If an average value of a is used, then:

In most cases total condensation occurs in the condenser, so that:

yB/n log an xa

and n is the required number of theoretical plates in the column.

It is important to note that, in this derivation, only the relative volatilities of two components have been used. The same relation may be applied to two components of a multicomponent mixture, as is seen in Section 11.7.6.

Example 11.9

For the separation of a mixture of benzene and toluene, considered in Example 11.7, xd = 0.9, xw = 0.1, and x/ = 0.4. If the mean volatility of benzene relative to toluene is 2.4, what is the number of plates required at total reflux?


The number of plates at total reflux is given by:

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