Abstract
Natural gas hydrates represent a valid opportunity in terms of energy supplying, carbon dioxide permanent storage and climate change contrast. Research is more and more involved in performing CO2 replacement competitive strategies. In this context, the inhibitor effect of sodium chloride on hydrate formation and stability needs to be investigated in depth. The present work analyses how NaCl intervenes on CO2 hydrate formation, comparing results with the same typology of tests carried out with methane, in order to highlight the influence that salt produced on hydrate equilibrium conditions and possibilities which arise from here for improving the replacement process efficiency. Sodium chloride influence was then tested on five CO2/CH4 replacement tests, carried out via depressurization. In relation with the same typology of tests, realised in pure demineralised water and available elsewhere in literature, three main differences were found. Before the replacement phase, CH4 hydrate formation was particularly contained; moles of methane involved were in the range 0.059–0.103 mol. On the contrary, carbon dioxide moles entrapped into water cages were 0.085–0.206 mol or a significantly higher quantity. That may be justified by the greater presence of space and free water due to the lower CH4 hydrate formation, which led to a more massive new hydrate structure formation. Moreover, only a small part of methane moles remained entrapped into hydrates after the replacement phase (in the range of 0.023–0.042 mol), proving that, in presence of sodium chloride, CO2/CH4 exchange interested the greater part of hydrates. Thus, the possibility to conclude that sodium chloride presence during the CO2 replacement process provided positive and encouraging results in terms of methane recovery, carbon dioxide permanent storage and, consequently, replacement process efficiency.
Highlights
Gas hydrates are ice-like crystalline solid compounds, where, in presence of suitable conditions of relatively low temperature and high pressure, small gaseous molecules, called “guests”, are trapped inside a hydrogen bonded network of water molecules, called “hosts” (Sloan and Koh 2008; Sloan Jr. 2003)
We investigated the effect produced by sodium chloride presence on carbon dioxide hydrate formation, in order to propose a new further possibility: the combination of CO2/CH4 replacement process with the use of chemical inhibitors
The capability of carbon dioxide to dissolve in water cannot be neglected; for this reason, the quantity of carbon dioxide involved into hydrates was calculated considering both the total amount of gas injected inside the reactor and the quantity dissolved in water, which was evaluated using the Henry law
Summary
Gas hydrates are ice-like crystalline solid compounds, where, in presence of suitable conditions of relatively low temperature and high pressure, small gaseous molecules, called “guests”, are trapped inside a hydrogen bonded network of water molecules, called “hosts” (Sloan and Koh 2008; Sloan Jr. 2003). Duranti 67, 06125 Perugia, PG, Italy water cage structures typical of hydrates and normal ice, is the formation of polyhedral structures, which hydrates guarantee enough space for containing non-polar guest molecules (Moridis et al 2009). Hydrate structures may involve lots of different “host” molecules; among them, the most investigated and researched are hydrocarbons, i.e. methane (CH4), ethane (C2H6) and propane (C3H8) (Sloan Jr. 2003). Natural gas hydrates (NGH) are diffused in deep oceans and in permafrost regions, or elsewhere, thermodynamic conditions are suitable and natural gas is present in presence of water (Mori 2003)
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