Methane ethane hydrates

Structural transitions (si and sll) have been experimentally determined in the methane + ethane + water system via Raman, NMR, and diffraction between 

As pressure is increased, the amount of si hydrate in the system relative to vapor becomes larger, enriching the vapor with methane. This can be seen by applying 

Effect of Temperature on Hydrate Structure in the Methane (0.73) + Ethane (0.27) + Water (Excess) System 

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Coexistence of si and sll carbon dioxide hydrate has been detected from x-ray diffraction measurements during hydrate growth (Staykova and Kuhs, 2003). Similarly, metastable sll hydrate phases were detected using NMR spectroscopy during si xenon hydrate formation (Moudrakovski et al., 2001a) and during si methane/ethane hydrate formation (Bowler et al., 2005 Takeya et al., 2003). 

FIGURE 3.36 Average rates for methane hydrate samples reaching 50% dissociation at 0.1 MPa, following destabilization by rapid release of P. The anomalous preservation regime is between 242 and 271 K. Square symbols experiments in which P is maintained at 2 MPa, Diamonds 0.1 MPa rapid depressurization tests on sll methane-ethane hydrate, showing no comparable preservation behavior at 268 K. (Reproduced from Stern, L.A., Circone, S., Kirby, S.H., Durhan, W., Can. J. Phys., 81, 271 (2003). With permission from the National Research Council.)... 

Figure 6.53 Methanol inhibition of methane + ethane hydrates.

Under certain conditions of temperature and pressure, and in the presence of free water, hydrocarbon gases can form hydrates, which are a solid formed by the combination of water molecules and the methane, ethane, propane or butane. Hydrates look like compacted snow, and can form blockages in pipelines and other vessels. Process engineers use correlation techniques and process simulation to predict the possibility of hydrate formation, and prevent its formation by either drying the gas or adding a chemical (such as tri-ethylene glycol), or a combination of both. This is further discussed in SectionlO.1. 

Handa, Y.P. (1986a). Compositions, enthalpies of dissociation, and heat capacities in the range 85 to 270 K for clathrate hydrates of methane, ethane, and propane, and enthalpy of dissociation of isobutane hydrate, as determined by a heat-flow calorimeter. J. Chem. Thermodynamics, 18 (10), 915-921. 

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

Lee, J.D. Susilo, R. Englezos, P. (2005c). Methane-ethane and methane-propane hydrate formation and decomposition on water droplets. Chem. Eng. Set., 60, 4203-4212. 

Ng, H.-J. Robinson, D.B. (1985). Hydrate Formation in Systems Containing Methane, Ethane, Propane, Carbon Dioxide or Hydrogen Sulfide in the Presence of Methanol. Fluid Phase Equilibria, 21, 145-155. 

Subramanian, S. Kini, R.A. Dec, S.F. Sloan, E.D. Jr. (2000a). Evidence of structure II hydrate formation from methane-ethane mixtures. Chem. Eng. Sci., 55, 1981-1999. 

Tsuji, H. Kobayashi, T. Ohmura, R. Mori, Y.H. (2005a). Hydrate Formation by Water Spraying in a Methane + Ethane + Propane Gas Mixture An Attempt at Promoting Hydrate Formation Utilizing LMGS for sH Hydrates. Energy and Fuel, 19, 869-876. 

Hydrates are crystalline compounds consisting of hydrocarbon molecules occupying voids within a framework of water molecules. Hydrocarbon molecules which are essential for the formation of hydrates are methane, ethane, propane and isobutane. Other molecules such as C0 and H-S are also suitable. The lattice structure formed by the water molecules varies depending on the guest molecules that are associated with the particular hydrate. 

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. 

Two French workers, Villard and de Forcrand, were the most prolific researchers of the period before 1934, with over four decades each of heroic effort. Villard (1888) first determined the existence of methane, ethane, and propane hydrates, de Forcrand (1902) tabulated equilibrium temperatures at 1 atm for 15 components, including those of natural gas, with the exception of iso-butane, first measured by von Stackelberg and Muller (1954). 

After Hammerschmidt s initial discovery, the American Gas Association commissioned a thorough study of hydrates at the U.S. Bureau of Mines. In an effort spanning World War II, Deaton and Frost (1946) experimentally investigated the formation of hydrates from pure components of methane, ethane, and propane, as well as their mixtures with heavier components in both simulated and real natural gases. 

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). 

Of the natural gas components that form simple hydrates, nitrogen, propane, and iso-butane are known to form structure II. Methane, ethane, carbon dioxide, and hydrogen sulfide all form si as simple hydrates. Yet, because the larger molecules of propane and iso-butane only fit into the large cavity of structure II, natural gas mixtures containing propane and iso-butane usually form structure II hydrate (see Section 2.1.3.3 in the subsection on structural changes in binary hydrate structure). 

In a review of the thermodynamics of water, Franks and Reid (1973) showed that the optimum molecular size range for maximum solubility was similar to hydrate stability. Franks and Reid noted, this is not intended to imply that long-lived clathrate structures exist in solution—only that the stabilization of the water structure by the apolar solutes resembles the stabilization of water in a clathrate lattice. Glew (1962) noted that, within experimental error, the heat of solution for ten hydrate formers (including methane, ethane, propane, and hydrogen sulfide) was the same as the heat of hydrate formation from gas and ice, thereby suggesting the coordination of the aqueous solute with surrounding water molecules. 

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. 

The data were modeled with one fitted parameter (K ) for hydrate growth of simple hydrate formers of methane, ethane, carbon dioxide. Since all these model components form si hydrate, the model should be used with caution for sll and sH. 

See Section 4.1.5 for other examples of how Gibbs Phase Rule works in the methane + water phase diagram. Section 5.2 shows the application of the Gibbs Phase Rule for hydrate guests of methane, ethane, propane, and their mixtures. 

The distribution coefficient method, often called the A -value method, was conceived by Wilcox et al. (1941) and finalized by Carson and Katz (1942). The best methane, ethane, and propane charts are from the latter reference. Updated charts are presented for carbon dioxide (Unruh and Katz, 1949), hydrogen sulfide (Noaker and Katz, 1954), nitrogen (Jhaveri and Robinson, 1965), isobutane (Wu et al., 1976), and n-butane (Poettmann, 1984), as well as for a method that is a function of hydrate structure (Mann et al., 1989). 

Holder, G.D., Multi-Phase Equilibria in Methane-Ethane-Propane-Water Hydrate Forming Systems, Ph.D. Thesis, University of Michigan, University Microfilms No. 77-7939, Ann Arbor, MI 48106, (1976). 

Section 5.2 shows the prediction method of phase diagrams of the major components of natural gas, namely methane, ethane, and propane hydrates and their mixtures at the common deep-ocean temperature of 277 K. Many of the commonly observed phenomena in natural gas systems are illustrated, while the power of the method is shown to go beyond that of Chapter 4, to illustrate future needs. 

Since mixtures of methane, ethane, and propane make up nearly 97 mol% of a typical natural gas mixture, the hydrate phase behavior of a natural gas mixture in contact with water will likely be approximated by that of a simple mixture of these three components in contact with water. 

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. 

To evaluate the phase equilibria of binary gas mixtures in contact with water, consider phase diagrams showing pressure versus pseudo-binary hydrocarbon composition. Water is present in excess throughout the phase diagrams and so the compositions of each phase is relative only to the hydrocarbon content. This type of analysis is particularly useful for hydrate phase equilibria since the distribution of the guests is of most importance. This section will discuss one diagram of each binary hydrate mixture of methane, ethane, and propane at a temperature of 277.6 K. 

Figure 5.16 is the pseudo-binary pressure versus excess water composition diagram for the methane + ethane + water system at a temperature of 277.6 K. In the diagram, pure ethane and pure methane both form si hydrates in the presence of water at pressures of 8.2 and 40.6 bar, respectively. Note that between the compositions of 0.74 and 0.994 mole fraction methane, sll hydrates form at the incipient formation pressure. Similar to the methane + propane + water system, only a small amount of ethane added to pure methane will form sll hydrates. 

For industrial applications, determining the stable hydrate structure at a given temperature, pressure, and composition is not a simple task, even for such a simple systems as the ones discussed here. The fact that such basic mixtures of methane, ethane, propane, and water exhibit such complex phase behavior leads us to believe that industrial mixtures of ternary and multicomponent gases with water will exhibit even more complex behavior. Spectroscopic methods are candidates to observe such complex systems because, as discussed earlier, pressure and temperature measurements of the incipient hydrate structure are not enough. 

The methane+ethane+propane+water system is the simplest approximation of a natural gas mixture. As shown in Figure 5.20, the phase equilibria of such a simple mixture is quite complicated at pressures above incipient hydrate formation conditions. One of the most interesting phenomenon is the coexistence of si and sll hydrates which occurs in the interior of some pseudo-ternary phase diagrams. 

A binary mixture of methane + ethane, which are both si hydrate formers, can form sll hydrate as the thermodynamically stable phase (Subramanian et al., 2000). 

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