Effect of a Dissolved Salt on Vapor-Liquid Equilibrium with Liquid Composition Held Constant

Abstract

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...

Salt Effect in Vapor—Liquid Equilibrium at Fixed Liquid Composition

“…previous studies(3,4) demonstrate clearly thatEquation1 can satis factorily correlate the salt effects of the inorganic salts tested at fixedliquid composition in ethanol-water, at all salt concentrations up to saturation. The R.A.A.D.…”

Analysis and extension of the Furter equation, and its application in the simulation of saline extractive distillation columns

“…The effect of the separating agent on the mixed solvent equilibrium is known as the salt effect. This salt effect of dissolved salts on liquid mixtures had already been studied and reported at the end of the nineteenth century (Kablukov, 1891;Miller, 1897) , then followed by systematic investigations on the subject from the 1950's (Cardoso and O'Connell, 1987;Jaques and Furter, 1974;Loehe and Donohue, 1997;Robinson and Stokes, 1959). The appearance of new materials that better withstand the highly corrosive medium associated with salt systems, the fact that the addition of salts to azeotropic mixtures avoids the requirement of a third solvent, which is usually harmful to the environment, and the increasing necessity of processes with low energy consumption, leads to a renewed interest in saline extractive distillation (SED).…”

Understanding membrane selectivity in pervaporation of water‐rich water:ethanol mixtures

“…It can also be seen that the pervaporation results reproduce previously observed results from VPE studies demonstrating that the vapour above a concentrated salt solution in water:ethanol is enriched in ethanol. [28,29,32] Time-Resolved Pervaporation through PDMS Time-resolved pervaporation measurements provide a means for determining if any kinetic effect exists in these systems. We postulated that a transient excess of ethanol would be expected early in the pervaporation process due to the greater diffusibility of ethanol through a hydrophobic PDMS membrane.…”

“…We have applied this apparatus to pervaporation through commercial PDMS membrane as received and after grafting with hydrophilic polymer, and to both simple water: ethanol solutions and water:ethanol solutions containing 3.0 mol/L of salts which have been demonstrated to disrupt water:ethanol hydrogen bonding and increase the mole fraction of ethanol in the vapour at a fixed feed composition. [26][27][28][29] We have also developed instrumentation for measurement of the VPE for the 3.0 mol/L salt solutions under conditions approximating those of the pervaporation experiments, in order to compare the apparent selectivity obtained with the selectivity that would be achieved in the absence of a membrane. Time-resolved observation of pervaporation behaviour in these systems (PDMS membranes grafted with hydrophilic polymer and to water: ethanol solutions containing salt) is novel and has the capacity to greatly increase understanding of the factors driving pervaporation performance.…”