Clathrate hydrate
Most low molecular weight gases, including O2, H2, N2, CO2, CH4, H2S, Ar, Kr, Xe, and Cl2 as well as some higher hydrocarbons and freons, will form hydrates at suitable temperatures and pressures.
Clathrate hydrates are not officially chemical compounds, as the enclathrated guest molecules are never bonded to the lattice.
[3][4][5] Clathrate hydrates were first documented in 1810 by Sir Humphry Davy who found that water was a primary component of what was earlier thought to be solidified chlorine.
Clathrates can also exist as permafrost, as at the Mallik gas hydrate site in the Mackenzie Delta of northwestern Canadian Arctic.
Hydrocarbon clathrates cause problems for the petroleum industry, because they can form inside gas pipelines, often resulting in obstructions.
Deep sea deposition of carbon dioxide clathrate has been proposed as a method to remove this greenhouse gas from the atmosphere and control climate change.
Clathrates are suspected to occur in large quantities on some outer planets, moons and trans-Neptunian objects, binding gas at fairly high temperatures.
[15] Clathrates were studied by P. Pfeiffer in 1927 and in 1930, E. Hertel defined "molecular compounds" as substances decomposed into individual components following the mass action law in solution or gas state.
Small molecules or gases (i.e. methane, carbon dioxide, hydrogen) can be encaged as a guest in hydrates.
Like ice, clathrate hydrates are stable at low temperatures and high pressure and possess similar properties like electrical resistivity.
The fast decomposition of such deposits is considered a geohazard, due to its potential to trigger landslides, earthquakes and tsunamis.
[25] In 2017, both Japan and China announced that attempts at large-scale resource extraction of methane hydrates from under the seafloor were successful.
This is highly undesirable, because the clathrate crystals might agglomerate and plug the line[29] and cause flow assurance failure and damage valves and instrumentation.
Triethylene glycol (TEG) has too low vapour pressure to be suited as an inhibitor injected into a gas stream.
Empty clathrate hydrates[30] are thermodynamically unstable (guest molecules are of paramount importance to stabilize these structures) with respect to ice, and as such their study using experimental techniques is greatly limited to very specific formation conditions; however, their mechanical stability renders theoretical and computer simulation methods the ideal choice to address their thermodynamic properties.
Starting from very cold samples (110–145 K), Falenty et al.[31] degassed Ne–sII clathrates for several hours using vacuum pumping to obtain a so-called ice XVI, while employing neutron diffraction to observe that (i) the empty sII hydrate structure decomposes at T ≥ 145 K and, furthermore, (ii) the empty hydrate shows a negative thermal expansion at T < 55 K, and it is mechanically more stable and has a larger lattice constant at low temperatures than the Ne-filled analogue.
[32] From a theoretical perspective, empty hydrates can be probed using Molecular Dynamics or Monte Carlo techniques.
Conde et al. used empty hydrates and a fully atomic description of the solid lattice to estimate the phase diagram of H2O at negative pressures and T ≤ 300 K,[33] and obtain the differences in chemical potentials between ice Ih and the empty hydrates, central to the van der Waals−Platteeuw theory.
Jacobson et al. performed[34] simulations using a monoatomic (coarse-grained) model developed for H2O that is capable of capturing the tetrahedral symmetry of hydrates.
Their calculations revealed that, under 1 atm pressure, sI and sII empty hydrates are metastable regarding the ice phases up to their melting temperatures, T = 245 ± 2 K and T = 252 ± 2 K, respectively.
Response to the applied (p, T) field was analyzed in terms of angle and distance descriptors of a classical tetrahedral structure and observed to occur essentially by means of angular alteration for (p, T) > (200 MPa, 200 K).
The length of the hydrogen bonds responsible for framework integrity was insensitive to the thermodynamic conditions and its average value is r(̅O H) = 0.25 nm.
[37] Researchers believed that oceans and permafrost have immense potential to capture anthropogenic CO2 in the form CO2 hydrates.
The utilization of additives to shift the CO2 hydrate equilibrium curve in phase diagram towards higher temperature and lower pressures is still under scrutiny to make extensive large-scale storage of CO2 viable in shallower subsea depths.
