F. V. Zhurko

Clathrate formation in water-noble gas (Hydrogen) systems at high pressures

Phase equilibria in helium-water, neon-water, and hxdrogen-water svstems were studied at pressures up to 15 kbar. The results are compared with the data for the previously investigated water systems with argon, crypton, and xenon. It is concluded that classical polyhedral clathrate hydrates are formed in all the systems, the stability of the hydrates diminishing from xenon to neon. In all the systems, except the xenon system, the hydrates are based on the crystalline framework of ice II. Their formation demands high pressures; the larger the guest molecule, the higher the pressure required. The xenon molecule seems to be too large to fit the cage of the ice II framework; therefore, the xenon hydrate CS-I remains stable up to at least 15 kbar.

Cubic structure II double clathrate hydrates with tetra(n-propyl)ammonium fluoride

A double clathrate hydrate with the composition THF·0.5(n-Pr)4NF·16H2O and cubic structure II (CS-II,a=17.67 Å) has been obtained. Its experimental density is 1.053±0.001 g/cm3; its melting point is 8.1°C, i.e. 3.1°C higher than that of the THF·17H2O hydrate. The double hydrates of acetone, 1,4-dioxan, trimethyleneoxide and 1,3-dioxolane with (n-Pr)4NF have melting points of −14.8, −5.5, −2.6 and −9.6°C, respectively. With pressure increase up to 6 kbar the melting points of the double hydrates increase monotonously in contrast to common CS-II hydrates. The friability of the structure of the hydrates (the packing coefficient) and their sensitivity to pressure (dT/dP) are compared.

Clathrate Hydrates of Sulfur Hexafluoride at High Pressures

The pressure dependence (0.4 Mpa–1.3 GPa) of the hydrate decomposition temperatures in the sulfur hexafluoride-water system has been studied. In addition to the known low-pressure hydrate SF6⋯17H2O of Cubic Structure II, two new high-pressure hydrates have been found. X-ray analysis in situ showed the gas hydrate forming in the sulfur hexafluoride-water system above 50 MPa at room temperature to be of Cubic Structure I. The ability of water to form hydrates whose structures depend on the guest molecule size under normal conditions and at high pressures is discussed.

CS-II binary clathrate hydrates at pressures of up to 15 kbar

The pressure dependence of the decomposition temperatures of binary clathrate hydrates of tetra-hydrofuran with xenon and methane as well as of chloroform and carbon tetrachloride clathrates with xenon has been studied. The absence of phase transitions at pressures from 1 to 15,000 bar indicates that the structure of all the hydrates remains constant (CS-II). The decomposition temperatures of the binary hydrates of tetrahydrofuran and carbon tetrachloride with xenon at 15 kbar (above 124ℴC) are exceedingly high for polyhedral clathrate hydrates because the guest molecules are highly complementary to the cavities of the clathrate lattice. The paper also considers the packing density effect in the crystal structure of hydrates on the behavior of the latter at elevated pressure.

Clathrate formation in water-cyclic ether systems at high pressures

Experimental data on the investigation of the water-trimethyleneoxide system,P, t, x phase diagram (up to 6 kbar) are presented. The results are compared with those on water systems with ethyleneoxide, 1,3- and 1,4-dioxane, 1,3-dioxolane and tetrahydrofuran, on the basis of which a summarizedP, t, x diagram is plotted for water-cyclic ether systems. It is shown that in all the systems in which a cubic structure II hydrate forms at 1 bar, it eventually turns to cubic structure I under pressure. The nature of high pressure hydrates is discussed.

Gas‐Hydrate Packing and Stability at High Pressures

The decomposition temperatures of double gas hydrates of tetrahydrofuran with noble gases from krypton to helium at pressures up to 15 kbar were found by differential thermal analysis. The stability of hydrates was shown to rise as their packing coefficient increases. Krypton and argon hydrates retain the original cubic structure II in the whole pressure range. In neon and helium systems, polyhedral double hydrates have upper stability limits at 7.4 and 6.0 kbar, respectively.

Clathrate formation in binary aqueous systems with CH2Cl2, CHCl3 and CCl4 at high pressures

P,T,X phase diagrams of the CH2Cl2-H2O, the CHCl3-H2O and the CCl4-H2) systems have been studied by DTA in the pressure range 10−3 to 5.0 kbar. Under pressure the cubic structure II (CS-II) hydrates forming in all the systems are replaced by hydrates with the composition M·7.3 H2O whose stoichiometry and positive dT/dP values of melting lead us to believe that they are CS-I hydrates.In the CH2Cl2 and CHCl3 systems the nonvariant point coordinates of the hydrate transformationQ2h (l1h17h7l2, where l1 and l2 are liquid phases abundant in water and hydrate former, respectively, h17 and h7 are hydrates with hydrate numbers 17 and 7, respectively) areP = 0.6 kbar, T = −1.5°C andP =2.65 kbar,T = −10.5°C, respectively. In the CCl4 system the 4-phaseQ3h point (l1h17h7s, where ‘s’ is crystalline CCl4) has coordinatesP = 0.75 kbar and T = 0.4°C.The main obstacle of the present study, the very slow achievement of equilibrium, has been eliminated by adding small amounts (0.25% by mass) of surfactants followed by ultrasonic mixing. We have shown that this accelerates the achievement of equilibrium without changing its position.

Phase Diagrams of the Ternary Gas Hydrate Forming Systems at High Pressures. Part 1. Propane–Methane–Water System

Abstract Cross sections of the ternary system propane–methane–water at pressure up to 15 kbar have been investigated by means of differential thermal analysis. It is stated that a double gas hydrate of the cubic structure II is formed in the system. It is stable within the whole pressure range investigated. It is most likely that at pressures above 3.4 kbar one more double hydrate exists in the system. The possibility of the formation of solid solutions of methane in propane hydrates and of propane in methane hydrates existing under different P–T conditions is discussed on the basis of P–T curves corresponding to monovariant equilibria in the system.

Clathrate formation in water-tetraalkyl ammonium iodide systems at high pressure

The phase diagrams of aqueous binary systems with PrBu3NI(I), Bu4NI(II), i-AmBu3NI(III) and i-Am4NI(IV) were studied at atmospheric and high pressures by DTA method. In systems III-H2O and IV-H2O at atmospheric pressure we observed polyhydrates melting incongruently at 7.1 and 14.7, correspondingly. In systems I-H2O and II-H2O hydrates form at higher pressure only, there are PrBu3NI (15–25) H2O at P≧0.13 kbar, Bu4NI (25–35) H2O at P≧0.4 kbar. In water systems with II–IV at pressure 1.2, 1.4 and 0.26 kbar correspondingly polyhydrates with smaller hydrate number form. Formation of hydrates in solution in the II-H2O system does not occur at pressure greater than 7 kbar. The summarized P, T, X-phase diagram is discussed.

Double Gas Hydrates at High Pressures: The Highest Decomposition Temperatures

Abstract: CS‐II double clathrate hydrates of tetrahydrofuran, CHCl3, CCl4, SF6, and xenon as a “help gas” keep their structures in the pressure interval 1215,000 bar. Decomposition temperatures of these hydrates rise monotonically in the pressure interval under consideration. The decomposition temperature of the double clathrate hydrate of SF 6 and xenon reaches 129.48C at 14.8 kbar.