High pressure hydrate phase

Less common clathrate hydrates formed by compounds other than natural gas guests (such as Jeffrey s structures III-VII, structure T, complex layer structures) and high pressure hydrate phases are also briefly described to provide a comprehensive account of clathrate hydrate structural properties. 

Of more than 130 compounds that are known to form clathrate hydrates with water molecules, the majority form either si, sll, or sH, with exceptions such as (1) bromine (Allen and Jeffrey, 1963 Dyadin et al., 1991), (2) dimethyl ether (Gough et al., 1974, 1975 Udachin et al., 2001a), (3) ethanol (Brownstein et al., 1967 Calvert and Srivastava, 1967), and (4) very high pressure hydrate phases (Dyadin et al., 1997 Loveday et al., 2001b, 2003b Kursonov et al., 2004). Detailed emphasis is given to si, sll, and sH hydrates since these are by far the most common natural gas hydrate structures. 

At very high pressures (0.3-2.1 GPa), gas hydrates undergo structural transitions to other hydrate phases and filled ice phases. Guests can multiply occupy the large cages of these high-pressure hydrate phases. 

In peridotites high-pressure hydrated magnesian silicate phases, named imaginatively after the letters of the alphabet, are capable of transporting water within a subducting slab deep into the lower mantle. Notable are the superhydrous phase B and phase D. This latter phase is stable to pressures of 50 GPa and can transport water deep into the lower mantle. If water penetrates even deeper than this it could be partitioned into the core as an iron hydride through reaction with native Fe metal in the lowermost mantle. 

C10H14O, Mr 150.22, pioi.3kPa 232.5 °C, df 0.9756, n53° 1.5227, is the main constituent of thyme and some origanum oils it also occurs in many other essential oils. It forms colorless crystals (mp 51.5 °C) with a spicy-herbal, slightly medicinal odor reminiscent of thyme. Thymol is prepared on a technical scale in a continuous high-temperature, high-pressure, liquid-phase, ort/io-alkylation process, from m-cresol and propylene, in the presence of activated aluminum oxide hydrate [163a],... 

In the liquid-phase process, high pressures in the range of 80-100 atmospheres are used. A sulfonated polystyrene cation exchange resin is the catalyst commonly used at about 150°C. An isopropanol yield of 93.5% can be realized at 75% propylene conversion. The only important byproduct is diisopropyl ether (about 5%). Figure 8-4 is a flow diagram of the propylene hydration process. ... 

FIG. 4 Phase diagram of fully hydrated DPPC bilayers. Different phases found are also schematically shown Lp, gel P, rippled gel L I, interdigitated gel and L , liquid crystalline phases. (From Ref. 50. Copyright 1999 The Japan Society of High Pressure Science and Technology.)... 

D. Gas-solid hydrate may occur in high pressure gas pipelines and processing facilities. They occur at temperatures below 285 K (55 F) and phase boundary pressures near 4100 kPa (40 Atm). 

I PA could always be made by direct hydration, but the severe operating conditions (high pressures and temperatures) and puny yields had always limited the economic enthusiasm for the process. Then catalysis research paid off with the development of a sulfonated polystyrene cationic exchange resin catalyst, a mouthful in itself. The breakthrough permitted reduced pressures and temperatures without loss of yield. The catalyst works in the vapor phase, the liquid phase, and the mixed phase. 

Kumazaki, T. Kito, Y. Sasaki, S. Kume, T. Shimizu, H. (2004). Single-crystal growth of the high-pressure phase II of methane hydrate and its Raman scattering study. Chem. Phys. Letter, 388 (1-3), 18-22... 

The oxo reaction (31) is carried out in the liquid phase at high pressure using a cobalt catalyst. A mixture of aldehyde isomers is always produced, each isomer being one carbon number higher than the starting olefin. As a group the oxygenated products of the hydrocarbon synthesis (Fischer-Tropsch) process and the oxo process are primary compounds and thus (except, of course, the methyl and ethyl derivatives) differ fundamentally from the products based on alcohols made by the hydration of olefins, which are always secondary or tertiary in structure. 

Originally, the hydration of olefins to alcohols was carried out with dilute aqueous sulphuric acid as the catalyst. Recently, the direct vapour phase hydration of olefins with solid catalysts has become the predominant method of operation. From the thermodynamic point of view, the formation of alcohols by the exothermic reaction (A) is favoured by low temperatures though even at room temperature the equilibrium is still in favour of dehydration. To induce a rapid reaction, the solid catalysts require an elevated temperatue, which shifts the equilibrium so far in favour of the olefin that the maximum attainable conversion may be very low. High pressures are therefore necessary to bring the conversion to an economic level (Fig. 11). To select an optimum combination of reaction conditions with respect to both rate limitation and equilibrium limitation,... 

Dyadin et al. discover a very high pressure phase of methane hydrate that is stable up to 600 MPa... 

While si, sll, and sH are the most common clathrate hydrates, a few other clathrate hydrate phases have been identified. These other clathrate hydrates include new phases found at very high pressure conditions (i.e., at pressures of around 1 GPa and higher at ambient temperature conditions). Dyadin et al. (1997) first reported the existence of a new methane hydrate phase at very high pressures (500 MPa). This discovery was followed by a proliferation in molecular-level studies to identify the structure of the high pressure phases of methane hydrate (Chou et al., 2000 Hirai et al., 2001 Kurnosov et al., 2001 Loveday et al., 2001, 2003). 

Trimethylene oxide (Hawkins and Davidson, 1966), cyclopropane (Hafemann and Miller, 1969 Majid et al., 1969), and ethylene sulfide (Ripmeester, Personal Communication, May 2,1988) are three molecules that can form in either the 51262 of structure I or the 51264 of structure II as simple hydrates. Raman spectroscopy measurements suggest that a low fraction of 512 cages may also be occupied by cyclopropane at high pressures (Suzuki et al., 2001). Such compounds change structures depending on the temperature and pressure of formation, and guest composition in the aqueous phase as discussed in Section 2.1.3. 

At very high pressures (in the GPa range), gas hydrates can undergo structural transitions to hydrate phases and filled ice structures. Figure 2.11 illustrates the structural changes that have been reported for gas hydrates at very high pressures at... 

Figure 2.11 Very high pressure (0.3-2.1 GPa) structural changes of gas hydrates at room temperature. Numerical values (adjacent to square boxes) indicate transition pressures. Hexagonal (sH ) and tetragonal (sT ) hydrate phases are distinct from sH and sT hydrate structures found at normal pressures. (Modified and redrawn from Hirai, H., Tanaka, H., Kawamura, K., Yamamoto, Y., Yagi, T., J. Phys. Chem. Solids, 65, 1555 (2004). With permission from Elsevier.)...

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. 

High-pressure differential scanning calorimetry (Handa, 1986d Le Parlouer et al., 2004 Palermo et al., 2005) Yes P, T Yes Hydrate phase vs. time Typically up to 5800 psi, 230 to 400 K 7 isS, heat capacities, heat of dissociation. Emulsion stability and hydrate agglomeration... 

Raman spectroscopy with high pressure windowed cell (Sum et al., 1997 Thieu et al., 2000) P, T and hydrate phase Yes P, P, hydrate phase vs. time (mins) Typically for sapphire window < 10,000 psi (for capillary tubes <60,000 psi diamond anvil cell GPa s) Guest occupancy ratios, structure, structural transitions... 

More recently, Tohidi and coworkers (Burgass et al., 2002 Mohammadi et al., 2003) have applied a novel method for measuring gas hydrate phase equilibria (Lw-H-V), which is based on a Quartz Crystal Microbalance (QCM). Figure 6.3 shows a schematic of the QCM set up and the QCM placed in a high pressure cell. The QCM consists of a thin disk of quartz sandwiched between two electrodes. The crystal will oscillate at a particular resonant frequency when an electric current is passed across the electrodes. This frequency is a function of the properties of the crystal. Any mass (from hydrate formation) attached to the surface of the crystal disk will cause a change in the resonant frequency, and hence be detected. The pressure and temperature of the system is measured using conventional methods, namely, a pressure transducer and a thermocouple in the high pressure cell. 

Hydrate Methane at High Pressure Reference Nakano et al. (1999) Phases Lw-H-V... 

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