Binary mixtures with azeotropic entrainer

Columns of extractive and autoextractive distillation are widely used in industry (Benedict Rubin, 1945 Drew, 1979 Happe, Cornell, Eastman, 1946 Hoffman, 1964 Kogan, 1971). The separation process of binary mixtures with azeotrope, having minimum bubble temperature, with the help of heavy entrainer brought into the column higher than the main feed was discovered empirically and started to be used in the 1940s in connection with military needs - in particular, for extraction of butadiene and toluene. 

Fig. 11.3-4 Process for fractionating a binary mixture with minimum azeotrope using a low boiling entrainer e (Stichlmair and Fair 1998)...

The azeotrope in the ethanol-water binary system has a composition of 89 mole per cent of ethanol(14). Starting with a mixture containing a lower proportion of ethanol, it is not possible to obtain a product richer in ethanol than this by normal binary distillation. Near azeotropic conditions exist at points marked in Figure 11.44. The addition of the relatively non-polar benzene entrainer serves to volatilise water, a highly polar molecule,... 

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 choice of an entrainer used to make a desired separation in an azeotropic distillation depends on the binary mixture being separated and the nonidealities of these components with the added entrainer. While several different entrainers might be used to provide a separation, the final selection may depend on the required purity of the product. If several entrainers can produce a product of desired purity, the final choice may depend on an economic evaluation of the several schemes. 

The composition of the ternary azeotrope and that of the feed determine the relative rates of entrainer and feed required to produce a pure product in the bottoms and the azeotropic composition in the overhead. Figure 10.5 shows a typical ternary diagram for ABE at expected process temperature and pressure. Point F represents the fresh feed, which is a binary mixture of A and B. Point Z represents the ternary azeotropic composition. A straight line drawn through E and F represents compositions obtained by mixing fresh feed with variable amounts of entrainer. The combined feed composition should be such that it would separate into pure component A and the ternary azeotrope. Therefore, the combined feed composition should fall on a straight line joining A and Z. The amount of entrainer added to the fresh feed should yield the composition represented by the intersection point F. ... 

It involves adding a further component, an entrainer, which forms an azeotrope with one of the members of an existing mixture and not the others (or the other, in the case of a binary mixture). Usually it is desirable for the entrainer to form an azeotrope with the small component rather than with the majority of the mixture since this, in most cases, reduces the amount of entrainer to be recycled. However, the entrainer s most important property is ease of separation from the solvent it is removing. 

When a homogeneous azeotrope is formed or the mixture is very close boiling, the procedures shown in Section 8.2 cannot be used. However, the engineer can add a solvent (or entrainer) that forms a binary or ternary azeotrope and use this to separate the mixture. The trick is to pick a solvent that forms an azeotrope that is either heterogeneous (then the procedures of Section 8.2 are useful) or easy to separate by other means such as extraction with a water wash. Since there are now three components, it is possible to have one or more binary azeotropes or a ternary azeotrope. The flowsheet depends upon the equilibrium behavior of the system, which can be investigated with distillation curves and residue curves rSection 8.51 A few typical exanples will be illustrated here. 

Kiva, V. N., Timofeev, V. S., Vizhesinghe, A. D. M. C., Chyue Vu Tam (1983). The Separation of Binary Azeotropic Mixtures with a Low-Boiling Entrainer. In The Theses of 5th Distillation Conference in USSR. Severodonezk (Rus.). Knapp, J. R, Doherty, M. F. (1994). Minimum Entrainer Flows for Extractive Distillation A Bifurcation Theoretic Approach. AlChE J., 40,243-68. Koehler, J., Aguirre, R, Blass, E. (1991). Minimum Reflux Calculations for Nonideal Mixtures Using the Reversible Distillation Model. Chem. Eng. Set, 46,3007-21. 

Kiva, V. N., Timofeev, V. S., Vizhesinghe, A. D. M. C., Chyue, Vu Tam (1983). The Separation of Binary Azeotropic Mixtures with a Low-Boiling Entrainer. Theses of 5th Distillation Conference in USSR, Severodonezk (Rus.). 

We next discuss the examples of application of semisharp extractive distillation (Petlyuk Danilov, 2000b) for separation of ternary mixture with two binary azeotropes, examples of application of two levels of pressure, and choice of entrainers that do not form heteroazeotrpes. 

Figure 8.25 shows separation of ternary mixture with two binary azeotropes. The sharp autoextractive distillation at split 2 1,3 is possible in the first column (entrainer - component 1) because at side 1-2 there is trajectory tear-off seg-... 

Figure 8.29. Phase equilibria map and sequence for extractive distillation of a binary azeotropic mixture (1,3) with a recycle of azeotropic entrainer (azeotrope 12) spht 1,2 3 in the first column.

If the bubble point of the entrainer is lower than that of the azeotrope then pure products cannot be maintained at the beginning of the process, but binary mixtures, without any azeotrope, can be produced. The separation schemes for minimum boiling azeotrope with light entrainer and for maximum boiling azeotrope with intermediate entrainer are... 

Distillation is still the most common unit operation to separate liquid mixtures in chemical and petroleum industry because the treatment of large product streams and high purities with a simple process design is possible. Despite of this the separation of azeotropic mixtures into pure components requires complex distillation steps and/or the use of an entrainer. Industrial applied processes are azeotropic, extractive or pressure swing distillation (Stichlmair and Fair, 1998). Another sophisticated method for the separation of binary or multicomponent azeotropic mixtures is the hybrid membrane process, consisting of a distillation column and a membrane unit. 

It is clear that the choice of entrainer is a most important consideration. The added substance should preferably form a low-boiling azeotrope with only one of the constituents of the binary mixture it is desired to separate, preferably the... 

In the first class, azeotropic distillation, the extraneous mass-separating agent is relatively volatile and is known as an entrainer. This entrainer forms either a low-boiling binary azeotrope with one of the keys or, more often, a ternary azeotrope containing both keys. The latter kind of operation is feasible only if condensation of the overhead vapor results in two liquid phases, one of which contains the bulk of one of the key components and the other contains the bulk of the entrainer. A t3q)ical scheme is shown in Fig. 3.10. The mixture (A -I- B) is fed to the column, and relatively pure A is taken from the column bottoms. A ternary azeotrope distilled overhead is condensed and separated into two liquid layers in the decanter. One layer contains a mixture of A -I- entrainer which is returned as reflux. The other layer contains relatively pure B. If the B layer contains a significant amount of entrainer, then this layer may need to be fed to an additional column to separate and recycle the entrainer and produce pure B. 

Nitromethane shows the simplest residue curve map with one unstable curved separatrix dividing the triangle in two basic distillation regions. Methanol and acetonitrile give rise two binary azeotropic mixtures and three distillation regions that are bounded by two unstable curved separatrices. Water shows the most complicated residue curve maps, due to the presence of a ternary azeotrope and a miscibility gap with both the n-hexane and the ethyl acetate component. In all four cases, the heteroazeotrope (binary or ternary) has the lowest boiling temperature of the system. As it can be seen in Table 3, all entrainers except water provide the n-hexane-rich phase Zw as distillate product with a purity better than 0.91. Water is not a desirable entrainer because of the existence of ternary azeotrope whose n-hexane-rich phase has a water purity much lower (0.70). Considering in Table 3 the split... 

The quantity of hexane necessary to entrain water, methanol, ethanol, acetone, and acetaldehyde dimethyl acetal to the top was estimated as the sum of the hexane quantities required to form the binary azeotropes with the quantities of water, methanol, ethanol, acetone, and acetaldehyde dimethyl acetal in the mixture. 

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