In the systems considered previously, the vapour becomes steadily richer in the more volatile component on successive plates. There are two types of mixture where this steady increase in the concentration of the more volatile component, either does not take place, or it takes place so slowly that an uneconomic number of plates is required.
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 other and exert a higher partial pressure than if their behaviour were ideal. This leads to the formation of a "minimum boiling" azeotrope shown in Figure 11.43a. For values of the activity coefficient less than unity, negative deviation from Raoult's law results in a lower partial pressure and the formation of a "maximum boiling" azeotrope, as shown in Figure 11.43b.
The second type of problem occurs where the relative volatility of a binary mixture is very low, in which case continuous distillation of the mixture to give nearly pure products will require high reflux ratios with correspondingly high heat requirements. In addition, it will necessitate a tower of large cross-section containing many trays. An example of the second type of problem is the separation of n-heptane from methyl cyclohexane in which the relative volatility is only 1.08 and a large number of plates is required to achieve separation.
The principle of azeotropic and of extraction distillation lies in the addition of a new substance to the mixture so as to increase the relative volatility of the two key components, and thus make separation relatively easy. Benedict and Rubin(43) have defined these two processes as follows. In azeotropic distillation the substance added forms an azeotrope with one or more of the components in the mixture, and as a result is present on most of the plates of the column in appreciable concentrations. With extractive distillation the substance added is relatively non-volatile compared with the components to be separated, and it is therefore fed continuously near the top of the column. This extractive agent runs down the column as reflux and is present in appreciable concentrations on all the plates. The third component added to the binary mixture is sometimes known as the entrainer or the solvent.
Young(44) , found in 1902, that if benzene is added to the ethanol-water azeotrope, then a ternary azeotrope is formed with a boiling point of 338.0 K, that is less than that of the binary azeotrope, 351.3 K. The industrial production of ethanol from the azeotrope, using this principle, has been described by Guinot and Clark(45) and the general arrangement of the plant is as shown in Figure 11.44. This requires the use of three atmospheric pressure fractionating columns, and a continuous two-phase liquid separator or decanter.
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, to a greater extent than ethanol, a moderately polar molecule, and a virtually pure ethanol product may be obtained. Equilibrium conditions for this system have been discussed by Norman(46) who shows how the number of plates required may be determined.
The first tower in Figure 11.44 gives the ternary azeotrope as an overhead vapour, and nearly pure ethanol as bottom product. The ternary azeotrope is condensed and splits into two liquid phases in the decanter. The benzene-rich phase from the decanter serves as reflux, while the water-ethanol-rich phase passes to two towers, one for benzene recovery and the other for water removal. The azeotropic overheads from these successive towers are returned to appropriate points in the primary tower.
Figure 11.45 shows a composition profile for the azeotropic distillation column in the process shown in Figure 11.44. This is taken from a solution presented by Robinson and Gilliland(1).
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