Batch distillation at atmospheric pressure

In a certain laboratory a considerable quantity (about 10 litres per week) of a benzene-n-heptane mixture is obtained as waste product, and this is to be separated into its components. The distillate is to contain 99 mol% of benzene, the residue not more than 2 mol%. 

At atmospheric pressure the difference in boiling point is 18.3 deg. C. A rough estimate, assuming ideal behaviour of the mixture, indicates that there will be required 

However, as will appear in paragraph 3, the mixture is not ideal and the number of theoretical stages needed will therefore be higher. The following example shows that only a strict calculation yields reliable values and that the difference in boiling point provides no more than a rough approximation. 

As it has been decided, for convenience, to carry out the separation at atmospheric pressure, the optimum pressure for distUlation need not be gone into. 

It should be noted, however, that the boiling point should be corrected to 760 mm (see Figs. 114, 115) and that temperatures should be corrected for emergent stem. 

---

Example 18.8. A batch of crude pentane contains 15 mol percent H-butane and 85 percent n-pentane. If a simple batch distillation at atmospheric pressure is used to remove 90 percent of the butane, how much pentane would be removed What would be the composition of the remaining liquid ... 

---