Distillation of azeotropic mixtures

Van Dongen, D. B. Distillation of Azeotropic Mixtures. The Application of Simple-Distillation Theory to the Design of Continuous Processes, Ph.D. Dissertation, University of Massachusetts, Amherst (1983). 

I am aware that various special methods have been proposed for separating these amines, such as fractional distillation of azeotropic mixtures under superatmospheric pressure, with or without the addition of large quantities of ammonia, and fractional crystallization of the hydrohalide salts from a solvent such as, alcohol. None of these methods is as simple or direct, nor does it produce as sharp a separation, as that which I am about to describe. 

Petlyuk, F. B. (1984). Necessary Condition of Exhaustion of Components at Distillation of Azeotropic Mixtures in Simple and Complex Columns. In The Calculation Researches of Separation for R ning and Chemical Industry, 3-22. Moscow Zniiteneftechim (Rus.). 

Petlyuk, F. B., Danilov, R. Yu. (1999). Sharp Distillation of Azeotropic Mixtures in a Two-Feed Column. Theor. Found. Chem. Eng., 33,233-42. 

Kondrat ev, A. A., Frolova, L. N., Serafimov, L. A., Hasanov, Z. K. (1977). Peculiarities of Distillation of Azeotropic Mixtures with Intersection of Boundaries of Distillation Regions. Theor. Found. Chem. Eng, 11,907-12. 

The breaking up of azeotropic mixtures. The behaviour of constant boiling point mixtures simulates that of a pure compound, because the composition of the liquid phase is identical with that of the vapour phase. The composition, however, depends upon the pressure at which the distillation is conducted and also rarely corresponds to stoichiometric proportions. The methods adopted in practice will of necessity depend upon the nature of the components of the binary azeotropic mixture, and include —... 

Another way to shift the position of equilibrium to favor the formation of ester is by removing water from the reaction mixture This can be accomplished by adding benzene as a cosolvent and distilling the azeotropic mixture of benzene and water... 

Of these five methods all but pressure-swing distillation can also be used to separate low volatiUty mixtures and all but reactive distillation are discussed herein. It is also possible to combine distillation and other separation techniques such as Hquid—Hquid extraction (see Extraction, liquid-liquid), adsorption (qv), melt crystallization (qv), or pervaporation to complete the separation of azeotropic mixtures. 

Obviously, the use of a nonvolatile ionic liquid simplifies the distillative workup of volatile products, especially in comparison with the use of low-boiling solvents, where it may save the distillation of the solvent during product isolation. Moreover, common problems related to the formation of azeotropic mixtures of the volatile solvents and the product/by-products formed are avoided by use of a nonvolatile ionic liquid. In the Rh-catalyzed hydroformylation of 3-pentenoic acid methyl ester it was even found that the addition of ionic liquid was able to stabilize the homogeneous catalyst during the thermal stress of product distillation (Figure 5.2-1) [21]. This option may be especially attractive technically, due to the fact that the stabilizing effects could already be observed even with quite small amounts of added ionic liquid. 

The major advantage of the use of two-phase catalysis is the easy separation of the catalyst and product phases. FFowever, the co-miscibility of the product and catalyst phases can be problematic. An example is given by the biphasic aqueous hydro-formylation of ethene to propanal. Firstly, the propanal formed contains water, which has to be removed by distillation. This is difficult, due to formation of azeotropic mixtures. Secondly, a significant proportion of the rhodium catalyst is extracted from the reactor with the products, which prevents its efficient recovery. Nevertheless, the reaction of ethene itself in the water-based Rh-TPPTS system is fast. It is the high solubility of water in the propanal that prevents the application of the aqueous biphasic process [5]. 

The breaking up of azeotropic mixtures Steam distillation. Distillation of a pair of immiscible liquids Distillation with superheated steam. ... 

Some of the oxygenates in the Fischer-Tropsch aqueous product could be recovered by distillation to improve the overall carbon efficiency of the Fischer-Tropsch refinery. However, it was pointed out that complete separation of the short-chain oxygenates was difficult due to the formation of azeotropic mixtures.23... 

If, for example, a mixture of ethanol and water is distilled, the concentration of the alcohol steadily increases until it reaches 96 per cent by mass, when the composition of the vapour equals that of the liquid, and no further enrichment occurs. This mixture is called an azeotrope, and it cannot be separated by straightforward distillation. Such a condition is shown in the y — x curves of Fig. 11.4 where it is seen that the equilibrium curve crosses the diagonal, indicating the existence of an azeotrope. A large number of azeotropic mixtures have been found, some of which are of great industrial importance, such as water-nitric acid, water-hydrochloric acid, and water-alcohols. The problem of non-ideality is discussed in Section 11.2.4 where the determination of the equilibrium data is considered. When the activity coefficient is greater than unity, giving a positive deviation from Raoult s law, the molecules of the components in the system repel each... 

At this time 270 ml. of toluene is added to the mixture and the condenser is changed for distillation. An azeotropic mixture of ethanol, toluene, and water is distilled at 75-78° with the bath at 105-110°. When the temperature begins to drop (Note 1), 525 ml. of commercial absolute ethanol is added and the mixture is again heated under reflux for 12-16 hours (Note 2). Again 270 ml. of toluene is added, and the azeotropic mixture is distilled until the vapor temperature falls to 68°. After the residue is cooled, the system is evacuated to 25-35 mm. and the remaining ethanol and toluene are distilled. 

Another field in which azeotropic distillation is finding application is in the separation of the complex mixtures of organic acids, aldehydes, ketones, and alcohols produced by the Hydrocol process. As petroleum stocks are utilized more and more for the production of chemicals, processing of azeotropic mixtures and the use of azeotropic separations should assume increasingly greater importance. 

Some solvent mixtures can be very difficult and energy intensive to separate because of the closeness of boiling points and the formation of azeotropic mixtures [45]. Azeotropic or extractive distiUation can be used for azeotropic solvent mixtures and solvents which have very low relative volatihties ]43, 45]. Azeotropic and extractive distillation involves the addition of another solvent, known as an entrainer, which will form its own azeotrope with one of the components to be separated ]45]. However, the additional solvent required for azeotropic and extractive distillation can also generate more wastes depending on how easily the entrainer itself can be recycled and reused. 

Separation of the aromatics from each other and from other hydrocarbons by distillation is not economical because of the limited boiling-point differences and the formation of azeotropic mixtures. Instead, extractive or azeotropic distillation and liquid-liquid extraction are applied.234,235 The latter process is by far the most often used technique. The three processes are applied according to the aromatic content of the gasoline source. p-Xylene, the most valuable of the isomeric xylenes, is isolated by freezing (crystallization) or solid adsorption. 

Data of Azeotropes. The choice of azeotropic entrainer for a desired separation is much more restricted than that of solvents for extractive distillation, although many azeotropic data are known. The most extensive compilation is that of Ogorodnikov, Lesteva, and Kogan (Handbook of Azeotropic Mixtures (in Russian), 1971). It contains data of 21,069 systems, of which 1274 are ternary, 60 multicomponent, and the rest binary. Another compilation Handbook of Chemistry and Physics, 60th ed., CRC Press, Boca Raton, FL, 1979) has data of 685 binary and 119 ternary azeotropes. Shorter lists with grouping according to the major substances also are available in Lange s Handbook of Chemistry... 

The anhydrous acetates of the rare earths have recently been prepared. Moeller et al. [355] obtained them for La, Dy, Ho, Er, Yb and Y by the azeotropic distillation of a mixture of hydrated acetates with N,N -dimethylformamide (DMF) and benzene. In the case of Ce, Pr, Nd, Sm, Eu and Gd the same method gave a monosolvated acetate, M(C2Hs02)3 DMF. However, the anhydrous acetates of Ce, Pr, Nd, Sm, Eu and Gd can be prepared [355] by vacuum desolvation of the monosolvated compounds. A direct desolvation of the acetates in vacuum at —150° C was attempted by Witt and Onstott [389] after dissolution of the rare earth oxides in 50 per cent acetic acid, and anhydrous acetates of definite composition were obtained for La, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu and Y. 

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