Vapor-liquid equilibrium salt effect

A salt dissolved in a mixed solvent is capable, through such effects on the structure of the liquid phase as preferential association and others, of altering the composition of the equilibrium vapor. Hence salt effect on vapor-liquid equilibrium relationships provides a potential technique of extractive distillation. A review is presented of the use of dissolved salts, rather than liquid solvents, as separating agents for extractive distillation. 

Ciparis, J. N., "Data of Salt Effect in Vapor-Liquid Equilibrium," Lithuanian Agricultural Academy, Kaunas, Lithuania, USSR, 1966. 

The topic covered in the 10 papers of the first section is commonly referred to as salt effect in vapor-liquid equilibrium and is potentially of great industrial importance. This salt effect leads to extractive distillation processes in which a dissolved salt replaces a liquid additive as the separating agent the replacement often results in a greatly improved separating ability and reduced energy requirements. Two papers in this volume, those by Sloan and by Vaillancourt, illustrate the use of such processing to concentrate nitric acid from its aqueous azeotrope. Nevertheless, the effect has not been exploited by industry to nearly the extent that would seem to be merited by its scientific promise. 

Correlation and Prediction of Salt Effect in Vapor-Liquid Equilibrium... 

A review is presented of techniques for the correlation and prediction of vapor-liquid equilibrium data in systems consisting of two volatile components and a salt dissolved in the liquid phase, and for the testing of such data for thermodynamic consistency. The complex interactions comprising salt effect in systems which in effect consist of a concentrated electrolyte in a mixed solvent composed of two liquid components, one or both of which may be polar, are discussed. The difficulties inherent in their characterization and quantitative treatment are described. Attempts to correlate, predict, and test data for thermodynamic consistency in such systems are reviewed under the following headings correlation at fixed liquid composition, extension to entire liquid composition range, prediction from pure-component properties, use of correlations based on the Gibbs-Duhem equation, and the recent special binary approach. 

The use of a dissolved salt in place of a liquid component as the separating agent in extractive distillation has strong advantages in certain systems with respect to both increased separation efficiency and reduced energy requirements. A principal reason why such a technique has not undergone more intensive development or seen more than specialized industrial use is that the solution thermodynamics of salt effect in vapor-liquid equilibrium are complex, and are still not well understood. However, even small amounts of certain salts present in the liquid phase of certain systems can exert profound effects on equilibrium vapor composition, hence on relative volatility, and on azeotropic behavior. Also extractive and azeotropic distillation is not the only important application for the effects of salts on vapor-liquid equilibrium while used as examples, other potential applications of equal importance exist as well. 

Such complexities tend to explain why progress has been relatively slow, at least until recently, in the formulation of effective relations and techniques for the representation of salt effect in vapor-liquid equilibrium. 

The original equation for salt effect in vapor-liquid equilibrium, proposed by Furter (7) and employed subsequently by Johnson and Furter (8), described the effect of salt concentration on equilibrium vapor composition under the condition of a fixed ratio of the two volatile components in the liquid phase. The equation, derived from the difference in effects of the salt on the chemical potentials of the two volatile components, with simplifying approximations reduces to the form... 

Pure-component properties from which prediction of salt effect in vapor-liquid equilibrium might be sought, include vapor pressure lowering, salt solubility, degree of dissociation and ionic properties (charges and radii) of the salt, polarity, structural geometry, and perhaps others. 

It has been generally held, at least until recently, that a salt dissolved in the liquid phase would enrich the equilibrium vapor in the component in which it was less soluble and impoverish it in the component in which it was more soluble. It was also assumed that the magnitude of the effect on vapor composition depended not only on how much salt was present but also on the degree of difference between the solubilities of the salt in the two liquid components taken separately. Various investigators, including Tursi and Thompson (28) and Fogg (29), have tried to relate the salt effect to this solubility difference alone, but their success... 

Over the years, various other theories and models have been proposed for predicting salt effect in vapor-liquid equilibrium, including ones based on hydration, internal pressure, electrostatic interaction, and van der Waals forces. These have been reviewed in detail by Long and McDevit (25), Prausnitz and Targovnik (31), Furter (7), Johnson and Furter (8), and Furter and Cook (I). Although the electrostatic theory as modified for mixed solvents has had limited success, no single theory has yet been able to account for or to predict salt effect on equilibrium vapor composition from pure-component properties alone. 

The research programs on extractive distillation by salt effect and on salt effect in vapor-liquid equilibrium at the Royal Military College of Canada are supported by the Defence Research Board of Canada, Grant No. 9530-142. 

A method of prediction of the salt effect of vapor-liquid equilibrium relationships in the methanol-ethyl acetate-calcium chloride system at atmospheric pressure is described. From the determined solubilities it is assumed that methanol forms a preferential solvate of CaCl296CH OH. The preferential solvation number was calculated from the observed values of the salt effect in 14 systems, as a result of which the solvation number showed a linear relationship with respect to the concentration of solvent. With the use of the linear relation the salt effect can be determined from the solvation number of pure solvent and the vapor-liquid equilibrium relations obtained without adding a salt. 

Then, we can obtain the preferential solvation number from the observed values of the salt effect. As the concentration of solvent is decreased by the number of solvated molecules, the actual solvent composition participating in the vapor-liquid equilibrium is changed. Assuming that a salt forms the solvate with the first component, the actual composition X a is given by... 

The establishment of the method of prediction has been attempted by the reverse calculation of the preferential solvation number from measured values, using Equations 4 and 7 which are based on the assumption that the salt effect in the vapor-liquid equilibrium is caused by the preferential solvation formed between a volatile component and a salt. The observed values were selected from Ciparis s data book (4), Hashitani s data (5-8), and the author s data (9-15). S was calculated by Equation 7 when the relative volatility as in the vapor-liquid equilibrium with salt is increased with respect to the relative volatility a in the vapor-liquid equilibrium with salt, but by Equation 4 when as is decreased. The results are shown in Figures 5-12. From these figures, it will be seen that the following three relations exist ... 

Prediction of salt effect. The procedure for calculation of the preferential solvation number S has been described above. By reversing this procedure, that is, by determining xia from S, we can estimate the salt effect using the vapor-liquid equilibrium without a salt. When the salt concentration is below saturation, the preferential solvation number S can be expressed as follows in cases where the solvation is formed with the first component. 

The salt effect is attributable to the formation of preferential solvation from the standpoint of molecular structure. In other words, when calcium chloride, which dissolves readily in methanol but very little in ethyl acetate, was added to the methanol-ethyl acetate system to saturation, calcium chloride formed with methanol the preferential solvate which may be written CaCl2 6CH30H. It was also shown from the observation of solubility that the solvated methanol molecules did not participate in the vapor-liquid equilibrium. 

Salt Effect on Isothermal Vapor-Liquid Equilibrium of 2-Propanol-Water Systems... 

Isothermal vapor-liquid equilibrium data at 75°, 50° and 25° C for the system of 2-propanol-water-lithium perchlorate were obtained by using a modified Othmer still. In the 2-propanol-rich region 2-propanol was salted out from the aqueous solution by addition of lithium perchlorate, but in the water-rich region 2-propanol was salted in. It is suggested from the experimental data that the simple electrostatic theory cannot account for the salt effect parameter of this system. 

Many papers concerning salt effect on vapor-liquid equilibrium have been published. The systems formed by alcohol-water mixtures saturated with various salts have been the most widely studied, with those based on the ethyl alcohol-water binary being of special interest (1-6,8,10,11). However, other alcohol mixtures have also been studied methanol (10,16,17,20,21,22), 1-propanol (10,12,23,24), 2-propanol (12,23,25,26), butanol (27), phenol (28), and ethylene glycol (29,30). Other binary solvents studied have included acetic acid-water (22), propionic acid-water (31), nitric acid-water (32), acetone-methanol (33), ethanol-benzene (27), pyridine-water (25), and dioxane-water (26). 

Although the third component of these systems is usually a single inorganic salt, mixtures of two or more salts have been studied, and some research has been done with third components of low vapor pressure (18,19). Some qualitative studies have been done on salt effect in vapor-liquid equilibrium with salts which are either soluble in only one or both components, hygroscopic or non-hygro-scopic, etc. 

The salt effects of potassium bromide and a series office symmetrical tetraalkylammonium bromides on vapor-liquid equilibrium at constant pressure in various ethanol-water mixtures were determined. For these systems, the composition of the binary solvent was held constant while the dependence of the equilibrium vapor composition on salt concentration was investigated these studies were done at various fixed compositions of the mixed solvent. Good agreement with the equation of Furter and Johnson was observed for the salts exhibiting either mainly electrostrictive or mainly hydrophobic behavior however, the correlation was unsatisfactory in the case of the one salt (tetraethylammonium bromide) where these two types of solute-solvent interactions were in close competition. The transition from salting out of the ethanol to salting in, observed as the tetraalkylammonium salt series is ascended, was interpreted in terms of the solute-solvent interactions as related to physical properties of the system components, particularly solubilities and surface tensions. 

The data in Tables I-XVI (see Appendix for all tables) show the isobaric vapor-liquid equilibrium results at the boiling point for potassium, ammonium, tetramethylammonium, tetraethylammonium, tetra-n-propylammonium, and tetra-n-butylammonium bromides in various ethanol-water mixtures at fixed liquid composition ratios. The temperature, t, is the boiling temperature for all solutions in these tables. In all cases, the ethanol-water composition was held constant between 0.20 and 0.35 mole fraction ethanol since it is in this range that the most dramatic salt effects on vapor-liquid equilibrium in this particular system should be observed. That is, previous data (12-15,38) have demonstrated that a maximum displacement of the vapor-liquid equilibrium curve by salts frequently occurs in this region. In the results presented here, it should be noted that Equation 1 has been modified to... 

Literature pertaining to salt effect in vapor-liquid equilibrium and to extractive distillation using salt effect was recently reviewed by Furter and Cook (10), and the theory and technical aspects were reviewed by Furter (II). Vapor-liquid equilibrium data for 188 systems containing salt were previously compiled by Ciparis (12), who has also published a recent book with Dobroserdov and Kogan on the theory and practice of extractive distillation by salt effect (13). 

In the equation, equilibrium vapor composition expressed in the form of an improvement factor, aB/a, is related to salt concentration z in the liquid phase by the salt effect parameter, k. According to the theoretical... 

---