Statistical study of clathrate-hydrate nucleation in a water/hydrochlorofluorocarbon system: Search for the nature of the "memory effect"

This paper describes an experimental study on the statistical nature of clathrate-hydrate nucleation in a quiescent hydrochlorofluorocarbon-in-water system in which a hydrate was once formed and then dissociated. The primary objective of the study is to investigate how hydrate nucleation in the system depends on its thermal history (i.e., the time evolution of system temperature), through which the preceding hydrate dissociation was carried out, and thereby better characterize the nature of the "memory effect" for hydrate formation. Isolated drops of HCFC-141b (CH₃CCl₂F) immersed in water were employed as test samples for use in detecting hydrate formation on the drop surfaces by video-monitoring. The thermal history of these samples was characterized by the temperature at which the hydrate covering the drops dissociated and also by the length of time for which the samples were held at that temperature before they were cooled again to a prescribed level to induce hydrate nucleation. Thirty to fifty samples were used in each thermal history program, to collect a sufficient amount of induction-time data for statistical data processing. The processed data indicated that hydrate nucleation in a system with a "memory" of prior hydrate formation/dissociation is an intrinsically stochastic event and that the rate of nucleation strongly depends on the thermal history of the system. A qualitative discrepancy is found between these data and predictions based on assumptions that the rate of nucleation is essentially the same for all samples subjected to the same thermal-history program and that the rate is held constant with time under constant hydrate-formable thermodynamic conditions. A hypothetical explanation is provided for this point, assuming that the "memory" of prior hydrate formation/dissociation that remains in individual samples could differ despite the samples having the same thermal history.