posted on 2021-10-13, 18:52authored byHannah
M. Stoner, Anh Phan, Alberto Striolo, Carolyn A. Koh
Although the wettability of hydrate
surfaces and hydrate film growth
are key to understanding hydrate agglomeration and pipeline plugging,
a quantitative understanding of the coupled behavior between both
phenomena is lacking. In situ measurements of wettability
coupled with film growth were performed for cyclopentane hydrate surfaces
in cyclopentane at atmospheric pressure and temperatures between 1.5–6.8
°C. Results were obtained as a function of annealing (conversion)
time and subcooling. Hydrate surface wettability decreased as annealing
time increased, while hydrate film growth rate was unaffected by annealing
time at any subcooling. The results are interpreted as a manifestation
of the hydrate surface porosity, which depends on annealing time and
controls water spreading on the hydrate surface. The wettability generally
decreased as the subcooling increased because higher subcooling yields
rougher hydrate surfaces, making it harder for water to spread. However,
this effect is balanced by hydrate growth rates, which increase with
subcooling. Also affecting the results, surface heating from heat
release (from exothermic crystallization) allows excess surface water
to promote spreading. The hydrate film growth rate on water droplets
increased with subcooling, as expected from a higher driving force.
At any subcooling, the instantaneous hydrate growth rate decreased
over time, likely from heat transfer limitations. A new phenomenon
was observed, where the angle at the three-phase point increases from
the initial contact angle upon hydrate film growth, named the crystallization
angle. This is attributed to the water droplet trying to spread while
the thin film is weak enough to be redirected. Once the hydrate film
grows and forms a “wall” around the droplet, it cannot
be moved, and further growth yields a crater on the droplet surface,
attributed to water penetrating the hydrate surface pore structures.
This fundamental behavior has many flow assurance implications since
it affects the interactions between the agglomerating hydrate particles
and water droplets.