(246e) Cyclopentane Hydrate Morphology From Brine: Heterogeneous Nucleation From Ice

Clathrate hydrate, a crystalline compound formed when a light hydrocarbon molecule stabilizes hydrogen-bonded water cages, poses a major problem in pipelines, including flow interruption, equipment damage, personnel injury and environmental pollution. In offshore drilling, the salinity of water in the aquifer becomes an important factor to be considered. In this study we have investigated the morphological properties of cyclopentane hydrate formed on a single aqueous drop immersed in cyclopentane at various NaCl (salt) and oil-soluble surfactant (Span 80) concentrations at different subcooling and temperature ramp rates. Observed morphologies are correlated with previously reported rheological properties such as viscosity and yield stress of cyclopentane hydrate slurries, at variable extent of water conversion to hydrate (controlled by salt concentration). In the absence of Span 80, a robust faceted shell formation occurs; thus further hydrate conversion is kinetically limited by mass diffusion through the shell. In the presence of salt and Span 80, a fractured hydrate shell formation occurs with residual brine inside the hydrate “particle”. The observations indicate that residual brine may pass through the fractured shell and form capillary bridges between hydrate particles as suggested by rheological studies. In the absence of salt, ice conversion to hydrate is kinetically limited by the availability of free liquid water as premelted ice; while in the presence of salt, ice to hydrate conversion is kinetically enhanced by the residual free liquid water present in brine. A remarkable observation showing the critical nature of heterogeneous nucleation is reported: The heterogeneous nucleation of hydrate from ice is found to be qualitatively controlled by the temperature ramp rate in the experiment. Apparently, a low temperature ramp rate results in a higher number of heterogeneous nucleation sites for hydrate, and thus a much higher surface area crystal structure than a rapid heating rate.