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Pounds of 62 per cent HNOs - 31.8(63) + 68.2(18) - 3,225 Pounds of 92 per cent H2S04 - 1.2(3,225) - 3,870

Mols HaO in bottoms - 3,87o(65a8)Q'35) " 106,6

Mols HaO in feed - 68.2 Mols H20 in overhead - 32.6(0.035) = 1.14 Calculating the steam rate, Sf by over-all water balance,

It is assumed that sufficient nitric acid will be mixed with the sulfuric acid at the top of the tower so that the combined stream contains 60 weight per cent sulfuric aoid.

Let xn = weight fraction of HNO« in top mixture and 0.4 — xn «- weight fraction of H20 in top mixture. H20 balance:

62 per cent UNO, added at top -

- 64(20.35 mols 100 per cent HN08)

Vapor and liquid rates on H2SC>4-free basis:

- It is interesting to note that for S = 22.2 an enthalpy balance gives Vn to the top plate of 53.2 as compared to the 58.2 obtained on the basis of constant 0 and V rates.

The mol fraction of sulfuric acid in the liquid phase is

For vapor-liquid equilibria the curve for a mol fraction of sulfuric acid equal to 0.25 given in Fig. 10-5 will be used. This curve is replotted in Fig. 10-6, and the operating lines for the calculated flow rates are given.

The upper operating line for nitric acid is

58.2yn - 106.8ztt+i + 31.5 - 20.35 - 106.8z»+i + 11.15

The lower operating line is

The plates are stepped* off starting at yr = 0.99 and continuing down to x - 0.003 at y = 0. Approximately eight theoretical plates are required.

A more conventional extractive distillation would be to add all of the feed in the middle region of the tower and to return a portion of the overhead product as reflux with the sulfuric acid. Figure 10-7 shows such a system.

An exact comparison with the system of Fig. 104 can not be made, but several cases will be evaluated using 25.4 mol per cent H2SO* in the liquid phase, and the same (O/V) ratio below the feed plate.

Fig. 10-6. Design diagram for nitric acid concentration.

Case I:

Feed all vapor and using 92 per cent H2SO4 at top. Basis: 100 mols of feed.

Composition of acid mixture refluxed to tower, 3,870 lb. 92 weight per cen H2SO4 -f 89.6 lb. mois 96.5 mol per cent HNOs

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