A thermodynamic study of the (fluoromethane + water) system
Highlights
► Solubility of fluoromethane in water as a function of (T, p) was observed. ► Liquid + hydrate + vapor phase equilibrium of the system was observed. ► Ice + hydrate + vapor equilibrium of the system was observed. ► Dissociation enthalpies were found for both equilibria. ► Stoichiometry of fluoromethane hydrate was determined.
Introduction
Fluoromethane readily forms hydrates of structure SI at moderate pressures. No complete study of the hydrate formation equilibria have been published to date. Also, there have been no reports of the vapor-liquid behavior (solubility) of fluoromethane in water near the hydrate formation region. All of these data sets are required in order to utilize the so-called “de Forcrand” method [1] to determine the hydration number of the hydrate at the lower quadruple point (Q1). In this work, the LHV measurements were repeated at high precision and agreed well with the previous studies. The IHV curve was determined for the first time. Extensive measurements of solubility were also made at various temperatures as a function of pressure, up to the hydrate formation pressure.
The equilibria of interest in this study are summarized in the following list.
Equation (R1) expresses the dissociation of hydrate to form a nearly pure fluoromethane vapor (the water vapor pressure being small) and a solution of liquid water saturated with fluoromethane. The hydration number n is unknown at first. Equation (R2) depicts the solution equilibrium in reverse. Adding (R1) and (R2) gives (R3), in which all components are in their standard states as pure phases. (R4) is analogous to (R3), except that it applies to temperatures below Q1, where water exists as a solid. The low vapor pressure of ice and the extremely low solubility of gases in ice guarantees that reactants and products in the actual system are in their standard states. (R5) is the difference (R4) – (R3) and simply shows the melting of n moles of ice. The essence of the “de Forcrand” method is that if the enthalpy changes of (R3), (R4), and hence (R5) are measured, n may be determined.
Section snippets
Experimental
All measurements were conducted in a stainless steel autoclave (see figure 1) with a volume of (6.33 ± 0.05)·10−4 m3. The autoclave was completely immersed in a thermostatic bath of 50% ethylene glycol in water. A magnetically coupled stirring bar provided mixing. The temperature was controlled by circulating the coolant with a Neslab® RTE 140 chiller. Temperature inside the autoclave was measured by an Omega® thermistor which had been calibrated against an ASTM 63C mercury thermometer. The
Solubility
The data were fitted to the Krichevsky–Kasarnovsky [3] equation, as given below
In equation (1), f represents the fugacity, X the dissolved mole fraction, KH the Henry’s law constant, and ps is the solvent vapor pressure, which will be neglected; is the partial molar volume of the solute at infinite dilution.
In figure 2, a plot of ln(f/X) vs p at each temperature yields from the slope and ln(KH) from the intercept.
The enthalpy of solution at infinite dilution
1Hydrate dissociation enthalpy and hydration number
Hydrate dissociation enthalpy and hydration number are related; lower hydration number means a lower enthalpy change per mole of guest molecules. As might be expected, fluoromethane hydrate falls between methane hydrate and difluoromethane hydrate. Results are given in table 4 below.
The de Forcrand method gives the hydration number with thermodynamic rigor but cannot shed light on the relative occupancy of the two different cages in the SI structure. One can say that the unit cell (46 water
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