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| The azeotrope separation scheme (Figure 4.10f) is a variation of the ¡backward feed arrangement suitable for azeotrope separation, and is ! illustrated with reference to a process for purifying ethanol (39). : In this case, the azeotrope increases in alcohol content as pressure is reduced. The HP column operates at atmospheric pressure, and produces 92 mol percent ethanol in the overheads, while the LP column operates at 95 mm Hg and produces 95 percent ethanol in the overheads. Note however, that although the multi-effect scheme reduces the energy consumption, the recycle of 92 percent ethanol and the difficulty of carrying out the separation near the azeotrope (where volatilities are low) act to increase energy consumption. In the case of alcohol production, the scheme in Figure 4.10f is considered energy inefficient (39) and is seldom used, because the heat requirements due to recycling and additional refluxing far outweighs the benefits from using multi-effect distillation.

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THEORY In all configurations Figure 4.10 a to f), energy input is made only into the HP column: r~e rest of the energy required fcr the separation is obtained by re-usmg the initial energy input. Ir. configurations 4.10a,b the energy introduced is what is needed to separate roughly only half the feed compared to a conventional column. In configuration 4.10 c - f, the energy introduced is what is required to perform the separation in the H? column, which is only a portion of the total energy required for the overall separation. This can be illustrated with the aid of x-v diagrams (35).

The prime factor that determines the feasibility of using multi-effect distillation is the ratio of the temperature difference between the highest and lowest economically usable temperatures to the temperature difference between the column top and bottom. This ratio usually needs to be well above 2. This is best illustrated by an example.

EXAMPLE 4.2 Is multi-effect distillation feasible for the benzene-toluene system in Example 2.1, assuming -

(i) The split-feed arrangement is to be used.

(ii) A minimum temperature approach of 20°F is to be used across each heat exchanger.

(iii) Cooling water is available at 90°F and steam is available at 15 psig.

(iv) Pressure drops are negligible.

(v) Constant latent heat of vaporization and the same reflux ratio throughout.

Determine -

(i) What is the maximum number of feasible effects?

(ii) What are the column pressures in each effect?

(iii) How does the energy usage of the multi-effect system compare with the single effect system in Example 2.1?

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