Dissociation procedures

The most accurate measurement of the gas content evolving from dissociating hydrate are those made in situ, in which no quenching, depressurization, room-pressure handling, or sample re-installment onto the apparatus is required following synthesis. Either of two general procedures can be followed to take the hydrate samples from post-synthesis conditions of elevated methane pressure down to 0.1 MPa methane pressure prior to dissociation.

The first method, referred to as "temperature-ramping", involves slow cooling of the external bath surrounding the samples while depressurizing the samples, remaining close to but just within the hydrate stability field (Fig. 6A, points 1-2-3). When the hydrate has cooled below 194 K, the remaining gas is vented to 0.1 MPa and the sample can be opened to the gas collection apparatus. Slowly warming the sample (by warming the external bath) above the methane hydrate dissociation temperature (194 K at 0.1 MPa) will then destabilize the hydrate, and the evolved gas is collected in the flow meter (Fig. 5). Warming the sample through the melting point of ice (273.15 K) ensures dissociation of the hydrate as well as melting of the ices. Previously-quenched samples can also be dissociated by this method, by first cooling the external bath (to T < 194 K), and loading the quenched sample into a cold (<194 K) vessel prior to reattachment to the apparatus.

Figure 6. P-T paths for destabilizing methane hydrate for measurement of dissociation rates, stoichiometry, or stability behavior. Samples can be processed in two manners: (A) the "temperature ramping" method, or (B) the "rapid depressurization" method, each of which are effective for different types of dissociation tests. Procedures are described in the text.

Temperature, K Temperature, K

Figure 6. P-T paths for destabilizing methane hydrate for measurement of dissociation rates, stoichiometry, or stability behavior. Samples can be processed in two manners: (A) the "temperature ramping" method, or (B) the "rapid depressurization" method, each of which are effective for different types of dissociation tests. Procedures are described in the text.

The second dissociation method, termed "rapid depressurization", involves first depressurizing the sample to a smaller overstep of the dissociation curve (Fig. 6B, pts. 1-2 or 3), then rapidly venting the pressure from several MPa above the equilibrium curve down to 0.1 MPa. The vent is then quickly closed while simultaneously opening the valve to the gas collection apparatus. The rapid depressurization is performed over about a 6 to 15 second interval, depending on the magnitude of the initial pressure overstep of the equilibrium curve. This technique is used to measure dissociation rates at a constant external bath temperature, and is particularly effective for isothermal warm-temperature tests to explore the P-T region where hydrate is predicted to dissociate to liquid water + gas. For samples tested at temperatures above 195 K but below 273 K, it is necessary to warm the sample through 273 K after the isothermal portion of dissociation has finished, as in the temperature-ramping tests discussed above.

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