Rectification with an Entrainer

More theoretical stages, and a reflux ratio which increases with time, are needed to separate a liquid mixture by rectification the closer the relative volatility approaches to unity. A simple separation is not possible if Ufj = 1. According to Colburn and ScHOENBORN the number of theoretical stages required is — 1) for a 99% separation. For a y = 1.05 80 theoretical stages are required, while for Uf j = 1.1 under the same operating conditions only 40 are required. 

An entrainer or auxiliary component must be added to mixtures with small dif- 

AC Atmospheric column VC Vacuum column HE Crude-oil heater, directly fired 

Choosing the entrainer to separate a certain mixture is a substantial problem. Properties of mainly polar entrainers (used in azeotropic and extractive rectification) are given in Table 2-3. 

Of the possible interacting forces between the liquid molecules of the mixture, hydrogen bonds are most important. A classification of important groups of substances with respect to the occurrence and strength of the hydrogen bonds has been done by Berg [2.27]. These groups are listed in Table 2-4. 

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