Azeotropic mixture separation entrainer

The extractive distillation profits from the capacity of an entrainer (solvent) to modify selectively the relative volatility of species. Normally, the entrainer is the highest boiler, while the component to be separated becomes heavier, being carried out in bottoms. For this reason, this operation may be regarded as an extractive absorption. Extractive distillation can be used for separating both zeo-tropic and azeotropic mixtures. The entrainer is fed near the top for a zeotropic mixture or a minimum-boiling azeotrope, or mixed with the feed for a maximumboiling azeotrope. The separation sequence normally has two columns, for extraction and solvent recovery [5]. 

On many occasions, general geometric theory of distillation allows development of flowsheets of multicomponent azeotropic mixture separation without using such special methods as distillation under two pressures or heteroazeotropic and extractive distillation with entrainers (i.e., with additional components injected into the unit). 

The feasibility of azeotropic mixture separation and bringing in fight entrainer in vapor phase into the cross-section lower than the main feed was shown along with regular extractive distillation (Kiva et al., 1983). 

In some cases, change of pressure very much influences the location of azeotrope points and sometimes even leads to transformation of azeotropic mixture into zeotropic. That uses change of pressure for separation of azeotropic mixtures without entrainers. The example is separation of mixture acetic add(l)-water(2)-formic acid(3) at two pressures at atmospheric pressure and at 200 nun Hg pressure (Kuschner et al., 1969). 

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. 

Fig. 19. Separation of ethanol and water from an ethanol—water—benzene mixture. Bottoms and are water, B is ethanol, (a) Kubierschky three-column sequence where columns 1, 2, and 3 represent the preconcentration, azeotropic, and entrainer recovery columns, respectively, (b) Material balance lines from the azeotropic and the entrainer recovery columns, A and E, respectively, where represents the overall vapor composition from the azeo-column, 2 1SP Hquid in equiUbrium with overhead vapor composition from the azeo-column, Xj, distillate composition from entrainer...

The choice of the appropriate azeotropic distillation method and the resulting flowsheet for the separation of a particular mixture are strong functions of the separation objective. For example, it may be desirable to recover all constituents of the original feed mixture as pure components, or only some as pure components and some as azeotropic mixtures suitable for recycle. Not every objective may be obtainable by azeotropic distillation for a given mixture and portfolio of candidate entrainers. 

The simplest case of combining T E and LLE is the separation of a binaiy heterogeneous azeotropic mixture. One example is the dehydration of 1-butanol, a self-entraining system, in which butanol (117.7°C) and water form a minimum-boiling heterogeneous azeotrope (93.0°C). As shown in Fig. 13-69, the fresh feed may be added... 

If the azeotrope is not sensitive to changes in pressure, then an entrainer can be added to the distillation to alter in a favorable way the relative volatility of the key components. Before the separation of an azeotropic mixture using an entrainer is considered, the representation of azeotropic distillation in ternary diagrams needs to be introduced. 

If the system forms azeotropes, then the azeotropic mixtures can be separated by exploiting the change in azeotropic composition with pressure, or the introduction of an entrainer or membrane to change the relative volatility in a favorable way. If an entrainer is used, then efficient recycle of the entrainer material is necessary for an acceptable design. In some cases, the formation of two liquid phases can be exploited in heterogeneous azeotropic distillation. 

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. 

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... 

Figure 13.27. Separation of the azeotropic mixture of acetonitrile and water which contains approximately 69 mol % or 79.3 wt % of acetonitrile. (Pratt, Countercurrent Separation Processes, Elsevier, New York, 1967, pp. 194, 497). (a) A dual pressure process with the first column at 100 Torr and the second at 760 Torr. (b) Process employing trichlorethylene as entrainer which carries over the water in a ternary azeotrope that in turn separates into two phases upon condensation.

R. Diissel, J. Stichlmair, Separation of azeotropic mixtures by batch distillation using an entrainer, Comp. Chem. Eng. 19 (1995) si 13— si 18. 

In contrast, when dealing with azeotropic mixtures the feasibility of separation is not guaranteed. Entrainer selection and feasibility is the central problem. The... 

Pervaporation surpasses conventional industrial separation in several respects thus, it makes more efficient use of energy, allows the ready separation of azeotropic mixtures and dehydration of multicomponent mixtures, avoids contamination of the product with entrained compounds and the environmental pollution usually resulting from treatment of entrained substances, uses little space and is easy to implement and install on-site as the pervaporator is skid-mounted [155]. 

Tables 4-1, 4-2, and 4-3 (L.H. Horsley, Anal. Chem. 21, 831, 1949) list some representative azeotropic mixtures. As you can see, an azeotrope is not a rare occurrence. As a chemist, you cannot assume that you have a pure liquid just because it distills at a constant temperature. Recall that an azeotropic distillation involves the formation of an azeotrope with at least one of the components of a liquid mixture which, thereby can be separated more readily because of the resulting increase in the difference between the volatilities of the components of the mixture. The third component added is often called an entrainer (see p. 47).

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