Energy science of clathrate hydrates: Simulation-based advances
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Clathrate hydrates simulations Oil and water do not mix. This we have learned from an early age and have been recently reminded of in the Gulf of Mexico oil spill in 2010. Oil and other non-polar compounds are thus said to be “hydrophobic.” Similarly, small hydrophobic, gaseous molecules, such as methane, have low solubility in water (typically on the order of one part in a thousand). It can strike us as counter-intuitive, then, that at low temperatures and/or high pressures, water and gas crystallize together into an “icy” solid with high gas composition (water to gas ratio of about 6:1). The explanation lies in the fact that hydrophobicity is not merely the tendency of dissimilar substances to demix, but also involves the tendency of water to form hydrogen-bond networks on a molecular scale. When the hydrophobic molecule is of the same size as the cells of the hydrogen-bond network, they can become trapped or “enclathrated” into a regular lattice of hydrogen-bond cages, forming a crystalline structure known as clathrate hydrate. (See the Rath article in the MRS Bulletin April 2008 issue.) These clathrate hydrates are often incorrectly termed “methane ice” or “icy crystals” as in, for example, popular media coverage of the Gulf of Mexico oil spill. In fact, the formation of gas hydrates has long been the foremost problem to the oil
and gas industry because of their tendency to plug pipelines. Large capital and financial resources are usually in place as remediation strategies to prevent, thermodynamically, or at least control kinetically the formation of hydrates in pipelines, such as the injection of inhibitors, insulation, and/or heating (to shift or avoid the phase boundary). Beyond their role in pipeline plugs, clathrate hydrates have been brought into focus in several areas of energy science. Broadly speaking, one can identify one category of areas as being concerned when the energy is contained in the guest molecules, which are thus a fuel. This is the case for the vast reserves of naturally occurring methane hydrate in the seafloor or permafrost (amount of methane in these hydrate deposits are on the order of 700,000 TCF (trillion cubic feet);1 as a base, the U.S. annual natural gas consumption is ~23 TCF1), where the methane is extracted and possibly replaced by its greenhouse gas byproduct, carbon dioxide.2 It is also the case for exploratory strategies envisioning molecular hydrogen as a fuel, where the clathrate water lattice acts as a storage medium to reversibly capture, concentrate, store, and release hydrogen fuel.3,4 More concrete is the storage and transportation of natural gas in clathrate hydrates, which is moving from laboratory to pilot scale to full industrial reality.5 Another major category of areas focuses
Amadeu K. Sum, Colorado School of Mines, USA; [email protected] David T. Wu, Colorado School of Mines, USA; [email protected] Kenji Yasuoka, [email protected] DOI: 10.1557/mrs.2011.33
© 2011 Materials Research Society
MRS BULLETIN • VOLUME 36 • MARCH 2011 • www.mrs.org/bulletin
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