Ethane propane hydrates

The pressure at which this dissociation is predicted to occur is called the hydrate pseudo-retrograde pressure at T. Pseudo-retrograde behavior is defined as the disappearance of a dense phase upon pressurization, which is counter-intuitive. This behavior resembles, but is not strictly the same as, vapor-liquid retrograde phenomena (de Loos, 1994). 

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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. 

M S. Zhurko, F.V. (2006a). Formation and decomposition of ethane, propane, and carbon dioxide hydrates in silica gel mesopores under high pressure. J. Phys. Chem. B, 110 (39), 19717-19725. 

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. 

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). 

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. 

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. 

In the discussion appendix of the original paper by Carson and Katz (1942), Hammerschmidt indicated that, while the method was acceptable for gases of normal natural gas composition, an unacceptable deviation was obtained for a gas rich in ethane, propane, and the butanes. More work is also required to revise the Kvs -value charts for two components, namely, carbon dioxide and nitrogen. In three-phase hydrate data for binary mixtures of carbon dioxide and propane, Robinson and Mehta (1971) determined that the Kvs method for carbon dioxide gave unsatisfactory results. The API Data Book shows the Kvs values for nitrogen to be only a function of pressure, without regard for temperature Daubert (Personal... 

It should be thermodynamically impossible for one set of Kvst charts to serve both hydrate structures (si and sll), due to different energies of formation. That is, the Kysi at a given temperature for methane in a mixture of si formers cannot be the same as that for methane in a mixture of sll formers because the crystal structures differ dramatically. Different crystal structures result in different xst values that are the denominator of Kvst (= yt/xSi). However, the Katz Kvst charts do not allow for this possibility because they were generated before the two crystal structures were known. The inaccuracy may be lessened because, in addition to the major component methane, most natural gases contain small amounts of components such as ethane, propane, and isobutane, which cause sll to predominate in production/transportation/processing applications. 

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. 

Figure 5.17 shows a predicted pressure versus excess water composition plot for the ethane+propane+water system at 274 K. At 0.0 mol fraction ethane (propane+ water) sll form at approximately 2 bar, and at 1.0 mol fraction ethane (ethane + water) si form at approximately 5 bar. At the intermediate composition of 0.78 mole fraction ethane, a quadruple point (Aq-sI-sII-V) exists in which both incipient hydrate structures are in equilibrium with vapor and aqueous phase. This point will be referred to as the structural transition composition the composition at which the incipient hydrate formation structure changes from sll to si at a given temperature. 

As the temperature is increased to 277.6 K the pressure versus composition diagram for the ethane + propane + water system changes drastically as shown in Figure 5.18 Between 0.0 and 0.6 mole fraction of ethane, the incipient hydrate structure is sll hydrate. However, if the pressure is increased to approximately 11.45 bar, between 0.3 and 0.6 mol fraction ethane, sll is predicted to dissociate to form an Aq-V-Lhc region. 

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. 

Structure identification, quantifying relative cage occupancies. 1II NMR has been used for ethane, propane, and isobutane hydrates (Davidson et al., 1977 Garg et al., 1977), while 2H, 19F, 31P, and 77 Se NMR have been used for several si guests (Collins et al., 1990). 13C cross-polarization and magic angle spinning (MAS) NMR techniques have been applied to study hydrates of carbon dioxide, methane, and propane (Ripmeester and Ratcliffe, 1988, 1999 Wilson et al., 2002 Kini et al., 2004). 

Kini, R.A., NMR Studies of Methane, Ethane, and Propane Hydrates Structure, Kinetics, and Thermodynamics, Ph.D. Thesis, Colorado School of Mines, Golden, CO (2002). With permission.)... 

While most of the simple hydrate data consist of the three-phase and quadruple point type, the available two-phase simple hydrate data are listed for methane, ethane, propane, and carbon dioxide. Plots of these data are not suitable for comparison between data sets and are therefore not provided. 

Hydrate Ethane + propane Reference Holder and Hand (1982)... 

Hydrates Ethane + propane Reference Song and Kobayashi (1994)... 

Hydrate Methane + ethane + propane + 2-methylpropane Reference Mei et al. (1998)... 

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