survival curve using the empirical model-independent method we had previously reported. The nucleation rate of carbon dioxide hydrate was found to be significantly greater than that of pure methane hydrate or that of natural gas hydrate over the entire range of subcooling investigated. We provide a new physical interpretation of an experimentally determined nucleation curve and, by doing so, solve one of the outstanding puzzles of the HP-ALTA technology.