Azeotropic and Extractive Distillations

Extractive distillation and azeotropic distillation share as a common characteristic, in that an auxiliary solvent is added to the crude aromatics fraction to achieve better separation by distillation. Extractive distillation takes place in the presence of an extractive material with high solvent power for aromatics, which has relatively low volatility compared with the compounds which are to be separated, and is constantly added at the top of the fractionation column. The purpose of the auxiliary solvent is to change the vapor pressures of the hydrocarbon components in such a way that they can be more easily separated by distillation e.g. the vapor pressure of benzene is lowered to the point when the accompanying non-aromatics can be distilled off as an overhead fraction. 

In azeotropic distillation, on the other hand, the additive and the component to be separated form an azeotrope i. e. a mixture boiling at a given temperature and with a constant composition. Azeotropic distillation can only be used to refine highly-enriched mixtures of aromatics, such as occur in coke-oven benzole, whereas extractive distillation can also be used to separate aromatics which are present in low concentrations. As early as World War I toluene used in the production of explosives was obtained by extractive distillation, using phenol as the extractive material. 

Operating the rectification columns under pressures up to 18 bar gives the overhead vapors a greater heat content these are used to heat the distillation feed. The yield of benzene is over 99%. 

1 Predistillation column 2 Extractive distillation column 3 Stripping column 4 Raffinate column 

In essentially all separations carried out to date by these techniques the effect of the added component is in the liquid phase, although it is possible to modify the vapor-phase properties for systems operating at high pressure. 

As the simplest case, consider a binary mixture of components 1 and 2 which is to be modified by the addition of component 3. 

Over moderate temperature ranges the ratio of the vapor pressures of the pure components does not change appreciably. The main possibility of modifying the relative volatility lies in altering the ratio of the activity coefficient. By subtracting the two activity coefficient equations, it is possible to obtain the ratio, 

Consider the case in which compounds 1 and 2 are similar, i.e., almost obey Raoult s law, e,g., ethanol and isopropanoL For this case, J3i2 will be small and Au will be approximately equal to unity. Equation (10-1) can be simplified for these conditions to the approximate relationship, 

The significance of this equation can be better shown by comparing it with the activity coefficient ratio, (71/72)0, for the binary mixture without the added agent. By Eqs. (3-44) and (3-45) 

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The principle of azeotropic distillation depends on the abiHty of a chemically dissimilar compound to cause one or both components of a mixture to boil at a temperature other than the one expected. Thus, the addition of a nonindigenous component forms an azeotropic mixture with one of the components of the mixture, thereby lowering the boiling point and faciHtating separation by distillation. The separation of components of similar volatiHty may become economical if an entrainer can be found that effectively changes the relative volatiHty. It is also desirable that the entrainer be reasonably cheap, stable, nontoxic, and readily recoverable from the components. In practice, it is probably the ready recoverabiHty that limits the appHcation of extractive and azeotropic distillation. 

Tassios, D. P. "Extractive and Azeotropic Distillation" Advances in Chemistry Series 115, American Chemical Society, Washington, D.C., 1972. 

Physical Methods of Organic Chemistry , Interscience, NY, Vol 4(1951), pp 1 to 174 (Theory of distillation) 175 to 316 (Ordinary fractional distillation) 317 to 387 (Extractive and azeotropic distillation) 389 to 461 (Distillation of liquefied gases and low-boiling liquids) 463 to 494 (Distillation under moder-... 

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. 

Chemical engineers have been solving distillation problems by using the equilibrium-stage model since 1893 when Sorel outlined the concept to describe the distillation of alcohol. Since that time, it has been used to model a wide variety of distillation-like processes, including simple distillation (single-feed, two-product columns), complex distillation (multiple-feed, multiple-product columns), extractive and azeotropic distillation, petroleum distillation, absorption, liquid-liquid extraction, stripping, and supercritical extraction. 

In conclusion, recent developments in solvent selection, phase nonideality description, and tray-to-tray calculation schemes have greatly facilitated the design of extractive and azeotropic distillation schemes, and use of salts give new methods for extractive distillation separations. Finally, the work of Gerster (30), Black and Ditsler (29), and Black et al. (25) compare these two schemes. 

Table VIII. Comparing Extractive and Azeotropic Distillation...

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