A semi-empirical test for the thermodynamic consistency of isobaric liquid-vapor equilibrium data for ethanol-water saturated with a salt is proposed. The test is based upon an adaptation of the Herington method ( I ) in which the ternary system is treated as a special binary. The test is applied both to full concentration range data and to data which are incomplete a t the high alcohol end. For data which are incomplete a t the high water end, a less severe test is employed.Literature data for 23 systems composed of ethanol, water, and an inorganic salt at saturation were tested. By the criterion of the proposed test, fourteen were deemed consistent, six were borderline, and three were pronounced inconsistent.All consistent isobaric liquid-vapor equilibrium data must satisfy the equation Generally heat of mixing data are unavailable, and so for binary isobaric systems Herington (1 ) suggested a semiempirical procedure in which the area above and the area below the abscissa are measured, the areas ( 2 ) are summed and are subtracted one from the other ( I ) . The percentage deviation is defined asHerington showed that if the data are consistent.* J is defined asThe only previous attempt to test systems containing two liquids and a salt was by the "slope" method. Kogan The calculated and experimental values should lie on two parallel curves. The method was tested by Kogan on isobaric data for three systems containing ethanol, water, and a salt at saturation. The biggest drawback of the "slope" method is that it is insensitive. An alternative method is presented for testing the consistency of salt data which is based upon Herington's test. METHOD OF TESTING ISOBARIC SALT DATAThe method makes the basic assumption that the ternary system can be treated as a binary composed of ethanol saturated with salt as one component and water saturated with salt as the other component. When these t w o solutions are mixed it is found in every case that, where the solubility of the salt in the boiling alcohol-water mixtures has been measured, precipitation occurs and hence the final solution is always saturated with salt. The activity coefficients of ethanol and water are calculated from Equation (6) : with one modification. The saturation vapor pressure pi0 is not applicable when the solvent is saturated with salt. Instead, for water the vapor pressure of the saturated salt solution pz' is substituted for pZo. For ethanol these vapor pressures are not available and so a correction to the saturation vapor pressure is applied by multiplying by the ratio of the vapor pressure of ethanol saturated with salt IIT to the vapor pressure of pure ethanol p l~O at the salt solution boiling point. This ratio c is assumed independent of temperature. Incomplete data are treated somewhat differently than complete data and so each will be considered separately. March, 1972
The original equation for correlating t h e effects of dissolved salts on vapor-liquid equilibrium, which has been employed extensively with data for entire systems, is tested for the first time under the conditions for which it was derived, namely, for predicting equilibrium vapor composition a s a function of salt concentration under t h e condition of a fixed ratio of the two volatile components in the liquid phase. Several sets of data were measured under such conditions in four different systems of the alcohol-water-salt type; good agreement with the equation was observed.The original equation for salt effect in vapor-liquid equilibrium (Furter, 1958; Johnson and Furter, 1960) related the change in vapor composition caused by dissolving a salt into the liquid phase to salt concentration in the liquid. The equation was derived from thermodynamic considerations, i e . , from the difference in effects of the salt on the chemical potentials of the two volatile components. It was demonstrated that the relation could also be derived from Gilliland's equation for liquid separating agents in extractive distillation, and again from the Gibbs free energy. When certain approximations are made, including the assumption of constant temperature and pressure, the equation reduces in its simplest form to In (cys/cy) = k z (1) In other words, the equation relates an "improvement factor," the ratio of relative volatilities with and without salt present, to salt concentration, z, in the liquid phase. It is important to note that the equation is rigorous only when the ratio of volatile components present in the liquid is held constant. This ratio is characterized by x, the mole fraction of the more volatile component in the liquid calculated on a salt-free basis. Salt effect in vapor-liquid equilibrium is believed to be a complex function of interactions and self-interactions between all system components, each of which is likely to be a function not only of composition but also of degree of salt dissociation (which itself is composition dependent), Therefore there are no theoretical considerations which would suggest that k should remain constant for a system if the equation were to be applied to the entire vapor-liquid equilibrium curve rather than under the more limited condition of constant x . Nevertheless, several investigators have employed the equation empirically in attempts to correlate data for systems over the entire x range from 0 to 1. The reasons for such attempts were no doubt due in part to the absence of a satisfactory alternative and also to the attractiveness of using a single-constant equation for representing such complex phenomena. Johnson and Furter (1960, 1965) and Furter (1958) tested eq 1 by applying it over the entire x range of vapor-liquid equilibrium data for 24 systems, each composed of an alcohol (methanol, ethanol, or 1-propanol), water, and an inorganic salt dissolved to saturation. The values of k were found to remain remarkably constant over the entire x range for most of the systems, a...
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