Non-stoichiometric clathrate compounds of water. Part 2.—Formation and properties of double hydrates

Abstract

An investigation has been made of type I1 double hydrates, involving organic species such as chloroform with Hz, Ne, N2, Ar, 0 2 , C&, Kr, Xe, C2H4, CzH6 and C02 as "help" gases. Among these, the heavier rare gases and methane showed considerable selectivity i n their affinity for the small cavities of the host lattice. All phases were non-stoichiometric in composition : the average fraction of the larger cavities of the host lattice occupied by the main hydrate formers CHC13 and CH31 were respectively 0.95 and 096 ; these fractions for the help gases in the smaller cavities were considerably smaller, ranging from 0.0046 (H2) to 0.64 (Xe) at 700 mm pressure and about 0°C. The amounts of help gases included in the water host lattice, in the molecular sieve zeolite 13X, and dissolved in liquid water, have been compared. The fractionation through hydrate f o r m a t h of mixtures of CHC13 with CH3I, C ~H S C H ~, C6H6, CCk and CH2C12, and of the " help " gas mhtures 02+N2, Ar+K.r and Kr+Xe has been measured and compared with theoretical estimates. Theoretical aspects of the stabilizing influence of help gases have been discussed in relation to the experimental results.

Mass fractionation of noble gases in synthetic methane hydrate: Implications for naturally occurring gas hydrate dissociation

“…These data confirm previous studies by Barrer and Ruzicka (1962), Barrer and Edge (1967) and Chersky and Tsarev (1999) demonstrating encapsulation of N 2 , O 2 , Ar, Kr and Xe and exclusion of He in gas hydrates. In contrast to Barrer and Ruzicka (1962) and Barrer and Edge (1967) A rough mass balance calculation from the initial gas pressure of 20 MPa (pressure at the time of sampling the initial headspace gas) down to a final pressure 17 MPa (post hydrate formation), at uniform temperature (254°K) and variable volume (0.9 to 0.759 to account for volume decrease with hydrate formation) yields a total loss of 2.4 mol of gas from the synthesis chamber to 120 g of water in the hydrate structure. In terms of mass balance of the gases with observed decreases (N 2 , O 2 , 20 Ne, 36,40 Ar, 84 Kr and 132 Xe) in mole fraction from initial to final headspace composition, all indicate loss (molar decrease) in the headspace; however, despite the noted increase in mole fraction of CH 4 , there is still a net loss of CH 4 from the headspace.…”

“…3). According to Barrer and Ruzicka (1962) and Barrer and Edge (1967), Ar, Kr, and Xe components are released from lattice sites of the methane hydrate, but He and Ne have small enough effective ionic radii that they should be absent. In our experiment, however, the concentrations of He and Ne are low, but not absent (Table 1), and the mass balance (Section 4.1) indicates that 20 Ne should be partially retained while 4 He should be low to absent.…”

“…Thus, the best hope for distinguishing methane derived from recently dissociated gas hydrate from other populations of methane is development of a technique that can exploit unique characteristics of the methane recently released from gas hydrates. Noble gases preferentially partition by molecular weight (Barrer and Ruzicka, 1962;Barrer and Edge, 1967) in the gas hydrate lattice, but do not have such a predictable relationship in other gas populations. Noble gas analyses might therefore be used to "fingerprint" methane emissions.…”

“…Selective enclathration of noble gases in synthetic gas hydrates has been recognized since the work of Barrer and Ruzicka (1962) and Barrer and Edge (1967). Their studies demonstrated that xenon (Xe) and krypton (Kr) were enriched relative to argon (Ar) in hydratederived gases and indicated that helium (He) and neon (Ne) could be removed relative to Ar at low temperature (Barrer and Edge, 1967).…”

Peculiarities of Methane Clathrate Hydrate Formation and Solid-State Deformation, Including Possible Superheating of Water Ice

“…Clathrate data points with arrows indicate faulting behavior. (6). Stressstrain curves of deformed methane clathrate (run 368) showing systematic strain hardening, compared with the sharp yielding, strain softening, and steady-state flow of "standard" polyctystalline H, O ice.…”