Salt-effect distillation

For a more complete discussion of salt-effect distillation see References 75—77. 

Salt distillation, of hafnium, 73 84 Salt domes, 22 798 Salt-dome sulfur deposits, 23 569 Salt effect distillation, 8 816—817 Salt flats, 5 786 Salt-fog unit, 78 72 Salt formation(s), 22 798 amino acids, 2 570 ammonia, 2 685—686 carboxylic acids, 5 40—41 citric acid, 6 637 cycloaliphatic amines, 2 501 fatty amines, 2 522 Salt industry... 

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. 

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. 

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. 

The addition of salts to a liquid mixture to aid in the separation of the components of that mixture by fractional distillation has important implications in terms of theoretical studies and practical applications. The complexity of the salt effect in vapor-liquid equilibria and the sparse conclusive work in this field are largely responsible for the limited applications it has received industrially, despite a potential for dramatically improved separation performance in certain systems. Not only is the effect of the salt on a system complex, but variations occur from system to system that is, each system is unique. In addition, the nature of the effects, as well as their magnitudes, tends to be com-... 

Although a few observations relevant to the mechanism have been made, it is difficult to offer a clear explanation at this time. Initial control experiments indicate that the reactions are kinetically controlled, i.e., they are irreversible under the conditions used. Furthermore, a salt effect is involved, because the distilled amino-titanium compound 102 is essentially non-selective (Table 3). Upon adding MgX2 salts, ketone selectivity increases to 70%. 

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. 

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). 

There are also a number of matrix issues that the analyst should bear in mind. In seawater there is a small salt effect, with lower absorbances observed in seawater versus distilled water for the same concentration of NH4 . Amino acids can cause an interference with the analysis, but at the concentrations of amino acids found in seawater the interference is generally insignificant (Solorzano, 1969) there is no interference with urea. Beware of analyzing samples with a pH higher than 11.0 because blanks become inconsistent a pH problem is indicated if the sample has a... 

In a procedure for the preparation of 2-thiophenealdehyde, the hexaminium salt is prepared in refluxing chloroform and steam distilled to effect the remaining steps... 

Furter [91] has analyzed the state of the art from the point of view of employing the salt effect in industrial processes, especially in extractive distillation. In addition, he ha.s made up a list of references covering the years 1966 to 1977 [91 a]. Schubert et al. [92] investigated the effect of some metal chlorides and other salts on the isothermal = 60°C) phase equilibrium behaviour of the systems n-propanol-water, n-butanol-water and methanol-water. Using CH30H/H20/NaBr as an example, the method of predicting salt effects for vapour-liquid equilibria as developed by Schuberth has been extended to uusaturated solutions [92a]. 

Fig. 12.4 Salt-effect distillation. 1, Feed stream 2, ED column 3, salt recovery operation 4, bottoms product 5, salt recycle 6, dissolving chamber 7, overhead product.

In certain systems where solubility considerations permit, it is possible to use a salt dissolved into the liquid phase as the separating agent in place of the normal liquid. The attraction of the salt-effect distillation technique lies in its potential for greatly reduced energy requirements compared with conventional extractive and azeotropic distillation processes. 

While it is easy to hnd liquid extraction solvents of the right volatility and polarity, there are fewer suitable salts with the right solubility except for solution in water. The need to avoid chloride and similar salts, unless the plant in contact with it is constructed in an exotic alloy, is another restriction which tends to mean salt-effect distillation requires special purpose plants. 

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