Phase equilibria hydrates

Two-Phase Equilibrium Hydrates with One Other Phase... 

U. Huetz and P. Englezos. Measurement of structure h hydrate phase equilibrium and the effect of electrolytes. Fluid Phase Equilibria, 117(1-2) 178-185,1995. 

Englezos, P. and S. Hull, Phase Equilibrium Data on Carbon Dioxide Hydrate in the Presence of Electrolytes, Water Soluble Polymers and Montmorillonite , CanJ. Chem. Eng, 72, 887-893 (1994). 

The initial hydration rate v and the equilibrium hydration amount were obtained as parameters reflecting the hydration behavior of LB films (see Figure 8). Temperature dependencies of the hydration behavior (v0and W ) of 10 layers of DMPE (Tc = 49 °C) LB films are shown in Figure 9. Large W and v0 values were observed only around the phase transition temperature (7C) of DMPE membranes. Thus, DMPE LB films were hydrated only near the Tc, but not in the solid state below the Tc and in the fluid state above the Tc. This indicates that the... 

Kang, S.P. Lee, H. (2000). Recovery of C02 from flue gas using gas hydrate Thermodynamic verification through phase equilibrium measurements. Environ. Sci. Technol., 34(20), 4397-4400. 

Kumar, R. Wu, H-J. Englezos, P. (2006). Incipient Hydrate Phase Equilibrium for Gas mixtures Containing Hydrogen, Carbon Dioxide Propane. Fluid Phase... 

Ohmura, R. Uchida, T. Takeya, S. Nagao, J. Minagawa, H. Ebinuma, T. Narita, H. (2003 a). Clathrate hydrate formation in (methane + water + methylcyclohexanone) systems the first phase equilibrium data. J. Chem. Thermodynamics, 35, 2045-2054. 

Ohmura, R. Matsuda, S. Takeya, S. Ebinuma, T. Narita, H. (2005c). Phase equilibrium for structure-H hydrates formed with methane and methyl-substituted cyclic ether. Int. J. Thermophysics, 26 (5), 1515-1523. 

Since around the mid-1990s, there has been a proliferation of hydrate time-dependent studies. These include macroscopic, mesoscopic, and molecular-level measurements and modeling efforts. A proliferation of kinetic measurements marks the maturing of hydrates as a field of research. Typically, research efforts begin with the consideration of time-independent thermodynamic equilibrium properties due to relative ease of measurement. As an area matures and phase equilibrium thermodynamics becomes better defined, research generally turns to time-dependent measurements such as kinetics and transport properties. 

This chapter focuses on the question, What are hydrates and Chapter 3 concerns the question, How do hydrates form Although a few thermodynamic properties are discussed in this chapter, the phase equilibrium conditions are considered in Chapters 4, 5, and 6. 

Figure 3.35 (See color insert following page 390.) X-ray CT imaging shows radial dissociation of a hydrate core. Image number 1 -8 (top number on each image) recorded over 0-245 min (bottom number on each image). The cell pressure was decreased from 4.65 to 3.0 MPa over 248 min. The hydrate core temperature decreased from 277 to 274 K with time, following the three-phase methane hydrate equilibrium line. (From Gupta, A., Methane Hydrate Dissociation Measurements andModeling The Role of Heat Transfer and Reaction Kinetics, Ph.D. Thesis Colorado School of Mines, Golden, CO (2007). With permission.)...

The calculation of two-phase (hydrate and one other fluid phase) equilibrium is discussed in Section 4.5. The question, To what degree should hydrocarbon gas or liquid be dried in order to prevent hydrate formation is addressed through these equilibria. Another question addressed in Section 4.5 is, What mixture solubility in water is needed to form hydrates ... 

Finally, Section 4.6 concerns the relationship of phase equilibrium to other hydrate properties. The hydrate application of the Clapeyron equation is discussed... 

The two calculation techniques in this chapter may be regarded as successive approximations to hydrate phase equilibrium, increasing both in accuracy and in... 

Two common misconceptions exist concerning the presence of water to form hydrates in pipelines, both of which are illustrated via the T-x phase equilibrium diagrams in Figure 4.3. The first and most common misconception is that a free water phase is absolutely necessary for the formation of hydrates. The upper three-phase (Lw-H-V) line temperature marks the condition of hydrate formation from free water and gas. Below that temperature and to the right of the hydrate line, however, are two-phase regions in which hydrates are in equilibrium only with hydrocarbon vapor or liquid containing a small (<1000 ppm) amount of water. 

Hydrate Enthalpy and Hydration Number from Phase Equilibrium... 

The intention of this section is to relate these enthalpies both to the phase equilibrium values and to show how these values relate to microscopic structure and to hydration numbers at the ice point. 

Table 4.10 shows the literature values for hydrate numbers, all obtained using de Forcrand s method of enthalpy differences around the ice point. However, Handa s values for the enthalpy differences were determined calorimetrically, while the other values listed were determined using phase equilibrium data and the Clausius-Clapeyron equation. The agreement appears to be very good for simple hydrates. Note also that hydrate filling is strongly dependent on... 

The object of this chapter is to provide the reader with a qualitative understanding of hydrate phase equilibrium. Such an understanding implies a historical overview, which also provides successive approximations to hydrate phase equilibrium in terms of accuracy. The accuracy of three-phase prediction is given below in the order of increasing accuracy ... 

In the following chapter, the most accurate method available is discussed for the determination of hydrate equilibrium—that of statistical thermodynamics. The consideration of this method ties the macroscopic phase equilibrium, such as has been discussed qualitatively in the present chapter, to the microscopic structure discussed in Chapter 2. 

The bridging of the microscopic and macroscopic phenomena is satisfying both from a theoretical and from a pragmatic standpoint. Mathematical bridges between the microscopic and macroscopic domains are the major focus of Chapter 6. Applications of the concepts of this chapter are also found in the final two chapters. Hydrates in the earth provide natural examples of phase equilibrium as detailed in Chapter 7. Applications to artificial hydrates and their problems in flow lines are presented in Chapter 8. 

Equation 5.23 may be used with Equation 5.22a to determine the chemical potential of water in hydrate /z, which is one of the major contributions of the model. The combination of these two equations is of vital importance to phase equilibrium calculations, since the method equates the chemical potential of a component in different phases, at constant temperature and pressure. 

Because there is a very large phase equilibrium data base, existing over 70 years as shown in Chapter 6, and because recent spectroscopic tools (e.g., Raman, NMR, and diffraction) have provided microscopic hydrate data, the latter approach was chosen in this monograph and the accompanying computer programs. While the latter method used in this book represents a theoretical advance, it is shown to compare favorably with the existing commercial hydrate programs in Section 5.1.8. 

The number of measurements for natural gas hydrate thermal properties is several orders of magnitude lower than that for phase equilibrium properties. The experimental difficulties in thermal measurements center on the determination of... 

In a thorough review of calorimetric studies of clathrates and inclusion compounds, Parsonage and Staveley (1984) presented no direct calorimetric methods used for natural gas hydrate measurements. Instead, the heat of dissociation has been indirectly determined via the Clapeyron equation by differentiation of three-phase equilibrium pressure-temperature data. This technique is presented in detail in Section 4.6.1. 

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