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it is therefore possible to proceed by the a method, as in the constant-pressure calculations, without troubling with the pressure variation.

Taj: Acid Fractionation. As another example of such calculations, consider the fractionation of a 35 mol per cent phenol, 15 mol per cent 0-dresol, 30 mol pet cent m-cresol, 15 mol per cent xylenols, and 5 mol per céftt heavier. The overhead is to be 95 mol per cent phenol, and the phenol recovery is to be 90 per cent. The still pressure will be 250 mm. Hg abs., and 4 mm. Hg pressure drop will be allowed per theoretical plate. A reflux ratio O/D equal to 10 will be employed.

The equilibrium data obtained by Rhodes, Wells, and Murray (Ref. 7) for this type of system indicate that Raoult's law is followed, and thus the relative volatilities are independent of the pressure and a function of the temperature only. Thus, the relative-volatility method will be most suitable for estimating the number of theoretical plates.

The result of over-all material balances is given in Fig. 9-8. The ratio of 0-C7 to m-C7 in the distillate was assumed as 30 to 1.

Figure 9-9 gives the volatilities relative to o-cresol as well as the vapor pressure of o-cresol. In the calculations, the temperature is checked occasionally by determining the vapor pressure of o-cresol, P0, on the plate. Since y0ir = P0x0, then

0 x Xax where t is corrected for pressure drop in the column. Below the feed, per mol of bottoms, the mols of vapor are

Fig. 9-9.

mols C6 35 oC7 15 mC7 30 C8 15 R 5

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