Extractive Distillation Using Salts
Extractive distillation with soluble salt is another method for dehydration of ~90% ethanol. When a salt is dissolved in a liquid mixture like ethanol and water consisting of two volatile miscible liquid components, the salt may affect the activity of the two volatile components through the formation of liquid phase associations or complexes. If the dissolved salt associates preferentially with the molecules of one component of the liquid solution compared to those of the other, the solubility relationship between the two volatile components is altered such that one component is "salted out" in respect to the other. In the case of water ethanol mixture, more polar water molecules interact strongly with anions and cations of the salt making hydration spheres around cations and anions of the salt. In such a case, the activities of the two volatile components of the liquid solution are altered relative to each other in a manner which results in a modification of composition of the equilibrium vapor phase, regardless of the fact that no salt is present in the vapor phase. Therefore, extractive distillation by the use of a dissolved salt as the separating agent can be applied to systems of low relative volatility or systems exhibiting azeotropic behavior in composition regions critical to separation. Most importantly, a comparatively small concentration of salt is capable of increasing the relative volatility of the more volatile component of the liquid solution to be distilled. This behavior is known as the salt effect, and is probably due to the preferential solvation of the ions by water molecules.
A number of common salts have been tested and shown effective for extractive distillation of the ethanol-water system. These include calcium chloride [28], calcium nitrate, sodium iodide, potassium iodide, cupric chloride, cobalt (II) chloride [29], nickel (II) chloride, strontium bromide [30], sodium and potassium acetates as well as glucose [31]. Even though glucose is not a salt, it may work similarly by interaction with water via hydrogen bonding. The distillation set-up for the extractive distillation with soluble salts is basically similar to the extractive distillation arrangement with high boiling liquid solvents. The salt, a non-volatile component, is introduced at the top or near the top tray of the distillation column, flows downward along the column, and is completely removed with the bottom product. There are a number of positive aspects of extractive distillation with soluble salts when compared to the extractive distillation with liquid solvents such as [2]:
1. High level of energy savings due to the absence of the vaporization-condensation cycle of the volatile liquid solvent (high boiling liquid separation agent) inside the column.
2. Production of a distillate totally free from the separation agent — salt.
3. Lower toxicity level of certain salts in comparison to previously cited liquids such as benzene used in the production of dry ethanol.
The technique of using inorganic salts to break the ethanol-water azeotrope for industrial production of anhydrous ethanol from dilute solutions dates back to patents registered in the period of 1932-1934 [32]. Salt-assisted extractive distillation has been used in industry as far back as the 1930s, and it is reported that over 100 plants based on the process with production capacities of up to 43,000 tonnes/year were built between 1930 and 1950 in Europe and elsewhere. The anhydrous ethanol produced was blended with gasoline to make gasohol containing 10% ethanol [33]. The last of these plants ceased operation in Brazil in 1965. Another example of salt-assisted extractive distillation is the HIAG process, which was developed in Germany in the 1930s and used a mixture of sodium and potassium acetates as the extractant. The users of the HIAG process claimed lower capital and operating costs in comparison with conventional azeotropic distillation using benzene or extractive distillation using ethylene glycol [33].