Methane propane hydrates

As indicated in Example 4.1, note the dramatic decrease in hydrate pressure caused by a small amount of propane added to methane, due to the structure change (si to sll). At pressures above incipient hydrate formation conditions, sll hydrates are predicted to be present throughout the entire composition range. 

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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. 

Figure 6.54 Methanol inhibition of methane + propane hydrates.

Barrer s discussion4 of his analog of Eq. 28 merits some comment. Equation 28 expresses the equilibrium condition between ice and hydrate. As such it is valid for all equilibria in which the two phases coexist and not only for univariant equilibria corresponding with a P—7" line in the phase diagram. (It holds, for instance, in the entire ice-hydratell-gas region of the ternary system water-methane-propane considered in Section III.C.(2).) In addition to Eq. 28 one has Clapeyron s equation... 

Methane hydrate and propane hydrate crystallize in Structures I and II respectively, their dissociation pressures at — 3°C have been determined and were found to be 23.1 and 1.48 atm. Above a... 

Carson and Katz5 studied another part of the methane-propane-water system. These authors investigated its behavior when an aqueous liquid, a hydrocarbon liquid, a gas, and some solid were present. It was found that the system was univariant so that the solid consisted of a single phase only. This phase is a hydrate which proved to contain methane and propane in various ratios. They then concluded that these hydrates behaved as solid solutions. It is clear that Carson and Katz measured a part of the four-phase line HllL1L2G. 

Uchida, T. Moriwaki, M. Takeya, S. Ikeda, I.Y. Ohmura, R. Nagao, J. Minagawa, H. Ebinuma, T. Narita, H. Gohara, K. Mae, S. (2004). Two-Step Formation of Methane-Propane Mixed Gas Hydrates in a Batch-Type Reactor. AIChE J. 50(2), 518-523. 

Fig. 17-4. Hydrate-formation conditions of methane-propane mixtures. (Deaton and Frost, U.S. Bureau of Mines, Monograph 8, 1946, 20.. Courtesy, Bureau of Mines, U.S. Department of the Interior.)...

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). 

Table 2.8 is a slight modification of a microscopic and macroscopic property summary by Davidson (1983) for ice and hydrate structures I and II. Although the values in the table were generally measured or estimated for methane or propane hydrates, the contribution of the guest molecule (other than causing the structure to exist) may be considered small for these properties, to a first approximation. 

Whalley (1980) presented a theoretical argument to suggest that both the thermal expansivity and Poisson s ratio should be similar to that of ice. With the above two estimates, Whalley calculated the compressional velocity of sound in hydrates with a value of 3.8 km/s, a value later confirmed by Whiffen et al. (1982) via Brillouin spectroscopy. Kiefte et al. (1985) performed similar measurements on simple hydrates to obtain values for methane, propane, and hydrogen sulfide of 3.3, 3.7, and 3.35 km/s, respectively, in substantial agreement with calculations by Pearson et al. (1984). 

Stoll and Bryan (1979) first measured the thermal conductivity of propane hydrates (0.393 Wm-1K-1 at T = 215.15 K) to be a factor of 5 less than that of ice (2.23 Wm-1K-1). The low thermal conductivity of hydrates, as well as similarities of the values for each structure (shown in Table 2.8) have been confirmed from numerous studies (Cook and Leaist, 1983 [0.45 Wm-1K-1 for methane hydrate at 216.2 K] Cook andLaubitz, 1981 Ross et al., 1981 Ross and Andersson, 1982 Asher et al., 1986 Huang and Fan, 2004 Waite et al., 2005). The thermal conductivity of the solid hydrate (0.50-0.58 W m-1 K-1) more closely resembles that of liquid water (0.605 W m-1 K-1). 

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. 

The best choice for the standard hydrate is one that is well-characterized and not too different from the real state of the system. If the standard state is well-defined, small perturbations from this standard state can be accounted for correctly. With this in mind, we turn to the three most well-known hydrates of si, sll, and sH, namely methane, propane, and methane+neohexane. Note that the standard states for si, sll, and sH are the empty hydrate lattices of these and not the actual hydrates. Therefore for the reference hydrates, the activity coefficients for methane, propane, and methane + neohexane hydrates will be unity. 

We define the molar volume of the standard hydrates of si and sll as the molar volumes of methane and propane hydrate, respectively. The molar volume of these hydrates, and therefore of the standard states, is well-characterized via diffraction data (Tse, 1990 Huo, 2002). Ballard proposed the following expression for the molar volume of water in hydrates ... 

The standard hydrates of si, sll, and sH were assumed to be the empty hydrates of methane, propane, and methane + neohexane, respectively. While the thermal expansion parameters are the same for the real and standard hydrates, the compressibility parameter and standard volume are not. The volumetric compressibilty of the standard hydrates of si, sll, and sH are 3E-5,3E-6, and 3E-7,... 

Figure 5.15 is the pseudo-binary pressure versus excess water composition diagram for the methane+propane+water system at a temperature of 277.6 K. At 277.6 K the hydrate formation pressures are 4.3 and 40.6 bar for pure propane (sll) and pure methane (si) hydrates, respectively, as shown at the excess water composition extremes in Figure 5.15. As methane is added to pure propane, there will be a composition at which the incipient hydrate structure changes from sll to si as seen in the inset of Figure 5.15, this composition is predicted to be 0.9995 mole fraction methane in the vapor—a very small amount of propane added to a methane+water mixture will form sll hydrates.

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. 

Half a century later, the work of Carson and Katz (1942) provided a second reason for considering the dissociation condition of the hydrate equilibrium point (see Chapter 3, Figure 3.1b for more details). Their work clearly showed the solid solution behavior of hydrates formed by gas mixtures. This result meant that hydrate preferentially encapsulated propane from a methane + propane gas mixture, so that a closed gas volume was denuded of propane (or enriched in methane) as more hydrates formed. On the other hand, upon hydrate dissociation, when the last crystal melted the initial gas composition was regained, minus a very small amount to account for solubility in the liquid phase. 

The most productive two-phase (H-V or H-Lhc) equilibrium apparatus was developed by Kobayashi and coworkers. The same apparatus has been used for two-phase systems such as methane + water (Sloan et al., 1976 Aoyagi and Kobayashi, 1978), methane + propane + water (Song and Kobayashi, 1982), and carbon dioxide + water (Song and Kobayashi, 1987). The basic apparatus described in Section 6.1.1.2 was used in a unique way for two-phase studies. With two-phase measurements, excess gas was used to convert all of the water to hydrate at a three-phase (Lw-H-V) line before the conditions were changed to temperature and pressures in the two-phase region. This requires very careful conditioning of the hydrate phase to prevent metastability and occlusion. Kobayashi and coworkers equilibrated the hydrate phase by using the ball-mill apparatus to convert any excess water to hydrate. 

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.)... 

Hydrate Methane + propane Reference Deaton and Frost (1946) Phases Lw-H-V... 

Hydrate Methane + propane Reference Song and Kobayashi (1982)... 

Hydrate Methane + propane + isobutane Reference Paranjpe et al. (1987)... 

Paranjpe, S.G., Patil, S.L., Kamath, V.A., Godbole, S.P., HydrateFormation in Crude Oils and Phase Behavior of Hydrates in Mixtures of Methane, Propane, Isobutane, and n-Butane, paper presented at the Third Chemical Congress of North America, Toronto, June 5-10 (1988). 

Verma, V.K., Hand, J.H., Katz, D.L., in Gas Hydrates from Liquid Hydrocarbons Methane-Propane-Water System, AIChE-VTG Joint Meeting, Munich, September, p. 106... 

In order to examine the effect of the Kihara parameters on the predicted hydrate equilibrium pressures, a sensitivity analysis was carried out (see also Cao et al. ). In this study we report results for methane (si hydrate former) and propane (sll hydrate former). The Kihara parameter values, as well as the thermodynamic property values, reported by Sloan were taken as the base-reference case and hydrate equilibrium pressures were calculated by perturbing the reference values in the range +(1%-10%). On the other hand, the reported thermodynamic parameters zl// and Ah have a wider range, but as it is going to be discussed later, have a less significant effect on the predictions. 

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