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High-pressure optical cell for studies on gas hydrate growth kinetics

On basis of an existing high-pressure cell a novel optical cell was developed and fabricated to determine the growth rate of gas hydrates in defined spaces referring to the condition in sediments. The cell is universally usable for studies on growth processes of gas hydrates and minerals under high pressure.

The narrow glass capillary with an inner diameter of 1.7mm is installed in a slotted brass container generating a defined constant temperature gradient along the capillary axis by heating at the top and cooling at the bottom. The capillary can be rotated on its vertical axis accurately by means of a play free gear at the top of the housing. Similarly, the microscope equipped with a photo camera is positioned vertically by means of a thread. A double layered window is moved mutually with this microscope.

The growth rate in the optical cell is controlled by diffusion along a temperature gradient to generate precisely adjusted steady state conditions suitable for measurements at slow linear rates in the range of 5 x 10-5 ¿ 5 x 10-4 m x s-1. Temperatures are measured by thermocouples at the circumference of capillary at three locations. The thermocouples are led through the acryl glass shield, which suppresses detrimental air convection at the glass capillary. The temperature deviation from linearity amounts to < 0.1K. The cell is installed in an insulated air bath, which atmosphere is kept dry by an influx of nitrogen. (Technical drawings are available.) In order to calculate the true distances from the microscopic photographs, the determined lengths are corrected numerically for distortion by the cylindrical optics of the glass capillary.

A gas water interface can be placed at a predetermined level by withdrawing liquid from the inner of the capillary using an installed needle. Fast formation of gas hydrate can be initiated at the gas liquid interface or at an additional vl- interface by placing a methane bubble e.g. at the bottom of the glass capillary.

The nucleation probability under ls-condition is low in comparison with the probability on the gas liquid interface. After long periods (> one month), gas hydrate crystals can be formed, adhering at the glass wall, without any contact to a gas liquid interface. The kinetic studies reveal, that structural changes from SII to SI were observed applying a gas with a composition of almost 0.5 % ethane and 99.5 % methane.

At low temperature <280K gas hydrates of the structuire SII are formed. The thermodynamically stable 100-crystal faces were rather exceptional, and predominantly 111-faces were found. Only after long residence time (>600h) hexaeder were observed, that means, 100-faces became evident at vanishing overall growth rate.

In a special case SII gas hydrate was transferred from a cold zone (276 K) to a hot zone (284 K) by buoyancy to a place 4.5mm below the hydrate free gas water interface. After this transfer a nearby competing SI crystal grew, whereas the SII crystal decomposed slowly. The horizontal growth rate determined under ls-condition at 9.49MPa and 283.0K, amounted to (1.01 +/-0.03) x 10-10 m x s-1, the vertical rate to (1.96 +/-0.08) x 10-10 m x s-1.

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