Film Growth Thickness

Micromechanical investigations of cyclopentane hydrate led to a technique to heterogeneously nucleate cyclopentane hydrate. This formation process involved a similar in situ formation method as that used to freeze water/THF droplets into THF hydrate. Using this technique, a water droplet was placed on a cantilever and submerged in cyclopentane. The water droplet was then nucleated with another hydrate particle. Nucleation occurred when the particles came into contact, and a thin hydrate shell formed around the water droplet over the course of a few minutes (Figure 1B). The hydrate film encompassed the entire surface of the water droplet. However, the interior of the particle still contained unconverted water. Once the hydrate layer spread over the surface of the water drop, the only way to maintain the clathration reaction is the transport of guest molecules through the intervening hydrate layer to the hydrate/water interface. Over time, cyclopentane diffused through the hydrate layer, and the water inside the hydrate shell was converted into hydrate. Dimples start to grow on the shell 0.5 hours after nucleation, indicating continued hydrate growth as shown in Figure 1C. Figure 1D shows further hydrate conversion, indicated by the hydrate phase darkening as light was scattered by the dense larger amounts of hydrate crystallites being formed. Finally, after seven hours the entire particle darkens due to further hydrate conversion (Figure 1E).

Figure 1: Nucleation of a water droplet immersed in cyclopentane. (A) Initial contact, (B) cyclopentane hydrate shell formed around the water droplet, (C) dimples formed on the hydrate shell, (D) conversion of interior water to hydrate, indicated by darkening, (E) almost completely converted hydrate.

As a further development of the above experiments, an apparatus was designed to investigate hydrate film/shell growth at a hydrocarbon/water interface. The initial intent of the study was to determine whether methane hydrate would exhibit a similar shell formation process as cyclopentane hydrate (a model sII hydrate which is stable at atmospheric pressure). The micromechanical experiments could be only performed at atmospheric pressure, while the interfacial film/growth apparatus could be used to study shell/film formation at pressures of up to 2000 psig.

The apparatus used to investigate interfacial hydrate film formation consisted of a brass cell (5.5 cm3) housing two sapphire windows (1.0 cm diameter, one on each side). The brass body of the cell was surrounded by a cooling jacket which was connected to a recirculating cooling bath (Figure 2). The cell was positioned on a vibration isolation table. A microscope (Microscope: Olympus SZ60, Objective: 100 AL 2X) was used to view the interface through the sapphire window. The cell was pressurized with gas from the top of the cell, and a pressure gauge and transducer were used to monitor the pressure inside the cell throughout the experiment. A T-type thermocouple on the outside of the brass cell was used to record the temperature. The pressure and temperature were monitored continually using a data acquisition system.

Figure 2: Schematic of the hydrate film growth apparatus

Multiple film growth experiments were performed for various hydrate guest molecules including cyclopentane and methane. The apparatus was designed to investigate the effect of temperature, pressure, diffusivity of hydrate guest, solubility of hydrate guest, and hydrate structure in effort to characterizing hydrate film growth and propose a mechanism based on data and observations.

A picture of the entire hydrate film was taken as shown in Figure 3. The hydrate film covered the entire vapor/water interface in the cell, which was 18 mm in diameter.

Figure 3: Picture of methane hydrate film encompassing entire cell.