j Benzene j Ethanol

Bottom 2 4 6 8 10 12 14 16 18 20 22 24 Top Liquid on equilibrium-stage number (from bottom)

Figure 11.45. Composition profile for the azeotropic distillation of ethanol and water, with benzene as entrainer

11.8.2. Extractive distillation

Extractive distillation is a method of rectification similar in purpose to azeotropic distillation. To a binary mixture which is difficult or impossible to separate by ordinary means, a third component, termed a solvent, is added which alters the relative volatility of the original constituents, thus permitting the separation. The added solvent is, however, of low volatility and is itself not appreciably vaporised in the fractionator. For a non-ideal binary mixture the partial pressure may be expressed as:

where yA and yB are the activity coefficients for the two components. The relative volatility a may thus be written as:

yb pb

The solvent added to the mixture in extractive distillation differentially affects the activities of the two components, and hence the relative volatility, a.

Such a process depends upon the difference in departure from ideality between the solvent and the components of the binary mixture to be separated. In the following example, both toluene and iso-octane separately form non-ideal liquid solutions with phenol, although the extent of the non-ideality with iso-octane is greater than that with toluene. When all three substances are present, therefore, the toluene and iso-octane themselves behave as a non-ideal mixture, and their relative volatility becomes high.

An example of extractive distillation given by Treybal(47) is the separation of toluene, boiling point 384K, from paraffin hydrocarbons of approximately the same molecular weight. This is either very difficult or impossible, owing to low relative volatility or azeotrope formation, yet such a separation is necessary in the recovery of toluene from certain petroleum hydrocarbon mixtures. Using iso-octane of boiling point 372.5 K, as an example of a paraffin hydrocarbon, Figure 11.46a shows that iso-octane in this mixture is the more volatile, although the separation is obviously difficult. In the presence of phenol, boiling point 454.6 K, however, the relative volatility of iso-octane increases, so that, with as much as 83 mole per cent phenol in the liquid, the separation from toluene is relatively

Toluene Octane Mixture
Figure 11.46. Extractive distillation of toluene-iso-octane with phenol

easy. A flowsheet of a process for accomplishing this is shown in Figure 11.46b, where the binary mixture is introduced more or less centrally into the extractive distillation tower (1), and phenol as the solvent is introduced near the top so as to be present in high concentration upon most of the trays in the tower. Under these conditions iso-octane is readily distilled as an overhead product, while toluene and phenol are removed as a residue. Although phenol is relatively high-boiling, its vapour pressure is nevertheless sufficient for some to appear in the overhead product. The solvent-recovery section of the tower, which may be relatively short, serves to separate the phenol from the iso-octane. The residue from the tower must be rectified in the auxiliary tower (2) to separate toluene from the phenol which is recycled, but this is a relatively easy separation. In practice, the paraffin hydrocarbon is a mixture rather than pure iso-octane, although the principle of the operation remains the same.

The solvent to be used is selected on the basis of selectivity, volatility, ease of separation from the top and bottom products, and the cost. The selectivity is most easily assessed by determining the effect on the relative volatility of the two key components of addition of the solvent. The more volatile the solvent, the greater the percentage of solvent in the vapour, and the poorer the separation for a given heat consumption in the boiler. It is important to note that the solvent must not form an azeotrope with any of the components. Some of the problems of selecting the solvent are discussed by Scheibel(48) who points out that use may be made of the fact that, when two compounds show deviations from Raoult's law, then one of these compounds shows the same type of deviation with any member of the homologous series of the other component. Thus the azeotropic mixture acetone (b.p. 329.6 K)-methanol (b.p. 337.9 K) has 20 mole per cent acetone and boils at 328.9 K., that is less than the boiling point of either component. Thus any member of the series ethanol (b.p. 357.5 K), propanol (b.p. 370.4 K), water (b.p. 373.2 K), butanol (b.p. 391.0 K) may be used as an extractive agent, or in the series of ketones, methyl n-propyl ketone (b.p. 375 K) and methyl iso-butyl ketone (b.p. 389.2 K). The advantage of using a solvent from the alcohol series is that the more volatile acetone will be taken overhead, though water would have the advantage of cheapness. Pratt(49) has given details of a method of calculation for extractive distillation, using the system acetonitrile-trichloroethylene-water as an example.

Extractive distillation is usually more desirable than azeotropic distillation since no large quantities of solvent have to be vaporised. In addition, a greater choice of added component is possible since the process is not dependent upon the accident of azeotrope formation. It cannot, however, be conveniently carried out in batch operations.

Azeotropic and extractive-distillation equipment may be designed using the general methods for multicomponent distillation, and detailed discussion is available elsewhere(1,42) and presented by Hoffman(50) and Smith(51).

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