Binary Hydrate Phase Equilibria

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. 

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Roberts et al. (1940), Barrer and Edge (1967), Skovborg and Rasmussen (1994) present similar, detailed derivations to consider the use of the Clapeyron equation for hydrate binary and multicomponent systems. The reader is referred to the work of Barrer and Edge (1967) for the precise meaning of dP/dT and the details of the derivation. Barrer and Stuart (1957) and Barrer (1959) point out that the problem in the use of the Clapeyron equation evolves from the nonstoichio-metric nature of the hydrate phase. Fortunately, that problem is not substantial in the case of hydrate equilibrium, because the nonstoichiometry does not change significantly over small temperature ranges. At the ice point, where the hydrate number is usually calculated, the nonstoichiometry is essentially identical for each three-phase system at an infinitesimal departure on either side of the quadruple point. 

It is convenient to use phase diagrams [46] to represent the thermodynamic properties that determine the stability and equilibrium composition of water-containing aerosols. The properties of interest are the temperature, the vapour pressure and composition of the various components in the condensed phases. This is particularly important with respect to the composition and stability of the various hydrates formed at low temperature in the nitric acid-water [47] and sulfuric acid-water binary systems [48], and the ternary systems HjSO/HNOj/HjO and HjSO/HCl/HjO [49],... 

In hydrate equilibrium, it may seem slightly unusual to apply it to binary systems (water and one guest component) of three-phase (Lw-H-V or I-H-V) equilibrium to obtain the heats of dissociation. As van der Waals and Platteeuw (1959b) point out, however, the application of the Clapeyron equation is thermodynamically correct, as long as the system is univariant, as is the case for simple hydrates. 

Lamivudine is an example of the effect of hydrates in nonaqueous solvents (Jozwiakowski et al., 1996). In distilled water at 25D, the anhydrate free base (form II) is 1.2 times as soluble as the 0.2 hydrate (form I). In ethanol at 26, the hydrate is 1.6 times as soluble as the anhydrate. The maximum solubility in ethanol-water mixtures was found to be at 40-60% water in ethanol, when form I is the most stable solid phase. The transition composition was with 18-20% water in ethanol in binary mixtures with more than 20% water, only the hydrate was found at equilibrium, and with less than 18% water, only the anhydrate was found at equilibrium. 

Dholabhai PD, Parent JS, Bishnoi PR (1997) Equilibrium conditions for hydrate formation from binary mixtures of methane and carbon dioxide in the presence of electrolytes, methanol and ethylene glycol. Fluid Phase Equilibria 141 235-246... 

The hydrate and phenol clathrate equilibrium data of the water-carbon dioxide, phenol-carbon dioxide, and water-phenol-carbon dioxide systems are presented in Table 1 and depicted in Figure 2. In order to establish the validity of the experimental apparatus and procedure the hydrate dissociation pressures of carbon dioxide measured in this work were compared with the data available in the literature (Deaton and Frost [7], Adisasmito et al. [8]) and found that both were in good agreement. For the phenol-carbon dioxide clathrate equilibrium results, as seen in Figure 2, the dramatic increase of the dissociation pressures in the vicinity of 319.0 K was observed. It was also found in the previous study (Kang et al. [9]) that the experimental phenol-rich liquid-phenol clathrate-vapor (Lp-C-V) equilibrium line of the binary phenol-carbon dioxide system could be well extended to the phenol clathrate-solid phenol-vapor (C-Sp-V) equilibrium line (Nikitin and Kovalskaya [10]). It is thus interesting to note that a quadruple point at which four individual phases of phenol-rich liquid, phenol clathrate, solid... 

An analysis of the experimental data was performed by consideration of the three binary systems water-NaCl, water-sucrose and NaCl-sucrose. Of these, the first system has been thoroughly studied, and the transition temperatures and compositions are included in Table 2. The situation is more complicated for water-sucrose mixtures, mainly because any crystallisation processes of sucrose or any of its hydrates from aqueous solutions are likely to be very slow, perhaps impossible to determine by DSC methods in real time. Some hydrates have, however, been identified by X-ray and chemical analytical methods they are also included in Table 2 and the (probable) equilibrium phase diagram is shown in Figure 8. As regards the anhydrous system NaCl-sucrose, no quantitative information exists. The crystallisation of NaCl from its solid solution in amorphous... 

Since we have only one non-stoichiometric solid phase, we will approach the study of this system by quasi-chemistiy of structure elements. However, this approach presents some difficulties. Indeed, hydrated salts are relatively conplex solids with at least three principal components the anion (itself often complex), the cation, and water. If salt admits several limiting hydrates, water molecules are not all equivalent. All these complexities require a simplification of the representation of solid. With this intention, we consider hydrated solids as pseudo-binary (see section 2.4.1) of which one of the components is the water concerned with dehydration and the other component is the skeleton of anhydrous salt or incorporates possible n molecules of water not implicated in the equilibrium under study. We will disregard specific defects related to the skeleton and thus take into account the following structure elements ... 

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