No abstract

in Wiley Online Library (wileyonlinelibrary.com) Induction time distributions for gas hydrate formation were measured for gas mixtures of methane 1 propane at pressures up to 11.3 MPa using a high-pressure automated lag time apparatus (HP-ALTA). Measurements were made at subcooling temperatures between 4.3 and 13.5 K and, while isothermal induction times between 0 and 15,000 s were observed, the median isothermal induction times for the distributions ranged from 100 to 4000 s. A hyperbolic relationship between median induction time and subcooling was used to correlate the data. A graphical interpretation is presented that relates the two types of data that can be acquired by using the HP-ALTA in one of two modes to study hydrate formation: induction time distributions at constant subcooling and formation temperature distributions observed during linear cooling ramps. The equivalence of these two modes provides a robust method for studying the variation of formation phenomena in different hydrate systems.

We studied the growth rate of methane−propane mixed gas hydrate films on the surface of water and up to 3 wt % aqueous solutions of a kinetic hydrate inhibitor, Luvicap EG. The greater the subcooling, the faster the growth rate of the hydrate film on water. Using Mori's heat-transfer-limited growth model, we estimated the initial thickness of the hydrate films on water to be between 1.3 and 25 μm. The addition of Luvicap EG inhibited both the nucleation and the growth of the hydrate films, because we observed that the films on Luvicap EG solutions grew slower and their nucleation required greater subcoolings than those on water. Notably, the hydrate films that grew on Luvicap EG solutions were more transparent and, thus, thinner than those on water. The thinner and yet more slowly growing hydrate films on Luvicap EG solutions suggest that the growth mechanisms had changed from heat-transfer-limited growth to mass-transfer-limited growth.

The effect of Polyvinylcaprolactam (PVCap), a commonly used kinetic hydrate inhibitor (KHI), on the cohesion force between cyclopentane hydrate particles was measured using a micromechanical force apparatus. The presence of PVCap in the aqueous bulk phase reduced the average hydrate cohesive force by 54% (from 1.49 to 0.69 mN/m). However, the cohesion forces did not vary significantly as a function of either the PVCap concentration (0.005−0.5 wt %) or the temperature (from 1.1 to 7.2 °C). When a layer of PVCap solution was applied to the surface of a pure cyclopentane hydrate particle in a bulk liquid cyclopentane phase, the interparticle cohesive force was reduced by 45% (from 4.3 to 2.4 mN/m). Hydrate growth on droplets of PVCap solutions was also studied by contacting a water droplet with a cyclopentane hydrate particle in a bulk cyclopentane phase. In cases where PVCap was absent, complete conversion of the water droplet to hydrate occurred within 30 s. However, when a water droplet of PVCap solution was brought into contact with a hydrate particle, hydrate film growth was significantly slowed, requiring over 2 h for complete conversion.