Columns with a Condenser and No Reboiler

Depending on the difference between the average boiling points of the feeds, the column, and mainly its rectifying section, may have either stronger distillative or absorption characteristics. Examples 9.1 and 9.2 apply to these two cases. 

Since this column has a reboiler but no condenser, and if the feeds are held at 

Component Upper Feed Lower Feed Overhead Bottoms 

Since no condenser is used, the colder feed, serving as external reflux, must be fed to the top tray. With no rectifying trays available above this feed, some of the heavy key component in this feed is lost in the overhead. In this example ethane is the heavy key (methane is the light key). Table 9.1 shows that, of the 250 kmol ethane in the upper feed, over 230 mol go overhead. 

The distribution of components in the main feed (stream 2) between the overhead and bottoms is consistent with the desired heavy key/light key separation. Most of the ethane in this stream goes with the bottoms and most of the methane with the overhead. Overall, this column may be considered not highly efficient from the standpoint of recovering ethane in the bottoms product but is very efficient in purifying it from the methane (hence its designation as a demethanizer). 

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