Azeotropic Distillation

The first commercial application of azeotropic distillation was the use of benzene by Young (Ref. 7) for the azeotropic dehydration of aqueous alcohol, which is still one of the most important applications of this type of operation.

It has been pointed out that this system differs from extractive distillation chiefly in the behavior of the agent. For example, consider the continuous dehydration of ethyl alcohol by the use of benzene as the azeotropic agent, as shown in Fig. 10-12. Tower 1 serves to remove the water from the alcohol, and tower 2 serves to recover the alcohol and benzene. Essentially anhydrous alcohol is produced as bottoms in tower 1, and sufficient plates are used above the feed plate to produce an overhead vapor that will give two liquid layers on condensation. The benzene-alcohol layer is used as reflux for tower 1, and the water layer containing small amounts of alcohol and benzene is stripped to recover these constituents. In such an operation, the agent, benzene, must vary from essentially zero in the still to a relatively high concentration in the tower. Thus there is a wide variation in the solvent concentration in the tower, and some of the approximations made for extractive distillation would lead to serious errors.

The benzene-alcohol-water system can produce an overhead vapor that will give two liquid phases on condensation which makes it possible to by-pass the azeotrope in a manner analogous to that which was shown for partly miscible binary distillations, and the same type of two-tower system is applicable. It should be noted that the system does not produce the ternary azeotrope as the overhead composition, but it is essential that the condensate is two layers. With some azeo-tropic systems the overhead will not give two liquid layers, and some other type of operation must be used to split the overhead, such as extraction or dilution.

In order to illustrate the phenomena involved in azeotropic distillation, an example will be considered first and then the various limiting

Azeotropic Distillation

alcohol

Fig. 10-12. Azeotropic system for the production of absolute ethanol using benzene.

alcohol

Fig. 10-12. Azeotropic system for the production of absolute ethanol using benzene.

conditions will be reviewed. Consider the production of anhydrous ethanol using benzene as the azeotroping agent. In such cases, it is found most economical to concentrate the alcohol by normal distillation to almost the binary azeotrope concentration before it is introduced into the azeotropic system.

Production of Absolute Alcohol by Azeotropic Distillation with Benzene.

For the purposes of this example it is assumed that the feed to the dehydration system contains 89 mol per cent alcohol and 11 mol per cent water A two-tower system will be employed similar to that illustrated in Fig. 10-12, and only a single liquid layer will be refluxed to each tower. Both towers will be designed for an overflow rate below the feed plate of 125 mois per 100 mois of vapor, and the usual simplifying assumptions will be made. The feed to the alcohol tower will be such that Vn — Vm, and it is assumed that any condensation due to the reflux liquids

Alcohol

Alcohol - water azeotrope----

Wafer

Alcohol

Alcohol - water azeotrope----

- *L ¡quid compositions below feed plate

Liquid compositions tower No. 2 V

xWj tower No 2

Ternary Water Ethanol Benzene

- *L ¡quid compositions below feed plate

-feed plate

---Liquid compositions above feed plate

Liquid compositions tower No. 2 V

xWj tower No 2

Fia. 10-13. Diagram for system, ethanol-benzene-water.

kBenzene

Fia. 10-13. Diagram for system, ethanol-benzene-water.

as OA 05 06 x! , *a , mols alcohol xa+*h mol alcohol * mols water

Fio. 10-14. Equilibrium data for system, ethanol-ibenaene-water.

as OA 05 06 x! , *a , mols alcohol xa+*h mol alcohol * mols water

Fio. 10-14. Equilibrium data for system, ethanol-ibenaene-water.

being at a lower temperature than their boiling point is negligible. The from the water tower are to contain not over 0.01 mol per cent alcohol, anhydrous alcohol is to contain not over 0.01 and 0.1 mol per cent and water, respectively. Theoretically, it is not possible to set the exact bottoms concentration because the composition of the refluxes is limited by the solubility relationships; however, it is found that rather wide latitude is possible in selecting the composition of the bottoms product. For this example it is assumed that the bottoms are 99.9, 0.01, and 0.09 mol per cent alcohol, benzene, and water, respectively.

The physical-chemical data for this system are taken from Cook (Ref. 3) and Barbaudy (Ref. 1). The solubility data for 25°C. are given in Table 10-4 and plotted in Fig. 10-13. The vapor-liquid equilibrium data for the system at atmospheric pressure are presented in Figs. 10-14 and 10-15.

It should be noted that equilibrium data available were not so complete or so consistent as would be desired, and that these two figures represent a smoothing, extrapolation, and interpolation of the data.

Solution. Basis: 100 mols of feed.

Alcohol balance,

Over-all balance,

bottoms and the benzene

Water Ethanol Balance

Fig. 10-15. Equilibrium data for system, ethanol-benzene-water.

Fig. 10-15. Equilibrium data for system, ethanol-benzene-water.

Table 10-4. Pairs op Tib-line Coordinates

Alcohol. Benzene Water..

0.225 andO.23 0.025 and 0.655 0.75 and 0.115

0.18 and 0.13 0.015 andO.82 0 805and0.05

0.08 andO.04 0.007 andO.94 0.914andO 02

Tower 1:

02~Vm+ 89.1 - 1257« Vm « 356 Om « 445 Vn » 356 On « 345

The calculations will be started from the still, using a basis of Om = 1.0, Vm « 0.8, and W ® 0.2. The results are given in Table 10-5.

The feed ratio of the key components is = 0.123, and the change from the lower to the upper section should be made at about this ratio. The ratios for plates 20, 21, and 22 are 0.075,0.0935, and 0.114, respectively, from which it would

Table 10-5 (B « Benzene, A = Alcohol, H Water)

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  • christin
    What is the relative volatility for azeotropes?
    3 years ago
  • laila
    How to find azeotropic compositions from reative volaitly?
    2 years ago
  • gary
    Why do commercial distillation use benzene for azeotropes?
    9 months ago

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