Hydrates phase diagram

Chapter 4 provides phase diagrams and simple prediction schemes for each of the hydrate phase diagram regions. 

Hydrate phase diagrams for water-hydrocarbon systems provide a convenient overview of the calculation types. These diagrams differ substantially from the normal hydrocarbon phase diagrams primarily due to hydrates and the hydrogen bonds inherent in aqueous systems. The phase diagrams of Section 4.1 provide an overview for the calculation methods in this chapter and the next. 

Hydrate Phase Diagrams for Water + Hydrocarbon Systems... 

To investigate this unexpected behaviour hydrate phase diagram was plotted for si and sll using our in-house thermodynamic model HydraFLASH. Figure 3 shows the prediction results. 

The strength of the vdWP theory is its ease of use and the speed with which predictions of the clathrate hydrate phase diagram can be generated. If the scope of validity of this theory is recognized, it can be a valuable tool in the hand of researchers in predicting hydrate stability conditions. 

Equilibrium hydrate formation condition for CO2 has been extensively investigated and available in the literature [32-44], A hydrate phase diagram of the CO2 + H2O system is presented in Fig. 10.5. As can be seen in the figure, equilibrium hydrate formation conditions above and below fi-eezing point of water is presented. 

The water-NaCl dihydrate in Figure 11.14 has a simple molecular formula and forms regular-structure crystals, as do most such compounds. The cage-structure hydrates discussed here do not, because the guest molecules do not bind to host molecules in a stoichiometric way, like water and NaCl [14, p. 313]. The S-L equilibria in Figures 11.13 and 11.14 are practically independent of pressure. Because the G in G-L-S has a much larger specific volume than liquids or solids, gas hydrates phase diagrams are pressure-dependent. 

Figure 6 Experimental and predicted methane hydrate phase diagrams computed using different potentials. (6(a) is a linear plot, with temperature below 250 K not shown. 6(b) is a log-linear plot, with entire temperature range shown.) (SOURCE Adapted with permission from refrence 9. Copyright 2001 American Chemical Society)...

For sodium palmitate, 5-phase is the thermodynamically preferred, or equiUbrium state, at room temperature and up to - 60° C P-phase contains a higher level of hydration and forms at higher temperatures and CO-phase is an anhydrous crystal that forms at temperatures comparable to P-phase. Most soap in the soHd state exists in one or a combination of these three phases. The phase diagram refers to equiUbrium states. In practice, the drying routes and other mechanical manipulation utilized in the formation of soHd soap can result in the formation of nonequilibrium phase stmcture. This point is important when dealing with the manufacturing of soap bars and their performance. 

The compound HI.H2O does not appear as a stable hydrate in the phase diagram, but the vibrational spectra of frozen solutions of this composition indicate the formulation [H30]+I . Higher hydrates appear at HI.2H2O (mp ... 

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... 

As already remarked in the introduction, the formulation of the laws governing heterogeneous equilibria by Bakhuis Roozeboom1 was partly based on his studies on gas hydrates. Although the general laws he derived are certainly correct, and have marked an important step in the development of physical chemistry, Roozeboom and his contempories were mistaken in the nature of the phase diagram of gas hydrates gas hydrates are not stoichiometric... 

In the discussion of phase diagrams involving hydrates, the following nomenclature will be used (if the structure of the hydrate is not specified the subscript is omitted) Hx is the hydrate of Structure I, Hu is the hydrate of Structure II, Lx is a nonaqueous liquid, L2 is an aqueous liquid, and G is a gas. 

If the critical temperature of the solute is below room temperature, the phase diagram is similar to the one described for the system hydroquinone-argon. No temperature can then be indicated above which hydrates cannot exist. This situation arises for the following solutes argon,48 krypton,48 xenon,48 methane,3 and ethylene.10... 

The effects of the intramicellar confinement of polar and amphiphilic species in nanoscopic domains dispersed in an apolar solvent on their physicochemical properties (electronic structure, density, dielectric constant, phase diagram, reactivity, etc.) have received considerable attention [51,52]. hi particular, the properties of water confined in reversed micelles have been widely investigated, since it simulates water hydrating enzymes or encapsulated in biological environments [13,23,53-59]. 

FIG. 4 Phase diagram of fully hydrated DPPC bilayers. Different phases found are also schematically shown Lp, gel P, rippled gel L I, interdigitated gel and L , liquid crystalline phases. (From Ref. 50. Copyright 1999 The Japan Society of High Pressure Science and Technology.)... 

Figure 104. Phase separation of a salt hydrate (e.g. CaCl2 6H20) into three distinct phases with different water concentration and density (right) and corresponding phase diagram (left)...

The well-known empirical Bancroft s rule [84] states that the phase in which the surfactant is preferentially soluble tends to become the continuous phase. An analogous empirical correlation has been reported by Shinoda and Saito [85]. Eor a nonionic surfactant of the polyethoxylated type [R-(CH2-CH2-0) -0H, where R is an alkyl chain], as temperature increases, the surfactant head group becomes less hydrated and hence the surfactant becomes less soluble in water and more soluble in oil. Its phase diagram evolves as schematically shown in Fig. 1.4. At low... 

Lokshin, K.A. Zhao, Y. (2006). Fast synthesis method and phase diagram of hydrogen clathrate hydrate. App. Phys. Lett., 88, 131909. 

RH (e.g., Oatis et al., 1998). In addition, there is evidence that new metastable solid states can form in these droplets (Tang et al., 1995). Similar studies of ammonium bisulfate, NH4HS04, have been carried out over a range of temperatures and relative humidities relevant to the atmosphere, and its phase diagram, including metastable states and a crystalline hydrate phase at low temperatures has been reported (Imre et al, 1997). 

Fig. 17-2. Hydrate portion of the phase diagram of a typical mixture of water and a light hydrocarbon.

The line of major interest on this phase diagram is the line Q]QZ, which represents the equilibrium between hydrocarbon gas, liquid water, and hydrate. 

---