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The relation between Db and R is shown in Figure 11.36 and the / R dDb is given by area

The relation between Db and R is shown in Figure 11.36 and the / R dDb is given by area

OABC as 96 kmol.

Assuming an average latent heat for the alcohol-water mixtures of 4000 kJ/kmol, the heat to be supplied to provide the reflux, QR is (96 x 4000)/1000 or approximately 380 MJ.

The heat to be supplied to provide the reflux per kmol of product is then (380/71.4) = 5.32 MJ and the total heat is (5.32 + 4.0) = 9.32 MJ/kmol product.

Example 11.13

If the same batch as in Example 11.12 is distilled with a constant reflux ratio of R = 2.1, what will be the heat required and the average composition of the distillate if the distillation is stopped when the composition in the still has fallen to 0.105 mole fraction of ethanol?

Figure 11.36. Graphical integration for Example 11.12

Solution

The initial composition of the top product will be 0.78, as shown in Figure 11.37, and the final composition will be 0.74. Values of xd, xs, xd — xs and of 1/(xd — xs) for various values of xs and a constant reflux ratio are:

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