Ethane gas hydrates

Englezos, P., Kalogerakis, N., Dholababhai, P.D. and Bishnoi, P.R., 1987a. Kinetics of fonuation of methane and ethane gas hydrates. Chemical Engineering Science, 42(11), 2647-2658. 

Subramanian, S. Ballard, A.L. Kini, R.A. Dec, S.F. Sloan, E.D. Jr. (2000b). Structural transitions in methane-ethane gas hydrates, Part I upper transition point and applications. Chem. Eng. Sci., 55, 5763-5771. 

For the gas hydrates it is not possible to make an entirely unambiguous comparison of the observed heat of hydrate formation from ice (or water) and the gaseous solute with the calculated energy of binding of the solute in the ft lattice, because AH = Hfi—Ha is not known. If one assumes AH = 0, it is found that the hydrates of krypton, xenon, methane, and ethane have heats of formation which agree within the experimental error with the energies calculated from Eq. 39 for details the reader is referred to ref. 30. 

A mechanistic model for the kinetics of gas hydrate formation was proposed by Englezos et al. (1987). The model contains one adjustable parameter for each gas hydrate forming substance. The parameters for methane and ethane were determined from experimental data in a semi-batch agitated gas-liquid vessel. During a typical experiment in such a vessel one monitors the rate of methane or ethane gas consumption, the temperature and the pressure. Gas hydrate formation is a crystallization process but the fact that it occurs from a gas-liquid system under pressure makes it difficult to measure and monitor in situ the particle size and particle size distribution as well as the concentration of the methane or ethane in the water phase. 

H. (2003). Dissociation Behavior of Pellet-Shaped Methane-Ethane Mixed Gas Hydrate Samples. Energy and Fuels, 17, 614-618. 

The best-known gas hydrates are those of ethane, ethylene, propane, and isobulaue. Others include methane and I butene, most of the fluorocarbon refrigerant gases, nitrous oxide, acetylene, vinyl chloride, carbon dioxide, methyl and ethyl chloride, methyl and ethyl bromide, cyclopropane, hydrogen sulfide, methyl mercaptan, and sulfur dioxide. 

Natural gas hydrates are crystalline solids composed of water and gas. The gas molecules (guests) are trapped in water cavities (host) that are composed of hydrogen-bonded water molecules. Typical natural gas molecules include methane, ethane, propane, and carbon dioxide. 

Englezos, P., A Model for the Formation Kinetics of Gas Hydrates from Methane, Ethane, and Their Mixtures, M.S. Thesis, University of Calgary, Alberta (1986). 

Although a typical natural gas is mainly comprised of the first three normal paraffins, the phase equilibria of each component with water will differ from that of a natural gas with water. However, a comparison of predictions with data for methane, ethane, and propane simple gas hydrates is given as a basis for understanding the phase equilibria of water with binary and ternary mixtures of those gases. 

For gases, such as methane, which are supercritical at hydrate forming temperatures, there is one quadruple point, as indicated by point Q1 in Fig. 1. At this point, ice, liquid water, gas and hydrate are in equilibrium. For gases that are subcritical at hydrate forming temperatures, such as ethane, ° there are two quadruple points (Q1 and Q2 in Fig. 2). While Q1 lies at approximately the freezing point of water, Q2 is at approximately the intersection of the hydrate-water-gas three-phase equilibrium curve with the vapor pressure curve. At this latter point, liquid water, gas, hydrate, and liquid hydrate former are all in equilibrium. As seen in Fig. 2, the hydrate... 

Clarke, M.A. Bishnoi, P.R. Measuring and modelling the rate of decomposition of gas hydrates formed from mixtures of methane and ethane. Chem. Eng. Sci. 2001, 56, 4715-4724. 

There is another, very important and large repository of methane methane hydrates (also known as gas hydrates or clathrates Kvenvolden 1988).They comprise ice in which the interstices of the lattice house small molecules, such as methane, ethane, carbon dioxide and hydrogen sulphide. In fact, enough gas needs to be present to fill 90% of the interstices in order for the hydrate to form, and it has a different crystal structure from normal ice (Sloan 1990). If fully saturated, the most common crystalline structure can hold one molecule of methane for every 5.75 molecules of water, so lm3 of hydrate can contain 164 m3 of methane at STP (see Box 4.8).The solubility of methane in water is insufficient to account for hydrate formation, and a major nearby source is required, typically methanogenesis, based on the dominance of methane (99%) and its very light isotopic composition (813C generally <—60%o see Section 5.8.2). 

The presence of water, as mentioned earlier, can have several detrimental results among which is the formation of gas hydrates—snowlike, crystalline compounds composed of small amounts of methane, ethane, propane, or isobutane and water. The formation of these hydrates is aided by the presence of liquid water and areas of turbulence. The formation of these hydrates increases the pressure drop along the pipeline, thereby decreasing its capacity the presence of liquid water also can contribute to some corrosion. The formation and inhibition of these hydrates will be discussed in Section XII. In this section about gas treatment, the removal of hydrogen sulfide and other sulfide forms from the natural gas is discussed along with removal of carbon dioxide. A number of processes have been commercialized in this area and a few of them will be described here. 

Raman spectroscopy provides a rapid and convenient tool for structure analysis without having to use diffraction techniques. Subsequently. Raman spectroscopy was used to study the structures, phase equilibria, and kinetics of gas hydrates at high pressure and close to ambient temperatures. By monitoring the Raman spectra of guest molecules, the phase transition from Stmcture I to II hydrate for an ethane-methane gas mixture was identified for the first time. This was an xmexpected result, because both methane and ethane form Structure I hydrate. ... 

The precise location of the base of the gas hydrate stability zone under known pressure/temperature conditions varies somewhat depending on several factors, most important of which is gas chemistry. In places where the gas is not pure methane, for example, in the Gulf of Mexico, at a pressure equivalent to 2.5 km water depth, the base of the gas hydrate stability zone will occur at about 21 °C for pure methane, but at 23" C for a typical mixture of approximately 93% methane, 4% ethane, 1% propane, and some smaller amounts of higher hydrocarbons. At the same pressure (2.5 km water depth) but for a possible mixture of about 62% methane, 9% ethane, 23% propane, plus some higher hydrocarbons, the phase limit will be at 28°C. These differences will cause major shifts in depth to the base of the gas hydrate stability zone as would be implied by Fig. 2B. Such mixtures of gases essentially make the formation of gas hydrate easier and therefore can result in the formation of gas hydrate near the seafloor at shallower depths (lower pressures) than for methane hydrate... 

Gas hydrates (clathrates) may technically be considered as an alternative form of ice that has the ability to entrap relatively large volumes of gas within cavities in the hydrate crystal matrix. The entrapped guest molecules (gas) stabilize the structure by means of van der Waals interactions, and combinations of the different unit cells give rise to structures I, II (175-177), and H (178). The most common gas to form gas hydrates is methane, but ethane, propane, butane, carbon dioxide, nitrogen, and many other types of gases may also give rise to gas hydrates. 

The size and shape of the gas molecules are the most important features decisive for what the structure of a gas hydrate will be like. For instance, both methane and ethane form hydrates of structure I, while propane forms a structure II hydrate. Neither ethane or propane are able to enter the smallest cavities in the hydrates these cavities may thus be empty. In the case of gas mixtures, small guest molecules may enter the smallest cavities while the larger guests are restricted to the larger ones. 

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