Rectifiers and Reboiled Strippers

A rectifier is a column with no stripping section, that is, the feed is sent to the reboiler or bottom tray. The feed could either be sent directly to the reboiler or, if sufficiently preheated, it could be sent to the bottom tray and no reboiler would be necessary. The purpose of such a column is to produce a distillate product that is enriched in lighter components from which the heavier components are refluxed in a condenser back to the column. Since no stripping section is used, a certain loss of light components in the bottoms should be expected. 

A reboiled stripper is the opposite of a rectifier in that it has no rectifying section. The feed is sent either directly to the condenser or, if sufficiently cold, to the top tray, requiring no condenser. The liquid portion of the feed acts as reflux. The main purpose of this type of column is to produce a bottoms product that is enriched in heavier components and stripped of the lighter ones. With no rectifying trays, a reboiled stripper could result in some heavies being lost in the overhead. 

Note that in reboiled strippers without a condenser or in rectifiers without a reboiler, there is only one degree of freedom or one independent variable. In a rectifier, for instance, varying the condenser duty directly affects both the reflux ratio and the overhead rate. These two variables can no longer be varied independently and, therefore, only one performance specification may be made. 

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The discussion in this chapter centers around multi-component distillation with conventional columns, that is, columns with a single feed and overhead and bottoms products. In general, such columns have a condenser and a reboiler, although the special cases of rectifiers and reboiled strippers are also considered. 

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