Methane Liquid structure

Mehta, A.P. Sloan, E.D. Jr. (1993). Structure H Hydrate Phase Equilibria of Methane + Liquid Hydrocarbon Mixtures. J. Chem. Eng. Data, 38, 580-582. 

As discussed in Sec. 4, the icomplex function of temperature, pressure, and equilibrium vapor- and hquid-phase compositions. However, for mixtures of compounds of similar molecular structure and size, the K value depends mainly on temperature and pressure. For example, several major graphical ilight-hydrocarbon systems. The easiest to use are the DePriester charts [Chem. Eng. Prog. Symp. Ser 7, 49, 1 (1953)], which cover 12 hydrocarbons (methane, ethylene, ethane, propylene, propane, isobutane, isobutylene, /i-butane, isopentane, /1-pentane, /i-hexane, and /i-heptane). These charts are a simplification of the Kellogg charts [Liquid-Vapor Equilibiia in Mixtures of Light Hydrocarbons, MWK Equilibnum Con.stants, Polyco Data, (1950)] and include additional experimental data. The Kellogg charts, and hence the DePriester charts, are based primarily on the Benedict-Webb-Rubin equation of state [Chem. Eng. Prog., 47,419 (1951) 47, 449 (1951)], which can represent both the liquid and the vapor phases and can predict K values quite accurately when the equation constants are available for the components in question. 

From a structural point of view the OPLS results for liquids have also shown to be in accord with available experimental data, including vibrational spectroscopy and diffraction data on, for Instance, formamide, dimethylformamide, methanol, ethanol, 1-propanol, 2-methyl-2-propanol, methane, ethane and neopentane. The hydrogen bonding in alcohols, thiols and amides is well represented by the OPLS potential functions. The average root-mean-square deviation from the X-ray structures of the crystals for four cyclic hexapeptides and a cyclic pentapeptide optimized with the OPLS/AMBER model, was only 0.17 A for the atomic positions and 3% for the unit cell volumes. 

In 1980 the BURRO test series was conducted. These were much larger, with spills up to 40 m at injection rates of 12-18 mVmin. In all cases, the LNG tank liquid had a composition of over 83 mole % methane. The first five tests were conducted with no indication of an RPT. In BURRO-6, motion pictures clearly showed several strong RPT events late in the spill and at a location near the outer edge of the ice which formed on the pond around the spill pipe. Three RPTs were particularly sharp and water and LNG were violently ejected into the air no instrumentation was operating to indicate oveipressures either in the water or air, and no structural damage was done to the test facility. 

This structural change is suppressed by the addition of tetrahydrothiophene (THT)19b. It prevents the formation of polymethylene zinc, i.e. (—CH2Zn—) . Without THT, a solution of 3 in THF yields polymethylene zinc at 60 °C. Monomeric bis(iodozincio)methane (3) is much more active for methylenation as compared to polymethylene zinc. As shown in Table 3 (entry 3), the addition of THT to the reaction mixture at 60 °C improved the yield of the alkene dramatically. Practically, however, its stinking property makes the experimental procedure in large scale uncomfortable. Fortunately, an ionic Uquid, l-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), plays the same role. Ionic liquid also stabilizes the monomeric structure of 3 even at 60 °C and maintains it during the reaction at the same temperature. The method can be applied to various ketones as shown in Scheme 14.4... 

Although polymorphism in plastic crystals is less frequent than in liquid crystals, it does exist. Tetrakis(methylmercapto)methane, C(SCH3)4, for example, has four crystal modifications of which the three high temperature forms have a high degree of plasticity 100). Also, it has been observed that plastic crystals are frequently mutually soluble 16b), a consequence of the less restrictive crystal structures. Phase separation of these solutions occurs often on transition to the fully ordered crystal, giving rise to quite complicated phase diagrams102). 

The thermodynamic and structural processes that occur when water molecules are in the vicinity of hydrophobic entities (water fearing, insoluble in water) are referred to collectively as hydrophobic hydration (Tanford, 1973 Privalov and Gill, 1988 Blokzijl andEngberts, 1993 Chau and Mancera, 1999). Hydrophobic hydration is important in gas hydrate formation, which usually starts with hydrophobic gas molecules (e.g., methane) being introduced into liquid water. 

In the Frank and Evans iceberg model, ice-like structures form around hydrophobic entities, such as methane. In this model, the hydrophobic molecules enhance the local water structure (greater tetrahedral order) compared with pure water. Ordering of the water hydration shell around hydrophobic molecules has been attributed to clathrate-like behavior, in which the water hydration shell is dominated by pentagons compared to bulk liquid water (Franks and Reid, 1973). 

Stoll and Bryan (1979) first measured the thermal conductivity of propane hydrates (0.393 Wm-1K-1 at T = 215.15 K) to be a factor of 5 less than that of ice (2.23 Wm-1K-1). The low thermal conductivity of hydrates, as well as similarities of the values for each structure (shown in Table 2.8) have been confirmed from numerous studies (Cook and Leaist, 1983 [0.45 Wm-1K-1 for methane hydrate at 216.2 K] Cook andLaubitz, 1981 Ross et al., 1981 Ross and Andersson, 1982 Asher et al., 1986 Huang and Fan, 2004 Waite et al., 2005). The thermal conductivity of the solid hydrate (0.50-0.58 W m-1 K-1) more closely resembles that of liquid water (0.605 W m-1 K-1). 

Englezos et al. (1987a,b) generated a kinetic model for methane, ethane, and their mixtures to match hydrate growth data at times less than 200 min in a high pressure stirred reactor. Englezos assumed that hydrate formation is composed of three steps (1) transport of gas from the vapor phase to the liquid bulk, (2) diffusion of gas from the liquid bulk through the boundary layer (laminar diffusion layer) around hydrate particles, and (3) an adsorption reaction whereby gas molecules are incorporated into the structured water framework at the hydrate interface. 

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