Lower operating line, same as before. Upper operating line,

122.2ft - 106.8zn+i + 31.5 ft - 0.873xn+i -f 0.2575

The upper operating line is shown on Fig. 10-6. it requires about one less theoretical plate, but the heat requirements would be greater because of the necessity of vaporizing all of the feed.

Fig. 10-7. Concentration of nitric acid.

Case II:

Feed 36 per cent vapor and using 92 per cent H2SOi at top. Basis: 100 mols of feed. In order to maintain the sulfuric acid concentration constant, 72 percent of it will be added at the feed plate.

Lower operating line, same as before but extends up the diagram farther because of the part liquid feed. Upper operating line,

This case requires the same heat and acid consumption as the original example but needs one additional theoretical plate. With the feed partly vaporized it would have been better to separate the liquid and vapor and introduce each at its optimum location. If this change is made, the system reduces to the original system of Fig. 10-4, because the optimum feed-plate composition for the liquid portion of the feed is approximately the same as the top plate composition.

Case III:

Feed 36 per cent vapor and using 85 weight per cent H2SO< at top. Basis: 100 mols of feed. The sulfuric acid will be split as for Case II. For 85 per cent acid,

Mol fraction H2S04 - 0.51 For bottom acid strength, ft 2iU__HjSOi_

and, for same slope of lower operating line,

106.8 (0,49/0.51)H2SQ4 + (100 ~ 32.6) + S 22.2 " S

S «■ 30.2 mols H2S04 - 49.4 mols '17 mols at top, 32.4 mols at feed)

HaSOrfree basis,

Vn - 30.2 + 36 - 66.2 0« - 49.95 Vm - 30.2 Om - 145.1

Lower operating line, same as before but the intersection of the two lines will be at a different position.

Upper operating line,

These operating lines are shown in Fig. 10-6. This case requires fewer plates than Case II but requires more steam. It requires both more plates and steam than the original example. The use of additional steam is objectionable both because of the increased steam consumption and because it must be removed from the sulfuric acid in the concentrator.

The original system is more desirable than any of the three cases. It is instructive to analyze the possibilities of improving the original distillation system. Below the feed plate it would be desirable to reduce the steam consumption as much as possible, but for the 25 mol per cent sulfuric acid, Fig. 10-6 indicates that the ratio of O/V for this section cannot be increased significantly without increasing the difficulty of fractionation excessively. Using this same slope (O/V »4.8), the steam consumption is a H20 (with H2S04) -f 67.4 (from feed)

For the same strength nitric acid feed and the given acid recovery and overhead concentration, the only way to reduce 8 is by reducing the water brought in by the sulfuric acid. Higher strength sulfuric acid would reduce S but increase the difficulty of reconcentrating the acid. A higher mol per cent sulfuric acid in the liquid phase would make it possible to reduce S but would increase the acid recirculation. None of these alternatives for the lower section appears to offer any great advantage over the original system. The concentration change per plate is less below the feed plate than above, and it is desirable to shift to the upper operating line at a low concentration. The original system accomplishes this result by using an all-vapor feed. If all the feed is added as a vapor (see Case I), the fractionation in the enriching section is very easy but requires vaporizing all of the feed. The original system reduces the heat consumption by vaporizing only a portion of the feed and adding the remaining feed as liquid at the top which is approximately the optimum feed-plate location for the liquid feed. This still gives the favorable intersection of the operating lines and reduces the heat required for vaporizing the

Relative Volatility Ethanol Water

0.4 06 mois C2H5 Oh

6 molsC2H50H + molsiC3H7 0H Fig. 10-8. Relative volatility of isopropanol to ethanol in presence of water.

0.4 06 mois C2H5 Oh

6 molsC2H50H + molsiC3H7 0H Fig. 10-8. Relative volatility of isopropanol to ethanol in presence of water.

feed. It makes the separation a little more difficult than using all of the feed as vapor, but it is obvious that the extra theoretical plate required is well justified by the savings of heat. On the basis of Fig. 10-6, it would appear that it might be advantageous to add more of the feed as liquid at the top and thereby reduce the amount to be vaporized. This would necessitate preheating the mixed acid added at the top by an amount equal to the reduced heat input with the vapor feed, but, low-pressure waste steam might be used for this purpose. Such a change would need to be carefully analyzed on an enthalpy basis in order to determine whether a pinched-in condition was being encountered at the top of the column. In the case of the original system, one portion of the feed was vaporized and the other added as liquid at the top. Some improvement would be obtained by vaporizing under equilibrium conditions such that the vapor feed would be more dilute in nitric' acid and the liquid stronger. These two fractions could then be added as in Fig. 10-4, and the operating lines would be more favorable.

In most extractive distillation cases the added component is volatile. The following example will illustrate the application of general equations developed in this chapter when using an extractive agent of appreciable volatility.

Separation of Ethanol and Isopropanol by Extractive Distillation. A mixture containing 20, 4, and 76 mol per cent of water, isopropyl alcohol, and ethyl alcohol, respectively, is to be separated into an ethyl alcohol product containing not over

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