Reboilers super fractionation stage

If the bottom product consisted of a mixture of isopentane, normal hexane, and normal heptane (i.e., C5S, Ces, and C7S) and the reboiler was a kettle-type reboiler (see Fig. 49.2), then the kettle reboiler would serve as a super-fractionation stage. The kettle reboiler would contribute far more to fractionation than any of the other stages in the lower portion of the distillation tower. 

Figure 49.2 A kettle reboiler serves as a super-fractionation stage.

Side reboilers and intercoolers also function as super-fractionation separation stages. They are just a subcase of once-through thermosyphon reboilers and partial overhead condensers. 

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