Rheology of Hydrate-Forming Emulsions Stabilized by Surfactant and Hydrophobic Silica Nanoparticles

Abstract

Observed effects of hydrophobic fumed silica nanoparticles (of average primary particle size 7 nm) on the rheological behavior of hydrate-forming emulsions are presented. Liquid cyclopentane (CP) is the hydrate former. The hydrate slurry is prepared in a Couette geometry at atmospheric pressure from a water-in-oil emulsion with the phases density matched to avoid segregation. Hydrates are formed upon quenching to a low temperature at a fixed shear rate. Dispersed water droplets convert to hydrate particles, leading to an effective viscosity increase by orders of magnitude. The hydrate inhibition by silica nanoparticles at the water oil interface, forming a Pickering type of emulsion, is characterized using the onset time of steep viscosity rise after seeding with small hydrate particles; this is termed the critical time. Seeding eliminates stochasticity associated with nucleation of the hydrate. The critical time is increased when the interface is covered with silica nanoparticles. For a particle concentration range of 0.05-0.5% (by weight based on total oil mass) at the interface, the hydrate crystallization process is delayed by 5 h in comparison to the particle-free case for a 20 vol % water-in-oil emulsion at T = -2 degrees C and shear rate of gamma = 100 s(-1). The final hydrate slurry viscosity was the same as observed in the slurry with no particles. At particle concentrations greater than 1 wt %, the viscosity increased abruptly and ultimately jammed the rheometer during hydrate formation. A hypothesis is presented to explain this latter behavior and indicates some of the limitations of this method of inhibition by nanoparticles. A discussion of factors which may complicate application of the method in the field is provided.