Super-fractionation separation stage

The concept of the super-fractionation separation stage in distillation is a key concept in process ojjerations and chemical engineering. Yet I have never seen this idea discussed in any 

The concept of a super-fractionation tray is important when separating products, if either the top or bottom product consists of a multi-component mixture. For example, if we are fractionating only a mixture of propane and isobutane, the super-fractionation concept would not be significant. However, if the overhead product consisted of a mixture of ethane, propane, and isobutane (i.e., C2S, C3S, and C4S) and the overhead condenser was a partial condenser (see Fig. 49.1), then the partial condenser would serve as a super-fractionation stage. The partial condenser would contribute far more to fractionation than any of the other stages in the upper portion of the distillation tower. 

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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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