Propane hydrates are rarely considered by scientists. Despite the narrow borders of the formation region, they can form during storage and transportation of the liquefied petroleum gases. Therefore, it is important to know the induction time for solid hydrate formation. The present paper has considered the formation both from melting ice (at 1 °C and 4 bar) and from water (at 2 °C and 3.6-4.8 bar) in a stirred vessel. The formation from ice was quite instantaneous, while the production from water took about 15 h to start (and about 3 days to be completed) and depended on the pressure: in fact, it was slower at low pressure. All hydrates contained a high amount of ice (75-80%). The modulated differential scanning calorimetry was used for hydrate characterization: the reversing (heat-capacity) curves permitted one to quickly distinguish between hydrate and ice, also allowing a semiquantitative evaluation of the hydrate content.
Modulated DSC has been applied to the study of methane, ethane and propane hydrates at different hydrate and ice concentrations. The reversing component of the TMDSC curves, makes it possible to characterize such hydrates. Methane and ethane hydrates show the melting-decomposition peak at a temperatures higher than the ice contained in the sample, while propane hydrate melts and decomposes at lower temperature than the ice present in the sample. The hydrate peaks tend to disappear if the hydrate is stored at atmospheric pressure. Guest size and cavity occupation fix the heat of dissociation and stability of the hydrates, as confirmed by parallel tests on tetrahydrofurane hydrates
For CO2 disposal in the form of hydrate it is important to know the decomposition kinetics at moderate pressures and temperatures, similar to those that could be realized in the storage systems. This paper has considered the preservation of CO2 hydrate containing different quantities of CO2, at pressures between 0.1 and 0.3 MPa and temperatures between −3 and 0 °C. At the conditions (P, T) of this work, CO2 hydrate does not present any anomalous self-preservation effect, and its dissociation is not affected by subcooling before storage. More than pressure, which is very important for methane hydrate, temperature affects the preservation. The temperature of −3 °C assures a good stability at atmospheric pressure, providing that CO2 saturation into the hydrate is not too high. CO2 hydrate is generally more stable than CH4 hydrate due to the different activation energy of decomposition.
A number of papers and research projects suggest that stranded natural gas can be transported in a solid hydrate state at higher temperatures or lower pressures compared to conventional transportation systems (LNG and CNG). The self-preservation effect of methane hydrate can probably be improved by the use of a third component besides CH4 and water. Tetrahydrofuran (THF) is a promoter that greatly reduces the required formation pressures. In the present work the influence of THF on the decomposition kinetics of mixed THF-CH4 hydrates was studied to evaluate the THF stabilization effect. The experimental work, carried out with the help of a reaction calorimeter, has revealed that the dissociation rate of mixed THF hydrates is lower (on average by one order of magnitude) than that of simple methane hydrates. Mixed hydrates can also be stored for short periods at temperatures over 0ıC. However, the best preservation conditions (among the experimented ones) are realized at 1ıC and 3 MPa. (about 66 days required for complete dissociation)
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